Generation of Nonequilibrium Nuclei of Magnetization in a Rapidly Increasing Magnetic Field in Pt/Co/Ir/Co/Pt Synthetic Ferrimagnets

Physics of the Solid State - In a Pt/Co/Ir/Co/Pt synthetic ferrimagnet with perpendicular magnetic anisotropy, a stochastically appearing magnetization peak was found in the field dependence of the...     

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.     

Interaction of Magnetization Centers of Different Signs as the Cause of the Nonmonotonic Field Dependence of the Domain Wall Velocity in Synthetic Pt/Co/Ir/Co/Pt Ferrimagnets

The purpose of the work is to find the dependence of growth rate of magnetization centers of various types on their surrounding by other nucleation centers in a synthetic Pt/Co/Ir/Co/Pt ferrimagnet with perpendicular magnetic anisotropy. The following four types of nucleation centers exist in a sample with two ferromagnetic layers of different thicknesses: P+ centers correspond to the regions where the magnetizations of the thick and thin Co layers are directed along an applied field (↑↑); P– centers are the regions where the magnetizations of the layers are opposite to an applied field (↓↓); and AP+ and AP– centers correspond to the regions where the magnetizations of the thick and thin Co layers are opposite to each other and the total magnetic moment is along (↑↓) or opposite to (↓↑) an applied field, respectively. P– nucleation centers are found to be surrounded by AP+ regions in any field and exhibit a monotonic field dependence of their boundary. The field dependence of the growth rate of AP– nucleation centers is nonmonotonic since, as the field increases, they are surrounded by AP+ nucleation centers, AP+ and P– regions, and only P– nucleation centers in strong fields.     

Conditions for the Occurrence of Spontaneous Oscillating Magnetic Relaxation in Synthetic Pt/Co/Ir/Co/Pt Ferrimagnets

Physics of the Solid State - It has been established that nonmonotonic magnetization relaxation in Pt/Co/Ir/Co/Pt synthetic ferrimagnets (SF) with perpendicular magnetic anisotropy is caused by the...     

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.     

Competition between domain walls and the reverse magnetization in the magnetic relaxation of a Pt/Co/Ir/Co/Pt spin switcher

A multilayer Pt/Co/Ir/Co/Pt/GaAs heterostructures demonstrates a long term (to several hours) magnetic relaxation between two stable states of the magnetization of the system. The magnetization reversal of the heterostructure layers occurs both due to the formation of nuclei of the reverse magnetization domains and as a result of their further growth by means of motion of domain walls. The competition between two these processes provides a nonexponential character of the magnetic relaxation. At 300 K, the contributions of these processes to the relaxation are commensurable, while, at temperatures lower than 200 K, the contribution of the nucleation is suppressed and the magnetic relaxation occurs as a result of motion of the domain walls.     

Spin pumping at the Co2FeAlo.sSio.5/Pt interface

Spin pumping at the Co2FeAlo.sSio.5/Pt and Pt/Co2FeAIo.sSio.5 interfaces has been studied by ferromagnetic res- onance technology （FMR）. The spin mixing conductance of the Co, FeA~o.sSio.s/Pt and Pt/Co2FeAlo.sSio.5 interfaces was determined to be 3.7~ lO19 m-2 and 2.1 ~ lO19 m 2 by comparing the Gilbert damping in a Co2FeAIo.sSio.5 sin- gle film, Co2FeAlo.sSio.s/Pt bilayer film and a Pt/Co2FeAIo.sSio.s/Pt trilayer film. Spin pumping is more efficient in the Co2FeAlosSios/Pt bilayer film than in permalloy/Pt bilayer film.     

Perpendicular magnetic anisotropy and thermal stability in Co2FeAl0.5Si0.5/Pt multilayers

Perpendicular magnetic anisotropy (PMA) and thermal stability of Co₂FeAl_0.5Si_0.5 (CFAS)/Pt multilayer structure prepared by dc magnetron sputtering have been studied. It is found that the strength of PMA depends on the thickness of CFAS layer, and the maximum effective thickness of CFAS with PMA is demonstrated to be about 1.5 nm. The interfacial anisotropy energy K _s is estimated to be 0.45 erg/cm², which is larger than those of Co/Pd and Co/Ni multilayers and responsible for the strong PMA of the CFAS/Pt multilayer. The annealing treatments will give rise to the loss of PMA of CFAS/Pt multilayer and has the relatively less influence with large period number N. With the increasing of the period number N, the CFAS/Pt multilayers tend to have wasp-waist-shaped easy-axis hysteresis loops and multidomain structures.     

Magnetization reversal in [Ni/Pt]6/Pt(x)/[Co/Pt]6 multilayers

The magnetization reversal is studied in magnetron sputtered artificial superstructures of the form [Ni/Pt]₆/Pt(x)/[Co/Pt]₆ with perpendicular anisotropy, in which the [Co/Pt]₆ stacks have higher coercivity than the [Ni/Pt]₆. For x≥2 nm the two stacks reverse separately and exhibit characteristic stepped loops with a “plateau” in the region between the two switching fields. First-Order Reversal Curves (FORCs) reveal that the maximum coupling is obtained for x=1.5 nm. While each of the Ni/Pt and Co/Pt stacks by itself is thin enough to reverse in large domains when they are coupled, formation of maze like domains is observed. In this case some reversibility of the demagnetization curves associated with interfacial domain wall pinning appears while in the rest of the cases the reversal mechanism is based on lateral domain wall pinning with low reversibility. In the loops monitored by Extraordinary Hall Effect (EHE) measurements this “plateau” appears as a hump due to the different sign of the EHE coefficient between the [Ni/Pt]₆ and [Co/Pt]₆.     

