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Dilute Magnetic Semiconductors

There are several naturally occuring magnetic elements: chromium, iron, nickel, cobalt and gadolinium. By introducing these into a materials as a dopant, they can introduce favourable magnetic properties in a controllable fashion.
In their elemental forms they have varying magnetic properties. Chromium is an antiferromagnet; iron, nickel and cobalt are all ferromagnets; and gadolinium is a ferromagnet below 300 K but is paramagnetic at room temperature.

In order for a material to exhibit magnetic properties, there must be some form of ordering amongst the magnetic moments. For an antiferromagnet the moments are aligned anti-parallel throughout the material, whilst for a ferromagnet the moments are parallel. An important parameter for this ordering is temperature, since if the thermal energy is larger than the ordering energy then the material will lose its magnetic properties. For antiferromagnets, this temperature is the Néel temperature (named for Louis-Eugène-Félix Néel) whereas for ferromagnets it is the Curie temperature (named for Pierre Curie).

Dilute magnetic semiconductors (DMS) are semiconductors doped with transition metal atoms. The primary aim of a DMS is compatibility with existing semiconductor structures and materials. Semiconductor-DMS interfaces can be made by introducing the dopant during the growth process, this controllability is especially evident in processes such as MBE, where it is easy to control the doping fluxes. The level of doping directly affects the magnetic properties of the DMS and affects the Curie temperature. A Curie temperature that is above room temperature is crucial in order to exploit spin polarisation in electronic devices. The main disadvantage of current DMS materials is that their Curie temperatures are below room temperature and result in a loss of magnetic ordering in everyday applications. For example, the highest Curie temperature obtained for Ga1-xMnxAs, a Mn-doped III-V DMS, is 150 K [1], rendering it unfeasible for inclusion in existing electronics. Another key issue with the introduction of these dopants is one of growth and chemical stability. Manganese has a low solubility in GaAs [2] and at large concentrations, x > 0.1, results in metallic-like conduction, eliminating the semiconducting behaviour [3].

Much work has been done on the introduction of transition and rare-earth atoms into semiconducting materials [2].

There is a large amount of literature on the introduction of magnetic ions into semiconducting materials [4,5,6], there is some evidence on room temperature ferromagnetism with controllable semiconducting properties [6]. In addition to the III-V semiconductors, some work has been performed on the II-VI semiconductors such as zinc oxide [7].

Finally, as an important concept for the introduction of controllable magnetic properties into existing electronic properties, spintronics (a portmanteau of spin and electronics) is the introduction of a spin-polarised current into silicon. A requirement of a spin-polarised current is to have a non-equilibrium imbalance in the spin population of an injected current. By manipulating this current, injected into silicon, the industrial standard material, the introduction of spin control into modern electronics is possible [8,9]. 

 

References

  1. K. C. Ku et al. (2003), Appl. Phys. Lett., 82, pp2302-2304
  2. R. P. Campion, K. W. Edmonds, L. X. Zhao, K. Y. Wang, C. T. Foxon, B. L. Gallagher and C. R. Staddon (2003), J. Crys. Growth., 247, pp42-48
  3. M. Ozeki, T. Haraguchi and A. Fujita, (2007), Phys. Stat. Sol. A., 204, pp992-997
  4. L. Gu, S. Y. Wu, H. X. Liu, R. K. Singh, N. Newman and D. J. Smith (2005), J. Magn. Magn. Mat., 290-291, pp1395-1397
  5. A. Ney, R. Rajaram, R. F. C. Farrow, J. S. Harris and S. S. P. Parkin (2005), J. Super, 18, pp41-46
  6. N. Teraguchi, A. Suzuki, Y. Nanishi, Y-K. Zhou, M. Hashimoto and H. Asahi (2002), Sol. Stat. Comm., 122, pp651-653
  7. S. A. Chambers (2009), Adv. Mat., 21, pp1-30
  8. S. P. Dash, S. Sharma, R. S. Patel, M. P. de Jong and R. Jansen (2009), Nature Letters, 462, pp491-494
  9. B. T. Jonker, G. Kioseoglou, A. T. Hanbicki, C. H. Li and P. E. Thompson (2007), Nature Materials, 31, pp542-546