MÖSSBAUER STUDIES IN FERROMAGNETIC Ru2FeSn

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MÖSSBAUER STUDIES IN FERROMAGNETIC Ru2FeSn

R. Pillay, R. Nagarajan, P. Tandon

To cite this version:

R. Pillay, R. Nagarajan, P. Tandon. MÖSSBAUER STUDIES IN FERROMAGNETIC Ru2FeSn.

Journal de Physique Colloques, 1979, 40 (C2), pp.C2-218-C2-220. �10.1051/jphyscol:1979278�. �jpa-

00218676�

JOURNAL DE PHYSIQUE

Colloque C2, supplément au n° 3, Tome 40, mars 1979, page C2-218

MOSSBAUER STUDIES IN FERROMAGNETIC Ru

FeSn

R.G. Pillay, R. Nagarajan and P.N. Tandon

Tata Institute of Fundamental Reeeavoh, Bombay 400 005, India

Résumé.- Nous avons mesuré les champs magnétiques hyperfins du fer et de l'étain dans un nouvel alliage Heusler Ru2FeSn, en utilisant l'effet Mossbauer. Les valeurs des champs magnétiques obtenus à 80 K sont 314,0 ± 1,5 et + 117,9 ± 1,5 kOe sur le fer et l'étain respectivement. Le champ sur l'étain est presque deux fois plus grand que dans RhjîeSn, alors que le champ au fer est approxima- tivement le même que dans le fer naturel.

Abstract.- We have measured the hyperfine magnetic fields at iron and tin in a new Heusler alloy Ru2FeSn using Mossbauer effect. The values for the hyperfine fields obtained at 80 K are 314.0 ± 1.6 and + 117.9 ± 1.5 kOe for iron and tin respectively. The field at tin is about a factor of two larger in comparison to Rh2FeSn while the field at iron is close to the natural iron field.

From the trends of the hyperfine fields at di- lute impurities in ferromagnetic hosts, like Fe, Co and Ni, it is known that a change in sign in the hyperfine field is observed when the s-p impurity has a valency between 3 and 4. In the case of tin, the hyperfine field is observed to be negative in iron and cobalt hosts but it is positive in nickel host /l/. It is the only known case where the sign of the hyperfine field changes in going from Fe, Co hosts to Ni host while all other impurities have the same sign in these hosts. Such a .behaviour is also exhibited in observed hyperfine fields at tin in ferromagnetic Heusler alloys containing Fe, Co and Ni. In the particular case of rhodium based Heusler alloys of the type Rh2TSn, where T = Mn, Fe, Co and Ni, the hyperfine field is positive at tin in Kh2MnSn and Rh2FeSn, +26 and +55 kOe respectively, while it is negative for Rh2CoSn and Rh2NiSn, -51 and -11 kOe respectively /2,3/. On the other hand, the field at rhodium in all these alloys is negative and roughly the same, about -200 kOe Ikl. Thus tin is a very sen- sitive probe for investigating' hyperfine interaction in these alloys. With an aim to study the systematics of the hyperfine fields at tin in similar systems, we have made a new Heusler alloy Ru2FeSn, by repla- cing rhodium by ruthenium in the already investiga- ted Rh2FeSn.

The alloy Ru2FeSn was prepared by powder metal- lurgical technique /5/. X-ray diffraction of the sample showed the formation of a unique cubic phase.

This is in contrast to Rh2FeSn and Rh2CoSn which are tetragonal /6/. The superstructure reflections from all odd hkl planes were present. The lattice cons- tant was found to be 6.202 (7) A.

The alloy was strongly magnetic at room tempe- rature. Its Curie temperature, T , was determined by measuring the susceptibility in a weak magnetic field of 0.5 gauss. The T was obtained to be 593 ± 5 K, quite close to that of Rh2FeSn, 583 K.

