Metal penetration induced interfacial challenges and engineering strategies in OSV devices

1 Introduction

1.1 Organic spin valves and interfacial challenges

Organic spintronics has attracted sustained interest in recent years, as it employs electron spin and the materials’ magnetic properties to carry information (Wolf et al., 2001). Among various device concepts, organic spin valves (OSVs) are widely regarded as a representative platform to explore and apply organic spintronics, spanning spin injection, transport, relaxation, and detection across organic semiconductor (OSC) spacers under certain conditions, such as operations in air and at room temperature (Sanvito, 2011; Devkota et al., 2016; Li and Yu, 2021; Zhu et al., 2023; Liu et al., 2025). The OSV devices are typically fabricated into a three-layered structure, where the OSC spacers are sandwiched between two ferromagnetic (FM) layers (Devkota et al., 2016). The two FM electrodes possess different coercive fields, which allow their magnetization directions to alternate between parallel or anti-parallel alignment when subjected to a sweeping external magnetic field (Xiong et al., 2004). With the resistance of electron spin, different alignments can adjust the resistance between the two electrodes (Xu, 2018), leading to the definition of magnetoresistance (MR): MR = (R _AP–R _P)/R _AP, where R _AP is the high-resistance state (antiparallel magnetization), and R _P is the low-resistance state (parallel magnetization) (Zhang et al., 2020).

A device concept closely related to OSV is the magnetic tunnel junction (MTJ), where two ferromagnetic electrodes are separated by an insulating metal oxide spacer layer, giving rise to tunneling magnetoresistance (TMR) (Elahi et al., 2022b). As reviewed by Elahi et al., the interface between the electrodes and the insulator spacers is critical in achieving a higher TMR value (Elahi et al., 2022b); indeed, as pointed out by both Sanvito (Sanvito, 2010) and Cinchetti (Cinchetti et al., 2017), the formation of a well-defined “spinterface” between the electrode and spacer layers is often critical in controlling electron spin injection and thus achieving stable spintronic devices. Despite decades of progress, the performance and reliability of OSVs remain strongly limited by interfacial effects, as noted by numerous previous studies and reviews (Sanvito, 2010; Szulczewski et al., 2011; Cinchetti et al., 2017; Yan et al., 2023; Gu et al., 2024). 2D materials, such as graphene, chromium trihalides (Elahi et al., 2022a), and MoS₂ (Dolui et al., 2014), have been applied in MTJs as spin filters or alternative spacers (Elahi et al., 2024) to enhance the performance and properties of MTJs by establishing accurate layer thickness control, spin filtering, as well as sharp spinterfaces (Elahi et al., 2022b). Analogously, controlled interfacial engineering is also indispensable for improving the robustness and MR performance in OSV devices.

1.2 Metal penetration as a multifactorial problem

Traditionally, OSV devices are fabricated by directly evaporatively depositing ferromagnetic electrode metals, such as Co and Ni₈₀Fe₂₀, onto the OSC spacer layers that are grown or spin-coated on a substrate (Morley et al., 2008). The top ferromagnetic electrodes of modern OSV devices are typically fabricated onto the spacer materials by e-beam evaporation, where a high-voltage cathode-anode system is employed to generate an electron beam. With the help of magnetic fields, the electron beam is then guided to the crucible containing the deposition material and bombards the deposition material, converting kinetic energy into heat to melt and evaporate the deposition metal, whose atoms then fly out and deposit onto the spacer layer. The deposited metal atoms hence possess residual kinetic energy when they collide with and finally create pinholes in the OSC layers, serving as the basis of the emergence of metal penetration.

The spacer materials, traditionally organic semiconductors (OSCs), are intrinsically soft, thus lacking efficient pathways to dissipate this excessive energy from the metal atoms. For some OSCs, such as F₁₆CuPc, polycrystalline morphology with grains (reflected by high peaks) and grain boundaries (reflected by deep valleys) is formed if grown at room temperature (Sun et al., 2016). The deposited metal atoms, motivated by residual kinetic energy, can therefore accumulate and permeate along the grain boundaries, morphological weak points, forming vertical conductive filaments. Once initiated, these metallic filaments provide preferential pathways for further metal diffusion, enabling deeper penetration development. For other OSCs, such as Alq₃, amorphous structures are formed after growing on the substrate, lacking well-defined grain structures and featuring randomly distributed free volumes across the layer material. If the spacer materials are improperly prepared, a high density of pinhole defects may be present before electrode deposition. During top electrode deposition, energetic metal atoms may preferentially permeate into these structural weak points, including grain boundaries, free volumes, and pre-existing pinholes. In these low-density regions, the mechanical softness and intrinsic flexibility facilitate further diffusion and aggregation, resulting in ill-defined interfaces that impede further research (Liu et al., 2025).

As shown in Figure 1, random void connectivity and mechanical softness in amorphous films, grain boundaries in polycrystalline films, energetic metal atoms during deposition, and poor spacer morphology collectively act as driving forces for metal penetration. As a result, regardless of whether in polycrystalline or amorphous OSCs, metal penetration is highly localized and intrinsically facilitated. Even worse, especially in thin-spacer-layer OSV devices, once the deposited metallic filament reaches the bottom electrode, a conductive path is then formed for the electrons, leading to short circuits, which is catastrophic for OSV devices. Consequently, no MR signals can be detected, and the performance of the devices is significantly undermined. Such interfacial failure modes remain a bottleneck in the field of spintronics (Liu et al., 2025).

To address the challenges above, a range of strategies has been explored with the development of spintronics, each targeting one or more of the underlying driving forces for metal penetration. These approaches include interlayer engineering (Hu et al., 2023; Morley et al., 2013; Szulczewski et al., 2011; Yang et al., 2012), alternative selection of spacer materials (Gobbi et al., 2011; Jang and Richter, 2017; Kaushik et al., 2025a; Majumdar et al., 2006), tuning deposition parameters such as rate, temperature, geometry (Hu et al., 2024; Kaushik et al., 2025b; Liu et al., 2023), and transferring pre-fabricated electrodes to avoid direct deposition onto the OSC layer fully (Ding et al., 2018; Luo et al., 2024; Zhang et al., 2025). Additionally, metal-organic frameworks (MOFs) are emerging as novel spacer materials for OSV devices (Song et al., 2020; 2024), which also exhibit potential to suppress metal penetration. While each approach can mitigate metal penetration to varying degrees, they entail distinct trade-offs among interfacial quality, spin-injection efficiency, fabrication complexity, and scalability; practical design principles that balance penetration suppression with device manufacturability remain a subject of future research.

