National Institute of Standards and Technology
Gaithersburg, Maryland | United States
Main Specialties: Other
Additional Specialties: Magnetism, Hysteretic Behavior, Nanofabrication, Neutron Scattering
Primary Affiliation: National Institute of Standards and Technology - Gaithersburg, Maryland , United States
PubMed Central Citations
University of Minnesota
NIST Center for Neutron Research
University of Gothenburg
Lawrence Berkeley National Laboratory
University of California
NIST Center for Neutron Research
51PubMed Central Citations
Phys. Rev. B 90, 104410 (2014)
Physical Review B
Combined first-order reversal curve (FORC) analyses of the magnetization (M-FORC) and magnetoresistance (MR-FORC) have been employed to provide a comprehensive study of the M-MR correlation in two canonical systems: a NiFe/Cu/FePt pseudo spin valve (PSV) and a [Co/Cu]8 multilayer. In the PSV, due to the large difference in switching fields and minimal interactions between the NiFe and the FePt layers, the M and MR show a simple one-to-one relationship during reversal. In the [Co/Cu]8 multilayer, the correlation between the magnetization reversal and the MR evolution is more complex. This is primarily due to the similar switching fields of, and interactions between, the constituent Co layers. The FORC protocol accesses states with much higher spin disorders and larger MRs than those found along the conventional major loop field cycle. Unlike the M-FORC measurements, which only probe changes in the macroscopic magnetization, the MR-FORCs are more sensitive to the microscopic domain configurations as those are most important in determining the resultant MR effect size. This approach is generally applicable to spintronic systems to realize the maximum spin disorder and the largest MR.
APL Mat. 2, 086106 (2014)
The A1-L10 phase transformation has been investigated in (001) FeCuPt thin films prepared by atomic-scale multilayer sputtering and rapid thermal annealing (RTA). Traditional x-ray diffraction is not always applicable in generating a true order parameter, due to non-ideal crystallinity of the A1 phase. Using the first-order reversal curve (FORC) method, the A1 and L10 phases are deconvoluted into two distinct features in the FORC distribution, whose relative intensities change with the RTA temperature. The L10 ordering takes place via a nucleation-and-growth mode. A magnetization-based phase fraction is extracted, providing a quantitative measure of the L10 phase homogeneity.
J. Appl. Phys. 116, 033922 (2014)
Journal of Applied Physics
The magnetization reversal process is investigated in perpendicular spring magnets of epitaxial L10 FePd/FePt (24 nm) thin films with varying FePd thickness. For thin FePd layers, the reversal is initiated by the nucleation of reversed bubble domains and is then mainly accomplished by the depinning and lateral movement of domain walls. For thick FePd layers, the magnetization reversal is predominantly governed by the nucleation process rather than wall motion, resulting in an increased density of bubble domains at more negative magnetic fields. The switching field is reduced significantly with increasing FePd thickness and exchange springs are formed locally due to a tilted magnetization in the FePd. These results arise from the interplay between differently strong perpendicular magnetic anisotropies in FePd and FePt and from layer dependent structural modifications, which is important for high density magnetic recording media.
Supercond. Sci. Technol. 26 085018 (2013)
Superconductor Science and Technology
The dynamics of the pinned vortex, antivortex and interstitial vortex have been studied in superconducting/magnetic hybrids consisting of arrays of Co/Pd multilayer nanodots embedded in Nb films. The magnetic nanodots show out-of-plane magnetization at the remanent state. This magnetic state allows for superconducting vortex lattices of different types in an applied homogeneous magnetic field. We experimentally and theoretically show three such lattices: (i) a lattice containing only antivortices; (ii) a vortex lattice entirely pinned on the dots; and (iii) a vortex lattice with pinned and interstitial vortices. Between the flux creep (low vortex velocity) and the free flux flow (high vortex velocity) regimes the interaction between the magnetic array and the vortex lattice governs the vortex dynamics, which in turn enables distinguishing experimentally the type of vortex lattice which governs the dissipation. We show that the vortex lattice with interstitial vortices has the highest onset velocity where the lattice becomes ordered, whereas the pinned vortex lattice has the smallest onset velocity. Further, for this system, we directly estimate that the external force needed to depin vortices is 60% larger than the one needed to depin antivortices; therefore we are able to decouple the antivortex–vortex motion.
