Metallic behavior induced by potassium doping of the trigonal antiferromagnetic insulator ${\mathrm{EuMn}}_2{\mathrm{As}}_2$

We report magnetic susceptibility $\chi$, isothermal magnetization $M$, heat capacity $C_{\mathrm{p}}$, and electrical resistivity $\rho$ measurements on undoped ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ and K-doped ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ and ${\mathrm{Eu}}_{0.93}{\mathrm{K}}_{0.07}{\mathrm{Mn}}_2{\mathrm{As}}_2$ single crystals with the trigonal ${\mathrm{CaAl}}_2{\mathrm{Si}}_2$-type structure as a function of temperature $T$ and magnetic field $H$. ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ has an insulating ground state with an activation energy of 52 meV and exhibits antiferromagnetic (AFM) ordering of the ${\mathrm{Eu}}^{+2}$ spins $S=\frac{7}{2}$ at $T_{\mathrm{N}1}=15$ K from $C_{\mathrm{p}}\left(T\right)$ and $\chi \left(T\right)$ data with a likely spin-reorientation transition at $T_{\mathrm{N}2}=5.0$ K. The ${\mathrm{Mn}}^{+2}3d^5$ spins-$\frac{5}{2}$ exhibit AFM ordering at $T_{\mathrm{N}}=142$ K from all three types of measurements. The $M\left(H\right)$ isotherm and $\chi \left(T\right)$ data indicate that the Eu AFM structure is both noncollinear and noncoplanar. The AFM structure of the Mn spins is also unclear. A 4% substitution of K for Eu in ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ is sufficient to induce a metallic ground state. Evidence is found for a difference in the AFM structure of the Eu moments in the metallic crystals from that of undoped ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ versus both $T$ and $H$. For metallic ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ and ${\mathrm{Eu}}_{0.93}{\mathrm{K}}_{0.07}{\mathrm{Mn}}_2{\mathrm{As}}_2$, an anomalous S-shape $T$ dependence of $\rho$ related to the Mn magnetism is found. Upon cooling from 200 K, $\rho$ exhibits a strong negative curvature, reaches maximum positive slope at the Mn $T_{\mathrm{N}}\approx 150$ K, and then continues to decrease but more slowly below $T_{\mathrm{N}}$. This suggests that dynamic short-range AFM order of the Mn spins above the Mn $T_{\mathrm{N}}$ strongly suppresses the resistivity, contrary to the conventional decrease of $\rho$ that is only observed upon cooling below $T_{\mathrm{N}}$ of an antiferromagnet.

Figure 1

Powder x-ray diffraction patterns of (a) ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ and (b) ${\mathrm{Eu}}_{0.93}{\mathrm{K}}_{0.07}{\mathrm{Mn}}_2{\mathrm{As}}_2$ recorded at room temperature. The solid lines through the experimental points are the Rietveld refinement profiles calculated for the ${\mathrm{CaAl}}_2{\mathrm{Si}}_2$-type trigonal structure (space group $P\overline{3}m1)$. The short vertical bars mark the Bragg peak positions. The lowermost curves represent the differences between the experimental and calculated intensities. The unindexed peaks marked with stars correspond to peaks from the residual flux on the surface of the samples.

Figure 2

(a) ${\mathrm{CaAl}}_2{\mathrm{As}}_2$-type trigonal crystal structure of ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ (space group $P\overline{3}m1$, No. 164). The spheres represent Eu, Mn, and As atoms in decreasing order of size. (b) Extended view of the ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ structure showing the Mn-As layers and ${\left[{\mathrm{Mn}}_2{\mathrm{As}}_2\right]}^{-2}$ networks. (c) A projection of the ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ structure onto the $ab$ plane showing the hexagonal arrangement of Eu, Mn, and As atoms. The Eu atoms form triangular-lattice layers and the Mn atoms form a corrugated honeycomb lattice.

Figure 3

Zero-field-cooled magnetic susceptibility $\chi$ (left ordinate) and its inverse ${\chi }^{-1}$ (right ordinate) for an ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ crystal versus temperature $T$ for $1.8\mathrm{K}\le T\le 300$ K in a magnetic field $H=3.0$ T applied (a) in the $ab$ plane $\left({\chi }_{ab},H\perp c\right)$ and (b) along the $c$ axis $\left({\chi }_c,H\parallel c\right)$. The solid red curves are fits of the ${\chi }^{-1}\left(T\right)$ data by the modified Curie-Weiss law (1) over the $T$ range 50 K $\le T\le$ 300 K and the dashed curves are extrapolations.

