Synthesis, magnetic properties and 57Fe Mössbauer study of the laves phase compound YbFe2

HAL Id: jpa-00208718

https://hal.archives-ouvertes.fr/jpa-00208718

Submitted on 1 Jan 1977

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.

Synthesis, magnetic properties and 57Fe Mössbauer study of the laves phase compound YbFe2

C. Meyer, B. Srour, Y. Gros, F. Hartmann-Boutron, J.J. Capponi

To cite this version:

C. Meyer, B. Srour, Y. Gros, F. Hartmann-Boutron, J.J. Capponi. Synthesis, magnetic properties

and 57Fe Mössbauer study of the laves phase compound YbFe2. Journal de Physique, 1977, 38 (11),

pp.1449-1455. �10.1051/jphys:0197700380110144900�. �jpa-00208718�

SYNTHESIS, ^MAGNETIC PROPERTIES AND 57Fe MÖSSBAUER STUDY

OF THE LAVES PHASE COMPOUND YbFe2

C. MEYER, ^B.

SROUR,

^Y.

GROS,

^F. HARTMANN-BOUTRON Laboratoire de

Spectrométrie Physique (*),

^B.P.

53,

^{38041 Grenoble}

Cedex,

^France

and J. J. CAPPONI

Laboratoire de

Cristallographie

du C.N.R.S.

(**),

^{B.P. 166}

X,

38042 Grenoble

Cedex,

^France

(Reçu

^{le 24}

juin 1977, accepté

^{le 13}

juillet 1977)

Résumé. ^- Le composé cubique YbFe2, ^{de structure} MgCu2, ^{a été} synthétisé ^{à 1 200°C sous}

80 kbar et identifié par diffraction des rayons X. La courbe d’aimantation ^en fonction de la tempéra-

ture présente ^un

point

^de

compensation

^{à T = 31 K. Les} spectres ^Mössbauer

indiquent

qu’à ^très

basse température l’aimantation est parallèle ^{à une direction 100} >. Quand ^la température s’élève, l’aimantation semble

légèrement

s’écarter de cette direction.

Abstract. ^- The cubic MgCu2-type compound YbFe2 ^was synthesised ^at 1 200 °C under 80 kbar pressure and characterized by ^means

of X-ray

diffraction. The temperature dependent magnetization

exhibits a

compensation point

^at 31 K. The Môssbauer spectra shows that at very low temperature the magnetization ^is parallel ^to a 100 > direction. As the temperature increases the

magnetization

probably

slightly

deviates from this direction.

Classification Physics ^Abstracts

76.80 ^- 75.30

1. Introduction. ^-

During

^the

past

^fifteen years, there has been a lot of interest in the cubic Laves

phase

intermetallic

compounds RFe2,

^{where R is a rare}

earth. Indeed some of these

ferrimagnetic materials, magnetized along 111 ),

^exhibit

giant magnetostric-

tions

(2

^{000 x 10- 6 in}

TbFe2

^{at 300}

K)

^{which have}

applications

ⁱⁿ

magnetostrictive

transducers

[1].

However some members of the series cannot be _pre-

pared

^at

atmospheric

pressure. In

particular YbFe2

was

prepared

^under pressure

only

^{a few} years ago

by

^{Cannon et al.}

[2]

^{and their}

samples

^were

apparently

contaminated with b.c.c. iron which

prevented

magne- tic and Môssbauer studies

being

^made

(1).

We

report

^{here the}

preparation

^under pressure of

samples of YbFe2

^{free from}

magnetic contaminants,

their Môssbauer

study

^with

^s7Fe

^{and some of their}

magnetic properties.

2.

Préparation

^and identification. ^- The

samples

were

prepared

^{in a}

high

pressure modified BELT

type apparatus designed by

^{M. Contré}

[3a].

Details of the

procedure

^{of such}

experiments

have been described elsewhere

[3b].

(*) Associé ^{au C.N.R.S.}

(**) Formerly « Laboratoire des Rayons ^{X ».}

e) ^{Cannon J.} F., Private communication.

