HYPERFINE FIELDS IN Co-BASED HEUSLER ALLOYS

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HYPERFINE FIELDS IN Co-BASED HEUSLER

ALLOYS

E. Baggio-Saitovitch, T. Butz, A. Vasquez, I. Vincze, F. Wagner, Keizo Endo

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 12, Tome 37, Dicembre 1976, page C6-417

HYPERFINE FIELDS IN CO-BASED HEUSLER ALLOYS

E. BAGGIO-SAITOVITCH (*), T. BUTZ, A. VASQUEZ (**)

I. VINCZE (***), F. E. WAGNER

Physik Department, Technis~he Universitat Miinchen, D-8046 Garching, Germany and

KEIZO E N D 0

Department of Physics, Tokyo Metropolitan University Setagaya-Ku, Tokyo, Japan

R6sum6. - Par spectroscopie Mossbauer avec 119Sn, 121Sb et 178Hf on a determine les champs hyperfins dans des alliages Heusler de la composition Co 2VSn, Co zNbSn, Co zHfSn, C ~ z - ~ F e ~ T i s n (x = 0.2, 0.4), C O ~ - ~ N ~ , T ~ S ~ (x = 0.1, 0.2) et Co~TiSno.sSb0.1. En outre on a

mesure le champ hyperfin du Ta au site du Hf dans I'alliage CozHfSn par la mkthode de perturba- tion de correlations angulaires differentielles. Les rksultats sont discutes par rapport A la structure electronique de ces alliages. En particulier les anomalies observees pour le champ hyperfin de 1'6tain sont expliquks par I'hypothbe que la polarisation des electrons-p de I'etain depend des atomes voisins de metal de transition non magnktiques.

Abstract. - From Mossbauer experiments with 19Sn, 121Sb and 178Hf we have determined the hyperfine fields at Sn, Sb and Hf, respectively, in Heusler alloys of the composition CozVSn, CozNbSn, CozHfSn, Co2-,FezTiSn (x = 0.2, 0.4), C O ~ - ~ N ~ ~ T ~ S ~ (x = 0.1, 0.2) and Co2TiSn0.9Sbo.l. Moreover, the hyperfine field at Ta on the Hf site in CozHfSn alloy has been measured by the time differential perturbed angular correlation method. The results are discussed in terms of the electronic structure of these alloys. In particular, the observed anomalies in the tin hyperfine field are explained by the assumption that the polarization of the p-electrons of tin depends decisively on the nature of the neighbouring nonmagnetic transition metal atoms.

Introduction.

-

In the ferromagnetic Heusler alloys Co2YSn, where the magnetic Y atoms are either the group IVa elements Ti, Zr and Hf or the group Va elements V and Nb, both the Co and Sn hyperfine fields have rather anomalous values. Thus the NMR value for the Co hyperfine field in Co2TiSn is

+

21 kOe [I], while a negative sign would be expected if the core polarization hypedine field were the dominant contribution, as it usually is. Mossbauer experiments, on the other hand, yield [l] large positive hyperline fields of+ 82,

+

88 and

+

106 kOe a t Sn in Co,TiSn, Co,ZrSn and Co2HfSn, respectively. These positive values of the Sn fieIds are suprising because the only magnetic atoms in these systems are the Co atoms, the contribution of which to the Sn hyperfine field should be negative according to the known systematics.

To obtain more information on the mechanism

(*) On leave from UFRGS, Brasil.

(**) On leave from the Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, Brasil.

