Hyperfine interaction and crystalline environment in NdTi(Fe1−xCox)11 compounds by 57Fe Mössbauer study

Physica B 179 (1992) North-Holland

PHYSICA L?

89-93

Hyperfine interaction and crystalline environment NdTi( Fe 1_$o,) 1 1 compounds by 57Fe Mijssbauer Jifan

Hu,

Ziwen

Dong,

Yinglie

Liu

and

Zhenxi

in study

Wang

Institute of Ph,ysics, Academia Sinica, Beijing 100080, China Received 3 December 1990 Final received I7 January 1992

In this paper, the Mossbauer spectra for NdTi(Fe I XCo,),, compounds have been investigated. The hyperfine interaction has been studied with respect to the crystalline environment. The results show that cobalt atom prefers to occupy the 8f site in the range of high iron content and iron atom prefers to occupy the 8j site in the range of high cobalt content. The variations of the magnetic moments for each iron site 8i, 8j and 8f are very small with increasing cobalt content in compounds. At the step of the initial replacement of the iron atom by cobalt, the quadrupole splittings QS and isomer shift IS for the 8f site increase and for 8i decrease corresponding to the occupation preference on the iron site for cobalt atoms. Similar composition dependences of QS and IS implies that there exists a shielding effect or coupling of s-d electrons in the compounds

1. Introduction The Fe-rich compounds of the type ,zPvMg (R = rare earth, M = Ti, Cr, V, MO, RFe W) are. of the body-centered tetragonal ThMn,, structure. The large solid solution range is achieved with M =V (1.4~~ < 3.5), but quite narrow for Ti (O.S
0

1992 - Elsevier

Science

fur Metall1, W-7000

Publishers

occupation of 16k2 [9], which has a relatively small interatomic distance between 3d atoms. Such a small interatomic distance is likely to favour an antiferromagnetic interaction. The preferential site occupation on 16k, of cobalt is attributed to the relatively strong initial increase of the Curie temperature with the replacement of iron by cobalt. The most interesting case is the 57Fe Mossbauer effect on Nd,(Fe, _,Co,),,B in the range of high cobalt content for which Van Noort and Buschow [lo] and Honma et al. [9] found strong evidence for a strong preference of the Fe atoms for the 8j, site, which has a relatively large number of 3d nearest neighbours. These results are in good agreement with a neutron diffraction study of Nd,(Fe, ,Co,) ,4B by Herbst and Yelon [ll]. As we know, we can understand something about the influence of the crystalline environment on the hyperfine interaction in the compounds via the hyperfine field, isomer shift and quadrupole splitting, and to compare them with the results of magnetic measurements, neutron diffractions and band structure calculations. To obtain the relationship between the local mag-

B.V. All rights

reserved

Experiment

2.

‘l‘hc NdTi( noniinnl pdrcd 00.5

Fc,

b>

with

,C‘oi ), , compounds

composition

the

~\-= 0.09LO.S26

LVCI-c prc-

the

of

melting

clcnients

purlt!

M’t’; or bcttcr under argon atmosphcrc.

button

ingots

wcrc

melted

then

scalecl

homogeneity.

annealed tion

of

in

timcs

ThMn

;I the

bc essentially

,? tetragonal

second

phase

The

Miissbaucr-

po~~nds.

using

to

;I constant

temperature. using

of The

cr-Fe

a11

sp&tr:r

“(‘0

The

amount

in

the

wcrc

scale

with dt

;I

room

calibrated

W;I~

at room

con-

mcasurcci

palladium

velocity

absorber

the phase

spectrometer

in

and

diffrac-

;i single

small

accclcration

50 niC~‘i source

tube

indicated

of

vcrv

CI~SLII-c

X-ray

structure.

is

The

to

quartz

;I

at 900°C for two weeks. ( Co-Ka radiation) patterns

compounds with

four

tcmperaturc.

1;lllcc

3.

hcthccI1

pcrfinc ‘I‘hc with

Miissbauer

spectra

.\-= O.(W.

mcasurcci

0.775.

room

at

of

NdTi(

0.454.

Fc,

O.h32

temperature

/ <‘oi ) , ,

and

O.S2h

arc shown

in fig.

kc

ne

timely.

