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The compound EuCo{2-y}As2 with the tetragonal ThCr2Si2 structure is known tocontain Eu{+2} ions with spin S = 7/2 that order below a temperature TN = 47 Kinto an antiferromagnetic (AFM) proper helical structure with the orderedmoments aligned in the tetragonal ab plane, perpendicular to the helix axisalong the c axis, with no contribution from the Co atoms. Here we carry out adetailed investigation of the properties of single crystals. Enhanced orderedand effective moments of the Eu spins are found in most of our crystals.Electronic structure calculations indicate that the enhanced moments arise frompolarization of the d bands, as occurs in ferromagnetic Gd metal. Electricalresistivity measurements indicate metallic behavior. The low-field in-planemagnetic susceptibilities chi{ab}(T < TN) for several crystals are reportedthat are fitted well by unified molecular field theory (MFT), and the Eu-Euexchange interactions Jij are extracted from the fits. High-field magnetizationM data for magnetic fields H||ab reveal what appears to be a first-orderspin-flop transition followed at higher field by a second-order metamagnetictransition of unknown origin, and then by another second-order transition tothe paramagnetic (PM) state. For H||c, the magnetization shows only asecond-order transition from the canted AFM to the PM state, as expected. Thecritical fields for the AFM to PM transition are in approximate agreement withthe predictions of MFT. Heat capacity Cp measurements in zero and high H arereported. Phase diagrams for H||c and H||ab versus T are constructed from thehigh-field M(H,T) and Cp(H,T) measurements. The magnetic part Cmag(T, H = 0) ofCp(T, H = 0) is extracted and is fitted rather well below TN by MFT, althoughdynamic short-range AFM order is apparent in Cmag(T) up to about 70 K, wherethe molar entropy attains its high-T limit of R ln8.

Enhanced moments of Eu in single crystals of the metallic helical antiferromagnet EuCo{2-y}As2