Magneto-Optical Spectroscopy of (CoFeB)x-(Al-O)100-x Nanocomposites: Evidence of Superferromagnetism

We present results of magneto-optical investigations of (CoFeB)
_x
-(Al-O)
_100-x
film nanocomposites in transverse Kerr effect (TKE) geometry in the spectral range 0.5-4.0 eV and magnetic field up to 3.1 kOe. Nanocomposites were deposited onto a glass-ceramic substrate by ion-beam sputtering.According to the data obtained from resistivity measurements,the percolation threshold is about x
_per
≈ 57 at.%,and the critical concentration for metal-insulator transition is about x
_c
≈ 48 at.%. The TKE response at room temperature strongly depends on the wavelength of light, applied magnetic field H, and the volume metallic fraction. The TKE field dependence is almost linear to H = 3.1 kOe and identical for 1.57 and 3.06 eV at x 30 at.%. It means that at x 30 at.%, nanocomposites consist of only superparamagnetic granules. With increasing concentration x, the TKE field dependence at 3.06 eV becomes more and more similar to the field dependence of ferromagnetic material,but it is not the case at 1.57 eV.For example,the TKE signal for x = 43 at.% measured at 3.06 eV saturates at H = 100 Oe, which indicates the presence of large ferromagnetic regions below xc. For the same composition, the TKE field dependence measured at 1.57 eV does not saturate and resembles more a superparamagnetic response. The same difference is observed also at x= 49 at.%.The presence of large ferromagnetic regions in completely insulating samples together with separate superparamagnetic granules can be considered as an evidence of superferromagnetic ordering in the vicinity of xc.