Magneto-Optical Spectroscopy of Nanocomposites (CoFeB)_x(LiNbO₃)_100-x with Concentrations up to the Percolation Threshold: From Superparamagnetism and Superferromagnetism to Ferromagnetism

Nanocomposites (CoFeB)(x)(LiNbO3)(100 -)(x) with x = 17-48 at % have been synthesized by ion beam sputtering of a composite target comprised of Co40Fe40B20 and LiNbO3 onto silicon substrates, and the transitions from the superparamagnetic state to the superferromagnetic and ferromagnetic states with an increase in the concentration of the magnetic component are studied by magneto-optical methods. The magneto-optical properties have been investigated in the geometry of the equatorial (transverse) Kerr effect (TKE). Magneto-optical spectra are recorded in the range of 0.5-4.0 eV in fields up to 2.5 kOe at 20-300 K, field and temperature dependences of the TKE at certain wavelengths are obtained, and the domain structure during magnetization reversal is visualized using a magneto-optical Kerr microscope. It is shown that the sample with x = 17 at % is superparamagnetic at temperatures above the blocking temperature (about 30 K). The interaction between the granules is considerable already at x = 20 at %, the transition to the superferromagnetic state occurs at x approximate to 32-36 at %, and the transition to the ferromagnetic state occurs at x approximate to 44 at % near the metal-dielectric transition, i.e., at a concentration below the percolation transport threshold.