Physics & Astronomy, Dept. of

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Anisotropic three-dimensional magnetism in CaFe2As2
(American Physical Society, 2008-11-28) McQueeney, R. J.; Diallo, S. O.; Antropov, V. P.; Samolyuk, G. D.; Broholm, C.; Ni, N.; Nandi, S.; Yethiraj, M.; Zaretsky, J.L.; Pulikkotil, J. J.; Kreyssig, A.; Lumsden, M.D.; Harmon, B. N.; Canfield, P.C.
Inelastic neutron scattering measurements of the magnetic excitations in CaFe2As2 indicate that the spin wave velocity in the Fe layers is exceptionally large and similar in magnitude to the cuprates. However, the spin wave velocity perpendicular to the layers is at least half as large that in the layer, so that the magnetism is more appropriately categorized as anisotropic three-dimensional, in contrast to the two-dimensional cuprates. Exchange constants derived from band structure calculations predict spin wave velocities that are consistent with the experimental data.
Asymmetric domain nucleation and unusual magnetization reversal in ultrathin Co films with perpendicular anisotropy
(American Physical Society, 2007-03-16) Iunin, Y. L.; Kabanov, Y. P.; Nikitenko, V.I.; Cheng, X.M.; Clarke, D.; Tretiakov, O. A.; Tchernyshyov, O.; Shapiro, A. J.; Shull, R. D.; Chien, C.L.
We report unexpected phenomena during magnetization reversal in ultrathin Co films and Co/Pt multilayers with perpendicular anisotropy. Using magneto-optical Kerr microscopy and magnetic force microscopy we have observed asymmetrical nucleation centers where the reversal begins for one direction of the field only and is characterized by an acute asymmetry of domain-wall mobility. We have also observed magnetic domains with a continuously varying average magnetization, which can be explained in terms of the coexistence of three magnetic phases: up, down, and striped.
Biological applications of multifunctional magnetic nanowires
(American Institute of Physics, 2003-05-15) Reich, D.H.; Tanase, M.; Hultgren, A.; Bauer, L. A.; Chen, C. S.; Meyer, G. J.
Magnetic particles that can be bound to cells and biomolecules have become an important tool for the application of force in biology and biotechnology. Multifunctional magnetic nanowires fabricated by electrochemical deposition in nanoporous templates are a type of magnetic carrier that offers significant potential advantages over commercially available magnetic particles. Recent experimental work aimed at developing these wires for this purpose is reviewed. Results on chemical functionalization of Au and Au/Ni wires and magnetic manipulation of wires in suspension are described. Fluorescence microscopy was used to demonstrate the covalent binding of thiol-terminated porphyrins to Au nanowires, and to optimize functionalization of two-segment gold-nickel nanowires for selectivity and stability of the nanowire-molecule linkages. Magnetic trapping is a technique where single nanowires are captured from fluid suspension using lithographically patterned micromagnets. The influence of an external magnetic field on this process is described. The dynamics of magnetic trapping is shown to be well described by a model based on the interplay of dipolar forces and viscous drag.
Bound spinons in an antiferromagnetic S=1/2 chain with a staggered field
(American Physical Society, 2004-07-02) Kenzelmann, M.; Chen, Y.; Broholm, C.; Reich, D.H.; Qiu, Y.
Inelastic neutron scattering was used to measure the magnetic field dependence of spin excitations in the antiferromagnetic S=1/2 chain CuCl2·2(dimethylsulfoxide) in the presence of uniform and staggered fields. Dispersive bound states emerge from a zero-field two-spinon continuum with different finite energy minima at wave numbers q=π and qi≈π(1-2). The ratios of the field dependent excitation energies are in excellent agreement with predictions for breather and soliton solutions to the quantum sine-Gordon model, the proposed low-energy theory for S=1/2 chains in a staggered field. The data are also consistent with the predicted soliton and n=1,2 breather polarizations and scattering cross sections.
Broken parity and a chiral ground state in the frustrated magnet CdCr2O4
(American Physical Society, 2006-08-01) Chern, Gia-Wei; Fennie, C. J.; Tchernyshyov, Oleg
We present a model describing the lattice distortion and incommensurate magnetic order in the spinel CdCr2O4, a good realization of the Heisenberg 'pyrochlore' antiferromagnet. The magnetic frustration is relieved through the spin-Peierls distortion of the lattice involving a phonon doublet with odd parity. The distortion stabilizes a collinear magnetic order with the propagation wave vector q=2π(0,0,1). The lack of inversion symmetry makes the crystal structure chiral. The handedness is transferred to magnetic order by the relativistic spin-orbit coupling: the collinear state is twisted into a long spiral with the spins in the ac plane and q shifted to 2π(0,δ,1).
