Using an atomistic spin model, we have simulated spin wave injection and propagation into antiferromagnetic IrMn from an exchange coupled CoFe layer. The spectral characteristics of the exited spin waves have a complex beating behavior arising from the non-collinear nature of the antiferromagnetic order. We find that the frequency response of the system depends strongly on the strength and frequency of oscillating field excitations. We also find that the strength of excited spin waves strongly decays away from the interfacial layer with a frequency dependent attenuation. Our findings suggest that spin waves generated by coupled ferromagnets are too weak to reverse IrMn in their entirety even with resonant excitation of a coupled ferromagnet. However, efficient spin wave injection into the antiferromagnet is possible due to the non-collinear nature of the IrMn spin ordering.
We investigate the impact of pinned antiferromagnetic order on the decoherence of spin current in polycrystalline IrMn. In NiFe/Cu/IrMn/CoFe multilayers, we coherently pump an electronic spin current from NiFe into IrMn, whose antiferromagnetic order is globally pinned by static exchange-bias coupling with CoFe. We observe no anisotropic spin decoherence with respect to the orientation of the pinned antiferromagnetic order. We also observe no difference in spin decoherence for samples with and without pinned antiferromagnetic order. Moreover, although there is a pronounced resonance linewidth …
We study the perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO bilayers in contact with W, Ta, IrMn and Ti which has been suggested as the spin-orbit-torque-related underlayers. The saturation magnetization of the CoFeB depends on the underlayer materials due to formation of a dead-layer, affecting PMA strength of each film. The x-ray magnetic circular dichroism measurement reveals that the interfacial intermixing suppresses only the perpendicular orbital moment of Fe, while the intermixing simultaneously suppresses both the perpendicular and in-plane orbital moments of Co.
We investigated a possible correlation between spin Hall angles and exchange bias in Ni$_{81}$Fe$_{19}$/IrMn samples by performing spin torque ferromagnetic resonance measurements. This correlation is probed by patterning of Ni$_{81}$Fe$_{19}$/IrMn bilayers in different relative orientations with respect to the exchange bias direction. The measured voltage spectra allow a quantitative determination of spin Hall angles, which are independent of the orientation around 2.8\pm0.3%.
The exchange interaction determines the ferromagnetic (FM) or antiferromagnetic (AFM) ordering of atomic spins. When ferromagnets and antiferromagnets are coupled together, they often exhibit the exchange bias effect, a unidirectional interface exchange field causing a shift of the magnetic hysteresis loop. The effective magnitude of this interface exchange field is at most a few percent of the bulk exchange, arising from pinned interfacial spins in the antiferromagnet. The pinned spins are known to comprise a small fraction of the total number of interface spins, yet their exact nature and ph…
Anti-ferromagnetic materials have the possibility to offer ultra fast, high data density spintronic devices. A significant challenge is the reliable detection of the state of the antiferromagnet, which can be achieved using exchange bias. Here we develop an atomistic spin model of the athermal training effect, a well known phenomenon in exchange biased systems where the bias is significantly reduced after the first hysteresis cycle. We find that the setting process in granular thin films relies on the presence of interfacial mixing between the ferromagnetic and antiferromagnetic layers. We sys…
Positive exchange bias has been observed in the Ni$_{81}$Fe$_{19}$/Ir$_{20}$Mn$_{80}$ bilayer system via soft x-ray resonant magnetic scattering. After field cooling of the system through the blocking temperature of the antiferromagnet, an initial conventional negative exchange bias is removed after training i. e. successive magnetization reversals, resulting in a positive exchange bias for a temperature range down to 30 K below the blocking temperature (450 K). This new manifestation of magnetic training is discussed in terms of metastable magnetic disorder at the magnetically frustrated inte…
Antiferromagnetic spintronic devices have the potential to outperform conventional ferromagnetic devices due to their ultrafast dynamics and high data density. A challenge in designing these devices is the control and detection of the orientation of the anti-ferromagnet. One of the most promising ways to achieve this is through the exchange bias effect. This is of particular importance in large scale multigranular devices. Due to the large system sizes, previously, only micromagnetic simulations have been possible, these have an assumed distribution of antiferromagnetic anisotropy directions. …
