We study dynamical and thermal effects that are induced in nanoparticle systems by a rotating magnetic field. Using the deterministic Landau-Lifshitz equation and appropriate rotating coordinate systems, we derive the equations that characterize the steady-state precession of the nanoparticle magnetic moments and study a stability criterion for this type of motion. On this basis, we describe ͑i͒ the influence of the rotating field on the stability of the small-angle precession, ͑ii͒ the dynamical magnetization of nanoparticle systems, and ͑iii͒ the switching of the magnetic moments under the action of the rotating field. Using the backward Fokker-Planck equation, which corresponds to the stochastic Landau-Lifshitz equation, we develop a method for calculating the mean residence times that the driven magnetic moments dwell in the up and down states. Within this framework, the features of the induced magnetization and magnetic relaxation are elucidated.

The investigation of a sizable thermal enhancement of magnetization is put forward for uniaxial ferromagnetic nanoparticles that are placed in a rotating magnetic field. We elucidate the nature of this phenomenon and evaluate the resonant frequency dependence of the induced magnetization. Moreover, we reveal the role of magnetic dipolar interactions, point out potential applications, and reason the feasibility of an experimental observation of this effect. DOI: 10.1103/PhysRevLett.97.227202 PACS numbers: 75.50.Tt, 05.40.ÿa, 75.60.Jk Presently, the study of magnetic nanoparticles and their structures is one of the most important research areas in nanoscale physics. The first reason is that such nanoparticles increasingly find numerous applications that range from medicine to nanotechnology. Another reason is that these systems exhibit a number of remarkable physical phenomena, such as quantum tunneling of magnetization [1], giant magnetoresistance [2], exchange bias [3], and finite-size and surface effects [4], to name but a few. Moreover, the study of fundamentals of magnetic behavior in these systems is also an important issue, especially for high-density data storage devices [5].From a practical point of view, the lifetime of stored data and the switching time (i.e., the time during which the reversal of the nanoparticle magnetic moments occurs) are salient characteristics of such devices. Now, thanks to the experimental discovery of fast switching of magnetization [6], the switching time reaches the fundamental (picosecond) limit for field-induced magnetization reversal. On the contrary, a feasible lifetime must cover up to 10 yr and beyond. Its value is usually limited by the superparamagnetic effect [7] and is defined by the probabilities p that the nanoparticle magnetic moment m stays in the up ( 1) and down ( ÿ1) equilibrium directions. These probabilities, which are also responsible for other thermal effects in such systems including magnetic relaxation [8], are very sensitive to small perturbations that change the static states of the magnetic moments. Namely, according to the Arrhenius law [9] the ratio p 1 =p ÿ1 is approximately given by exp E=kT , where E E 1 ÿ E ÿ1 , E is the potential barrier for the reorientation ! ÿ , k is the Boltzmann constant, and T is the absolute temperature. Therefore, if without perturbations E 0, then p 1 =p ÿ1 1 and the nanoparticle system is demagnetized. But due to the exponential dependence on E and T, the ratio p 1 =p ÿ1 can drastically be changed by small perturbations. In particular, a static magnetic field H applied along the nanoparticle easy axis of magnetization yields E 2Hm (m jmj), and so p 1 strongly differs from p ÿ1 if jHj=H a 1=4a, where a H a m=2kT and H a is the anisotropy field. This means that even small magnetic fields (in comparison with H a ) almost fully magnetize the nanoparticle systems when a 1. In the case of time-periodic perturbations the situation is not settled yet and far less researched. On the one hand, these perturbations generate ...

We study, both analytically and numerically, the phenomenon of energy dissipation in single-domain ferromagnetic nanoparticles driven by an alternating magnetic field. Our interest is focused on the power loss resulting from the Landau-Lifshitz-Gilbert equation, which describes the precessional motion of the nanoparticle magnetic moment. We determine the power loss as a function of the field amplitude and frequency and analyze its dependence on different regimes of forced precession induced by circularly and linearly polarized magnetic fields. The conditions to maximize the nanoparticle heating are also analyzed.

We study the slow phase of thermally activated magnetic relaxation in finite two-dimensional ensembles of dipolar interacting ferromagnetic nanoparticles whose easy axes of magnetization are perpendicular to the distribution plane. We develop a method to numerically simulate the magnetic relaxation for the case that the smallest heights of the potential barriers between the equilibrium directions of the nanoparticle magnetic moments are much larger than the thermal energy. Within this framework, we analyze in detail the role that the correlations of the nanoparticle magnetic moments and the finite size of the nanoparticle ensemble play in magnetic relaxation.Comment: 21 pages, 4 figure

We study the deterministic and stochastic rotational dynamics of ferromagnetic nanoparticles in a precessing magnetic field. Our approach is based on the system of effective Langevin equations and

The coupled magnetic and mechanical motion of a ferromagnetic nanoparticle in a viscous fluid is considered within the dynamical approach. The equation based on the total momentum conservation law is used for the description of the mechanical rotation, while the modified Landau-Lifshitz-Gilbert equation is utilized for the description of the internal magnetic dynamics. The exact expressions for the particles trajectories and the power loss are obtained in the linear approximation. The comparison with the results of other widespread approaches, such as the model of fixed particle and the model of rigid dipole, is performed. It is established that in the small oscillations mode the damping precession of the nanopartile magnetic moment is the main channel of energy dissipation, but the motion of the nanoparticle easy axis can significantly influence the value of the resulting power loss.

a b s t r a c tUsing 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.

We present a comprehensive study of the magnetization switching of a uniaxial nanoparticle driven by a circularly polarized magnetic field rotated in the plane perpendicular to the easy axis. The conditions for the existence of the uniform and non-uniform precessions of the nanoparticle magnetic moment are derived. In addition, the differences between switchings via uniform and non-uniform precession are determined, and the essential role of field polarization is demonstrated. The dependence of the switching time on the field amplitude and frequency are calculated numerically. We show that a permanent magnetic field can reduce the amplitude and frequency of the switching rotating field, and that the combined action of these fields is characterized by an extremely strong dependence of the switching time on the field parameters. We also demonstrate that the transition process caused by an external magnetic field pulse can decrease the switching amplitude in comparison with the value predicted from analysis of the stability criterion. We discuss the advantages of switching the magnetization by means of the action of a rotating field over the magnetization switching using a steady field applied perpendicular to the easy axis.