Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U12: Focus Session: Diluted Magnetic Semiconductors II |
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Sponsoring Units: GMAG DMP FIAP Chair: George Kioseoglou, Naval Research Laboratory Room: Colorado Convention Center Korbel 3C |
Thursday, March 8, 2007 8:00AM - 8:36AM |
U12.00001: High Curie temperature ferromagnetism in self-organized GeMn nanocolumns. Invited Speaker: We have recently evidenced high Curie temperature (T$_{C}>$400 K) Mn-rich nano-columns self-assembled in a diluted Ge$_{0.94}$Mn$_{0.06}$ film. Their composition is close to Ge$_{2}$Mn as given by nanoscale chemical analysis (Electron Energy Loss Spectroscopy). Their average diameter, height and spacing are 3 nm, 80 nm and 10 nm respectively. Their volume fraction in the GeMn film is almost 16 {\%}. From Transmission Electron Microscopy (TEM) cross sections we could clearly evidence that nano-columns are crossing the whole GeMn film. SQUID measurements reveal a very high Curie temperature ($>$ 400 K) and Zero Field Cooled-Field Cooled (ZFC-FC) data rule out the presence of superparamagnetic nanoparticles. Growth is performed by simultaneously evaporating Ge and Mn atoms from standard effusion cells on Ge(001) single crystal substrates. The growth temperature is varied from 80 to 200\r{ }C. In the whole temperature range, Mn-rich nano-columns are clearly observed by TEM. However magnetic properties depend on the growth temperature and high T$_{C }$columns are only obtain in a very narrow temperature range around 130\r{ }C. Magnetotransport measurements have been performed with magnetic fields applied perpendicular to the film plane. A large positive magnetoresistance (up to 7000 {\%} at 30 K) in contradiction with the negative MR in granular systems or other ferromagnetic semiconductors is measured. Taking MR effects into account we have evidenced a large Anomalous Hall Effect (AHE) up to room temperature despite of the low volume fraction of nano-columns in GeMn films. The presence of AHE proves that holes are spin-polarized by crossing nano-columns. In this presentation, we will discuss the kinetic mechanisms leading to the columns formation, the composition and crystal structure of the columns as well as their magnetic properties as a function of the growth temperature and Mn concentration. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U12.00002: Dopant segregation and giant magnetoresistance in manganese-doped germanium A.P. Li, C. Zeng, K. van Benthem, M.F. Chisholm, J. Shen, S.V.S. Nageswara Rao, S.K. Dixit, L.C. Feldman, A.G. Petukhov, M. Foygel, H.H. Weitering Dopant segregation in Mn$_{x}$Ge$_{1-x}$ dilute magnetic semiconductor leads to a remarkable self-assembly of Mn-rich nanocolumns, embedded in a fully compensated Ge matrix. Samples grown at 80 \r{ }C display a giant positive magnetoresistance that correlates directly with the distribution of magnetic impurities. Annealing at 200 \r{ }C increases Mn substitution in the host matrix above the threshold for the insulator-metal transition, while maintaining the columnar morphology, and results in global ferromagnetism with conventional negative magnetoresistance. The qualitative features of magnetism and transport in this nanophase material are thus extremely sensitive to the precise location and distribution of the magnetic dopants. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U12.00003: Ferromagnetism in Mn doped Ge thin films Jiani Yu, Jiwei Lu, Kevin West, Li He, Robert Hull, Stu Wolf Exploring ferromagnetism in Group IV semiconductors is of great interest due to their potential application to spintronics. In this presentation, we discuss the ferromagnetism induced in thin Ge films by Mn$^{+}$ ion implantation as well as the correlation between their magnetism and their transport properties. The as-received Germanium on insulator (GOI) wafer consists of 200nm of (100) oriented Ge on 400nm of oxide both on a Si wafer. Mn ions were implanted at 300 $^{o}$C into the Ge layer at 200 KeV. The ferromagnetism has been observed in Ge with a range of Mn concentration from 0.5 to 2 atom {\%}. The sample with 2 {\%} Mn doping has a Curie temperature near 300K and has a moment of $\sim $ 0.7 $\mu _{B}$/Mn at 10 K. Transmission electron microscopy (TEM) reveals the formation of second phase clusters of which are probably responsible for the majority of the magnetism in this sample. In contrast, the 0.5 {\%} and 1 {\%} Mn as implanted Ge thin films behave like diluted ferromagnetic semiconductors, both have Curie temperatures is around 100 $\sim $150 K and the 0.5{\%} sample doesn't show TEM evidence of a second phase. Our data indicates that the transport properties of Mn doped Ge correlates with the magnetism. