Antiferromagnetic-ferromagnetic phase transition in FeRh probed by x-ray magnetic circular dichroism

The phase transition from antiferromagnetic to ferromagnetic ordering in FeRh is investigated in an element specific way by means of x-ray absorption spectroscopy. Dichroism sum rules allow us to determine spin and orbital moments of the two elements as the system is driven through the phase transition. Increasing the temperature from 300 to 450 K, the magnetic moments in Fe and Rh both evolve from zero to their final value, while the ratio of Rh to Fe moments stays constant. Our experimental findings are supported by the results of fully relativistic ab initio calculations.

Figure 1

(Color online) VSM measurement of the saturated magnetic moment as a function of temperature is shown in (a). $●$ and $○$ denote measurements while increasing (decreasing) the temperature, respectively (see the arrows). The sample cross section is sketched in (b).

Figure 2

(Color online) Comparison of measured XAS (a) and XMCD (b) of FeRh (black line, $T=420\text{K}$) and pure Fe (bright red line, $T=300\text{K}$). The absorption spectra in (a) are normalized to an edge jump equal to 1. The dichroism in (b) is normalized to $-1$ at the $\text{Fe}{L}_3$ peak. The geometry of the transmission measurement is drawn in the inset.

Figure 3

(Color online) Calculated XAS and XMCD spectra (lines) of FeRh are compared to the measured ones (dots). In (a) and (c), the isotropic spectra $\left(I_{+}+I_{-}\right)/2$ are drawn for the Fe and Rh absorption edges, respectively. Dichroism $\left(I_{+}-I_{-}\right)$ at the Fe edge is plotted in (b) and the Rh edge in (d).

Figure 4

(Color online) Magnetic hysteresis loops of the Fe and Rh components, measured at the $\text{Fe}{L}_3$ and $\text{Rh}{M}_3$ edges, are plotted in (a) and (b), respectively. The transmitted x-ray intensity was measured as a function of the applied field, normalized by the incident flux, and the resulting curves were centered on zero by subtracting the mean intensity.

Figure 5

(Color online) XMCD of the FeRh film at fixed temperatures between 300 and 450 K. Measured data at the $\text{Fe}L$ and $\text{Rh}M$ absorption edges are displayed in (a) and (b), respectively. A magnetic field of $\pm 0.5\text{T}$ was applied along the x-ray propagation direction, which enclosed an angle of $30°$ with the sample surface normal. Due to the low absorption cross section of the $\text{Rh}M$ edges, two consecutive Rh spectra were averaged.

Figure 6

(Color online) XMCD sum rule analysis of the FeRh spectra in Fig. 5. In (a), the spin ($m_S$, squares) and orbital ($m_L$, circles) magnetic moments of Fe (filled symbols) and Rh (open symbols) are plotted as a function of temperature. In (b), the ratio of the Rh to Fe spin moments is plotted as circles and the orbital to spin moment ratio for Fe (Rh) is plotted as filled (open) squares, respectively. The lines reflect their average value.