Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D10: Focus Session: Hybrid Magnetic-Superconducting Systems I |
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Sponsoring Units: DMP Chair: Lance Delong, University of Kentucky Room: Morial Convention Center RO8 |
Monday, March 10, 2008 2:30PM - 3:06PM |
D10.00001: Odd-frequency pairing state in superconducting junctions Invited Speaker: We have theoretically studied the induced odd-frequency pairing states in ballistic normal metal/superconductor (N/S) junctions where a superconductor has even-frequency symmetry in the bulk. We demonstrate that the pair amplitude in the junction has an admixture of an odd-frequency component due to the breakdown of translational invariance near the N/S interface [1]. We have also studied about the proximity effect in junctions between diffusive normal metals (DN) and superconductors. It is revealed for spin-triplet superconductor that the resulting symmetry in DN is always odd-frequency spin-triplet [2]. The resulting quasiparticle density of state in DN has a zero energy peak [3]. This unusual proximity effect due to the generation of odd-frequency state is also expected in ferromagnet / superconductor junctions [4]. \newline [1] Y. Tanaka, et al, Phys. Rev. Lett. 99, 037005 (2007), M. Eschirig, et al, J. Low Temp. Phys. \textbf{147} 457 (2007). \newline [2] Y. Tanaka and A.A. Golubov, Phys. Rev. Lett. 98 037003 (2007). \newline [3] Y. Tanaka and S. Kashiwaya, Phys. Rev. B, 70, 012507 (2004). \newline [4] Y. Asano, Y. Tanaka and A. A. Golubov, Phys. Rev. Lett., 98, 107002 (2007). [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D10.00002: Superfluid density in the ferromagnetic layers of superconductor-ferromagnet hybrid structures Thomas Lemberger, Michael Hinton, Adam Hauser, Julian Hetel, Fengyuan Yang, Julia Meyer We have measured the areal superfluid density of superconductor-ferromagnet bilayers and trilayers. Samples are made by sputtering Nb and Ni films sequentially in an ultrahigh vacuum chamber with base pressure $<$10$^{-9}$ torr. Interfaces are cleaner when Ni is sputtered onto Nb for reasons related to disorder at the initial growth of Nb films. Superfluid density is measured using a low-frequency (50 kHz) two-coil technique with coils on opposite sides of the sample. These measurements provide the true T$_{C}$, i.e., the temperature below which superfluid exists. We find a nonmonotonic dependence of T$_{C}$ on ferromagnetic layer thickness, in agreement with resistive measurements of T$_{C}$. The superfluid density is also nonmonotonic. Even at large ferromagnetic layer thicknesses where T$_{C}$ is essentially constant, the areal superfluid density continues to increase, indicating that superfluid extends deeply into the ferromagnet layers. We will discuss these measurements in the context of theory of the superconductor-ferromagnet proximity effect. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D10.00003: Study of Inverse Proximity Effect in Ferromagnet/ Superconductor Bilayers Using a Sagnac Interferometer Jing Xia, A. Palevski, A. Kapitulnik It was recently proposed theroretically that ferromagnetic order can be induced in the superconductor in a ferromagnet/superconductor ($S/F$) bilayer structure through a so called inverse proximity effect. The proposal predicts a sizable magnetic moment in the ``S" layer that couples antiferromagnetically to the moment in the ``F" layer due to Cooper pairs near the interface formed with one electron in the F layer and one in the S layer. The induced magnetic moment is expected to penetrate the superconductor over a size of the Cooper pairs, i.e. $\xi_s$. In order to directly test this interesting scenario, we fabricated Ni/Pb and Ni/Al bilayer samples and probed the possible induced magnetic moments in the ``S" layer through high resolution Surface Magneto Optical Polar Kerr Effect (PKE) measurements on the ``S" layer side through the bilayer's $T_C$ using a Sagnac interferometer. The thickness of the ``S" layer was fabricated to be larger than the optical skin depth in order to make sure that our experiment doesn't pickup up any magnetic moment from the ``F" layer. $\xi_s$ dependence of the effect is studied by comparing the results in Ni/Pb and Ni/Al samples. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D10.00004: Investigation of the role of Al/CoFe interface in proximity effect of Nb/Al/CoFe K. Char, Junhyung Kwon, Wenjian Lu When a few nm-thick Al layer is inserted between Nb and ferromagnetic (F) layers such as CoFe, Ni, or CuNi, the superconducting critical temperature of the trilayers increase rapidly almost to a level of Nb/Al bilayers. In order to understand the role of Al/CoFe interface in the proximity effect of Nb/Al/CoFe, we have added Mg and Au scattering centers and found different behavior. The Mg scattering center did not change the critical temperature behavior of Nb/Al/CoFe, while the Au scattering centers reduced the critical temperature. The results point toward the importance of spin-orbit scattering. In addition, tunneling spectroscopy data on Nb/Al/F vs. on Nb/F will be presented in order to further characterize the role of Al/F interfaces. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D10.00005: Magnetic property change by superconductivity in Py-Nb Hybrid system David Murakami, Michael Hetman, Jiyeong Gu Recently ferromagnet/superconductor systems have attracted a great attention due to their scientific interest and potential for the technological application. So far, most of the work focused on the superconducting property change by magnetism in the hybrid system, and only few researches focused on the magnetic property change by superconductivity. In this presentation we will focus on the magnetic property change of the system by superconductivity when the system goes through the superconducting transition. We have investigated different types of hybrid structures including Py/Nb bilayer, multilayer, and composite systems. To separate out the signal from superconductor, we also measured the single Nb film. We measured the magnetization as a function of temperature and compared it between in a normal state and a superconducting state for the different types of structures. It showed strong magnetic history dependence. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D10.00006: Proximity effect in MgB$_{2}$-Permalloy hybrid system April O'Brien, Jiyeong Gu Recently ferromagnet/superconductor systems have attracted a great attention due to their scientific interest and potential for the technological application. So far, most of the work has been done for Nb with ferromagnetic metals or high temperature oxide superconductors with magnetic oxides. In this presentation we will discuss the proximity effect in magnesium diboride (MgB$_{2})$/Py thin films, where the superconducting property changes due to the adjacent Py layer. Even though the good electronic properties such as large critical current density and large coherence length of MgB$_{2}$ make it a good candidate for superconducting electronic applications, depositing a good-quality \textit{in-situ} MgB$_{2}$ thin film is still challenging. Especially for the multilayer deposition, it is ideal to deposit all the layers \textit{in-situ} without \textit{ex-situ} treatment. We used a sputtering to deposit the MgB$_{2}$ layer \textit{in-situ} and made a hybrid system with Py. We found that the substrate temperature during the MgB$_{2}$ deposition is the most important parameter to determine the superconducting transition temperature of the sample. In addition to the proximity effect we will also discuss the problem with a sputtering method for MgB$_{2}$ deposition. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D10.00007: Spin-switch effect in Permalloy-Niobium hybrid system Jiyeong Gu, Michael Hetman Proximity effect in ferromagnet ($F)$/superconductor ($S)$ systems has become a center of attention recently. It has been well known that the superconducting property is modified when the magnetization of the adjacent $F$ layer changes. Especially when the superconducting transition temperature changes as a function of the magnetization of the $F$ layers, the system can work as a switching device. In this presentation we compare three different Py-Nb hybrid structures; Py/Nb bilayer, Py/Nb/Py/FeMn spin valve, and Nb/Py/Nb/Py/FeMn ($S-F$-\textit{F$'$}) structure. We discuss the similarity and the difference in magnetic and transport properties of these three structures. When the Nb layer is thick in bilayer or $S-F-F'$ structure, the device showed a change between zero resistance (superconducting state) and finite resistance (normal state) by applying a small external magnetic field, where the shift in transition temperature is greater than the transition width, and this gave a huge magnetoresistance effect ([$R$(0)-$R(H)$]/$R$(0)=$\infty )$. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D10.00008: Anomalous transport property in the single-crystal Co nanowire with superconducting electrodes Jian Wang, Nitesh Kumar, Mingliang Tian, Qi Zhang, Jainendra Jain, Thomas Mallouk, Moses H.W. Chan Transport measurements were made on individual single-crystal Co nanowire with four focused ion beam (FIB) deposited tungsten (W) electrodes, which are superconducting below 5 K. It was found that the 2 microns long Co nanowire shows a sharp and large resistance peak near the onset transition temperature (T$_{C})$ of W and a rapid resistance drop below T$_{C}$. The large, 50{\%} resistance drop at low temperature suggests the proximity effect from superconducting W electrodes extends to a long fraction of the ferromagnetic Co nanowire. The resistance peak is not seen in the Au nanowire with same superconducting W electrodes. Measurements on a Co nanowire contacted with FIB deposited non-superconducting Pt electrodes show no change in resistance. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D10.00009: Transport properties of hybrid superconductor/ferromagnet nanowires fabricated by electrodeposition. Nitesh Kumar, Jian Wang, Qi Zhang, Mingliang Tian, Moses H.W. Chan We have fabricated multilayer nanowires with alternating superconducting and ferromagnetic segments using template-based electrodeposition. Nanowires are fabricated with different diameters and length, with individual segments on the length scale of few hundreds of nm to few microns, using both porous polycarbonate and anodized alumina membranes. We have used Pb as the superconducting and Co or Ni as the ferromagnetic components. Structural characterizations done with X-ray diffraction and Transmission Electron Microscope demonstrated that Pb and Co segments are good single crystal whereas Ni segments are polycrystalline. We have done electrical transport measurements on arrays of multilayer nanowires (embedded inside the template) showing interesting magnetoresistance behaviors below the superconducting transition temperature of Pb. Four terminal electrical measurements on a single multilayer nanowire are in progress. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D10.00010: Ferromagnetic Josephson Resonance Ivana Petkovic, Marco Aprili Ferromagnetic Josephson junctions with negative ($\pi -)$ coupling behave as phase sources, a potentially very useful component of quantum electronics. In order to elaborate sophisticated circuits, it is crucial to understand the interplay between spin- and superconducting phase dynamics. For that purpose, we fabricated strongly underdamped sub-micron Josephson junctions. We measured the critical current at zero voltage as a function of the applied magnetic field. The finite magnetization in the junction induces a shift in the Fraunhofer pattern which is invariant under time reversal. For a voltage such that the Josephson frequency matches the ferromagnetic resonance, we observe a reduction of the critical current due to the absorption of the Josephson radiation by the ferromagnetic layer. We have investigated the effect of an external microwave radiation and of the magnetic field. The resonances appear as satellites at every Shapiro step and they are shifted in energy by the magnetic field as expected. The high sensitivity of the ac Josephson effect to a small amount of spins opens up new routes for ESR in nano-magnetism. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D10.00011: Theory of spin wave excitation by Josephson current in a superconductor/ferromagnet/superconductor junction Shin-ichi Hikino, Michiyasu Mori, Saburo Takahashi, Sadamichi Maekawa The Josephson effect in a superconductor/ferromagnet/superconductor junction has been of considerable interest in recent years. Current-voltage (I-V) characteristics of superconducting weak links are studied by the resistively shunted junction (RSJ) model, which describes phase dynamics of superconductors (SC). The ferromagnet (F) has spin waves (SW). Therefore, in an S/F/S junction, it is important to treat the spin- and phase dynamics in an equal footing. However, the spin dynamics has not received much attention in the study of an S/F/S junction. ~We study the effect of the spin dynamics on the phase dynamics in an S/F/S junction. The RSJ model is extended to include the spin dynamics using gauge invariant phase difference between superconducting leads. We find that the I-V characteristics show step structures. The voltage at the steps is proportional to the SW energy in F. The origin of step structures will be discussed. [Preview Abstract] |
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