A STUDY OF THE MAGNETIC AFTER-EFFECT IN A DILUTE Fe-N SYSTEM

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A STUDY OF THE MAGNETIC AFTER-EFFECT IN

A DILUTE Fe-N SYSTEM

Di-Hua Ding, Jin Wu, Y. Chow

To cite this version:

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A STUDY OF THE MAGNETIC AFTER-EFFECT IN A DILUTE Fe-N SYSTEM

DI-HUA DING, JIN

WU

AND Y.S. CHOW

Department of Physics, Wuhan University, Wuhan, China

Abstract

-

MAE of a dilute Fe-N system in the range of 200480K was investi-

gated

by

measuring the disaccommodation of its initial susceptibility at

vac

rious temperature T with or without preceding demagnetization of specimen.

The results suggest that magnetic relaxation spectrum in the range of 350-

480K is induced

by

the metastable Fe8N phase transformation. It is composed

of a structural relaxation induced

by

pinning and depinning of the domain

walls through precipitation and resolution of FegN and a thermal-activated

relaxation due to N atoms $lose to the precipitates. A single cold work m

w

netic peak was observed at L430K from a cold worked specimen.

I

-

INTRODUCTION

Magnetic after-effect(MAFi) is a very interesting property, from which one may ob-

tain valuable informations of micraprocesses in ferromagnetic materials since

MAE

is generated

3 the interaction between domain walls and various internal stress

sources. Recently,

MBE

measurements have been uaed to investigate phase transform+

tion/l,2/, behavior of point defects and dislocations/3,4,5,6,~/ invarious f

err*

magnetic materials. The original purpose of the present work was to apply this method

to investigate the Snoek-K8ster effect in F8-N system, which manifests itself as a

sipificant peak at 4 9 0 K in low frequency internal friction measurements. It is

necessary to investigate the magnetic speotrum of Fe-N system in different conditions

and wider temperature range.

Specimens were pr ared from electrolytially refined Fe wire(~d.~lg, Mn-0.03,

S-O.of5$,

Si-trac30& $lm.

Interstitial impurities were removed

by

annealing the

specimen in wet hydrogen atomsphere at 1000K for 35h. Kitrogen was introduced

by

annealing the specimen in

an atomsphere of

2.5%

NHI

+

%

at 800-860K for different

-

time intervals. All specimens were water quenched and kept at 200K ready for mea-

surements.

bUE

of the N-charged ~pecimen

was studied with a LC-oscillator e dpment

which allows to measure the time-dependence of the initial susceptibilif xIqt) at

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JOURNAL

DE

PHYSIQUE

various temperature T within time interval

tt<t&t2

with o r without preceding demagnetization of the specimen. Results are represented a s isochronal ourves of r e

luctivity r ( ~ , t )

&(T,t) r 0 , t 2 )

-

( ~ , t , )

- -

r _ro(To~tl) 9

0

where t l = l s , t2=180s, To=200K. The heating r a t e was about 1~/3min. The frequency and amplitude of the AC magnetization f i e l d used aere 980% and lOmOe respectively. I11

-

JBSULTS and DISCUSSION

3.1 The nagnetic relaxation spectrum of a specimen as-quenched

fig. 1 shows a typical isochronalmagnetic relaxation spectrum of an Be-N specimen as-quenched, obtained from KSiE measurements w i t h preoiding demagnetization. It con- s i s t s of two parts. The lower t e q e r a t u r e part i s a large peak situated ath250K, of which theheightincreases and the temperature of i t s maximun decreases of

t2.

This relaxation i s attributed to the Richter relaxation corresponding to the well &own Snoelc effect of

B

atoms in ~e-matrix/8/, more exactly, within the domain walls.

A more complex isochronal relaxation spectrum appears i n the range of 350- 480K, with a small peak across 35040K followed 'Irg a low plateau or a broad low peak across 400-480K. It i s well laown that in Pe-IT system a metastable phase Fe8N precipitates and redisolves across 350-400 and 400-480~ respectivety, so that t h i s

complex relaxation spectrum might be associated with this phase.transfonnation.

me.

