Magnetic susceptibility and optical study of the organic-intercalated two-dimensional ionic ferromagnet bis(benzylammonium) tetrachlorochromate(II)

Abstract

The powder a.c. magnetic susceptibility of bis(benzylammonium) tetrachlorochromate(II) has been measured from 4.2 to 150 K and a transition to three-dimensional ferromagnetic order observed at 37.0 K. From 80 to 150 K the susceptibility follows the Curie-weiss law with the Curie and weiss constants respectively 4.80 and 58 K. The susceptibility from 40 to 150 K is fitted to a hightemperature series expansion for a square two-dimensional lattice to give the near neighbour exchange constant J/k= 10.6(2) K (ℋ=JS₁·S₂) and g= 2.1 0(5). The divergence of the ‘energetic’ initial susceptibility χ_OT/C as T_C is approached from above is fitted to a power law in (T–T_c)/T to yield an exponent γ varying from 2.2 (far from T_c) to 1.87 (within 4 K of T_c). The ligand-field band system near 626 nm shows more structure than in corresponding Group 1 A or n-alkylammonium salts. Temperature dependence of the intensity from 1.5 to 15 K shows that the zone centre magnon gap is smaller than in any of the other salts since the intensity closely follows a T² law down to 1.5 K. Minor differences in the intensity behaviour of the components of the 621 nm band system suggest that the ground state consists of three closely spaced levels. Comparison with other tetrachlorochromate(II) salts shows no clear correlation of either kT_c/θ or kT_c/J with interlayer separation.