Angle-dependent electron magnetic resonance was performed on 4.9, 8.0, and 19 nm iron oxide nanoparticles encapsulated within protein capsids and suspended in water. Measurements were taken at liquid nitrogen temperature after cooling in a 1 T field to partially align the particles. The angle dependence of the shifts in the resonance field for the iron oxide nanoparticles (synthesized within Listeria-Dps, horse spleen ferritin, and cowpea chlorotic mottle virus) all show evidence of a uniaxial anisotropy. Using a Boltzmann distribution for the particles’ easy-axis direction, we are able to use the resonance field shifts to extract a value for the anisotropy energy, showing that the anisotropy energy density increases with decreasing particle size. This suggests that surface anisotropy plays a significant role in magnetic nanoparticles of this size. 相似文献
Magnetic properties of core–shell cobalt ferrite nanoparticles prepared by co-precipitation route in the range 15–48 nm have been studied. It is shown that the coercivity follows non-monotonic size dependence and exhibits a peak at around 26 nm. Field-cooled magnetization exhibited both horizontal (exchange bias) and vertical shifts. The exchange bias is understood as originating at the interface between a surface region (with structural and spin disorder) and a core ferrimagnetic region. The dependence of the exchange bias and vertical shift on the particle size and cooling field is found to have significant differences. These differences are explained in the light of recent results that suggest that there is a variation of the pinning strength amongst the interface spins and the vertical shift is affected by the more strongly pinned uncompensated spins. 相似文献
Superparamagnetic properties of self-aggregated cobalt nanoparticles in the perfluorinated sulfo-cation membrane (MF-4SK) prepared by ion-exchange method were investigated by transmission electron microscopy (TEM) and superconducting quantum interference device (SQUID) magnetometry at various temperatures. Our experimental results show that cobalt nanoparticles in MF-4SK exhibit superparamagnetic properties above the blocking temperature (TB), which varies from ∼80 to ∼300 K depending on the cobalt concentration at 100 Oe applied field. The average particle radius of 3.8 nm inferred from Langevin function fit for the concentration of 7.8×1019 cobalt atoms per 1 g of polymer film is in good agreement with TEM observation. This experimental evidence suggests that cobalt nanoparticles in the polymer film obey a single-domain theory. The results are discussed in the light of current theory for the superparamagnetic behavior of magnetic nanoparticles. 相似文献
The N-soliton solutions of magnetization in uniaxial anisotropic ferromagnet on the background of spin wave are presented by using the effective Darboux transformation method. With the analytical solutions new properties of magnon density is studied in detail. On the ground state background the magnon density is constant for the spin wave solution and the magnetic soliton, respectively. However, on the spin wave background the magnon density possesses of temporal or spatial periodic oscillation. Moreover, the soliton solution possess the breather character in its propagation along the ferromagnet. These results show that during soliton propagation a periodic magnon exchange occurs between the magnetic soliton and the spin wave background. 相似文献
Gilbert's damping constants, α, of Co(tCo)/Pt (1.4 nm) multilayer thin films are investigated by Q-band FMR analysis. α is calculated from the resonance width of the FMR spectrum. With decreasing tCo, the α value decreases from 0.034 (tCo=8.7 nm) to 0.023 (tCo=1.8 nm), and then increases to 0.037 (tCo=1.0 nm). The decrease of α with tCo>1.8 nm is probably due to the eddy current loss effects. The increase of α with tCo<1.8 nm would be caused by the increase of the distortion between the Co and the Pt layers at the interface. When the magnetic field direction was changed from θ=90° (parallel to the specimen) to θ=0° (perpendicular to the specimen), the α of all the specimens increased, and a sharp step in α was observed around θ=40°, where the α has the maximum value. 相似文献
