Magnetic, heat capacity, and magnetoresistance measurements on II-VI and III-V magnetic semiconductors

Thomas Mark Pekarek, Purdue University

Abstract

We have taken magnetization and calorimetric measurements on $\rm Zn\sb{1-{\it x}}Cr\sb{{\it x}}Te\ ({\it x}=0.003)$ and $\rm Cd\sb{1-{\it x}}Cr\sb{{\it x}}Te\ ({\it x}=0.0035).$ Magnetic measurements on single crystal $\rm Zn\sb{1-{\it x}}Cr\sb{{\it x}}Te$ samples are anisotropic ($\sim$7% at 3.67 K in 6 T) with the (111) direction having the greatest value. The theory predicts an anisotropy of only $\sim$0.4% for $\rm Cd\sb{1-{\it x}}Cr\sb{{\it x}}Te$ which is within the uncertainty level of the magnetometer (the experimental results show a slight anisotropy of 0.8 $\pm$ 0.9% at 5.2 K in 6 T.). A Curie-Weiss fit to the magnetization's temperature dependence in $\rm Cd\sb{1-{\it x}}Cr\sb{{\it x}}Te$ yields an effective nearest-neighbor exchange of $J\sb{eff}/k\sb{B}={-}0.3\pm1$ K. This is the lowest reported value for $J\sb{eff}/k\sb{B}$ to date for a II-VI DMS system and the only reported value for a chromium based II-VI DMS system. The magnetization data for both $\rm Zn\sb{1-{\it x}}Cr\sb{{\it x}}Te$ and $\rm Cd\sb{1-{\it x}}Cr\sb{{\it x}}Te$ were fit with a model including a static Jahn-Teller distortion proposed previously in these materials (J. T. Vallin, G. A. Slack, S. Roberts, and A. E. Hughes, Phys. Rev, B 2, 4313 (1970)). We have found that the magnetic behavior in fields up to 6 Tesla and temperatures between 1.5 and 324 K for both systems is well described by a theoretical model with crystal field, Jahn-Teller, spin-orbit, and spin-spin terms including ligand field corrections to the spin-orbit and spin-spin parameters. Good agreement was found between the experimental results and the theoretical calculations for a Jahn-Teller splitting of 370 and 320 cm$\sp{-1}$ and effective spin-orbit parameters of $-$49.9 and $-$59.4 cm$\sp{-1}$ for $\rm Cd\sb{1-{\it x}}Cr\sb{{\it x}}Te$ and $\rm Zn\sb{1-{\it x}}Cr\sb{{\it x}}Te,$ respectively. The key feature is an orbital singlet, S = 2 ground state with a small splitting (1.5 K for $\rm Cd\sb{1-{\it x}}Cr\sb{{\it x}}Te$ and 12.4 K for $\rm Zn\sb{1-{\it x}}Cr\sb{\it x}Te)$ of the $m\sb{s}$ states. Heat capacity data for $\rm Cd\sb{1-{\it x}}Cr\sb{{\it x}}Te$ reveal a significant excess even at 1 K which cannot be accounted for by the lattice or electronic states of the chromium. The heat capacity measurements on $\rm Zn\sb{1-{\it x}}Cr\sb{{\it x}}Te$ show a Schottky peak indicating an energy level splitting of 3.1 K between the ground and first excited states. We also observe an excess in the heat capacity of $\rm Zn\sb{1-{\it x}}Cr\sb{{\it x}}Te$ above 1.5 K. The excess in both systems is believed to arise from additional low energy excitations. We also present magnetization measurements on Fe$\sb3$GaAs clusters distributed throughout a layer 170 nm thick at the surface of a GaAs wafer. The clusters have 1.5, 4.4, and 28 nm mean diameters and effective moments of 240, 6 000, and 10 000 Bohr magnetons. We have obtained the first magnetoresistance data on low iron-concentration samples ($\sim$1%) showing a large negative magnetoresistance (3.2% at 5 K in 0.5 T) attributed to the imbedded clusters in $\rm In\sb{0.53}Ga\sb{0.47}As.$ Magnetic measurements confine these clusters, obtained by ion implanting $\rm In\sb{0.53}Ga\sb{0.47}As$ followed by a rapid thermal anneal, are superparamagnetic. There are strong similarities between this system and the GMR system containing Co precipitates in Cu. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Crooker, Purdue University.

Subject Area

Condensation|Electrical engineering

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