SPIN ARRANGEMENT IN Na3Fe5O9

(1)

HAL Id: jpa-00214520

https://hal.archives-ouvertes.fr/jpa-00214520

Submitted on 1 Jan 1971

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

SPIN ARRANGEMENT IN Na3Fe5O9

P. Schurer

To cite this version:

P. Schurer. SPIN ARRANGEMENT IN Na3Fe5O9. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-278-C1-279. �10.1051/jphyscol:1971193�. �jpa-00214520�

(2)

JOURNAL DE PHYSIQUE Colloque C I , suppliment au no 2-3, Tome 32, Fiivrier-Mars 1971, page C 1

-

278

SPIN ARRANGEMENT IN Na,Fe,O,

(*) P. J. SCHURER

Solid State Physics Laboratory, University of Groningen, The Netherlands

RburnB. - Des mesures par effet Mossbauer montrent l'existence de trois sites magnetiques differents pour les ions de fer dans la combinaison antiferromagnetique Na 3Fe 5 0 9. Les valeurs des champs magnetiques hyperfins extrapoles a 0 OK aux noyaux Fe57 dans les sites Fe(l), Fe(2) et Fe(3) s'C1Bve a 485, 506 et 539 kOe. Les champs magnetiques hyperiins

Hene aux noyaux dans les deux sites tetraedriques (Fe(l), Fe(2)) montrent la m6me dependance en temperature, mais la d6croissance de Heer, lorsque la temperature croit, s'effectue plus vite pour le site octaedrique (Fe (3)). Nous proposons un arrangement de spins en accord avec ce comportement. Les positions theoriques des raies Mossbauer pour les directions differentes du champ magnktique sont calculkes et par comparaison avec les positions des raies experimentales on trouve que la direction de Heme est (0,03, 0,82, - 0,53).

Abstract. - Mossbauer effect measurements show that there are three different magnetic positions for iron ions in the antiferromagnetic compound Na3Fe509. The hyperfine magnetic fields extrapolated to 0 OK at Fe57 nuclei in the Fe(l), Fe(2) and Fe(3) position are 485, 506 and 539 kOe. The temperature dependence of the hyperfine magnetic field

H e ~ f at iron nuclei in the two tetrahedral positions (Fe(1) and ~ e ( 2 ) ) are the same but ^{H e f f}at iron nuclei in the octahedral position (Fe(3)) decreases more rapidly with increasing temperature. A spin arrangement is proposed which agrees with this behaviour. The theoretical Mossbauer line positions for different directions of the magnetic field are calculated and by comparison with the experimental line positions we find that the magnetic field points in the (0.03, 0.82, - 0.53) direction.

Introduction. - Na3Fe50, is an antiferromagnetic compound [I]. The crystal structure is C2/c with a monoclinic unit cell and j3 = 89O lo' 121. There are two Fe(1) and two Fe(2) positions for every Fe(3) position. Fe(1) and Fe(2) are surrounded by slightly distorted oxygen tetrahedra, while the Fe(3) ions are surrounded by slightly distorted octahedra.

With the point ion model we have calculated the directions of the principal axes of the E. F. G. and the asymetry parameters y = [Wxx - Wyy]/WZZ for the iron positions, assuming that the Fe-ions are in the 6 S g state and the crystal structure is orthorhombic.

For the three values of 11 we find y, = 0.84, q2 = 0.64 and q, = 0.06.

These values of y will be used for the analysis of the recorded spectra.

Experiments and analysis. - The susceptibility has been measured with a Faraday balance as function of temperature. Below 180 ^{O K}

x

increases with decreasing temperature and at the NCel temperature there is a broad maximum as can be seen in figure 1. A similar dependence of

x

on the temperature has been observed in other antiferromagnetic compounds [4].

FIG. 1. - Magnetic susceptibility as function of the temperature for Na3Fe509.

(*) Supported by the Foundation for Fundamental Research of Matter (FOM) of the Netherlands.

For the Mossbauer experiments we used a 14 mg/cm2 thick absorber of Na3Fe50, and a 5 mC source of Co57 in Pd. The spectrum above the N6el temperature TN = 381.1 ^O^K shows an asymmetrical two line pat- tern (Fig. 2), which we analysed as consisting of a single line, the contribution of Fe(3), and a quadrupole doublet attributed to Fe(1j and Fe(2). The spectra below TN (Fig. 2) have been fitted with three six lines spectra.

