Publications by authors named "Dustin A Gilbert"

21 Publications

  • Page 1 of 1

Reconstructing phase-resolved hysteresis loops from first-order reversal curves.

Sci Rep 2021 Feb 17;11(1):4018. Epub 2021 Feb 17.

Physics Department, University of California, Davis, CA, 95616, USA.

The first order reversal curve (FORC) method is a magnetometry based technique used to capture nanoscale magnetic phase separation and interactions with macroscopic measurements using minor hysteresis loop analysis. This makes the FORC technique a powerful tool in the analysis of complex systems which cannot be effectively probed using localized techniques. However, recovering quantitative details about the identified phases which can be compared to traditionally measured metrics remains an enigmatic challenge. We demonstrate a technique to reconstruct phase-resolved magnetic hysteresis loops by selectively integrating the measured FORC distribution. From these minor loops, the traditional metrics-including the coercivity and saturation field, and the remanent and saturation magnetization-can be determined. In order to perform this analysis, special consideration must be paid to the accurate quantitative management of the so-called reversible features. This technique is demonstrated on three representative materials systems, high anisotropy FeCuPt thin-films, Fe nanodots, and SmCo/Fe exchange spring magnet films, and shows excellent agreement with the direct measured major loop, as well as the phase separated loops.
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http://dx.doi.org/10.1038/s41598-021-83349-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7889904PMC
February 2021

3D Nanomagnetism in Low Density Interconnected Nanowire Networks.

Nano Lett 2021 Jan 10;21(1):716-722. Epub 2020 Dec 10.

Physics Department, University of California, Davis, California 95618, United States.

Free-standing, interconnected metallic nanowire networks with densities as low as 40 mg/cm have been achieved over centimeter-scale areas, using electrodeposition into polycarbonate membranes that have been ion-tracked at multiple angles. Networks of interconnected magnetic nanowires further provide an exciting platform to explore 3-dimensional nanomagnetism, where their structure, topology, and frustration may be used as additional degrees of freedom to tailor the materials properties. New magnetization reversal mechanisms in cobalt networks are captured by the first-order reversal curve method, which demonstrate the evolution from strong demagnetizing dipolar interactions to intersection-mediated domain wall pinning and propagation, and eventually to shape-anisotropy dominated magnetization reversal. These findings open up new possibilities for 3-dimensional integrated magnetic devices for memory, complex computation, and neuromorphics.
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http://dx.doi.org/10.1021/acs.nanolett.0c04366DOI Listing
January 2021

Termination switching of antiferromagnetic proximity effect in topological insulator.

Sci Adv 2020 Aug 12;6(33):eaaz8463. Epub 2020 Aug 12.

Department of Electrical and Computer Engineering, University of California, Los Angeles, CA 90095, USA.

This work reports the ferromagnetism of topological insulator, (Bi,Sb)Te (BST), with a Curie temperature of approximately 120 K induced by magnetic proximity effect (MPE) of an antiferromagnetic CrSe. The MPE was shown to be highly dependent on the stacking order of the heterostructure, as well as the interface symmetry: Growing CrSe on top of BST results in induced ferromagnetism, while growing BST on CrSe yielded no evidence of an MPE. Cr-termination in the former case leads to double-exchange interactions between Cr surface states and Cr bulk states. This Cr-Cr exchange stabilizes the ferromagnetic order localized at the interface and magnetically polarizes the BST Sb band. In contrast, Se-termination at the CrSe/BST interface yields no detectable MPE. These results directly confirm the MPE in BST films and provide critical insights into the sensitivity of the surface state.
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http://dx.doi.org/10.1126/sciadv.aaz8463DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7423361PMC
August 2020

Optical and Magnetic Properties of Ag-Ni Bimetallic Nanoparticles Assembled via Pulsed Laser-Induced Dewetting.

ACS Omega 2020 Aug 21;5(30):19285-19292. Epub 2020 Jul 21.

Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States.

