It was been proved that besides with the isotropical magneto-optical Kerr effect (EKE) in the cubic single crystal Ni there is a strongly anisotropic magneto-optic orientation effect 60, which is quadratic in magnetization. The dependence of 60, (w) for the main crystallographical directions in the interval 0.17 < hw < 3.25 eV and the anisotropy of !or in the plane (110) at fixed 9 have been studied. The dependence of 60,(w) is more complicated than for EKE, whch is accounted for by the chang~ng of the band structure of a ferromagnetic metal under the influence of the spin-orbit interaction due to the rotation of I. On the basis of the above assumption the peculiarities of the curve 60,(w) are related to the definite interband transitions according to the existing models of the electronic structure. The investigation of the anisotropy and frequency dependence of magnetooptical orientation effect (MOE) [l, 21 consisting in even in magnetization change of intensity of p-wave of lineary polarized light is the most urgent task. The study of MOE anisotropy is important for the understanding of the nature of this magneto-optical phenomenon, while the knowledge of its frequency dependence will allow us to make the next step in ascertaining the structure of electronic energy spectrum of the ferromagnetic metals. In this paper the investigation of MOE anisotropy of single crystals of ferromagnetic nickel is discussed. The measurements were carried out in the energy interval of light quanta from 0.17 to 3.25 eV on the magneto-optical setup described before . The sample of the single crystal of nickel was cut out of a rod by the spark method in the plane (110) and had the form of a disc with the diameter of 10 mm and thickness 0.5 mm. The sample was subjected first, to mechanical and then to electrical polishing. The thickness of the removed layer was greater than 100 microns. The measurements of MOE and equatorical Kerr effect (EKE) were carried out simultaneously. The measurements of EKE, which is the odd function of magnetization, were carried out, as before,  by the commutation dynamic method with periodically changing magnetic field H = Ho sin cot (Horn,, = f 2 700 Oe). EKE corresponding to the saturation magnetization is 6, = (AR/R,),=,s (R, the intensity of light reflected by the ferromagnetic mirror in demagnetized state, A R the change of reflected light intensity in result of magnetizing the sample). The change of the magnetic field fromOto + a and from H t o O ( 1 HI = 2000Oe) causes the corresponding change in magnetization from I, to I, and from I, to I, which results in the effects 6, in the formed and 6, in the latter case. *' defines the even in magnetizaThe difference 2 + defines the tion MOE 60, while the sum --2 odd EKE-6. Figure 1 represents the results of the measurements of the MOE frequency dependence 6b",mn1(co) carried out FIG. 1. Orientation magneto-optical effect for single crystal of Ni ;me-H parallel to [I 111, AAA-H parallel to , 0 0 H parallel to [loo] ; light incidence angle y, = 800. on a single crystal of Ni being magnetized along different crystallographical directions [l, m, n]. The positive sign of MOE corresponds to the increase of the reflected light intensity, while the sample is being magnetized. The values of the effect given in the picture are normalized according to  in order that the values of MOE should corresponds to the transition Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711381 C 1 1062 G. S. KRINCHIK, E. A. GANSHINA, V. S. GUSHCHIN of the sample from the quite demagnetized state to that of magnetization saturation. The curves of MOE dependence on the angle $ between the direction of magnetization field and the crystallographical axis , given in figure 2, display FIG. 2. Angular variation of the MOE (solid curves) and EKE (dashed curves) in the (110) symmetry plane at following energies : 44-0.35 eV, 00-0.69 eV, 00-0.89 eV light incidence angle y, = 80°. strong anisotropy of this effect where as EKE is quite isotropic within the limits of the error the measurement experiment. The anisotropy of magneto-optical effects in ferromagnetic metals was discussed by Donovan and Medcalf . Having made the calculation according to the scheme of Argyres , taking into account the terms of the second order in spin-orbit interaction, these authors showed that in cubic ferromagnetic metals, as well as in cubic semiconductors with anisotropic Fermi-surface, the phenomenon of anisotropy of magneto-optical effect is possible because of the effects of the second