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 Introduction to SP-STM Why study Cr? Sample Preparation Growing Cr(001) Scanning with a W tip Preparing an Fe coated tip Scanning with an Fe coated tip Anything to conclude? 21/03/2005

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 21/03/2005

1.1 - Introduction to SP-STM Spin Polarised STM is analogous to tunnelling magnetoresistance (TMR). 21/03/2005

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. Ferromagnetic tip scanned over alternately magnetised terraces. Wiesendanger et al. J. Vac. Sci. Technol B, Vol. 9, No. 2, (1991) pp 519-524. 21/03/2005

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. 21/03/2005

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. 21/03/2005

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. Ferromagnetic tip scanned over alternately magnetised terraces. Wiesendanger et al. J. Vac. Sci. Technol B, Vol. 9, No. 2, (1991) pp 519-524. 21/03/2005

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 monoatomic steps 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. 21/03/2005

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) 21/03/2005

3.1 – Sample Preparation Samples grown in UHV MBE chamber and transferred under vacuum to UHV STM. 21/03/2005

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 21/03/2005

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. 21/03/2005

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 21/03/2005

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 21/03/2005

LEED of Fe surface taken at 90 eV. 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. 21/03/2005

LEED of Cr surface taken at 90 eV. 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. 21/03/2005

Tunneling parameters: U=0.5 V and I=1.00 nA 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 21/03/2005

Tunnelling parameters: U=0.5 V and I=1.00 nA 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 21/03/2005

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. Thin Film tips of different thicknesses. Anisotropy forces their magnetic moments in different directions. 21/03/2005

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 21/03/2005

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? 21/03/2005

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. 21/03/2005

Acknowledgements Supervisors: Dr Sarah Thompson and Dr Steve Tear. Post Doc: Dr Chris Bonet. Additional members of the Surface and Magnetism groups. EPSRC. 21/03/2005

1.3 – Introduction to SP-STM Intending to use the technique to study spin injection and transport. 21/03/2005