Pulsed laser-induced dewetting (PLiD) of AgNi thin films results in phase-separated bimetallic nanoparticles with size distributions that depend on the initial thin film thickness. Co-sputtering of Ag and Ni is used to generate the as-deposited (AD) nanogranular supersaturated thin films. The magnetic and optical properties of the AD thin films and PLiD nanoparticles are characterized using a vibrating sample magnetometer, optical absorption spectroscopy, and electron energy loss spectroscopy (EELS). Magnetic measurements demonstrate that AgNi nanoparticles are ferromagnetic at room temperature when the nanoparticle diameters are >20 nm and superparamagnetic <20 nm. Optical measurements show that all nanoparticle size distributions possess a local surface plasmon resonance (LSPR) peak that red-shifts with increasing diameter. Following PLiD, a Janus nanoparticle morphology is observed in scanning transmission electron microscopy, and low-loss EELS reveals size-dependent Ag and Ni LSPR dipole modes, while higher order modes appear only in the Ag hemisphere. PLiD of Ag-Ni thin films is shown to be a viable technique to generate bimetallic nanoparticles with both magnetic and plasmonic functionality.
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http://dx.doi.org/10.1021/acsomega.0c02894DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7409265PMC
August 2020

Observation of Quantum Anomalous Hall Effect and Exchange Interaction in Topological Insulator/Antiferromagnet Heterostructure.

Adv Mater 2020 Aug 21;32(34):e2001460. Epub 2020 Jul 21.

Department of Electrical and Computer Engineering, University of California, Los Angeles, CA, 90095, USA.

Integration of a quantum anomalous Hall insulator with a magnetically ordered material provides an additional degree of freedom through which the resulting exotic quantum states can be controlled. Here, an experimental observation is reported of the quantum anomalous Hall effect in a magnetically-doped topological insulator grown on the antiferromagnetic insulator Cr O . The exchange coupling between the two materials is investigated using field-cooling-dependent magnetometry and polarized neutron reflectometry. Both techniques reveal strong interfacial interaction between the antiferromagnetic order of the Cr O and the magnetic topological insulator, manifested as an exchange bias when the sample is field-cooled under an out-of-plane magnetic field, and an exchange spring-like magnetic depth profile when the system is magnetized within the film plane. These results identify antiferromagnetic insulators as suitable candidates for the manipulation of magnetic and topological order in topological insulator films.
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http://dx.doi.org/10.1002/adma.202001460DOI Listing
August 2020

Two-way magnetic resonance tuning and enhanced subtraction imaging for non-invasive and quantitative biological imaging.

Nat Nanotechnol 2020 06 25;15(6):482-490. Epub 2020 May 25.

Department of Biochemistry and Molecular Medicine, UC Davis Comprehensive Cancer Center, University of California Davis, Sacramento, CA, USA.

Distance-dependent magnetic resonance tuning (MRET) technology enables the sensing and quantitative imaging of biological targets in vivo, with the advantage of deep tissue penetration and fewer interactions with the surroundings as compared with those of fluorescence-based Förster resonance energy transfer. However, applications of MRET technology in vivo are currently limited by the moderate contrast enhancement and stability of T-based MRET probes. Here we report a new two-way magnetic resonance tuning (TMRET) nanoprobe with dually activatable T and T magnetic resonance signals that is coupled with dual-contrast enhanced subtraction imaging. This integrated platform achieves a substantially improved contrast enhancement with minimal background signal and can be used to quantitatively image molecular targets in tumours and to sensitively detect very small intracranial brain tumours in patient-derived xenograft models. The high tumour-to-normal tissue ratio offered by TMRET in combination with dual-contrast enhanced subtraction imaging provides new opportunities for molecular diagnostics and image-guided biomedical applications.
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http://dx.doi.org/10.1038/s41565-020-0678-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7307456PMC
June 2020

Record thermopower found in an IrMn-based spintronic stack.

Nat Commun 2020 Apr 24;11(1):2023. Epub 2020 Apr 24.

