Frustrated magnetism in the tetragonal CoSe analog of superconducting FeSe

Recently synthesized metastable tetragonal CoSe, isostructural to the FeSe superconductor, offers a new avenue for investigating systems in close proximity to the iron-based superconductors. We present magnetic and transport property measurements on powders and single crystals of CoSe. High field magnetic susceptibility measurements indicate a suppression of the previously reported 10 K ferromagnetic transition with the magnetic susceptibility, exhibiting time dependence below the proposed transition. Dynamic scaling analysis of the time dependence yields a critical relaxation time of ${\tau }^{*}=0.064\pm 0.008$ s which in turn yields activation energy $E_a^{*}=14.84\pm 0.59$ K and an ideal glass temperature $T_0^{*}=8.91\pm 0.09$ K from Vogel-Fulcher analysis. No transition is observed in resistivity and specific heat measurements, but both measurements indicate that CoSe is metallic. These results are interpreted on the basis of CoSe exhibiting frustrated magnetic ordering arising from competing magnetic interactions. Arrott analysis of single crystal magnetic susceptibility has indicated the transition temperature occurs in close proximity to previous reports and that the magnetic moment lies solely in the $ab$ plane. The results have implications for understanding the relationship between magnetism and transport properties in the iron chalcogenide superconductors.

Figure 1

Crystal structures of ${\mathrm{KCo}}_2{\mathrm{Se}}_2$ and CoSe.

Figure 2

Inverse magnetic susceptibility of CoSe vs temperature measured in an applied field of 100 Oe. The inverse magnetic susceptibility is fit from 100 to 300 K to the Curie-Weiss law plus a temperature-independent term. The inset shows inverse magnetic susceptibilities for different applied dc fields (0.2, 1, and 5 T) to emphasize the change in slope near 82 K.

Figure 3

Magnetic susceptibility of CoSe vs temperature measured in various applied fields. The insets show the zoomed region close to the transition temperature; ZFC (zero-field-cooled) and FC (field-cooled) curves are shown by arrows which indicate the irreversibility of the magnetic ordering in the system at low fields. The bifurcation of ZFC-FC curves at low applied field (a) = 0.01 T and (b) = 0.2 T is destroyed with high applied fields (c) = 1 T and (d) = 5 T turning the system into a paramagnetic state with no irreversibility.

Figure 4

Ac magnetic susceptibility measured with various driving frequencies. The applied ac field was 10 Oe and the residual dc applied field due to internal instrumentation was 40 to 100 Oe. (a) The real parts of magnetic susceptibility $\left({\chi }^{\prime }\right)$ and (b) the imaginary parts $\left({\chi }^{\prime \prime }\right)$ parts. (c) Temperature dependence of ${\chi }^{\prime \prime }$ peaks at various driving frequencies (100 to 1200 Hz) and a fit with the Volger-Fulcher law.

Figure 5

Electrical transport measurements of CoSe single crystals obtained through de-intercalation of ${\mathrm{KCo}}_2{\mathrm{Se}}_2$. (a) Temperature dependence of longitudinal resistivity at various applied fields with the inset around the transition temperature. (b) Normalized longitudinal magnetoresistance up to 31 T with different applied field directions by sample rotation. (c) Angular dependence of longitudinal magnetoresistance at an applied field of 31 T. The magnetoresistance is fit with a sinusoidal dependence to the field angle.

Figure 6

Temperature-dependent specific heat of a pressed pellet of CoSe from 150 K to base temperature. Upper inset shows the temperature dependence near the transition temperature as well as a fit to a specific heat model accounting for electronic and vibrational components in the range 1.8 to 15 K.

Figure 7

Magnetic measurements of CoSe crystals mounted on a quartz paddle with orientations relative to the applied field direction as listed. (a) Temperature-dependent magnetic susceptibility for a 100 Oe field applied in two different orientations. (b) Field-dependent magnetization for both field orientations. (c) Arrott plots constructed from $M\left(H\right)$ curves from 2 to 12 K for $H\parallel ab$, which indicate a ferromagnetic transiton within the 8–10 K range. (d) Arrott plots constructed for $H\parallel c$ axis showing no spontaneous magnetization in the $c$ direction for any temperature.