Magnetic ordering effects in the ultra-violet reflectance of EuS and EuSe

Solid State Communications,

Vol. 7,

pp. 1685—1690, 1969.

Pergamon Press.

Printed in Great Britain

MAGNETIC ORDERING EFFECTS IN THE ULTRA-VIOLET REFLECTANCE OF EuS AND EuSe* W.J. Scouler,

J.

Feinleib and J.O. Dimrnock

Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02173 and C.R. Pidgeon Francis Bitter National Magnet Laboratory,t Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

(Received 22 September 1969 by J.A. Krumhansl)

The u.v. reflectance spectra of the ferromagnetic semiconductors, EuS and EuSe have been studied for the first time as a function of temperature and polarization in a high magnetic field. The behavior of the main ultraviolet peak, E 2, near 4eV appears more complicated than that of the infrared peak, E1. Splitting of E2 into right and left circularly polarized components 7(8S however indicates 6(7F~)5d(e that this peak is due, at least partly, to 4f 712) -~ 4f 0) 2the obserPREVIOUSLY, we ordering have reported ‘ in the low vation of magnetic effects energy reflectance peak, E 1, in the magnetic semiconductors EuO, EuS and EuSe. These measurements, usinglight unpolarized as well magnetic as circularly polarized in an orienting

This peak shows temperature dependent effects around the Curie point, and is separated from the

E1 peak by about the same energy as the

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ligand fieldin splitting (1 2eV) observed 2~ions other compounds.3 Polarizedfor light Eu measurements of E 2 with a magnetic field, however, were not made to verify expected polarization effects. In this paper we report on the temperature dependence of the E2 reflectance peak for EuS and EuSe and associated polarization effects in an orienting magnetic field. The results indeed sugges~tthat the E2 peak 65d(eis at least partially composed of 4f ~ -+ 4f 0) transitions.

field at temperatures above and below the Curie point, have provided direct evidence that the conduction band in spin-polarized in the ferromagnetic state. These results also support the model that the absorption edge associated with E1, is due to a transition from a localized 7(8S europium 4f (129)712) ground state to a final 7p ) 5d configuration.

4f (

We have previously suggested’ that the E

2

In order to show the relationship of E1 and E2 for reference, Fig. 1 shows the normal

peak observed in EuO around might be due 7( 8S 6(7F 5eV )5d(e in part to 4f 7,2) -~ 4f 0) transitions, *

incidence reflectance of cleaved single crystal surfaces of EuS and EuSe at room temperature from 2 to 12eV. Below 2eV, the reflectance 4 In is calculatedto from addition E index of refraction data. 1 and E2 several higher energy peaks are observed.

The Lincoln Laboratory portion of this work was Force, sponsored by the Department of the Air Sponsored by the U.S. Air Force Office of Scientific Research. 1685

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MAGNETIC ORDERING EFFECTS OF EuS AND EuSe

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FIG. 1. Near normal incidence reflectance of cleaved single crystal EuS and EuSe at room temperature. Below 2eV, index of refraction values (ref. 4) were used to calculate the reflectance.

Using a variable temperature cryostat, we measured the reflectance at 45°angle incidence at several temperatures overofthe range from 2 to 7eV with no applied magnetic field.* Except for peaks E, and E 2, no significant temperature effects in the reflectance throughout this range were observed. Figures 2a and b show the temperature dependence of E 2 for EuS and EuSe respectively. In EuS, we observe a barely resolved doublet at room temperature which is easily seen at 50°K, where the separation is *

The curve shape is slightly different at 45° angle of incidence compared to near-normal incidence, as can be seen by comparing E2 at room temperature in Fig. 1 and 2a, b.

0.16eV. As we the Curie t5he approach double peaks seem ten)perature, to merge, T~ = 16°K~ becoming a sharp single peak at T~and broadening again at lower temperatures. At 8°K and below, a hint of further splitting is seen. 1 As Similar Fig. 2b effects in the EuOsharp near doublet = 69°K. shows, occur in EuSe at 50°K becomes less resolved as the temperature decreases, the splitting decreasing from 0.32eV at 50°K to 0.20eV at 1.5°K. In 7 EuSe, T~~ 3°K in zero applied magnetic field.

