Coexisting magnetic phases on the frustrated pyrochlore sublattice of the mixed Jahn-Teller system Ni1-xCuxCr2O4

Abstract

The magnetic order and phase transitions in the normal spinel system Ni_1-xCu_xCr₂O₄ are studied by powder neutron and x-ray diffraction, as well as by magnetization measurements to get a complete magnetic phase diagram. For chromites with x(Cu) > 0.5 a canted antiferromagnetic phase of Cr appears first, followed by the onset of ferromagnetism in the Cu sublattice at lower temperature forming a ferrimagnetic lattice. Conversely, with x(Cu) < 0.5 the ferrimagnetic order between the Cr and Cu spins occurs at the higher ordering temperature followed by the onset of antiferromagnetic order in the Cr sublattice. Apart from the crossing of the two boundary lines of the transition temperatures at x(Cu) = 0.50 a compensation point of the ferrimagnetic moments is determined at x(Cu) = 0.60, where the spontaneous magnetization almost completely vanishes. Most remarkable is the antiferromagnetic Cr ordering on the orthorhombically distorted pyrochlore lattice for samples in the x(Cu) range from 0 to 0.12 due to the large variety of coexisting commensurate and incommensurate magnetic phases. The different magnetic phases are discussed qualitatively based on the lattice dimensions depending on the concentration ratio of two Jahn-Teller ions at the tetrahedral A site, where Ni²⁺ causes elongated and Cu²⁺ compressed tetragonal lattice distortions.

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