Magnetic structure and analysis of the exchange interactions inBiMO(PO4) (M = Co, Ni)

Abstract

The magnetic structures of the two bismuth oxy-phosphate compoundsBiMPO5 (M2+ = Ni2+,Co2+) have been determined by neutron powder diffraction using group theory analysis as apreliminary tool. Both compounds adopt a monoclinic crystal structure (S.G.P 21/n,a = 7.1642(2) Å,b = 11.2038(3) Å,c = 5.1740(2) Åand β = 107.296(2)° for Ni2+ and a = 7.2441(1) Å, b = 11.2828(1) Å, c = 5.2258(1) Åand β = 107.841(1)° for Co2+). The refinement of the magnetic structures belowTN = 17.5 and 15 K,respectively, for both compounds show that the magnetic structure is characterized by the propagationvector k = (−1/2, 0, 1/2), with components given with respect to the reciprocal lattice of the nuclear structure. Thismeans a magnetic unit cell that is a multiple of the nuclear cell. The magnetic structure isconstituted of ferromagnetic pairs of metal ions antiferromagnetically coupled withindouble chains. The relative strength of the intra and inter double chains exchangeinteractions has been examined by establishing a theoretical magnetic phase diagram. Mostof the interactions come from M–O–O–M super–super-exchange paths. At its ground state,BiNiPO5 shows a nearly collinear arrangement of magnetic moments withm1.5 K = 2.13(3) μB/Ni. Due to the strongmagnetic anisotropy of Co2+ (m1.5 K = 3.52(3) μB/Co), the collinear character is largely lost while the magnetic structure remainsanalysable on the basis of the greatest isotropic component of the local moments.