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For the spin-to-charge conversion (SCC) in heavy metal/ferromagnet (HM/FM) heterostructure, the contribution of interfacial spin-orbit coupling (SOC) remains controversial. Here, we investigate the SCC process of the Pt/NiFe heterostructure by the spin pumping in YIG/Pt/NiFe/IrMn multilayers. Due to the exchange bias of NiFe/IrMn structure, the NiFe magnetization can be switched between magnetically unsaturated and saturated states by opposite resonance fields of YIG layer. The spin-pumping signal is found to decrease significantly when the NiFe magnetization is changed from the saturated state to the unsaturated state. Theoretical analysis indicates that the interfacial spin absorption is enhanced for the above-mentioned NiFe magnetic state change, which results in the increased and decreased spin flow in the Pt layer and across the Pt/NiFe interface, respectively. These results demonstrate that in our case the interfacial SOC effect at the Pt/NiFe interface is dominant over the bulk inverse spin Hall effect in the Pt layer. Our work reveals a significant role of interfacial SOC in the SCC in HM/FM heterostructure, which can promote the development of high-efficiency spintronic devices through interfacial engineering.

Thanks to the strong perpendicular magnetic anisotropy (PMA), excellent processing compatibility as well as novel spintronic phenomenon, Co/Pt multilayers have been attracting massive attention and widely used in magnetic storage. However, reversed magnetic domains come into being with the increasing layer repetition ‘N’ to reduce magneto-static energy, resulting in the remarkable diminishment of the remanent magnetization (M r). As a result, the product of M r and thickness (i.e., the remanent moment-thickness product, M r t), a key parameter in magnetic recording for reliable data storing and reading, also decreases dramatically. To overcome this issue, we deposit an ultra-thick granular [Co/Pt]80 multilayer with a total thickness of 68 nm on granular SiN x buffer layer. The M r t value, M r to saturation magnetization (M s) ratio as well as out of plane (OOP) coercivity (H coop) are high up to 2.97 memu/cm2, 67%, and 1940 Oe (1 Oe = 79.5775 A⋅m−1), respectively, which is remarkably improved compared with that of continuous [Co/Pt]80 multilayers. That is because large amounts of grain boundaries in the granular multilayers can efficiently impede the propagation and expansion of reversed magnetic domains, which is verified by experimental investigations and micromagnetic simulation results. The simulation results also indicate that the value of M r t, M r/M s ratio, and H coop can be further improved through optimizing the granule size, which can be experimentally realized by manipulating the process parameter of SiN x buffer layer. This work provides an alternative solution for achieving high M r t value in ultra-thick Co/Pt multilayers, which is of unneglectable potential in applications of high-density magnetic recording.

Antiferromagnetic insulators have recently been proved to support spin current efficiently. Here, we report the damping-like spin- orbit torque (SOT) in Pt / NiO / CoFeB has a strong temperature dependence and reverses the sign below certain temperatures, which is different from the slight variation with temperature in the Pt / CoFeB bilayer. The negative damping-like SOT at low temperatures is proposed to be mediated by the magnetic interactions that tie with the “exchange bias” in Pt / NiO / CoFeB, in contrast to the thermal-magnon-mediated scenario at high temperatures. Our results highlight the promise to control the SOT through tuning the magnetic structure in multilayers.

Ferromagnetic noncollinear spin valve effect has been observed in a magnetic multilayer structure of IrMn/NiFe/Cu/Y3Fe5O12 (YIG). In this structure, the detected amplitude of spin current emitted by spin pumping depends on the NiFe magnetic states (saturation or unsaturation). This yields a spin valve-like behavior with reliable amplitude changes up to 110% and the behavior may originate from the difference of spin absorption when the NiFe layer is magnetically saturated or not. Our results provide a ferromagnetic noncollinear spin valve, paving the road towards ultralow-dissipation spintronic devices based on pure spin current.

We present an experimental observation of interfacial spin accumulation induced by anomalous Hall effect (AHE) in ferromagnets (FMs) in a multilayer structure of FM/YIG/Pt, where the direction of charge current injection in FM layer is perpendicular to the direction of voltage detection in Pt layer. In this structure, the magnon-mediated drag voltage (Vdrag) due to the interfacial spin accumulation induced by AHE, can be unambiguously separated from spin Seebeck voltage (VSSE) by sweeping or rotating the applied magnetic field. Field-dependent spin accumulation induced by AHE has been observed by comparison of nonlocal voltages between Ni/Cu/YIG/Pt and Pt/YIG/Pt samples. Furthermore, we demonstrate that the AHE voltage strongly depends on the spin polarization of conductivity and spin Hall angles of electrons with opposite spins in FMs. Our results show a prospect for FMs to be field-control spin generators via AHE, and provide a new viewpoint to realize the AHE.

