Perpendicular Magnetic Films Research Articles

Disorder-induced domain wall velocity shift at high fields in perpendicularly magnetized thin films

Domain wall dynamics in a perpendicularly magnetized system is studied by means of micromagnetic simulations in which disorder is introduced as a dispersion of both the easy-axis orientation and the anisotropy constant over regions reproducing a granular structure of the material. High field dynamics show a linear velocity-field relationship and an additional grain size dependent velocity shift, weakly dependent on both applied field and intrinsic Gilbert's damping parameter. We find the origin of this velocity shift in the nonhomogeneous in-plane effective field generated by the tilting of anisotropy easy axis introduced by disorder. We show that a one-dimensional analytical approach cannot predict the observed velocities and we augment it with the additional dissipation of energy arising from internal domain wall dynamics triggered by disorder. This way we prove that the main cause of higher velocity is the ability of the domain wall to irradiate energy into the domains, acquired with a precise feature of disorder.

In Situ TEM Scratch Testing of Perpendicular Magnetic Recording Multilayers with a Novel MEMS Tribometer

Utilizing a newly developed two-dimensional (2D) transducer designed for in situ transmission electron microscope (TEM) nanotribology, deformation mechanisms of a perpendicular magnetic recording film stack under scratch loading conditions were evaluated. These types of films are widely utilized in storage devices, and loss of data by grain reorientation in the recording layers is of interest. The observed deformation was characterized by a stick–slip mechanism, which was induced by a critical ratio of lateral to normal force regardless of normal force. At low applied normal forces, the diamond-like carbon (DLC) coating and asperities in the recording layer were removed during scratching, while, at higher applied forces, grain reorientation and debonding of the recording layer was observed. As the normal force and displacement were increased, work for stick–slip deformation and contact stress were found to increase based upon an Archard’s Law analysis. These experiments also served as an initial case study demonstrating the capabilities of this new transducer.

Continuous Tuning of the Magnitude and Direction of Spin‐Orbit Torque Using Bilayer Heavy Metals

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 magnetic random access memory (MRAM). The effective manipulation of SOT has thus become an important step toward 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‐orbit effective fields can be continuously tuned by changing the Pt layer thickness, consistent with the current‐induced magnetization switching data. The ratio of longitudinal to transverse spin‐orbit effective fields is found to be determined by the Ta/CoFeB interface and can be continuously tuned by changing the Pt layer thickness. The Dzyaloshinskii–Moriya interaction (DMI) is found to be weak and shows an insignificant variation with the Pt thickness. The results demonstrate an effective method to tune SOTs utilizing bilayer heavy metals without affecting the DMI, a desirable feature which will be useful for the design of SOT‐based devices.

Energetic molding of chiral magnetic bubbles

Topologically protected magnetic structures such as skyrmions and domain walls (DWs) have drawn a great deal of attention recently due to their thermal stability and potential for manipulation by spin current, which is the result of chiral magnetic configurations induced by the interfacial Dzyaloshinskii-Moriya Interaction (DMI). Designing devices that incorporate DMI necessitates a thorough understanding of how the interaction presents and can be measured. One approach is to measure growth asymmetry of chiral bubble domains in perpendicularly magnetized thin films, which has been described elsewhere by thermally activated DW motion. Here, we demonstrate that the anisotropic angular dependence of DW energy originating from the DMI is critical to understanding this behavior. Domains in Co/Ni multi-layers are observed to preferentially grow into non-elliptical teardrop shapes, which vary with the magnitude of an applied in-plane field. We model the domain profile using energetic calculations of equilibrium shape via the Wulff construction, which explains both the teardrop shape and the reversal of growth symmetry at large fields.

Optically induced spin-dependent diffusive transport in the presence of spin-orbit interaction for all-optical magnetization reversal

We have considered the effect of different spin-orbit interaction mechanisms on the process of demagnetization under the influence of short-pulse lasers. All-optical magnetization reversal of perpendicularly magnetized thin films can occur if there are sufficient strong spin-Hall, skew scattering, and Rashba interactions. In the presence of spin-orbit interactions, the transient charge currents provide the generation of transverse-spin currents and accumulations, which eventually exert spin-transfer torque on the magnetization. By combining the optically excited spin-dependent diffusive transport with the spin and charge currents due to skew scattering, spin-Hall, inverse spin-Hall and Rashba interactions, into a numerical model, we demonstrate a possibility of ultra-fast all-optical magnetization reversal. This understanding provokes intriguing, more in-depth experimental studies on the role of spin-orbit interaction mechanisms in optimizing structures for all-optical magnetization reversal.

In-plane current-driven spin-orbit torque switching in perpendicularly magnetized films with enhanced thermal tolerance

We study spin-orbit-torque (SOT)-driven magnetization switching in perpendicularly magnetized Ta/Mo/Co40Fe40B20 (CoFeB)/MgO films. The thermal tolerance of the perpendicular magnetic anisotropy (PMA) is enhanced, and the films sustain the PMA at annealing temperatures of up to 430 °C, due to the ultra-thin Mo layer inserted between the Ta and CoFeB layers. More importantly, the Mo insertion layer also allows for the transmission of the spin current generated in the Ta layer due to spin Hall effect, which generates a damping-like SOT and is able to switch the perpendicular magnetization. When the Ta layer is replaced by a Pt layer, i.e., in a Pt/Mo/CoFeB/MgO multilayer, the direction of the SOT-induced damping-like effective field becomes opposite because of the opposite sign of spin Hall angle in Pt, which indicates that the SOT-driven switching is dominated by the spin current generated in the Ta or Pt layer rather than the Mo layer. Quantitative characterization through harmonic measurements reveals that the large SOT effective field is preserved for high annealing temperatures. This work provides a route to applying SOT in devices requiring high temperature processing steps during the back-end-of-line processes.

Anomalous Nernst Effect of Perpendicularly Magnetic Anisotropy TbFeCo Thin Films

In this study, we investigated anomalous Nernst effect (ANE) of perpendicularly magnetized TbFeCo thin films with various Tb content, and especially studied the relation between ANE and anomalous Hall effect. As a result, the hysteresis of anomalous Nernst coefficient showed the same behavior as that of anomalous Hall resistivity, and the sign of anomalous Nernst coefficient was consistent with that of anomalous Hall voltage in any Tb content, whereas the Seebeck coefficient and the resistivity were almost constant even if the applied magnetic field was varied. Taking into account of thermoelectric coefficient tensor, it was revealed that the off-diagonal thermopower corresponding to the ANE in TbFeCo thin films is the product of Hall angle and Seebeck coefficient.

Effects of grain size and disorder on domain wall propagation in CoFeB thin films

Micromagnetic simulations are used to investigate the effect of disorder on field-driven domain wall motion in perpendicularly magnetized CoFeB thin films. It is found that some degree of inhomogeneity in the form of an irregular grain structure needs to be introduced in the model in order to account for the domain wall velocities measured experimentally, even for applied fields much larger than the finite propagation field induced by weak disorder in the film. Moreover, the details of this grain structure have a large impact on domain wall motion in this flow regime. In particular, it is found that, for a fixed applied field, domain wall velocity rapidly increases with grain size up to a diameter of 40 nm, above which it slowly decreases. This is explained showing that the grain structure of the material introduces a new form of dissipation of energy via spin wave emission during domain wall propagation. We focus on the relation between grain size and domain wall velocity, finding that the frequency of emission of spin waves packets during domain wall motion depends on the grain size and affects directly the domain wall velocity of propagation.

Strain-induced magnetic domain wall control by voltage in hybrid piezoelectric BaTiO3 ferrimagnetic TbFe structures.

This paper reports on the voltage dependence of the magnetization reversal of a thin amorphous ferromagnetic TbFe film grown on a ferroelectric and piezoelectric BaTiO3 single crystal. Magneto-optical measurements, at macroscopic scale or in a microscope, demonstrate how the ferroelectric BaTiO3 polarisation history influences the properties of the perpendicularly magnetized TbFe film. Unpolarised and twinned regions are obtained when the sample is zero voltage cooled whereas flat and saturated regions are obtained when the sample is voltage cooled through the ferroelectric ordering temperature of the BaTiO3 crystal, as supported by atomic force microscopy experiments. The two steps involved in the TbFe magnetization reversal, namely nucleation and propagation of magnetic domain walls, depend on the polarisation history. Nucleation is associated to coupling through strains with the piezoelectric BaTiO3 crystal and propagation to pinning with the ferroelastic surface patterns visible in the BaTiO3 topography.

