DIELECTRIC AND TRANSPORT-PROPERTIES OF MAGNETIC INSULATORS IRRADIATEDWITH GEV HEAVY-IONS

The dielectric and ac/dc transport properties of single crystals of yt trium iron garnet (Y3Fe5O12 and Y3Fe5O12:Si), and barium hexaferrite ( BaFe12O19 and BaFe12O19:Co,Ti) were investigated after irradiations wi th Xe and Pb ions in the GeV range. In the virgin n-type samples (Y3Fe 5O12:Si and BaFe12O19:Co,Ti), the strong dielectric relaxation below 1 00 kHz is found to correspond to a space-charge polarization at the bl ocking metal/insulator contacts yielding a nonohmic de conductivity. T he relaxation frequency decreases with increasing amorphization yield in relation to the decrease of the insulators bulk de conductivity whi ch becomes ohmic in the amorphous phases. The ac conductivity data of both crystalline and amorphous Y3Fe5O12:Si above 100 kHz and for 100K< T<300K exhibit two contributions: (i) that of carrier transport in a d isordered or inhomogeneous medium varying as nu(s), with S similar or equal to 0.8, (ii) and that of a two-site polaron hopping process of c harge transfer between Fe2+ and Fe3+ with an activation energy of 0.29 eV for T > 180 K. The dc conductivity data of crystalline Y3Fe5O12:Si for 80 K<T<300 K are discussed on the basis of a small polaron hoppin g conduction mechanism between Fe2+ and Fe3+ with an activation energy around 0.28 eV for T>125 K, in agreement with the activation energy a round 0.28 eV of the space-charge dielectric relaxation frequency for T>180 K. All amorphous phases data are consistent with the picture of hopping conduction between gap states in a disordered medium with (i) an nu S dependence for the ac conductivity above a critical frequency proportional to the de conductivity, (ii) and an exp(-T-1/4) law for t he dc conductivity. (C) 1997 American Institute of Physics.