A mixture of

approximately

¹⁰⁰ mg of Yb and Fe

powders

^is

packed

^{into a} boron nitride

capsule

^and

subjected

^to

high

pressure ^at

temperatures comprised

between 1150 and 1 300

OC,

for between five minutes and one hour. Seventeen

attempts

^at

preparing YbFe2

^{were made. A}

preliminary attempt

^{at 35 kbar} and 1300 OC

completely

^{failed. For} the sixteen other

attemps,

^the

highest

pressure available

(80 kbar)

was used. One

experiment

failed. The

remaining

fifteen resulted in

small,

^brittle

ingots weighing

80 _mg each.

X-ray powder analysis

^{of these}

products

^showed

the characteristic lines of

YbFe2

^{as tabulated}

by

Cannon et al.

[2],

with very

nearly

^{the same lattice}

parameter, ^7.244

Á.

Eleven out of fifteen

samples

also contained ^a small amount of

Yb6Fe23 (together

with some Fe and Yb in two of

them). Finally

^some

weak broad lines have remained unidentified.

3. Môssbauer

spectroscopy of 5’ Fe.

^-

Spectra

^have

been obtained on four different

samples :

1

unenriched,

free from

Yb6Fe23,

^{but with a}

little non

magnetic impurity

^{in the center of the}

Môssbauer

spectrum ;

II enriched in

1 ’Fe, apparently

very pure, which

unfortunately

^{exhibited some Môssbauer satu-}

ration ;

^_

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197700380110144900

1450

III

unenriched,

^{with some}

Yb6Fe23 (according

to the

X-ray spectrum)

^{which did not show} up in the Môssbauer

spectra ;

IV

unenriched,

^{free from}

Yb6Fe23.

Most of the Môssbauer

study

^was

performed

^on

samples

^{1 and II}

(Fig. 1

^and

2).

FIG. 1. ^- Môssbauer spectra ^of YbFe2 (sample I, unenriched).

At 5.2 K the full-line is the theoretical fit mentioned in the text,

taking ^{account of the} tipping and the second order effect of the quadrupole interaction (Hn ^{= 204} kOe, H dip ^{= 7.3 k0e,}

e2 qQ/2

^{= - 0.5} mm/s). At the other temperatures, the full-line

serves only ^to join ^the experimental points and the bars represent the _average positions of the lines for the best fit to a pure Zeeman

spectrum.

At 5

K,

^the _spectrum ^{is a}

simple symmetric

^Zeeman

pattern.

^{When the}

temperature

^is

increased,

^{two of}

the

lines,

^{a and}

b’, gradually

^broaden up ^to 85 K.

Then, except

for the slow decrease of the

hyperfine field,

the

spectrum

^remains

essentially

^{the same up to}

295 K. The

broadening

^{of lines a} and b’ was observed

on all four

samples.

At any

temperature

^the average

positions

of the six Môssbauer lines are well fitted to within 1

% by

^a pure Zeeman

spectrum.

^{The corres-}

ponding

values of the

hyperfine

^field

H.

^are

given by

table

1 ; they

^are

quite comparable

^to those obtained in other

RFe2’ compounds.

^These results will be discussed further in

paragraph

^5.

FIG. 2. ^- Môssbauer spectra ^of YbFe2 (sample II, enriched) :

Same remarks as for figure ^{1. Due to} Mossbauer saturation the

integrated ^areas of the lines are not quite proportional ^{to 3. 2. 1.}

TABLE I

Temperature

^variation

of

^the

hyperfine field

at the

5 7Fe

nucleus

as fitted

^with a pure Zeeman spectrum

An

attempt

^to determine the

quadrupolar splitt- ing e2 qQ12 1 at

^{650 K} i.e. above the Curie

tempera-

ture,

by heating

^a

sample

^{in an} argon ^stream in order to avoid

oxydation, only

^{resulted in a}

rapid

destruction of the

sample.

4.

Magnetic properties.

^{- These} measurements were

performed

in the Service Général de Mesures d’Aimantation du Laboratoire de

Magnétisme (Mr. Barlet).

