Magnetic Resonance Force Microscopy

Towards Three-Dimensional Sub-Surface Atomic-Scale Imaging of Magnetic Materials.



H A M M E L   G R O U P     Los Alamos

R O U K E S   G R O U P      Caltech
P. Chris Hammel
Condensed Matter and Thermal Physics
MS K764, Los Alamos National Laboratory, 
Los Alamos, NM 87545  USA
Contact:  Chris Hammel  + 505 665 0759
Michael L. Roukes
Condensed Matter Physics, 
Caltech 114-36, 
Pasadena CA 91125  USA 
Contact: Michael Roukes  + 626 395 2933

The magnetic resonance force microscope (MRFM) is a novel scanned probe instrument which combines the three-dimensional imaging capabilities of magnetic resonance imaging with the high sensitivity and resolution of atomic force microscopy. It will enable non-destructive, chemical-specific, high-resolution microscopic studies and imaging of subsurface properties of a broad range of materials. This technology holds clear potential for atomic-scale resolution.

New magnetic materials and devices with unprecedented capabilities and levels of performance are now being created by tailoring the structure and composition of multi-component materials at the nanometer scale. The buried interfaces between the various components of these new materials play a central role in determining their functional behavior. We are presently applying the emerging capabilities of the MRFM to the study of layered magnetoelectronic materials. The MRFM will enable very high resolution studies of the structural and magnetic properties of the buried interfaces.

High Resolution Force Detected Magnetic Resonance Imaging
Force Detection of Magnetic Resonance
The magnetic resonance force microscope (MRFM) is a microscopic imaging instrument that mechanically detects magnetic resonance signals by sensitively measuring the force F=ÑB between a permanent magnet that provides ÑB, and the spin magnetization m. Periodically modulating this force by modulating m alters the oscillation amplitude of a high Q, low spring-constant micro-mechanical resonator (cantilever or bridge) such as is used presently in AFM. 

High Resolution Magnetic Resonance Imaging

The magnetic field gradient of the small magnetic tip serves a second key role: it defines the spins (electronic or nuclear) which will be probed in the magnetic resonance experiment. The resonance frequency of a spin is proportional to the magnetic field it experiences: w=gB(r), only the resonant spins are effected by the magnetic resonance experiment.  As in conventional  magnetic resonance imaging (MRI) the spatial variation of B allows us to probe only those spins whose resonance frequency matches the frequency wo of the applied rf field, that is those spins residing in the "sensitive slice."

Magnetic Multilayer Materials
High sensitivity magnetic field detectors for high density magnetic information storage devices.
The field of magnetoelectronics is experiencing a huge resurgence of interest and a new influx of research effort. The driving force is the  technological quest of the $100B/yr magnetic recording industry for increased storage density in recording media. Further impetus also comes from the semiconductor industry, in the area of memory: ideal non-volatile memory elements are sought that can be produced, and addressed, in massive arrays.  The growth of high quality magnetic multilayer materials is providing the foundation for a range of important modern advances in both fields,  and offers a new range of possibilities. The quality of such materials, upon which the performance of these devices so strongly depends, is affected not only by the purity of the materials grown, but, crucially, by the properties of the interfaces between different layers.  For example, in magnetic multilayer materials displaying "giant''  magnetoresistance  (GMR), device performance is critically dependent on the microscopic characteristics  of the buried interfaces.  GMR-based read heads have enabled up to a 50% increase in magnetic storage density and have thus become a central new technology for the industry. A second example is the recently developed spin injection devices, which offer a new approach to non-volatile magnetic memory.   Although the physics of these new types of magnetic systems is highly dependent upon interfacial characteristics, remarkably little is known about the microscopic morphology of these interfaces.  Nor are  the microscopic mechanisms that determine interfacial magnetic properties well understood. MRFM studies will enable  unprecedented understanding of structural and magnetic properties of buried interfaces and microstructure; this will enable development of materials with improved properties.

Ferromagnetic Resonance Using MRFM
We have been performing MRFM detected FMR as a microscopic probe of magnetic thin film materials.  MRFM can enable microscopic characterization of magnetic homogeneity and interlayer exchange coupling.  Please see some of our recent publications.

"Imaging mechanisms of force detected FMR microscopy"
       J. Appl. Phys., 87, 6493 (2000)
      (LAUR 00-2495)

"Observation of Ferromagnetic Resonance in a Microscopic
Sample Using Magnetic Resonance Force Microscopy"   
      Appl. Phys. Lett. 68, 2005 (1996)

"Ferromagnetic Resonance Force Microscopy on Microscopic
Co Single Layer Films,"
       Appl. Phys. Lett. 73, 2036 (1998)

"Ferromagnetic Resonance Imaging of Co Films Using
Magnetic Resonance Force Microscopy"
      J. Vac. Sci. Tech. B 16, 2275 (1998)

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