(***) On leave from the Central Research Institute for Phy-

sics, Budapest, Hungary.

producing these anomalous positive fields we have measured :

i) the Sn hyperfine field in Co2-,Fe,TiSn ( x = 0.2, 0.4), Co2-,Ni,TiSn ( x = 0.1, 0.2), Co2TiSno.,Sbo.,, Co,VSn and Co,NbSn by Mossbauer spectroscopy with the 23.9 keV gamma-rays of '19Sn,

ii) the Sb hyperfine field in Co2TiSno.,Sbo., by Mossbauer spectroscopy with the 37.15 keV gamma- rays of 12'Sb,

iii) the Hf hypefine field in Co2Hf Sn by Mossbauer spectroscopy with the 93.3 keV gamma-rays of 178Hf and

iv) the hyperfine field at Ta on the Hf site in Co2HfSn using the time dependent perturbed angular correlation (TDPAC) of the 482-133 keV cascade in 18'Ta following the decay of 18'Hf.

2. Experimental.

-

The studied Co,VSn and Co,NbSn samples have already been used in previous investigations [2]. The others were prepared in

a

similar way as described in ref. [2].

The Mossbauer spectra were measured with a

conventional transmission spectrometer consisting

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C6-418 E. BAGGIO-SAITOVITCH et al.

of a liquid He cryostat with a vertical beam geometry and a sine-wave velocity spectrometer. Both the absorber and the source were kept a t 4.2 K except for the measurements with '"Sb, which were performed at liquid N, temperature. The sign of the hyperfine field at Sn and Hf was determined by applying a longitudinal magnetic field of 56 kOe produced by means of a superconducting solenoid.

For the ll'Sn measurements a 15 mCi BaSnO, source was used. The spectra showed the presence of a certain amount of disorder and impurity phase content in the sample. This was taken into account in the least-squares fitting of the spectra. In the Co2TiSn alloys substituted by Fe, Ni and Sb, only the average tin hyperfine field was determined. The substitution resulted in a line broadening of about 30

%

over the linewidth observed for the pure Co2TiSn alloy. The shift in the substituted alloys remained unchanged at the value of 1.40 (2) mm/s found in Co,TiSn.

The 12'Sb measurements were carried out with a source of 121rnSb in CaSnO,. The Mossbauer spectrum of C O , T ~ S ~ ~ ~ ~ S ~ ~ ~ , was fitted with a sum of 18 Lorent- zian lines. The relative line intensities were restricted to the theoretical values expected for a M 1 transition between nuclear levels with spins 7/2' and 5/2+.

The 178Hf spectra were taken with a single line source of 178W in Ta metal produced by the 181~a(d, 5 n)178W reaction with 45 MeV deuterons. The magnetic hyperfine patterns of Co2HfSn measured without external field were fitted with a superposition of 5 lines of equal intensity as is adequate for a transition between spins 2' and 0'. With a longitudinal magnetic field applied to either the Co,HfSn absorber alone or both the source and absorber, the spectra consist of two lines only 131, which results in an improv- ed resolution. The magnitude of the hyperfine field was calculated from the observed hyperfine splitting with g(2') =

+

(0.263 _$ 0.015) [4]. The linewidth obtained with the Co2HfSn absorber was about 3.2 mm/s, i. e. 60

%

larger than the natural linewidth. This probably reflects the incomplete crystallographic order observed earlier in the l19Sn data on the same sample [I]. A small amount of an impurity phase present in the sample caused a slight asymmetry of the 17'Hf Mossbauer spectra 'but did not seriously influence the value derived for the Hf hyperfine field. The TDPAC spectrum of lS1Ta in Co2HfSn was measured with a conventional fast-slow coincidence setup having a time resolution of about 1 ns. The experiments were carried out with the detectors under angles of 900 and 1800 and a magnetizing field at an angle of 450 in the detector plane. In this geometry one observes the Larmor frequency rather than its first harmonic 151, which renders the observation of high precession frequencies easier.

The source used for the TDPAC measurements was made by neutron irradiation of Co,HfSn. The I8lTa hyperfine field was determined from the Larmor preces-

sion frequencies with g(5/2-) = 1.30 (1) for the g-factor of the 482 keV state [6]. The TDPAC pattern was strongly damped. This again reflects the partial disorder and the presence of an impurity phase.

