Yong Hu

Oscillation frequency of magnetic vortex induced by spin-polarized current in a confined nanocontact structure

We studied the oscillation frequency associated with gyrotropic motion of magnetic vortex in a Permalloy nanodot driven by an out-of-plane spin-polarized current injected through a nanocontact by micromagnetic simulations and analytical calculations. The analytical results were calculated by the Thieles equation, where both the forces corresponding to the spin-transfer-torque and the Oersted field accompanying the current were taken into account. Variation curves of oscillation frequency with vortex core position, nanocontact radius, and current density were given. Good agreement with analytical calculations and micromagnetic simulations was found.

Cooling field and temperature dependent exchange bias in spin glass/ferromagnet bilayers.

We report on the experimental and theoretical studies of cooling field (HFC) and temperature (T) dependent exchange bias (EB) in FexAu1u2009−u2009x/Fe19Ni81 spin glass (SG)/ferromagnet (FM) bilayers. When x varies from 8% to 14% in the FexAu1u2009−u2009x SG alloys, with increasing T, a sign-changeable exchange bias field (HE) together with a unimodal distribution of coercivity (HC) are observed. Significantly, increasing in the magnitude of HFC reduces (increases) the value of HE in the negative (positive) region, resulting in the entire HEu2009∼u2009T curve to move leftwards and upwards. In the meanwhile, HFC variation has weak effects on HC. By Monte Carlo simulation using a SG/FM vector model, we are able to reproduce such HE dependences on T and HFC for the SG/FM system. Thus this work reveals that the SG/FM bilayer system containing intimately coupled interface, instead of a single SG layer, is responsible for the novel EB properties.

Exchange bias and its propagation in ferromagnetic/antiferromagnetic/ferromagnetic trilayers

We establish a simple ferromagnetic/antiferromagnetic/ferromagnetic trilayer model, where the lower-interface coupling is varied and study its combined effect with cooling field or temperature on the upper-interface exchange bias effect. Exchange bias field is almost independent of cooling field, if the lower-interface coupling is ferromagnetic and large. Otherwise, its value becomes sensitive to cooling field, and the evolution trend of exchange bias with increasing cooling field depends on the sign and strength of lower-interface coupling. On the other hand, the blocking temperature, below which exchange bias emerges, is reduced (or elevated) in the trilayers with a large ferromagnetic (or antiferromagnetic) lower-interface coupling. The numerical results obtained are interpreted well in terms of the magnetization behaviors in the respective layers. A cooling field creates a low-temperature frozen configuration in the antiferromagnetic layer after cooling to generate exchange bias, while such a configur...

Field-induced transitions from negative to positive exchange bias in nanoparticles with inverted ferromagnetic-antiferromagnetic core-shell morphology

In an antiferromagnetic (core)/ferromagnetic (shell) nanoparticle, the transition behaviors from negative (NEB) to positive exchange bias (PEB) at low temperature after field cooling are studied in detail. The NEB field may exhibit an oscillatory behavior due to the competition between interfacial coupling and cooling field. The critical cooling fields, at which the transitions occur, exhibit a logarithmic decrement with the decrease of interfacial coupling, but indicate a linear decrease with the decrease of antiferromagnetic coupling or with the further dilution in the antiferromagnetic core. With the further increase of cooling field, the PEB field increases linearly and finally levels off. Moreover, the weaker antiferromagnetic coupling may enhance the NEB field value, whereas the suppression of PEB may be observed by diluting the antiferromagnetic core. The magnetization reversal by coherent rotation strongly depends on the variation of the magnetic parameters, because its occurrence just needs to co...

Exchange bias training relaxation in spin glass/ferromagnet bilayers

A canonical spin glass (SG) FeAu layer is fabricated to couple to a soft ferromagnet (FM) FeNi layer. Below the SG freezing temperature, exchange bias (EB) and training are observed. Training in SG/FM bilayers is insensitive to cooling field and may suppress the EB or change the sign of the EB field from negative to positive at specific temperatures, violating from the simple power-law or the single exponential function derived from the antiferromagnet based systems. In view of the SG nature, we employ a double decay model to distinguish the contributions from the SG bulk and the SG/FM interface to training. Dynamical properties during training under different cooling fields and at different temperatures are discussed, and the nonzero shifting coefficient in the time index as a signature of slowing-down decay for SG based systems is interpreted by means of a modified Monte Carlo Metropolis algorithm.