In-situ, device level measurement of thermal mag-noise spectral linewidths in 60nm diameter CPP-GMR spin-valve stacks of IrMn/ref/Cu/free, with reference and free layer of similar CoFe/CoFeGe alloy, are used to simultaneously determine the intrinsic Gilbert damping for both magnetic layers. It is shown that careful alignment at a "magic-angle" between free and reference layer static equilibrium magnetization can allow direct measurement of the broadband intrinsic thermal spectra in the virtual absence of spin-torque effects which otherwise grossly distort the spectral line shapes and require l…
In spintronics applications, ultrafast spin dynamics have to be controlled at femtosecond (fs) timescales via fs-laser radiation. At such ultrafast timescales, the effect of the Gilbert damping factor α on ultrafast demagnetization time should be considered. In previous explorations for the relationship between these two parameters, it was found that the theoretical calculations based on the local spin-flip scattering model do not agree with the experimental results. Here, we find that in Fe81Ga19(FeGa)/Ir20Mn80(IrMn) bilayers, the unconventional IrMn thickness dependence of α results from the…
We present a comprehensive study of the exchange bias effect in a model system. Through numerical analysis of the exchange bias and coercive fields as a function of the antiferromagnetic layer thickness we deduce the absolute value of the averaged anisotropy constant of the antiferromagnet. We show that the anisotropy of IrMn exhibits a finite size effect as a function of thickness. The interfacial spin disorder involved in the data analysis is further supported by the observation of the dual behavior of the interfacial uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry …
Pure spin current based research is mostly focused on ferromagnet (FM)/heavy metal (HM) system. Because of the high spin orbit coupling (SOC) these HMs exhibit short spin diffusion length and therefore possess challenges for device application. Low SOC (elements of light weight) and large spin diffusion length make the organic semiconductors (OSCs) suitable for future spintronic applications. From theoretical model it is explained that, due to $π$ - $σ$ hybridization the curvature of the C$_{60}$ molecules may increase the SOC strength. Here, we have investigated spin pumping and inverse spin …
A modified effective field model was developed to quantitatively interpret the angular dependent magnetization reversal processes in exchange biased Fe/IrMn bilayers. Several kinds of multi-step loops with distinct magnetization reversal routes were observed for the samples measured at various field orientations. Two types of angular dependent switching fields are observed and their transitions are investigated, which are found to be driven by both Fe and IrMn layer thicknesses. Our modified effective field model can nicely describe all the switching field behaviors including the critical effe…
We demonstrate an anomalous spin-orbit torque induced by the broken magnetic symmetry in the antiferromagnet IrMn. We study the magnetic structure of three phases of IrMn thin films using neutron diffraction technique. The magnetic mirror symmetry M' is broken laterally in both L10-IrMn and L12-IrMn3 but not γ-IrMn3. We observe an out-of-plane damping-like spin-orbit torque in both L10-IrMn/permalloy and L12-IrMn3/permalloy bilayers but not in γ-IrMn3/permalloy. This is consistent with both the symmetry analysis on the effects of a broken M' on spin-orbit torque and the theoretical predictions…
We report an investigation of temperature and IrMn layered thickness dependence of anomalous-Hall resistance (AHR), anisotropic magnetoresistance (AMR), and magnetization on Pt/Ir20Mn80/Y3Fe5O12 (Pt/IrMn/YIG) heterostructures. The magnitude of AHR is dramatically enhanced compared with Pt/YIG bilayers. The enhancement is much more profound at higher temperatures and peaks at the IrMn thickness of 3 nm. The observed spin-Hall magnetoresistance (SMR) in the temperature range of 10-300 K indicates that the spin current generated in the Pt layer can penetrate the entire thickness of the IrMn layer…
Iridium is a very promising material for spintronic applications due to its interesting magnetic properties such as large RKKY exchange coupling as well as its large spin-orbit coupling value. Ir is for instance used as a spacer layer for perpendicular synthetic antiferromagnetic or ferrimagnet systems. However, only a few studies of the spintronic parameters of this material have been reported. In this paper, we present inverse spin Hall effect - spin pumping ferromagnetic resonance measurements on CoFeB/Ir based bilayers to estimate the values of the effective spin Hall angle, the spin diffu…
