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Magnetic ordering effects on the reflectivity of EuS and EuSe
Solid State Communications,
Vol. 7, pp. 1323—1326, 1969.
Pergamon Press.
Printed in Great Britain
MAGNETIC ORDERING EFFECTS ON THE REFLECTIVITY OF EuS AND EuSe* C.R. Pidgeon Francis Bitter National Magnet Laboratoryt, Massachusetts Institute of Technology, Cambridge, Massachusetts
J.
Feinleib, W.J. Scouler,
J.
Hanust, J.O. Dimmock and T.B. Reed
Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts (Received 18 July 1969 by J.A. Krumhansl)
The reflectivity of the ferromagnetic semiconductors EuS and EuSe has been measured as a function of temperature and polarization in a domain orienting magnetic field. The triplet structure below T~. and the doublet above 4f6(7F~)5d(i are evidence that the absorption edge transition is 4f~(~S712)-. 2~).
the edges EuS (~ 1.6eV)behavior and EuSeabsorption (‘-‘- 1.8eV).2 We of observe similar among all Eu ferromagnetic semiconductors in support of the proposed model.
RECENT observations of the effect of ferro-1 magnetic ordering on evidence the reflectivity Eu0 have provided direct for the of spin polarization of the conduction band in this ferromagnetic semiconductor and suggest that the optical absorption edge is due to a transition from the localized 4f7 ground state on the Eu2 ion to a final state configuration comprising an excited 4f~ multiplet and an electron in an apparently narrow 5d(t 2g) band. Since the transition in this model involves primarily the electrons of the Eu 2+ ion, we would predict that similar effects to those observed in EuO should exist in the other Eu chalcogenide semiconductors, EuS and EuSe. We here report measurements of the temperature dependent unpolarized reflectivity, and the reflectivity of circularly polarized light in an orienting magnetic field of theE1 reflectivity peak associated with
Figure 1 shows the temperature dependence of the reflectivity from a cleaved surface of single crystal EuS with no applied magnetic field. As the temperature is decreased from
This work sponsored by the Department of the Air Force.
room temperature, theE1 reflectivity peak sharpens at the maximum and a second peak is observed on the high energy side for temperatures well above the Curie temperature, T~,of 16.5°K. In a narrcw temperature range near T~ the E1 peak moves to lower energies qualitatively the way the absorption edge has been observed to move. ~ In addition, the structure of the E1 peak changes such that a third peak E1’ moves from theE1 peak to lower energies. At 1.5°K the E1 peak has a triplet composition, as is clearly seen in the magnetic field data below.
Supported by the U.S. Air Force of Scientific Research.
A similar effect in the temperature dependent reflectivity of the E1 peak is
* Present address: Groupe de Physique des Solides de l’Ecole Normale Superieure, Paris, France.
observed from a cleaved single crystal of EuSe as shown in Fig. 2. EuSe is metamagrietic with an antiferromagnetic ordering temperature,
___________
*
1323
1324
REFLECTIVITY OF EuS AND EuSe
r
Vol. 7, No. 18
I
I
I
E~Se
P~4OTONENERGY 1ev)
FIG. 1. Unpolarized reflectivity of theE
1 peak of EuS as a function of temperature.
T~ = 4.8°K. However, in a magnetic45field above it becomes a critical valueWith of no8koe at 1.9°K ferromagnetic. magnetic field applied, we observe that the E 1 peak is partly resolved into a doublet structure above the magnetic ordering temperature. As the temperature is reduced through TN, the E 1 structure shifts to lower energy and is broadened. Measurements in a magnetic field described below clearly resolve the 1.5°K data into the triplet labeled here. In both these materials the effects are similar to those previously observed in EuO. The red shift of the central E1 peak, as the temperature decreases from about 15°above T~ to 1.5°K, is 0.06eV for EuS and 0.05eV for EuSe. The red shift of the absorption edges over a similar temperature rangefor measured 3 is 0.18eV EuS andby Busch and Wachter 0.335eV for EuSe. Since the absorption edge is most likely caused by the onset of the E, transition, the additional red shift is attributed to the splitting off of the E transition. The unpolarized reflectivity structure at
PHOTON ENERGY 1eV)
FIG. 2. Unpolarized reflectivity of theE1
peak of EuSe as a function of temperature.
