Surface studies of the ferromagnetic alloy Pt3Cr

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ELSEVIER

Surface

Science 307-309

(1994) 450-454

Surface studies of the ferromagnetic A.J. Liddiard, Department

alloy Pt,Cr

R.B. Patel, M.D. Crapper

of Physics,Uni~~ersity of

Technology,

(Received

Loughborough,

20 August

Lelcestwshire

* LEI I 3Tl.J. UK

1993)

Abstract The crystallographically ordered Pt,Cr (Liz structure) is a ferromagnetic alloy. We have made the first direct measurements of its electronic structure using ultraviolet photoelectron spectroscopy from the (I 10) surface. WC find that the measurements are consistent with previous calculations of the density of states (DOS). The DOS near the Fermi level is dominated by the chromium contribution. Studies of the (110) surface using Auger electron spectroscopy are consistent with stoichiometry. Low energy electron diffraction studies indicate that the surface forms a (2 X 1) reconstruction with the repeat length in the [OOl] direction being twice that expected from a simple termination.

1. Introduction The investigation of the surfaces of metallic elements is highly developed and there has been a drive in the past ten years to extend this to the study of alloys and their properties. One area which has received a lot of attention is the study of transition metal alloys (e.g. Cu/Au [1,2], Cu/Pd [3,4] etc). The main aims of these investigations has been to compare surface ordering with bulk ordering and to use surface sensitive techniques such as ultraviolet photoelectron spectroscopy (UPS) to elucidate the electronic structure of the materials. Indeed such alloys form a useful testing ground for band structure calculation schemes and for theories of ordering. We present here the first investigation of the surface properties of a transition metal alloy which has

* Corresponding

author.

Fax: + 44-509-219702.

0039.6028/94/$07.00 0 1994 Elsevier SSDI 0039-6028(93)E0938-Q

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the added interest of having unusual magnetic properties. The transition metal alloy Pt, _.I01 is one of that interesting family of materials which demonstrates ferromagnetic behaviour despite having component elements which are not themselves ferromagnetic [5]. The structural and magnetic properties of the alloy have been studied by several authors for a wide range of the values of x. It has been found [6,71 that the average magnetic moment of the alloy is a maximum when x = 0.25 and that for this case, the alloy is ordered. This crystallographically orderded Pt ,Cr has the Ll 7 structure (Cu,Au-type structure) with four interpenetrating simple cubic sublattices; one of Cr and three of Pt.0 The lattice parameter of the material is 3.87 A [6]. The magnetic moment is owing to Cr, but each Cr atom has twelve Pt atoms as nearest neighbours and so there is no direct Cr-Cr interaction. As a necessary extension of the magnetic characterisation of the Pt,Cr

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A.J. Liddiard et al. /Surface Science 307-309 (1994) 450-454

system, theoretical calculations of the electronic structure of the alloy have been carried out [S], but as yet there have been no direct investigations of this electronic structure. We have carried out the first investigation of the surface and electronic properties of this material. The structure and composition of the (110) surface of the alloy has been investigated using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). These investigations have been supplemented by UPS to form a first preliminary investigation of the electronic structure.

2. Experimental The experimental measurements were performed in two ultrahigh vacuum systems, both with base pressures around 2 X lO_‘” mbar. The initial experiments were performed on a diffusion/titanium sublimation pumped system at Loughborough. This was equipped with an argon ion bombardment gun (500 eV energy) and heating stage for sample cleaning and a VG reverse view three grid LEED system which can also be used for AES. LEED patterns were recorded using a Staib Rheed-Vision frame grabbing system. The second system was a turbo/TSP pumped system on station 6.2 on the SRS at Daresbury. This was equipped with a heated stage, argon ion gun (O-10 kV), three grid LEED system and cylindrical mirror analyser (CMA) for AES and UPS. The UPS measurements utilised the synchrotron radiation from the SRS. The line has a toroidal grating monochromater and is equipped with two gratings; 1800 lines/cm giving a photon energy range of 40-140 eV and 700 lines/cm giving 15-60 eV. The former was used for the work reported here. The sample was a single crystal of Pt,Cr previously prepared from high purity metals. The crystal (irregularly shaped but of dimensions approximately 5 mm x 5 mm x 3 mm) was oriented using Laue X-ray diffraction and cut to the (110) surface using spark erosion. The resulting face was carefully polished using diamond paste to 0.1 km

451

and stress relieved by heating to 400°C for 4 h in high vacuum. In situ preparation comprised many cycles of sputtering (at 500 eV, 10 PA or 2 keV, 5 PA depending on the experimental system) and annealing to temperatures between 400°C and 800°C.

