Phase transitions of europium valency and manganese magnetic order and thermal hysteresis phenomena ins

The compounds (Formula presented) have been studied by (Formula presented) and dilute (Formula presented) Mössbauer spectroscopy, x-ray diffraction, magnetization, and resistivity measurements. In (Formula presented), Eu is divalent and orders magnetically at 13 K. The Mn sublattice orders magnetically at (Formula presented) =302 K. In (Formula presented) the Eu ion is in a valence fluctuating state, at 90 K it is trivalent and at 650 K it is almost completely divalent. The Mn sublattice orders antiferromagnetically at (Formula presented) =395 K, but at lower temperatures, 107, 65, and 32 K, undergoes spin reorientation transitions with a ferromagnetic component. Replacing ∼15% of Si by Ge in (Formula presented) converts all Eu ions into their divalent state. In (Formula presented), x=0.1, 0.2, at 90 K both divalent and trivalent Eu ions are present. As the temperature is raised towards a critical temperature a first order valence phase transition occurs, all (Formula presented) ions convert to (Formula presented) in a short range of temperatures. This first order valence phase transition is accompanied by the antiferromagnetic phase transition of the Mn sublattice. The formation of a single first order phase transition from two interacting order parameters, each of which by itself leads to an independent second order phase transition, is predicted by theory. This phase transition has a wide thermal hysteresis loop observed by Mössbauer studies (both (Formula presented) and (Formula presented)), magnetization, x-ray diffraction, and resistivity measurements. For x=0.1, x=0.2 the transition temperatures are 370, 330 K with increasing temperature and 320, 275 K with decreasing temperature, respectively.