Calculations of magnetic susceptibility of magnetotactic bacteria culture

Calculations of magnetic susceptibility of magnetotactic bacteria (MTB) culture are reported. The model was elaborated with regard to the effect of chemotaxis for two different geometrical compositions of the experiment. The results obtained allow one to determine concentration of MTB from magnetic measurements. It was shown that the characteristic parameters of chemotaxis can be extracted from the dependencies of the susceptibility on magnetic field strength.     

MFM study of magnetic vortex cores in circular permalloy dots: behavior in external field

In a circular dot of permalloy with an appropriate size, a vortex structure with perpendicular (turned-up) magnetization at the core is realized. The existence of the perpendicular magnetization spot has been confirmed and the direction of the magnetization, up or down, has been determined by magnetic force microscopy (MFM) for permalloy dots with the diameter of 0.1–1 μm. The switching field of turned-up magnetization is determined by applying external fields perpendicularly and in tilted directions to the plane. By comparing the MFM results and the magnetization curves measured by a SQUID magnetometer, the switching process of turned-up magnetization is argued.     

Magnetization switching in single-domain particles and a response to a field pulse

An equation in the Gilbert form that describes the motion of the magnetization vector in intense high-frequency magnetic fields is solved numerically. The solution obtained is used to study switching of the magnetization of a single-domain ferromagnetic particle that has the shape of an ellipsoid of revolution and possesses cubic anisotropy from the position parallel to an easy axis to the position normal to this axis. The ranges of amplitudes and frequencies of the magnetic field where magnetization switching is observed are determined. An expression for the response of an ensemble of variously oriented particles is derived. It is shown that a particle ensemble generated by an rf field may serve as a data carrier on which information is written and read out by means of nonlinear and linear ferromagnetic resonances.     

Magnetic elements for switching magnetization magnetic force microscopy tips

Using combination of micromagnetic calculations and magnetic force microscopy (MFM) imaging we find optimal parameters for novel magnetic tips suitable for switching magnetization MFM. Switching magnetization MFM is based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within the technique the sum of the scanned data with reversed tip magnetization depicts local atomic forces, while their difference maps the local magnetic forces. Here we propose the design and calculate the magnetic properties of tips suitable for this scanning probe technique. We find that for best performance the spin-polarized tips must exhibit low magnetic moment, low switching fields, and single-domain state at remanence. The switching field of such tips is calculated and optimum shape of the Permalloy elements for the tips is found. We show excellent correspondence between calculated and experimental results for Py elements.     

Precessional switching of thin nanomagnets: analytical study

We study analytically the precessional switching of the magnetization of a thin macrospin. We analyze its response when subjected to an external field along its in-plane hard axis. We derive the exact trajectories of the magnetization. The switching versus non switching behavior is delimited by a bifurcation trajectory, for applied fields equal to half of the effective anisotropy field. A magnetization going through this bifurcation trajectory passes exactly along the hard axis and exhibits a vanishing characteristic frequency at that unstable point, which makes the trajectory noise sensitive. Attempting to approach the related minimal cost in applied field makes the magnetization final state unpredictable. We add finite damping in the model as a perturbative, energy dissipation factor. For a large applied field, the system switches several times back and forth. Several trajectories can be gone through before the system has dissipated enough energy to converge to one attracting equilibrium state. For some moderate fields, the system switches only once by a relaxation dominated precessional switching. We show that the associated switching field increases linearly with the damping parameter. The slope scales with the square root of the effective anisotropy. Our simple concluding expressions are useful to assess the potential application of precessional switching in magnetic random access memories.Received: 2 October 2003, Published online: 19 November 2003PACS: 75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.) - 75.60.Jk Magnetization reversal mechanisms - 75.75. + a Magnetic properties of nanostructures     

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.     

Response of Mn overlayers on Fe to external magnetic fields: Electronic structure calculations

The behavior of Mn overlayers on Fe(0 0 1) under the influence of external magnetic fields is investigated. The electronic charge distribution, local magnetic moments as well as their couplings are determined as a function of the external field by solving self-consistently a tight binding Hamiltonian, parameterized to ab initio TBLMTO calculations. Our method allows to trace back the field-dependent average magnetization of the system to its electronic structure and magnetic configuration. We show how in the non-collinear framework the response of the system is markedly different to what is found in the collinear framework. If metastable magnetic configurations exist, the external field can be used for tuning the system between some of them because the system stays in some of those metastable states even after switching off the external field.     

