矩形磁性纳米点动力学反磁化过程的微磁学研究

采用微磁学模拟方法研究了初始态为C形磁结构的矩形CoFe纳米点在方波脉冲场作用下的动力学反磁化过程.研究发现,随着脉冲场强的增强,磁体的反磁化模式发生了改变.当场强较弱时反磁化过程通过畴壁移动-单涡旋的形成和移动来完成;当场强较大时反磁化过程模式转变为畴壁移动-双涡旋的形成与移动;在更强的场强下反磁化过程通过畴壁的移动-多涡旋的形成与湮没来实现.由于反磁化模式随场强的变化而改变,反磁化时间随场强的增大出现振荡变化现象. 关键词： 动力学反磁化过程 反磁化时间 微磁学模拟     

Ultrafast nanomagnetic toggle switching of vortex cores

We present an ultrafast route for a controlled, toggle switching of magnetic vortex cores with ultrashort unipolar magnetic field pulses. The switching process is found to be largely insensitive to extrinsic parameters, like sample size and shape, and it is faster than any field-driven magnetization reversal process previously known from micromagnetic theory. Micromagnetic simulations demonstrate that the vortex core reversal is mediated by a rapid sequence of vortex-antivortex pair creation and annihilation subprocesses. Specific combinations of field-pulse strength and duration are required to obtain a controlled vortex core reversal. The operational range of this reversal mechanism is summarized in a switching diagram for a 200 nm Permalloy disk.     

磁涡旋极性翻转的局域能量

针对坡莫合金纳米圆盘中的单个磁涡旋结构,采用微磁学模拟研究了磁涡旋极性翻转过程中的局域能量密度.磁涡旋的极性翻转通过与初始涡旋极性相反的涡旋与反涡旋对的生成,以及随后发生的反涡旋与初始涡旋的湮没来实现.模拟结果显示当纳米圆盘样品中局域能量密度的最大值达到一临界值时,磁涡旋将会实现极性翻转,其中交换能起主导作用.基于涡旋极性翻转过程中出现的三涡旋态结构,应用刚性磁涡旋模型对局域交换能量密度进行了理论分析.通过刚性磁涡旋模型得到的磁涡旋极性翻转所需的局域交换能量密度的临界值与模拟结果符合得较好.     

Edge-soliton-mediated vortex-core reversal dynamics

We report an additional reversal mechanism of magnetic vortex cores in nanodot elements driven by currents flowing perpendicular to the sample plane, occurring via dynamic transformations between two coupled edge solitons and bulk vortex solitons. This mechanism differs completely from the well-known switching process mediated by the creation and annihilation of vortex-antivortex pairs in terms of the associated topological solitons, energies, and spin-wave emissions. Strongly localized out-of-plane gyrotropic fields induced by the fast motion of the coupled edge solitons enable a magnetization dip that plays a crucial role in the formation of the reversed core magnetization. This work provides a deeper physical insight into the dynamic transformations of magnetic topological solitons in nanoelements.     

True single domain and configuration-assisted switching of submicron Permalloy dots observed by electron holography

The switching behavior of submicron circular Permalloy nanomagnets has been investigated. Electron holography provides a magnetic resolution of down to 10 nm. This allows us to observe in detail the switching and to measure the induction within single nanodots with diameters down to 150 nm at a thickness of 6 nm. Particles of these dimensions show a single domain state during the whole switching process which takes place at external fields of only a few 100 A/m. For larger or thicker particles the magnetization reversal runs via the formation of a C state or an intermediate vortex state.     

Magnetoresistance measurement of permalloy thin film rings with triangular fins

Magnetization reversals in permalloy rings controlled by nucleation sites using triangular fins at the same side and diagonal with respect to the field direction are demonstrated by magnetoresistance measurement and micromagnetic simulation. In the ring with triangular fins at the same side, there exists two-step reversal from onion to flux-closure state (or vortex state) and then from flux-closure (or vortex state) to reverse onion state; in the ring with diagonal triangular fins, one-step reversal occurs directly from onion to reverse onion state. The reversal processes are repeatable and controllable in contrast to an ideal ring without triangular fins where one-step and two-step reversals occur randomly in sweep-up and sweep-down processes.     