Orbit-Transfer Torque Driven Field-Free Switching of Perpendicular Magnetization

he reversal of perpendicular magnetization （PM） by electric control is crucial for high-density integration of low-power magnetic random-access memory.Although the spin-transfer torque and spin-orbit torque technologies have been used to switch the magnetization of a free layer with perpendicular magnetic anisotropy,the former has limited endurance because of the high current density directly through the junction,while the latter requires an external magnetic field or unconventional configuratio...     

Preparation of Fe/Pt Films with Perpendicular Magnetic Anisotropy

We have investigated the microstructures and magnetic properties of L1₀ ordered equi-atomic FePt thin films prepared by ion beam sputtering and subsequent annealing. It is observed from X-ray reflectivity and X-ray diffraction measurements that the mixing at Fe/Pt interfaces starts to occur with annealing and leading to the FePt alloy phase formation. The rapid increase in the coercivity values above 275°C, obtained from vibrating sample magnetometer (VSM) measurements, confirms the formation of the ordered L1₀ FCT FePt phase.     

Pt/Co/Pt/Ni多层膜的磁光特性

用磁控溅射法制备了Pt/Co/Pt/Ni系列样品,其中Ni层具有不同的厚度.通过测量样品的Kerr转角、椭偏率、折射率和吸收率,推算出了四个等效电导张量元σ_1xx,σ_2xx,σ_1xy,σ_2xy随Ni层厚度的变化情况.再结合电导张量元与Kerr角的理论公式,分析了在Ni层厚度的变化过程中,每一个电导张量元对Kerr角的贡献;发现在短波段σ_2xy起主导作用,而在长波段四个电导张量元共同起作用.经分析认为这是由于在Ni层的增厚过程中,Pt 5d带劈裂程度逐渐减小.与纯Pt/Co膜相比,在Ni层的厚度为0.10—0.23nm的范围内样品具有较低的居里温度和较高的Kerr角,这说明Pt/Co/Pt/Ni多层膜具有较高的应用价值. 关键词：     

Field-driven creep motion of a composite domain wall in a Pt/Co/Pt/Co/Pt multilayer wire

We have studied the field-driven motion of a pair of coupled Bloch domain walls in a perpendicular magnetic anisotropy Pt/Co/Pt/Co/Pt multilayer Hall bar. The nucleation of an isolated but coincident pair of walls in the two Co layers, observed by Kerr microscopy, took place at an artificial nucleation site created by Ga⁺ ion irradiation. The average velocity v of the wall motion was calculated from time-resolved magnetotransport measurements at fixed driving field H, where the influence of the extraordinary Hall effect leads to the observation of voltages at the longitudinal resistance probes. We observed a good fit to the scaling relation lnv∝H^−1/4, consistent the motion of a single 1-dimensional wall moving in a 2-dimensional disordered medium in the creep regime: the two walls are coupled together into a 1-dimensional composite object.     

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.     

Expansion and relaxation of magnetic mirror domains in a Pt/Co/Pt/Co/Pt multilayer with antiferromagnetic interlayer coupling

We detail measurements of field-driven expansion and zero-field relaxation of magnetic mirror domains in antiferromagnetically coupled perpendicularly magnetized ultrathin Co layers. The zero-field stability of aligned ('mirror') domains in such systems results from non-homogeneous dipolar stray fields which exist in the vicinity of the domain walls. During field-driven domain expansion, we evidence a separation of the domain walls which form the mirror domain boundary. However, the walls realign, thereby reforming a mirror domain, if their final separation is below a critical distance at the end of the field pulse. This critical distance marks the point at which the effective net interaction between the walls changes from attractive to repulsive.     

CoFeB/Pt多层膜的垂直磁各向异性研究

具有垂直磁各向异性的磁性纳米结构是自旋转移力矩器件的重要研究内容, 本文采用反常霍尔效应系统地研究了磁控溅射法制备的[CoFeB/Pt]_n多层膜的垂直磁各向异性. 当CoFeB的厚度小于0.6 nm时, 可以在[CoFeB/Pt]_n多层膜中观察到清晰的垂直磁各向异性, 其垂直磁各向异性强烈依赖于CoFeB和Pt层厚度及多层膜周期数. 当多层膜周期数n ≥ 5时, 出现零剩磁现象. 另外, [CoFeB/Pt]_n多层膜的矫顽力均小于2 kA·m^-1, 有望作为垂直自由层的重要侯选材料应用于垂直磁纳米结构中.     

Impact of the Field of a Magnetic Force Microscope Probe on the Skyrmion State in a Modified Co/Pt Film with Perpendicular Anisotropy

Physics of the Solid State - We present the results of experiments on the local magnetization reversal of artificial magnetic skyrmions in a Co/Pt multilayer film with a perpendicular magnetization...     

Thermosensitive Spin Valve Based on an Artificial Ferrimagnet: Magnetization Process in a Wide Range of Fields

Physics of the Solid State - Film structures of the Gd–Co/Co/Cu/Co type having the properties of heat-sensitive spin valves have been developed, fabricated, and investigated. The results of...