ELECTRON SPIN POLARIZATION IN PHOTOEMISSION FROM DYSPROSIUM AND GADOLINIUM AND THE ROLE OF ELECTRON-MAGNON SCATTERING

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ELECTRON SPIN POLARIZATION IN

PHOTOEMISSION FROM DYSPROSIUM AND GADOLINIUM AND THE ROLE OF

ELECTRON-MAGNON SCATTERING

U. Bänninger, G. Busch, M. Campagna, H. Siegmann

To cite this version:

U. Bänninger, G. Busch, M. Campagna, H. Siegmann. ELECTRON SPIN POLARIZATION IN PHO- TOEMISSION FROM DYSPROSIUM AND GADOLINIUM AND THE ROLE OF ELECTRON- MAGNON SCATTERING. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-290-C1-292.

�10.1051/jphyscol:1971198�. �jpa-00214525�

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JOURNAL DE PHYSIQUE Colloque C I, suppl6ment au no 2-3, Tome 32, Fe'urier-Mars 1971, page C 1 - 290

ELECTRON SPIN POLARIZATION IN PHOTOEMIS SION FROM DYSPROSIUM AND GADOLINIUM AND THE ROLE

OF ELECTRON-MAGNON SCATTERING

U. BANNINGER, G. BUSCH, M. CAMPAGNA and H. C , SIEGMANN Laboratorium fiir Festkorperphysik, ETH, Zurich, Switzerland

R6sum6. - La polarisation des photoelectrons provenant de couches minces de Dy et Gd, evaporees sur des substrats tenus a differentes temperatures, a ete mesurBe A 4.2 OK. Nous avons observe que la polarisation est plus grande pour les photoBIectrons 6mis des Btats de conduction du Dy que pour ceux Bmis du Gd, bien que le nombre des spins dans la couche Blectronique 4 f du Dy soit plus petit. Cela nous montre que la polarisation des Btats Blectroniques dans le solide joue un role dominant sur la polarisation des photo6lectrons, contrairement aux effets de diffusion electron-magnon tels qu'ils ant kt6 suggBr6s kcemment par De Wames et Vredevoe.

Abstract. - Photoelectron spin polarization from films of Dy and Gd, prepared on both hot and cold substrates, was measured at 4.2 OK. We observed more polarisation of electrons from the conduction states of Dy than for those from Gd, despite the lower number of spins in the 4 f shell of Dy. This indicates that the spin polarization of the electron states in the solid has the dominating influence as opposed to the effects of electron-magnon scattering in the manner suggested recently by De Wames and Vredevoe.

Ferromagnetism means spontaneous interatomic Integration of (1) yields spin alignment of electrons in open shells. Experi-

ments have shown [I] that a considerable degree of P(s) = n(l - exp(- as)) + P, exp(- as) .

electron spin polarization (ESP) is conserved-in the

process of photoemission. The question arises, whether We "OW assume = cx, with a 'Onstant and one obtains useful information on ferromagnetism the distance of the point of excitation from the surface.

by measurement of ~ h o t o - ~ s p for instance as a func- For the density of photo-emitted electrons from a tion of magnetic field .photon enerw, or depth X, n(x), we make the following usual << ~ n s a t z >> :

- ^{. *}

conditions of sample preparation. We can here further evidence that in fact the measured Photo-ESP, P, gives information on the polarization Po of the electron states in the material. This is in contrast to a recent suggestion of De Wames and Vredevoe [2]

according to which P is strongly influenced by ine- lastic electron-magnon scattering processes. In view of the difficulties of a theoretical calculation, we examine first a possible experimental way to detect any contribution of P', the ESP generated by electron- magnon scattering ; in the second part experimental results obtained from Dy- and Gd-films are reported, from which we conclude that Po has the main influence on P.