^{Most of the} measurements were

perform-

ed on

sample

VI which contained a small amount

of

Yb6Fe23. Later,

^our

attempts

^to obtain purer

compounds

resulted in

samples

^IV

(also

^{used in the}

Môssbauer

study)

^and

V,

^{free from}

Yb6Fe23,

^which

were also used for

magnetic

measurements.

In order to find the

compensation point

^{we have}

measured the residual

magnetization

Ur of

YbFe2

in a weak field of 130 Oe as a function of

increasing temperature (Fig. 3).

^{For the}

samples

^{V and}

VI,

^the

magnetization changes

^its

sign

^{at 36 K and 38 K}

respectively ;

^{it reaches a}

negative

^{minimum at 50 K}

and then increases to zero and becomes

positive again.

This

dependence

is characteristic of

compensation [4] :

as

long

^{as the}

applied

field is smaller than the coercive field

H,, ,,

^{which is} maximum in the

vicinity

^{of the}

compensation point 0,, ,,

^the

change

ⁱⁿ

sign

^of 6r will occur without reversal. As the

temperature

increases above

0,, ,, H,,,

decreases and when it is

equal

to the

applied field,

u, ^reverses and becomes

positive again.

^For

sample IV, however,

^it

only

goes

through

a minimum near 50

K,

^without

sign change.

FIG. 3. ^- Residual magnetization ^of YbFe2 measured in a field of 130 Oe as a fonction of temperature. ^The compensation point

is located between the value 0,’, measured with increasing tempe-

rature and the value 0,’,’ measured with decreasing temperature.

We deduce from the curves

of figure

^{3 that}

sample

^V

is the best

(as

^{was to be}

expected

^on the basis of the

X-ray data)

^and

that, unexpectedly, sample

^{IV is the}

worst,

perhaps

because of the _presence of a small amount of

magnetic impurities

different from

Yb6Fe23

and which don’t have a

compensation point

in this temperature range

(notice

^that

they

^{do not}

show _{up in} the Môssbauer

spectra...) (2).

^Such

impu-

(2) Notice however that near a compensation point, ^{a few 10-4}

of ferromagnetic impurities ^are sufficient two completely ^{alter the} magnetization curves, while X-ray ^{and Môssbauer} spectra ^are only ^{sensitive to a few 10-2} of impurity.

rities would

globally

increase the

magnetization

^and

give

^{a more or less}

deep

minimum in the curve and

similarly

^{for the}

compensation point.

Structural inho-

mogeneities

could also

play

^{a role}

[4,

p.

172].

According

^{to the curves of}

figure 3,

^the

purest sample

^{is V whose curve} would lead to

0.

^{= 36 K.}

But

strictly speaking

^{an accurate} determination also

requires

measurement of the

magnetization

^{curve as}

a function of

decreasing température ;

^this

gives

another and lower value

0"

and the real

compensation température

^{is the} average of the two determinations

(0’

⁺

0")/2.

^This

procedure

^{was used with}

sample

VI and leads

to 0,,,

^{= 31 K ± 7 K}

(in

^view

of the

similarity

between V and

VI, sample

^{V would}

have

probably

^{lead to the same}

value).

Notice that the

magnetization

^{curve at} 26 kOe also leads to a

minimum around 30 K

(Fig. 4).

^The existence of a

compensation point

shows that Yb is trivalent in

YbFe2

^as in irradiated

TmFe2 [5] (this

^was

expected

on the basis of the lattice

cell parameters).

FIG. 4. ^- Temperature dependence ^{of the} magnetization of YbFe2

in an external field of 26 kOe.

We have also determined ^some

magnetization

curves with

sample

^VI

(Fig.

⁵

(3). They

show that the

anisotropy

^of

YbFe2

^{is not as}

large

^as for the other members of the series. In

agreement

with this observa- tion our attempts ^to

align

^a

powder

in 18 kG between

(3) ^{A few} magnetization curves were also obtained for samples ^IV

and V. They ^{are similar to} those of VI.

1452

FIG. 5. ^- Isothermal magnetization ^{curves of} YbFe2.

