Magnetic double resonance in force microscopy

Magnetic-resonance force microscopy is combined with cross-polarization and spin-decoupling NMR techniques to obtain double-resonance NMR signals of micrometer-scaled objects. The effective one-dimensional spatial resolution obtained in our experiments performed on a KPF6 single crystal sample is approximately 0.5 microm. The spectral linewidth of 900 Hz is sample limited. The described double-resonance techniques can introduce new chemical specificity to the magnetic-force sensor.     

铁磁共振磁交换力显微镜

原子间的自旋相互作用力对原子级别磁性纳米构造体的表面磁性质的理解是极为重要的. 磁交换力显微镜是测量表面自旋作用力的重要手段, 但它的缺点一是需要加外部强磁场, 二是不能分离表面形貌和自旋信息, 这就导致材料表面受外部磁场的影响, 而且表面形貌和自旋信息之间相互影响, 使自旋间的相互作用力的检测和成像研究受到限制.为了解决上述问题, 利用原子力显微镜, 并采用微波照射的方法, 根据铁磁共振原理, 分别独立提取磁性材料表面形貌和自旋信息, 称之为铁磁共振磁交换力显微镜, 理论和实验结果表明此方法可以有效地分离两种信息. 铁磁共振磁交换力显微镜可以促进对原子级磁性材料机能的理解以及磁性相关科学领域的进步, 特别是对自旋电子元件的发展有很大的促进作用, 是新世纪高度信息化社会不可缺少的测量技术. 关键词： 原子力显微镜 磁交换力显微镜 自旋 铁磁共振     

Magnetic resonance force microscopy combined with surface topography

A new method of surface microscopy is proposed, which combines three-dimensional electron spin resonance imaging by magnetic resonance force microscopy (MRFM) and topographic imaging of the sample surface by scanning force microscopy (SFM). In order to demonstrate its potential for the identification of microscale objects, the individual and combined images are used to provide the locations, shapes and spin density distributions of target phantom objects. We report spatial resolution in MRFM of 2.8 x 2.8 x 2.0 microm(3). This could be improved to the theoretical limit of 0.08 x 0.08 x 0.04 microm(3) through reduction of the thermal noise by cooling to cryogenic temperatures approximately 0.5K. We believe that this type of microscopy will become a very useful tool for the investigation of anomalies induced in surfaces by materials buried below the surface.     

Magnetic force microscopy of GaAs:Mn ferromagnetic semiconductors

The dependence of magnetic properties of GaAs:Mn and MnAs epitaxial films grown on GaAs (001) by laser ablation of Mn and undoped GaAs in a hydrogen atmosphere under the growth conditions has been studied by magnetic force microscopy (MFM). Magnetic probe calibration for quantitative MFM measurements was performed by scanning across the slit of the magnetic-head of a tape recorder through which controlled direct current was passed. The dipole approximation was used to describe the magnetic properties of the MFM probe. Nonuniformity of the magnetization of GaAs:Mn films related to the formation of MnAs nanoinclusions, which are ferromagnetic at 300 K, has been observed. The typical scales of the spatial nonuniformity of the magnetization of GaAs:Mn films were varied from 270 to 550 nm depending on the film-growth conditions. The MnAs phase was identified by MFM measurements at an elevated temperature (up to 80°N).     

Magnetic elements for switching magnetization magnetic force microscopy tips

Using combination of micromagnetic calculations and magnetic force microscopy (MFM) imaging we find optimal parameters for novel magnetic tips suitable for switching magnetization MFM. Switching magnetization MFM is based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within the technique the sum of the scanned data with reversed tip magnetization depicts local atomic forces, while their difference maps the local magnetic forces. Here we propose the design and calculate the magnetic properties of tips suitable for this scanning probe technique. We find that for best performance the spin-polarized tips must exhibit low magnetic moment, low switching fields, and single-domain state at remanence. The switching field of such tips is calculated and optimum shape of the Permalloy elements for the tips is found. We show excellent correspondence between calculated and experimental results for Py elements.     

Magnetic force microscopy applied in magnetic data storage technology

Microstructured thin-film elements with critical dimensions of 1 μm or less play an increasingly important role in magnetic components for information technology applications. Devices that are directly based on such microstructures are key components in magnetoelectronics for storage and sensor applications as well as modern concepts which are likely to substitute today’s hard disk drives. Basic research on magnetic materials as well as industrial applications create an increasing demand for high-resolution magnetic imaging methods. One such method is magnetic force microscopy (MFM). In spite of considerable achievements, MFM also has some serious shortcomings, which have not been overcome to date. Under normal circumstances, the method yields only qualitative information about the magnetic object and it is difficult to improve the resolution to values below 100 nm. In this paper, we will report on advanced MFM probe preparation, based on electron beam methods, and discuss the possibilities for batch fabrication of such advanced MFM tips. We show that the advanced probes allow high-resolution imaging of fine magnetic structures within thin-film permalloy elements without perturbing them. Additionally, we present high-frequency MFM measurements on a hard disk write head. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-681/302-3790, E-mail: m.koblischka@mx.uni-saarland.de     

Magnetic force microscopy study of domain structures in magnetic films

Magnetic force microscopy is used to investigate two different types of samples: thin metal films and ferrite garnet films. It is pointed out for garnet films that magnetic force microscopy allows us only to judge the domain structure of surface layers. Problems associated with conducting measurements in external magnetic fields, the effect of the magnetic field of the probe on the investigated domain structure, and using magneto-polarized optics in combination with magnetic force microscopy are considered.     

Magnetic field of an isolated flux line in a type-II superconductor at finite temperature

The exact BCS-solution for an isolated vortex shows field reversal at large distances for small ?.     

