Co纳米线磁矩反转动态过程的有限元微磁学模拟

采用有限元微磁学模拟方法研究了Co纳米线在不同外加恒磁场下磁矩的翻转过程.研究结果表明在直径为10 nm的Co纳米线内,经过一定的形核时间将在其一端形成一个反向磁畴.磁畴壁的类型为横向畴壁,该畴壁将在一外加恒定磁场的驱动下匀速地从一端运动到另一端.畴壁的运动速度与外加磁场大小呈线性关系.在H为1000 kA/m时,发现在纳米线的两端均会形成一个“头对头”的反向磁畴.计算结果表明,畴壁内磁矩的方向旋转一个周期所导致的畴壁运动的距离相同,与外加磁场强度无关. 关键词： 磁性纳米线 微磁学模拟 磁畴 横向畴壁     

Microscale localized spectroscopy with a magnetic resonance force microscope

Magnetic resonance force microscopy is combined with spin-echo spectroscopy to obtain spatially and spectrally resolved NMR signals of micrometer-scale objects. The experimental spatial resolution for the demonstration experiment on a sample consisting of Ba(ClO3)2.H2O and (NH4)2SO4 single crystals is 3.4 microm. The spectral resolution of 3.4 kHz is sample limited. Improvements in resolution and extensions of the method to more than one spatial dimension and to multidimensional spectroscopy are possible.     

Interaction of a magnetic vortex with the probe field of a magnetic force microscope

The interaction of a magnetic vortex in a circular ferromagnetic nanoparticle with the probe field of a magnetic force microscope (MFM) is theoretically investigated. In the calculations, the probe field is approximated by the point dipole field. The rigid magnetic vortex model is used to describe the vortex state of magnetization. It is found that the effect of the probe field on the rigid magnetic vortex shell is similar to the effect of a uniform magnetic field parallel to the particle plane. The effect of the Z component of the probe field on the core of the vortex results in mutual probe-vortex attraction or repulsion. It is shown that the magnetization direction of the core of the vortex in the MFM probe field can be changed without a change in the shell vorticity direction.     

Semiconductor acousto-electric potential detection using a force microscope

Acousto-electric force microscopy (AEFM), developed as a technique for high-resolution mapping of carrier characteristics in solids is described. This technique is based on an acousto-electric voltage (AEV) detection scheme where AEV pulses are obtained from the cantilever deflection caused by the electrostatic force acting between the cantilever tip and the sample surface. AEFM appears to be feasible, but an improved AEV detection scheme is required.     

Deforming and moving a vortex by the tip of a magnetic force microscope

Recent experiments [O.M. Auslaender et al., Nat. Phys. 5 (2009) 35] use a magnetic force microscope not only to image but also to move and deform an individual vortex line in a bulk YBCO type-II superconductor. The theory of this experiment is presented accounting for pinning and curving of the vortex and for the full three-dimensional anisotropy of pinning and of vortex line tension in this material.     

In situ magnetic hysteresis measurement of magnetic tips in magnetic force microscope

In situ magnetic hysteresis measurements of magnetic tips in a magnetic force microscope (MFM) are demonstrated using alternating gradient force magnetometry. The measured magnetic moments of MFM tips are estimated in the range from 10⁻⁶ to 10⁻⁵ emu by this technique and the whole MFM tips in cantilevers are considered to be measured from the value of measured magnetic moments. The relationship between the magnetic hysteresis loops of MFM tips and those of coated magnetic films is discussed.     

