Anand, V.K.; Johnston, D.C.: Metallic behavior induced by potassium doping of the trigonal antiferromagnetic insulator EuMn2As2. Physical Review B 94 (2016), p. 014431/1-19
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We report magnetic susceptibility χ, isothermal magnetization M, heat capacity Cp, and electrical resistivity ρ measurements on undoped EuMn2As2 and K-doped Eu0.96K0.04Mn2As2 and Eu0.93K0.07Mn2As2 single crystals with the trigonal CaAl2Si2-type structure as a function of temperature T and magnetic field H. EuMn2As2 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 TN1 = 15 K from Cp(T ) and χ(T ) data with a likely spin-reorientation transition at TN2 = 5.0 K. TheMn+2 3d5 spins- 5/2 exhibit AFM ordering at TN = 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 Eu0.96K0.04Mn2As2 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 EuMn2As2 versus both T and H. For metallic Eu0.96K0.04Mn2As2 and Eu0.93K0.07Mn2As2, 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 TN ≈ 150 K, and then continues to decrease but more slowly below TN. This suggests that dynamic short-range AFM order of the Mn spins above the Mn TN strongly suppresses the resistivity, contrary to the conventional decrease of ρ that is only observed upon cooling below TN of an antiferromagnet