Magnetic behavior of $\mathrm{Eu}{\mathrm{Cu}}_2{\mathrm{As}}_2$: A delicate balance between antiferromagnetic and ferromagnetic order

The Eu-based compound, $\mathrm{Eu}{\mathrm{Cu}}_2{\mathrm{As}}_2$, crystallizing in the $\mathrm{Th}{\mathrm{Cr}}_2{\mathrm{Si}}_2$-type tetragonal structure, has been synthesized and its magnetic behavior has been investigated by magnetization $\left(M\right)$, heat-capacity $\left(C\right)$, and electrical resistivity $\left(\rho \right)$ measurements as a function of temperature $\left(T\right)$ and magnetic field $\left(H\right)$ as well as by ${}^{151}\mathrm{Eu}$ Mössbauer measurements. The results reveal that Eu is divalent, ordering antiferromagnetically below $15\mathrm{K}$ in the absence of a magnetic field, apparently with the formation of magnetic Brillouin-zone boundary gaps. A fascinating observation is made in a narrow temperature range before antiferromagnetism sets in: That is, there is a remarkable upturn just below $20\mathrm{K}$ in the plot of magnetic susceptibility versus $T$, even at low fields, as though the compound actually tends to order ferromagnetically. There are corresponding anomalies in the magnetocaloric effect data as well. In addition, a small application of magnetic field (around $1\mathrm{kOe}$ at $1.8\mathrm{K}$) in the antiferromagnetic state causes the spin-reorientation effect. These results suggest that there is a close balance between antiferromagnetism and ferromagnetism in this compound.

Figure 1

(Color online) ${}^{151}\mathrm{Eu}$ Mössbauer spectra of $\mathrm{Eu}{\mathrm{Cu}}_2{\mathrm{As}}_2$ at low temperatures. The continuous lines represent least-squares fit of the data to a single Lorentzian line at $25\mathrm{K}$ and to a hyperfine-split pattern at 12 and $4.2\mathrm{K}$.

Figure 2

(Color online) Magnetic susceptibility obtained in a field of $5\mathrm{kOe}$ and $100\mathrm{Oe}$ for $\mathrm{Eu}{\mathrm{Cu}}_2{\mathrm{As}}_2$. The plot of inverse $\chi \left(T\right)$ $(H=5\mathrm{kOe})$ is also shown and the continuous line represents linear region. The continuous line through the data points in the case of $H=100\mathrm{Oe}$ represents the behavior for the field-cooled condition of the specimen; otherwise all the data points correspond to the zero-field-cooled state of the specimen.

Figure 3

(Color online) (a) Isothermal magnetization behavior $\mathrm{Eu}{\mathrm{Cu}}_2{\mathrm{As}}_2$ at 1.8, 10, 14.5, and $17\mathrm{K}$. The lines through the data points serve as a guide to the eyes. The inset shows the low-field data in an expanded region to highlight metamagnetic transition and the dashed line is obtained by linear extrapolation of the data near the zero-field region, (b) hysteresis loops at 5 and $17\mathrm{K}$, and (c) Arrott plot, $M^2$ vs $H∕M$, for $\mathrm{Eu}{\mathrm{Cu}}_2{\mathrm{As}}_2$.

Figure 4

(Color online) (a) Heat capacity, (b) entropy change, and (c) adiabatic change in temperature, as a function of temperature for $\mathrm{Eu}{\mathrm{Cu}}_2{\mathrm{As}}_2$.

Figure 5

(Color online) (a) Electrical resistivity $\left(\rho \right)$ as a function of temperature $(1.8–50\mathrm{K})$ in the presence of various externally applied magnetic fields and (b) magnetoresistance, defined as $[\rho \left(H\right)-\rho \left(0\right)]∕\rho \left(0\right)$, as a function of external field at selected temperatures for $\mathrm{Eu}{\mathrm{Cu}}_2{\mathrm{As}}_2$.