We report that the unusual behavior of anomalous Hall effect (AHE) in permalloy (Fe20Ni80, Py) thin film with silicon oxide (SiO2) as an adjacent layer. It is found that sign of AHE changes from positive to negative with decrement in thickness of Py. Through the AHE measurement, we demonstrate that this unusual behavior originates from the interface of Py/SiO2 rather from the structural inversion asymmetry. In the Pt/YIG/Cu/Py/SiO2 multilayer structure with a charge current injected in the Py layer, the information on spin accumulation at the YIG/Cu interface induced by the AHE in Py, can be obtained from the nonlocal voltage measured in the Pt layer. Through the nonlocal measurement, it is found, however, the electrons with same spin for interfacial Py near the SiO2 layer and bulk Py are scattered to the same direction. Hence we can deduce that the sign of spin polarization of the conductivity closely related to the asymmetry in density of states at the Fermi level for interfacial Py and bulk Py should be opposite to each other, which is responsible for the abnormal AHE observed. Our results demonstrate that oxide layer can drive the change of sign of AHE of Py, which provide an alternative way of manipulating the spin-related transport.

The exchange bias field (Hex) and tunneling magnetoresistance (TMR) ratio play an important role for the applications of magnetic tunnel junction (MTJ) in spintronic devices. Here the structure, magnetic properties, and TMR ratio are systematically investigated for the IrMn-based MTJ (IrMn/CoFe/Ru/CoFe/CoFeB/MgO/CoFeB) with seed layers of Ta, Ta/Ru, NiFeCr and Ta/NiFe. The MTJs with seed layers of Ta/Ru and Ta/NiFe present high Hex values, and the MTJ with seed layer of Ta/Ru shows the highest TMR ratio. We found that the Hex is positively correlated with the (111) texture quality of the IrMn layer while the interdiffusion at the interfaces of CoFeB/MgO/CoFeB has significant impact on the TMR ratio. These results highlight the crucial role of seed layers on the Hex and TMR ratio, and suggest that the Ta/Ru seed layer is beneficial to developing MTJ-based spintronic devices such as magnetic field sensor towards high sensitivity and high stability.

Effective probing current-induced magnetization switching is highly required in the study of emerging spin–orbit torque (SOT) effect. However, the measurement of in-plane magnetization switching typically relies on the giant/tunneling magnetoresistance measurement in a spin valve structure calling for complicated fabrication process, or the non-electric approach of Kerr imaging technique. Here, we present a reliable and convenient method to electrically probe the SOT-induced in-plane magnetization switching in a simple Hall bar device through analyzing the MR signal modified by a magnetic field. In this case, the symmetry of MR is broken, resulting in a resistance difference for opposite magnetization orientations. Moreover, the feasibility of our method is widely evidenced in heavy metal/ferromagnet (Pt/Ni20Fe80and W/Co20Fe60B20) and the topological insulator/ferromagnet (Bi2Se3/Ni20Fe80). Our work simplifies the characterization process of the in-plane magnetization switching, which can promote the development of SOT-based devices.

The magnetization-direction-dependent inverse spin Hall effect (ISHE) has been observed in NiFe film during spin Seebeck measurement in IrMn/NiFe/Cu/yttrium iron garnet (YIG) multilayer structure, where the YIG and NiFe layers act as the spin injector and spin current detector, respectively. By using the NiFe/IrMn exchange bias structure, the magnetization direction of YIG (M YIG) can be rotated with respect to that of NiFe (M NiFe) with a small magnetic field, thus allowing us to observe the magnetization-direction-dependent inverse spin Hall effect voltage in NiFe layer. Compared with the situation that polarization direction of spin current (σ s) is perpendicular to M NiFe, the spin Seebeck voltage is about 30% larger than that when σ s and M NiFe are parallel to each other. This phenomenon may originate from either or both of stronger interface or bulk scattering to spin current when σ s and M NiFe are perpendicular to each other. Our work provides a way to control the voltage induced by ISHE in ferromagnets.

The low Gilbert damping factor, which is usually measured by ferromagnetic resonance, is crucial in spintronic applications. Two-magnon scattering occurs when the orthogonality of the ferromagnetic resonance mode and other degenerate spin wave modes was broken by magnetic anisotropy, voids, second phase, surface defects, etc., which is important in analysis of ferromagnetic resonance linewidth. Direct fitting to linewidth with Gilbert damping is advisable only when the measured linewidth is a linear function of measuring frequency in a broad band measurement. We observe the nonlinear ferromagnetic resonance linewidth of Co2MnSi thin films with respect to measuring frequency in broad band measurement. Experimental data could be well fitted with the model including two-magnon scattering with no fixed parameters. The fitting results show that two-magnon scattering results in the nonlinear linewidth behavior, and the Gilbert damping factor is much smaller than reported ones, indicating that our Co2MnSi films are more suitable for the applications of spin transfer torque.

Co2MnSi thin films are made by magnetron sputtering onto MgO (001) substrates. The crystalline quality is improved by increasing depositing temperature and/or annealing temperature. The sample deposited at 550°C and subsequently annealed at 550°C (sample I) exhibits a pseudo-epitaxial growth with partially ordered L21 phase. Sample I shows a four-fold magnetic anisotropy, in addition to a relatively weak uniaxial anisotropy. The Gilbert damping factor of sample I is smaller than 0.001, much smaller than reported ones. The possible reasons responsible for the small Gilbert damping factor are discussed, including weak spin-orbit coupling, small density of states at Fermi level, and so on.