Fert Beijing Institute, BDBC, School of Microelectronics, Beihang University, 100191, Beijing, China.

The Seebeck effect converts thermal gradients into electricity. As an approach to power technologies in the current Internet-of-Things era, on-chip energy harvesting is highly attractive, and to be effective, demands thin film materials with large Seebeck coefficients. In spintronics, the antiferromagnetic metal IrMn has been used as the pinning layer in magnetic tunnel junctions that form building blocks for magnetic random access memories and magnetic sensors. Spin pumping experiments revealed that IrMn Néel temperature is thickness-dependent and approaches room temperature when the layer is thin. Here, we report that the Seebeck coefficient is maximum at the Néel temperature of IrMn of 0.6 to 4.0 nm in thickness in IrMn-based half magnetic tunnel junctions. We obtain a record Seebeck coefficient 390 (±10) μV K at room temperature. Our results demonstrate that IrMn-based magnetic devices could harvest the heat dissipation for magnetic sensors, thus contributing to the Power-of-Things paradigm.
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http://dx.doi.org/10.1038/s41467-020-15797-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7181642PMC
April 2020

Correlation-driven eightfold magnetic anisotropy in a two-dimensional oxide monolayer.

Sci Adv 2020 Apr 10;6(15):eaay0114. Epub 2020 Apr 10.

Hefei National Laboratory for Physical Sciences at the Microscale, National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, China.

Engineering magnetic anisotropy in two-dimensional systems has enormous scientific and technological implications. The uniaxial anisotropy universally exhibited by two-dimensional magnets has only two stable spin directions, demanding 180° spin switching between states. We demonstrate a previously unobserved eightfold anisotropy in magnetic SrRuO monolayers by inducing a spin reorientation in (SrRuO)/(SrTiO) superlattices, in which the magnetic easy axis of Ru spins is transformed from uniaxial 〈001〉 direction ( < 3) to eightfold 〈111〉 directions ( ≥ 3). This eightfold anisotropy enables 71° and 109° spin switching in SrRuO monolayers, analogous to 71° and 109° polarization switching in ferroelectric BiFeO. First-principle calculations reveal that increasing the SrTiO layer thickness induces an emergent correlation-driven orbital ordering, tuning spin-orbit interactions and reorienting the SrRuO monolayer easy axis. Our work demonstrates that correlation effects can be exploited to substantially change spin-orbit interactions, stabilizing unprecedented properties in two-dimensional magnets and opening rich opportunities for low-power, multistate device applications.
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http://dx.doi.org/10.1126/sciadv.aay0114DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7148107PMC
April 2020

Interfacial-Redox-Induced Tuning of Superconductivity in YBaCuO.

ACS Appl Mater Interfaces 2020 Jan 10;12(4):4741-4748. Epub 2020 Jan 10.

Physics Department , Georgetown University , Washington, D.C. 20057 , United States.

Solid-state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here, we report a simple, scalable approach allowing for manipulation of the superconducting transition in optimally doped YBaCuO (YBCO) films via a chemically driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. This work demonstrates an effective step toward total ionic tuning of superconductivity in oxides, an interface-induced effect that goes well into the quasi-bulk regime, opening-up possibilities for electric field manipulation.
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http://dx.doi.org/10.1021/acsami.9b18820DOI Listing
January 2020

Hydrogen finds a home in ionic devices.

Nat Mater 2019 01;18(1):7-8

NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD, USA.

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http://dx.doi.org/10.1038/s41563-018-0247-6DOI Listing
January 2019

Exchange-biasing topological charges by antiferromagnetism.

Nat Commun 2018 07 17;9(1):2767. Epub 2018 Jul 17.

Department of Electrical and Computer Engineering, Department of Physics and Astronomy, Department of Materials Science and Engineering, University of California, Los Angeles, CA, 90095, USA.

