21/03/2005

Spin-Polarised Scanning Tunnelling Microscopy of Thin

Film Cr(001)?

Debra Schofield,

Chris Bonet, Steve Tear, and Sarah Thompson.Department of Physics, The University of York,

Heslington, York, YO10 5DD, UK.

21/03/2005

Outline

1) Introduction to SP-STM2) Why study Cr?3) Sample Preparation4) Growing Cr(001)5) Scanning with a W tip6) Preparing an Fe coated tip7) Scanning with an Fe coated tip8) Anything to conclude?

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1.1 - Introduction to SP-STM

• The distance an electron can travel before it spin-flips is long, and depends upon the medium in which the electron is travelling.

• Spin↑ and spin↓ electrons can be modelled independently.

Spin-split DOS for Fe and CrO2, respectively

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1.1 - Introduction to SP-STM

• Spin Polarised STM is analogous to tunnelling magnetoresistance (TMR).

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1.2 - Introduction to SP-STM

In constant current mode the tip has to withdraw/ approach the surface depending on its polarisation to observe the same tunnelling current.

)1(0 PGG

1)exp(

1)exp(

sA

sAP

Ferromagnetic tip scanned over alternately magnetised terraces.

Wiesendanger et al. J. Vac. Sci. Technol B, Vol. 9, No. 2, (1991) pp 519-524.

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2.1 – Why Study Cr(001)?

• Chromium is an AFM.• bcc structure.• Magnetic moment of

atoms alternate layer by layer.

Spin-lattice of antiferromagnetic Cr.

Figure from: Wiesendanger et al. J. Vac. Sci. Technol B, Vol. 9, No. 2, (1991) pp 519-524.

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2.2 – Why Study Cr(001)?

• Cr forms ferromagnetic terraces separated by monoatomic steps.

• Terraces are alternately magnetised in opposite directions.

Topological antiferromagnetic order of a Cr(001) surface.

Blügel et al. Phys. Rev. B, Vol. 39, No. 2, (1989) pp-1392-1394.

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1.2 - Introduction to SP-STM

In constant current mode the tip has to withdraw/ approach the surface depending on its polarisation to observe the same tunnelling current.

)1(0 PGG

1)exp(

1)exp(

sA

sAP

Ferromagnetic tip scanned over alternately magnetised terraces.

Wiesendanger et al. J. Vac. Sci. Technol B, Vol. 9, No. 2, (1991) pp 519-524.

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2.3 - Why Study Cr(001)?

• Using a tip of CrO2 a periodic alteration of the measured step height was observed.

Arbitrarily chosen (not successive) single line scans over the same three monoatomic steps. For

comparison a single line scan over two monoatomicsteps taken with a tungsten tip. I=1 nA and U=+2.5 V.

Wiesendanger et al. J. Vac. Sci. Technol B, Vol. 9, No. 2, (1991) pp 519-524.

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2.4 - Why Study Cr(001)?

• All previous work done on bulk single crystal Cr.• Months of repeated cycles of Ar+ bombardment

and annealing required.• Thin film freshly grown likely to take less time to

clean.• In-plane magnetisation, used as a pinning layer

in magnetic hard disks.• Method developed from several sources:

– R. Bertacco et al. J. Vac. Sci. Technol A, Vol. 16, No. 4, (1998) pp 2277-2280.– G. Isella et al. Solid State Comm. Vol. 166 (2000) pp 283-286.– Personal communications with F. Almeida, Instituut voor Kern-en Stralingsfysica,

KuLeuven, Belgium. (J. Meersschaut)

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3.1 – Sample Preparation

Samples grown in UHV MBE chamber and transferred under vacuum to UHV STM.

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3.2 – Sample Preparation

• Freshly cleaved MgO.• MgO fairly good

lattice match to Cr, and Fe.

• MgO – 2.97 Å• Cr – 2.88 Å• Fe – 2.87 Å RHEED of MgO Substrate

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3.3 – Sample Preparation (1)

1st method:• 1000 Å Fe

deposited.• Annealed at 500 °C

for 10 mins.• Deposited 50-300 Å

Cr.• Transfer to STM.

RHEED of Cr on Fe on MgO

Image is 400 x 400 Å. Tunneling parameters: U=0.5 V and I=0.5 nA.

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3.4 – Sample Preparation (2)

2nd Method:• 200 Å Cr deposited

with the substrate held at 200 °C

• This acts as a buffer layer to help stop the diffusion of oxygen from the substrate.

• Also reduces strain when depositing Fe.

RHEED of Cr on MgO

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3.5 – Sample Preparation (2)

• 1000 Å Fe deposited, with substrate held at 100 °C.

• Transfer to STM after Fe deposition.

RHEED of Fe on Cr on MgO

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4.1 – Growing Cr(001)

• Ar+ bombardment and annealing of the Fe surface.

• Number of cycles depends on success of transfer.

• Good 1x1 LEED observed.

LEED of Fe surface taken at 90 eV.

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4.2 – Growing Cr(001)

• 20 Å Cr evaporated.

• Quick anneal at 500 °C. For 5 Mins.

• Good 1x1 LEED obtained.

LEED of Cr surface taken at 90 eV.

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5.1 - Scanning with a W tip

• Monoatomic terraces formed.

• Closer together than on bulk single crystal samples.

• Frustration effects from lattice mismatch.

Image is 1000 x 1000 Å. Tunneling parameters: U=0.5 V and I=1.00 nA

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5.2 - Scanning with a W tip

• Taking a cross section.

• Observed step height is 1.44 ± 0.01 Å.

• Z calibration taken from step height of Si

Image is 400 x 290 Å. Tunnelling parameters: U=0.5 V and I=1.00 nA

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6.1 – Preparing an Fe Coated Tip

• Thin film tips are versatile.• Fe film >10ML sensitive to in-

plane components of magnetisation.

• Antiferromagnetic tips possible.• Rounder tips allow better film

coverage.• Need to think about the in-plane

angle between the magnetisation of the sample and the tip.

W W

Fe Fe

Thin Film tips of different thicknesses. Anisotropy forces their magnetic moments in different directions.

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7.1 – Scanning with an Fe coated tip

• Monoatomic terraces still observed.

• Still very close together. Would be better slightly wider.

Image is 800 x 800 Å. Tunnelling parameters: U=0.5 V and I=1.00 nA

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7.2 – Scanning with an Fe coated tip

• Taking a cross-section.• Observed step heights are

difficult to measure.• About the right height but

any observed alteration?• Many more results

needed!• Orientation of the tip?• Texturing?

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8.1 – Anything to conclude?

• Clean single crystal Cr(001) has been grown and imaged in an STM.

• The surface has been imaged with both conventional W and Fe tips.

• No comment.

• Will continue with Mn(001) and Fe3O4 thin films.

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Acknowledgements

• Supervisors: Dr Sarah Thompson and Dr Steve Tear.

• Post Doc: Dr Chris Bonet.• Additional members of the Surface and

Magnetism groups.• EPSRC.

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1.3 – Introduction to SP-STM

Intending to use the technique to study spin injection and transport.