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Negative differential resistance state in the free-flux-flow regime of driven vortices in a single crystal of 2H-NbS$_2$
Authors:
Biplab Bag,
Sourav M. Karan,
Gorky Shaw,
A. K. Sood,
A. K. Grover,
S. S. Banerjee
Abstract:
Time series measurements in 2H-NbS$_2$ crystal had unravelled a drive induced transition wherein the critical current (Ic) changes from a low to a high Ic jammed vortex state, via a negative differential resistance (NDR) transition. Here, using multiple current (I) - voltage (V) measurement cycles, we explore the statistical nature of observing the NDR transition in the free-flux-flow (FF) regime…
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Time series measurements in 2H-NbS$_2$ crystal had unravelled a drive induced transition wherein the critical current (Ic) changes from a low to a high Ic jammed vortex state, via a negative differential resistance (NDR) transition. Here, using multiple current (I) - voltage (V) measurement cycles, we explore the statistical nature of observing the NDR transition in the free-flux-flow (FF) regime in a single crystal of 2H-NbS$_2$. The probability of observing the NDR transition always remains finite for a vortex state created with either fast or slow rate of magnetic field. The probability of observing the NDR transition in the FF regime is found to systematically increase with magnetic field (B) in weak collective pinning regime. In the strong pinning regime, the said probability becomes B-independent. We show that the higher Ic state is unique and cannot be accessed via any conventional route. While the I-V curves do not distinguish between zero field cooled (ZFC) and field cooled (FC) modes of preparing the vortex state, the probability for observing an NDR transition has different B-dependences for the vortex matter prepared in the ZFC and FC modes. We find that the NDR occurs in a high dissipation regime, where the flow resistivity is well above the theoretical value expected in the FF regime. We understand our results on the basis of a rapid drop in vortex viscosity at high drives in 2H-NbS$_2$, which triggers a rapid increase in the vortex velocity and reorganization in the moving vortex matter leading to a dynamical unstable vortex flow. This dynamical instability leads to the NDR transition into a high entropy vortex state with high Ic.
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Submitted 3 November, 2021; v1 submitted 18 October, 2021;
originally announced October 2021.
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Spin dynamics and a nearly continuous magnetic phase transition in an entropy-stabilized oxide antiferromagnet
Authors:
Benjamin A. Frandsen,
K. Alec Petersen,
Nicolas A. Ducharme,
Alexander G. Shaw,
Ethan J. Gibson,
Barry Winn,
Jiaqiang Yan,
Junjie Zhang,
Michael E. Manley,
Raphaël P. Hermann
Abstract:
The magnetic order and the spin dynamics in the antiferromagnetic entropy-stabilized oxide (Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$)O (MgO-ESO) have been studied using muon spin relaxation ($μ$SR) and inelastic neutron scattering. We find that antiferromagnetic order develops gradually in the sample volume as it is cooled below 140 K, becoming fully ordered around 100 K. The spin dynami…
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The magnetic order and the spin dynamics in the antiferromagnetic entropy-stabilized oxide (Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$)O (MgO-ESO) have been studied using muon spin relaxation ($μ$SR) and inelastic neutron scattering. We find that antiferromagnetic order develops gradually in the sample volume as it is cooled below 140 K, becoming fully ordered around 100 K. The spin dynamics show a critical slowing down in the vicinity of the transition, and the magnetic order parameter grows continuously in the ordered state. These results indicate that the antiferromagnetic transition is continuous but proceeds with a Gaussian distribution of ordering temperatures. The magnetic contribution to the specific heat determined from inelastic neutron scattering likewise shows a broad feature centered around 120 K. High-resolution inelastic neutron scattering further reveals an initially gapped spectrum at low temperature which sees an increase in a quasielastic contribution upon heating until the ordering temperature.
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Submitted 11 July, 2020; v1 submitted 8 April, 2020;
originally announced April 2020.
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Quantitative magneto-optical investigation of superconductor/ferromagnet hybrid structures
Authors:
G. Shaw,
J. Brisbois,
L. B. G. L. Pinheiro,
J. Müller,
S. Blanco Alvarez,
T. Devillers,
N. M. Dempsey,
J. E. Scheerder,
J. Van de Vondel,
S. Melinte,
P. Vanderbemden,
M. Motta,
W. A. Ortiz,
K. Hasselbach,
R. B. G. Kramer,
A. V. Silhanek
Abstract:
We present a detailed quantitative magneto-optical imaging study of several superconductor/ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB, and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data is complemented with and compared to scanning Hall probe microscopy measu…
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We present a detailed quantitative magneto-optical imaging study of several superconductor/ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB, and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data is complemented with and compared to scanning Hall probe microscopy measurements. Comprehensive protocols have been developed for calibrating, testing, and converting Faraday rotation data to magnetic field maps. Applied to the acquired data, they reveal the comparatively weaker magnetic response of the superconductor from the background of larger fields and field gradients generated by the magnetic layer.
