Magnetisability anisotropy

This method is based on the Villari effect applying a uniaxial stress to a ferromagnetic substance induces a magnetoelastic anisotropy which may modify all the parameters of its magnetisation curve, e.g. magnetic susceptibility, coercive force, and so on. Some experimental techniques to measure the strain-induced anisotropy are discussed shortly below. 

An original technique was developed by Konishi et al. (1969) and extended later on by Narita et al. (1980). This method is known as the small-angle magnetisation rotation (SAMR) method a static bias field H and a tensile stress (o) are applied in the direction of the film a small-amplitude ac driven field H = W max sin(wf) is applied perpendicular to H. It is this ac magnetic field that induces a magnetisation rotation, which can be detected as an induced voltage in a sensor coil wound around the film axis. This response is measured as a function of the applied stress, i.e. of the strain-induced anisotropy. An experimental SAMR set-up is illustrated in fig. 5. The sensitivity of this method was 2 x 10-7 (Narita et al. 1980) and even much higher, namely 10-9 (Hernando et al. 1983). 

For thin films with a perpendicular anisotropy (easy axis normal to the film plane), Huang et al. (199S) determined magnetostriction (As) and anisotropy (Iff) values, by using the so-called initial-susceptibility method. An ac magnetic field was applied in the film direction, and the induced magnetisation component in the field direction was measured. These authors deduced K and As from the x 1 vs. o plots, using ... 

Here, Ms is the saturation magnetisation. Notice, that apparently, the stress transfer from the kapton substrate to the deposited film was ignored (compare eq. (IS) with eq. (1 Id)). A consistent result was obtained by determining the anisotropy from the hysteresis loops measured in the hard and easy magnetisation directions, using a vibrating sample magnetometer. 

The normalised magnetostriction as a function of normalised magnetisation for films with perpendicular and parallel anisotropy is plotted in fig. 27(a, b) for (Tbo.27Dyo.73)o.42Feo.58 (Schatz et al. 1994). The films with in-plane anisotropy appear to show high magnetostriction and magnetisation at low fields (the coercivity is less than 0.01 T), due to the easy rotation of the spins in the isotropic plane. The motion of 180°... 

A simple way to account for this particle volume-dependent narrowing of the SPR spectra, suggested by De Biasi and Devezas [18], consists in averaging the magnetisation and magnetic anisotropy over the thermal fluctuations of the magnetic moment as follows ... 

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