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Development of New Method for Evaluating Spin Polarization – New method for evaluation of spintronic materials - (Press Release)

Release Date
20 Nov, 2009
  • BL08W (High Energy Inelastic Scattering)
Japan Synchrotron Radiation Research Institute (JASRI), jointly with Bristol University (UK) Rutherford Appleton Laboratory (UK), the University of Warwick (UK), and the University of Minnesota (US), has developed a new method of evaluating the spin polarization of spintronic materials, which will benefit the next-generation information industry, using the synchrotron radiation at SPring-8. 

Japan Synchrotron Radiation Research Institute

Japan Synchrotron Radiation Research Institute (JASRI) (Shirakawa Tetsuhisa, President), jointly with Bristol University (UK) Rutherford Appleton Laboratory (UK), the University of Warwick (UK), and the University of Minnesota (US), has developed a new method of evaluating the spin polarization*1 of spintronic*2 materials, which will benefit the next-generation information industry, using the synchrotron radiation at SPring-8.  The development of this method was realized through experiments on high-precision magnetic Compton scattering*3 (Fig. 1) using the stable and high-intensity high-energy X-rays produced at SPring-8.     

The spin polarization of the interior of a material, which is difficult to measure using conventional methods, can be evaluated using the new spin polarization evaluation method because high-energy X-rays are used, and the evaluation results are obtained irrespective of the material's surface condition.  For Co1-xFexS2, a candidate spintronic material, an experiment on magnetic Compton scattering was performed by changing the Fe concentration to determine the Fe-concentration dependence of the spin polarization. 

The achievement of this study is expected not only to contribute to improving the evaluation and design methods of spintronic materials but also to be applied in fundamental research for the development of more precise models of the giant magnetoresistive effect,*4 in which electron spin is strongly related to the phenomenon of electric conduction.

These results were obtained by Masayoshi Ito, an associate senior scientist, and Yoshiharu Sakurai, an associate chief scientist of JASRI with cooperation from overseas institutions.  The research achievements were published online in the American scientific journal, Physical Review Letters, on 25 November 2009.

Publication:
"A new approach to determine bulk spin polarization applied to Co(1-x)FexS2"
C. Utfeld, S. R. Giblin, J. W. Taylor, J. A. Duffy, C. Shenton-Taylor, J. Laverock, S. B. Dugdale, M. Manno, C. Leighton, M. Itou and Y. Sakurai
Physical Review Letters 103, 226403 (2009), published online 25 November 2009.

<Figure>

 

Fig. 1 Schematic of Compton scattering

Fig. 1 Schematic of Compton scattering
Compton scattering occurs as a result of an elastic collision between an electron and an X-ray photon, similar to the collision of two billiard balls.  By measuring the X-ray photon energy after Compton scattering, the momentum (or velocity) of the electron before Compton scattering can be measured.  In magnetic Compton scattering, a method using Compton scattering, the momentum of spin-polarized electrons can be measured by aligning the directions of electron spins (micromagnets) in a magnetic field induced in a superconducting electromagnet using circularly polarized incident X-rays.

 

 

Fig. 2 Schematic of half metal

Fig. 2 Schematic of half metal
In the electronic state shown in the figure, spin-up electrons exist at the Fermi level, and there are no spin-down electrons.  Namely, only spin-up electrons contribute to the conduction of electricity, while spin-down electrons do not contribute to the conduction of electricity; therefore, this state is known as a half metal.

 

 

Fig. 3 Densities of states of spin-up and spin-down electrons of CoS2 obtained by theoretical band calculation with linear muffin-tin orbital (LMTO) method

Fig. 3 Densities of states of spin-up and spin-down electrons of CoS2 obtained by theoretical band calculation with linear muffin-tin orbital (LMTO) method
Densities of states of spin-up and spin-down electrons exist at the Fermi level.  Spin polarization is negative because the density of states of spin-down electrons is larger than that of spin-up electrons.  When Co atoms are replaced with Fe atoms, the distribution of the density of states (areas filled with blue or red) remains the same; however, the Fermi level (vertical dotted line) shifts to a lower energy (to the left in the figure), and the densities of states of spin-up and spin-down electrons at the Fermi level change.  Accordingly, the spin polarization of Co0.9Fe0.1S2 becomes positive because the density of states of spin-up electrons becomes larger than that of spin-down electrons.

 

 

Fig. 4 Change in spin polarization of Co1-xFexS2 with Fe concentration

Fig. 4 Change in spin polarization of Co1-xFexS2 with Fe concentration
Experimental results: results obtained in this study
Theoretical results: results obtain by theoretical band calculation with LMTO method

 

<Glossary>

*1 Spin polarization
The electrons in a metal first occupy the lower energy levels.  The highest occupied energy level is called the Fermi level.  The rotation (spin) of an electron acts as a micromagnet, and the direction of the spin at the Fermi level determines the magnetic and electric properties of the material.  Spin polarization is defined as the difference between the numbers (density of states) of spin-up electrons and spin-down electrons.

*2 Spintronics
Electrons have two degrees of freedom: one is the electric charge and the other is the spin, which acts as a micromagnet.  In conventional electronics, only the characteristics of electric charge are considered, whereas in spintronics, the characteristics of both electric charge and spin are used.  In spintronics, the direction of the electron spin is reversed by applying a current or light, and the current is controlled by aligning its direction with the application of a magnetic field.  The development of spintronics-related technologies to realize ultrahigh-density memory and quantum-computer devices is underway.

*3 Magnetic Compton scattering
Compton scattering is considered to occur as a result of an elastic collision between an electron and an X-ray photon, similar to the collision of billiard balls.  By measuring the X-ray photon energy after Compton scattering (after collision), the momentum (or velocity) of an electron before Compton scattering can be determined.  In magnetic Compton scattering, a method using Compton scattering, the momentum of spin-polarized electrons can be measured by aligning the directions of the electron spins (micromagnets) in a magnetic field induced by a superconducting electromagnet using circularly polarized incident X-rays.

*4 Giant magnetoresistive (GMR) effect
The GMR effect is a phenomenon in which the electric resistivity of a material markedly changes when it is placed in a magnetic field.  This effect is used, for example, in the driving principle of magnetic heads in hard drives.

 

 

For more information, please contact:
Dr. Masayoshi Ito (JASRI/SPring-8)
e-mail: mail

or
Dr. Yoshiharu Sakurai (JASRI/SPring-8)
e-mail: mail.