Spin reversal and ferrimagnetism in (Gd,Ca)MnO3

Abstract

The gadolinium-based manganite GdMnO₃, partially substituted by calcium at the cationic site, has been investigated through its crystallochemical and magnetic properties. The Gd_1−xCa_xMnO₃ solid solution crystallises in an orthorhombic perovskite-type structure (S.G. Pbnm) in the range 0 ≤ x ≤ 0.45. With increasing x, the structure evolves towards a quasi-tetragonal symmetry, with c/√2 ≈ a < b, while the orthorhombicity factor b/a decreases. The magnetisation in the ordered state was studied as a function of temperature and applied field. ZFC + FC cycles show that the solid solution can be described as a ferrimagnetic-like system, in which negatively-polarised gadolinium moments behave as free spins under the internal field of the ordered Mn sublattice. As a result, the spontaneous magnetisation changes sign upon cooling (FC process), reaching similar values as those obtained upon reversal of the magnetic field. Results are explained in terms of two interacting magnetic sublattices: a Mn-based ferromagnetic one and a negatively-aligned gadolinium network. The local field at a given site depends on the exchange interaction between these two sublattices, leading to a spin reversal when the magnetic moment of the gadolinium sublattice is larger than the ferromagnetic network. This interpretation is confirmed through the thermal evolution of the remanent magnetisation M_rem(T) after freezing the spins using a high magnetic field.