1.3 Scope and organization of this review

Distinct from prior spinterface reviews that broadly study spin transport phenomena or mainly emphasize interfacial electronic and chemical mechanisms, this review concentrates specifically on metal penetration as a device-limiting interfacial failure mode. By centering the experimental observations of metal penetration processes and their suppression, we provide a focused and solution-oriented analysis of systematic engineering approaches, spanning interlayer design, spacer selection, electrode preparation, assisted electrode transfer, high-quality spacer layer preparation, geometry and junction-area engineering. In particular, we highlight the emergence of metal-organic frameworks (MOFs) as new spacer materials. The suppression techniques reviewed are summarized in Figure 2. Finally, we summarize the key design criteria for achieving metal-penetration-suppressed yet spin-injection-compatible spinterfaces, and outline future directions for next-generation interfacial research.

2 Observations of metal penetration in OSV devices

2.1 Characterization techniques of metal penetration

The direct signatures of metal penetration are often revealed by various characterization techniques, such as cross-sectional transmission electron microscopy (TEM) and elemental mapping analysis.

The cross-sectional TEM images, or high-resolution TEM (HRTEM) images, appear to be the most visually intuitive evidence of metal penetration. Simon et al. used electron holography, which was based on cross-sectional TEM, to observe the non-homogeneous diffusion of gold into organic layers with higher resolution, providing another possible use of TEM in studying metal penetration (Simon et al., 2003). In recent years, however, with the improvement of device fabrication techniques, the cross-sectional TEM is often implemented to show the successful suppression of metal penetration when clear and sharp boundaries between the ferromagnetic electrode and the spacer layer are observed (Liu et al., 2023). For example, in the study of D. Li et al., the researchers observed only a Co metal penetration of less than 5 nm into the organic spacer layer, indicating a high-quality FM-organic interface (Li et al., 2022); in the study of Zhang et al., the sharp and smooth interfaces under TEM indicated a high-quality interface (Zhang et al., 2025).

In scientific research, TEM is typically combined with elemental analysis, like energy dispersive X-ray spectroscopy (EDS or EDX) and electron energy loss spectroscopy (EELS). In the field of spintronics, EDS and EELS are also regarded as useful characterization techniques for metal penetration. In 2003, Huang et al. combined HRTEM, EDX, and EELS to analyze interdiffusion effects in thin films and spin-valve devices, which served as a pioneering work in such combined characterization (Huang et al., 2003). In the work of Vinzelberg et al., EELS data were applied to confirm the existence of Co traces in the Alq₃ layer in OSV devices, highlighting metal penetration as a dominant conduction mechanism (Vinzelberg et al., 2008). Until recently, these characterization techniques were still actively used by researchers. For instance, Zhang et al. also used EDS to show that, within the detection limits, the exposed P3HT spacer layer revealed no interdiffusion of the Co element, as very little Co was detected in the P3HT layer (Zhang et al., 2025).

Beyond bulk element analysis, surface elemental examination techniques, such as X-ray photoelectron spectroscopy (XPS), also remain an effective approach for detecting metal penetration. XPS is also able to indicate chemical reactions between the electrode metal and the spacer layer at the interface, which can be a new kind of evidence of metal penetration. Early in 2008, Zhan et al. observed interdiffusion of cobalt atoms into the Alq₃ spacer layer using core-level XPS (Zhan et al., 2008). In their study, a sample where Co was deposited onto a 15 nm thick Alq₃ overlayer and then inverted was constructed, and the surface of the Alq₃ layer underwent an XPS examination. Theoretically, for such an overlayer, the Co signal should be nearly undetectable, but the actual Co 2p peak appeared to retain 20% of the intensity of a bare Co surface in this inverted sample, suggesting that the Co atoms were not buried 15 nm deep but are distributed closer to the surface (Zhan et al., 2008). Then, in 2009, they applied XPS again to examine the effect of Al₂O₃ interlayer on the spinterface and observed a clear interaction between Co and Alq₃ at the interface, indicating Co penetration into the Alq₃ layer (Zhan et al., 2009). In the work of F. Li et al., in 2015, the researchers discovered that in their devices, the intensity of Co (2p)_3/2 decayed with the thickness of the spacer layer, indicating Co penetration into the spacer layer (Li et al., 2015). Recently, D. Li et al. also utilized XPS to verify the polymer-Co interface, showing interfacial reaction at the PIID-TVTCN-Co interface but suppressed Co penetration at the PIID-TVTCN-AlO_*x*-Co interface, proving the function of the inserted AlO_*x* layer (Li et al., 2022).

Recently, direct structural probes such as combined X-ray reflectometry (XRR) and grazing incidence X-ray standing wave (GIXSW) have also revealed metal penetration into organic spacers, reaching several tens of nanometers in depth. For example, in the study of Mondal et al., the combined XRR and GIXSW studies exhibited the depth of diffusion of Co ranging from 160 Å to 300 Å, depending on the deposition rate (Mondal et al., 2021). Zhang et al. employed XRR to confirm the layer thickness data with TEM observations, showing consistent layer thickness and thus implying high-quality fabrication and achieving reliable device performances (Zhang et al., 2025).

Figure 3 presents some representative characterization techniques and results, and Table 1 summarizes the characterization techniques commonly used in the studies of spinterface and typical signatures of metal penetration. Taken together, these techniques characterize metal penetration at different scales and sensitivities; therefore, reliable identification of metal penetration requires the combination of multiple characterization approaches rather than reliance on a single technique.

TABLE 1. Summary of characterization techniques of metal penetration in spintronics.

2.2 Experimental phenomena and detrimental effects of metal penetration

As metal penetration can cause severe damage to the performance of the OSV devices, a number of experimental phenomena also manifest the occurrence of metal penetration. Early in 2004, Xiong et al. observed a lack of magnetic resonance and electroluminescence in devices with thin spacer layers, and then defined the formation of an “ill-defined” layer caused by pinholes and Co penetration (Xiong et al., 2004). Such a layer reduces the effective thickness of the devices’ spacer layer, and in catastrophic situations, leading to short-circuiting and a complete erasure of the MR signal (Zhu et al., 2023).

