RECENT MÖSSBAUER INVESTIGATIONS ON THE RFe2 COMPOUNDS

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RECENT MÖSSBAUER INVESTIGATIONS ON THE RFe2 COMPOUNDS

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

To cite this version:

C. Meyer, F. Hartmann-Boutron, Y. Gros, J. Capponi. RECENT MÖSSBAUER INVESTIGATIONS ON THE RFe2 COMPOUNDS. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-191-C1-192.

�10.1051/jphyscol:1980157�. �jpa-00219726�

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JOURNAL DE PHYSIQUE Colloque

Cl,

suppl&ment au n ^O

1,

Tome

41,

janvier

1980,

page

C1-191

RECENT

MSSBAUER

I

NVESTI

GAT1 ONS ON M E RFe2

COMPOUNDS

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

+

Laboratoire de Spectrom6trie Physique, B.P. 53 X, 38041 GRENOBLE-CEDEX, France.

Laboratoire de CristaZZographie, B.P. 266 X, 38042 GRENOBLE-CEDEX, France.

By using high pressures and temperatures have succeeded i n preparing three new Laves phase caw pounds W e 2 , NdFe2, PrFe2, which have been inves- tigated both by magnetic masu-nts and by

'

Massbauer spectroscopy on 5 7 ~ e and 174ib[ 1

I

[ 2

I

[ 3 1 . These studies raise questiom concerning a)

the

magnitude of

the

crystalline f i e l d a t

the

R.E.

ion

in

the We2 series and b)

the

origin of

the

"dipolar f i e l d splittings" observed in the 5 7 ~ e spectra of the We2. &re we w i l l discuss p i n t a ) while g i n t b)

w i l l

be treated i n a separate

cam

m i c a t i o n

to

ICM 79. We

w i l l

report som prelimi- nary results, concerning the Curie t-ratures Tc

,

and e l e c t r i c quadruple couplings e 2 qQ a t the iron

above Tc, for W e 2 , NdFe2 and PrFe2.

CFESTAL FIELD l3FEETS I N THE me2

Wehave' performed the Massbauer study of W e 2 between 4.2 and 60 K [ 2 ]

.

^The^hyperfine

f i e l d and e l e c t r i c quadruple coupling a t

the

=170 nucleus are directly related

to

<JZ> and

<uZ2 -

^J(J+l)>. In turn the t h e m 1 variations of these quantities are controlled by

the

Yb-Fe

ex-

change f i e l d (Hex) and by the crystalline f i e l d

(a4<r4>, ~ ~ < r ~ > )

a t the Yb3+ ion. L a s t f i t s of our ~Essbauer

data

$mw t h a t pg Hex/kB 100 K , which is cmqarable t o other We2, while the crys- t a l l i n e f i e l d is much d l e r than t h a t expected on

the

basis of current ideas ( IA4<r 4

> I kg,

b6<r6> I 4

⁴4 K instead of the expectation

4 6

A 4 < r

>/IS

^%³⁶K t A6<r

>/kg

^%

-

³^{K ) .} ^Indeed,^up^to

now it was mre o r l e s s acimitted, on the Sasis of low T e x t r a p l a t i o n s of anisotropy curves (Tm Fe

2' Er Fe2)

,

and of the phase diagrams of ternary ccan- pounds Ccbx D Y ~ - ~ Fe2

Hex

Tbl-x Fe2, e t c )

,

^{t h a t}

one had single ion anisotropy due to the c r y s t a l f i e l d a t

t k

R.E. and t h a t A4<r 4 > was essentially constant and large i n

the

We2 series. Our results raise the general questions :

I ) I£ one neglects the small iron contribution, i s the magnetocrystalline anisotropy of the We2 enti- rely due t o

the

crystal f i e l d

( ~ ~ < r ~ > , ~ ~ < r ~ > )

a t the R.E. ?

2) ~s concerns

the "true"

crystal f i e l d (A4, As) i n

the 9

(M'= Fe, Co, N i ) , does

it

:

a) depend on the RE

and

on M b) d e ~ n d . o n the RE but not on M c) depend on M but not on the W d) depend neither on the RE nor on M.

As concerns p i n t 1). mgnetostrictive effects have recently been invoked i n order to improve

the

in- terpretation of ternary phase diagrams [ 4 ]

.

^The

theoretical analysis of Ref. [ 2

I

shows t h a t , on

the

one hand, magneimstriction brings

to

the fourth order anisotrom constant K1 a contribution

%.

which adds t o t h a t of

~ ~ < r ~ >

; on the other hand, its effec; on the R.E. levels

(and

hence on

the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980157

(3)

(21-192 JOURNAL DE PHYSIQUE

Mijssbauer spectra)

is the

s n a s that of a second order crystalline potential V2 ^%

&.

