Inter-network magnetic interactions in GdMexMn1-xO3 perovskites (Me = transition metal)

The gadolinium-based manganite GdMnO3 of perovskite structure has been partially substituted at the manganese site by transition metal elements Me like Cu, Ni and Co, leading to a general formula GdMexMn1-xO3 in which different magnetic entities (e.g., Gd3+, Cu2+, Ni2+, Co2+, Co3+, Mn3+, Mn4+) can coexist, depending on charge equilibrium conditions. For divalent cations such as Cu2+ and Ni2+, the solid solution extends from x(Me) = 0-0.5, with O-type orthorhombic symmetry (a < c/root 2 < b). When the substituting element is cobalt, the solid solution extends over the whole range [0 < x < 1], changing from O'-type symmetry (c/root 2 < a < b) to O-type for x > 0.5. In this latter case, the synthesis is performed under oxygen flow, which allows the cobalt ion to take a 3 + oxidation state. Magnetic properties were studied through susceptibility and magnetization measurements. A paramagnetic-ferromagnetic transition occurs at Tc, due to double-exchange interactions between transition metal ions (Mn3+-Mn4+, Ni2+-Mn4+, Co2+-Mn4+), leading to an optimum value at x(Me) = 0.50 (Tc = 145 and 120 K, for GdNi0.5Mn0.5O3 and GdCo0.5Mn0.5O3, respectively). Different situations were identified, among them, a spin reversal in GdNi0.3Mn0.7O3, strong ferromagnetic interactions in GdNi0.5Mn0.5O3, large coercive fields in GdCo0.5Mn0.5O3 or Co3+-Mn4+ antiferromagnetic interactions in GdCo0.9Mn0.1O3. Most of these situations are explained by a phenomenological model of two magnetic sublattices: a transition-metal | Me + Mn| network which orders ferromagnetically at Tc and a gadolinium sublattice, composed of independent Gd3+ ions. These networks are antiferromagnetically coupled through a negative exchange interaction. The local field created by the ferromagnetic |Me + Mn| lattice at the gadolinium site polarizes the Gd moment in a direction opposite to the applied field. When the magnetization of paramagnetic gadolinium, which varies as T-1, gets larger than the ferromagnetic magnetization of the transition metal, which is 'frozen' at T < Tc, then the total magnetic moment changes its sign, leading to an overall ferrimagnetic state.