Interfacial Spin Research Articles

Exchange bias effect in hierarchical NiO/NiFe2O4 sub-microcubes fabricated via a self-sacrifice template process

In this study, we report a facile in situ synthesis of hierarchical NiO/NiFe2O4 sub-microcubes by annealing Ni3[Fe(CN)6]2 sub-microcubes fabricated via a one-step solution-precipitation method. The temperature-dependent magnetization curves show that there exists a collective freezing of the moments of interface spin clusters at TB≈296K, which further indicates the appearance a coupling interaction between the ferrimagnetic NiFe2O4 and antiferromagnetic NiO nanoparticles. Furthermore, hysteresis loops measurements reveal the existence of an exchange bias effect in the NiO/NiFe2O4 composite system. Interestingly, no training effect of exchange bias is observed, which indicates that the magnetic pinning interaction between NiFe2O4 and NiO is stable and robust. Exchange bias field shows an exponential dependence on temperature and vanishes at about 150 K, which reveals the strong coupling interaction does not improve effectively the thermal stability of exchange bias effect in the current system.

Longitudinal spin Seebeck effect in pyrochlore iridates with bulk and interfacial Dzyaloshinskii-Moriya interaction

The longitudinal spin-Seebeck effect (SSE) in magnetic insulator$|$non-magnetic metal heterostructures has been theoretically studied primarily with the assumption of an isotropic interfacial exchange coupling. Here, we present a general theory of the SSE in the case of an antisymmetric Dzyaloshinskii-Moriya interaction (DMI) at the interface, in addition to the usual Heisenberg form. We numerically evaluate the dependence of the spin current on the temperature and bulk DMI using a pyrochlore iridate as a model insulator with all-in all-out (AIAO) ground state configuration. We also compare the results of different crystalline surfaces arising from different crystalline orientations and conclude that the relative angles between the interfacial moments and Dzyaloshinskii-Moriya vectors play a significant role in the spin transfer. Our work extends the theory of the SSE by including the anisotropic nature of the interfacial Dzyaloshinskii-Moriya exchange interaction in magnetic insulator$|$non-magnetic metal heterostructures and can suggest possible materials to optimize the interfacial spin transfer in spintronic devices.

Interface hybridization and spin filter effect in metal-free phthalocyanine spin valves.

Spin-orbit coupling (SOC) has long been regarded as the core interaction to determine the efficiency of spin conserved transport in semiconductor spintronics. In this report, a spin-valve device with a Co/metal-free phthalocyanine (H2Pc)/Co stacking structure is fabricated. The magnetoresistance effect was successfully obtained in the device. It is also found that the magnetoresistance response is relatively smaller than that of metallic phthalocyanines, clearly implying that SOC is not the key factor to affect the magnetoresistance in phthalocyanine spin-valves. The dominant mechanism that determines the spin transport efficiency in the present H2Pc devices was systemically explored by combining both experimental measurements and first-principles calculation analysis. It was noticed that both the crystalline structure and molecular orientation of the H2Pc layer could be modified by the contact under-layer materials, which changes the magnetization intensity of the ferromagnetic metallic electrode due to the strong interface hybridization of Co/H2Pc. Meanwhile, the theoretical calculations clearly demonstrated that the spin filter effect from the second H2Pc layer should be responsible for the decrease of the magnetoresistance response in the present spin-valves compared to those using metallic phthalocyanine layers. This investigation may trigger new insights into the role of SOC strength and interface hybridization in organic spintronics.

Modulation of field-like spin orbit torque in heavy metal/ferromagnet heterostructures.

Spin orbit torque (SOT) has drawn widespread attention in the emerging field of magnetic memory devices, such as magnetic random access memory (MRAM). To promote the performance of SOT-MRAM, most efforts have been devoted to enhance the SOT switching efficiency by improving the damping-like torque. Recently, some studies noted that the field-like torque also plays a crucial role in the nanosecond-timescale SOT dynamics. However, there is not yet an effective way to tune its relative amplitude. Here, we experimentally modulate the field-like SOT in W/CoFeB/MgO trilayers through tuning the interfacial spin accumulation. By performing spin Hall magnetoresistance measurement, we find that the CoFeB with enhanced spin dephasing, either generated from larger layer thickness or from proper annealing, can distinctly boost the spin absorption and enhance the interfacial spin mixing conductance Gr. While the damping-like torque efficiency increases with Gr, the field-like torque efficiency is found to decrease with it. The results suggest that the interfacial spin accumulation, which largely contributes to the field-like torque, is reduced by higher interfacial spin transparency. Our work shows a new path to further improve the performance of SOT-based ultrafast magnetic devices.

