arXiv subjects

Ivan K. Schuller

Publications and source records attributed to Ivan K. Schuller.

At least 73 records · Page 4Linked to original sources

arXiv2012

Deconvoluting Reversal Modes in Exchange Biased Nanodots

Ensemble-averaged exchange bias in arrays of Fe/FeF2 nanodots has been deconvoluted into local, microscopic, bias separately experienced by nanodots going through different reversal modes. The relative fraction of dots in each mode can be modified by exchange bias. Single domain dots exhibit a simple loop shift, while vortex state dots have asymmetric shifts in the vortex nucleation and annihilation fields, manifesting local incomplete domain walls in these nanodots as magnetic vortices with tilted cores.

cond-mat.mtrl-sci↗

arXiv2012

Exchange Bias Induced by the Fe3O4 Verwey transition

We present a study of the exchange bias in different configurations of V2O3 thin films with ferromagnetic layers. The exchange bias is accompanied by a large vertical shift in the magnetization. These effects are only observed when V2O3 is grown on top of Ni80Fe20 permalloy. The magnitude of the vertical shift is as large as 60% of the total magnetization which has never been reported in any system. X-Ray diffraction studies show that the growth conditions promote the formation of a ferrimagnetic Fe3O4 interlayer. The change in the easy magnetization axis of Fe3O4 across the Verwey transition at 120 K is correlated with the appearance of exchange bias and vertical shift in magnetization. Both phenomena disappear above 120 K, indicating for the first time a direct relationship between the magnetic signature of the Verwey transition and exchange bias.

cond-mat.mtrl-sci↗

arXiv2012

Temperature dependence of in-plane correlation lengths in exchange biased Co/FeF2

We have measured resonant soft x-ray diffuse magnetic scattering as a function of temperature in a positively exchange biased Co/FeF2 bilayer and analyzed the data in the distorted wave Born approximation to obtain in-plane charge and magnetic correlation lengths associated with the Co and FeF2 layers and estimate interfacial roughness. Tuning to the Fe and Co L3 edges reveals significantly different temperature trends in these quantities in the antiferromagnetic and ferromagnetic layers, respectively. While the magnetic correlation length of the uncompensated interfacial spins in FeF2 layer increase as temperature decreases, these quantities remain unchanged in the Co layer. Our results indicate that uncompensated Fe spins order within a range of few hundred nanometers in otherwise randomly distributed uncompensated magnetic moments, giving rise to spin clusters in the antiferromagnet whose size increase as the temperature decrease.

cond-mat.mtrl-sci↗

arXiv2011

Upper limit to magnetism in LaAlO3/SrTiO3 heterostructures

Using polarized neutron reflectometry (PNR) we measured the neutron spin dependent reflectivity from four LaAlO3/SrTiO3 superlattices. This experiment implies that the upper limit for the magnetization induced by an 11 T magnetic field at 1.7 K is 2 emu/cm3. SQUID magnetometry of the superlattices sporadically finds an enhanced moment, possibly due to experimental artifacts. These observations set important restrictions on theories which imply a strongly enhanced magnetism at the interface between LaAlO3 and SrTiO3.

cond-mat.mtrl-sci↗

arXiv2011

Asymmetric magnetic dots: A way to control magnetic properties

We have used Monte Carlo simulations to investigate the magnetic properties of asymmetric dots as a function of their geometry. The asymmetry of round dots is produced by cutting off a fraction of the dot and is characterized by an asymmetry parameter $α$. This shape asymmetry has interesting effects on the coercivity ($H_{c}$), remanence ($M_{r}$), and barrier for vortex and C- state formation. The dependences of $H_{c}$ and $M_{r}$ are non monotonic as a function of $α$ with a well defined minima in these parameters. The vortex enters the most asymmetric part and exits through the symmetric portion of the dot. With increasing $α$ the vortex formation starts with a C-state which persists for longer fields and the barrier for vortex exit diminishes with increasing asymmetry, thus providing control over the magnetic chirality. This implies interesting, naively-unexpected, magnetic behavior as a function of geometry and magnetic field.

cond-mat.mes-hall↗

arXiv2011

Evidence for a capacitor network near the metal insulator transition in VO2 thin films probed by in-plane impedance spectroscopy

Impedance spectroscopy measurements were performed in high quality Vanadium dioxide (VO2) thin films. This technique allows us investigate the resistive and capacitive contribution to the dielectric response near the metal-insulator transition (MIT). A non ideal RC behavior was found in our films from room temperature up to 334 K. A decrease of the total capacitance was found in this region, possibly due to interface effects. Above the MIT, the system behaves like a metal as expected, and a modified equivalent circuit is necessary to describe the impedance data adequately. Around the MIT, an increase of the total capacitance is observed.

