Magnetic thermal hysteresis in dysprosium, gadolinium and gadolinium/dysprosium/gadolinium nanolayers and experimental determination of thermodynamic properties of thin film systems

Magnetic thermal hysteresis (MTH) is observed when the temperature dependent magnetic properties of a material are reliant on the starting point of the measurement. MTH was observed in thin film Dy samples at low values of constant external magnetic field strengths using a Superconducting Quantum Interference Device (SQUID magnetometer). The temperature is changed from 20K to 300K back to 20K under a constant field. In these temperature sweeps differences in magnetic moment were observed near the low end of the temperature range starting around 150K. However, when starting at the high end of the temperature scale and making the same measurements this separation is not observed. The large difference in moment at low temperature can also be controlled by the magnitude of the external field strength. In the Dy films the existence of Alternate Helicity (AH-state) a shifting of the easy magnetization axis and a Helical (H-state) are the primary causes for the existence of the observed MTH. In Gd helical magnetization states do not exist but to a small extent the Gd films also show thermal hysteresis, over a smaller temperature range. Measurements of three layer systems which consist of a Dy film between two Gd films showed noticeable differences in the forward and return paths of the moment vs. temperature M(T) graphs consistent with the effects seen in the individual films. The effects of the individual films are easily distinguished in the three layer system. This cumulative effect can be controlled by modifying the layer thickness and therefore changing the shape and size of the observed magnetic thermal hysteresis. In addition to this a series of measurements was done to find a simplified means for calculating otherwise difficult thermodynamic properties of a system such heat capacity and entropy. A dataset consisting of multiple field hysteresis M(H) loops was made and by using the well known Maxwell relationships, for a magnetic system, the necessary calculations are done to find these otherwise intricate functions.