Skyrmion driven by rotary magnetic field on the surface of magnetic nanotube: a Monte Carlo study.

We report a Monte-Carlo simulation of the formation of skyrmions under a rotary magnetic field on a nanotube. The zero-field magnetic state is characterized as helical stripe domains swirling on the nanotube, with one to three periods depending on the ratio of Dzyaloshinskii-Moriya to ferromagnetic interaction and tubular size. Under a rotary magnetic field, the formation of skyrmions is in pair and the skyrmion number can be tuned. The movement of skyrmions is neither synchronous along with the rotary field, nor along a helical trajectory perpendicular to the rotary field. It is ascribed to that within a skyrmion pair, on one hand, the coupling between skyrmions is nonnegligible; on the other hand, different skyrmion pairs side by side are decoupled. This work predicts a way of nanotube-based skyrmion manipulation, and might develop the rotary information storage on energy- and space-saving modes or an edgeless racetrack memory.

Improvement and stabilization of exchange bias in ferromagnet/antiferromagnet/ferromagnet trilayers.

Exchange bias (EB) in ferromagnet/antiferromagnet bilayers, which has been extensively studied and applied for several decades, is sensitive to many factors such as layer thickness, texture and crystallization. Various factors in an antiferromagnet may counterbalance each other to limit and deteriorate EB. We used an unbiased Monte-Carlo method based on a modified Metropolis algorithm to predict that dependence of EB properties on antiferromagnetic anisotropy (K AF) are highly improved by attaching a soft ferromagnet on the other side of the antiferromagnet. On one hand, target ferromagnet in trilayers displays a pronounced and stabilized EB plateau at high K AF, on the contrary, EB is completely removed and instead a high coercivity is observed at low K AF, exhibiting a roughly K AF-modulated EB switching effect. On the other hand, EB is identified with no training in trilayers and well axially symmetric about antiferromagnetic easy axis. Meanwhile, in trilayers EB versus angle (θ) which is between antiferromagnetic easy axis and the direction of magnetic field is a roughly linear relationship in the intermediate θ range. Microscopic explorations found that a fully uncompensated magnetization in antiferromagnet may appear in trilayers and its rotatability is precisely controlled by K AF, designating that the antiferromagnet/seed-ferromagnet bilayers resemble a ferromagnet with changeable hardness to induce a maximized coercivity of target ferromagnet at low K AF to a maximized EB at high K AF. Finally, a phenomenological model reveals that antiferromagnetic spins change from a fluctuated state to a blocked state due to the existence of seed ferromagnet, and thus in this work we conceived an artificial pinning layer to establish and regulate EB.

Spin glass properties mapped by coercivity in ferromagnet/spin glass bilayers.

We report on a study on the spin glass (SG) anisotropy (K SG) and interfacial exchange coupling (J IF) dependent coercivity (H C) at the ferromagnet/SG interface, based on a modified Monte Carlo Metropolis algorithm. It is shown that K SG and J IF are interdependent while taking effect on different magnetic degrees of freedom and different time scales, resulting in complicated H C behaviors. By means of a micromagnetic approximation approach, we analytically explain the H C behaviors with respect to K SG and J IF. The dynamic SG surplus magnetization and the SG spin rotatability at the interface, hard to be detected experimentally, have proven to play crucial roles. This paper elucidates the weak anisotropy dependence of SG magnetic properties, and predicts that the SG features can be tunable at will by precisely controlling the magnetic parameters.

Cooling-field dependence of dipole-induced loop bias.

In ferromagnet/antiferromagnet bilayers and core/shell nanoparticles, an exchange-bias-like loop bias phenomenon in the ferromagnet is observed solely due to the long-range dipolar interactions between ferromagnet and antiferromagnet. With increasing cooling field, the loop bias field may increase from zero in the bilayers or from a negative value in the core/shell nanoparticles to a positive saturated value, depending on the interfacial dipolar interaction and/or ferromagnetic/antiferromagnetic thickness. Using a modified Monte-Carlo method and the Meiklejohn-Bean model, the interfacial dipole fields (up to several teslas) and the domain sizes imprinted on the interfacial antiferromagnet are explicitly calculated to elucidate the cooling field dependence of loop bias, which is governed by distinct mechanisms at the flat and curved interfaces. Finally, through simply discussing the roles of lattice structure, ferromagnetic dipolar interaction, and simulation time, it is evidenced that the dipole-induced loop bias is ubiquitous and applicable for stabilizing a ferromagnet, irrespective of the interface mismatch and the undeterministic diffusion between different ingredients. This work helps us to develop the spintronic devices with nonatomic-contact nanostructure assemblies.

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 0 K, 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.