Metallic behavior induced by potassium doping of the trigonal antiferromagnetic insulatorEuMn2As2

Abstract

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. The Mn +2 3d 5 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 Sshape 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.

Exploration of magnetism in 3D and 2D systems

“…The observation of long-range antiferromagnetic order at low temperature in EuMn 2 As 2 is consistent with a previous report, based on magnetic and thermodynamic measurements [51]. However, direct evidence to magnetic transition at T ∼ 140 K is lacking.…”

“…Thus, exploring the magnetic ordering in the system can provide important insight into the properties of the material. [51]. Copyright 2019 by the American Physical Society.…”

“…Previously electrical, magnetic and heat capacity measurements have been performed where two magnetic transition has been reported [51], however, the correlation of magnetic moments is not well understood. We have performed detailed neutron scattering measurement to understand this correlation.…”

Complex magnetic phase diagram in noncentrosymmetric EuPtAs

“…In addition, extrapolation of the c axis susceptibility χ c (T ) down to zero temperature gives a finite χ c (0) of about 0.168 emu/mol, yielding the ratio χ c (0)/χ (T N ) = 0.44. This feature is inconsistent with a collinear AFM structure of the Eu spins along the c axis, for which χ c (0) should be zero [29]. A possible explanation is that the Eu spins tilt away from the c axis, forming a noncollinear magnetic structure.…”

Geometrically frustrated magnetism in the pyrochlore Er>sub<2>/sub<Ti>sub<2-x>/sub<Sn>sub<x>/sub<O>sub<7>/sub< and in the disordered fluorites R>sub<2>/sub<Zr>sub<2>/sub<O>sub<7>/sub< (>i<R >/i<= Dy, Ho, Tb)

“…For Nd 2 Zr 2 O 7 , neutron-scattering results revealed the coexistence of a fluctuating Coulomb phase with an ordered one [17]. Another examples are Tb 2 Hf 2 O 7 and Pr 2 Hf 2 O 7 in which long-range ordered ground states are prevented down to 100 mK [18,19].…”