E 2 was also examined in a domain orienting magnetic field of 40 koe using circularly polarized light in the Faraday configuration (E i. H) at near

Vol.7, No.23

MAGNETIC ORDERING EFFECTS OF EuS AND EuSe

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FIG.2. E 2 reflectance of EuS (a) and EuSe (b) at various temperatures at 45°angle of incidence. For EuS T~~ 16°Kand for EuSe T~~ 3°K. E2 reflectance of EuS (c) and EuSe (d) at 1.5°Kat near normal incidence using right, CR, and left, CL, circularly polarized light in a 4OkOe orienting field, E .1. H. normal incidence at 1.5°K. The results are shown in Figs. 2c and d. In EuS, the lower energy portion is predominantly CR, and the higher energy portion, CL. In EuSe, the lower energy portion of E2 is an almost equal mixture of CR and CL, while the higher energy portion is predominantly CL again. In EuSe, a very similar curve is obtained even at 20°K,well above T0. Formuch E,, more the ~Rpronounced dominance for on both the low side is EuSenergy and EuSe

as Fig. 3 indicates. That the polarization behavior of E1 and E2 is different should not be surprising since E2 in the unpolarized reflectance measurements is more complicated than E, and also because different transitions are involved.

We have previously discussed the temperature dependence of the polarized reflectivity in the 2 vicinity of the E1 peak for EuO, EuSe” as an effect arising partially fromEuS theand preferential

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MAGNETIC ORDERING EFFECTS OF EuS AND EuSe I

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Vol.7, No.23 I

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FIG. 3. Reflectance of EuS and EuSe at 1.5°Kat near normal incidence using right, a~,and left, UL, circularly polarized light in 40 koe field, E ~. H, showing E 1 and E2 polarization dependence. 7(8S population of the all spin-down 4f 7/2) localized ground state configuration, which causes the polarization dependence above T~in a magnetic field. A spin-splitting of the 5d(t29) conduction band occurs below T~giving rise to the additional splitting observed at low temperatures forE1. In this model, the band splitting does not occur until the temperature is near or below T,~ whereas the preferential population of the spin-down M~= — 7/2 localized 4f (es7,2) state is directly proportional to the sample magnetization andexternal may occur well above T~in the presence of large magnetic fields.1’7

Thus, if this model is correct, the observation of polarization dependent reflectivity associated with the E2 peak in EuSe at temperatures as high as 20°K, well above T~,indicates that this peak involves, 6S at least in part, transitions from the 4f~6(7F)5d(e ( 7/2) ground state presumably to the 4f 9) excited state configuration. However, since the energy of the E2 peak is above the fundamental energy gap of 3.1eV separating the conduction bands from the chalcogenide p-valence bands in EuS and 8EuSe, it is as quite determined from possible that thisphotoemission transition is data, mixed with a

Vol.7, No.23

MAGNETIC ORDERING EFFECTS OF EuS AND EuSe

p- to d, or p- to s, band-to-band transition.

1689

In contrast to E,, however, E2 shows no drastic change in the polarization dependence above and below T~. This is consistent with 6 5d(e assigning E2 to 4f ~ 4f 9) transitions and can spin-orbit be understood in termsinof difference in the interaction thethe5d(t 20) and 5d(e9) bands. The 5d(129) band is split by spin orbit coupling whereas the 5d(e9) band is not, at least at k 0. Below T~,the e9 band becomes exchange split into pure spin-up and pure spindown states, in contrast to the ~2g band, where the spin states are mixed by spin-orbit coupling. The absence of spin-orbit mixihg effects in the 5d(e0) band would 7(8Smean that transitions from the all spin-down 4f 712) ground state to the spin up component of the 5d(e9) band is not allowed

The effect of temperature on E2 in zero magnetic field, shown in Figs. 2a, b, appears to be somewhat different in EuS and EuSe since the doublet does not merge in EuSe as in EuS. This may be due in part to the fact that the peaks are more separated in EuSe abovenoticeable T9. The change 1is most around in T~, E2 in EuS and EuO indicating that the effect is connected with magnetic ordering in some manner. At first glance. one might be led to believe that the doublet above T0 is due to spin-orbit splitting in the p-valence band since the splitting is of the right order of magnitude for the anion and increases in EuSe compared to EuS. However, the fact that the spacing between the peaks has a strong temperature dependence (and disappears in EuS) argues against this. The spin-orbit interaction is localized around the core and we would not expect

