Magnetization reversal in perpendicular exchange-biased multilayers

Co/Pt multilayers with perpendicular magnetic anisotropy exhibit an exchange bias when covered with an IrMn layer. The exchange bias field, which is about 7 mT for 3 Co/Pt bilayer repetitions and a Co layer thickness of 5 Å, can be increased up to 16.5 mT by the insertion of a thin Pt layer at the Co/IrMn interface. The interfacial magnetic anisotropy of the Co/IrMn interface (KSCo/IrMn =-0.09 mJ/m2) favours in-plane magnetization and tends to tilt the Co spins away from the film normal. Dynamical measurements of the magnetization reversal process reveal that both thermally activated spin reversal in the IrMn layer and domain wall nucleation in the Co/Pt multilayer influence the interfacial spin structure and therefore the strength of the perpendicular exchange bias field.     

Localized spin-wave resonance modes of ferromagnetic microstrips in the field of a magnetic probe

Some results of the micromagnetic modeling of forced magnetization oscillations in planar microstrips of NiFe with easy plane anisotropy and Co/Pt with perpendicular easy axis anisotropy in the field of a magnetic spherical probe are considered. It has been shown that the probe field provokes the appearance of a hedgehog–antivortex coupling state in the NiFe strips, due to its lateral components and a skyrmion magnetization state in the Co/Pt layer. These effects destroy spatial magnetization oscillations in the microstrips and lead to the appearance of additional resonances in the spectrum of oscillations corresponding to the modes localized in the probe field.     

Perpendicular exchange bias of Co/Pt multilayers

Exchange bias measurements of ferromagnetic/antiferromagnetic (F/AF) bilayers are typically performed with the magnetization of the F layer parallel to the AF interface. We describe measurements of Co/Pt multilayers with out-of-plane magnetic easy axis that are exchange biased with CoO. Field-cooling experiments with the applied field perpendicular and parallel to the sample plane exhibit loop shifts and enhanced coercivities. Modeling and comparison to biasing of samples with planar easy axis suggests such measurements provide a way to probe the spin projections at F/AF interfaces.     

First principles studies of Fe/Co superlattices and multilayers with bcc (0 0 1) and (1 1 0) orientations

The layer resolved magnetic moments and magnetic anisotropy energy of Fe/Co superlattices and multilayers with bcc (0 0 1) and (1 1 0) orientations obtained from first principles simulations are reported here. The magnetic moment of Fe atoms are found to depend on the geometry, coordination number and proximity to Co atoms, whereas that of Co remains almost constant in the superlattices and multilayers. Mixing of atoms at the interface resulted in enhanced Fe magnetic moment while that of Co is unaffected. The magnetic anisotropy energy in superlattices and multilayers are found to be larger than the corresponding values of bulk counterparts. Calculated easy axis of magnetization is in the plane for all superlattice compositions considered in the study, while that in multilayers, changes with crystalline orientation and thickness of Co layers.     

Anisotropic interface magnetoresistance in Pt/Co/Pt sandwiches

We report on an effect of reduced dimensionality on the magnetotransport in cobalt layers sandwiched by platinum. In a current in-plane geometry it is found that the resistivity depends on the magnetization orientation within the plane perpendicular to the current direction. The resistivity shows a symmetry adapted cos(2) dependence on the angle to the surface normal, with the maximum along the surface normal. The Co thickness dependence of the effect in Pt/Co/Pt sandwiches clearly points out that the mechanism behind this effect originates at the Co/Pt interfaces and is disparate to the texture induced geometrical size effect.     

