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  Reehuis, M.; Tovar, M.; Klemke, B.; Többens, D.M.; Hoser, A.; Hoffmann, J.-U.: Coexisting antiferromagnetic phases on the frustrated pyrochlore sublattice of the mixed Jahn-Teller system Ni_1-xCu_xCr₂O₄. Physical Review Materials 8 (2024), p. 054414/1-12

10.1103/PhysRevMaterials.8.054414
Open Access Version (available 01.01.2025)

Abstract:
The magnetic order and phase transitions in the normal spinel system Ni1-xCuxCr2O4 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 has almost completely vanished. Most remarkable is the antiferromagnetic Cr ordering on the orthorhombic distorted pyrochlore lattice for samples in the x(Cu) range from 0 to 0.12 due to the large variety of coexisting magnetic phases. In the magnetic ground state of NiCr2O4 two commensurate antiferromagnetic structures with the propagation vectors kAF = (0,0,1) and (½,½,½) coexist. With increasing Cu content from x(Cu) = 0 to 0.09 these phases undergo a transition to another commensurate structure with k = 0 via two coexisting incommensurate magnetic phases with the vectors kIC1 = (0,0,kz) and kIC2 = (0,ky,kz). 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. Further, magnetoelasticity studies on selected samples indicate that the magnetically induced lattice strains follow the symmetry of the underlying Jahn-Teller distorted lattices.