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Submitted 15 February, 2018; v1 submitted 11 December, 2017;
originally announced December 2017.
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A Scanning Hall Probe Microscope for high resolution, large area, variable height Magnetic Field Imaging
Authors:
Gorky Shaw,
R. B. G. Kramer,
N Dempsey,
K Hasselbach
Abstract:
We present a Scanning Hall Probe Microscope operating in ambient conditions. One of the unique features of this microscope is the use of the same stepper motors for both sample positioning as well as scanning, which makes it possible to have a large scan range (few mm) in x and y directions, with a scan resolution of 0.1 $μ$m. Protocols have been implemented to enable scanning at different heights…
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We present a Scanning Hall Probe Microscope operating in ambient conditions. One of the unique features of this microscope is the use of the same stepper motors for both sample positioning as well as scanning, which makes it possible to have a large scan range (few mm) in x and y directions, with a scan resolution of 0.1 $μ$m. Protocols have been implemented to enable scanning at different heights from the sample surface. The z range is 35 mm. Microstructured Hall probes of size 1-5 $μ$m have been developed. A minimum probe-sample distance \textless{} 2 $μ$m has been obtained by the combination of new Hall probes and probe-sample distance regulation using a tuning fork based force detection technique. The system is also capable of recording local B(z) profiles. We discuss the application of the microscope for the study of micro-magnet arrays being developed for applications in micro-systems. * gorky.shaw@ulg.ac.be.; Present address:
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Submitted 24 October, 2016;
originally announced October 2016.
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Commensurate - Incommensurate vortex phase in a nanopatterned superconductor
Authors:
Gorky Shaw,
S. S. Banerjee,
T. Tamegai,
Hermann Suderow
Abstract:
Magneto-optical imaging studies on a high-quality Bi2Sr2CaCu2O8 single crystal partially patterned with a triangular array of holes reveal enhanced flux shielding in the patterned region of the sample. By mapping local magnetic field and shielding current density distributions at different applied magnetic fields and temperatures we determine the regime where pinning from the patterned holes domin…
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Magneto-optical imaging studies on a high-quality Bi2Sr2CaCu2O8 single crystal partially patterned with a triangular array of holes reveal enhanced flux shielding in the patterned region of the sample. By mapping local magnetic field and shielding current density distributions at different applied magnetic fields and temperatures we determine the regime where pinning from the patterned holes dominates over the intrinsic pinning in the sample. In this regime, the flux density near the center of the patterned region is observed to increase when the applied field is varied from below the matching field to just above it, while significant magnetic field gradients are sustained in the patterned region. Our measurements indicate heterogeneous pinning properties of the vortex population, exhibiting signatures of both weak and strong pinning, in the nanopatterned region of the superconductor.
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Submitted 30 May, 2016;
originally announced May 2016.
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Metastable inhomogeneous vortex configuration with non-uniform filling fraction inside a blind hole array patterned in a BSCCO single crystal and concentrating magnetic flux inside it
Authors:
Gorky Shaw,
S S Banerjee,
T Tamegai,
Hermann Suderow
Abstract:
Using magneto-optical imaging technique, we map local magnetic field distribution inside a hexagonally ordered array of blind holes patterned in BSCCO single crystals. The nature of the spatial distribution of local magnetic field and shielding currents across the array reveals the presence of a non-uniform vortex configuration partially matched with the blind holes at sub-matching fields. We obse…
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Using magneto-optical imaging technique, we map local magnetic field distribution inside a hexagonally ordered array of blind holes patterned in BSCCO single crystals. The nature of the spatial distribution of local magnetic field and shielding currents across the array reveals the presence of a non-uniform vortex configuration partially matched with the blind holes at sub-matching fields. We observe that the filling fraction is different in two different regions of the array. The mean vortex configuration within the array is described as a patchy vortex configuration with the patches having different mean filling fraction. The patchy nature of the vortex configuration is more pronounced at partial filling of the array at low fields while the configuration becomes more uniform with a unique filling fraction at higher fields. The metastable nature of this patchy vortex configuration is revealed by the application of magnetic field pulses of fixed height or individual pulses of varying height to the array. The metastability of the vortex configuration allows for a relatively easy way of producing flux reorganization and flux focusing effects within the blind hole array. Effect of the magnetic field pulses modifies the vortex configuration within the array and produces a uniform enhancement in the shielding current around the patterned array edges. The enhanced shielding current concentrates magnetic flux within the array by driving vortices away from the edges and towards the center of the array. The enhanced shielding current also prevents the uninhibited entry of vortices into the array. We propose that the metastable patchy vortex configuration within the blind hole array is due to a non-uniform pinning landscape leading to non-uniform filling of individual blind holes.
