Abstract:
An apparatus for scanning over a surface of an arbitrarily sized sample in magnetic resonance force microscopy comprising a cantilever for holding a magnetic particle at the cantilever tip, an RF antenna, positioned around the cantilever, for emitting an RF magnetic field across a portion of the sample causing spin of particles in the sample to reverse attracting and opposing the magnetic particle at the cantilever tip, an optical fiber, positioned close to the cantilever tip, for measuring displacements of the cantilever tip where the RF antenna, cantilever, magnetic particle and optical fiber are in fixed positions relative to each other and the sample is positionable according to a sample stage.

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used and licensed by or for the U.S. Government. 
    
    
     FIELD OF INVENTION 
     Embodiments of the present invention generally relate to magnetic imaging and, more particularly, to an apparatus for mechanically robust thermal isolation of components in an imaging device. 
     BACKGROUND OF THE INVENTION 
     Magnetic resonance force microscopy (MRFM) is an imaging technique that acquires magnetic resonance images (MRI) at nanometer scales, and possibly at atomic scales in the future. An MRFM system comprises a probe, method of applying a background magnetic field, electronics, and optics. The system measures variations in a resonant frequency of a cantilever or variations in the amplitude of an oscillating cantilever. The changes in the characteristic of the cantilever being monitored are indicative of the tomography of the sample. More specifically, as depicted in  FIG. 1 , an MRFM probe  100  comprises a base  102  with a cantilever  104  tipped with a magnetic (for example, Samarium Cobalt) particle  106  to resonate as the spin of the electrons or nuclei in the sample  101  are reversed. There is a background magnetic field  108  generated by a background magnetic field generator  110  which creates a uniform background magnetic field in the sample  101 . As the magnetic tip  106  moves close to the sample  101 , the atoms&#39; electrons or nuclear spins become attracted (force detection) to the tip and generate a small force on the cantilever  104 . Using a radio frequency (RF) magnetic field applied by an RF antenna  117  through the RF source  105 , the spins are then repeatedly flipped at the cantilever&#39;s resonant frequency, causing the cantilever  104  to oscillate at its resonant frequency. In the geometry shown, when the cantilever  104  oscillates, the magnetic particle&#39;s  106  magnetic moment remains parallel to the background magnetic field  108 , and thus it experiences no torque. The displacement of the cantilever is measured with an optical sensor  114  comprised of an interferometer (laser beam)  116  and an optical fiber  118  to create a series of 2-D images of the sample  101  held by sample stage  120 , which are combined to generate a 3-D image. The interferometer measures the time dependent displacement of the cantilever  104 . Smaller magnetic particles and softer cantilevers increase the signal to noise ratio of the sensor. 
     Nano-MRI and nano NMR spectroscopy are both performed at a temperature of 4 Kelvin, or colder, to improve signal-to-noise ratio (SNR) over room temperature. The large RF magnetic fields required frequently come with large amounts of heat (1 Watt) that must be conducted out of the base  102  without heating the rest of the apparatus  100 . At 4 K, 1 Watt is a large amount of heat for these small probes, typically only 5 to 10 cm in diameter, that often heat the rest of the probe  100  reducing the signal to noise ratio. 
