Magnetic behavior of EuCu<SUB>2</SUB>As<SUB>2</SUB>: delicate balance between antiferromagnetic and ferromagnetic order

The Eu-based compound, EuCu2As2, crystallizing in the ThCr2Si2-type tetragonal structure, has been synthesized and its magnetic behavior has been investigated by magnetization (M), heat-capacity (C), and electrical resistivity (ρ) measurements as a function of temperature (T) and magnetic field (H) as well as by 151Eu Mössbauer measurements. The results reveal that Eu is divalent, ordering antiferromagnetically below 15 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 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 kOe at 1.8 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