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.

Films of cerium-doped LaMnO3, which has been intensively discussed as an electron-doped counterpart to hole-doped mixed-valence lanthanum manganites during the past decade, were analyzed by x-ray photoemission spectroscopy with respect to their manganese valence under photoexcitation. The comparative analysis of the Mn 3s exchange splitting of La0.7Ce0.3MnO3 (LCeMO) films in the dark and under illumination clearly shows that both oxygen reduction and illumination are able to decrease the Mn valence towards a mixed 2+/3+ state, independently of the film thickness and the degree of CeO2 segregation. Charge injection from the photoconductive SrTiO3 substrate into the Mn eg band with carrier lifetimes in the range of tens of seconds and intrinsic generation of electron-hole pairs within the films are discussed as two possible sources of the Mn valence shift and the subsequent electron doping.

We report on imaging of the nonuniform superconducting states in a Pb thin-film bridge on top of a ferromagnetic BaFe 12 O 19 single crystal with a single straight domain wall along the center of the bridge by low-temperature scanning laser microscopy. We have visualized domain-wall superconductivity (DWS) close to the critical temperature of Pb, when the Pb film above the domain wall acts as a superconducting path for the current. The evolution of the DWS signal with temperature and the external-field-driven transition from DWS to reverse-domain superconductivity was visualized. It is well known that so-called surface or bound states can be generated by the presence of boundaries in a material. For example, the formation of surface states for a single electron wave function in a semi-infinite crystalline lattice due to the modification of the boundary conditions was described by Tamm 1 and by Shockley. 2 Other examples of bound states are surface plasmons, propagating along the interface between a dielectric and a metal, [3][4][5] and surface acoustic waves traveling along the surface of a material exhibiting elasticity. 6,7 In both latter cases these waves are confined in the direction perpendicular to the wave vector, i.e., their amplitudes decay exponentially far from the interface or surface. The formation of surface bound states for the superconducting order-parameter wave function was considered by Saint-James and de Gennes. 8,9 They showed that localized superconductivity at a superconductor (S)/vacuum or S/insulator interface can appear at an applied magnetic field H ext above the upper critical field H c2 for bulk superconductivity. Similarly to this surface superconductivity, localized superconductivity can also nucleate near the sample edge in a thin semi-infinite superconducting film 10 or in a thin superconducting disk of very large diameter 11 in a perpendicular magnetic field. Such so-called edge superconductivity (ES), with transition temperature T c2 . 15For flux-coupled S/F structures of finite lateral size the localized states of ES and DWS may compete as illustrated in Fig. 1 for the case of a thin-film S strip of finite width above a F substrate with a domain wall along the center of the bridge, for H c2 < B 0 < H c3 . For a domain structure with steplike b z (x) profile and H ext = 0, ES and DWS nucleate simultaneously in the S strip as shown in Fig. 1(a). Figure 1(b) shows the case of a domain wall with finite width and H ext = 0. Here, DWS becomes energetically more favorable compared to ES and only DWS nucleates. 16 For H ext = 0, the local field is compensated above the domain with magnetization direction opposite to H ext . If ||H ext | − B 0 | < H c2 superconductivity is turned on above this reverse domain while it is still suppressed above the parallel domain [cf. Fig. 1(c)]. This effect is termed reverse-domain superconductivity (RDS). 17,18 We note that when H c2 becomes larger than |H ext | + B 0 (e.g., upon cooling) above the parallel domain, superconductivity may also nucleate...

The resistance R versus perpendicular external magnetic field H was measured for superconducting Nb thin film microbridges with and without microholes (antidots, ADs). Well below the transition temperature, integral R(H) measurements of the resistive transition to the normal state on the plain bridge show two distinct regions, which can be identified as bulk and edge superconductivity, respectively. The latter case appears when bulk superconductivity becomes suppressed at the upper critical field H c2 and below the critical field of edge superconductivity H c3 ≈ 1.7 H c2 . The presence of additional edges in the AD bridge leads to a different shape of the R(H) curves. We used low-temperature scanning laser microscopy (LTSLM) to visualize the current distribution in the plain and AD bridges upon sweeping H. While the plain bridge shows a dominant LTSLM signal at its edges for H > H c2 the AD bridge also gives a signal from the inner parts of the bridge due to the additional edge states around the ADs. LTSLM reveals an asymmetry in the current distribution between the left and right edges, which confirms theoretical predictions. Furthermore, the experimental results are in good agreement with our numerical simulations (based on the time-dependent Ginzburg-Landau model), yielding the spatial distribution of the order parameter and current density for different bias currents and H values.