Search

Magnetic resonance force microscopy (MRFM) images the three-dimensional spatial distribution of resonant spins by mechanical force detection. Image reconstruction in MRFM is challenging because the resonance occurs in a strongly curved shell that extends beyond the scan range. In contrast with conventional magnetic resonance imaging, where Fourier techniques work well, the curved-shell resonant geometry inherent to MRFM requires novel reconstruction methods. Here, we show the application of iterative reconstruction in an electron spin resonance imaging experiment with 80 nm voxels. The reconstructed image has a total scan volume of 0.5 cubic micrometers, and was generated by a magnetic resonant shell with a curvature radius of 2.3 μm. The imaged object was a paramagnetically doped solid with an obliquely tilted surface. The reconstructed image correctly identified the location and orientation of the surface, and mapped the spin distribution within the solid. Applications of MRFM include three-dimensional nanometer-scale mapping of dopant distributions in semiconductors, studies of magnetism of thin films, and spin diffusion physics. An ultimate goal of MRFM is the direct observation of molecular structure at the atomic scale.

Early work in force-detected nuclear magnetic resonance (FD-NMR) and magnetic resonance force microscopy was restricted to nuclei with a relatively large gyromagnetic ratio γ. Increasingly, as researchers look to apply FD-NMR to practical problems, observing isotopes with a small γ is becoming necessary. The small γ of these isotopes places severe restrictions on the radio frequency field strength necessary to flip the sample spins at practical cantilever frequencies by adiabatic rapid passage. These restrictions led us to investigate the feasibility of observing FD-NMR by flipping sample spins at a rate well below the cantilever frequency. In this article we show that there is no increase in thermomechanical force noise in off-cantilever-resonance FD-NMR relative to on-cantilever-resonance work. Further, we show that working off-cantilever resonance can reduce artifacts and decrease data acquisition time. The major disadvantage to working off-cantilever resonance—reduced cantilever response—increases the importance of low noise detection of cantilever oscillation.

Motivated by the difficulty of obtaining quantitative micromagnetic results using current magnetic force microscope imaging techniques, we have employed an imaging mode where the oscillation amplitude of the tip was recorded versus the tip–sample separation as the tip was raster scanned over a magneto-optic sample. The mechanical response of the cantilever depends on the magnetic, but also on topographic, interferometric and nonmagnetic dissipative interactions between the tip and the sample. We separated the magnetic signal from the other interactions and analyzed it in terms of a refined theory of magnetic force microscope response. The extracted magnetic signal, which we refer to as a force gradient map, showed some features not apparent in conventional magnetic force microscope images and was well fit by a simple micromagnetic model of the magneto-optic sample.

The continuous wave ferromagnetic resonance (FMR) spectrometer operating in multioctave (0.05–40 GHz) frequency range has been built to investigate the magnetic properties of thin ferromagnetic films in the temperature range of 4–420 K. The spectrometer has two probeheads: one is the X-band microwave reflection cavity used to perform express room temperature measurements and the other is an in-cryostat microstrip line probe to carry out FMR experiments covering the entire frequency range offered by the microwave source. Very uniform and stable magnetic field up to 2.4 T, temperature 4 K–420 K, and continuous frequency scan performed by an HP8722D vector network analyzer provide various modes of operation. Both probe heads are equipped with two-circle high precision goniometers to ensure accurate characterization of magnetic anisotropy and magnetostatic waves spectra recording. Use of the phase sensitive detection, utilized by magnetic field modulation at audio frequency and computer triggering of the netw...

We have fabricated a superconducting resonator capable of generating a strong microwave magnetic field in a small (100 μm) volume for low temperature magnetic resonance experiments. While the resonator was specifically developed for use at millikelvin temperatures in a dilution refrigerator, where the total cooling power is limited to a few hundred microwatts, it is also useful at temperatures up to 5 K. The resonator consists of a 220 μm diameter, 2-1/2 turn niobium coil resonating with a short section of niobium microstripline. At a resonance frequency of 3 GHz, the loaded Q of the resonator was 780. The field strength was characterized by performing electron spin nutations. Operating at 100 mK with 320 μW of dissipated power, the resonator generated a field of 4 G at a distance of 100 μm from the coil.

