Anisotropic uniaxial pressure response of the Mott insulator Ca${}_2$RuO${}_4$

We have investigated the in-plane uniaxial pressure effect on the antiferromagnetic Mott insulator Ca${}_2$RuO${}_4$ from resistivity and magnetization measurements. We succeeded in inducing the ferromagnetic metallic phase at lower critical pressure than by hydrostatic pressure, indicating that the flattening distortion of the RuO${}_6$ octahedra is more easily released under in-plane uniaxial pressure. We also found a striking in-plane anisotropy in the pressure responses of various magnetic phases: although the magnetization increases monotonically with pressure diagonal to the orthorhombic principal axes, the magnetization exhibits peculiar dependence on pressure along the in-plane orthorhombic principal axes. This peculiar dependence can be explained by a qualitative difference between the uniaxial pressure effects along the orthorhombic $a$ and $b$ axes, as well as by the presence of twin domain structures.

Figure 1

Temperature dependence of the resistance $R_{\left[010\right]\mathrm{T}}$ of Ca${}_2$RuO${}_4$ under $P$${}_{\parallel \left[100\right]\mathrm{T}}$ measured in a cooling process. The inset represents the temperature dependence of $R_{\left[010\right]\mathrm{T}}$ normalized by that at 20 K. The decrease of resistance below 12 K indicates an emergence of a FM order.

Figure 2

Temperature dependence of the resistance $R_{\left[010\right]\mathrm{T}}$ of Ca${}_2$RuO${}_4$ at $P$${}_{\parallel \left[100\right]\mathrm{T}}$ of 0.4 GPa under magnetic field $H$${}_{\parallel \left[100\right]\mathrm{T}}$. The sample is different from that of Fig. 1. All the resistivity data were taken with the field-cooling process, but reproducible under different cooling processes. We have checked that $R\left(T\right)$ does not differ by cooling processes. The top inset displays the difference of resistance between 0 and 6 T. The peak indicates $T_{\text{FM}}\sim 12$ K. The bottom inset displays the magnetic field $H$${}_{\parallel \left[100\right]\mathrm{T}}$ dependence of the magnetization $M$${}_{\parallel \left[100\right]\mathrm{T}}$ at 2 K under $P$${}_{\parallel \left[100\right]\mathrm{T}}$ of 0.4 GPa. It exhibits a typical hysteresis corresponding to a FM ordering.

Figure 3

Development of the magnetization $M$ measured with a field of 10 mT parallel to the pressure for (a) $P$${}_{\parallel \left[100\right]\mathrm{T}}$, warming after zero field cooling (ZFC), (b) $P$${}_{\parallel \left[110\right]\mathrm{T}}$, warming after ZFC, (c) $P$${}_{\parallel \left[100\right]\mathrm{T}}$, field cooling (FC), and (d) $P$${}_{\parallel \left[110\right]\mathrm{T}}$, FC. The insets represent the pressure dependence of $\Delta M=M\left(2\mathrm{K}\right)-M\left(30\mathrm{K}\right)$ in FC for (e) $P$${}_{\parallel \left[100\right]\mathrm{T}}$ and (f) $P$${}_{\parallel \left[110\right]\mathrm{T}}$. The units of the vertical and horizontal axes are 10${}^{-3}$ emu/mm${}^3$ and GPa, respectively.

Figure 4

Temperature dependence of the magnetization measured with a field of 5 T parallel to the pressure in the field-cooling process for (a) $P$${}_{\parallel \left[100\right]\mathrm{T}}$ and (b) $P$${}_{\parallel \left[110\right]\mathrm{T}}$. The closed arrows around 115 K and 140 K indicate the peak and shoulder structures, corresponding to the A-centered and B-centered AFM transitions, respectively. The open arrows present the onset of the AFM orders.

Figure 5

Pressure dependence of $T_{\mathrm{FM}}$ and $T_{\mathrm{AFM}}$ for (a) $P$${}_{\parallel \left[100\right]\mathrm{T}}$ and (b) $P$${}_{\parallel \left[110\right]\mathrm{T}}$. Diamonds and reversed triangles denote $T_{\mathrm{FM}}$ determined from magnetoresistance and magnetization, respectively. Triangles, open circles, and closed circles indicate $T_{\mathrm{AFM}}^{\mathrm{onset}}$, $T_{\text{A-AFM}}$, and $T_{\text{B-AFM}}$, respectively. The insets schematically describe the RuO${}_2$ plane with the uniaxial pressure. The broken lines denote the tilting axes. Because of the tilting, half of the O ions described with the open circles are located above the undistorted RuO${}_2$ plane, while the others with the closed circles below the plane. Dotted lines indicate a unit cell.