We report on structural, magnetic and electrical properties of Sr-doped LaMnO3 single crystals for doping levels 0.4 ≤ x ≤ 0.85. The complex structural and magnetic phase diagram can only be explained assuming significant contributions from the orbital degrees of freedom. Close to x = 0.6 a ferromagnetic metal is followed by an antiferromagnetic metallic phase below 200 K. This antiferromagnetic metallic phase exists in a monoclinic crystallographic structure. Following theoretical predictions this metallic antiferromagnet is expected to reveal an (x 2 -y 2 )-type orbital order. For higher Sr concentrations an antiferromagnetic insulator is established below room temperature.

X-ray diffraction, magnetic susceptibility, electron-spin resonance, and heat-capacity investigations were performed on MAl 2 O 4 compounds with M = Co, Fe, Mn, and Zn. All compounds crystallize in cubic spinel structure AB 2 O 4 with minor inversion between A and B sites. CoAl 2 O 4 and FeAl 2 O 4 reveal spin-glass-like ground states with freezing temperatures well below the Curie-Weiss temperatures indicating strong geometric frustration. Below the freezing temperatures, the heat capacities show a T 2 temperature dependence for T → 0 K as has recently been observed in some related geometrically frustrated magnets, in contrast to the linear dependence expected for common spin glasses. The heat capacity of FeAl 2 O 4 exhibits an additional orbital contribution. With the absence of a cooperative Jahn-Teller distortion, this points toward an orbital freezing. MnAl 2 O 4 orders antiferromagnetically below T N = 40 K with a reduced value of the ordered moment and a large paramagnetic component.

No abstract

No abstract