Figure 4

(a) Zero-field-cooled (ZFC) magnetic susceptibility $\chi$ of an ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ single crystal as a function of temperature $T$ for $1.8\mathrm{K}\le T<30$ K measured in a magnetic field $H=0.05$ T applied in the $ab$ plane $\left({\chi }_{ab},H\perp c\right)$ and along the $c$ axis $\left({\chi }_c,H\parallel c\right)$. Insets: expanded views of $\chi \left(T\right)$ showing the anomalies near 5 K. (b), (c) ZFC ${\chi }_{ab}\left(T\right)$ and ${\chi }_c\left(T\right)$, respectively, for $1.8\mathrm{K}\le T\le 25$ K measured at the indicated $H$ values.

Figure 5

Isothermal magnetization $M$ of an ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ single crystal as a function of applied magnetic field $H$ measured at the indicated temperatures for $H$ applied (a) in the $ab$ plane $\left(M_{ab},H\perp c\right)$ and (b) along the $c$ axis $\left(M_c,H\parallel c\right)$.

Figure 6

High-field isothermal magnetization $M$ of an ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ single crystal as a function of applied magnetic field $H$ measured at the indicated temperatures for $H$ applied (a) in the $ab$ plane $\left(M_{ab},H\perp c\right)$ and (b) along the $c$ axis $\left(M_c,H\parallel c\right)$. (c) A comparison of $H\perp c$ and $H\parallel cM\left(H\right)$ isotherms at 2 K.

Figure 7

$H\text{−}T$ phase diagram for the Eu magnetic ordering in an ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ single crystal determined from the $H$ dependence of $T_{\mathrm{N}}$ from the magnetic susceptibility $\chi \left(T\right)$ data in Fig. 4 and $T$ dependence of isothermal magnetization $M\left(H\right)$ data in Fig. 6 for $H$ applied (a) in the $ab$ plane $\left(H\perp c\right)$ and (b) along the $c$ axis $\left(H\parallel c\right)$. The solid curves are the fits to the data by $H=H_0{(1-T/T_{\mathrm{N}})}^{\alpha }$. The data points at $H=0.05$ T and $T=5$ K are for the spin-reorientation transition seen in Fig. 4. The stars correspond to the point obtained from $M\left(H\right)$ measurements.

Figure 8

The quantity $(\chi -{\chi }_0)(T-{\theta }_{\mathrm{p}})$ versus temperature $T$ in $H=3$ T for ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ with $H\perp c$ and $H\parallel c$. The Néel temperature $T_{\mathrm{N}}=142$ K of the Mn sublattice is apparent from the changes in slope of $(\chi -{\chi }_0)(T-{\theta }_{\mathrm{p}})$ upon traversing $T_{\mathrm{N}}$ with increasing $T$.

Figure 9

Heat capacity $C_{\mathrm{p}}$ of an ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ crystal and nonmagnetic reference compound ${\mathrm{SrZn}}_2{\mathrm{As}}_2$ versus temperature $T$ in zero magnetic field. The blue curve is a fit of $C_{\mathrm{p}}\left(T\right)$ of ${\mathrm{SrZn}}_2{\mathrm{As}}_2$ over the whole $T$ range by the Debye model of lattice heat capacity with ${\mathrm{\Theta }}_{\mathrm{D}}=267\left(2\right)$ K.

Figure 10

(a) Magnetic heat capacity $C_{\mathrm{mag}}$ of ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ versus temperature $T$ measured in $H=0$. (b) $C_{\mathrm{mag}}/T$ versus $T$. (c) Magnetic entropy $S_{\mathrm{mag}}$ versus $T$. The AFM ordering temperatures $T_{\mathrm{N}}$ of the Eu and Mn spins are indicated in each panel. The entropy of disordered Eu spins $Rln\left(8\right)$ is indicated by the horizontal dashed line. The continuing increase of $S_{\mathrm{mag}}\left(T\right)$ for $T>50$ K is due to the release of magnetic entropy of the long-range AFM-ordered Mn spins for $T<142$ K and of short-range-ordered Mn spins for $T>142$ K.