By comparing the recorded spectra below TN and calculated spectra, we find for the direction r of Heft with respect t o the crystal axes r ⁼(0.03, 0.82, 0.53).

The positions and intensities of these calculated spectra have been determined as a function of R =

1

eQVz,/2g+pN He.,

1,

using a computer pro- gram [3], for different values of the angles O and @, where O and @ specify the direction of Her, with respect to the electrical field gradient.

The isomer shift with respect to N~,[F~(CN),NO]

2 H20 at room temperature is 0.67 mm/s for Fe(3) and 0.48 mm/s for Fe(1) and Fe(2) ; this difference can be explained from the difference in F e - 0 distances.

There is more charge transferred in s orbitals of the Fe(1) and Fe(2) ions than in those of the Fe(3) ions because the average Fe(1,2)- 0 distance of 1.86

A

is smaller than the average Fe(3j- 0 distance of 2.03

A.

The effective magnetic field extrapolated to T ⁼0 ^O^K ( He,, (0 ^OK)( at Fe57 nuclei in octahedral sites is 539 +_ 2 kOe and the fields

I

He,, (0 OK)

I

at the ~e~~ nuclei in the two tetrahedral sites are 506 $- 4 kOe and 485

+

4 kOs. The reason that the effective magnetic field at iron nuclei in octahedral Fe(3) sites is larger than the fields at iron nuclei in the tetrahedral Fe(1) and Fe(2) sites can also be found in the larger Fe(3)-0 distance compared to the Fe(1)-0 and Fe(2)-0 distance.

I

^Heff

1

at an iron nucleus will be reduced through transfer of oxygen electrons into iron 3d- and 4s-orbitals and through

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971193

(3)

SPIN ARRANGEMENT IN Na3Fes09

FIG. 2. - Mossbauer absorption spectra of Na3Fes09 at different temperatures.

overlap distortion of the closed shell s orbitals ; all these contributions are expected to increase with decreasing Fe-0 distance. This also explains why

I

He,

I

in tetrahedral sites in Na,Fe,O, is smaller than

I

He,,

I

in tetrahedral sites in ferrites. The average iron-oxygen distances for tetrahedral iron sites are 1.86

A

and 1.90

A

respectively.

Figure 3 shows He, as function of temperature.

When we write

H,f,(T)IH,f,(O) = D

we find j?, =

P,

⁼0.22 for the iron ions in the two tetrahedral sites and

P,

⁼0.28 for the iron ions in the octahedral sites. The temperature dependence of He,, is the same for the two tetrahedral sites but He,, decreases more rapidly with temperature for iron ions in octahedral sites. The strength of the antiferromagnetic superexchange is therefore the same

500

400

300

A Octahedral

1

²⁰⁰

FIG. 3. - The effective magnetic fields as function of the tern- perature of the Fe57 nuclei at the three different crystallographic

positions.

for the tetrahedral Fe(1) and Fe(2) ions with their neighbours, but is stronger than that of the octahedral Fe(3) ions with its neighbours.

From these considerations we suggest a spin arrangement as shown in figure 4, which is also in agree-

FIG. 4. - Schematic projection along [001] of a part of the unit cell, as reproduced from figure 3 in ref. [2] with the pro- posed spin structure. FiIIed circles : Fe3+ ions Oxygen ions at the corners of the octahedra and tetrahedra. The numbers are the z parameters of the atoms ; i : in version center.

ment with the usually accepted rules about the dependence of superexchange strength . on cation-anion- cation distances and angles.

We like to acknowledge Dr. C . B. van den Berg for suggesting this investigation of Na,Fe,O,. We like to thank Dr. G. A. Sawatzky and Dr. F. van der Woude for fruitful discussions and for the help with the interpretation of the experimental data.

References

[I] ROOYMANS (C. J. M.), J . Phys. Soc. Japan, 1962, [3] KUNDIG (W.), Nucl. Instr. and Methods, 1967, 48,

17, 722. 219.

121 ROMERS (C.), ROOYMANS (C. J. M.), DE GRAAF (R. A. G.), [4] MORRIS (B. L.), RUSSO (P.), WOLD (A.), J. Phys.

Acfa Cryst., 1967, 22, 766. Chem. Solids, 1970, 31, 635.