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Title: Enhanced moments of Eu in single crystals of the metallic helical antiferromagnet EuCo 2 - y As 2

EuCo 2 - y As 2 with the tetragonal ThCr 2Si 2 structure is known to contain Eu +2 ions with spin S=$$\frac{7}{2}$$that order below a temperature T N ≈ 47 K into an antiferromagnetic (AFM) proper helical structure with the ordered moments aligned in the tetragonal ab plane, perpendicular to the helix axis along the c axis, with no contribution from the Co atoms. Here we carry out a detailed investigation of the properties of single crystals. We consistently find about 5% vacancies on the Co site from energy-dispersive x-ray analysis and x-ray diffraction refinements. Enhanced ordered and effective moments of the Eu spins are found in most of our crystals. Electronic structure calculations indicate that the enhanced moments arise from polarization of the d bands, as occurs in ferromagnetic Gd metal. Electrical resistivity measurements indicate metallic behavior. The low-field in-plane magnetic susceptibilities $$\chi_{ab}$$ (T < T N) for several crystals are reported that are fitted well by unified molecular field theory (MFT), and the Eu–Eu exchange interactions J ij are extracted from the fits. High-field magnetization M data for magnetic fields H ∥ ab reveal what appears to be a first-order spin-flop transition followed at higher field by a second-order metamagnetic transition of unknown origin, and then by another second-order transition to the paramagnetic (PM) state. For H ∥ c , the magnetization shows only a second-order transition from the canted AFM to the PM state, as expected. The critical fields for the AFM to PM transition are in approximate agreement with the predictions of MFT. Heat capacity C p measurements in zero and high H are reported. Phase diagrams for H ∥ c and H ∥ ab versus T are constructed from the high-field M (H ,T) and C p (H,T) measurements. Lastly, the magnetic part C mag (T,H = 0) of C p (T,H= 0) is extracted and is fitted rather well below T N by MFT, although dynamic short-range AFM order is apparent in C mag (T) up to about 70 K, where the molar entropy attains its high- T limit of R ln 8.
Authors:
 [1] ;  [1] ;  [2] ;  [3] ;  [3] ;  [4]
  1. Ames Lab. and Iowa State Univ., Ames, IA (United States)
  2. Swiss Federal Laboratories for Materials Science and Technology (Empa), Dubendorf (Switzerland)
  3. Stockholm Univ. (Sweden). Dept. of Materials and Environmental Chemistry
  4. Ames Lab. and Iowa State Univ., Ames, IA (United States); Iowa State Univ., Ames, IA (United States). Dept. of Physics and Astronomy
Publication Date:
Report Number(s):
IS-J-9613
Journal ID: ISSN 2469-9950; PRBMDO
Grant/Contract Number:
AC02-07CH11358
Type:
Accepted Manuscript
Journal Name:
Physical Review B
Additional Journal Information:
Journal Volume: 97; Journal Issue: 14; Journal ID: ISSN 2469-9950
Publisher:
American Physical Society (APS)
Research Org:
Ames Laboratory (AMES), Ames, IA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY
OSTI Identifier:
1433661
Alternate Identifier(s):
OSTI ID: 1432098