U.W. Bangor - School of Informatics - Mathematics Preprints 1999
Computational Applied Mathematics
99.08 : RIDLEY, P.H.W., ROBERTS, G.W., WONGSAM, M.A. & CHANTRELL, R.W.
Finite element modelling of nanoelements
The numerical simulation of the magnetisation structure of two-dimensional permalloy nanoelements is described. The nanoelements have a uniform magnetisation structure and consist of bars with flat and/or pointed ends. The Landau-Lifschitz-Gilbert equation of motion is used to described the time evolution of the system and the numerical simulation consists of a finite element spatial discretisation coupled with a time-stepping scheme. The Poisson equation for the demagnetising field scalar potential phi is solved by a standard finite element variational or stationary functional formulation. The jump condition in grad phi at a nanoelement boundary is integrated into the finite element formulation so that phi is obtained globally, both inside and outside the nanoelement, thus allowing interactions between two or more nanoelements to be modelled easily. The work will be compared to previously published experimental and numerical results for single nanoelements.
Micromagnetics; Field calculations; Finite element method; Permalloy
J. Magn. Magn. Mater. 193:423-426 (1999).
Elsevier JMMM (pdf)
99.34 : WONGSAM, M.A., HANNAY, J.D., ROBERTS, G.W. & CHANTRELL, R.W.
Recent studies in thermal activation and spin waves
Some recent results in computational approaches to thermally activated fast reversal in magnetic recording media are reviewed. In particular, recent results reported in the simulation of pulsed-field-induced magnetisation reversal and thermal activation of spin waves are described. The short time scale breakdown of the Arrhenius-Néel law for a single moment is demonstrated and explained in terms of the dynamics of the precessional motion. The variation in response as a function of the damping parameter is found to be an important factor determining the remanent magnetisation for a given pulse width. The effects of interactions between moments are described, including the apparent increase in effective damping. It is shown that interactions between moments can be described in terms of thermally excited spin waves. The spectrum of relaxation times for systems consisting of coupled moments is explained in terms of the thermal excitation of spin waves.
Thermal activation; Spin waves; Magnetic recording media; Magnetisation-reversal.
J. Magn. Magn. Mater. 200:649-663 (1999).
Elsevier JMMM (pdf)
99.35 : RIDLEY, P.H.W., ROBERTS, G.W., WONGSAM, M.A., CHANTRELL, R.W., GORE, J. & MAYLIN, M.
Modelling the evolution of domains in nanoelements using finite elements
Investigation into the theoretical magnetic behaviour of permalloy is described. The material is discretised into a structure of nanoelements, so that we may employ micromagnetic simulations in order to investigate the material behaviour. In this paper we describe the formation and structure of domains in an array of interacting nanoelements with varying space between them. The simulations begin at saturation and end when the nanoelements are in a zero field equilibrium state. The time evolution of the system is described by the Landau-Lifschitz equation of motion and the field calculations are carried out by the use of a finite element spatial discretisation scheme.
IEEE Trans. Mag. 35(5):3874-3876 (1999).
99.36 : CHANTRELL, R.W., HANNAY, J.D., COVERDALE, G.N., ROBERTS, G.W. & LYBERATOS, A.
The physics of the fluctuating field and activation volume
The definition and use of the fluctuation field Ht and activation volume Vac are reviewed. A simple model system of non-interacting Stoner-Wohlfarth particles is used to demonstrate the complex effect of material properties on Ht. Initial computational studies also emonstrate that interparticle interactions play an important role in determinaing Ht. In the case of a system with energy barriers linear in H, Vac can be considered as the volume interacting with the applied field to determine the energy barrier during magnetisation reversal; a definition useful for M-O media. Finally th relationship between Vac and the stability of stored information in conventional recording is discussed.
Magnetic viscosity; Activation volume; Fluctuation field.
J. Magn. Soc. Japan 23(11):2058-2064 (1999).