THE INFLUENCE OF THE DEMAGNETIZING FIELD ON THE MAGNETIC SPLITING OF MÖSSBAUER SPECTRA

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THE INFLUENCE OF THE DEMAGNETIZING FIELD ON THE MAGNETIC SPLITING OF

MÖSSBAUER SPECTRA

J. Knudsen, S. Mørup

To cite this version:

J. Knudsen, S. Mørup. THE INFLUENCE OF THE DEMAGNETIZING FIELD ON THE MAG-

NETIC SPLITING OF MÖSSBAUER SPECTRA. Journal de Physique Colloques, 1980, 41 (C1),

pp.C1-155-C1-156. �10.1051/jphyscol:1980140�. �jpa-00219709�

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JOURNAL DE PHYSIQUE Colloque C1, suppl&ment au n ^O1, Tome 41, janvier 1980, page C1-155

M E INFLUENCE OF M E DEWWETIZING FIEUI ON THE MAGNETIC SPLIlTING OF M~SBAUER SPECTRA

J.E. Knudsen and S. Mbrup

h b o ~ a t o r y of Applied Physics 11, Technical University of Denmark, DK-2800 Lyngby, Denmark.

In a ferromagnetic material the arrangement of the atomic magnetic dipoles may give r i s e t o uncompensated magnetic poles a t the surface, and these create the so-call ed demagnetizing f i e l d . This f i e l d adds to the magnetic hyperfine field a t the Mossbauer nucleus, and i t therefore contributes t o the magnetic s p l i t t i n g of the Mossbauer spectrum.

If the sample i s macroscopically non-magnetized, the magnetization i s s p l i t up into domains of homogeneous magnetization, arranged in a way which minimizes the magnetic energy, including that of the demagnetizing f i e l d . In the i n t e r i o r of the material, f i e l d s from uncompensated magnetic poles a t the domain boundaries are avoided by an arrangement of the magnetization of neighbouring domains, so that the component of magnetization perpendicular to the boundary i s constant across the boundary. A t the surface the domain magnetization may be parallel to the surface, or neighbouring surface domains can have poles of opposite signs, to reduce the demagnetizing f i e l d . Thus in a multi-domain sample the demagnetizing f i e l d i s negligible.

If the sample i s magnetized ( f o r instance by the application of an external magnetic f i e l d ) the domain structure i s broken down and a t magnetic sa- turation each particle i s a single domain with homogeneous magnetization

fi.

The uncompensated poles a t the particle surface then give r i s e t o a demagnetizing f i e l d

$=-D$

i n the i n t e r i o r of the par- t i c l e ( t h i s i s s t r i c t l y correct only for a particle of ellipsoidal shape magnetized in a principal di- r e c t i ~ n ) . In metallic iron the demagnetizing f i e l d therefore increases the magnetic s p l i t t i n g of the tliissbauer spectrum. The value of D depends on the shape of the sample. For a spherical particle D=41r/3 so t h a t HD=-7kr;in spherical particles of metal 1 ic iron. For a foi 1 magnetized parallel or perpendicular t o the f o i l plane, D=O and D=4r, respectively. Thus f o r a magnetized sample the magnetic s p l i t t i n g of a Mdssbauer spectrum i s expected

t o depend on the geometrical shape and the direction of magnetization.

In order to examine the effect we have mea- sured the magnetic s p l i t t i n g in room temperature MSssbauer spectra of three different types of me- t a l l i c iron absorbers with various magnetic f i e l d s applied in the absorber plane which i s perpendicu- l a r t o the y-ray direction:

( a ) a thin f o i l of metallic iron (10 mg Fe/

cm2 )

(b) fine iron powder (Koch-Light, 3N) uniformly dispersed in a r a l d i t , thickness ca. 1 mm (10 mg Fe/cm2)

( c ) a thin layer of the iron powder suspen- ded on an o i l film, and firmly pressed between two plates of plexiglass (20 mg Fe/cm2).

Scanning electron micrographs of the powder showed that i t consists of particles of nearly perfect spherical shape with diameters of 1-10 u.

Some of the Mdssbauer spectra of absorbers ( a ) and ( b ) without an applied f i e l d and with a f i e l d H=12.3 kG applied, are shown in figure 1.

