Experimental study of the photoinduction mechanism in silicon-doped yttrium iron garnet by means of "first-order" circular dichroism

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Abstract
It has recently been proposed that the photosensitive impurity centers in silicon-doped yttrium iron garnet, which are the active sites in the processes giving rise to photoinduced dichroism, should also exhibit magnetic circular dichroism (MCD). This property of the photomagnetic centers was labeled "first order" in opposition to the photoinduced effects, which are second order in the difference of absorption cross section between inequivalent centers. We have observed and measured first-order circular dichroism in YIG(Si) in the wavelength range 1.00-1.60 μm at 300, 77, and 4.2°K. Experimental evidence of the connection between the concentration of photosensitive impurity centers and the magnitude of the circular dichroism has been found. The anisotropy of these centers has been found not to change with dopant concentration in the range 0.006-0.045 atoms/(formula unit). As an application, MCD spectra provide a convenient nondestructive method for determining relative amounts of active centers. Using MCD, we have measured a dimensionless parameter of the photoinducing cross section at room temperature. Finally, the photoinduction mechanism has been found to be weakly dependent on temperature, thus confirming the previous theoretical result that the steep temperature dependence of photoinduced effects is to be ascribed mainly to relaxation mechanisms.