3. LEED and AES measurements Following several cleaning cycles, AES spectra were collected from the Pt,Cr crystal and from pure polycrystalline Pt and Cr in situ standards. The main contaminant on all three samples was found to be carbon, which decreased with successive cleaning. Quantification of the AES spectra was carried out using the Cr 529 eV and Pt 237 eV peaks. The as sputtered surface showed some evidence of preferential sputtering with an apparent surface Pt: Cr ratio of 4 to 1. Prolonged annealing reduced this effect and for the annealed surface, repeated spectra under various conditions show an apparent surface ratio of around 7 to 3 rather than the expected 3 to 1. However, accurate quantification was hampered by an inability to achieve a clean Cr standard surface. It was found to be impossible to achieve a Cr surface without a significant carbon peak which would affect the sensitivity factors obtained and thus lead to an overestimate of the level of Cr present in the alloy. It is not clear if the ratio of Pt to Cr obtained by AES really indicates surface segregation or a termination which enhances the Pt contribution to the spectra or is an artifact of the quantification process. A typical AES spectrum is shown in Fig. 1. The Cr peaks around 500 eV, and the Pt peaks around 150-250 eV and 2000 eV can be seen along with the C contaminant at 272 eV. The C could be mostly removed by sputtering and annealing to 400°C but reappeared to a level great enough to interfere with the experiments in around two hours, suggesting contamination from the residual vacuum. This was confirmed by the presence of CO peaks in the UPS measurements. Thus data collection time was severely restricted and regular cleaning of the sample a necessity. LEED measurements of the surface were car-

Electron 1 OOeV Fig. 1. Auger

electron

spectrum

Energq of the Pt,Cr(l

IO) surface,

collected

ried out after annealing at 400°C. The surface gave a sharp LEED pattern indicating a high degree of surface order. A typical diffraction pattern is shown in Fig. 2. Alongside it is an indication of the real space azimuthal directions (in the correct relative orientation compared with the LEED optics) of the crystal as determined by X-ray diffraction The characteristic rows of spots associated with the “long bridge” [OOl] can be clearly seen. It can also be seen that there is some streaking of the spots along these rows, implying that there is some disorder in the [OOll direction. There is no similar streaking in the [liO] azimuth, suggesting that this disorder is one-dimensional. Superimposed on the LEED pattern is the reciprocal unit mesh of an fcc(ll0) surface with a unit cell size of 3.87 A. This structure represents that which would be present if all the atoms were of the same species and there was no reconstruction. However, if the surface is a simple termination of the alloy bulk structure, then we would expect the true repeat distance in the [liOl azimuth to be twice this owing to the alternation between Pt and Cr atoms (Fig. 3). Indeed, “extra” rows of spots are present in the LEED pattern to indicate that this is the

900eV with 3 keV primary

electron

energy

and IS IJ-A hum

current.

case (these rows of spots are much weaker than the main ones). It is also found that “extra” spots are present at half the expected repeat unit within the rows of spots. This indicates that the true repeat distance in the [OOl] azimuth is twice that expected, suggesting that we have a (2 x 1) reconstruction of the surface. A reconstruction of the “missing row” variety would be consistent with this, but there are many other possibilities involving atomic displacements or substitutions between Pt and Cr atoms. At the present, we have no evidence for the nature of the reconstruction.

4. Ultraviolet

photoelectron

spectroscopy

The results of UPS measurements carried out on the Pt,Cr crystal are shown in Fig. 4. These measurements were performed using the CMA for various photon energies. The angle of incidence was 45” and the direction of polarisation of the synchrotron radiation was in the plane defined by the surface normal and a direction 45” between the [OOll and [liO] azimuths. From the spectra it can be seen that the valence band of Pt,Cr extends to around 7 eV below the Fermi

A.J. Liddiard et al. /Surface

Science 307-309

(1994) 450-454

(4

0

Cr

@I L

PO11

Pt

Real Space Directions Fig. 3. Schematic representation of two possible terminations ((a) and (b)) of the Pt,Cr(llO) surface, demonstrating the alternation of Cr and Pt atoms in the [liO] direction.