Effect of surface defects on vortex penetration in small superconducting squares

We report the dependence of the vortex penetration on the number and positions of surface defects in mesoscopic superconducting squares. The vortex penetration is detected by the change in the transport properties of small tunnel junctions attached to the sample (small-tunnel-junction method). We find that for vorticities L = 0 and 1, a defect at the center of a side facilitate the vortex penetration, while a defect at a corner does not affect the vortex penetration field. For higher vorticities, the influence of defects becomes complicated, indicating that not only the existence of surface defects but also the vortex arrangement influences the vortex penetration. Simple explanation for this mesoscopic vortex penetration effect is given.     

Topological defects and magnetization processes for the triangular lattice of ising particles with an antiferromagnetic interaction

For the frustrated triangular lattice of Ising magnetic moments with an antiferromagnetic interaction, which is in a state with two sublattices, a new type of topological defects with zero energy in the approximation of the interaction between only the nearest-neighbors has been found. These defects have a nonzero magnetic moment, and the magnetization in a low field occurs via the formation of a system of such defects. These properties are valid for a 2D superstructure in the form of a triangular lattice of single-domain magnetic particles with perpendicular anisotropy and dipole coupling.     

Field evolution of tilted vortex cores in exchange-biased ferromagnetic dots

We developed an analytical model for the magnetization reversal via vortex nucleation and annihilation in double-layer ferromagnetic/antiferromagnetic cylindrical dots. The coupling of the ferromagnet to the antiferromagnet is modeled by means of an interfacial exchange field. The nonuniformity of the magnetization reversal mode perpendicular to the layers is explicitly included and results in a tilted vortex core (tilted Bloch line). The vortex core tilt results in an asymmetry of the nucleation and annihilation fields, which are calculated as a function of the dot geometry.     

Controlling double vortex states in low-dimensional dipolar systems

The reversal process of the chirality of each opposite vortex belonging to a double vortex state in ferromagnetic hysterons, via the application of in-plane magnetic fields, is reported. Simulations reveal that such a process involves the formation of four intermediate states, including original ones. Hysteresis loops can occur only in a counterclockwise fashion because of one of these intermediate states. Double vortex states can also be controlled by electric fields in ferroelectric nanostructures of different shapes, but with some key differences with respect to the ferromagnetic case.     

Magnetization of two-dimensional superconductors with defects

A new method is developed for numerical simulation of the magnetization of layered superconductors with defects that is based on the Monte Carlo algorithm. The minimization of the free energy functional of a two-dimensional vortex system enables one to obtain equilibrium configurations of vortex density and calculate the magnetization of a superconductor with arbitrary distribution of defects in a wide temperature range. Magnetization curves are obtained for the first time for a defective superconductor under conditions of cyclic variation of the external magnetic field for different temperatures. The magnetic induction profiles and the magnetic flux distribution inside a superconductor are calculated, which support the validity of Bean’s model. It is demonstrated that the process of magnetization reversal is accompanied by the emergence of an annihilation wave, i.e., the motion of a zone with zero magnetic induction at the leading front of the incoming magnetic flux.     

Magnetization reversal in submicron disks: exchange biased vortices

Submicron, circular, ferromagnetic-antiferromagnetic dots exhibit different magnetization reversal mechanisms depending on the direction of the magnetic applied field. Shifted, constricted hysteresis loops, typical for vortex formation, are observed for fields along the exchange bias direction. However, for fields applied close to perpendicular to the exchange bias direction, magnetization reversal occurs via coherent rotation. Magnetic force microscopy imaging together with micromagnetic simulations are used to further clarify the different magnetic switching behaviors.     

Dynamic origin of vortex core switching in soft magnetic nanodots

The magnetic vortex with in-plane curling magnetization and out-of-plane magnetization at the core is a unique ground state in nanoscale magnetic elements. This kind of magnetic vortex can be used, through its downward or upward core orientation, as a memory unit for information storage, and thus, controllable core switching deserves some special attention. Our analytical and micromagnetic calculations reveal that the origin of vortex core reversal is a gyrotropic field. This field is induced by vortex dynamic motion and is proportional to the velocity of the moving vortex. Our calculations elucidate the physical origin of the vortex core dynamic reversal, and, thereby, offer a key to effective manipulation of the vortex core orientation.     

