MÖSSBAUER EFFECT IN Fe-DOPED KMgF3

HAL Id: jpa-00214371

https://hal.archives-ouvertes.fr/jpa-00214371

Submitted on 1 Jan 1971

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.

MÖSSBAUER EFFECT IN Fe-DOPED KMgF3

J. Chappert, J. Regnard, R. Frankel, A. Misetich, N. Blum

To cite this version:

J. Chappert, J. Regnard, R. Frankel, A. Misetich, N. Blum. MÖSSBAUER EFFECT IN Fe-DOPED KMgF3. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-941-C1-942.

�10.1051/jphyscol:19711337�. �jpa-00214371�

JOURNAL DE PHYSIQUE

Colloque C I , supplkment au no 2-3, Tome 32, Fhorier-Mars 1971, page C 1 - 941

MOSSBAUER EFFECT IN Fe-DOPED KMgF3

J. CHAPPERT, J. R. REGNARD.

Centre dYEtudes NuclCaires, Grenoble, France R. B. FRANKEL and A. MISETICH

Francis Bitter National Magnet Laboratory

(*),

M. 1. T., Cambridge, Mass, U. S. A.

and N. A. BLUM

(**)

Nasa Electronics Research Center, Cambridge, Mass, U. S. A.

Resumb. - Un monocristal de KMgF, dope avec du fer a et6 6tudi6 par effet Mossbauer. Les deux etats de valence Fez+ et Fe3f sont observb. En dessous de 12

OK

un doublet quadrupolaire apparait pour l'etat Fezb. Utilisant le modkle de contraintes propost5 par Ham, on deduit que le premier Btat excite dil A I'interaction spin-orbite se trouve a - ^{120 cm-1}

au-dessus du niveau fondamental Tss.

Abstract.

-

An iron-doped KMgF, single crystal has been studied with the Mossbauer technique. The two charge states, Fez+ and Fe3+, are observed. Below 12 OK an Fez+ quadrupole doublet appears. Using Ham strain model we calculate that the first excited spin-orbit level lies - ¹²⁰ cm-1 above the

Tsg

ground state.

In order to explain the appearance of a quadrupole doublet in the Mossbauer spectrum of the system MgO

:

Fez+ at temperatures below 14 OK [l, 21. Ham developped a model [3] including random strains in the crystal. These strains can lift the threefold degene- racy of the r,, spin-orbit triplet (effective J

=

1) and a quadrupole doublet with a splitting AE is to be expected at temperatures sufficiently low that the rela- xation time

^7,

for electronic transitions among the three states is long compared to

^{7, =}

h/AE (Fig. 1).

'D

S,

cubic s p i n st,ains

f i e l d o r b i t

t,

= f i / ~ E

F A S T RELAXATION

t, <t2

( T > I Z K ) SLOW RELAXATION

t,

>

t2

(1 < 1 2 ~ )

FIG.

1. - On

the

left

is shown

^the

energy-level scheme of the Fez+ 5D term, on the

right

the partial energy

level

scheme of the 57Fe nucleus. The nuclear excited state is split

by the

el- tric field gradient due to the random strain splitting of the

electronic ground state.

above the r,, ground state, in contrast to the free ion value of 200 cm-'. This has been confirmed by far infra-red measurements [4]. Ham et al. concluded that this reduction in 2 1 is too large to be attributed to covalency and is predominantly the result of the Jahn-Teller (J. T.) coupling [5].

We present here the preliminary results of the Mossbauer study of Fe-doped KMgF,. This system is of interest because of its similarity to Fe-doped MgO.

However the strain coupling coefficients for Fe2+ in KMgF,, GI,

=

1 340 cm-' and Gq4

=

1 000 cm-I [6] are nearly twice as large as the MgO : Fe2+ values.

Since the J. T energy is proportional t o the square of the coupling coefficients, one expects that KMgF,

:

Fez+ may show larger J. T. effects than MgO

:

Fez+

[5]. Covalency effects can also be important, but fortunately the isomer shift (T. S.) of the Mossbauer spectrum provides a measure of the covalcnt character and thus allows a comparison of the covalency and J. T. effects.

Single crystals of KMgF, doped with 0.05

at

% of

"Fe were grown by the Czochralski method [7]. The room temperature Mossbauer spectrum of a 0.8 mm slice [I001 is shown in (Fig. 2). It can be interpreted as resulting from two valence states, Fe3+ and ~ e " , having I. S. of 0.77 mm/s and 1.42 mm/s respectively (relative to sodium ferrocyanide). These I. S. values are interesting in regard to the problem of the I. S.

calibration. In the Walker et al. scale [8], the purely ionic Fe3+ configuration was assumed t o correspond to an I. S. of 0.55 mm/s (relative to stainless steel).