Consider up spin ( + ) and down spin ( - ) electrons of density ni after excitation to an escape level by absorption of a photon : on their way to the surface of length s, (+) electrons will become (-) electrons by magnon absorption and vice versa by magnon creation. The inverse mean free paths for these processes are o- and o", respectively. We obtain dn+ = o+ n- d s - a- n+ ds. We assume that the escape probability from the material is not changed appreciably by magnon scattering. This gives n+ + ^n-⁼const. Note that in reference 2 the scattered electrons, whose spin direction has been changed, were not allowed to contribute to the emerging beam, and only a lower bound of the effect of electron- magnon scattering was established. We obtain

with I7 = (0' - c-)/(a+ + c-) and o = o' + ^a-.

n(x) = a, I exp(- ax). G exp(- xlx,) with a, electron excitation probability, a optical absorption constant, I intensity of incident light and G exp(- xlx,) probability of escape from depth x.

For a thick sample we obtain :

The constants a,, I, and G have considerable influence in the usual photoemission studies, i. e. intensity measurements, whereas in Photo-ESP analysis they do not appear ; P will differ significantly from Po if ca s (a + llx,). In metals x, is governed by electron-electro~z scattering processes ; these processes will not alter P because in most cases a scattered electron has lost so much energy that it cannot escape over the surface barrier potentials. We suppose

- ¹

x, 4 a and x,-' < co. At temperatures far below T, no magnons are present, hence o- = 0 and from (2)

De Wames and Vredevoe [2] have given a formula for a'. Baltensperger [3] calculated the relaxation rate of an electron state in the center of the Brillouin- zone and in the (-) spin band into a state of the (+)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971198

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ELECTRON SPIN POLARIZATION IN P HOTOEMISSION FROM DYSPROSIUM C 1-291 band and a magnon ; the calculation can be extended to

the higher k-values that occur in photoemission [4].

The result is : o+(T = 0) = C(Q/2 7 ~ ) . ( r n / E ~ ) ~ . sJ2 ^;

Q volume and S number of spins per magnetic atom ; nz effective mass and JS exchange energy of the photo- excited electron. C = 1 in reference 3 and C = 4 in reference 2. Besides this uncertainty J(k) can only be esimated ; furthermore x, and c in (3) are not known accurately. Therefore, numerical computation cannot decide whether effects of electron-magnon scattering make a sizable contribution to P. For Gd, P' can be anything below about 10 %.

In the heavy rare earth metals (REM) the conduction states are polarized via an indirect exchange mechanism with the localized mametic 4f-states. The number of spins S i n the 4f-states can be varied without significant alteration of the conduction states. Additio- nally, the 4f-conduction state exchange integral J is believed to be constant within the series of thz heavy REM. Gd has the highest possible S of 7/2(4f7), for Dy S = 5/2(4f9). It follows pl,,/P;, = 715 and PGd > PDY if the contribution of P' is significant.

Both metals are efficient photoemitters with work functions of - 3 eV and Curie-temperatures of 85 and 290 OK and therefore suitable also from a tech- nical point of view.

The experimental technique is fully described in reference 1. The Dy- and Gd- films were prepared from 99.9 % pure metal and evaporated, by electron bom- bardment, onto a stainless steel substrate kept at 4.2 OK. During evaporation of 30 s duration the pressure rose to lo-' - lo-' torr and almost immediately afterwards fell to the lo-'' torr range, at which pressure the measurements were performed. In figure 1 we show the ESP measured a t 4.2 OKfor a Dy- and Gd-film prepared onto substrate at 350 OK. The full spectrum of a Xenon-Hg arc was used. The magnetic field B is applied perpendicular to the film, so that

FIG. 1 and 2. - Dependence of Photo-ESP for Dy- and Gd- lilms on the magnetic field strength, measured at liquid-He temperature. , (a) lilm prepared on substrate at -- 350 ^{O K ,} (b) lilm prepared on substrate at 4.2 OK. Vertical width of the points gives statistical uncertainty, horizontal width the range of the magnetic field in which the single measurements were taken to give the average value shown. The preferential direc- tion of the magnetic moment of the electrons is parallel to the magnetization. The full spectrum of a XeHg high pressure

arc was used.

the demagnetization factor is near to 1. Note that the saturation magnetization M , of Gd and Dy are 25 and 37 kG respectively and we expect to reach saturation-ESP only at B of this respective order of magnitude ; with Dy even higher B should be applied because of the large anisotropy in some crystallo- graphic directions, see J. J. Rhyne et al. [5]. Since we measure the expectation value of the spin direction along B, the curves must have the shape of Mvs.B- curves. This is the case apart from deviations at small B in the case of Dy and at higher B in the case of Gd.

