Session Y19: Focus Session: Novel Ferromagnetic Semiconductors I

Mn dopant distribution and magnetic ordering in Mn-doped Ge ferromagnetic semiconductor

We report the spatial distribution of Mn dopant atoms in germanium, grown with molecular-beam epitaxy. Cross-sectional transmission electron microscopy shows the stripe-shaped nanostructures with 2 nm in diameter, which arise from the chemical phase separation of the Mn dopants. Electron energy loss spectroscopy and ion-channeling spectroscopy indicate that these nano-stripes have $\sim $5 times higher Mn concentration than in the dilute doped Ge host. Only about 20{\%} of total Mn atoms are incorporated in Ge lattice sites. Ferromagnetism in the doped Ge is characterized by two different ordering temperatures. A critical temperature $T_{C}$* is obtained as ferromagnetic spin clusters form, and a second transition occurs at much lower temperature \textit{Tc} at the onset of global ferromagnetic ordering. Both transition temperatures can be enhanced significantly by an appropriate post-annealing process. The Mn aggregated clusters provide seeds of magnetic spin clusters, these spin clusters expand in size with lowering temperature and percolate eventually to form infinite magnetic clusters at low temperature.\newline [1] A. P. Li, et al., Appl. Phys. Lett. \textbf{86}, 152507 (2005).\newline [2] A. P. Li, et al., Phys. Rev. B 72, 195205 (2005).   

Epitaxial Growth and Properties of Fe and Co Co-doped Ge (100) Magnetic Semiconductor Films

Structural, magnetic and electronic properties of Fe and Co co-doped Ge (100) epitaxial films, grown by combinatorial molecular beam epitaxy techniques, have been studied systematically using diffraction, magnetometry, magnetotransport techniques. These experiments reveal that this epitaxial ternary system can be grown coherently on Ge (100) substrates for total doping concentrations of the transition metal dopants as high as 14 at. {\%}, where high quality p-type magnetic semiconductor films have been synthesized with Curie temperature as high as 270 K. Above this concentration rough disordered growth occurs, which is characterized by a large number of stacking faults along the $<$111$>$ directions; no phase separation has been detected for doping concentrations as high as 20 at. {\%}. The observed magnetotransport properties exhibit large anomalous Hall coefficients and large magnetoresistance (MR) with MR ratios $>$100. They also indicate the presence of an acceptor level at $\sim $70 meV above the valence band with the resulting hole concentrations that can be controlled up to 10$^{19}$ cm$^{-3 }$by the doping concentrations. Ferromagnetic ordering, specifically the Curie temperature is shown to depend on the carrier concentration.   

Optimal doping control of magnetic semiconductors

Dilute magnetic semiconductors (DMS) with high ferromagnetic ordering temperatures ($T_{C})_{ }$have vast potential for advancing spin-based electronics or ``spintronics''. To date, achieving high-$T_{C}$ DMS typically required doping levels of order 5{\%}. Such high doping levels inevitably compromise the structural homogeneity and carrier mobility of the DMS. Here, we establish ``\textit{subsurfactant epitaxy}'' as a novel kinetic pathway for synthesizing Mn-doped germanium with $T_{C}$ much higher than room temperature, at dramatically reduced doping levels. This is accomplished by optimal control of the diffusion kinetics of the dopant atoms near the growth front in two separate deposition steps. The first involves a submonolayer dose of Mn on Ge(100) at low temperature, which populates subsurface interstitial sites with Mn while suppressing lateral Mn diffusion and clustering. The second step involves epitaxial growth of Ge at elevated temperature, taking advantage of the strong floating ability of the interstitial Mn dopants towards the newly defined subsurface sites at the growth front. Most remarkably, the Mn dopants trapped inside the film are uniformly distributed at substitutional sites, and the resulting film exhibits ferromagnetism above 400 K at the nominal doping level of only 0.2{\%}.   

Room-temperature ferromagnetism in (Zn,Cr)Te

Ferromagnetic diluted magnetic semiconductors (DMSs) are the key material to developing semiconductor spintronic devices. One of the most characteristics physical phenomena in DMS is a strong interaction between \textit{sp}-carriers and localized $d$-spins (\textit{sp}-$d$ exchange interaction) [1]. Confirmation of this interaction is essential to prove a synthesis of real DMS, and can be done directly by the magneto-optical studies such as a magnetic circular dichroism (MCD) measurement [2]. Here, we report room-temperature (RT) ferromagnetism with the \textit{sp}-$d$ exchange interaction in Zn$_{1-x}$Cr$_{x}$Te (x=0.20) [3]. Zn$_{1-x}$Cr$_{x}$Te films with x$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $0.20 were grown on GaAs (001) substrates by a molecular beam epitaxy method. No sign of a secondary phase was detected in any films by the reflection high-energy electron and X-ray diffractions. MCD spectra were measured in a transmission mode. Magnetization ($M)$ measurements were carried out using a SQUID. The $M-H $curves of Zn$_{1-x}$Cr$_{x}$Te (x=0.20) showed a ferromagnetic behavior up to about RT. Curie temperature $T_{C}$ was estimated to be 300$\pm $10 K by the Arrott plot analysis. A strong enhancement of the MCD signal at the optical transition energies of critical points of host ZnTe was observed in Zn$_{1-x}$Cr$_{x}$Te, indicating a strong \textit{sp}-$d$ exchange interaction. The MCD spectra of Zn$_{1-x}$Cr$_{x}$Te at any magnetic field could be superposed upon a single spectrum, indicating that the observed MCD signals come from a single material, that is, Zn$_{1-x}$Cr$_{x}$Te. The magnetic field dependence of MCD intensity showed the ferromagnetic feature, which coincides with the $M-H$ curves measured using a SQUID. Furthermore, the MCD data showed the same $T_{C}$ as that obtained from magnetization data. These results indicate that Zn$_{1-x}$Cr$_{x}$Te (x=0.20) is an intrinsic DMS with RT ferromagnetism. References [1] J. K. Furdyna, J. Appl. Phys. \textbf{64}, R29 (1988). [2] K. Ando, in \textit{Magneto-Optics, Springer Series in Solid-State Science,} edited by S. Sugano and N. Kojima (Springer, Berlin, 2000), Vol.128, p. 211. [3] H. Saito, V. Zayets, S. Yamagata, and K. Ando, Phys. Rev. Lett., \textbf{90} 207202 (2003).   