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Submitted 25 May, 2016;
originally announced May 2016.
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A self-calibrating optomechanical force sensor with femtonewton resolution
Authors:
John Melcher,
Julian Stirling,
Felipe Guzman Cervantes,
Jon R. Pratt,
Gordon A. Shaw
Abstract:
We report the development of an ultrasensitive optomechanical sensor designed to improve the accuracy and precision of force measurements with atomic force microscopy. The sensors reach quality factors of 4.3x10^6 and force resolution on the femtonewton scale at room temperature. Self-calibration of the sensor is accomplished using radiation pressure to create a reference force. Self-calibration e…
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We report the development of an ultrasensitive optomechanical sensor designed to improve the accuracy and precision of force measurements with atomic force microscopy. The sensors reach quality factors of 4.3x10^6 and force resolution on the femtonewton scale at room temperature. Self-calibration of the sensor is accomplished using radiation pressure to create a reference force. Self-calibration enables in situ calibration of the sensor in extreme environments, such as cryogenic ultra-high vacuum. The senor technology presents a viable route to force measurements at the atomic scale with uncertainties below the percent level.
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Submitted 21 October, 2014;
originally announced October 2014.
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MicroSQUID Force microscopy in a dilution refrigerator
Authors:
Danny Hykel,
Zhao-Sheng Wang,
Pauline Castellazzi,
Thierry Crozes,
Gorky Shaw,
Klaus Hasselbach,
K. F. Schuster
Abstract:
We present a new generation of a scanning MicroSQUID microscope operating in an inverted dilution refrigerator. The MicroSQUIDs have a size of 1.21$ \ μ$m\textsuperscript{2} and a magnetic flux sensitivity of 120 $μΦ_{0} / \sqrt{\textrm{Hz}}$ and thus a field sensitivity of %$550^{-6} \ Φ_{0} / \sqrt{\textrm{Hz}}$ 550$ \ μ\textrm{G}/ \sqrt{\textrm{Hz}}$. The scan range at low temperatures is about…
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We present a new generation of a scanning MicroSQUID microscope operating in an inverted dilution refrigerator. The MicroSQUIDs have a size of 1.21$ \ μ$m\textsuperscript{2} and a magnetic flux sensitivity of 120 $μΦ_{0} / \sqrt{\textrm{Hz}}$ and thus a field sensitivity of %$550^{-6} \ Φ_{0} / \sqrt{\textrm{Hz}}$ 550$ \ μ\textrm{G}/ \sqrt{\textrm{Hz}}$. The scan range at low temperatures is about 80 $μ$m and a coarse displacement of 5 mm in x and y direction has been implemented. The MicroSQUID-to-sample distance is regulated using a tuning fork based force detection. A MicroSQUID-to-sample distance of 420 nm has been obtained. The reliable knowledge of this distance is necessary to obtain a trustworthy estimate of the absolute value of the superconducting penetration depth. An outlook will be given on the ongoing direction of development.
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Submitted 23 March, 2014;
originally announced March 2014.
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Anomalous local magnetic field distribution and strong pinning in CaFe1.94Co0.06As2 single crystals
Authors:
Pabitra Mandal,
Gorky Shaw,
S. S. Banerjee,
Neeraj Kumar,
S. K. Dhar,
A. Thamizhavel
Abstract:
Magneto-optical imaging of a single crystal of CaFe1.94Co0.06As2, shows anomalous remnant magnetization within Meissner like regions of the superconductor. The unconventional shape of the local magnetization hysteresis loop suggests admixture of superconducting and magnetic fractions governing the response. Near the superconducting transition temperature, local magnetic field exceeds the applied f…
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Magneto-optical imaging of a single crystal of CaFe1.94Co0.06As2, shows anomalous remnant magnetization within Meissner like regions of the superconductor. The unconventional shape of the local magnetization hysteresis loop suggests admixture of superconducting and magnetic fractions governing the response. Near the superconducting transition temperature, local magnetic field exceeds the applied field resulting in a diamagnetic to positive magnetization transformation. The observed anomalies in the local magnetic field distribution are accompanied with enhanced bulk pinning in the CaFe1.94Co0.06As2 single crystals. We propose our results suggest a coexistence of superconductivity and magnetic correlations.