Canparison of

-

the magnetocrystalline anisotropy with Mtjssbauer data on the R.E. could therefore be a way to get in- f o m t i o n on

k .

^Ourestjmates

seem

t o suggest that, while not negligible, mt explain

im-

p r t a n t variations of the apparent'crystal field.

Anisotropic R.E.-Fe exchange might also contribute to the apparent crystal field,but its magdtude is d i f f i c u l t to estimate. As concerns m i n t 2)

,

^the

f a c t t h a t for given R the magnetization directions

i n

RFe2 and W302

are

often

similar

would tend t o favor b) o r d). The usual a s s a p t i o n of constancy of A4 and As i n

the

We2 requires c) o r d). On the contrary, the m l l n e s s of A 4 < r 4 > i n YbFe2,cmpred with the large e x t r a p l a t e d anisotropy of We2

,is

not mnpatible with c ) o r d)

.

In addition, while i n the second half of the RFe2 it is usually asfllmed t h a t A6<r 6 > < 0 and / A <r6>

1

^<<~ ~ < r ~ > . on the con-

6

trary the [I101 magnetization direction observed i n

6 6

NdFe2 and SMe2 requires A 6 < r > < 0 and

%<r

> %

A4<r 4 > ; this is also incompatible with c ) o r d).

It therefore seems that we have situation b) (or a ) ^?)

.

In connection with t h i s it is interesting to note that when the low anisotropy ccmpund We2 is doped with a few percent of Nd o r E r o r Dy, the magnetization direction beccanes t h a t of NdE'e2 o r

&Fez o r DyFe2 [5]

.

In conclusion, it seems t h a t Essbauer e x p e r b n t s on

the

R.E. in the We2 ampunds could yield interesting information on

the

c r y s t a l f i e l d a t the R.E.. A s a a t t e r of f a c t , in DyFe2 and Tm ¹⁶⁹ i n TmFe2 were already studied [6]

,

but

the

r e s u l t s e r e interpreted with pure exchange only : indeed

CURIE TEMPEFWURES OF We2, NdE'e2 AND PrFe2 Curie temperatures f o r previously known RFe2 cam pounds a r e l i s t e d in p. 1242 of Buschow's review

[7]

.

^{A s}concerns m e 2 , NGe2 and PrFe2 we have perfonred both magnetization masurements and Mijssbauer studies. Magnetization masurements w a e done i n the "Service @rial de Xesures d 1 ~ t a - tion" of the Louis N&l Laboratory ( M r Barlet) ; the ingots were contained i n copper tubes sealed under vacuum a t

r c o m

temperature by pinching off [B] ; t h i s was sufficient t o prevent any oxydation during the duration of the masurements (1-2 hours).

On the contrary, up t o now we did not have much success with the Mijssbauer study, which was performed on pllets made of a mixture of We2 and aluminum m e r , and sealed under vacuum in aluminum capsules ; t h i s

i s

probably due t o oxydation of

the

We2 by alumina a t the surface of the aluminum grains. k k intend t o r e p a t the experiment indif- ferent conditions.

The magnetization data lead to

the

following Curie t-ratures :

We2 : Tc = 275 t 5OC (548 K) Nme2 : Tc = 304 1: 5OC (577 K) PrFe2 : Tc = 270 ^ts 0 C (543 K)

Acceptable Mtjssbauer spectra could only be obtain

-

ed with YbFe2. A t 30O0CI / e 2 g Q/2I

*

0.37 m/ss.

MEYER C. et a l , J. Physique (E'r)

38

(1977) 1449 MEYER C. e t

a l ,

J. Physique (Fr) 40 (1979) 403 Note t h a t z c z r d i n g

to

Abbundi

set&,

^J.^Apfi.

Phys.

50

(1979) 1671, t h e i r previous assumption A , , , >>A,,

is

not verified i n HoFe

MEYER C.

--

e% dl, J. Physique Coll.

&

(1979) C5 191

-

( S a i n t - p i e r r e - d e r e u s e )

KCCN N.C.

&

^@, J. Appl. Phys.

49

(1978) 1948 MIZOGUCHI T.

et

a l l J. Phys. Coll.

40

(1979) C2

-

²¹¹^(KYO=)-

See Refs [23] and [24] of [2]

BUSCFDW K.H.J., Rep. Prog. _Phys.

40

(1977) 1179 We

are

indebted t o D r Francis IWUWdW for .sug-

- -

> 150 K i n these

canpunds,

which is large gesting this method.

b H e x ^'L

capaxed to crystal f i e l d s effects even i f A 4 < r 4 >

*

30

-

40 K. High precision experiments w u l d therefore be necessary.