The nflection of light from a medium with ordered spin smclure characterized by the breakdown of time-reversal and parity s y " e v y is exgected to be non-reciprocal even if the net magnetic moment of the medium equals Zem. We repoa on the fust experimental observation of sponmwnk non-recipmcal mtarion and circular dichroism in spin magnetoelectric Cr203. Nokrecipmcal effects were obmved M o w the antifemmagnetic tansition temperahlre Tw = M7 U and their temperahre behaviour roughly obrresponds to that of the order parameter. Observed values of (1 -4) x for the magnetoelecuic susceptibility in the optical range are several orden of magnitude higher than predicted earlier. This increase of the susceptibility is presumably atuibutable lo eleclmnic dipole transitions in the optical range. tutroductionAmong different optical phenomena in crystals a particular type of effect may be assigned to the so-cklled non-reciprocal (NR) effects. They are characterized by different phase velocities and/or attenuations for light waves travelling via the same optical path but in opposite directions. The most typical exaniples of these effects are the Faraday rotation observed in transmission and the Ken effect observed in reflection (see e.g. [l]). As far as we know, up until now non-reciprocal optical effects were observed exclusively in media possessing a magnetic moment. This moment can be induced by a magnetic field in dia-and paramagnets or can arise spontaneously, ai in feqoor ferrimagnets. A magnetic moment induced by an electric current may also give rise to non-recipmcal phenomena [2]. In all these cases the timereverd symmetry (1') is broken; that is, the crystal may be in two different states converted into one another by the operation 1' .There exists a special class of magnetically ordered materials in which below the magnetic transition temperature TN there is no net magnetic moment, but in addition to the timereversal symmetry breaking the parity symmetry (7) is also broken. At the same time the combined symmetry operation i' is retained. Magnetoelectrics (ME) are the most well known and widely studied materials [ 3 4 belonging to this class. Soon after the discovery of ME, theoretical analysis of light propagation in magnetoelectric antiferromagnets showed 17-10] that new optical phenomena should be found, when the spatial dispersion is taken into account. These phenomena, though beiig in some manifestations similar to those observed in media with a net magnetic moment, may exhibit some important differences. Some of these effects have been observed iecently in "ission in 1111. Since these new phenomena are related to the spatid~dispepion they have smaller values as compared with
Pulsed laser deposition has been used to grow thin (10–84 nm) epitaxial layers of Yttrium Iron Garnet Y3Fe5O12 (YIG) on (111)–oriented Gadolinium Gallium Garnet substrates at different growth conditions. Atomic force microscopy showed flat surface morphology both on micrometer and nanometer scales. X-ray diffraction measurements revealed that the films are coherent with the substrate in the interface plane. The interplane distance in the [111] direction was found to be by 1.2% larger than expected for YIG stoichiometric pseudomorphic film indicating presence of rhombohedral distortion in this direction. Polar Kerr effect and ferromagnetic resonance measurements showed existence of additional magnetic anisotropy, which adds to the demagnetizing field to keep magnetization vector in the film plane. The origin of the magnetic anisotropy is related to the strain in YIG films observed by XRD. Magneto-optical Kerr effect measurements revealed important role of magnetization rotation during magnetization reversal. An unusual fine structure of microwave magnetic resonance spectra has been observed in the film grown at reduced (0.5 mTorr) oxygen pressure. Surface spin wave propagation has been demonstrated in the in-plane magnetized films.
Second-harmonic genention (SHG) at normal incidence in transmission geometry was studied in Bi-containing magnetic gamet thin films on gadolinium-gallium garnet (GGG) substrates oF(l1I) and (210) types. A Q-switched Nd-YAG h e r was used as a radiation source.The intensity of the SHG w35 lower than that in crjstalline quam but it was nevertheless reliably detectable. The study of the SHG intensity as a function of the rotation angle of the films around their normals showed that the signal is well described by phenomenoiog?cal expressions derived under the assumption that the point symmetry of the films is Cu, ((210) substrate) or C2, ((111) subsmte). The SHG intensity was found to be independent of temperature in the range 290-405 K. thus excluding a 'magnetic' origin of the SHG. These experiments as well as those previously published on the linear magnetoelectric effect prove that there is no inversion centre in magnetic gamet thin films
The magnetization dynamics of single-crystalline Fe͑001͒ thin films with Cr cap layers has been studied by an all-optical time-resolved pump-probe technique. The system is characterized by a fourfold in-plane magnetic anisotropy. We observed long-lived ͑ϳ1 ns͒ magnetization oscillations caused by the ultrafast ͑ϳ0.15 ps͒ optical pulse excitation. The oscillations are associated with the temporal variation of the magnetization component M z normal to the film surface. The phase of the oscillations is independent of the polarization state of the pump beam giving evidence for a predominantly thermal origin of the excitation. The amplitude of the oscillations considerably depends on the in-plane orientation and magnitude of the magnetic field. The azimuthal variation of the oscillation frequency at constant magnetic field follows the fourfold in-plane magnetic anisotropy. Angle and field variations of the frequency are well described by a uniform precession mode known from the theory of ferromagnetic resonance. Our analysis indicates that the precession amplitude is determined by the frequency of the uniform mode and an in-plane tilting of the effective magnetic field directly caused by the pumping light beam.
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