Pinning force in thin superconductors with small number of flux line rows

It is shown that the volume pinning force (and hence the critical current density) of thin superconducting slabs decreases strongly after the transition of the one-dimensional flux line lattice into the two-dimensional structure. The results are applied to superfine filamentary super-conductors (diameterd0·1 m) and they are in accordance with experiments on NbTi superfine filaments, as well as on weakly reacted Nb₃Sn multifilamentary superconductors. The maximum of the volume pinning force appears therefore at the transition fieldB ₁₂ from the one-dimensional to the two-dimensional flux line lattice. This field is much smaller than the fieldB _m at whichF _p is maximum for macroscopic superconductors. So, e.g. for filament diameterd=60 nm of NbTi superconductor,B ₁₂1·2 T, whereas theB _m value for macroscopic NbTi is about 5 T.     

Magnetic force signal of vortex creation in type-II superconducting thin films

The possibility of the formation of a vortex in thin film due to the field of magnetic dipole moment when a point dipole is vertically approaching the surface of a thin film is discussed. Calculations are performed for certain discrete values of the critical position of the point dipole for creating the vortex in the thin film and the equilibrium positions of the vortex, both of which depend on the strength of magnetic dipole moment. The creation of a new single vortex in the thin film causes an abrupt change in the vertical force.     

Magnetic resonance force microscopy quantum computer with tellurium donors in silicon

We propose a magnetic resonance force microscopy (MRFM)-based nuclear spin quantum computer using tellurium impurities in silicon. This approach to quantum computing combines well-developed silicon technology and expected advances in MRFM. Our proposal does not use electrostatic gates to realize quantum logic operations.     

Magnetic resonance imaging of isolated single liposome by magnetic resonance force microscopy

Magnetic resonance imaging (MRI) is very useful spectroscopy to visualize a three-dimensional (3D) real structure inside the sample without physical destruction. The spatial resolution of the readily available MRI spectrometer is, however, limited by a few ten to hundreds of microns due to a technological boundary of generating larger magnetic field gradient and to the insensitivity inherent to the inductive signal detection. Magnetic resonance force microscopy (MRFM) is new alternative MRI spectroscopy which is anticipated to significantly surpass the conventional MRI in both resolution and sensitivity. We report two imaging experiments on our MRFM spectrometer operated at room temperature and in vacuum approximately 10(-3)Pa. One is for approximately 20 microm liposome membrane labeled entirely by a nitroxide imaging agent and the other for approximately 15 microm DPPH particles, both are nearly the same size as that of human cell. The reconstructed images at spatial resolution approximately 1 microm were in satisfactory agreement with the scanning electron microscope images. The potential capability of visualizing intrinsic radicals in the cell is suggested to investigate redox process from a microscopic point of view.     

Magnetic force microscopy studies of domain walls in nickel and cobalt films

A magnetic force microscopy is used to examine the domain walls in nickel and cobalt films deposited by argon ion sputtering. Thin nickel films deposited at high substrate temperatures exhibit coexistent Bloch and Neel walls. Films grown at room temperature display alternative Bloch lines with cap switches. These films agglomerate to form grains after annealed at high temperatures. The film composed of larger grains behaves better nucleation implying magnetic domains of closure, while the film composed of smaller grains exhibits more defects implying alternative Bloch lines. We have also observed domain displacements and cap switches, which occur due to precipitation of particles in small grain size films. Stripe domains are observed for film thicknesses larger than 100 nm. They become zigzag cells when an external field of 1.5 T is applied perpendicular to the surface of the films. This experiment indicates that the domain sizes in thin films and the strip widths for thick films both depend on the square-root of the film thickness, which varies from 5 to 45 nm and from 100 to 450 nm, respectively.     

Asymptotic survival probability of random walks on a semi-infinite line

Symmetric and asymmetric random walks on a segment (−∞,T>0) of the real line are considered. There is a non-zero probability for the random walk to get absorbed at a site it visits. We derive for such random walks, expressions for survival probabilities in the asymptotic limit ofT→∞. An application of this asymptotic formulation to the problem of radiation transport through thick shields is presented.     

Magnetic flux distribution in type II superconductors with large demagnetization and a high edge barrier

The trapped flux distribution in thin wafers of both polycrystalline and granular superconductors having large demagnetization and edge barriers of different heights is measured by means of polarized neutrons. It is shown that the nature of the critical state in polycrystalline wafers, unlike that in a ceramic wafer, is not described by the Bean model. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 12, 771–775 (25 December 1999)     

Magnetic force microscopy investigation of the magnetization reversal of permalloy particles at high temperatures

The magnetization reversal of an array of permalloy particles formed by scanning probe lithography on the silicon dioxide surface has been investigated in the temperature range from room temperature to 800 K. Using scanning magnetic force microscopy and numerical calculations of the magnetic anisotropy field of a particle at different temperatures, it has been shown that an increase in the temperature leads to a decrease in the external magnetic field required to reverse the magnetization direction of the particle. From the obtained results, it has been concluded that the magnetization reversal of the studied particles is accompanied by the formation of an intermediate state with an inhomogeneous magnetization structure.     

The volume pinning force for periodical interaction forces between defects and the flux line lattice

The volume pinning force for some forms of the interaction potential defect-flux line is calculated without restrictions on the vortex lattice distance and the interaction range of defects. It is shown that for larger maximum elementary interaction forces, the direct summation of pinning forces is realistic. However, if the interaction range of the defects is smaller than the vortex lattice distance, one obtains a region (about one order of magnitude) in which Labusch's quadratic dependence of the volume pinning force on the elementary interaction force is valid. In the region where the direct summation of pinning forces occur, the volume pinning force is proportional the vortex lattice distance and one obtains an additional magnetic field dependence of the volume pinning force.