Figure 11

Magnetic heat capacity $C_{\mathrm{mag}}$ of an ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ crystal divided by temperature $T$ versus $T$ below 40 K. The $T^3$ extrapolation to $T=0$ is the dashed red line. The blue curve is the MFT prediction for Eu spin $S=\frac{7}{2}$ and $T_{\mathrm{N}}=14.8$ K [55].

Figure 12

(a) $ab$-plane electrical resistivity $\rho$ of an ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ single crystal as a function of temperature $T$ for $1.8\mathrm{K}\le T\le 350$ K measured in zero magnetic field. Inset: expanded view of $\rho \left(T\right)$ data at $T>100$ K. (b) The $\rho \left(T\right)$ data in (a) plotted on a semilog scale. The red curve is a fit of $\rho \left(T\right)$ data with 50 K $\le T\le 100$ K by Eq. (2). Inset: ${log}_{10}\rho$ versus $1000/T$. The straight red line is the same fit as in the main figure. The vertical arrows in the main panel (b) and its inset indicate a feature in the data at the Mn AFM ordering temperature $T_{\mathrm{N}}=142$ K.

Figure 13

(a) Zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility $\chi$ of an ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ crystal versus temperature $T$ for $1.8\mathrm{K}\le T\le 40$ K measured in a magnetic field $H=0.1$ T applied in the $ab$ plane $\left({\chi }_{ab},H\perp c\right)$ and along the $c$ axis $\left({\chi }_c,H\parallel c\right)$. Inset: expanded plot of $\chi \left(T\right)$ for $H\parallel c$ showing the anomaly near 9 K. (b) ZFC $\chi \left(T\right)$ for $1.8\mathrm{K}\le T\le 40$ K measured at the indicated $H\perp c$. Inset: expanded plot of $\chi \left(T\right)$. (c) ZFC $\chi \left(T\right)$ for $1.8\mathrm{K}\le T\le 40$ K measured at the indicated $H\parallel c$ values.

Figure 14

Zero-field-cooled magnetic susceptibility $\chi$ (left ordinate) and its inverse ${\chi }^{-1}$ (right ordinate) of an ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ single crystal versus temperature $T$ for $1.8\mathrm{K}\le T\le 350$ K measured in a magnetic field $H=1.0$ T applied (a) in the $ab$ plane $\left({\chi }_{ab},H\perp c\right)$ and (b) along the $c$ axis $\left({\chi }_c,H\parallel c\right)$. The red solid straight lines are the fits of ${\chi }^{-1}\left(T\right)$ data to the modified Curie-Weiss behavior in the $T$ range 50 K $\le T\le 125$ K and the red dashed straight lines are extrapolations. Arrows mark anomalies near 150 and 275 K. Insets: expanded plots of $\chi \left(T\right)$ for $100\mathrm{K}\le T\le 350$ K.

Figure 15

Magnetization $M$ versus applied magnetic field $H$ isotherms for an ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ crystal measured at the indicated temperatures for $H$ applied (a) in the $ab$ plane $\left(M_{ab},H\perp c\right)$ and (b) along the $c$ axis $\left(M_c,H\parallel c\right)$. (c) Comparison of $H\perp c$ and $H\parallel cM\left(H\right)$ isotherms at 1.8 K. Inset: $dM\left(H\right)/dH$ versus $H$ at $T=1.8$ K.

Figure 16

(a) Heat capacity $C_{\mathrm{p}}$ of an ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ crystal versus temperature $T$ in $H=0$ for $1.8\mathrm{K}\le T\le 300$ K. Insets: expanded views of $C_{\mathrm{p}}\left(T\right)$ of (a) ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ and ${\mathrm{SrZn}}_2{\mathrm{As}}_2$ for $T<40$ K and (b) ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ for $120\mathrm{K}\le T\le 180$ K. (b) Magnetic contribution $C_{\mathrm{mag}}$ to $C_{\mathrm{p}}$ plotted as $C_{\mathrm{mag}}\left(T\right)/T$ versus $T$ per mole of Eu spins. Inset: magnetic contribution $S_{\mathrm{mag}}\left(T\right)$ to the entropy per mole of Eu spins. The dashed curve is the extrapolation to $T=0$ and the solid red curve is the MFT prediction for $S=\frac{7}{2}$ and $T_{\mathrm{N}}=13.5$ K.

