Separation of magnetic properties at uranium and cobalt sites in UCoAl using soft x-ray magnetic circular dichroism

Temperature ($T$) and magnetic field ($H$) dependence of the magnetic properties in metamagnetic UCoAl have been investigated using a soft x-ray magnetic circular dichroism (XMCD). In order to extract element-specific magnetic properties at the U and Co sites, the XMCD experiment has been performed at the U 4$d$-5$f$ ($N_4,5$) and Co 2$p$-3$d$ ($L_2,3$) absorption edges, respectively. Directions of magnetic moments at the U and Co sites have been determined from shapes of the XMCD spectra. The directions of the total magnetic moments at the U and Co sites are parallel to the $H$ direction ($c$ axis), but the direction of the spin magnetic moment at the U site is opposite to that at the Co site. The XMCD intensities at both the U and Co sites at $T=5.5$ K increase steeply at $H=0.77$ T ($H_{\mathrm{m}}$), corresponding to the metamagnetic transition. The XMCD intensities do not saturate, even in the field-induced ferromagnetic state above $H_{\mathrm{m}}$. In addition, the ratio of the increase of the XMCD intensity at the Co site is smaller than that at the U site. From comparison of the $H$ dependence of the XMCD intensities at $T=25$ and 5.5 K, we found that the magnetic behavior of the Co atom has a stronger $T$ dependence than that of the U atom.

Figure 1

The XAS and XMCD spectra of UCoAl at the U $N_4,5$ and the Co $L_2,3$ edges at $T=5.5$ K and $H=7$ T. The U $N_4$ edge overlaps with the Co $L_3$ edge. (a) The ${\mu }^{+}$ (${\mu }^{-}$) refers to the absorption spectrum for the photon helicity parallel (antiparallel) to the magnetization direction. The XAS spectrum is defined as ${\mu }^{+}+{\mu }^{-}$. Inset: The magnified figure of the ${\mu }^{+}$ and ${\mu }^{-}$ absorption spectra around $h\nu =778$ eV. (b) The XMCD spectrum defined as ${\mu }^{+}-{\mu }^{-}$. These spectra are normalized so that the intensity of the XAS (${\mu }^{+}+{\mu }^{-}$) spectrum at the U $N_5$ edge ($h\nu =736.6$ eV) becomes 1.

Figure 2

The $H$ dependence of the XMCD spectra of UCoAl at the U $N_4,5$ and Co $N_2,3$ edges. These spectra are normalized as in Fig. 1. (a) At $T=5.5$ K. Inset: The magnified figure of the XMCD spectra at the U $N_5$ edge. These spectra at each magnetic field are normalized so that the negative peak of the XMCD spectra at $h\nu =735.0$ eV becomes $-1$. (b) At $T=25$ K.

Figure 3

Decision of the direction of the magnetic moments at the U and Co sites. (a) The expanded plot of the XMCD spectrum around the U $N_4$ and Co $L_3$ edges. The red dashed line shows the deduced XMCD spectrum at the U $N_4$ edge. See text for details. (b) A schematic picture of the directions of the magnetic moments (total magnetic moment $M_{\mathrm{total}}$, spin magnetic moment $M_{\mathrm{S}}$, and orbital magnetic moment $M_{\mathrm{L}}$) deduced from the shape of the XMCD spectrum at the U $N_4,5$ and Co $L_2,3$ edges. The size of the arrows is drawn schematically because the magnitude of magnetic moments cannot be estimated quantitatively. However, the U 5$f$ moments are drawn in accordance with the result from the XMCD experiment at the U $M_4,5$ edge[20] so that the ratio $-M_{\mathrm{L}}/M_{\mathrm{S}}$ of the U 5$f$ electrons at $H=7$ T and $T=10$ K is estimated to be 1.95 in the case of the U 5$f^3$ configuration.

Figure 4

The $H$ dependence of the XMCD intensities (M-H curve) at the U $N_5$ ($h\nu =735.0$ eV) and Co $L_2$ ($h\nu =794.5$ eV) edges at $T=5.5$ and 25 K. The dashed straight lines are obtained by fitting the M-H curves between $H=1$ and 7 T. These straight lines are guides to the eye. Inset: M-H curves taken at $h\nu =715$ eV, where there is no absorption edge, indicating the precision of the M-H curve measurements.