Observation of a metamagnetic phase transition in an itinerant $4f$ system via the magneto-optic Kerr effect: ${\mathrm{C}\mathrm{e}(\mathrm{F}\mathrm{e}}_{1-x}{\mathrm{Co}}_x{)}_2$

The optical and magneto-optical properties of single crystals of ${\mathrm{YFe}}_2,$ ${\mathrm{CeFe}}_2,$ and $\mathrm{Ce}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2$ $(x\simeq 0.1)$ were measured between 1.4 and 5.0 eV using a rotating-analyzer ellipsometer and a normal-incidence polar Kerr spectrometer. The electronic structures and optical properties of ${\mathrm{YFe}}_2$ and ${\mathrm{CeFe}}_2$ were calculated using the tight-binding linear muffin-tin orbital method in the atomic-sphere approximation. For ${\mathrm{YFe}}_2$ the calculations reproduce the experimental spectra. Alloying ${\mathrm{CeFe}}_2$ with small amounts of Co leads to an electronic instability which is evidenced by a low-temperature antiferromagnetic (AF) phase. The Kerr effect was measured in the AF and field-induced ferromagnetic (FM) regime. A remarkably large Kerr rotation was measured even in the AF state (up to $-1°).\mathrm{}$ The phase transition from AF to FM order was observed in the Kerr rotation spectra at 5 and 50 K at photon energies of 1.8 and 4.0 eV.