Low-temperature magnetization and thermoremanence of CoCl2⋅H2O

Abstract

Reported here are field-cooled (FC) and zero-field-cooled (ZFC) magnetizations (MFC and MZFC, 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 MFC and MZFC develops near 9 K, well below the 16.1 K magnetization maximum or the ∼14 K antiferromagnetic transition; thus, reentrant behavior occurs as for the higher fields used previously. Maxima in MZFC(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 MFC(T) and MZFC(T); the separation between these develops at somewhat higher temperatures than 9 K and no maximum appears in MZFC. The field dependence of (MIRR/H)MAX, where MIRR=MFC−MZFC, 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 MFC and MZFC. 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.