A summary of all results obtained in this work is given in table I and figure 1.

FIG. 1.

-

The average tin hyperfine field Hsn in C02-~l?e~TiSn, Coz-sNizTiSn and Co2TiSno.$3bo.l Heusler alloys as a function of the average magnetic moment

L.

The dotdashed line

represents the simple dilution.

3. Discussion. - As has been mentioned above, the positive hyperfine fields observed at Co and Sn [I] in various Heusler alloys of the type studied here are an outstanding feature calling for an explanation. We shall try to discuss the present results together with known data in terms of possible models for the des- cription of the magnetic hyperfine interactions in such systems.

Obviously, the positive hyperfine fields could summa- rily be explained by the assumption of a positive net conduction electron polarization. The Co and Sn atoms could then be considered as probes merely sensing this. The same would be expected for any other nonmagnetic probe. The negative hyperfine field at Hf in Co2HfSn (Table I) therefore rules out this possibility. Campbell and Blandin [7] discuss the hyperfine fields at non-magnetic atoms in different metallic systems in terms of a conduction electron polarization model in which the screening on the sp site is expli- citly included. They suggest that the contributions from the magnetic neighbours depend not only on the magnetic moments of the magnetic constituents, but also on the number of conduction electrons (B,) in the sense that the contributions from the first and second neighbours would be shifted towards the positive side when n, increases. The Sn fields in Ni2MnSn and

Cu,MnSn(+ 90 kOe and

+

200 kOe, respectively [8]) are, indeed, in agreement with this model, since n,

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HYPERFINE FIELDS IN CO-BASED HEUSLER ALLOYS C6-419

Properties of the studied Heusler alloys : the lattice parameters and cobalt magnetic moments are taken from ref. [2].

The hyper$ne$eields at Sn and Hf were measured at 4.2 K , those at Sb and Ta at 77 K

and room temperature, respectively

Lattice

Alloy parameter

(A)

PC~(PB) Hsn (koe) Other hf. fields (kOe)

-

- - Co2VSn 5.98 0.28

+

lO(1) Co2NbSn a = 6.21 (*) 0.32

+

15(1) c = 6.06 Co2TiSn 6.06 0.98

+

82(1) Hco =

+

21.2 C O ~ T ~ S ~ ~ . ~ S ~ ~ . , 6.05 0.94 77(1) Hsb= 125(8) Co2HfSn 6.20 0.75

+

106(2) HHf =

-

470(20) H T a = 347(8) (*) Tetragonally distorted.

and about 40 kOe/p, for Co2(V, Nb) Sn. If the assumed correlation between hyperfine field and n, is to hold, these data can only be explained by the assumption that V contributes less to n, than Ti due to a better localization of its d electrons. This explanation, however, cannot easily be reconciled with the '19Sn isomer shifts observed in Co2TiSn and Co2VSn, which are the same and thus indicate that the substitu- tion of Ti by V does not substantially change n,.

In this context it is interesting to consider the effects of substituting Fe and Ni for Co in Co2TiSn. The Fe and Ni substituents can be assumed to be nonmagnetic since both Fe2TiSn and Ni2TiSn are parama- gnets 121. According to the magnetization measurements, the effect of substituting Fe is close to a mere dilution, whereas the substitution of Ni results in a decrease of the average Co moment. 57Fe Mossbauer measurements in Co,-,Fe,TiSn support this assumption since the observed Fe hyperfine fields are less than 20 kOe and they presumably originate from the second neighbour Co atoms. In figure 1 we have plotted the average Sn-hyperfine field Hsn as a function of the average magnetic moment ji in the first coordination shell. The dot-dashed line in figure 1 corresponds to a simple proportionality of Hsn with ji. The substantial deviations from this correlation exhibited by the data for both Co2-,Ni,TiSn and Co2-,Fe,TiSn reveals an explicit dependence of the hyperfine field on the nature of the elements occupying the nearest-neighbour shell. The fact that we observed no change of the isomer shift of Sn in these alloys upon substitution of Fe or Ni, on the other hand, seems to rule out a change in the number n, of conduction electrons as the source of these deviations.

