30 research outputs found
Dynamics of Vortex Nucleation in Nanomagnets with Broken Symmetry
We investigate fundamental processes that govern dynamics of vortex
nucleation in sub-100 nm mesoscopic magnets. We focus on a structure with
broken symmetry - Pacman-like nanomagnet shape - in which we study
micromagnetic behavior both by means of a simple model and numerically. We show
that it is possible to establish desired vortex chirality and polarity by
applying only quasi-static in-plane magnetic field along specific directions.
We identify the modes of vortex nucleation that are very robust against
external magnetic field noise. These vortex nucleation modes are common among
wide range of sub-100 nm magnets with broken rotational symmetry.Comment: 5 pages, 4 figure
The chiral phase transition in charge ordered 1T-TiSe2
It was recently discovered that the low temperature, charge ordered phase of
1T-TiSe2 has a chiral character. This unexpected chirality in a system
described by a scalar order parameter could be explained in a model where the
emergence of relative phase shifts between three charge density wave components
breaks the inversion symmetry of the lattice. Here, we present experimental
evidence for the sequence of phase transitions predicted by that theory, going
from disorder to non-chiral and finally to chiral charge order. Employing X-ray
diffraction, specific heat, and electrical transport measurements, we find that
a novel phase transition occurs ~7 K below the main charge ordering transition
in TiSe2, in agreement with the predicted hierarchy of charge ordered phases.Comment: 5 pages, 3 figures; includes additional experimental and theoretical
results; fixed typo
Flux coupled tunable superconducting resonator
We present a design and implementation of frequency-tunable superconducting
resonator. The resonance frequency tunability is achieved by flux-coupling a
superconducting LC-loop to a current-biased feedline; the resulting screening
current leads to a change of the kinetic inductance and shift in the resonance
frequency. The thin film aluminum resonator consists of an interdigitated
capacitor and thin line inductors forming a closed superconducting loop. The
magnetic flux from the nearby current feedline induces Meissner shielding
currents in the resonator loop leading to change in the kinetic part of the
total inductance of the resonator. We demonstarte continuous frequency tuning
within 160 MHz around the resonant frequency of 2.7 GHz. We show that: (1)
frequency upconversion is achieved when kHz AC modulation signal is
superimposed onto the DC bias resulting in sidebands to the resonator tone; (2)
three-wave mixing is attained by parametrically pumping the nonlinear kinetic
inductance using a strong RF pump signal in the feedline. The simple
architecture is amenable to large array multiplexing and on-chip integration
with other circuit components. The concept could be applied in flux
magnetometers, upconverters, and parametric amplifiers operating above 4 Kelvin
cryogenic temperatures when alternative high critical temperature material with
high kinetic inductance is used
Imaging the spontaneous formation of vortex-antivortex pairs in planar superconductor/ferromagnet hybrid structures
Low-temperature magnetic force microscopy has been used to visualize spontaneous formation of vortex-antivortex pairs in hybrid ferromagnet/superconductor systems. Vortex-antivortex pairs are induced by the periodic stray field of the ferromagnet. We find general equilibrium conditions for which spontaneous vortex-antivortex pairs are formed during zero-field cooling of the hybrid ferromagnet/superconductor bilayers. Vortices can be generated by the ferromagnet domains in the absence of an external field and they are thermodynamically stable for values of the stray field and the period of the stripe magnetic domains that exceed a certain threshold
Effect of Hf alloying on magnetic, structural, and magnetostrictive properties in FeCo films for magnetoelectric heterostructure devices
Materials with high magnetoelectric coupling are attractive for use in engineered multiferroic heterostructures with applications such as ultra-low power magnetic sensors, parametric inductors, and non-volatile random-access memory devices. Iron-cobalt alloys exhibit both high magnetostriction and high saturation magnetization that are required for achieving significantly higher magnetoelectric coupling. We report on sputter-deposited (Fe0.5Co0.5)1-xHfx (x = 0 - 0.14) alloy thin films and the beneficial influence of Hafnium alloying on the magnetic and magnetostrictive properties. We found that co-sputtering Hf results in the realization of the peening mechanism that drives film stress from highly tensile to slightly compressive. Scanning electron microscopy and x-ray diffraction along with vibrating sample magnetometry show reduction in coercivity with Hf alloying that is correlated with reduced grain size and low film stress. We demonstrate a crossover from tensile to compressive stress at x âŒ0.09 while maintaining a high magnetostriction of 50 ppm and a low coercive field of 1.1 Oe. These characteristics appear to be related to the amorphous nature of the film at higher Hf alloying
Superconducting VortexâAntivortex Pairs: Nucleation and Confinement in Magnetically Coupled SuperconductorâFerromagnet Hybrids
Superconducting vortices are a well known class of vortices, each of them carrying a single magnetic flux quantum. In this chapter the authors present the results of low temperature Magnetic Force Microscopy experiments to investigate the nucleation and dynamics of superconducting vortices in magnetically coupled Superconductor/Ferromagnet (S/F) heterostructures made by Nb/Py. It is here shown that by controlling the thicknesses of both S and F layer, the formation of spontaneous vortex-antivortex pairs (V-AV) can be favored and their confinement and mobility can be tuned. The experimental results are compared with two theoretical models dealing with the spontaneous nucleation of V/AV pairs in the limits of S thickness respectively greater and smaller than the London penetration depth. It is shown that vortex nucleation and confinement is regulated by the intensity of the out-of-plane component of the magnetization with respect to a critical magnetization set by the thickness of both S and F layers. Additionally, external field cooling processes were used to probe in-field vortex nucleation and V-AV unbalancing, whereas the sweeping of an external magnetic field when below the superconducting critical temperature was used to force the vortex into motion, probing the vortex mobility/rigidity and the vortex avalanche events
Optical detection and manipulation of spontaneous gyrotropic electronic order in a transition-metal dichalcogenide semimetal
The observation of chirality is ubiquitous in nature. Contrary to intuition,
the population of opposite chiralities is surprisingly asymmetric at
fundamental levels. Examples range from parity violation in the subatomic weak
force to the homochirality in essential biomolecules. The ability to achieve
chirality-selective synthesis (chiral induction) is of great importance in
stereochemistry, molecular biology and pharmacology. In condensed matter
physics, a crystalline electronic system is geometrically chiral when it lacks
any mirror plane, space inversion center or roto-inversion axis. Typically, the
geometrical chirality is predefined by a material's chiral lattice structure,
which is fixed upon the formation of the crystal. By contrast, a particularly
unconventional scenario is the gyrotropic order, where chirality spontaneously
emerges across a phase transition as the electron system breaks the relevant
symmetries of an originally achiral lattice. Such a gyrotropic order, proposed
as the quantum analogue of the cholesteric liquid crystals, has attracted
significant interest. However, to date, a clear observation and manipulation of
the gyrotropic order remain challenging. We report the realization of optical
chiral induction and the observation of a gyrotropically ordered phase in the
transition-metal dichalcogenide semimetal -TiSe. We show that shining
mid-infrared circularly polarized light near the critical temperature leads to
the preferential formation of one chiral domain. As a result, we are able to
observe an out-of-plane circular photogalvanic current, whose direction depends
on the optical induction. Our study provides compelling evidence for the
spontaneous emergence of chirality in the correlated semimetal TiSe. Such
chiral induction provides a new way of optical control over novel orders in
quantum materials.Comment: 22 pages, 5 figure
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