Field:
Material/Biomaterial Science
Session Topic:
Magnetic Interactions in Nano-materials
Speaker:
Gen Tatara/Tokyo Metropolitan University
Nano magnetism and electron transport theory:
Physics and application to information technology
1. Introduction
Our life relies much on information technology. In most of devices such as computers,
video recorder and car navigation systems, high density storages use magnetism to
store information. At present, the size of each magnet carrying one bit of information
is less than micron meter (=1/1000 mm), and it will rapidly decrease to be in nano
meter scale as storage density becomes higher. For higher density storage, downsizing
of magnets is of course necessary, but development of new write-read mechanism with
higher efficiency is also essential. One possiblity to realize efficient ultra high density
magnetic memories is to use switching by direct electric current, instead of using
magnetic field as done so far. This mechanism is based on microscopic and strong
coupling between magnetization and electron. In my talk, I will explain what kind of
physics lies under present information device, and also discuss newly found
phenomena and their possible application to future devices.
2. Spin
Spin is a quantum mechanical degress of freedom attached to most elementally
particles such as electron, neutron and proton. Exisitence of spin is a natural
consequence of relativistic quantum mechanics (Dirac equation). Spin is a kind of
atomic magnet with N and S poles, but is quantum mechanical object subject to
uncertainty principle. Ferromagnets we are familiar with such as iron magnet is in
fact a mocroscopic (of number of about 10^{23}) sum of spins aligning in the same
direction. The spin can be coupled with each other by quantum mechanical strong
interaction called exchange interaction, and can be coupled to orbital motion of
electron, namely with current via relativistic interaction called spin-orbit interaction.
3. Magnetic devices
Present magnetic devices uses submicron scale ferromagnets as unit and write the
information by use of magnetic field. To read out, the magnetic information needs to
be converted into electric current by use of resistivity change by magnetization, called
magnetoresistance, by use of spin-orbit interaction (anisotropic magnetoresistance) or
much more efficiently in thin multilayer systems (giant- or tunneling
magnetoresistance). In magnetic devices, spin-orbit interaction is quite important
because of another reason that it determines magnetic anisotropy energy which
governs the stability of stored information. Therefore, our technology is based on
emsenble of quantum mecanical object, spin, and quantum and relativistic effects.
4. Magnetization switching by use of electric current
In macroscopic world, applying an electric current through a magnet does not flip
magnetization. In nanoscale, in contrast, magnetization is small and thus can be
flipped by use of spin (tiny magnetization) of the electron carrying the current. This
mechanism could be quite efficient in future high-density devices. The interaction
responsible is the exchange interaction. We will discuss how exchange interaction
drives magnetization switching, and mention possible applications to magnetic
memories.
5. Other new spin-dependent transport
We will also discuss some other new spin-dependent transport properties, such as spin
Hall effect and inverse spin Hall effect. Again relativistic spin-orbit interaction is the
origin of these effects. In spin Hall effect, spin-orbit interaction causes deviation of the
electron orbit depending on its spin, and in the inverse spin Hall effect, the
interaction converts the flow of spin (spin current) into electric (charge) current.
6. Conclusion
As we saw above, our present technology and perhaps also future nano technology are
based on novel phenomena arising from quantum mechanics and relativity. Solid state
physics is an exciting and attractive field.
References
Conversion of spin current into charge current at room temperature: Inverse
spin-Hall effect
E. Saitoh, M. Ueda, H. Miyajima, and G. Tatara, Appl. Phys. Lett. 88,
182509-1-182509-3 (2006).
Effect of Spin Current on Uniform Ferromagnetism: Domain Nucleation
J. Shibata, G .Tatara and H. Kohno, Phys. Rev. Lett. 94, 076601-1-076601-4 (2005).
Current-induced resonance and mass determination of a single magnetic domain wall
E. Saitoh, H. Miyajima, T. Yamaoka and G. Tatara, Nature 432, 203-206 (2004).
Theory of Current-Driven Domain Wall Motion: Spin Transfer versus Momentum
Transfer
Gen Tatara and Hiroshi Kohno, Phys. Rev. Lett. 92, 086601-1-086601-4 (2004).
Domain wall scattering explains 300% ballistic magnetoconductance of nanocontacts
Gen Tatara, Yuwen-W. Zhao, Manuel Munoz, and Nicolas Garcia, Phys. Rev. Lett. 83,
2030-2033 (1999).
Resistivity due to a Domain Wall in Ferromagnetic Metal
Gen Tatara and Hidetoshi Fukuyama, Phys. Rev. Lett. 78, 3773-3776 (1997).