OPTICAL TRANSITIONS BETWEEN SPIN-POLARIZED BANDS IN MAGNETIC CHROMIUM SPINELS

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OPTICAL TRANSITIONS BETWEEN SPIN-POLARIZED BANDS IN MAGNETIC

CHROMIUM SPINELS

H. Lehmann, G. Harbeke, H. Pinch

To cite this version:

H. Lehmann, G. Harbeke, H. Pinch. OPTICAL TRANSITIONS BETWEEN SPIN-POLARIZED BANDS IN MAGNETIC CHROMIUM SPINELS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-932-C1-933. �10.1051/jphyscol:19711333�. �jpa-00214367�

JOURNAL DE PHYSIQUE Colloque C I, supplkment au no 2-3, Tome 32, Fe'urier-Mars 1971, page C 1

-

⁹³²

OPTICAL TRANSITIONS BETWEEN SPIN-POLARIZED BANDS IN MAGNETIC CHROMIUM SPINELS

H. W. LEHMANN and G . HARBEKE Laboratories RCA Ltd., Ziirich, Switzerland

and H. PINCH

RCA Laboratories, Princeton, USA

Rhumb. - On a mesure I'absorption optique des monocristaux de ZnCr2Se4 et de CdCrzSe4 en lumiere polarisk circulaire et lintaire entre 4,2 OK et 300 OK dans des champs magnetiques de 0 a 90 kOe. On observe une structure triplet des bandes d'absorption qui correspondent aux transitions optiques avec ^Am =

+

1,0, - 1. Nous proposons d'expliquer cette structure avec des transitions de la bande de valence dans un etat singlet et spin-polarise. La degenerescence quadru- ple de la bande de valence est annulee par le champ magnetique. La variation de la bande d'absorption avec la temp6rature et le champ magnetique indique une transition de la structure helimagnetique en une structure ferromagnetique.

Abstract. - Measurements of the optical absorption of single crystal ZnCr2Se4 and CdCr2Se4 in circularly and linear- ly polarinxl light between 4.2 and 300 OK in fields of up to 90 kOe are reported. In both materials we observe a triplet structure with peaks separated by about equal amounts for optical transitions with change in magnetic quantum number Am =

+

^1, 0, - ^1, respectively. We propose to explain the structure with transitions from a fourfold valence band state with degeneracy lifted by the magnetic field to a spin-polarized singlet state. The temperature and field dependence of the absorption peak reflects the transition from helical to parallel spin structure in rnetamagnetic ZnCr2Se4.

I. Introduction. - The strong influence of magne- tic ordering on the position of electronic levels in the ferromagnetic chromium spinels [ I ] has been well documented by optical absorption measurements [2-51.

We have recently also demonstrated the close relation between the lowest-energy absorption edge and the detailed helical or conical spin structure in meta- magnetic HgCr2S4 [6]. In this communication we report on measurements on metamagnetic ZnCr2Se4 and ferromagnetic CdCr,Se, (T, = 130 OK) in circu- larly and linearly polarized light. ZnCr,Se4 orders antiferromagnetically at about 20 OK and has a helical spin structure up to fields of about 64 kOe at which field ferromagnetic alignment is enforced at 4.2 OK [7]. The asymptotic Curie-Weiss temperature is

-

^{= 1 15 OK [8].}

11. Results and discussion. - The optical transmis- sion of thin platelets (thickness between 14 and 50 p) of single crystal ZnCr,Se4 grown by vapor phase transport reactions and CdCr2Se4 [2] has been measur- ed in the range from 0.8 to 1.4 eV. Data havc been taken between 4.2 OK and 300 OK in magnetic fields of up to 90 kOe with circularly polarized light in the Faraday configuration and with linearly polarized light in the Voigt configuration.

Figure 1 shows initial absorption data for H = 0 in unipolarized light (obtained on a sample of thickness t = 41 p) in order to illustrate the ternperaturc depen- dence of the lowest-energy absorption edge. One notcs a slight blue shift between 298 OK and 181 OK followed by a strong red shift of a precursor band. The strength of this precursor absorption increases with decreasing temperature. It can also vary in intensity from sample to sample. All properties are nearly identical to those of the lowest-energy absorption band in CdCr,Se, [5]

and we have again to conclude that the band is of non- intrinsic origin. The peak energy of the precursor band for H ⁼ 0 (obtained from samples of thickness t = 14 or 20 p) is plotted vs. temperature in figure 2 (upper continuous curve). The curve represents the influence of the spontaneous magnetic ordering. The red shift is qualitatively similar to the nearest neighbor ferro-

I I I 1

1.20 1.30

ENERGY (eV)

FIG. 1. - Absorption cocfficient vs. energy of ZnCr2Se4 at diirerent temperatures with unpolarized light.

magnetic spin correlation function as in many other cases [6]. It shows that despite of the non-intrinsic origin of the band the electronic states involved in the optical transition are evidently strongly coupled to the spin system. Such a strong interaction resulting in red shifts of a few tenths of an eV is most probably due to intraatomic exchange at the magnetic ion [9].

