ИСТИНА

Ca-Doped lanthanum manganites La1-xCaxMn0.98Fe0.02O3+δ (где x=0.05, 0.10, 0.20) of both nonstoichiometric and stoichiometric compositions have been studied in a wide temperature range by Mossbauer spectroscopy and magnetic methods. As-prepared samples are single-phase (rhombohedral R-3c phase for x=0.05 and 0.10, and orthorhombic PnmaI phase for x=0.20) and have a non-stoichiometric composition (δ > 0). As the temperature decreases all the samples investigated demonstrate superparamagnetic behavior.With increasing Ca concentration the distortions of nearest environment of Fe atoms in the samples of non-stoichiometric composition decrease by virtue of decreasing the excess oxygen content occupying interstitial sites [1]. When the temperature decreases the ferromagnetic clusters appear in the samples.There are two ways to obtain lanthanum manganites of a stoichiometric composition ((δ = 0): the first one is the replacement of La3+ by a divalent element (Ca, Sr, Ba), the second one is removal of excess interstitial oxygen by annealing in vacuum. The first way is usually used in literature [2,3]. In this case a stoichiometric composition can be obtained only when a doping element concentration is large. The works on the investigation of lanthanum manganites of a stoichiometric compositions with a small concentration of a doping element are virtually absent. In the present work the second way (annealing in vacuum) is used to obtain the samples with a stoichiometric composition that allowed us to extend the concentration range of a doping element.The structural phase separation was revealed in the samples of a stoichiometric composition (δ = 0) at room temperature representing itself the co-existence of three orthorhombic phases. With decreasing the temperature, one paramagnetic orthorhombic PnmaI phase transfers to ferromagnetic state, two the other PnmaII and PnmaII* phases, to antiferromagnetic state. When a Ca concentration increases, a relative content of ferromagnetic phase increases, and the temperature of its magnetic transition increases. It follows from the obtained experimental results that both the structural and magnetic phase separation exists in the Ca-doped lanthanum manganite La1-xCaxMn0.98Fe0.02O3 of a stoichiometric composition which is most pronounced at a small concentration of a doping element.