NON-COLLINEARITY AS A CRYSTALLITE-SIZE EFFECT OF γ-Fe2O3 SMALL PARTICLES

(1)

HAL Id: jpa-00219716

https://hal.archives-ouvertes.fr/jpa-00219716

Submitted on 1 Jan 1980

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.

NON-COLLINEARITY AS A CRYSTALLITE-SIZE EFFECT OF γ -Fe2O3 SMALL PARTICLES

A. Morrish, K. Haneda

To cite this version:

A. Morrish, K. Haneda. NON-COLLINEARITY AS A CRYSTALLITE-SIZE EFFECT OF γ- Fe2O3 SMALL PARTICLES. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-171-C1-172.

�10.1051/jphyscol:1980147�. �jpa-00219716�

(2)

JOURNAL DE PHYSIQUE

Colloque

C1,

suppl&ment

au

n "

1,

Tome

41, janvier 1980, page C1-171

NON-COUI NEARITY AS

A CRYSTALLI

TE-S I

ZE

EFFECT OF

Y-Fe20 SWLL

PARTICLES

A.H. Morrish and K. Haneda

*

Department of Physics, Vrzioersity o f Manitoba, Fdnnipeg, MB, Canada R3T 2N2.

Absfract.- The s u r f a c e w g n e t i c s t r u c t u r e o f y-Fez03 p a r t i c l e s , known t o b e n o n - c o l l i n e a r , i s e x p l o r e d i n terms o f t h e s i z e of the c r y s t a l l i e e s t h a t make up t h e p a r t i c l e . MGssbauer s p e c t r o s c o p y i s employed.

1. I n t r o d u c t i o n .

-

The a p p l i c a t i o n of t h e ~ 6 ~ s - pure Y-FezOa.

-

The shape of t h e p a r t i c l e s i s c l o s e

bauer e f f e c t h a s y i e l d e d a number o f i n t e r e s t i n g t o s p h e r i c a l , a s determined by e l e c t r o n microscopy.

r e s u l t s concerning t h e microscopic magnetic proper- Information on t h e p a r t i c l e s i z e s was o b t a i n e d by t i e s of f i n e p a r t i c l e s . Gamma f e r r i c o x i d e t h e BET n i t r o g e n a d s o r p t i o n method, and o n t h e (y-Fe203) i s a f e r r i m a g n e t w i t h t h e s p i n e l s t r u c - c r y s t a l l i t e s i z e from X-ray l i n e broadening. The t u r e , and is u s u a l l y assumed t o have a c o l l i n e a r p a r t i c l e s i z e ranged from around l o 2 t o l o 3 and magnetic s t r u c t u r e c o n s i s t i n g of two s u b l a t t i c e s . t h e c r y s t a l l i t e s i z e from about 70 t o a few hundred However e a r l i e r M"dssbauer s t u d i e s of s m a l l Y-Fen03 angstroms; t h e d a t a a r e l i s t e d i n t a b l e I.

p a r t i c l e s [1,21 and micron-sized Y-FezO3 p a r t i c l e s _Table I ! Parameters f o r the Y-Fe20s samples.

wich s u r f a c e c a t i o n s e n r i c h e d i n t h e i s o t o p e FeS7 [ 3 ] , have revealed t h a t a non-collinear s p i n

arrangement, p r i m a r i l y a t o r n e a r t h e s u r f a c e , o c c u r s f o r p a r t i c l e s i z e s l e s s t h a n about 10'

x.

T h i s c o n c l u s i o n was based on t h e presence of t h e second and f i f t h l i n e s of t h e ~ e " MZjssbauer s p e c t r a o b t a i n e d w i t h a l a r g e magnetic f i e l d a p p l i e d a l o n g t h e propagation d i r e c t i o n of t h e y-ray. Moreover, t h e a v e r a g e s p i n c a n t i n g a n g l e i s l a r g e a t l i q u i d helium t e m p e r a t u r e s , and g r a d u a l l y d e c r e a s e s w i t h i n c r e a s i n g temperatures.

Meanwhile, i t h a s been g e n e r a l l y a c c e p t e d t h a t a micron- o r submicron-sized y-Fe203 p a r t i c l e i s composed of f i n e l y d i v i d e d c r y s t a l l i t e s [ 4 - 6 1 . Thus, t h e r e l a t i o n s h i p between t h e c r y s t a l l i t e s i z e and t h e s p i n s t r u c t u r e , i f any, needs t o b e ex- p l o r e d b e f o r e f u r t h e r p r o g r e s s can b e made.

2. Experiments and Result$.

-

S e w r a l sample* w i t h v a r i o u s p a r t i c l e and c r y s t a l l i t e s i z e s were exam- i n e d . X-ray d i f f r a c t i o n p a t t e r n s using Co-Ka r a d i - a t i o n t o g e t h e r w i t h S s s b a u e r s p e c t r a a t room tem- p e r a t u r e c l e a r l y i n d i c a t e t h a t t h e s e samples a r e

X

P r e s e n t a d d r e s s : Research I n s t i t u t e f o r S c i e n t i f - i c Measurements, Tohoku U n i v e r s i t y , Sendai, Japan.

