Magnetic bistability and controllable reversal of asymmetric ferromagnetic nanorings

Magnetization reversals through the formation of a vortex state and the rotation of an onion state are two processes with comparable probabilities for symmetric magnetic nanorings with a radius of about 50 nanometers. This magnetic bistability is the manifestation of the competition between the exchange energy and the magnetostatic energy in nanomagnets. The relative probability of the two processes in symmetric nanorings is dictated by the ring geometry and cannot be altered after fabrication. In this work, we report a novel type of nanorings--asymmetric nanorings. By tuning the asymmetry, we can control the fraction of the vortex formation process from about 40% to nearly 100% by utilizing the direction of the external magnetic field. The observed results have been accounted for by the dependence of the domain-wall energy on the local cross-section area for which we have provided theoretical calculations.     

Fabrication of Ni nanodots templated by nanoporous polysulfone membrane: Structural and Magnetic properties

Two-dimensional nickel nanodots were prepared using a simple polymer based lithography process on silicon substrates. The nanoporous polysulfone membranes were fabricated using a phase inversion polymerization process. Nickel nanodots were then grown using the polysulfone membrane as a mask. The structures were written by depositing a certain thickness of nickel using electron beam evaporation. After lift off, the structural properties of the samples were studied using atomic force microscopy and grazing incidence X-ray diffractometry. The dots were found to have diameters in the range of 75–120 nm and heights of 3–5 nm. The magnetization and magnetic domain arrangement of the Ni nanodots were analyzed using vibrating sample magnetometer and magnetic force microscopy respectively. The nanodots were found to exhibit excellent soft ferromagnetic properties with a preferred easy axis of magnetization.     

Low-Temperature Magnetic Properties of Co Antidot Array

Cobalt antidot arrays with different thicknesses are fabricated by rf magnetron sputtering onto porous alumina substrates. Scanning electron microscopy and grazing incidence x-ray diffraction are employed to characterize the morphology and crystal structure of the antidot array, respectively. The temperature dependence of magnetic properties shows that in the temperature range 5K--300K, coercivity and squareness increase firstly, reach their maximum values, then decrease. The anomalous temperature dependences of coercivity and squareness are discussed by considering the pinning effect of the antidot and the magnetocrystalline anisotropy.     

Magnetic anisotropy study of triangular-shaped Co nanostructures

Atomic force microscopy (AFM), X-ray magnetic circular dichroism (XMCD), magnetic force microscopy (MFM) and vibrating sample magnetometry (VSM) have been used to measure the magnetic and geometrical characteristics of triangular-shaped Co structures of lateral size 730 nm and thickness 32 nm, prepared by nanosphere lithography (NSL). Evidence of in-plane six-fold magnetic anisotropy induced by the symmetry of the structure has been found. By means of XMCD measurements, performed at remanence after applying a pulsed field, a structure rotation angle-dependent oscillation of about 15% with a periodicity of 60° has been observed for both the orbital and spin moments. Furthermore, the system exhibits the angular hysteresis effect. The magnetic measurements performed by MFM show a reduction of the magnetic configurations to only two states, one quasi-single domain Y state and second, a combination of vortex and Y state.     

Magnetic properties of arrays of interacting Co nanocrystals

Monodisperse Co FCC nanocrystals with 12 nm diameter were self-assembled into regular quasi-two-dimensional triangular periodic arrays on carbon substrates from a toluene-based colloidal suspension. At 300 K the regular arrays show a collective magnetic behaviour due to dipolar coupling. A remanent magnetization with an easy axis in the film-plane and an uniaxial in-plane anisotropy field of 0.037 T were determined by SQUID magnetometry and angular dependent ferromagnetic resonance.     

Magnetic properties of Co nanoparticles in zirconia matrix

Granular films composed of nanometric Co particles embedded in an insulating ZrO₂ matrix were prepared by pulsed laser deposition in a wide range of Co volume concentrations (0.06<x_v<0.42). High-resolution electron microscopy (HREM) shows very sharp interfaces between the crystalline particles and the amorphous matrix, with no evidence of intermixing. The mean particles size and width of the distribution determined by fitting the low-field magnetic susceptibility and magnetization curves in the paramagnetic regime to a distribution of Langevin functions are in agreement with the parameters extracted from direct TEM observations. Ferromagnetic correlations between Co particles are evident in the field-cooled state when increasing Co concentration. The effective anisotropy constant estimated from magnetic measurements is about two orders of magnitude larger than the bulk value, and decreases as particle size increases.     

磁光光学双稳态

本文报道一种磁光光学双稳态装置。该装置使用的磁光调制器是由新型磁性玻璃并经特殊设计而成,用该装置完成了光学双稳态实验,并对双稳特性及其有关参数进行了测量,测量结果与理论分析基本相符。     

Magnetic anisotropy of Co/Cu/Co films with indirect exchange coupling

The effect of isothermal annealing on the magnetic anisotropy, bilinear and biquadratic exchange coupling energies, and domain structure of Co/Cu/Co trilayer fiilms with d_Co=6 nm and d_Cu=1.0 and 2.1 nm prepared by magnetron sputtering has been studied. It is shown that, under isothermal annealing, the biquadratic coupling energy decreases by more than an order of magnitude in films with d_Cu=1.0 nm and increases in films with d_Cu=2.1 nm. The fourth-order magnetic anisotropy is shown to be related to the existence of biquadratic exchange energy.     

