Thermally assisted switching in FePt single-domain particles with a Gaussian distribution of temperature

Magnetic nanoscale systems,including nanodots,nanofibers,nanowires and nanoparticles,are currently attracting great interest due to their interesting physical and promising applications in various fields,such as magnetic recording,sensors,target drugs and catalysts,as well as others.To achieve ultrahigh recording density,the method of heat assisted magnetic recording(HAMR) has been introduced.In this work,with the help of a Monte Carlo method,the mechanisms of thermally assisted magnetization switching in F...     

Magnetization of an ensemble of noninteracting single-domain particles with uniform magnetization rotation

The limiting hysteresis loops are constructed for an ensemble of single-domain uniaxial particles for various types and degrees of order among the easy-magnetization directions of these particles. The influence of an inhomogeneity of the particles with respect to magnitude of the anisotropy field is evaluated.     

Theoretical limit of the minimal magnetization switching field and the optimal field pulse for Stoner particles

The theoretical limit of the minimal magnetization switching field and the optimal field pulse design for uniaxial Stoner particles are investigated. Two results are obtained. One is the existence of a theoretical limit of the smallest magnetic field out of all possible designs. It is shown that the limit is proportional to the damping constant in the weak damping regime and approaches the Stoner-Wohlfarth (SW) limit at large damping. For a realistic damping constant, this limit is more than 10 times smaller than that of so-called precessional magnetization reversal under a noncollinear static field. The other is on the optimal field pulse design: if the magnitude of a magnetic field does not change, but its direction can vary during a reversal process, there is an optimal design that gives the shortest switching time. The switching time depends on the field magnitude, damping constant, and magnetic anisotropy.     

Distributions of switching times of single-domain particles using a time quantified Monte Carlo method

Using a time quantified Monte Carlo scheme we performed simulations of the switching time distribution of single mono-domain particles in the Stoner–Wohlfarth approximation. We considered uniaxial anisotropy and different conditions for the external applied field. The results obtained show the switching time distribution can be well described by two relaxation times, either when the applied field is parallel to the easy axis or for an oblique external field and a larger damping constant. We found that in the low barrier limit these relaxation times are in very good agreement with analytical results obtained from solutions of the Fokker–Planck equation related to this problem. When the damping is small and the applied field is oblique the shape of the distribution curves shows several peaks and resonance effects.     

Variation in the magnetic structure of a single-domain particle ensemble and its response to an rf pulse

A set of equations in the Gilbert form that describes the motion of the magnetization vector in an ensemble of interacting magnetic nanoparticles is numerically solved for the case of high-amplitude rf pulses. Based on the numerical solution, the magnetic structure of spherical particle ensembles showing cubic anisotropy at certain parameters of the variable field is studied. This phenomenon is shown to have a threshold. The dependence of the field threshold amplitude on the acting pulse repetition rate and amount of magnetic interaction is determined. It is demonstrated that a change in the magnetic structure of the interacting particle ensembles causes a change in the spectrum of the response. This fact can be used for pulsed rf writing and readout of information based on ferromagnetic resonance.     

Magnetization reversal of a single cobalt cluster using a RF field pulse

Technological improvements require the understanding of dynamical magnetization reversal processes at the nanosecond time scales. In this paper, we present the first magnetization reversal measurements performed on a single cobalt cluster (counting only a thousand of spins), using the micro-superconducting quantum interference device (SQUID) technique by applying a constant magnetic field combined with a radio-frequency (RF) field pulse. First of all, we present the different technical steps necessary to detect the magnetic reversals at low temperature (T=35 mK) of a well-defined nanoparticle prepared by low energy clusters beam deposition (LECBD). We previously showed that the three-dimensional (3D)-switching Stoner-Wohlfarth astroid represents the magnetic anisotropy of the nanoparticle. Then, an improved device coupled with a gold stripe line, allow us to reverse such macrospin, using a RF pulse. A qualitative understanding of the magnetization reversal by non-linear resonance has been obtained with the Landau-Lifschitz-Gilbert (LLG) equation.     

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 ...     

Magnetization reversal by a single pulse of magnetic field or spin-polarized current

One of the important requirements for spintronic devices concerns an efficient magnetization reversal, which may eventually lead to ultra-fast non-volatile magnetic memory applications. In particular, it is necessary to achieve stable sub-nanosecond switching times and to reduce magnetization “ringing”, so that the reversal will proceed along the shortest ballistic path connecting the initial and the target magnetization states. This paper is dedicated to the numerical simulations of a mono-domain ferromagnetic particle, described by the Landau–Lifshitz–Gilbert equation. We study the general case of arbitrary orientation of the applied field/current pulses, constructing dynamic diagrams for the reversal time. We have found that even short 50 ps pulses, applied at a proper angle, will induce magnetization reversal with minimal ringing effects.     

