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Electronic Structure and Magnetism in Diluted Magnetic Semiconductors Print

The possibility of using electrons' spins in addition to their charge in information technology has created much enthusiasm for a new field of electronics popularly known as "spintronics." An intensely studied approach to obtaining spin-polarized carriers for data-storage devices is the use of diluted magnetic semiconductors created by doping ions like Mn, Fe, or Co having a net spin into a semiconducting host such as GaAs, ZnO, or GaN. The interaction among these spins leads to ferromagnetic order at low temperatures, which is necessary to create spin-polarized carriers. A research team working at ALS Beamline 4.0.2 and European Synchrotron Radiation Facility Beamline ID8 made a big leap forward in clarifying the microscopic picture of magnetism and anisotropy in Mn-doped GaAs by resolving localized and hybridized d states using angle-dependent x-ray magnetic circular dichroism (XMCD) measurements.

Three types of semiconductors: (a) nonmagnetic semiconductor, which contains no magnetic ions; (b) diluted magnetic semiconductor (DMS), i.e., a cross between a nonmagnetic semiconductor and a magnetic transition-metal (TM) element, in a paramagnetic state; (c) DMS with ferromagnetic order mediated by charge carriers (holes).

Spin and Charge in Dilute Magnetic Semiconductors

"When will this finally be as easy as switching on the light?" many people ask themselves every time they turn on their computer then wait for the operating system and programs to load from the hard drive into random access memory (RAM). The answer might be "not far in the future." Currently, most information is stored in a nonvolatile way in magnetic bits on hard drives that make use of electrons' spin (their orientation up or down), while semiconductor devices like RAM operate by manipulating electron charge. But the ability to manipulate electrons' spin together with controlling their charge flow could create a host of new capabilities for computer technology—like eliminating the need for lengthy boot-up times. This is the topic of an exciting research field known as "spintronics."

Manipulating an electron's magnetic state in a semiconductor device is the key to successful spintronics, and the simplest way to do that is by using a semiconductor material such as gallium arsenide (GaAs), which incorporates magnetic elements like manganese (Mn). A major obstacle to developing such devices is creating magnetic semiconductor materials that work at room temperature. In their research, Edmonds et al. correlate the electronic and magnetic characteristics of electrons in manganese-doped gallium arsenide. Their results are crucial for understanding and further development of a new class of semiconductors—dilute magnetic semiconductors.

Mn-doped GaAs, in which the Mn dopant provides both a magnetic moment and a spin-polarized charge carrier, has attracted considerable interest as spintronics material. However, the microscopic picture of magnetism and magnetic anisotropy (direction dependence of the magnetic properties) in this system is still hotly disputed. Are the Mn states localized, strongly hybridized with the GaAs valence band, or do they form a separate impurity band? To further understand this system, researchers used x-ray absorption spectroscopy (XAS) and XMCD to study (Ga,Mn)As samples. XAS measures excitation from the Mn 2p to 3d levels, thus probing the unoccupied valence states with Mn 3d character. XMCD measures the difference (dichroism) between absorption spectra obtained with opposite alignments of the sample magnetization direction and x-ray helicity vector.

Top: Mn L2,3 absorption spectra for parallel and antiparallel alignment of polarization and magnetization when the magnetization is aligned along the [001] (black) and [111] (green) directions. Bottom: XMCD spectra for magnetization along [001] (black) and [111] (green). The pronounced differences between the absorption spectra and the observed anisotropy in the pre-edge features in the XMCD signal (shown in the inset) are most remarkable.

XAS and XMCD spectra measured along two directions—[111] and [001]—show pronounced differences. Detailed study of the angular dependence illustrates that almost all spectral features, including pre-edge feature A (peak A), exhibit cubic symmetry about the crystalline axes. Only the pre-edge feature B (peak B) shows a gradual increase going from out-of-plane to in-plane magnetization in both the (100) and (110) planes—i.e., uniaxial symmetry.

In annealed (Ga,Mn)As, Mn occupies Ga sites with tetrahedral symmetry. However, the (Ga,Mn)As/GaAs(001) films are placed under compressive strain, breaking the symmetry between in-plane and out-of-plane directions. This leads to a large uniaxial magnetic anisotropy. Thus, while almost all spectral features share the cubic symmetry of the Mn site, peak B reflects uniaxial symmetry of the strain field.

To determine the origin of peak B, the researchers compared experimental results to atomic multiplet calculations, which reproduce almost all of the multiplet structure of the Mn L2,3 XMCD and correctly predict the angular dependence of those features, with one notable exception. The calculated spectra only show a single peak in the pre-edge region, peak A. Peak B in the experimental spectrum is not reproduced by the atomic calculation, so it must be of a different origin. Studying the size of peak B versus hole concentration, ρ, obtained from Hall measurements, shows a clear correlation, with peak B becoming more negative with increasing ρ. The intensity of peak B is thus dependent on the Fermi level position (level of the least tightly held electrons), indicating that this feature corresponds to transitions to states at or just above Fermi energy (EF).

Dependence of the XMCD signal of spectral features A and B on the out-of-plane (001) angle θ. Open circles represent the angular dependence in the (110) plane, solid symbols indicate the results for the (100) plane. Dashed lines show the angular dependence expected in cubic (uniaxial) anisotropy. At right is the dependence of the pre-edge features in Mn L2,3 XMCD spectra on the hole density obtained from Hall measurements.

The uniaxial anisotropy and the correlation with hole density indicate peak B is due to hybridization of Mn d states with strain-split GaAs valence states at EF. The results are clear evidence of a small, but finite, density of unoccupied Mn d states close to EF. Thus, both localized atomic-like states lying far above the Fermi level and Mn 3d states strongly hybridized with the valence bands of the GaAs host are observed. The ability to separately resolve localized and hybridized d states makes angle-dependent XMCD a powerful method for determining the electronic structure of magnetic semiconductors.

Research conducted by K.W. Edmonds, A.A. Freeman, N.R.S. Farley, R.P. Campion, C.T. Foxon, and B.L. Gallagher (University of Nottingham, UK); G. van der Laan and T.K. Johal (Daresbury Laboratory, UK), and E. Arenholz (ALS).

Research funding: U.K. Engineering and Physical Sciences Research Council, U.K. Council for the Central Laboratory of the Research Councils, Royal Society, and U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: K.W. Edmonds, G. van der Laan, A.A. Freeman, N.R.S. Farley, T.K. Johal, R.P. Campion, C.T. Foxon, B.L. Gallagher, and E. Arenholz, "Angle-dependent x-ray magnetic circular dichroism from (Ga,Mn)As: Anisotropy and identification of hybridized states," Phys. Rev. Lett.96, 117207 (2006).