A MÖSSBAUER SPECTROSCOPY STUDY OF SUPERPARAMAGNETISM IN THE IRON-MERCURY SYSTEM

HAL Id: jpa-00229093

https://hal.archives-ouvertes.fr/jpa-00229093

Submitted on 1 Jan 1988

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished 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.

A MÖSSBAUER SPECTROSCOPY STUDY OF

SUPERPARAMAGNETISM IN THE

IRON-MERCURY SYSTEM

S. Linderoth, S. Mørup, A. Meagher, S. Wells, J. van Wonterghem, H.

Rasmussen, S. Charles

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment au no 12, Tome 49, dkcembre 1988

A

MOSSBAUER SPECTROSCOPY STUDY OF

IN THE

IRON-MERCURY SYSTEM

S. Linderoth (I), S. Mdrup (I), A. Meagher (2), S. Wells ( 2 ) , J. van Wonterghem (3), H. K. Rasmussen (I) and S. W. Charles (3).

(I) Laboratory of Applied Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark

(2) Department of Physics, University College of North Wales, Bangor LL57 ZUW, G.B.

(3) Haldor Topsfle Equipment Division, DK-3600 Frederikssund, Denmark

Abstract. - Utrasmall particles in the iron-mercury system have been found to exhibit superparamagnetic relaxation above 150 K. At 12 K the average hyperfine field is significantly larger than that of bulk or-Fe. The results suggest that most of the iron atoms are influenced by mercury in a surface or bulk phase.

Metallic iron particles in mercury have been stud- ied extensively in the past [l-31. It was one of the first systems in which superparamagnetic relaxation was studied in detail. Since iron and mercury do not form stable alloys 141 it has been assumed that the mag- netic consist of pure a-Fe. Suspensions of ultrasmall a-Fe particles in mercury constitute electri- cally conducting ferrofluids which may have interesting technological applications [5].

In a previous study [6] we used Mossbauer spec- troscopy to characterize iron particles in mercury. The spectra were magnetically split up to the melting point of mercury and contained an a-Fe component and a second component which could be attributed to sur- face atoms or to a metastable iron-mercury alloy. In this paper we present results of a study of a similar system in which the magnetic particles are so small that they are superparamagnetic above approximately 200 K.

A sample of mercury containing a few per cent 5 7 ~ e was prepared by an electrolysis method described else- where [6]. A droplet of the ferrofluid was squeezed into

a thin film and frozen in liquid nitrogen.

The Mossbauer measurements were carried out with a constant acceleration spectrometer using 5 7 ~ o in rhodium as the source. Isomer shifts are given rela- tive to a-Fe at 295 K. External magnetic fields up to 0.6 T could be applied perpendicular to the direction of the y-rays.

The Mossbauer spectra obtained a t different tem- perature are shown in figure 1. At 12 K the spectrum is magnetically split. The lines are broad, indicating that more than one component is present. In a fit with two magnetically split components (sextets) magnetic hyperfine fields of about 39 T and 36 T, isomer shifts of about 0.36 mms-' and 0.16 mms-' and negligible quadrupole shifts were found. The magnetic hyperfine fields are significantly larger than that of the bulk of single-domain a-iron particles (= 34.7 T). The isomer shifts and the magnetic hyperfine fields show that the

1 : : : : : : : I -16 -12 -8 -4 0 4 8 12 16

Velocity (mm sl)

Fig. 1. - Mijssbauer spectra for the Fe-Hg system obtained at different temperatures.

iron is not present as Fe(I1) or Fe(III), but rather in the metallic state.

At higher temperatures the magnetic hyperfine split- ting collapses in a way typical of superparamagnetic particles [7, 81. At 150 K about 50 % of the mag- netically split components has collapsed. At 217 K essentially all the particles are superparamagnetic.

In figure 2 is shown the spectra obtained at 217 K at different applied magnetic fields. The appearence of a substantial magnetic hyperfine splitting when a field is applied is a well-known feature of superpara- magnetic particles of ferromagnetic materials [7, 81. From the field dependence of the induced hyperfine splitting the saturation hyperfine field, Bo, and the magnetic moment of the superparamagnetic particles,

p, can be determined [7-91. Using this method we find that Bo = 31 f 1 T , which is smaller than the value for bulk a-Fe at 217 K, and p = (1.8 f 0.4) x 10-l9 JT-l. Assuming that each iron atom possesses a magnetic moment of about 2 Bohr magnetons we find that each particle contains about 9500 iron atoms. If the mag- netic interaction between the particles is not negligible

JOURNAL DE PHYSIQUE

-16 -12 -8 -4 0 4 8 12 16 Velocity (mm s")

Fig. 2.

-

the magnetic moment is smaller than the value given above [9]. If the particles can be considered as isolated, spherical and consisting of a-Fe the diameter would be about 6 nm. The spectrum of silica-supported 6 nm a -

Fe particles consists of a "bulk" a-Fe component and a surface component with a hyperfine field of about 37 T at 80 K and with a relative area of about 15 %

[lo]. Thus the Mcssbauer parameters of the present particles are considerably different from those of the

6 nm a-Fe particles studied earlier. The reason for the difference could be that the particles in mercury are, for example, needle-shaped and therefore have a large fraction of the atoms in the first surface layer.

Another possibility is that the iron atoms are present in an alloy. The only other metal present is mercury which, however, does not form stable alloys with iron, but it is possible that a metastable Fe-Hg alloy has formed during the sample preparation.

Acknowledgements

Financial support from the Danish Technical Re- search Council and the Danish Natural Science Re- search Council is gratefully acknowledged.

[I] Mayer, A. and Vogt, E., 2. Naturforsch. 7A (1952) 334.

[2] Bean, C. P. and Jacobs, I. S., J. Appl. Phys. 27 (1956) 1448.

[3] Luborsky, F. E., J. Appl. Phys. 33 (1962) 1909. [4] Lihl, F., 2. MetalH. 44 (1953) 160.

[5] Charles, S. W. and Popplewell, Ferromagnetic Materials, Vol. 2, Ed. Wohlfarth, E. P. (North Holland, Amsterdam) 1980, 509.

[6] van Wonterghem, J., ~ 0 k u ~ , S. Charles, S. W.

and Wells, S., J. Magn. Magn. Mater. 65 (1987) 276.

[7] Marup, S., Durnesic, J. A., Topsae, H., Appli- Ations of Miissbauer Spectroscopy, Ed. Cohen, R. L. (Academic Press, New York) 2 (1980) 1. [8] Motup, S., Clausen, B. S. and Topsm, H., Phys.

Scr. 25 (1982) 713.

[9] Memp, S., Christensen, P. H. and Clausen, B. S.,

J. Magn. Magn. Mater. 68 (1987) 160.