Room-Temperature Femtosecond Faraday Effect in CdMnTe Single Crystals

We report the subpicosecond Faraday effect, measured in high quality Cd$_{1-x}$Mn$_{x}$Te (x = 0.12 and x = 0.09) single crystals at room temperature. Using a femtosecond pump-probe technique, we were able to generate sub-picosecond current pulses by illuminating a free-standing LT-GaAs photoswitch, couple those pulses to the CdMnTe probe crystal using a coplanar transmission line, and, finally, optically sample the temporal evolution of the resulting magnetic transients with subpicosecond resolution and the excellent signal-to-noise ratio. The ultrafast (below 600 fs) Faraday rotation, responsible for the observed magneto-optical effect, has been attributed to the ultrafast spin dynamics of holes in our p-type CdMnTe crystals. The observed femtosecond Faraday effect can be the basis for a development of a magneto-optical sampling system for ultrafast, time-resolved characterization of current transients in novel electronic and spintronic devices.   

Spin-Polarization of a 2DEG in Cd$_{1-x}$Mn$_{x}$Te Quantum Wells

Low energy excitations of a spin-polarized electron gas embedded in Cd$_{1-x}$Mn$_{x}$Te quantum wells are studied by magneto-resonant Raman scattering and magneto-photoluminescence at superfluid helium temperature in the Voigt geometry. From Raman, we measured the one electron spin flip energy as a function of the bare Zeeman splitting originating from exchange interaction between $s$ electrons and localized Mn d electrons. The ratio between the spin flip energy and the bare Zeeman splitting shows a strong dependence on the electron density. From photoluminescence measurements, we extract the spin flip energy and the spin-polarization rate. Comparison of the measured spin flip energy with our theoretical model [1] shows that the Raman values are in better agreement with the theory. The photoluminescence lineshape, however, gives estimation of the spin-polarization rate that is in good agreement with our model. [1] F. Perez \textit{et al.} ACTA Physics Polonica A, \textbf{106}, 311 (2004).   

Theory of the spin EPR shift and application to Pb$_{1-x}$Mn$_{x}$Te

We consider a system with a periodic potential, spin-orbit interaction, conduction electron-local moment interaction and an applied magnetic field. We derive a theory for the spin-contribution to the electron-paramagnetic resonance shift (P$_{s})$ by considering an effective equation of motion of the Green's function in a representation defined by the periodic part of the Bloch function. The spin-EPR shift is expressed as a function of the matrix elements of the momentum, Pauli spin-operators, and conduction electron-local moment interactions. We apply the theory to calculate P$_{s}$ at Mn$^{2+}$ ion in the diluted magnetic semiconductor Pb$_{1-x}$Mn$_{x}$Te, as a function of the carrier concentration. Contributions from band-edge interactions as well as from far bands are included and their relative strengths are analyzed. P$_{s}$ is found to be anisotropic arising mainly due to spin-orbit interactions. Our results of P$_{s}$ for two typical hole densities agree fairly well with the recent experimental results for p-Pb$_{1-x}$Mn$_{x }$Te.   

Density functional study of manganese doped bulk silicon

Using a 250 atom Si supercell with two manganese impurities at the substitutional and tetrahedral interstitial positions, we find, using the density functional projector-augmented wave method, that the ferromagnetic arrangement of manganese atoms at the tetrahedral interstitial position is energetically more stable compared to that at the substitutional position. We find a half-metallic density of states (integer spin magnetic moment over the supercell) for both the interstitial and substitutional manganese. However, the total energy difference (or magnetic energy difference) between interstitial manganese in the ferro and anti-ferro spin-alignments are not large enough to conclude whether ferromagnetic or antiferro-magnetic stability is preferred in the pure Si samples doped with manganese.   

Static and dynamic magnetic response in Mn-doped InP diluted magnetic semiconductor (DMS) nanoparticles

Soft chemical synthesis of nanostructured diluted magnetic semiconductor (DMS) materials is promising for achieving single phase, ferromagnetic materials. We report the first observation of ferromagnetic ordering at 25K in Mn-doped InP nanoparticles of average size 3 nm. A hot colloidal method without any external surfactant was used to synthesize the nanoparticles. Structural and elemental characterizations established the occurrence of the zinc-blende phase of the DMS without any impurity phases. DC, AC and RF susceptibility measurements were done over a wide range in temperature and magnetic fields to probe the static and dynamic magnetic response. The samples showed a ferromagnetic transition at 25K and frozen spin state below 15K. Transverse susceptibility experiments done using a resonant radio-frequency method revealed a strong temperature-dependent effective anisotropy. Frequency dependence observed in the AC susceptibility was reminiscent of spin-glass characteristics.