Probe--sample coupling in the magnetic resonance force microscope

The magnetic resonance force microscope (MRFM) provides a route to achieving scanned probe magnetic resonance imaging with extremely high spatial resolution. Achieving this capability will require understanding the force exerted on a microscopic magnetic probe by a spatially extended sample over which the probe is scanned. Here we present a detailed analysis of this interaction between probe and sample. We focus on understanding the situation where the micromagnet mounted on the mechanical resonator generates a very inhomogeneous magnetic field and is scanned over a sample with at least one spatial dimension much larger than that of the micromagnet. This situation differs quite significantly from the conditions under which most MRFM experiments have been carried out where the sample is mounted on the mechanical resonator and placed in a rather weak magnetic field gradient. In addition to the concept of a sensitive slice (the spatial region where the magnetic resonance condition is met) it is valuable to map the forces exerted on the probe by spins at various locations; this leads to the concept of the force slice (the region in which spins exert force on the resonator). Results of this analysis, obtained both analytically and numerically, will be qualitatively compared with an initial experimental finding from an EPR-MRFM experiment carried out on DPPH at 4 K.     

Magnetization reversal of ferromagnetic nanoparticles under inhomogeneous magnetic field

We investigated remagnetization processes in ferromagnetic nanoparticles under inhomogeneous magnetic field induced by the tip of magnetic force microscope (MFM) in both theoretical and empirical ways. Systematic MFM observations were carried out on arrays of submicron-sized elliptical ferromagnetic particles of Co and FeCr with different sizes and periods. It clearly reveals the distribution of remanent magnetization and processes of local remagnetization of individual ferromagnetic particles. Modeling of remagnetization processes in ferromagnetic nanoparticles under magnetic field induced by MFM probe was performed on the base of Landau–Lifshitz–Gilbert equation for magnetization. MFM-induced inhomogeneous magnetic field is very effective to control the magnetic state of individual ferromagnetic nanoparticles as well as to create different distribution of magnetic field in array of ferromagnetic nanoparticles.     

Dynamical friction coefficient maps using a scanning force and friction microscope

A novel method to analyze and to distinguish the non-dissipative component from the dissipative frictional component of the lateral force in a Scanning Force and Friction Microscope (SFFM) is worked out and applied to images acquired on various samples. We have determined the twisting spring constants of the cantilevers by computer statical analysis, since they are essential for the quantitative determination but are not provided by the manufacturer. Quantitative results for the _D dynamical friction coefficient images are reported. Comparison with the forward-backward lateral force image subtraction model is made.     

Structural and magnetic properties of polymer-stabilized tetragonal Ni nanoparticles

Ni nanoparticles (Ni-NPs), with diameter (D) ranging 5–30 nm, were synthesized by reducing nickel chloride with NaBH₄ in the presence of polymer molecules of poly-vinyl alcohol (PVA) in cold water. Nickel chloride was dispersed in the PVA molecules which stabilized the resulting Ni-NPs. Experiments were carried out with and without PVA to elucidate the effect of PVA molecules on the structural and magnetic properties of Ni-NPs. It was found that both uncoated (uc) and PVA-coated (pc) Ni-NPs exhibit a tetragonal (t) crystal structure, i.e. different from the cubic (fcc) structure of bulk nickel. pc Ni-NPs (paramagnetic in nature) converted to fcc Ni (spherical shape, D ～ 12 nm) on annealing at 573 K in air, exhibiting a saturation magnetization M _s = 20.5 emu/g, squareness ratio M _r /M _s = 0.48 and coercivity H _c = 248 Oe, which is higher than the bulk Ni (0.7 Oe). uc Ni-NPs showed little improvement in M _s and H _c on air annealing. The core–shell structure resulted in a high H _c value in stable pc Ni-NPs in air. Electron magnetic resonance revealed exchange interaction between the core and shell, which changes on annealing in air.     

Element-selective mapping of magnetic moments in ultrathin magnetic films using a photoemission microscope

We combine X-ray magnetic circular dichroism (XMCD) and photoelectron emission microscopy to obtain locally resolved magnetic information on a microscopic scale. Scanning the photon energy across elemental absorption edges and recording microscopic images of the local secondary electron intensity for both photon helicities at each photon energy step allows to analyze local XMCD spectra at any position of the imaged area of the sample. With the help of magnetic sum-rules local quantitative information about magnetic moments can be extracted from such microspectroscopic measurements. The full power of XMCD as a spectroscopic tool is so maintained, while microscopic spatial resolution is added.     