Cell manipulation using magnetic nanowires
(American Institute of Physics, 2003-05-15) Hultgren, A.; Tanase, M.; Chen, C. S.; Meyer, G. J.; Reich, D.H.
The use of magnetic nanowires is demonstrated as a method for the application of force to mammalian cells. Magnetic separations were carried out on populations of NIH-3T3 mouse fibroblast cells using ferromagnetic Ni wires 350 nm in diameter and 35 μm long. Separation purities in excess of 90% and yields of 49% are obtained. The nanowires are shown to outperform magnetic beads of comparable volume.
Central mode and spin confinement near the boundary of the superconducting phase in YBa2Cu3O6.353 (Tc=18 K)
(American Physical Society, 2006-03-01) Birgeneau, R. J.; Broholm, C.; Liang, R.; Yamani, Z.; Bonn, D.; Tun, Z.; Chung, J.-H.; Hardy, W. N.; Stock, C.; Buyers, W.J.L.
We have mapped the neutron scattering spin spectrum at low energies in YBa2Cu3O6.353 (Tc=18 K), where the doping ~0.06 is near the critical value (pc=0.055) for superconductivity. No coexistence with long-range-ordered antiferromagnetism is found. The spins fluctuate on two energy scales: one a damped spin response with a ≃2 meV relaxation rate and the other a central mode with a relaxation rate that slows to less than 0.08 meV below Tc. The spectrum mirrors that of a soft mode driving a central mode. Extremely short correlation lengths, 42±5 Å in-plane and 8±2 Å along the c direction, and isotropic spin orientations for the central mode indicate that the correlations are subcritical with respect to any second-order transition to Neel order. The dynamics follows a model where damped spin fluctuations are coupled to the slow fluctuations of regions with correlations shortened by the hole doping.
Comparison of surface spin wave modes at Fe/MnF2 and Fe/Mn interfaces
(American Institute of Physics, 2000-11-01) Siu, I. L.; Islam, M. N.; Walker, J. C.
We have studied the thermal demagnetization in semi-infinite ferromagnets in Fe/MnF2 and Fe/Mn bilayers using Mössbauer spectroscopy. We find that the hyperfine field at the Fe/MnF2 interface follows a quasilinear temperature dependence, which reverts to a T3/2 dependence further into the bulk. The region in which linear temperature dependence was observed also showed significantly higher spin canting than in the film's bulk layers. The interface in the Fe/Mn system immediately showed a T3/2 dependence which persisted deeper into the bulk. We attribute the linear temperature behavior to surface spin wave modes created by a perpendicular surface anisotropy at the interface. This behavior diminishes farther away from the interface, until the hyperfine field goes like T3/2 as expected for bulk, 3D spin waves. We conclude that the perpendicular surface anisotropy is much stronger at the Fe/MnF2 than the Fe/Mn interface.
Comparison of the electron-spin-resonance linewidth in multilayered CuMn spin glasses with insulating versus conducting interlayers
(American Institute of Physics, 1994-05-15) Leslie-Pelecky, D. L.; VanWijland, F.; Hoff, C. N.; Cowen, J.A.; Gavrina, A.; Chien, C.-L.
The temperature-dependent electron-spin-resonance linewidth ΔH(T) may be used to investigate the effect of the geometry and interlayer material on the magnetic properties of multilayered systems. We compare ΔH(T) in CuMn/Al2O3 multilayers with previous measurements of CuMn/Cu samples. CuMn/Al2O3 samples with CuMn thicknesses, WSG, from 40 Å to 20000 Å obey the same form as the CuMn/Cu system, but show quantitative differences in the fitting parameters. The linewidths of the CuMn/Al2O3 samples, even in the bulk, are systematically larger than the linewidths for the CuMn/Cu samples, suggesting that the ESR linewidth is sensitive to differences in sample growth and structure. The value of the minimum linewidth decreases with decreasing WSG in the CuMn/Al2O3 series, but remains constant in the CuMn/Cu series. Although susceptibility measurements of the freezing temperature Tf do not differentiate between samples with WSG≥5000 Å, the ESR linewidth is sensitive to changes at larger length scales. This experiment emphasizes the importance of considering both the total sample thickness, as defined by the range of the conduction electrons, and the spin-glass layer thickness in analyzing the ESR linewidth in multilayers.
Composite domain walls in flat nanomagnets: the magnetostatic limit
(American Institute of Physics, 2006-04-15) Youk, H.; Chern, G.-W.; Merit, K.; Oppenheimer, B.; Tchernyshyov, O.