In fact, the detrimental influence of the “ill-defined” layer on the MR signal can be quantitatively analyzed by the modified Jullière equation (Sun et al., 2021):

M R = Δ R R = 2 P 1 P 2 e − d − d 0 / λ s 1 + P 1 P 2 e − d − d 0 / λ s

In which P ₁ and P ₂ represent the injected and detected effective spin polarization, respectively; d is the thickness of the spacer, λ _*S* is the spin diffusion length in the spacer layer, and d ₀ is the “dead layers” that reduce effective spin polarizations at the interfaces. The spin diffusion length is not changed as an intrinsic property of the spacer materials. When metal atoms permeate into the spacer layer, not only will the ill-defined layer have the thickness of the dead layer d ₀ increased, but the effective spin polarizations are also negatively impacted, resulting in an overall small MR signal. In addition, fitting the data of MR versus d according to this modified Jullière equation and then obtaining the value of d ₀ is also a useful technique in examining the extent of metal penetration.

3 Early strategies for suppressing metal penetration

3.1 The addition of interlayers

In a number of previous studies on OSV devices, interlayers are added between the OSC spacer and the top ferromagnetic electrode. While the major purpose of introducing the interlayers is to facilitate polarized spin injection of the electrons (Santos et al., 2007), the interlayers can also mitigate metal penetration by interrupting localized diffusion pathways at the interfaces, rather than by fundamentally modifying the mechanical properties of the spacer layers themselves.

Among various interlayer materials, AlO_*x* has been extensively employed in OSV devices (Santos et al., 2007; Shim et al., 2008; Sun et al., 2013; 2017; Li et al., 2022; Qin et al., 2023). Depending on their thickness and stoichiometry, AlO_*x* can be categorized into two types: leaky and non-leaky (Sun et al., 2013). Intuitively, such an interlayer deposited on top of the organic spacer is expected to physically obstruct the top ferromagnetic atoms with residual kinetic energy during electrode evaporation. However, it has been shown that the AlO_*x* layers themselves induce metallic filamentation pervasively across diverse organic layers (Cölle et al., 2006), particularly owing to an applied bias voltage (Yang et al., 2012). Once formed under a biased voltage beyond a certain limitation, such penetration can lead to instability in the devices’ resistance, eventually percolating through the organic spacer and reaching the bottom ferromagnetic electrode, severely undermining the MR performance. Nevertheless, as recently reported by D. Li et al., under carefully optimized deposition conditions, AlO_*x* interlayers are able to partially suppress metal penetration (Li et al., 2022). They have demonstrated that the insertion of an ultrathin AlO_*x* layer between the cobalt electrode and a 40 nm thick polymer spacer prevented cobalt penetration during low-rate evaporation, as evidenced by cross-sectional TEM, leading to stabilized magnetoresistance behavior. However, they also discovered negative MR in thin spacer films (25 nm), indicating a smaller transport channel led by conductive filaments and pinholes (Li et al., 2022).

Another type of interlayer is the ultrathin buffer layers, epitomized by inorganic compounds like LiF, which are used mainly to lower the energy barrier of electron injection between the organic spacer and the electrode. Apart from this function, LiF interlayer has been shown to be effective in blocking the local diffusion of Co into the Alq₃ organic spacer film and the reaction involving Co at the interface via PNR and XAS measurements (Szulczewski et al., 2011). However, in terms of MR performance, LiF was pointed out to diminish MR due to the paramagnetic defects in the LiF tunnel barriers, which can degrade spin polarization (Szulczewski et al., 2011). Morley et al. also observed similar phenomena in their study, where the LiF layer restrained the penetration of Co into Alq₃, but no MR was measured in the devices (Morley et al., 2013).

Nonetheless, recent representative research featuring the LiF buffer layer was reported by Hu et al., in 2023 (Hu et al., 2023). In this study, the LiF coverage was not only found to help achieve a reversed magnetoresistance at room temperature but also discovered to form bonds with the Co electrode, as confirmed by XPS spectra and electron localization function (ELF) maps (Hu et al., 2023). This phenomenon of the affinity between Co and LiF indirectly indicates the possibility that the LiF buffer layers are not able to suppress the Co atoms from penetrating through.

Self-assembled monolayers (SAMs) have also been employed as interlayers to regulate the surface chemistry of ferromagnetic electrodes. As reviewed by Jang and Richter, the hydrophobicity of the SAMs is subject to change by altering the molecular head groups, thereby strongly influencing the growth mode of the deposited organic layers (Jang and Richter, 2017). Such a property also shows the potential of restraining metal penetration into the organic layers. In particular, as revealed by X-ray absorption spectroscopy (XAS), the Alq₃/MHA/Co sample exhibited the largest [Al]:[Co] ratio, indicating the smallest amount of Co penetration in the Alq₃ spacer layer. This phenomenon can be attributed to the hydrophilic nature of MHA, as MHA’s wetting property enables the Alq₃ layer to grow smoothly and uniformly on the Co substrate (Jang and Richter, 2017), suppressing the entrance of Co into the spacer.

To conclude, the interlayer-based strategies are useful in several contexts but still may not be the optimal choice for achieving both metal penetration control and excellent magnetoresistance performance in OSV devices under certain circumstances.

3.2 Polymer and single-crystalline spacers

Beyond the addition of interlayers, alternative choices and adjustments of the organic spacers in spin-valve devices have been explored as another intuitive strategy to suppress metal penetration. This strategy mainly focuses on addressing the problems brought by the properties of amorphous or polycrystalline spacer materials.

Early experimental evidence has already hinted that the metal penetration can be potentially mitigated by substituting small-molecule spacers with large-molecule polymeric spacers. Xiong et al. constructed devices in which Co penetration during evaporation went up to 100 nm in the Alq₃ film (Xiong et al., 2004), rendering small-molecule spacers Alq₃ inadequate in suppressing metal penetration. Later, Majumdar et al. systematically compared the behavior of Alq₃ and conjugated polymer poly(3-hexylthiophene) (P3HT) spacers (Majumdar et al., 2006). In their study, polymeric spin-valves (PSV) were fabricated with P3HT as the spacer layer. Compared with Alq₃-based devices, P3HT-based devices exhibited enhanced tolerance against metal penetration, as the devices were not shorted even with the thickness of the spacer reaching 80 nm (Majumdar et al., 2006). However, the researchers still identified the formation of Co-doped “dirty layers” within the organic spacer, which introduced additional spin scattering and limited spin transport (Majumdar et al., 2006).

In recent years, fullerene-based spacers have provided another representative example of attempts to mitigate metal penetration-induced failure. In C₆₀-based spin-valves, Gobbi et al. demonstrated room-temperature MR with molecular spacer thicknesses up to tens of nanometers, indicating the absence of catastrophic metallic short circuits (Gobbi et al., 2011). The well-defined packing and chemical stability of C₆₀ enable it to tolerate metal penetration to some extent. However, the disadvantage of C₆₀ lies in its mechanical softness, as in the recent study of Kaushik, Priyadarsini, et al., deep penetration of the ferromagnetic atoms was still observed inside the C₆₀ film (Kaushik et al., 2025a).

