Nonmagnetic Layer Research Articles

Thermodynamic properties and magnetocaloric effect of a graphdiyne bilayer with RKKY interaction

Based on the Monte Carlo method, a ferromagnetic mixed spin-3/2 and spin-5/2 Ising model is extended to research the thermodynamic and magnetocaloric properties of the graphdiyne bilayer under the presence of the external magnetic field. The results of the finite-size effect show that the thickness of the nonmagnetic layers and the total number of the atoms have the significant influence on the studied quantities including magnetization, susceptibility, internal energy, specific heat, magnetic entropy, and magnetic entropy change. In addition, the maximum of the magnetic entropy change -ΔSm can be improved by increasing the exchange interaction between atoms b in the bottom magnetic surface Jb/Ja and adding the nonmagnetic layers L. We also obtain the relative cooling power RCP under the change of the external field Δh/Ja.

Observation of Magnetism-Induced Topological Edge State in Antiferromagnetic Topological Insulator MnBi4Te7.

Breaking time reversal symmetry in a topological insulator may lead to quantum anomalous Hall effect and axion insulator phase. MnBi4Te7 is a recently discovered antiferromagnetic topological insulator with TN ∼ 12.5 K, which is composed of an alternatively stacked magnetic layer (MnBi2Te4) and nonmagnetic layer (Bi2Te3). By means of scanning tunneling spectroscopy, we clearly observe the electronic state present at a step edge of a magnetic MnBi2Te4 layer but absent at nonmagnetic Bi2Te3 layers at 4.5 K. Furthermore, we find that as the temperature rises above TN the edge state vanishes, while the point defect induced state persists upon an increase in temperature. These results confirm the observation of magnetism-induced edge states. Our analysis based on an axion insulator theory reveals that the nontrivial topological nature of the observed edge state.

Spin anomalous-Hall unidirectional magnetoresistance

We predict a spin anomalous-Hall unidirectional magnetoresistance (AH-UMR) in conducting bilayers composed of a ferromagnetic layer and a nonmagnetic layer, which does not rely on the spin Hall effect in the normal metal layer---in stark contrast to the well-studied unidirectional spin-Hall magnetoresistance---but instead arises from the spin anomalous Hall effect in the ferromagnetic layer. Physically, it is the charge-spin conversion induced by the spin anomalous Hall effect that conspires with the structural inversion asymmetry to generate a net nonequilibrium spin density in the ferromagnetic layer, which, in turn, modulates the resistance of the bilayer when the direction of the applied current or the magnetization is reversed. The dependencies of the spin AH-UMR effect on materials and geometric parameters are analyzed and compared with other nonlinear magnetoresistances. In particular, we show that, in magnetic bilayers where spin anomalous Hall and spin Hall effects are comparable, the overall UMR may undergo a sign change when the thickness of either layer is varied, suggesting a scheme to quantify the spin Hall or spin anomalous Hall angle via a nonlinear transport measurement.

Magnetic properties of bilayer nano-stanene-like structure with Ruderman–Kittel–Kasuya–Yoshida coupling

A bilayer nano-stanene-like structure with Ruderman–Kittel–Kasuya–Yoshida (RKKY) coupling described by the Ising model is proposed. The magnetic and thermodynamic properties are studied using the effective-field theory with correlations. The exchange coupling, longitudinal magnetic field, number of non-magnetic layers, and anisotropies had major influences on the magnetization, specific heat, and internal energy. Different saturation magnetizations are observed on the magnetization curve. The variation in the system blocking temperature is studied. The results provide theoretical guidance for the magnetic investigation of nanomaterials with RKKY coupling.

Magnetization Reversal and Domain Structures in Perpendicular Synthetic Antiferromagnets Prepared on Rigid and Flexible Substrates

Ferromagnetic (FM) layers separated by nonmagnetic metallic spacer layers can exhibit Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, which may lead to a stable synthetic antiferromagnetic (SAF) phase. In this article, we have studied magnetization reversal by varying the number of bilayer stacks [Pt/Co] as well as thicknesses of Ir space layer t\(_{Ir}\) on rigid Si(100) and flexible polyimide substrates. The samples with t\(_{Ir}\) = 1.0 nm show a FM coupling, whereas samples with t\(_{Ir}\) = 1.5 nm show an antiferromagnetic (AFM) coupling between the FM layers. At t\(_{Ir}\) = 2.0 nm, it shows a bow-tie shaped hysteresis loop indicating a canting of magnetization at the reversal. Higher anisotropy energy compared to the interlayer exchange coupling (IEC) energy is an indication of the smaller relative angle between the magnetization of lower and upper FM layers. We have also demonstrated the strain-induced modification of IEC as well as magnetization reversal phenomena. The IEC shows a slight decrease upon application of compressive strain and increase upon application of tensile strain, which indicates the potential of SAFs in flexible spintronics.