Laser heating and oxygen partial pressure effects on the dynamic magnetic properties of perpendicular CoFeAlO films

The impact of oxidation and laser heating on the dynamic magnetic properties of perpendicularly magnetized Co50Fe25Al25O films has been studied by time-resolved magneto-optical Kerr effect in a fs-laser pump-probe setup. We find that pump laser fluence Fp can affect the effective magnetic anisotropy field and thus the precession frequency f seriously, leading to an increased dependence of effective magnetic damping factor αeff on the external field at higher fluences. Moreover, the αeff increases with increasing the oxygen partial pressure PO2 while the uniaxial anisotropy energy Ku and Landau factor g decrease, owing to the increased proportion of superparamagnetic CoFe oxides formed by over-oxidation. By optimizing both the Fp and PO2, the intrinsic damping factor is determined to be lower than 0.028 for the perpendicular film showing a uniaxial anisotropy energy as high as 4.3×106erg/cm3. The results in this study provide a promising approach to manipulate the magnetic parameters for possible applications in spintronic devices.

Enhanced perpendicular coercivity of ultrathin perpendicularly magnetized Tb–Fe–Co films on silicon substrates using a thin Pt underlayer

Abstract We have studied the effect of an additional thin Pt underlayer on the magnetic properties of ultrathin perpendicularly magnetized TbFeCo films grown on silicon substrates. We have demonstrated that the presence of a thin Pt underlayer can enhance the perpendicular coercivity of ultrathin TbFeCo films with thickness down to 1 nm while preserving their low saturation magnetization. This can be attributed to the hybridization of Pt and Co orbitals, which induces the strong interfacial perpendicular anisotropy of Co/Pt and/or CoFe/Pt. These characteristics are potentially useful to reduce the critical current density induced magnetization switching and enhance the thermal stability of a magnetic tunnel junction and the current-induced domain wall motion in a nanowire made out of the ultrathin TbFeCo magnetic layer.

Modeling of Temperature Dependence of Magnetization in TbFe Films — An Atomistic Spin Simulation Study

In this paper, we performed spin simulations at atomistic level to study the temperature dependent properties of perpendicularly magnetized TbFe thin films. The crystallographically amorphous feature of TbFe ferrimagnetic alloys is modeled by using a lattice system with disordered site occupation of rare earth (RE) and transition metal (TM) spins. The simulated Curie temperature ([Formula: see text]) is consistent well with the mean-field approximation theory. With the increase of Tb concentration, the [Formula: see text] decreases almost linearly, whereas the magnetization compensation temperature ([Formula: see text]) increases gradually until the [Formula: see text] value is reached. The inter-sublattice exchange coupling strength [Formula: see text] between the RE and TM atoms can significantly affect [Formula: see text], but has less impact on [Formula: see text]. With the increase of Tb concentration, the TbFe sample of high [Formula: see text] exhibits a much faster increase in [Formula: see text] than the sample with low [Formula: see text]. Moreover, we have tested the simulation code to model the laser pulse induced ultrafast nonequilibrium spin dynamics. As an example, the femto-second pulse laser induced demagnetization and recovery process is clearly reproduced. These features are in a good agreement with the experiments, indicating that the simulation model can capture the basic physics in describing the high temperature dependent magnetic property as well as the ultrafast spin dynamics.

Atomic spin resonance in a rubidium beam obliquely incident to a transmission magnetic grating

We studied atomic spin resonance induced by atomic motion in a spatially periodic magnetostatic field. A rubidium atomic beam, with a velocity of about 400 m s−1, was obliquely incident to a transmission magnetic grating that produced a spatially periodic magnetic field. The magnetic grating was formed by a magnetic thin film on a polyimide substrate that had multiple slits at 150 μm intervals. The atoms experienced field oscillation, depending on their velocity and the field period when passing through the grating, and underwent magnetic resonance. Resonance spectra obtained with a perpendicular magnetization film were in clear contrast to ones obtained with an in-plane magnetization film. The former exhibited resonance peaks at odd multiples of the frequency, determined by the velocity over the period, while the latter had dips at the same frequencies.

