Domain-wall pinning and nucleation in SmCo5 sintered magnet alloys

Abstract

The magnetization curve and the magnetic viscosity in reverse fields are investigated in sintered SmCo₅ magnet alloys. The initial magnetization curve is shown to be close to that of an ideal reversible magnetic material until a field greater than 2500 Oe is applied. At greater forward fields significant pinning takes place at grain boundaries and coercive forces exceeding 10 kOe are observed. Barkhausen discontinuities corresponding to compete reversal of the magnetization of individual grains are found on applying reverse fields. After magnetization in a forward field the magnetization falls with time in a fixed reverse field according to the relation I = const. – S ₀ In t. Values of S ₀ and the susceptibility χ₀ are measured in a fixed reverse field. The calculation of the derivative of the activation energy with respect to effective field, is discussed. Values of (δE/δH) _T from 4 × 10⁻¹⁶ to 40 × 10⁻¹⁶ erg Oe⁻¹ are determined at temperatures from 77 to 205 K. Viscosity effects at 4.2 K were too small to measure. (δE/δH) _T was shown to be independent of magnetic history in these materials. Theoretical estimates of (δE/δH) _T are found to be in reasonable agreement with extrapolated experimental observations. The activation due to quantum mechanical tunnelling is shown to be less than thermal activation down to 0.02 K so that quantum effects were not observed in these alloys. The range of coercive fields and the related values of (δE/δH) _T are explained in terms of the variation in anisotropy constant across the boundary between the matrix SmCo₅ and the inhomogeneities responsible for 360° wall pinning. The theoretical calculations assume inhomogeneities of Sm₂Co₁₇ although this has not yet been clearly detected by electron microscopy. Another grain boundary material of a similar anisotropy constant lower than that of SmCo₅, could explain the observations.