Magnetic Force Images __TOP_

The effect of magnetization reversal of magnetic force microscope (MFM) tips based on low coercive thin-films on MFM images has been studied both experimentally and theoretically. By analyzing the MFM images obtained on structures with high magnetic stray fields we show that during the imaging process the magnetic state of the probe is modified anisotropically: the horizontal component of the magnetization follows the external field, whereas the vertical component of the magnetization stays almost constant. The observed complex magnetic behavior of the tip is explained theoretically based on the shape anisotropy of the tip. The obtained results are important for interpretation of MFM images of structures with high magnetic moment. Moreover, these results can be used for characterization of both laboratory-made and commercially available MFM tips.

Magnetic Resonance Imaging (MRI) is a non-invasive imaging technology that produces three dimensional detailed anatomical images. It is often used for disease detection, diagnosis, and treatment monitoring. It is based on sophisticated technology that excites and detects the change in the direction of the rotational axis of protons found in the water that makes up living tissues.

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MRIs employ powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field. When a radiofrequency current is then pulsed through the patient, the protons are stimulated, and spin out of equilibrium, straining against the pull of the magnetic field. When the radiofrequency field is turned off, the MRI sensors are able to detect the energy released as the protons realign with the magnetic field. The time it takes for the protons to realign with the magnetic field, as well as the amount of energy released, changes depending on the environment and the chemical nature of the molecules. Physicians are able to tell the difference between various types of tissues based on these magnetic properties.

To obtain an MRI image, a patient is placed inside a large magnet and must remain very still during the imaging process in order not to blur the image. Contrast agents (often containing the element Gadolinium) may be given to a patient intravenously before or during the MRI to increase the speed at which protons realign with the magnetic field. The faster the protons realign, the brighter the image.

Although MRI does not emit the ionizing radiation that is found in x-ray and CT imaging, it does employ a strong magnetic field. The magnetic field extends beyond the machine and exerts very powerful forces on objects of iron, some steels, and other magnetizable objects; it is strong enough to fling a wheelchair across the room. Patients should notify their physicians of any form of medical or implant prior to an MR scan.

Replacing Biopsies with Sound

Chronic liver disease and cirrhosis affect more than 5.5 million people in the United States. NIBIB-funded researchers have developed a method to turn sound waves into images of the liver, which provides a new non-invasive, pain-free approach to find tumors or tissue damaged by liver disease. The Magnetic Resonance Elastography (MRE) device is placed over the liver of the patient before he enters the MRI machine. It then pulses sound waves through the liver, which the MRI is able to detect and use to determine the density and health of the liver tissue. This technique is safer and more comfortable for the patient as well as being less expensive than a traditional biopsy. Since MRE is able to recognize very slight differences in tissue density, there is the potential that it could also be used to detect cancer.

(a) Circularly averaged TF of the subimage average in Fig. 3. (b) Cross sections of TFs without average, TF averaged over subimages, and TF obtained through final circular average. (c) Decay of the three TFs in Fourier space with decreasing wavelengths.

Demonstration of surface magnetization retrieval in the case of large spatial wavelengths. (a) Real magnetization pattern of the calibration sample. (b) Actual measurement contrast. (c) Deconvoluted surface magnetization map from (b). (d) Generic magnetization map with the same magnetic parameters as in (a) but with a domain periodicity of the full image size (2m). (e) Simulated measurement contrast based on (d). (f) The deconvolution of (e) for retrieving the magnetization in (d).

Because the stray magnetic field from the sample can affect the magnetic state of the tip, and vice versa, interpretation of the MFM measurement is not straightforward. For instance, the geometry of the tip magnetization must be known for quantitative analysis.

Atomic force microscopy (AFM) 1986, forces (atomic/electrostatic) between the tip and sample are sensed from the deflections of a flexible lever (cantilever). The cantilever tip flies above the sample with a typical distance of tens of nanometers.

