FMR measurements in YIG single crystals doped with Mn have for a long time been of great interest due to uncertainties in positions and valencies of Mn ions. The influence of a small number of impurities with orbital degenerate ground state on the electron spectrum can be studied especially on single crystals with extremely small FMR linewidth [l]. Even in lightly doped YIG crystals the Jahn-Teller effect leads to large low-temperature acoustic and microwave losses [2], anomalous changes in anisotropy constants [3], magnetostriction [4], and their temperature dependences.Some investigations of this problem were made on spherical samples [2], others on thin films [l]. Most interesting effects were studied on single crystals [3, 41. It is likely that the attribution of peculiarities in the temperature dependence of relaxation processes caused by Jahn-Teller distortion may help to understand the state of Mn ions in the single crystals.We have studied angular and temperature dependences of FMR resonance field and linewidth for YIG single crystals doped with Mn (x = 0.23) of rectangular shape and approximately 1 mm3 volume. Unfortunately, we have no possibility to operate on spherical samples, so FMR signals were reliably detected in some directions of the external magnetic field close to symmetry directions as discussed further. The samples were rotated in a magnetic field oriented in (110) and (111) planes. The temperature was set and stabilized by an Oxford Instruments ESR-900 helium cryostat. The FMR spectra were registered on a commercial homodyne spectrometer ESR-230 in X-band (9.45 GHz) with 100 kHz modulation as derivative of the absorbed power. Resonance field H , and linewidth AH were detected in a standard way [4]. The general view of the FMR spectrum, being rather complex consists of the low-field signal with H , M 2850 G, AH M 40 G (wide signal), the most intensive high-field signal with H , M 3900 G, AH M 1 G (narrow signal), and its subharmonics when the magnetic field takes a direction close ( M 15') to one of the sample edges. The orientation dependence of FMR parameters leads to the conclusion that well-defined values of H , and AH can be obtained for the narrow signal. For the directions of magnetic field far enough from the sample edges the FMR line exhibits a large growth of AH (Fig. I). This is a consequence of the broadening of the FMR signal due to inhomogeneities of magnetic field in a rectangular sample in an external field of arbitrary direction. So further we consider the narrow FMR signal at various temperatures for directions of the external magnetic field 9 = 0, 90", 180" (where 9 describes the inclination angle in the sample plane). The H , values for 9 = 90" are less by 35 G than the corresponding values for 9 = 0, and this may be attributed to the difference in demagnetization factors along a and b edges of the single crystal (equal I) Kovalevskaya 18, 620 219 Ekaterinburg, GSP-170, Russia.
