Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M30: Focus Session: Magnetism in Carbon and Nanoclusters |
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Sponsoring Units: GMAG DMP Chair: Enrique Cobas, Naval Research Laboratory Room: 206B |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M30.00001: Synthesis of graphene like graphite sheets and its ferromagnetism Rakesh Chaudhary, "Max" Qinhong Hu, Ali R. Koymen Scientific reports of experimental studies on magnetism of graphite have suggested that disorder and hydrogen adsorption on the surface in graphite may trigger ferromagnetism. In this study we report magnetism of few layers of graphene like graphite sheets prepared by plasma in the cavitation field of toluene due to ultrasonication. Transmission electron microscopy (TEM) observations show transparent planar graphene like sheets that are few microns in size. Number of graphene layers observed using high resolution transmission electron microscopy (HRTEM) are $\sim $10 to 35. X-ray diffraction (XRD) and Raman spectroscopy of the powder sample suggest some degree of disorder in the crystal structure. Magnetic properties of synthesized powder measured using vibrating sample magnetometer (VSM) showed a saturation magnetization of $\sim $0.05 emu/g and coercivity of $\sim $75 Oe at room temperature. We explore the uncommon and new, top down synthesis route of few layers of graphene sheets and their magnetic properties. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M30.00002: Edge Magnetism in Graphene Nanoflakes Tanusri Saha-Dasgupta, Mukul Kabir We will discuss the manipulation of magnetism in small sized hexagonal graphene nanoflakes. We explore possible ways to manipulate the intrinsic edge magnetism in hexagonal graphene nanoflake with zigzag edges, using density functional theory supplemented with on-site Coulomb interaction. The effect of carrier doping, chemical modification at the edge, and finite temperature on the edge magnetism has been studied. The magnetic phase diagram with varied carrier doping, and on-site Coulomb interaction is found to be complex. Chemical modification of the edge atoms by hydrogen leads to partial quenching of local moments, giving rise to a richer phase diagram. We further report the influence of temperature on the long-range magnetic ordering at the edge using ab initio molecular dynamics. These findings will have important implications in controlling magnetism in graphene based low dimensional structures for technological purpose, and in understanding varied experimental reports.[*] \\ $^*$ Mukul Kabir and T. Saha-Dasgupta, Phys. Rev. B 90, 035403 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M30.00003: Determination of superexchange correlations in magnetically substituted graphene C.B. Crook, C. Constantin, T. Ahmed, J.-X. Zhu, A.V. Balatsky, J.T. Haraldsen We investigate the electronic and magnetic properties between two homogeneous magnetic impurities (vanadium, chromium, or manganese) in a 128-atom graphene superlattice. With varying the impurity distance, we calculate these properties using a first principles approach. For each configuration, we determine the electronic bandstructure and density of states, along with the Mullikan populations for each atom. Furthermore, we calculate the exchange parameter between the two magnetic ions through the analysis of the change in total energy for different magnetic configurations. We found that the magnetic impurities induce a mangetic moment in the graphene superlattice, helping to meditate the superexchange between the impurities. Depending on the choice of ion used, the interactions between the two ions can exhibit either a ferromagnetic or an antiferromagnetic behavior. These correlations indicate an RKKY-like behavior in the system. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M30.00004: ABSTRACT MOVED TO V1.00139 |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M30.00005: Magnetic Interaction Study on Multi-Walled Carbon Nanotubes filled with Core/Shell Iron/Iron Oxide Nanoparticles K. Stojak Repa, D. Israel, H. Khurshid, J. Alonso, M.H. Phan, H. Srikanth 1-dimensional magnetic nanostructures composed of carbon nanotubes (CNTs) filled with magnetic nanoparticles (NPs) are promising for a variety of applications such as electromagnetic interference shielding and biomedical engineering. Here, we present results from CNTs synthesized using commercial 200nm pore-size templates filled with core/shell (CS) Fe/$\gamma $-Fe$_{2}$O$_{3}$ NPs of $\sim$ 10 nm diameter. CS NPs were synthesized using thermal decomposition; CNTs were made by