Stability of Magnetic States in Patterned Materials
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Stability of Magnetic States in Patterned Materials Martha Pardavi-Horvath The George Washington University, Department of Electrical and Computer Engineering, Washington, D.C. 20052, U.S.A.
ABSTRACT The stability of the individual elements of a two-dimensional (2D) regular array of single domain particles is investigated. The variance in the statistical distribution of up- and down switching fields of the elements leads to premature switching for the low coercivity particles, and incomplete overwriting for the high end of the distribution. The distribution of the interaction fields from surrounding elements on a 2D array depends the saturation magnetization of the elements, their packing density, and the recorded information. The non-ellipsoidal shape of the elements leads to reduced switching fields as a result of non-collinear magnetization around the corners and edges. The thermal stability of 2D arrays, switching by (incoherent) rotation of the magnetization, is enhanced compared to bulk/contiguous media, due to the lack of low energy barrier domain wall motion processes. However due to the fast decrease of the anisotropy, stability at elevated temperatures is still a problem. Experimental data for a model 2D square array of single crystalline, strongly uniaxial, single domain garnet particles illustrate the effects on stability of statistics, shape, and thermal excitation.
INTRODUCTION The interest in regular two-dimensional (2D) arrays of small magnetic particles is motivated by their potential as the next generation of high density magnetic recording media. The stability of the magnetic states of a 2D patterned magnetic system is an important practical problem with interesting fundamental aspects. The stability of the magnetic state of the individual elements and the system, as a whole, is determined by material parameters, the statistical distribution of the se parameters, shape and size of the elements, geometry of the array, and by the reversal mode. The shape of the elements depends on the technology of preparation, it can be elliptical [1] , or flat rectangular [2], or even conical [3]. The elements (bits in recording) should have two stable magnetic states, separated by an energy barrier, high enough to prevent erroneous switching, but low enough to make (over)writing possible. The easy direction can be either inplane, or normal to the plane of the array. Perpendicular recording mode is preferred because of higher packing density. The elements of a 2D array are single domain particles. Ideal single domain particles would switch by coherent rotation, similar to the Stoner–Wohlfarth mode [4]. However, although the elements on the 2D arrays do not have any domain walls, the magnetization state is not uniform due to the presence of corners and edges. As a result, the switching mode is of incoherent rotation. Any interaction between the elements will influence the switching field. The elements are separated by nonmagnetic areas, thus the interaction between them is purely magnetostatic, and it depends on
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