150 K and 300

K,

^{in order to} determine the direction of the

magnetization

^{in this}

temperature

range ^were unsuccessful. The saturation

magnetizations

^{at room}

temperature,

measured in 26

kG,

^{were found to be}

respectively

⁴⁵

e.m.u./g,

³⁵

e.m.u./g

^{and 40}

e.m.u./g

for

samples IV,

^{V and}

VI,

^{i.e. an} average value of 40 ± ⁵

e.m.u./g. (Measurements

ⁱⁿ

higher

^{fields are}

planned

ⁱⁿ the future in order to elucidate the dis-

crepancies

^{between different}

samples).

^{The above}

magnetization

data will be reexamined later in the text.

We have not

yet

determined the Curie

temperature

of

YbFe2 ;

it should be

approximately

^{610 K as in}

TmFe2 and LuFe2 [6].

5. Discussion. ^- The cubic cell of the Laves

phase

structure

(group

^{Fd3m n°}

227)

^contains

eight equiva-

lent iron atoms

(site 8a)

and sixteen

equivalent ytter- .

bium atoms

(site 16d).

^{The local}

symmetry

^{at an iron} is

trigonal

^{around one} of the four

directions

¹¹¹

).

The local

symmetry

^{at an}

ytterbium

^{is cubic.}

5. 1 MOSSBAUER SPECTRA. ^- 5. 1. 1 General _pro-

perties

^{in the}

RFe2 series.

^{- In the} presence of _magne- tic order the iron atoms are no

longer equivalent

because of both

quadrupolar

effects and

dipolar

fields which add to the

isotropic hyperfine

^field.

The

quadrupole

effect in the

RFe2

series is rather small

(e2 qQ12 - -

^0.5

mm/s [7-10])

and for each iron atom it can be characterized

by

^the

quadrupole parameter :

...

where

0,

^{is the}

angle

^{of the}

magnetic

^moment J.1Fe and

hyperfine

^field

H.

^{oc -} J.1Fe of the iron atom with

respect

^{to its local}

trigonal symmetry

^{axis i.}

In addition there is a

dipolar

^field

Hdip,

^{created at}

. one atom

by

all the other atoms, which adds to the intrinsic

hyperfine

^field

Hn. However, |Hdip ^{1 « 1 H. 1}

and

therefore,

^{to a}

good approximation,

^{the total}

internal field seen

by

^the nucleus of an iron atom with

trigonal

^{axis i is :}

where :

is the

component

^{of the}

dipolar

^field

along

^the

hyper-

fine field

H. (which

^is

opposite

^to

PF,,)-

The lattice sums necessary for the evaluation of

Hd;p

^{have been}

computed by

various authors

[11-12]

who assumed a colinear

ferrimagnetic

^structure.

We have checked that the

dipolar

^{field is a function}

only

^{of the sums}

YB E

³

^xzlr’

whose values are

(ao

⁼

edge

of the unit

cell) :

The various

bHi

^{are then}

simply

^{related to} the para-

meter (4) :

In the _presence of

dipolar

^and

quadrupolar

^effects

and for an

arbitrary

direction of the

magnetization M,

the iron atoms

separate

^{into four}

inequivalent

^Môss-

bauer sites.

If M is

along 100 )

^these sites reduce to one

with e ⁼

0,

^{ôH =}

0,

which leads to a

single

pure Zeeman

spectrum.

If M is

along

^{a direction}

[uv0]

^{in the}

(001) plane

^at

an

angle

ç from

[100],

the four sites reduce to two groups of two for which :

In

particular, along the 110 >

directions :

( - corresponds

^to

Oi

⁼

^{9Qo ;}

^{+ to}

Oi

⁼

35°) (8)

ôH =

+ A.

(4) YbFe2 is in the second half of the RFe2 series where the R.E.

moment is antiparallel ^{to the iron moment. In} the first half it is

parallel ^and (-) ^- (+ ).

If M is

along

^{a direction}

[u,

^u,

w]

^{at an}

angle §

from

[001],

^the four sites separate ^into groups of _one,

one and two for which :

with :

In

particular along

^a

[111]

^direction

(cos t/J

⁼

1/ fi),

the last three sites become

equivalent

^{and we have}

that

More

generally

^{it can} be shown that for a colinear

magnetic

^{structure with an}

arbitrary

direction of the

magnetization :

i.e. if the lines are not

split,

^their average

positions correspond

^{to a} pure Zeeman

spectrum.

