MÖSSBAUER STUDY OF CUBIC IRON-DOPED KMgF3 IN THE PRESENCE OF A MAGNETIC FIELD

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MÖSSBAUER STUDY OF CUBIC IRON-DOPED KMgF3 IN THE PRESENCE OF A MAGNETIC

FIELD

J. Regnard

To cite this version:

J. Regnard. MÖSSBAUER STUDY OF CUBIC IRON-DOPED KMgF3 IN THE PRESENCE OF A MAGNETIC FIELD. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-181-C6-183.

�10.1051/jphyscol:1974616�. �jpa-00215773�

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MOSSBAUER STUDY OF CUBIC IRON-DOPED K M ~ F , IN THE PRESENCE OF' A MAGNETIC FIELD

J. R. REGNARD

Centre #Etudes NuclCaires de Grenoble, DRF/Groupe #Interactions Hyperfines B. P. 85, Centre de Tri, 38041 Grenoble, France

R6sum6. - Des monocristaux de KMgF3 dopb avec du 57Fer ont ttC Btudies par spectroscopie Mossbauer avec et sans champ magnetique. L'interaction quadrupolaire induite par un champ exterieur de 50 kOe est

0,36 f 0,03 mm/s et 0,65 i 0,05 mmjs lorsque le champ est parallkle respectivement aux directions (100) et (1 11) du cristal. Utilisant le modkle de Ham, on en conclut que le couplage de l'orbitale blectronique 3d avec les modes de vibration de type Tag est plus impor- tant qu'avec les modes de type Eg. Des experiences recentes du msme type ont montr6 la situation inverse dans le cas du systkme CaO ^: Fez+.

Abstract. - Iron-doped KMgF3 single crystals have been studied by the Mossbauer technique in the presence of a magnetic field. The quadrupole interaction induced by a 50 kOe external field is

0.36 ^f 0.03 mm/s and + 0.65 + 0.05 mm/s when the field is successively oriented parallel to the (100) and (111) directions of the cubic crystal. Using Ham's model, one can deduce that the coupling of the electronic triplet state with the Tzg modes of vibration is more important than coupling with the Eg modes. Recent experiments of the same type with the system CaO

Fez+ have shown the inverse of the above behavior.

A comparison of Mossbauer spectra in the absence of a magnetic field and with a magnetic field oriented respectively in the (100) and (1 11) directions of ~ e ~ + - doped cubic crystals can give useful information concerning the coupling of the 3d electron orbital of Fe2+ with the E, and Tzg modes of vibration of the ligand complex. According to HAM'S theory [I, 21, the quadrupole splitting, due to random strain-field effects and with suitably long relaxation times, obtained at low temperatures for Fe2+ impurities in cubic crystals in the absence of a magnetic field is :

where KE and KT are the reduction factors relative to the coupling of the 3d electron orbital with the E, and Tz,-type modes of vibration of the ligand complex. ai, b , ci are the real coefficients which account for the mixing of the three states of the fundamental spin-orbit triplet, T,,, due to the presence of a weak internal strain field, and

Application of an external magnetic field parallel to the (100) direction of the crystal will induce a quadru- pole interaction which is opposite in sign to the inter- action induced when the field is applied parallel to the (1 11) direction. In the former case the interaction is proportional to K,, while in the latter it is propor-

tional to KT. The expressions (for the quadrupole interaction in the two directions are :

and

In addition, if we represent the energies of the Moss- bauer transitions in the (100) orientation by El, E2, E,, E, and in the (111) orientation by el, e,, e,, e,, it is possible to assign a positive energy shift to lines I and 4 and a negative shift to lines 2 and 3 :

For the cubic crystal MgO ^: Fez+, Chappert et al. [3]

have measured the same quadrupole interaction indu- ced by a magnetic field in the (100) and (I 11) direction :

I AEQ I

0.32 ^f 0.04 mm/s. If one measures in this case

I ei - E, I as previously defined, one obtains a value of about 0.165 mm/s. From eq. (2) and (3), it can be seen immediately that the electronic triplet state of the Fe2+

impurity in MgO is equally coupled with the E, and T,, modes (K, ⁼ KT) consistent with the conclusions of Ham et al. 121. In the case of Fe2+-doped KMgF,, isomer shift measurements indicate a greater ionicity in this system than in the system MgO : Fe2+. Howe- wer the electronic observables g and 2 I. [4, 51 are more strongly reduced than for MgO : Fe2+, indicating the importance of dynamic Jahn-Teller coupling. Simi- larly the strain coupling coefficients for Fe2+ in

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974616

C6-182 J. R. REGNARD KMgF,, GI,

1 340 cm-' and G4, = 1 000 cm-l,

are nearly twice as large as the values observed in the system MgO : Fez+. A Mossbauer study in the presence of a magnetic field for Fez+-doped KMgF, is of interest because of its similarity to Fez+-doped MgO and in the context of the preced- ing discussion. The present study constitutes a preli- minary report of the observations in the system KMgF, : ~ e " obtained both with and without the application of an external magnetic field. In order to evaluate eq. (2) and (3) experimentally single crystals of KMgF, doped with 0.05 at % 57Fe have been used. The crystals were grown by the Czochralski method and cleaved in 0.7 mm slices parallel to the (100) plane. For orientation in the (1 11) direction a sample holder having an orientation of 540 with respect to the (100) plane has been used. In this case, the geometry was less favourable (reduced area of the crystal and more significant absorption). The Moss- bauer spectrum at 4.2 K without a magnetic field consists of a quadrupole doublet with a splitting of 0.44 mm/s 171. Mossbauer experiments have also been carried out at 4.2 K with the same crystal successively oriented in the two directions in an external magnetic field parallel to the direction of emission of the y-rays.

