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A comparative study of the Czochralski crystal growth of Ca and Sr fluoro-phosphate, Ca and Sr fluoro- and chloro-vanadate is reported. Sr fluoro-phosphate and fluoro-vanadate demonstrate the best overall properties and crystal quality. Codoping of the fluoro-apatite crystals with Si4+ or Ge4+ ions increases the neodymium distribution coefficient but somewhat deteriorate the Nd3+ emission.
We have grown Mn:LaGaO3 crystals (Mn=0.5%, 2%, 10%, and 50%) and studied their properties with the methods of optical and EPR spectroscopy. We have found Mn2+, Mn4+, and Mn5+ ions in low doped crystals (Mn=0.5% and 2%) and Mn3+ ions in crystals with higher Mn concentrations (10% and 50%). The formation of Mn3+ valence state in Mn:LaGaO3 is discussed.
Microwave response near zero magnetic field was observed in YAlO3 and CaYAlO4 crystals dilutely doped with Mn in concentration ranging from 0.05 to 2 atomic %. The response is due to non-resonant microwave absorption, which co-exists with normal electron paramagnetic resonance (EPR) absorption due to different paramagnetic valence states of manganese. Mn2+ and Mn4+ charge states were identified in Mn-doped YAlO3, and Mn2+, Mn4+ and Mn5+ in Mn-doped CaYAlO4. The low field response has the opposite phase with respect to the paramagnetic absorption. This shows that Mn-doped YAlO3 and CaYAlO4 exhibit magnetically induced microwave absorption, which has a minimum at zero magnetic field and increases with the applied magnetic field. This effect is similar to microwave magneto-resistance effects observed in manganite perovskites, where spin-dependent electron tunneling occurs between ferromagnetically coupled manganese ions in different valence states. We show, however, that in the present case of diluted paramagnetic systems, magneto-induced microwave losses are due to intramolecular spin-dependent tunneling, where central paramagnetic ion does not change its charge state and spin-dependent charge migration occurs in the first coordination sphere of paramagnetic ion. Evidences are presented that this ion is Mn2+ exhibiting the highest electron spin S = 5/2.
Manganese doped perovskites are promising materials for non-magnetic data storage. Systems with low doping concentration, such as Mn:YAlO3 are high-quality optical crystals, demonstrating significant photorefractive effect. Optically and electrically induced quasi-permanent change in low-field conductivity was observed in the materials with high concentration of manganese such as single crystals and films of LaGa1−xMnxO3 (x= 0.1–1). These memory effects can last for a long time at room temperature and can be easily erased by heating up to 230 C. We explain our experimental results by photoinduced or thermoinduced local phase transition with the oxidation of Mn ions.
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