We have measured positive exchange bias in a Ni80Fe20/NixFe1−xO thin-film nanocrystallite system. A series of solid solution NixFe1−xO 40 nm thick films capped with 25 nm thick Ni80Fe20 were deposited using a range of %O2/Ar bombardment energies (i.e. End-Hall voltages). Proper tuning of the deposition conditions results in a Ni80Fe20/NixFe1−xO (30%O2/Ar) based bilayer that exhibits a positive exchange bias loop shift of Hex∼60 Oe at 150 K. 相似文献
We have investigated the ferromagnetic resonance (FMR) response of as-made and temperature annealed FePt magnetic nanoparticles. The as-made nanoparticles, which have been fabricated by a chemical route, crystallize in the low magnetic anisotropy fcc phase and have a diameter in the range of 2-4 nm. The annealing of the particles at high temperatures (TA=550, 650 and C) in an inert Ar atmosphere produces a partial transformation to the high magnetocrystalline anisotropy L10 phase, with a significant increase in particle size and size distribution. FMR measurements at X-band (9.5 GHz) and Q-band (34 GHz) show a single relatively narrow line for the as-synthesized particles and a structure of two superimposed lines for the three annealed samples. The origin of this line shape has been attributed to the presence of the disordered fcc phase. Assuming that the system consists of a collection of identical particles with a random distribution of easy axes, we have been able to estimate a mean value for the magnetic anisotropy constant of the particles in the fcc phase, K∼2×106 erg/cm3. The measured line shape in the annealed samples can be explained if we consider that the magnetic anisotropy of the particles has a gaussian distribution with a relatively broad width. 相似文献
For CuO nanocrystals of size 6.6-37 nm, the exchange bias Heb and coercivity Hc are measured at 5 K in zero-field-cooled (ZFC) and field-cooled (FC at 50 kOe) samples and their variations investigated as a function of particle size D. The similar 1/D variations observed for the difference coercivity ΔHc=Hc(FC)−Hc(ZFC) and the interfacial exchange energy Δσ=HebMfD are discussed in terms of the ferromagnetic magnetization Mf being produced by the uncompensated surface Cu2+ spins in the otherwise antiferromagnetically ordered CuO nanoparticles. This leads to the observation that the experimentally measured ΔHc provides a good measure of Δσ in nanoparticle systems, with Heb/ΔHc varying as 1/MfD. 相似文献
We investigate the mutual interaction between superconductivity and ferromagnetism in a Nb/Ni81 Ee19 multilayer by ac susceptibility measurements. Compared with a pure superconducting Nb film, the critical current density of the multilayer is apparently enhanced in a low magnetic field region but remains nearly the same in high magnetic fields, which indicates that a continuous ferromagnetic layer with in-plane magnetization can produce strong vortex pinning in a low field region. We interpret this unusual vortex-pinning phenomenon as a consequence of dynamic spin vortex interaction which induces a spin rotation following vortex movement. In addition, we propose that this dynamic interaction could be used for spin manipulation via a superconductor. 相似文献
We demonstrate that a current pulse of a non-uniform spin-polarized current density in a nanomagnet can drive, apart from magnetization reversal a static magnetic vortex. This vortex configuration can be achieved in low shape anisotropy spin valves of elliptical cross-sectional area. These non-uniform configurations exist also in presence of either ion mill damages below the nano-aperture or thermal effects at low temperature. We performed a numerical experiment of spin-torque driven ferromagnetic resonance in a magnetic vortex configuration, our results predict a frequency response with a few maxima and minima related to small oscillation of the vortex state around its equilibrium configuration. 相似文献
An original approach is proposed to study the magnetic phase separation phenomenon. It is based on the registration of the noise‐like FMR Fine Structure (FMR FS) caused by the magnetic interparticle dipole–dipole interaction between spatially separated ferromagnetic regions. Data obtained for a La0.7Pb0.3MnO3 single crystal point to the existence of spatially separated ferromagnetic regions. It is shown that FMR FS of the La0.7Pb0.3MnO3 single crystal is temperature reversible and disappears at the maximum of magnetoresistance.