We observe the magnetic proximity effect (MPE) in Pt/CoFe2O4 bilayers grown by molecular beam epitaxy. This is revealed through angle-dependent magnetoresistance measurements at 5 K, which isolate the contributions of induced ferromagnetism (i.e. anisotropic magnetoresistance) and spin Hall effect (i.e. spin Hall magnetoresistance) in the Pt layer. The observation of induced ferromagnetism in Pt via AMR is further supported by density functional theory calculations and various control measurements including insertion of a Cu spacer layer to suppress the induced ferromagnetism. In addition, ano…
We demonstrate that an antiferromagnet can be employed for a highly efficient electrical manipulation of a ferromagnet. In our study we use an electrical detection technique of the ferromagnetic resonance driven by an in-plane ac-current in a NiFe/IrMn bilayer. At room temperature, we observe antidamping-like spin torque acting on the NiFe ferromagnet, generated by the in-plane current driven through the IrMn antiferromagnet. A large enhancement of the torque, characterized by an effective spin-Hall angle exceeding most heavy transition metals, correlates with the presence of the exchange-bias…
We report an exchange bias of more than $2700\,$Oe at room temperature in MnN/CoFe bilayers after high-temperature annealing. We studied the dependence of exchange bias on the annealing temperature for different MnN thicknesses in detail and found that samples with $t_{\text{MnN}}>32\,$nm show an increase of exchange bias for annealing temperatures higher than $T_{\text{A}}=400\,^{\circ}$C. Maximum exchange bias values exceeding $2000\,$Oe with reasonably small coercive fields around $600\,$Oe are achieved for $t_{\text{MnN}}= 42, 48\,$nm. The median blocking temperature of those systems is de…
Spin-to-heat current conversion effects have been investigated in bilayer films consisting of a paramagnetic metal (PM; Pt, W, or Ta) and a ferromagnetic metal (FM; CoFeB or permalloy). When a charge current is applied to the PM/FM bilayer film, a spin current is generated across the PM/FM interface owing to the spin Hall effect in PM. The spin current was found to exhibit cooling and heating features depending on the sign of the spin Hall angle of PM, where the spin-current-induced contribution is estimated by subtracting the contribution of the anomalous Ettingshausen effect in FM monolayer …
Non-collinear antiferromagnets can have additional spin Hall effects due to the net chirality of their magnetic spin structure, which provides for more complex spin-transport phenomena compared to ordinary non-magnetic materials. Here we investigated how ferromagnetic resonance of permalloy ($Ni_{80}Fe_{20}$) is modulated by spin Hall effects in adjacent epitaxial $IrMn_{3}$ films. We observe a large dc modulation of the ferromagnetic resonance linewidth for currents applied along the [001] $IrMn_{3}$ direction. This very strong angular dependence of spin-orbit torques from dc currents through…
The effect of nonmagnetic metal additives in metallic antiferromagnets (AFMs) on the exchange bias (EB) has been investigated from a structural, magnetic and Monte Carlo simulation point of view in bilayers of CoFe/IrMn:Cu. Dilution by Cu atoms throughout the volume of the AFM IrMn give rise to an enhanced EB field for temperatures between 5 K and 300 K. The thermoremanent magnetization of the AFM-only is also enhanced with dilution, shows qualitatively the same temperature dependence as EB field and is at the origin of the EB mechanism. Monte Carlo simulations based on a Heisenberg model and …
CoFe/FeMn, FeMn/CoFe bilayers and CoFe/FeMn/CoFe trilayers were grown in magnetic field and at room temperature. The exchange bias field $H_{eb}$ depends strongly on the order of depositions and is much higher at CoFe/FeMn than at FeMn/CoFe interfaces. By combining the two bilayer structures into symmetric CoFe/FeMn($t_\mathrm{FeMn}$)/CoFe trilayers, $H_{eb}^t$ and $H_{eb}^b$ of the top and bottom CoFe layers, respectively, are both enhanced. Reducing $t_\mathrm{FeMn}$ of the trilayers also results in enhancements of both $H_{eb}^b$ and $H_{eb}^t$. These results evidence the propagation of exc…
We show how compensated antiferromagnetic spins can be implemented in the micromagnetic simulation program MuMax3. We demonstrate that we can model spin flop coupling as a uniaxial anisotropy for small canting angles and how we can take into account the exact energy terms for strong coupling between a ferromagnet and compensated antiferromagnet. We also investigate the training effect in biaxial antiferromagnets and reproduce the training effect in a polycrystalline IrMn/CoFe bilayer.
