Session A19: Magnetic and Superconducting Properties

Transport in ultrathin gold films decorated with magnetic Gd atoms

We have performed four-probe transport measurements of ultrathin Au films decorated with Gd ad-atoms. The samples were prepared by quench condensation, i.e., sequential evaporation on a cryogenically cooled substrate under UHV conditions while monitoring the film thickness and resistance. Electrically continuous Au films at thickness of about 2 mono-layers of material are grown on an amorphous Ge wetting layer. The quench condensation method provides a sensitive control on the sample growth process, allowing us to tune the morphological and electrical configuration of the system. The ultrathin gold films develop from an insulating to a metallic state as a function of film thickness. The temperature dependence of the Au conductivity for different thickness is studied. It evolves from hopping transport for the insulating films, to a ln T dependence for thicker films. For gold films in the insulating regime we found a decreasing resistance by adding Gd. This is in agreement with a decreasing tunneling barrier height between metallic atoms. The Gd magnetic moments are randomly oriented for isolated atoms. This magnetic disorder leads to scattering of the charge carriers and a reduced conductivity compared to nonmagnetic materials.   

From Order to Disorder and Back: Co on Narrow Stepped Cu

Bimetallic surface systems allow a ready template to explore the compositional dependence of surface phases. When these systems involve a vicinal substrate, the surface also becomes a template for nanoscale-phase formations. In this regard, we examine the bimetallic system of Co on Cu(775), due the wide-spread interest in the magnetic phenomena of the related Co/Cu(111). We present an STM imaging study of this surface to show that it is possible to observe self-assembly of reduced-dimension quantum structures. These observations show a rich set of bimetallic phase transitions as a function of coverage - moving from wires at low coverage to step-induced ordered islands at high coverage. At coverage of less than 0.1ML, we observe growth of sharp, straight 2-atom-wide Co wires; topographic measurements suggest an interesting interpretation of recent DFT computations on such a system. Increasing coverage causes a marked change of the step spacing and causes the surface to be covered with an ordered array of 2-D islands beyond a critical deposition amount. Thus, as coverage increases, the Cu step structure evolves from straight ordered step edges to concave-shaped edges and then from disordered to ordered islands.   

Electron charge and spin pairing (pseudo)gaps and Nagaoka instabilities in nanoclusters

The electron pairings, phase separation and magnetism in various frustrated Hubbard clusters are studied exactly with emphasis on tetrahedron and octahedron under doping, magnetic field and temperature. Small clusters yield intriguing insight into charge spin separation and invoked thermal condensation of electron charge and spin in more than one bosonic mode. The spin saturated phase in so called Nagaoka state is found equivalent to ferromagnetic Mott-Hubbard like insulator with (negative) spin pairing gap, while non maximum spin ground state is of BCS-like metallic origin with equal charge (negative) and spin (positive) pairing gaps. The calculated phase diagrams resemble a number of inhomogeneous coherent and incoherent paired phases in high T$_c$ cuprates, fullerene molecules, Co and Nb nanoparticles.   

Superconductivity of Ultra-thin Pb films on Semiconductor Substrates: A Scanning Tunneling Microscopy/Spectroscopy Study

Ultra-thin Pb films on semiconductor substrates have exhibited many intriguing phenomena manifested by the quantum confinement of electronic states. Quantum stability has been a topic of interest for many years. Recently, it was shown that quantum confinements also play an interesting role on superconductivity. Oscillations of superconductivity gap and Tc as a function of film thickness have been observed in Pb/Si(111) and Pb/Ge(111) systems. Moreover, it is found that the superconductivity remains very robust even for films as thin as 5 ML. An interesting question arises as to what extent the robustness of superconductivity remains in even thinner regime. By using a different surface template, namely $\surd $3x$\surd $3-$\alpha $ Pb/Si(111) surface, we have grown uniform Pb films down to 2 ML. The film shows preferred thicknesses of 2ML and 4ML, presumably a manifestation of the quantum stability. We find that the superconducting gap remains robust down to 4ML and shows BCS-like temperature dependence. For 2ML films, we find a much smaller gap at 4.2K. Whether or not it corresponds to superconducting gap is under investigation.   

Magnetic field dependence of Interface Superconductivity in LSCO/LCO bilayers

Interface superconductivity (IS) with a high T$_{c}$ has been discovered recently in bi-layer films consisting of a thin layer of La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ (overdoped and metallic but not superconducting) covered with a thin layer of La$_{2}$CuO$_{4}$ (undoped, insulating, and antiferromagnetic) grown by molecular beam epitaxy (MBE)$^{1}$. Here we report on a study of magneto-transport properties in such IS systems. By measuring the magnetic-field dependence of in-plane longitudinal and Hall resistivities we find the temperature dependence of the upper critical field (H$_{c2})$. Other findings and inferences on the nature of IS and T$_{c}$ enhancement will be discussed as well. $^{1}$A. Gozar, G. Logvenov, A. T. Bollinger and I. Bozovic, ``Interface superconductivity between a metal and a Mott insulator'', submitted for publication.   