A 50 kOe magnetic field has been used to obtain a

VELOCITY ( r n m l s )

FIG. 1.

Mossbauer spectra of KMgF3 : Fez+ at 4.2 K in the presence of a 50 kOe magnetic field parallel to the (100) and (111) directions of the crystal. The solid line is a least square fit of the

data.

94 % contribution of the J'

1 state of the

r,, spin-orbit triplet (effective J

1) thus simplifying the Mossbauer spectra to only four lines (Fig. 1).

From the separation between lines 1 and 4, one can deduce the value of the Fez+ hyperfine field H,, = - 115 kOe, which is slightly different in the two orientations. The quadrupole interaction in the (100) direction (E4 - E,) - (E, - El) has been measured to be

0.36 + 0.03 mm/s and the (111) direction(e,

e,)

(e, - el) ⁼ + 0.65 + 0.05 mm/s.

The mean value of the energy shift of the Mossbauer lines, as defined in eq. (3), is

lei - E,li=,,,,

0.25 + ^0.04mm/s.

In constrast to the MgO : Fez+ results the present data imply a very different value for the induced quadrupole interaction in the two crystallographic directions in the case of KMgF, : Fez+. These results can be accounted for in two ways : (i) the coup- ling of 3d orbital with the E, and T,, modes may not be equal (ii) the difference in the two values may be attributed to the presence of second-order terms in the quadrupolar Hamiltonian for example due to a pseudo-quadrupole interaction. In the case of (ii), one can calculate the induced quadrupole interaction in the two directions :

AE~(H/(~OO))

(E, - E,) - ( E , - El)

-

- -

12PK,+-- A: for (100) A

for where A, is the hyperfine constant and A the Zeeman splitting between the J'

- 1, 0, + 1 spin-orbit states. The second term 2 A ~ / A , which is always positive, can diminish AEQ in the (100) direction and increase BEQ in the (1 11) direction by the same amount.

Unfortunately, since the magnitude of this term is on the order of 2 x lo-, mm/s it cannot account for the observed difference in AEQ in the two directions which amounts to 2.9 x lo-' mmls. One way to account for our results consists in an unequal coupling (KE # KT) of the 3d orbital with the E, and T,, modes. If one assumes K, # KT, using eq. (2) and (3), one can deduce 3 PK,

0.09 f 0.02 mm/s and

With these values of 3 PKE and 3 PK., one can calculate the mixing (a; b: + b;

+

defined in eq. (1) of the three fundamental spin-orbit states by the strain field. This mixing is found to be 0.065 in the case of KMgF, : Fe2+. In order to evaluate KE and KT quantitatively, it is now necessary to have a reliable value of the term < r - , >,,in this system. Hazony [a]

has recently noted a correlation between Hc, < r - > ,,

MOSSBAUER STUDY OF CUBIC IRON-DOPED KMgF3 IN THE PRESENCE OF A MAGNETIC FIELD C6-183

and the isomer shift for various ferrous compounds.

As suggested by the isomer shift and Hc values for KMgF, : Fez+ [7, 91, one can use for this calculation the free-ion value < r-3

of 5.1 a. U. as approxi- matively correct. A reasonable value for Q is taken to be 0.21 + 0.03 barn. It is now possible to give an approximate evaluation of KE and KT within the spin-orbit triplet: K, - ^{0.058 and KT}

0.10. Accord- ing to HAM'S theory, it is concluded that the coupling of the electronic triplet state with the T,, mode is more important than the coupling with the E, mode

>

Recently these same types of experiments have been carried out in the CaO : Fe2+ system. The quadrupole interaction induced by a field of 55 kOe is found to be

0.74 + 0.04 and + 0.35 -+ 0.04 mm/s in the (100) and (111) orientations, respectively.

Contrary to the KMgF, : Fe2

case, in the CaO : Fez '

system the Jahn-Teller coupling seems to be more important with the E, modes of vibration of the complex (K, > KT). As a complementary study the

far infra-red spectra of pure and 150 ppm Fe-doped CaO have been obtained and it is found the electronic transitions T,,

T,, and T,,

T,, are located at 30 cm-' and 32.6 cm-' respectively (the energy distance 2 1 between r,, and T,,, T3, is 200 cm-I for the free ion). This result is in good agreement with the value proposed by Wilkinson 2 A

23 cm-' [lo].

In conclusion, Mossbauer experiments with a magne- tic field for KMgF, : Fez+ and CaO : Fez+ indicate an unequal coupling of the fundamental electronic triplet state with the E, and T,, modes of the complex.

For KMgF, : Fez' there is an important Jahn- Teller interaction with a dominant coupling of the electronic triplet with the T,, modes (KT > KE) while for CaO : Fe2+ an even stronger Jahn-Teller interac- tion with a dominant coupling with the E, modes (KE > KT).

Acknowledgments.

The author would like to thank Dr. T. Ray for helpful discussions and Dr. J. Chappert for having suggested this problem.

References

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