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Paramagnetic resonance of Fe2+ in KMgF3
Solid State Communications,
Vol.
~,
PP. 823—825, 1971.
Pergamon Press.
Printed in Great Britain
PARAMAGNETIC RESONANCE OF F&~IN KMgF
3
J.T. Vallin* and W.W. Piper General Electric Company Research and Development Center Schenectady, New York, 12301, U.S.A.
(Received 25 February 1971 by E. Burstein)
2~inKMgF An isotropic paramagnetic resonance of Fe 3 has been observed at low temperature. From the relatively sharp resonances associated with the double quantum transition and with the half field line it is deduced that the g-value is 3.36 instead of the value of 3.53 predicted by second order crystal field theory. The difference is attributed to a combination of the dynamic Jahn—Teller effect and the effect of covalency both of which reduce the orbital contribution to the g-value.
TECHNIQUES other than EPR used 2~in thehave past.been 1—3 These for studying KMgF3 earlier studies have Fe concluded that the Fe2~
Detection was with a The balanced bridge tuned to made absorption. powerbolometer was modulated at 80Hz with peak-to-peak intensity of 12G. The spectrum is interpreted in the same way as McMahon ~ interpreted the MgO: Fe2~spectrum.
ions enter the lattice substitutionally for Mg3~ thus occupying sites of octahedral symmetry. The electronic ground state is then a spin-orbit triplet, r’ 59, originating from the orbital ~7’2~ term. The EPR results reported in this paper
______________________________________ I
I
HFL
of the F&~ ion and the ground state assignment. The sample used to obtain the data in this support these earlier conclusions on the position paper was a single crystal containing about 0.1 per cent Fe which was grown from the melt q’136
at the University of Aberdeen. The EPR spectrum obtained from this sample is shown in Fig. 1. The spectrometer was operated at a microwave frequehcy of 14 GHz and has been described in 4 the literature elsewhere. The resonance spectrum of interest could be observed most clearly near lambda high pointincident of heliummicrowave (about 2°K) underthe relatively power. *
2 3 MAGNETIC FIELD (KILOGAUSS)
4
2~at 14GHz andof2°K. FIG. 1. The EPR absorption spectrum KMgF3 :0.1 per cent Fe
Permanent address: Dept. of Physics, Chalmers
The broad line is anto allowed = 1 transition which is sensitive strain inAM first order. The
University of Technology, S—402, 20 Gothenburg
line marked DQL is a double quantum transition
5, Sweden. 823
where two photons are absorbed in rapid succession. The DQL line is broadened only by second order strain shifts. The line at half field (HFL)
24 IN KMgF
824
PARAMAGNETIC RESONANCE OF Fe
is a true by AMthe= 2strain transition. Thisof transition is allowed admixture wave functions. Since DQL and NFL can occur with finite probability only at fields less than the zero strain value of the magnetic field, the high field edge of these two transitions was measured to determine the g-value. Assuming an effective spin Hamiltonian of the form
3
Vol. 9, No. 11
8 From our experimental completely quenched. result, g = 3.36, we conclude that the Jahn—Teller coupling for KMgF 24 is not nearly so strong 3 : Fe as was indicated by W 2 igmore et al. If the coupling had been so strong as to lead to a static Jahn—Teller effect, we could of course have seen an anistropic EPR spectrum (a superposition of the axial spectra of the several dis-
H
3
=
g13 H S (S
1)
(1)
torted configurations) with g-factors near two. In contrast, the spectrum observed was completely
the data which is reproduced in Fig. 1 yields a g-value of 3.36 ±0.02. On the other hand, from second-order crystal 67 field theory a g-value of 3.53 is predicted.
isotropic.
We attribute the difference between experimental and theoretical g-values to the reduction
had failed~ ~ We conclude from our experiments that Fe substitutes for Mg in KMgF 3 with a 2+ charge state and that in the Zeeman splitting fromthe the reduction crystal field value is
of the orbital contribution caused by result covalency 7’8 This is and the to Jahn—Teller effect. for Fe24 in Mg07. similar the result obtained Also Chappert et ~ had to assume a reduction in the spin-orbit parameter when they interpreted the Mossbauer spectrum of KMgF 4. However 3 : F& our results are in disagreement with the results of Wigmore et al. 2 From acoustic paramagnetic resonance measurements these authors obtained a g of approximately 3.0. This is the value one would obtain for a dynamic Jahn—Teller effect if the orbital contribution to the g-factor was almost
Optical spectra in24the near infrared for MgO:absorption F&4 and KMgF are very Fe experiments similar ‘which suggested the3:EPR
the result of the combination of a dynamical Jahn—Teller effect and covalency. Although the spin-orbit reduction for Fe24 in KMgF 3 is less 3 reducthan that for F&4 in MgO, 10 the g-factor tion is larger. This is an interesting difference for which we have no explanation at the present time. .4cknowledgements — We are indebted to F.S. Ham for helpful comments and to G. A. Slack for providing the sample.
