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Far-infrared absorption of MgO: Fe2+ in magnetic fields
Solid State Communications,Vol. 16, pp. 105—107, 1975.
Pergamon Press.
Printed in Great Britain
FAR-INFRARED ABSORPTION OF MgO: Fe2~IN MAGNETIC FIELDS A. Hjortsberg, B. Nygren and J.T. Vallin Department of Physics, Chalmers University of Technology, S-402 20 Gothenburg, Sweden (Received 19 September 1974 by S. Lundqvist)
We have measured the far-infrared absorption of iron-doped MgO in the wavenumber region 10—200 cm~1and in magnetic fields up to 6 T. Absorption peaks found at 107.0 and 110.5 cm’ are assigned to magnetic dipole transitions between the spin—orbit I’~,groundstate (J = 1) and the r~,r~excited states (J = 2) of the Fe2 ~4on at a cubic site: The observed magnetic field dependence shows that r~is the higher excited level, so that the crystal field order of the levels is not changed by the reduction of the spin—orbit splitting attributed to a dynamic Jahn—Teller effect. An additional absorption peak at 33.4 cm~1is found to split in magnetic field. In iron-doped KMgF 3 absorption peaks at 52 and 87 cm~ 2 + arethat found have previously beenand attributed same transitions to remain unshifted unsplit to in the magnetic fields up toof6 Fe T.
THE Fe2~-IONin MgO substitutes for Mg2~and occupies sites of cubic symmetry and octahedral coordination. Its energy-level diagram is shown in Fig. 1.
__________________________________
5E 9
The lowest orbital term ~T~is split by spin—orbit interaction, the ground state being a r~g spin—orbit triplet (J = 1). The lowest excited state comprises a I’.~triplet and a r~ doublet, which are split in second order the spin—orbit coupling. free ion value of the by spin—orbit parameter ~yof—With 100acm~,crystal field theory predicts transitions rsg I’~and I’~~ at approximately 12X1 or 192 and 198 cm~,respectively.
/ 5D
/
Fe~
I 10
10850 cm-’
\
—~
Wong1 has found an absorption peak with a width
~
5T
\
2g of9 cm~at 105 cm~in MgO: Fe, but no absorption at 200 cm’. Accordingly the 105 cm~peak has been attributed to transitions from r5~ to I’~and r~ which were not resolved. The discrepancy from crystal field theory has by Ham, Schwarz andspin— 2 asbeen due explained to the partial quenching of the O’Brien orbit interaction by the Jahn—Teller coupling. Meyer, Regis and Farge have recently shown that this absorpon can be resolved into two peaks at 106.9 and 110.5 cm_i They conclude from the peak absorption
{~R
/
(3)
FREE ION
CUBIC FIELD
.
SPIN-ORBIT 1ST ORDER
_j_)3)
I’
SPIN-ORBIT 2NOORDER
FIG. 1. Energy level diagram of (3a )-ion Fe given by crystal field theory.
.
105
2+
as
FAR-INFRA1~EDABSORPTION OF MgO: Fe2~
106
MgO:Fe
Vol. 16, No. 1
MgO:Fe 1)5
/
.
/
1)0
0.5
,/~ERGV
105
tb ENERGY [cm~]
115
~,
SHIFT
—
TRANSITION
Z
FIG. 2. Measured optical absorption of MgO: Fe vs photon wavenember in different magnetic fields (displaced).
7
PROBABILITIES ‘
0
2
3
5
‘
-
5
MAGNETIC FIELD [T]
ratio assuming equal halfwidth that the F~state is the higher excited state. The purpose of this communication is to report the far infrared absorption spectra of Fe2~in MgO in magnetic fields up to 6T. From the splitting of the absorption peaks we are able to show that the peak at 107.0 cm_i in zero field is due to the r’ 1 due to the 5g F5g F3g r transition, 110.5 cm of the assignment 4g transition. that This at is the reverse proposed by Meyer et a!. * —~
Our absorption measurements were made using one interferometer of michelson type in the wavenumber region 40—200 cm~,and one lamellar interferometer from l0—8Ocm1. Both are commercial Beckman instruments. The resolution is I cm~or better in the whole region. The radiation is in both cases transmitted through a 0 1 cm brass light pipe into a dewar where sample and detector are mounted. The detector is a germanium bolometer at 1.3 K. The sample is mounted in the light pipe and can be kept either in liquid helium at 1.5 K or in a special temperature control unit at temperatures from 5—50 K. The light passes through a superconducting magnet giving maximum field of 6T at the sample.