Additionally, as a great substitution for polycrystalline OSC spacer materials with extensive grain boundaries, single-crystalline OSCs have entered the vision of spintronic scientists. As reviewed by Jang and Richter, single-crystalline OSCs, such as rubrenes, are not only able to offer a cleaner spin transport medium, which is advantageous for reducing spin scattering, but also capable of preventing metals from penetrating the OSCs (Jang and Richter, 2017), serving as a potential substitution for polycrystalline materials. Despite these advantages, rubrene single-crystalline OSC spacers often suffer from inefficient spin injection at the FM/OSC interface, leading to a shortened spin diffusion length (Jang and Richter, 2017). Also, it has been reported that rubrene single crystals are prone to oxidation, inducing a stricter usage environment for OSV devices (Bisri et al., 2010). Therefore, the practical direct implementation of single-crystalline spacer materials is still subject to further development and research.

Taken together, polymeric spacers represent a conceptual advance over interlayer-only strategies by addressing bulk metal penetration. However, their intrinsic softness or potential harm to spin injection still render them non-ideal for application in OSV devices.

3.3 Process-level control

Beyond material selection, process-level optimization of OSV device fabrication has been explored as a practical approach to mitigate metal penetration as well. In the following sections, this review discusses the optimization of three key fabrication-related steps in OSV devices. Better electrode preparation and innovative electrode transfer aim to suppress metal penetration associated with energetic deposition, whereas higher-quality spacer preparation intends to prevent polycrystalline growth as well as improve the spacer surface morphology. Conversely, geometry and junction-area engineering primarily address the statistical and device-level consequences of metal penetration, rather than its microscopic origins.

3.3.1 Electrode preparation

By tuning evaporation parameters such as deposition rate and substrate temperature, thus adjusting the kinetic energy of incoming metal atoms, the probability of fatal metal penetration can be reduced. A representative example is provided by the research of Majumdar et al., where two sets of OSV devices underwent different Co evaporation rates (Majumdar et al., 2009). Merely 20–30 nm of metal penetration was found in the set with a deposition rate of 0.2–0.5 Å/s, resulting in larger effective thicknesses, while no accurate boundary was found between metal electrodes and spacers in the set with a deposition rate of 1.5–2 Å/s (Majumdar et al., 2009). Mondal et al. also provided a typical study of deposition rate and metal penetration (Mondal et al., 2021). More recently, Liu et al. have also achieved interfaces with better quality using a low deposition rate (Liu et al., 2023). In their work, the OSV devices were fabricated based on acid-exfoliated two-dimensional covalent organic framework (COF) thin films using vacuum thermal evaporation of Co top electrodes (Liu et al., 2023). By deliberately employing a low deposition rate, they successfully prevented the penetration of energetic metal atoms into the COF spacer, as the cross-sectional TEM image and the elemental mapping confirmed that there existed only a slight penetration of Co atoms into the COF layer (Liu et al., 2023). Under these conditions, the devices exhibited large negative MR values. Another work with the control of deposition rate was conducted by Hu et al. (2023) In their devices employing PC₇₁BM as the molecular spacer, the first 2 nm of the Ni₈₀Fe₂₀ ferromagnetic electrode were deposited at the rate of 0.1 Å/s, and the remaining 10 nm were deposited at 1 Å/s (Hu et al., 2023). Additionally, the deposition was further conducted in a low-temperature environment; the integration of these two methods is referred to as the low-temperature rate control fabrication technique (Hu et al., 2023). As uncovered by the cross-sectional TEM image, the Ni₈₀Fe₂₀/PC₇₁BM interface was clear and sharp, implying high quality of the devices. These two works demonstrate that controlling evaporation speed and temperature can significantly suppress metal penetration and the catastrophic consequences that come after.

Besides the control of deposition rate and temperature, the optimization of the metal deposition geometry appears to be another effective way of limiting metal penetration in OSV devices. For instance, glancing angle deposition (GLAD) has been implemented to induce columnar growth and self-shadowing effects, thereby controlling the metal atom effective penetration along the depth of the spacer layer owing to the inclined projection (Kaushik et al., 2025b). In the study of Kaushik, Raj, et al., GLAD was observed to control the penetration of deposited Co₂FeAl into the C₆₀ molecular layer, which presents a new pathway for developing high-performance organic spintronic devices by tailoring deposition geometry to suppress the metal electrode/spacer interface penetration.

In order to fundamentally avoid penetration associated with electrode evaporation, alternative preparation strategies have been developed, including non-evaporative deposition of magnetic electrodes. Li et al. prepared the ferromagnetic electrodes of their devices, V[TCNE]_*x* (x ≈ 2, TCNE: tetracyanoethylene), using chemical vapor deposition (CVD) at low temperature (≈40 °C) instead of thermal deposition (Li et al., 2011). In this way, the formation of ballistic channels due to pinholes was precluded, and thus the metal penetration, resulting in spin-valve effects observed in most of their fabricated devices (Li et al., 2011).

3.3.2 Electrode transfer

Given the disadvantages of directly depositing ferromagnetic electrodes onto the devices, numerous recent studies are exploring electrode transfer techniques as an alternative strategy for constructing OSV devices. In this approach, ferromagnetic electrodes are prepared separately on inert substrates and subsequently laminated onto the OSC spacer layers, thereby avoiding the residual kinetic energy caused by evaporative deposition.

In 2018, Ding et al. proposed a reliable water-auxiliary mechanical transfer strategy for the top electrode that could achieve a damage-free interface and be subject to generalization (Ding et al., 2018). They fabricated a self-encapsulated metal stack on a sacrificial PSS layer, which was subsequently released in water and laminated onto the organic P3HT layer using polydimethylsiloxane (PDMS) hollow molds as transfer holders; then, the device was heated to 70 °C for 10 min to achieve a tighter combination. As proven by the excellent MR performance and great reproducibility, the top electrode transfer technique can suppress metal penetration effectively. In 2024, Guo et al. proposed a strain-restricted transfer of ferromagnetic electrodes for high-quality OSV device fabrications (Guo et al., 2024), in which the ferromagnetic electrodes are transferred with the support of a PS film. By fitting the MR signal values according to the modified Jullière equation, the dead layer thickness at the interface was only 0.01 nm, indicating almost no metal penetration (Guo et al., 2024), offering another successful example of suppressing metal penetration using polymer-assisted mechanical transfer. The study of Zhang et al. also exemplifies such an electrode-transfer technique (Zhang et al., 2025). Similar to the study of Ding et al., in their work, the researchers also employed a PDMS-assisted mechanical transfer method to assemble AlO_*x*/Co/Au top electrodes onto P3HT spacer layers (Zhang et al., 2025). In particular, the previously fabricated PSS/AlO_*x*/Co/Au/PSS self-encapsulated structure was carefully laminated onto the spin-coated P3HT spacer layer using the PDMS transfer stamp (Zhang et al., 2025). The cross-sectional TEM images and elemental mapping confirmed sharp interfaces between Co and P3HT, demonstrating a successful suppression of metal penetration.