Results and discussion

ticid

Si

1.c

ha\c

the

the

\itc

4incc

ncarcst

ncighbour

Iarget

avci-age

smallest

e;Ich

dlou1d

;ltolll\

site

IOI

Si

nunibcl-

ot iron

ticlci should

corrq~ond

\ifC hince it ha4 the dlortcst

atomic

clistancc.

distribution

iron

hypcrfinc

broader

line

the site

environment.

shape comes

do111

distribution

has

an influence

neighhour knowlccigc

that ha5

ncighbour In this for NciTi( ments

or

Fe, were

ncighbour

the

atom

the

site.

hypedine

titanium

:I

, Co, ) , ,

of

compou~lds.

performed environment

or

the r;u-

in

shells

used

for

anal!,sis, such

and

xpectra

KFc

the

continuou\ a

assign-

table

the

nearest

ments

dis-

ing

the

had been

INII

the St

“Fc

a11d

With

Miissbaucl

successf’ully

cur\ c through

tit to the of

quadrupole

i-clati\e

both

caih

In

fitted

[ 121.

reprcscntk

tf,,,.

,,,\I,

tor

for the Si 5itc.

of’ assignment.

for

previously

~uby~ectrum

one

kind

a

t Iic

r:e- l-‘c Intel

in fitting.

arc for

the> 10

5ubspectra

to be introduced

~MO subhpcctra

\itt‘\

Xj

site

the interatomic

haI

parameters

[ 12. 1.31. was

Gtc

The

nearest

spectra The

the

decreaw~ the

method

the

which

next-nearest

ticki

considering and

;I

01

considering

atoms

least-squares

fitting

from

at the tii site

nenrcst

iron

Such

the multiplicit?

next-neighbour

work.

computer

on each

when

from

[4],

It originates

of Ti

for

iron

fields

II\-

largc>\t

atom5 2nd the

St’ iron

of

the

nieanwhilc

1. The outer lines of the spectra arc broadci- than t hc inner 011~5. indicating that there exists ;I

I ip to tucl

rc~pcc

Iargc41

has

fccaPte cli4t;incc.

hi pcrtinc

4ltC’.

intcnsit\

the

for

OS. cad1

I. Wc try to derive of the F;e \ublatticcs

hvpcrfinc

The

\ubspectra.

splitting I

the data point\

spectrum.

tietcia. ‘l‘hc

isomer iron

site

average from

hypcrlinc

hyperfine

licld

diift

IS

and

art‘ listed

niiignctic

ii) ilio-

the corrcspon&

approximation

xhoulrl

.I. Hu et al.

I ‘?Fe Miissbauer studies on NdTi(Fe,

be made for the hyperfine interaction constant, the proportional factor between the average iron moment of the alloy and the average hyperfine field. Here, we adopt a proportional factor of 15.6T/p., derived from ref. [13]. The composition dependence of the average hyperfine field and the average magnetic moment of iron atoms for NdTi(Fe,_.Co,), , system are shown in fig. 2. One can clearly see that with the replacement of the iron by cobalt, the average hyperfine field as well as the average iron magnetic moment increases at first. and then decreases. The peak point appears at about x = 0.27. It is interesting to point out that a maximum at about x = 0.27 for the saturation magnetization in RTi(Fe,_,Co,),, has also been found by Sinha et al. from magnetization measurements [a]. It implies that besides the effect of the relative small magnetic moment of the cobalt sublattice. the change of the magnetic moment of the iron sublattice also plays an important role in the total saturation magnetization M, of the RTi(Fe, _rC~x)l, system. A maximum when present at low temperature T = 4.2 K for Fe-Co based magnetic materials can be easily explained by the process of electron transfer between the 3d spin up and spin down energy band. However, at high temperature, such as room temperature, the effect of electron transfer

>Co,),,

0.00

0.20

compounds

0.40

X

91

0.60

x = 0.092

x = 0.275

x = 0.459

x = 0.642

x = 0.826

8i Xj Xf Xi 8j 8f 8i 8j 8f 8i *j 8f 8i 8j 8f

-0.024 -0.134 -0.124 -0.038 -0.131 -0.111 -0.004 -0.045 -0.086 -0.100 -0.022 -0.135 -0.050 -0.007 -0.094

for a similar maximum is not so clear due to the complicated temperature dependence of the 3d spin up and spin down energy band. When considering the composition dependence of the magnetic moment for each iron site 8i, Sj and 8f, shown in fig. 3, it seems that the variation of the magnetic moment is very small, which can be only called as some kind of fluctuation. In such case, it is not easy to understand why there is such a maximum. We can only say that the maximum may be originates from the counteracting effects of the increase of Curie temperature T, and the decrease of 3d sublattice magnetization with increasing the cobalt content. The composition dependence of the iron occu-

QS (mm/s)

-0.124 -0.175 -0.118 -0.024 -0.084 0.010 -0.064 -0.050 -0.031 -0.032

1.00

Fig. 2. The composition (x) dependence of the average hyperfine field (H,,) and the average iron atom magnetic moment (MFc) for the NdTi(Fe,_,c~~),, system.

