Magnetic behaviour in Dy1 − xMmxCo2 compounds
The magnetic behaviour in Dy1 − xMmxCo2 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5; Mm = mischmetal) compounds is reported using temperature and field dependence of magnetization (M–T and M–H respectively) measurements. A strong composition dependent irreversibility is observed in both the M–T and M–H scans below the magnetic ordering temperature (TC). A clear change of the first-order magnetic transition of DyCo2 to a second-order one in Dy0.5Mm0.5Co2 is evidenced by M–T and a series of Arrott (M2 versus H/M) plots, obtained from the M–H isotherms around TC. The variation in induced moments of the Co sublattice is estimated. It is found that the Mm substitution can only lead to a considerable reduction in the TC, saturation magnetization, and Co moment. The observed behaviour of M–T and M–H plots with increasing Mm content is discussed in detail.
Magnetic structure, magnetostriction, and magnetic transitions of the Laves-phase compound NdCo2
The crystal and the magnetic structures as well as magnetostriction of the Laves-phase compound NdCo2 are investigated by means of temperature-dependent high-resolution neutron-powder diffraction. The compound crystallizes in the cubic Laves phase C15 structure at high temperature, undergoes a tetragonal distortion (space group I4l/amd) around T(C)approximate to 100 K and an orthorhombic distortion (space group Fddd) at T 42 K. The temperature dependence of lattice constants, magnetostriction constant, and magnetic moment indicate that the magnetic and structural transitions are second order in the vicinity of T-C and are first-order around 42 K. Refinements of magnetic structure reveal that the Nd moment distinctly exhibits an abrupt increase at the first-order transition and the easy magnetization direction of the compound changes from [001] in the tetragonal lattice to [011] in the orthorhombic lattice, indicating a strong coupling between crystal structure and magnetic properties at zero field. Analysis of the temperature dependence of bondlength suggests a strong magnetic exchange striction in the tetragonal structure and that the abrupt increase of the Nd moment is attributed essentially to a change in crystal electric field. Field-dependent neutron diffraction reveals a decoupling of the magnetic and structural transitions under relatively modest magnetic fields.
Study of the d-magnetism instability in the system around the critical concentration by means of resistivity measurements
The substitution of Gd for Tm in the pseudobinary system gives rise to the onset of long-range magnetic order in the itinerant Co sublattice above a critical concentration . The features of the Co magnetism around the critical concentration were studied by means of resistivity measurements in magnetic fields up to 10 T and pressures up to 15 kbar. An extremely large drop of the resistivity at the ordering temperature measured for is explained by a suppression of the spin-fluctuation contribution to the resistivity in the vicinity of due to the intersublattice molecular field. The analysis of the magnetoresistance revealed that the spin fluctuations above are additionally enhanced due to short-range correlations within the f - d-electron system in a limited temperature region above . The application of external pressure results in a suppression of both the spin fluctuations and the onset of the long-range order in the Co sublattice. In the Co sublattice remains paramagnetic below the Curie temperature for . The magnetic Gruneisen parameter was found to substantially increase as the critical concentration was approached.