Randy K. Dumas

Magnetic/luminescent core/shell particles synthesized by spray pyrolysis and their application in immunoassays with internal standard

Many types of fluorescent nanoparticles have been investigated as alternatives to conventional organic dyes in biochemistry; magnetic beads also have a long history of biological applications. In this work we apply flame spray pyrolysis in order to engineer a novel type of nanoparticle that has both luminescent and magnetic properties. The particles have magnetic cores of iron oxide doped with cobalt and neodymium and luminescent shells of europium-doped gadolinium oxide (Eu:Gd(2)O(3)). Measurements by vibrating sample magnetometry showed an overall paramagnetic response of these composite particles. Luminescence spectroscopy showed spectra typical of the Eu ion in a Gd(2)O(3) host-a narrow emission peak centred near 615 nm. Our synthesis method offers a low-cost, high-rate synthesis route that enables a wide range of biological applications of magnetic/luminescent core/shell particles. Using these particles we demonstrate a novel immunoassay format with internal luminescent calibration for more precise measurements.

Memory effect in magnetic nanowire arrays.

www.advmat.de www.MaterialsViews.com Xiaoming Kou , Xin Fan , Randy K. Dumas , Qi Lu , Yaping Zhang , Hao Zhu , Xiaokai Zhang , Kai Liu , and John Q. Xiao* Magnetic materials are widely used for information storage because of their large capacity and low cost. [ 1 ] Storage medium technologies have evolved from analog recording with mag- netic tapes to high fidelity digital recording with magnetic hard disks. Nevertheless, both techniques use a magnetic medium consisting of magnetic particles, whose sizes have also evolved from micrometers in magnetic tapes to nanometers in modern hard disks. In analog recording, signals are converted into mag- netic fields which change the magnetization of a group of mag- netic particles (bit). The magnetization variations represent the stored information which can subsequently be read out. The magnetization, and therefore the stored information, could be changed by an external magnetic field and/or thermal effects. In digital recording, the bit magnetization can be aligned either left or right in parallel recording or up and down in perpen- dicular recording. [ 2 ] The information is stable as long as the medium is not subjected to a magnetic field higher than the coercivity, or a temperature higher than the superparamagnetic limit, of the constituent magnetic particles. In order to clearly distinguish one bit from another it is advantageous to minimize the dipolar interaction among magnetic particles, which is typi- cally achieved by creating boundaries between particles. Since the magnetic dipolar interaction is particularly pronounced in a collection of magnetic entities, such as magnetic particles and nanowires, it is scientifically interesting to question whether such a degree of freedom can be exploited in order to create additional memory functions. To answer this question, one needs a magnetic system with a sizable and preferably control- lable dipolar interaction. The magnetic nanowire array is an ideal system for this purpose. Magnetic nanowire arrays embedded in an insulating Al 2 O 3 matrix have been intensively studied. [ 3–12 ] When the magnetoc- rystalline anisotropy is negligible, the magnetization direction of the nanowires is preferably aligned along the length of the nanowire because of the shape anisotropy. When nanowires are very close to each other, dipolar interactions play a significant X. Kou, Dr. X. Fan, Q. Lu, Y. Zhang Prof. J. Q. Xiao Department of Physics and Astronomy University of Delaware Newark, DE, 19716, USA E-mail: jqx@udel.edu Dr. R. K. Dumas, Prof. K. Liu Department of Physics University of California Davis, CA, 95616, USA Dr. H. Zhu, Dr. X. Zhang Spectrum Magnetics LLC, 1210 First State Blvd, Wilmington, DE, 19804, USA DOI: 10.1002/adma.201003749 Adv. Mater. 2011, 23, 1393–1397 role in the magnetic behavior of the nanowire array, leading to rich physical phenomena and great application potentials. [ 7–12 ] Recently, it was demonstrated that the dipolar interaction among magnetic nanowires could provide zero field ferromagnetic res- onance (FMR) tunability, which has potential applications in a variety of microwave devices. A double FMR feature caused by the dipolar interaction in a magnetic nanowire array was also predicted [ 13 ] and verified. [ 14–17 ] In this manuscript, we demon- strate how dipolar interactions can induce an analog memory effect in magnetic nanowire arrays. Through this effect, the magnetic nanowire array has the ability to ‘memorize’ the maximum magnetic field that the array has been exposed to. A novel, low cost, and robust electromagnetic pulse detecting method is proposed based on this memory effect. Nanowire arrays of Ni 90 Fe 10 and Ni were synthesized by elec- trodeposition into anodized alumina templates. The diameter, center-to-center interpore distance, and length of the nanowires are 35 nm, 60 nm, and 30 μ m, respectively. Figure 1 a shows the hysteresis loop, with a coercivity of 1080 Oe, of a Ni 90 Fe 10 nanowire array with a magnetic field parallel to the wire (open squares). The loop with the field perpendicular to the wire is shown in the inset. Clearly, a well defined easy axis exists along the wire axis because of the dominant shape anisotropy. The memory effect was demonstrated using a vibrating sample magnetometer. The Ni 90 Fe 10 nanowire array was satu- rated along the wire prior to the measurement. The magnetic moment of the array was monitored as a series of magnetic field pulses were applied parallel to the nanowires. Figure 1b displays the series of magnetic pulses with different magni- tudes and directions. The corresponding change of the mag- netic moment is illustrated in Figure 1c. We find that the magnetic moment decreases monotonically as the magnitude of the negative pulses increases, while the moment remains the same after the positive pulses. This demonstrates that the maximum negative magnetic field can be recorded into the nanowire array. However, this is violated for the 800 and 900 Oe field pulses, and this discrepancy will be explained later. The result is also plotted in the magnetic moment verses applied field ( M – H ) graph, displayed in Figure 1d. Similar prop- erties are also observed in Ni nanowire arrays. This phenomenon is attributed to the dipolar interactions among the nanowires. Previously, using a theoretical model, two assumptions were proposed. [ 13 ] First, each nanowire is a single domain cylinder with a uniform magnetization pointing up or down parallel to the wire. The second assumption is that the number of nanowires with up magnetizations ( N ↑ ) and down magnetizations ( N ↓ ) is determined by the total magnetization M(H) , i.e. (N ↑ – N ↓ )/(N ↑ + N ↓ ) = M(H)/M s , where M s is the saturation magnetization. According to these assumptions, the dipolar field among the nanowires can be written as [ 13 ]

