We fabricated YBa2Cu3O7 (YBCO) direct current (dc) nano superconducting quantum interference devices (nanoSQUIDs) based on grain boundary Josephson junctions by focused ion beam patterning. Characterization of electric transport and noise properties at 4.2 K in magnetically shielded environment yields a very small inductance L of a few pH for an optimized device geometry. This in turn results in very low values of flux noise < 50 nΦ0/Hz 1/2 in the thermal white noise limit, which yields spin sensitivities of a few µB/Hz 1/2 (Φ0 is the magnetic flux quantum and µB is the Bohr magneton). We observe frequency-dependent excess noise up to 7 MHz, which can only partially be eliminated by bias reversal readout. This indicates the presence of fluctuators of unknown origin, possibly related to defect-induced spins in the SrTiO3 substrate. We demonstrate the potential of using YBCO nanoSQUIDs for the investigation of small spin systems, by placing a 39 nm diameter Fe nanowire, encapsulated in a carbon nanotube, on top of a non-optimized YBCO nanoSQUID and by measuring the magnetization reversal of the Fe nanowire via the change of magnetic flux coupled to the nanoSQUID. The measured flux signals upon magnetization reversal of the Fe nanowire are in very good agreement with estimated values, and the determined switching fields indicate magnetization reversal of the nanowire via curling mode.

Superconductivity in the cuprate YBa(2)Cu(3)O(7) (YBCO) persists up to huge magnetic fields (B) up to several tens of Teslas, and sensitive direct current (dc) superconducting quantum interference devices (SQUIDs) can be realized in epitaxially grown YBCO films by using grain boundary Josephson junctions (GBJs). Here we present the realization of high-quality YBCO nanoSQUIDs, patterned by focused ion beam milling. We demonstrate low-noise performance of such a SQUID up to B = 1 T applied parallel to the plane of the SQUID loop at the temperature T = 4.2 K. The GBJs are shunted by a thin Au layer to provide nonhysteretic current voltage characteristics, and the SQUID incorporates a 90 nm wide constriction which is used for on-chip modulation of the magnetic flux through the SQUID loop. The white flux noise of the device increases only slightly from 1.3 μΦ(0)/(Hz)(1/2) at B = 0 to 2.3 μΦ(0)/(Hz))(1/2) at 1 T. Assuming that a point-like magnetic particle with magnetization in the plane of the SQUID loop is placed directly on top of the constriction and taking into account the geometry of the SQUID, we calculate a spin sensitivity S(μ)(1/2) = 62 μ(B)/(Hz))(1/2) at B = 0 and 110 μ(B)/(Hz))(1/2) at 1 T. The demonstration of low noise of such a SQUID in Tesla fields is a decisive step toward utilizing the full potential of ultrasensitive nanoSQUIDs for direct measurements of magnetic hysteresis curves of magnetic nanoparticles and molecular magnets.

We investigate electric transport and noise properties of microstrip-type submicron direct current superconducting quantum interference devices (dc SQUIDs) based on Nb thin films and overdamped Josephson junctions with a HfTi barrier. The SQUIDs were designed for optimal spin sensitivity S 1/2 µ upon operation in intermediate magnetic fields B (tens of mT), applied perpendicular to the substrate plane. Our so far best SQUID can be continuously operated in fields up to B ≈ ±50 mT with rms flux noise S 1/2 Φ,w ≤ 250 nΦ0/Hz 1/2 in the white noise regime and spin sensitivity S 1/2 µ ≤ 29 µB/Hz 1/2 . Furthermore, we demonstrate operation in B = 0.5 T with high sensitivity in flux S 1/2 Φ,w ≈ 680 nΦ0/Hz 1/2 and in electron spin S 1/2 µ ≈ 79 µB/Hz 1/2 . We discuss strategies to further improve the nanoSQUID performance.

We present an optimization study of the spin sensitivity of nanoSQUIDs based on resistively shunted grain boundary Josephson junctions. In addition the dc SQUIDs contain a narrow constriction onto which a small magnetic particle can be placed (with its magnetic moment in the plane of the SQUID loop and perpendicular to the grain boundary) for efficient coupling of its stray magnetic field to the SQUID loop. The separation of the location of optimum coupling from the junctions allows for an independent optimization of the coupling factor φµ and junction properties. We present different methods for calculating φµ (for a magnetic nanoparticle placed 10 nm above the constriction) as a function of device geometry and show that those yield consistent results. Furthermore, by numerical simulations we obtain a general expression for the dependence of the SQUID inductance on geometrical parameters of our devices, which allows to estimate their impact on the spectral density of flux noise SΦ of the SQUIDs in the thermal white noise regime. Our analysis of the dependence of SΦ and φµ on the geometric parameters of the SQUID layout yields a spin sensitivity S 1/2 µ = S 1/2 Φ /φµ of a few µB/Hz 1/2 (µB is the Bohr magneton) for optimized parameters, respecting technological constraints. However, by comparison with experimentally realized devices we find significantly larger values for the measured white flux noise, as compared to our theoretical predictions. Still, a spin sensitivity on the order of 10 µB/Hz 1/2 for optimized devices seems to be realistic.

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