Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex ... - Physics

More documents

Recommendations

Info

Confinement of Spin Waves in Grids of Permalloy Nanowires D. Venkateswarlu Department of Physics, Indian Institute of Science, Bangalore 560012, India Key words—electron beam lithography, shape anisotropy, nanowires, FMR, demagnetization fields, micromagnetics. Recent advances in magnonics highlight its importance in microwave frequency applications [1] . This involves the use of spin waves for device applications. Here one needs to look for better control over the spin waves and their band structure [2-3] . The periodic variation in the effective magnetic field in ferromagnetic materials is the basis for the spin wave confinement and their propagation. Since the effective field is the combination of exchange, external, anisotropy and demagnetization (dipolar/ stray) fields; one can achieve the periodic condition in many ways. In our study, we employed geometry engineering by which the shape anisotropy was tailored in obtaining the periodicity in demagnetization fields. Soft magnetic material permalloy (Ni 80 Fe 20 ) was made into grid like structure using a top down approach. Ferromagnetic resonance (FMR) measurements give the information about the non travelling spin waves (k=0 modes). In general, one can see only single resonance mode in FMR spectra in the case of thin films of permalloy. This is due to its weak crystalline anisotropy even in epitaxial conditions unlike the Fe thin films [4] . When the permalloy film is made into nanowires with high aspect ratio, it still gives the single mode but resonance condition varies with applied field angles. This is due to the fact that the shape anisotropy governs the condition for resonance. This advantage of the shape anisotropy points towards the usefulness of engineering the network structures. Fundamentally one needs to understand the dynamics of these network structures in order to meet the required conditions for spin wave band structures in order to use them in magnonics devices. The grid structures were fabricated using electron beam lithography followed by DC magnetron sputtering and liftoff technique. The permalloy deposited on Si wafers was 20nm thick and is capped with 4nm gold to avoid oxidation. The thickness of the deposited materials was confirmed with the help of pre calibrated digital thinness monitor. The lateral dimensions of the grid structures were obtained using Scanning Electron Microscopy (SEM). The permalloy wires in the grids found with widths about 140- 160nm. To understand the spin wave confinement effect we varied the periodicities along horizontal and vertical directions in the grid: (i) 560x560 (G1), (ii) 800x400 (G2) and (iii) 1000x500 (G3) (all in nm units). The FMR spectra on all the three grids were compared with a reference sample, a continuous thin film grown under same conditions during the fabrication. The continuous film showed an uni-axial anisotropy. This was induced due to the in-situ applied field during the growth. But the magnitude is very small when compared to that of structured samples. The quantitative comparison of anisotropy constants was obtained with the help of fitting done with theoretical equations involved in FMR phenomena. Two well resolved modes were observed in all three grids whereas the reference sample gave only one mode. These two modes corresponds to the spin waves confined in horizontal and vertical sections of the grids. Spin wave confinement was understood with the help of the mode dependency on the in-plane angle of the applied magnetic field with the grid. There were multiple peaks observed at some angles in the G2, G3 which is not seen in G1. The origin for these multiple peaks is understood with the help of static micromagnetic simulations (MMS). Dynamics of G1 using MMS followed by Discrete Fast Fourier Transformations were used to correlate FMR spectra. Our simulations methodology gave more insight into the understanding of spin waves confinement. The implementation of 2D-PBC [5] in our MMS made it possible to study the real systems in realistic time. Fig.1 Spin wave confinement in the horizontal and vertical sections of the G1. REFERENCES [1] V. V. Kruglyak et al., J. Phys. D: Appl. Phys., vol. 43, p. 264001, 2010. [2] Z. K. Wang et al., Acs Nano, vol. 4, p. 643, 2010. [3] K. S. Lee et al., Phys. Rev. Lett., v. 102, p. 127202, 2009. [4] S. Sakshath et al., J. Supercond. Nov. Magn., DOI:10.1007/s10948-011-1269-3 [5] D. Venkateswarlu et al., IEEE Trans. Magn., vol. 48, no. 11, Nov. 2012. FMR experiments were done in collaboration with Prof. S. V. Bhat D.Venkat@physics.iisc.ernet.in
