Magnetic properties of PrCo2 and its ternary hydride PrCo2H4

Magnetization and susceptibility data on PrCo₂ and PrCo₂H₄ are presented. The ac susceptibility of PrCo₂ measured in zero dc field displays a sharp and high peak at T_c = (39.9 ± 0.2) K. The magnetization versus temperature curves show ferromagnetic behaviour for B >1 T, but display a maximum at lower values of the applied field. These results, together with the behaviour of the hysteresis loops at different temperatures below T_c, indicate that PrCo₂ orders ferromagnetically, the magnetic hardness increasing strongly for T → 0. The saturation moment at 4.2 K equals 3.9 μ_B per formula unit, as found from the magnetization curve measured in a pulsed-field magnet up to B = 30 T.Similar experiments on PrCo₂H₄ provide evidence that the introduction of hydrogen in PrCo₂ not only destroys the long-range atomic order, but also considerably reduces the ferromagnetic interactions. Such an effect of the hydrogen is commonly observed in cobalt intermetallics. Part of the PrCo₂H₄ is found to have decomposed into PrH₂ and free Co. The clusters of free Co atoms give rise to a maximum in the zero-field ac susceptibility versus temperature curves, similar as observed in spin glasses or magnetic glasses. By increasing the ac frequency, the maximum shifts to higher temperatures. The behavior can be explained in terms of the Néel model for superparamagnetic particles with randomly oriented local anisotropy axes.