Electricity: measuring and testing – Magnetic – Magnetic test structure elements
Reexamination Certificate
1998-02-10
2001-04-03
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetic test structure elements
C324S260000, C360S110000, C360S123090, C360S125020
Reexamination Certificate
active
06211673
ABSTRACT:
BACKGROUND OF THE INVENTION
TECHNICAL FIELD
This invention refers to a magnetic-field-detecting and/or generating apparatus. More particularly, the invention relates to a scanning superconducting quantum interference device (SQUID) microscope (SSM) and a read/write head for use in magnetic storage devices.
PRIOR ART
An AFM is a device which uses a tip with a very sharp apex and low-range atomic forces to investigate the surface of a sample down to atomic dimensions. Such a device is described in U.S. Pat. No. 4,724,318 by Binnig et al. and in various other patent publications. Basically, this device has the tip mounted at a flexible cantilever which is attached to a probe holder and the vertical deflection of the tip is sensed by a detector. A commonly used operation mode of this microscope is to scan the tip over the sample while keeping the force between the tip and the surface constant by moving either the sample or the tip up and down, such that the deflection of the cantilever remains constant. In this way, the topography of the sample can be examined keeping record of the vertical motion of the probe holder. This data can be used to create 3-dimensional images of the topography of the surface. AFMs are used in many applications and embodiments and also with special tips, as disclosed for example in U.S. Pat. No. 5,331,589. Appropriate measurement devices, methods for sensing and methods for processing the resulting data are also known and disclosed in the referenced prior art and in various other publications, e.g. U.S. Pat. No. 5,224,376.
Another kind of microscope is disclosed in Appl. Phys. Lett. 66,1138 (1995) by J. R. Kirtley et al. using a scanning superconducting quantum interference device (SQUID). Such a Scanning SQUID microscope (SSM) is a very sensitive sensor for magnetic flux. A SQUID is described e.g. in NATO ASI series, Series E Applied Sciences 251 by J. Clark, H. Weinstock, W. Ralston in “The new superconducting electronics” or in other introductory textbooks about superconductivity and superconducting electronics. The SQUID itself consists of a superconducting ring interrupted by one (rf-SQUID) or two (dc-SQUID) Josephson junctions. The electrical properties of the ring (effective impedance of rf-SQUID or current-voltage characteristic of dc-SQUID) depend strongly on the magnetic flux through the superconducting ring. In order to efficiently couple the signal to be measured to the SQUID ring, often use is made of a so-called flux transformer. The flux transformer is a closed superconducting structure consisting of a pick-up loop which picks up the signal and an input coil which couples the signal to the superconducting SQUID ring. The output signal of the SQUID is measured and processed with appropriate electronic equipment.
U.S. Pat. No. 5,491,411 discloses a magnetic-flux microscope that measures the magnetic filed about a sample surface by using a thin-film SQUID as the scanning device.
Further, the SSM has been shown to be a highly valuable tool to image small magnetic fields of various types of samples such as inorganic materials, living tissues or bacteria. Though the magnetic-field sensitivity of a SSM is outstanding, its spatial resolution is up to now limited by the diameter of the superconducting ring of the SQUID or by the diameter of the pick-up loop of the flux-transformer, which at the present is 4 &mgr;m at best. In a similar way, the resolution of read/write heads for magnetic storage devices is limited because of the mechanical properties.
SUMMARY OF THE INVENTION
In accordance with the present invention, a magnetic-field-detecting and/or generating apparatus is described, comprising a tip of non-ferromagnetic material with an at least partly embedded longitudinal member of ferromagnetic material and magnetic-shield means around the tip and a magnetic-field device, e.g. a SQUID, positioned above the tip for detecting and/or generating a magnetic field. In this sense one embodiment of the invention is an improved SSM. Another embodiment is a magnetic read/write head.
The SSM arrangement has a significantly enhanced spatial resolution, which is no longer limited by the diameter of the superconducting ring of a SQUID or by the diameter of the pick-up loop of a flux-transformer. In order to optimize the resolution, the longitudinal member should have a high permeability and should preferably be made of a softferromagnetic material. In a variation of the invention, the tip comprises a longitudinal member extending from the apex of the tip along the central axis to its upper side, and protruding through the tip apex. The longitudinal member may be a narrow filament of a material having preferably a high bulk-permeability value, e.g. above 10
4
. In a further embodiment of the invention, the magnetic-shield means comprises a conductive layer, e.g. a metallic film, on the non-ferromagnetic material of the tip with a slit oriented essentially parallel to the axis of the longitudinal member to reduce the disturbing influence of magnetic fringe fields. The magnetic field should reach the ferromagnetic filament only at its apex. That shielding layer also can be made of a superconducting material. The slit serves to avoid circulating eddy currents that would diminish the coupling of the magnetic flux from the sample to the filament apex. An optimal coupling between the filament and the magnetic-field device exists when the magnetic-field lines in the filament also thread through the magnetic-field device. The term magnetic-field device is here to be understood as at least a magnetic-field-sensitive or a magnetic-field-generating element. Therefore, in an advantageous embodiment, the magnetic-field device is centrally placed over the tip, especially above the filament.
Above the filament, a superconducting pick-up loop of a SQUID can be arranged as the magnetic-field device to form a scanning SQUID microscope. The tip then has an effective apex radius that equals the radius of the filament apex, e.g. in the range of 10 nm, and is scanned across the sample. Hence, the filament will pick up the magnetic field of the sample with a spatial resolution given by the size of its apex radius and, due to the large permeability, transmits the field into the SQUID pick-up loop.
The principle also can be used for magnetic-storage devices. Therefore, the invention also proposes e.g. an induction coil as the magnetic-field device for retrieving magnetically stored data, and, furthermore, an inductive element, e.g. a coil, for generating a magnetic field usable to magnetically record data on a recording means. Hence, a write/head for use in a magnetic-storage device is proposed.
With the principle of the invention, the spatial resolution of a SSM as well as of magnetic-storage devices is significantly enhanced.
REFERENCES:
patent: 4724318 (1988-02-01), Binnig
patent: 5224376 (1993-07-01), Elings et al.
patent: 5331491 (1994-07-01), Hayakawa et al.
patent: 5331589 (1994-07-01), Gambino et al.
patent: 5491411 (1996-02-01), Wellstood
J.R. Kirtley et al. (1995) “High-resolution Scanning SQUID Microscope”Appl. Phys. Letts.66 (9): 1138-1140.
J. Clarke (1993) “Squids: Theory and Practice”The New Superconducting Electronics(Kluwer Academic Publisher: The Netherlands): 123-180.
Gerber Christoph
Hilgenkamp Johannes W.
Mannhart Jochen
International Business Machines - Corporation
Sbrollini, Esq. Jay P.
Scully Scott Murphy & Presser
Snow Walter E.
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