We discuss the structure of the so-called 'vortex' domain walls in soft magnetic nanoparticles. A wall of this kind is a composite object consisting of three elementary topological defects: two edge defects with winding numbers -1/2 and a vortex with a winding number +1 between them. We provide a qualitative model accounting for the energetics of such a domain wall.
Condensation of magnons and spinons in a frustrated ladder
(American Physical Society, 2006-06-01) Fouet, J.-B.; Mila, F.; Clarke, D.; Youk, H.; Tchernyshyov, O.; Fendley, P.; Noack, R. M.
Motivated by the ever-increasing experimental effort devoted to the properties of frustrated quantum magnets in a magnetic field, we present a careful and detailed theoretical analysis of a one-dimensional version of this problem, a frustrated ladder with a magnetization plateau at m=1/2. We show that even for purely isotropic Heisenberg interactions, the magnetization curve exhibits a rather complex behavior that can be fully accounted for in terms of simple elementary excitations. The introduction of anisotropic interactions (e.g., Dzyaloshinskii-Moriya interactions) modifies significantly the picture and reveals an essential difference between integer and fractional plateaus. In particular, anisotropic interactions generically open a gap in the region between the plateaus, but we show that this gap closes upon entering fractional plateaus. All of these conclusions, based on analytical arguments, are supported by extensive density matrix renormalization group calculations.
Coupled Magnetic and Ferroelectric Domains in Multiferroic Ni3V2O8
(American Physical Society, 2009-08-21) Chen, W. C.; Kenzelmann, M.; Gentile, T. R.; Chen, Y.; Cabrera, I.; Rogado, N.; Lawes, G.; Broholm, C.; Lynn, J. W.; Erwin, R.; Cava, R.J.; Leao, J. B.
Electric control of multi-ferroic domains is demonstrated through polarized magnetic neutron diffraction. Cooling to the cycloidal multi-ferroic phase of Ni3V2O8 in an electric field E causes the incommensurate Bragg reflections to become neutron spin polarizing, the sense of neutron polarization reversing with E. Quantitative analysis indicates the E-treated sample has a handedness that can be reversed by E. We further show a close association between cycloidal and ferroelectric domains through E-driven spin and electric polarization hysteresis. We suggest that a definite cycloidal handedness is achieved through magneto-elastically induced Dzyaloshinskii-Moriya interactions.
Current and shot noise measurements in a carbon nanotube-based spin diode
(American Institute of Physics, 2009-04-01) Merchant, Christopher A.; Markovic, Nina
Low-temperature measurements of asymmetric carbon nanotube (CNT) quantum dots are reported. The CNTs are end-contacted with one ferromagnetic and one normal-metal electrode. The measurements show a spin-dependent rectification of the current caused by the asymmetry of the device. This rectification occurs for gate voltages for which the normal-metal lead is resonant with a level of the quantum dot. At the gate voltages at which the current is at the maximum current, a significant decrease in the current shot noise is observed.
Dependence of exchange coupling on antiferromagnetic layer thickness in NiFe/CoO bilayers
(American Institute of Physics, 1998-06-01) Ambrose, T.; Chien, C.-L.
A systematic study of the dependence of exchange coupling in NiFe/CoO bilayers on CoO layer thickness tAF from 5 to 500 Å has been made. For large CoO thicknesses (tAF>100 Å), the exchange field varies as 1/tAF, whereas for small CoO thicknesses (tAF<100 Å), finite-size scaling of the Néel temperature TN and also the blocking temperature TB dominate.
Direct transition from a disordered to a multiferroic phase on a triangular lattice
(American Physical Society, 2007-06-29) Kenzelmann, M.; Lawes, G.; Harris, A. B.; Gasparovic, G.; Broholm, C.; Ramirez, A. P.; Jorge, G.A.; Jaime, M.; Park, S.; Huang, Q.; Shapiro, A. Y.; Demianets, L. A.
We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet RbFe(MoO4)2. Ferroelectricity is observed only when the magnetic structure has chirality and breaks inversion symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that chiral magnetic order can give rise to a pseudoelectric field, whose temperature dependence agrees with experiment.
Doubly exchange-biased NiCoO/NiFe/Cu/NiFe/NiCoO spin valves
(American Institute of Physics, 1999-04-15) Ambrose, T.; Liu, K.; Chien, C.-L.
