Spin-glass to ferromagnet transition in LaMn1−xScxO3

Abstract

Abstract 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/ 2 >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 antiferromagnetic phase. A similar spin-glass to ferromagnet transition in the mixed-valent La1−xSrxMnO3 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.