Irreversible field-induced magnetic phase transitions and properties of Ho3CO

The results of magnetic susceptibility, magnetization, electrical resistivity and specific heat measurements performed on Ho3CO single crystals show that this compound exhibits two different antiferromagnetic structures: AF(II) at 8 K < T < 22 K and AF(I) below T-t approximate to 8K. Below the Neel temperature. T-N = 22 K the application of a magnetic field along the main crystallographic directions induces magnetic phase transitions which are accompanied by giant magnetoresistance. At T < Tt the field-induced phase transitions along the c- and b-axes are found to be irreversible, and a small ferromagnetic component is observed along the a-axis. These peculiarities are associated with the non-Kramers character of the Ho ion and with the presence of a complex incommensurate magnetic structure of Ho3Co below TN. The temperature coefficient of the electrical resistivity for Ho3Co above TN over a wide temperature range is found to differ from that observed for other R3CO compounds. Such a behaviour is attributed to the presence of an additional contribution to the conduction electron scattering by spin fluctuations induced by f-d exchange in the itinerant d-electron subsystem. The value of this extra contribution and its temperature range is suggested to depend on the spin value of the R ion. The excess of the effective magnetic moment per R ion, which is observed in Ho3CO and in other R3M type compounds, is also attributed to spin fluctuations induced by f-d exchange.