FIG. 3. Upper: Shift of absorption peaks in magnetic fields. Lines: theory. Points: experiments. Lower: Calculated transition probabilities of the corresponding transitions. Two different samples were used in the experiments. One sample was doped with 0.2 wt. % Fe, and the other with 0.6 percent. The thickness of the samples was2 with typically 1 cmparallel and the was 0.5direction. cm a [1001 axis to cross-section the light propagation ‘~-~
The absorption coefficient versus photon wavenumber in different fields is shown in Fig. 2. In magnetic field the peak positions shift. The 107 cm’ peak has a g-factor of 3.5. The magnetic field dependence is shown in Fig. 3. At lower frequency an absorption peak at 33.4 cm’ was observed, which also splits in a magnetic field. One peak remained at 33.4 cm’ ,and one went down with g = 1.0. The strength of these peaks was more strongly dependent on the Fe-concentration than were the peaks at —~ 110 cm1. In order to find out if this absorption is due to a lower energy level of the isolated Fe2~-ion, the sample was heated to 50 K. No hot lines were observed at—S 75 cm’, which might represent transitions ‘~
-
Vol. 16, No.1
FAR-INFRARED ABSORPTION OF MgO: Fe2~
from this level to those at 107 and 110 cm_i. This is in agreement with similar measurements made by Wong. The magnetic field dependence of the P~,r~ levels can easily be calculated if the Zeeman perturba. tion is weak enough that coupling with levels in the ground state and other excited levels can be ignored. In Fig. 3 we show the result of such a calculation where we have subtracted the g-factor 3.428 of the ground state, to give the theoretical shift of the peak positions in magnetic fields. We have adjusted the r~—r~ separation to the experimentally found zerofield value of 3.5 cm~’,and assumed F~to be lower than r’~. Calculated probabilities from the ground state are also transition shown in Fig. 3.
107
The fact that the order of the levels F~and F~ is not changed from the crystal field case, is consistent with the conclusion from MOssbauer studies4 that the Jahn—Teller coupling with Eg and T~modes is approximately equal. From a perturbation calculation2 of the shift in r~ and I’~from the crystal field values we get (EJT)E = 110 cm_i and ~(EJT)r 1, 2 = cm 125 cm~ assuming an effective frequency hw = 400 of the coupling modes. Similar experiments were performed with KMgF 3: Fe. Ourthe results support the observation 5 that 87 cm~ absorption in KMgFof Ray et al. 2~.However we disagree with their3: Fe is not due tothat Fe the 52 cm_i peaks are associated to conclusions r~g [‘v, F~transitions of Fe2’. There are three reasons for this: -*
If we assume I’~to be lower than F~,we should get two lines starting from 110.5 cm , one with g 3.5 and one withg increasing from 3.5 to 6.5. Clearly the experimental points in Fig. 3 favor the case with r3g lower in energy than F4g. The calculated lines fit the experimental points very well except at high fields where the assumption of no interaction with other states is not valid. At 5.5 T coupling with the ground state levels will displace the level represented by the straight line in Fig. 3 with 1 cm’. Coupling with higher excited levels should decrease this value, but the coupling is weaker since these levels are farther away, so the observed departure from the line at high fields is expected. —~
(a) The oscillator strength does not scale with Fe -concentration as measured by near infrared absorption. +
(b) The line does not split in a magnetic field. (c) The line does not shift in energy with magnetic field although the ground state has a g-factor of 3.36.6
Acknowledgements We wish to acknowledge Prof. G. Brogren for placing experimental facilities to our disposal, and for his interest in this work. —
REFERENCES 1.
WONG J.Y.,Phys. Rev. 168, 337 (1968).
2.
HAM F.S., SCHWARZ W.M. and O’BRIEN M.C.M., Phys. Rev. 185, 548 (1969).
3.
MEYER P., REGIS M. and FARGE Y.,Phys. Lett. 48A, 41(1974).
4. 5.
CHAPPERT J., FRANKEL R.B., MISETICH A. and BLUM N.A.,Phys. Rev. 179,578(1969). RAY T., REGNARD J.R., LAURANT J.M. and RIBEYRON A., Solid State Commun. 13, 1959 (1973).
6.