Magnetic Force Microscopy (MFM), 1987[7] Derives from AFM. The magnetic forces between the tip and sample are sensed.[8][9] Image of the magnetic stray field is obtained by scanning the magnetized tip over the sample surface in a raster scan.[10]

Often, MFM is operated with the so-called "lift height" method.[14] When the tip scans the surface of a sample at close distances (< 10 nm), not only magnetic forces are sensed, but also atomic and electrostatic forces. The lift height method helps to enhance the magnetic contrast through the following:

The stray field from the sample exerts a force on the magnetic tip. The force is detected by measuring the displacement of the cantilever by reflecting a laser beam from it. The cantilever end is either deflected away or towards the sample surface by a distance z = Fz/k (perpendicular to the surface).

If an external oscillating force Fz is applied to the cantilever, then the tip will be displaced by an amount z. Moreover, the displacement will also harmonically oscillate, but with a phase shift between applied force and displacement given by:[5][6][9]

Dynamic mode of operation refers to measurements of the shifts in the resonance frequency.The cantilever is driven to its resonance frequency and frequency shifts are detected.Assuming small vibration amplitudes (which is generally true in MFM measurements), to a first-order approximation, the resonance frequency can be related to the natural frequency and the force gradient. That is, the shift in the resonance frequency is a result of changes in the spring constant due to the (repelling and attraction) forces acting on the tip.

Theoretically, the magneto-static energy (U) of the tip-sample system can be calculated in one of two ways:[1][5][6][17]One can either compute the magnetization (M) of the tip in the presence of an applied magnetic field ( H {\displaystyle H} ) of the sample or compute the magnetization ( M {\displaystyle M} ) of the sample in the presence of the applied magnetic field of the tip (whichever is easier).Then, integrate the (dot) product of the magnetization and stray field over the interaction volume ( V {\displaystyle V} ) as

and compute the gradient of the energy over distance to obtain the force F.[18] Assuming that the cantilever deflects along the z-axis, and the tip is magnetized along a certain direction (e.g. the z-axis), then the equations can be simplified to

The MFM can be used to image various magnetic structures including domain walls (Bloch and Neel), closure domains, recorded magnetic bits, etc. Furthermore, motion of domain wall can also be studied in an external magnetic field. MFM images of various materials can be seen in the following books and journal publications:[5][6][19] thin films, nanoparticles, nanowires, permalloy disks and recording media.

There are some shortcomings or difficulties when working with an MFM, such as: the recorded image depends on the type of the tip and magnetic coating, due to tip-sample interactions. Magnetic field of the tip and sample can change each other's magnetization, M, which can result in nonlinear interactions. This hinders image interpretation. Relatively short lateral scanning range (order of hundreds micrometers). Scanning (lift) height affects the image. Housing of the MFM system is important to shield electromagnetic noise (Faraday cage), acoustic noise (anti-vibration tables), air flow (air isolation), and static charge on the sample.

There have been several attempts to overcome the limitations mentioned above and to improve the resolution limits of MFM. For example, the limitations from air flow has been overcome by MFMs that operate at vacuum.[20] The tip-sample effects have been understood and solved by several approaches. Wu et al., have used a tip with antiferromagnetically coupled magnetic layers in an attempt to produce a dipole only at the apex.[21]

The Lift Mode allows the imaging of relatively weak but long-range magnetic interactions while minimizing the influence of topography. Measurements are taken in two passes across each scan line (Interleave Mode); each pass consists of one trace and one retrace. In the first pass, topographical data is collected in the Tapping Mode. The tip is then raised to the lift scan height and a second trace and retrace cycle is performed while maintaining a constant separation between the tip and the local surface topography. Magnetic interactions are detected during this second pass. Since the surface profile data is not included in the frequency signal, the topography is virtually absent from the MFM image.

The above settings merely set the machine up to make magnetic field images. The quality of the images relies on the operator to fine adjust the magnitude of key parameters to reduce noise and obtain a high resolution image. The following comments should help guide the new user to make good images. As experience increases, this adjustment procedure will advance in creativity and speed. e24fc04721