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Spin polarisation

Spin polarisation and spin transmission in ultrathin magnetic structures (Polarimeter)

Spin electronics is based on utilising the electron spin for novel and improved electronic devices. The success of spintronics ultimately depends on our ability to precisely control the polarisation of electrons transported within the actual thin film structure. Knowledge of and the ability to control the surface or interface polarisation in thin ferromagnetic films is thus a crucial issue for spintronics. At present electrical measurements are principally used to assess the quality and performance of magnetic thin film devices. However, this is an indirect way of understanding the effect of structural and interfacial properties which ultimately determine the spin plarisation and thus the performance. In a revolutionary approach we measure the spin polarisation of freely scattered electrons from atomically clean surfaces and interfaces using spin polarimetry.

Spin Transport

Figure 1: Spin polarimetry - An unpolarised electron beam is scattered at the magnetic film. The resulting beam is polarised and its polarisation is determined with the 25 kV Mott polarimeter.

Spin polarimetry, i.e. the determination of the polarisation of a reflected, diffracted or low energy secondary electron beam generated from a magnetic structure, provides a direct and highly sensitive probe of the surface polarisation. This technique is highly advantageous over other more widely used methods such as the magneto optic Kerr effect (MOKE), which originates from the spin orbit interaction. Recent studies of a spin polarised electron beam transmitted through a freestanding magnetic layer (the electron analogue of Faraday rotation) have indeed revealed that the direct coupling of electrons to the sample polarisation is two orders of magnitude larger than the corresponding magneto optical effect [1].

We have set up and commissioned a 25 kV micro-Mott spin analyser in a UHV chamber with high purity metals deposition, in-situ MOKE and surface analysis capabilities such as low energy electron diffraction (LEED) and Auger electron spectroscopy.


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Figure 2: A schematic diagram and photograph of the present Mott polarimeter are shown. The schematic is reproduced from [2].

We employ a state of the art, compact, retarding potential Mott Polarimeter to perform in-situ studies of surface magnetization effects. Particular systems of interest include Co thin films grown epitaxially on various single crystal copper surfaces. By carefully admitting specific gases to the growth chamber and employing surfactant effects we are able to create highly reproducible adsorbate layers on our films and the resulting changes to the surface magnetism can be probed. In a collaboration with Dr S.Jenkins from the Chemistry department here at Cambridge our macroscopic measurements can be compared directly with the predictions of microscopic DFT calculations. We are able to complement our polarimetry measurements with transverse MOKE as an independent verifier of the magnetic behaviour as well as standard AES and LEED surface science techniques.

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A 3D representation of the polarimeter showing the path of the electron beam and the four electron counters which allow measurement of the two in-plane components of polarisation.

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A photo showing the inside of the polarimeter, taken during assembly of the instrument. At the top of the picture the entrance aperture for the electron beam can be seen. The four metal cylinders surrounding the central column house the channeltron electron counters.

 

Published work:

Chemically selective gas-induced spin polarization changes in ultrathin fcc Co films

K. P. Kopper, D. Küpper, R. Reeve, T. Mitrelias, and J. A. C. Bland

JOURNAL OF APPLIED PHYSICS 103, 07C904 (2008)

 

Chemically selective modification of spin polarisation in ultrathin ferromagnetic films

K. P. Kopper, D. Küpper, R. Reeve, T. Mitrelias, D.S.D.Gunn and S.J.Jenkins

To be published.

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