Using an optimally coupled nanometer-scale superconducting quantum interference device, we measure the magnetic flux originating from an individual ferromagnetic Ni nanotube attached to a Si cantilever. At the same time, we detect the nanotubes volum
e magnetization using torque magnetometry. We observe both the predicted reversible and irreversible reversal processes. A detailed comparison with micromagnetic simulations suggests that vortex-like states are formed in different segments of the individual nanotube. Such stray-field free states are interesting for memory applications and non-invasive sensing.
Nanoscale magnets might form the building blocks of next generation memories. To explore their functionality, magnetic sensing at the nanoscale is key. We present a multifunctional combination of a scanning nanometer-sized superconducting quantum int
erference device (nanoSQUID) and a Ni nanotube attached to an ultrasoft cantilever as a magnetic tip. We map out and analyze the magnetic coupling between the Ni tube and the Nb nanoSQUID, demonstrate imaging of an Abrikosov vortex trapped in the SQUID structure - which is important in ruling out spurious magnetic signals - and reveal the high potential of the nanoSQUID as an ultrasensitive displacement detector. Our results open a new avenue for fundamental studies of nanoscale magnetism and superconductivity.
We have investigated asymmetrically shunted Nb/Al-AlO$_x$/Nb direct current (dc) superconducting quantum interference devices (SQUIDs). While keeping the total resistance $R$ identical to a comparable symmetric SQUID with $R^{-1} = R_1^{-1} + R_2^{-1
}$, we shunted only one of the two Josephson junctions with $R = R_{1,2}/2$. Simulations predict that the optimum energy resolution $epsilon$ and thus also the noise performance of such an asymmetric SQUID can be 3--4 times better than that of its symmetric counterpart. Experiments at a temperature of 4.2,K yielded $epsilon approx 32,hbar$ for an asymmetric SQUID with an inductance of $22,rm{pH}$. For a comparable symmetric device $epsilon = 110,hbar$ was achieved, confirming our simulation results.
We investigated, at temperature $4.2,mathrm{K}$, electric transport, flux noise and resulting spin sensitivity of miniaturized Nb direct current superconducting quantum interference devices (SQUIDs) based on submicron Josephson junctions with HfTi ba
rriers. The SQUIDs are either of the magnetometer-type or gradiometric in layout. In the white noise regime, for the best magnetometer we obtain a flux noise $S_{Phi}^{1/2}=250,mathrm{n}Phi_0/mathrm{Hz}^{1/2}$, corresponding to a spin sensitivity $S^{1/2}_mu,ge,29,mu_B/mathrm{Hz}^{1/2}$. For the gradiometer we find $S_{Phi}^{1/2}=300,mathrm{n}Phi_0/mathrm{Hz}^{1/2}$ and $S^{1/2}_mu,ge,44,mu_B/mathrm{Hz}^{1/2}$. The devices can still be optimized with respect to flux noise and coupling between a magnetic particle and the SQUID, leaving room for further improvement towards single spin resolution.
We propose to couple a trapped single electron to superconducting structures located at a variable distance from the electron. The electron is captured in a cryogenic Penning trap using electric fields and a static magnetic field in the Tesla range.
Measurements on the electron will allow investigating the properties of the superconductor such as vortex structure, damping and decoherence. We propose to couple a superconducting microwave resonator to the electron in order to realize a circuit QED-like experiment, as well as to couple superconducting Josephson junctions or superconducting quantum interferometers (SQUIDs) to the electron. The electron may also be coupled to a vortex which is situated in a double well potential, realized by nearby pinning centers in the superconductor, acting as a quantum mechanical two level system that can be controlled by a transport current tilting the double well potential. When the vortex is trapped in the interferometer arms of a SQUID, this would allow its detection both by the SQUID and by the electron.
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.
We experimentally demonstrate the occurrence of negative absolute resistance (NAR) up to about $-1Omega$ in response to an externally applied dc current for a shunted Nb-Al/AlO$_x$-Nb Josephson junction, exposed to a microwave current at frequencies
in the GHz range. The realization (or not) of NAR depends crucially on the amplitude of the applied microwave current. Theoretically, the system is described by means of the resistively and capacitively shunted junction model in terms of a moderately damped, classical Brownian particle dynamics in a one-dimensional potential. We find excellent agreement of the experimental results with numerical simulations of the model.
