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EuO: Surface depolarization of photoelectrons
Journal of Magnetism and MagneticMaterials 6 (1977) 283- 284 © North-Holland Publishing Company
EuO: SURFACE DEPOLARIZATION OF PHOTOELECTRONS F. MEIER and P. Z(JRCHER Eidgen6ssische Technische Hochschule Zfirich, Hanggerberg, CH.8093 Zf~rich, Switzerland
Received 30 March 1977
The spin polarization P(T) of 4f-photoelectrons emitted from Eu 1-xGdxO (x ~ 0.01) was measured at temperatures 20 K < T < Tc. The depolarization caused by spin-flip scattering during the photoemission process is studied by comparing P(T) with the static magnetization of the same sample.
It has been shown before [1 ] that the spin polarization P = (N'~ - N4,)/(Nt + N.~) of the 4f photoelectrons emitted from pure or doped EuO is not identical with the reduced static magnetization o = M(T)/M(O); Art (N~) denotes the number of photoelectrons emitted with magnetic moment parallel (antiparaUel) to the direction of magnetization of the sample and M(73, M(0) are the static magnetization at temperature T and T = 0, respectively. The setup of the experiment is described elsewhere [2]. The magnetization of the ferromagnetic semiconductor EuO is solely caused by the 7 4f electrons (half-filled shell) of the Eu 2+ ions. As the 4f states lie in the band gap well separated from the valence and conduction band [3], they can be studied alone without need of energy analysis. At the light energies used (hv <~ 5 eV), the escape depth of the 4f electrons is ~100 A. This means that most of them come from the bulk of the sample. Consequently, the discrepancy between P(73 and o(7)cannot be attributed to anomalous magnetic properties of the surface but must be due to depolarization of bulk excited electrons. Because, even at T < < To, the depolarization typically amounts to 30% in an applied field of 10 kG, the spin exchange must take place at the surface. At these temperatures all 4f spins are aligned and spin-flips are impossible by conservation of angular momentum. Fig. 1 shows a measurement ofF(T) together with the reduced static magnetization o(T) of the same sample. By analyzing the two curves, one finds that
the depolarization is constant between 22 < T < 40 K, but becomes stronger when the Curie temperature is approached. The following model is able to account for the depolarization. At low temperatures the depolarization is caused by paramagnetic centres as was suggested by Helman and Siegmann [4]. Such centres ~P(%)
ioo 90 80
70 6o 50 40 30 zo Io 0
I0
20
30
40
50
60
70
80 T(K)
Fig. 1. Spin polarization P and reduced magnetization o o f a
1% Gd-doped EuO crystal. Tc: Curie temperature. 283
284
F. Meier, P. Ziircher /EuO: surface depolarization of photoelectrons
are most likely to exist on certain sites of a nonperfect, rough surface, e.g. lattice steps and edges where the coupling to the bulk is very weak. As T ~ To, the characteristic properties of the regular parts of the surface become more prominent, and cause additional depolarization. Mills [5] found that near ire the surface magnetization of a Heisenberg ferromagnet is a linear function of temperature. This temperature dependence is in perfect agreement with the T-dependence of the additional depolarization near ire. Therefore, we conclude that two depolarizing mechanisms are effective in spin polarized photoemission: depolarization by paramagnetic moments at anomalous surface sites and by the moments at regUlar sites which
give rise to a surface magnetization different from bulk.
References [1] K. Sattler and H.C. Siegmann, Phys. Rev. Lett. 29 (1972) 1565; F. Meier and H. Ruprecht, Commun. Phys. 1 (1976) 137. [2] M. Campagna, D.T. Pierce, F. Meier, K. Sattler and H.C. Siegmann, Advan. Electron. Electron Phys. 41 (1976) 113. [3] P. Wachter, Crit. Rev. Solid State Sci. 3 (1972) 189. [4] J.S. Helman and H.C. Siegman, Solid State Commun. 13 (1973) 891. [5] D.L. Mills, Phys. Rev. B3 (1971) 3887.