Geometric Hall effect is induced by the emergent gauge field experienced by the carriers adiabatically passing through certain real-space topological spin textures, which is a probe to non-trivial spin textures, such as magnetic skyrmions. We report experimental indications of spin-texture topological charges induced in heterostructures of a topological insulator (Bi,Sb)Te coupled to an antiferromagnet MnTe. Through a seeding effect, the pinned spins at the interface leads to a tunable modification of the averaged real-space topological charge. This effect experimentally manifests as a modification of the field-dependent geometric Hall effect when the system is field-cooled along different directions. This heterostructure represents a platform for manipulating magnetic topological transitions using antiferromagnetic order.
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http://dx.doi.org/10.1038/s41467-018-05166-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6050290PMC
July 2018

Growth-Induced In-Plane Uniaxial Anisotropy in VO/Ni Films.

Sci Rep 2017 10 18;7(1):13471. Epub 2017 Oct 18.

Physics Department, Colorado State University, Fort Collins, Colorado, 80523, United States.

We report on a strain-induced and temperature dependent uniaxial anisotropy in VO/Ni hybrid thin films, manifested through the interfacial strain and sample microstructure, and its consequences on the angular dependent magnetization reversal. X-ray diffraction and reciprocal space maps identify the in-plane crystalline axes of the VO; atomic force and scanning electron microscopy reveal oriented rips in the film microstructure. Quasi-static magnetometry and dynamic ferromagnetic resonance measurements identify a uniaxial magnetic easy axis along the rips. Comparison with films grown on sapphire without rips shows a combined contribution from strain and microstructure in the VO/Ni films. Magnetization reversal characteristics captured by angular-dependent first order reversal curve measurements indicate a strong domain wall pinning along the direction orthogonal to the rips, inducing an angular-dependent change in the reversal mechanism. The resultant anisotropy is tunable with temperature and is most pronounced at room temperature, which is beneficial for potential device applications.
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http://dx.doi.org/10.1038/s41598-017-12690-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5647448PMC
October 2017

Magnetic Yoking and Tunable Interactions in FePt-Based Hard/Soft Bilayers.

Sci Rep 2016 09 8;6:32842. Epub 2016 Sep 8.

Dept. of Physics, University of California, Davis, California 95616, USA.

Magnetic interactions in magnetic nanostructures are critical to nanomagnetic and spintronic explorations. Here we demonstrate an extremely sensitive magnetic yoking effect and tunable interactions in FePt based hard/soft bilayers mediated by the soft layer. Below the exchange length, a thin soft layer strongly exchange couples to the perpendicular moments of the hard layer; above the exchange length, just a few nanometers thicker, the soft layer moments turn in-plane and act to yoke the dipolar fields from the adjacent hard layer perpendicular domains. The evolution from exchange to dipolar-dominated interactions is experimentally captured by first-order reversal curves, the ΔM method, and polarized neutron reflectometry, and confirmed by micromagnetic simulations. These findings demonstrate an effective yoking approach to design and control magnetic interactions in wide varieties of magnetic nanostructures and devices.
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http://dx.doi.org/10.1038/srep32842DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5015099PMC
September 2016

Structural and magnetic depth profiles of magneto-ionic heterostructures beyond the interface limit.

Nat Commun 2016 07 22;7:12264. Epub 2016 Jul 22.

NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, 20899, Maryland, USA.

Electric field control of magnetism provides a promising route towards ultralow power information storage and sensor technologies. The effects of magneto-ionic motion have been prominently featured in the modification of interface characteristics. Here, we demonstrate magnetoelectric coupling moderated by voltage-driven oxygen migration beyond the interface in relatively thick AlOx/GdOx/Co(15 nm) films. Oxygen migration and Co magnetization are quantitatively mapped with polarized neutron reflectometry under electro-thermal conditioning. The depth-resolved profiles uniquely identify interfacial and bulk behaviours and a semi-reversible control of the magnetization. Magnetometry measurements suggest changes in the microstructure which disrupt long-range ferromagnetic ordering, resulting in an additional magnetically soft phase. X-ray spectroscopy confirms changes in the Co oxidation state, but not in the Gd, suggesting that the GdOx transmits oxygen but does not source or sink it. These results together provide crucial insight into controlling magnetism via magneto-ionic motion, both at interfaces and throughout the bulk of the films.
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http://dx.doi.org/10.1038/ncomms12264DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4961844PMC
July 2016

Controllable positive exchange bias via redox-driven oxygen migration.

