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).