Magnetization Steps from Cd -Diluted TMMC at 20 mK: Effects of Weak Intra-cluster Interactions

The infinite spin chains in tetramethylammonium manganese chloride (TMMC) (chemical formula: (CH_3)_4NMnCl_3 ) have long been thought to be the best example of a one-dimensional antiferromagnet with nearest-neighbor (NN) Heisenberg exchange interaction. The present work continues the study of magnetization steps (MSTs) from the infinite spin chains in Cd-diluted TMMC (chemical formula: (CH_3)_4NMn_xCd_1-xCl_3 ). MSTs from a sample with x = 0.50 were studied in magnetic fields up to 17 T. These MSTs originated from chains with the following numbers of NN spins: n = 2, 4, 6 and 8. An earlier study of MSTs from samples with 0.16 ≤ x ≤ 0.50 was carried out at T = 0.55 K. In that earlier lower-resolution study, the observed MSTs were from chains with n = 2 and 4. The 0.55 K data seemed to agree with the isotropic NN Heisenberg (INNH) model, which includes only the NN Heisenberg exchange and the Zeeman energy. The present higher-resolution data at 20 mK show clear discrepancies with this model, indicating that some weak interactions that are not included in the INNH model are not insignificant. Discrepancies with the INNH model in magnetic fields B ≈ 1 T are attributed to the dipole-dipole anisotropy. Of greater significance are discrepancies in the ratios of the magnetic field at different MSTs. These discrepancies are observed above 2 T. The discrepancies in the field ratios imply that the energies of the short spin chains deviate from those calculated from the INNH model. Similar deviations were noted a few earlier studies of spin clusters in other materials. In these earlier studies the deviations were attributed to biquadratic exchange. Although the known properties of Cd-diluted TMMC are consistent with the requirements for a significant biquadratic exchange, the present data are insufficient for concluding that biquadratic exchange is definitively the cause of the observed discrepancies in the field ratios.