Magnetic first-order phase transition in MnAs films

For polycrystalline and epitaxial films of MnAs, the magnetic phase transformation from the ferromagnetic to the paramagnetic phase was studied. It was found that the films which are removed from the substrate undergo an abrupt loss of magnetization near 38°C within a temperature interval of 0.2°C. They exhibit a temperature hysteresis of 5°C. Films clamped to thick inflexible substrates show a second-order phase transition and a shift in Curie temperature of 35°C per 1-percent volume change due to a difference in thermal expansion coefficient between film and substrate. MnAs films on highly flexible thin plastic foils exhibit a first-order-like phase transition. First-order transition films show a drastic change in resistivity ρ due to microcracks. At 20°C ρ of the virgin film is 1.93 × 10^-4Ω cm and increases irreversibly to 6.0 × 10^-4Ω cm after 15 transitions. The reversible change varies between a factor of 2.5-3.5. The Hall voltage was measured and the Hall constants R0and R1were calculated to be 2 × 10^-3and 0.33 cm³/A.s respectively, at 20°C.