Evolution of the magnetic and structural properties ofFe1−xCoxV2O4

Abstract

The magnetic and structural properties of single crystal Fe1−xCoxV2O4 samples have been investigated by performing specific heat, susceptibility, neutron diffraction, and X-ray diffraction measurements. As the orbital-active Fe 2+ ions with larger ionic size are gradually substituted by the orbital-inactive Co 2+ ions with smaller ionic size, the system approaches the itinerant electron limit with decreasing V-V distance. Then, various factors such as the Jahn-Teller distortion and the spin-orbital coupling of the Fe 2+ ions on the A sites and the orbital ordering and electronic itinerancy of the V 3+ ions on the B sites compete with each other to produce a complex magnetic and structural phase diagram. This phase diagram is compared to those of Fe1−xMnxV2O4 and Mn1−xCoxV2O4 to emphasize several distinct features.

Suppression of phase transitions at low temperature by chromium substitution in vanadium spinel Fe(V1−xCrx)2O4

“…The temperature T 1 has little effect on the amount of the substitution of V 3+ ions by Cr 3+ ions. This behaviour is significantly different from that observed in samples doped by Mn 2+ and / or Co 2+ ions, as the transition disappears for Fe 1−x Mn x V 2 O 4 [20,21] and Fe 1−x Co x V 2 O 4 [22] systems. The reason for this behaviour is that the phase transition at T 1 originates from the cooperative Jahn-Teller effect of the Fe 2+ ions located at the A-site.…”

“…Firstly, the trivalent Cr 3+ ions at the A-site is inconsistent with the results of the bond valence sum. In addition, it is surmised that the structural transition temperature from the cubic to the HTT phase significantly decreases with increasing orbital-inactive Cr 3+ ions because the Jahn-Teller effect of the Fe 2+ ions at the A-site is diluted by the orbital-inactive ions [20][21][22][23].…”

“…, have been extensively investigated [20][21][22][23][24][25][26]. However, although orbital degrees of freedom exist in the V 3+ ions, little effort has been focused on the study of the substitution effect for the B-sites.…”

Orbital competition of Mn3+ and V3+ ions in Mn1+x V2-x O4

“…As Mn1-xCoxV2O4 [20] and Fe1-xCoxV2O4 [43], it is also needed to be pointed out the decoupling of lattice and magnetic transitions in Mn1+xV2-xO4 due to the induced competition between the orbital ordering and electronic itinerancy.…”

“…Moreover, a Mn 3+ ions noncollinear magnetic ordering phase was observed at about 15 K for x 0.5. As Mn 1-x Co x V 2 O 4 [20] and Fe 1-x Co x V 2 O 4 [43], the decoupling of lattice and magnetic transitions in Mn 1+x V 2-x O 4 can be explained by the induced competition between the orbital ordering and electronic itinerancy.…”