Weat tribute the deviations to electron-optical effects as discussed in reference 1 and reference 6. From figure 1 : PDy = (7.33 + ^0,35)^% ^{for 30}< B < 35 kG and PGd ⁼(5.70 + ^0.21)% for 15 < ^B< 25 kG. There is an additional systematical error in both cases of about 0.8 %. The result can only be understood by putting P' = 0 and assuming a difference in band structure, Po

.,

^#^Po^{G d .}This is confirmed by thefollow- ing additional observation : for photons with energy smaller than 4 eV, PD, increased by about 1 % whereas P,, remained constant within statistical uncertainty.

This indicates that conduction states with higher than average ESP lie in the neighborhood of the Fermi-energy in the case of Dy whereas in Gd such states do not exist. Films prepared on warm substrate (a) did show only slightly higher ESP than films pre- pared on substrate at 4.2OK (b) ; for Gd :

and

The results for Dy are shown in figure 2 : (b) is magne- tically harder than (a) but also seems to reach a similar saturation value. Tn the case of the 3d-ferromagnets Fe, Ni and Co and for the magnetic insulator EuS we observed large differences between the two types of films. Therefore, disordered structure is not easy to produce with Dy and Gd and/or does not induce a large effect on magnetism.

From the independence of the results on substrate temperature during film preparating and from the absence of effects of electron-magnon scattering it follows that our earlier conclusions 161 drawn from Photo-ESP of Gd are valid. Hence either the sharp decrease of the density of states near the vacuum potential as suggested by Blodgett et al. [8] is not present in this metal or the splitting of the conduction states into the two spin states and their relative shift due to the indirect exchange mechanism is not an allowed procedure. We believe further, that effects of electron-magnon scattering are unimportant not only in REM, but also in the Europium-chalcogenides and- pnictides and in Fe, Co and Ni : in EuS, EuSe, Eu,P, and Eu,As, [9] we have observed meanwhile a strong dependence of Photo-ESP on photon energy, that speaks against the importance of these effects ; in the 3d-ferromagnets, Photo-ESP was also not proportional to the number of spins per atom.

This work has received financial support from the Schweizerischer Nationalfonds and the Verein zur Forderung der Festkorperphysik an der ETH.

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C 1 - 292 U. BANNINGER, G. BUSCH, M. CAMPAGNA AND H. C. SIEGMANN

References

[I] B u s c ~ (G.), CAMPAGNA (M.) and SIEGMANN (H. C.), [6] BUSCH (G.), CAMPAGNA (M.), COTTI (P.), and SIEG- J. Appl. Phys., 1970, 41, 1044-1051. ^MAW (H. C.), Phys. Rev. Letters, 1969, 22, [2] DEWAMES (R. E.), VREDEVOE (L. A.), Phys. Rev. Letters, 597-600.

1969, 23, 123-126. [7] B~NNINGER (U.), BUSCH (G.), CAMPAGNA (M.) and f3] BALTENSPERGER (W.), J. Appl. PhyS., 1970, 41, 1052- SIEGMANN (H. C.), Phys. Rev. Letters, 1970,

1054. 25, 585.

[8] BLODGETT (A. J.), SPICER (W. E.), and Yu (A. Y. C.), [4] We are indebted to D. MAISON for assistance with Optical Propert~es and Electronic Structure of the calculation. Metals and Alloys, 1966, North Holland, Ams- [5] RHYNE (J. J.), FONER (S.), MCNIFF (E. J., Jr), and terdam.

DOKLO (R.), J. Appl. Phys., 1968, 39, 892-893. 191 To be published in J. Phys. and Chem. Solids.