Magnetic properties of Co films and Co/Pt multilayers deposited on PDMS nanostructures

The magnetic properties of magnetron sputtered Co films (with in-plane anisotropy) and Co/Pt multilayers (with perpendicular anisotropy) deposited on elastomeric poly-dimethylsiloxane (PDMS) films and nanostructured templates are presented. Apart from the etched nanosize features, maze-like submicron features develop after the sputter deposition as a result of film buckling due to thermal contraction of the underlying PDMS layer. The nanostructured templates can physically isolate the magnetic entities but magnetic correlations within the range of the buckling features remain. By using oblique deposition geometries in-plane anisotropy develops and it is transferred to the PDMS buckling patterns. In the case of Co/Pt multilayers deposition on PDMS nanostructured templates results in a loss of the perpendicular anisotropy, attributed to the acuteness of the PDMS nanostructures.     

Template synthesis of monodisperse carbon nanodots

Monodisperse carbon nanodots in pores of mesoporous silica particles are obtained by template synthesis. This method is based on introducing a precursor (organosilane) into pores, its thermal decomposition with formation of carbon nanodots, and the template removal. Structural analysis of the nanomaterial has been performed, which showed that carbon nanodots have an approximately spherical form and a graphite-like structure. According to dynamic light scattering data, the size of carbon nanodots is 3.3 ± 0.9 nm.

     

Magnetic coupling between Co layers separated by Cu

We have grown on a Cu(100) substrate epitaxial Co-monolayers separated by Cu. When the thickness of the Cu-spacer is increased, we observe the magnetic coupling between the Co-monolayers to change from ferromagnetic to antiferromagnetic and back to ferromagnetic before vanishing.     

Co/Fe Multilayers as Planar Magnetic Nanocomposites

Co/Fe multilayers were electron beam evaporated in ultra-high vacuum and analyzed by Alternating Gradient Force Magnetometry, Magnetic Force Microscopy, Conversion Electron Mössbauer Spectroscopy, and Transmission Electron Microscopy. The multilayer of 10 nm Co and 30 nm Fe layer thickness showed a single-phase magnetic behavior because of a strong exchange coupling established between the layers. The system exhibits stripe domains which were correlated to the presence of a perpendicular magnetic anisotropy. The study performed on multilayers where Co was intercalated by very thin ⁵⁷Fe layers showed that the interfaces were very clean and sharp.     

Magnetic Anisotropy and the Appearance of Stripe Magnetic Domains in Co/Fe Multilayers

Thin Co/Fe multilayers were e-beam evaporated in ultra-high vacuum, keeping constant the Co layer thickness and varying that of Fe in the 0.5÷15 nm range. By increasing the Fe layer thickness, a component of magnetization perpendicular to the film plane rises up, and long and parallel magnetic domains (stripe domains) appear. The phenomenon is explained on the basis of the competition between the magnetoelastic anisotropy induced by stresses at the interfaces, and the shape anisotropy constraining the magnetization in the film plane. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic properties of dilute Co impurities in liquid Sn

The susceptibility of dilute Co impurities in liquid Sn is reported. It displays an almost linear increase from 1.8 × 10^?4 cm³/mole at 240°C to 4.5 × 10^?4 cm³/mole at 1000°C. These results are compared with other experimental information and are shown to be consistent with a recently proposed ionic impurity model if an interaction parameter is assumed to be temperature dependent.     

Magnetic properties of Zr doped Y9Co7

Detailed measurements of magnetization and ac susceptibility at low temperatures of 1% Zr-substituted Y₉Co₇ are presented. All results are indicative of itinerant weak ferromagnetism withT _c ∼ 9.5 K. The zero-field magnetizations followT ² orT ^4/3 behaviour as in the Ni-substituted system. The estimated critical exponents areβ=0.38±0.03,γ=1.16±0.05. It is argued that the main effect of the non-magnetic Zr-substitution in Y₉Co₇ is to stabilize the ferromagnetic ordering by suppressing the ‘hopping’ of Co atoms along thec-axis sites of the hexagonal structure.     

Magnetic properties of Gd-substituted Y2Co7 hydrides

The magnetization of (Y_1−yGd_y)₂Co₇H_x(y = 0.1, 0.2, 0.4 and 1; 0 ⩽ x ⩽ 3) has been measured at 4.2K up to 30T. The exchange field introduced by the Gd substitution enhances the Co moment in the hydrides. The origin of the disappearance of ferromagnetism in the β_L hydride (y = 0; x = 1.7) has been discussed.     

Magnetic properties of DNA-templated Co/Cu naonoparticle chains

<正>According to ultraviolet(UV)-vis absorption spectra recorded in the DNA metallization process,DNA-templated Co/Cu binary nanoparticle chains are fabricated by incubating genome DNA of paralichthys olivaceus muscle in CoCl_2 and CuCl_2 mixture solution for 20 hours and reducing the complex for 2 hours.Transmission electron microscopy observation indicates that Co and Cu nanoparticles with 20 nm in diameter were randomly dispersed on the DNA template. The superconducting quantum interference device(SQUID) measurements display that the magnetic interaction between cobalt particles is greatly decreased by the copper particle.With increasing copper content,the coercivity of the systems enhance from 9 Oe to 100 Oe(1 Oe=79.5775 A/m).