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Browse Prior Art Database

High Uniaxial Anisotropy in Ferrimagnetic Materials

IP.com Disclosure Number: IPCOM000089187D
Original Publication Date: 1977-Sep-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 5 page(s) / 71K

Publishing Venue

IBM

Related People

Cargill, GS: AUTHOR [+2]

Abstract

A mechanism is described for using anisotropic composition inhomogeneities to obtain large uniaxial magnetic anisotropies in ferrimagnetic materials, e.g., anisotropy energies K(u) larger than 2 Pi M/2/(s), where M(s) is the net saturation magnetization.

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High Uniaxial Anisotropy in Ferrimagnetic Materials

A mechanism is described for using anisotropic composition inhomogeneities to obtain large uniaxial magnetic anisotropies in ferrimagnetic materials, e.g., anisotropy energies K(u) larger than 2 Pi M/2/(s), where M(s) is the net saturation magnetization.

Several mechanisms have been proposed to account for magnetic anisotropy observed in ferrimagnetic materials of interest for magnetic bubble applications, e.g., rare earth metal-transition metal based amorphous alloy films and single crystal, cubic garnet films, with Q = K(u)/(2 Pi M/2/(s)) > 1 in a temperature interval containing the materials' compensation temperature. These mechanisms include anisotropic short range ordering (pair ordering or pseudocrystalline anisotropy), stress-induced (magnetoelastic) anisotropy, and internal shape anisotropy.

It is commonly believed that shape anisotropy cannot be the predominant source of the observed anisotropy in such materials since its theoretical upper limit has been thought to be K(u) = Pi M/2/(s)/4, i.e., Q = 1/2, whereas typically observed anisotropies in the ferrimagnetic materials of interest for magnetic bubble applications have anisotropies corresponding to Q > 1 near room temperature. This theoretical limit is actually applicable only to shape effects produced by anisotropic voids in ferrimagnetic or ferromagnetic materials or by anisotropic regions of low (or high) magnetization in ferromagnetic materials.

The mechanism described here involves anisotropically shaped regions of composition inhomogeneity in ferrimagnetic materials; the anisotropically shaped regions are exchange coupled to their surroundings by the dominant sublattice (or subnetwork) magnetic interactions. Consider, for example, a ferrimagnet containing two types of magnetic atoms (or ions) A and B which interact antiferromagnetically, giving rise to two sublattices which are antiferromagnetically coupled with one another, but for which the A atoms' sublattice is ferromagnetically ordered, as is the B atoms' sublattice, e.g., amorphous Gd(20) Co(80). The net magnetization M(s) is then the sum of the two oppositely directed sublattice contributions M(s) = M(A) + M(B), and for certain alloy compositions the temperature dependence of M(s) is like that shown in Fig. 1.

As an example of the proposed mechanism, consider a film of thickness t composed of two types of square columns of compositions <x> +/- Delta x at. %A and of edge length d, such that t>>d and d<< typical domain wall widths in materials of interest for magnetic bubble applications, as illustrated in Fig. 2. Under some conditions, the temperature dependence of magnetization for the two types of columns will be very different. For example, this is illustrated by curves 1 and 2 in Fig. 3, although the temperature dependence of M(s)(T) for the composite film will correspond to curve 3 of Fig. 3, since the magnetizations in regions (<x> + Delta x at. %A)...