REFERENCES 1.
JONES GD., Phys. Rev. 155, 259 (1967).
2.
WIGMORE J.K., ROSENBERG H.M. and GARROD D.K., J. appi. Phys. 39, 682 (1968).
3.
CHAPPERT
4.
Abstract JeG1O, Grenoble (1970). LUDWIG G.W. and WOODBURY H.H., Solid State Phys. 13, p.239 Academic Press, New York, (1962).
5.
MCMAHON D.H., Phys. Rev. 134, A128 (1964).
6.
LOW W. and WEGER M., Phys. Rev. 118, 1119, 1130 (1960).
7.
HAM F.S., SCHWARZ W.M. and O’BRIEN M.C.M., Phys. Rev. 185, 548 (1969).
.8. 9. 10.
J.,
BLUM N.A., FRANKEL R.B. and MISETICH A., 1970
ml. Conf. on Magnetism,
HAM F.S., Phys. Rev. 138, A1727 (1965). HALL T.P.P., Thesis, Clarendon Laboratory, Oxford, 1962. (unpublished), WONG J.Y., Phys. Rev. 168, 337 (1968).
Vol. 9, No. 11
2~IN KMgF
PARAMAGNETIC RESONANCE OF Fe
3
Le spectre de r~sonanceparamagnétique électronique a basse 24 temperature est isotrope. d’un On observe monocristal 2 raies de KMgF9 étroites,contenant l’une se des trouvant ions ~ Feune valeur moitié en chamy magnéti 3ue de l’autre. En associant la raie êtroite ~ champ magnetique superieur avec la transition quantique double, on déduit un facteur spectroscopique g de 3,36 au lieu de 3,53, valeur pr~ditepar la théorie du champ cristallin de second ordre. On attribue la difference ~ une superposition de l’effet Jahn_ Teller dynamique a l’êtat de covelence, qui tous deux diminuent la valeur deu facteur g.
825
Vol.
~,
PP. 823—825, 1971.
Pergamon Press.
Printed in Great Britain
PARAMAGNETIC RESONANCE OF F&~IN KMgF
3
J.T. Vallin* and W.W. Piper General Electric Company Research and Development Center Schenectady, New York, 12301, U.S.A.
(Received 25 February 1971 by E. Burstein)
2~inKMgF An isotropic paramagnetic resonance of Fe 3 has been observed at low temperature. From the relatively sharp resonances associated with the double quantum transition and with the half field line it is deduced that the g-value is 3.36 instead of the value of 3.53 predicted by second order crystal field theory. The difference is attributed to a combination of the dynamic Jahn—Teller effect and the effect of covalency both of which reduce the orbital contribution to the g-value.
TECHNIQUES other than EPR used 2~in thehave past.been 1—3 These for studying KMgF3 earlier studies have Fe concluded that the Fe2~
Detection was with a The balanced bridge tuned to made absorption. powerbolometer was modulated at 80Hz with peak-to-peak intensity of 12G. The spectrum is interpreted in the same way as McMahon ~ interpreted the MgO: Fe2~spectrum.