Magnetohydrodynamic stability of stochastically driven accretion flows Sujit Kumar Nath 1 , Banibrata Mukhopadhyay 1 , Amit K. Chattopadhyay 2 1. Department of Physics, Indian Institute of Science, Bangalore 560 012, India; sujitkumar@physics.iisc.ernet.in ; bm@physics.iisc.ernet.in 2. Aston University, Non-linearity and Complexity Research Group, Engineering and Applied Science, Birmingham B4 7ET, UK; a.k.chattopadhyay@aston.ac.uk Abstract We investigate the origin of magnetohydrodynamic turbulence in rotating shear flows. The particular emphasis is the flows whose angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows, which are extensively seen in astrophysics, are Rayleigh stable, but must be turbulent in order to explain observed data. The present work explores the effect of stochastic noise on such magnetohydrodynamic flows. We essentially concentrate on a small section of such a flow which is nothing but a plane shear flow supplemented by the Coriolis effect. This also mimics a small section of an astrophysical accretion disk. It is found that such stochastically driven flows exhibit large temporal and spatial correlations of perturbation velocities, and hence large energy dissipations of perturbation increasing indefinitely with time, which presumably generates instability. A range of specific angular momentum (λ) profiles, as functions of radial coordinate of background flow, starting from Keplerian to constant specific angular momentum is explored. However, all the background profiles exhibit identical growth and roughness exponents with similar amplitude of perturbations of energy, revealing a unique universality class for the stochastically forced magnetohydrodynamics of rotating shear flows. This work, is an attempt to understand origin of instability and turbulence in the three-dimensional Rayleigh stable rotating shear flows. This has important implications to resolve the turbulence problem in astrophysical magnetohydrodynamic flows such as accretion disks.
Page 3: FOREWORD As a periodic review of it
Page 6 and 7: Session II 11:00-1:00 T07 11:00-11:
Page 9 and 10: List of Posters No. Presenter Title
Page 11: TALK ABSTRACTS
Page 14 and 15: Possible nature of internal disorde
Page 16 and 17: A new model of flow induced voltage
Page 18 and 19: Landau Level Spectroscopy of Broken
Page 20 and 21: Evidence of gradient in dynamics of
Page 22 and 23: A reversible shear-induced crystall
Page 26 and 27: Flow Enhanced Pairing and Other Sto
Page 28 and 29: Heteronuclear Double Quantum Correl
Page 30 and 31: Probing superconductivity in the 2D
Page 32 and 33: Freezing of the octahedral tilt nea
Page 35: POSTER ABSTRACTS
Page 38 and 39: Unzipping Force Analysis to determi
Page 40 and 41: Can interaction between emergent ex
Page 42 and 43: High mobility graphene devices Pari
Page 44 and 45: Ultrafast NMR Techniques in Inhomog
Page 46 and 47: Investigating DNA hybridization thr
Page 48 and 49: Title : Measurement of Forces appli
Page 50 and 51: Superconducting fluctuations, Anoma
Page 52 and 53: Title of abstract: Transport proper
Page 54 and 55: Random matrix theory and gene corre
Page 56 and 57: Study of effect of alkali mixture o
Page 58 and 59: Magnetic, Dielectric and Transport
Page 60 and 61: Thermoelectric properties of chalco
Page 62 and 63: Realization of Fermionic Superfluid
Page 64 and 65: Flow Induced Alignment of water mol
Page 66 and 67: Universal Conductance Fluctuations
Page 68 and 69: Evolution of fermionic superfluid a
Page 70 and 71: Microwave assisted synthesis of sin
Page 72 and 73: Using cis peptide containing fragme
Page 74 and 75:
Optoelectronic properties of graphe
Page 76 and 77:
Violation of Chandrasekhar mass lim
show all

Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex ... - Physics

Create successful ePaper yourself

Delete template?

Save as template?