PHOTOEMISSION FROM Ni, Co AND Fe FROM 10 TO 41 eV

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PHOTOEMISSION FROM Ni, Co AND Fe FROM 10 TO 41 eV

D. Eastman

To cite this version:

D. Eastman. PHOTOEMISSION FROM Ni, Co AND Fe FROM 10 TO 41 eV. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-293-C1-295. �10.1051/jphyscol:1971199�. �jpa-00214526�

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JOURNAL DE PHYSIQUE Colloque C 1, supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page Cj 1

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²⁹³

PHOTOEMISSION FROM Ni, Co AND Fe FROM 10 TO 41 eV

by D. E. EASTMAN

IBM Thomas J. Watson Research Center, Yorktown Heights, New York

R6sum6. - Pour les matkriaux ferromagnktiques Ni, Co et Fe les distributions'd'6nergie ont kte mesurks par spec- troscopie d'emission ultraviolette (SEW. Les mesures ont kt6 faites pour des Cnergies de 10,7, 16,8, 21,2, 26,9 et 40,8 eV.

Les donnees indiquent des structures non stationnaires (variations en forme, amplitude et en position des maxima). Ceci sugg6re des transitions d'intra-bande et ne peut pas gtre dkrit par une densit6 d'ktat optique simple. Nos rksultats montrent des largeurs de bande d occup6es de 3,3 f 0,3, 3,6 ^f0,3 et 3,8 0,3 eV pour le Ni, le Co et le Fe respectivement. Les rksultats SEU sont compar6s avec ceux obtenus par 6rnission B rayons X.

Abstract. - Ultraviolet photoemission spectroscopy (UPS) energy distributions have been measured for Ni, Co and Fe at photon energies hv of 10.7, 16.8, 21.2, 26.9 and 40.8 eV. The data show nonstationary structure (variation in peak locations, shapes and amplitudes) indicative of direct interband transition and cannot be described by a simpleoptical density.of states. Our new data show occupied d-band widths of 3.3 ^f0.3,3.6 k 0.3, and 3.8 & 0.3 eV for Ni, Co and Fe respectively. UPS data are compared with X-ray photoemiss~on data (XPS).

I. Introduction and Summary. ..- We report high resolution (cz 0.2 eV) UPS measurements for Ni, Fe and Co at energies hv = 16.8, 21.2, 26.9 and 40.8 eV which have been made using a windowless, differen- tially pumped photoemission spectrometer. Energy distribution curves (EDC's) for Ni, Co and Fe for 10 ,( hv < 40.8 eV show variations in peak locations, amplitudes, and shapes which can not be described by an ⁽⁽optical density of states ^D(i. e. the nondirect transition model). [l-31 Rather, observed structure is indicative of direct interband transitions. [4-61 Since in this case the joint interband density of states is invol- ved instead of the band density of states, optical densities of states determined for transition and noble metals [2-41 with photon energies hv

<

11.6 eV should not be expected-to show ditailed correlation with band densities of states.

prepared by evaporation (-- 5 A/s) onto smooth Mo substrates using an electron beam gun. Possible conta- mination effects were studied by deliberately exposing the films to low pressures of 0,, N,, H,, Co, etc. In particular, Ni, Co, and Fe were all found to be very sensitive to O,, with a large emission peak appearing at

-

5.5 eV below EF due to chemisorbed 0,.

111. Data and Discussion. - Several EDC's for Ni are shown in figure 1. Shifts in peak locations and amplitude changes are observed in the EDC's. This structure cannot be described by a simple optical density of states. [I-31 Rather, it is indicative of direct interband transitions. [4-61 In this case, a theoretical description of the data requires a detailed energy

NICKEL

UPS

BANDS^

Occupied d-band widths of 3.3

+

^0.3,3.6

+

^{0.3 and}

3.8

+

0.3 eV have been determined for Ni, Co, and Fe respectively. These widths are somewhat narrower than those given by current energy band calculations, which give widths of approximately 4.3, 4.4 and 4.6 eV for Ni, [7] Co, [8] and Fe, [9] respectively, and are in reasonable agreement with values of 3.0,

4.0 and 4.2 eV for Ni, Co, and Fe determined via 5

x-ray photoemission spectroscopy (XPS). [lo] Charac- $

teristic peaks are observed near the Fermi energy EF ^z in the EDC's for Ni, Co, and Fe at all photon energies. 0

E

These peaks are consistent with ferromagnetic energy band exchange splittings of 6Ee, < 0.6 eV for Ni,

6Ee,<1.1eVforCoand6Ee,cz1.8+0.4eVforFe.

-

_z

11. Experimental Methods. - Photoemission mea- % ⁰

surements above 11.6 eV were made using a windowless spectrometer with a 900 cylindrical electrostatic

5

analyzer. [6] Radiation a t 21.2 eV and 40.8 eV was provided by the He1 and He11 lines of a cold-cathode He resonance lamp operated a t low pressures (- 30 m torr). Likewise, radiation at 16.8 eV and 26.9 eV was provided by the NeI and NeII lines.

Two stages of differential pumping between the lamp and sample chamber are used to minimize gas through- put. The sample chamber is pumped with a Hg diffusion pump and Ti sublimation pump. Pressures of

-

⁵^x ^torr^weremaintained in the sample chamber with the lamp operating ; these conditions

were found to ~ i e l d reproducible results. P~lycrY~talline m ~ . 1. - Emrgy distributions for ferromagnetic Ni, Energies films of Ni, CO and Fe about 1000 A thick were are measured relative to the Fermi level EP = 0.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971199

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C 1 - 294 D. E. EASTMAN band calculation together with a calculation of the

joint density of states and matrix elements for the relevant hv's. Such calculations have not yet been made for Ni, Co and Fe.