Spin-Current Oscillations in Diluted Magnetic Semiconductor Multibarrier GaMnAs/GaAs: Role of Temperature and Bias Voltage

This paper has studied the electronic properties of multi-diluted magnetic semiconductor (DMS) layers Ga(1 − x)MnxAs interposed between nonmagnetic GaAs layers. The asymmetry of confining potential on the transmission coefficient by tuning the temperature and the size of the (DMS) layers was discussed. The diluted magnetic layers Ga(1 − x)MnxAs behave as barriers for spin-up holes and quantum wells for spin-down holes. Furthermore, we have addressed the impact of an applied bias voltage and the temperature on the variation of the spin-polarization and spin current densities. Our findings reveal that the transmission coefficients present an oscillating behavior due to the resonant states and strongly depend on the temperature of the system and the number of magnetic layers. Furthermore, the obtained results demonstrated that the number of these states is multiplied by augmenting the magnetic layers. Moreover, we demonstrate that the asymmetric structure presents a completely different transmission of holes than the symmetric structure. Furthermore, the negative differential resistance (NDR) is demonstrated in the current density variations. Especially, this (NDR) was more intense for spin-up holes than spin-down holes. The findings in the present paper can be useful in manufacturing spin-filters by adjusting the values of the temperature and the external voltages.

Seebeck effect and Joule heating in CoFeB/MgO/CoFeB-based perpendicular magnetic tunnel junctions with low resistance area product

Perpendicular magnetic tunnel junctions (p-MTJs) have attracted great interest due to their excellent performance in spin-transfer-torque magnetic random access memories (STT-MRAMs). Here, the resistance states can be manipulated by an applied current in the order of 109–1010 A m−2, yet the appearance of a heating influence must be understood. In this work, we systematically study the Seebeck effect in nano scale p-MTJs induced due to Joule heating by the tunneling current. The CoFeB/MgO/CoFeB-based p-MTJs were nanofabricated and the current-induced switching was characterized. We find a sign change of the thermovoltage (ΔV) between AP (positive) and P (negative) states, indicating a significant dependence of the Seebeck effect on the magnetic state of the p-MTJ. The temperature distribution in the stack was simulated, by which the Seebeck coefficient (S) and the tunnel magneto-Seebeck ratio were calculated. Our further study indicates that the thermal STT can reduce the switching currents, showing the possibility to re-use this dissipative heating energy. To improve the efficiency of the energy re-use, a method is proposed through the materials optimization of the non-magnetic layers but still retaining high tunneling magnetoresistance effect. Our study shows that the magneto-Seebeck effect plays an important role in the p-MTJs, which can be crucial and must be considered in the design of the high performance p-STT-MRAMs and thermal-assisted MRAMs.

Voltage Manipulation of Synthetic Antiferromagnetism in CoFeB/Ta/CoFeB Heterostructure for Spintronic Application

AbstractThe synthetic antiferromagnets (SAFs) with inserted heavy metal tantalum (Ta) are attracting an increasing interest due to the relatively large spin Hall angles, resulting in a much lower driven current for low‐power consumption, and the conduciveness to high magnetoresistance, showing more suitability for an application. Here, stable and reversible switching behavior is experimentally realized between antiferromagnetic (AFM) coupling and ferromagnetic (FM) coupling in Co40Fe40B20/Ta/Co40Fe40B20/(011) Pb(Mg1/3Nb2/3)O3‐PbTiO3 (PMN‐PT) multiferroic heterostructures after applying an external voltage, proved by vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) measurements. The indirect interaction shows no periodical oscillation with the layer thicknesses variation of FM layer or nonmagnetic (NM) layer experimentally, which is consistent with the previous reports in theoretical calculation. This work is instructive and guiding for a better understanding of SAFs and realizing the next generation of AFM spintronic devices.

Perpendicular Anisotropy Controlled by Seed and Capping Layers of Heusler-Alloy Films

Half-metallic Heusler alloys typically have in-plane magnetic anisotropy, which can be converted to perpendicular by attaching MgO or heavy metal, e.g., Pt, layers as similarly applied for conventional ferromagnets. Recently we have found body-centered cubic (bcc) seed layers, e.g., V and W, to induce perpendicular anisotropy in Heusler-alloy films above, however, they show small giant magnetoresistive (GMR) ratios in spin-valve structures to date. This is partially because of the large resistivity of the seed layer and the nonmagnetic layer in the spin-valve. In this study, we have systematically investigated nonmagnetic overlayers and have found that a Ag layer best maintains the perpendicular anisotropy. The corresponding GMR devices have then been fabricated and characterized, achieving the GMR ratio of ~0.03% at room temperature. Such bcc seed layers can offer an alternative method for perpendicularly magnetized GMR junctions for applications.