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Submitted 4 December, 2012;
originally announced December 2012.
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Generating strong magnetic flux shielding regions in a single crystal of Bi2Sr2CaCu2O8 using a blind hole array
Authors:
Gorky Shaw,
Biplab Bag,
S. S. Banerjee,
Hermann Suderow,
T. Tamegai
Abstract:
Magneto-optical imaging studies in a single crystal of Bi2Sr2CaCu2O8 partially patterned with a hexagonal array of pinning centers (blind holes) reveals local features in the patterned region which are distinct compared to the pristine unpatterned regions in the sample. The patterned area exhibits a strongly diamagnetic local magnetization response and is characterized by a local penetration field…
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Magneto-optical imaging studies in a single crystal of Bi2Sr2CaCu2O8 partially patterned with a hexagonal array of pinning centers (blind holes) reveals local features in the patterned region which are distinct compared to the pristine unpatterned regions in the sample. The patterned area exhibits a strongly diamagnetic local magnetization response and is characterized by a local penetration field enhanced by a factor of three. We show that strong shielding currents around the periphery of the nanopatterned region create a barrier which prevents vortex entry into the patterned region thus sustaining an effectively flux-free state upto the enhanced penetration field.
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Submitted 14 August, 2012;
originally announced August 2012.
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Critical behavior at de-pinning of a driven disordered vortex matter in 2H-NbS2
Authors:
Gorky Shaw,
Pabitra Mandal,
S. S. Banerjee,
A. Niazi,
A. K. Rastogi,
A. K. Sood,
S. Ramakrishnan,
A. K. Grover
Abstract:
We report unusual jamming in driven ordered vortex flow in 2H-NbS2. Reinitiating movement in these jammed vortices with a higher driving force, and halting it thereafter once again with a reduction in drive, unfolds a critical behavior centered around the de-pinning threshold via divergences in the lifetimes of transient states, validating the predictions of a recent simulation study, which also p…
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We report unusual jamming in driven ordered vortex flow in 2H-NbS2. Reinitiating movement in these jammed vortices with a higher driving force, and halting it thereafter once again with a reduction in drive, unfolds a critical behavior centered around the de-pinning threshold via divergences in the lifetimes of transient states, validating the predictions of a recent simulation study, which also pointed out a correspondence between plastic de-pinning in vortex matter and the notion of random organization proposed in the context of sheared colloids undergoing diffusive motion.
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Submitted 5 April, 2012;
originally announced April 2012.
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Visualizing a dilute vortex liquid to solid phase transition in a Bi2Sr2CaCu2O8 single crystal
Authors:
Gorky Shaw,
Pabitra Mandal,
S. S. Banerjee,
T. Tamegai
Abstract:
Using high sensitivity magneto-optical imaging we find evidence for a jump in local vortex density associated with a vortex liquid to solid phase transition just above the lower critical field in a single crystal of Bi2Sr2CaCu2O8. We find the regions of the sample where the jump in vortex density occurs are associated with low screening currents. In the field - temperature vortex phase diagram we…
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Using high sensitivity magneto-optical imaging we find evidence for a jump in local vortex density associated with a vortex liquid to solid phase transition just above the lower critical field in a single crystal of Bi2Sr2CaCu2O8. We find the regions of the sample where the jump in vortex density occurs are associated with low screening currents. In the field - temperature vortex phase diagram we identify phase boundaries demarcating a dilute vortex liquid phase and the vortex solid phase. The phase diagram also identifies a coexistence regime of the dilute vortex liquid and solid phases and shows the effect of pinning on the vortex liquid to solid phase transition line. We find the phase boundary lines can be fitted to the theoretically predicted expression for the low-field portion of the phase boundary delineating a dilute vortex solid from a vortex liquid phase. We show that the same theoretical fit can be used to describe the pinning dependence of the low-field phase boundary lines provided a dependence of the Lindemann number on pinning strength is considered.
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Submitted 11 October, 2012; v1 submitted 18 January, 2012;
originally announced January 2012.