In

simple

^cases

however,

^{such as}

TbFe2, ErFe2, magnetized along ( 111 )

^or

SmFe2

^{at 77 K}

magnetiz-

ed

along ( 110 ) [11],

^the

dipolar

^{field is}

large

^and

two

of the lines, corresponding

^{to a and b’ in}

figure 1-2,

are

completely split

^{into two}

components

with relative intensities 3 : 1 or 2 : 2.

5.1.2 Môssbauer spectra

of YbFe2.

^{- At 5 K}

we have a

symmetric

^Zeeman

pattern

with unbroaden- ed lines which indicates that the

magnetization

^is

along

^{a direction}

(

¹⁰⁰

).

^The

positions

of the lines

are fitted to within 1

% by

^a pure Zeeman spectrum with

Hn

⁼ 206 kOe. However in order to obtain a

good computer

^{fit we were}

obliged

^{to take account of}

the

tipping

of the total

hyperfine

^field

Ht

^{due to the}

dipolar

^field

Hdlp perpendicular

^to

Hn,

^{and of the}

second order effect of the

quadrupole

interaction

[8]

(all

these small effects were

neglected

^{in the}

preceding paragraph).

^The values used in the fit were

Hn

^{= 204}

kOe, Hd;p

⁼

J2 A

^{= 7.3 kOe}

(see below) and, tentatively, ^e2 qQ/2

^{= - 0.50}

mm/s

^{as in other}

RFe2.

When the

temperatures

rises lines a and b’ broaden

progressively

^{but do not}

split

^{and their} average

posi-

tions still fit well with a pure Zeeman

pattern

^as

expected

^{for a colinear} structure. The spectra ^{at 85 K} and 300 K are reminiscent of those of

GdFe2

^which

have been

interpreted

^{in a} number of different _ways

[13,14,15],

^{and of those}

of HoFe2

^{below 15}

K,

^which

were

adjusted successively

^{in two} different fashions

by assuming

^the

magnetization, first,

^{to be}

along [u,

v,

0]

^with ç - 200

[16] and, second,

^{to be}

along [u,

^u,

w] [17].

Before

going

^{further we must} first estimate the

dipolar

^{field at} the iron sites in

YbFe2.

^{For this we}

will use the results obtained in

(5) :

this work is relative to the Môssbauer effect

of 17°Yb

as a trivalent

impurity

ⁱⁿ

TmFe2 (magnetized along (

¹¹¹

»)

^and

in

HOO.2Tmo.8Fe2 (magnetized along 110 >

^below

30 K and

along 001 )

^{above 40}

K).

^{From the values}

of the electric field

gradient

^and

hyperfine

^{field at the}

Yb nucleus in these two

compounds,

the authors have deduced the values of the

crystalline

field and Fe-Yb

exchange

^field

[18]. Using

these data in

YbFe2,

^we

can

calculate 1 (

^J

^)Yb 1.

^We find that it is not very

anisotropic;

^{therefore we can use its value}

along ( 100 >

^{in order to estimate}

A R.E.-

On the other hand

we know the

compensation temperature

^{31 K.}

Then, assuming

^{that the}

magnetization

^{is not far from}

( 100 )

^{at 31 K and}

using

^{the values}

(deduced

^{from a} calculation based on the above

data),

^we find for the

magnetic

^{moment of iron :}

This is to be

compared

with the values

given

^{in the}

literature for other

RFe2 compounds,

^which

depend

on the rare earth and also on the authors and which vary between

1.4 /ÀB

^and

2.1 MB.

Let us then assume that _IlFe ⁼ 1.8 _IÀB and

neglect

its small variation between 4 and 85 K. Let us also

assume that the

magnetization

^is

along [100].