Separation of the strain and finite size effect on the ferromagnetic properties of La$_{0.5}$Sr$_{0.5}$CoO$_3$ thin films

The ferromagnetic properties of epitaxial La$_{0.5}$Sr$_{0.5} $CoO$_3$ thin films have been studied. The magnetic transition is affected by both strain and finite thickness. We have used a series of films of different thickness and on different substrates in order to quantitatively determine the change in Curie temperature contributed by each effect. The phase diagram of T$_C$ versus in-plane strain suggests that the ferromagnetic transition temperature is suppressed by tensile strain and enhanced by compressive strain. The general method of separating strain and finite thickness effects should be applicable to any ordering phase transition in thin films. A leading theory for the ferromagnetism in La$_{0.5}$Sr$_{0.5} $CoO$_3$ is the double exchange mechanism. This model relies upon Co-O-Co electron hopping so that a strong dependence on bond length is expected. Our recent EXAFS results will examine whether the double exchange mechanism quantitatively predicts the strain dependence we have measured.   

Giant Magnetoelectric Coupling in Asymmetric Ferromagnet-Ferroelectric-Metal Trilayers: Toward Electric Control of Magnetization

By examining electron screening at the surface of ferromagnetic metals and semiconductors, we propose an asymmetric ferromagnet-ferroelectric-metal trilayer to realize giant magnetoelectric effect. The origin is the strengthened accumulation of spin-polarized screening electrons at the ferromagnet-ferroelectric interface. Including the electrostatic energy from such spin-polarized charges in the Laudau-Devibshire free energy for ferroelectics, a magnetoelectric coupling term $\chi P^2M^2$ is derived, and consequently an electric control of magnetization hysteries is observed. The dependence of $\chi$ as well as the ME effect on the choice of materials is discussed for some real ferromagnets.   

Room temperature ferromagnetism in as-deposited and post-annealed Co-doped ZnO films

The Co-doped ZnO thin films were prepared on $c$-cut sapphire substrates by magnetron co-sputtering, and then annealed at various temperatures in vacuum. Magnetic measurements indicate that all the films are ferromagnetic at room temperature and the magnetization of the annealed Zn$_{0.88}$Co$_{0.12}$O films is increased about one order of magnitude in comparison with the corresponding as-deposited one. Both X-ray diffraction and TEM results show that there are not any Co and Co oxides secondary phases. Optical spectrometry indicates that Co$^{2+}$enters the tetrahedral sites of the wurtzite structure of ZnO host and substitutes for Zn$^{2+}$.   

Ferromagnetic $Mn_{3-\delta } Ga$ On Wurtzite GaN: Initial Stages Of Growth By Molecular Beam Epitaxy

Ferromagnetic (FM) metal/wide band-gap bilayers are of great interest due to their potential for novel spintronics applications, such as blue and ultra-violet spin light-emitting diodes$^{[1]}$. It has been reported$^{[2]}$ that $Mn_{3-\delta } Ga$, a promising FM alloy, can be grown epitaxially on top of w-GaN(0001) with controllable magnetism via controlling of the Mn:Ga flux ratio. Here we report studies on the initial stages of growth of MnGa on w-GaN. Growth experiments were carried out in a UHV chamber using molecular beam epitaxy with rf (N$_{2})$-plasma, on both N- and Ga-polar substrates. Reflection high-energy electron diffraction (RHEED) data suggest that at the initial stages of growth, the surface structures depend on the substrate polarity. This may be due to the structural differences between the N-polar and the Ga-polar GaN surfaces. Stoichometry dependence of initial stages of growth is also being investigated. This work has been supported by DOE (Grant No.DE-FG02-06ER46317) and NSF (Grant No.0304314). Equipment support from ONR is also acknowledged. [1] S.A.Wolf \textit{et al}, Science \textbf{294}, 1488 (2001) [2] E.Lu \textit{et al}, Phys.Rev.Lett. \textbf{97}, 146101 (2006)   

Molecular Beam Epitaxial Growth of Iron Nitrides on Zinc-Blende Gallium Nitride(001)

Iron nitrides are attractive materials for their high magnetic moments, corrosion, and oxidation resistance. We present the successful epitaxial growth of iron nitride on zinc-blende gallium nitride (c-GaN) in order to develop a novel magnetic transition metal nitride/semiconductor system. First, GaN is grown on magnesium oxide (MgO) substrates having (001) orientation using rf N$_{2}$-plasma molecular beam epitaxy. Then we grow FeN at substrate temperature of $\sim $ 210 $^{\circ}$C up to a thickness of $\sim $ 10.5 nm. \textit{In-situ} reflection high-energy electron diffraction (RHEED) is used to monitor the surface during growth. Initial results suggest that the epitaxial relationship is FeN[001] $\vert \vert $ GaN[001] and FeN[100] $\vert \vert $ GaN[100]. Work in progress is to investigate the surface using \textit{in-situ} scanning tunneling microscopy (STM) to reveal the surface structure at atomic scale, as well as to explore more Fe-rich magnetic phases.   

