Effect of damping on the time variation of fields produced by a small pole tip with a soft under layer

The time variation of magnetostatic fields generated by space and time varying magnetization configurations in small perpendicular pole tips is studied. The magnetization configurations are a response to external fields driving the pole tip and soft under layer (SUL). When the system damping is sufficiently small the magnetization excitations persist for a long time after reversal. The effects of damping parameter, position in the media, and discretization cell size on the magnitude of the time varying magnetostatic fields will be given. Decreasing the damping parameter increases the magnitude of the magnetostatic field variation

Implementation of the "hyperdynamics of infrequent events" method for acceleration of thermal switching dynamics of magnetic moments

For acceleration of the calculations of thermal magnetic switching, we report the use of the Voter method, recently proposed in chemical physics (also called "hyperdynamics of the infrequent events"). The method consists of modification of the magnetic potential so that the transition state remains unchanged. We have found that the method correctly describes the mean first passage time even in the case of small damping (precessional case) and for an oblique angle between the anisotropy and the field directions. Due to the costly evaluation of the lowest energy eigenvalue, the actual acceleration depends on its fast computation. In the current implementation, it is limited to intermediate time scale and to small system size

Parametric optimization for terabit perpendicular recording

The design of media for ultrahigh-density perpendicular recording is discussed in depth. Analytical and semianalytical models are developed to determine the constraints upon the media to fulfill requirements of writability and thermal stability, and the effect of intergranular exchange coupling is examined. The role of vector fields during the write process is examined, and it is shown that one-dimensional models of perpendicular recording have significant deficiencies. A micromagnetic model is described and the results of simulations of recording undertaken with the model are presented. The paper demonstrates that there is no physical reason why perpendicular recording should not be possible at or above 1 Tb/in(2)

Theory of rotational processes in perpendicular media and application to anisotropy measurement

Numerical and analytical calculations of rotational process in perpendicular recording media are presented. The work supports recent experimental studies that suggest that the measurement of rotational magnetization processes can be used to determine the value of the anisotropy constant. An expression for the rotational magnetization for a noninteracting system is derived taking into account the dispersion of K and the easy-axis orientation. The calculations show that the experiments determine the mean value of H-K, essentially independent of the angular dispersion. A numerical (Monte-Carlo based) micromagnetic model is used to study the effects of magnetostatic and exchange interactions at nonzero temperatures. It is shown that for small values of KV/kT, irreversible magnetization processes take place, which precludes the use of the rotational magnetization method to determine K values. This effect is enhanced by the presence of the magnetostatic interaction. However, the presence of exchange interactions is found to enforce coherent rotation in small fields, reducing the irreversible processes and allowing the determination of H-K. Under these circumstances, it is shown that the exchange does not significantly affect the value of HK, and that a well-defined demagnetization correction of 4piM is appropriate. Finally, a comparison with experimental data gives good agreement for multilayer and granular media and shows the role of domain formation on the rotational magnetization process

Unified model of hyperthermia via hysteresis heating in systems of interacting magnetic nanoparticles

We present a general study of frequency and magnetic field dependence of the specific heat power produced during field-driven hysteresis cycles in magnetic nanoparticles with relevance to hyperthermia applications in biomedicine. Employing a kinetic Monte-Carlo method with natural time scales allows us to go beyond the assumptions of small driving field amplitudes and negligible inter-particle interactions, which are fundamental to applicability of the standard approach based on linear response theory. The method captures the superparamagnetic and fully hysteretic regimes and the transition between them. Our results reveal unexpected dipolar interaction-induced enhancement or suppression of the specific heat power, dependent on the intrinsic statistical properties of particles, which cannot be accounted for by the standard theory. Although the actual heating power is difficult to predict because of the effects of interactions, optimum heating is in the transition region between the superparamagnetic and fully hysteretic regimes

Electronic and magnetic properties of bimetallic L10_0 cuboctahedral clusters by means of a fully relativistic density functional based calculations

By means of density functional theory (DFT) and the generalized gradient approximation (GGA) we present a structural, electronic and magnetic study of FePt, CoPt, FeAu and FePd based L1$_0$ ordered cuboctahedral nanoparticles, with total numbers of atoms, N$_{tot}$ = 13, 55, 147. After a conjugate gradient relaxation, the nanoparticles retain their L1$_0$ symmetry, but the small displacements of the atomic positions tune the electronic and magnetic properties. The value of the total magnetic moment stabilizes as the size increases. We also show that the Magnetic Anisotropy Energy (MAE) depends on the size as well as the position of the Fe-atomic planes in the clusters. We address the influence on the MAE of the surface shape, finding a small in-plane MAE for (Fe,Co)$_{24}$Pt$_{31}$ nanoparticles

Controlling the Polarity of the Transient Ferromagnetic-Like State in Ferrimagnets

After the application of an ultrashort laser pulse, the antiferromagnetic alignment in rare earth-transition metal alloys can temporarily become ferromagnetic with the rare-earth polarity. Proposed models merely describe this effect, without showing the route for its manipulation. Here we use extensive atomistic spin model simulations and micromagnetic theory for ferrimagnets at elevated temperatures to predict that the polarity of this transient ferromagnetic-like state can be controlled by initial temperature. We show that this arises because the magnetic response of each lattice has a different temperature dependence, at low temperatures the transition metal responds faster than the rare earth, while at high temperatures this role is interchanged. Our findings contribute to the physical understanding and control of this state and thus open new perspectives for its use in ultrafast magnetic devices