Domain wall motion in ferromagnetic systems with perpendicular magnetization

Although we lack clear experimental evidence, apparently out-of-plane magnetized systems are better suited for spintronic applications than the in-plane magnetized ones, mainly due to the smaller current densities required for achieving domain wall motion. [Co/Pt] multilayers belong to the first category of materials, the out-of-plane magnetization orientation arising from the strong perpendicular magnetocrystalline anisotropy. If the magnetization arranges itself out-of-plane narrow Bloch walls occur. In the present paper, both field and current-driven domain wall motion have been investigated for this system, using micromagnetic simulations. Three types of geometries have been taken into account: bulk, thin film and wire, and for all of them a full comparison is done between the effect of the applied field and injected current. The reduction of the system's dimension induces the decrease of the critical field and the critical current, but it does not influence the domain wall displacement mechanism.     

自旋轨道矩调控的垂直磁各向异性四态存储器结构

利用氧化钽缓冲层对垂直各向异性钴铂多层膜磁性的影响,构想并验证了一种四态存储器单元.存储器器件包含两个区域,其中一区域的钴铂多层膜[Pt(3 nm)/Co(0.47 nm)/Pt(1.5 nm)]直接生长在热氧化硅衬底上,另一个区域在磁性膜和衬底之间沉积了一层氧化钽作为缓冲层[TaO x(0.3 nm)/Pt(3 nm)/Co(0.47 nm)/Pt(1.5 nm)],缓冲层导致两个区域的垂直磁各向异性不同.在固定的水平磁场下对器件施加与磁场同向的电流,由于电流引起的自旋轨道耦合力矩,两个区域的磁化取向均会发生翻转,且拥有不同的临界翻转电流.改变通过器件导电通道的电流脉冲形式,器件的磁化状态可以在4个态之间切换.本文器件的结构为设计自旋轨道矩存储器件提供了新的思路.     

The role of nucleation field of media in heat-assisted magnetic probe recording

We characterize a method of heat-assisted magnetic probe recording on perpendicular media. Heating source is field emission current from a scanning tunneling microscope (STM) tip. Recording media are three kinds of magnetic films, Co/Pt, CoNi/Pt, and Co/Pd multilayers with different nucleation fields. Pulses with amplitude of 5 V were applied between the STM tip and the recording medium. Experiments show that magnetic marks with an average size of 180 nm were formed on both Co/Pt and CoNi/Pt films whose nucleation fields are greater than their saturation magnetization. No marks were observed on the Co/Pd film whose nucleation field is smaller than its saturation magnetization. A model is built to simulate the dynamic process of domain formation in probe-based magnetic recording system. Simulation results agree with experiments and it explains the effect of the nucleation field of medium in perpendicular recording.     

Switching plasticity in compensated ferrimagnetic multilayers for neuromorphic computing

Current-induced multilevel magnetization switching in ferrimagnetic spintronic devices is highly pursued for the application in neuromorphic computing. In this work, we demonstrate the switching plasticity in Co/Gd ferrimagnetic multilayers where the binary states magnetization switching induced by spin-orbit toque can be tuned into a multistate one as decreasing the domain nucleation barrier. Therefore, the switching plasticity can be tuned by the perpendicular magnetic anisotropy of the multilayers and the in-plane magnetic field. Moreover, we used the switching plasticity of Co/Gd multilayers for demonstrating spike timing-dependent plasticity and sigmoid-like activation behavior. This work gives useful guidance to design multilevel spintronic devices which could be applied in high-performance neuromorphic computing.     

Magnetization reversal and domains structures in (Co/Ni/Co/Pt) multilayers

Multilayers of [Co/Ni(t_Ni)/Co/Pt]_×4 are investigated for different Ni insertion layer thicknesses. The resulting magnetic properties and magnetic domain structures are compared with [Co/Ni]_×8 multilayers. As determined by magneto-optical Kerr effect microscopy and a vibrating sample magnetometer measurements, all multilayers exhibited a perpendicular magnetic anisotropy. It is found that the nucleation field and magnetic coercivity of [Co/Ni(t)/Co/Pt]_×4 multilayers are lower than (Co/Ni)_×8 and decreased with Ni thickness. Magnetization decay measurements reveal that these multilayers did not show an exponential decay behavior as was observed in rare earth transition metal alloys. Very narrow wires will remain stables for several hours even with an applied magnetic field closer to the coercivity. Insertion of very thin Ni in (Co/Pt) multilayers offers a good way to optimize the magnetic properties of the material and adjust the domain size for nanowire-based devices.     

