Magnetic specific heats of four spin {5}/{2} linear chain compounds of formula MnX 2L 2 ( X = Cl, Br; L = N2C3H4, NC5H5)

The specific heats of the compounds MnX ₂L ₂ with X = Cl or Br and L = N₂C₃H₄ (pyrazole) or NC ₅H ₅ (pyridine), have been measured in the 1.3-80 K temperature range. The well-resolved broad maxima in the magnetic specific heat curves of all the compounds can be interpreted in terms of linear chain exchange interactions. The measured specific heats are compared with the isotropic antiferromagnetic Heisenberg linear chain model, but a model assuming, in addition, a moderately large single-ion anisotropy gives a much better agreement with experiment. The values obtained for the intrachain exchange parameter are in the range 0.5 K < | J/ k| < 1 K, and for the uniaxial single-ion anisotropy parameter, 0.2 K < | D/ k| < 0.5 K. The lambda-like anomalies observed in MnBr ₂.2pz, MnCl ₂.2py and MnBr ₂.2py at T = 1.930 K, 1.380 K and 2.055 K, respectively, are attributed to long-range ordering due to the influence of weak interchain interactions. The strength of the interchain interactions has been estimated as about 1% of the intrachain interactions. The compound MnCl ₂.2pz does not exhibit a transition to a long-range ordered state down to 1.3 K, which suggests that the interactions between the chains are very weak.