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Journal of Magnetism and Magnetic Materials 140-144 (1995) 2047-2048 ~ 4 Journal of

magnetism and magnetic materials

The d - f exchange interaction constants of localized electrons in EuSe

Koji Yamada, Kohichi Satoh, Akira Kowata, Katsuhiko Yamaguchi Department of Materials Science, Saitama University, Urawa, Saitama 338, Japan

Abstract The activation energies of the localized states in magnetic semiconductors exhibit the magnetization dependences due to

the d - f exchange interactions. We derived the d - f exchange interaction constants of the local states for EuSe in high magnetic fields up to 30 T between 10-77 K by the magneto-transport and luminescence on this semiconductors.

The d - f exchange interaction and the bound magnetic polaron effect (BMP) [1] among the lattice site spins and a localized electron, give rise to the energy level shifts for the local states with the magnetization changes in magnetic semiconductors of RE compounds. The difference between an extended electron in a band and a localized electron near the conduction band bottom, lies in the extension of the wave function size adopting the hydrogen-like model, like as in the impurity states of the usual semiconductors. Therefore, the level shift of a localized electron is the direct measure of the wavefunction extension. In this study, we investigated the magneto-transport and the opti- cal properties of single crystal EuSe, aiming at the wave- function extension dependence of the d- f exchange inter- action together with the BMP effect. For this purpose, the magnetically stimulated current (MSC) [2], the magneto- conductivity [3], the luminescence and the optical trans- mission were observed for different crystals of EuSe in high magnetic fields up to 30 T and in the temperature range between 4.2 and 77 K.

The red-transparent single crystals were grown from Eu-rich melt. The as-grown sample (S-1) showed n-type and contained carriers of 1018 cm -3 at RT. Well annealed samples at 1600°C for 24 h, contained background states formed by the large scale voids of micron sizes with the same nature of the states as those at the crystal surface. Two types of red-transparent samples were obtained. One (S-2) contained 1011 cm-3 background states obtained by the hot-press process before the crystal growth, and the other (S-3) 1014 cm -3 obtained without the hot-press process. The local states inside of the crystals are uni- formly formed in S-2 and S-3 [4]. The magneto-conductiv- ity was measured for S-1. The conductivity change was in

* Corresponding author.

a range between 10 - 4 and 1 S/cm in high magnetic fields up to 30 T at the paramagnetic temperature range: T N = 4.6 K. The activation energy decreased from 24 to 12 meV with the magnetization changes from 0 to full 7/.t n induced by the Fermi energy change at T = 18 K. It also changed from 10 to 5 meV at T = 77 K in the same magnetic field change up to 30 T. These are explained exactly by the d- f exchange interaction and the BMP effect with the magneti- zation dependence of the activation energy as follows [3]:

E A = EAO -- S (J~ - J d ) M / 2 M o (1)

where, arc and Jd denote the exchange interaction constant for a conduction electron and that for a donor, M and M o the relative magnetization and the saturated magnetization normalized to 1 and S = 7/ZB/2. It was found that all the conductivity changes in T = 18-77 K and in B = 0-30 T were explained by J¢ --Jd = 9 meV where the effects of BMP on the conductivity change are included altogether, in this analysis. It must be noted that the exchange interac- tion for the conduction electron is larger than that for the

10 -~

A 169

lO 11

Magnetic Field B (T)

Fig. 1. The magnetically stimulated current of single crystal EuSe.

0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)01208-3

2048 K. Yamada et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 2047-2048

doner. Therefore, the activation energy of a donor as a local state becomes smaller with the magnetization in- crease as expressed in Eq. (1).

The magneto-conductivity experiments of MSC were performed for S-2 and S-3 in dc magnetic fields up to 10 T and in a temperature range between 15 and 70 K. Fig. 1 shows the experimental results of MSC. Note here that the currents in MSC change with the activation energy con- trolled by exchange interactions. On the other hand, ther- mally stimulated current (TSC) is controlled by the ther- mal energy, or equivalently by ambient temperatures. Therefore, the same method is applicable to determine the activation energy both for MSC and TSC. The exchange interaction constants were thus determined by Eq. (1). The results were expressed in the difference forms of J c - J t for several trapped states t. They ranged between 6 meV (E A = 11 meV measured at T = 15 K) and 9 meV (E A = 24 meV at T = 77 K) which coincided with that for the donor measured for S-1. It was found that the exchange interac- tion of a localized electron increases with decreasing acti- vation energy to the conduction band bottom, and the value coincided with that for the conduction electron at the shallowest limit. The knowledge of the wavefunction sizes of each of the local states of different activation energies will inform us their extension dependence of the exchange interaction of localized electrons with the lattice site spins.

For the deep electron spectroscopy, the experiment of the luminescence were performed for S-2 in magnetic fields up to 22 T at T = 77 K. The magnetic field depen- dence of the luminescence spectra were obtained by a H e - N e laser irradiation of 1 mW at the sample surface. Fig. 2 shows a magnetic field dependence of the lumines- cences through the filters of 701 (half-width = 7 nm), 726

726

m 752

i i I I I t I i ~ t 10 20

v~aetic r~ed BtT)

Fig. 2. The magnetic field dependence of the luminescences of different wavelengths at T = 77 K in high magnetic fields up to 22 T.

hv 1

I l f tna. l s t a t e

, M a g n e t i z a t i o n M

Fig. 3. The illustration of the local state controlled luminescence of different wavelengths. The final state is supposed constant for the magnetization change only for illustration. A similar magneti- zation dependence of luminescences occurs between the two wavelengths close to each other as shown in Fig. 2. The shallow local state exhibits the large magnetization dependence both for up and down spin states due to the relatively large wavefunction extension.

(6 nm) and 752 nm (9 nm), respectively. By these curves, the exchange constants of the deep local states could be determined as illustrated in Fig. 3. As shown in this figure, the initial state from trap1 forms the luminescence extreme A for hv2, and it shifts t o / ( for h~, l with the magnetiza- tion increase. The exchange constant could be derivable because the luminescences of the adjacent two curves show similar magnetization dependence with a constant shift. Fig. 2 shows the similar curves. However the energy differences were too large to obtain the similar magnetiza- tion in this experiment.

In conclusion, the exchange interactions of the local- ized electrons become smaller for the electrons with larger activation energy measured from the conduction band bot- tom. The influence of BMP might be the direct origin of the extension dependence of the activation energy due to the same dependence of BMP size on the wavefunction extension [5].

References

[1] P. Leroux-Hugon, Phys. Rev. Lett. 29, 14 (1972) 939. [2] K. Yamada, J. Heleskivi and A. Salin, Solid State Commun.

37 (1981) 957. [3] K. Yamada and N. Kamata, J. Mag. Magn. Mater. 104-107

(1992) 991. [4] K. Yamada, J. Heleskivi and H. Stubb, Kotai Buturi (Solid

State Physics in Japan) 20 (1985) 101. [5] P. Kuivalainen, Doctor thesis, Helsinki University of Technol-

ogy, Feb. 1980.