Gallium manganese arsenide (abbreviated GaMnAs, or (Ga,Mn)As, or Ga1-xMnxAs) is a dilute III-V magnetic semiconductor based on doping gallium arsenide (GaAs) with manganese (Mn). Magnetic semiconductors are materials that exhibit both Ferromagnetism (or a similar response and useful Semiconductor properties Gallium arsenide ( GaAs) is a compound of two elements Gallium and Arsenic. Manganese (ˈmæŋgəniːz is a Chemical element, designated by the symbol Mn. Manganese atoms are substituted for gallium, creating both a local moment and acting as an acceptor, leading to an electron hole. In Physics, Astronomy, Chemistry, and Electrical engineering, the term magnetic moment of a system (such as a loop of Electric current An electron hole is the conceptual and mathematical Opposite of an Electron, useful in the study of Physics and Chemistry.
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. Spintronics (a Neologism meaning "spin transport electronics" also known as magnetoelectronics is an Emerging technology which exploits the intrinsic
In order for practical use, the curie temperature (Tc) needs to be above room temperature. The Curie point ( Tc) or Curie temperature, is a term in Physics and Materials science, named after Pierre Curie (1859-1906 Currently, the best samples have a Tc of approximately 200K. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic
Contents |
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. Ferromagnetism is the basic mechanism by which certain materials (such as Iron) form Permanent magnets and/or exhibit strong interactions with Magnets it To overcome this, a lower growth temperature of around 250C is used, allowing for non-equilibrium growth. The Celsius Temperature scale was previously known as the centigrade scale. 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 led to a record Tc of 173K in 2005. Molecular beam Epitaxy (MBE, is one of several methods of depositing Single crystals It was invented in the late 1960s at Bell Telephone Laboratories [1] During the experimental breakthroughs, increasingly more accurate and powerful models were developed. [2]
 A graph showing the highest Curie temperature attained as a function of the date for GaMnAs. |
Theoretical Background
The direct dipole-dipole magnetic interaction of Mn atoms is far too weak in GaMnAs to provide any reasonable ferromagnetic behavior. In physics there are two kinds of dipoles ( Hellènic: di(s- = two- and pòla = pivot hinge An electric dipole is a In physics there are two kinds of dipoles ( Hellènic: di(s- = two- and pòla = pivot hinge An electric dipole is a 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. In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules usuallyrelated to the spins of Electrons align in a regular pattern with neighboring RKKY stands for Ruderman-Kittel-Kasuya-Yosida and refers to a coupling mechanism of nuclear magnetic moments or localized inner d or f shell electron spins in a metal by means 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). A Many-body system with interactions is generally very difficult to solve exactly except for extremely simple cases ( Gaussian field theory, 1D Ising model. The local-density approximation (LDA is an approximation of the exchange - correlation (XC energy functional in Density functional theory (DFT by taking 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. A Many-body system with interactions is generally very difficult to solve exactly except for extremely simple cases ( Gaussian field theory, 1D Ising model. [3]
Experimental Techniques
GaMnAs is grown like most semiconductors, using molecular beam epitaxy (MBE). A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that Molecular beam Epitaxy (MBE, is one of several methods of depositing Single crystals It was invented in the late 1960s at Bell Telephone Laboratories Using high purity materials under ultra high vacuum, high quality epitaxial growth can be achieved. Epitaxy refers to the method of depositing a Monocrystalline film on a monocrystalline substrate
The standard approach to growing samples involves first raising the temperature to 600C to allow the desorption of the oxide layer. The Celsius Temperature scale was previously known as the centigrade scale. Desorption is a Phenomenon whereby a substance is released from or through a surface 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. The Celsius Temperature scale was previously known as the centigrade scale. Most MBE systems have in-situ RHEED guns that can be used to determine growth rate and verify growth is epitaxial. Reflection high-energy electron diffraction ( RHEED) is a technique used to characterize the surface of crystalline materials 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). Squid are marine Cephalopods of the order Teuthida, which comprises around 300 species These instruments have the ability to measure magnetization as a function of both temperature and applied magnetic field. Typical graphs are shown below:
 Graph of magnetization versus temperature for GaMnAs as measured by a Quantum Design SQUID |
 Graph of magnetization versus applied magnetic field (a hysteresis loop) for GaMnAs as measured by a Quantum Design SQUID |
Magnetotransport
Studies of electrical transport through GaMnAs under applied magnetic field, have helped characterize important material properties. A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges For most samples with a Mn concentration above 1 to 2 percent, the GaMnAs films show metallic behavior. The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across 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. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is Hall effect measurements are an easy and effective method for measuring carrier concentration, an important semiconductor property. The Hall effect refers to the Potential difference ( Hall voltage) on the opposite sides of an Electrical conductor through which there is an Electric
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. Magnetic semiconductors are materials that exhibit both Ferromagnetism (or a similar response and useful Semiconductor properties
References
- ^ Wang, K. Y. ; R. P. Campion, K. W. Edmonds, M. Sawicki, T. Dietl, C. T. Foxon, B. L. Gallagher (2005). "Magnetism in (Ga,Mn)As Thin Films With TC Up To 173K". ICPS-27 proceedings 772: 333–334. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
- ^ a b 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. The Reviews of Modern Physics is a journal of the American Physical Society. American Physical Society. The American Physical Society was founded in 1899 and is the World 's second largest organization of physicists behind the Deutsche Physikalische Gesellschaft. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
- ^ MacDonald, A. H. ; P. Schiffer, N. Samarth (2005-03-08). "Ferromagnetic Semiconductors: Moving Beyond (Ga,Mn)As" (HTML abstract). Nature Materials 4: 195. Nature Materials is a monthly multi-disciplinary journal aimed at bringing together cutting-edge research across the entire spectrum of materials science. Nature Publishing Group. Nature Publishing Group (NPG is an international Publishing company that publishes Scientific journals It is a division
- ^ 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. Applied Physics Letters is a weekly Peer-reviewed Scientific journal published by the American Institute of Physics devoted to the publication of new experimental American Institute of Physics. The American Institute of Physics (AIP is an international body representing Physicists and publishing physics related journals
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