This paper reports a brief overview on recent developments regarding the high-frequency response in the GHz range of nanostructured magnetic materials. Emphasis is placed on the linear regime in the frequency domain characterized by the dynamic susceptibility spectrum. Some modeling tools and experimental probes allowing determination of the dynamic susceptibility spectrum are first rapidly reviewed and their respective advantages and disadvantages are discussed. Next, some illustrative examples of the high-frequency response of nanopatterned materials based on recent works are presented. The role played by the shape of the element on the characteristics of excitation spectrum is underlined. Lastly, some prospects are proposed and promising trends are highlighted. 相似文献
In magnetic nanoparticles the uniform precession
(q = 0 spin wave) mode gives the predominant contribution
to the magnetic excitations. We have calculated the energy of the
uniform mode in antiferromagnetic nanoparticles with uncompensated
magnetic moments, using the coherent potential approximation. In
the presence of uncompensated moments, an antiferromagnetic
nanoparticle must be considered as a kind of a ferrimagnet. Two
magnetic anisotropy terms are considered, a planar term confining
the spins to the basal plane, and an axial term determining an
easy axis in this plane. Excitation energies are calculated for
various combinations of these two anisotropy terms, ranging from
the simple uniaxial case to the planar case with a strong
out-of-plane anisotropy. In the simple uniaxial case, the
uncompensated moment has a large influence on the excitation
energy, but in the planar case it is much less important. The
calculations explain recent neutron scattering measurements on
nanoparticles of antiferromagnetic α-Fe2O3 and NiO. 相似文献
The magnetization reversal of electrodeposited CoNi/Cu multilayer nanowires patterned in an array using a hole template has been investigated. The reversal mode is found to depend on the CoNi layer thickness t(CoNi); with increasing t(CoNi) a transition occurs from coherent rotation to a combination of coherent and incoherent rotation at around t(CoNi)=51 nm. The reversal mode has been identified using the magnetic hysteresis loops measured at room temperature for CoNi/Cu nanowires placed at various angles between the directions of the nanowire axis and external fields using a vibrating sample magnetometer. The nanowire samples have a diameter of ∼250 nm and constant Cu layer thickness of 4.2 nm with various t(CoNi) ranging from 6.8 nm to 7.5 μm. With increasing t(CoNi), the magnetic easy axis moves from the direction perpendicular to nanowires to that parallel to the nanowires at around t(CoNi)=51 nm, indicating a change in the magnetization reversal mode. The reversal mode for the nanowires with thin disk-shaped CoNi layers (t(CoNi)=6.8, 12 and 17 nm) is of a coherent rotation type, while that for long rod-shaped CoNi layers (t(CoNi)=150 nm, 1.0, 2.5 and 7.5 μm) can be consistently explained by a combination of coherent rotation and a curling mode. The effects of dipole–dipole interactions between nanowires and between adjacent magnetic layers in each nanowire on the reversal process have been discussed. 相似文献
Magnetic nanoparticles with different compositions (CoxFe3−xO4, 0?x?0.1) were synthesized from metal salts using a coprecipitation technique to produce magnetic fluids following a peptization technique. The liquid carrier was the hydrocarbon Isopar M and the surfactant was oleic acid. The colloidal-sized ferrimagnetic nanoparticles produced were found to be superparamagnetic. Measurements of the complex magnetic susceptibility were carried out to evaluate the resonant frequency fres, the anisotropy constant K, and anisotropy field HA. fres was found to be a linear function of the cobalt content of the magnetic nanoparticles over the range of cobalt content studied. 相似文献
We theoretically investigate the spin-dependent electron transport properties in a magnetic superlattice (MSL) with broken two-fold symmetry. An abnormal barrier in the MSL can break the two-fold symmetry of the system when it is not located at the two-fold symmetry center. A two-fold symmetry breaking factor is introduced to describe the two-fold symmetry breaking degree. Our numerical calculations show that the transmission, the conductance and the spin polarization are non-trivially dependent on the two-fold symmetry breaking factor. When the factor is large enough, the polarization almost approaches 100% in a proper Fermi energy range. However, for two mutually mirror-symmetric MSLs with the same factor, their polarizations may be either similar or distinct. These features provide some clues to the design and applications of MSL-based spin filters or spin-dependent tunneling electron devices. 相似文献
