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Cyclotron Resonance Studies in Ferromagnetic Semiconductors

G. A. Khodaparast and Y. H. Matsuda

The carrier-induced ferromagnetism in magnetic III-V semiconductors [1,2] has opened up several opportunities for device applications, as well as fundamental studies of a material system in which itinerant carriers interact with the localized spins of magnetic impurities. The origin of carrier-induced ferromagnetism is still controversial where reasonable agreement between theory and experiments has been noted for GaMnAs, but in contrast, the observation of Tc of 10 K for InMnAs films with carrier concentration of 1018cm-3 is inconsistent with theory [3]. Based on theoretical calculations, InMnAs with a hole concentration of 1018cm-3should have a Tc of 8 K instead of the experimentally observed 40~90 K [2]. The case for MOVPE grown films is even more complex. Films with a carrier concentration of 1018cm-3 have a Tc of 330 K and the Tc is nearly independent of carrier concentration [3]. In order to understand hole mediated ferromagnetism, probing the band structure in these materials are crucial. Cyclotron Resonance (CR) spectroscopy is an extremely powerful tool for the study of electronic states in semiconductors and here we present and compare the experimental and theoretical studies of the magneto-optical properties of p-type In1-xMnxAs and In1-xMnxSb ferromagnetic semiconductors films in ultrahigh magnetic fields oriented along [001].

Fig. 1. a) CR spectra for InMnAs and InMnSb films. The CR of InMnSb at 295 K, was measured at 10.6 μm; whereas, the other three resonances were measured at 10.7 μm. b) Calculated CR spectra for InMnAs and InMnSb comparing to the experimental results in Fig.1a. The CR resonance in InMnAs originates from a single (HH only) transition; whereas, in InMnSb it arises from multiple (both LH and HH) transitions.

Since these magnetic semiconductors usually have low carrier mobilities (typically of the order of 100 cm2/Vs), a very high magnetic field is necessary to observe CR (i.e., ωcτ > 1) where ωc is the cyclotron frequency. Since CR directly provides the effective masses and scattering times of carriers, we can obtain detailed information on the itinerancy of the carriers while in transport measurements it is normally difficult to deduce the contributions of the mass and scattering time independently. In addition, our observations provide information on the band structure, sp-d exchange parameters (α, β), and the position of the Fermi level in these material systems. Recently in GaMnAs, the tunability of the Fermi level within the impurity band can be used to control the Tc.

CR measurements were performed using CO2, H2O, and CO lasers, providing laser radiation at 10.6, 10.7, and 16.9 and 5.53 μm. Magnetic fields exceeding 100 T were generated by a single turn coil technique. The external magnetic field was applied along the growth direction and measured by a pick-up coil around the sample, placed inside a continuous flow helium cryostat. Several IR wavelengths were used as the excitation source and the transmitted signal through the sample was collected using a fast liquid-nitrogen-cooled HgCdTe detector.

Figure 1a shows the results of the CR measurements for MBE grown InMnSb (sample A) and MOVOPE grown InMnAs with 2% Mn content. The observed cyclotron mass in the InMnSb(A) is larger than that in MOVPE grown InMnAs with similar Mn contents (x ~ 0.02). The larger hole density in the InMnSb compared to the InMnAs, can shift the Fermi energy to a much higher level. The cyclotron mass can be enhanced when the resonance transition takes place between the Landau levels with higher indices. The CR of the holes in InMnSb has been observed for the first time and in the case of Fig. 1a, the CR masses are 0.057m0 at room temperature (RT) and 0.051m0 at 121 K. These are much smaller than the band edge HH mass in InSb (0.32m0). The cyclotron mobility, μCR, was extracted from the width of the resonance peaks. We have μCR = 4.8 × 102 cm2/(V s) at RT and μCR = 6.0 × 102 cm2 /(V s) at 121 K. Lowering the temperature in InMnSb increased the mobility and reduced the effective mass, where the InMnAs resonance was not affected significantly by lowering the temperature. The CR absorption spectra shown in Fig.1b, is obtained from the calculated magneto-optical absorption due to transitions between different Landau levels. From Fermi's golden rule, the magneto-optical absorption coefficients at a given photon energy and for a magnetic field perpendicular to the sample can be calculated.

The observed effective mass in the InMnAs studied here is consistent with the heavy hole (HH) mass reported in an earlier CR study of p-type MBE grown InMnAs grown on GaAs [4] but different from the observation reported in InMnAs with the GaSb buffer layer [5]. The MOVPE grown InMnSb structure shows a much higher hole mobility and a factor of 100 less hole carrier density compared to the MBE grown structures.


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Authors
  • G. A. Khodaparasta,Y. H. Matsuda, T. R. Merritta, H. Saito, S. Takeyama, D. Sahab, G. D. Sandersb, C. J. Stantonb, C. Feeserc, B. Wesselsc, X. Liud, and J. Furdynad
  • aVirginia Tech
  • bDepartment of Physics, University of Florida
  • cNorthwestern University
  • dUniversity of Notre Dame