the field up the to 100 koe, but below field of 2Okoe reflectivities tend atocritical have the unresolved features of the H = 0 curves as the presence of random magnetic domains wash out the optical structure. The reflectivity of EuS and EuSe were examined above and below their respective Curie points using circularly polarized light in the Faraday configuration (E ~. H), at near normal incidence. At 1.5°K, EuS and EuSe both have a triplet structure, in which the lowest energy peak E has a pure aR polarization, the E~peak is nearly pure CL, while the central E1 peak has both polarizations, although ~R appears somewhat stronger than CL. The results at 1.5°K for all three compounds are very similar except for different Thethe central E position of the E1 structure. 1 peak is at 1.24eV in EuO, 1.89eV in EuS, and 2.09eV in EuSe. For both EuO and EuS the splitting of the E and E ‘~ peaks from E is nearly symmetrical with AE = ~iE ~‘ = 0.25 eV. However, in EuSe the splitting is asymmetrical about E,, AE = 0.19eV and AE~= 0.26eV.
1.5°K for both EuS and EuSe (Figs. 3 and 4) resolves into a sharp triplet structure with 40koe applied. The structure is not changed by increasing
The polarization dependence of the reflectivity was also measured at 22°Kwhich is only
Vol. 7, No. 18
REFLECTIVITY OF EuS AND EuSe
:
__________________________ UNPOLARIZED
I
I
:~:2K
I
UNPOLARIZED
14.O I
1325
I
I
I
~
_
PHOTON ENERGY I.V)
PHOTON
ENERGY Ccv)
FIG. 3. Polarized and unpolarized reflectivity of EuS in a magnetic field at 1.5°K and 22°K.
FIG. 4. Polarized and unpolarized reflectivity of EuSe in a magnetic field at 1.5°K and 22°K.
slightly above T~in EuS but is more than four times T~(at H = 40koe) for EuSe. In both materials the E 1 peak is resolved into a CR and CL doublet structure. This splitting is about 0.25eV as in EuO, but structure is about is 0.33eV in EuSe. inAtEuS, 70°K,the doublet still
we would expect to find the doublet structure that is observed with polarized light in all three
clearly resolved in EuS although T > 4 T~,but there is only a slight remnant of the doublet structure in EuSe for which T > 15 T~. At room temperature, no splittings were observed in E, for either material, As previously discussed for EuO, these results can be understood in terms of the transition probabilities for left and right circularly polarized light for the transition which is
Eu chalcogenides. It might be surprising that this doublet structure5 exists upmagnetization to at least 4 T~. that the is However, found still aboutit25is per cent of the saturation value in a field of 40 koe at temperatures -‘~ 4 T~.Since the preferential population of down over up spin states is directly proportional to the magnetization, the J = — 7/2 state of the octet ground state is still preferentially populated causing the doublet structure to persist at these ternperatures. Below the magnetic ordering temperature, the doublet structure is split into a triplet
6(7F~) 5d(t
described as 4f7(8S 712
) 4f
29). Our initial calculation showed that 7F~ the transition states -.
to the manifold of spin—orbit should show a resolution into split right and left circularly polarized portions when the octet
ground state is split in a magnetic field so that the lowest state, J 7/2, is preferentially populated at low temperatures. We have since —
included the appropriate spin—orbit splitting of the Sd (t 29) final state, and have found that this does not qualitatively change the situation, Just above the magnetic ordering temperature,
structure by the exchange splitting of the narrow Sd sub-bands. triplet appearssplitting rather than quartet becauseA the exchange of a ‘-~~0.25 eV in all these materials is nearly equal to the resultant splitting of the spin—orbit split
state and the 4f~multiplet. Thus the two center lines of the quartet overlap which also
accounts for the mixed polarization of the central peak. The fact that the splitting in EuSe is not symmetric may arise from an incomplete resolution of the quartet. The exchange splitting of the excited state of this transition, i.e., the
1326
REFLECTIVITY OF EuS AND EuSe
d band, has been previously inferred from magnetic dichrojsm measurements in EuSe6 and from absorption in thin films of EuSY However, our polarized reflectivity measurements provide a direct observation of the exchange splitting
Vol. 7, No. 18
of the 5d conduction band. Our optical data are consistent with other evidence for a narrow Sd conduction band, but our measurements do not distinguish this situation from that of localized Sd states lying near a 6s derived conduction band.
