Size effects in the magneto-optical response of Co nanoparticles

A study of the magneto-optical (MO) spectral response of Co nanoparticles embedded in MgO as a function of their size and concentration in the spectral range from 1.4 to 4.3 eV is presented. The nanoparticle layers were obtained by sputtering at different deposition temperatures. Transmission electron microscopy measurements show that the nanoparticles have a complex structure which consists of a crystalline core having a hexagonal close-packed structure and an amorphous crust. Using an effective-medium approximation we have obtained the MO constants of the Co nanoparticles. These MO constants are different from those of continuous Co layers and depend on the size of the crystalline core. We associate these changes with the size effect of the intraband contribution to the MO constants, related to a reduction of the relaxation time of the electrons into the nanoparticles.

Figure 1

EFTEM (left) and BFTEM (right) plan-view micrographs of the samples grown at 200 (a), 400 (b), 550 (c), and 700 °C (d). EFTEM images are obtained from the $M_{2,3}$ line of Co, revealing all the Co present in the sample, while BFTEM images display only the crystalline Co.

Figure 2

(a) Concentration of Co nanoparticles and (b) nanoparticle mean diameter measured by EFTEM and BFTEM as a function of the Co deposition temperature. (c) Sketch of the sample structure.

Figure 3

Experimental MO constants of the different layers consisting of Co nanoparticles deposited at 200, 400, 550, and 700 °C and embedded in MgO.

Figure 4

Experimental (symbols) and fits for the MO constants corresponding to the nanoparticle layers of samples grown at 200 (continuous line), 400 (dashed line), and 550 °C (dotted line) calculated by means of (a) the Bruggeman and (b) the Maxwell-Garnett approximation.

Figure 5

MO constants of the Co that is part of the different nanoparticle thin layers deposited at 200, 400, 550, and 700 °C, together with the MO constants of a 420-Å-thick cobalt sample grown at RT (divided by 2). The continuous, dotted, and dashed lines correspond to the intraband contribution obtained using the Drude model and with $\tau =0.632$, 0.32, and 0.25 eV.