VALLIN J.T. and PIPER W.W., Solid State Commun. 9, 823 (1971).
Pergamon Press.
Printed in Great Britain
FAR-INFRARED ABSORPTION OF MgO: Fe2~IN MAGNETIC FIELDS A. Hjortsberg, B. Nygren and J.T. Vallin Department of Physics, Chalmers University of Technology, S-402 20 Gothenburg, Sweden (Received 19 September 1974 by S. Lundqvist)
We have measured the far-infrared absorption of iron-doped MgO in the wavenumber region 10—200 cm~1and in magnetic fields up to 6 T. Absorption peaks found at 107.0 and 110.5 cm’ are assigned to magnetic dipole transitions between the spin—orbit I’~,groundstate (J = 1) and the r~,r~excited states (J = 2) of the Fe2 ~4on at a cubic site: The observed magnetic field dependence shows that r~is the higher excited level, so that the crystal field order of the levels is not changed by the reduction of the spin—orbit splitting attributed to a dynamic Jahn—Teller effect. An additional absorption peak at 33.4 cm~1is found to split in magnetic field. In iron-doped KMgF 3 absorption peaks at 52 and 87 cm~ 2 + arethat found have previously beenand attributed same transitions to remain unshifted unsplit to in the magnetic fields up toof6 Fe T.
THE Fe2~-IONin MgO substitutes for Mg2~and occupies sites of cubic symmetry and octahedral coordination. Its energy-level diagram is shown in Fig. 1.
__________________________________
5E 9
The lowest orbital term ~T~is split by spin—orbit interaction, the ground state being a r~g spin—orbit triplet (J = 1). The lowest excited state comprises a I’.~triplet and a r~ doublet, which are split in second order the spin—orbit coupling. free ion value of the by spin—orbit parameter ~yof—With 100acm~,crystal field theory predicts transitions rsg I’~and I’~~ at approximately 12X1 or 192 and 198 cm~,respectively.
/ 5D
/
Fe~
I 10
10850 cm-’
\
—~
Wong1 has found an absorption peak with a width
~
5T
\
2g of9 cm~at 105 cm~in MgO: Fe, but no absorption at 200 cm’. Accordingly the 105 cm~peak has been attributed to transitions from r5~ to I’~and r~ which were not resolved. The discrepancy from crystal field theory has by Ham, Schwarz andspin— 2 asbeen due explained to the partial quenching of the O’Brien orbit interaction by the Jahn—Teller coupling. Meyer, Regis and Farge have recently shown that this absorpon can be resolved into two peaks at 106.9 and 110.5 cm_i They conclude from the peak absorption
{~R
/
(3)
FREE ION
CUBIC FIELD
.
SPIN-ORBIT 1ST ORDER
_j_)3)
I’
SPIN-ORBIT 2NOORDER
FIG. 1. Energy level diagram of (3a )-ion Fe given by crystal field theory.
.
105
2+
as
FAR-INFRA1~EDABSORPTION OF MgO: Fe2~
106
MgO:Fe
Vol. 16, No. 1
MgO:Fe 1)5
/
.
/
1)0
0.5
,/~ERGV
105
tb ENERGY [cm~]
115
~,
SHIFT
—
TRANSITION
Z
FIG. 2. Measured optical absorption of MgO: Fe vs photon wavenember in different magnetic fields (displaced).
7
PROBABILITIES ‘
0
2
3
5
‘
-
5
MAGNETIC FIELD [T]
ratio assuming equal halfwidth that the F~state is the higher excited state. The purpose of this communication is to report the far infrared absorption spectra of Fe2~in MgO in magnetic fields up to 6T. From the splitting of the absorption peaks we are able to show that the peak at 107.0 cm_i in zero field is due to the r’ 1 due to the 5g F5g F3g r transition, 110.5 cm of the assignment 4g transition. that This at is the reverse proposed by Meyer et a!. * —~
Our absorption measurements were made using one interferometer of michelson type in the wavenumber region 40—200 cm~,and one lamellar interferometer from l0—8Ocm1. Both are commercial Beckman instruments. The resolution is I cm~or better in the whole region. The radiation is in both cases transmitted through a 0 1 cm brass light pipe into a dewar where sample and detector are mounted. The detector is a germanium bolometer at 1.3 K. The sample is mounted in the light pipe and can be kept either in liquid helium at 1.5 K or in a special temperature control unit at temperatures from 5—50 K. The light passes through a superconducting magnet giving maximum field of 6T at the sample.
