MÖSSBAUER EFFECT : A TEST FOR CHECKING THE NÉEL MODEL IN FERRIMAGNETIC RARE-EARTH IRON COMPOUNDS

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MÖSSBAUER EFFECT : A TEST FOR CHECKING THE NÉEL MODEL IN FERRIMAGNETIC

RARE-EARTH IRON COMPOUNDS

D. Barb, E. Burzo, M. Morariu

To cite this version:

D. Barb, E. Burzo, M. Morariu. MÖSSBAUER EFFECT : A TEST FOR CHECKING THE NÉEL MODEL IN FERRIMAGNETIC RARE-EARTH IRON COMPOUNDS. Journal de Physique Collo- ques, 1974, 35 (C6), pp.C6-625-C6-627. �10.1051/jphyscol:19746136�. �jpa-00215750�

JOURNAL DE PHYSIQUE Colloque C6, supplgment au no 12, Tome 35, Dkcembre 1974, page C6-625

MOSSBAUER EFFECT : A TEST FOR CHECKING THE NEEL MODEL IN FERRIMAGNETIC RARE-EARTH IRON COMPOUNDS

D. BARB, E. BURZO and M. MORARIU

Institute for Atomic Physics, P. 0 . Box 35, Bucharest, Romania

R6sumC. - Nous ktudions la variation thermique des champs internes du 57Fe dans les composCs ferrimagnktiques RFez (R = Gd, Tb et Ho). Nous comparons la dkpendance avec la tempkrature du champ interne moyen avec Mpe(T) I'aimantation du sous-rkseau du fer. Les courbes Mpe(T) ont ktk calculkes dans le modele de Nee1 a partir des mesures magnktiques. On remarque un bon accord avec les valeurs obtenues par E. M. Le modele de NQl decrit bien le comportement magnetique de ces alliages metalliques.

Abstract. - We study the thermal variation of 57Fe hyperfine fields in some ferrimagnetic RFez (R ⁼ Gd, Tb and Ho) compounds. We compare the temperature dependence of the mean hyperfine field with Mpe(T) iron sublattice magnetization. The Mpe(T) was computed - according to Nkel model - from magnetization data. The calculated curves agree with the values obtained from M. E.

studies. We conclude that the Nee1 model may be used to describe the behaviour of these metallic alloys.

1. Introduction. - The NCel model (1) is successful in describing the magnetic behaviour of ferrimagnetic insulators, where the magnetic moments are well- localized. It is also of interest to analyse to what extent this model may take into account the magnetic properties of ferrimagnetic metallic systems.

The heavy rare-earth (R)-iron compounds are typical alloys, where the moments of the two sublat- tices ^- corresponding to R and Fe respectively -

are antiparallel oriented. The ferrimagnetic RFe, sys- tems (R = Gd to Tm) are very attractive among these compounds because of their high crystalline symmetry. They crystallize in MgCu, type-structure.

The point symmetry of rare-earth is cubic (43 m) and that of iron is trigonal (3 m). All R and Fe atoms, respectively, are equivalent.

The 4f electrons responsible for the magnetism of the rare-earths are well-localized. This cannot be asserted about the 3d magnetic shell of iron. Thus, with the increase of rare-earth content, a decrease of the iron magnetic contributions is evidenced [2]. It is also of interest to analyse the magnetic behaviour of iron moments in these systems.

Making use of our magnetic data [3] we calculated the thermal variation of the spontaneous magnetiza- tion of the R and Fe sublattices - according to NCel model - in some RFe, compounds. In order to check if these solutions are unique we used the Mossbauer data, comparing the thermal variation of 57Fe hyper- fine field with the calculated iron sublattice magne- tization.

2. The NBel model of ferrimagnetic RFe, compounds.

- The temperature dependence of the magnetic moment of binary compounds, M(T), according to NCel model of ferrimagnetism is given by :

M(T) ⁼ MR(T) - MF,(T) (1) where

MR(T) = MR(0) B,,(XR)

(2) MFe(T) = MFe(o) B~F,(X~e)

Bj,(XR) and BsFe(XFe) are the Brillouin functions and

J~ gR PI3 XR = ----

k g T (NRR M~ - N ~ MFe) ~ e

(3)

Here Nij (i, j = R, Fe) are the molecular field coefficients characterizing the magnetic interactions in compounds.

Using the magnetization data [3] we computed the thermal variation of sublattice magnetization. We used the Nij values deduced both from saturation measurements - by fitting the experimental sponta- neous magnetization data - and from paramagnetic studies. No essential differences have been observed [4]

though the determination of molecular field coeffi- cients from susceptibility data may lead t o some errors [5]. The accuracy in determining the Nij values

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746136

C6-626 D. BARB, E. BURZO AND M. MORARIU from measurements in the ordered range is better.

The above values are used in our calculation.

We present in figure 1, for example, the computed curves for GdFe,, compared with the experimental results.

Temperature (K)

FIG. 1. - Thermal variation of spontaneous magnetization of Gd and Fe sublattices and the GdFez magnetization. Experi-

mental data and calculated curves by solid lines.

