Two-dimensional antiferromagnetism and weak ferromagnetism in some anilinium salts of the types A2MnCl4 and A2FeCl4

Abstract

Magnetic susceptibility measurements have been made between 4.2 and 160 K on the powdered salts (4-XC₆H₄NH₃)₂MnCl₄(X = F, Cl, Br, or Buⁿ). These compounds have layer structures of the K₂NiF₄ type, with interlayer spacings of 15.8, 16.6, 18.8, and 23.5 Å, respectively. Broad susceptibility maxima are observed at ca. 70 K, and can be fitted on the assumption of a two-dimensional exchange interaction of Heisenberg (–2JS_i. S_j) form, with –J/k= 4.25, 4.50, 4.00, and 3.75 (±0.1) K, respectively. All four compounds show weak ferromagnetism below the Néel temperature (T_Nca. 40 K), which is attributed to spin canting in the antiferromagnetically ordered state. The transition temperature is practically independent of the layer spacing. Measurements on a single crystal of the 4-chloro-compound indicate a canting angle of ca. 0.06°, and show that the resulting magnetisation lies in the layer plane. Powder susceptibility measurements on (4-ClC₆H₄NH₃)₂FeCl₄ indicate that this compound is also a two-dimensional antiferromagnet, with –J/k= 10.7 ± 0.5 K (T_Nca. 87 K) and a canting angle ca. 0.8° in the ordered state. Below T_N the magnetic behaviour shows unusual dependence on the thermal and magnetic history of the sample, possibly due to a nearly degenerate magnetostrictive distortion.