following a catalyst-free CVD. CNTs were filled with CS NPs using magnetically-assisted capillary action. TEM results indicate spherical NPs with core/shell morphology inside CNTs. Magnetometry results indicate that free-standing CS NPs and filled CNTs share a similar magnetic interaction mechanism. However, the overall magnetic properties appear to have been enhanced after filling CNTs with CS NPs. This is observed from increased blocking temperature ($\sim$ 50K) when CS NPs are enclosed within CNTs, increased anisotropy and longer relaxation time due to enhanced dipolar interparticle interactions as probed by Vogel-Fulcher fitting of AC susceptibility data. Both 0- and 1-dimensional structures retain room-temperature superparamagnetism. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M30.00006: Synthesis and properties of ferromagnetic nanostructures embedded within a high-quality crystalline silicon matrix Girish Malladi, Mengbing Huang, Thomas Murray, Steven Novak, Akitomo Matsubayashi, Vincent LaBella, Hassaram Bakhru Ferromagnetism in transition metal implanted Si has been reported earlier but unavoidably high density of structural defects in such materials render the realization of spintronic devices unviable. We report an implantation approach enabling the synthesis of a ferromagnetic layer within a relatively defect free Si environment using an additional implant of hydrogen (range: $\sim$ 850 nm) in a region much below the metal implanted layer (range: $\sim$ 60 nm). Upon annealing at different temperatures, nanocavities created within the H-implanted region act as gettering sites for the implanted metal, forming metal nanoparticles ($\sim$ 10-25nm and density $\sim$ 10$^{11}$/cm2) in Si region of excellent crystal quality. The magnetization properties measured using a SQUID magnetometer show transition from superparamagnetism to ferromagnetism-like, with ferromagnetism persisting at 300K. This transition is attributed to changes in both the amount of Ni in the nanoparticles and inter-particle distances using SIMS and RBS. RBS/Channeling and TEM show a fully recovered crystalline Si adjacent to these Ni nanoparticles. The magnetic switching energy barrier, $\sim$ 0.86 eV, is about order of magnitude higher compared to nanoparticles on Si surface or silica matrices. Preliminary electrical measurements on these devices show $\sim$ 10{\%} MR at 1T, 300K. These results show promise towards implementing spintronic devices in Si. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M30.00007: Helical spin order in Fe nanoislands Soo-hyon Phark, Kohji Nakamura, Jaison Fischer, Marco Corbetta, Safia Ouazi, Dirk Sander, Juergen Kirschner We report a spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM/S), with an atomic scale resolution, of individual nanostructures of biatomic-layer-high Fe on Cu(111). SP-STM/S of the Fe nanoislands reveals a magnetic stripe phase with a period of 1.28 nm, which is identified as a one-dimensional helical spin order [1]. \textit{Ab initio} calculations identify reduced-dimensionality-enhanced long range antiferromagnetic interactions as the driving force of this spin order, whereas the contribution of the spin-orbit coupling is negligible. In addition, energy-resolved SP-STS mapping provide a spatially-resolved and spin-dependent electronic structure of this helical spin order. The wave vector describing the spin order remains constant in the energy range --0.8 to $+$0.6 eV, whereas the spin contrast shows dissipation features around and sign change across the Fermi energy. We discuss the results in view of an energy gap opening associated with the non-collinear spin order. Our result identifies a novel aspect of SP-STM/S to characterize complex spin order with respect to the corresponding spin-dependent electronic band structure. \\[4pt] [1] S. Phark et al., \textit{Nat}. \textit{Commun}. 5 DOI: 10.1038/ncomms6183 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M30.00008: Unconventional top down synthesis of FeNi core and C shell magnetic nanoparticles Rakesh Chaudhary, Ali R. Koymen Carbon encapsulated FeNi nanoparticles were prepared by a top down approach using plasma in organic solvents. FeNi core-Carbon shell morphology of nanoparticles have been observed using transmission electron microscopy (TEM). FeNi nanoparticles prepared in toluene are of diameter 3-820 nm and encapsulated by a 3-60 nm shell. FeNi nanoparticles prepared in ethanol are of diameter 15-820 nm and encapsulated by a 4-34 nm shell. Using x-ray diffraction (XRD) the core crystal structure of the nanoparticles showed an increase in lattice constant from a$=$3.575 {\AA} to a$=$3.6 {\AA} in both cases. The FeNi nanoparticles have face centered cubic (FCC) crystal structure. No oxide formation in these core-shell nanoparticles was observed using energy dispersive x-ray spectroscopy (EDX) and XRD. Synthesized nanoparticles using toluene and ethanol showed a saturation magnetization of $\sim $1 emu/g and $\sim $3 emu/g and moderate coercivity $\sim $40 Oe and $\sim $10 Oe respectively at room temperature. We explore the uncommon and new, top down synthesis route of bimetallic nanoparticles of FeNi phase using plasma. These carbon-encapsulated FeNi nanoparticles could have potential applications in biomedicine, especially hyperthermia. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M30.00009: Facile Synthesis of L10 FePt/Reduced-Graphene Oxide Nanocomposites Xiaocao Hu, Vasilis Tzitzios, David Sellmyer, George Hadjipanayis FePt nanoparticles (NPs) have attracted much attention recently for applications in high density recording media and catalysis. Usually the as-made FePt NPs are in the fcc phase and must be annealed at high temperatures to be converted to the L10 (fct) structure and because of this the particles agglomerate. In this paper, we used a novel fabrication method to directly prepare FePt NPs in fct phase on the surface of R-GO (Reduced Graphene oxide). A layered bimetallic precursor Fe(H2O)6]PtCl6 was mixed in water environment with exfoliated GO.The precipitates were annealed under a forming gas (5{\%} H2 and 95{\%} Ar)atmosphere at different temperatures varying from 500 $^{\circ}$C to 950 $^{\circ}$C. During the annealing process, the layered precursor [Fe(H2O)6]PtCl6 was reductively decompose directly to the L10 FePt crystal structure while the GO reduced by H2 atmosphere. Transmission electron microscopy (TEM) results showed that the FePt NPs have the L10 structure and a uniform size distribution with their average particle size in the range from 5 nm to 28 nm depending on the annealing temperature used. The fct particles had coercivity values in the range of 6 kOe-9 kOe. Furthermore,, it is clear that the FePt nanoparticles are formed only on the surface of the graphene and no individual particles were observed off the graphene. The L10 FePt NPs are still isolated on the surface of the graphene and the particle size remains quite small (8 nm) even at annealing temperatures as high as 750 $^{\circ}$C. Work supported by DOE-BES-DMSE (Grant No. DE-FG02-04ER4612) [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M30.00010: Ferromagnetic to antiferromagnetic transition in the Fe$_{1/3}$(Ta$_{1-x}$Nb$_{x}$)S$_{2}$ layered dichalcogenides Chih-Wei Chen, Jesse Adams, Will Hardy, Douglas Natelson, Emilia Morosan Fe$_{1/3}$TaS$_{2}$ is known to order ferromagnetically with $T_{C}$ = 40 K, while the isostructural Nb compound orders antiferromagnetically below $T_{N}$ = 40 K. The Ta-Nb solid solution provides an opportunity to search for a quantum phase transition (QPT) as we tune the magnetic order in Fe$_{1/3}$TaS$_{2}$ by doping between T = Ta and T = Nb. We will analyze magnetization, specific heat, and resistivity data to search for signatures of a $T$ = 0 transition (QPT). Additionally, we will explore the magnetoresistive effects in these Fe-intercalated TS$_{2}$ (T = Ta, Nb) and compare with our previous results on Fe$_{x}$TaS$_{2}$ ($x$ = 0.25, 0.28). Small departures from the superstructure Fe compositions ($x$ = 0.25, 0.33) in Fe$_{x}$TaS$_{2}$ resulted in two orders of magnitude increase in the magnetoresistance. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M30.00011: Influence of interparticle interactions on the blocking temperature and high frequency permeability of Fe$_{3}$O$_{4}$ nanoparticle systems Md Ehsan Sadat, David B. Mast, Donglu Shi, Sergey L. Bud'ko In N\'{e}el's model of superparamagnetism, magnetization and relaxation dynamics depend on the magnetic anisotropy constant of the non-interacting, individual nanoparticles (NPs). However, NP interactions modify these dynamics in real systems. To investigate the influence of magnetic interactions on NP anisotropy, we compare the results from measurements of the blocking temperature of Fe$_{3}$O$_{4}$ NP systems with very different average interparticle separations, one with uniformly dispersed NPs and the other with the NPs tightly confined in a polystyrene matrix. The blocking temperature for the confined NPs (263K) was substantially higher than for the uniformly dispersed NPs (131K), which we attribute to stronger dipolar interactions. The relaxation times are determined from peaks in the imaginary part of the complex permeability versus frequency from 10 MHz - 3 GHz. For uniformly dispersed Fe$_{3}$O$_{4}$ NP (in hydrocarbon carrier), the N\'{e}el relaxation and the gyromagnetic resonance were observed at 41 MHz and 1.2 GHz, respectively, which corresponds to an anisotropy constant of $\sim$ 15 KJ/m3 compared to 12 kJ/m3 predicted by N\'{e}el's relaxation model for 10 nm diameter Fe$_{3}$O$_{4}$ NPs. Details of this correlation between the blocking temperature and high frequency permeability will be discussed. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M30.00012: Novel magnetic and electric properties of small pure gold clusters Lei Ma, Ramiro Moro, Baiqian Zhang, John Indergaard, Ilia Larkin, Hannu H\"akkinen, Markus Kindermann, Walt de Heer Cryogenic molecular beam magnetic and electric deflections of neutral gold clusters AuN (up to N$=$25), demonstrate novel properties. In contrast to normal spin half clusters that exhibit symmetric Stern-Gerlach deflections with a high-field-seeking and a low-field-seeking component, here we show that small odd-N gold clusters that are cryogenically fully cooled exhibit only the high-field-seeking (paramagnetic) component. In contrast, the magnetization of undercooled, metastable gold clusters almost exactly reverses so that they are highly diamagnetic. Electric deflections of gold clusters are also anomalous. Analogously, while the polarizabilities of equilibrated clusters are normal, in the undercooled condition they are essentially equal and opposite (with an offset) to the fully cooled case. Some clusters even have negative polarizabilities so that they are repelled by the electric field. Giant diamagnetism and negative polarizabilities, reported here for the first time, are observed in gold clusters, but not in copper clusters. A simple two-level model explains some of the effects and suggests laser applications. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M30.00013: Effect of confinement on spin polarization and magnetism of Co$_{2}$Si nanoclusters David Sellmyer, Balamurugan Balasubramanian, Priyanka Manchanda, Ralph Skomski, Pinaki Mukherjee, Bhaskar Das, George Hadjipanayis Size-modified electronic structure and surface effects can lead to unusual magnetic ordering, modified ordering temperatures, and different spin structures in nanoclusters as compared to the corresponding bulk alloys. Thus nanoclusters can be used as building blocks to create new complex magnetic nanostructures for potential applications.\footnote{B. Balamurugan, D.J. Sellmyer et al. \textit{Sci. Rep}. \textbf{4}, 6265 (2014); \textit{Adv. Mater.} \textbf{25}, 6089, (2013); \textit{Appl. Phys. Lett.} \textbf{101}, 122407 (2012).} We show room-temperature ferromagnetic ordering in Co$_{2}$Si nanoclusters with relatively large magnetic moments (0.49 $\mu_{\mathrm{B}}$/Co at 300 K and 0.70 $\mu_{\mathrm{B}}$/Co at 10 K) and magnetocrystalline anisotropy ($K_{1} \approx $ 4 Mergs/cm$^{3}$ at 10 K), as contrasted to very weak itinerant magnetism in bulk Co$_{2}$Si (0.001 $\mu_{\mathrm{B}}$/Co at 300 K and 0.072 $\mu_{\mathrm{B}}$/Co at 10 K). The DFT and analytical model calculations explain the size-dependence of the observed magnetic moments on the size of Co$_{2}$Si nanoclusters, which vary from 0.6 to 30 nm, by a surface-induced spin polarization of nanoclusters. [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M30.00014: Magnetic properties of ternary and quaternary transition metal nanoclusters Jaime Souto Casares, James Chelikowsky The magnetic properties of transition-metal nanostructures is a topic that has attracted much interest, mainly because of the dramatic difference that exists in comparison with the bulk. Without a proper knowledge of the fundamental mechanism behind the magnetic phenomena, it is hard to predict the properties of complex materials. We investigate the electronic and magnetic properties of nanoclusters made of ternary and quaternary compounds of transition metals (Fe, Co, and Ni) using PARSEC, a real-space implementation of pseudopotentials within density functional theory. Our code is well suited for the study of isolated structures because of its implementation of very efficient techniques for solving the Kohn-Sham equations. The real-space nature of PARSEC allows the use of proper boundary conditions, imposing the electronic wave functions to vanish outside a spherical domain. With these computational tools we can cover a wide spectrum of magnetic alloys and look for specific magnetic properties. This work is supported by NSF grant DMR 14-35219. [Preview Abstract] |
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