Appl. Phys. Lett. 103, 022409
Applied Physics Letters
We have observed distinct temperature-dependent magnetization reversal modes in a perpendicular (Co/Pd)4/Co/Cu/(Co/Ni)4/Co pseudo-spin-valve, which are correlated with spin-transport properties. At 300 K, magnetization reversal occurs by vertically correlated domains. Below 200 K the hysteresis loop becomes bifurcated due to laterally correlated reversal of the individual stacks. The magnetic configuration change also leads to higher spin disorders and a significant increase in the giant magnetoresistance effect. First order reversal curve measurements reveal that the coupled state can be re-established through field cycling and allow direct determination of the interlayer coupling strength as a function of temperature.
J. Appl. Phys. 113, 203910 (2013)
Journal of Applied Physics
We present a comprehensive investigation of the size-dependent switching characteristics and spin wave modes of FePt nanoelements. Curved nanomagnets (“caps”) are compared to flat disks of identical diameter and volume over a size range of 100 to 300 nm. Quasi-static magnetization reversal analysis using first-order reversal curves shows that spherical caps have lower vortex nucleation and annihilation fields than the flat disks. As the element diameter decreases, the reversal mechanism in the caps crosses over sooner to coherent rotation than in the disks. The magnetization dynamics are studied using optically induced small angle precession and reveal a strong size dependence that differs for the two shapes. Flat disks exhibit well-known center and edge modes at all sizes, but as the diameter of the caps increases from 100 to 300 nm, additional oscillation modes appear in agreement with dynamic micromagnetic simulations. In addition, we show that the three-dimensional curvature of the cap causes a much greater sensitivity to the applied field angle, which provides an additional way for controlling the ultrafast response of nanomagnetic elements.
Appl. Phys. Lett. 102, 132406 (2013)
Applied Physics Letters
We have achieved (001) oriented L10 (Fe1−x Cu x)55Pt45 thin films, with magnetic anisotropy up to 3.6 × 107 erg/cm3, using atomic-scale multilayer sputtering and post annealing at 400 °C for 10 s. By fixing the Pt concentration, structure and magnetic properties are systematically tuned by the Cu addition. Increasing Cu content results in an increase in the tetragonal distortion of the L10 phase, significant changes to the film microstructure, and lowering of the saturation magnetization and anisotropy. The relatively convenient synthesis conditions, along with the tunable magnetic properties, make such materials highly desirable for future magnetic recording technologies.
Appl. Phys. Lett. 102, 052601 (2013)
Applied Physics Letters
Hybrid superconducting/magnetic nanostructures on Si substrates have been built with identical physical dimensions but different magnetic configurations. By constructing arrays based on Co-dots with in-plane, out-of-plane, and vortex state magnetic configurations, the stray fields are systematically tuned. Dissipation in the mixed state of superconductors can be decreased (increased) by several orders of magnitude by decreasing (increasing) the stray magnetic fields. Furthermore, ordering of the stray fields over the entire array helps to suppress dissipation and enhance commensurability effects increasing the number of dissipation minima.
J. Mater. Chem., 2012, 22, 8449
Journal of Materials Chemistry
The surface modification of various nanoparticles with silica has been exploited to increase their utility for bioapplications. However, silica encapsulation through conventional methods requires long reaction times (hours to days). Herein, we demonstrated that uniform and spherical silica encapsulation of magnetic nanoparticles can be achieved within 10 min via microwave irradiation after phase transferring monodisperse magnetic nanoparticles from organic to water phase. In addition, we showed that silica shell addition through microwave synthesis is more effective than conventional heating methods, such as a hot plate. The approach that we propose may be useful in preparing multifunctional nano-probes, particularly for radiolabeling, which requires fast preparation times.
IEEE Trans. Magn. 47, 2988 (2011)
IEEE Transactions on Magnetics
Understanding how interactions within magnetic systems affect the reversal process is critical to the development of magnetic recording media. Using the first-order reversal curve (FORC) method we demonstrate an ability to map out and uniquely identify magnetic recording media. We evaluate the interactions and intrinsic switching field distribution (SFD) in single layer and exchange coupled composite granular recording media. Landau-Lifshitz-Gilbert-based simulations are used to probe the roles lateral and vertical exchange interactions have on the distribution of reversal events. We find that there is an optimal value for these terms which maximizes homogeneity of the reversal behavior.
Phys. Rev. B 83, 060415 (2011)
Phys. Rev. B
Reproducible control of the magnetic vortex state in nanomagnets is of critical importance. We report on chirality control by manipulating the size and/or thickness of asymmetric Co dots. Below a critical diameter and/or thickness, chirality control is achieved by the nucleation of a single vortex. Interestingly, above these critical dimensions, chirality control is realized by the nucleation and subsequent coalescence of two vortices, resulting in a single vortex with the opposite chirality as found in smaller dots. Micromagnetic simulations and magnetic force microscopy highlight the role of edge-bound half vortices in facilitating the coalescence process.