Spin-glass to ferromagnet transition in LaMn1-xScxO3

Dilution of LaMn1-xMxO3 with a main-group trivalent M=Sc or Al atom is compared to the case M=Ga in order to confirm the generality of a vibronic ferromagnetic Mn(III)-O-Mn(III) superexchange interaction and stabilization of orbital disorder by a magnetizing field. A spin-glass to ferromagnet transition in a modest applied magnetic field has been found in the single-valent perovskite system LaMn1-xScxO3 as in the analogous system LaMn1-xGaxO3; the coexistence of a ferromagnetic phase and an antiferromagnetic phase at low temperatures extends to compositions where the orthorhombic room-temperature axial ratio is c/root2 > a. The Sc(III) ion is more resistant to a cooperative, static ordering of the Mn(III) e-orbitals than is Ga(III); the Al(III) ion is less resistant than Ga(III). The data for these insulators confirm the existence of a ferromagnetic vibronic-superexchange interaction between octahedral-site, high-spin Mn(III) with fluctuating e-orbitals and stabilization by an applied magnetic field of the orbitally disordered, ferromagnetic phase relative to the orbitally ordered anti ferromagnetic phase. A similar spin-glass to ferromagnet transition in the mixed-valent La1-xSrx MnO3 system at compositions exhibiting the colossal magnetoresistance {CMR) phenomenon supports the coexistence of ferromagnetic vibronic-superexchange and double-exchange interactions in the ferromagnetic phase and correlation bags rather than conventional magnetic polarons in the paramagnetic phase of the CMR manganites. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.