Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W19: Focus Session: Novel Magnetic Devices |
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Sponsoring Units: GMAG DMP Chair: Sujoy Roy, Lawrence Berkeley National Laboratory Room: D170 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W19.00001: Novel Spintronic Device-Terahertz Magnon-Photon Laser Boris Tankhilevich A novel spintronic -based method of generating THz radiation is proposed. The method is based on pumping of non-equilibrium electrons into the upper (spin-down) sub-band of spin-polarized half-metallic ferromagnets or ferromagnetic semiconductors, which makes it possible to build tunable, narrow-band, high-power THz sources. Non-equilibrium electrons pumped into the spin-down subband rapidly emit non-equilibrium magnons with THz frequency, pass into highly excited states of the spin-up subband, and fall into the ground state due to interaction with the equilibrium spin-up electrons or by emitting optical phonons. The mechanism of magnon generation is similar to a three-level conventional laser, and at a critical pumping intensity, which depends on the magnon damping, magnon lasing begins. In this regime the number of excited magnons increases exponentially with time. Merging of two THz magnons with frequency f generates a THz photon with frequency 2f. Thus, a magnon laser becomes a THz photon laser. The proposed one-stage device is capable of generating THz power being of orders of milliwatt and is tunable by tuning the magnetic field and/or the bias. The device has nano-dimensions and can be mass produced on a large scale. Recently THz radiation by spin-polarized current in a ferromagnetic structure was observed. However, the material used in this experiment has not met the conditions for magnon lasing. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W19.00002: Micro-Structured Ferromagnetic Tubes for Spin Wave Excitation Alexander Kozhanov, Daniel Ouellette, Mark Rodwell, Dok Won Lee, Shan X. Wang, S. James Allen Small scale magnetostatic spin wave devices are potentially important for on-chip filters for communication systems and spin wave logic devices. Low efficient coupling the electronic signals into the spin waves as well as coupling-out makes it difficult to build logical circuits especially when structures are scaled down to nanometer sizes. In this work we study the effect of external biasing magnetic field on the propagation of backward volume magnetostatic spin waves (BVMSW) in ferromagnetic CoTaZr stripe with micron sized ferromagnetic tubes fabricated at the ends. Spin waves are excited by shorted coplanar waveguides signal line of which is placed inside the tubes. Ferromagnetic tubes placed at the ends of the stripe form closed magnetic circuit that traps the RF magnetic field produced by the coupling loop. Transmission S-parameters of fabricated structures were measured using a vector network analyzer in the frequency range (0.5-20) GHz and biasing magnetic fields (0-1000) Oe. Experimental data is analyzed with use of theoretical model for BVMSW in ferromagnetic stripe. This work is supported by the Nanoelectronics Research Initiative (NRI) - Western Institute of Nanoelectronics (WIN). [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W19.00003: Ferromagnetic STM tip operating as a Spin-diode Poliana H. Penteado, Fabricio M. Souza, Ant\^onio C. Seridonio, Renato M. Coutinho, Edson Vernek, J. Carlos Egues We study spin-dependent transport in a system composed of a ferromagnetic STM tip coupled to an adsorbed atom (adatom) and to a host metallic (non-magnetic) surface. Electrons can tunnel directly from the tip to the surface or through the adatom. Our calculation is based on the nonequilibrium Green functions technique (Keldysh formalism). We self-consistently calculate the adatom spin occupation and its magnetization as a function of the tip position. We find that the adatom becomes magnetized when the tip approaches it; this magnetization switches sign as the voltage changes from forward to reverse bias. We also calculate the spin-resolved currents. If the tip is near the adatom, we obtain the spin-diode effect [PRB \textbf{75}, 165303 (2007)] - i. e., unpolarized current for positive bias and polarized current for reverse bias - when the adatom is singly occupied. We also observe Friedel oscillations in the current as the tip-adatom distance increases [F. M. Souza, P. H. Penteado, et al. - to be submitted]. