Zero-field signal in the electron paramagnetic resonance spectrum of Mn+2 in silicate glasses

A 9.4-9.8 GHz electron paramagnetic resonance (EPR) study of Mn+2-doped Na2 O-CaO-MgO-SiO2 glasses has revealed a nonresonant microwave magneto-absorpt ion near zero magnetic field in addition to normal paramagnetic absorption due to Mn+2 ions, electron spin S = 5/2. The low-field response has an oppo site phase relative to paramagnetic signal and is independent of the mutual orientation of the magnetic field of the microwave H-1 and static magnetic field H. In contrast, the paramagnetic signal is different for perpendicul ar H(1)perpendicular to H and parallel H(1)parallel to H polarization of th e microwave field, which is attributed to enhancement of forbidden magnetic dipolar transitions and suppression of the allowed transitions for paralle l polarization. The low-field response is described in terms of microwave d ielectric losses that derive from the magneto-induced charge migration in t he first coordination sphere of Mn+2. As opposed to the spin-polarized tunn eling that was described in ferromagnets between different valence forms of Mn, the observed effect is due to spin-dependent tunneling that occurs in the vicinity of Mn+2 in a diluted paramagnetic system. (C) 2000 American In stitute of Physics. [S0021-9606(00)50128-7].