Without an applied f i e l d absorbers of a l l three types have Mtissbauer spectra with identical magne- t i c s p l i t t i n g s and with line intensities approximately in the r a t i o 3:2:1:1:2:3. However, with applied f i e l d s the spectra of the powdered absorber (b) show larger s p l i t t i n g than the corresponding spectra of the foi 1 ( a ) . For '.1 it 6 k6 the difference in magnetic s p l i t t i n g of the powder and f o i l samples are nearly independent of H , and the relative l i n e areas are approximately given by 3:4:1:1:4:3, indicating that the magnetization i s para1 l e l to the applied f i e l d . A t H=12.3 kG the deviations in magnetic s p l i t t i n g f o r the three types of absorbers are I ~ ~ l - ( H ~ l = 6 k l kG and IHcI-IHal=3+1 kG. These results are in accordance with the expected influence of the demagnetizing f i e l d . In zero applied f i e l d the demagnetizing f i e l d i s negligible be-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980140

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c1-156 JOURNAL DE PHYSIQUE

VELOCITY (MM/S>

Fig. 1 : Mb'ssbauer spectra of metallic iron samples a t room temperature without an applied f i e l d and with H-12.3 kG applied perpendicularly t o the y-ray d i r e c t i o n . Only part of t h e velocity s c a l e i s shown. ( a ) iron f o i l ( b ) powder sample.

cause both t h e f o i l and t h e p a r t i c l e s a r e s p l i t up i n t o magnetic domains. In t h e applied f i e l d of 12.3 kG the f o i l as well a s t h e p a r t i c l e s of t h e powder samples become s i n g l e domains wi t h homogeneous magnetization parallel t o the applied f i e l d . In the f o i l we find HD=O because D=O, and t h e s p l i t - t i n g i s determined by the f i e l d a t the nucleus a t H=O (330.2 kG) minus t h e applied f i e l d H (12.3 kG).

In t h e powder sample ( b ) t h e s p l i t t i n g i s enhanced by the contribution from t h e demagnetizing f i e l d HD ( 7 kG).

W i t h the absorber ( c ) t h e enhancement i s smaller. However, i n t h i s absorber the individual iron p a r t i c l e s a r e not well separated, and f o r a given p a r t i c l e t h e magnetic moments of the other p a r t i c l e s contribute a dipolar f i e l d which i s opposite t o the demagnetizing f i e l d of t h e p a r t i c l e

-f

proximately given by +$ fM, where f i s the pack- ing f r a c t i o n .

The present investigation shows t h a t t h e magnetic s p l i t t i n g i n Mossbauer spectra of f e r r i - o r ferromagnetic samples i n applied magnetic f i e l d s depends c r i t i c a l l y on the sample preparation (e.g. t h e p a r t i c l e shape and the separation of the grains in a powder sample) because of t h e influence of demagnetizing and dipolar f i e l d s .

I t i s of p a r t i c u l a r i n t e r e s t t h a t i n magne- t i c microcrystals, which a r e s i n g l e domain p a r t i - c l e s , the demagnetizing f i e l d contributes t o t h e magnetic s p l i t t i n g , even i n the absence of an applied f i e l d . In spherical s i n g l e domain p a r t i c l e s of metallic iron this contribution equals 7 kG / I / .

In studies of very thin films the e f f e c t may a l s o be of g r e a t i,mportance. In most thin film studies the magnetization has been found t o be pa- r a l l e l t o the film plane, and i n t h i s case HD=O.

However, in recent Mossbauer studies of very t h i n iron f i lms covered by Cu /2/, Ag / 2 / , and MgO /3/, t h e r e l a t i v e i n t e n s i t i e s of the absorption 1 ines indicated a magnetization which i s nearly perpendi- c u l a r t o the film plane. In t h i s case a s i g n i f i c a n t contribution from the demagnetizing f i e l d t o the magnetic s p l i t t i n g of about 21 kG i s expected and t h i s must be taken i n t o account in the interpreta- t i o n of the spectra.

The authors a r e indebted t o F. Kragh f o r ob- taining the scanning electron micrographs and t o t h e Danish Natural Science Research Council f o r f i - nancial support.

References

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i t s e l f . For a sample t h a t c o n s i s t s of p a r t i c l e s uniformly d i s t r i b u t e d in a layer para1 l e l t o the magnetization d i r e c t i o n , t h i s dipolar f i e l d i s ap-