REFERENCES 1. 2.
FEINLEIB J., SCOULER W.J., DIMMOCK J.O., HANUS J., REED T.B. and PIDGEON C.R., Phys. Rev. Left. 22, 1385 (1969). For a recent review of experimental data see METHFESSEL S. and ~MATTISD.C., Handbuch der Physik XVIII, Pt. 1, p. 389. Springer, Berlin (1968).
3.
BUSCH G. and WACHTER P., Phys. cond. Matter 5, 232 (1966).
4.
FREISER M.J., HOLTZBERG F., METHFESSEL S., PETTIT G.B., SHAFER M.W. and SUITS J.C., Helv. Phys. Acta, Amsi. 41, 832 (1968).
5.
SUITS J.C. and ARGYLE B.E., Phys. Rev. Let:. 14, 687 (1965); MCGUIRE T.R. and SHAFER M.W., J. appi. Phys. 35, 984 (1964); BUSCH G., JUNOD P., RISI M. a~idVOGT B., Proc. mt. Conf. Semiconductors, Exeter (1962). 6. ARGYLE B.E., SUITS J.C. and FREISER M.J., Phys. Rev. Lett.15, 822 (1965). 7.
WILD R.L., SHINMEI M. and ANDERSEN A.L., Proc. In:. Conf. Semiconductors, Moscow (1968).
La réflectivité des semiconducteurs ferromagnetiques EuS et EuSe a été mesurée en fonction de la temperature et de la polarization dans Un champ magnetique que oriente les domains. La structure de triplet en dessous de T~et celle de doublet en dessus indiguent que la transition du bord de l’absorption est 4f7(8S 6(7F~)5d(t 712)-~4f 2~).
Vol. 7, pp. 1323—1326, 1969.
Pergamon Press.
Printed in Great Britain
MAGNETIC ORDERING EFFECTS ON THE REFLECTIVITY OF EuS AND EuSe* C.R. Pidgeon Francis Bitter National Magnet Laboratoryt, Massachusetts Institute of Technology, Cambridge, Massachusetts
J.
Feinleib, W.J. Scouler,
J.
Hanust, J.O. Dimmock and T.B. Reed
Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts (Received 18 July 1969 by J.A. Krumhansl)
The reflectivity of the ferromagnetic semiconductors EuS and EuSe has been measured as a function of temperature and polarization in a domain orienting magnetic field. The triplet structure below T~. and the doublet above 4f6(7F~)5d(i are evidence that the absorption edge transition is 4f~(~S712)-. 2~).
the edges EuS (~ 1.6eV)behavior and EuSeabsorption (‘-‘- 1.8eV).2 We of observe similar among all Eu ferromagnetic semiconductors in support of the proposed model.
RECENT observations of the effect of ferro-1 magnetic ordering on evidence the reflectivity Eu0 have provided direct for the of spin polarization of the conduction band in this ferromagnetic semiconductor and suggest that the optical absorption edge is due to a transition from the localized 4f7 ground state on the Eu2 ion to a final state configuration comprising an excited 4f~ multiplet and an electron in an apparently narrow 5d(t 2g) band. Since the transition in this model involves primarily the electrons of the Eu 2+ ion, we would predict that similar effects to those observed in EuO should exist in the other Eu chalcogenide semiconductors, EuS and EuSe. We here report measurements of the temperature dependent unpolarized reflectivity, and the reflectivity of circularly polarized light in an orienting magnetic field of theE1 reflectivity peak associated with
Figure 1 shows the temperature dependence of the reflectivity from a cleaved surface of single crystal EuS with no applied magnetic field. As the temperature is decreased from
This work sponsored by the Department of the Air Force.
room temperature, theE1 reflectivity peak sharpens at the maximum and a second peak is observed on the high energy side for temperatures well above the Curie temperature, T~,of 16.5°K. In a narrcw temperature range near T~ the E1 peak moves to lower energies qualitatively the way the absorption edge has been observed to move. ~ In addition, the structure of the E1 peak changes such that a third peak E1’ moves from theE1 peak to lower energies. At 1.5°K the E1 peak has a triplet composition, as is clearly seen in the magnetic field data below.