A new type of doubly exchange-biased Ni50Co50O/NiFe/Cu/NiFe/Ni30Co70O spin-valve structure, with two ferromagnetic layers exchange biased in opposite directions by two different antiferromagnetic layers is demonstrated. By field cooling in magnetic fields of opposite directions, the resultant hysteresis loop consists of two loops shifted in opposite directions from the zero magnetic field. The NiFe layers are in the antiparallel state in zero magnetic field, and the switching to the parallel state can be tuned by varying the exchange bias. The modified spin valves also show potential for suppressed Barkhausen noise.
Dynamics of a vortex domain wall in a magnetic nanostrip: application of the collective-coordinate approach
(American Physical Society, 2008-10-01) Clarke, D.J.; Tretiakov, O. A.; Chern, G.-W.; Bazaliy, Ya.B.; Tchernyshyov, O.
The motion of a vortex domain wall in a ferromagnetic strip of submicron width under the influence of an external magnetic field exhibits three distinct dynamical regimes. In a viscous regime at low fields the wall moves rigidly with a velocity proportional to the field. Above a critical field the viscous motion breaks down, giving way to oscillations accompanied by a slow drift of the wall. At still higher fields the drift velocity starts rising with the field again but with a much lower mobility dv/dH than in the viscous regime. To describe the dynamics of the wall, we use the method of collective coordinates that focuses on soft modes of the system. By retaining two soft modes, parametrized by the coordinates of the vortex core, we obtain a simple description of the wall dynamics at low and intermediate applied fields that applies to both the viscous and oscillatory regimes below and above the breakdown. The calculated dynamics agrees well with micromagnetic simulations at low and intermediate values of the driving field. In higher fields, additional modes become soft and the two-mode approximation is no longer sufficient. We explain some of the significant features of vortex-domain-wall motion in high fields through the inclusion of additional modes associated with the half antivortices on the strip edge.
Dynamics of domain walls in magnetic nanostrips
(American Physical Society, 2008-03-28) Tretiakov, O. A.; Clarke, D.; Chern, Gai-Wei; Bazaliy, Ya.B.; Tchernyshyov, O.
We express the dynamics of domain walls in ferromagnetic nanowires in terms of collective coordinates, generalizing Thiele[psila]s steady-state results. For weak external perturbations the dynamics is dominated by a few soft modes. The general approach is illustrated on the example of a vortex wall relevant to recent experiments with flat nanowires. A two-mode approximation gives a quantitatively accurate description of both the steady viscous motion of the wall in weak magnetic fields and its oscillatory behavior in moderately high fields above the Walker breakdown.
Effect of spacer material on the magnetic surface anisotropy in ultrathin Fe70B30 multilayer films
(American Institute of Physics, 1991-04-15) Hicken, R. J.; Rado, G. T.; Chien, C.-L.
It has been found recently that the magnetic surface anisotropy Ks in Fe70B30/Ag multilayer films decreases monotonically with magnetic layer thickness (2L) for 2L<16.5 Å. In order to determine possible effects of the spacer material on the surface anisotropy in the aforementioned system, Ag has been replaced with Al2O3 and ferromagnetic resonance (FMR) measurements have been made on these films. These Fe70B30/Al2O3 films were fabricated by magnetron sputtering and were characterized by X-ray-diffraction and vibrating sample magnetometer (VSM) measurements in addition to FMR. In the region where Ks depends upon 2L, the data is insufficient to confirm the thickness dependence of Ks that was observed in Fe70B30/Ag, while in the region where Ks is independent of 2L, the values of Ks deduced for Fe70B30/Ag and Fe70B30/Al2O3 are in good agreement. The latter is particularly interesting in light of the enormous difference in conductivity between Ag and Al2O3.
Effects of carrier mobility and morphology in organic semiconductor spin valves
(American Institute of Physics, 2009-05-01) Liu, Yaohua; Lee, Taegweo; Katz, Howard E.; Reich, Daniel H.
We studied spin transport in four organic semiconductors (OSCs) with different electronic properties. Magnetoresistance (MR) effects were observed up to room temperature in junctions based on an electron-carrying OSC, tris(8-hyroxyquinoline) aluminum (Alq3) and a hole-carrying OSC, copper phthalocyanine (CuPc). The MR shows similar temperature dependence for these two OSCs. We also investigated junctions based on two high lateral mobility electron-carrying OSCs, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N,N'-bis(4-trifluoromethylbenzyl)-1,4,5,8-naphthalenetetracarboxylic diimide (CF3-NTCDI). However, these junctions showed much weaker spin transport effects. Morphological studies suggest that these high mobility OSC films have much rougher surfaces than either Alq3 or CuPc, therefore the degradation of spin transport may originate from enhanced scattering due to the rougher FM/OSC interfaces. Our study shows that FM/OSC interfaces play an important role for spin transport in organic devices and need further exploration.

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