Magnetization switching modes in nanopillar spin valve under the external field

The current-induced magnetic switching is studied in Co/Cu/Co nanopillar with an in-plane magnetization traversed under the perpendicular-to-plane external field.Magnetization switching is found to take place when the current density exceeds a threshold.By analyzing precessional trajectories,evolutions of domain walls and magnetization switching times under the perpendicular magnetic field,there are two different magnetization switching modes:nucleation and domain wall motion reversal;uniform magnetization ...     

Direct observation of dendritic domain growth in perpendicular magnetic anisotropy CoFe/Pt multilayers

We present the experimental results on thermally activated magnetization reversal for [Co_0.9Fe_0.1(5.0 Å)/Pt(20 Å)]₄ multilayer. Direct domain observations show that magnetization reversal is initiated with rare nucleation and followed by dendritic growth of domain walls. Based on macroscopic magnetic parameters from experimental data, the dendritic domain growth mode is qualitatively interpreted by Monte Carlo simulations in terms of a simple uniaxial magnetic anisotropy model. Moreover, both time evolution of domain growth observation and magnetic relaxation measurements reveal that CoFe/Pt multilayer has a relatively large activation volume compared with Co/Pt multilayers.     

Magnetic properties of Co films and Co/Pt multilayers deposited on PDMS nanostructures

The magnetic properties of magnetron sputtered Co films (with in-plane anisotropy) and Co/Pt multilayers (with perpendicular anisotropy) deposited on elastomeric poly-dimethylsiloxane (PDMS) films and nanostructured templates are presented. Apart from the etched nanosize features, maze-like submicron features develop after the sputter deposition as a result of film buckling due to thermal contraction of the underlying PDMS layer. The nanostructured templates can physically isolate the magnetic entities but magnetic correlations within the range of the buckling features remain. By using oblique deposition geometries in-plane anisotropy develops and it is transferred to the PDMS buckling patterns. In the case of Co/Pt multilayers deposition on PDMS nanostructured templates results in a loss of the perpendicular anisotropy, attributed to the acuteness of the PDMS nanostructures.     

Dependence of Interlayer AF Coupling on Ferromagnetic Layer Thickness in [Pt/Co]5/Ru/[Co/Pt]5 Multilayers

We study magnetization reversal in the interlayer coupled [Pt/Co]5/Ru/[Co/Pt]5 multilayers （MLs） by means of the measurement of extraordinary Hall effect （EHE）. Fitting experimental data to a simple model, we determine the interlayer coupling strength for various thicknesses of the ferromagnetic layers at a fixed Ru spacer thickness. It is found that the dependence of interlayer coupling strength on the Pt layer thickness is much stronger than the previous report in the ferromagnetic/nonmagnetic/ferromagnetic multilayers.     

Resolving magnetic and chemical correlations in CoPtCr films using soft X-ray resonant scattering

Resonant small-angle scattering at the 2p levels of 3d transition elements strongly enhances scattering from both magnetic and chemical structure in the plane of thin films, as recently demonstrated for Co/Pt multilayers having perpendicular anisotropy. Here this resonant enhancement is demonstrated for CoPtCr films having in-plane magnetic anisotropy. A simple formalism describing the spectral dependence of the kinematical scattering provides a means to distinguish between magnetic and charge scattering and to probe the chemical segregation processes yielding charge scattering, thereby providing new information about this structure. It is found that correlation lengths of magnetic scattering are roughly 5 times larger than those for chemical scattering in the as-deposited CoPtCr film studied, consistent with significant exchange-coupling between polycrystalline grains.     

