ABO2 type materials, which is one of the “triangular lattice antiferromagnet” (TLA) models with delafossite crystal structure, provide a good example of TLA and present an opportunity to study the geometrical frustration. One of the ABO2 TLA model materials, CuFeO2, is of great interest because of the appearance of “dual character” with both quasi-Ising magnetic ground state and Heisenberg magnetic field-induced states, in spite of the orbital singlet Fe3+ (S = 5/2 , L = 0). Although there were previously a great deal of works for CuFeO2, the origin of the unusual spin behaviors has not been clarified.
We conceived, as a way to study the unusual spin behaviors, that disturbing the delicate balance of the competing exchange interactions by substituting nonmagnetic Al3+ for magnetic Fe3+ should retrieve the original Heisenberg spin character of the orbital singlet Fe3+. In this study, therefore, in order to open the way to study the unusual spin behaviors in CuFeO2, we have performed the neutron scattering, magnetization, specific heat, and synchrotron radiation x-ray diffraction (SR-XRD) measurements using slightly diluted CuFe1−xAlxO2 (0≤ x ≤ 0.05) single crystals. As the results, we found dramatic changes of quasi-Ising to Heisenberg character in magnetic orderings and excitations by a small amount of nonmagnetic impurity. In addition, we have also revealed a “scalene triangle model” crystal lattice distortion for the 4-sublattice ground state of CuFeO2.
In this presentation, I will separately talk about “nonmagnetic impurity effect on magnetic orderings”, “spontaneous crystal distortion” and “magnetic excitation spectrum” in CuFe1−xA l x O2
