Inter-network magnetic interactions in GdMe xMn 1−xO 3 perovskites (Me=transition metal)

Abstract

The gadolinium-based manganite GdMnO 3 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 GdMe x Mn 1− x O 3, in which different magnetic entities (e.g., Gd 3+, Cu 2+, Ni 2+, Co 2+, Co 3+, Mn 3+, Mn 4+) can coexist, depending on charge equilibrium conditions. For divalent cations such as Cu 2+ and Ni 2+, the solid solution extends from x(Me)=0–0.5, with O-type orthorhombic symmetry ( a < c / √ 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 / √ 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 T c, due to double-exchange interactions between transition metal ions (Mn 3+–Mn 4+, Ni 2+–Mn 4+, Co 2+–Mn 4+), leading to an optimum value at x(Me)=0.50 ( T c=145 and 120 K, for GdNi 0.5Mn 0.5O 3 and GdCo 0.5Mn 0.5O 3, respectively). Different situations were identified, among them, a spin reversal in GdNi 0.3Mn 0.7O 3, strong ferromagnetic interactions in GdNi 0.5Mn 0.5O 3, large coercive fields in GdCo 0.5Mn 0.5O 3 or Co 3+–Mn 4+ antiferromagnetic interactions in GdCo 0.9Mn 0.1O 3. Most of these situations are explained by a phenomenological model of two magnetic sublattices: a transition-metal |Me+Mn| network which orders ferromagnetically at T c and a gadolinium sublattice, composed of independent Gd 3+ 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< T c, then the total magnetic moment changes its sign, leading to an overall ferrimagnetic state.