Effect of Ta capping layer on spin dynamics in Co50Fe50 thin films

Spin pumping induced non-equilibrium spin accumulation at the interface shows an anomalous anti-damping effect for heavy metal nonmagnetic layer deposited on a ferromagnetic layer at the spin diffusion length regime without application of any external electrical current. In this work, we report a comprehensive study of static and dynamic magnetic properties of Co50Fe50 (20 nm)/Ta (t nm) thin films where the thickness of the Ta layer was varied as t = 0, 2, 5, 10, 20 and 30 nm. Ferromagnetic resonance measurements revealed a significant reduction of Gilbert damping parameter for CoFe (20 nm)/Ta (2 nm) sample in comparison to bare CoFe (20 nm) sample due to interfacial Rashba spin-orbit interaction accompanied by the formation of Ta2O5 as evidenced by x-ray photoelectron spectroscopy. The extrinsic contributions to Gilbert damping were ruled out in our samples with the invariance of resonance fields and linear increment of linewidth with frequency for different thicknesses of Ta. The saturation magnetization of the bare Co50Fe50 (20 nm) sample was found out to be 1831 ± 54 mT without any annealing effect or seed layer. Maximum spin mixing conductance of −31.46 × 1018 m−2 was found for CoFe (20 nm)/Ta (2 nm) sample, whereas it was nearly constant for all other samples.

Enhanced subterahertz spin-current transients via modulation of cross-sublattice damping in uniaxial antiferromagnets

We present an analytical model to compute the subterahertz (sub-THz) spin current transients injected from the insulating uniaxial antiferromagnet (AFM) into the adjacent nonmagnetic layer excited by spin pumping under the antiferromagnetic resonance condition, where both intra- and cross-sublattice damping parameters are treated on an equal footing. As expected, the sub-THz spin-pumping signal decreases with larger intra-sublattice damping dissipation. Interestingly, it is found that the amplitude of the spin current transient is enhanced with increasing cross-sublattice damping. On the other hand, the spin pumping is reduced by increasing the cross-sublattice spin-mixing conductance. These trends indicate that the intrinsic origin of the cross-sublattice damping in the bulk AFM enhances the spin current transients while its extrinsic origin, directly related to the interfacial cross-sublattice spin-mixing conductance, has the opposite effect. Our results suggest the important role of the previously neglected cross-sublattice damping in modulating the sub-THz spin current pulses for ultrafast spintronic applications.

High-coercivity SmFe10V2 powder with Sm-rich layers prepared by a reduction-diffusion process

The remarkable intrinsic hard magnetic properties of SmFe12-based powders are difficult to transform into extrinsic properties for practical applications; in particular, the coercivity is insufficient because the conventional magnetic isolation structure, which relies on nonmagnetic intergranular phases and is critical for inducing high coercivity in commercial Nd2Fe14B systems, has not yet been perfected. In this study, we present a new approach for achieving magnetic isolation with a reduction-diffusion process that endows SmFe10V2 powder with nonmagnetic Sm-rich layers. The optimal sample of this Sm-rich layer surrounding SmFe10V2 powder has a high coercivity of 744.9 kA/m and a moderate maximum magnetization of 61.7 Am2/kg. Based on phase characterization and microstructure observation, the principal SmFe10V2 magnetic phase is well maintained, and the surrounding Sm-rich layers separate the magnetic phases, weakening the ferromagnetic interaction of neighboring magnetic phases and contributing to the high coercivity. Our study presents a new method for developing high-coercivity magnetic powders by introducing nonmagnetic layers.

Correlation between the average composition of coherent superlattice and the GMR properties of electrodeposited Co–Cu/Cu multilayers

Abstract In contrast to vacuum-deposited Co/Cu multilayers the electrodeposited ones do not exhibit any oscillation behaviour in magnetic resistance properties as a function of the thickness of non-magnetic Cu layers. According to the magnetic investigations they show ferromagnetic and superparamagnetic character as a consequence of the lack of antiparallel coupling of neighbouring magnetic layers. Magnetic resistance observed on electrodeposited Co– Cu/Cu multilayers is believed to originate from the randomly antiparallel oriented magnetic domains. In the present paper the evolution of magnetic resistance properties is studied against the average Cu concentration of multilayers prepared under a Cu deposition potential of –0.25 and – 0.62 V, and the best magnetic resistance properties were found in the films with composition of 52% and 60% Cu, respectively. The average concentration was determined by electron microscopic and X-ray diffraction measurements.

