High-Moment-Low-Moment Description of Magnetovolume Effects in Y(Mn x Al 1− x ) 2 and Y x Sc 1− x Mn 2
Based on the assumption of a high-moment-low-moment instability of the Mn atom, we construct a simple spin model with coupled magnetic and spatial degrees of freedom to describe the Laves phase systems Y(Mn x Al 1 m x ) 2 and Y x Sc 1 m x Mn 2 . Monte Carlo simulations of this model qualitatively reproduce anomalies observed in these materials like a discontinuous giant volume change and anomalous thermal expansion behavior.
Characteristic spin fluctuations in Y(Mn1-xAlx)2
The thermal expansion and magnetic susceptibility have been measured for Y(Mn1-xAlx)2 pseudobinary Laves phase intermetallic compounds. A large volume change at TN and a remarkable enhancement of the thermal expansion coefficient above TN were observed in YMn2. The substitution of Al for Mn makes these anomalies less clear. The temperature dependence of the susceptibility, being almost temperature independent for YMn2, becomes of the Curie-Weiss type as the Al content increases. Such behaviour is explained as the characteristic features of spin fluctuations. The amplitude of local spin fluctuations at the Mn site has been estimated on the basis of the phenomenological theory of magnetovolume effects. It has been revealed that the magnetic ground state of Al-substituted compounds is a spin glass.
Recent progress in the theory of itinerant electron magnetism
Abstract A review is given of recent theoretical investigations toward unified understanding of magnetism in narrow-band electron systems. It is emphasized that the classical controversy between the itinerant and localized models have been resolved into a more general and well-defined problem of spin density fluctuations in a general sense. The local moment picture is a limiting form of general spin fluctuations; and in its opposite limit we have weakly ferro- and antiferromagnetic metals. As an approach from the latter limit, the self-consistent renormalization theory of spin fluctuations is shown to have been quite successful in explaining and predicting a number of qualitatively new physical properties of this class of materials. More recent theoretical studies of spin fluctuations from a general point of view interpolating between the above-mentioned two limits seem to lead to a unified picture of magnetism in narrow-band electron systems including 3d transition metals and magnetic compounds.