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W19.00004: Spin voltage generation across rare earth spin filter barriers Guoxing Miao, Joonyeon Chang, Jagadeesh Moodera When a metal is in close contact with a rare-earth based magnetic compound, strong exchange interaction exists between the localized 4f electrons and the free moving conduction electrons. One important consequence is that the spin degeneracy among the conduction electrons is lifted, showing up as an effective Zeeman splitting higher than tens of Tesla in low dimensional systems such as graphene and other 2DEG. We perform our work using a vertical transport geometry, which consists of double spin filtering barriers based on a ferromagnetic Eu chalcogenide - EuS. A thin Al metallic layer is sandwiched in the middle and its conduction electrons thus experience the strong spin splitting, which is subsequently detected via the spin filtering effect. A spontaneous spin dependent voltage appears across such a device, and its polarity is directly determined by the EuS/Al interface. The voltage level difference between the spin-parallel and -antiparallel configurations is as large as a few mV. Such spin splitting also induces a clear universal behavior in the observed TMR bias dependence. Such spin voltage effect offers a possibility of directly converting magnetic exchange energy into electrical power. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W19.00005: Generation of spin currents due to mechanical rotation Mamoru Matsuo, Jun'ichi Ieda, Eiji Saitoh, Sadamichi Maekawa In the frontier of spintronics, much attention is paid on the control and generation of spin currents. Due to the exciting progress of nanomechatrononics, the importance of mechanical manipulation of electron spin will increase. We discuss theoretically effects of mechanical rotation on spin currents using generally covariant Dirac equation with gauge fields in the non-relativistic limit. We derive semi-classical equations of motion for a wavepacket of electrons in two dimentional planes subject to the spin-orbit interaction argumented by a mechanical rotation. We show that a circular spin current is created by the mechanical rotation with a magnetic field. The magnitude of the spin current becomes $10^8 \mbox{A/m}^2$ in Pt with the magnetic field $\approx 1$T and the rotational velocity $\approx 1$kHz. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W19.00006: Magnetic Field Effects on Mechanical Cantilevers with Deposited Thin Film Micromagnets Rosa Elia C\'{a}rdenas, Francisco M\'{a}rquez, John T. Markert We report on the techniques used to deposit magnetic material onto mechanical cantilevers. The deposition of the magnetic material, permalloy, onto the cantilevers was achieved by using a precise masking technique before mounting the cantilevers inside an electron beam evaporation chamber. This method resulted in a mechanical cantilever with a deposited micromagnet on its tip. A typical size of the resulting micromagnet is 200 nm thick by 20 microns wide by 10 microns in height. Using a laser interferometer, the driven response of the cantilevers with the deposited micromagnets are currently being studied in vacuum as a function of the external magnetic field. We will analyze the magnetic-field-dependent changes in the resonant frequencies and the quality factors of the cantilevers to determine the micromagnet net moment and anisotropy constants. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W19.00007: Torque magnetometry of permalloy-coated microcantilevers using higher order vibrational modes Joseph Losby, Jacob A.J. Burgess, Douglas Vick, John P. Davis, Wayne K. Hiebert, Mark R. Freeman There has been an accumulation of recent interest in the development of magnetometry techniques facilitating the use of nano- and micro-resonators. A finite element model describing the interaction of a magnetic cantilever driven at its fundamental resonance frequency by an external field is described and illustrated for the simple case of a straight domain wall propagating across the cantilever during magnetization reversal. The experimental results are compared to the finite element mechanical transformation of Landau-Lifshits-Gilbert based micromagnetic simulations. This idea is then extended to higher order (flexural and torsional) modes, with the intent of moving towards increased sensitivity and functionalization of magnetometers for the observation of quasi-static magnetization processes. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W19.00008: Forces due to Patterned Magnetic Traps within Microfluidic Channels M. Howdyshell, G. Vieira, A. Chen, M. Simon, M. Poirier, R. Sooryakumar An array of microscopic ferromagnetic disks patterned onto a silicon surface has been previously utilized to trap and transport magnetic microspheres as well as magnetically labeled biological cells across the surface. The transport is activated through programmable weak external magnetic fields that do not damage the cells and enable remote control on the magnitude and direction of the fields. In this talk we present results in which the array of magnetic bits is imprinted within microfluidic channels where now competing hydrodynamic drag forces come into play. The trapping forces on individual microspheres are directly determined from the flow rates required to overcome the local magnetic forces. These findings are compared to results derived from micromagnetic simulations of the magnetic profile of individual disks. The fluid flow within the channel is also used to stretch DNA molecules tethered between two microparticles. With one of the ends trapped on a magnetic disk, the extension is controlled by the fluid flow rate. Comparisons to DNA stretching achieved with conventional magnetic tweezers reported in the literature serve as an additional calibration of the measured forces. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W19.00009: Giant Magnetioimpedance in Co-Based Amorphous Ribbons Coated in Magnetic Nanoparticles for Biosensing Applications N. Laurita, A. Chaturvedi, K. Stojak, S. Chandra, M.H. Phan, H. Srikanth Giant magnetoimpedance (GMI) is a large change in the ac impedance of a ferromagnetic conductor subject to a dc magnetic field. It forms the basis for developing highly sensitive magnetic sensors. We report studies aimed at developing GMI as a magnetic biosensing technique. We have investigated the GMI effect and its field sensitivity in Co-based amorphous alloys with and without coated magnetic nanoparticles. Fe$_{3}$O$_{4}$ and CoFe$_{2}$O$_{4}$ nanoparticles (mean size, 5-10 nm) were patterned onto the ribbon surfaces and the number of particle layers was varied from 10 to 80. The influences of particles size, concentration, and layer thickness on the GMI and field sensitivity have been investigated systematically. The coating of the nanoparticles has been shown to enhance the GMI and field sensitivity, both of which increase with increase of particle concentration and layer thickness. Overall, our studies demonstrate the possibility of using GMI as a magnetic biosensor with high sensitivity for applications in biomolecular detection. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W19.00010: Correlation between magnetic softness, sample surface and magnetoimpedance in Co$_{69}$Fe$_{4.5}$\textit{X}$_{1.5}$Si$_{10}$B$_{15}$ (\textit{X} = Ni, Al, Cr) amorphous ribbons A. Chaturvedi, T. Dhakal, S. Witanachchi, M.H. Phan, H. Srikanth, A.T. Le In this work we have studied the giant magnetoimpedance (GMI) effect and its field sensitivity ($\eta )$ in Co$_{69}$Fe$_{4.5}$X$_{1.5}$Si$_{10}$B$_{15}$ (X = Ni, Al, Cr) amorphous ribbons in the frequency (f) range of 0.1 to 10 MHz. We find that at f $<$ 5 MHz, the GMI effect and $\eta $ reach the largest values for the Al-containing sample and the smallest values for the Ni-containing sample, while an opposite trend is observed at f $>$ 5 MHz. Magnetization and atomic force microscopy (AFM) experiments reveal that the largest values of the low-frequency GMI effect and $\eta $ for the Al-containing sample result from the largest value of magnetic permeability, while the largest values of the high-frequency GMI effect and $\eta $ for the Ni-containing sample are attributed to the smallest surface roughness of this sample. These results point to the importance of the sample surface in determining high-frequency GMI behavior. A correlation between the sample surface and high-frequency GMI is established in the investigated ribbons. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W19.00011: Modulating the Magnetic Field to Improve Magnetic Sensors Alan Edelstein, Jonathan Petrie, Jonathan Fine, Greg Fischer, James Burnette, Gopal Srinivasan, Sanjay Mandal The sensitivity of most magnetic sensors is affected by 1/$f$ noise. Modulating the magnetic field to be detected by magnetic sensors can improve their performance by minimizing the effect of this 1/$f$ noise and, in some cases, also have them operate in a narrow frequency band where they have higher sensitivity. We present approaches for modulating the field. One approach is the MEMS flux concentrator can be used with small magnetic sensors and another, based on using a rotating disc containing flux concentrators that can be used with large magnetic sensors, such as magnetoelectric sensors, that have an increased sensitivity at their mechanical resonance frequency. Sidebands observed around the modulation frequency demonstrate the applicability of these approaches. The MEMS flux concentrator has improved the signal to noise ratio in the power spectrum by a factor of 15. The sensors have the potential to achieve sensitivities of a few pT/Hz$^{1/2}$ at 1 Hz. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W19.00012: Ultra high sensitivity, room temperature magneto-optic field sensor made of ferromagnetic bismuth rare-earth iron garnet thick films Dong Ho Wu, Anthony Garzarella, Vince Fratello The ferrimagnetic bismuth rare-earth iron garnet (BiGdLu)$_{3}$(FeGa)$_{5}$O$_{12}$ thick film has a specific Faraday rotation $\theta _{S}$ of 0.09 \r{ }/mm at 1550 nm and excellent transparency at infrared wavelengths. Using the thick film we recently have demonstrated a magneto-optic (MO) field sensor with a sensitivity of about 10$^{-14}$T/Hz$^{1/2}$, comparable with SQUID. The sensor is made of all dielectric materials including the bismuth rare-earth iron garnet and optical fibers, and is operated at room temperature without any cooling requirement. The MO field sensor is capable to measure a magnetic field over a very large dynamic range (from a very weak field to a very high magnetic field exceeding several hundred Tesla) and over a very wide frequency range, which may be from DC to a few hundred GHz. However, presently, our MO sensor's frequency range is limited from DC to 2 GHz. We think that this limited frequency range is due to the presence of magnetic domains in the bismuth rare-earth iron garnet film. In this presentation we will report our experimental results obtained from this MO field sensor as well as the effect of magnetic domains. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W19.00013: Spin-Flipping at Sputtered Co(90)Fe(10)/Cu Interfaces Hoang Yen Thi Nguyen, Rakhi Acharyya, William P. Pratt Jr., Jack Bass Knowledge of the spin-flipping probability, P$_{F/N}$ = 1 -- exp[-$\delta $(F/N)], at ferromagnetic/non-magnetic (F/N) interfaces in the Current-Perpendicular-to-Plane (CPP) geometry is minimal [1,2]. We use a new technique [2] to find $\delta $(CoFe/Cu) at 4.2K, where CoFe = Co(90)Fe(10). With thin (3 nm) CoFe layers, the spin-diffusion length of CoFe $\sim $ 12 nm doesn't mask spin-flipping due to $\delta $. Our most important samples, sensitive to $\delta $(CoFe/Cu), have the form FeMn/Py/Cu/X/Cu/Py/FeMn. Here Py = Ni(80)Fe(20), the antiferromagnet FeMn pins the two Py layers to flip at a much higher field than X = [CoFe(3)/Cu(1.4)]$_{n}$CoFe(3), and 1.4 nm of Cu couples the CoFe layers ferromagnetically so X reverses as a unit. We measure, versus the number of repeats $n$, the change in specific resistance, A$\Delta $R = AR(AP) -- AR(P), between states where the X moment is anti-parallel (AP) or parallel (P) to the pinned Py moments. CPP current flows through area A. Our resulting best estimate is $\delta $(CoFe/Cu) $\approx $ 0.2. [1] J. Bass and W.P.Pratt Jr., J. Phys. Cond. Matt. \textbf{19}, 183201 (2007). [2] B. Dassonneville et al., Appl. Phys. Lett. \textbf{96}, 022509 (2010). [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W19.00014: Magnetic Properties of Niobium-Permalloy hybrid system Jiyeong Gu, Jill Pestana, David Christiansen Ferromagnet/superconductor hybrid system has been studied intensively due to the proximity effect between ferromagnetism and superconductivity. Not only the superconducting property changes because of the different magnetic configuration, the presence of the superconductivity can also often drastically change the magnetic properties of the ferromagnets. In our current work we focused on the investigation of the magnetic property change of the Nb-Py (Permalloy; NiFe) hybrid system through the superconducting transition. Nb-Py bilayer and trilayers were fabricated using Multi-target sputtering system. Magnetization was measured by Physical Property Measurement System using the Vibrating Sample Magnetometer (VSM) or Alternating Current Measurement System (ACMS) options. In addition to the magnetometry measurement, we also measured magneto optical Kerr effect. Magnetization changed when the system goes through the superconducting transition; however the result varied depending on many parameters, such as magnetic measurement history, cooling or warming rate, and measurement method. Magnetic responses obtained from different methods would be compared and discussed. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W19.00015: Localized magnetism on the surface of niobium: experiments and theory Thomas Proslier, John Zasadzinski, Gianluigi Ciovati, Mike Pellin The presence of magnetic impurities in native niobium oxides have been confirmed by Point contact spectroscopy (PCT), SQUID magnetometry and Electron paramagnetic resonance (EPR). All niobium (Nb) samples displayed a small impurity contribution to the magnetic susceptibility at low temperatures which exhibited Curie-Weiss behavior, indicative of weakly coupled localized paramagnetic moments. By examining Nb samples with widely varying surface-to-volume ratios it was found that the impurity contribution is correlated with surface area. Tunneling measurements which use the native oxide layers as barriers exhibit a zero-bias conductance peak which splits in a magnetic field $>$ 4T, consistent with the Appelbaum model for spin flip tunneling. Viewed together the experiments strongly suggest that the native oxides of Nb are intrinsically defective, and consistently exhibit localized paramagnetic moments caused by oxygen vacancies in Nb$_{2}$O$_{5}$. The computation of the surface impedance (R$_{S})$ in presence of magnetic impurities in the Shiba approximation reveals the saturation at low temperature of Rs, suggesting that magnetic impurities are responsible for the so-called residual resistance. [Preview Abstract] |
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