Effect of process temperature on structure and magnetic properties of perpendicularly magnetized D022-Mn3Ge thin films on a Cr buffer layer

We investigated the effect of post-annealing on the perpendicular magnetic anisotropy constant (Ku) and surface roughness (Ra) of Mn3Ge thin films grown at comparatively low temperatures (room temperature, 150, 200, and 250 °C) on Cr buffer layers. The films grown at ≥200 °C exhibit a D022-ordered crystal structure in an as-deposited state. The post-annealing process demonstrates differences in trends between the 200-°C-grown film and the 250-°C-grown film. The 200-°C-grown film displays significant degradation of Ku and an increase in Ra upon annealing at >300 °C because of its poor thermal durability, while the 250-°C-grown film is still intact even at 500 °C. The 250-°C-grown film post-annealed at 300 °C displays relatively high Ku while Ra remains low. It may be possible to grow D022-Mn3Ge with higher Ku and low Ra using a buffer-layer material with a lattice-matched crystal structure with D022-Mn3Ge and higher thermal durability than Cr.

Material dependence of anomalous Nernst effect in perpendicularly magnetized ordered-alloy thin films

Material dependence of the anomalous Nernst effect (ANE) in perpendicularly magnetized ordered-alloy thin films is systematically investigated. The ANE was found to have a tendency to increase simply as uniaxial magnetic anisotropy increased at room temperature. The ANE increases as temperature increases from 10 to 300 K for all the materials. However, the signs of the ANE in Fe-based ordered-alloys (L10-FePt and L10-FePd) and in a Co/Ni multilayer are opposite to those in Mn-based ordered-alloys (L10-MnGa and D022-Mn2Ga). Ordered-alloys with larger uniaxial magnetic anisotropies reveal larger ANE and might be desirable for thermoelectric applications.

Growth of perpendicularly magnetized thin films on a polymer buffer and voltage-induced change of magnetic anisotropy at the MgO|CoFeB interface

We show that perpendicularly magnetized thin films can be grown onto polyimide, a potentially flexible substrate. With polar Kerr magnetometry, we demonstrate that the coercive field of CoFeB thin film can be modulated by applying a back gate voltage. Our proposed multi-layered structure is suitable for surface-sensitive measurements of the voltage-induced change in anisotropy, and could be used to realize flexible spintronics devices.

磁気コンプトン散乱で調べる化学結合の形とその磁場依存性～垂直磁化膜の起原～

磁気コンプトン散乱で調べる化学結合の形とその磁場依存性～垂直磁化膜の起原～

Critical behavior of zero-field magnetic fluctuations in perpendicularly magnetized thin films

Magneto-optic Kerr effect is used to study the statistical properties of thermally driven magnetic domain walls in a thin film at zero magnetic field.

Gilbert damping and critical real-space trajectory of L10-ordered FePt films investigated by magnetic-field-induction and all-optical methods

The magnetization dynamics of perpendicularly magnetized FePt films is studied using both magnetic-field-induction and all-optical methods. A critically damped trajectory was observed in this system, where the precession ended within subnanoseconds after a single large oscillation. Using the Landau–Lifshitz–Gilbert (LLG) calculation with an experimental configuration, the effective anisotropy and damping constant were obtained. A damping constant of approximately 0.2 was determined after both a magnetic field and a laser pulse were used. The laser-induced real-space trajectory was well explained by the modified LLG calculation taking into account the demagnetization and time-dependent anisotropy.

Magnetization reversal in ferromagnetic thin films induced by spin-orbit interaction with Slonczewski-like spin transfer torque

We theoretically investigate the dynamics of magnetization in ferromagnetic thin films induced by spin-orbit interaction with Slonczewski-like spin transfer torque. We reproduce the experimental results of perpendicular magnetic anisotropy films by micromagnetic simulation. Due to the spin-orbit interaction, the magnetization can be switched by changing the direction of the current with the assistant of magnetic field. By increasing the current amplitude, wider range of switching events can be achieved. Time evolution of magnetization has provided us a clear view of the process, and explained the role of minimum external field. Slonczewski-like spin transfer torque modifies the magnetization when current is present. The magnitude of the minimum external field is determined by the strength of the Slonczewski-like spin transfer torque. The investigations may provide potential applications in magnetic memories.