This Phase I SBIR project will evaluate the feasibility of developing ultra-low-noise, micron-scale magnetometers based on the novel high-transition temperature (high-Tc) superconducting quantum interference devices (SQUIDs) the UCSD group has developed. During the past few years, a group at J?lich, Germany, has succeeded in reducing the noise of a magnetometer based on a high-Tc SQUID operating at 77 Kelvin by about an order of magnitude, to a level (~5 fT/?Hz) comparable to the noise level of low-Tc SQUIDs operating at liquid helium temperature (4.2 K). Although this is an important breakthrough, the SQUID junction is very difficult to fabricate with a very low yield, greatly limiting the availability of a large number of such SQUIDs for magnetometry. Recently, one of us (Cybart at UCSD) has shown that high-Tc SQUIDs can be constructed with a junction noise matching that of the J?lich SQUIDs using a radically different approach that greatly eases the fabrication process with a very high yield. In this approach, very small nanometer-scale Josephson junctions are made in a YBCO high-Tc superconductor with a focused helium ion beam with a precision of 0.5 nm. It enables mass fabrication of highly reliable, low-noise SQUIDs with a very high yield, unlike any other previous approaches. This nano-junction oxide technology is useful in numerous areas of science and technology. In Phase I of this project, we will focus on developing the best design for ultra low- noise micron-scale magnetometers with applications in biology and neuroscience. We will evaluate three magnetometer designs: (1) new washer-coupled SQUID sensor; (2) Transformer-injected SQUID sensor; and (3) Serially connected direct-injection SQUID sensor. Design 1 uses an improved type of washer to collect the magnetic flux more effectively than our previous versions. Design 2 uses inductive coupling to concentrate the magnetic flux efficiently into the SQUID. Design 3 uses a large number of direct injection SQUIDs connected in series to increase the signal-to-noise ratio (SNR) of the sensor. The focused helium beam technique will provide the precision needed for producing nano-junction oxide SQUIDs and the other components connected to the Josephson junctions. We will select the best design in terms of SQUID junction noise and slope of the voltage-vs-magnetic flux transfer function. The sensitivity of an array of such sensors will be evaluated in a SQUID microscope that Tristan and Moment have previously built. The design of the microscope enables us to adjust the operating temperature of the sensors between 4 K and 77 K. Since the sensitivity of the high-Tc SQUIDs depends on temperature, its operating temperature will be varied to determine the best temperature. This will provide the benchmark for going to the Phase II portion of this project to develop a magnetic microscope. As the deliverable, we will provide the best sensor design and its noise and sensitivity characteristics.