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.
Butterflylike specific heat, magnetocaloric effect, and itinerant metamagnetism in (Er,Y)Co2 compounds
The field-induced first-order phase transition in (Er{sub 1-x}Y{sub x})Co{sub 2} with the yttrium concentration x = 0.45 is observed to be accompanied by a butterflylike behavior and significant irreversibility of the specific heat. The coefficient {gamma} of the T-linear specific heat decreases by {approx}48% under application and removal of a magnetic field up to 20 kOe. This behavior is attributed to the itinerant electron metamagnetism of Co 3d electrons. The isothermal magnetic entropy change {Delta}S{sub m} in Er{sub 0.55}Y{sub 0.45}Co{sub 2} includes a large contribution associated with spin fluctuations induced by the f-d exchange interaction in the hybridized 3d-5d-electron subsystem. These spin fluctuations are suggested to contribute substantially to the magnetocaloric effect of the RCo{sub 2} type compounds. The maximal {Delta}S{sub m} value observed for ErCo{sub 2} just above the Curie temperature is ascribed to the closeness of the T{sub c} value to the spin-fluctuation temperature T{sub sf} of itinerant Co 3d electrons. The nonmonotonous change in {Delta}S{sub m} with the Curie temperature of R{sub 1-x}R{sub x}{prime}Co{sub 2} compounds is explained by the temperature variation in the spin-fluctuation contribution to the magnetocaloric effect.
Thermodynamic and hydrogen-induced structural properties of Ho1−xMmxCo2-hydrides
Thermodynamic and hydrogen-induced structural properties of hydrogenated Ho1−xMmxCo2 (x = 0, 0.1, 0.2, 0.3 and 0.4, Mm = mischmetal) alloys have been investigated. The hydrogen absorption pressure-composition isotherms were studied in the temperature range 50–200 °C and pressure range 0.001–1 bar using a Sieverts-type apparatus. The effect of Mm on the equilibrium plateau pressure and hydride stability has been discussed. Structural analyses and hydrogen-induced amorphization of Ho1−xMmxCo2hydrides were performed using powder x-ray diffraction. The structural stability of these alloys upon repeated hydrogenation and dehydrogenation cycles has been investigated. At lower hydriding pressure and temperatures, Ho1−xMmxCo2-hydrides retain their Laves C15 structure; however, the large lattice volume expansion about 24% is observed in the β-phase. The effect of hydrogenation pressure and temperature and Mm concentration on the hydrogen-induced transformation from crystalline Ho1−xMmxCo2-H to amorphous Ho1−xMmxCo2-H and recrystallization into elemental hydrides have been studied and discussed.