The present invention relates generally to a sensor and a method for indicating the presence of a low magnetic field and, more particularly, to such a sensor and method for indicating the presence of a magnetic field in the range of about 10.sup.-10 tesla, utilizing a sensor consisting essentially of a high critical temperature superconductor ceramic material.
It is generally well known that superconducting materials or superconductors have a critical temperature which varies, depending upon the particular superconductor. When a superconductor is at a temperature which is higher than its critical temperature, it does not conduct electricity in an efficient manner in that some of the electrical energy is converted to heat. When the temperature of the superconductor is reduced to below its critical temperature, it becomes the most efficient conductor of electricity. In the past, only superconductors having a relatively low critical temperature (near 20.degree. Kelvin) were generally available. Use of superconductors of this type was very limited because it was difficult and expensive to maintain the temperature of such superconductors below their critical temperatures. More recently, ceramic superconductors have been developed which have a relatively high critical temperature (about 90.degree. Kelvin), which is above the temperature of liquid nitrogen (77.degree. Kelvin), thereby permitting such superconductors to be inexpensively maintained below their critical temperatures using inexpensive liquid nitrogen for cooling. Such high critical temperature ceramic superconductors are practical for a wide variety of applications.
In the past, high critical temperature superconductors have been employed in connection with the detection of low magnetic fields. One such prior device, referred to as a "superconducting quantum interference device" (SQUID) uses a pellet formed of a bulk, high critical temperature superconductor. The pellet is secured to a special breaking fixture and is fractured or broken along a predetermined line in a controlled manner at a temperature substantially below its critical temperature. The fractured portions of the pellet are then brought back together to form a "crack" SQUID having one or more Josephson junctions. The pellet is then inductively coupled to a cooled, tuned circuit driven at a predetermined resonant frequency. The SQUID extracts energy from the resonant circuit when the presence of a magnetic field drives the Josephson junction into a dissipative state. Other prior art devices employ a thin film of the superconducting material to form a similar SQUID.
While the prior art SQUID-type devices are adequate for detecting the presence of low magnetic fields, the special processes necessary for forming either the crack SQUID pellet or the thin film SQUID are time consuming and expensive and require reasonably sophisticated and expensive processing equipment. In addition, the SQUID pellet and/or thin film are not very stable and have a tendency to degrade rapidly after exposure to the atmosphere and/or after a relatively small number of thermal cycles.
The present invention overcomes many of the problems and drawbacks associated with such prior art low magnetic field detection devices by providing a sensor having a sensing element consisting essentially of a sample or chunk of a high critical temperature superconductor ceramic material without the need for fracturing, formation into a thin film or any further forming, processing or the like. Alternatively, the sensing element could be formed of high critical temperature superconducting ceramic material in a powder form which, if desired, may be embedded in a nonmagnetic resin epoxy formed into a desired shape or structure, again, without fracturing, thin film formation or any other processing.