U.W. Bangor - School of Informatics - Mathematics Preprints 2000
Computational Applied Mathematics
00.22 : RIDLEY, P.H.W.
Finite Element Simulation of the Micromagnetic Behaviour of Nanoelements
Over recent years the investigation into the magnetic behaviour of nanostructured permalloy has become more advanced due to improvements in numerical micromagnetic methods on the theoretical side and high accuracy electron-beam lithography methods experimentally. The interest in such structures of magnetic material is increasing mainly due to the possible potential use in future high-density magnetic storage media applications.
When the material is discretised into a nanoelement structure at the sub micron level theoretical micromagnetic techniques may be employed in order to investigate the magnetization behaviour. This thesis describes a theoretical study of the hysteresis and domain behaviour in thin film permalloy nanoelements. To carry out our investigations we have developed a dynamical micromagnetic model based on the use of the finite element method. The results presented in this thesis begin with a test of the performance of our model. We then proceed with an investigation into the effect of size, elongation and geometry on the transition states for single nanoelements. The investigation is then extended to look at the magnetization behaviour of arrays of interacting nanoelements in relation to their separation and material properties.
The reversal mechanism of the arrays is very sensitive to the degree of disorder. In the case of an aligned uniaxial anisotropy a highly symmetric cooperative switching mechanism is observed. A large anisotropy has the effect of stabilizing states during the reversal process leading to distinctive switching. A random anisotropy breaks this high symmetry sufficiently to reduce the cooperative switching leading to a relatively random reversal of individual elements. The theoretical predictions are compared with experimental observations.
U W Bangor PhD thesis (March 2000)
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00.23 : RIDLEY, P.H.W., ROBERTS, G.W. & CHANTRELL, R.W.
Investigation of magnetisation behaviour in nanoelements using the finite element method
A model of thin film permalloy using an efficient finite element variational approach to the magnetostatic field calculation is described. The material is discretised into a nanoelement structure at the micron level which enables us to investigate material properties due to patterning. Predicted domain structures agree well with experimental data. Interactions between elements are significant it will be shown that the domain structure in the central element differs from that of its neighbours. As expected the addition of pointed ends stabilises the single domain state. Elements with two pointed ends exhibit pseudo single domain behavior. We have studied the single domain / pseudo single domain transition for permalloy platelets, results for non interacting platelets are given as a function of the elongation. Interactions are shown to increase or decrease the critical size depending on the geometry.
J. Appl. Phys. 87(9):5523-5525 (2000)
00.24 : RIDLEY P.H.W., KIRK K.J., ROBERTS, G.W., CHANTRELL, R.W. & CHAPMAN, J.N.
Computational and experimental micromagnetics of arrays of 2-D platelets
2-D regular nanoelements are of interest as micromagnetic model systems and in a number of sensor applications. In this paper we concentrate on a recent development in the form of experimental structures of arrays of small nanoelements which are 300nm long and between 50-80nm wide in small arrays which are amenable to computational studies. A direct comparison of theoretical and experimental hysteresis loops gives good quantitative agreement and suggests that both interactions and variations in intrinsic properties contribute significantly to the width of the loops. The experimental samples were produced by electron beam lithography and consisted of either a 6 X 3 array or a 6 element row. The intra-row spacing was 50nm or 80nm and the inter-row spacing was 100nm. Magnetic images were obtained by Lorentz microscopy, from which the magnetization curves were determined. Computational studies were carried out using a finite element method with magnetostatic field calculations via the maximisation of the scalar potential. The technique is computationally efficient and allows the calculation of the properties of interacting elements.
Finite element method; Interactions; Nanoelements; Magnetic-structures; Elements.
IEEE Trans. Magn. 36(5):3161-3163 (2000)