^Then

(according

^to

calculation) :

JUYB ⁼ 3.96

IlB at

⁴

K,

JlYb ⁼ 2.03 ,uB at ^{85 K and the}

dipolar parameter

^A

(eq. (6))

^{is :}

At 85

K, where [ (

^J

)Yb 1

^is

fairly isotropic,

^these

values lead to a difference AH between the

dipolar

fields bH

corresponding

^{to different}

^sites, equal

^to

2 A-7.8 k0e in

thé 110

directions and to

(8/3)

A - 10.4 k0e in the

111 )

directions.

Let us now compare with

experiment.

^{Since the}

lines a’ and b are

fairly symmetric,

^we may

try

^to

interpret

^the

spectrum

^{at 85 K}

by assuming

^{for exam-}

ple

^{that the}

magnetization

^{is in the}

(001) plane,

^i.e.

we have two

equally populated inequivalent

^sites

with

opposite quadrupole

^{effects e and}

dipolar

^fields

1454

ÔH,

whose effects cancel for lines a’ and b and add for lines a and b’. From the

broadening

of these lines

or from a two spectra

fit,

^we deduce that :

e2 qQ

⁰

and that _very

roughly :

while the values

expected

ⁱⁿ

the ( 110 >

^{direction are}

respectively

^{7.8 kOe and}

(if

^{we assume that} e2

qQ/2 N -

^0.5

mm/s

^{as in other}

RFe2),

^0.25

mm/s,

that is

approximately

^{two and 3.5 times}

larger.

^A

possible explanation

^{could be}

that,

^{as in}

HoFe2

^at

4

K,

^the

magnetization

^is

along

^{a direction}

[u,

^v,

0] :

this has the effect of

reducing

^the

splittings

^{AH and}

As

by

^a factor sin _{2 ({J} with respect ^to

[110]

^direction.

If we assume that sin 2 _ç ⁼

1/3

^{we arrive at}

ç ⁼ 90 42’. The

progressive broadening

of the lines a

and b’ when T increases from 0 to 85 K would then

correspond

^{to a small and}

progressive

deviation of M from

[100]

^{in the}

(001) plane. However,

^{as shown}

by

the different reduction factors obtained for AH and

A8,

^this

interpretation

^{is somewhat}

speculative

^since

the values of _JlFe and _IÀYB are

only

estimates and since

we also do not know the value

of e2 qQ/2

ⁱⁿ

YbFe2 (5).

Finally

^{we cannot rule out a structure}

_[u,

u,

w]

^which

for small

#,

would also

produce symmetrically

^broa-

dened lines.

We note that in reference

[15]

^{where the Môss-}

bauer

spectra

^of

GdFe2

^were

interpreted by

^assum-

ing a ( 110 > direction,

^the value obtained

for e2 qQ/2,

i.e. - 0.18

mm/s,

^was also much smaller than the usual value - 0.50

mm/s, perhaps

because the true structure is

[u,

^v,

0],

with its reduction factors sin _{2 ço.}

In order to check the above considerations we have used our

previous crystalline

^field calculation for the

Yb’ ’

in order to evaluate its free _energy as a

function of temperature and of the orientation of the iron

exchange

^field

(assumed

^{to be}

isotropic

^{as in}

reference

[5]), both for

^the

^{high symmetry}

^directions

and for the

(001) (110) planes.

Indeed it is well known that in the

RFe2 compounds,

^the

anisotropy

^is

mainly

due to the rare earth ions

[17, 19].

^{We find}

that ( 100 ) always gives

the lowest free _energy

FYb. However,

when T

rises, FYb/kB

^becomes

rapidly isotropic (to

within 2 K above 60

K)

and other factors such as the iron

anisotropy,

^which

probably favors (

¹¹¹

)>,

will then be of

importance (in YFe2

^this

anisotropy

is estimated to about _{1 K per} formula unit

[20]).