Mapping the near-field spin angular momenta in the structured surface plasmon polariton field.

Optical spin angular momenta in a confined electromagnetic field exhibit a remarkable difference with their free space counterparts; in particular, the optical transverse spin that is locked with the energy propagating direction lays the foundation for many intriguing physical effects such as unidirectional transportation, quantum spin Hall effects, photonic Skyrmions, etc. In order to investigate the underlying physics behind the spin-orbit interactions as well as to develop the optical spin-based applications, it is crucial to uncover the spin texture in a confined field, yet it faces challenges due to their chiral and near-field vectorial features. Here, we propose a scanning imaging technique which can map the near-field distributions of the optical spin angular momenta with an achiral dielectric nanosphere. The spin angular momentum component normal to the interface can be uncovered experimentally by employing the proposed scanning imaging technique and the three-dimensional spin vector can be reconstructed theoretically with the experimental results. The experiment is demonstrated on the example of surface plasmon polaritons excited with various vector vortex beams under a tight-focusing configuration, where the spin-orbit interaction emerges clearly. The proposed method, which can be utilized to reconstruct the photonic Skyrmion and other photonic topological structures, is straightforward and of high precision, and hence it is expected to be valuable for the study of near-field spin optics and topological photonics.

Temperature dependence of spin pumping and inverse spin Hall effect in permalloy/Pt bilayers

The temperature dependence of the spin pumping process in Fe80Ni20 (Py)/Pt bilayers is analyzed for different Pt capping layer thickness. Samples have been grown by means of dc magnetron sputtering, at room temperature, on top of Si substrates. In order to get an optimized interface between the Py and the Pt layers, the Pt capping layers were deposited In situ. The values of the damping constant, α, obtained for the Py alone samples, around 0.007 at RT, make evident the good quality of the samples. The increase of α due to spin pumping into the Pt layer is clearly appreciated; α values in Py/Pt bilayers are twice the value in Py alone. Spin pumping is confirmed by direct measurements of the inverse spin Hall effect (ISHE) voltage signal (VISHE) measured across the samples. To separate ISHE and spin rectification contributions to VISHE the line shape analysis method has been used and the conditions for its applicability thoroughly discussed. VISHE increases almost linearly on lowering temperature for all the frequencies investigated and smoothly increases as frequency decreases showing roughly a linear dependence with the inverse of the frequency. The Pt layer thickness dependence of α and VISHE suggest a low interfacial spin mixing conductance and high spin memory loss.

Temperature induced interface roughness and spin reorientation transition in Co/Au multilayers thin films

Spin reorientation transition (SRT) due to high temperature has been studied in Co/Au multilayered thin films. The effect of temperature on structural and magnetic properties of the thin films has been investigated. Film thickness and interface roughness has been studied by measuring x-ray reflectivity. Multilayers are thermodynamically stable, and thickness of the layers and interface roughness remains unchanged over longer periods of time as well as up to a temperature of 300 °C. An increase in interface roughness at higher temperatures indicates intermixing of Co and Au into each other. Behavior of coercive field for both the samples is quite different. However, the spin reorientation transition occurred in both the samples around 350 °C. Here we are reporting our study on temperature induced spin reorientation transition in Co/Au multilayers. A spin reorientation transition is observed from out-of-plane to in-plane direction for 12 Å thick Co layer sample at 350 °C and a reverse transition is observed for 25 Å thick Co layer sample. Results were obtained using ex situ and in situ Magneto-Optic Kerr Effect (MOKE) measurements.

Dysprosium Iron Garnet Thin Films with Perpendicular Magnetic Anisotropy on Silicon

AbstractMagnetic insulators, such as the rare‐earth iron garnets, are promising materials for energy‐efficient spintronic memory and logic devices, and their anisotropy, magnetization, and other properties can be tuned over a wide range through selection of the rare‐earth ion. Films are typically grown as epitaxial single crystals on garnet substrates, but integration of these materials with conventional electronic devices requires growth on Si. The growth, magnetic, and spin transport properties of polycrystalline films of dysprosium iron garnet (DyIG) with perpendicular magnetic anisotropy (PMA) on Si substrates and as single crystal films on garnet substrates are reported. PMA originates from magnetoelastic anisotropy and is obtained by controlling the strain state of the film through lattice mismatch or thermal expansion mismatch with the substrates. DyIG/Si exhibits large grain sizes and bulk‐like magnetization and compensation temperature. Polarized neutron reflectometry demonstrates a small interfacial nonmagnetic region near the substrate. Spin Hall magnetoresistance measurements conducted on a Pt/DyIG/Si heterostructure demonstrate a large interfacial spin mixing conductance between the Pt and DyIG comparable to other garnet/Pt heterostructures.