     Therefore, there is a need in the art for an apparatus mechanically robust thermal isolation of components in an imaging device preventing other probe components from overheating and reducing the signal to noise ratio. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the present invention relate to an apparatus for thermally isolating components in an imaging device comprising a mount for mounting the components; a clamp for holding the mount, and an accompanying first plate on the opposite side of the mount, for preventing rotation of the mount; particulate matter, positioned between the clamp and the mount, and the first plate and the mount, for absorbing heat generated by the components and isolating the mount thermally from the rest of the apparatus; and a second plate coupled to the first plate and a third plate coupled to the clamp, both coupled to a securing mechanism for compressing the apparatus and preventing breakage of the first plate and the clamp. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  depicts a conventional MRFM system known to those of ordinary skill in the art; 
         FIG. 2  depicts a block diagram of an MRFM system in accordance with an exemplary embodiment of the present invention; 
         FIG. 3  is an illustration of a thermal isolation apparatus in accordance with exemplary embodiments of the present invention; 
         FIG. 4  is an illustration of another configuration of a thermal isolation apparatus in accordance with exemplary embodiments of the present invention; 
         FIG. 5  is an end-view of the thermal isolating apparatus in accordance with exemplary embodiments of the present invention; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention comprise a mechanically robust thermal isolation mechanism for components to absorb heat and avoid heat transfer to thermally-sensitive components in a probe head. Often the RF antenna of a probe head, as described in related U.S. patent application Ser. No. 13/361,056, hereby incorporated by reference in its entirety, generates heat and transfers this heat undesirably to electrical components or sensitive components such as an optical fiber or cantilever, skewing accuracy of measurements or causing dysfunction. As such, the RF antenna component must be kept thermally isolated from other parts of a probe head. Therefore, the thermal isolation apparatus discussed herein is a sub-component of the apparatus disclosed in U.S. patent application Ser. No. 13/361,223, which issued as U.S. Pat. No. 8,549,661, hereby incorporated by reference in its entirety, to prevent heat from the RF component mounting. 
       FIG. 2  depicts a block diagram of an MRFM system  200  in accordance with an exemplary embodiment of the present invention. The system  200  generally has an RF source  202  coupled to a probe  204 . The probe  204  is coupled to an interferometer  206  for performing optical measurements using the optical sensor  216  in the probe  204  of sample  201 . The interferometer  206  transmits the measurements to a processor  208 . The processor  208  generates an output image  210  based on the measurements or oscillations of portions of the probe  204 . The probe  204  comprises a magnetic sensor  212 , an RF antenna  214  and an optical sensor  216 . The apparatus  200  is kept in a spatially homogeneous background magnetic field  217  (approximately 9 T) generated by a background magnetic field generator  218 . In an exemplary embodiment, the background magnetic field generator  218  comprises two one inch diameter Samarium Cobalt (SmCo) magnets. In an exemplary embodiment, the magnetic sensor  212  is comprised of a bridge coupled with a smaller SmCo particle (for example, 10 μm in diameter) which generates a spatially inhomogeneous field. The magnetic field experienced at a particular point in the sample  201  is the sum of the background magnetic field and the magnetic field generated by the magnetic sensor  212 . The RF antenna  214  at least partially circumscribes the magnetic sensor  212 . The RF antenna  214  generates an RF magnetic field which causes the spin in the particles of the sample  201  to reverse and oppose the SmCo particle on the bridge of the magnetic sensor  212 . This repeated reversal of the spin of the particles in sample  201  causes the magnetic sensor  212  to oscillate at a particular frequency. The interferometer  206  senses oscillation of the magnetic sensor  212  using optical sensor  216  by using optical fiber  216  to reflect a laser off of the magnetic sensor  212 . In another exemplary embodiment, the sample  201  is directly coupled to the bridge comprising the magnetic sensor  212  and a magnetic particle array of SmCo particles is proximate the magnetic sensor  212 . According to an exemplary embodiment, the optical fiber is 125 microns in diameter and is within approximately 1/10 of a millimeter of the magnetic sensor  212 . In an exemplary embodiment, the optical sensor  216  is an optical fiber approximately twenty five times greater in diameter than the width of the bridge of the magnetic sensor  212 . The gap between the optical fiber and the magnetic sensor  212  is fixed at a particular distance in this embodiment. Thermal isolation apparatus  300  isolates the heat generated by the RF antenna  214  from other probe components so accuracy of measurements is not skewed, or the signal to noise ratio is not significantly reduced due to Brownian motion in the magnetic sensor  212 . 
