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

Abstract

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 $\mathrm{CsC}\overrightarrow{l}\mathrm{FeB}$ takes place at around 250 K propagating down to 120 K. After a thermal cycle through the forward and the reverse transformation, at room temperature the percentage of both phases is found to be ∼25% for the CsCl-type structure and ∼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 $T_N^{\mathrm{CsCl}}=150\mathrm{}\mathrm{K}$ is most consistent with a simple antiferromagnetic one with a propagation vector ${\mathbf{Q}}^{\mathbf{CsCl}}=(\frac{1}{2},\frac{1}{2},0),$ the magnetic moments lying along the c direction. However, for the FeB-type structure below $T_N^{\mathrm{FeB}}=45\mathrm{}\mathrm{K},$ the situation is more complex: a helimagnetic structure with a propagation vector ${\mathbf{Q}}^{\mathbf{FeB}}=(0,\frac{1}{4},\frac{1}{4})$ is proposed. Furthermore, it is concluded that while $R\mathrm{Cu}$ cubic magnetic structures could be understood within a simple isotropic free-electron Ruderman-Kittel-Kasuya-Yosida model, an exchange anisotropy is needed in the orthorhombic ${\mathrm{GdNi}}_{1-x}{\mathrm{Cu}}_x$ compounds to account for the evolution of the magnetic structures. Finally, an insight into the mechanism of the martensitic transformation is also discussed.