Magnetic structure of GdCu through the martensitic structural transformation: A neutron-diffraction study

We report investigations on the magnetic structure through the martensitic structural transformation in the GdCu system obtained by means of neutron-diffraction experiments. At room temperature, the as-cast bulk samples adopt a CsCl-type crystallographic structure, but when the temperature is lowered a martensitic structural transformation CsCl-->FeB takes place at around 250 K propagating down to 110 K. After a thermal cycle through the forward and the reverse transformation, at room temperature the percentage of both phases is found to be similar to 25% for the CsCl-type structure and similar to 75% for the FeB-type one. In contrast, in powdered samples the CsCl-type phase is stable at any temperature. A comparative neutron thermodiffractometric study in both types of samples allows us to separate and investigate the magnetic behavior of these phases. The magnetic structure of the CsCl-type phase below T-N(CsCl) = 150 K is most consistent with a simple antiferromagnetic one with a propagation vector Q(CsCl)=(1/2, 1/2, 0), the magnetic moments lying along the c direction. However, for the FeB-type structure below T-N(FeB)=45 K, the situation is more complex: a helimagnetic structure with a propagation vector Q(FeB)=(0, 1/4, 1/4) is proposed. Furthermore, it is concluded that while RCu cubic magnetic structures could be understood within a simple isotropic foe-electron Ruderman-Kittel-Kasuya-Yosida model, an exchange anisotropy is needed in the orthorhombic GdNi1-xCux compounds to account for the evolution of the magnetic structures. Finally, an insight into the mechanism of the martensitic transformation is also discussed. [S0163-1829(99)08401-5].