Besides polymer-assisted mechanical transfer, van der Waals magnetic electrode transfer, which is solvent-free, thus contamination-free and nondestructive, has also been developed to avoid metal penetration caused by evaporative deposition, as shown in the work of Luo et al. (2024). In their work, the ferromagnetic electrode comprised multilayer graphene (MLG), FM metal, and Au, which were first deposited in sequence onto a sacrificial substrate of SiO₂/Si; then, as the adhesion force between the MLG and the substrate is weaker than the van der Waals interaction between the MLG and the targeted 2D spacer material, the FM electrode could be easily peeled off from the substrate and closely attached to the spacer layer (Luo et al., 2024). The high-resolution TEM image revealed a clean electrode/spacer interface.

Admittedly, various electrode transfer techniques can result in effective metal penetration suppression that is sufficient for the formation of high-quality interfaces and spin injection. However, compared to direct deposition, the multi-step nature of such techniques poses challenges in terms of reproducibility and scalability in industrial production.

3.3.3 Preparation of OSC spacers

When growing on the substrates, the OSC spacers are prone to becoming polycrystalline, where grain boundaries serve as potential penetration pathways for the metal atoms, facilitated by the intrinsic softness of the OSC materials. Therefore, another way of mitigating the penetration, focusing on the preparation of OSC spacers themselves, was proposed by Sun et al. (2016). In their study, active morphology control was applied by employing a low-temperature growth technique enabled by liquid nitrogen (LN). The organic spacer layer F₁₆CuPc could be transitioned from a rough polycrystalline state into an atomically smooth amorphous phase (Sun et al., 2016), thereby providing a physical barrier to suppress metal penetration. As a result, the OSV devices exhibited more desirable rectangular MR signals.

More recently, Mondal et al. presented in their study the influence of the spacer deposition rate on the spacer quality and thus electrode penetration depth (Mondal et al., 2021). With the deposition rates of 5.0 Å/s, 2.5 Å/s, and 1.0 Å/s of the Alq₃ spacer layer, the GISAXS calculation showed that the average distance between the pores of the film increased from 500 Å to 120 Å, while the pore depth decreased from 300 Å to 120 Å. As a result, the depth of Co penetration reduced from 300 Å to 160 Å, suggesting that the metal penetration can be significantly suppressed with lower rates of deposition and the high quality of the spacer film that ensues (Mondal et al., 2021). Nevertheless, lowered deposition rates inevitably induce lowered fabrication efficiency, which is undesirable for large-scale production of proper OSV devices.

3.3.4 Geometry and junction-area engineering

Another engineering approach to mitigate metal penetration relies on reducing the effective junction area. In particular, researchers used nano-indentation lithography to fabricate magnetic tunneling junctions (MTJs) with junction areas on the order of tens of square nanometers, significantly lowering the probability of penetration-induced short circuits (Ding et al., 2021). Barraud et al.’s MTJ devices even achieved an MR of 300% with the absence of metal penetration and short circuit (Barraud et al., 2010). Although these studies were conducted in MTJ systems rather than OSVs, the underlying principle of lowering catastrophic filament formation probability is applicable to OSV devices. However, this technique poses a high requirement on precise nano-etching, while inherently low reproducibility limits its applicability to large-scale OSV device fabrications (Ding et al., 2021).

Figure 4 is a summary of the previously discussed strategies aligned to the driving forces. Among these techniques, the choice of polymeric spacer materials appears to be the weakest solution for metal penetration, because although the no necessity for assembling interlayers or multiple steps makes it simple and thus appealing for large-scale fabrication, the intrinsic mechanical softness, porous nature, and potential reactive sites of polymeric materials render them incapable of blocking metal penetration effectively. Interlayers such as leaky/non-leaky AlO_*x* and buffer layers manage to restrain metal penetration to some extent, but the desired spin injection from the electrodes to the spacer layers remains complicated to adjust and maintain. The adjustment of deposition parameters like speed and temperature is also of great potential, but the root cause of metal penetration–the residual kinetic energy ensuing evaporative deposition–is not eliminated, thus the risk of metal penetration still exists, although not catastrophic or fatal for the spin-valve devices. On the contrary, the assisted transfer of top ferromagnetic electrodes presents to be the strongest solution, in that completely no direct deposition eradicates the possibility of any residual kinetic energy, leading to clean and high-quality spinterfaces and thus excellent MR performances. The disadvantages of such techniques lie in the multi-step nature, which is detrimental to device reproducibility and large-scale production, as well as potential contamination brought by additional solutions involved. The proper preparation of high-quality spacer layers, either by LN cooling or lowering deposition rates, also presents great potential, while not being industrially favorable as well. An ideal strategy for inhibiting metal penetration in the OSV devices should simultaneously combine effective blockage of energetic metal atoms, well-defined and tunable spinterfaces, and compatibility with simple and scalable device fabrication. In particular, as reviewed in the following sections, emerging porous crystalline frameworks provide a promising material for spacers and a platform to engineer metal/organic interfaces with enhanced resistance to penetration while preserving efficient spin injection.

4 MOFs as emerging spacers for metal penetration control

Metal-organic frameworks (MOFs) are a material class developed more than 3 decades ago, but continue to attract significant attention from researchers nowadays (Gobbi et al., 2011; Santos et al., 2007; Shim et al., 2008; Sun et al., 2017; Qin et al., 2023). These open crystalline frameworks, created by combining metal-containing units with organic ligands using strong chemical bonds (Furukawa et al., 2013), feature excellent crystalline order, rigid coordination networks, and chemically modifiable pore surfaces, and thus serve as a promising candidate for the spacers of the spin-valve devices.

4.1 Experimental demonstrations of MOF-based spacers

To date, several experimental studies have incorporated MOFs as spacer materials in OSV devices (Yan et al., 2023). However, the improvements in MR performance and interfacial stability have been primarily demonstrated in proof-of-concept devices under controlled laboratory conditions. In this section, representative experimental demonstrations are reviewed with emphasis on device fabrications, penetration-related evidence, and the resulting MR performances.

Song et al. first introduced MOFs into OSV devices in 2020, in which MOFs led to enhanced device stability and improved reproducibility (Song et al., 2020). Due to the insolubility of the extended 2D skeleton, the conventional techniques of applying organic spacers onto the bottom substrates, such as spin-coating, were not realistic for MOFs. Therefore, Song et al. employed a layer-by-layer (LBL) assembly approach to grow Cu₃(HHTP)₂ (HHTP: 2,3,6,7,10,11-hexahydroxytriphenylene) films on La_0.7Sr_0.3MnO₃ (LSMO) substrates, utilizing a 3-aminopropyltrimethoxysilane (3-APTMS) self-assembled monolayer to facilitate and optimize the uniformity of the MOF deposition (Song et al., 2020). The Cu₃(HHTP)₂ framework here exhibited remarkable thickness adaptability, as distinct MR effects were consistently reported with the MOF thickness varied across a wide range of 30–100 nm (Song et al., 2020), which indicates that the penetration depth of thermally evaporated cobalt atoms was effectively limited by the MOF structure, preventing catastrophic short circuits even in thin spacers. Furthermore, the implementation of the 100 nm-thick MOF spacer achieved a high device reproducibility of over 70%, with MR values reaching approximately 25% at 10 K (Song et al., 2020). This implies that although some degree of Co penetration was observed under cross-sectional TEM and XPS measurements, the MOF spacers were able to suppress the occurrence of metal filament percolation in most devices, even without additional buffer layers.

In their subsequent work (Song et al., 2024), the researchers further demonstrated that MOFs can not only inhibit metal penetration but also reshape it into a functional interfacial feature. The PcM-Cu MOF thin films were fabricated via a programmed LBL assembly in ethanol solutions, where paddle-wheel Cu(II) nodes and PcM – (OH)₈ ligands were deposited in automated cycles. To form the vertical OSV structure, these frameworks were grown in situ on a patterned LSMO substrate, providing a stable platform for the subsequent thermal evaporation (Song et al., 2024). Here, the 2D c-MOF spacer was found to act as a robust reinforcement layer to systematically suppress metal penetration under cross-sectional TEM images, where the boundary between the Co electrode and the MOF spacer was relatively clear and definite. The XPS depth profiling also revealed that the penetrated Co atoms were chemically localized at the interface. The coordination between these penetrated Co metal atoms and the ligand structure results in a strong hybridization (Mugarza et al., 2011), while, as revealed by previous studies, integrating cobalt into the MOF layers can form an antiferromagnetic (AFM) layer (Kurmoo, 2009). The coupling between the metal-ligand hybridization and the AFM layer further combines to achieve a pronounced exchange bias effect, which enhances the coercivity of the upper Co layer and facilitates a well-defined antiparallel alignment over a broad region, thereby benefiting the magnetoresistance performance of the OSV devices (Song et al., 2024). Under an applied current of 0.1 μA, the resulting OSV devices showed a remarkable MR value of −19%, which remained stable below 50 K, and such negative MR effects were also reported in devices with different MOF thicknesses (Song et al., 2024).

Figure 5 demonstrates the process and results of Song et al.’s studies. Collectively, Song et al.’s experiments confirm that MOFs provide a promising engineering solution to the interfacial metal failure bottleneck, not by eliminating metal penetration, but by transforming the unintended metal penetration into a controllable and, in some cases, even functionally beneficial hybrid interface.

4.2 Proposed mechanisms and structural advantages of MOFs for diffusion blocking

Apart from all the experimental studies, the underlying mechanisms remain an active area of investigation. In this review, based on the structural, physical, and chemical features of MOFs, three key mechanisms contributing to metal diffusion blocking are hypothesized, ranging from geometric regulation, physical suppression, and chemical localization. These potential capabilities suggest that MOFs are not merely passive spacers, but can function as active penetration barriers, rendering them valuable for potential applications in OSV devices.

4.2.1 Geometric regulation of metal penetration

Conventional OSCs implemented as spacer or buffer layer materials, such as Alq₃ and SME1, are typically amorphous (Hu et al., 2024; Sun et al., 2013), which contain a broad and poorly defined distribution of free volume that facilitates the penetration of guest molecules (Budd et al., 2005). Under the context of metal deposition in spin-valve device fabrication, during which metal atoms are deposited onto the surface of the spacer with residual kinetic energy, such voids can randomly connect into continuous penetration pathways for their deep diffusion and eventually induce interfacial failure.

The MOFs, by contrast, exhibit a different geometry. MOFs are pure crystalline frameworks constructed by interconnecting metal-containing secondary building units through rigid, specific geometrical shapes such as squares and octahedra (Furukawa et al., 2013), thereby proving that MOFs have long-range crystalline order with periodic pore networks. Additionally, MOFs like Cu₃(HHTP)₂, which have been used in previous spin-valve devices, possess extended 2D sheets stacked along the crystallographic c direction (De Gonzalez-Juarez et al., 2022). The GIXRD patterns characterizing the geometric features of the MOF spacers confirm that on a spin-valve device, the MOF film on the ferromagnetic electrodes presents a preferred face-on orientation, in which 2D honeycomb layers stack perpendicularly to the substrate in a slipped-parallel AB configuration (Song et al., 2020). It’s worth noting, however, that the vertically geometrically connected channels of the periodic pores should not be interpreted as more convenient pathways for the metal atoms to facilely penetrate through the MOF spacer. Unlike the free volume in amorphous films, the pores in MOFs are constructed by layers of rigid coordination frameworks. From a physical perspective, such an ordered internal structure of MOFs is expected to disrupt the connectivity necessary for the deep and percolative penetration of the metal atoms. Consequently, when metal atoms attempt to penetrate vertically through these voids, they are forced to repetitively conquer pore apertures defined by the coordination lattice and unique chemical environment of MOFs, the latter of which will be discussed in subsequent sections. Up till now, direct experimental evidence on metal penetration with MOF pore networks under evaporative deposition remains limited; future in situ studies are still required to fully resolve this process.

Nevertheless, the geometry alone only constitutes the first level of metal interdiffusion control. The robust skeleton of MOFs can effectively restrain other possibilities of forming alternative penetration pathways, leading to a discussion of physical suppression and percolative diffusion behavior in the following section.