The effects of inserting impurity $δ$-layers of various elements into a Co/IrMn exchange biased bilayer, at both the interface, and at given points within the IrMn layer a distance from the interface, has been investigated. Depending on the chemical species of dopant, and its position, we found that the exchange biasing can be either strongly enhanced or suppressed. We show that biasing is enhanced with a dusting of certain magnetic impurities, present at either at the interface or sufficiently far away from the Co/IrMn interface. This illustrates that the final spin structure at the Co/IrMn i…
Antiferromagnetic spintronics is an emerging field; antiferromagnets can improve the functionalities of ferromagnets with higher response times, and having the information shielded against external magnetic field. Moreover, a large list of aniferromagnetic semiconductors and metals with Néel temperatures above room temperature exists. In the present manuscript, we persevere in the quest for the limits of how large can anisotropic magnetoresistance be in antiferromagnetic materials with very large spin-orbit coupling. We selected IrMn as a prime example of first-class moment (Mn) and spin-orbit…
We report on the new polycrystalline exchange bias system MnN/CoFe, which shows exchange bias of up to 1800Oe at room temperature with a coercive field around 600Oe. The room temperature values of the interfacial exchange energy and the effective uniaxial anisotropy are estimated to be $J_\mathrm{eff} = 0.41\,\mathrm{mJ}/\mathrm{m}^2$ and $K_\mathrm{eff} = 37\,\mathrm{kJ}\,/\,\mathrm{m}^3$. The thermal stability was found to be tunable by controlling the nitrogen content of the MnN. The maximum blocking temperature exceeds $325^\circ$C, however the median blocking temperature in the limit of t…
Pure spin currents hold promises for an energy-friendlier spintronics. They can be generated by a flow of charge along a non-magnetic metal having a large spin-orbit coupling. It produces a spin accumulation at its surfaces, controllable by the magnetization of an adjacent ferromagnetic layer. Paramagnetic metals typically used are close to a ferromagnetic instability and thus magnetic proximity effects can contribute to the observed angular-dependent magnetoresistance (ADMR). As interface phenomena govern the spin conductance across the metal/ferromagnetic-insulator heterostructures, unraveli…
The antiferromagnet (AFM) / ferromagnet (FM) interfaces are of central importance in recently developed pure electric or ultrafast control of FM spins, where the underlying mechanisms remain unresolved. Here we report the direct observation of Dzyaloshinskii Moriya interaction (DMI) across the AFM/FM interface of IrMn/CoFeB thin films. The interfacial DMI is quantitatively measured from the asymmetric spin wave dispersion in the FM layer using Brillouin light scattering. The DMI strength is enhanced by a factor of 7 with increasing IrMn layer thickness in the range of 1- 7.5 nm. Our findings p…
The spin Hall magnetoresistance (SMR) phenomenon includes the fundamental physics of spin current, and originates from spin accumulation at an interface owing to the spin Hall effect. Although bilayers are the simplest structure exhibiting SMR, these exploit spin accumulation at only one side of a layer. Herein, trilayers of CoFe2O4/Pt/CoFe2O4 were fabricated and their spin Hall magnetoresistance was investigated. The trilayer structure featuring a thin Pt layer exhibited an SMR ratio four times that of a CoFe2O4/Pt bilayer. Further, the SMR ratio exhibited a dependence on Pt layer thickness t…
Exchange bias in ferromagnetic/antiferromagnetic systems can be explained in terms of various interfacial phenomena. Among these spin glass frustration can affect the magnetic properties in exchange bias systems. Here we have studied a NiMn/CoFeB exchange bias system in which spin glass frustration seems to play a crucial role. In order to account the effect of spin glass frustration on magnetic properties, we have performed the temperature and cooling field dependence of exchange bias. We have observed the decrease of exchange bias field (HEB) with cooling field (HFC) whereas there is not sig…
In this study we have compared magnetic, magnetostrictive and piezomagnetic properties of isotropic and anisotropic cobalt ferrite pellets. The isotropic sample was prepared by the ceramic method while the sample exhibiting uniaxial anisotropy was made by reactive sintering using Spark Plasma Sintering (SPS). This technique permits to induce a magnetic anisotropy in cobalt ferrite in the direction of the applied pressure during SPS process. Sample with uniaxial anisotropy revealed a higher longitudinal magnetostriction and piezomagnetism compared to the isotropic sample, but the transversal ma…