EuO: SURFACE DEPOLARIZATION OF PHOTOELECTRONS F. MEIER and P. Z(JRCHER Eidgen6ssische Technische Hochschule Zfirich, Hanggerberg, CH.8093 Zf~rich, Switzerland
Received 30 March 1977
The spin polarization P(T) of 4f-photoelectrons emitted from Eu 1-xGdxO (x ~ 0.01) was measured at temperatures 20 K < T < Tc. The depolarization caused by spin-flip scattering during the photoemission process is studied by comparing P(T) with the static magnetization of the same sample.
It has been shown before [1 ] that the spin polarization P = (N'~ - N4,)/(Nt + N.~) of the 4f photoelectrons emitted from pure or doped EuO is not identical with the reduced static magnetization o = M(T)/M(O); Art (N~) denotes the number of photoelectrons emitted with magnetic moment parallel (antiparaUel) to the direction of magnetization of the sample and M(73, M(0) are the static magnetization at temperature T and T = 0, respectively. The setup of the experiment is described elsewhere [2]. The magnetization of the ferromagnetic semiconductor EuO is solely caused by the 7 4f electrons (half-filled shell) of the Eu 2+ ions. As the 4f states lie in the band gap well separated from the valence and conduction band [3], they can be studied alone without need of energy analysis. At the light energies used (hv <~ 5 eV), the escape depth of the 4f electrons is ~100 A. This means that most of them come from the bulk of the sample. Consequently, the discrepancy between P(73 and o(7)cannot be attributed to anomalous magnetic properties of the surface but must be due to depolarization of bulk excited electrons. Because, even at T < < To, the depolarization typically amounts to 30% in an applied field of 10 kG, the spin exchange must take place at the surface. At these temperatures all 4f spins are aligned and spin-flips are impossible by conservation of angular momentum. Fig. 1 shows a measurement ofF(T) together with the reduced static magnetization o(T) of the same sample. By analyzing the two curves, one finds that
the depolarization is constant between 22 < T < 40 K, but becomes stronger when the Curie temperature is approached. The following model is able to account for the depolarization. At low temperatures the depolarization is caused by paramagnetic centres as was suggested by Helman and Siegmann [4]. Such centres ~P(%)
ioo 90 80
70 6o 50 40 30 zo Io 0
I0
20
30
40
50
60
70
80 T(K)
Fig. 1. Spin polarization P and reduced magnetization o o f a
1% Gd-doped EuO crystal. Tc: Curie temperature. 283
284
F. Meier, P. Ziircher /EuO: surface depolarization of photoelectrons
are most likely to exist on certain sites of a nonperfect, rough surface, e.g. lattice steps and edges where the coupling to the bulk is very weak. As T ~ To, the characteristic properties of the regular parts of the surface become more prominent, and cause additional depolarization. Mills [5] found that near ire the surface magnetization of a Heisenberg ferromagnet is a linear function of temperature. This temperature dependence is in perfect agreement with the T-dependence of the additional depolarization near ire. Therefore, we conclude that two depolarizing mechanisms are effective in spin polarized photoemission: depolarization by paramagnetic moments at anomalous surface sites and by the moments at regUlar sites which
give rise to a surface magnetization different from bulk.
References [1] K. Sattler and H.C. Siegmann, Phys. Rev. Lett. 29 (1972) 1565; F. Meier and H. Ruprecht, Commun. Phys. 1 (1976) 137. [2] M. Campagna, D.T. Pierce, F. Meier, K. Sattler and H.C. Siegmann, Advan. Electron. Electron Phys. 41 (1976) 113. [3] P. Wachter, Crit. Rev. Solid State Sci. 3 (1972) 189. [4] J.S. Helman and H.C. Siegman, Solid State Commun. 13 (1973) 891. [5] D.L. Mills, Phys. Rev. B3 (1971) 3887.