Table 1 Isomer shift IS, quadrupole splitting QS, hyperfine field H,, and relative intensity I of each of the subspectra sites Si, Sj and 8f in the NdTi(Fe,_,Co,),, compounds with x = 0.092, 0.275, 0.642 and 0.826. IS (mm/s)

0.80

0.098 0.056 0.093 0.089 0.002 0.098 0.027 0.100 0.166 -0.154 0.016 0.078 0.058 -0.053 0.168

0.086 0.279 0.103 0.166 0.042 0.065 0.074 0.232

30.6 27.6 22.4 30.3 27.8 23.0 31.2 28.5 22.7 31.1 28.3 23.3 31.3 28.7 23.1

iron

I (%

H,,(T) 0.080 0.026

for the three

25.2 18.9 25.7 20.5 25.5 20.2 26.1 20.3 25.8 19.7

36 20 20 37 13 17 20 29 12 10 27 30 5 13 28

21 3 2x 5 34 5 26 7 49 5

‘I he

‘xullposItlon

depe11dc11ce

splitting

rup’)Ic

(C)S)

qC’(jt 1,

1hc

‘,I

col~~llt

MC’

~211

I-;ingc 0,.?75.

\

cx~ntrar\

. QS( SI )

1

only range

content

l’or

three

ircjn

i),(145. WC thinh. cxntial in fig.

4.

the

We

Sf Gte

f-c prcfcI-s the

each iron

A

result

interaction. the

the

(‘ohnit. ;I

1:ehavioIof

the St atoms the distance

a

larger of

relative

i4

ma>

the

increases. increases

rapidly

of iron

lw cobalt.

WC

antiferromagncti~,

Thercforc.

interaction

the with

the

Curie initial

tenrc‘-

on

of the

St

QS

clecrcascs.

With

tent.

Si

the

tion NdTi(

PC,

the

the

XI

atom

will

ix

1’

that

\alcncc

the

contribution

QS

t’)

>ysem

cjt‘ CjS Iv

incrcaes

sites

coI;p’ment

lcurthcrniorc. 01 clcctric

hb to

-incre;isc tountl conIp<>\~-

the niainl\~

/I1

field

the

clrtcI-Ininccl

of the- c.lcctric wc want

III

replaccIncIIt atoll1

term

CX)I~

cobalt.

The

cobalt

is

SI and Si

been

[“I.

with ;I

to the clcctric

cbcculxclu;~ci~-upolc

of the cxAxrlt

have

I

i\ I’) w\

(Kf).

bcgina

data )i ,B

4plittIn~

That the

the

and thc*lI

the prelcl-

occupied

, (‘0, ) i ! 4vsteni

principal

with

hits.

cohall

O.S1h. first

\ =z 0.002

cohalt.

‘111~1

In the

t hc preferential

QS( Si )

gradient

111c1-c;Iw

accord

i1.011 \Itc.

5itc

Fe / ,(‘o, iron

\

to \

the increasing

clep~ndencc the

I\1

from

ot

experimental

Nd,(

the

1177.q

tkm

t”)r the unoccupied

~oI-I-cspondin~l~. Similar

the

In

01 the cluaclrupolc

to

cotwIt

On

dccreaw

)

0 IFI

‘wulxrtion

Meanwhile.

t)l

ciecre~rscs and

fcrromagnctic

hchavior

site.

01

4plittIng

‘)t

excha~igc

the Xf iron

tion

the

amdlcr

magnetic

on the Xf site

the

intc‘r-

of an antiferromagneti~

pcrature placement

Xj)

to occupy

prcqx)rtion

Fe-Fc

In

4itc

c’~rresponciin~

t hal.

and

content.

t-‘c-Fe distance

with

and

prefers

pl-oportion

content

between

;I short

i\ shoun to occup>

iron

shortest

A.

occurrcncc

radius.

interaction

of high the

is 2.392

Such

interaction.

thinh

Co prefers

St‘) and Ftz( Xf)-Fe( 141.

in the

atomic

Sj and tif.

in the high cobalt

4ysteni.

distance

I;c( St’-Fc( 7.454

that

range

to Sj site

KFc,,I‘i

atomic

can SW

in the

Si.

site,

1’1

quadrupol~~

ttlc

4itc4 decrease\

pcrhap\

III

incrcax.\.

CjS(Si)

\ite\

III

5.

