more to the right of the periodic table than Y), and Z stands for an sp element. With respect to the magnetic sublattice, Heusler alloys split up into two classes: ...
Hyperfine Interactions 28 (1986) 603-606
603
MOSSBAUER STUDY OF HYPERFINE FIELD DISTRIBUTION IN Co2TiSn E.A. G O R L I C H 2, R. KMIE(~ I, K. L ~ T K A 2 and W. Z A J ~ C l
tInstitute of Nuclear Physics, Krak6w, Poland 2Institute of Physics, Jagiellonian University, Krak6w, Poland
The CozTiSn Heusler-type alloy Ccrystal structure L2~, ferromagnetic with Tc = 364(r was investigated by means of magnetometric and 119-Sn N~ssbauer techniques in the temperature range from 4.2K to 37OK. The study was supplemented by similar examination of off-stoichiometric samples: Co z~Ti,;zSn. The temperature evolution of the shape of hyperfine field distribution leads to the conclusion that the process of demagnetization takes place in a non-uniform fashion within the sample volume. It is further claimed that local, short-range interactions are dominant both in magnetic and hyperfine couplings in this IIeusler-type Co-based alloy.
I.
INTRODUCTION
Heusler alloys focus considerable attention as a convenient systems to study non-magnetic-site hyperfine fields in ferromagnets. Besides, the details and the very nature of magnetic coupling within these compounds are not fully understood, as yet. The crystal structure of IIeusler alloys (those with general formula XzYZ),L2~, may be thought of as composed of four interpenetrating fcc lattices. Usually, X and Y are transition metals C X being more to the right of the periodic table than Y), and Z stands for an sp element. With respect to the magnetic sublattice, Heusler alloys split up into two classes: most widely investigated X ~ Z Cmagnetic moment localized on ~ atoms), and CozYZ (magnetic moment carried by Co atoms). As it was shown in a number of papers (cf. eg./ 1,2,3/ the two classes, are by no means identical in terms of their ~icroscopic magnetic properties. The Co-Co distance in CozYZ is t/2 smaller than that of Mn-Mn in X2~mZ Cno direct Fm-Mn interactions are therefore possible in the latter case). Magnetic moment confined to the Co site is reported to vary with the alloy composition from ca .23#~ to ca I#B. On the contrary, the Fm moment always takes on values close to 4 ~ . Furthermore, the general trends of sp-site hy~erfine fields are correctly predicted by either Blandin-Campbell /4/ or JenaGeldart /5/ model for the XzMnZ compounds. These models fail, however, if applied to the COzYZ alloys. It happens that the field of wrong sign is sometimes predicted. The source of these difficulties may be either inherent in the models (approximations) or may lay in uncertainty of parameters estimation (e.g. kF, no). Thus the natural way to approach the question concerning magnetic and related hyperfine interactions in Co-based Heusler alloys, is to gain more experimental knowledge on the physical background (also in descriptive, qualitative form). At this point local, microscopic information provided by the M~ssbauer effect, is of special importance. The present contribution presents analysis of the temperature evolution of the hyperfine field distribution detected by 119-Sn MSssbauer
9 J.C. Baltzer A.G., Scientific Publishing Company
604
E.A. GOrlich,et al., MOssbauer study of hyperfine field distribution in Co2TiSn
spectroscopy in the CozTiSn Heusler alloy. The latter has been chosen as a representative of Co-based Heusler phases because: I~ it contains a convenient MSssbauer element (tin) as a constituent, 2 ~ it exhibits relatively large hyperfine field at tin site. Besides, there is a lot of information on CozTiSn available in literature. 2.
EXPERIMENTAL
The samples of Co2_~Ti,+ ~Sn (6 = -0.1, O, +O.1) were prepared from appropriate amounts of high purity elements by arc melting in an argon atmosphere, and then annealed for 100 h at 800~ in evacuated quartz tubes. X-ray diffraction proved that within the accuracy of a conventional powder experiment, all samples were single L2~ phases with the following lattice constants: 6.056(5)~, 6.073C9)~, and 6 . 0 4 6 ( 2 ~ , respectively (at room temperature). The bulk magnetic measurements were performed with a Faraday balance. Their results are listed in table I. Hyperfine structure of the 119-Sn MSssbauer spectra was investigated in the temperature range from 4.2K to 370K. 3.
RESULTS AND DISCUSSION
Fig.1 shows the 23.8keV resonance spectra for CofTiSn and Co1~Ti~fSn, taken at 4.2K. The corresponding hyperfine field distribution curves are given in fig.2. They were obtained through a formal deconvolution of the M@ssbauer spectra by means of the extended Uesse-R~bartsch method /6,7/. Much attention has been payed to selecting and testing the applied smoothing criteria. Reliability checks were performed for a series of simulated cases. As a result, we can claim that the local maxima in the distribution curves do have physical meaning. They systematically appear while raising temperature, and are even more pronounced in magnetically diluted alloys Co2~Ni~TiSn /8/. Our spectra of Co~TiSn were contaminated by an unidentified component originating from an unknown ferromagnetic phase. Such "impurity lines" are reported by almost all authors presenting their spectra of C % Y Z alloys. In our case they did not exceed 6% of the main spectrum intensity, and were always appropriately accounted for. Hyperfine field distribution curves respond to growing tem0erature first by decreasing the mean (and modal) value, and becomin9 more asymmetric. At ca O.9T~ they develop another maximum, centered at about 0.6T. The latter p~ak than gains area at the cost of the "high field" one. It can be inferred that in these compounds the Table I Lattice parametez, Curie temperature, saturation magnetic moment per cobalt atom, mean valne of the hf field distribution at 4.2K and the room temperature isomer tel. BaSnO 3 source for Co2_s Compound
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E.A. G6rlich, et al., M6ssbauer study of hyperfine field distribution in Co2TiSn
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Fig.3. Hyperfine field distributions at 119-Sn in Co~TiSn derived from the spectra at 358K Ctop) and 370K (bottom). Horizontal scales: Bhf in IT].
606
E.A. G6rlich,et aL, Mi~ssbauer study of hyperfine field distribution in Co2TiSn
Curie point is not a well-defined quantity, being smeared over a certain temperature interval. Within this interval, the Sn nuclei in already paramagnetic regions can still detect a transfer-type hf field from Q& the neighbouring, magnetically ordered volumes. The "low field" part of the distributions in figs. 2 and 3 represents positive fields but the sign was not determined experimentally, as yet. It should be noticed that the total distribution's mean (points in fig.4) changes with temperature exactly as the macroscopic Fig.4. See text. magnetization (solid line)does, in a manner close to that given by the molecular field theory (dashed line). The above described behaviour is indicative for the short range of the magnetic ions interaction (Co-Co), much alike in non-conducting materials. The inspection of the off-stoichiometric samples provides further support for such a statement. For Co~,TioqSn the distribution reduces practically to a single discrete hf field value being equal to~B~ in Co2TiSn at saturation. The spread in the hf field values in the latter compound (which is believed to result from Co-Ti disorder) is inhibited by the enough number of excess cobalt atoms to assure full occupation of a regular Co-sublattices. This not only leads to non-perturbed nearest neighbourhoods of tin atoms but also enables Co atoms to develope magnetic moment according to the concept of Jaccarino and Walker /9/. A given Co atom can develope magnetic moment only if it is surrounded by at least "z" other Co atoms within its sublattice, out of possible Z (for L2, structure Z = 6). The analysis of the magnetically diluted system C o ~ N i • TiSn leads to the value 4