Dynamically stabilized magnetic skyrmions

Magnetic skyrmions are topologically non-trivial spin textures that manifest themselves as quasiparticles in ferromagnetic thin films or noncentrosymmetric bulk materials. So far attention has focused on skyrmions stabilized either by the Dzyaloshinskii–Moriya interaction (DMI) or by dipolar interaction, where in the latter case the excitations are known as bubble skyrmions. Here we demonstrate the existence of a dynamically stabilized skyrmion, which exists even when dipolar interactions and DMI are absent. We establish how such dynamic skyrmions can be nucleated, sustained and manipulated in an effectively lossless medium under a nanocontact. As quasiparticles, they can be transported between two nanocontacts in a nanowire, even in complete absence of DMI. Conversely, in the presence of DMI, we observe that the dynamical skyrmion experiences strong breathing. All of this points towards a wide range of skyrmion manipulation, which can be studied in a much wider class of materials than considered so far.

[Co/Pd]―NiFe exchange springs with tunable magnetization tilt angle

We investigate exchange coupled [Co/Pd](5)-NiFe thin films. Due to competition between the in-plane shape anisotropy of the NiFe and strong perpendicular magnetic anisotropy of the [Co/Pd](5) multi ...

Quantitative Decoding of Interactions in Tunable Nanomagnet Arrays Using First Order Reversal Curves

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.

Spin-wave-beam driven synchronization of nanocontact spin-torque oscillators

The synchronization of multiple nanocontact spin-torque oscillators (NC-STOs) is mediated by propagating spin waves (SWs). Although it has been shown that the Oersted field generated in the vicinity of the NC can dramatically alter the emission pattern of SWs, its role in the synchronization behaviour of multiple NCs has not been considered so far. Here we investigate the synchronization behaviour in multiple NC-STOs oriented either vertically or horizontally, with respect to the in-plane component of the external field. Synchronization is promoted (impeded) by the Oersted field landscape when the NCs are oriented vertically (horizontally) due to the highly anisotropic SW propagation. Not only is robust synchronization between two oscillators observed for separations larger than 1,000 nm, but synchronization of up to five oscillators, a new record, has been observed in the vertical array geometry. Furthermore, the synchronization can no longer be considered mutual in nature.

Spin wave mode coexistence on the nanoscale: a consequence of the Oersted field-induced asymmetric energy landscape

It has been argued that if multiple spin wave modes are competing for the same centrally located energy source, as in a nanocontact spin torque oscillator, that only one mode should survive in the steady state. Here, the experimental conditions necessary for mode coexistence are explored. Mode coexistence is facilitated by the local field asymmetries induced by the spatially inhomogeneous Oersted field, which leads to a physical separation of the modes, and is further promoted by spin wave localization at reduced applied field angles. Finally, both simulation and experiment reveal a low frequency signal consistent with the intermodulation of two coexistent modes.

Temperature induced single domain–vortex state transition in sub-100nm Fe nanodots

Magnetization reversal in nanomagnets via a vortex state, although often investigated at the remanent state, may not necessarily display a zero remanence or a highly pinched hysteresis loop. In contrast, the irreversible nucleation/annihilation events are clear indications of a vortex state. In this work, temperature induced single domain–vortex state transition has been investigated in 67nm Fe nanodots using a first-order reversal curve (FORC) technique. The two phase coexistence is manifested as different features in the FORC distribution. At lower temperatures, it becomes harder to nucleate and annihilate vortices and the amount of single domain dots increases.

Identifying reversible and irreversible magnetization changes in prototype patterned media using first- and second-order reversal curves

Arrays of nanomagnets have important potential applications as future generation ultrahigh-density patterned magnetic recording media, in which each nanomagnet constitutes a single bit. We introduce a powerful technique to identify and quantify reversible and irreversible magnetization changes, a key challenge in characterizing these systems. The experimental protocol consists of measuring a few families of second-order reversal curves along selected profiles in the first-order-reversal-curve diagram, which then can be decomposed into truly irreversible switching events and reversible magnetization changes. The viability of the method is demonstrated for arrays of sub-100-nm Fe nanomagnets, which exhibit complex magnetization reversal processes.

Tuning carrier type and density in Bi2Se3 by Ca-doping

The carrier type and density in Bi2Se3 single crystals are systematically tuned by introducing a calcium (Ca) dopant. A carrier density of ∼1×1017 cm−3 which corresponds to ∼25 meV in the Fermi energy is obtained in both n- and p-type materials. Electrical transport properties show that the insulating behavior is achieved in low carrier density crystals. In addition, both the band gap and reduced effective mass of carriers are determined.