We report on experimental advances in scanning electron microscopy with polarization analysis that allow the observation of ferromagnetic domains in external magnetic fields of about 0.1 T. This is achieved by using a modified electron optics that produces a magnetic field at the sample surface that is spatially confined on the length scale of 0.1 mm. During imaging, primary and secondary electrons pass through the magnetic field without significant disturbance. We demonstrate that the primary electron beam may be used to keep track of the generated magnetic field. As an exemplary application, the switching processes of rectangular Permalloy elements are analyzed.

We present the design of a scanning force microscope, which is optimized for magnetic force microscopy experiments. It can be operated at temperatures down to 5.2 K, in ultrahigh vacuum, and in magnetic fields of up to 5 T. Cooling is provided by a liquid helium bath cryostat, and the magnetic field is generated by a superconducting split-coil magnet. The design allows easy access from the side through a shutter system for fast in situ tip and sample exchange, while the microscope stays at temperatures below 25 K. The microscope itself features an all-fiber interferometric detection system, a 7.5×7.5 μm2 scan area at 5.2 K, and an xy table. The topographic resolution is demonstrated by imaging monoatomic steps on a nickel oxide surface, while magnetic contrast is shown on cobalt platinum multilayers and on a manganite perovskite film.

Images of a thin film permalloy element taken with Lorentz and magnetic force microscopies are compared with those from a simulation of the expected magnetic structure of the element. Measurements taken from the domain walls present in the element allow a quantitative comparison to be made. In the case of magnetic force microscopy, quantification is made possible by using a nonperturbative approach based on an extended charge model for the magnetic probe. Excellent agreement between experiment and simulation is observed for both imaging techniques.

In this article, we present the first fully integrated nuclear magnetic resonance probe ever realized. Planar spiral coils, a radio-frequency preamplifier, a mixer, and an audio-frequency amplifier are integrated using a standard complementary metal–oxide–semiconductor technology on a single silicon chip of 10 mm2. A 1 mm3 solid sample of cis-polyisoprene, placed over one of the integrated coils, is used as resonating material. The realized integrated probe allows measurements of static magnetic fields from 0.7 to 7 T with a magnetic-field resolution better than 1 ppm/Hz1/2, an absolute accuracy better than 3 ppm, and a spatial resolution of about 1 mm.

It is shown that by varying pulse lengths and delay times in electron spin resonance microwave pulse sequences, designed to enhance 13C nuclear magnetic resonance signals in diamond, and measuring the resulting 13C nuclear magnetic resonance signal, the paramagnetic impurity concentration, electron spin–spin and spin-lattice relaxation times in the laboratory and rotating frames, as well as the electron-nuclear interaction time, can be obtained. It is also shown that the maximum 13C polarization at a specific microwave frequency is dependent on the electron spin-lattice relaxation times in the laboratory and rotating frames.

In atomic force microscopy cantilevers are used to detect forces caused by interactions between probing tip and sample. The minimum forces which can be detected with commercial sensors are typically in the range of 10−12 N. In the future, the aim will be to construct sensors with improved sensitivities to detect forces in the range of 10−18 N. These sensors could be used for mass spectroscopy or magnetic resonance force microscopy. Achieving this goal requires smaller sensors and increased quality factor Q. In this article we describe a model to characterize the dynamics of cantilevers of each eigenmode. In contrast to previous models, the damping is treated rigorously in the calculations.

We have developed a mechanical method for modulating sample magnetization in magnetic resonance force microscopy experiments. A small-amplitude, low-frequency dithering of a magnetic field gradient source relative to a sample is used to create an oscillating magnetic field at the sample, which leads to a modulation of magnetization in rf-irradiated samples by magnetic resonance. Using this technique in concert with modulating rf power, we have achieved a 10 000× decrease in a sensitivity-limiting spurious response of microcantilevers to rf fields in sample-on-cantilever magnetic resonance force microscopy experiments. This mechanical implementation of “anharmonic modulation” is especially well suited for low power and vacuum applications, and is unique in its ability to modulate sample magnetization in magnet-on-cantilever experiments where the cantilever frequency is found to be a strong function of magnetic field.

