Metallic behavior induced by potassium doping of the trigonal antiferromagnetic insulator EuMn 2 As 2
Here, we report magnetic susceptibility χ, isothermal magnetization M, heat capacity C p , and electrical resistivity ρ measurements on undoped EuMn 2 As 2 and K-doped Eu 0.96 K 0.04 Mn 2 As 2 and Eu 0.93 K 0.07 Mn 2 As 2 single crystals with the trigonal CaAl 2 Si 2 -type structure as a function of temperature T and magnetic field H. EuMn 2 As 2 has an insulating ground state with an activation energy of 52 meV and exhibits antiferromagnetic (AFM) ordering of the Eu +2 spins S=7/2 at T N1 =15 K from C p (T) and χ(T) data with a likely spin-reorientation transition at T N2 =5.0 K. The Mn +2 3d 5 spins-5/2 exhibit AFM ordering at T N =142 K from all three types of measurements. The M(H) isotherm and χ(T) 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 Eu 0.96 K 0.04 Mn 2 As 2 is sufficient to induce a metallic ground state. We found evidence for a difference in the AFM structure of the Eu moments in the metallic crystals from that of undoped EuMn 2 As 2 versus both T and H. For metallic Eu 0.96 K 0.04 Mn 2 As 2 and Eu 0.93 K 0.07 Mn 2 As 2 , an anomalous S-shape T dependence of ρ related to the Mn magnetism is found. Upon cooling from 200 K, ρ exhibits a strong negative curvature, reaches maximum positive slope at the Mn T N ≈150 K, and then continues to decrease but more slowly below T N . Finally, this suggests that dynamic short-range AFM order of the Mn spins above the Mn T N strongly suppresses the resistivity, contrary to the conventional decrease of ρ that is only observed upon cooling below T N of an antiferromagnet.