Yu. Obukhov

Magnetization reversal in an individual 25 nm iron-filled carbon nanotube

The magnetization reversal and switching behavior of an individual Fe-filled carbon nanotube has been measured using vibrating cantilever magnetometry. We report measurements of the magnetic field at which the 25 nanometer diameter iron core inside the nanotube reverses. The fields at which reversal occurs, characterized by an exceptionally narrow distribution (σH≤1 G at 6.3 K), are determined by thermally activated excitation over a field dependent barrier. The high precision achievable by virtue of measuring individual nanowires allows detailed quantitative understanding of magnetization reversal.

Real time cantilever signal frequency determination using digital signal processing

We describe a digital signal processing method for high precision frequency evaluation of approximately sinusoidal signals based on a computationally efficient method. We demonstrate frequency measurement enabling sensitive measurement of the oscillatory force exerted on a micromechanical cantilever. We apply this technique to detection of the force signal arising in a micromechanically detected magnetic resonance force microscopy electron spin resonance signal. Our frequency detection measurements agree well with the theoretical noise analysis presented here, and we find that due to the excellent sensitivity of optical displacement detection, our sensitivity is limited only by the thermal displacement noise of the cantilever.

Quantitative magnetic force microscopy on permalloy dots using an iron filled carbon nanotube probe

An iron filled carbon nanotube (FeCNT), a 10-40 nm ferromagnetic nanowire enclosed in a protective carbon tube, is an attractive candidate for a magnetic force microscopy (MFM) probe as it provides a mechanically and chemically robust, nanoscale probe. We demonstrate the probes capabilities with images of the magnetic field gradients close to the surface of a Py dot in both the multi-domain and vortex states. We show the FeCNT probe is accurately described by a single magnetic monopole located at its tip. Its effective magnetic charge is determined by the diameter of the iron wire and its saturation magnetization 4πM(s) ≈ 2.2 × 10(4)G. A magnetic monopole probe is advantageous as it enables quantitative measurements of the magnetic field gradient close to the sample surface. The lateral resolution is defined by the diameter of the iron wire and the probe-sample separation.

The “Paramagnetic” Meissner Effect in Superconductors

There are a number of publications concerning the observation of a paramagnetic signal in HTSC ceramic samples during field-cooling procedures. Some authors assume that this signal is connected with spontaneous currents and discuss the possibility of the existence of π-junctions in Josephson media. In this work, it is shown that the paramagnetic signal could appear in the case of capture and compression of the magnetic flux in a sample during field-cooling procedures. The results of numerical calculation of the magnetic moment for different configurations of the sample and flux compression are presented. The unusual behavior of longtime relaxation of the magnetic moment in experiments on capture of magnetic flux is discussed.

Effect of localized magnetic field on the uniform ferromagnetic resonance mode in a thin film

We theoretically analyze the influence of the micromagnetic probe used in ferromagnetic resonance force microscopy (FMRFM) on the ferromagnetic resonance (FMR) modes in a thin ferromagnetic film. Our analysis of the FMRFM force response reveals three regimes defined by the extent to which the probe perturbs the uniform FMR mode. With closer approach, the FMRFM force grows more slowly because the strengthening probe field suppresses the FMR response. Our analysis agrees well with experimental data and provides theoretical foundations for FMRFM imaging.

Detection of higher order modulation harmonics in magnetic resonance force microscopy

Magnetic resonance force microscopy measurements of the electron spin resonance of a thin film of 2,2-diphenyl-1-picrylhydrazyl were performed using a low doped silicon cantilever with a high coercivity SmCo particle glued to its end. The low doping level enables amplitude modulation of the microwave field with only small spurious driving of the cantilever. Besides amplitude modulation we use frequency modulation of the microwave field at integer fractions of the cantilever resonance frequency leading to derivative signals up to the fourth derivative of the amplitude modulation response signal. The influence of the modulation depth on the line shape of the first derivative response is also presented.

London penetration depth measurement using a SQUID microscope

A method for the solution of the problem of a superconductor in an external magnetic field is developed. The method allows to convert a three dimensional vector problem into a two dimensional surface problem. Using this method, the Meissner behaviour of thin film and bulk superconductors in an external magnetic field is considered. A scheme for measurement of the local London penetration depth using a SQUID microscope of appropriate design is proposed. It is shown that the signal of such a microscope is linear with the change of the London depth λ and it is possible to attain a resolution of 1 nm of the λ measurement.

Critical currents in YBaCuO ceramics

Magnetization measurements on a ceramic YBaCuO sample were carried out. TheM(H, T) dependence has demonstrated some peculiarities which we assume are connected withHj1,Hj2,Hc1 fields. By analyzing the magnetization in the field region betweenHj1 andHj2 within the framework of the critical state model, one obtains the dependence of the intergranular critical current density on field and temperature.

Manipulating spins by cantilever synchronized frequency modulation: A variable resolution magnetic resonance force microscope

We report a new spin manipulation protocol for periodically reversing the sample magnetization for magnetic resonance force microscopy (MRFM). The protocol modulates the microwave excitation frequency synchronously with the position of the oscillating detection cantilever, thus allowing manipulation of the spin magnetization independent of both magnetic field gradient strength and cantilever response time. This allows continuous variation of the detected sample volume and is effective regardless of spin relaxation rate. This enhanced flexibility improves the utility of MRFM as a generally applicable imaging and characterization tool.

Localized ferromagnetic resonance force microscopy in Permalloy-cobalt films

We report the Ferromagnetic Resonance Force Microscopy (FMRFM) experiments on a combined permalloy-cobalt continuous film. Our studies demonstrate the capability of FMRFM to perform local spectroscopy of different ferromagnetic materials. Theoretical analysis of the uniform resonance mode at the edge of the film provides good quantitative agreement with the experimental data. Our experiments demonstrate the micron scale lateral resolution and allow to extract local magnetic properties in continuous ferromagnetic samples.