
%0 Journal Article
%A Goodenough, J. B. 
%T Colossal Magnetoresistance in Ln<emph tye="8">1-</emph><emph tye="8"><emph type="2">x</emph></emph> Ax MnO<emph tye="8">3</emph> Perovskites
%J Australian Journal of Physics
%D 1999/04/28
%V 52
%N 2
%P 155-186
%K 
%R https://doi.org/10.1071/P98070
%U https://www.publish.csiro.au/paper/P98070 
%X In the Ln1-xAxMnO3 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&#8242;-orthorhombic/O-orthorhombic phase boundary. For a fixed ratio Mn(IV)/Mn = 0&#183;3, the transition through the O phase from localised-electron behaviour and orbital ordering in the O&#8242; phase to itinerant-electron behaviour in an R-rhombohedral phase occurs with increasing geometric tolerance factor t &#8801; &lang;A-O&rang;/&radic;2&lang;Mn-O&rang;, where &lang;A-O&rang; and &lang;Mn-O&rang; are mean equilibrium bond lengths. The CMR occurs in the temperature interval Tc &#x2264; T &#60; Ts 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 Tc, to an applied magnetic field and, below Tc, 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 Tco &#x2264; Tc is found in compositions with x &#x2248; &frac18; or x &#x2248; &frac12;. With x &#x2248; &frac12;, 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 &#x2248; 0&#183;3, there is no static charge and orbital ordering; but for smaller t, strong electron-lattice coupling gives a &#8216;bad metal&#8217; behaviour below Tc indicative of a dynamic phase segregation as in a traveling charge-density wave. In La1-xCaxMnO3 with &frac12; &#x2264; x &#x2264; &frac78;, segregation of the CE x = 0&#183;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. 
