The large variety of magnetic phenomena observed in the Co based Laves phases are reviewed. Following the band structure calculations it is argued that the outstanding magnetic features of the RCo2 intermetallics are intimately related to the position of the Fermi level, which is near to a local peak in N(e). This is why the Co 3d-electron system reacts sensitively either to the molecular field of the R partner element or to the changes of external parameters such as a magnetic field or pressure. Magnetic, magnetoelastic and transport measurements of RCo2 compounds and related pseudobinaries such as R(Co1-xAlx)2 with R either magnetic or nonmagnetic rare earth element are shown and discussed. The conditions for the appearance of itinerant electron metamagnetism and spin fluctuations are outlined. In particular, the influences of spin fluctuations on physical properties, e.g. the susceptibility, thermal expansion and transport phenomena, are demonstrated.

https://iopscience.iop.org/article/10.1088/0953-8984/13/23/202/pdf

Physical properties of RCo2 Laves phases

Magnetisation and magnetoresistance isotherms were measured for a number of (R,R′)Co2, (R,Y)Co2 and R(Co,Si)2 (R,R′=rare earth) compounds. A metamagnetic transition is observed just above the Curie temperature (TC) of compounds having a first order phase transition, i.e. ErCo2-, HoCo2-, and DyCo2-based ones. Both 4f- and 3d-sublattice magnetic moments contribute to a sharp change of the magnetisation at this transition. The concurring suppression of the magnetoresistance can be considered to be due to quenching of spin fluctuations. In addition, the magnetic entropy change ΔSm is estimated from the magnetisation data by using a Maxwell equation. The resulting giant magnetocaloric effects are discussed in terms of the 4f(R)-localised spin and the 3d(Co)-spin fluctuations as well as the nature of the phase transition.

Metamagnetism, giant magnetoresistance and magnetocaloric effects in RCo2-based compounds in the vicinity of the Curie temperature

The dependence on pressure (P) and field (H) of the magnetic phase diagram of MnSi has been investigated by means of measurements of the AC susceptibility up to 16 kbar and 7 T at temperatures down to 30 mK. For ambient pressure, we report on a peak in above at a characteristic temperature that rises quickly with field. The pressure dependence of is found to be strongly analogous to that of the zero-field transition to long-range order at . Features of in the field versus temperature (T) phase diagram may be viewed as `fingerprint' evidence of a field-induced crossover at of the itinerant magnetism from a non-polarized regime at high T and low H to a polarized regime at low T and high H. The long-wavelength spin spiral, present at low H, appears to be supported only in the polarized regime, so in the immediate vicinity of the critical pressure for which , a small pocket exists as a re-entrant state.

https://iopscience.iop.org/article/10.1088/0953-8984/9/31/019/pdf

Field dependence of the magnetic quantum phase transition in MnSi

The electronic structures and transport properties of a series of actinide monocarbides, mononitrides, and dioxides are studied systematically using a combination of density-functional theory and dynamical mean-field theory. The studied materials present different electronic correlation strength and degree of localization of 5f electrons, where a metal-insulator boundary naturally lies within. In the spectral function of Mott-insulating uranium oxide, a resonance peak is observed in both theory and experiment and may be understood as a generalized Zhang-Rice state. We also investigate the interplay between electron-electron and electron-phonon interactions, both of which are responsible for the transport in the metallic compounds. Our findings allow us to gain insight in the roles played by different scattering mechanisms, and suggest how to improve their thermal conductivities.

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Electronic correlation and transport properties of nuclear fuel materials

Close correlation among lattice, spin, and charge in the manganese-based antiperovskite material

Susceptibility and high field magnetization have been measured for Y(Co1−xMx)2 (M=Al, Fe, and Ni). YCo2 exhibits a metamagnetic transition at 69 T. Ni doping in YCo2 increases the critical field (Bc), while Fe doping decreases it. These changes can be elucidated with the band picture of metamagnetism. Bc of YCo2 exhibits a positive shift proportional to the square of temperature. In the paramagnetic region of Y(Co1−xAlx)2 with x≤0.11, the susceptibility is enhanced with Al and a sharp metamagnetic transition with lower Bc is observed. The susceptibility becomes maximum at a finite temperature Tmax. Bc in the ground state is proportional to Tmax. These experimental results are discussed with a new theory based on a spin fluctuation model. The susceptibility and the metamagnetic transition are found to be very sensitive to pressure.

Itinerant electron metamagnetism and related phenomena in Co‐based intermetallic compounds (invited)

X-ray photoemission and bremsstrahlung isochromat spectra are measured for UC. Valence state spectra are compared with the relativistic APW band calculation. Shape analysis of the U 4f core spectrum is made by Doniach-Sunjic lineshape with broadening. The discrepancies between valence spectra and the APW calculations, and presence of U 4f satellites show a correlation effect between U 5f electrons.

Electronic structure of UC studied by X-ray photoemission and bremsstrahlung isochromat spectroscopy

High field magnetizations up to 100 T, lattice constants and electronic specific heat coefficients have been measured for the Laves phase Lu(Co 1− x M x ) 2 (M = Ga, Sn and Si) compounds which exhibit itinerant electron metamagnetism for small x . The lattice constant increases with increasing x for M = Ga, or Sn, while it decreases linearly in the case of M = Si. The metamagnetic characteristics strongly depend on the substitutional element M. The critical field for the metamagnetic transition decreases with increasing x for M = Ga and Si, whereas it remains constant around 40 T above x = 0.06 for M = Sn. The magnetization jump due to the transition increases for M = Sn and rapidly decreases for M = Si. The different characteristics may result from the difference in the shape of the density of states modified by the substitutional element.

K. Murata

Papers

Itinerant electron metamagnetism and related phenomena in Co‐based intermetallic compounds (invited)

Electronic structure of UC studied by X-ray photoemission and bremsstrahlung isochromat spectroscopy

Metamagnetic transition in LuCo2 and its pseudo-binary compounds