Xiaoming Kou

Physiological effects of magnetite (Fe3O4) nanoparticles on perennial ryegrass (Lolium perenne L.) and pumpkin (Cucurbita mixta) plants.

Abstract To date, knowledge gaps and associated uncertainties remain unaddressed on the effects of nanoparticles (NPs) on plants. This study was focused on revealing some of the physiological effects of magnetite (Fe3O4) NPs on perennial ryegrass (Lolium perenne L.) and pumpkin (Cucurbita mixta cv. white cushaw) plants under hydroponic conditions. This study for the first time reports that Fe3O4 NPs often induced more oxidative stress than Fe3O4 bulk particles in the ryegrass and pumpkin roots and shoots as indicated by significantly increased: (i) superoxide dismutase and catalase enzyme activities, and (ii) lipid peroxidation. However, tested Fe3O4 NPs appear unable to be translocated in the ryegrass and pumpkin plants. This was supported by the following data: (i) No magnetization was detected in the shoots of either plant treated with 30, 100 and 500 mg l−1 Fe3O4 NPs; (ii) Fe K-edge X-ray absorption spectroscopic study confirmed that the coordination environment of Fe in these plant shoots was similar to that of Fe-citrate complexes, but not to that of Fe3O4 NPs; and (iii) total Fe content in the ryegrass and pumpkin shoots treated with Fe3O4 NPs was not significantly increased compared to that in the control shoots.

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 ]

Rapid thermal annealing study of magnetoresistance and perpendicular anisotropy in magnetic tunnel junctions based on MgO and CoFeB

The tunneling magnetoresistance and perpendicular magnetic anisotropy in CoFeB(1.1-1.2 nm)/MgO/CoFeB(1.2-1.7 nm) junctions were found to be very sensitively dependent on annealing time. During annealing at a given temperature, decay of magnetoresistance occurs much earlier compared to junctions with in-plane magnetic anisotropy. Through a rapid thermal annealing study, the decrease of magnetoresistance is found to be associated with the degradation of perpendicular anisotropy, instead of impurity diffusion as observed in common in-plane junctions. The origin of the evolution of perpendicular anisotropy as well as possible means to further enhance tunneling magnetoresistance is discussed.

Tunable ferromagnetic resonance in NiFe nanowires with strong magnetostatic interaction

Magnetic materials with tunable ferromagnetic resonant (FMR) frequencies are highly desirable in microwave devices. In this manuscript, we demonstrate that the natural FMR of Ni90Fe10 nanowire array can be tuned continuously from 8.2 to 11.7 GHz by choosing different remanent states. Theoretical model based on magnetostatic interaction among nanowires has been developed to explain the observed phenomena.

Study and tailoring spin dynamic properties of CoFeB during rapid thermal annealing

We studied the real-time evolution of magnetic dynamic and static properties of 20 nm CoFeB thin film during annealing at 380 °C. The ferromagnetic resonance linewidth quickly reduces by 30% within 300 s annealing, and monotonically increases upon longer annealing. The magnetic static coercivity shows similar trend. The underlying physical relation between linewidth and anisotropy can be connected by the two-magnon scattering theory. By doping of Nb into CoFeB films, the damping was maintained at a low value within 2000 s annealing. This method to tailor the dynamic properties of CoFeB may benefit the development of magnetics and spintronics based microwave devices.

Magnetic tunneling junction based magnetic field sensors: Role of shape anisotropy versus free layer thickness

Al2O3- and MgO-based magnetic tunnel junction (MTJ) sensors were designed and fabricated using microfabrication techniques. This study revealed that in the case of Al2O3-based sensors, the shape anisotropy in the free NiFe electrode resulted in a linear and hysteresis-free tunneling magnetoresistance (TMR) curve. These sensors exhibited TMR values between 27% and 30% and sensitivity up to 0.4%/Oe over a magnetic field range of − 40 to 40 Oe. In the case of CoFeB/MgO/CoFeB MTJ sensors, shape anisotropy alone was not sufficient to achieve a linear and hysteresis-free MR response. A superparamagnetic free layer was used to achieve the desired sensor response. MgO-based sensors had about 90% TMR and 1.1%/Oe sensitivity over the same field range as Al2O3-based MTJs.

Microwave Permeability and Tunable Ferromagnetic Resonance in Cobalt Nanowire Arrays

A simple theoretical analysis indicates that high permeability is possible in nanowire arrays with the magnetocrystalline anisotropy comparable to the demagnetization energy and its easy axis perpendicular to the nanowire. Both conditions can be satisfied in Co nanowire arrays. With proper fabrication conditions, we have fabricated Co nanowire arrays with crystalline easy axis perpendicular to the nanowire. For Co nanowire arrays with diameters of 50 nm and inter-pore distances of 90 nm, the real part of the permeability is about 3.5 and the loss tangent is about 0.045 below 4.5 GHz. We have also shown that by magnetizing the sample to different remanent states, its ferromagnetic resonance frequency can be tuned between 4.5 and 6.5 GHz.

Robust and Tunable One-Way Magnetic Surface Plasmon Waveguide: An Experimental Demonstration

We have demonstrated experimentally a one-way magnetic surface plasmon (MSP) electromagnetic (EM) waveguide in the microwave range based on the magnetic photonic crystals (MPCs). The waveguide exhibits asymmetric transmission of EM waves in the frequency range near the MSP resonance for an MPC, such that a significant one-way propagation can be observed in the channel between the two MPC slabs, each in an external static magnetic field (ESMF) of opposite directions. The one-way waveguide is not only immune to interstitial metal defects but also robust against the disorder of rod position. Furthermore, its working frequency can be flexibly tuned by an ESMF, which makes it more favorable for the design of EM devices. The physics is related to the broken time-reversal symmetry of the MSP band states and the excitation of a giant circulation of the energy flow, similar to the case in the quantized Hall effect.

Spin wave resonance detection using magnetic tunnel junction structure

We have demonstrated that spin wave resonance in a permalloy microstrip can be detected by an electrical method based on magnetic tunnel junction structures. The detection method promises high spatial resolution and sensitivity. Both even and odd spin wave resonance modes can be clearly observed in a permalloy microstrip. The spin wave induced voltage is proportional to the input microwave power at each resonance mode. Data analysis using the model of quantized dipole-exchange spin wave resonance suggests the edge pinning of spin wave sensitively depends on the order of the spin wave mode, as well as on the excitation frequency for modes of the higher order.

Electrical detection of nonlinear ferromagnetic resonance in single elliptical permalloy thin film using a magnetic tunnel junction

A quantitative method to detect ferromagnetic resonance using magnetic tunnel junction structure has been developed. Experimental results reveal three distinct regions for single elliptical permalloy film of micrometer lateral size. Above the spin wave instability threshold, the experimental results show a linear response of the longitudinal magnetization component to the microwave field amplitude over a large range rather than a lock-up phenomenon appeared in macroscopic permalloy films and then a phase limiting behavior. The linear behavior can be described by the theoretical model describing subsidiary resonance.