Creation and Annihilation of Skyrmions in the Frustrated Magnets with Competing Exchange Interactions

In triangular-lattice magnets, the coexistence of third-neighbor antiferromagnetic and nearest-neighbor ferromagnetic exchange interactions can induce rich magnetic phases including noncoplanar skyrmion crystals. Based on Monte Carlo simulation, we studied the dependence of magnetic phase transition on exchange interaction strength. Under the consideration of uniaxial anisotropy and magnetic field both perpendicular to the film plane, a large antiferromagnetic exchange interaction induces a high frustration. When the value of antiferromagnetic exchange interaction is one and a half times larger than the ferromagnetic one, a magnetic phase composed of canting spin stripes, never observed in the chiral magnets, forms. Interestingly, different canting spin stripes along three 120 degree propagation directions may coexist randomly in a magnetic phase, attesting that the canting spin stripes are three-fold degenerate states akin to helices and the multiple state of canting spin stripes is a circular configuration with zero skyrmion charge number. Moreover, skyrmions and antiskyrmions can be observed simultaneously in the configuration at the low temperature nearly close to 0u2009K, and their configuration and diameter properties are discussed. Finally, the mechanisms of skyrmion creation and annihilation are properly interpreted by comparing exchange and Zeeman energy terms.

Full Electric Control of Exchange Bias at Room Temperature by Resistive Switching

Electric control of exchange bias (EB) is of vital importance in energy-efficient spintronics. Although many attempts have been made during the past decade, each has its own limitations for operation and thus falls short of full direct and reversible electrical control of EB at room temperature. Here, a novel approach is proposed by virtue of unipolar resistive switching to accomplish this task in a Si/SiO2 /Pt/Co/NiO/Pt device. By applying certain voltages, the device displays obvious EB in the high-resistance-state while negligible EB in the low-resistance state. Conductive filaments forming in the NiO layer and rupturing near the Co-NiO interface are considered to play dominant roles in determining the combined resistive switching and EB phenomena. This work paves a new way for designing multifunctional and nonvolatile magnetoelectric random access memory devices.

Low-field magnetocaloric effect in single crystals controlled by magnetocrystalline anisotropy

The influence of magnetocrystalline anisotropy (K) on the magnetic entropy change (ΔSM) in single crystals under low fields (H) is studied based on unbiased Monte Carlo simulations. When H is applied perpendicular to the easy axis, the positive and negative ΔSM may coexist. The positive ΔSM peak value and its maximum value optimized by the field and the threshold of switching to negative ΔSM depend on K. Moreover, K linearly enhances the blocking temperature where the negative ΔSM maximum is obtained, associated with the decrease in the peak value with increasing K. Finally, the rotating ΔSM under a constant H may be much larger than ΔSM produced by moving the model in and out of the same H, and the best response to K is found at 86° between K and H, independent of the anisotropy magnitude.The influence of magnetocrystalline anisotropy (K) on the magnetic entropy change (ΔSM) in single crystals under low fields (H) is studied based on unbiased Monte Carlo simulations. When H is applied perpendicular to the easy axis, the positive and negative ΔSM may coexist. The positive ΔSM peak value and its maximum value optimized by the field and the threshold of switching to negative ΔSM depend on K. Moreover, K linearly enhances the blocking temperature where the negative ΔSM maximum is obtained, associated with the decrease in the peak value with increasing K. Finally, the rotating ΔSM under a constant H may be much larger than ΔSM produced by moving the model in and out of the same H, and the best response to K is found at 86° between K and H, independent of the anisotropy magnitude.

Exchange bias mechanism at the ferromagnetic/antiferromagnetic interface with rotatable antiferromagnetic spins: A Monte Carlo study

We perform modified Monte Carlo simulations on a ferromagnetic/antiferromagnetic bilayer structure with adjustable antiferromagnetic anisotropy and degree of exchange coupling. Generally, both the antiferromagnetic anisotropy and the degree of exchange coupling at the ferromagnetic/antiferromagnetic interface are difficult to be directly detected experimentally. However, they may play crucial roles in establishing the exchange bias properties through determining whether the antiferromagnetic spins at the interface are rotatable or pinned. Therefore, we precisely calculated the numbers of rotatable and pinned antiferromagnetic spins at the interface and analyzed their contribution to exchange bias and coercivity in the specified ranges of antiferromagnetic anisotropy and degree of exchange coupling. The simulation results may help to clarify the experimental controversies concerning the occurrence of exchange bias effect prior to the detection of pinned uncompensated antiferromagnetic spins. They can also ...

Spin-transfer force acting on vortex induced by current gradient in a planar polarizer geometry

We discuss a new mechanism of changing the magnetic vortex gyrotropic motion in a permalloy/nonmagnet bi-layers system. In this system, a spin current characterized by an in-plane polarizer is injected from the nonmagnetic layer to the permalloy disk. We introduce current density gradient to the spin current, and find that the interplay between the planar polarizer and current gradient can change the damping of the vortex motion. This change originates from a spin-transfer force acting on the vortex. The influence of the spin-transfer force on the vortex motion is dependent on the direction of the planar polarizer, the orientation of the current density gradient, and the vortex state.