Supported by the U.S. Air Force of Scientific Research.
A similar effect in the temperature dependent reflectivity of the E1 peak is
* Present address: Groupe de Physique des Solides de l’Ecole Normale Superieure, Paris, France.
observed from a cleaved single crystal of EuSe as shown in Fig. 2. EuSe is metamagrietic with an antiferromagnetic ordering temperature,
___________
*
1323
1324
REFLECTIVITY OF EuS AND EuSe
r
Vol. 7, No. 18
I
I
I
E~Se
P~4OTONENERGY 1ev)
FIG. 1. Unpolarized reflectivity of theE
1 peak of EuS as a function of temperature.
T~ = 4.8°K. However, in a magnetic45field above it becomes a critical valueWith of no8koe at 1.9°K ferromagnetic. magnetic field applied, we observe that the E 1 peak is partly resolved into a doublet structure above the magnetic ordering temperature. As the temperature is reduced through TN, the E 1 structure shifts to lower energy and is broadened. Measurements in a magnetic field described below clearly resolve the 1.5°K data into the triplet labeled here. In both these materials the effects are similar to those previously observed in EuO. The red shift of the central E1 peak, as the temperature decreases from about 15°above T~ to 1.5°K, is 0.06eV for EuS and 0.05eV for EuSe. The red shift of the absorption edges over a similar temperature rangefor measured 3 is 0.18eV EuS andby Busch and Wachter 0.335eV for EuSe. Since the absorption edge is most likely caused by the onset of the E, transition, the additional red shift is attributed to the splitting off of the E transition. The unpolarized reflectivity structure at
PHOTON ENERGY 1eV)
FIG. 2. Unpolarized reflectivity of theE1
peak of EuSe as a function of temperature.
the field up the to 100 koe, but below field of 2Okoe reflectivities tend atocritical have the unresolved features of the H = 0 curves as the presence of random magnetic domains wash out the optical structure. The reflectivity of EuS and EuSe were examined above and below their respective Curie points using circularly polarized light in the Faraday configuration (E ~. H), at near normal incidence. At 1.5°K, EuS and EuSe both have a triplet structure, in which the lowest energy peak E has a pure aR polarization, the E~peak is nearly pure CL, while the central E1 peak has both polarizations, although ~R appears somewhat stronger than CL. The results at 1.5°K for all three compounds are very similar except for different Thethe central E position of the E1 structure. 1 peak is at 1.24eV in EuO, 1.89eV in EuS, and 2.09eV in EuSe. For both EuO and EuS the splitting of the E and E ‘~ peaks from E is nearly symmetrical with AE = ~iE ~‘ = 0.25 eV. However, in EuSe the splitting is asymmetrical about E,, AE = 0.19eV and AE~= 0.26eV.
1.5°K for both EuS and EuSe (Figs. 3 and 4) resolves into a sharp triplet structure with 40koe applied. The structure is not changed by increasing
The polarization dependence of the reflectivity was also measured at 22°Kwhich is only
Vol. 7, No. 18
REFLECTIVITY OF EuS AND EuSe
:
__________________________ UNPOLARIZED
I
I
:~:2K
I
UNPOLARIZED
14.O I
1325
I
I
I
~
_
PHOTON ENERGY I.V)
PHOTON
ENERGY Ccv)
FIG. 3. Polarized and unpolarized reflectivity of EuS in a magnetic field at 1.5°K and 22°K.
