Magnetic and powder neutron-diffraction studies of hexagonal -NaCoPO4: A possible geometrically frustrated magnet

Magnetic and powder neutron-diffraction studies were performed on a polycrystalline hexagonal compound β-NaCoPO 4 with space group P6 5 and lattice constants, a = 10.167 Å and c = 23.785 Å (1Å = 10 −10 m). The susceptibility data display a magnetic transition below about 3.2 K and follow the Curie–Weiss law in the temperature range 5–300 K with a paramagnetic Curie temperature, θ p , of −16.6 K indicating that the spins are strongly frustrated. The Co 2+ ions form a helix about the c direction but due to the topology of the structure the helix can be decomposed into sets of six roughly triangular layers per unit cell. Thus, nearest-neighbor antiferromagnetic interactions can give rise to frustration within the pseudolayers. Low-temperature neutron-diffraction data reveal magnetic Bragg peaks below the ordering temperature. A Rietveld refinement of the data suggests a 120° magnetic structure of Co 2+ spins within each pseudolayer.