Band-Structure Spin-Filtering in Vertical Spin Valves Based on Chemical Vapor Deposited WS2.

We report on spin transport in WS2-based 2D-magnetic tunnel junctions (2D-MTJs), unveiling a band structure spin filtering effect specific to the transition metal dichalcogenides (TMDCs) family. WS2 mono-, bi-, and trilayers are derived by a chemical vapor deposition process and further characterized by Raman spectroscopy, atomic force microscopy (AFM), and photoluminescence spectroscopy. The WS2 layers are then integrated in complete Co/Al2O3/WS2/Co MTJ hybrid spin-valve structures. We make use of a tunnel Co/Al2O3 spin analyzer to probe the extracted spin-polarized current from the WS2/Co interface and its evolution as a function of WS2 layer thicknesses. For monolayer WS2, our technological approach enables the extraction of the largest spin signal reported for a TMDC-based spin valve, corresponding to a spin polarization of PCo/WS2 = 12%. Interestingly, for bi- and trilayer WS2, the spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction. With the support of ab initio calculations, we propose a model to address the experimentally measured inversion of the spin polarization based on the change in the WS2 band structure while going from monolayer (direct bandgap) to bilayer (indirect bandgap). These experiments illustrate the rich potential of the families of semiconducting 2D materials for the control of spin currents in 2D-MTJs.

Asymmetric spin dependent scattering at the interfaces of Si/La0.7Sr0.3MnO3/ZnO heterostructures

A ferromagnetic 120 Å thick La0.7Sr0.3MnO3 (LSMO) film grown on (001)Si using the sputtering deposition technique demonstrates a large positive in-plane magnetoresistance (MR) at 10 K, in the field window of ±0.084 kG to±0.405 kG, although the bulk LSMO exhibits negative MR. Around the coercive field (∼179 G), the positive MR becomes ∼ 11%. The positive MR of the LSMO thin film is explained by the charge transfer driven localized strong antiferromagnetic coupling at the Si−LSMO interface, which favors the reduction of the Curie temperature TC of LSMO compared to that of its bulk value. The construction of the interface on the top surface of LSMO with ZnO thin films further reduces TC ∼ 30 K and the positive MR decreases to ∼ 1% for 45° oriented in-plane current with the in-plane field. The coupling through Mn−O−Zn at the LSMO−ZnO interface preserves the charge state, and the weak exchange coupling at the (La/Sr)O−ZnO interface reduces the spin-dependent scattering process under the field and thereby, the negative MR. The reduced TC and in-plane low-field MR at 10 K of a series of Si/LSMO/ZnO are the same irrespective of the ZnO thickness, which confirms their interfacial origin. The presence of interfacial spin disorder at the Si−LSMO interface is further confirmed from the increase in resistance at low temperatures, which is explained by the Kondo like effect and quantum interference effect. Our investigations show that the technologically important interfacial magnetic coupling and magnetoresistance could be achieved and manipulated by the selective interfacial exchange coupling.

Influence on the Gilbert damping of yttrium-iron-garnet films by the spin-pumping effect

The effect of a platinum (Pt) capping layer on the magnetization dynamics in polycrystalline yttrium iron garnet (YIG) films of various thicknesses has been studied in order to understand the influence on the Gilbert damping of YIG films by the spin-pumping at the YIG/Pt interface. The Gilbert damping in each YIG film with the Pt capping layer is found to be much higher than that of the YIG film without the Pt capping layer, as expected due to the spin-pumping effect. In comparison with the samples without capping layer, an enhanced value of interfacial spin mixing conductance can also be observed for films with the capping layer. These results demonstrate the existence of spin-pumping effect at the YIG/Pt interface which plays an important role for the enhancement of Gilbert damping.

Spin-orbit torque manipulated by fine-tuning of oxygen-induced orbital hybridization.

Current-induced spin-orbit torques provide an effective way to manipulate magnetization in spintronic devices, promising for fast switching applications in nonvolatile memory and logic units. Recent studies have revealed that the spin-orbit torque is strongly altered by the oxidation of heterostructures with broken inversion symmetry. Although this finding opens a new field of metal-oxide spin-orbitronics, the role of the oxidation in the spin-orbit physics is still unclear. Here, we demonstrate a marked enhancement of the spin-orbit torque induced by a fine-tuning of oxygen-induced modification of orbital hybridization. This is evidenced by a concomitant enhancement of the interface spin-orbit torque, interface spin loss, and interface perpendicular magnetic anisotropy within a narrow range of the oxidation level of metallic heterostructures. This result reveals the crucial role of the atomic-scale effects in the generation of the spin-orbit torques, opening the door to atomic-level engineering of the spin-orbit physics.