Mössbauer study of the fast magnetization reversal in FeBO3 induced by external RF magnetic fields

The effect of fast magnetization reversal induced by external radio frequency (rf) fields has been studied in FeBO₃ using the Mössbauer technique. The rf collapse and sideband effects were investigated as a function of intensity for two rf field frequencies: 62 and 36 MHz. The switching times estimated for magnetization reversal are of the same order of magnitude as in amorphous metals and Fe-Ni alloys. Because of the relatively short switching times the magnetization reversal must be of rotational character.     

Field-dependent evolution of magnetic states in Co ring arrays

We present a systematic study of field-dependent evolution in the magnetization reversal process of elongated Co ring arrays using in-situ field magnetic force microscopy. We observed that, the rings typically undergo a uniform→onion→vortex→reverse onion→reverse uniform spin-state transition as the field is swept along the major axis of the rings. However, the switching field distribution in the arrays generally leads to each transition occupying a wide range of fields, which results in the observation of coexistence of different magnetization states at lower field. The magnetization reversal sequence has also been verified by micromagnetic simulations, which show good agreement with our experimental data.     

Step feature observed in the angular dependence of magnetization switching fields in GaMnAs micro-device

The magnetization switching phenomena of GaMnAs Hall devices have been investigated by using the planar Hall effect (PHE) measurement. Though two different sizes of Hall bar devices, width of 300 and of 10 μm, show very similar Curie temperature, their magnetization switching fields behave significantly different. While the angle dependence of magnetization switching field of the 300 μm device showed typical rectangular shape behavior with an applied magnetic field angle in the polar plot, that of the 10 μm device exhibited large step at 〈1 1 0〉 crystallographic directions, breaking the continuity of the switching field in angle dependence. Such unusual phenomenon observed in the 10 μm device was discussed in terms of the change in magnetic anisotropy by the fabrication of micro-device.     

Magnetization switching modes in nanopillar spin valve under the external field

The current-induced magnetic switching is studied in Co/Cu/Co nanopillar with an in-plane magnetization traversed under the perpendicular-to-plane external field.Magnetization switching is found to take place when the current density exceeds a threshold.By analyzing precessional trajectories,evolutions of domain walls and magnetization switching times under the perpendicular magnetic field,there are two different magnetization switching modes:nucleation and domain wall motion reversal;uniform magnetization ...     

Phase coherent precessional magnetization reversal in microscopic spin valve elements

We evidence multiple coherent precessional magnetization reversal in microscopic spin valves. Stable, reversible, and highly efficient magnetization switching is triggered by transverse field pulses as short as 140 ps with energies down to 15 pJ. At high fields a phase coherent reversal is found revealing periodic transitions from switching to nonswitching under variation of pulse parameters. At the low field limit the existence of a relaxation dominated regime is established allowing switching by pulse amplitudes below the quasistatic switching threshold.     

Tunable Magnetoresistance in InAs/Ferromagnet Hybrid Devices

Metal-oxide semiconductor field-effect transistors with ferromagnetic source- and drain-electrodes are fabricated on InAs. Tailored micromagnetic behavior of the ferromagnetic electrodes is achieved by use of a specific shape as well as in-plane external magnetic fields and is measured by magnetic-force microscopy. The field-effect in transistors with electrodes of defined magnetization is determined as a function of the gate voltage. The amplitude and the sign of abrupt resistance changes observed at points of irreversible switching of the ferromagnetic contacts' magnetization can be tuned by the gate voltage providing evidence for spin-polarized transport.     

Current-driven magnetic excitations in permalloy-based multilayer nanopillars

We study current-driven magnetization switching in nanofabricated Ni(84)Fe(16)/Cu/Ni(84)Fe16 trilayers at 295 and 4.2 K. The shape of the hysteretic switching diagram at low magnetic field changes with temperature. The reversible behavior at higher fields involves two phenomena, a threshold current for magnetic excitations closely correlated with the switching current, and a peak in differential resistance characterized by telegraph noise, with an average period that decreases exponentially with current and shifts with temperature. We interpret both static and dynamic results at 295 and 4.2 K in terms of thermal activation over a potential barrier, with a current-dependent effective magnetic temperature.     