Samples A , B, and C a r e prepared by chemical p r e c i p i t a t i o n O £ ' F ~ , O ~ and a r e converted by subse- quent heat-treatment t o y-Fez03. Sample D i s prepared by a n o r d i n a r y powder m e t a l l u r g y method.

For a l l t h e s e samples, ~ S s s b a u e r s p e c t r a were taken a t 4.2 K i n a magnetic f i e l d of 50 kOe ap- p l i e d p a r a l l e l t o t h e d i r e c t i o n of t h e y-ray, and a r e shown i n f i g u r e 1. The d a t a were computer- f i t t e d w i t h two s i x - l i n e p a t t e r n s ; t h e i r sum i s shown a s f u l l curves. The 2 and 5 s p e c t r a l l i n e s a r e c l e a r l y v i s i b l e f o r each sample; t h e i n t e n s i t y is g r e a t e s t f o r sample A. The r e l a t i v e i n t e n s i t y of t h e 2, 5 l i n e a r e a s compared t o t h e 1, 6 l i n e a r e a s was determined f o r each sample ( t a b l e I).

From t h e s e v a l u e s , t h e t h i c k n e s s of t h e spin-cantqd s u r f a c e l a y e r of each c r y s t a l l i t e was e v a l u a t e d assuming a s p h e r i c a l shape, no c a v i t i e s , v a r i a b l e o r random c a n t i n g a n g l e s between 0 and 90°, t h e

Sample

A B C D

s ( ~ , ~ )

a

^{1 9 6}

0.213 0.127 0.137 0.091

Canted-layer Thickness

(1)

5 4 7 7 Partic e

S e

65 175 300 955

Crystal1 t e Size

(1)

70 97 209 303

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980147

(3)

C1-172 JOURNAL bl! PHYSIQUE

maintenance of local magnetic order, and a 3:2:1 line area ratio. The random-angle assumption is equivalent to an average canting angle of 54.5O, which is consistent with earlier results [3]. The values of the canted-layer thickness deduced are remarkably similar considering the experimental errors. An analysis based on particle size does not give consistent results for the thicknew of the canted layer.

In addition, for another sample with 2. 27

8

dxameter crystallites of cubic iron oxides formed on the shrface of 2. 293

2

diameter Fe particles, an extremely large spin canting is observed, as shown in figure 2 [7]. The relative area intensity of the 2, 5 lines to the 1, 6 iines for the iron- oxides is 0.690, and indicates that the spin canting extends over almost the entire crystallite. This result is compatible with the data of table I.

I I I I

0 5 10

VELOCITY

(rnrnls)

Fig. 2 : ~gssbauer spectrum of the surface oxidized iron powder at 4.2 K in an external longitudinal magnetic field of 25 kOe. The full curve is a least-squares computer fit of an iron four-line pat- tern plus two iron-oxide six-line patterns.

Earlier, Berkowitz et al, [51, to account for the dependence of the saturation magnetization of y-Fe203 on the crystallite size, postulated that the crystallites were separated by a non-magnetic grain boundary 6 in width. In view of the pre- sent results, it seems that the non-magnetic grain boundary may instead be a magnetic boundary in which the spin suffers considerable canting.

To conclude, the data imply that the crystallite rather than the particle size is the important factor for spin canting. Therefore, the particle

VELOCITY (mm/s) [6] Kishimoto, M. and Wakai, K., Japan J. Appl.

Phys. 16 (1977) 2059.

morphology of y-Fe203 particles, which is largely controlled by the starting raw materials and the heat-treatment conditions [6], appears to be a deci- sive factor in determining the magnetic properties.

References

[l] Coey, J.M.D. and Khalafalla, D., Phys. Stat.

Sol. (a)

11

(1972) 229.

[2] Haneda, K. and Morrish, A.H., Solid State Commun.

22

(1977) 779.

[3] Morrish, A.H., Haneda, K. and Schurer, P.J., J. Physique Colloq.

37

(1976) C6-301.

[4] Bando, Y., Kiyama, M., Takada, T. and

Kachi, S., Japan J. Appl. Phys.

4

(1965) 240.

[ 5 ] Berkowitz, A.E., Schuele, W.J. and Flanders,

P.J., J. Appl. Phys.

2

(1968) 1261.

Fig. 1 : Mijssbauer spectra at 4.2 K in an external

longitudiaal magnetic field of 50 kOe for y - ~ e ~ O ,

[TI

Haneda, K. and MorriGh, A.H., Surf, Sci.

samples A, B, C, and D. (1978) 584.