Theoretical Prediction of Ferromagnetism in Semiconductor Dot Array

Hiroyuki Tamura, Kenji Shiraishi, and Hideaki Takayanagi
Materials Science Laboratory

Ferromagnetism in non-magnetic semiconductor dot arrays is theoretically predicted. It has been known that a ferromagnetic spin-state is induced by electron-electron interactions in a certain type of lattice structure having dispersion-less band, so called flat band [1]. However, there has been no clear evidence of observing flat-band ferromagnetism so far, because it is difficult to synthesize or fabricate materials in such a way that the flat band is formed. The present work proposes that the use of semiconductor-dot array (Fig. 1) would be the most promising way to realize flat-band ferromagnets (Fig. 2) [2].
Ferromagnets of non-magnetic semiconductor-dots have several great advantages in application. First, one can make ferromagnets just by utilizing existing LSI fabrication techniques. For example, conventional MOS transistors and magnetic devices, such as sensors or memories, made by semiconductor-dot ferromagnets can be fabricated on the same silicon chip. This allows us to reduce the chip size and the number of fabrication processes. Second, one can modulate the electron filling by changing the gate voltage, and ferromagnetic and paramagnetic states can be freely switched by changing the gate voltage. This controllability of magnetic states can lead to the possibility of ultra-fast switching devices or ultra-sensitive sensors made by the proposed semiconductor-dot ferromagnets.
[1] E. H. Lieb, Phys. Rev. Lett. 62 (1989) 1201.
[2] H. Tamura, K. Shiraishi, and H. Takayanagi, Jpn. J. Appl. Phys. 39 (2000) L241.

Fig. 1. Schematic two-dimensional dot array exhibiting ferromagnetism.

Fig. 2. Example of flat-band (in the middle of three subbands).