4.2.2 Physical suppression of percolative diffusion

Catastrophic metal penetration in the spin-valve devices arises from the formation of percolating metallic pathways that electrically bridge the two ferromagnetic electrodes, causing short-circuit-dominated charge transport, eliminating the chance of discovering MR.

In conventional amorphous or polycrystalline OSCs, mechanical softness or grain boundaries permit deposition-induced structural rearrangement, enabling random voids to coalesce into percolative metal pathways under deposition. In contrast, MOFs are endowed with rigid lattice structures and sturdy coordination networks, which enable them to maintain the ordered porous layered skeleton even when the metal atoms are resisted by the repetitive voids. As revealed by previous research, MOFs are composed primarily of strong chemical bonds (Furukawa et al., 2013), and, in particular, robust metal-ligand interactions that contribute significantly to MOFs’ stability (Denawi, 2026). Through bottom-up synthetic approaches, a large degree of structural control and compositional modularity can be achieved (Ko et al., 2018). While the geometric hindrance enabled by the periodically stacked porous layers suppresses interdiffusion, such excellent mechanical robustness impedes large-scale structural rearrangement, thereby inhibiting large-scale structural rearrangement that would enable alternative percolation pathways. As a result, the rigid framework of MOFs limits percolative diffusion, hence diminishing deposition-induced damages.

Notably, the metal filament suppression through MOFs’ physically rigid frameworks has been reported in other research fields. For example, in lithium metal batteries, modified MOFs, such as NH₂-MOF, have been shown to mitigate Li dendrite growth by significantly reinforcing the mechanical strength of the composite electrolytes owing to MOFs-enhanced tensile strength (Li et al., 2024). These studies offer analogous evidence that MOFs can effectively suppress percolative filament growth, further providing mechanistically relevant evidence in their applications in controlling metal penetration through structural rigidity in OSV devices.

Beyond the geometric regulation and physical robustness, the unique chemical environment within MOFs also plays a critical role in mitigating the metal penetration behavior. In the next section, the role of chemical interactions between penetrating metal atoms and the MOF framework is discussed as an additional mechanism.

4.2.3 Chemical localization and energetic trapping

Chemical interactions at the MOF/metal interface also play a critical role in stabilizing metal penetration. The rich chemical environments of MOF provide abundant active coordination sites that can directly interact with penetrating metal atoms, thereby localizing them at the interface. Extensive chemical studies have demonstrated that MOFs possess well-defined coordination sites (Ko et al., 2018), including exposed metal nodes and ligand-based donor groups, exhibiting strong affinity towards metal species. As a result, the mobility of metal atoms entering the MOF frameworks is effectively restricted not only through geometric and physical structural restrictions but also through coordination bonding and local hybridization with framework constituents. In lithium batteries, certain MOFs have been shown to regulate interfacial chemistry and suppress parasitic reactions by providing strong ionic selectivity and dissociating metal-solvent complexes (Yuan et al., 2023), supporting the role of MOFs as chemically active platforms capable of reshaping metal-interface interactions. Therefore, rationally designed MOF spacer materials may similarly mitigate undesirable reactions between the Co metal and the spacer layers in OSV devices, thereby controlling metal penetration. The AFM layer reported by Song et al. (Song et al., 2024) reveals a desirable reaction between the Co metal and the MOF spacer layer, which is not unique to MOF systems. The formation of the AFM can be understood within the effective electrode framework proposed by Barraud et al., where the coupling-induced broadening (Γ↑/↓) and the relative position of molecular levels with respect to the Fermi level govern spin polarization (Barraud et al., 2010). However, such a beneficial effect emerges only when the metal penetration is spatially confined, and the reaction between Co and the spacer layers remains chemically localized by MOF layers. In contrast, if metal diffusion proceeds uncontrollably, as in ordinary polymer spacers, excessive undesirable reactions and percolative pathways would dominate the transport behavior, leading to spin scattering, short-circuiting, and eventually device degradation. Therefore, the beneficial role of interfacial reactions depends critically on whether the spacer architecture can suppress penetration to an interfacial level.

Figure 6 schematically shows the three-level mechanisms of MOF regulating metal penetration. Consequently, chemical trapping serves both as the third and final level of metal penetration restriction and a potential factor tuning interfacial magnetic coupling (Song et al., 2024), complementing geometric constraint and physical rigidity in MOF-based spacers.

While the penetration-suppression capabilities of MOFs are promising, it is also important to take process-related contributions into account, in that the preparation of MOF interlayers is greatly different from traditional approaches, such as spin-coating and deposition. The LBL growth is less aggressive and intense than direct deposition, which can intrinsically contribute to a more uniform spacer layer with reduced defect density, thus suppressing metal penetration. Therefore, part of the observed suppression effect may arise from optimized processing conditions of MOF spacers. However, several available experimental observations suggest that the suppression cannot be fully attributed to process-related factors. For example, the Co-ligand coordination and formation of AFM layers revealed in the work of Song et al. provides us with direct evidence for Co-MOF chemical interactions (Song et al., 2024). Such a phenomenon is difficult to explain solely by the growing mechanisms or film-smoothness effects and instead points toward the existence of MOF’s ability of chemical localization.

It should also be emphasized that the potential structural advantages discussed in the preceding sections are based on a rational analysis of MOF geometry, rigidity, and coordination chemistry. These hypotheses only highlight the intrinsic capability of MOFs to regulate metal diffusion; nevertheless, systematic experiments that independently vary deposition kinetics, interfacial oxidation, and framework chemistry are still required to fully decouple process factors from MOF-specific mechanisms.

4.3 Comparison with conventional interfacial control strategies

Table 2 summarizes the features of traditional strategies implemented to control metal penetration and MOF spacers for comparison, and Table 3 presents a quantitative summary of representative systems. Different spacer and buffer layer strategies exhibit distinct penetration control effects, ranging from partial suppression to complete block but high transport resistance. In contrast, MOF-based spacers do not entirely block metal penetration but instead restrain it and reshape it into localized and non-percolative interfacial reconstruction.

TABLE 2. Summary of metal-penetration control strategies.

TABLE 3. Quantitative summary of metal-penetration-suppressed representative OSV systems.

4.4 Current limitations and practical challenges

In spite of the excellent metal-penetration control performance and promising capability of reshaping the interface, systematic studies on long-term operational stability in terms of metal penetration suppression and MR performances of MOF-based OSVs under electrical bias, thermal cycling, and ambient exposure remain scarce. Additionally, several practical limitations and challenges ought to be addressed before the broad integration of MOFs into OSV devices can be realized.

