YBa 2 Cu 3 O 7 24 • (30 •) bicrystal grain boundary junctions (GBJs), shunted with 60 nm (20 nm) thick Au, were fabricated by focused ion beam milling with widths 80 nm w 7.8 μm. At 4.2 K we find critical current densities j c in the 10 5 A cm −2 range (without a clear dependence on w) and an increase in resistance times junction area ρ n with an approximate scaling ρ n ∝ w 1/2. For the narrowest GBJs j c ρ n = I c R n ≈ 100 μV (with critical current I c and junction resistance R n), 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 S 1/2 ≈ 4 μ 0 Hz −1/2 in the white noise limit. We also derive an expression for the spin sensitivity S 1/2 μ , which depends on S 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 unoptimized SQUIDs presented here, we estimate S 1/2 μ = 390 μ B Hz −1/2 , which could be further improved by at least an order of magnitude.

Heteroepitaxially grown bilayers of ferromagnetic La0.7Ca0.3MnO3 (LCMO) on top of superconducting YBa2Cu3O7 (YBCO) thin films were investigated by focusing on electric transport properties as well as on magnetism and orbital occupation at the interface. Transport measurements on YBCO single layers and on YBCO/LCMO bilayers, with different YBCO thickness dY and constant LCMO thickness dL = 50 nm, show a significant reduction of the superconducting transition temperature Tc only for dY < 10 nm,with only a slightly stronger Tc suppression in the bilayers, as compared to the single layers. X-ray magnetic circular dichroism (XMCD) measurements confirm recently published data of an induced magnetic moment on the interfacial Cu by the ferromagnetically ordered Mn ions, with antiparallel alignment between Cu and Mn moments. However, we observe a significantely larger Cu moment than previously reported, indicating stronger coupling between Cu and Mn at the interface. This in turn could result in an interface with lower transparency, and hence smaller spin diffusion length, that would explain our electric transport data, i.e. smaller Tc suppression. Moreover, linear dichroism measurements did not show any evidence for orbital reconstruction at the interface, indicating that a large change in orbital occupancies through hybridization is not necessary to induce a measurable ferromagnetic moment on the Cu atoms.

Cerium-doped manganite thin films were grown epitaxially by pulsed laser deposition at 720 • C and oxygen pressure pO 2 = 1 − 25 Pa and were subjected to different annealing steps. According to x-ray diffraction (XRD) data, the formation of CeO2 as a secondary phase could be avoided for pO 2 ≥ 8 Pa. However, transmission electron microscopy shows the presence of CeO2 nanoclusters, even in those films which appear to be single phase in XRD. With O2 annealing, the metal-toinsulator transition temperature increases, while the saturation magnetization decreases and stays well below the theoretical value for electron-doped La0.7Ce0.3MnO3 with mixed Mn 3+ /Mn 2+ valences. The same trend is observed with decreasing film thickness from 100 to 20 nm, indicating a higher oxygen content for thinner films. Hall measurements on a film which shows a metal-toinsulator transition clearly reveal holes as dominating charge carriers. Combining data from x-ray photoemission spectroscopy, for determination of the oxygen content, and x-ray absorption spectroscopy (XAS), for determination of the hole concentration and cation valences, we find that with increasing oxygen content the hole concentration increases and Mn valences are shifted from 2+ to 4+. The dominating Mn valences in the films are Mn 3+ and Mn 4+ , and only a small amount of Mn 2+ ions can be observed by XAS. Mn 2+ and Ce 4+ XAS signals obtained in surface-sensitive total electron yield mode are strongly reduced in the bulk-sensitive fluorescence mode, which indicates hole-doping in the bulk for those films which do show a metal-to-insulator transition.

Knowledge of the electron sampling depth and related saturation effects is important for quantitative analysis of X-ray absorption spectroscopy data, yet for oxides with the perovskite structure no quantitative values are so far available. Here we study absorption saturation in films of two of the moststudied perovskites, La 0.7 Ca 0.3 MnO 3 (LCMO) and YBa 2 Cu 3 O 7 (YBCO), at the L 2,3 edge of Mn and Cu, respectively. By measuring the electron-yield intensity as a function of photon incidence angle and film thickness, the sampling depth d, photon attenuation length λ and the ratio λ/d have been independently determined between 50 and 300 K. The extracted sampling depth d LCMO ≈ 3 nm for LCMO at high temperatures in its polaronic insulator state (150 -300 K) is not much larger than values reported for pure transition metals (d Co or Ni ≈ 2 -2.5 nm) at room temperature, but is smaller than d YBCO ≈ 3.9 nm for metallic YBCO that is in turn smaller than the value reported for Fe 3 O 4 (d Fe3O4 ≈ 4.5 nm). The measured d LCMO increases to 4.5 nm when LCMO is in the metallic state at low temperatures. These results indicate that a universal rule of thumb for the sampling depth in oxides cannot be assumed, and that it can be measurably influenced by electronic phase transitions that derive from strong correlations.

We report resistance versus magnetic field measurements for a La0.65Sr0.35MnO3/SrTiO3/La0.65Sr0.35MnO3 tunnel junction grown by molecular-beam epitaxy, that show a large field window of extremely high tunneling magnetoresistance (TMR) at low temperature. Scanning the in-plane applied field orientation through 360 • , the TMR shows 4-fold symmetry, i.e. biaxial anisotropy, aligned with the crystalline axes but not the junction geometrical long axis. The TMR reaches ∼ 1900 % at 4 K, corresponding to an interfacial spin polarization of > 95 % assuming identical interfaces. These results show that uniaxial anisotropy is not necessary for large TMR, and lay the groundwork for future improvements in TMR in manganite junctions.