Phase diagrams for the precession states of the nanoparticle magnetization in a rotating magnetic field

Using the analytical and numerical solutions of the Landau–Lifshitz equation, we calculate the phase diagrams for the precession states of the nanoparticle magnetization in a rotating magnetic field. We show that there are three different scenarios for the magnetization switching. The bias magnetic field applied antiparallel to the nanoparticle magnetization strongly decreases the switching amplitudes and frequencies of the rotating field.     

Switching plasticity in compensated ferrimagnetic multilayers for neuromorphic computing

Current-induced multilevel magnetization switching in ferrimagnetic spintronic devices is highly pursued for the application in neuromorphic computing. In this work, we demonstrate the switching plasticity in Co/Gd ferrimagnetic multilayers where the binary states magnetization switching induced by spin-orbit toque can be tuned into a multistate one as decreasing the domain nucleation barrier. Therefore, the switching plasticity can be tuned by the perpendicular magnetic anisotropy of the multilayers and the in-plane magnetic field. Moreover, we used the switching plasticity of Co/Gd multilayers for demonstrating spike timing-dependent plasticity and sigmoid-like activation behavior. This work gives useful guidance to design multilevel spintronic devices which could be applied in high-performance neuromorphic computing.     

Effect of carbon substitution on low magnetic field AC losses in MgB2 single crystals

The DC magnetization and AC magnetic susceptibilities were measured for MgB₂ single crystals, unsubstituted and carbon substituted with the composition of Mg(B_0.94C_0.06)₂. AC magnetic losses were derived from the AC susceptibility data as a function of the AC amplitude and the DC bias magnetic field. From the DC magnetization loops critical current densities were derived as a function of temperature and DC field. Results show that the substitution with carbon decreases critical current densities at low external magnetic fields, in contrast to the well known effect of an increase of the critical current densities at higher magnetic fields.     

Quasiballistic magnetization reversal

We demonstrate a quasiballistic switching of the magnetization in a microscopic magnetoresistive memory cell. By means of time resolved magnetotransport, we follow the large angle precession of the free layer magnetization of a spin valve cell upon application of transverse magnetic field pulses. Stopping the field pulse after a 180 degrees precession rotation leads to magnetization reversal with reversal times as short as 165 ps. This switching mode represents the fundamental ultrafast limit of field induced magnetization reversal.     

Tb0.3Dy0.7Fe2合金磁畴偏转的滞回特性研究

研究了不同载荷作用下Tb_0.3Dy_0.7Fe₂合金在压磁和磁弹性效应中磁畴偏转的滞回特性. 基于Stoner-Wolhfarth模型的能量极小原理, 采用绘制自由能-磁畴偏转角度关系曲线的求解方法, 研究了压磁和磁弹性效应中载荷作用下的磁畴角度偏转和磁化过程, 计算分析了不同载荷作用下磁畴偏转的滞回特性. 研究表明, 压磁和磁弹性效应中磁畴偏转均存在明显的滞回、跃迁效应, 其中磁化强度的滞回效应来源于磁畴偏转的角度跃迁; 压磁效应中预加磁场的施加将增大磁化强度的滞回, 同时使滞回曲线向大压应力方向偏移; 磁弹性效应中磁畴偏转的滞回存在两个临界磁场强度, 不同磁场强度下合金具有不同的磁畴偏转路径和磁化滞回曲线, 临界磁场强度的大小取决于预压应力的施加. 理论分析对类磁致伸缩材料磁畴偏转模型的完善和材料器件的设计应用非常有意义.     

Magnetization switching and microwave oscillations in nanomagnets driven by spin-polarized currents

A novel theoretical approach to magnetization dynamics driven by spin-polarized currents is presented. Complete stability diagrams are obtained for the case where spin torques and external magnetic fields are simultaneously present. Quantitative predictions are made for the critical currents and fields inducing magnetization switching, for the amplitude and frequency of magnetization self-oscillations, and for the conditions leading to hysteretic transitions between self-oscillations and stationary states.