A charge flow through a magnetic tunnel junction (MTJ) leads to the generation of a spin-polarized current which exerts a spin-transfer torque (STT) on the magnetization. When the density of applied direct current exceeds some critical value, the STT excites high-frequency magnetization precession in the "free" electrode of MTJ. Such precession gives rise to microwave output voltage and, furthermore, can be employed for spin pumping into adjacent normal metal or semiconductor. Here we describe theoretically the spin dynamics and charge transport in the CoFeB/MgO/CoFeB/Au tunneling heterostruct…
We study the interaction of surface acoustic waves with spin waves in ultra-thin CoFeB/Pt bilayers. Due to the interfacial Dzyaloshinskii-Moriya interaction (DMI), the spin wave dispersion is non-degenerate for oppositely propagating spin waves in CoFeB/Pt. In combination with the additional nonreciprocity of the magnetoacoustic coupling itself, highly nonreciprocal acoustic wave transmission through the magnetic film is observed. We systematically characterize the magnetoacoustic wave propagation in a thickness series of CoFeB($d$)/Pt samples as a function of magnetic field magnitude and dire…
There has been considerable interest in spin-orbit torques for the purpose of manipulating the magnetization of ferromagnetic (FM) films or nano-elements for spintronic technologies. Spin-orbit torques are derived from spin currents created from charge currents in materials with significant spin-orbit coupling that diffuse into an adjacent FM material. There have been intensive efforts to search for candidate materials that exhibit large spin Hall angles, i.e. efficient charge to spin current conversion. Here we report, using spin torque ferromagnetic resonance, the observation of a giant spin…
We report the experimental observation of the spin Seebeck effect (SSE) in Ag/CoFe noble metal/magnetic metal bilayers with a longitudinal structure. Thermal voltages jointly generated by the anomalous Nernst effect (ANE) and the SSE were detected across the Ag/CoFe/Cu strip with a perpendicular thermal gradient. To effectively separate the SSE and the ANE part of the thermal voltages, we compared the experimental results between the Ag/CoFe/Cu strip and Cu/CoFe/Cu strip, where two samples processed with the heating power instead of the temperature difference through the thin CoFe film. The re…
Polar and quadratic magneto-optical Kerr effects in nonmagnetic/ferromagnet bilayers for spin-orbit torque measurements
The University of Tokyo（东京大学）；Research Institute of Electrical Communication (RIEC), Tohoku University（东北大学电气通信研究所）；Mie University（三重大学）；Trans-scale quantum science institute (TSQS), The University of Tokyo（东京大学跨尺度量子科学研究所）
Estimation of spin-orbit torques in the presence of current-induced magnon creation and annihilation
Université de Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux de Strasbourg（斯特拉斯堡大学，法国国家科学研究中心，斯特拉斯堡材料与化学物理研究所）；ETH Zurich（苏黎世联邦理工学院）；Politecnico di Milano（米兰理工大学）
In this work, IrMn$_{3}$/insulating-Y$_{3}$Fe$_{5}$O$_{12}$ exchange-biased bilayers are studied. The behavior of the net magnetic moment $Δm_{AFM}$ in the antiferromagnet is directly probed by anomalous and planar Hall effects, and anisotropic magnetoresistance. The $Δm_{AFM}$ is proved to come from the interfacial uncompensated magnetic moment. We demonstrate that the exchange bias and rotational hysteresis are induced by the irreversible switching of the $Δm_{AFM}$. In the training effect, the $Δm_{AFM}$ changes continuously. This work highlights the fundamental role of the $Δm_{AFM}$ in th…
Reducing critical current for spin-transfer-torque-induced magnetization reversal in CPP-GMR devices: effect of low damping and enhanced spin scattering asymmetry in $Co_2FeGa_{0.5}Ge_{0.5}$ Heusler alloy
We have studied the spin Hall magnetoresistance (SMR), the magnetoresistance within the plane transverse to the current flow, of Pt/Co bilayers. We find that the SMR increases with increasing Co thickness: the effective spin Hall angle for bilayers with thick Co exceeds the reported values of Pt when a conventional drift-diffusion model is used. An extended model including spin transport within the Co layer cannot account for the large SMR. To identify its origin, contributions from other sources are studied. For most bilayers, the SMR increases with decreasing temperature and increasing magne…