The influence of orbital moments in anomalous Hall effect in Co/Ni multilayers with perpendicular magnetic anisotropy

We focus on the anomalous Hall effects (AHEs) of Co/Ni multilayers with perpendicular magnetic anisotropy (PMA) by using Ta, Nb, and Cu as the buffer and top layer. An un-conventional AHE behavior was found in which the AH resistance exhibits two anti-symmetric peaks in the presence of a magnetic field. Moreover, Co/Ni multilayers with a Ta neighboring layer show reverse AH resistance compared to the Nb and Cu neighboring layers, except Ta bottom and Cu capping layers. The former can be explained by considering the influence of the external magnetic field on the interfacial spin orbit interaction due to spontaneous symmetry breaking at the ferromagnetic (FM)/FM layer interface. Furthermore, the reverse Co/Ni AHE with a Ta adjacent layer can be interpreted as the leakage spin current of proximity effects from Ta due to its larger spin–orbit coupling, and finally, taking the shunting action of the Cu layer into account, the Co/Ni AHE with Ta and Cu adjacent layers can also be explained. Our results provide a clear physics picture of the AHE in a two-dimensional nano-scaling FM/FM interface with PMA; in particular, this work shows that the non-magnetic adjacent layer with large spin–orbit coupling will play an important role in the understanding of AHE in two-dimensional FM multilayers.

Electromagnetic Field Analysis and Shielding Method of Underground Variable Frequency Power Cable

The transmission and radiation of underground variable frequency electromagnetic waves will seriously interfere with the operation of the power cable and its surrounding environment. At present, the test methods for power cables basically require the impedance of the test system to match the characteristic impedance of the cable. The defect is that the process of designing and making the impedance matching impedance network is relatively complex and requires high manufacturing accuracy. In order to solve these problems, this paper puts forward the electromagnetic field fast detection formula and electromagnetic field shielding method of underground variable frequency power cable. The research method of this paper is the principle of shielding electromagnetic field materials and the suppression principle of shielding layer for electromagnetic coupling. The function of the two principles is to study the reflection, absorption, and multiple reflection of electromagnetic waves and to study the cut-off frequency of the nonmagnetic shielding layer. These two principles guide the experiment. In this paper, the measurement formula of the shielding performance of mismatched cables is derived through experiments. The results show that the error of the measurement formula is no more than 8 dB. Then, through the experiment of restraining the interference of magnetic materials on the electromagnetic field, it is concluded that the magnetic field shielding performance can reach 20 dB. Then, through the performance test of electromagnetic field shielding materials, the shielding efficiency of metal fiber antiradiation materials is the largest, and the average efficiency reaches 76.4 dB.

Magnetic modulation of Fano resonances in optically thin terahertz superlattice metasurfaces

Optically thin metasurfaces operating at sub-skin depth thicknesses are intriguing because of their associated low plasmonic losses (compared to optically thick, beyond skin-depth metasurfaces). However, their applicability is restricted largely because of reduced free space coupling with incident radiations resulting in limited electromagnetic responses. To overcome such limitations, we propose enhancement of effective responses (resonances) in sub-skin depth metasurfaces through incorporation of a magneto-transport (giant magneto resistance) concept. Here, we experimentally demonstrate dynamic magnetic modulation of structurally asymmetric metasurfaces (consisting of superlattice arrangement of thin (∼10 nm each) magnetic (Ni)/nonmagnetic (Al) layers) operating in the terahertz (THz) domain. With increasing magnetic field (applied from 0 to 30 mT approximately, implies increasing superlattice conductivity), we observe stronger confinement of electromagnetic energy at the resonances (both in dipole and Fano modes). Therefore, this study introduces a unique magnetically reconfigurable ability in Fano resonant THz metamaterials, which directly improves their performances operating in the sub-skin depth regime. Our study can be explained by spin-dependent THz magneto-transport phenomena in metals and can stimulate the paradigm for on-chip spin-based photonic technology enabling dynamic magnetic control over compact, sub-wavelength, sub-skin depth metadevices.

