Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X22: Focus Session: Magnetism in II-VI and IV Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Igor Zutic, SUNY Buffalo Room: 324 |
Thursday, March 19, 2009 2:30PM - 2:42PM |
X22.00001: Magnetization studies of II-VI semiconductor columnar quantum dots with type-II band alignment M. Eginligil, I.R. Sellers, B.D. McCombe, W-C Chou, I.L. Kuskovsky We report SQUID magnetization measurements of MBE-grown type-II, II-VI semiconductor quantum dot (QD) samples, with and without Mn incorporation. In all samples, the easy axis is out-of-plane, possibly due to columnar QD formation that arises from strain interaction between adjacent \textit{thin} dot-containing layers. In addition, both types of QDs display a non-zero spontaneous magnetic ordering at 300 K. One set of samples consists of five-layers of (Zn,Mn)Te/ZnSe with a nominal (Zn,Mn)Te thickness of 3 nm, and ZnSe spacer thickness of 5 nm and 20 nm. These \textit{magnetic} QD samples show magnetization vs. temperature behavior that can be interpreted in terms of two independent FM phases characterized by transition temperatures T$_{C1}$ $<$ T$_{C2}$. A sample containing no Mn consists of 130 ZnTe/ZnSe layers, which forms Zn(Se,Te) QD layers separated by ZnSe spacers. Evidence of ferromagnetism is also seen in this structure, but the spontaneous magnetization is much weaker. For this sample only \textit{one phase} is seen with T$_{C}$ above 300 K. Results will be discussed in terms of magneto-polaronic effects and defect-level induced ferromagnetism. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X22.00002: Transition metal-doped Sb$_{2}$Te$_{3}$ magnetic semiconductor thin films. Chang-Peng Li, Yi-Jiunn Chien, Lynn Davies Endicott, Ctirad Uher With the doping of vanadium in tetradymite-based Sb$_{2}$Te$_{3}$, magnetic semiconductor thin films Sb$_{2-x}$V$_{x}$Te$_{3}$ have been prepared on (0001) sapphire substrates by low-temperature molecular beam epitaxy. X-ray diffraction measurements and RHEED patterns confirm single crystalline films growing along the c-axis direction. Magnetic and anomalous Hall measurements clearly show stable ferromagnetic ordering with the easy axis along c-axis direction up to Curie temperature, which increases nearly linearly with the content of V incorporated in the lattice. So far, a high Curie temperature of 213 K has been achieved for the composition of Sb$_{1.55}$V$_{0.45}$Te$_{3}$. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X22.00003: MCD investigation on Mn doped CdSe Quantum Ribbons Kritsanu Tivakornsasithorn, Xinyu Liu, Margaret Dobrowolska, Jacek K. Furdyna, Jung H. Yu, Jin Joo, Dong W. Lee, Jae S. Son, Taeghwan Hyeon, Jiwon Park, Young-Woon Kim We have successfully incorporated manganese ions into 1.4 nm thick CdSe nanoribbons, thus generating 1-dimensionally quantum confined diluted magnetic semiconductor nanostructures. A series of CdSe:Mn nanoribbons with Mn concentration ranging from 0.7 to 6.4\% have been investigated using magnetic circular dichroism (MCD) spectroscopy in order to study spin effects in these CdSe:Mn quantum ribbons. In all samples, a strong MCD signal was found at the energy about 2.9 eV (435 nm), which corresponds to the 1S$_h$-1S$_e$ transition, indicating a strong enhancement of the exciton Zeeman splitting due to the $sp-d$ exchange interaction between the CdSe nanoribbon host and the incorporated Mn$^{2+}$ ions. In addition, the magnetic field dependence of the MCD signal obeys a Brillouin function. Finally, we found that the Zeeman splitting energy $\Delta$E calculated from the MCD signal increases with the Mn concentration, which clearly demonstrates that Mn$^{2+}$ ions are indeed present in the CdSe nanoribbons. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X22.00004: Fine Structure of PbSe Colloidal Nanostructures J.G. Tischler, T.A. Kennedy, E.R. Glaser, E.E. Foos, T.J. Zega, R.M. Stroud, Al.L. Efros, S.C. Erwin Although much work has been done on PbSe nanocrystals in order to understand excited states, little is known of the ground state fine structure. Bulk PbSe is an unusual semiconductor material with a direct band gap at the L point. The band structure at this symmetry point is four-fold degenerate for both electrons and light holes, and conduction and valence bands possess similar effective masses and g-factors. In this work, we synthesized high quality PbSe nanocrystals and characterized them using transmission electron microscopy and optical methods. We probed the g-factors and fine structure of excitons in undoped PbSe quantum dots using optically detected magnetic resonance (ODMR) at 24 GHz and polarized photoluminescence in a magnetic field. The results show that the dark states in this system are active even in the absence of external magnetic fields mainly due to thermal population. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X22.00005: Tunable magnetic exchange interactions in manganese-doped inverted core-shell ZnSe-CdSe nanocrystals David Bussian, Scott Crooker, Ming Yin, Marcin Brynda, Alexander Efros, Victor Klimov Magnetic doping of semiconductor nanostructures is pursued for applications in magnetic memory and spin-based electronics. A primary goal is to control interactions between carriers (electrons and holes) and the embedded magnetic atoms. We have demonstrated$^{\ast }$ a tunable magnetic \textit{sp-d} exchange interaction between electron-hole excitations and paramagnetic Mn$^{2+}$ ions using `inverted' core-shell nanocrystals composed of Mn$^{2+}$-doped ZnSe cores overcoated with undoped shells of narrower-gap CdSe. Magnetic circular dichroism studies reveal giant Zeeman spin splittings of the band-edge exciton that are tunable in magnitude \textit{and sign}. Effective exciton g-factors are controllably tuned from -200 to +30 at 1.6 K by increasing the CdSe shell thickness, demonstrating that strong quantum confinement and wavefunction engineering in heterostructured nanocrystals can be utilized to manipulate carrier-Mn$^{2+}$ wavefunction overlaps \textit{and} the \textit{sp-d }exchange parameters themselves. $^{\ast }$D. Bussian et al., Nature Materials, \textit{in press}. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X22.00006: Observation of Exchange Enhanced Zeeman Splitting in (Zn,Mn)Se Nanowires B. J. Cooley, T. Clark, B. Liu, C. Eichfeld, E. Dickey, S. Mohney, N. Samarth, S. A. Crooker, C. E. Pryor, M. E. Flatte Magnetic semiconductor nanowires (NWs) are of potential interest as model systems for studying the physics of spin polarized 1D Fermi liquids. A high degree of spin polarization is anticipated from the sp-d exchange-enhanced Zeeman splitting of band edge states. Here, we report the vapor-liquid-solid growth of magneto-optically active (Zn,Mn)Se nanowires that show large Zeeman shifts in band edge photoluminescence (PL) at low temperatures. Transmission electron microscopy reveals the formation of single crystal wurtzite NWs oriented along the c-axis and with diameters as narrow as $\sim 10$ nm (approaching the 1D regime). Low temperature magneto-PL measurements of as- grown NW ensembles show Zeeman shifts of $\sim 5$ meV/T. The Zeeman shifts show a Brillouin-like functional dependence on magnetic field and temperature, consistent with mean field expectations for an exchange-enhanced spin splitting. We discuss the magnetic field dependence of the Zeeman shifts and the modified selection rules for PL polarization resulting from quantum confinement and the NW geometry. Supported by NSF MRSEC and the NNIN. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X22.00007: Influence of antiferromagnetic interactions on ferromagnetic properties of p-(Cd,Mn)Te quantum wells T. Dietl, A. Lipi\'nska, C. Simserides, K. N. Trohidou, A. Majhofer Modulation-doped p-type (Cd,Mn)Te/(Cd,Mg,Zn)Te quantum wells (QWs) remain a unique medium allowing to probe carrier-induced Ising-like ferromagnetism in the two-dimensional case, as in this system the mean free path is longer than the QW width [1]. However, a surprising result is the absence of hysteresis loops below the Curie temperature. In order to obtain information on mechanisms controlling spin dynamics, we have extended our previous Monte Carlo (MC) simulations combining Metropolis algorithm with the determination of hole eigenfunctions at each MC sweeps [2], and found that short range spin-spin antiferromagnetic (AFM) interactions play a crucial role in the accelerating magnetization dynamics. Moreover, we reveal that the effect of AFM interactions becomes much reduced if the thickness of the layer containing Mn spins is narrower than the extend of the hole wave function. This implies that magnetic hysteresis should be recovered in quantum wells, in which the thickness of the Mn layer would be smaller than the region visited by the holes. [1] H. Boukari et al., Phys. Rev. Lett. 88, 207204 (2002); [2] D. Kechrakos et al., ibid 94, 127201 (2005). [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X22.00008: Effects of defects on the half metallicity of a Mn/Si digital ferromagnetic heterostructure Michael Shaughnessy, Ryan Snow, Ching Fong The effects of defects on the half-metallic properties of the Mn/Si digital ferromagnetic heterostructure (DFH) (PRL \textbf{96}, 027211 (2006)) are investigated using a first principles density-functional theory approach. The half metallicity is retained when the $\delta $-layer of the Mn atoms has a vacancy or a Si imperfection. However, the the half metallicity is destroyed if the $\delta $-layer spreads because of the increased d-d bonding of the defect makes the d-p hybridization imperfect. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X22.00009: Magnetism of Mn doped in $a$-Si and $a$-Ge Juexian Cao With experimental studies and density function theory calculations, we report on the properties of Mn-doped amorphous Si and Ge which are designed to understand the fundamentals of cooperative phenomena in highly correlated electronic and magnetic systems. We observed a striking difference in Mn local moment when doped in $a-$Si and $a-$Ge matrices, in great contrast to the previous speculation that these two should behave very similar as the semiconductor host for transition metals. While we observed a large local moment of Mn in $a-$Ge, Mn moment is quenched in $a-$Si. The large difference of local magnetic moment of Mn in $a-$Si and $a-$Ge can be understood by the local atomic environment at the magnetic dopant sites, that is, the bond length and the coordination. Statistical DFT calculations shows that the magnetic dopant Mn with less coordination and large bond length hold large magnetic moment. Otherwise, the magnetic moment would be killed. In $a-$Ge, dopant Mn favours less coordination and large bond while more coordination and small bond length in $a-$Si, which result in the enhancement/quenchement of local magnetic moment Mn in $a-$Si/$ a-$Ge. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X22.00010: Local structure and spin states of Mn-Co and Fe-Co codoped Ge epitaxial films of dilute magnetic semiconductors Brian Collins, Liang He, Frank Tsui, Yong Chu, Daniel Haskel, Evgeny Kravtsov Codoping with Co has been shown to promote successful dilution of magnetic dopants of Mn and Fe into Ge up to a combined doping level of 15 at. {\%}. Local structure, magnetic moment, and spin-dependant states of these diluted magnetic semiconductors (DMS) have been probed as a function of composition using X-ray and tunneling spectroscopy techniques. Extended X-ray Absorption Fine Structure (EXAFS) measurements reveal an increased level of substitution for each dopant species with respect to traditional single dopant DMS materials. A codoped environment results in 70{\%} of Mn and 90{\%} of Co successfully incorporating substitutionally into the Ge lattice. The remaining dopants are seen to form interstitial dimmers and trimers rather than metallic precipitates. Tunneling spectroscopy measurements on these systems through Al$_{2}$O$_{3}$ barriers using both Nb and Fe as spin detectors will be discussed in their correlation with the structural results. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X22.00011: Ferromagnetic ordering of Mn-As co-doped Ge as diluted magnetic semiconductors studied within a Heisenberg model Hua Chen, Wenguang Zhu, Zhenyu Zhang To achieve room temperature ferromagnetism in group-IV-based dilute magnetic semiconductors, it is required to increase the concentration of the magnetic dopants in the host semiconductors. A recent theoretical study has suggested that the concentration of substitutional Mn in Ge can be greatly enhanced upon co-doping with As, an n-type electronic dopant [1]. Using Monte Carlo simulations based on a classical Heisenberg model with the magnetic coupling parameters calculated from first principles, we study the magnetic property of Mn and As co-doped Ge at low doping concentrations. The estimated Curie temperature increases almost linearly with increasing Mn doping concentration and reaches 264K at 5\% Mn. In contrast, at the same doping level pure Mn doped Ge does not show any finite temperature ferromagnetic ordering. [1] W. G. Zhu, Z. Y. Zhang, and E. Kaxiras, Phys. Rev. Lett. 100, 027205 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X22.00012: Pulsed Laser Melting and Resolidification of GeMn Thin Films Melissa Dolph, Wenjing Yin, Jiwei Lu, Stuart Wolf, Taeseok Kim, Michael Aziz The effect of pulsed laser melting (PLM) and resolidification on the Group-IV diluted magnetic semiconductor (DMS) GeMn is investigated. (100) Ge thin films of thickness 200nm (the top layer of a Germanium on Insulator Wafer) were ion implanted with Mn at concentrations of 0.5 - 4 atomic {\%}. Mn implantation caused amorphization of the near-surface region of the Ge film as well as the formation of Mn nanoclusters. The studies reported here focus on the use of pulsed laser melting to restore crystallinity in the Ge and to redistribute the Mn more uniformly and increase its substitutionality in the Ge lattice. The single crystal Ge at the interface between the Ge device layer and the SiO$_{2}$ acts as a seed layer for single crystal solidification. X-ray diffraction (XRD) data showed a very strong diffraction from (400) Ge at the optimum laser beam fluence. Laser melted films were also found to be ferromagnetic. The effects on the ferromagnetic behavior due to the Mn concentration and post-PLM thermal annealing conditions will also be reported and correlated with transmission electron microscopy (TEM) structure measurements. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X22.00013: Mn-implanted GeC: An Amorphous Ferromagnetic Material Samaresh Guchhait, M. Jamil, D. Ferrer, E. Tutuc, J. Markert, S. Banerjee, A. Li-Fatou, L. Colombo High energy (20 keV) Mn ions were implanted in two samples: 1) bulk Ge (100) and 2) a 250 nm thick epitaxial GeC film, grown on a Si (100) wafer. The GeC thin film was grown by UHV chemical vapor deposition using a mixture of germane (GeH$_4$) and methylgermane (CH$_3$GeH$_3$) gases and contains about 1\% carbon. The Mn implant dose was $1.1\times10^{16}$/cm$^2$ at a temperature of 300$^{\circ}$C for both samples. A SQUID magnetometer study shows ferromagnetism in both samples. The Curie temperature of the first samples is about 150 K, while that of the second sample is about 170 K. The in-plane saturated magnetic moment for the first sample is about $2.2\times10^{-5} $emu/cm$^2$ and that for the second sample is about $3.0\times10^{-5} $emu/cm$^2$. These results show clear enhancement of magnetic properties of the Mn-implanted GeC thin film over the identically implanted Ge layer due to the presence of a small amount of carbon. Further, high-resolution transmission electron microscopy showed that Mn implanted region is amorphous, without any precipitates. It is believed that it is this amorphous phase that is responsible for ferromagnetism. [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X22.00014: Magnetic transitions and giant magnetocaloric effect in Eu$_{8}$Ga$_{16}$Ge$_{30 }$clathrates A. Chaturvedi, S. Stefanoski, M.H. Phan, G.T. Woods, G.S. Nolas, H. Srikanth Semiconductors with the clathrate hydrate crystal structure have demonstrated interesting physical properties that are directly related to the fact that ``guest'' atoms reside inside ``host'' polyhedra that are formed by other species. These materials are well known for their excellent thermoelectric properties. One of the interesting ``guests'' in the clathrate structure is europium. Since the magnetic moment of Eu is large and the Eu moments order at low temperatures in Eu$_{8}$Ga$_{16}$Ge$_{30}$ clathrates, these materials are expected to exhibit interesting magnetic and magnetocaloric properties. In this work, we report on the systematic studies of magnetic and magnetocaloric properties of Eu$_{8}$Ga$_{16}$Ge$_{30}$ clathrates. The magnetic entropy change was numerically calculated from the magnetization isotherms using the Maxwell relation and giant magnetocaloric effect (GMCE) was observed. Experimental results reveal a coherent correlation between the structure, magnetic property and the GMCE in Eu$_{8}$Ga$_{16}$Ge$_{30}$ clathrates. The low-field GMCE, in addition to the absence of thermal hysteresis and field hysteresis, makes this material an attractive candidate for active magnetic refrigeration at low temperatures. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X22.00015: Cr Moment Coupling in Cr-containing Diamond-like Carbon Thin Films Juan Colon, Varshni Singh, Vadim Palshin, Andre Petukhov, Yaroslav Losovyj, Andrei Sokolov, Peter Dowben, Ihor Ketsman Structural and magnetic properties of the Cr-doped hydrogenated diamond-like carbon (Cr-DLC) and chromium carbide hydrogenated diamond-like carbon alloy thin films were investigated. Results showed promising magnetic and spintronics application. Diodes were constructed using the silicon substrate as the n-type material and Cr-DLC film as the p-type material for a wide range of Cr concentrations and temperatures. At low chromium content a high capacitance was observed, limiting the properties of the heterojunction device, however, at high Cr concentration, a large coefficient of negative magneto-resistance was observed even at room temperature, suggesting the possibility for a spintronic application. This negative magneto-resistance effect may be related to the coexistence of various material phases including chromium carbide. [Preview Abstract] |
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