Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W30: Focus Session: Frontiers in Magnetism I |
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Sponsoring Units: GMAG DMP Chair: Christian Binek, University of Nebraska Room: 206B |
Thursday, March 5, 2015 2:30PM - 3:06PM |
W30.00001: Antiferromagnetic coupling in ferrimagnetic hard-soft core/shell nanoparticles Invited Speaker: Josep Nogues The coupling between different magnetic layers in thin film bi-layers and multilayer systems is usually ferromagnetic (FM) (layers parallel to each other). However, other types of couplings such as antiferromagnetic (AFM) (i.e., antiparallel layers) have also been reported. In contrast, the magnetic properties of bi-magnetic core/shell nanoparticles remain relatively unexplored. While Monte Carlo simulations have probed the effects of different types of interface couplings from the theoretical point of view (e.g., FM vs. AFM coupling), experimental work so far has only reported ferromagnetic coupling between the counterparts. Here we present the existence of an interfacial AFM coupling in ferrimagnetic (FiM) soft/hard and hard/soft core/shell nanoparticles based on iron and manganese oxides [1]. Narrow size distributed Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$/Mn$_{\mathrm{3}}$O$_{\mathrm{4}}$ and Mn$_{\mathrm{3}}$O$_{\mathrm{4}}$/Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ core/shell, soft/hard and hard/soft, were synthesized by seeded growth. In contrast to conventional systems, the temperature dependence of the magnetization, M, and the ferromagnetic resonance field, H$_{\mathrm{R}}$, show a downturn at the magnetic ordering temperature of the hard Mn$_{\mathrm{3}}$O$_{\mathrm{4}}$ phase (T$_{\mathrm{C}}$(Mn$_{\mathrm{3}}$O$_{\mathrm{4}})=$40 K). This decrease in M and H$_{\mathrm{R}}$ can be linked to an antiferromagnetic coupling between both phases. Moreover, element selective X-ray magnetic circular dichroism (XMCD) spectra and hysteresis loops confirm that the magnetization of the Mn-containing phase lies opposite to the Fe-containing phase. Magnetometry hysteresis loops show that for small cooling fields the loop shifts towards negative fields similar to exchange bias in conventional FM/AFM systems. However, for large cooling fields the loops shift to the opposite direction, i.e., positive exchange bias. Finally, Monte Carlo simulations clearly confirm that an AFM interface coupling leads to a magnetization decrease at low temperatures and a positive exchange bias for large cooling fields.\\[4pt] [1] M. Estrader et al. Nat. Commun. 4, 2960 (2013) [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W30.00002: Electronic, magnetic and oxidation properties of Co and Co$_{\mathrm{1-x}}$Ni$_{\mathrm{x}}$ subnanoscopic cylinders Eugenio Vogel, J. Martin Montejano-Carrizales, Fautino Aguilera-Granja One of the most convenient shapes among magnetic nanoparticles are cylinders (solid or hollow). They can be made out of different materials, they are easy to produce in different sizes, and they are relatively easy to handle. One of the present aims at present it to make these particles as small as possible. Which is the minimum stable cylindrical shape possible? Can they grow from a seed to make large particles? What are their physical properties at this scale where quantum mechanics operate? Some of these questions have been recently answered for the case of cylinders formed by Co atoms [F. Aguilera-Granja, J.M. Montejano-Carrizales, E.E. Vogel, Eur. Phys. J. D (2014) \textbf{68}:38]. In this presentation we want to get deeper into this problem in particular considering binary magnetic alloys like it is the case when Ni atoms substitute for Co in the original stable structures. We invoke program SIESTA to study different configurations. Among the results to be reported are the following: cylindrical clusters are stable to any length at this scale; there is an insulator-conductor transition at a certain length; magnetic moments are not uniformly distributed; Ni atoms tend to go to the periphery; Ni alloys tend to be more resistant to oxidation. [Preview Abstract] |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W30.00003: Structure and magnetic properties of Co/CoO core-shell nanowires Kinjal Gandha, Kevin Elkins, Narayan Poudyal, J. Ping Liu Cobalt nanowires with high coercivity have been synthesized via a solvothermal chemical process. A record high room-temperature coercivity value of 12.5~kOe has been measured in aligned Co nanowires with a diameter of about 15~nm and a mean length of 200~nm. When the surface of the Co nanowires were oxidized, exchange-bias (EB) was detected at low temperatures owing to the exchange coupling between the ferromagnetic (FM) Co core and the antiferromagnetic (AFM) CoO shell of the nanowires. EB fields of $\sim $2.0 kOe were measured at 10 K, along the parallel direction of nanowires. Manipulation and control of the EB in the nanowires may lead to a better understanding of the EB effect and the applications of the nanowires in for future permanent magnets and recording media. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W30.00004: Chemical Synthesis of Iron-Nickel Nanoparticles Frank Abel, Vasilias Tzitzios, George Hadjipanayis Equiatomic FeNi alloys undergo a phase transformation, like FePt, from a disordered fcc structure to an ordered fct structure. However, unlike FePt in Fe-Ni this transformation is very sluggish and has been only observed in heavily irradiated thin films and in meteorite samples as was recently reported.$^{\mathrm{1,2}}$ In this study, we used a high temperature chemical synthesis route to investigate the possibility of fabricating fct FeNi nanoparticles. The Iron Nickel Boron nanoparticles were made using anhydrous Iron (II) Chloride and Nickel (II) Chloride using Sodium borohydrite as a reducing agent in tetraglyme under a nitrogen hydrogen atmosphere. The high temperature of the reaction allowed for the formation of as made crystalline Iron Nickel nanoparticles without additional annealing. By changing the concentration of sodium borohydrite we were able to prepare nanoparticles either in the pure fcc phase, or in a new mixed phase. The magnetic properties were improved by increasing the concentration of Iron precursor. We obtained FeNi nanoparticles with saturation magnetization of (56 emu/g) and coercivity of (190 Oe). The particle size distribution of the FeNi particles ranged from several hundred nanometers to a half micron. \textbf{References: 1. }L Neel, et al., \textit{Journal of Applied Physics}, Volume \textbf{35}, No. 3 (1964)\textbf{ 2. }M Kotsugi, et al., \textit{Journal of Physics: Condensed Matter}\textbf{ , 26 064206 }(2014) [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W30.00005: Voltage-Tunable Magnetic Stability in a Ni Nanoparticle Patrick Gartland, Wenchao Jiang, Dragomir Davidovic We study single nickel particles $\approx 2$nm in diameter using single electron tunneling spectroscopy and find that such particles lie at the threshold of stable ferromagnetic order. We find that the application of a bias voltage can precisely tune the conditions for a stable magnetization orientation, and simulate the experimental configuration using a master equation. Due to the addition of anisotropy from a single electron, a new energy scale emerges which governs the stability of magnetization as a function of voltage bias conditions. [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W30.00006: Engineered diamond nanopillars as mobile probes for high sensitivity metrology in fluid P. Andrich, C.F. de las Casas, F.J. Heremans, D.D. Awschalom, B.J. Aleman, K. Ohno, J.C. Lee, E.L. Hu The nitrogen-vacancy (NV) center`s optical addressability and exceptional spin coherence properties at room temperature, along with diamond`s biocompatibility, has put this defect at the frontier of metrology applications in biological environments. To push the spatial resolution to the nanoscale, extensive research efforts focus on using NV centers embedded in nanodiamonds (NDs). However, this approach has been hindered by degraded spin coherence properties in NDs and the lack of a platform for spatial control of the nanoparticles in fluid. In this work\footnote{P. Andrich \emph{et al.}, Nano Lett. 14, 4959 (2014).}, we combine the use of high quality diamond membranes with a top-down patterning technique to fabricate diamond nanoparticles with engineered and highly reproducible shape, size, and NV center density. We obtain NDs, easily releasable from the substrate into a water suspension, which contain single NV centers exhibiting consistently long spin coherence times (up to 700 $\mu$s). Additionally, we demonstrate highly stable, three-dimensional optical trapping of the nanoparticles within a microfluidic circuit. This level of control enables a bulk-like DC magnetic sensitivity and gives access to dynamical decoupling techniques on contactless, miniaturized diamond probes. [Preview Abstract] |
Thursday, March 5, 2015 4:06PM - 4:18PM |