Manipulation and soldering of carbon nanotubes using atomic force microscope

Manipulation of carbon nanotubes (CNTs) by an atomic force microscope (AFM) and soldering of CNTs using Fe oxide nanoparticles are described. We succeeded to separate a CNT bundle into two CNTs or CNT bundles, to move the separated CNT to a desirable position, and to bind it to another bundle. For the accurate manipulation, load of the AFM cantilever and frequency of the scan were carefully selected. We soldered two CNTs using an Fe oxide nanoparticle prepared from a ferritin molecule. The adhesion forces between the soldered CNTs were examined by an AFM and it was found that the CNTs were bound, though the binding force was not strong.     

Electric polarization of magnetic textures: New horizons of micromagnetism

A common scenario of magnetoelectric coupling in multiferroics is the electric polarization induced by spatially modulated spin structures. It is shown in this paper that the same mechanism works in magnetic dielectrics with inhomogeneous magnetization distribution: the domain walls and magnetic vortexes can be the sources of electric polarization. The electric field driven magnetic domain wall motion is observed in iron garnet films. The electric field induced nucleation of vortex state of magnetic nanodots is theoretically predicted and numerically simulated. From the practical point of view the electric field control of micromagnetic structures suggests a low-power approach for spintronics and magnonics.     

Nanomechanical probing of microbubbles using the atomic force microscope

Atomic force microscopy (AFM) is a versatile mechanical nanosensor that can be used to quantify the mechanical properties of microbubbles (MBs) and the adhesion mechanisms of targeted MBs.

Mechanical properties were investigated using AFM tipless cantilevers to microcompress the MBs. The range of compressive stiffness for biSphere^® was found to be between 1 and 10 N m⁻¹ using a cantilever with a spring constant of 0.6 N m⁻¹. This stiffness was shown to decrease with the MB size in a non-linear fashion. It is also possible to calculate a theoretical Young’s modulus of the shell.

The adhesion properties of targeted lipid based MBs that use avidin–biotin chemistry for the attachment of targeting ligands were also studied. The MBs were attached to poly-l-lysine treated tipless cantilevers with spring constants ranging from 0.03 to 0.1 N m⁻¹. This system interrogated individual cells with pulling cantilever distance of 15 μm, and scan rate at 0.2 Hz. The depth of contact was not larger than 0.4 μm. The targeted MBs provided a significantly larger adhesion to the cells compared to control ones. Average adhesion force was dependent on depth of contact. Analysis of the data demonstrated a single distribution of adhesion events with median at 89 pN, which is in agreement with the literature for such interactions.

The nanointerrogation of MBs using AFM provides new insight into their mechanical properties, and should be of assistance to MB design and manufacture.     

Synthesis and characterization of Ni magnetic nanoparticles in AlMCM41 host

Superparamagnetic nickel nanoparticles were prepared by incorporating nickel ion into AlMCM41 as a nanoreactor and then reduced with sodium borohydride or H₂ gas. Products were characterized by elemental analysis, transmission electron microscopy, X-ray powder diffraction, and magnetic susceptibility. The nickel particle size and blocking temperature depend on the reduction method.     

The shape dependence of magnetic and microwave properties for Ni nanoparticles

The magnetic and microwave properties of Ni nanospheres and conical nanorods have been investigated through experimental and theoretical methods. Ni nanospheres and conical nanorods have the same crystal structure and close particle size, whereas the remanence ratio, coercivity, dynamic permeability and microwave absorbing properties show great dependence on their shape. Ni conical nanorods self-assembled into urchin-like structure have higher natural resonance frequency due to the large shape anisotropy compared to the Ni nanospheres. Supposing random spatial distribution of magnetic easy axes and using the Landau-Lifshitz-Gilbert equation associated with the Bruggeman's effective medium theory, we simulate the complex permeability of Ni nanoparticles, which agrees well with the experimental results.