cond-mat.mtrl-sci↗

arXiv2011

Vortex ratchet reversal: The role of interstitial vortices

Triangular arrays of Ni nanotriangles embedded in superconducting Nb films exhibit unexpected dynamical vortex effects. Collective pinning with a vortex lattice configuration different from the expected fundamental triangular "Abrikosov state" is found. The vortex motion which prevails against the triangular periodic potential is produced by channelling effects between triangles. Interstitial vortices coexisting with pinned vortices in this asymmetric potential, lead to ratchet reversal, i.e. a DC output voltage which changes sign with the amplitude of an applied alternating drive current. In this landscape, ratchet reversal is always observed at all magnetic fields (all numbers of vortices) and at different temperatures. The ratchet reversal is unambiguously connected to the presence of two locations for the vortices: interstitial and above the artificial pinning sites.

cond-mat.supr-con↗

arXiv2011

Controllable manipulation of superconductivity using magnetic vortices

The magneto-transport of a superconducting/ferromagnetic hybrid structure consisting of a superconducting thin film in contact with an array of magnetic nanodots in the so-called "magnetic vortex-state" exhibits interesting properties. For certain magnetic states, the stray magnetic field from the vortex array is intense enough to drive the superconducting film into the normal state. In this fashion, the normal-to-superconducting phase transition can be controlled by the magnetic history. The strong coupling between superconducting and magnetic subsystems allows characteristically ferromagnetic properties, such as hysteresis and remanence, to be dramatically transferred into the transport properties of the superconductor.

cond-mat.supr-con↗

arXiv2009

Rocking ratchet induced by pure magnetic potentials with broken reflection symmetry

A ratchet effect (the rectification of an ac injected current) which is purely magnetic in origin has been observed in a superconducting-magnetic nanostructure hybrid. The hybrid consists of a superconducting Nb film in contact with an array of nanoscale magnetic triangles, circular rings or elliptical rings. The arrays were placed into well-defined remanent magnetic states by application of different magnetic field cycles. The stray fields from these remanent states provide a magnetic landscape which influences the motion of superconducting vortices. We examined both randomly varying landscapes from demagnetized samples, and ordered landscapes from samples at remanence after saturation in which the magnetic rings form parallel onion states containing two domain walls. The ratchet effect is absent if the rings are in the demagnetized state or if the vortices propagate parallel to the magnetic reflection symmetry axis (perpendicular to the magnetic domain walls) in the ordered onion state. On the other hand, when the vortices move perpendicular to the magnetic reflection symmetry axis in the ordered onion state (parallel to the domain walls) a clear ratchet effect is observed. This behavior differs qualitatively from that observed in samples containing arrays of triangular Ni nanostructures, which show a ratchet of structural origin.

cond-mat.supr-con↗

arXiv2009

Enhanced spin-flip scattering at the surface of copper in lateral spin valves

We performed non-local electrical measurements of a series of Py/Cu lateral spin valve devices with different Cu thicknesses. We show that both the spin diffusion length of Cu and the apparent spin polarization of Py increase with Cu thickness. By fitting the results to a modified spin-diffusion model, we show that the spin diffusion length of Cu is dominated by spin-flip scattering at the surface. In addition, the dependence of spin polarization of Py on Cu thickness is due to a strong spin-flip scattering at the Py/Cu interface.

cond-mat.mtrl-sci↗

arXiv2009

Angular Dependence of Vortex Annihilation Fields in Asymmetric Co Dots

Shape asymmetries in nominally circular nanomagnets provide a potential means for vortex chirality control. However, in realistic arrays their effects are challenging to probe since asymmetric magnetization reversal processes are often averaged to include distributions over all angles. Here we investigate how shape asymmetry influences the vortex reversal in arrays of sub-micron edge-cut Co dots. We find that the vortices can be manipulated to annihilate at particular sites under different field orientations and cycle sequences. The vortex annihilation field and degree of chirality control depend sensitively on the angular position of the applied field relative to the flat edge of the dots. For small angles, the major loop annihilation field is significantly larger than that found from the half loop and the vortex chirality can be well controlled. At intermediate angles the chirality control is lost and an interesting crossover in the annihilation field is found: the half loop actually extrudes outside of the major loop, exhibiting a larger vortex annihilation field. At large angles the annihilation fields along major and half loops become degenerate.

cond-mat.mtrl-sci↗

arXiv2009

Control of spin injection by direct current in lateral spin valves

The spin injection and accumulation in metallic lateral spin valves with transparent interfaces is studied using d.c. injection current. Unlike a.c.-based techniques, this allows investigating the effects of the direction and magnitude of the injected current. We find that the spin accumulation is reversed by changing the direction of the injected current, whereas its magnitude does not change. The injection mechanism for both current directions is thus perfectly symmetric, leading to the same spin injection efficiency for both spin types. This result is accounted for by a spin-dependent diffusion model. Joule heating increases considerably the local temperature in the spin valves when high current densities are injected ($\sim$80--105 K for 1--2$\times10^{7}$A cm$^{-2}$), strongly affecting the spin accumulation.

