arXiv subjects

V. Novosad

Publications and source records attributed to V. Novosad.

87 records · Page 5Linked to original sources

arXiv2012

Design and characterization of 90 GHz feedhorn-coupled TES polarimeter pixels in the SPTpol camera

The SPTpol camera is a two-color, polarization-sensitive bolometer receiver, and was installed on the 10 meter South Pole Telescope in January 2012. SPTpol is designed to study the faint polarization signals in the Cosmic Microwave Background, with two primary scientific goals. One is to constrain the tensor-to-scalar ratio of perturbations in the primordial plasma, and thus constrain the space of permissible inflationary models. The other is to measure the weak lensing effect of large-scale structure on CMB polarization, which can be used to constrain the sum of neutrino masses as well as other growth-related parameters. The SPTpol focal plane consists of seven 84-element monolithic arrays of 150 GHz pixels (588 total) and 180 individual 90 GHz single-pixel modules. In this paper we present the design and characterization of the 90 GHz modules.

astro-ph.IM↗

arXiv2012

Feedhorn-coupled TES polarimeter camera modules at 150 GHz for CMB polarization measurements with SPTpol

The SPTpol camera is a dichroic polarimetric receiver at 90 and 150 GHz. Deployed in January 2012 on the South Pole Telescope (SPT), SPTpol is looking for faint polarization signals in the Cosmic Microwave Background (CMB). The camera consists of 180 individual Transition Edge Sensor (TES) polarimeters at 90 GHz and seven 84-polarimeter camera modules (a total of 588 polarimeters) at 150 GHz. We present the design, dark characterization, and in-lab optical properties of the 150 GHz camera modules. The modules consist of photolithographed arrays of TES polarimeters coupled to silicon platelet arrays of corrugated feedhorns, both of which are fabricated at NIST-Boulder. In addition to mounting hardware and RF shielding, each module also contains a set of passive readout electronics for digital frequency-domain multiplexing. A single module, therefore, is fully functional as a miniature focal plane and can be tested independently. Across the modules tested before deployment, the detectors average a critical temperature of 478 mK, normal resistance R_N of 1.2 Ohm, unloaded saturation power of 22.5 pW, (detector-only) optical efficiency of ~ 90%, and have electrothermal time constants < 1 ms in transition.

astro-ph.IM↗

arXiv2012

SPTpol: an instrument for CMB polarization measurements with the South Pole Telescope

SPTpol is a dual-frequency polarization-sensitive camera that was deployed on the 10-meter South Pole Telescope in January 2012. SPTpol will measure the polarization anisotropy of the cosmic microwave background (CMB) on angular scales spanning an arcminute to several degrees. The polarization sensitivity of SPTpol will enable a detection of the CMB "B-mode" polarization from the detection of the gravitational lensing of the CMB by large scale structure, and a detection or improved upper limit on a primordial signal due to inflationary gravity waves. The two measurements can be used to constrain the sum of the neutrino masses and the energy scale of inflation. These science goals can be achieved through the polarization sensitivity of the SPTpol camera and careful control of systematics. The SPTpol camera consists of 768 pixels, each containing two transition-edge sensor (TES) bolometers coupled to orthogonal polarizations, and a total of 1536 bolometers. The pixels are sensitive to light in one of two frequency bands centered at 90 and 150 GHz, with 180 pixels at 90 GHz and 588 pixels at 150 GHz. The SPTpol design has several features designed to control polarization systematics, including: single-moded feedhorns with low cross-polarization, bolometer pairs well-matched to difference atmospheric signals, an improved ground shield design based on far-sidelobe measurements of the SPT, and a small beam to reduce temperature to polarization leakage. We present an overview of the SPTpol instrument design, project status, and science projections.

astro-ph.IM↗

arXiv2012

Evidence of Vortex Jamming in Abrikosov Vortex Flux Flow Regime

We report on dynamics of non-local Abrikosov vortex flow in mesoscopic superconducting Nb channels. Magnetic field dependence of the non-local voltage induced by the flux flow shows that vortices form ordered vortex chains. Voltage asymmetry (rectification) with respect to the direction of vortex flow is evidence that vortex jamming strongly moderates vortex dynamics in mesoscopic geometries. The findings can be applied to superconducting devices exploiting vortex dynamics and vortex manipulation, including superconducting wires with engineered pinning centers.

cond-mat.supr-con↗

arXiv2009

Temperature dependent nucleation and annihilation of individual magnetic vortices