FIG. 4. Polarized and unpolarized reflectivity of EuSe in a magnetic field at 1.5°K and 22°K.
slightly above T~in EuS but is more than four times T~(at H = 40koe) for EuSe. In both materials the E 1 peak is resolved into a CR and CL doublet structure. This splitting is about 0.25eV as in EuO, but structure is about is 0.33eV in EuSe. inAtEuS, 70°K,the doublet still
we would expect to find the doublet structure that is observed with polarized light in all three
clearly resolved in EuS although T > 4 T~,but there is only a slight remnant of the doublet structure in EuSe for which T > 15 T~. At room temperature, no splittings were observed in E, for either material, As previously discussed for EuO, these results can be understood in terms of the transition probabilities for left and right circularly polarized light for the transition which is
Eu chalcogenides. It might be surprising that this doublet structure5 exists upmagnetization to at least 4 T~. that the is However, found still aboutit25is per cent of the saturation value in a field of 40 koe at temperatures -‘~ 4 T~.Since the preferential population of down over up spin states is directly proportional to the magnetization, the J = — 7/2 state of the octet ground state is still preferentially populated causing the doublet structure to persist at these ternperatures. Below the magnetic ordering temperature, the doublet structure is split into a triplet
6(7F~) 5d(t
described as 4f7(8S 712
) 4f
29). Our initial calculation showed that 7F~ the transition states -.
to the manifold of spin—orbit should show a resolution into split right and left circularly polarized portions when the octet
ground state is split in a magnetic field so that the lowest state, J 7/2, is preferentially populated at low temperatures. We have since —
included the appropriate spin—orbit splitting of the Sd (t 29) final state, and have found that this does not qualitatively change the situation, Just above the magnetic ordering temperature,
structure by the exchange splitting of the narrow Sd sub-bands. triplet appearssplitting rather than quartet becauseA the exchange of a ‘-~~0.25 eV in all these materials is nearly equal to the resultant splitting of the spin—orbit split
state and the 4f~multiplet. Thus the two center lines of the quartet overlap which also
accounts for the mixed polarization of the central peak. The fact that the splitting in EuSe is not symmetric may arise from an incomplete resolution of the quartet. The exchange splitting of the excited state of this transition, i.e., the
1326
REFLECTIVITY OF EuS AND EuSe
d band, has been previously inferred from magnetic dichrojsm measurements in EuSe6 and from absorption in thin films of EuSY However, our polarized reflectivity measurements provide a direct observation of the exchange splitting
Vol. 7, No. 18
of the 5d conduction band. Our optical data are consistent with other evidence for a narrow Sd conduction band, but our measurements do not distinguish this situation from that of localized Sd states lying near a 6s derived conduction band.
REFERENCES 1. 2.
FEINLEIB J., SCOULER W.J., DIMMOCK J.O., HANUS J., REED T.B. and PIDGEON C.R., Phys. Rev. Left. 22, 1385 (1969). For a recent review of experimental data see METHFESSEL S. and ~MATTISD.C., Handbuch der Physik XVIII, Pt. 1, p. 389. Springer, Berlin (1968).
3.
BUSCH G. and WACHTER P., Phys. cond. Matter 5, 232 (1966).
4.
FREISER M.J., HOLTZBERG F., METHFESSEL S., PETTIT G.B., SHAFER M.W. and SUITS J.C., Helv. Phys. Acta, Amsi. 41, 832 (1968).
5.
SUITS J.C. and ARGYLE B.E., Phys. Rev. Let:. 14, 687 (1965); MCGUIRE T.R. and SHAFER M.W., J. appi. Phys. 35, 984 (1964); BUSCH G., JUNOD P., RISI M. a~idVOGT B., Proc. mt. Conf. Semiconductors, Exeter (1962). 6. ARGYLE B.E., SUITS J.C. and FREISER M.J., Phys. Rev. Lett.15, 822 (1965). 7.
WILD R.L., SHINMEI M. and ANDERSEN A.L., Proc. In:. Conf. Semiconductors, Moscow (1968).
La réflectivité des semiconducteurs ferromagnetiques EuS et EuSe a été mesurée en fonction de la temperature et de la polarization dans Un champ magnetique que oriente les domains. La structure de triplet en dessous de T~et celle de doublet en dessus indiguent que la transition du bord de l’absorption est 4f7(8S 6(7F~)5d(t 712)-~4f 2~).