ions enter the lattice substitutionally for Mg3~ thus occupying sites of octahedral symmetry. The electronic ground state is then a spin-orbit triplet, r’ 59, originating from the orbital ~7’2~ term. The EPR results reported in this paper
______________________________________ I
I
HFL
of the F&~ ion and the ground state assignment. The sample used to obtain the data in this support these earlier conclusions on the position paper was a single crystal containing about 0.1 per cent Fe which was grown from the melt q’136
at the University of Aberdeen. The EPR spectrum obtained from this sample is shown in Fig. 1. The spectrometer was operated at a microwave frequehcy of 14 GHz and has been described in 4 the literature elsewhere. The resonance spectrum of interest could be observed most clearly near lambda high pointincident of heliummicrowave (about 2°K) underthe relatively power. *
2 3 MAGNETIC FIELD (KILOGAUSS)
4
2~at 14GHz andof2°K. FIG. 1. The EPR absorption spectrum KMgF3 :0.1 per cent Fe
Permanent address: Dept. of Physics, Chalmers
The broad line is anto allowed = 1 transition which is sensitive strain inAM first order. The
University of Technology, S—402, 20 Gothenburg
line marked DQL is a double quantum transition
5, Sweden. 823
where two photons are absorbed in rapid succession. The DQL line is broadened only by second order strain shifts. The line at half field (HFL)
24 IN KMgF
824
PARAMAGNETIC RESONANCE OF Fe
is a true by AMthe= 2strain transition. Thisof transition is allowed admixture wave functions. Since DQL and NFL can occur with finite probability only at fields less than the zero strain value of the magnetic field, the high field edge of these two transitions was measured to determine the g-value. Assuming an effective spin Hamiltonian of the form
3
Vol. 9, No. 11
8 From our experimental completely quenched. result, g = 3.36, we conclude that the Jahn—Teller coupling for KMgF 24 is not nearly so strong 3 : Fe as was indicated by W 2 igmore et al. If the coupling had been so strong as to lead to a static Jahn—Teller effect, we could of course have seen an anistropic EPR spectrum (a superposition of the axial spectra of the several dis-
H
3
=
g13 H S (S
1)
(1)
torted configurations) with g-factors near two. In contrast, the spectrum observed was completely
the data which is reproduced in Fig. 1 yields a g-value of 3.36 ±0.02. On the other hand, from second-order crystal 67 field theory a g-value of 3.53 is predicted.
isotropic.
We attribute the difference between experimental and theoretical g-values to the reduction
had failed~ ~ We conclude from our experiments that Fe substitutes for Mg in KMgF 3 with a 2+ charge state and that in the Zeeman splitting fromthe the reduction crystal field value is
of the orbital contribution caused by result covalency 7’8 This is and the to Jahn—Teller effect. for Fe24 in Mg07. similar the result obtained Also Chappert et ~ had to assume a reduction in the spin-orbit parameter when they interpreted the Mossbauer spectrum of KMgF 4. However 3 : F& our results are in disagreement with the results of Wigmore et al. 2 From acoustic paramagnetic resonance measurements these authors obtained a g of approximately 3.0. This is the value one would obtain for a dynamic Jahn—Teller effect if the orbital contribution to the g-factor was almost
Optical spectra in24the near infrared for MgO:absorption F&4 and KMgF are very Fe experiments similar ‘which suggested the3:EPR
the result of the combination of a dynamical Jahn—Teller effect and covalency. Although the spin-orbit reduction for Fe24 in KMgF 3 is less 3 reducthan that for F&4 in MgO, 10 the g-factor tion is larger. This is an interesting difference for which we have no explanation at the present time. .4cknowledgements — We are indebted to F.S. Ham for helpful comments and to G. A. Slack for providing the sample.
REFERENCES 1.
JONES GD., Phys. Rev. 155, 259 (1967).
2.
WIGMORE J.K., ROSENBERG H.M. and GARROD D.K., J. appi. Phys. 39, 682 (1968).
3.
CHAPPERT
4.
Abstract JeG1O, Grenoble (1970). LUDWIG G.W. and WOODBURY H.H., Solid State Phys. 13, p.239 Academic Press, New York, (1962).
5.
MCMAHON D.H., Phys. Rev. 134, A128 (1964).
6.
LOW W. and WEGER M., Phys. Rev. 118, 1119, 1130 (1960).
7.
HAM F.S., SCHWARZ W.M. and O’BRIEN M.C.M., Phys. Rev. 185, 548 (1969).
.8. 9. 10.
J.,
BLUM N.A., FRANKEL R.B. and MISETICH A., 1970
ml. Conf. on Magnetism,
HAM F.S., Phys. Rev. 138, A1727 (1965). HALL T.P.P., Thesis, Clarendon Laboratory, Oxford, 1962. (unpublished), WONG J.Y., Phys. Rev. 168, 337 (1968).
Vol. 9, No. 11
2~IN KMgF
PARAMAGNETIC RESONANCE OF Fe
3
Le spectre de r~sonanceparamagnétique électronique a basse 24 temperature est isotrope. d’un On observe monocristal 2 raies de KMgF9 étroites,contenant l’une se des trouvant ions ~ Feune valeur moitié en chamy magnéti 3ue de l’autre. En associant la raie êtroite ~ champ magnetique superieur avec la transition quantique double, on déduit un facteur spectroscopique g de 3,36 au lieu de 3,53, valeur pr~ditepar la théorie du champ cristallin de second ordre. On attribue la difference ~ une superposition de l’effet Jahn_ Teller dynamique a l’êtat de covelence, qui tous deux diminuent la valeur deu facteur g.
825