The average Co moment in Co2TiSno.,Sb0., is slightly smaller than that in Co2TiSn, but the data point for the Sb substituted alloy in figure 1 does not deviate from the cc mere dilution )) line. The Sb field in

the former alloy is 125 kOe and probably positive like

in all measured cases,

e.

g. the Sb field in Ni2MnSb (H,, = 307 kOe [9]). The ratio of the Sb and Sn hyperfine fields in Ni,MnSb (H,,/H,, x 5.6 [lo]) and in Co2TiSno ,,Sb0., (Hsb/Hs, x 1.5) are, however, substantially different.

This state of affairs may be compared with our results for Hf and Ta in Co,HfSn. In this system we find HT,/HH, = 0.91 (6) if we assume the Ta hyperfine field to be negative and correct the experimental value (Table I) for the fact that it has been obtained at room temperature instead of 4.2 K. This correction corresponds to an increase of the Ta field by a factor of 1.23, as can be concluded from the temperature dependence of the Sn field in Co2Hf Sn [I]. For the hyperfine fields at Hf and Ta in an iron matrix one obtains [3, 111-796 (47) kOe and 596 (18) kOe, respectively. This gives the ratio HF,/HHf = 0.77 (9) if one again applies a small (3

%)

temperature correction to the TaFe field value. Unless it is accidental, the fair agreement between HT,/HHf ratios in the Fe and Co2HfSn matrices indicates that the mechanism producing these fields in the two matrices is largely the same. Although more data will be needed before definite conclusions can be drawn, the data discussed above seem to reveal two different types of behaviour for nonmagnetic impurities in the same Heusler alloy matrices :

i) Nonmagnetic transition elements like Hf or Ta behave like in other magnetic environments (e. g. a-iron). This suggests that the mechanism producing the hyperfine field is also the same.

ii) Nonmagnetic sp group elements like Sn or Sb do not follow this pattern. They rather turn out to be- sensitive to the nature of the neighbouring nonmagnetic transition metal atoms.

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C6-420 E. BAGGIO-SAITOVITCH et al.

increases with the number of valence electrons of the sp element, has been explained by Blandin and Camp- bell [12] as a consequence of the charge screening at the sites of the sp elements. In this model the s and the p electrons are assumed to be alike and only their total number matters.

A possible explanation of the discussed anomalies in the Sn hyperfine field, however, seems to be that the s and p electrons play different roles, and that mainly the polarization of the p-electrons is responsible for the hyperfine fields. By this assumption one could explain that the tin fields were strongly affected by different substitutions in the Co,TiSn system while the isomer shifts remained virtually unchanged. An important consequence of this concept is that the contributions of the magnetic atoms to the tin hyperfine field may depend on the specific nature of the transition element, rather than merely on the magnitude of the polarizing

magnetic moment as has usually been assumed in the discussion of tin hyperfine field data [13].

Acknowledgment.

-

We should like to thank Prof. G. M. Kalvius, Dr. I. A. Campbell and Dr. G. Wortmann for stimulating discussions. Three of us (E. B.-S., A. V. and I. V.) wish to thank Prof. G. M. Kalvius for his hospitality at the Technical University of Munich. The reactor and cyclotron irradiations performed by the Gessellschaft fiir Kernforschung mbH, Karlsruhe, are also gratefully acknowledged. We are much indebted to Dr. J. M. Friedt for his interest and help in the I2'Sb Mossbauer experiments, which were performed at the Laboratoire de Chimie Nuclkaire, Strasbourg. The financial support from the CNPq (Brasil) E. B.

-

S.

+

A. V.) the KFA (Jiilich) (E. B.

-

S.) is also gratefully acknowledged.

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