Nat Commun 2016 Mar 21;7:11050. Epub 2016 Mar 21.

Physics Department, University of California, Davis, One Shields Avenue, Davis, California 95616, USA.

Ionic transport in metal/oxide heterostructures offers a highly effective means to tailor material properties via modification of the interfacial characteristics. However, direct observation of ionic motion under buried interfaces and demonstration of its correlation with physical properties has been challenging. Using the strong oxygen affinity of gadolinium, we design a model system of GdxFe1-x/NiCoO bilayer films, where the oxygen migration is observed and manifested in a controlled positive exchange bias over a relatively small cooling field range. The exchange bias characteristics are shown to be the result of an interfacial layer of elemental nickel and cobalt, a few nanometres in thickness, whose moments are larger than expected from uncompensated NiCoO moments. This interface layer is attributed to a redox-driven oxygen migration from NiCoO to the gadolinium, during growth or soon after. These results demonstrate an effective path to tailoring the interfacial characteristics and interlayer exchange coupling in metal/oxide heterostructures.
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http://dx.doi.org/10.1038/ncomms11050DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4802176PMC
March 2016

Realization of ground-state artificial skyrmion lattices at room temperature.

Nat Commun 2015 Oct 8;6:8462. Epub 2015 Oct 8.

Department of Physics, University of California, Davis, California 95616, USA.

The topological nature of magnetic skyrmions leads to extraordinary properties that provide new insights into fundamental problems of magnetism and exciting potentials for novel magnetic technologies. Prerequisite are systems exhibiting skyrmion lattices at ambient conditions, which have been elusive so far. Here, we demonstrate the realization of artificial Bloch skyrmion lattices over extended areas in their ground state at room temperature by patterning asymmetric magnetic nanodots with controlled circularity on an underlayer with perpendicular magnetic anisotropy (PMA). Polarity is controlled by a tailored magnetic field sequence and demonstrated in magnetometry measurements. The vortex structure is imprinted from the dots into the interfacial region of the underlayer via suppression of the PMA by a critical ion-irradiation step. The imprinted skyrmion lattices are identified directly with polarized neutron reflectometry and confirmed by magnetoresistance measurements. Our results demonstrate an exciting platform to explore room-temperature ground-state skyrmion lattices.
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http://dx.doi.org/10.1038/ncomms9462DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4633628PMC
October 2015

A new reversal mode in exchange coupled antiferromagnetic/ferromagnetic disks: distorted viscous vortex.

Nanoscale 2015 Jun;7(21):9878-85

Physics Department, University of California, Davis, CA, USA.

Magnetic vortices have generated intense interest in recent years due to their unique reversal mechanisms, fascinating topological properties, and exciting potential applications. In addition, the exchange coupling of magnetic vortices to antiferromagnets has also been shown to lead to a range of novel phenomena and functionalities. Here we report a new magnetization reversal mode of magnetic vortices in exchange coupled Ir20Mn80/Fe20Ni80 microdots: distorted viscous vortex reversal. In contrast to the previously known or proposed reversal modes, the vortex is distorted close to the interface and viscously dragged due to the uncompensated spins of a thin antiferromagnet, which leads to unexpected asymmetries in the annihilation and nucleation fields. These results provide a deeper understanding of the physics of exchange coupled vortices and may also have important implications for applications involving exchange coupled nanostructures.
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http://dx.doi.org/10.1039/c5nr01856kDOI Listing
June 2015

Quantitative decoding of interactions in tunable nanomagnet arrays using first order reversal curves.

Sci Rep 2014 Feb 26;4:4204. Epub 2014 Feb 26.

Dept. of Physics, University of California, Davis, California, 95616, USA.

