SPIN PINNING EFFECT IN THE RF COLLAPSE STUDIES

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SPIN PINNING EFFECT IN THE RF COLLAPSE STUDIES

M. Kopcewicz, G. Karczewski

To cite this version:

M. Kopcewicz, G. Karczewski. SPIN PINNING EFFECT IN THE RF COLLAPSE STUDIES. Journal

de Physique Colloques, 1980, 41 (C1), pp.C1-215-C1-216. �10.1051/jphyscol:1980168�. �jpa-00219737�

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JOURNAL Dl3 PHYSIQUE

ColloqueCl, supplkment au no 1, Tome41, janvier 1980,page C1-215

SPIN

PINNING EFFECT I N THE RF

COLLAPSE

STUDIES

M. Kopcewicz and G. Karczewski

I n s t i t u t e of ExperimentaZ Physics, Warsaw University, 00-681 Warszawa, Hoza 69, Poland.

The M6ssbauer studies of the surface effects i n ferromagnetic

materials

performed

i n

l a s t few years show t h a t

the

magnetic properties of the surface layers may differ considerably f m that observed for the bulk samples (see e.g. / I / ) . One of the m s t interesting effects is the "spin pinning" of

the

surface spins, observed by Eerkowitz e t al. /2/.

Spin pinning is due to the very large anisotrow f i e l d acting on the d a c e s p h . It seems that this effect may considerably affect the magnetization reversal i n

thin

f o i l s of the soft f-gnet.

In the Mossbauer studies /3, 4 , 5/ of the

r£

collapse effect, which is strongly dependent on the

ani-

sotropy field acting on the spins and which should be particulary sensitive to the s p i n pinning,

in-

fluence of the surface effects on the magnetization reversal forced by the e x t e n d . r f field was not discussed.

Tb study these effects an experiment was per-

£0- for invar w l e s of the canpsition:

61.98%Fe-38.00%Ni-O.O2%C. 'lbm samples were prepared: the bulk one of the thickness of 12

p m , and the

sandwich consisting of 5 layers of 2.5 in- var f o i l s separated frcnn each other by

thin

mica layers. Invar f o i l s were prepared by cold rolling

and then

annealed in hydrogen a t 1400

K

for 4 hours

atad

rapidly cooled to tanperatwe during 1 mi- nute. The Mssbauer measurenents were performed a t room temperature w i t h the rf field frequency of 50 MHz

and

intensity varying £ran 3 to 11 Ce. The samples were placed as stationary absorbers within

*'helical coil of the

r£

generator in such a way that the r f field was applied i n the plane of the sample. In all

experiments

the samples were water cooled to keep their -ature ell

klow

the Cwie point which f o r these samples is about 750 K.

Preliminary results obtained for

the

rf coll- apse effect in

the

_{bulk and} samples

are

presented i n Fig. 1. Spectra la, Ib, Ic w x e observed for the sandwich sample a t 7.4, 9.3 and 11

Oe,

respectively, and IA, IB, 1C

-

f o r the bulk sample a t the s a m r f field intensities. The spec-

I I I I

- 8 -1 o ^L B Fig. 1

VELOCITY [rnrnls 1

t r a obtained a t 11 Oe (Figs. l a

and

1A) are very similar. The width of the

central

collapsed l i n e is almost the same for both samples. Hobever, for

lowsr

r f f i e l d intensities the collapsed l i n e cor- responding to

6

sandwich sample is considerably broader

than

that observed for the bulk sample

(Figs. Ib and IB). The spectra I c and 1C differ ampletely £rum

each

other: the

spectrum

of the

bulk

sample a t 7.4 Oe

is

collapsed

tn

a single li- ne (Fig. 1C)

,

while that of the sandwich sample

is

much less collapsed

and

consists of unresolved hfs lines (Fig. Ic)

.

^{I n}^this^case^when^ther f field

is

only marginally larger

than

the anisotropy field in

the

bulk sample

even smll

increase of

the

ani- sotropy f i e l d due

to the

surface effects may affect considerably the mgnetization reversal. The results presented suggest t h a t the spins in the sand- wich sample, i n which the contribution

£ram t k

surface spins

is much

larger

than

in

the

bulk

saq-

l e , a r e

mre

reluctant

to rotate

a t a given r f field applied. This effect can be related

to

t k large anisotropy f i e l d acting on

the

near surface spins what induces

the

spin pinning. Since

the rf

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980168

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C1-216 JOURNAL DE PHYSIQUE

collapse e f f e c t

is

connected with the r o t a t i o n of a l l spins in t h e whole volume of the sample it seems that t h e anisotropy f i e l d which is very l a r g e only a t t h e surface layer does not decrease sharply within few surface atcanic layers, but decreases gradualy to the value c h a r a c t e r i s t i c f o r t h e bulk material.

I t should be noticed t h a t the results obtained

in

the present study do not correspond direct- l y to t h e studies of

the

surface e f f e c t s in very

thin

films since our s m p l e s are much thicker

than

t h a t used i n e.g. / I , 6, 7/. m v e r , the

r£

collapse e f f e c t seems

to

be much mre s e n s i t i v e t o small changes of the

anisotqy

f i e l d s

than

the e f f e c t s studied in e.g. /I

,

6, 7/ such a s the orientation of t h e magnetization a t

the

surface o r the magni- tude of the hyperfine f i e l d in t h i n films. I n t h e discussion of our r e s u l t s

we mst

take i n t o account the f a c t t h a t the suzfaces Of t h e 2.5pm invar f o i l m e not perfect what increase t h e area of t h e surface and may cause

me

increase of the influence of t h e surface effects on the r f collapse e f f e c t .

The r e s u l t s presented i n Fig. 1

show

a l s o t h a t t h e r f sidebands e f f e d increases in t h e sandwich sample a s a m p r e d

to the

bulk one

(ccanpare

Figs. l a and IA, I b and 1B)

.

^Thisobservation supports the modified magnetostriction model o f the sidebands formation p r o p s & by P f e i f f e r e t al. /8/ and discussed a l s o in /9/. The &el

assu-

m e s t h a t the acoustic vibrations,responsible f o r t h e nxlulation of t h e Mijssbauer g mrays, w h i c h are induced by t h e

rf

f i e l d a t f i r s t i n t h e plane of thc? sample a r e scattered £ran t h e defects, surfaces, grain boundaries, etc.. Hence t h e large ccmpnent of vibrational amplitude m y be produced along t h e g m rays. Since t h e sandwich sample contains more surfaces the scattering of t h e acoustic wdw

&uld

be more e f f e c t i v e what w i l l lead to t h e increase of the r f sidebands e f f e c t , a s observed in the present study.

Acknowledgements. The authors

are

iMepted to Dr. I. S o s n m k a f o r her interest in t h i s study.

One of the authors (MK)

is

deeply grateful t o Professor U. Gonser f o r f r u i f f u l discussions.

Thanks are due lm M r . S. Fijdlkowski f o r

the

con- s t r u c t i o n of t h e r f genera& and technical assis- tance.

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