Colossal Magnetoresistance in Ln1-x Ax MnO3 Perovskites

Abstract

In the Ln_1-xA_xMnO₃ pseudoperovskites in which Ln is a lanthanide and A an alkaline-earth atom, an intrinsic colossal magnetoresistance (CMR) occurs in an O-orthorhombic phase near an O′-orthorhombic/O-orthorhombic phase boundary. For a fixed ratio Mn(IV)/Mn = 0·3, the transition through the O phase from localised-electron behaviour and orbital ordering in the O′ phase to itinerant-electron behaviour in an R-rhombohedral phase occurs with increasing geometric tolerance factor t ≡ ⟨A-O⟩/√2⟨Mn-O⟩, where ⟨A-O⟩ and ⟨Mn-O⟩ are mean equilibrium bond lengths. The CMR occurs in the temperature interval T_c ≤ T < T_s where there is a segregation, via cooperative oxygen displacements, into a Mn(IV)-rich ferromagnetic phase imbedded in a paramagnetic phase. The volume of the ferromagnetic, more conductive clusters increases from below to beyond a percolation threshold in response, above T_c, to an applied magnetic field and, below T_c, to a Weiss molecular field. In the O phase, the magnetic transition at Tc decreases on the exchange of ¹⁸O/¹⁶O and increases under hydrostatic pressure. Charge and orbital ordering below a T_co ≤ T_c is found in compositions with x ≈ ⅛ or x ≈ ½. With x ≈ ½, the charge-ordered phase CE is tetragonal and antiferromagnetic. An applied magnetic field stabilises the ferromagnetic, conductive phase relative to the insulator phase CE to give a second type of intrinsic CMR. For _x ≈ 0·3, there is no static charge and orbital ordering; but for smaller t, strong electron-lattice coupling gives a ‘bad metal’ behaviour below T_c indicative of a dynamic phase segregation as in a traveling charge-density wave. In La_1-xCa_xMnO₃ with ½ ≤ x ≤ ⅞, segregation of the CE x = 0·5 phase and the all-Mn(IV) x = 1 phase has been reported to take the form of a static charge-density wave. The origins of this complex behaviour are discussed.