Magnetization reversal of small particles in an oscillating magnetic field

The nonlinear behavior of single spherical particles and many-particle ensembles under an external oscillating magnetic field was studied using the Landau-Lifshitz-Gilbert approach. The exact analytical formulae for calculating field frequency and the dependence of the configuration of the initial moment on the system mean magnetization were obtained. Using asymptotic decomposition we obtained simple expressions for describing the nonlinear dynamics of spherical particles in the cases of large and small frequencies. These solutions are in good agreement with numerical calculations of the Landau-Lifshitz-Gilbert equation for the considering systems.     

Noncollinear states in a chain of single-domain magnetic particles

The ground state of a chain of single-domain magnetic particles has been theoretically analyzed. The conditions under which this state corresponds to a noncollinear structure in zero external magnetic field are determined. Such noncollinear states are due to the features of the long-range magnetostatic interactions in the systems without the center of inversion.     

Tunable spectral switching in the far field with a chirped cosh-Gaussian pulse

We have shown the possibility of tunable spectral switching in the far field with an apertured chirped cosh-Gaussian pulse by varying its cosh parameter (Ω). We have also shown that the switching frequency and both blue and red shifted frequencies can be also tuned by varying Ω. Possible application of our results in optical space communications has been highlighted.     

Long-range order and magnetic relaxation in a system of single-domain particles

The paramagnetic-ferromagnetic phase transition and distinctive characteristics of relaxation of the magnetization in a system of interacting single-domain ferromagnetic particles distributed randomly in a nonmagnetic matrix are investigated in the mean-field approximation. Fiz. Tverd. Tela (St. Petersburg) 41, 1822–1827 (October 1999)     

Hyperfine field response to RF excitation in superparamagnetic particles

Small magnetic particles with uniaxial magnetic anisotropy are under investigation. A possible mechanism of amplification of the RF field acting on nuclei in fine magnetic particle systems is described. The calculations are based on the exact solution of the proper Fokker-Planck equation using the matrix continued fractions method. This more general approach expands and reaffirms results first obtained with the discrete orientation approximation.     

Effect of a dc magnetic field on the magnetization relaxation of uniaxial single-domain ferromagnetic particles driven by a strong ac magnetic field

The nonlinear ac stationary response of the magnetization of noninteracting uniaxial single-domain ferromagnetic particles acted on by superimposed dc and ac magnetic fields applied along the anisotropy axis is evaluated from the Fokker-Planck equation, expressed as an infinite hierarchy of recurrence equations for Fourier components of the relaxation functions governing longitudinal relaxation of the magnetization. The exact solution of this hierarchy comprises a matrix continued fraction, allowing one to evaluate the ac nonlinear response and reversal time of the magnetization. For weak ac fields, the results agree with perturbation theory. It is shown that the dc bias field changes substantially the magnetization dynamics leading to new nonlinear effects. In particular, it is demonstrated that for a nonzero bias field as the magnitude of the ac field increases the reversal time first increases and having attained its maximum at some critical value of the ac field, decreases exponentially.     

On magnetic ordering in colloids of single-domain particles

Observed data supporting the possibility of magnetic ordering in magnetic colloids are reported. Conditions under which magnetized structures arise in magnetic colloids, initial samples in which these magnetized structures can be formed, and colloids with a developed system of magnetized aggregates are analyzed.     

Magnetization vector field in a uniaxial ferromagnetic film

Technical Physics - Analytical models for the magnetization vector field B m in a uniaxial ferromagnetic film are studied. Some of them are found to closely approximate B m even if the quality...     

Dependence of prefactor on the angle between an applied field and the easy axis for single-domain particles

The prefactor (attempt frequency) of particles with uniaxial anisotropy in an applied field at an oblique angle to the easy axis is studied systematically by calculating the smallest nonvanishing eigenvalue of the Fokker-Planck equation for several values of parameters under typical conditions in practical recording media. In the range of the damping constant 0.001-0.01, the prefactor increases with increasing the oblique angle from 0° to 30°, but for that angle of over 30° the prefactor increases very little. It is shown that the dependence of the prefactor on the energy barrier height is almost the same in the range of the oblique angle 5-60° of the applied field to the easy axis. It is confirmed that the prefactor under the conditions above takes from 10⁸ to 10¹⁰ per second.     

Nonlinear response of superparamagnetic particles to a sudden change of a high constant magnetic field

For a system of superparamagnetic particles in a high external constant magnetic field, a technique for calculating the nonlinear response to a sudden change in the field direction and magnitude is proposed. A set of momentary equations for the averaged spherical harmonics, which is derived from the Fokker-Planck equation for the magnetization-orientation distribution function is the basis of this technique. As an example, the nonlinear response of a system of particles with anisotropy of the easy-axis type is examined. For this case, a solution to the momentary equations is obtained by using matrix continued fractions. The magnetization relaxation time and the spectrum of the relaxation function are calculated for typical values of anisotropy, dissipation, and nonlinearity parameters. It is shown that the magnetization kinetics is essentially dependent on these parameters.