Figure 17

$ab$-plane electrical resistivity $\rho$ of an ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ crystal versus temperature $T$ measured in $H=0$. Inset: expanded view of $\rho \left(T\right)$ data at $T<40$ K. The bumps in $\rho \left(T\right)$ at $\sim 210$ K are experimental artifacts.

Figure 18

(a) Zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility $\chi$ of an ${\mathrm{Eu}}_{0.93}{\mathrm{K}}_{0.07}{\mathrm{Mn}}_2{\mathrm{As}}_2$ crystal versus temperature $T$ for $1.8\mathrm{K}\le T\le 40$ K measured in a magnetic field $H=0.1$ T applied in the $ab$ plane $\left({\chi }_{ab},H\perp c\right)$ and along the $c$ axis $\left({\chi }_c,H\parallel c\right)$. (b) ZFC $\chi \left(T\right)$ for $1.8\mathrm{K}\le T\le 40$ K measured at the indicated $H\perp c$. Inset: expanded plot of $\chi \left(T\right)$. (c) ZFC $\chi \left(T\right)$ for $1.8\mathrm{K}\le T\le 40$ K measured at the indicated $H\parallel c$.

Figure 19

Zero-field-cooled magnetic susceptibility $\chi$ (left ordinate) and its inverse ${\chi }^{-1}$ (right ordinate) of an ${\mathrm{Eu}}_{0.93}{\mathrm{K}}_{0.07}{\mathrm{Mn}}_2{\mathrm{As}}_2$ crystal versus temperature $T$ for $1.8\mathrm{K}\le T\le 350$ K measured in a magnetic field $H=1.0$ T applied (a) in the $ab$ plane $\left({\chi }_{ab},H\perp c\right)$ and (b) along the $c$ axis $\left({\chi }_c,H\parallel c\right)$. The solid curves are fits of the ${\chi }^{-1}\left(T\right)$ data by the modified Curie-Weiss behavior in the $T$ range 75 K $\le T\le 225$ K and the dashed curves are the extrapolations. Insets: expanded plots of $\chi \left(T\right)$.

Figure 20

Magnetization $M$ versus applied magnetic field $H$ isotherms for an ${\mathrm{Eu}}_{0.93}{\mathrm{K}}_{0.07}{\mathrm{Mn}}_2{\mathrm{As}}_2$ crystal measured at the indicated temperatures for $H$ applied (a) in the $ab$ plane $\left(M_{ab},H\perp c\right)$ and (b) along the $c$ axis $\left(M_c,H\parallel c\right)$. (c) Comparison of $H\perp c$ and $H\parallel cM\left(H\right)$ isotherms at 1.8 K. Left inset: $M(H<1$ T). Right inset: $dM/dH$ versus $H$.

Figure 21

(a) Heat capacity $C_{\mathrm{p}}$ of an ${\mathrm{Eu}}_{0.93}{\mathrm{K}}_{0.07}{\mathrm{Mn}}_2{\mathrm{As}}_2$ crystal versus temperature $T$ for $1.8\mathrm{K}\le T\le 300$ K measured in zero magnetic field. Inset: expanded plot of $C_{\mathrm{p}}\left(T\right)$ along with the $C_{\mathrm{p}}\left(T\right)$ of the nonmagnetic reference compound ${\mathrm{SrZn}}_2{\mathrm{As}}_2$. (b) Magnetic contribution $C_{\mathrm{mag}}$ to $C_{\mathrm{p}}$ plotted as $C_{\mathrm{mag}}\left(T\right)/T$ versus $T$ per mole of Eu. Inset: magnetic contribution $S_{\mathrm{mag}}\left(T\right)$ to the entropy per mole of Eu. The dashed curve is the extrapolation to $T=0$ and the solid red curve is the MFT prediction for $S=\frac{7}{2}$ and $T_{\mathrm{N}}=14.5$ K.

Figure 22

$ab$-plane electrical resistivity $\rho$ of an ${\mathrm{Eu}}_{0.93}{\mathrm{K}}_{0.07}{\mathrm{Mn}}_2{\mathrm{As}}_2$ crystal versus temperature $T$ for $1.8\mathrm{K}\le T\le 395$ K measured in zero magnetic field. Inset: expanded plot of $\rho \left(T\right)$ data for $T<35$ K.