M. van Zalk

Bio: M. van Zalk is an academic researcher from MESA+ Institute for Nanotechnology. The author has contributed to research in topics: Magnetoresistance & Ferromagnetism. The author has an hindex of 5, co-authored 8 publications receiving 2411 citations.

Magnetic effects at the interface between non-magnetic oxides

Abstract: The electronic reconstruction at the interface between two insulating oxides can give rise to a highly conductive interface. Here we show how, in analogy to this remarkable interfaceinduced conductivity, magnetism can be induced at the interface between the otherwise non-magnetic insulating perovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the interface is found, together with a logarithmic temperature dependence of the sheet resistance.At lowtemperatures, the sheet resistance reveals magnetic hysteresis.Magnetic ordering is a key issue in solid-state science and its underlying mechanisms are still the subject of intense research. In particular, the interplay between localized magnetic moments and the spin of itinerant conduction electrons in a solid gives rise to intriguingmany-body effects such as Ruderman–Kittel–Kasuya–Yosida interactions3, the Kondo effect4 and carrier-induced ferromagnetism in diluted magnetic semiconductors5. The conducting oxide interface now provides a versatile system to induce and manipulate magnetic moments in otherwise non-magnetic materials.

Magnetic effects at the interface between nonmagnetic oxides

Abstract: The electronic reconstruction at the interface between two insulating oxides can give rise to a highly-conductive interface. In analogy to this remarkable interface-induced conductivity we show how, additionally, magnetism can be induced at the interface between the otherwise nonmagnetic insulating perovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the interface is found, together with a logarithmic temperature dependence of the sheet resistance. At low temperatures, the sheet resistance reveals magnetic hysteresis. Magnetic ordering is a key issue in solid-state science and its underlying mechanisms are still the subject of intense research. In particular, the interplay between localized magnetic moments and the spin of itinerant conduction electrons in a solid gives rise to intriguing many-body effects such as Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, the Kondo effect, and carrier-induced ferromagnetism in diluted magnetic semiconductors. The conducting oxide interface now provides a versatile system to induce and manipulate magnetic moments in otherwise nonmagnetic materials.

Magnetization-induced resistance-switching effects in La 0.67 Sr 0.33 MnO 3 /YBa 2 Cu 3 O 7-δ bi- and trilayers

Abstract: We have studied the influence of the magnetization on the superconducting transition temperature (Tc) in bi- and trilayers consisting of the half-metallic ferromagnet La0.67Sr0.33MnO3 and the high-temperature superconductor YBa2Cu3O7â��I´ (YBCO). We have made use of tilted epitaxial growth in order to achieve contacts between the two materials that are partly in the crystallographic ab plane of the YBCO. As a result of uniaxial magnetic anisotropy in the tilted structures, we observe sharp magnetization-switching behavior. At temperatures close to Tc, the magnetization-switching induces resistance jumps in trilayers, resulting in a magnetization dependence of Tc. In bilayers, this switching effect can be observed as well, provided that the interface to the ferromagnetic layer is considerably rough. Our results indicate that the switching behavior arises from magnetic stray fields from the ferromagnetic layers that penetrate into the superconductor. A simple model describes the observed behavior well. We find no evidence that the switching behavior is caused by a so-called superconducting spin switch, nor by accumulation of spin-polarized electrons. Observation of magnetic coupling of the ferromagnetic layers, through the superconductor, supports the idea of field-induced resistance switching.

Interface resistance of YBa2Cu3O7−δ/La0.67Sr0.33MnO3 ramp-type contacts

Abstract: We fabricated and characterized YBa2Cu3O7−δ/La0.67Sr0.33MnO3 (YBCO/LSMO) ramp-type contacts and junctions. An interlayer technique was applied to repair the ramp stoichiometry after etching. It was found that, typically, the resistance of the YBCO/LSMO interface is high compared to the resistances of YBCO interfaces to Au, Pt and the epitaxially grown ferromagnetic oxide SrRuO3. The YBCO/LSMO interfaces were characterized electrically and were found to show a large negative, linear magnetoresistance. Electron energy loss spectroscopy experiments do not show a significant oxygen depletion near the YBCO/LSMO interface. Our results indicate that the high interfacial resistance is caused by the effect of charge transfer across the interface. The magnetoresistance suggests that part of the interface resistance is of magnetic origin.