I

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range of 350-400K, followed by a monotonous r i s e i n the range of @0-480K. Since susceptibility of a ferromagnetic material represents the mobility of i t s domain walls, the f a l l and r i s e of $ ( ~ , t ) are pressumably due to pinning and depinning of

the domain walls through precipitation and resolution of Fe8N respectively. secon- dly, a p r e c i p i t a t i o n treatment of the specimen should eliminate the hWs due t o precipitation of Fe ₈1H. Fig. 2 shows the isochronal relaxation spectrum of a specimen, annealed slowly on heating from 300 to 420K and then on cooling from 420 t o

300K a t a r a t e of 201(/h, in which, the u250K peak and the 350-400K peak disappeared, and the plateau or the broad peak remained, r i s i n g from -370IG Its

$(T,

t ) curve shows a monotonous slow r i s e with temperature. A l l these confirm t h a t the-250K peak i s due to the Snoek e f f e c t , the 350-400K peak i s due t o precipitation and the 400-480K broad relaxation plateau i s due t o resolution of Fe8N respectively. 3.2 The s t r u c t u r a l relaxation spectrum induced by phase transformation.

It i s interesting t o note t h a t from an as-quenched specimen, with

t2

increasing, the strength of the relaxation spectrum inoreases and the temperature of the m a x i -

rmun

i s s l i g h t l y lowered, a s shown in Fig. 1, and that from a precipitated specimen the broad peak i s l e f t , a s shown i n Fig. 2, so t h a t there may be some s o r t of the- mal-activated relaxation process superimposed upon that of Fe8N transformation. Since the purpose of demagnetization preceding MAE measurement i s t o randomize the distribution of N atoms within the domain walls with respect t o the spontaneous magnetization direction so t h a t they may a c t with moving domain walls of the spon- taneously magnetized specimen and redistribute i n order to take preferential f a t t i c e s i t e so t o minimize the f r e e ener,g of the system, causing disaccommodation of permeability. T h i s kind of relaxation i s induced by diffusion of

W

atoms. As f o r phase transformation, the precipitates would cause a decrease and the resolution would lead t o an increase of susceptibili-&. This i s a kind of s t r u c t u r a l relam- tion, irrespective of preceding demagnetization. Therefore, hUE measurements wi- thout preceding demagnetization were carried on a specimen as-quenched. The iso-

chronal relaxation spectrum thus obtained a s shows in Fig. 3, consists of a p a i r of positive and ne t i v e peaks only, situated across 350-400K and 400-480K respectively, and i t s

$c,

t ) curve shows a

minimum

a t -400K. 'This i s then the exact st-

t u r a l relaxation induced by Fe8N phase transformation. After correction f o r this relaxation the resulting 350-480K spectrum would be more o r l e s s a plateau which i s

induced by diffusion of N atoms bound t o the mu*faces of the precipitates according t o Walz/~/.

Fig. 2

z & d

Fig. 2

-

MAE egectrum of a precipitation treated specimen.

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3.3 The cold work magnetic relaxation.

The stress field induced

by

magnetostriction will also interact with dislocatTons

in the specimen, which will

be

displaced with the drag of

N

atoms within the domain

falls to lower the free energy of the system causing aisaccommodation/5/. This mi-

croprocess should manifist in an isochronal relaxation in

MAE

measurements as a

Koster peak in internal friction measurements. Hence,

M A 3

measurements were carried

out on heating from 200 to

@OK with preceding demagnetization on a specimen as-

quenched

and

then cold drawn from $lmm to

@.9mm.

The result is shovm in Fig.

4 curve (b), the shape of which shows little difference from that of

an

unworked

spe-

cimen (curve a), but lower in strength. According to the relation between the tem-

perature of

maximum

of the magnetic peak Tm and that of the internal friction peak

Ti, Tm=Ti(l-O. 15)

/6/. the cold work magnetic peak m w

be

sheded ly the relaxation

- -

-due to precipitation. It seems necessary to avoid the interference of the &IAEs in-

duced

by

the phase transformation. The cold worked specimen was then

up

quenched to

480K and remained for Ih.

lI11E

measurements with preceding demagnetization were then

carried out on cooling right from 480 to 300K, and a single peak was obtained at

-430K. This peak is reproducible only when measured on cooling from 480K, so that

it must

be

attributed to cold work magnetic relaxation corresponding to the Snoek-

K6ster effect.

. .

2oo

250

300

350

400

450 T(K)

Fin.

4

Fig.

4 -

Effect of cold work: curve (a), as-quenced; curve (b), cold worked after

quenching; curve (c), cold worked after quenching measured on cooling from 480K.

/I/

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