Magnetic Circular Dichroism in X-Ray Emission from Ferromagnets

The existence of novel magnetic circular dichroism in core-level x-ray emission is reported. By means of circular polarization analysis, the dichroic effect of the Fe ${K\alpha }_1$ emission spectrum is measured on an Fe single crystal. The observed dichroic effect (12%) is remarkably large, if one takes into account the small dichroic effect (about 0.5%) in the conventional $K$-edge absorption spectroscopy of $3d$ transition metal elements. The mechanism is ascribed to exchange splitting of the $2p$ level possessing large spin-orbit coupling. This new magnetooptical effect enables us to explore a variety of new research subjects in the magnetism of $3d$ transition metals and their compounds by fully utilizing its large dichroic effect, the true bulk sensitivity of hard x rays, and the element selectivity of core-level spectroscopy.

Figure 1

(a) Energy-level diagram showing the origin of circular dichroism in x-ray emission. For clarity, two transitions are indicated among four ${K\alpha }_1$ transitions. The magnetic quantum number is shown in $2p_{3/2}$ and $1s$ sublevels. $\mathrm{\Delta }m$ is the change in magnetic quantum number at the transition.

Figure 2

Top view of the experimental layout. QWP: A diamond phase retarder that acts as a quarter-wave plate. Analyzer: A Ge(400) single crystal that is both an energy and polarization analyzer.

Figure 3

(a) Fe $K{\alpha }_1$ emission spectra $I^{+}$ and $I^{-}$ for a phase shift of $+\pi /2$ (red open circles) and $-\pi /2$ (blue solid circles), respectively. The lines connect the data points to guide the eye. The magnetic field is applied as shown in the inset and the direction is defined as positive. (b) Difference spectra between $I^{+}$ and $I^{-}$ normalized by the peak intensity with two antiparallel directions of magnetic field. Magenta solid circles and green open circles are measured when the magnetic field is directed along the positive and negative direction, respectively. Solid lines are a guide to the eye. Note that the ordinate is not exactly the degree of circular polarization.

Figure 4

Calculated Fe $K{\alpha }_1$ emission spectra. The red (blue) line corresponds to the sum of intensities for the right (left) circularly polarized x rays and one-half of the linearly polarized x rays. Dichroism is obvious in the theoretical calculations.