Analysis of current-in-plane giant magnetoresistance using Co2FeAl0.5Si0.5 half-metallic Heusler alloy

There has been renewed interest in current-in-plane giant magnetoresistance (CIP-GMR) devices for high-sensitivity magnetic sensors. However, further improvement in magnetoresistance (MR) ratio is necessary to achieve sufficient magnetic field sensitivity. Use of a half-metallic Co-based Heusler alloy ferromagnetic (FM) layer has been demonstrated to be effective in enhancing GMR in the configuration with current perpendicular to the plane; however, only small MR ratios are obtained in the CIP configuration. To understand the origin of the disappointingly low MR in the CIP configuration when using Heusler alloy FM layers, we investigated the magnetotransport properties of CIP-GMR devices using half-metallic Co2FeAl0.5Si0.5 (CFAS) Heusler alloy and conventional CoFe alloy as the FM layers in combination with Ag or Cu as a nonmagnetic (NM) spacer layer. Regardless of the high lattice and electronic band matching at the CFAS/Ag interface, CFAS/Ag CIP spin valves (SVs) show a MR ratio of only 1.2% at room temperature, which is much smaller than those of reference CoFe/Cu and CoFe/Ag SVs (21.6% and 8.4%, respectively). Current density distribution simulations suggest that large current shunting occurs in the Ag layer due to the significant resistivity gap between CFAS and Ag, which limits the generation of highly spin-polarized current from the CFAS layer, resulting in the very small MR ratios. To enhance the MR ratio in CIP-GMR using half-metallic materials, resistivity matching between FM layers and the NM layer is required, in addition to the high electronic band match that has been considered, as a key factor to obtain a high MR ratio in CIP-GMR devices.

GdV6Sn6: A Multi-carrier Metal with Non-magnetic 3d-electron Kagome Bands and 4f-electron Magnetism

Electronic properties of the single crystal of GdV6Sn6, where non-magnetic V-kagome layers are separated by magnetic Gd-triangular lattice, are investigated. GdV6Sn6 exhibits unique magnetotransport properties at low-temperature such as non-linear Hall resistivity and increase of resistance R in magnetic field H as R ~ H^0.75 up to 56 T with Shubnikov-De Haas oscillations. Investigation of the non-magnetic analogue YV6Sn6 and the first principles calculations reveal these properties are relevant to the bands arising from the V-kagome layer. A magnetic transition at 5 K in GdV6Sn6 modifies the transport properties, pointing to a coupling between Gd-spins on the triangular lattice and carriers in the V-kagome layer.

Systemic consequences of disorder in magnetically self-organized topological MnBi2Te4/(Bi2Te3) n superlattices

MnBi2Te4/(Bi2Te3) n materials system has recently generated strong interest as a natural platform for the realization of the quantum anomalous Hall (QAH) state. The system is magnetically much better ordered than substitutionally doped materials, however, the detrimental effects of certain disorders are becoming increasingly acknowledged. Here, from compiling structural, compositional, and magnetic metrics of disorder in ferromagnetic (FM) MnBi2Te4/(Bi2Te3) n it is found that migration of Mn between MnBi2Te4 septuple layers (SLs) and otherwise non-magnetic Bi2Te3 quintuple layers (QLs) has systemic consequences—it induces FM coupling of Mn-depleted SLs with Mn-doped QLs, seen in ferromagnetic resonance as an acoustic and optical resonance mode of the two coupled spin subsystems. Even for a large SL separation (n 4 QLs) the structure cannot be considered as a stack of uncoupled two-dimensional layers. Angle-resolved photoemission spectroscopy and density functional theory studies show that Mn disorder within an SL causes delocalization of electron wave functions and a change of the surface band structure as compared to the ideal MnBi2Te4/(Bi2Te3) n . These findings highlight the critical importance of inter- and intra-SL disorder towards achieving new QAH platforms as well as exploring novel axion physics in intrinsic topological magnets.

Antiferromagnetic spin pumping via hyperfine interaction

Spin pumping is an interfacial spin current generation from the ferromagnetic layer to the non-magnetic metal at its interface. The polarization of the pumped spin current $$\mathbf {J}_s \propto \mathbf {m}\times \dot{\mathbf {m}}$$ depends on the dynamics of the magnetic moment $$\mathbf {m}$$ . When the materials are based on light transition metals, mechanism behind the spin current transfer is dominated by the exchange interaction between spin of localized d-electrons and itinerant conduction electrons. In heavier transition metals, however, the interaction is not limited to the exchange interaction. The spin of the conduction electron can interact to its nuclear spin by means of hyperfine interaction, as observed in the shift of NMR frequency. By studying the spin polarization of conduction electron of the non-magnetic metallic layer due to a nuclear magnetic moment $$\mathbf{I}$$ of the ferromagnetic layer, we show that the hyperfine interaction can mediate the spin pumping. The polarization of the spin current generation is shown to have a similar form $${J}_s\propto \mathbf {I}\times \dot{\mathbf {I}}$$ .