We studied the temperature dependence of the magnetization reversal in individual submicron permalloy disks with micro-Hall and bend-resistance magnetometry. The nucleation field exhibits a nonmonotonic dependence with positive and negative slopes at low and high temperatures, respectively, while the annihilation field monotonically decreases with increasing temperature, but with distinctly different slopes at low and high temperatures. Our analysis suggests that at low temperatures vortex nucleation and annihilation proceeds via thermal activation over an energy barrier, while at high temperatures they are governed by a temperature dependence of the saturation magnetization.

cond-mat.mes-hall↗

arXiv2009

Transverse instabilities of multiple vortex chains in superconductor-ferromagnet bilayers

Using scanning tunneling microscopy and Ginzburg-Landau simulations we explore vortex configurations in magnetically coupled NbSe$_2$-Permalloy superconductor-ferromagnet bilayer. The Permalloy film with stripe domain structure induces periodic local magnetic induction in the superconductor creating a series of pinning-antipinning channels for externally added magnetic flux quanta. Such laterally confined Abrikosov vortices form quasi-1D arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. At high fields we show that the system becomes non-linear due to a change in both the number of vortices and the confining potential. The longitudinal instabilities of the resulting vortex structures lead to vortices `levitating' in the anti-pinning channels.

cond-mat.supr-con↗

arXiv2009

Low Temperature Thermal Transport in Partially Perforated Silicon Nitride Membranes

The thermal transport in partially trenched silicon nitride membranes has been studied in the temperature range from 0.3 to 0.6 K, with the transition edge sensor (TES), the sole source of membrane heating. The test configuration consisted of Mo/Au TESs lithographically defined on silicon nitride membranes 1 micron thick and 6 mm^2 in size. Trenches with variable depth were incorporated between the TES and the silicon frame in order to manage the thermal transport. It was shown that sharp features in the membrane surface, such as trenches, significantly impede the modes of phonon transport. A nonlinear dependence of thermal resistance on trench depth was observed. Partial perforation of silicon nitride membranes to control thermal transport could be useful in fabricating mechanically robust detector devices.

physics.ins-det↗

arXiv2008

Guiding superconducting vortices by magnetic domain walls

We demonstrate a unique prospect for inducing anisotropic vortex pinning and manipulating the directional motion of vortices using the stripe domain patterns of a uniaxial magnetic film in a the superconducting/ferromagnetic hybrid. Our observations can be described by a model, which considers interactions between magnetic charges of vortices and surface magnetic charges of domains resulting in the enhanced pinning of vortices on domain walls.

cond-mat.supr-con↗

arXiv2006

Origin of four-fold anisotropy in square lattices of circular ferromagnetic dots

We discuss the four-fold anisotropy of in-plane ferromagnetic resonance (FMR) field $H_r$, found in a square lattice of circular Permalloy dots when the interdot distance $a$ gets comparable to the dot diameter $d$. The minimum $H_r$, along the lattice $<11>$ axes, and the maximum, along the $<10>$ axes, differ by $\sim$ 50 Oe at $a/d$ = 1.1. This anisotropy, not expected in uniformly magnetized dots, is explained by a non-uniform magnetization $\bm(\br)$ in a dot in response to dipolar forces in the patterned magnetic structure. It is well described by an iterative solution of a continuous variational procedure.

cond-mat.mtrl-sci↗

arXiv2006

Soliton pair dynamics in patterned ferromagnetic ellipses

Confinement alters the energy landscape of nanoscale magnets, leading to the appearance of unusual magnetic states, such as vortices, for example. Many basic questions concerning dynamical and interaction effects remain unanswered, and nanomagnets are convenient model systems for studying these fundamental physical phenomena. A single vortex in restricted geometry, also known as a non-localized soliton, possesses a characteristic translational excitation mode that corresponds to spiral-like motion of the vortex core around its equilibrium position. Here, we investigate, by a microwave reflection technique, the dynamics of magnetic soliton pairs confined in lithographically defined, ferromagnetic Permalloy ellipses. Through a comparison with micromagnetic simulations, the observed strong resonances in the subgigahertz frequency range can be assigned to the translational modes of vortex pairs with parallel or antiparallel core polarizations. Vortex polarizations play a negligible role in the static interaction between two vortices, but their effect dominates the dynamics.

