STRUCTURAL "TUNING" OF MAGNETISM IN A_1-xA'_xMnO₃ (A = La, Pr; A' = Ca, Sr, Ba). P.G. Radaelli^a, D.E. Cox^b, M. Marezio^c,d, S-W. Cheong^d, P. E. Schiffer^d, A.P. Ramirez^d, D.N. Argyriou^e and J.D. Jorgensen^e, ^aInstitut Max Von Laue-Paul Langevin, BP156, 38042 Grenoble Cedex FRANCE, ^bPhysics Dept., Brookhaven National Lab, Upton, NY, ^cLab de Cristallographie, CNRS/UJF, BP166, 38042 Grenoble Cedex FRANCE, ^dAT&T Bell Labs, Murray Hill, NJ, ^eMaterials Science Div., Argonne National Lab, Argonne, IL 60439

Manganese oxide perovskites with general formula A_1-xA'_xMnO₃ (A = La, Pr, Y..; A' = Ca, Sr, Ba..) have been the subject of renewed interest, due to the giant magnetoresistance (GMR) exhibited near the ferromagnetic (FM) spin ordering temperature T_C. In fact, for values of the electronic doping x~0.30, the high-temperature paramagnetic state is electrically insulating, whereas the low-temperature FM state is metallic. The Curie temperature can be raised upon application of an external magnetic field, thereby producing the GMR effect. At higher doping levels (x > 0.50) the system is antiferromagnetic at low temperatures, and, in a narrow region of composition around x=0.50, both types of magnetic order occur. The presence of structural anomalies associated with the magnetic transitions in A_1-xA'_xMnO₃ (x=0.25, x=0.30 and x=0.50) has been evidenced by high-resolution synchrotron x-ray and neutron powder diffraction. In all cases, the lattice parameter anomalies are associated with a significant rearrangement of the Mn-O bond lengths, so that the MnO₆ octahedra are Jahn-Teller-distorted in the insulating state, and almost undistorted in the metallic state. These results provide strong experimental evidence for the importance of static/dynamic Jahn-Teller distortions as a charge carrier localization mechanism. Very recently, it has been shown that, at a constant value of the electronic doping level x, the Curie temperature can be "tuned" by changing the average radius <r_A> of the A-site ion or by applying external pressure (dT_C/dP is always positive, although its value changes significantly as a function of <r_A>). To study this effect, the structural phase diagram of the A_0.7A'_0.3MnO₃ system (A = La, Pr; A' = Ca, Sr, Ba) was determined by neutron powder diffraction as a function of temperature, pressure and <r_A>. In addition to confirming that the overall increase of T_C as a function of <r_A> is associated with a reduction of the structural distortion from the cubic symmetry, this study has evidenced the extreme sensitivity of T_C to the average Mn-O distance. In fact, as a function of <r_A>, <Mn-O> has a minimum that coincides with the maximum of the Curie temperature. Furthermore, the structure responds to external pressure by a compression of the Mn-O bond lengths, while the Mn-O-Mn bond angles are only slightly pressure-dependent.