The hyperfine fields in iron alloys originate fromathe moments [6], therefore they contain also information about the iron magnetic contribution. The M. E.

measurements will be used in such a way as to check if the above decomposition of magnetization is real and unique.

3. Experimental. - The Mossbauer measurements were made between 4.2 K and the Curie temperatures by a standard ELRON-type equipment. A 57Co source in copper matrix was used. The data storage was performed for the velocity range of k 10 mm/s by means of DIDAC-4000 analyser. The spectra were fitted using a FORTRAN programme, supposing a Lorentzian shape of the lines.

The thermal variation of the hyperfine fields is pre- sented in figures 2a-4a. In case of GdFe, the measure- ments were made only up to 650 K ; at higher tempe- ratures the spectra could not be analysed.

Although the iron atoms occupy only one crystallo- graphic site, in the case of GdFe, and TbFe, two six-line patterns are evidenced. The presence of different spectra in RFe, systems was attributed to the different orientations of the easy magnetization axis relative to the crystallographic axis 17, 81. When the system magnetizes along [OOl] direction, all the iron atoms are magnetically equivalent and a six-line spectrum is observed, as in case of HoFe,. If the easy direction of magnetization is along [ I l l ] direction two magneti- cally unequivalent iron sites are present. The spectra consist of a superposition of two six-line patterns with the intensities in 1 : 3 ratio. Generally the direction of magnetization is essentially determined by rare- earth anisotropy, since the iron has its orbital moment quenched [8,9]. When R = Gd - which is in S-state -

the competition between anisotropies of the iron and

FIG. 2-4. - ^(a) Thermal variation of hyperfine fields in GdFe2, TbFe2 and HoFe2 compounds, respectively. (b) Comparison between the temperature dependence of iron sublattice magneti- zation (solid line) and the values obtained from hyperfine fields.

the gadolinium sublattices (which are comparable) in.

determining the easy direction of magnetization may be considered. The spectra consist of two six-line pat- terns with the intensities in 1 : 1 ratio. The hyper- fine parameters at 4.2 K are presented in Table I.

MOSSBAUER EFFECT : A TEST FOR CHECKING THE NEEL MODEL c6-627

TABLE I sublattice magnetization with the mean hyperfine The hyperjne parameters at 4.2 K

Hypefine Isomer Quadrupole Intensities The fields shift splitting ratio compound (kGs) (mm/s) (mm/s) hl111

- ^{- - -} -

4. Discussion. - Since in case of TbFe, and GdFe, there are two values of the hyperfine field we have to use their mean value gn, where = Cn, Hni/Zni and ni is the number of the atoms with the hyperfine field Hni. The average procedure is justified in this situation, whenever we want to correlate @,, data with the magnetization [9].

We assume a proportionality between the hyperfine field and magnetization

KIM,,

The obtained value A = (150 f 5) kGs p i 1 is close t o that observed in ternary iron systems [9] or in iron metal. The proportionality is also evidenced in GdFe, compound. Since gadolinium - unlike other R ele- ments - is in the S-state, its moment is not affected by the crystalline field. Supposing the ferrimagnetic coupling of the sublattices, from magnetization measu- rements at 4.2 K, one obtains p,, = 1.60 p,. From the average hyperfine field - using the above A value

- we obtain 1.56 pB, close to the previous values.

An analysis of the temperature dependence of A for iron showed that the fractional change in the hyperfine coupling constant is less than 1 % between 0 and 400 K [ll].

It seems thus reasonable to compare the iron

- - -

field. For simplicity we use the reduced values : -

MF, . Hn ^rn

- - _and 1 ,

MFe(") ' Tc

where K ( 0 ) is the mean hyperfine field at T = 0 K and Tc the Curie temperature.

As seen in figures 2b-4b a rather good agreement between the calculated values and the experimental M. E. data is observed. The agreement is better in case of HoFe,, for GdFe, and TbFe, the experimental data being slightly different from the calculated curves.

This may be justified by the fact that we have consi- dered a mean contribution of the iron moments, the interactions being actually more complex as a result of the two different iron magnetic sites.

One notes that the calculated values are unique, our test proving that the Ntel model describes rather well the magnetic properties of the considered systems.

The iron atoms present the features described by the two rather different magnetic models : band and loca- lized, respectively. The localized behaviour is suggested by our discussion which evidences a linearity between the iron moment and the hyperfine field. At the same time, the thermal variation of the reciprocal suscepti- bility obeys a Ntel law [3]. The values of the magnetic interactions deduced from the saturation measure- ments do well agree with those obtained from the paramagnetic studies. Using the latter values, we described the thermal variation of the spontaneous magnetization obtaining good agreement with the experimental data [4] as in the case of the magnetic insulators. The band behaviour is reflected by the sensibility of the iron moment to the rare-earth content. Thus, as the molar fraction of the rare-earth is increasing, a decrease of the iron magnetic moment, deduced from both saturation and paramagnetic measurements, is evidenced 121. It is expected that the conduction electrons contributed by the rare-earth will gradually fill the iron 3d magnetic shell [2, 31.

The above characteristics may be considered in the frame of the existing magnetic models [12, 131.

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