The material properties, which arise from holes in an impurity band, created by manganese doping, depletes the valence band and shifts the Fermi level . This should enable a boost in the materials' spintronics performance

Scientists say they have solved a long-standing controversy regarding gallium manganese arsenide ((Ga,Mn)As)), a material which shows great potential for spintronic applications.

Researchers at the U.S. DOE's Lawrence Berkeley National Laboratory and the University of Notre Dame have determined the origin of the charge-carriers responsible for the ferromagnetic properties that make (Ga,Mn)As such a hot commodity in spintronics. Such devices utilise electron spin rather than charge to read and write data, resulting in smaller, faster and much cheaper data storage and processing.

Wladek Walukiewicz, a physicist with Berkeley Lab’s Materials Sciences Division and Margaret Dobrowolska, a physicist at Notre Dame, led a study that showed that the holes (positively-charged energy spaces) in (Ga,Mn)As control the Curie temperature.

This is the temperature at which magnetism is lost, and located in an impurity energy band rather than a valence energy band, as many scientists have argued. This finding opens up the possibility of fabricating (Ga,Mn)As so as to expand the width and occupation of the impurity band and thereby boost the Curie temperature to improve spintronic potential.

 

 

Schematic illustrating the presence of the impurity band between the valence and conduction band

“Our results challenge the valence band picture for gallium manganese arsenide and point to the existence of an impurity band, created by even moderate to high doping levels of manganese,” Walukiewicz says. “It is the location and partially localised nature of holes within this impurity band that drives the value of the Curie temperature.”

As a commercial semiconductor, GaAs is second only to silicon. Substitute some of the gallium atoms with atoms of manganese and you get a ferromagnetic semiconductor that is well-suited for spintronic devices. While it has been established that the ferromagnetism of (Ga,Mn)As is hole-mediated, the nature of the hole-states, which has a direct and crucial bearing on its Curie temperature, has been vigorously debated.

In semiconductors and other solid-state materials, the valence band is the range of energies in which the movement of charge is determined by availability of holes. Doping gallium arsenide with manganese can create an impurity band that depletes the valence band and shifts the Fermi level, the energy level at which the electronic states below are filled and those states above are empty.

 “The question has been whether the holes mediating the interactions of manganese spins reside in a delocalised valence band, or in a manganese-derived partially localised impurity band,” Walukiewicz says. “The valence band model assumes that a separate impurity band does not exist for manganese concentrations higher than about two-percent.”

Walukiewicz and his co-authors addressed the issue through channelling experiments that measured the concentrations of manganese atoms and holes relevant to the ferromagnetic order in (Ga,Mn)As. These experiments were carried out at Berkeley Lab’s Rutherford Backscattering facility, which is operated under the direction of co-author Kin Man Yu. The results of these experiments were then combined with magnetisation, transport and magneto-optical data performed at the University of Notre Dame.

 

 

Wladek Walukiewicz and Kin Man Yu at Berkeley Lab’s Rutherford Backscattering Spectrometry laboratory

“We were able to determine where the manganese atoms were located, what fraction of this total replaced gallium and acted as electron acceptors (meaning they created ferromagnetic-mediating holes), and what fraction was in the interstitial sites, acting as positively-charged double donors compensating for a fraction of manganese acceptors,” Walukiewicz says.

“Taking all our data together, we find that the Curie temperature of gallium manganese arsenide can be understood only by assuming that its ferromagnetism is mediated by holes residing in the impurity band, and that it is the location of the Fermi level within the impurity band that determines the Curie temperature.”

Electron spin is a quantum mechanical property arising from the magnetic moment of a spinning electron. Spin carries a directional value of either “up” or “down” and can be used to encode data in the 0s and 1s of the binary system. Walukiewicz says that understanding the factors that control the Curie temperature can serve as a guide for strategies to optimise ferromagnetic materials for spintronic applications.

“For example, with appropriate control of the manganese ions, either co-doping with donor ions, or modulation doping, we can engineer the location of the Fermi level within the impurity band to best the advantage,” he points out.

Walukiewicz says the findings of this study further suggest that it should be possible to optimise magnetic coupling and the Curie temperature for the whole family of ferromagnetic semiconductors by tuning the binding energy of the acceptor ions.

The results of this study have been published in the paper, “Controlling the Curie temperature in (Ga,Mn)As through location of the Fermi level within the impurity band” by M. Dobrowolska et al in Nature Materials, published online on 19 Feb 2012. DOIi:10.1038/nmat3250

This research was supported in part by the DOE Office of Science, and by grants from the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, and the Canadian Institute for Advanced Research.