On the basis of molecular orbital calculations, Danon 191 vrovosed a free ion I. S. of 1.0 mm/s. Our experi- mental-value for KMgF,

:

Fe3+ indicatks that K M ~ F , Ham showed that the temperature is purel ionic and corresponds to a configu- of the quadrupole splitting could be explained if the ration 3d5

4 s % ~ 7

on Danon,s scale. The transitions between the states went via an Orbach I indicates a negligible amount of covalency process involving the first excited spin-orbit level and (L > ^{ds sy.}

calculated that this level must lie at 2 1

=

95 cm-' When the temperature of the sample is decreased, the Fez+ line undergoes a transition-at 12 OK to a

(*)

Supported

^by

the

U.

S. Air Force Office

of

Scientific quadrupole doublet with a splitting AE

⁼

0.44 mm/s Research.

(**)

Present address

:

Applied Physics Laboratory,

The

(Fig. 2). This splitting corresponds to John Hopkins University, Silver Spring, Md.,

U.

S. A.

^{7, =}

h/AE

=

3

x

s .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711337

C 1

-

942 J. CHAPPERT, J. R. REGNARD, R. B. FRANKEL AND A. MISETICH AND N. A. BLUM

- 2 0

2

VELOCITY (mm/s)

FIG.

2.

-',Experimental Mossbauer spectra of Fedoped

KMgFs

at

300 OK

and

4.2 OK

showing the onset of an Fez+

quadrupole doublet a low temperature. The source

is

57C0

in

a chromium matrix.

At 12 OK then we have

z, = z,.

Assuming an Orbach type relaxation process, the temperature dependence of

^T,

is [3]

:

where A

=

2 1 A I, p

(=

3.15 g/cm3) is the density of KMgF, and V, and V, are the velocities of propa- gation of transverse and longitudinal acoustic waves.

Using V,

=

6.47 x 105cm/s and VT

=

3.94

x

10' cm/s [lo] and G , ,

=

1 000 cm-'/unit strain [6], we derive from eq. 1 that A - 120 cm-'. This value is some- what higher than the value (95 cm-') found for MgO

:

Fez+, but still represents a strong reduction from the free ion value (200 cm-I). Such a reduction can have two origins : covalency or dynamic J. T. [ll].

From the I. S. value, one can rule out any significant covalency effect. Therefore the reduction can be attributed to a J. T. coupling, similar in strength to that existing in MgO

:

Fez+. Although we expected a larger J. T. coupling in KMgF,

:

Fez+ from the strain coupling coefficients [5, 61, the similarity between the two systems is indicated by the reduced orbital contri- bution to the g-factors of 3.37 for KMgF,

:

Fez+ [12]

and 3.428 for MgO

:

Fez+ [13] compared to the crystal field theoretical value of 3.5.

Further information can be obtained from the magnetic hyperfine interaction induced by application of an external magnetic field at low temperature. The same effect of covalency and J. T. that reduces the orbital contribution to the g-factor should also reduce the orbital contribution to the magnetic hyperfine field. Experiments are underway to check this point.

Discussions with M. Belakhovsky, J. Danon and F. S. Ham are gratefully acknowledged.

References

[I] LEIDER

(H.

R.) and PIPKORN

(D.

N.),

Phys. Rev.,

[8] WALKER (L. R.), WERTHEIM (G. K.) and JACCA- 1968, 165, 494. R l ~ o

(V.), Phys. Rev. Letters,

1961, 6, 98.

121 CHAPPERT (J.),

FRANKEL (R. B.),

MISF,TICH (A.)

and

[9] DANON (J.), in Chemical Applications of Mossbauer BLUM ( N .

A.), Phys. Rev.,

1969, 179, 578. Spectroscopy, ed. by Goldanskii

V. I.

and Her- [3] HAM

(F.

s.),

Phys. Rev.,

1967, 160, 328. ber

R.

H., Academic Press, N.

Y.

1968.

[4] WONG (J.

Y.), Phys. Rev.,

1968, 168, 337.

[lo]

ROSENBERG

(H.

M.) and WIGMORE (J.

K.), Phys.

[5] HAM

( F .

S.), SCHWARZ (W.

M.)

and O'BRIEN MARY

Letters,

1967, 24A, 317.

(C.

M.),

Phys. Rev.,

1969, 185, 548. [ll] I ~ A M

(F.

S.),

Phys. Rev.,

1965, 138A, 1727.

[6] WIGMORE

^(J.

K.), ROSENBERG

(H. M.)

and GARROD [12] VALLIN (J.

T.)

and PIPER (W. W.), Quoted in ref.

[5].

@. K.), J. Appl. Physics,

1968, 39, 682. [13] LOW (W.) and WEGER

(M.), Plrys. Rev..

1960, 118, [7] Crystals provided by Cristal Tech, Centre d'Etudes 1130.

Nuclbaires, Grenoble.