Spin Hall magnetoresistance (SMR) is studied in metallic bilayers that consist of heavy metal (HM) layer and a ferromagnetic metal (FM) layer. We find nearly a ten-fold increase of SMR in W/CoFeB compared to previously studied HM/ferromagnetic insulator (FI) systems. The SMR increases with decreasing temperature despite the negligible change in the W layer resistivity with temperature. A model is developed to account for the absorption of the longitudinal spin current to the FM layer, one of the key characteristics of a metallic ferromagnet. We find that the model not only quantitatively descr…
Using a three-dimensional focused-ion beam lithography process we have fabricated nanopillar devices which show spin transfer torque switching at zero external magnetic fields. Under a small in-plane external bias field, a field-dependent peak in the differential resistance versus current is observed similar to that reported in asymmetrical nanopillar devices. This is interpreted as evidence for the low-field excitation of spin waves which in our case is attributed to a spin-scattering asymmetry enhanced by the IrMn exchange bias layer coupled to a relatively thin CoFe fixed layer.
Spin-orbit torques (SOTs) have opened a new path to switch the magnetization in perpendicularly magnetized films and are of great interest due to their potential applications in novel data storage technology, such as the magnetic random access memory (MRAM). The effective manipulation of SOT has thus become an important step towards these applications. Here, current induced spin-orbit effective fields and magnetization switching are investigated in Pt/Ta/CoFeB/MgO structures with bilayer heavy metals. With a fixed thickness (1 nm) of the Ta layer, the magnitude and sign of current induced spin…
While the principal features of the exchange bias between a ferromagnet and an antiferromagnet are believed to be understood, a quantitative description is still lacking. We show that interface spin disorder is the main reason for the discrepancy of model calculations versus experimental results. Taking into account spin disorder at the interface between the ferromagnet and the antiferromagnet by modifying the well known Meiklejohn and Bean model, an almost perfect agreement can be reached. As an example this is demonstrated for the CoFe/IrMn exchange biased bilayer by analyzing the azimuthal …
The electronic and optoelectronic properties of two dimensional materials have been extensively explored in graphene and layered transition metal dichalcogenides (TMDs). Spintronics in these two-dimensional materials could provide novel opportunities for future electronics, for example, efficient generation of spin current, which should enable the efficient manipulation of magnetic elements. So far, the quantitative determination of charge current induced spin current and spin-orbit torques (SOTs) on the magnetic layer adjacent to two-dimensional materials is still lacking. Here, we report a l…
We investigate the effect of a direct current on propagating spin waves in a CoFeB/Ta bilayer structure. Using the micro-Brillouin light scattering technique, we observe that the spin wave amplitude may be attenuated or amplified depending on the direction of the current and the applied magnetic field. Our work suggests an effective approach for electrically controlling the propagation of spin waves in a magnetic waveguide and may be useful in a number of applications such as phase locked nano-oscillators and hybrid information processing devices.
An in-field interaction plot, delta-M_R, has been recently introduced, presenting important advantages over the classical remanence plots. Here a general delta-M_R is proposed, allowing to assess interactions even in systems with shifted and asymmetric major loops. To construct such a plot, a recoil loop and the position of the center of the major loop are only needed. Applying the method on exchange-biased Co/IrMn bilayer gives two types of delta-M_R obtained for measuring field either parallel or antiparallel to the exchange-bias direction. This provides valuable information on the reversal …
Ultrafast manipulation of magnetic order has challenged our understanding the fundamental and dynamic properties of magnetic materials. So far single shot magnetic switching has been limited to ferrimagnetic alloys and multilayers. In ferromagnetic (FM)/antiferromagnetic (AFM) bilayers, exchange bias (He) arises from the interfacial exchange coupling between the two layers and reflects the microscopic orientation of the antiferromagnet. Here we demonstrate the possibility of single shot switching of the antiferromagnet (change of the sign and amplitude of He) with a single femtosecond laser pu…
