Magnetic properties of Sr 1- x Ca x RuO 3 have been investigated at temperatures from 4.2 up to 300 K and in magnetic fields up to 44 Tesla. The results of high-field magnetization process are consistent with the interpretation that this system ranges from an intermediate itinerant ferromagnetic metal to a strongly exchange-enhanced Pauli paramagnet (a nearly ferromagnetic metal) with increase of x . Although all the samples have indicated very large high-field magnetic susceptibility, no symptoms of the occurrence of itinerant-electron metamagnetism, which is seen in some nearly ferromagnetic metals like YCo 2 , have been observed.

High-field magnetization of Sr1-xCaxRuO3

We propose a phenomenological picture for the mechanism of the metamagnetic transition often observed in intermetallic alloy compounds near the paramagnetic side of the ferromagnetic instabilities such as YCo2 with Al doping. The present treatment assumes the magnetization process is described in terms of the spin fluctuation spectrum, in clear contrast with the previous approaches based on the Hartree-Fock approximation assuming the fine structure of the density of states curve around the Fermi energy.

https://iopscience.iop.org/article/10.1088/0953-8984/7/31/011/pdf

Theory of itinerant electron metamagnetism

All salient features of paramagnetic MnSi under high pressures, i.e., a broad gigantic susceptibility maximum, a pronounced steep peak of the field-dependent differential susceptibility, and the nonquadratic temperature dependence of the electrical resistivity, can be explained on the basis of the Fermi liquid model in terms of the logarithmic dependence of the free energy and the density of states function with respect to temperature, magnetization and quasiparticle energy.

Fermi Liquid Description for Paramagnetic MnSi under High Pressures

Isothermal magnetizations of itinerant metamagnet Y(Co 1- x Al x ) 2 , x =0.08∼0.11, were measured in static fields up to 23 T at various temperatures 3.6∼30 K. Temperature variation of the magnetization M is found to change dramatically around the metamagnetic field H c , from ∂ M /∂ T >0 ( H H c , gradual decrease of γ continues to higher fields. These results suggest that the spin fluctuation behavior of this system is strongly affected by the metamagnetic transition.

Finite Temperature Study of the Itinerant Metamagnetic Transition in Y(Co1-xAlx)2

We perform a microscopic study of itinerant ferromagnetic systems. We reveal a very rich phase diagram in the three-dimensional space spanned by the chemical potential, a magnetic field, and temperature beyond the Landau theory analyzed so far. Besides a generic wing structure near a tricritical point upon introducing the magnetic field, we find that an additional wing can be generated close to a quantum critical end point (QCEP) and also even from deeply inside the ferromagnetic phase. A tilting of the wing controls the entropy jump associated with the metamagnetic transition. Ferromagnetic and metamagnetic transitions are usually accompanied by a Lifshitz transition at low temperatures, i.e. a change of Fermi surface topology including the disappearance of the Fermi surface. In particular, the Fermi surface of either spin band vanishes at the QCEP. These rich phase diagrams are understood in terms of the density of states and the breaking of particle-hole symmetry in the presence of a next nearest-neighbor-hopping integral tʹ, which is expected in actual materials. The obtained phase diagrams are discussed in a possible connection to itinerant ferromagnetic systems such as UGe2, UCoAl, ZrZn2, and others including materials exhibiting the magnetocaloric effect.

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Ferromagnetic and metamagnetic transitions in itinerant electron systems: a microscopic study

On the basis of the Fermi-liquid model the magnetic susceptibility for paramagnetic or ferromagnetic metals carrying itinerant electrons is evaluated as a function of temperature. It is shown that for the paramagnetic regime, at low temperatures, the susceptibility definitely shows a maximum which is responsible for universal phenomena observed in 4d and 5d metals, Laves phase compounds and heavy-fermion compounds. The present model explains the concurrence of the susceptibility maximum and Curie-Weiss susceptibility in nearly ferromagnetic metals, the appearance of thermally induced ferromagnetism and the precise Curie-Weiss law for relatively weak ferromagnets. The Fermi-liquid effect produces logarithmic terms in the free energy with respect to temperature, magnetization M etc. Because of the M 4 In M term of the energy, some Laves phase compounds are shown to exhibit a metamagnetic transition. This term is also the origin of peculiar nonlinear magnetization curves observed for CeRu2Si2.

S. Misawa

Papers

Fermi-liquid description for itinerant-electron magnetism