141 research outputs found
Modeling of long-time thermal magnetization decay in interacting granular magnetic materials
We present a general method to evaluate the long-time magnetization decay in granular magnetic systems. The method is based on Arrhenius-Neel kinetics with the evaluation of the energy barriers in a multidimensional space. To establish a possible reversal mode, we suggest the use of Metropolis Monte Carlo and for the mode statistical sampling-the kinetic Monte Carlo criteria. The examples considered include long-time magnetization decay in CoCrPt low-magnetization longitudinal recording media and in a collection of Co particles with different concentrations
Ultrafast relaxation rates and reversal time in disordered ferrimagnets
In response to ultrafast laser pulses, single-phase metals have been classified as “fast” (with magnetization quenching on the time scale of the order of 100 fs and recovery in the time scale of several picoseconds and below) and “slow” (with longer characteristic time scales). Disordered ferrimagnetic alloys consisting of a combination of “fast” transition (TM) and “slow” rare-earth (RE) metals have been shown to exhibit an ultrafast all-optical switching mediated by the heat mechanism. The behavior of the characteristic time scales of coupled alloys is more complicated and is influenced by many parameters such as the intersublattice exchange, doping (RE) concentration, and the temperature. Here, the longitudinal relaxation times of each sublattice are analyzed within the Landau-Lifshitz-Bloch framework. We show that for moderate intersublattice coupling strength both materials slow down as a function of slow (RE) material concentration. For larger coupling, the fast (TM) material may become faster, while the slow (RE) one is still slower. These conclusions may have important implications in the switching time of disordered ferrimagnets such as GdFeCo with partial clustering. Using atomistic modeling, we show that in the moderately coupled case, the reversal would start in the Gd-rich region, while the situation may be reversed if the coupling strength is larger
Moving toward an atomistic reader model
With the move to recording densities up to and beyond 1 Tb/in/sup 2/, the size of read elements is continually reducing as a requirement of the scaling process. The expectation is for read elements containing magnetic films as thin as 1.5 nm, in which finite size effects, and factors such as interface mixing might be expected to become of increasing importance. Here, we review the limitations of the current (micromagnetic) approach to the theoretical modeling of thin films and develop an atomistic multiscale model capable of investigating the magnetic properties at the atomic level. Finite-size effects are found to be significant, suggesting the need for models beyond the micromagnetic approach to support the development of future read sensors
Self-consistent description of spin-phonon dynamics in ferromagnets
Several recently reported exciting phenomena such as spin caloritronics or ultrafast laser-induced spin dynamics
involve the action of temperature on spin dynamics. However, the inverse effect of magnetization dynamics on
temperature change is very frequently ignored. Based on the density matrix approach, in this work we derive
a self-consistent model for describing the magnetization and phonon temperature dynamics in ferromagnets
in the framework of the quantum Landau-Lifshitz-Bloch equation. We explore potential applicability of our
approach for two cases, inspired by magnetocaloric effect and magnetic fluid hyperthermia. In the first case, the
spin-phonon dynamics is governed by the longitudinal relaxation in bulk systems close to the Curie temperature;
while in the second case it is described by the transverse relaxation during the hysteresis cycle of individual
nanoparticles well below the Curie temperature
Micromagnetic modelling of magnetic domain walls and domains in cylindrical nanowires
Magnetic cylindrical nanowires are very fascinating objects where the curved
geometry allows many novel magnetic effects and a variety of non-trivial
magnetic structures. Micromagnetic modelling plays an important role in
revealing the magnetization distribution in magnetic nanowires, often not
accessible by imaging methods with sufficient details. Here we review the
magnetic properties of the shape anisotropy-dominated nanowires and the
nanowires with competing shape and magnetocrystalline anisotropies, as revealed
by micromagnetic modelling. We discuss the variety of magnetic walls and
magnetic domains reported by micromagnetic simulations in cylindrical
nanowires. The most known domain walls types are the transverse and vortex
(Bloch point) domain walls and the transition between them is materials and
nanowire diameter dependent. Importantly, the field or current-driven domain
walls in cylindrical nanowires can achieve very high velocities. In recent
simulations of nanowires with larger diameter the skyrmion tubes are also
reported. In nanowires with large saturation magnetization the core of these
tubes may form a helicoidal ('corkscrew') structure. The topology of the
skyrmion tubes play an important role in the pinning mechanism, discussed here
on the example of FeCo modulated nanowires. Other discussed examples include
the influence of antinotches ('bamboo' nanowires) on the remanent magnetization
configurations for hcp Co and FeCo nanowires and Co-Ni multisegmented
nanowires.Comment: 24 pages, 17 figures, 1 tabl
Ultra-fast magnetisation rates within the Landau-Lifshitz-Bloch model
The ultra-fast magnetisation relaxation rates during the laser-induced
magnetisation process are analyzed in terms of the Landau-Lifshitz-Bloch (LLB)
equation for different values of spin . The LLB equation is equivalent in
the limit to the atomistic Landau-Lifshitz-Gilbert (LLG)
Langevin dynamics and for to the M3TM model [B. Koopmans, {\em et al.}
Nature Mat. \textbf{9} (2010) 259]. Within the LLB model the ultra-fast
demagnetisation time () and the transverse damping ()
are parameterized by the intrinsic coupling-to-the-bath parameter ,
defined by microscopic spin-flip rate. We show that for the phonon-mediated
Elliott-Yafet mechanism, is proportional to the ratio between the
non-equilibrium phonon and electron temperatures.
We investigate the influence of the finite spin number and the scattering
rate parameter on the magnetisation relaxation rates. The relation
between the fs demagnetisation rate and the LLG damping, provided by the LLB
theory, is checked basing on the available experimental data. A good agreement
is obtained for Ni, Co and Gd favoring the idea that the same intrinsic
scattering process is acting on the femtosecond and nanosecond timescale.Comment: 9 pages, 7 figure
- …