We present experimental results on the characterization of commercially available magnetic force microscopy (MFM) thin film tips as a function of an external magnetic field. Well defined magnetic stray fields are produced using current carrying rings with radii ranging between 603 and 2369 nm fabricated by electron-beam lithography directly imaged by MFM. Treating the MFM tip as a point probe, the analysis of the image contrast as a function of both the magnetic stray field and the lift height allows for a quantitative determination of effective magnetic dipole and monopole moments of the tip as well as their imaginary location within the real physical tip. Our systematic study gives a quantitative relationship on how absolute values of the magnetic dipole and monopole moments and their location within the tip depend on a characteristic decay length of the z component of the magnetic field being detected. From this we can estimate the effective tip volume of the real physical thin film tip relevant in MFM...

Three methods for two-dimensional correlation nuclear magnetic resonance spectroscopy at zero field are discussed. All three involve coherence transfer via longitudinal polarization, double quantum coherence, or both in parallel. The double quantum pulse sequences exploit the spinor property of spin states. These sequences have been applied to connected Δm=1 transitions, as well as for the indirect detection of forbidden or nearly forbidden Δm>1 transitions.

We have developed magnetic resonance force microscopy for quantitative measurements of magnetic fields. A microscopic particle attached near the end of a microcantilever serves as the field sensing probe. We have demonstrated two-dimensional field mapping with a lateral resolution of 3.2 μm and a field resolution of 0.19 mT (1.9 G). The instrument holds considerable promise for field mapping with spatial resolution better than 0.1 μm at room temperature. Applications include field mapping of magnetic recording heads.

We report on the design and performance of an atomic force microscope which operates at temperatures down to 20 mK and in magnetic fields up to 9 T. The scan range at low temperatures is 4 μm×4 μm. The instrument features a piezoelectric linear motor for vertical coarse approach, and a horizontal sample translation stage with a 2 mm×2 mm range. A fiber interferometer is used to detect the force-sensing cantilever displacement. The performance demonstrated includes the ability to detect single atomic steps on a graphite surface at 4.2 K and the ability to locate and image nanometer scale electronic devices at millikelvin temperatures.

Magnetic resonance experiments were conducted on optically pumped Cs to investigate the use of atomic magnetic resonance in Cs atoms for precision magnetometry. The resonance frequencies agreed within 4 ppm with the theoretically calculated ones. The different amplitudes of the separate signals were ascribed to differences in the population densities in the sublevels. The noise level for low-field measurements was 10 pT/Hz.

The profile of the magnetic field distribution over the surface of Bi2212 superconducting samples is studied by scanning with an electron paramagnetic resonance (EPR) microprobe. The magnetic field distribution is mapped from the measurements of the spin probe EPR-signal shift. The investigations were carried out for both zero field cooling and field cooling procedures and showed different profile shapes in reversible and irreversible magnetization ranges of the superconducting material. It has been found that this method is particularly suitable to study the field profiles above the sample surface where other techniques are not so successful.

Time-resolved microscopy enables valuable new measurements of the dynamics of resonance and relaxation in a range of magnetic systems. An overview of the scope of applications to ferromagnetic microstructures is presented. These include observations of ferromagnetic resonance and spatially nonuniform modes of oscillation, studies of magnetization reversal, and characterizations of the speed of magnetic recording devices.

We present our new setup of longitudinal nuclear magnetic resonance detected by a superconducting quantum interference device. It was built specifically for the study of metallic single crystals at low frequency. We report the main difficulties encountered with such measurements, mainly thermal effects which are usually barely discussed in the literature although they put the true limit of such measurements. We also illustrate in detail the actual performances of our experiment on different test samples up to the first measurement of a metallic single crystal with such a technique.

A novel miniature superconducting quantum interference device magnetometer with direct readout electronics has been developed. A high flux-to-voltage transfer factor of up to 2500 μV/Φ0 is achieved without additional positive feedback (Φ0 is the flux quantum). A flux resolution of 8×10−7 Φ0/Hz1/2, corresponding to a magnetic moment sensitivity of approximately 2×10−20 A m2/Hz1/2, has been measured. This magnetometer can be used to study the dynamic magnetization properties (including nuclear magnetic resonance and magnetic resonance imaging) of micron and submicron size particles and for high-resolution surface magnetometry.

Detecting the tiny magnetic field from a few thousand atomic nuclei is a first step toward imaging molecular structures directly. Detecting the tiny magnetic field from a few thousand atomic nuclei is a first step toward imaging molecular structures directly.

In magnetic resonance force microscopy (MRFM) experiments, magnetic forces couple to the motion of microscale cantilever beams. Extension of MRFM to the detection of single electrons will require both unprecedented force sensitivity and motional stability of the cantilever. We describe the principles and performance of optimal cantilever motion control. The method accounts for inherent noise processes and practical application of control forces. We show that active feedback control improves cantilever motional stability, enabling instrument designs of much higher sensitivity and faster imaging than passive designs. Experimental results of implemented cantilever control systems are presented in Part II.

We describe the implementation of optimal controllers for damping the motion of cantilevers used in magnetic resonance force microscopy. We demonstrate that optimal control is achievable and that torsional magnetic coupling provides an effective actuation method. Cantilever Brownian vibrational amplitude was reduced from 2 to 0.16 A and resonant quality was reduced from 2000 to 5. Applied control fields were sufficiently small that they would not affect magnetic resonance phenomena.

A magnetic resonance force microscope was used to demonstrate three‐dimensional nuclear magnetic resonance imaging with micrometer‐scale spatial resolution. The sample was mounted on a silicon nitride cantilever that served as a micromechanical force sensor. A nearby magnetic tip generated a field gradient of 22 G/μm. A three‐dimensional magnetic resonance force map of the 1H spins in the sample was produced by lateral scanning of the magnetic tip relative to the sample and by varying the rf frequency of the spin excitation. The real‐space spin density of the sample was reconstructed from the force map by means of a deconvolution technique. The spatial resolution achieved in the experiment was ∼3 μm in the axial direction.

Using the techniques of microelectromechanical systems, we have constructed a small low-power magnetic sensor based on alkali atoms. We use a coherent population trapping resonance to probe the interaction of the atoms’ magnetic moment with a magnetic field, and we detect changes in the magnetic flux density with a sensitivity of 50pTHz−1∕2 at 10Hz. The magnetic sensor has a size of 12mm3 and dissipates 195mW of power. Further improvements in size, power dissipation, and magnetic field sensitivity are immediately foreseeable, and such a device could provide a hand-held battery-operated magnetometer with an atom shot-noise limited sensitivity of 0.05pTHz−1∕2.

Ferromagnetic resonance measurements were performed on CoO∕Co exchange biased microstructures with a low-temperature magnetic resonance force microscope (MRFM). The MRFM instrument relies on piezoresistive force detection, and the magnetic tip attached to the cantilever acts as a field gradient source. These features extend the applicability of the MRFM that can also be used as a conventional magnetic force microscope. Spatial variations of the MRFM signal, which are induced by a spatially modulated exchange bias, are monitored with a lateral resolution of about 5μm.

A three-dimensional (3D) electron spin resonance (ESR) microimaging system, operating in pulse mode at 9GHz is presented. This microscope enables the acquisition of spatially resolved magnetic resonance signals of free-radicals in solid or liquid samples with a resolution of up to ∼3.5×7×11.4μm in 20min of acquisition. The detection sensitivity at room temperature is ∼1.2×109spins∕√Hz, which enables the measurement of ∼2×107 spins in each voxel after 60min of acquisition. The resolution and detection sensitivity are the best obtained so far for ESR at ambient conditions of temperature and pressure. This ESR microscope can be employed in the investigation of a variety of samples in the fields of botany, life sciences, and materials science.

We have detected nuclear magnetic resonance in GaAs by selectively inverting 71Ga spins to create a local force gradient which shifts the mechanical resonance frequency of a nearby magnet tipped microcantilever. Employing a low spring constant cantilever (60 μN/m) at 4.4 K and 7 T, we demonstrate a magnetic moment sensitivity of 7.5×10−21 J/T, equal to the Curie Law magnetization from 7.1×108 71Ga spins. The method obviates the need to spin-lock sample magnetization during detection, allowing the signal to be collected for a time approaching the full spin-lattice relaxation time.

Point-contact junctions were fabricated by contacting an atomic force microscope (AFM) cantilever with a soft magnetic material coating on to [Al-O barrier/Fe3O4 half-metal] films using a tunneling AFM. The observed distribution of tunneling current through a thin barrier of Al-O in specimen point-contact junctions was influenced by the strength of the applied magnetic field and by the quality of the barrier at a measured position. Magnetization of the magnetically soft coating on the AFM cantilever was easily affected with alignment along the applied H direction, even when the contact size was as small as several tens of nanometers.

A method to characterize the magnetic moment of a magnetic force microscopy (MFM) tip is presented. The response of the MFM while imaging a superparamagnetic film is compared to a nonmagnetic reference. A simple model is used to compare the experimental results with a calculation of the response of the MFM tip due to the interactions with the magnetization of a superparamagnetic film.

We model and analyze the effect of particle shape on the signal amplification in inductive coil magnetic resonance detection using the reversible transverse magnetic susceptibility of oriented magnetic nanostructures. Utilizing the single magnetic domain Stoner-Wohlfarth model of uniform magnetization rotation, we reveal that different ellipsoidal particle shapes can have a pronounced effect on the magnetic flux enhancement in detection configurations typical of magnetic resonance settings. We compare and contrast the prolate ellipsoids, oblate ellipsoids, and exchange-biased spheres and show that the oblate ellipsoids and exchange-biased spheres have a significantly higher flux amplification effect than the prolate ellipsoids considered previously. In addition, oblate ellipsoids have a much broader polarizing magnetic field range over which their transverse flux amplification is significant. We show the dependence of transverse flux amplification on magnetic resonance bias field and discuss the resulting signal-to-noise ratio of inductive magnetic resonance detection due to the magnetic nanoparticle-filled core of the magnetic resonance detection coil.

Electron spin resonance (ESR) spectroscopy has been performed for gold nanorods (AuNRs) of four different sizes covered with a diamagnetic stabilizing component, cetyltrimethylammonium bromide. ESR signals were detected in AuNRs except the largest one. Two smallest AuNRs showed an abrupt change in the temperature dependence of resonance field and line width at around 60 K, indicating ferromagnetic phase transition. In medium-size AuNRs, the resonance with a large shift was observed below 100 K. The resonance field shifts at the lowest temperature exhibit systematic variation with the system size, which is explained by considering magnetic anisotropy for the ferromagnetic resonance.

This article presents a new modulation technique for noise reduction in magnetic resonance force microscopy. Applied fields are modulated at frequencies that are not rational fractions of the cantilever resonant frequency, thus avoiding overtones that contribute to the noise level. An on‐resonance signal is obtained because the nonlinear sample magnetization acts as a frequency mixer of the two modulation frequencies, producing a net force modulation at the cantilever resonant frequency. These techniques are experimentally demonstrated using electron spin resonance in a

We performed X-ray magnetic circular dichroism (XMCD) photoemission electron microscopy imaging to reveal the magnetic domain structure of anti-perovskite nitride Co3FeN exhibiting a negative spin polarization. In square and disc patterns, we systematically and quantitatively determined the statistics of the stable states as a function of geometry. By direct imaging during the application of a magnetic field, we revealed the magnetic reversal process in a spatially resolved manner. We compared the hysteresis on the continuous area and the square patterns from the magnetic field-dependent XMCD ratio, which can be explained as resulting from the effect of the shape anisotropy, present in nanostructured thin films.

An advanced scanning magnetoresistive microscopy (SMRM) - a robust magnetic imaging and probing technique - will be presented, which utilizes state-of-the-art recording heads of a hard disk drive as sensors. The spatial resolution of modern tunneling magnetoresistive sensors is nowadays comparable to the more commonly used magnetic force microscopes. Important advantages of SMRM are the ability to detect pure magnetic signals directly proportional to the out-of-plane magnetic stray field, negligible sensor stray fields, and the ability to apply local bipolar magnetic field pulses up to 10 kOe with bandwidths from DC up to 1 GHz. Moreover, the SMRM can be further equipped with a heating stage and external magnetic field units. The performance of this method and corresponding best practices are demonstrated by presenting various examples, including a temperature dependent recording study on hard magnetic L1(0) FeCuPt thin films, imaging of magnetic vortex states in an in-plane magnetic field, and their controlled manipulation by applying local field pulses.

A magnetic resonance force microscope is used to measure ΔMz, the change of longitudinal magnetization associated with the resonant absorption of microwave power in a ferromagnetic crystal. It is demonstrated that quantitative measurements of ΔMz can be obtained by analyzing the dipolar force exerted between a disk sample and a cylindrical probe magnet aligned and magnetized along their axis. Finite size effects are found to be important when probing the resonance in a micron-sized sample.

We study the resonant frequency shift of CoPt-capped single-crystal-silicon micro-oscillators when a magnetic field is applied perpendicular to the magnetic film, as required for application to nuclear magnetic resonance force microscopy. The oscillator resonant frequencies show two distinct regimes of behavior. At low fields, when the magnetic moment is nearly perpendicular to the external field, the frequency decreases sharply with field, while at high fields, when the moment and field are nearly aligned, the frequency increases. We present models that accurately describe both behaviors. The transition point between these two regimes scales with the volume of the micromagnets.

We demonstrate temperature measurement of a sample attached to the end of a cantilever using cantilever magnetometry of solid air ``contamination'' of the sample surface. In experiments like our Magnetic Resonance Force Microscopy (MRFM), the sample is mounted at the end of a thin cantilever with small thermal conductance. Thus, the sample can be at a significantly different temperature than the bulk of the instrument. Using cantilever magnetometry of the oxygen paramagnetism in solid air provides the temperature of the sample, without any modifications to our MRFM (Magnetic Resonance Force Microscopy) apparatus., Comment: Submitted to J of Applied Physics

A magnetic force microscope is used as a highly localized probe of the magnetic properties of Co‐Pd multilayer films. The stray field from the magnetic tip is used to induce magnetization changes in the sample on a scale of the order of ∼100 nm. These changes are characterized by discontinuous jumps in the force or force gradient acting on the tip due to its interaction with the magnetostatic stray field from the sample. Both reversible and irreversible domain‐wall motion is seen.

We describe the principles and imaging characteristics of a new type of high resolution magnetic resonance microscopy. Magnetic resonance is detected by measuring a small oscillating magnetic force acting between the spins in a sample and a nearby ferromagnetic particle. The oscillating magnetic force is generated by polarizing the spins in the magnetic field and then modulating the sample magnetization using magnetic resonance techniques. The oscillating magnetic force is detected by sensing the angstrom‐scale vibration of a micromechanical cantilever on which the sample is mounted. High spatial resolution is achieved as a result of the narrowness of the magnetic resonance spectral response and the large magnetic field gradient generated by the ferromagnetic particle. Electron paramagnetic resonance images are presented that demonstrate axial resolution on the order of 1 μm and lateral resolution on the order of 5 μm. Submicron resolution can be expected with straightforward technical improvements.

We have constructed a scanned stylus atomic force microscope (AFM) with direct force modulation and integrated microfluorescence optics. The instrument was designed to image the surface of massive samples under various ambient conditions. In force modulation microscopy the imaging force is modulated during the scanning process via an external magnetic field that acts directly on the magnetic AFM tip. Polymeric Langmuir–Blodgett films on silicon oxide were imaged to evaluate the application range of the instrument. We demonstrate that direct force modulation microscopy permits the quantitative recording of the local complex compliance both as a function of the location and as a function of the frequency. In a novel imaging mode referred to as sample resonance mode, the contrast of the image can be selectively enhanced based on local elasticity differences.

We describe a technique for imaging magnetic flux density by measuring lateral Lorentz force generated by current through a nonmagnetic tip in a stray magnetic flux density emanating from a sample. We demonstrate its usefulness by showing Lorentz force images of recorded tracks in a magnetic hard disk together with the topographic image. The lateral resolution of the technique is also discussed.

A focused ion beam process was utilized to physically define a “point dipole” magnetic force microscopy (MFM) tip with cylindrical dimensions as small as 50 nm in diameter and 10 nm in height. By controlling the preferred orientation of the magnetization, it is possible to define directional sensitivity. The preferred orientation is controlled either by choosing a proper magnetic material with preferred crystalline anisotropy or by applying a sufficiently strong external magnetic field. The signal selectivity with respect to the imaged field orientation allows more direct information to be obtained from the imaged object, as compared to the information obtained from the same object when using a conventional MFM tip.

We compare mechanically detected ferromagnetic resonance spectra for different separations h between the magnetic tip and sample surface. When the bias field generated by the tip is smaller than a few hundred gauss, the prominent changes are shifts of the entire spectrum (without line shape distortions) to higher frequency as h decreases. These results are in agreement with the Damon and Eshbach model for spin waves propagating in a potential perturbed by the additional field of the probe magnet. It is used to predict the spatial resolution limit for magnetostatic modes bounded by the stray field of the tip. The answer is ∼4 μm for yttrium iron garnet.

Using a low-field magnetic resonance scanner, we have obtained images of gaseous polarized 129Xe and water cells at room temperature. This potentially low-cost imaging technique offers the possibility of high-resolution imaging using both polarized noble gas and proton magnetic resonance imaging of tissues in the same scanner.

We applied the recently developed technique of force-detected nuclear magnetic resonance (NMR) to observe 71Ga, 69Ga, and 75As in GaAs. The nuclear spin-lattice relaxation time is 21$\pm$5 min for 69Ga at $\sim 5$ K and 4.6 Tesla. We have exploited this long relaxation time to first create and then observe spatially varying nuclear spin polarization within the sample, demonstrating a new form of contrast for magnetic resonance force microscopy (MRFM). Such nuclear spin contrast could be used to indirectly image electron spin polarization in GaAs-based spintronic devices., Comment: 3 pages, 2 figures

We report on magnetic force microscopy images of data tracks written in recording media taken using Pt-coated tips. The force acting on the tip when immersed in the magnetic field of the substrate was modeled assuming diamagnetic and paramagnetic interactions. The sample scanning using Pt-coated tips shows an improvement in the image delineation of the magnetic field distribution compared to the images taken using Co-coated hard magnetic tips and the display of at least a twice higher data track density.

A commercial atomic force microscope/magnetic force microscope (MFM) was modified to cool magnetic samples down to around 100 K under a high vacuum while maintaining its routine imaging functionality. MFM images of a 120 nm thick La0.7Ca0.3MnO3 film on a LaAlO3 substrate at low temperature show the paramagnetic-to-ferromagnetic phase transition. Evolution of magnetic domains and magnetic ripples with decreasing temperature are also observed near the edge of a 20 nm thick patterned Co film on a Si substrate.

A new technique of magnetic imaging on a spin-stand [Mayergoyz et al., J. Appl. Phys. 87, 6824 (2000)] is further developed and extensively tested. The results of successful imaging of digital patterns overwritten with misregistration ranging from 0.3 to 0.07 μm are reported. The results are compared with magnetic force microscopy (MFM) images and the conclusion is reached that the spin-stand imaging technique can provide (at least) the same level of resolution and accuracy as the MFM imaging technique.