We report on the fabrication and characterization of nanopatterned superconducting quantum interference devices (SQUIDs) based on grain boundary Josephson junctions in epitaxially grown multilayer YBa2Cu3O7 (YBCO)/SiTrO3 (STO) thin films. Nanopatterning is performed by Ga+ focused ion beam milling. The evolution of the electric transport and noise properties of the YBCO nanoSQUIDs over a time span of more than one year are recorded and analyzed. We find that the multilayer YBCO/STO nanoSQUIDs show stable and high performance over time. The critical current decreases within ∼40 days by 30%–50% and then remains almost constant without obvious decline trend for nanoSQUIDs with STO layer as the interface even more than 380 days, which demonstrates the superiority of STO as the capping layer. Moreover, we find that the multilayer nanoSQUIDs have about an order of magnitude smaller low-frequency excess flux noise (compared to similar single layer devices) with root-mean-square spectral density ∼5–6 µΦ0/Hz1/2 at 1 Hz. For one device, we show that the low-frequency excess noise does not degrade within three months.
We report on the fabrication and characterization of nanopatterned superconducting quantum interference devices (SQUIDs) based on grain boundary Josephson junctions in epitaxially grown multilayer YBa2Cu3O7 (YBCO)/SiTrO3 (STO) thin films. Nanopatterning is performed by Ga+ focused ion beam milling. The evolution of the electric transport and noise properties of the YBCO nanoSQUIDs over a time span of more than one year are recorded and analyzed. We find that the multilayer YBCO/STO nanoSQUIDs show stable and high performance over time. The critical current decreases within ∼40 days by 30%–50% and then remains almost constant without obvious decline trend for nanoSQUIDs with STO layer as the interface even more than 380 days, which demonstrates the superiority of STO as the capping layer. Moreover, we find that the multilayer nanoSQUIDs have about an order of magnitude smaller low-frequency excess flux noise (compared to similar single layer devices) with root-mean-square spectral density ∼5–6 µΦ0/Hz1/2 at 1 Hz. For one device, we show that the low-frequency excess noise does not degrade within three months.
We report on nanopatterned YBa2Cu3O7-δ (YBCO) direct current superconducting quantum interference devices (SQUIDs) based on grain boundary Josephson junctions. The nanoSQUIDs are fabricated by epitaxial growth of 120 nm-thick films of the high-transition temperature cuprate superconductor YBCO via pulsed laser deposition on MgO bicrystal substrates with 24° misorientation angle, followed by sputtering of dAu = 65 nm thick Au. Nanopatterning is performed by Ga focused ion beam (FIB) milling. The SQUID performance is comparable to devices on SrTiO3 (STO), as demonstrated by electric transport and noise measurements at 4.2 K. MgO has orders of magnitude smaller dielectric permittivity than STO; i.e., one may avoid Au as a resistively shunting layer to reduce the intrinsic thermal flux noise of the nanoSQUIDs. However, we find that the Au layer is important for avoiding degradation during FIB milling. Hence, we compare devices with different dAu produced by thinning the Au layer via Ar ion milling after FIB patterning. We find that the reduction of dAu yields an increase in junction resistance, however at the expense of a reduction of the critical current and increase in SQUID inductance. This results in an estimated thermal flux noise that is almost independent of dAu. However, for two devices on MgO with 65 nm-thick Au, we find an order of magnitude lower low-frequency excess noise as compared to nanoSQUIDs on STO or those on MgO with reduced dAu. For one of those devices we obtain with bias-reversal readout ultra-low flux noise of ∼175 nΦ0 Hz-1/2 down to ∼10 Hz.
NanoSQUIDs from YBa2Cu3O7/SrTiO3 superlattices with bicrystal grain boundary Josephson junctions
We report on the fabrication and characterization of nanopatterned dc SQUIDs with grain boundary Josephson junctions based on heteroepitaxially grown YBa2Cu3O7 (YBCO)/ SiTrO3 (STO) superlattices on STO bicrystal substrates. Nanopatterning is performed by Ga focused-ion-beam milling. The electric transport properties and thermal white flux noise of superlattice nanoSQUIDs are comparable to single layer YBCO devices on STO bicrystals. However, we find that the superlattice nanoSQUIDs have more than an order of magnitude smaller low-frequency excess flux noise, with root-mean-square spectral density SΦ1/2∼5−6μΦ0/Hz at 1 Hz (Φ0 is the magnetic flux quantum). We attribute this improvement to an improved microstructure at the grain boundaries forming the Josephson junctions in our YBCO nanoSQUDs.
YBa2Cu3O7 24∘ (30∘) bicrystal grain boundary junctions (GBJs), shunted with 60\,nm (20\,nm) thick Au, were fabricated by focused ion beam milling with widths 80nm≤w≤7.8μm. At 4.2\,K we find critical current densities jc in the 105A/cm2 range %\dkc{\#1} (without a clear dependence on w) and an increase in resistance times junction area ρ with an approximate scaling ρ∝w1/2. For the narrowest GBJs jcρ≈100μV, 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 w≈100\,nm wide GBJs we find an rms magnetic flux noise spectral density of SΦ1/2≈4μΦ0/Hz1/2 in the white noise limit. We also derive an expression for the spin sensitivity Sμ1/2, which depends on SΦ1/2, 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 Sμ1/2=390μB/Hz, which could be further improved by at least an order of magnitude.
We have fabricated and measured the properties of micron-size single grain boundary junctions and dc superconducting quantum interference devices (SQUIDs). The width of the junctions was varied between 0.25 and 2 μm. The junctions were formed in c-axis oriented epitaxial films with a single 24° tilt boundary. They were fabricated using electron beam lithography and ion milling. All of the junctions operate at 77 K. The noise properties of SQUIDs made of these junctions have been characterized. They show a significant 1/f component which is comparable to published values. However, the white noise level, 3×10−31 J/Hz measured at 77 K, is the lowest ever reported.
In this work, we demonstrate the ability to fabricate superconducting quantum interference devices (SQUIDs) by directly writing Josephson junctions into the plane of YBa2Cu3O7−δ thin films with a focused helium ion beam. This technique allows for the control of the Josephson barrier transport properties through the single parameter, ion dose. SQUIDs written with a dose of 4 × 1016 ions/cm2 had metallic barrier junctions that exhibited nearly ideal electrical transport characteristics at 50 K and a flux noise of 20 μΦ0/Hz at 10 Hz. At higher irradiation doses, the SQUIDs had insulating barrier Josephson junctions with a quasi particle energy gap edge at 20 meV.
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 Sμ1/2 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 SΦ,w1/2≤250 nΦ0/Hz1/2 in the white noise regime and spin sensitivity Sμ1/2≤29 μB/Hz1/2. Furthermore, we demonstrate operation in B = 0.5 T with high sensitivity in flux SΦ,w1/2≈680 nΦ0/Hz1/2 and in electron spin Sμ1/2≈79 μB/Hz1/2. We discuss strategies to further improve the nanoSQUID performance.