Correlated magnetic vortex chains in mesoscopic cobalt dot arrays

Nucleation and annihilation of vortex states have been studied in two-dimensional arrays of densely packed cobalt dots. A clear signature of dipolar interactions both between single-domain state dots and vortex state dots has been observed from the dependence of vortex nucleation and annihilation fields on interdot separation. A direct consequence of these interactions is the formation of vortex chains as well as dipole chains aligned along the direction of the external field. In addition, short range correlation of chiralities within vortex chains has been observed using magnetic force microscopy imaging and has been attributed to cross-talking between adjacent elements.     

Reversal of coherent‐state spins in square magnetic nanodots induced by a magnetic microwave field

We study the coherent state excitation of spins in square nanodots induced by a magnetic microwave field. We present a new mechanism of spin reversal in nanodots. That is, the microwave field directly induces the reversal of the coherent‐state spins instead of indirectly through the magnetic vortex. We obtain the space distribution of coherent‐state spins in terms of a quantum theory, and calculate the time of spin reversal. This spin‐reversal process may be used to serve as a storage mechanism of binary information.     

Electronic structure of edge and vortex states in chiral mesoscopic superconductor

A subgap quasiparticle spectrum in a mesoscopic disk of a chiral superconductor is studied. An exact expression has been derived for the spectrum of surface states localized at the disk edge. For an Abrikosov vortex placed at the center of a superconducting disk, the spectrum transformation near the intersection points of the surface and vortex anomalous energy branches is investigated. The resulting splitting of the anomalous branches due to the hybridization of the edge and vortex states is determined by an external magnetic field and can lead, in particular, to the formation of a set of minigaps in the quasiparticle spectrum. Tuning the external magnetic field makes it possible to control the width of the minigaps and the positions of the corresponding density of state singularities at the minigap edges. The text was submitted by the author in English.     

Thermally assisted magnetization reversal in submicron-sized magnetic thin films

We have measured the rate of thermally assisted magnetization reversal of submicron-sized magnetic thin films. For fields H just less than the zero-temperature switching field H(C), the probability of reversal, P(exp)(s)(t), increases for short times t, achieves a maximum value, and then decreases exponentially. Micromagnetic simulations exhibit the same behavior and show that the reversal proceeds through the annihilation of two domain walls that move from opposite sides of the sample. The behavior of P(exp)(s)(t) can be understood through a simple "energy-ladder" model of thermal activation.     

Vortex formation and annihilation in three textures of rotating superfluid 3He-A

Textures, textural transformation, and formation and annihilation of a single vortex were investigated in narrow cylinders with 100 microm radius in A-phase under rotation up to 6.28 rad/sec. Three textures were found, depending on the cooling conditions of the sample through the superfluid transition temperature T(c). We found the gyromagnetic effect of textures; that is, two textures (A or B) could be selected either by applying a magnetic field in parallel or anti-parallel to the rotation axis. The critical angular speed of a single vortex formation Omega(f) and that of annihilation Omega(a) for each texture were measured. The textural transformation in type A texture was induced by rotation. Both type A and B textures held macroscopic angular momentum along the rotation axis. We identified the texture for type A, B, and C as Mermin-Ho, radial disgyration, and a soliton type of defect along the axis, respectively.     

Ferroelectric Properties of Polycrystalline Ceramics with Dipolar Defect Simulated from the Potts--Ising Model

Physical properties of polycrystailine ferroelectrics including the contributions of the fixed dipolar defects and the average grain size in the Potts-Ising model are simulated by using the Monte Carlo method. Domain pattern, hysteresis loop and switching current of the polarization reversal process are obtained. Two processes are considered in our simulation. In the first one, the grain texture of ferroelectric ceramics are produced from the Ports model, and then the Ising model is implemented in the obtained polycrystailine texture to produce the domain pattern, hysteresis loop and switching current. It is concluded that the defect has the ability to decrease the remnant polarization P~ as well as the coercive field E~. The back switching is obviously observed after the electric field is off, and it shows some variation after introducing the fixed dipolar defect. Meanwhile, the spike of the switching current is found to lower with the increasing defect concentration and the decreasing average grain size.