FIG. 3. Upper: Shift of absorption peaks in magnetic fields. Lines: theory. Points: experiments. Lower: Calculated transition probabilities of the corresponding transitions. Two different samples were used in the experiments. One sample was doped with 0.2 wt. % Fe, and the other with 0.6 percent. The thickness of the samples was2 with typically 1 cmparallel and the was 0.5direction. cm a [1001 axis to cross-section the light propagation ‘~-~
The absorption coefficient versus photon wavenumber in different fields is shown in Fig. 2. In magnetic field the peak positions shift. The 107 cm’ peak has a g-factor of 3.5. The magnetic field dependence is shown in Fig. 3. At lower frequency an absorption peak at 33.4 cm’ was observed, which also splits in a magnetic field. One peak remained at 33.4 cm’ ,and one went down with g = 1.0. The strength of these peaks was more strongly dependent on the Fe-concentration than were the peaks at —~ 110 cm1. In order to find out if this absorption is due to a lower energy level of the isolated Fe2~-ion, the sample was heated to 50 K. No hot lines were observed at—S 75 cm’, which might represent transitions ‘~
-
Vol. 16, No.1
FAR-INFRARED ABSORPTION OF MgO: Fe2~
from this level to those at 107 and 110 cm_i. This is in agreement with similar measurements made by Wong. The magnetic field dependence of the P~,r~ levels can easily be calculated if the Zeeman perturba. tion is weak enough that coupling with levels in the ground state and other excited levels can be ignored. In Fig. 3 we show the result of such a calculation where we have subtracted the g-factor 3.428 of the ground state, to give the theoretical shift of the peak positions in magnetic fields. We have adjusted the r~—r~ separation to the experimentally found zerofield value of 3.5 cm~’,and assumed F~to be lower than r’~. Calculated probabilities from the ground state are also transition shown in Fig. 3.
107
The fact that the order of the levels F~and F~ is not changed from the crystal field case, is consistent with the conclusion from MOssbauer studies4 that the Jahn—Teller coupling with Eg and T~modes is approximately equal. From a perturbation calculation2 of the shift in r~ and I’~from the crystal field values we get (EJT)E = 110 cm_i and ~(EJT)r 1, 2 = cm 125 cm~ assuming an effective frequency hw = 400 of the coupling modes. Similar experiments were performed with KMgF 3: Fe. Ourthe results support the observation 5 that 87 cm~ absorption in KMgFof Ray et al. 2~.However we disagree with their3: Fe is not due tothat Fe the 52 cm_i peaks are associated to conclusions r~g [‘v, F~transitions of Fe2’. There are three reasons for this: -*
If we assume I’~to be lower than F~,we should get two lines starting from 110.5 cm , one with g 3.5 and one withg increasing from 3.5 to 6.5. Clearly the experimental points in Fig. 3 favor the case with r3g lower in energy than F4g. The calculated lines fit the experimental points very well except at high fields where the assumption of no interaction with other states is not valid. At 5.5 T coupling with the ground state levels will displace the level represented by the straight line in Fig. 3 with 1 cm’. Coupling with higher excited levels should decrease this value, but the coupling is weaker since these levels are farther away, so the observed departure from the line at high fields is expected. —~
(a) The oscillator strength does not scale with Fe -concentration as measured by near infrared absorption. +
(b) The line does not split in a magnetic field. (c) The line does not shift in energy with magnetic field although the ground state has a g-factor of 3.36.6
Acknowledgements We wish to acknowledge Prof. G. Brogren for placing experimental facilities to our disposal, and for his interest in this work. —
REFERENCES 1.
WONG J.Y.,Phys. Rev. 168, 337 (1968).
2.
HAM F.S., SCHWARZ W.M. and O’BRIEN M.C.M., Phys. Rev. 185, 548 (1969).
3.
MEYER P., REGIS M. and FARGE Y.,Phys. Lett. 48A, 41(1974).
4. 5.
CHAPPERT J., FRANKEL R.B., MISETICH A. and BLUM N.A.,Phys. Rev. 179,578(1969). RAY T., REGNARD J.R., LAURANT J.M. and RIBEYRON A., Solid State Commun. 13, 1959 (1973).
6.
VALLIN J.T. and PIPER W.W., Solid State Commun. 9, 823 (1971).