Evaluating Methods for Identifying the Spin Glass Transition Temperature: A Comparative Study

The spin glass transition temperature, often denoted as T_g, marks the point where a material undergoes a phase transition from a high-temperature paramagnetic state to a low-temperature spin glass state. There are several experimental signatures that have been used to determine this transition temperature. Spin glasses were first discovered from a peak in the ac susceptibility and the low frequency ac susceptibility peak is often used as the transition temperature. Researchers have also used the peak of the zero-field cooled (ZFC) magnetization [Phys. Rev. B 40, 869 (1989)] or the peak in the field-cooled (FC) magnetization [Phys. Rev. Lett. 54, 150 (1985)] as the transition temperature. However, others have taken the onset of irreversibility as the transition temperature [Phys. Rev. Lett. 66, 2923 (1991)]. If these techniques truly determined the spin glass phase transition temperature T_c, we would expect the measured T_g to be consistent across all methods, with similar behavior, such as a uniform response to varying magnetic fields. This study on a single crystal Cu_0.94Mn_0.06 sample presents the first comparative analysis of these different techniques for determining the spin glass transition temperature through static and dynamic measurements. We observe that the peak of the ZFC, FC, and ac susceptibility curves remain invariant as a function of field (for low fields) whereas the onset of irreversibility starts at a temperaure above T_g and as the magnetic field is increased, it shifts through T_g to lower temperatures.