-

‘fee’ reciprocal unit mesh

- - - - true reciprocal unit mesh Fig. 2. Low energy electron diffraction pattern from the Pt,Cr(llO) surface collected at a primary energy of 137 eV. The legend below the Fig. indicates the real space orientation of the crystal in front of the LEED optics. The pattern shows weak spots at half the periodicity of that expected for a single element fee crystal showing that the true repeat length is twice that in both principal azimuths. Superimposed on the diffraction patterns are the reciprocal unit mesh of the surface and that for the fictitious single element fee crystal.

edge and without correcting for transition probabilities it is clear that it corresponds roughly with the double peaked DOS calculated by Jezierski [8], which is also shown in Fig. 4. The extra broadening in the experimental data is owing to the finite instrumental resolution. An interesting change in the shape of the spectra as a function of the photon energy is observed. The strength of the photoemission from the region near the Fermi edge increases relative to that from the rest of the spectrum as the photon energy increases. This can be understood by considering the photoemission cross sections [9] of the two elements as a function of photon energy (Fig. 5). At 50 eV photon energy, the cross section of the Pt 5d electrons is twice that of the Cr 3d electrons, but at 100 eV the ratio is re-

versed with a strong enhancement of Cr over Pt. Thus, from these measurements, we can conclude that the electronic structure near the Fermi level is dominated by electrons associated with the Cr

c

5

’ I

’

4

3

2

1

0

-1

I

-2 -8

-6 Binding

-4

-2 Energy

0

2 (eV)

4

6

Fig. 4. Ultraviolet photoelectron spectra from the Pt,Cr(llO) surface for photon energies between 45 and 105 eV, compared with the density of states calculated by Jezierski [S].

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A.J. Liddiard rt al. /Surface

Scirtw

307-NV

(1994) 4X-454

lated by Jezierski [S]. The electronic structure at the Fermi level is dominated by electrons associated with the Cr atoms.

6. Acknowledgements 0

50

Photon

100

Energy

150

200

(eV)

Fig. !i. Graph of the photo-ionisation cross sections Cr as a function of photon energy [9].

of Pt and

component of the alloy. This is consistent with the work of Jezierski as the main difference between the spin up and spin down components of the DOS occurs near the Fermi level (Fig. 41, and it is owing to the Cr atoms which are resposible for the magnetic behavior-u. The cross sections of Cr and Pt are identical near to 60 eV photon energy (Fig. 51, so the spectrum at this energy will be the most representative of the DOS.

5. Conclusions We have reported the first study of a surface of the Pt,Cr ordered alloy. The (110) surface is found to be ordered with a (2 X 1) reconstruction over and above that which would be expected from the Pt-Cr substitution. The surface concentrations of Pt and Cr as measured by AES are not inconsistent with a stoichiometric surface. UPS studies as a function of photon energy are in broad agreement with the theoretical DOS calcu-

The support of the SERC is acknowleged for the provision of a studentship for A.J.L., a research assistantship for R.B.P. and for supporting this work through grant number GR/F49040. We are indebted to David Williams for the Pt,Cr sample and for many valuable discussions, to Gary Critchlow for assistance with the Auger analysis and to Paul Bailey, Dave Teehan and Bruce Cowie for their support at the SRS. Spccial thanks arc due to Mr. James Brown.

7. References [I] E.G. McRae and R.A. Malik. Phys. Rev. B 42 (IYYO) l13)O. [2] Z.Y. Wang, SC. Wu, .I. Quinn. C.K.C. Lok. Y.S. Li. F Jona and J.W. Davenport. Phys. Rev. B 3X (1088) 7441. [3] H. Wright, P. Weightman. P.T. Andrews. W. Folkerth, C.F.J. Flipse, G.A. Sawatzky, D. Norman and H. Padmore, Phys. Rev. B 35 (1987) 519. [4] D.J. IIolmes. D.A. King and C.J. Barnes. Surt. Sci. 777 Cl’)901 179. [Sl D.E.G. Williams and B.G. Lewis, Z. Metallk. 70 (lY7Y) 441. [h] D.E.G. Williams and A. Jezierski, J. Magn. Magn. Mater. 59 (1986) 41. [7] A. Jezierski, Physica B 141) (1988) 244. [8] A. Jezierski, J. Phys. F 13 (1983) LI Il. [9] J.J. Yeh and I. Lindau, At. Data Nucl. Data Tables 32 (19X.5) I.