Pressure-induced structural phase transitions in UIr

Strain and resistivity measurements in UIr showed the occurrence of two structural phase transitions under high pressure, indicating the existence of three phases I–III, with slightly different crystal structures. It is suggested that ferromagnetic phases FM1–FM3 emerge in phases I–III, respectively. The pressure-temperature phase diagram was sensitive to the hydrostaticity of pressure. FM3 and superconductivity are observed in the measurements in which Daphne 7373 is used as a pressure-transmitting medium; however they are both absent in the case where petroleum ether is used as a pressure-transmitting medium. These results indicate a close relationship between ferromagnetism and superconductivity.

Figure 1

Crystal structure of UIr. The crystal does not possess the inversion center in any direction. The easy axis of the magnetic moment is along the [1 0 $\overline{1}$] direction.

Figure 2

Pressure dependence of resistance along [0 1 0] and that of $\Delta b/b$, at room temperature. First-order transitions with clear hysteresis were observed in both measurements. We define these phases as I–III.

Figure 3

Pressure dependences of the lattice constants and the angle $\beta$ between the $a$ and $c$ axes at room temperature. Each lattice constant and $\beta$ changed without any anomaly at around 2.0–2.2 GPa within the experimental accuracy. The compressibility along the $b$ axis is roughly twice those of other axes.

Figure 4

Temperature dependence of $\rho$ along [0 1 0] using (a) petroleum ether and (b) Daphne 7373. The results on cooling are plotted. The inset shows the hysteresis between $T_S^c$ and $T_S^h$, which denote the midpoints in the jumps on cooling and heating, respectively.

Figure 5

Pressure-temperature phase diagrams and pressure dependences of ${\rho }_0$. (a) In the case of the measurement using petroleum ether, the phase transition from III to II occurs with decreasing temperature, and both FM3 and the superconducting phases simultaneously disappear. (b) In the case of the measurement using Daphne 7373, phase III survives even at low temperatures, and the transition to FM3 and the emergence of superconductivity are observed at around 2.7 GPa. The boundary between phases I and II is $\sim$2.0 GPa at room temperature. The pressure dependence of ${\rho }_0$ suggests that the boundary between phases I and II is vertical against pressure.

Figure 6

Temperature dependence of $\rho$ for different media at nearly the same pressure of 2.2–2.3 GPa. In the case of Daphne 7373, ${\rho }_0$ is low and a kink is observed at $T_{C3}$; however, the $\rho$ for petroleum ether shows no anomaly.

Figure 7

Temperature dependence of $\rho$ below 0.4 K for $J\parallel$ [0 1 0] using petroleum ether and Daphne 7373. In the case of petroleum ether, ${\rho }_0$ is high and superconductivity is absent. In the case of Daphne 7373, ${\rho }_0$ decreases in this pressure range and superconductivity appears. Zero resistance is clearly observed in the case of $J\parallel$ [1 0 $\overline{1}$].[2, 3, 11]

Figure 8

Temperature dependence of ac susceptibility. The diamagnetic signal can be observed clearly, but the volume fraction is $\sim$19%.