GaMnAs

Gallium manganese arsenide (or GaMnAs) is a dilute III-V magnetic semiconductor based on doping gallium arsenide (GaAs) with manganese (Mn). Manganese atoms are substituted for gallium, creating both a local moment and acting as an acceptor, leading to an electron hole.

GaAs is already in wide use as a semiconductor in many applications. Adding magnetic moments allows for an additional degree of freedom due to spin. An entire field, spintronics, has been growing rapidly dealing partly with magnetic semiconductors like GaMnAs.

In order for practical use, the curie temperature (Tc) needs to be above room temperature. Currently, the best samples have a Tc of approximately 200K.

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History

A ternary alloy of GaMnAs under equilibrium growth condition is limited to roughly 0.1% manganese. Beyond that, the more energetically favorable ferromagnetic metal MnAs forms. To overcome this, a lower growth temperature of around 250C is used, allowing for non-equilibrium growth. The Ohno group reported the first ferromagnetic GaMnAs with a Tc of around 7.5K in 1992. In 1998, breakthroughs in growth techniques allowed for a 110K curie temperature. Further progress in MBE techniques, and the addition of post-growth annealing has lead to a record Tc of 173K in 2005. During the experimental breaktroughs, increasingly more accurate and powerful models were developed. ^[1]

Theoretical Background

The direct dipole-dipole magnetic interaction of Mn atoms is far too weak in GaMnAs to provide any reasonable ferromagnetic behavior. Instead, the origins of the ferromagnetism are due to the antiferromagnetic coupling of holes to the Mn spins which can be understood using RKKY theory and Zener’s kinetic-exchange mechanism. These models accurately predict that Curie temperature monotonically increases with substitutional Mn and hole concentration.^[2]

Most models of GaMnAs utilize the mean field theory and local spin density approximation (LSDA) calculations, which is a specific application of local density approximation (LDA). These calculations have had success in modeling important properties in GaMnAs, and lead to the expectation of room temperature ferromagnetism. Nevertheless, the effectiveness of mean field theory to model GaMnAs, especially at the high Mn region, is starting to be questioned.^[3]

Experimental Techniques

GaMnAs is grown like most semiconductors, using molecular beam epitaxy (MBE). Using high purity materials under ultra high vacuum, high quality epitaxial growth can be achieved.

The standard approach to growing samples involves first raising the temperature to 600C to allow the desorption of the oxide layer. Then a high temperature GaAs buffer layer is grown on top, usually about 100nm. This creates a smooth surface for the GaMnAs, which is then grown after lowering the temperature to around 250C. Most MBE systems have in-situ RHEED guns that can be used to determine growth rate and verify growth is epitaxial. Post-growth annealing near the growth temperature then can be used to enhance the Tc^[4]

Characterization of the GaMnAs properties can be done by measuring magnetization, magneto-transport or magneto-optics.

Magnetization

Magnetization measurements can be done by vibrating sample magnetometer but is more commonly accomplished by using a Superconducting Quantum Interference Device (SQUID). These instruments have the ability to measure magnetization as a function of both temperature and applied magnetic field. Typical graphs are shown below:

Magnetotransport

Studies of electrical transport through GaMnAs under applied magnetic field, have helped characterize important material properties. For most samples with a Mn concentration above 1 to 2 percent, the GaMnAs films show metallic behavior. In optimum annealing condition, the GaMnAs resistivity decreases compared to the as grown sample. Also, there is usually a peak in the longitudinal electrical resistance near the curie temperature. Hall effect measurements are an easy and effective method for measuring carrier concentration, an important semiconductor property.

Magneto-Optics

Optical measurements have been instrumental in determining the true nature of the ferromagnetic behavior in GaMnAs, namely the p-d exchange coupling. Besides characterization, there is an enormous amount of interest in the practical applications of using optical devices to control magnetic semiconductors.

References

1. ^ Jungwirth, T; Jairo Sinova, J. Mašek, J. Kučera, A. H. MacDonald (2006-08-11). "Theory of ferromagnetic (III,Mn)V semiconductors" (HTML abstract). Reviews of Modern Physics 78 (3): 809–864. American Physical Society. doi:10.1103/RevModPhys.78.809. Retrieved on 2007-11-09.
2. ^ Jungwirth, T; Jairo Sinova, J. Mašek, J. Kučera, A. H. MacDonald (2006-08-11). "Theory of ferromagnetic (III,Mn)V semiconductors" (HTML abstract). Reviews of Modern Physics 78 (3): 809–864. American Physical Society. doi:10.1103/RevModPhys.78.809. Retrieved on 2007-11-09.
3. ^ MacDonald, A.H.; P. Schiffer, N. Samarth (2005-03-08). "Ferromagnetic Semiconductors: Moving Beyond (Ga,Mn)As" (HTML abstract). Nature Materials 4: 195. Nature Publishing Group. Retrieved on 2007-12-12.
4. ^ Potashnik, S; K. C. Ku, S. H. Chun, J. J. Berry, N. Samarth, and P. Schiffer (2001-05-29). "Effects of annealing time on defect-controlled ferromagnetism in GaMnAs" (HTML abstract). Applied Physics Letters 78 (3): 809–864. American Institute of Physics. Retrieved on 2007-11-11.