Synthesis of Co$_{1-x}$Fe$_{2+x}$O$_{4}$: Towards Spin Polarized Ferrites

Ferrites are promising materials for spintronic devices, since they are predicted to exhibit high spin polarizations [1]. Thin-film cobalt ferrite (CoFe$_{2}$O$_{4})$ has a large saturation magnetization and magnetic coercivity, but is insulating [2]. In this work, epitaxial Co$_{1-x}$Fe$_{2+x}$O$_{4}$ thin films are grown by MBE on Fe$_{3}$O$_{4}$(001) and MgO(001), where a fraction of the Co$^{2+}$ ions are replaced with Fe$^{2+}$. LEED, RHEED and XRD confirm the crystal structure. Stoichiometry and cation valence states are ascertained by XPS, and the electronic structure near the Fermi level is determined by UPS. We show that, by varying the stoichiometry of Co$_{1-x}$Fe$_{2+x}$O$_{4}$, we can tailor its electronic properties, which may lead to a conductive, spin polarized ferrite. [1] J. Cibert, et al., C.R. Physique \textbf{6} (2005) 977. [2] W. Huang, et al., J. Crystal Growth \textbf{300} (2007) 426.   

A surface-driven route to novel magnetic structures: Manganese on Si(100)(2x1)

The combination of Silicon with an element with a large magnetic moment such as Manganese is highly desirable for the development of novel spintronics devices. We present a study on the surface-driven synthesis of Mn-nanostructures on the Si(100) (2x1) surface using STM and photoelectron spectroscopy. The Si-surface functions as a template and monoatomic Mn-nanowires are formed, which always run perpendicular to the Si-dimer rows. Their length and spatial distribution is used to derive a model for the wire formation. The bonding of Mn to the Si, which is decisive for the resultant magnetic properties, are presented. The transition to a silicide is kinetically hindered and controlled by the Si- mobility. In the next step a Ge-overlayer is deposited, and analyzed with voltage dependent STM. In the low-adatom-mobility regime the Ge-growth is unperturbed by the presence of Mn, and the Mn-nanostructure is preserved and embedded.   

High-resolution ARPES studies of atomically uniform Pb films on Si(111).

Investigations on the fundamental physics evolved in the low-dimensional systems are of great interests both in basic research and in potential applications. Atomically uniform metal thin films on semiconductor substrate are the simplest quasi-2D electron systems, which demonstrate quantum confinement and form the quantum well states (QWS). Recently, atomically uniform Pb films on Si(111) have demonstrated novel properties induced by the quantum size effects. We have manufactured atomically uniform Pb films on Si(111)-7x7 surface. The dependence of the Pb films band structures on thickness has been studied by high-resolution angle-resolved photoemission spectroscopy (ARPES). In addition, we have investigated the superconductivity properties in such a quasi-2D electron systems by measuring the ARPES below superconducting transition temperature.   

Unusual Hall effect due to carrier delocalization.

Recently, an unusual Hall effect (HE) in antiferromagnetic YbRh$_{2}$Si$_{2}$ was reported.$^{1}$ Here, we describe the observation of a similar HE in ferromagnetic EuB$_{6}$. The unusual HE is characterized by two distinct slopes in the Hall resistivity as a function of applied magnetic field: a small slope in the ferromagnetic state and a large slope at high temperatures. In the paramagnetic state just above the Curie-Weiss temperature $\theta $ (intermediate temperatures), the Hall resistivity switches from the large slope at low fields to the small slope at high fields. The phenomenon cannot be attributed to the anomalous HE since the change in the Hall slope is not accompanied by saturation of magnetization. Moreover, the switching field was found to depend linearly on temperature and vanish right at $\theta $. We show that the switching occurs at a certain magnetization at which carriers are delocalized due to the overlapping of magnetic polarons. A quantitative fit to the HE data has been obtained based on this model of carrier delocalization. The model and analysis were successfully applied to the published HE data on YbRh$_{2}$Si$_{2}$, which suggests a possible relation between carrier delocalization and quantum criticality. This work was supported by a FSU Research Foundation PEG, NSF DMR 0710492 and 0503360 grants. $^{1}$S. Paschen et al., Nature \textbf{432}, 881 (2004).   

Spin-dependent band structure of the ferromagnetic semimetal EuB$_{6}$

The spin polarization of EuB$_{6}$ crystals has been measured using Andreev reflection spectroscopy. The conductance spectra of the EuB$_{6}$/Pb junctions are well-described by the spin-polarized BTK model, which yields a spin polarization of about 56{\%}. The results demonstrate that ferromagnetic EuB$_{6}$ is not half-metallic. Further analyses of the Hall effect and magnetoresistivity indicate a semi-metallic band structure with complete spin polarization for the hole band only. The values and the spread of the measured spin polarization are \textit{quantitatively} consistent with Fermi surface determined by quantum oscillation measurements$^{1}$ and carrier densities obtained from standard two-band model fits to the low temperature magnetoresistivity and Hall resistivity. This work was supported by a FSU Research Foundation PEG, NSF DMR 0710492 and 0503360 grants. $^{1}$R. Goodrich et al., PRB \textbf{58}, 14896 (1998); M. Aronson et al., PRB \textbf{59}, 4720 (1999).