Incidentally,

^another

interesting

^{result of our} crys- talline

plus exchange

field calculation is that at low

temperatures,

^{when the}

exchange

field created

by

the iron at the

ytterbium

deviates from a

high

symme-

try direction,

^{the moment of the}

ytterbium

^{is not}

(5) Even if we had succeeded in measuring ^{it above} T,, ,, ^{one should}

not forget ^{that it} might depend ^on temperature, ^{as in} ErFe2 [7].

parallel

^{to the}

exchange field,

^{i.e. to the iron moment.}

That

is,

^the

magnetic

^{structure would not be coli-}

near. However this effect decreases

rapidly

^with

temperature

^{and is no}

longer important

^{above 60 K}

(for example,

^if PFe and

HeX

^{are at} l{J ⁼ 200 of

[100]

ⁱⁿ

the

(001) plane,

^then PYb is _{at l{J} = 7° 47’ when T ⁼ 4 K and

at ç

^{= 180 04’ when T = 60}

K).

^Notice

that these results

imply

^{that at 4 K our}

expressions

for the

dipolar splittings

^{are not}

strictly

valid outside of the

high symmetry

directions.

5.2 MAGNETIZATION MEASUREMENTS. ^- If we assume

that 1 PFe 1

^is

proportional

^to the measured

hyperfine

^{field of the}

iron,

^{its value at 295 K should}

be 1.6 _YB. At this same temperature ^the

ytterbium magnetization

^is

isotropic and, assuming

^{that the}

Fe-Yb

exchange

^{field is}

proportional to 1 PFe I

we find

by

the calculation

that 1 PYb 1

^{= 0.59} YB- For a colinear

ferrimagnetic

^{structure the}

expected

saturation moment per molecule is :

to be

compared

^{with the}

experimental

^value

2.0 ± 0.25 _,uB. The agreement ^{is not} very

good,

^which

is

probably

^{due to} the uncertainties in both the theo- retical and

experimental

^estimates.

We

hope

^that

forthcoming

^Môssbauer

experiments

on

1 7°Yb

between 4 and 60 K will

provide

^direct

information on the thermal variation

of (

^J

)yb,

thus

giving

^a check of the

crystal

^{field and}

exchange

parameters

[18]

determined in reference

[5].

^In

TmFe2, magnetized along

¹¹¹

),

^J

)Yb

^is

along

¹¹¹

),

instead

of ( 100 )

^{as in}

YbFe2 :

^different magneto- strictive effects in

TmFe2

^and

YbFe2 might

^{be a}

source of

discrepancy (see [21]).

6.

Summary. -

^{At 4 K the}

magnetization of YbFe2

is

along 100 ).

^{When the}

temperature

^{rises it} pro-

gressively

^and

slightly

deviates from this

direction,

but we cannot assert with

certainty

^{its new direction}

([u,

^v,

0], [u,

^u,

w],

^or

possibly other).

Acknowledgments.

^{- It is a}

pleasure

^{for us to}

thank Dr J. F. Cannon for

correspondence containing interesting

information and advice. Mr.

Perroux,

^of the Laboratoire de

Cristallographie,

assisted with the

preparation

^{of the}

samples.

^{We are}

greatly

^indebted

to Dr. R. Lemaire and his _group of the Laboratoire de

Magnétisme

for their kind

help

^{in the}

magnetic

measurements and their

interpretation.

^{We also}

thank Drs. C.

Cohen-Addad,

^D. Bordeaux and Mr. M. Chamel for their assistance in our

attempts

to

produce

^oriented

samples. Finally,

^{we are indebted}

to Pr. R. A. B. Devine for

checking

^the

English ‘of the

manuscript

^{and for comments.}

References

[1] CLARK, A. E., BELSON, H. S., TAMAGAWA, ^N. and CALLEN, E., Proc. Int. Conf. on Magnetism, ^Moscow (1973)

Vol. IV, p. 335 (Publishing ^house Nauka) ^1974.

[2] CANNON, J. F., ROBERTSON, ^{D. L. and} HALL, ^H. T., Mater.

Res. Bull. 7 (1972) ^5.

[3a] CONTRÉ, M., ^{French Patent C.E.A. n°} 1457690, 23-9-65 and 26-9-66.

[3b] CAPPONI, J. J., ^Thèse d’Etat, ^Grenoble (1973), ^C.N.R.S.

AO 9139.

[4] BELOV, ^K. P., Magnetic transitions, Consultants Bureau,

New York, ^1961.