cond-mat.mtrl-sci↗

arXiv2005

Magnetic Vortex Resonance in Patterned Ferromagnetic Dots

We report a high-resolution experimental detection of the resonant behavior of magnetic vortices confined in small disk-shaped ferromagnetic dots. The samples are magnetically soft Fe-Ni disks of diameter 1.1 and 2.2 um, and thickness 20 and 40 nm patterned via electron beam lithography onto microwave co-planar waveguides. The vortex excitation spectra were probed by a vector network analyzer operating in reflection mode, which records the derivative of the real and the imaginary impedance as a function of frequency. The spectra show well-defined resonance peaks in magnetic fields smaller than the characteristic vortex annihilation field. Resonances at 162 and 272 MHz were detected for 2.2 and 1.1 um disks with thickness 40 nm, respectively. A resonance peak at 83 MHz was detected for 20-nm thick, 2-um diameter disks. The resonance frequencies exhibit weak field dependence, and scale as a function of the dot geometrical aspect ratio. The measured frequencies are well described by micromagnetic and analytical calculations that rely only on known properties of the dots (such as the dot diameter, thickness, saturation magnetization, and exchange stiffness constant) without any adjustable parameters. We find that the observed resonance originates from the translational motion of the magnetic vortex core.

cond-mat.mtrl-sci↗

arXiv2005

Dynamics of coupled vortices in layered magnetic nanodots

The spin dynamics are calculated for a model system consisting of magnetically soft, layered nanomagnets, in which two ferromagnetic (F) cylindrical dots, each with a magnetic vortex ground state, are separated by a non-magnetic spacer (N). This permits a study of the effects of interlayer magnetostatic interactions on the vortex dynamics. The system was explored by applying the equations of motion for the vortex core positions. The restoring force was calculated taking into account the magnetostatic interactions assuming a realistic surface charge free spin distribution. For tri-layer F/N/F dots with opposite chiralities and the same core polarizations (lowest energy state), two eigenmodes are predicted analytically and confirmed via micromagnetic simulations. One mode is in the sub-GHz range for submicron dot diameters and corresponds to quasi-circular rotation of the cores about the dot center. A second mode is in the MHz range corresponding to a small amplitude rotation of the mean core position. The eigenfrequencies depend strongly on the geometrical parameters of the system, suggesting that magnetostatic effects play a dominant role in determining the vortex dynamics.

cond-mat.mes-hall↗

arXiv2004

Vortex state oscillations in soft magnetic cylindrical dots

We have studied magnetic vortex oscillations in soft sub-micron cylindrical dots with variable thickness and diameter by an analytical approach and micromagnetic simulations. We have considered two kinds of modes of the vortex magnetization oscillations: 1) low-frequency translation mode, corresponding to the movement of the vortex as a whole near its equilibrium position; 2) high-frequency vortex modes, which correspond to radially symmetric oscillations of the vortex magnetization, mainly outside the vortex core. The vortex translational eigenmode was calculated numerically in frequency and time domains for different dot aspect ratios. To describe the discrete set of vortex high-frequency modes we applied the linearized equation of motion of dynamic magnetization over the vortex ground state. We considered only radially symmetric magnetization oscillations modes. The eigenfrequencies of both kinds of excitation modes are determined by magnetostatic interactions. They are proportional to the thickness/diameter ratio and lie in the GHz range for typical dot sizes.

cond-mat.mtrl-sci↗

arXiv2004

Oscillatory Thickness Dependence of the Coercive Field in Magnetic 3D Anti-Dot Arrays

We present studies on magnetic nano-structures with 3D architectures, fabricated using electrodeposition in the pores of well-ordered templates prepared by self-assembly of polystyrene latex spheres. The coercive field is found to demonstrate an oscillatory dependence on film thickness reflecting the patterning transverse to the film plane. Our results demonstrate that 3D patterned magnetic materials are prototypes of a new class of geometrical multilayer structures in which the layering is due to local shape effects rather then compositional differences.

cond-mat.mes-hall↗

arXiv2004

Magnetic properties and magnetostructural phase transitions in Ni2+xMn1-xGa shape memory alloys

A systematic study of magnetic properties of Ni2+xMn1-xGa (0 \le x \le 0.19) Heusler alloys undergoing structural martensite-austenite transformations while in ferromagnetic state has been performed. From measurements of spontaneous magnetization, Ms(T), jumps ΔM at structural phase transitions were determined. Virtual Curie temperatures of the martensite were estimated from the comparison of magnetization in martensitic and austenitic phases. Both saturation magnetic moments in ferromagnetic state and effective magnetic moments in paramagnetic state of Mn and Ni atoms were estimated and the influence of delocalization effects on magnetism in these alloys was discussed. The experimental results obtained show that the shift of martensitic transition temperature depends weakly on composition. The values of this shift are in good correspondence with Clapeyron-Clausius formalism taking into account the experimental data on latent heat at martensite-austenite transformations.

cond-mat.mtrl-sci↗