Study of the Low Field Microwave Response in Yttrium Aluminates Dilutely Doped with Manganese
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Study of the Low Field Microwave Response in Yttrium Aluminates Dilutely Doped with Manganese Rakhim R. Rakhimov, David E. Jones, and George B. Loutts. Center for Materials Research, Norfolk State University, Norfolk, VA 23504, U.S.A
ABSTRACT 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 Mndoped 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. INTRODUCTION Recent discoveries of manganese based single molecular magnets, exhibiting magnetization tunneling, inspire the search for manganese doped insulating materials, where similar spin-dependent tunneling may occur within paramagnetic dopant ion and be accompanied by charge displacement current. Since the displacement current is one of the sources of microwave losses in materials, the search for spin-dependent tunneling in insulating materials can be realized through the investigation of their microwave dielectric properties. Study of microwave dielectric properties of rare-earth aluminates is of interest for their potential applications as dielectric resonators and substrates for high temperature superconductors (HTSC) [1]. In particular, yttrium orthoaluminate YAlO3 was suggested as a substrate material for microstrip antennas utilizing HTSC thin films [1]. Study of the mechanism of the charge carrier mobility is important in understanding of unique dielectric properties of insulators like rare-earth aluminates. YAlO3 doped with manganese has been shown to be a potential material for holographic and data storage [2]. The high holographic recording efficiency in Mn-doped YAlO3 may imply non-centrosymmetric charge distribution. Then microwave properties associated with charge displacement current can be expected in the crystal. Microwave losses associated with spin-dependent tunneling can be measured by means of mi
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