Pressure-induced first-order magnetic phase transition and Yb ordered moment in ${\mathrm{YbCu}}_2{\mathrm{Si}}_2$

We report on high-pressure x-ray absorption near edge structure and x-ray magnetic circular dichroism (XMCD) measurements at the Yb $L_3$ edge on ${\mathrm{YbCu}}_2{\mathrm{Si}}_2$. This intermetallic is a well-known intermediate valence compound which shows a pressure-induced paramagnetic-ferromagnetic phase transition at about 8 GPa. The pressure dependence of the Yb magnetization has been determined by following the evolution with pressure of the maximum of the dichroic signal at the energy corresponding to the electric quadrupole ($2p_{3/2}-4f$) transitions. The observed XMCD deduced magnetization curves confirm the ferromagnetic nature of the high-pressure phase as well as the first-order nature of the transition. The Yb ordered moment deduced from XMCD which amounts to about $1.25{\mu }_B$ at 8.7 GPa and 2.7 K is in very good agreement with the one estimated by Mössbauer spectroscopy for the magnetically ordered component of the spectra. This observation indicates that the paramagnetic and ferromagnetic phases coexist only in a narrow pressure range in our XMCD experiment. This is not the case for the Mössbauer measurements, which show that the paramagnetic component vanishes only at 20 GPa. Our results clarify the existing discrepancy with dc magnetization measurements that reported values of the Yb moment more than two times smaller. We propose that these apparent contradictory results are due to different experimental conditions which influence the range of coexistence of the paramagnetic and ferromagnetic phases, as found in many similar situations and frequently addressed in the literature.

Figure 1

(a) Yb $L_3$ edge XANES spectra at 2.7 K recorded at different pressures. The $L_3$ absorption edge consists of contributions due to electric dipolar transitions of both ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Yb}}^{2+}$ and together with the one due to the electric quadrupolar transition of ${\mathrm{Yb}}^{3+}$. The inset shows the pressure dependence of XANES spectra [9] recorded in the partial fluorescence mode with improved resolution (see text). (b) Difference of XANES spectra at 2.7 K recorded at 0 and 12 GPa. Deconvolution of the experimental Yb $L_3$ edge XANES spectrum at 0 GPa into a sum of model spectra for ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Yb}}^{2+}$ is presented in the inset. (c) XMCD spectra recorded at 2.7 K at different pressures and applied magnetic fields along the $c$ axis.

Figure 2

Yb XMCD magnetization at 2.7 K (left axis, circles) recorded at the maximum of the dichroic spectra due to the electric quadrupolar transition (8.9385 keV) with the magnetic field applied along the $c$ axis together with the macroscopic magnetization curve at 2 K (right axis, broken black line) with $H$ parallel to the $c$ axis [10].

Figure 3

XANES and XMCD spectra recorded at the Cu $K$ edge at 2.7 K at different pressures and magnetic fields along the $c$ axis. The XMCD signal has been multiplied by a factor of 40.

Figure 4

Yb XMCD magnetization curves measured at 2.7 K and different pressures. The saturation tendency at high pressures indicates the FM nature of the magnetically ordered phase above $P_c$. The rough data have been corrected to take into account the demagnetization field. This allows a direct comparison with the dc magnetization data by Tateiwa et al. [17].

Figure 5

(a) Yb magnetization at 2.7 K (solid red circles) together with dc magnetization [17] at 2 K (open black squares) as a function of the pressure under 10 kOe magnetic field applied along the $c$ axis. (b) Left scale: Yb initial magnetic susceptibility (solid green circles) as a function of the pressure extracted from Fig. 4. Right scale: Yb valence-pressure dependence (open black triangles) at 7 K taken from Ref. [9]. The dashed lines are guides to the eyes.