Abstract:
The results of synchrotron x-ray-diffraction and magnetization measurements on a triangular lattice antiferromagnet CuFeO2 under pulsed high magnetic fields are reported. This material exhibits a distorted triangular lattice structure below 11 K to relieve partially the geometric frustration. We find stepwise changes in the lattice constants, associated with the magnetization changes parallel H and perpendicular H to the trigonal c axis. The relative changes in the lattice constant b with H are reproduced by a calculation based on a model in which the number of bonds connecting two parallel spins along the b axis increases with increasing field and the lattice contracts to gain the ferromagnetic direct and antiferromagnetic superexchange energies. For H , we find a discontinuous change in b and c at 24 T, and a plateau in b and c at 24 H 30 T. The change in b with increasing H agrees also with the same calculation. We discuss the anisotropic behavior in CuFeO2 observed at the low fields and find that the anisotropy is closely correlated with the lattice distortion.
I. INTRODUCTION
In many magnetic materials, the crystal lattice is coupled with the magnetic moments through magnetoelastic interaction. Typical examples are magnetostriction1 and spin-Peierls transition.2 In recent years, magnetic materials with geometric spin frustration, such as triangular and pyrochlore lattice antiferromagnets, have been attracting much attention, in which the magnetoelastic interaction also plays an important role. When the nearest-neighbor exchange interaction between magnetic moments on a tetrahedron of the pyrochlore lattice is antiferromagnetic, strong geometric frustration results. For classical spins, there are infinite ways of arranging spins on the vertices of a tetrahedron3 with total spin Stot = 0. Due to this vast degeneracy, a classical Heisenberg antiferromagnet on the pyrochlore lattice is predicted to be disordered down to absolute zero temperature.4 In real pyrochlore magnets, however, a magnetic order has been observed at a finite temperature by breaking lattice symmetry.5–9 Theory predicts that a Heisenberg antiferromagnet on the triangular lattice will show a noncollinear magnetic structure.10 On the contrary, in CuFeO2, which is an archetype triangular lattice antiferromagnet TLA, a collinear magnetic structure has been observed.11,12 The interrelation between the magnetic properties and the magnetoelastic interaction in CuFeO2 is the subject of this paper.
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