To develop a full understanding of interactions in nanomagnet arrays is a persistent challenge, critically impacting their technological acceptance. This paper reports the experimental, numerical and analytical investigation of interactions in arrays of Co nanoellipses using the first-order reversal curve (FORC) technique. A mean-field analysis has revealed the physical mechanisms giving rise to all of the observed features: a shift of the non-interacting FORC-ridge at the low-HC end off the local coercivity HC axis; a stretch of the FORC-ridge at the high-HC end without shifting it off the HC axis; and a formation of a tilted edge connected to the ridge at the low-HC end. Changing from flat to Gaussian coercivity distribution produces a negative feature, bends the ridge, and broadens the edge. Finally, nearest neighbor interactions segment the FORC-ridge. These results demonstrate that the FORC approach provides a comprehensive framework to qualitatively and quantitatively decode interactions in nanomagnet arrays.
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http://dx.doi.org/10.1038/srep04204DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3935205PMC
February 2014

Rapid microwave-assisted synthesis of dextran-coated iron oxide nanoparticles for magnetic resonance imaging.

Nanotechnology 2012 Jun 3;23(21):215602. Epub 2012 May 3.

Department of Chemistry, University of California, Davis, CA 95616, USA.

Currently, magnetic iron oxide nanoparticles are the only nanosized magnetic resonance imaging (MRI) contrast agents approved for clinical use, yet commercial manufacturing of these agents has been limited or discontinued. Though there is still widespread demand for these particles both for clinical use and research, they are difficult to obtain commercially, and complicated syntheses make in-house preparation unfeasible for most biological research labs or clinics. To make commercial production viable and increase accessibility of these products, it is crucial to develop simple, rapid and reproducible preparations of biocompatible iron oxide nanoparticles. Here, we report a rapid, straightforward microwave-assisted synthesis of superparamagnetic dextran-coated iron oxide nanoparticles. The nanoparticles were produced in two hydrodynamic sizes with differing core morphologies by varying the synthetic method as either a two-step or single-step process. A striking benefit of these methods is the ability to obtain swift and consistent results without the necessity for air-, pH- or temperature-sensitive techniques; therefore, reaction times and complex manufacturing processes are greatly reduced as compared to conventional synthetic methods. This is a great benefit for cost-effective translation to commercial production. The nanoparticles are found to be superparamagnetic and exhibit properties consistent for use in MRI. In addition, the dextran coating imparts the water solubility and biocompatibility necessary for in vivo utilization.
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http://dx.doi.org/10.1088/0957-4484/23/21/215602DOI Listing
June 2012

Rapid size-controlled synthesis of dextran-coated, 64Cu-doped iron oxide nanoparticles.

ACS Nano 2012 Apr 30;6(4):3461-7. Epub 2012 Mar 30.

Department of Chemistry, University of California, Davis, California 95616, United States.

Research into developing dual modality probes enabled for magnetic resonance imaging (MRI) and positron emission tomography (PET) has been on the rise recently due to the potential to combine the high resolution of MRI and the high sensitivity of PET. Current synthesis techniques for developing multimodal probes is largely hindered in part by prolonged reaction times during radioisotope incorporation--leading to a weakening of the radioactivity. Along with a time-efficient synthesis, the resulting products must fit within a critical size range (between 20 and 100 nm) to increase blood retention time. In this work, we describe a novel, rapid, microwave-based synthesis technique to grow dextran-coated iron oxide nanoparticles doped with copper (DIO/Cu). Traditional methods for coprecipitation of dextran-coated iron oxide nanoparticles require refluxing for 2 h and result in approximately 50 nm diameter particles. We demonstrate that microwave synthesis can produce 50 nm nanoparticles with 5 min of heating. We discuss the various parameters used in the microwave synthesis protocol to vary the size distribution of DIO/Cu and demonstrate the successful incorporation of (64)Cu into these particles with the aim of future use for dual-mode MR/PET imaging.
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http://dx.doi.org/10.1021/nn300494kDOI Listing
April 2012