Colossal magnetoresistance in Ln1-xAxMnO3 perovskites

In the Ln(1-x)A(x)MnO(3) 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 equivalent to [A-O]/root 2[Mn-O], where [A-O] and [Mn-O] are mean equilibrium bond lengths. The CMR occurs in the temperature interval T-c less than or equal to 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,, to an applied magnetic field and, below T-c, to a Weiss molecular field. In the O phase, the magnetic transition at T, decreases on the exchange of O-18/O-16 and increases under hydrostatic pressure. Charge and orbital ordering below a T-co less than or equal to T-c is found in compositions with x approximate to 1/8 or x approximate to 1/2. With x approximate to 1/2, 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 approximate to 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, indicative of. dynamic phase segregation as in a traveling charge-density wave. In La1-xCaxMnO3 with 1/2 less than or equal to x less than or equal to 7/8, 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.