Microscopic Properties of  -FeMn Studied by Mo ¨ssbauer Spectroscopy Krzysztof SZYMAN ´ SKI  , Wojciech OLSZEWSKI, Dariusz SATULA, Ludwik DOBRZYN ´ SKI 1 , Katarzyna BRZO ´ ZKA 2 , and Joanna JANKOWSKA-KISIELIN ´ SKA 3 Faculty of Physics, University of Bial ystok, Lipowa 41, 15-424 Bial ystok, Poland 1 The Andrzej Sol tan Institute for Nuclear Studies, 05-400 Swierk/Otwock, Poland 2 Department of Physics, Technical University of Radom, Malczewskiego 29, Radom 26-600, Poland 3 Institute of Atomic Energy, 05-400 Otwock-S ´ wierk, Poland (Received May 29, 2009; accepted October 16, 2009; published December 10, 2009) Detailed analysis of hyperfine interactions in  -Mn 60 Fe 37 Cu 3 is presented. Isotropic three-dimensional Gaussian shape was introduced for modeling the distribution of hyperfine magnetic field derived from Mo ¨ssbauer measurements performed in a broad temperature range. Electrical quadrupole interactions resulting from broken local symmetry, which cannot be treated as small perturbation in the vicinity of transition temperature, were treated by exact method of invariants. Obtained values of quadrupolar interactions are small and cause only spectra broadening. Small values of measured electric field gradient are confirmed by empirically found correlation between known quadrupolar splitting in selected class of systems and an estimation based on postulated pseudopotential. The effects of anharmonicity were considered in the interpretation of the temperature dependence of shift of the spectrum centre. KEYWORDS: -FeMn, antiferromagnets, Mo ¨ssbauer spectroscopy DOI: 10.1143/JPSJ.78.124708 1. Introduction Magnetic moments in cubic gamma Fe–Mn with anti- ferromagnetic order could be parallel to [100], [110] or [111] directions. The neutron diffraction is unable to determine the magnetic moment directions because the diffraction pattern in all three cases is the same. 1) Attempts of the determination of the magnetic moment directions by hyperfine interaction measurements 1,2) were unsuccessful as well. The only sound information we have is that the combined neutron diffrac- tion, susceptibility and resistivity measurements showed that magnetic structure of given composition of Fe–Mn depends on temperature, 3) and that the nuclear spin orientation measurements indicated [111] orientation of the magnetic moments in isostructural Mn–Ni. 4) Mo ¨ssbauer spectroscopy is useful tool for investigation of hyperfine interactions. Unfortunately, Mo ¨ssbauer spectra of gamma Fe–Mn are poorly resolved. 5) This fact makes the spectra analysis difficult and usually ambiguous, because electric monopole, magnetic dipolar and electric quadrupolar interactions may contribute simultaneously to the spectra. In the case of alloys like Fe–Mn local symmetry breaking can appear because of chemical disorder. Any departure from local cubic symmetry may in turn result in appearance of the electric field gradient (EFG). For example, EFG resulting from chemical disorder was detected in bcc V-rich magneti- cally disordered Fe–V alloys. 6) The influence of the EFG on the shape of Mo ¨ ssbauer spectrum collected for cubic and hcp nonmagnetic chemically disordered structures was analyzed in ref. 7. It is interesting to note, that among all regular (simple cubic, bcc, fcc) structures, fcc produces mostly doublet-like spectrum for equiatomic binary disordered alloy. 7) Results of Mo ¨ssbauer measurements carried out on  -Fe–Mn alloys in external magnetic field 8) were tried to be interpreted with the electric field gradient taken into account. However, this attempt was only partly successful. Also, the interpretation of the broadening of the resonance line observed at high temperatures in Mo ¨ssbauer spectrum, 5) could not be unambiguous in this respect because no resolved doublet was observed. In principle, the electric field gradient can be determined when the spectra are measured in a broad temperature range. In the case of metals one expects that changes of isomer shift and electric field gradient are small in contrast to changes of magnetic interaction. This gives a hope for separation of both interactions. Nevertheless, the task is difficult because the description of the Mo ¨ ssbauer spectra in the low hyperfine magnetic field (HMF) region in the presence of EFG requires strict approach, because quadrupole electric and dipole magnetic interactions are of the same order. This itself is a challenging task. In addition, the studies of Fe–Mn alloys are of importance because of their application as pinning and exchanged bias layers, 9) and general interest in systems with mixed magnetic interactions. The structure of these layers and their physical properties could be modified by advanced technology. 10) Mo ¨ssbauer spectroscopy, which can deliver information even about a single atomic layer 11) can serve as sensitive tool for detection of those properties on atomic level. The present paper presents results of exact treatment of the mixed hyperfine interactions and simultaneous fitting of data measured at various temperatures in  -FeMn. Although expected EFG is small in comparison with typical values detected in metallic noncubic systems, it can not be treated as small perturbation when the sample temperature is close to the magnetic transition temperature. In this temperature range, the usually used first order perturbation approxima- tion (small quadrupolar electric interaction with respect to the magnetic one) can not be valid. Then, in order to solve the full Hamiltonian problem, it is necessary to assume an orientation of hyperfine magnetic field with respect to the principal axes of the local electric field gradient. This has to  E-mail: kszym@alpha.uwb.edu.pl Journal of the Physical Society of Japan Vol. 78, No. 12, December, 2009, 124708 #2009 The Physical Society of Japan 124708-1