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U12.00004: Effects of complementary doping of transition metals into Ge epitaxial films Brian Collins, Liang He, Frank Tsui, Yuncheng Zhong, Stefan Vogt, Yong Chu We report structural and magnetic properties of epitaxial films of Co and Mn co-doped Ge, grown by combinatorial molecular beam epitaxy on Ge (001) substrates. A ternary epitaxial phase diagram has been determined for total doping concentrations up to 30 at. \%, where regions of coherent epitaxy and associated strain states, and regions of rough disordered growth and the nature of the disorders have been examined and identified. In the phase diagram, there are two adjacent regions in composition, one Co-rich and another Mn-rich, where pseudomorphic epitaxial growth can be achieved at combined doping concentrations as high as 17 at. \%. These values are significantly higher than those from using either of the dopants individually. The lattice constants of the Co-rich films obey the Vegard's law, i.e. a linear dependence on the concentration, while the Mn-rich counterparts do not. This finding indicates that two transition metal dopants can compensate for the internal stress caused by the individual dopants in the host lattice. Our results also show that the presence of a second dopant can significantly reduce the tendency for phase separation and disorder, especially when Mn is the primary dopant. A ternary magnetic phase diagram has been determined using the magnetooptic Kerr effect, within which there exit high quality epitaxial films of magnetic semiconductors. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U12.00005: Dopant-assisted Concentration Enhancement of Substitutional Mn in Si and Ge Wenguang Zhu, Zhenyu Zhang, Efthimios Kaxiras Incorporation of Mn atoms as magnetic impurities in bulk Si and Ge is of great importance for integrating magnetism with existing device technology. Here, we study the influence of p- and n-type electronic dopants on Mn incorporation in bulk Si and Ge, using first-principles calculations within density functional theory. We find that in bulk Si, the site preference of a single Mn atom is changed from interstitial to substitutional in the presence of a neighboring n-type dopant (P, As, Sb). In bulk Ge, a Mn atom is more easily incorporated into the lattice when an n-type dopant is present in its immediate neighborhood, forming a stable Mn/dopant dimer with both impurities at substitutional sites. A detailed analysis of magnetic exchange interactions between such dimers reveals that magnetic properties are not degraded when Mn atoms coexist with n-type dopants. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U12.00006: Interstitial Mn in Si: half-metallic heterostructures studied by density-functional theory Peter Kratzer, Hua Wu, Matthias Scheffler Adding magnetic functionality to the most common semiconductor, Si, is in its infancy. So far, research on Mn-doped Si has concentrated on substitutional Mn (Mn$_{sub})$ as done for Mn-doped GaAs and Ge, although Mn$_{sub}$ impurities in Si are energetically less stable than interstitial Mn (Mn$_{int})$. In this work, we investigate the role of Mn$_{int}$ impurities for ferromagnetism in Si, and propose a novel type of heterostructures with Mn$_{int} \quad \delta $-doping. Using density-functional theory within the generalized gradient approximation, we show that Si-based heterostructures with 1/4 layer $\delta $-doping of Mn$_{int}$ are half-metallic. For Mn$_{int}$ concentrations of 1/2 or 1 layer, the $\delta $-doped heterostructures still display a high spin-polarization of conduction electrons, about 85{\%} and 60{\%}, respectively. The proposed heterostructures are more stable than previously assumed $\delta $-layers of Mn$_{sub}$. Contrary to wide-spread belief, the present study demonstrates that interstitial Mn can be utilized to tune the magnetic properties of Si, and thus provides a new clue for Si-based spintronics materials. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U12.00007: Ge-Based Diluted Magnetic Semiconductor films on Si Zuoming Zhao, Xiaoyu Zhou, Kang L. Wang Ge-based diluted magnetic semiconductor (DMS) films with 4{\%} manganese (Mn) are grown on Si (001) substrates using molecular beam epitaxy (MBE). Surface morphology is measured by atomic force microscopy (AFM). For a 24-nm thick film, surface roughness around is around 1nm. Structure properties of the film are characterized by X-ray diffraction (XRD) and single crystal film quality by the results which only (004) Ge peak is observed. Magnetic properties are measured by a superconducting quantum interference device (SQUID). Clear hysteresis is observed at room-temperature. The results indicate that high quality Ge-based DMS can be grown on Si with good crystal quality and magnetic properties. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U12.00008: Local moment and interaction of manganese moment in amorphous silicon thin films Li Zeng, Frances Hellman, Robert Culbertson, Rafiqul Islam, David Smith, Erik Helgren Highly homogeneous Mn doped amorphous silicon ($a-$Mn$_{x}$Si, x = 1$\sim $18 at.{\%}) samples are prepared by e-beam co-evaporation to explore the local Mn moment in $a-$Mn$_{x}$Si. The relationship between the microstructure and the magnetic properties has been investigated. HR-XTEM shows that Mn atoms are uniformly distributed inside the amorphous silicon matrix, greatly exceeding the Mn solubility in crystalline silicon. Rutherford backscattering (RBS) is used to verify the film composition and the oxygen impurity. The effective moment of Mn has a nontrivial Mn composition dependence. The smaller the Mn concentration, the larger the Mn effective moment is found. The saturation moment of the sample is small, indicating a small Mn local moment. Possible explanations of the small effective moment and its concentration dependence are discussed, including direct antiferromagnetic interaction, s-d interaction. Magnetotransport is also studied for this system and magnetoresistance is found very small comparing to other magnetically doped amorphous semiconductor, such as $a-$Gd$_{x}$Si$_{1-x}$. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U12.00009: Spin waves in Mn-doped Si: exchange interactions from first-principles calculations G. Rao and J. E. Raynolds College of Nanoscale Science and Engineering, University at Albany, State University of New York Gayathri Rao, James Raynolds There has been considerable interest in magnetic semiconductors in recent years for potential applications in the field of spintronics.The present work was motivated by recent experimental achievement of above-room- temperature magnetism in Mn doped silicon (Phys.Rev.B \textbf{71}, 033302 (2005)).We present the results of Density Functional calculations that have been carried out to determine the strength of the distance-dependant exchange interaction in Mn-doped Si.The exchange interaction determines the energy difference between ferro-magnetic and anti-ferromagnetic spin configurations and as such it provides for a prediction of spin-wave velocities.Such spin waves, if they exist, are of interest in that they may provide means for transmitting spin-based information. Comparison of the relative energy differences between ferromagnetic and anti-ferromagnetic configurations for a series of Mn locations yielded the distance dependant exchange interaction J(R).Interestingly we find that the exchange interaction is negative (anti-ferromagnetic) for short and long distances and is positive (ferromagnetic) for intermediate distances.This talk will present these findings along with estimates of spin-wave velocities,densities of states,band structure and spin-density distributions. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U12.00010: Controlling magnetic, magnetotransport and optical properties of Al codoped Zn-Co-O thin films Plamen Stamenov, M. Venkatesan, L. Dorneles, J.M.D. Coey Thin films of 5{\%} Co doped ZnO were grown on C- and R-cut sapphire substrates by pulsed-laser deposition, with and without Al codoping ($x$=0--1{\%} Al). Al-doped films retain significant magnetization while exhibiting degenerate semiconductor behaviour. Magnetoresistance of these novel Co-doped ZnO semiconductor films is found to be highly dependent on Al doping and is vanishingly small at ($x>$0.2 {\%}) Large ($\sim $20 {\%} at 2 K) in-plane anisotropy of the magnetoresistance is observed, resembling an AMR effect, which is attributed to Fermi surface anisotropy and most of it has no ``ferromagnetic'' origin. The field dependence of the magnetoresistance can be explained in terms of two-band model and ionised impurity scattering. Hall-effect data indicates completely degenerate electron gas at ($x \quad >$ 0.5 {\%}). High resolution x-ray scattering and magnetisation data on samples with ($x$=0) reviles the presence of Co metal clusters ($\sim $8nm in size) that account for much or all the ferromagnetic magnetisation and exhibit temperature activated decrease of the coercive field. A huge band gap shift is observed with Al doping as a result of \textit{Burstein-Moss} effect. In view of the vanishing magnetoresistive effects at room temperature, it is clear that these Co doped ZnO samples are not dilute magnetic semiconductors. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U12.00011: Electrical transport and magnetic properties of sputtered Co-doped indium-tin oxide films Jolanta Stankiewicz, Francisco Villuendas We report results of electrical resistivity, Hall effect and magnetization measurements in Co-doped indium-tin oxide films, in a temperature range from 5 to 400 K and in magnetic fields of up to 5 T. The films were grown on fused quartz substrates, by magnetron sputtering. ITO (In$_2$O$_3$ with 10 wt \% Sn) homogeneous films doped with less than 20 at.\% of Co seem to show intrinsic FM behavior. Magnetic hysteresis loops with coercive fields of up to 100 Oe at room temperature, as well as a ferromagnetic contribution in the difference between field-cooled and zero field-cooled magnetization, are observed in these films. We find that post-growth treatment strongly affects the electrical and magnetic properties of our films. This allows us to control the electron concentration of the films by varying the temperature and/or changing the ambient gas in the annealing process. A clear correlation between the values of the magnetic moment and of the electron concentration found for the ITO films doped with 10 at.\% of Co seems to follow the predictions for a bound magnetic polaron percolation model. This suggests a carrier-mediated ferromagnetic mechanism. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U12.00012: Investigation of Mn incorporation on GaN(0001) by spin-polarized STM Yun Qi, M. Weinert, L. Li We investigate the Mn substitution of Ga on GaN(0001) by spin-polarized scanning tunneling microscopy (SP-STM) using a Fe coated W tip. The GaN films are grown by plasma-assisted MBE on 6H-SiC(0001), with a metallic pseudo-1x1 (denoted ``1x1'') surface, consists of 2.3 ML Ga on top of the Ga-terminated GaN. Mn deposition on this surface results in the formation of domains of 5x5 and $5\sqrt 3 \times 5\sqrt 3 $ structures. First principles calculations show that Mn substitution of Ga leads to virtual bound states with bandwidth of $\sim $1.5 eV, indicating significant Mn-Ga interactions. We propose that Mn substitution of Ga freezes the Ga motion in the adlayer of the ``1x1'', forcing the extra Ga atoms of the top layer to ``pop up'' and reside at the T$_{4}$ sites, forming the (5x5) reconstruction similar to the DAS structure observed on Si(111) surface. With the Fe/W tip, the regions that contain the incorporated Mn would give rise to the extra bright features that form the $5\sqrt 3 \times 5\sqrt 3 $ structure, due to the higher spin DOS at Mn sites. Implications of these results for understanding the magnetic coupling between Mn atoms in GaN will be discussed at the meeting. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U12.00013: Electrical manipulation of non-volatile spin cell based on diluted magnetic semiconductor quantum dots Ki Wook Kim Electrical manipulation of a memory cell based on a semiconductor nanostructure consisting of a diluted magnetic semiconductor quantum dot (QD) and a reservoir of itinerant holes separated by an energy barrier is investigated theoretically. The operating principle takes advantage of the paramagnetic-ferromagnetic phase transition mediated by the itinerant holes in the diluted magnetic semiconductor QD that can lead to electrically controlled Write/Erase operations. Non-volatility can be achieved when the structure is properly designed to reach a thermodynamic equilibrium at both the PM and FM configurations (i.e., bistability). Assuming a parabolic confining potential in the QD, the performance characteristics of the proposed nanostructure are analyzed including the scalability and the lifetime. An advantage of this memory concept is the extremely small dissipative energy for Write/Erase functions due to the open circuit nature of the process. A readout scheme enabling electrical detection with the repetition rate up to the 10 - 100 MHz range is also explored by utilizing only two contacts. [Preview Abstract] |
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