Structure and magnetic properties of the rhombohedral-structure compound Co2 (OH)3 Br, a member of the geometrically frustrated series of the compounds M2 (OH)3 X where the magnetic ions form a deformed pyrochlore lattice, were studied using dc and ac magnetic susceptibilities, heat-capacity, neutron powder-diffraction, and muon-spin-rotation/-relaxation (μSR) measurements. The structure of Co 2 (OH)3 Br is featured by alternatively stacked layers of perfect kagome-lattice planes and triangular-lattice planes with a 10% distortion along the stacking direction (the c axis). Despite a very small difference in the distortion [0.42% larger in Co2 (OH)3 Br], Co2 (OH)3 Br was found to show contrasting antiferromagnetism that is strikingly different from the previously reported ferromagnetic Co2 (OH)3 Cl. Successive antiferromagnetic transition was observed at TN1 =6.2 K and TN2 =4.8 K, respectively. The antiferromagnetic ground state is metastable and an intermediate magnetic phase was induced by applying a relatively low magnetic field of H∼5 kOe. When the field was further increased above H∼20 kOe spin reorientation occurred to form a configuration similar to ferromagnetic Co2 (OH)3 Cl. The successive antiferromagnetic transitions in zero field were found to occur with propagation vector of k1 = (0 -1/2 1/2) at TN1 and an additional k2 = (0 0 3/2) at TN2. Refinement of the neutron powder-diffraction patterns revealed an unconventional multi- k and noncoplanar spin structure for the antiferromagnetic phases. Multiple measurements, in particular, the μSR study, consistently demonstrated magnetic coupling at high temperatures, and persistent fluctuations well below the TN. This work presents a unique system to investigate the orbital effect and the critical role of lattice distortion in geometric frustration, and provides a single material system to study multiple phase transitions and competing exchange interactions.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - Dec 29 2010|
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics