Denis V. Pelekhov

Magnetic Force Microscopy of Superparamagnetic Nanoparticles

The use of magnetic force microscopy (MFM) to detect probe-sample interactions from superparamagnetic nanoparticles in vitro in ambient atmospheric conditions is reported here. By using both magnetic and nonmagnetic probes in dynamic lift-mode imaging and by controlling the direction and magnitude of the external magnetic field applied to the samples, it is possible to detect and identify the presence of superparamagnetic nanoparticles. The experimental results shown here are in agreement with the estimated sensitivity of the MFM technique. The potential and challenges for localizing nanoscale magnetic domains in biological samples is discussed.

Nanoscale scanning probe ferromagnetic resonance imaging using localized modes

The discovery of new phenomena in layered and nanostructured magnetic devices is driving rapid growth in nanomagnetics research. Resulting applications such as giant magnetoresistive field sensors and spin torque devices are fuelling advances in information and communications technology, magnetoelectronic sensing and biomedicine. There is an urgent need for high-resolution magnetic-imaging tools capable of characterizing these complex, often buried, nanoscale structures. Conventional ferromagnetic resonance (FMR) provides quantitative information about ferromagnetic materials and interacting multicomponent magnetic structures with spectroscopic precision and can distinguish components of complex bulk samples through their distinctive spectroscopic features. However, it lacks the sensitivity to probe nanoscale volumes and has no imaging capabilities. Here we demonstrate FMR imaging through spin-wave localization. Although the strong interactions in a ferromagnet favour the excitation of extended collective modes, we show that the intense, spatially confined magnetic field of the micromagnetic probe tip used in FMR force microscopy can be used to localize the FMR mode immediately beneath the probe. We demonstrate FMR modes localized within volumes having 200 nm lateral dimensions, and improvements of the approach may allow these dimensions to be decreased to tens of nanometres. Our study shows that this approach is capable of providing the microscopic detail required for the characterization of ferromagnets used in fields ranging from spintronics to biomagnetism. This method is applicable to buried and surface magnets, and, being a resonance technique, measures local internal fields and other magnetic properties with spectroscopic precision.

Imaging mechanisms of force detected FMR microscopy

We demonstrate spatial resolution of ferromagnetic resonance in a microscopic sample of YIG using ferromagnetic resonance force microscopy (FMRFM). Measurements were performed on a small single crystal YIG film grown on a GGG substrate, roughly rectangular in shape 20 µm×~150 µm and 3 µm thick. The perpendicular and parallel force geometries of FMRFM, in conjunction with an external bias field both parallel and perpendicular to the film, were used to scan the sample. This enabled the detection of strong signals, even at atmospheric pressure and room temperature. The fundamental and higher-order magnetostatic modes were observed to have 26–29 Gauss separation. The intensity of these modes exhibited spatial variation as the magnetic tip was scanned over the sample, and this behavior is qualitatively explained by DE theory. An improved fabrication method for magnet on cantilever was employed, which yielded a spatial resolution of 15 µm. These results demonstrate the potential of FMRFM for investigating the spatial dependence of ferromagnetic resonance, and for studying the anisotropy fields and exchange coupling effects within multilayer films and small magnetic systems.

Design of a variable temperature scanning force microscope.

We have developed the variable temperature scanning force microscope capable of performing both magnetic resonance force microscopy (MRFM) and magnetic force microscopy (MFM) measurements in the temperature range between 5 and 300 K. Modular design, large scanning area, and interferometric detection of the cantilever deflection make it a sensitive, easy to operate, and reliable instrument suitable for studies of the dynamic and static magnetization in various systems. We have verified the performance of the microscope by imaging vortices in a Nb thin film in the MFM mode of operation. MRFM spectra in a diphenyl-picryl-hydrazyl film were recorded to evaluate the MRFM mode of operation.

Theory of spin relaxation in magnetic resonance force microscopy

We study relaxation of a spin in magnetic resonance force microscopy (MRFM) experiments. We evaluate the relaxation rate for the spin caused by high-frequency mechanical noise of the cantilever under the conditions of adiabatic spin inversion. We find quantitative agreement between the obtained relaxation time and the experimental results of B. C. Stipe, H. J. Mamin, C. S. Yannoni, T. D. Stowe, T. W. Kenny, and D. Rugar [Phys. Rev. Lett. 87, 277602 (2001)]. Based on our analysis, we propose a method for improving the MRFM sensitivity by engineering cantilevers with reduced tip positional fluctuations.

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≤1u2002G 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.