[5] YANOVSKY, R., BAUMINGER, E. R., LEVRON, D., NOWIK, ^I.

and OFER, S., Solid State Commun. 17 (1975) ^1511.

[6] BUSCHOW, ^{K. H. J. and VAN} STAPELE, ^R. P., ^J. Appl. Phys.

41 (1970) ^4066.

[7] GUBBENS, P. C. M., Thesis (Delft University Press) ^1977.

See _p. 59, data relative to

ErFe2 : |e2 qQ/2|

decreases

from 0.70 mm/s ^{at 0 K to 0.35} mm/s ^at Tc, ^then jumps

up to 0.50 mm/s. ^In YFe2, |e2 qQ/2| ^is only ^0.40 mm/s.

[8] BOWDEN, G. J., ^J. Phys. ^{F : Metal} Phys. ³ (1973) ²²⁰⁶ (TbFe2).

[9] WIESINGER, G., Proc. Int. Conf. on the Applications ^{of the}

Môssbauer Effect, Corfu, 1976, J. Physique ^{Coll. 37} (1976) C6-585. Direct measurement on the figure ^seems

to indicate that in TmFe2, e2 qQ/2 ~ - ^0.66 mm/s.

[10] SARKAR, D., SEGNAN, R., CORNELL, ^E. K., CALLEN, E., HAR- RIS, R., PLISCHKE, ^{M. and} ZUCKERMAN, ^M. J., Phys. ^Rev.

Lett. 32 (1974) ^542.

This _paper gives the values of |e2 qQ/2| above Tc ^for

two componds, DyFe2, HoFe2, magnetized along [100].

These values are very similar to those obtained at 0 K

in compounds magnetized along 111 > ; ^{this seems to}

indicate that e2 qQ/2 ^{is not} very much affected by ^the important magnetostriction ^{of these} compounds.

[11] BOWDEN, ^G. J., BUNBURY, ^{D. ST.} P., GUIMARAES, ^{A. P. and} SNYDER, ^R. E., ^J. Phys. ^{C 2} (1968) ^1376.

[12] DARIEL, ^M. P., ATZMONY, U. and LEDENBAUM, D., Phys.

Status Solidi 59 (1973) ^615.

[13] ATZMONY, U. and DARIEL, ^M. P., Phys. ^{Rev. B 10} (1974)

2060.

[14] ^{VAN DER} VELDEN, J. N. J., VAN DER KRAAN, A. M., GUB- BENS, P. C. M. and BUSCHOW, ^{K. H.} J., Proc. Int. Conf.

on Mössbauer Spectroscopy, Cracow, Poland, ^Vol. 1, p. 2 C-9 (1975).

[15] MORARIU, M., BURZO, E. and BARB, D., Proc. Int. Conf.

on the Applications of the Môssbauer Effect, Corfu, 1976, ^J. Physique Colloq. ³⁷ (1976) ^C6-615.

[16] ROSEN, M., KLIMKER, H., ATZMONY, ^{U. and} DARIEL, ^M. P., J. Phys. Chem. Solids 37 (1976) 513, See _{p. 517.}

[17] ATZMONY, U. and DARIEL, ^M. P., Phys. ^{Rev. B 13} (1976) 4006, See p. 4008.

[18] ^{At 0 K} these values are (in temperature units) : A4 r4 > ^{= 36} K,

A6 r6 > = - 3 K, 03BCB Hex ^{= 116 K.}

[19] ATZMONY, U., DARIEL, M. P., BAUMINGER, E. R., LEDEN- BAUM, D., NOWIK, ^{I. and} OFER, S., Phys. ^{Rev. B 7} (1973)

4220.

[20] ^VAN DIEPEN, A. M., DE WIJN, H. W. and BUSCHOW, ^{K. H.} J., Proc. Int. Conf. on Magnetism, Moscow, 1973, Vol, I, p. 227 (Publishing ^house Nauka, Moscow) ^1974.

[21] CULLEN, J. R. and CLARK, ^A. E., Phys. ^{Rev. B 15} (1977) ^4510.