Spin Seebeck Imaging of Spin-Torque Switching in Antiferromagnetic Pt/NiO Heterostructures

As electrical control of N\'eel order opens the door to reliable antiferromagnetic spintronic devices, understanding the microscopic mechanisms of antiferromagnetic switching is crucial. Spatially-resolved studies are necessary to distinguish multiple nonuniform switching mechanisms; however, progress has been hindered by the lack of tabletop techniques to image the N\'eel order. We demonstrate spin Seebeck microscopy as a sensitive, table-top method for imaging antiferromagnetic order in thin films, and apply this technique to study spin-torque switching in NiO/Pt and Pt/NiO/Pt heterostructures. We establish the interfacial antiferromagnetic spin Seebeck effect in NiO as a probe of surface N\'eel order, resolving antiferromagnetic spin domains within crystalline twin domains. By imaging before and after applying current-induced spin torque, we resolve spin domain rotation and domain wall motion, acting simultaneously. We correlate the changes in spin Seebeck images with electrical measurements of the average N\'eel orientation through the spin Hall magnetoresistance, confirming that we image antiferromagnetic order.

Ferromagnetic resonance of perpendicularly magnetized Tm3Fe5O12/Pt heterostructures

Broadband ferromagnetic resonance is used to investigate magnetization dynamics, damping, interfacial spin transport, and perpendicular magnetic anisotropy (PMA) of (111)-oriented epitaxial thin films of the ferrimagnetic insulator Tm3Fe5O12 (TmIG) on substrates of (111)-oriented Gd3Ga5O12. A PMA field of ∼162 mT is found at 350 K, in the temperature range where spin–orbit torque switching was previously reported [Avci et al., Nat. Mater. 16, 309–314 (2017)]. A Landé g-factor of 1.56 strongly supports large intrinsic spin–orbit coupling due to the presence of the heavy rare earth Tm. Gilbert damping coefficients α are compared for three samples: a 28 nm thin TmIG film (α ∼ 0.014), a TmIG (28 nm)/Pt (6 nm) bilayer (α ∼ 0.022), and a TmIG (28 nm)/Cu (3 nm)/Pt (6 nm) trilayer (α ∼ 0.024). Applying the spin pumping formalism, we find that the real part of the effective interfacial spin mixing conductance Geff↑↓ = 5.7 × 1014 Ω−1 m−2 is comparable to that of well-studied garnet/Pt interfaces. Our work strengthens the candidacy of TmIG for spintronics applications requiring PMA in insulating thin films.

Tunable damping-like and field-like spin-orbit-torque in Pt/Co/HfO2 films via interfacial charge transfer

The spin–orbit-torque (SOT) consists of dampinglike torque (DLT) and fieldlike torque (FLT), where the combined effects of these two torques need further consideration for efficient SOT switching. Here, the tunable correlation between the DLT and FLT is investigated in Pt/Co/HfO2 multilayers with different annealing temperatures (Ta). With increasing Ta, the FLT decreases monotonously, while both the sign and the magnitude of DLT are changed. Interfacial analysis results reveal that the tunable correlation of them is strongly dependent on the interfacial electron structure between the Co and HfO2 layer. The interfacial charge transfer between the Co, O, and Hf atoms could modify interfacial spin–orbit coupling and the crystal electric field (ECEF), which promotes the interface-generated SOT. This work demonstrates an effective method to tune the correlation of the two SOT components, a desirable feature which will be beneficial for the design of SOT-based devices.

Drop of Spin Polarization of Half Metal due to Interfacial Spin Mixing

Half metal is a ferromagnetic metal that act as a conductor to electrons with of one spin orientation, while act as an insulator for the opposite spin. Because of this high spin-dependent property, half metals are widely studied in spiritroriics area. This property can be represented by the spin polarization P = (G↑ — G↓) / (G↑ + G↓) of its spin-dependent conductances G↑,↓. The value of P of an ideal half metal is 1. However, the value of P is reduced when it is attached to other material. We study how the spin current generations that occurs at the interface of half metal and normal metal reduces the value P. At the interface of a half metal and a normal metal a spin current can be pumped from the ferromagnetic layer to the normal metal. This spin pumping effect is characterized by spin-mixing conductance, which represents the magnitude of the generated spin current due to the mixing of electrons with spin up and down. By analyzing the coupling of spin and electric currents in the interface of half metal and normal metal, we show that the spin mixing strongly influences the spin-dependent transport near the interface, and therefore reduces the spin polarization of the half metal.