Further evidence of this coupling is provided by the results obtained in magnetic fields. At a given tempera- ture the precursor band shifts to lower energies with increasing field for circularly polarized radiation caus- ing transitions with change in magnetic quantum number Am = - 1. This is illustrated in the lower continuous curve in figure 2 for a field N = 67 kOe.

For radiation with the opposite sense of rotation (Am =

+

1) we observe at low fields a decrease and broadening of the precursor absorption. At fields higher than 30 kOe a well-pronounced band is again formed at lower energies. Its oscillator strength is only about one third of that for Am = - 1. The position of this peak is also given in figure 2 (dashed curve). Note that the energy difference for Am =

+

or - 1 isabout

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711333

OPTICAL TRANSITIONS BETWEEN SPIN-POLARIZED BANDS I N MAGNETIC CHROMIUM SPINELS C 1 - 933

0 . 9 ~ " " " " " ~

0 20 40 60 80.. 100

T (OK

1

FIG. 2. - Absorption peak position vs. temperature for H = 0 ( A ) and H

-

^{67 kOc} for optical transition with

Ant

- ⁺

^{1 (0) and A m = - 1 (0) in ZnCrzSe4.}

40 meV for the temperature range over which the peak for Am ⁼

+

1 can be observed. Measurements in the Voigt configuration for E

//

H (Am = Oj indicate peak positions about midway between the two lower curves in figure 2.

The same triplet configuration with nearly iden- tical splittings for H = 67 kOe was also observed in CdCr,Se,. Also the oscillator strength is again lowest for Am =

+

1. We propose t o explain these results in both materials with transitions arising from valence bands split by the magnetic field. It is most likely that the valence band maximum is composed out of Se 4 p wave functions [lo, 51. This assumption is strongly supported by the hole mobility in CdCr,Se, [I I]. The sixfold degeneracy (including spin) of the p-state is split by spin-orbit irlteraction into a fourfold and a twofold degenerate statc. In the presence of a magnetic field the degeneracy is completely lifted. The higher lying fourfold state we are concerned with splits up into four separate levels with mj-values

3,

^{f , - f}

and - .$ respectively. Optical transitions into a singlet m, =

+ 4

state are thcn allowed [I21 from r n j = - ^f, , .;

3

states for An7 ⁼ m, - r n j =

+

1, 0,

- 1, respectively, as obscrved in the experiment.

The singlct nature of the final state supports our previous conjecture of an anion vacancy filled with a 4 s electron from neighboring Cr-atoms. In ZnCr,Se4

we found indications of transitions into the upper final m, = - f state. These transitions d o not form a band which could be well separated from higher transitions. If, however the photon energies at which K = 1 200 cm-I is reached are plotted versus tempera- ture for H = 0 and H = 67 kOe we obtain in addition to the two full curves in figure 2 (with a slight vertical displacement, of course) a Am =

+

1 curve at lugher energies. This curve is nearly the mirror image of the Am = - 1 curve relative to the H = 0 curve so that the total splitting at 4.2 OK amounts to about 0.3 eV.

The connection between the band shift and the magnetic properties is illustrated in figure 3. It shows

FIG. 3. - Absorption peak position vs. magnetic field for

T = 4.2 OK ( A ) , 30 O K (0) and 58OK (0) in circularly polarized light, ^{Am =} - 1.

the peak position for Am = - 1 transitions versus magnetic field at three different temperatures. The 4.2OK curve is nearly identical to the magnetization curve obtained by Allain et al. [I31 and reflects the gradual spin alignment until a field of 67 kOe is reached.

The spin alignment adds to the ferromagnetic nearest neighbor spin correlation which in turn determines the band shift. At 30 OK and even stronger at 58 OK the curves are washed out and d o not reach saturation within our present field range. The temperature beha- vior of the spiral spin structure manifests itself in a very similar way as in HgCr,S, (sec Fig. 3 and 4 in ref. 6).

Minima in optical transmission are observed a t those temperatures and magnetic fields a t which the spin correlation reaches its maximum.

References

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Phys. Rev. Letters. 1965, 15, 493. [8] LOTGERING (F. K.), Solid State Comm~micatiotis, [2] HARBEKE ( G . ) and PINCH (H.). Phys. Rev. Letters, 1965, 3, 347.

1966. 17, 1090. [9] RYS (F.), HELMAN (J.) and BALTENSPERGFR (W.),

[3] B u s c ~ (G.), MAGYAR (B.) and WACHTER (P.), Phy- Phys. Cond. Matter. 1967, 6 , 105.

sics Letters. 1966, 23, 438. [lo] REHWALD (W.). Phys. Rev.. 1967, 155, 861.

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