cond-mat.mes-hall↗

arXiv2009

Signature of persistent metallic domains in FORC measurements of the VO$_2$ metal-insulator transition

We have performed first order reversal curve measurements of the temperature-driven metal-insulator transition in VO$_2$ thin films, which enable quantitative analysis of the hysteresis behavior. An unexpected tail-like feature in the contour plot of the reversal curve distribution indicates the existence of metallic domains, even at temperatures below the closing of the hysteresis. These domains interact with the surrounding medium and change the reversal path relative to a path from a \emph{fully} insulating state. With this in mind, and assuming that such interaction persist through the entire phase transition, we develop a model where the driving force (or energy barrier) in charge of opening a hysteresis in VO$_2$ are inter-domain interactions. This model is intrinsically different from the Preisach model usually used to describe hysteresis; given that it looks for the microscopic origin of the hysteresis, and provides physical parameters to characterize it.

cond-mat.mtrl-sci↗

arXiv2008

Switchable collective pinning of flux quanta using magnetic vortex arrays

We constructed a superconducting/ferromagnetic hybrid system in which the ordering of the pinning potential landscape for flux quanta can be manipulated. Flux pinning is induced by an array of magnetic nanodots in the magnetic vortex state, and controlled by the magnetic history. This allows switching on and off the collective pinning of the flux-lattice. In addition, we observed field-induced superconductivity that originates from the annihilation of flux quanta induced by the stray fields from the magnetic vortices.

cond-mat.supr-con↗

arXiv2008

Multiple Avalanches Across the Metal-Insulator Transition of Vanadium Oxide Nano-scaled Junctions

The metal insulator transition of nano-scaled $VO_2$ devices is drastically different from the smooth transport curves generally reported. The temperature driven transition occurs through a series of resistance jumps ranging over 2 decades in amplitude, indicating that the transition is caused by avalanches. We find a power law distribution of the jump amplitudes, demonstrating an inherent property of the $VO_2$ films. We report a surprising relation between jump amplitude and device size. A percolation model captures the general transport behavior, but cannot account for the statistical behavior.

cond-mat.mes-hall↗

arXiv2007

Dynamic Spin-Polarized Resonant Tunneling in Magnetic Tunnel Junctions

Precisely engineered tunnel junctions exhibit a long sought effect that occurs when the energy of the electron is comparable to the potential energy of the tunneling barrier. The resistance of metal-insulator-metal tunnel junctions oscillates with an applied voltage when electrons that tunnel directly into the barrier's conduction band interfere upon reflection at the classical turning points: the insulator-metal interface, and the dynamic point where the incident electron energy equals the potential barrier inside the insulator. A model of tunneling between free electron bands using the exact solution of the Schroedinger equation for a trapezoidal tunnel barrier qualitatively agrees with experiment.

cond-mat.mtrl-sci↗

arXiv2007

Magnetic Fingerprints of sub-100 nm Fe Nanodots

Sub-100 nm nanomagnets not only are technologically important, but also exhibit complex magnetization reversal behaviors as their dimensions are comparable to typical magnetic domain wall widths. Here we capture magnetic "fingerprints" of 1 billion Fe nanodots as they undergo a single domain to vortex state transition, using a first-order reversal curve (FORC) method. As the nanodot size increases from 52 nm to 67 nm, the FORC diagrams reveal striking differences, despite only subtle changes in their major hysteresis loops. The 52 nm nanodots exhibit single domain behavior and the coercivity distribution extracted from the FORC distribution agrees well with a calculation based on the measured nanodot size distribution. The 58 and 67 nm nanodots exhibit vortex states, where the nucleation and annihilation of the vortices are manifested as butterfly-like features in the FORC distribution and confirmed by micromagnetic simulations. Furthermore, the FORC method gives quantitative measures of the magnetic phase fractions, and vortex nucleation and annihilation fields.

cond-mat.mtrl-sci↗

arXiv2006

Dynamics of Spontaneous Magnetization Reversal in Exchange Biased Heterostructures

The dependence of thermally induced spontaneous magnetization reversal on time-dependent cooling protocols was studied. Slower cooling and longer waiting close to the Nèel temperature of the antiferromagnet ($T_N$) enhances the magnetization reversal. Cycling the temperature around $T_N$ leads to a thermal training effect under which the reversal magnitude increases with each cycle. These results suggest that spontaneous magnetization reversal is energetically favored, contrary to our present understanding of positive exchange bias.

cond-mat.mtrl-sci↗