EVIDENCE FOR POLARISED BORON IN FeB and COB ALLOYS

Y.B. Xu. D. Greig and P.K. Hucknall, Department of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK

E.A Seddon, CLRC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK

J.AD. Matthew, Department of Physics, University of York, York YO1 SDD, UK

Backeroun4

Iron-boron and cobalt-boron amorphous alloys are archetypal transition metal- metalloid alloys and their magnetic, electronic and mechanical properties have been extensively studied over the last 20 years. In recent years there has been much progress in understanding the electronic structure and magnetic properties of these amorphous systems within an itinerant electron approach [I-31 Self consistent calculations of ferromagnetic transition metal boron (TM-B) alloys suggest thai non bonding transition metal d electrons dominate at the Fermi level but that strong covalent bonding between TM d electrons and B p like electrons occurs in states at higher binding energy. However, the details of this interaction depends both on the topological disorder in the system and on the model used. In this paper we show that by exploiting the tuneabiity of synchrotron id i t i on in spin-resolved photoemission studies of amorphous FeB and COB alloys it has proved possible to obtain insight into the partial spindependent density of states of both the transition metals andof boron.

Emeriment and analwis

Fe,,B2, and C*B2, amorphous alloys were prepared as -3 mm wide ribbons by melt spinning in a helium atmosphere. The samples were formed into a closed loop, shiny side outermost, that could be magnetised by passing a small current through an insulated wire wrapped around the rear. Magnetc-optical K m effect measurements showed that they could be magnetised to saturation with 1OoOh remanence. The photoemission measurements were made on remanently magnetised samples on Station 1.2 of the UK Synchrotron (SRS) operating at a base pressure of better than ~ X I O - ' ~ mbar. This station is designed for spin polarised photoemission work across the photon energy range 5-95 eV and has been described in detail elsewhere [4].

Corresponding Author Professor D Greig, Department of Physics & Astronomy, University of Leeds,

Tele: (UK)-113-233-3880; FAX: (UK)-113-233-3900 E-mail: d.grei&leeds.ac.uk

Leeds LS2 9JT. UK

Figl: Polarisation sp&a of F q & for photons of 15 and 35 eV

8 6 4 2 E F Binding energy (ev)

Figure 1 shows the spin-polarisation spectra at 15 and 35 eV photon energies of Fa,B2, with similar results obtained for C%BZ3. The polarisation spectra collected aa the lower photon energy show distinct minima at around 2 eV that are not ob& at higher photon energies. A key issue in the interpretation of this effect is the comparative mapitodes of the scattering cross sections of the iron and boron atoms. For photon energies greater than -25 eV the 2F and 2p cross sections of boron are much smaller than that of iron 3d. However as the photon energy drops below 20 eV the boron Zp cross section increases sharply and for 15 eV photons is comparable with that of Fe 3d. The same interpretation applies to measurements on COB. This photon energy depmdence indicates that the polarisation dips are associated with photoemission from boron states. In principle, the polarisation dip could be explained by unpolarised B-Zp mntfibutions but the fact that the polarisation changes differ sigoificantly across the B-Zp region implies that there are differences in the state of the B polarisation across the band and that the minority spin is dominant around the binding energy of the polarisation minimum (-I .7 eV). This is to be compared with Hafner et U/. 's [3] prediction that the density of states in Fer& has a substantial B-Zp component between 2 and 7 eV binding energy with a modest excess of down spin density peaked at slightly above a binding energy of 3 eV. Similar spectroscopic behaviour was observed in Co,,B,,.

References: [ I ] A M Bratkovsky and A V Smirnov, J. Phys. Condens. Matter, 5,3203, (1993) [2] H Tanaka. S Takayama, M Hasegawa, T Fulomnaga, U MizutN A Fujita and W Fukamichi, Phys. Rev B 47,2671 (1993) [3] J Hafner, M Tegze and Ch Becker, Phys. Rev. B 49,285 (1994) [4] F M Quinn, E A Seddon and I W Kirkman, Rev. Sci. Inst., 66, 1564 (1995)

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