Grain size effects on magnetic and transport properties for heavily Sr-doped A-type antiferromagnetic La0.4Sr0.6MnO3 ceramics were studied. It was observed that with decrease in grain size, surface ferromagnetism could be introduced due to bond-breaking at surfaces. With decrease in grain size, the surface ferromagnetism was enhanced, and the phase transition order distinguished from the Arrott plot was a second one. The surface-induced ferromagnetism was insulating as judged from transport properties. With decrease in grain size, magnetoresistance was largely improved for both high magnetic and low magnetic fields. Under a 500 Oe magnetic field, the magnetoresistance is improved from 0.2%, 0.1%, 0.03% and 0.02% for the sample with grain size of 150 nm at 10, 100, 200 and 300 K, respectively, to 3%, 2.3%, 0.43% and 0.12% for the sample with grain size of 20 nm at 10, 100, 200 and 300 K. It was interesting to find that large magnetoresistance could be induced due to the surface ferromagnetism in A-type antiferromagnetic La0.4Sr0.6MnO3 nanoparticles, which suggested that it was possible to search for manganites with relatively high low-field magnetoresistance in nanostructured A-type antiferromagnetic materials. 相似文献
We report enhancement of magnetization below the antiferromagnetic ordering temperature TN in nanoparticles of two antiferromagnets, viz CoRh2O4 and Cr2O3. The enhancement of magnetization below TN is systematic, being larger for sample with smaller particle size. Scaling analysis showed that such enhancement of magnetization in CoRh2O4 nanoparticles is due to the superparamagnetic type contribution of surface (shell) spins. The present work shows that similar analysis can also be applied in Cr2O3 nanoparticles. 相似文献
Thin ferromagnetic films with in-plane magnetic anisotropy are promising materials for obtaining high microwave permeability. To produce a bulk massive sample from thin films, multi-layer films made of a number of many thin films may be used. The paper reports on an experimental study of microwave and static magnetic properties of multi-layer iron-based films and composite samples made of these. The multi-layer films under study are deposited onto a mylar substrate by a magnetron-sputtering process performed in Ar atmosphere with controlled N2 admixture. The sputtered iron layers are alternated with SiO2 layers. The measured static and microwave magnetic properties of the bulk sample are found to differ from the properties of constituent films. This is an evidence for strong interactions between the magnetic layers in the sample, which interact at distances exceeding greatly the distance between adjacent magnetic layers. A theoretic model is developed to account for magneto-dipole interactions between iron films in a multi-layer system. The model explains the anomalously high demagnetization field of the sample observed in the measurements. 相似文献
The present article reports studies on structural and magnetic properties of nanostructured Fe/MnO2 materials prepared by mechanosynthesis method, with Fe to MnO2 ratios of 20/80, 50/50 and 60/40. X-ray diffraction patterns indicate that the milled materials have crystalline grain size in the nanoscale region. Mössbauer spectra of the milled materials suggest the presence of two Fe phases for each sample: a nanocrystalline α-Fe phase with a high degree of disorder/defects and small Fe-oxide particles. The magnetic hysteresis (M(H)) loops, measured at 4.2 K, after the samples were cooled from 300 K in ±10 kOe fields, show unexpected large shifts in both horizontal and vertical directions for the 20/80 sample, while only horizontal shift was detected in the samples with higher Fe concentration. The anomalous vertical shift of the M(H) loop for the 20/80 sample, observed at low cooling field (10 kOe), is being associated with a large contribution from non-collinear magnetic structure of the particles surface. This surface magnetic contribution is strongly influenced by the field cooling magnitude. A simple model is proposed to interpret this result. 相似文献