多层膜［Co85Cr15/Pt］20的磁性、垂直磁记录特性和微结构的关系

采用磁控溅射法制备了性能优良的以Pt为缓冲层的［Co85₈₅Cr15_{15/Pt］2020 多层膜，研究了溅射气压对［Co8585Cr1515/Pt］2020多层膜微结构和磁性的 影响.研究结果表明，Ar溅射气压对［Co8585Cr1515/Pt］2020多层膜的微结构 、垂直磁各向异性和矫顽力有重要的影响 关键词： 溅射气压 多层膜 垂直磁各向异性 有效磁各向异性常数}     

Antisymmetric magnetoresistance in magnetic multilayers with perpendicular anisotropy

While magnetoresistance (MR) has generally been found to be symmetric in applied field in nonmagnetic or magnetic metals, we have observed antisymmetric MR in Co/Pt multilayers. Simultaneous domain imaging and transport measurements show that the antisymmetric MR is due to the appearance of domain walls that run perpendicular to both the magnetization and the current, a geometry existing only in materials with perpendicular magnetic anisotropy. As a result, the extraordinary Hall effect gives rise to circulating currents in the vicinity of the domain walls that contributes to the MR. The antisymmetric MR and extraordinary Hall effect have been quantitatively accounted for by a theoretical model.     

High-frequency dynamics of Co/Fe multilayers with stripe domains

Within the framework of two-dimensional (2D) numerical micromagnetic simulations, the equilibrium magnetization configuration and the high-frequency (0.1–30 GHz) linear response of Co/Fe multilayers have been investigated in detail. Due to the perpendicular anisotropy of Co layers, a stripe domain pattern can develop through the whole multilayer, the characteristics of which depend on the magnitude of the perpendicular anisotropy, the respective thicknesses of Co and Fe layers and the number of Co/Fe bilayers in the stack. One of the most striking features associated with the layering effect is the ripening aspect of the static magnetization configuration across the multilayers which induces complicated dynamic susceptibility spectra including surface modes and volume modes strongly confined within the inner Fe layers. The effect of the cubic magnetocrystalline anisotropy of Fe layers and the influence of a nonuniform perpendicular magnetic anisotropy within the Co layers on the static and dynamic magnetic properties of Co/Fe multilayers are then analyzed quantitatively.     

Magnetic properties of a Pt/Co₂ FeAl/MgO structure with perpendicular magnetic anisotropy

Microstructures and magnetic properties of Ta/Pt/Co 2 FeAl(CFA)/MgO multilayers are studied to understand perpendicular magnetic anisotropy(PMA) of half-metallic full-Heusler alloy films.PMA is realized in a 2.5-nm CFA film with B2-ordered structure observed by a high resolution transmission electron microscope.It is demonstrated that a high quality interface between the ferromagnetic layer and oxide layer is not essential for PMA.The conversions between in-plane anisotropy and PMA are investigated to study the dependence of magnetic moment on temperature.At the intersection points,the decreasing slope of the saturation magnetization(M s) changes because of the conversions.The dependence of M s on the annealing temperature and MgO thickness is also studied.     

Investigation of the magnetization reversal of a magnetic dot array of Co/Pt multilayers

The magnetization reversal behavior of a dot array consisting of Co/Pt multilayers with perpendicular magnetic anisotropy was investigated. The size of the dots was varied from 200 nm down to 40 nm, while keeping the filling factor constant at about 0.16. The structural properties were determined by scanning electron microscopy, whereas the magnetic investigation was performed using SQUID and MFM techniques. It was observed that the dot size has a severe impact on the magnetization reversal mechanism where only the smallest dots with a size of 40 nm are found to be in a magnetic single-domain state. Moreover, the patterning process leads to a degradation of the multilayer, leading to a reduction of the switching field and an increase of the switching field distribution with decreasing dot size. In addition, micromagnetic simulations were performed to understand the magnetization reversal mechanism in more detail.