Although a detailed comparison with energy band calculations is not yet possible, several conclusions can be drawn from the data. The occupied d-band width of Ni is assumed to coincide with the high primary emission region (3.3 +_ 0.3 eV just below EF, Fig. 1).

The EDC for hv = 10.7 eV does not permit the deter- mination of the d-band width, a common limitation of UPS for hv < 11.6 eV. The estimated contribution of secondary emission is shown in figure 1 by the dashed lines for hv = 21.2 and 40.8 eV. The shape of these secondary emission curves was calculated using the inelastic pair-production model [I]. It is seen that secondary emission within -- 8 eV of EF becomes weaker with increasing photon energy. This permits a more reliable separation of primary and secondary emission at higher energies.

For Ni, a peak is observed in the EDC's at -- 0.3 eV below EF for all photon energies. Because this peak is seen for such a wide range of energies, we expect with considerable certainty that it corresponds to a peak in the band density of states. Comparison with ferromagnetic band calculations for Ni [7] indicates that this peak is consistent with an average exchange splitting of the d-bands of 6Eex 5 0.6 eV (see Fig. 2a).

UPS measurements for Ni, Co and Fe at hv = 40.8 eV are compared with x-ray photoemission (XPS) spectra [lo] in figure 2. Ferromagnetic energy band calculations are also shown. The UPS and XPS curves have been corrected for secondary emission

FIG. 2. - UPS Spectra atx hv = 40.8 eV, XPS spectra at hv = 1.25 keV [lo], and band densities of states for Ni [71, Co [8] and Fe [12]. The UPS and XPS spectra have been cor-

rected for inelastic scattering.

effects. The XPS spectra are all

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1.5 eV wider than the UPS spectra. This can be mainly attributed to the

N 1 eV XPS resolution compared to the 0.25 eV UPS resolution. Both UPS at 40.8 eV and XPS show similar overall triangular shapes for Ni, Co and Fe which are in rough agreement with the overall shape of the band densities of states.

An approximate comparison of d-band widths is made as follows. For the UPS and band density of states curves, we define the d-band width as the full width at 15 % of maximum amplitude. For the XPS curves, Fadley and Shirley define the d-band width at

+

^maximum^;this appears t o be reasonable in view of the

-

¹eV resolution. The above definitions were used to determine the d-band widths summarized in Section I. All three measures show the same trend, with the width increasing slightly from Ni to Co to Fe.

For Co, a peak has been observed in the EDC's at

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0.35 eV below EF for all photon energies. We associate this peak with a peak in the density of states. Comparison with the band density of states by Wong et al. [8] in figure 2 for hcp Co with 6Eex = 1.1 eV shows good agreement in peak location. This exchange splitting is the preferred value suggested by Wohlfarth [ll]. For 6Eex 2 1.1 eV, the d-band peak shifts to lower energy and results in poorer agreement with experiment.

For Fe, several characteristic features have been observed in the EDC's for photon energies from 16.8 through 40.8 eV. Namely, a shoulder is seen at EF, a peak at N 0.6 eV below EF, and a dip at 21 1.8 eV below EF. This is shown for hv = 16.8 eV in figure 3 as well as for hv = 40.8 eV in figure 2. This structure is seen to correlate very well with the ferromagnetic density of states [12] in figure 3 but not with the

-1

^IRON

t2 -

Z 3

> ^EDC,^hv⁼^{16.8 eV}

a K

>

-6 -5 -4 -3 -2 -1 O=EF ENERGY (eV)

FIG. 3. - UPS energy distribution for Fe at hv = 16.8 eV and band density of states for ferromagnetic Fe [12] and nonmagne-

tic Fe 1131.

nonmagnetic density of states [13]. From such a comparison, we conclude that an exchange splitting of 6Ee, 2: 1.8

+

0.4 eV for Fe is consistent with the UPS data. As seen in figure 3, the large 6Eex for Fe indicates that it would be informative to measure the UPS spectra of Fe above the Curie temperature T,

--

^{750 OC.}

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PHOTOEMISSION FROM Ni, Co AND Fe FROM 10 TO 41 eV C1-295

References

[I] BERGLUND (C. N.) and SPICER (W. E.), Phys. Rev., [8] WONG (K. C.), W O H L F A R ~ (E. P.) and HUM (D. M.),

1964, 136, A1030. Phys. Letters, 1969, 29 A, 452.

[2] KROLKOWSKI (W. F.) and SPICER (W. E.), Phys. [9] WAKOH (S.) and YAMASHITA (J.), J. Phys. Soc. Japan,

Rev., 1969, 185, 882. 1966, 21, 1712.

[3] EASTMAN (D. E.), J. Appl. Phys., 1969, 40, 1387. [lo] FADLEY (C. S.) and SHIRLEY (D. A.), Symposium of [4] SMITH (N. V.) and SPICER (W. E.), Optics Commun., Electronic Density of States, Gaithersburg, Md.,

1969, 1, 157. Nov. 3-6, 1969.

[5] JANAK (J. F.), EASTMAN (D. E.) and WILLIAMS (A. R.), [ l l ] WOHLFARTH (E. P.), J. Appl. Phys. 1970, 41,1205.

Sol. State Commun., 1970, 8 , 271. [12] CONNOLLY (J.), Symposium of Electronic Density of [6] EASTMAN (D. E.) and CASHION (J. K.), Phys. Rev. States, Gaithersburg, Md., Nov. 3-6, 1969.

Letters, 1970, 24, 310. [13] CORNWELL (J. F.), Hum (D. M.) and WONG (K. G.), [7] ZORNBERG (E. I.), Phys. Rev., 1970, B1, 244. Phys. Letters, 1968, 26 A, 365.