by electric dipole interaction. Consequently the

the splitting to be grossly affected by a change

polarization dependence should not change drastically at T~,since transitions to only one of the exchange split bands are allowed, in agreement with our observations. Some shift of the polarization

in temperature or lattice constant. Recent results of measurements on EuTe show’°a larger zero field splitting of E2, of about 0.4eV, with each peak of the doublet showing right and left

peaks is possible but this is not observed. The absence of spin-orbit interaction in the Sd(e0) band is also supported by the experimental observation of seven resolved peaks in the absorption spectra of EuF2 due to europium 65d(e 41 ~ -* 4f 9) transitions which would be obscured if spin-orbit mixing effects were

circular polarization dependence in large magnetic fields. We possibly do not observe this in EuS and EuSe since the peak separation is smaller and the polarization effects are more mixed. It is also possible that the E2 doublet is due to two separate transitions perhaps to two different critical points in the 5d(e0) band.

significant.’° The results in EuF2 indicate that 6(7J3) the interaction between the final state 4f 2~is not configuration and the 5d(e~)states of Eu

Although E 2 is obviously more complicated than E 1, the results of these optical studies are most consistentofwith a model for the europium chalcogenides strongly crystal field split,

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large. The ladder structure observed in 9is the not transitions to the 5d(e~) states in EuF2 observed in E 2 probably because the 5d(e9) band in the chalcogenides has greater width which broadens65d(e the structure. In addition, the 4f ~ -, 4f 9) transitions in the chalcogenides occur at higher energies than the 4f ~ -~ 4[5d(s29)

relatively narrow, 5d(t29) and 5d(e9) bands, and localized 41 states. of this model, 7(8SOn the basis 6(7F E2 consists of 4f 712) -* 41 3)5d(e9) transitions possibly in~ombination with other transitions.

transitions which is in contrast to the situation in EuF2 where 5d(e9) lies below 5d(129). REFERENCES

1.

FEINLEIB J., SCOULER W.J., DIMMOCK J.O., HANUS Rev. Lets. 22, 1385 (1969).

J., REED T.B. and PIDGEON C.R. Pliys.

2.

PIDGEON C.R., FEINLEIB J., SCOULER W.J., HANUS Solid Slate Cominun. (in press).

J., DIMMOCK J.O. and REED T.B.

1690 3. 4. 5.

MAGNETIC ORDERING EFFECTS OF EuS AND EuSe

FREISER M.J., HOLTZBERG F., METHFESSEL S., PETTIT G.D., SHAFER M.W. and SUITS J.C., Helv. phys. Acta 41, 832 (1968). WACHTER P., Phys. condensed Matter 8, 80 (1968). BUSCH G., RISI M. and VOGT 0., Proc. inS. Confere~iceon Semiconductors, Exeter, 1962, p. 727; McGUIRE T.R., ARGYLE B.E., SHAFER M.W. and SMART J.S., App!. Phys. Leti. 1, 17 (1962);

ENZ U., FAST j.F., VAN HOUTEN S. and SMIT 6. 7.

Vol.7, No. 23

J., Philips Res. Rep., 17, 451 (1962).

SCHWAB P. and VOGT 0., Phys. LetS. 22, 374 (1966); BUSCH G. and WACHTER P., Phys. condensed Matter 5, 232 (1966). HANUS J., DIMMOCK J.O. and FEINLEIB J., (Ann. Conf. on Magnetism and Magnetic Materials, Philadelphia, Pa., 1969).

8.

EASTMAN D.E., HOLTZBERG F. and METHFESSEL S., Phys. Rev. Leit. 23, 226 (1969).

9.

FREISER M.J., METHFESSEL S. and HOLTZBERG F., J. app!. Phys. 39, 900 (1968).

10.

FEINLEIB

J. and PIDGEON C.R. (to be published).

La spectre de reflection ultra-violette des semi-conducteurs ferromagnétiques, EuS et EuSe a été étudié pour la premiere fois en fonction de la temperature et de la polarisation dans un champs magnétique de haute intensité. Le comportement du pic ultra-violet principal, E 2, aux environs de 4eV semble plus compliqué que celui du pic infra-rouge, E1. La separation de E2 en ses deux composantes polarisées circulairement droite et gauche indique 7(8S cependant ~ue 7Fce pic est du, au moms en partie, aux transitions 4f 7,2) -~ 4f ( 3)

5d(e9).