The magnetic-resonance force microscope: a new tool for high-resolution, 3-D, subsurface scanned probe imaging

The magnetic-resonance force microscope (MRFM) is a novel scanned probe instrument which combines the three-dimensional (3-D) imaging capabilities of magnetic-resonance imaging with the high sensitivity and resolution of atomic-force microscopy. It will enable nondestructive, chemical-specific, high-resolution microscopic studies and imaging of subsurface properties of a broad range of materials. The MRFM has demonstrated its utility for study of microscopic ferromagnets, and it will enable microscopic understanding of the nonequilibrium spin polarization resulting from spin injection. Microscopic MRFM studies will provide unprecedented insight into the physics of magnetic and spin-based materials. We will describe the principles and the state-of-the-art in magnetic-resonance force microscopy, discuss existing cryogenic MRFM instruments incorporating high-Q, single-crystal microresonators with integral submicrometer probe magnets, and indicate future directions for enhancing MRFM instrument capabilities.

The effect of spin transport on spin lifetime in nanoscale systems

Spin transport electronics – spintronics – focuses on utilizing electron spin as a state variable for quantum and classical information processing and storage [1]. Some insulating materials, such as diamond, offer defect centers whose associated spins are well-isolated from their environment giving them long coherence times [2–4]; however, spin interactions are important for transport [5], entanglement [6], and read-out [7]. Here, we report direct measurement of pure spin transport – free of any charge motion – within a nanoscale quasi 1D ‘spin wire’, and find a spin diffusion length ∼ 700 nm. We exploit the statistical fluctuations of a small number of spins [8] ( √ N < 100 net spins) which are in thermal equilibrium and have no imposed polarization gradient. The spin transport proceeds by means of magnetic dipole interactions that induce flip-flop transitions [9], a mechanism that can enable highly efficient, even reversible [10], pure spin currents. To further study the dynamics within the spin wire, we implement a magnetic resonance protocol that improves spatial resolution and provides nanoscale spectroscopic information which confirms the observed spin transport. This spectroscopic tool opens a potential route for spatially encoding spin information in long-lived nuclear spin states. Our measurements probe intrinsic spin dynamics at the nanometre scale, providing detailed insight needed for practical devices which seek to control spin.Spintronics use the electron spin as a state variable for information processing and storage. This requires manipulation of spin ensembles for data encoding, and spin transport for information transfer. Because of the central importance of lifetime for understanding and controlling spins, mechanisms that determine this lifetime in bulk systems have been extensively studied. However, a clear understanding of few-spin systems remains challenging. Here, we report spatially resolved magnetic resonance studies of electron spin ensembles confined to a spin nanowire formed by nitrogen ion implantation in diamond. We measure the spin lifetime of the ensemble--that is, its spin autocorrelation time--by monitoring the statistical fluctuations of its net moment, which is in thermal equilibrium and has no imposed polarization gradient. We find that the lifetime of the ensemble is dominated by spin transport from the ensemble into the adjacent spin reservoir that is provided by the remainder of the nanowire. This is in striking contrast to conventional spin-lattice relaxation measurements of isolated spin ensembles. Electron spin resonance spectroscopy performed on nanoscale spin ensembles by means of a novel spin manipulation protocol corroborates spin transport in strong field gradients. Our experiments, supported by microscopic Monte Carlo modelling, provide a unique insight into the intrinsic dynamics of pure spin currents needed for nanoscale devices that seek to control spins.

Ferromagnetic resonance force microscopy on a thin permalloy film

Ferromagnetic resonance force microscopy (FMRFM) offers a means of performing local ferromagnetic resonance. The authors have studied the evolution of the FMRFM force spectra in a continuous 50nm thick permalloy film as a function of probe-film distance and performed numerical simulations of the intensity of the FMRFM probe-film interaction force, accounting for the presence of the localized strongly nonuniform magnetic field of the FMRFM probe magnet. Excellent agreement between the experimental data and the simulation results provides insight into the mechanism of FMR mode excitation in a FMRFM experiment.

Temperature-dependent magnetic resonance force microscopy studies of a thin Permalloy film

We used magnetic resonance force microscopy (MRFM) to study a 50nm thick continuous Permalloy film. We mechanically measured the ferromagnetic resonance signal in the temperature range between 10 and 70K in the presence of a static magnetic field applied normal to the surface of the film. The measurements show a decrease of the ferromagnetic resonance field with increasing temperature. We attribute this behavior to the temperature-dependent changes of the saturation magnetization. Our experiments demonstrate the potential of MRFM to perform quantitative ferromagnetic resonance measurements as a function of temperature.