Thermal anomalies hysteresis

Scientific awareness of a low-temperature transition in magnetite began in 1929 with the observation of a A-type anomaly in the specific heat at about 120 K. The anomaly was typical of an order-disorder transition, but it was well below the magnetic-ordering temperature Tc = 850 In 1931, Okamura observed an abrupt semiconductor-semiconductor transition near 120 K. The transition exhibits no thermal hysteresis, but the transition temperature is sensitive to the oxygen stoichiometry. More recent specific-heat measurements show the presence of two resolvable specific-heat peaks at the transition temperature the lower-temperature peak near 110 K appears to be due to a spin reorientation. 

The complete monolayer is modeled by an NVT ensemble with again deformable periodic boundary conditions, where fixed particle number N and fixed area V results in a constant coverage, which was chosen to be unity [102, 301]. This models the situation where either the physical or the grain boundaries prohibit a thermal expansion of the complete monolayer in the graphite plane. Also in this case, a melting transition to a modulated fluid occurs, but the transition is now located at about 87 K see the heat capacity anomaly in Fig. I9b. The transition does not show any noticeable hysteresis nor appreciable size effects, and the modulated fluid persists up... 

The susceptibility of Er, measured by an AC technique with peak helds of 10" T, reveals anomalies (hg. 6.9) at 27 K and 34 K in addition to those at Tn, Tnx and Tc (Taylor et al, 1975). Habenschuss et al. did not observe magnetic transitions at these temperatures but suggested instead that the low held susceptibility will exhibit singularities when the magnetic and crystallographic periodicities become commensurate. The extra susceptibility anomalies and their associated thermal hysteresis correspond to the regions of eight and hfteen layer periodicity. 

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