Phase transitions of europium valency and manganese magnetic order and thermal hysteresis phenomena inEuMn2Si2−xGexs

Abstract

The compounds EuMn{sub 2}Si{sub 2{minus}x}Ge{sub x} have been studied by {sup 151}Eu and dilute {sup 57}Fe M{umlt o}ssbauer spectroscopy, x-ray diffraction, magnetization, and resistivity measurements. In EuMn{sub 2}Ge{sub 2}, Eu is divalent and orders magnetically at 13 K. The Mn sublattice orders magnetically at T{sub c}=302 K. In EuMn{sub 2}Si{sub 2} 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 T{sub N}=395 K, but at lower temperatures, 107, 65, and 32 K, undergoes spin reorientation transitions with a ferromagnetic component. Replacing {approximately}15{percent} of Si by Ge in EuMn{sub 2}Si{sub 2} converts all Eu ions into their divalent state. In EuMn{sub 2}Si{sub 2{minus}x}Ge{sub x}, 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 Eu{sup 3+} ions convert to Eu{sup 2+} 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 ofmore » 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 M{umlt o}ssbauer studies (both {sup 151}Eu and {sup 57}Fe), 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. {copyright} {ital 1997} {ital The American Physical Society}« less