O.O’C

\

tlecre~iacs.

\

ir’)n

‘I‘hc trends

the

Si

:incf

QS( Si) trc)ni

CJS ot all three incI-c;isc\.

C_K(

c0nte11t

QS( Xi)

11.159.

only

Gtc

iu liy.

fr0111

that

quad

iron

I\ 4houn

OS(H).

:incl

~x,h;llt

the

incanwhilc

for

~c’c’

ol the St \itc.

ot

each

c011te11t

splitting

l~allgc

pation

\v\lcni

NdI’i(I-c!

the

ot

tor

to p’jirit gI\c\

grxlient

field ou1


(El-G I

J. Hu et ul. I ‘7Fe Mhshauer

studies on NdTi(Fe,

-01

Fig. 6. The composition (x) dependence of isomer for each iron site 8i. 8j and 8f in the NdTi(Fe, system.

shift IS rCo,),,

shift (IS) for each iron site in the system are shown in fig. 6. NdTi(Fe,_,Co,),, The isomer shift IS of the Mossbauer spectrum for “Fe is proportional to the local electronic charge density p(O) at the “Fe nuclear position and therefore is related to the spatial distribution of the electrons surrounding the nuclei. The electron distribution can change not only with a change in the number and character of electrons but also with a surrounding the 57Fe nuclei, change in the distribution of the nearest neighbours of the iron atom. The isomer shift is essentially determined by the number of s-electrons of the nearest-neighbor iron atom. From fig. 6, one can easily see that the isomer shift of 8f increases (the absolute value of the negative IS decreases) with the initial preferential occupation by cobalt atom on the 8f site, which is very similar to the results of calculations and experiments on the system of Co-impurity in Fe [14]. It may be implies that the number of s-like electrons at the 8f iron site decreases in this case. Meanwhile, from fig. 6 we can also see that the IS of 8i decreases and the IS of 8j increases slightly with the initial increase of the cobalt content in the compound. We do not know

I Co<),, compounds

93

exactly in this case how much the local d charge or the intra-atomic screening affect the IS. Comparing the composition dependence of quadrupole splitting QS and isomer shift IS in the range of small content of cobalt atoms in compounds, it is very interesting to find that two dependences are somewhat similar. It implies that there exists a shielding effect or a coupling of s-d electrons. However, since the relative accuracy of the values of the quadrupole splitting and the isomer shift are less than that of the hyperfine held and there exist four types of atoms (Nd, Ti, Fe and Co) in compounds, the complications of the shielding effect of s-d electrons increase. Speaking frankly, to give an exact analysis on the density of s or d electric charge at such case is almost impossible.

References

[II

D.B. de Mooij and K.H.J. Buschow, J. Less-Common Metals 136 (1988) 207. PI Jifan Hu, Tao Wang. Shougong Zhang, Yizhong Wang and Zhenxi Wang, J. Magn. Magn. Mater. 74 ( 1988) 22. PI R.B. Helmholdt, J.J.M. Vleggaar and K.H.J. Buschow. J. Less-Common Metals 138 (1988) Ll 1. [41 0. Moze, L. Pareti, M. Solzi and W.I.F. David. Solid State Commun. 66 (1988) 465. D.B. de Mooij and [51 F.R. de Boer. Huang Ying-Kai. K.H.J. Buschow, J. Less-Common Metals 135 (1987) 199. VI Jifan Hu, Yizhong Wang, Ruwen Zhao, Taishan Ning, Zhenxi Wang, Solid State Commun. 70 (1989) 983. [71 K.H.J. Buschow. J. Appl. Phys. 63 (1988) 3130. [81 V.K. Sinha. S.F. Cheng, W.E. Wallace and S.G. Sankar. J. Magn. Magn. Mater. 81 (1989) 227. PI H. Honma and H. Ino, IEEE Trans. Magn. MAC-23 (1987) 3116. [lOI H.M. Van Noort and K.H.J. Buschow. J. Less-Common Metals 113 (1985) L9. [Ill J.F. Herbst and W.B. Yelon. J. Appl. Phys. 60 (19X6) 4224. iI21 A. Deriu. G. Leo. 0. Moze, L. Pareti. M. Solzi and K.H. Xue, Hyperfine Interactions 45 (1989) 241. [13] Bo-Ping Hu. Hong-Shou Li and J.M.D. Coey, Hyperfine Interactions 45 (1989) 233. [ 141 P.H. Dederichs, R. Zeller, H. Akai and H. Ebert. J. Magn. Magn. Mater. 100 (1991) 241.