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Direct evidence of magnetic correlations deep inside the superconducting state of CaFe1.94Co0.06As2
Authors:
Pabitra Mandal,
Gorky Shaw,
S. S. Banerjee,
Neeraj Kumar,
S . K. Dhar,
A. Thamizhavel
Abstract:
Imaging local magnetic field distribution in a single crystal of superconductor CaFe1.94Co0.06As2 shows the presence of anomalous remnant magnetization within Meissner like regions of the sample. Close to the superconducting transition temperature we find a coexistence of superconductivity and magnetic correlations. While the magnetic correlations are enhanced with the lowering of temperature, int…
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Imaging local magnetic field distribution in a single crystal of superconductor CaFe1.94Co0.06As2 shows the presence of anomalous remnant magnetization within Meissner like regions of the sample. Close to the superconducting transition temperature we find a coexistence of superconductivity and magnetic correlations. While the magnetic correlations are enhanced with the lowering of temperature, interestingly the area over which the magnetic phase exists shrinks at the expense of a growing superconducting phase in the sample. The coexistence of two phases deep inside the superconducting phase in this compound is maintained within the temperature range that we have explored in our experiment.
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Submitted 23 December, 2012; v1 submitted 18 January, 2012;
originally announced January 2012.
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Driven weak to strong pinning crossover in partially nanopatterned 2H-NbSe2 single crystal
Authors:
Gorky Shaw,
Jaivardhan Sinha,
Shyam Mohan,
S. S. Banerjee
Abstract:
Investigations into the heterogeneous pinning properties of the vortex state created by partially nano-patterning single crystals of 2H-NbSe2 reveal an atypical magnetization response which is significantly drive dependent. Analysis of the magnetization response shows non-monotonic behavior of the magnetization relaxation rate with varying magnetic field sweep rate. With all the patterned pinning…
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Investigations into the heterogeneous pinning properties of the vortex state created by partially nano-patterning single crystals of 2H-NbSe2 reveal an atypical magnetization response which is significantly drive dependent. Analysis of the magnetization response shows non-monotonic behavior of the magnetization relaxation rate with varying magnetic field sweep rate. With all the patterned pinning centers saturated with vortices, we find that the pinning force experienced by the vortices continues to increase with increasing drive. Our studies reveal an unconventional dynamic weak to strong pinning crossover where the flow of the vortex state appears to be hindered or jammed as it is driven harder through the interstitial voids in the patterned pinning lattice.
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Submitted 31 May, 2010;
originally announced May 2010.
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Interacting nanopatterned pins: barriers towards creating interstitial vortices
Authors:
Gorky Shaw,
Shyam Mohan,
Jaivardhan Sinha,
S. S. Banerjee
Abstract:
We show that by nano-patterning a superconductor (NbSe2 single crystal) with an array of blind holes produces significant magnetic field sweep rate dependent metastable magnetization response. Our results are explained on the basis of a unique collective action of the blind holes pins which creates a barrier against vortex redistribution inside the sample. We propose that this barrier leads to a…
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We show that by nano-patterning a superconductor (NbSe2 single crystal) with an array of blind holes produces significant magnetic field sweep rate dependent metastable magnetization response. Our results are explained on the basis of a unique collective action of the blind holes pins which creates a barrier against vortex redistribution inside the sample. We propose that this barrier leads to a phase separation creating distinct population of vortices viz., those pinned on blind holes and those confined in the interstitials between the holes.
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Submitted 10 November, 2008;
originally announced November 2008.
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Effects of hydrostatic pressure on the magnetic susceptibility of ruthenium oxide Sr3Ru2O7: Evidence for pressure-enhanced antiferromagnetic instability
Authors:
Yuri V. Sushko,
Bruno DeHarak,
Gang Cao,
G. Shaw,
D. K. Powell,
J. W. Brill
Abstract:
Hydrostatic pressure effects on the temperature- and magnetic field dependencies of the in-plane and out-of-plane magnetization of the bi-layered perovskite Sr3Ru2O7 have been studied by SQUID magnetometer measurements under a hydrostatic helium-gas pressure. The anomalously enhanced low-temperature value of the paramagnetic susceptibility has been found to systematically decrease with increasin…
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Hydrostatic pressure effects on the temperature- and magnetic field dependencies of the in-plane and out-of-plane magnetization of the bi-layered perovskite Sr3Ru2O7 have been studied by SQUID magnetometer measurements under a hydrostatic helium-gas pressure. The anomalously enhanced low-temperature value of the paramagnetic susceptibility has been found to systematically decrease with increasing pressure. The effect is accompanied by an increase of the temperature Tmax of a pronounced peak of susceptibility. Thus, magnetization measurements under hydrostatic pressure reveal that the lattice contraction in the structure of Sr3Ru2O7 promotes antiferromagnetism and not ferromagnetism, contrary to the previous beliefs. The effects can be explained by the enhancement of the inter-bi-layer antiferromagnetic spin coupling, driven by the shortening of the superexchange path, and suppression, due to the band-broadening effect, of competing itinerant ferromagnetic correlations.
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Submitted 11 March, 2003;
originally announced March 2003.