Raman spectroscopy of magnetic excitations in diluted magnetic II -VI semiconductors and in ruby

Xiangshun Lu, Purdue University

Abstract

Raman electron paramagnetic resonance(Raman-EPR) of the transitions due to the spin-flip of the 3d electrons of Cr+ in Zn1-xCr xTe and Cd1-x CrxTe are observed at ħωPM=g( Cr+)μBB with g(Cr+)=2.0041 ± 0.0095 and 2.0039 ± 0.0093, respectively. Raman lines appear at ω LO ± nωPM, n=1,2 and 3, resulting from the strong Fröhlich interaction with the zone center longitudinal optic phonon LO. The intensity of Raman-EPR of Cr+ can be enhanced through the photo-generation process by the simultaneous illumination with photons having an energy close to the excitonic band gap; photoluminescence spectra reveal signatures of excitons bound to Cr+ acceptors in Zn 1-xCrxTe specimens. The resonance profile of Cr+ Raman-EPR shows that the strong resonant enhancement is mediated via an exciton bound to a neutral acceptor. Spin-flip Raman scattering (SFRS) from donor-bound electrons in II-VI diluted magnetic semiconductors containing 3d transition-metal ions, e.g., Cd1-xCrxSe and Cd1-x VxSe, allows one to explore the exchange interaction between the conductor band or donor electrons with the 3d magnetic ions, so called s-d exchange interaction. The s-d exchange energy, being the SFRS Raman shift after subtracting the intrinsic Zeeman splitting characterized by g*=0.52 for CdSe, together with the magnetization measurements of those DMSs, allow one to deduce the s-d exchange constant N0α. For Cd1-x CrxSe and Cd1- xVxSe, the N 0α values are 210 ± 13 and 269 ± 10 meV, respectively. Zn1-xFexTe, a zinc blende II-VI diluted magnetic semiconductor(DMS), exhibits a temperature independent magnetization at low temperature (van Vleck paramagnetism) as a consequence of the electronic structure of Fe2+ in its site symmetry as an isoelectronic replacement of Zn2+. The lowest level of its 5Γ 3 ground state multiplet has a Γ1 nonmagnetic level, with a Γ4 magnetic level 2.26 meV above it. The Raman spectrum of this DMS displays the Γ1 → Γ4 electronic transition (labeled in this paper Γ1→4), whose Zeeman splitting is interpreted in terms of symmetry considerations and numerical calculations. The magnetic field and temperature dependence of the spin-flip Raman line (ωSFR) of the donor-bound electron in Zn1-xFexTe exhibit characteristics typical of the van Vleck paramagnetism and, in combination with magnetization measurements, yield the s-d exchange constant N0α = 239.0 ± 10 meV. The Raman spectra also show Γ1→4 in combination with the LO phonons of Zn 1-xFexTe as a ternary alloy with an intermediate mode behavior. We have observed the Raman-EPR of the Zeeman split 4 A2 ground state of the Cr3+ ion in Al 2O3: Cr, i.e., ruby, exploiting the resonance conditions associated with the R1 line. Employing a tunable dye laser with a photon energy ħω in the vicinity of the Zeeman component of the R1 luminescence, the Stokes and anti-Stokes Raman transitions are observed with shifts corresponding to the transitions within the Zeeman levels of the 4A2 ground state split by an external magnetic field (B). The proximity of the incident and the scattered radiation to the Zeeman components of R1 leads to selective dramatic resonance enhancements of the intensities of Raman-EPR transition brought about as a function of B and ħω L. The Raman shifts in both B::ĉ and B⊥ĉ fit very well with the numerical calculations using the spin Hamiltonian for the 4A 2 ground state characterized by g::=1.982, g⊥=1.987, and 2D=0.39 cm-1 (the zero field splitting of 4 A2 state). The microscopic mechanism for the resonance enhancement involves the 'in resonance' and 'out resonance' conditions fulfilled by the virtual transitions from the sublevels of 4 A2 ground state to the sublevels of Ē (2E) by the incident and the scattered radiation in a two step process. Raman-EPR of the Ē ( 2E) excited state of R1 is also observed with g=2.43 for B::ĉ.

Degree

Ph.D.

Advisors

Ramdas, Purdue University.

Subject Area

Condensed matter physics

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