       FIG. 3  is an illustration of a thermal isolation apparatus  300  in accordance with exemplary embodiments of the present invention. The apparatus  300  is one embodiment of isolation apparatus  215  of  FIG. 2 . The isolation apparatus  300  thermally isolates a component mount  302 , using plates  304 ,  306 , clamp  308  and plate  310 , from mechanical mount  312 . The isolated component mount  302  is made of a very hard surface material that small objects are not able to penetrate. In an exemplary embodiment, the component mount  302  is made of sapphire and RF antenna components  214  which generate heat are mounted on component mount  302 . Plates  304  and  306  also have a hard surface material and are located on either side of isolated component mount  302 . In an exemplary embodiment, plates  304  and  306  are made of Macor®, a machineable glass-ceramic with excellent thermal characteristics, acting as efficient insulation, and stable up to temperatures of 1000° C., with very little thermal expansion or out-gassing. 
     Plates  306  and  310  are located adjacent to plates  304  and  306 , coupled to their out-facing surfaces. In exemplary embodiments, plates  306  and  310  are made of more malleable material than plates  304  and  308 , particularly metals such as copper and the like, that can better withstand mechanical compression caused by screw  314 . The screw  314  passes through component mount  302  through a hole (not shown) that is larger than the diameter of screw  314 , therefore the component mount  302  is thermally isolated from the screw  314 . In another embodiment, the screw  314  is not a screw but another securing mechanism under tension to compress all the components together such as a wire pulled tight and attached to  306  and  312  under tension, a rod under tension attached to  306  and  312  via glue, welding, epoxy, swaging, soldering and other methods know to experts in the art. In an exemplary embodiment, mechanical mount  312  is made of titanium or any non-magnetic material and is responsible for positioning the RF antenna components mounted on component mount  302 . In other embodiments, plates  306  and  310  are washers instead of plates. Though not shown in the figure, component mount  302 , plate  304 , clamp  308  and plate  310  have a large hole bored into them allowing the screw  314  to pass through these components without any contact with  302 ,  304 ,  308 , and  310 . 
     Particulate matter, or, dust,  316  is held by a securing substance  318 , which in an exemplary embodiment, is comprised of wax. In exemplary embodiments, the dust  316  is composed of hard material such as sapphire pieces, ceramics, glass, semiconductors or other hard and poor thermal conductivity material. In this exemplary embodiment, the dust  316  is 75 μm diameter glass spheres. The dust  316  covers the surface of plate  304 , clamp  308  and component mount  302  well enough so even when the components are compressed together, the surfaces of component  304  and  302 , and  308  and  302  do not come into direct contact with each other. The dust  316  enclosed in a securing material  318  is sandwiched between the two elements  308  and  302 , therefore there is no surface contact between element  308  and  302 . This is also true for the upper portion of the clamp  308 , where the top lip of clamp  308  touches the component mount  302 . A layer of dust  316  is suspended by securing substance  318  for eliminating the surface area contact between the lip of clamp  308  and component mount  302  at the top and bottom of clamp  308 , though not shown in the Figure. 
     By prohibiting the large flat surfaces of plates  304 ,  302  and  308  from coming into direct contact with each other, heat generated by component mount  302  can only be conducted to plates  304  and  306  through the dust  316 , as disclosed in a paper entitled “Thermal Impedance of pressed contacts at temperatures below 4° K” by Yoo and Andersen, hereby incorporated by reference. Since the dust  316  and its mating surfaces are all hard materials, under compression, deformation of the plates and components is minimized and the contact area between the two remains miniscule. Reduction of the contact area between two objects directly results in lesser thermal conductivity between the two. It is necessary to provide a mechanism to remove the heat generated in the components mounted on the component mount  302  or the component mount  302  will rapidly overheat. Heat generated in the components mounted on the component mount  302  is removed from  302  by conduction through a thermal link with one end attached to  302  and the other end attached to a heat sink in the system. To help mechanically stabilize the apparatus  300  from rotation with respect to mechanical mount  312 , plate  310  is attached to clamp  308  using a securing material such as glue or epoxy. Similarly, plates  304 ,  306  and  314  are optionally attached to each other with glue, epoxy or other securing material. 
