Low-temperature magnetization and thermoremanence of CoCl2•H2O

Reported here are field-cooled (FC) and zero-field-cooled (ZFC) magnetizations (M-FC and M-ZFC, respectively) in smaller and larger fields than used in previous work, and thermoremanent magnetization data (TRM) for various temperatures and wait times. For fields of 12.8 and 0.1 G a separation between M-FC and M-ZFC develops near 9 K, well below the 16.1 K magnetization maximum or the similar to 14 K antiferromagnetic transition; thus, reentrant behavior occurs as for the higher fields used previously. Maxima in M-ZFC(T) are observed at 7.2 and 8.0 K for 12.8 and 0.1 G, respectively. Similar measurements in fields of from 3.0 to 15.5 kG show an evolution in the form of M-FC(T) and M-ZFC(T); the separation between these develops at somewhat higher temperatures than 9 K and no maximum appears in M-ZFC. The field dependence of (M-IRR/H)(MAX), where M-IRR = M-FC-M-ZFC, is determined and shows a change in regime between 2 and 3 kG. The TRM as a function of time is measured for temperatures of 1.6, 4.3, and 5.8 K, with wait times ranging from 10 to 500 min. The relaxation rate is quite small, consistent with glassy dynamics with a wide distribution of relaxation times, and increases with increasing temperature. Remarkably, however, aging effects are absent even at 5.8 K, although this is 69% of the splitting temperature between M-FC and M-ZFC. An explanation may reside in randomized antiferromagnetic couplings between chemical chains which are individually ordered ferromagnetically, and in the formation of a structure of almost independent clusters. (C) 1999 American Institute of Physics. [S0021-8979(99)74608-6].