Browsing by Author "Goodenough, J.B."

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Magnetic properties of R2Mn2O7 pyrochlore rare-earth solid solutions
(2010) Imamura, N.; Karppinen, M.; Yamauchi, H.; Goodenough, J.B.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Three (R,R′)2Mn2O7 ferromagnetic pyrochlore systems were studied to investigate the role of the R3+ ionic size versus 4f moment on the magnetic properties of the Mn2O7 sublattice. The Curie temperature TC=18±1 K for R=Y and Lu remained nearly constant for (Y1−xLux)2Mn2O7 but the magnetization data show characteristics of spin-glass behavior in low magnetic fields, and at 5 T, the magnetization fails to reach the expected 3 μB/Mn4+ found by 0.5 T in Tl2Mn2O7 and In2Mn2O7. A frustrated, minor antiferromagnetic component apparently competes with the ferromagnetic component of the Mn4+-O-Mn4+ interactions to give a minor antiferromagnetic component to the major ferromagnetic spin alignment of the Mn4+ ions. A TC=42±1 K for R=Dy and Yb remains nearly constant in the (Dy1−yYby)2Mn2O7 system and TC increases systematically from 19 to 42 K with Dy concentration in (Dy1−zLuz)2Mn2O7, which clearly shows that a ferromagnetic interaction between the R3+-ion and Mn4+-ion spins favors ferromagnetic alignment of the Mn4+-ion spins to double the magnitude of TC by relieving the frustration of the Mn2O7 sublattice. The R3+-ion size has little effect as the Mn-O-Mn bond angle changes by no more than ∼1° with changing ionic radius of the R3+ ion.
Structural transformation and magnetic competition in Yb(Mn1−xFex)O3
(2007) Huang, Y.H.; Karppinen, Maarit; Imamura, N.; Yamauchi, H.; Goodenough, J.B.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Structural and magnetic properties of the Yb(Mn1−xFex)O3 (0⩽x⩽1) system have been systematically investigated. Initial samples were prepared via a sol-gel method. A pure hexagonal phase was only obtained for samples with x⩽0.5. With high-pressure annealing, a pure orthorhombic perovskite phase was achieved for all the compositions. The 57Fe Mössbauer spectrum for x=0.5 shows that only Fe3+ ions exist in the system; there was no evidence of chemical inhomogeneities. With increasing x, the Néel temperature TN increases for both hexagonal and orthorhombic phases. The orthorhombic Yb(Mn0.5Fe0.5)O3 shows an interesting weak ferromagnetic state in the temperature range of 239–298K, the ferromagnetism disappearing abruptly on cooling below Tt=239K. The transition at Tt appears to be a reorientation of the spin axis of a type-G antiferromagnetic order from the orthorhombic a axis to the b axis in the (010) plane.