LiHoF4 : Cuboidal demagnetizing factor in an Ising ferromagnet

Abstract

The demagnetizing factor can have an important effect on physical properties, yet its role in determining the behavior of nonellipsoidal samples remains to be fully explored. We present a detailed study of the role of spin symmetry in determining the demagnetizing factor of cuboids, focusing, as a model example, on the Ising dipolar ferromagnet LiHoF 4. We distinguish two different functions: the demagnetizing factor as a function of intrinsic susceptibility N (χ) and the demagnetizing factor as a function of temperature N (T). For a given nonellipsoidal sample, the function N (χ) depends only on dipolar terms in the spin Hamiltonian, but apart from in the limits χ → 0 and χ → ∞, it is a different function for different spin symmetries. The function N (T) is less universal, depending on exchange terms and other details of the spin Hamiltonian. We apply a recent theory to calculate these functions for spherical and cuboidal samples of LiHoF 4. The theoretical results are compared with N (χ) and N (T) derived from experimental measurements of the magnetic susceptibility of corresponding samples of LiHoF 4 , both above and below its ferromagnetic transition at T c = 1.53 K. Close agreement between theory and experiment is demonstrated, showing that the intrinsic susceptibility of LiHoF 4 and other strongly magnetic systems can be accurately estimated from measurements on cuboidal samples. Our results further show that for cuboids, and implicitly for any sample shape, N (χ) below the ordering transition takes the value N (∞). This confirms and extends the scope of earlier observations that the intrinsic susceptibility of ferromagnets remains divergent below the transition, in contradiction to the implications of broken symmetry. We discuss the topological and microscopic origins of this result.

Field-induced magnetic states in geometrically frustrated SrEr2O4

“…With the demagnetisation effects being significant, ideally one has to correct for them by subtracting the demagnetising field from the applied field. This procedure is straightforward for ellipsoidal samples for which the demagnetising factor depends only on sample shape, but is much more involved for non-ellipsoidal samples [38]. Faced with the necessity to use cuboid samples because of the low-temperature thermalisation requirements we report the uncorrected data.…”