To start with, the efficient fabrication approaches of MOF spacers are still of great potential for future exploration. As mentioned in the pioneering works by Song et al. (2020); Song et al. (2024), the extended 2D skeleton of MOFs renders themselves hard to solve in conventional organic solvents like chloroform, which means traditional coating strategies like spin-coating are not suitable for MOF films to grow on the substrates in order. At present, the mainstream and most reliable approach of growing MOFs is the layer-by-layer (LBL) method, which can guarantee the in-plane order and controlled thickness of MOF films, but is extremely time-consuming and tedious. In order to obtain an MOF spacer layer with an ordered structure and sufficient thickness, an anchor layer should be self-assembled onto the substrate in advance, and then tens to hundreds of repetitive dipping-rinsing cycles are often required (Song et al., 2020). Such fabrication procedures are compatible with laboratory-scale research but are not desirable for high-throughput factory manufacturing, in that the efficiency is far lower than that of spin-coating.

On top of that, the uniformity and edge effects should also be taken into account. With an expansion of the substrate area, the non-uniformity of precursor liquid diffusion on the anchor film may induce a difference in thickness between the central part and the edge regions of the spacer. Such non-uniformity can be accumulated and amplified because of a defect in the anchor film. As a result, the smoothness of the 2D MOF film cannot be guaranteed, whereas low roughness is imperative to advance the performance of OSV devices (Yan et al., 2023). Controlling thickness homogeneity remains a nontrivial challenge for MOF-based spacers.

Finally, the application of MOF materials may further influence the spin transport in OSV devices. While conventional polymer spacers are composed of light elements like carbon and hydrogen, the metallic atom nodes coordinated in MOFs usually possess large atomic weights, which can result in a strengthened spin-orbit coupling (SOC), promoting the undesirable spin relaxation during spin transport (Yang et al., 2024) in disordered or hopping-dominated transport regimes, and thus shortening the spin lifetimes. Nonetheless, several studies have shown that owing to mechanisms such as chirality-induced spin selectivity (CISS), such an SOC effect could be beneficial in spin filtering and promote spin transport when the MOF material exhibits structural chirality (Huizi-Rayo et al., 2020). In terms of processing, the MOF films grown on the substrate are prone to chemical contamination and solvent residue. In a liquid environment required for the LBL growth, the solvent molecules (e.g., ethanol or DMF) may be trapped in the voids of the MOF layers during fabrication. Such contamination may not be detrimental to the metal penetration-controlling effect, but could serve as spin-scattering centers, thereby shortening the spin path length and decreasing the relaxation time. Consequently, the magnetoresistance performance of the devices will be undermined. Systematic studies on the long-term impact of chemical residues and interfacial cleanliness are therefore necessary for future applications of MOF spacers.

5 Conclusion and future outlook

5.1 Summary of strategies for suppressing metal penetration in OSV devices

This review introduced the common problem existing in spintronic research–top ferromagnetic electrode metal penetration during fabrication. Metal penetration is initialized by residual kinetic energy of the metal atoms during evaporative deposition, facilitated by the intrinsic properties of the spacer layers, and results in undermined performances and even complete erasure of MR signals due to catastrophic short-circuiting. Modern characterization techniques, ranging from direct observation to elementary analyses, can all showcase the phenomena of metal penetration. Then, several strategies that have been employed in different research to suppress metal penetration were systematically reviewed. Early strategies largely relied on patch solutions, including adding oxide or inorganic buffer interlayers and selecting optimized polymeric spacers. Meanwhile, process-based improvements, spanning electrode and spacer preparation, electrode transfer, and junction-area engineering, were also widely explored, among which electrode transfer presents as the most effective one. While these approaches have achieved partial success, they are often highly process-dependent, lacking either robustness, pureness, reproducibility, or efficiency, which might be undesirable when it comes to thin-spacer-layer fabrication or large-scale production. Some of the methods even appeared to be unfavorable for spin injection at the metal/spacer interface.

Within this context, metal-organic frameworks (MOFs), 2D c-MOFs in particular, offer a specifically promising spacer material for next-generation interface engineering in OSV devices. Possessing an intrinsic structural order and well-defined pore architectures with a unique coordinative local environment, MOFs are capable of suppressing metal penetration hierarchically by their geometric regulation, physical robustness, and chemical localization. Recent applications of MOFs in spintronics by Song et al. have proved their ability to suppress metal penetration and thus achieve a more stable MR performance, although limited to low temperatures. Nevertheless, applying MOFs as a spacer material faces certain limitations as well, including great difficulties in uniform film formation and complexity in fabrication procedures.

5.2 Future challenges and research directions

Looking forward, a fundamentally attractive route for addressing metal penetration lies in the optimized selection and design of spacer materials themselves. Compared with auxiliary strategies such as additional buffer layers or electrode transfer, spacer-centered approaches preserve interfacial cleanliness by avoiding extra spin scattering centers and enable simpler fabrication workflows without introducing additional processing steps. MOFs emerge as a highly competitive leading role. In future research, MOFs’ geometric blocking against metal penetration can be strengthened with the help of their designable pore architectures, while their chemical tunability allows further tailored interfacial interactions with ferromagnetic electrodes without compromising spin transport. With the inspiration from lithium batteries, high-mechanical-strength MOFs can be designed and constructed such that the metal electrode atoms can no longer form chemical bonds with the spacers easily, thus suppressing the formation and growth of metal filaments. However, several open questions are preventing researchers from moving beyond proof-of-concept demonstrations of MOF spacer layers. First, the fabrication of high-quality MOF spacer layers often involves multi-step growth procedures, which can be time-consuming for both laboratory and industrial fabrications. Second, the structural diversity and tunability of MOFs also introduce significant complexity in understanding spin transport mechanisms. The wide variations in metal nodes, ligands, chirality, defect densities, and film morphologies make it difficult to establish simple structure-property correlations. As a result, theoretical studies in the mechanisms of spin transport within MOF layers remain limited.

Beyond materials design, future progress will critically depend on the development of scalable and reproducible MOF film formation strategies. For instance, specific substrates on which MOF layers are able to grow directly and smoothly can be devised to get rid of the pre-grown SAM layers. Supplementary to this, simplified deposition schemes or lithography-compatible patterning methods can be explored in the future as well. Advancing these directions will be essential for translating MOF-based spacer materials from proof-of-concept demonstrations into practical and manufacturable organic spintronic devices.

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Footnotes

Edited by: Kathiresan Murugavel, Central Electrochemical Research Institute (CSIR), India

Reviewed by: Ehsan Elahi, Sejong University, Republic of Korea

Neha Jha, Fritz Haber Institute, Max Planck Society, Germany