Fabrication of multiband MgB2 tunnel junctions for transport measurements

Abstract: The coexistence of multiple superconducting condensates in a material gives rise to intriguing phenomena, such as the possible presence of number-phase fluctuations. In this work, tunnel junctions were fabricated, aiming for the detection of such phenomena. Planar junctions with normal conducting and superconducting counter-electrodes have been realized. It was found from spectroscopic investigations that the MgO barrier used for the planar junctions is not ideal. To optimize for the two-band behaviour of MgB2, ab-plane ramp-type MgB2 tunnel junctions were fabricated. The use of Al2O3 barriers substantially improved the homogeneity of the junctions, as was concluded from the nearly ideal magnetic field dependence of the critical current and excellent scaling of the junction properties with the junction widths. The current–voltage characteristics show clear Shapiro steps under microwave irradiation. However, the spectrum showed a degradation of the MgB2 in the ramp region and the signature of the σ-gap was absent. For tunnel junction spectroscopy in the ab-direction and the detection of new, two-band related modes, the fabrication process still has to be further optimized.

Superconducting Interfaces Between Insulating Oxides

Abstract: At interfaces between complex oxides, electronic systems with unusual electronic properties can be generated. We report on superconductivity in the electron gas formed at the interface between two insulating dielectric perovskite oxides, LaAlO3 and SrTiO3. The behavior of the electron gas is that of a two-dimensional superconductor, confined to a thin sheet at the interface. The superconducting transition temperature of ≅ 200 millikelvin provides a strict upper limit to the thickness of the superconducting layer of ≅ 10 nanometers.

Emergent phenomena at oxide interfaces

Abstract: Recent technical advances in the atomic-scale synthesis of oxide heterostructures have provided a fertile new ground for creating novel states at their interfaces. Different symmetry constraints can be used to design structures exhibiting phenomena not found in the bulk constituents. A characteristic feature is the reconstruction of the charge, spin and orbital states at interfaces on the nanometre scale. Examples such as interface superconductivity, magneto-electric coupling, and the quantum Hall effect in oxide heterostructures are representative of the scientific and technological opportunities in this rapidly emerging field.

Electric field control of the LaAlO3/SrTiO3 interface ground state.

Abstract: Interfaces between complex oxides are emerging as one of the most interesting systems in condensed matter physics. In this special setting, in which translational symmetry is artificially broken, a variety of new and unusual electronic phases can be promoted. Theoretical studies predict complex phase diagrams and suggest the key role of the charge carrier density in determining the systems' ground states. A particularly fascinating system is the conducting interface between the band insulators LaAlO(3) and SrTiO(3) (ref. 3). Recently two possible ground states have been experimentally identified: a magnetic state and a two-dimensional superconducting condensate. Here we use the electric field effect to explore the phase diagram of the system. The electrostatic tuning of the carrier density allows an on/off switching of superconductivity and drives a quantum phase transition between a two-dimensional superconducting state and an insulating state. Analyses of the magnetotransport properties in the insulating state are consistent with weak localization and do not provide evidence for magnetism. The electric field control of superconductivity demonstrated here opens the way to the development of new mesoscopic superconducting circuits.

Interface Physics in Complex Oxide Heterostructures

Abstract: Complex transition metal oxides span a wide range of crystalline structures and play host to an incredible variety of physical phenomena. High dielectric permittivities, piezo-, pyro-, and ferroelectricity are just a few of the functionalities offered by this class of materials, while the potential for applications of the more exotic properties like high temperature superconductivity and colossal magnetoresistance is still waiting to be fully exploited. With recent advances in deposition techniques, the structural quality of oxide heterostructures now rivals that of the best conventional semiconductors, taking oxide electronics to a new level. Such heterostructures have enabled the fabrication of artificial multifunctional materials. At the same time they have exposed a wealth of phenomena at the boundaries where compounds with different structural instabilities and electronic properties meet, giving unprecedented access to new physics emerging at oxide interfaces. Here we highlight some of these exciting...

Electric-field-induced superconductivity in an insulator

Abstract: Increasing the carrier density of a material to the limit at which superconductivity can be induced has been a long-standing challenge. This is now realized in an insulator by using an electric-double-layer gate in an organic electrolyte. Electric field control of charge carrier density has long been a key technology to tune the physical properties of condensed matter, exploring the modern semiconductor industry. One of the big challenges is to increase the maximum attainable carrier density so that we can induce superconductivity in field-effect-transistor geometry. However, such experiments have so far been limited to modulation of the critical temperature in originally conducting samples because of dielectric breakdown1,2,3,4. Here we report electric-field-induced superconductivity in an insulator by using an electric-double-layer gating in an organic electrolyte5. Sheet carrier density was enhanced from zero to 1014 cm−2 by applying a gate voltage of up to 3.5 V to a pristine SrTiO3 single-crystal channel. A two-dimensional superconducting state emerged below a critical temperature of 0.4 K, comparable to the maximum value for chemically doped bulk crystals6, indicating this method as promising for searching for unprecedented superconducting states.