Isotropic non-local Gilbert damping driven by spin pumping in epitaxial Pd/Fe films on MgO(001) substrates

Although both theoretical predictions and experimental observations have demonstrated that the Gilbert damping is anisotropic at ferromagnet/semiconductor interface possessing robust interfacial spin–orbit coupling, it is not well understood whether non-local Gilbert damping driven by spin pumping in heavy metal/ferromagnetic metallic bilayers is anisotropic or not. Here, we investigated the angular and frequency dependence of magnetic relaxation in epitaxial Pd/Fe films on MgO(001) substrates. After disentangling parasitic contributions, we unambiguously observe that the non-local Gilbert damping is isotropic in the Fe(001) plane, suggesting that the spin transport across the Pd/Fe interface is independent of the Fe magnetization orientation. First principles calculations reveal that the effective spin mixing conductance of the Pd/Fe interface is nearly invariant for different magnetization directions, in good agreement with the experimental observations. These results offer valuable insight into spin transport in metallic bilayers, and facilitate the development of next-generation spintronic devices.

Composition dependent magnetic and microwave properties of exchange-coupled hard/soft nanocomposite ferrite

In the present work, exchange-coupled BaFe12O19 (BaM)/Ni0.5Zn0.5Fe2O4 (NZFO) nanocomposites with different weight ratio were prepared by sol-gel autocombustion method. The effect of relative weight fraction on structural, morphological, magnetic and microwave properties were investigated. X-ray diffraction patterns of composites confirmed the co-existence of BaM and NZFO phases without any secondary phase. Smooth hysteresis loops without kink confirmed that BaM/NZFO phases were well exchange-coupled. Non linear change in magnetization (Ms) and coercivity (Hc) with NZFO content were observed. The drastic increase in Ms with corresponding sharp decrease in Hc for 70/30 wt ratio was observed. The change in Ms and Hc was explained with the schematic model of interfacial spin interactions. Complex permeability and permittivity in Ku-band showed oscillatory behaviour for higher weight fraction of NZFO. Reflection loss frequency found to depends on NZFO content and maximum loss (−41 dB) was observed for 70/30 wt ratio at 13.8 GHz frequency.

Spin-orbit torques and their angular dependence in ferromagnet/normal metal heterostructures

Spin-orbit torques (SOTs) are investigated for various elements of X (Pt, Ru, Pd, Cu, Mo, W, and Ta) in Pt/Co/X heterostructures. We find that both the angle-independent and angle-dependent torque components are strongly dependent on the choice of X. Large negative anisotropic SOTs are observed for elements exhibiting high SOT efficiencies (such as X = Ta or W), the behavior of which is detrimental to device applications, thus requiring us to understand the underlying physics behind the anisotropic torques. Our careful analysis shows that angle-dependent torques are closely correlated with the work function difference between Co and X, indicating that the interfacial Rashba spin–orbit coupling plays a role in the anisotropy of SOTs.

Single crystal epitaxial [Co/CoO]n superlattices consisting of ferromagnetic metal and antiferromagnetic insulator

Heterostructures consisting of ferromagnetic metal and antiferromagnetic insulator have exhibited novel properties in interface exchange coupling, magnetoresistance and spin injection due to their fundamental physics of magnetism. However, as compared with bilayer heterostructures, it is very challenging in experiments to prepare single crystal epitaxial ultrathin superlattices consisting of ferromagnetic metal and antiferromagnetic insulator, where the interface exchange coupling, magnetic order, and electrical transport properties are expected to be greatly modulated due to the multiple interfaces and periodical structures. Here we prepared single crystal epitaxial [Co/CoO]n superlattices consisting of ferromagnetic metal Co and antiferromagnetic insulator CoO by molecular beam epitaxy. It is found that the saturation magnetization of [Co 0.6 nm/CoO 1.2 nm]5 superlattice is robust against temperature increase, which only shows a slight decrease of 1.5% from 5 K to 300 K. Moreover, we found that the longitudinal resistivity and anomalous Hall resistivity of [Co/CoO]n superlattices are strongly modulated by the short-period Co/CoO heterostructure, which are quite distinguished from the electrical transport properties of Co bulk ferromagnetic metal. The [Co/CoO]n superlattices provide a new insight into heterostructures consisting of ferromagnetic metal and antiferromagnetic insulator, which are beneficial for designing future spintronics devices.