Metastability of the Topological Magnetic Orders in the Chiral Antiferromagnet EuPtSi

TL;DR

This work demonstrates that EuPtSi hosts multiple metastable topological magnetic orders whose stability strongly depends on field orientation: a skyrmion-lattice A-phase for $H \parallel [111]$ persists down to $T<0.1$ K via field cooling, while the conical phase for $H \parallel [100]$ hosts A'- and B-phases that exhibit pronounced metastability and hysteresis. The authors map these states using resistivity and topological Hall effect measurements, revealing a highly anisotropic $H$–$T$ phase diagram and confirming a topological origin for the observed phases through a finite topological Hall signal. Angular dependence shows the spin textures are stabilized by crystallographic direction, highlighting the interplay between topology, quantum effects, and lattice symmetry. Overall, EuPtSi emerges as a rich platform to study metastable quantum skyrmion states in a centrosymmetric, antiferromagnetic system with strong thermal and quantum fluctuations.

Abstract

We report resistivity and Hall effect measurements in the chiral antiferromagnet EuPtSi. Depending on the magnetic field orientation with respect to the crystallographic axes, EuPtSi presents different topological magnetic phases below the Néel temperature $T_N=4.05$K. In particular, for a field $H \parallel $ [111], it exhibits the well known skyrmion lattice A-phase inside the conical phase between $T=0.45$K and $T_N$ in the field range from 0.8T to 1.4T. Remarkably, the skyrmion lattice state in EuPtSi, composed of nanoscale skyrmions, can be extended down to very low temperature (lower than 0.1K) through field-cooling regardless of the cooling rate and of the magnetic history. Similarly the metastability of the A'- and B-phases ($H \parallel $ [100]) at low temperature is evidenced by our measurements. These results suggest that EuPtSi is a peculiar example where the competition between the topological stability and the thermal agitation can lead to metastable quantum skyrmion state.

Figures (8)

• Figure 1: (a) Field dependence of $\rho$ at various temperatures above 0.175 K, for increasing magnetic field sweeps between 0 T and 3 T along the [111] direction with the current $J \parallel [\Bar{1}\Bar{1}2]$. The 0.175 K curve is shifted vertically by $-0.5~\mu\Omega\cdot$cm for clarity. The magnetic transitions are indicated by colored triangles on the 1 K data. (b) Comparison between increasing (full lines) and decreasing (dashed lines) $\rho(H)$ field sweeps for $T = 2$ and 0.6 K. The arrow indicates the jump $\Delta\rho$ at 0.6 K. (c) The amplitude of the jump $\Delta\rho$ in the resistivity as a function of the temperature. (d) Magnetic phase diagram obtained for the [111] direction by resistivity measurements. Full squares (open triangles) are transition fields from increasing (decreasing) field sweeps. Circles denote transitions obtained by increasing temperature sweeps of $\rho$. The two open squares correspond to an extrapolation at 0.22 K of the phase boundaries / respective critical field lines of the thermodynamic equilibrium A-phase ($H_{A1}$ and $H_{A2}$), see text about the metastable SkL regime and also Fig. \ref{['Fig2']} (b).
• Figure 2: (a) Temperature dependence $\rho(T)$ for various fixed magnetic fields along the [111] and current $J \parallel [\Bar{1}\Bar{1}2]$ after initial zero field cooling (ZFC). Solid lines (dashed lines) denote increasing (ZFC) (decreasing, (field cooled (FC)) temperature sweeps between 0.25 K and 4.5 K. Arrows indicate the variation of the temperature for the 1.37 T curve. The blue triangles indicate the entering into the A-phase on heating, and red triangles indicate transitions from the A-phase to the conical state. Green triangles denote $T_N$ at 0 T and 1.37 T). The black star indicates the metastable starting point in panel (b). (b) Field sweeps performed from the metastable state at $T= 0.22$ K, with the starting point at $H =1.04$ T indicated by a (black) star. Open blue (red) squares indicate the sharp transitions back to the conical state on decreasing (increasing) the field from the starting point. The dashed line shows the field sweep after zero field cooled at $T = 0.175$ K for comparison (blue arrows indicate the direction of the field).
• Figure 3: (a) $\rho(H)$ for $H \parallel [100]$ between 0 and 3.5 T. Colored arrows denote the anomalies corresponding to transitions $H_{D}$ (orange), $H_{A1}$ (blue), $H_{A2}$ (red), $H_{B}$ (pink) and $H_{C}$ (green) in the order of increasing field. (b) $\rho(H)$ in increasing (full lines) and decreasing (dashed lines) fields for 1 K (almost no hysteresis) and 0.35 K (strong hysteresis). Also shown are the additional scattering contributions $\Delta \rho_{A'}$ and $\Delta \rho_B$ to the resistivity appearing in the A' and the B-phase, respectively. (c) $\rho(H)$ for $T \leq 0.3$ K, in the conical order between 0.7 T and 2 T. The triangles indicate the different anomalies. $H_{A1}$ disappears below 0.225 K as the A'-phase closes. $H_{A2}$ and $H_B$ are observed at all temperatures. (d) Amplitude of the contributions $\Delta \rho_{A'}$ and $\Delta \rho_B$ in the resistivity as a function of the temperature (from increasing field sweeps). (e) Magnetic phase diagram obtained for the [100] direction by resistivity measurements. Full squares (open triangles) are anomalies from increasing (decreasing) field sweeps. Full circles are anomalies from $\rho(T)$. Colored areas indicate the various magnetic structures.
• Figure 4: (a) Temperature dependence of the resistivity $\rho(T)$ for $H \parallel [100]$ between 0.1 K and 4.1 K for different fields. Increasing $T$ is plotted in full lines and decreasing fields in dashed lines. Colored arrows denote the anomalies corresponding to transitions $H_{A1}$ (blue), $H_{A2}$ (red), $H_{B}$ (pink) and $H_{C}$ (green). (b) Field dependence of $\rho$ from the metastable A'- and B-phases induced by field cooling, for $T=0.11$ K. Stars indicate the starting point of the $H$-sweeps (respectively at 1.18, 1.35 and 1.6 T), and the direction of the sweep is given by the black arrows. Anomalies in the metastable $H$-sweeps are labeled with numbers from 1 to 5. (c) and (d) Low temperature $H-T$ phase diagram of EuPtSi with $H\parallel$ [100] for increasing and decreasing magnetic field, respectively. Full squares (open triangles) are transitions from increasing (decreasing) $H$-sweeps in the equilibrium state, and the shaded areas indicate the A'- and B-phases. Full circles are transitions from (a). Anomalies in the $H$-sweeps from the metastable states are represented with half-filled markers, with the same numbering as in (b).
• Figure 5: angular dependence of the resistivity (a) $\rho(H)$ for applied fields up to 5 T at 1.75 K for various angles between $H\parallel [111] (\theta=0$°) and $H\parallel [\bar{1}\bar{1}2]$ ($\theta=90$°), sequence ⓐ. Data are shifted vertically for clarity. Colored triangles indicate the respective transitions $H_D$ (brown), $H_{A1}$ (blue), $H_{A2}$ (red), $H_B$ (pink) and $H_c$ (green). (b) Simplified cubic cell indicating the principal crystallographic directions. The two rotation studies are represented by the blue arrow ⓐ and the orange arrow ⓑ, respectively. (c) Angle-dependent phase diagram obtained from sequence ⓐ. The color strength of the transition markers correlates with the size of the anomaly in $\rho$.