order. In paper  discussing the anisotropy of quadratic magneto-optical effects for Voigt configuration (E I I) in the plane (100). The observed MOE is shown to correspond to the Voigt configuration for the case of the crystallographical plane (110). Having made the calculations analogous to  for the plane (110), we obtain the angle dependence dielectric permeability tensor components A E!;) quadratic in magnetization : 8%' = 8%) = 3 sin 2 $ sin cp(cos 2 $ + cos2 $) d2) where $ the angle between the direction of the magnetic field H and the axis  ; cp the angle of light incidence. It follows, from the received expressions, that when $ = 0 i. e. for the case when the crystal is magnetized along the x axes [OOl], all 8:;' components become zero, and consequently, the linear double refracting in Voigt configuration is absent. This conclusion is quite contradictory to the above experimental results, as well as the analogous measurements on the single crystal of silicon iron  and consequently, the observed even in magnetization change of reflected light intensity (MOE), comparable in magnitude with the usual EKE, is not reduced to the Voigt effect in cubic crystals obtained by the calculations taking into account the terms of the second order in spin-orbit interaction . Besides strong anisotropy the orientation effect differs from the equatorial effect by a peculiar dependence on the wavelength which is evidently, connected with the orientation effect << localization >> in the vicinity of those points and lines of Brillouin zone where the energy band degeneration is removed by the spinorbit interaction. In this respect the obtained experimental results are in qualitative agreement with the electron structure model of Ni with the reverse order of levels [4, 71. In papers Falicov and Ruvalds  and Zornberg  a detailed analysis of the problem of spin-orbit removing degeneration occurring at the intersection of oppositely directed band spins is given. Figure 1 in paper  reproduced the scheme of such removing of band degeneration L32, t and L328 J. due to the change of magnetization vector orientation I. Let us consider the following identification of the peculiarities of So, curves with the interband transitions B, E, F and G. The broad maximum on the curves 6Cd,0O1 and Sbt lO1 in the region of 0.7-0.9 eV (Fig. 1) may be connected with the transition F, while the maximum 6bt111 in the interval of 0.9-1.2 eV with the transition E (MOE of the second type ). Using the additional data on these maxima, obtained from FIG. 3. Orientation magneto-optical effect and equatorial Kerr effect for single crystal of Ni in the visible region of the spectrum. MOE : 00-H // [Ill], Ad-H // , 00-H// [I001 ; XX-EKE ; H // [Ill] light incidence angle q = 70°. ANISOTROPY OF MAGNETO-OPTICAL ORIENTATION EFFECT IN SINGLE CRYSTAL C 1 1063 the low temperature measurements  receive that haF = 0.76 eV and ho, = 1.1 eV. The maxima in the region tio s 0.3 eV on the curves 60, may be connected with the transitions B and G. To eliminate ambiguity, let us assume ho, = 0.35 eV (maximum on the curve 6b$111) and ho, = 0.3 eV (maxima on the curves 6b:001 and 66t101). The transitions C and D should be less influenced by the spin-orbit interaction, though it is possible, that the low energy maximum on the curve 6b;lt1 is due to C transitions i. e. ho, = 0.2 eV. Applying the set VIT of band parameters from , obtain the following values for the above interband transitions : L o B E 0.4 eV, tiw, E 0.25 eV, fro, E 0.7 eV, ho, z 1.0 eV. Figure 3 shows the results of MOE frequency dependence measurements for three main crystallographical directions in the visible region of the spectrum (2.0-3.25 eV) where, according to the magnetooptical observations  the anomaly connected with the interband transitions X, f + Xk f in the vicinity of point X occurs. The measurements of the EKE in this energy interval have shown that, within the exactness of the measurements, this effect is isotropic (one curve 6,(co) for the axis [ I l l ] is given in the figure 3 for the sake of illustration), while MOE in contrast to EKE displays strong anisotropy. It is most probable, that such a behavior of MOE in the vicinity of point X, is connected with the anisotropic spin-orbit splitting of X, band due to the magnetization vector rotation. It is also worthwhile to note, that the above identification of the transition X5 T. + X i f (hw = 2.4 eV) is in a considerably better agreement with band calculations according to the model with the reverse order of levels in point L [7, 91.