Polarized neutron diffraction study of the magnetization density distribution in Rb 2 CrCl 4 : a two-dimensional ionic ferromagnet

Abstract

The magnetization density distribution in the ionic ferromagnet Rb 2 CrCl 4 has been determined by polarized neutron diffraction from a single crystal at 4.5 K. Magnetic structure factors F M were derived from the flipping ratios R of 660 main structure reflections (189 independent) and 57 super-structure reflections with sin θ /λ ≼ 0.8 Å -1 (λ = 0.90 Å). The main structure reflections were corrected for extinction using a secondary extinction parameter. The F M were modelled by representing the mag­netization density by a superposition of densities from orbitals centred at each atom : Cr(3d, 4s) and Cl(3s, 3p). The results verify the presence of orbital ordering arising from the cooperative Jahn-Teller distortion in the basal plane, previously postulated as responsible for the ferromagnetic exchange interaction. The proportion of the Cr 2+ moment occupying each 3d orbital is 0.26(2)( yz ), 0.25(2)( xz ), 0.25 0.25(2)( z 2 ) and — 0.01 (2)( x 2 — y 2 ), where z is the principal axis of Jahn-Teller elongation at each Cr site. The optimum refined distribution of the magnetic moment (μ B ) was Cr(3d) 3.16(8), Cr(4s) 0.6(1), equatorial Cl(3s) 0.05(2), Cl(3p) 0.06(3) and axial Cl(3s) 0.03(3), Cl(3p) 0.08(5).