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Register a new userA Directly Coupled Superconducting Quantum Interference Device Magnetometer Fabricated in Magnesium Diboride by Focused Ion Beam
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G.Burnell
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We report the fabrication of a directly coupled superconducting quantum interference device (SQUID) magnetometer in MgB2 using a focused ion beam (FIB) to create Josephson junctions in a 70 nm thick film of MgB2. The SQUID shows a voltage modulation (deltaV) of 175 mV at a temperature of 10 K and operates over a temperature range from 10 K to 24 K. We find excellent agreement between the measured maximum transfer functions and those predicted by theory. We have measured the magnetic flux noise at 20 K to be as low as 14 micro Phi-0 per root Hz.
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We have used a neon focused-ion-beam to fabricate both nanoscale Nb Dayem bridges and NbN phase-slip nanowires located at the short-circuited end of quarter-wavelength coplanar waveguide resonators. The Dayem bridge devices show flux-tunability and intrinsic quality factor exceeding 10,000 at 300 mK up to local fields of at least 60 mT. The NbN nanowires show signatures of incoherent quantum tunnelling of flux at 300 mK.
We explore novel junction configurations as an extension of our established Focused Ion Beam-based low TC SNS Junction fabrication technique. By milling a circular trench (diameter 1 micron, width 50 nm) in a 125 nm Nb 75 nm Cu bilayer we define a superconducting island connected to the bulk of the film by a normal metal barrier and entirely enclosed in-plane by the superconducting film. The circular junction properties can be probed by depositing an insulating layer over the device and drilling a 0.3 micron diameter hole down to the island to allow a Nb via to be deposited. Device behavior has been studied at 4.2 K. An SNS-like current voltage characteristic and Shapiro steps are observed. It is in terms of magnetic field behavior that the device exhibits novel characteristics: as the device is entirely enclosed in type II superconductor, when a magnetic field is applied perpendicular to the plane of the film, only quantized flux can enter the junction. Hence as applied magnetic field is increased the junction critical current is unchanged, then abruptly suppressed as soon as a flux quantum enters (close to the expected value of lower critical field for the film).
Making use of focused Ga-ion beam (FIB) fabrication technology, the evolution with device dimension of the low-temperature electrical properties of Nb nanowires has been examined in a regime where crossover from Josephson-like to insulating behaviour is evident. Resistance-temperature data for devices with a physical width of order 100 nm demonstrate suppression of superconductivity, leading to dissipative behaviour that is shown to be consistent with the activation of phase-slip below Tc. This study suggests that by exploiting the Ga-impurity poisoning introduced by the FIB into the periphery of the nanowire, a central superconducting phase-slip nanowire with sub-10 nm dimensions may be engineered within the core of the nanowire.
In a recent paper Tettamanzi et al (2009 Nanotechnology bf{20} 465302) describe the fabrication of superconducting Nb nanowires using a focused ion beam. They interpret their conductivity data in the framework of thermal and quantum phase slips below $T_c$. In the following we will argue that their analysis is inappropriate and incomplete, leading to contradictory results. Instead, we propose an interpretation of the data within a SN proximity model.
YBa$_2$Cu$_3$O$_7$ 24$^circ$ (30$^circ$) bicrystal grain boundary junctions (GBJs), shunted with 60,nm (20,nm) thick Au, were fabricated by focused ion beam milling with widths $80,{rm nm} le w le 7.8,mu$m. At 4.2,K we find critical current densities $j_c$ in the $10^5,{rm A/cm^2}$ range %dkc{#1} (without a clear dependence on $w$) and an increase in resistance times junction area $rho$ with an approximate scaling $rhopropto w^{1/2}$. For the narrowest GBJs $j_crhoapprox 100,mu$V, which is promising for the realization of sensitive nanoSQUIDs for the detection of small spin systems. We demonstrate that our fabrication process allows the realization of sensitive nanoscale dc SQUIDs; for a SQUID with $wapprox 100$,nm wide GBJs we find an rms magnetic flux noise spectral density of $S_Phi^{1/2}approx 4,muPhi_0/{rm Hz}^{1/2}$ in the white noise limit. We also derive an expression for the spin sensitivity $S_mu^{1/2}$, which depends on $S_Phi^{1/2}$, on the location and orientation of the magnetic moment of a magnetic particle to be detected by the SQUID, and on the SQUID geometry. For the not optimized SQUIDs presented here, we estimate $S_mu^{1/2}=390,mu_B/sqrt{rm{Hz}}$, which could be further improved by at least an order of magnitude.
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