W30.00007: Controlling Quantum Nanomagnets with Atomic Exchange Bias Shichao Yan, Deung-Jang Choi, Jacob Burgess, Steffen Rolf-Pissarczyk, Sebastian Loth Miniaturizing spintronic devices to the point where magnetization of the device's elements becomes quantized is a possible avenue to achieving quantum computation with magnetic elements. Critical to such an approach is the ability to exert local control over the quantum nanomagnets . Atomic exchange bias field has been proposed as a mechanism for localized control of individual nanomagnets. Here we demonstrate that exchange coupling with the magnetic tip of a scanning tunnelling microscope provides continuous tuning of spin dynamics in an individual nanomagnet. By directly measuring spin relaxation time with electronic pump-probe spectroscopy, we find that the exchange interaction acts analogously to a local magnetic field that can be applied to a specific atom. It can be tuned in strength up to several teslas and cancel external magnetic fields, thereby demonstrating the feasibility of complete control over individual quantum magnets with atomically localized exchange coupling. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W30.00008: High temperature magnetic phase transitions and exchange bias effect of FeSiCr alloy Xin Wang We present magnetic properties of FeSiCr alloy which was synthesized by melting and crushing method followed by milling and heat treatment. The samples under different heat treatment temperature were characterized by using X-ray diffractometer, scanning electron microscopy, vibrating sample magnetometer and M\"ossbauer spectroscopy. The micro-structure undergo crystallization and grain growth process starting from as-crushed state, and gradually transform to B2 atomic disordered crystal phase. B2 phase is formed into DO3 atomic ordered phase with the increasing temperature. M\"ossbauer spectroscopy are well fitted into one or two subspectras correspond to DO3 structure and two subspectras to Fe(7) and Fe(4) structures. Only one DO3 subspectra with hyperfine magnetic field increasing from 26.8T to 30.4T with temperature is observed under 450${^\circ}$. The shift of hysteresis loop exhibits exchange bias effect based on zero-field cooling and FC measurement. The bias effect and DO3 ordered phase are simultaneously formed and changed, suggesting the exchange interaction between Fe-Cr crystal planes and Fe-Fe crystal planes contribute to unidirectional anisotropy in powder. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W30.00009: Determination of the magnetocrystalline anisotropy constant from the frequency dependence of the specific absorption rate in a frozen ferrofluid Nathaniel Mosher, Emily Perkins-Harbin, Brandon Aho, Lihua Wang, Ronald Kumon, Corneliu Rablau, Prem Vaishnava, Ronald Tackett Colloidal suspensions of superparamagnetic nanoparticles, known as ferrofluids, are promising candidates for the mediation of magnetic fluid hyperthermia (MFH). In such materials, the dissipation of heat occurs as a result of the relaxation of the particles in an applied ac magnetic field via the Brownian and Neel mechanisms. In order to isolate and study the role of the Neel mechanism in this process, the sample can be frozen, using liquid nitrogen, in order to suppress the Brownian relaxation. In this experiment, dextran-coated Fe$_3$O$_4$ nanoparticles synthesized via co-precipitation and characterized via transmission electron microscopy and dc magnetization are used as MFH mediators over the temperature range between -70 $^{\circ}$C to -10 $^{\circ}$C (Brownian-suppressed state). Heating the nanoparticles using ac magnetic field (amplitude $\sim$300 Oe), the frequency dependence of the specific absorption rate (SAR) is calculated between 150 kHz and 350 kHz and used to determine the magnetocrystalline anisotropy of the sample. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W30.00010: Ballistic Anisotropic Magnetoresistance of Single-Atom Contacts F. Otte, J. Sch\"oneberg, A. Weismann, R. Berndt, S. Heinze, N. N\'eel, J. Kr\"oger, Y. Mokrousov It has been predicted that the anisotropic magnetoresistance (AMR) is greatly enhanced in the ballistic transport regime. \footnote{J. Velev \textit{et al.}, Phys. Rev. Lett. \textbf{94}, 127203 (2005)} Results from break junctions in a magnetic field \footnote{A. Sokolov \textit{et al.}, Nature Nano. \textbf{2}, 171 (2006)}$^{,}$\footnote{M. Viret \textit{et al.}, Eur. Phys. J. B \textbf{51}, 1 (2006)} can be explained in terms of this ballistic AMR (BAMR), although the interpretation is controversal due to the unknown atomic geometry of the junction. Here, we demonstrate the emergence of BAMR in single-atom contacts. Single Co and Ir atoms are deposited on domains and domain walls of ferromagnetic Fe layers on W(110), which is used to control their magnetization directions. They are contacted with nonmagnetic tips in a low-temperature scanning tunneling microscope to measure the junction conductances. AMR is observed and changes drastically between tunneling and the ballistic regime. First-principles calculations and tight-binding modeling demonstrate that this change is due to a competition of delocalized and localized $d$ states of different orbital symmetry. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W30.00011: Quantum Molecular Magnetism Sylvain Brechet, Francois Reuse, Klaus Maschke, Jean-Philippe Ansermet Our theoretical description of quantum molecular magnetism is based on the quantum master equations, where the system consists of the electronic spin degrees of freedom and the bath consists of the remaining degrees of freedom. The system is weakly coupled and weakly correlated to the bath, which is at equilibrium on an appropriate time scale. The electrons satisfy the exclusion principle, which requires the tensorial product of the spin and orbital parts of the state to be antisymmetric under permutation. However, the symmetries of the parts of the state taken separately are determined by the irreducible unitary representations of the permutation group. The structure of the quantum master equations is also determined by these representations. The coupling between different isotypic components of the permutation group appearing in the quantum master equations leads to a description of magnetic dissipation at the molecular level and defines molecular spin selection rules. Thus, this theoretical description is expected to bring new and fundamental insight for molecular magnetism. In particular, it is expected to predict the non-trivial deflection of molecular clusters in a field gradient. [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W30.00012: Using the binding site to control the magnetic and spintronic properties of a single magnetic molecule in a tunnel junction Ben Warner, Fadi El Hallak, Henning Prueser, Tobias G. Gill, John Sharp, Andrew J. Fisher, Mats Persson, Cyrus F. Hirjibehedin Many proposals outline the use of single magnetic molecules in new applications in information technology and spintronics, with the intention of creating new devices based on phenomena that only manifest at the atomic scale. To create these devices it will be necessary to engineer the required properties, whether through controlling the molecule's chemical makeup or its interaction with the external surroundings. The latter may involve using interactions with the supporting substrate surface, which have been shown to not only modify the molecule properties [1] but also create effects such as chirality [2]. Here we utilize the surface interaction to modify the properties of FePc on copper nitride, a thin insulator, above bulk Cu(001). Using scanning tunneling microscopy we show that the interaction with the surface is defined by the binding site of the central Fe atom in the molecule. By performing elastic and inelastic tunneling spectroscopy and comparing the results to DFT modeling, we explore how coupling to the surface can be used to control the molecular orbitals and the accessibility of the spin excitations. This demonstrates the importance of controlling molecule-substrate coupling down to the atomic scale for the development of single molecule devices. \\[4pt] [1] N. Tsukahara et al., Phys. Rev. Lett. 102, 167203 (2009)\\[0pt] [2] A. Mugarza et al. Phys. Rev. Lett, 105, 115702 (2010). [Preview Abstract] |
Thursday, March 5, 2015 5:18PM - 5:30PM |
W30.00013: Stability of spin-electric coupling in triangular single-molecule magnets under external contacts Fhokrul Islam, Javier Nossa, Carlo Canali, Mark Pederson Triangular single molecule magnets (SMMs) with antiferromagnetic exchange coupling exhibit Kramer degenerate chiral spin-doublets ground states, which can be efficiently coupled by an electric field, even in the absence of spin-orbit interaction. Recent first-principles calculations [1] show that unsupported V$_{\mathrm{3}}$~SMM has giant spin-electric coupling corresponding to dipole moment of about one tenth of the water-molecule dipole moment. The corresponding Rabi time for electric switching between two chiral states can be on the order of one nano-second for reasonable electric fields, which makes these molecules very attractive candidates for storing and manipulating pairs of coupled spin-chiral qbits. However, for device applications of the spin-electric coupling, these frustrated SMMs need to be supported on a surface or between metallic leads. Preserving this effect in an external environment is a challenging problem requiring appropriate functionalization. In this talk we will discuss the stability of the spin-electric coupling in V$_{\mathrm{3}}$~SMM when coupled to gold leads or deposited on a graphene surface. [1] J. F. Nossa \textit{et al}., Electric control of spin states in frustrated triangular molecular magnets (unpublished) [Preview Abstract] |
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