Magnetic behavior of Eu Cu 2 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 $(M)$, heat-capacity $(C)$, and electrical resistivity $(\ensuremath{\rho})$ measurements as a function of temperature $(T)$ and magnetic field $(H)$ as well as by $^{151}\mathrm{Eu}$ M\"ossbauer measurements. The results reveal that Eu is divalent, ordering antiferromagnetically below $15\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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 $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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 $1\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ at $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) 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.