Nano-sized magnetic particles in non-magnetic matrices attract considerable attention because of their fundamental and applied importance. Here the results are presented of the investigation of the magneto-optical effects in potassium-alumino-borate glasses containing several mass percents of Fe₂O₃ and MnO. The glass preparation was presented in [1]. In spite of a low paramagnetic elements concentration, magnetic nano-particles were formed in the glasses as a result of an additional thermal treatment. So the samples undergoing the treatment could be considered as composites. The particles size, structure and composition were studied with the help of x-ray diffraction and Mössbauer spectroscopy, and the correlation was obtained between the additional thermal treatment conditions and the particle characteristics. Particles of the maximal average dimension of about 250 Å had crystal structure close to the structure of manganese ferrite. The magneto-optical Faraday rotation (FR) dependencies of the composite samples on the light wavelength, on the external magnetic field and on temperature as well as their magnetic moment field and temperature dependencies were due to the particle dimensions. But for all samples investigated they were typical for substances possessing a magnetic order, i.e., the non-linear FR and magnetisation field dependencies were observed with hysteresis and magnetic saturation. The FR value exceeded the paramagnetic FR for the same paramagnetic ion concentration more than two orders of value. Such kind of magnetic properties of the glasses in combination with their transparency in the part of the visible and near IR spectral ranges allowed to consider them as a candidate for application in different magneto-optical devices. On the other hand, these glasses are of interest from the fundamental point of view because of several size effects revealed in the particles ensembles. First of all, the character of the FR and the magnetisation field dependencies were different for samples with different average particle dimensions. It changed from the almost rectangular hysteresis loop for the particles average diameter of about 250 Å to the Langevene type curves for the smallest particles. In the first case the large remnant magnetisation and FR were observed, i.e. the glasses were transparent permanent magnets, what could be interesting for applications too. Further, the strong magnetisation and FR increase were observed when the temperature decrease. This increase was in some cases more than twice larger in comparison with the temperature magnetisation increase of the manganese ferrite, and its value depended on the particle size: the smaller were particles the higher was the temperature increase. The large high field magnetic susceptibility which value depended also on the particles size was the next peculiarity of the glasses considered. And, at last, the relation of the FR value to the magnetization depended on the particles size too. All these peculiarities were associated with the inhomogeneities of the spin directions near the particles surface, which contribution became higher for the particles of lower dimensions.

The highest magneto-optical figure of merit observed in the samples investigated for the wave length = 1.5 m important for novel practical application was equal to 12.5 grad in the magnetic field 1.0 kOe. The highest FR in zero magnetic field at = 1.5 m was equal to 5.3 grad * cm^-1. I.Edelman, R.Ivantsov, A.Vasiliev, S.Stepanov, E.Kornilova, T.Zarubina Physica B: condenced matter 301(2001) 203-211.