     In this exemplary embodiment clamp  308  is in a C shape for holding component mount  302  from rotating around the axis created by the screw  314 , along with plate  304 . In other embodiments, other shapes for the clamp  308  are possible as long as component mount  302  is not allowed to rotate around the axis defined by  314  or translate in the plane defined by the dust  316 . According to another exemplary embodiment, the clamp  308  has a pronged portion at the top and the bottom so that the securing material only need be placed at the contact points between clamp  308  and component mount  302 . As disclosed in related U.S. patent application Ser. No. 13/361,223 a cantilever is also an important part of an MRFM imaging apparatus probe head along with the RF antenna. The component mount  302  upon which the RF antenna  214  is mounted must not rotate relative to the cantilever of the probe head  204 , otherwise the RF magnetic field from the RF antenna  214  will be distorted and measurements of structure of the sample  201  will be inaccurate. In this exemplary embodiment, one translational and two rotational degrees of freedom for component mount  302  are very well constrained by the compression of materials by screw  314 , while one translational degree of freedom is constrained by the friction between the components of the apparatus  300 . In this exemplary embodiment, one rotational and one translational degree of freedom for component mount  302  are very well constrained by the top and bottom of the clamp  308 . In other exemplary embodiments, the number of translational and rotational degrees of freedom that component mount  302  possesses must be constrained depending on the application and the direction of the applied torque and forces on the apparatus  300 . 
       FIG. 4  is an illustration of another configuration of a thermal isolation apparatus  200  in accordance with exemplary embodiments of the present invention.  FIG. 4  is largely similar to  FIG. 1 , with the exception that in this embodiment, plate  310  is formed into a C clamp as opposed to clamp  308  in  FIG. 1 . In this embodiment, clamp  308  is separated into bars  308   a ,  308   b  and plate  308   c . The machining of plates  308   a ,  308   b , and  308   c  is easy and the design is more robust as clamp  310  is made of metal instead of a brittle material. Therefore clamp  310  is physically stronger and able to withstand more stress and compression caused by screw  314  and the component mount  302 . In this embodiment,  308   a ,  308   b  and  308   c  are under compressive forces and not sheer forces as in  FIG. 1 . Plates  308   a ,  308   b  and  308   c  are coated with securing material  318  within which dust  316  is suspended, to minimize contact between the plates and the component mount  302 . If the component mount  302  is not a hard surfaced material, a securing material such as glue or epoxy is used to attach a hard material to the component mount  302  to stabilize it for better grip and to prevent the dust  316  from penetrating the surface of component mount  302 . This apparatus  400 , as compared to previous embodiments, is heavier but more robust. 
     In other exemplary embodiments, apparatus  300  and  400  both contain thermal guard rings to capture any residual heat that does leak out of the compressed component plates. In an exemplary embodiment, plates  306  and mechanical mount  312  are made of a good thermal conductor such as copper and via the same or separate connections, are thermally grounded to an appropriate cold point (heatsink) in the system. In this configuration, any heat absorbed into the screw  314  and through screw  314  into mechanical mount  312  is dissipated into the thermal ground before being absorbed by the other components of the apparatuses  300  or  400 . Thermally grounding plate  306  causes the heat from the compressed plates to be removed before the heat might be transferred to screw  314 , reducing the overall temperature for some components in apparatuses  300  and  400 . 
       FIG. 5  is an end-view of thermal isolating apparatus  300  in accordance with exemplary embodiments of the present invention. Dust  316  is shown secured by a securing material  318  such as wax between components  302  and plates  304  and  308 . As discussed above, the dust  316  reduces the contact surface area between the component mount  302  and the plates  304  and  308  and therefore reducing the heat transferred from components  302  to plates  308  and  304 . 
     The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the present disclosure and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as may be suited to the particular use contemplated. 
     Various elements, devices, modules and circuits are described above in associated with their respective functions. These elements, devices, modules and circuits are considered means for performing their respective functions as described herein. While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.