Uniaxial-Pressure Effects on Spin-Driven Lattice Distortions in Geometrically Frustrated Magnets CuFe_(1-x)Ga_xO_2 (x = 0, 0.035)

摘要

The triangular lattice antiferromagnet CuFeO_2 (CFO) is a spin-lattice coupled system, whose magnetic phase transitions are accompanied by crystal lattice distortions to relieve the geometrical spin frustration. Recent neutron diffraction and magnetic susceptibility measurements on CFO have revealed that the application of uniaxial pressure(up to 100 MPa) in the triangular lattice plane shifts the magnetic phase transition temperatures to higher values[Nakajima et al.: J. Phys. Soc. Jpn. 81 (2012) 094710]. In the present study, we have performed synchrotron radiation x-ray diffraction measurements on CFO and CuFe_(1-x)Ga_xO_2 (CFGO) with x = 0.035 under applied uniaxial pressure in order to directly observe the uniaxial-pressure effects on the lattice. As a result, we have revealed that the applied uniaxial pressure certainly affects the crystal structure so that the structural transition temperatures increase as well as the magnetic phase transition temperatures. We have also found that CFO exhibits a pronounced lattice instability in the vicinity of the phase transition from the paramagnetic phase with a trigonal structure to an incommensurate magnetic phase with a monoclinic structure. In addition, the present results for CFGO (x = 0.035) have revealed that the lattice instability is suppressed by the substitution of a small amount of nonmagnetic Ga~(3+) ions for the magnetic Fe~(3+) ions. This indicates that the spin degree of freedom plays an important role in the lattice instability in this system.