The Mossbauer effect in tin and iron was ob- served using single line sources of Ca 1 1 9Sn0 and

57Co(Pd) respectively. The spectra were taken on a constant acceleration drive using a sequential data acquisition system. Figure lb shows the six line spectrum obtained with a 57Co(Pd) source and Ru.FeSn absorber at liquid air temperature. The hyperfine magnetic field values obtained at iron from the least squares fit to the data are 282.5 ± 1.6 and 314.0 ±

1.6 kOe at room and liquid air temperature respecti- vely. The intensities ratios of the six absorption lines are consistent with that from a randomly pola- rized sample.

The Mossbauer spectrum observed with the same absorber, taken with a Ca 119SnO source, shown in figure la indicates a partially resolved six finger pattern with an additional doublet in the centre.

The line intensities of this spectrum suggest the sample to have significant polarization. However, the iron data with the same, absorber shows the sam- ple to be unpolarized (Fig. lb). This is suggestive of at least two distinct magnetic hyperfine fields seen by tin. In addition there is a central doublet arising from some unidentified impurity phase. The least squares fit was done for two distinct magnetic fields and a impurity doublet. The hyperfine field values obtained are summarized in the following

table. Of the two components, the one which has the largest intensity and which also happens to be the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979278

from the change in splittings in an applied trans- verse magnetic field of 5 kG. The sign was uniquely determined to be positive for the largest field com- ponent. It was not unambiguously possible to deter- mine the sign of the magnetic field of the other

component.

Table I1 shows the hyperfine fields at iron and tin sites in Ru2FeSn and compared with the known values in Rh,FeSn.

Table I1

A comparison of the hyperfine field data on Ru,FeSn and Rh,FeSn

i

i

i

i

:

i Iron r Tin

i

ⁱ

V E L O C I T Y ( m m / s )

Fig. I (a) : The Massbauer spectrum obtained with a Ca 19sn0, source and Ru,FeSn absorber at 80 K. The dashed line is the Least squares fit to the data.

The solid lines show splittings due to two distinct magnetic fields and a doublet (see text).

(b) : The Mgssbauer spectrum obtained with a

"Co(pd) source and Ru2FeSn absorber at 80 K.

largest field (site I) is assigned to the tin atoms occupying the proper site.

Table I

Summary of tin hyperfine fields measured in Ru,FeSn Ssomer shift;

f Hyp. fields (kOe) :Relative

i

i

ⁱ i i

i

i

i

~ R U ~ F ~ S ~ : 314.0+1.6~+117.9t1.5

i

i

i

i

As one sees from this table, the net effect of re- placing rhodium by ruthenium resulted in an increase in the hyperfine field, both at iron and tin sites.

At iron the field approaches that of natural iron, assuming a negative sign for the field in Ru2FeSn.

The tin site is more sensitive and the field, though still remains positive increases by a factor of two.

An attempt is made to understand the field at tin using Blandin-Campbell model /8/ in which the spin polarization is treated to arise from double reso- nance scattering off the magnetic atoms. The summa- tion for the spin polarization, both for Ru,FeSn and Rh2FeSn, has been carried out to within a radius of

0

100 A. The net spin polarization at tin site in both This is further supported from the X-ray diffraction

photograph which showed the presence of ordered lines indicating the presence of reasonable order in the alloy. From the iron Mgssbauer spectrum, it is evi- dent that iron is occupying a unique site suggesting that the smaller hyperfine field on tin is due to tin occupying the ruthenium site indicating the pre- sence of partial disorder between Ru and Sn sites.

Such a disorder has also been observed in Pd,MnSb by Malik et al. /7/. Annealing the sample for two weeks at 500'~ in argon atmosphere did not in any way affected the shape and splittings of the Hgss- bauer spectrum.

The sign of the hyperfine field was measured

these alloys were computed for iron moments ranging from 2 to 4 pB, and conduction electron contribution from ruthenium and rhodium ranging from 0 to I elec- tron per atom. This always leads to a positive field at tin site in both these alloys. However the magni- tude of the net spin polarization is rather sensiti- ve to the choice of both the moment at iron and the number of conduction electrons from ruthenium or rhodium. Within the scope of this model, one canex- plain the sign of the field observed at tin. However it is difficult to generate the exact magnitude of the hyperfine field from the net spin polarization because of the uncertainties in the hyperfine cou- pling constants

.

JOURNAL DE PHYSIQUE

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