Ivan K. Schuller

Ordered magnetic nanostructures: fabrication and properties

The fabrication methods and physical properties of ordered magnetic nanostructures with dimensions on the submicron to nanometer scale are reviewed. First, various types of nanofabrication techniques are described, and their capabilities and limitations in achieving magnetic nanostructures are discussed. Specifically, we address electron beam lithography, X-ray lithography, laser interference lithography, scanning probe lithography, step growth methods, nanoimprint, shadow masks, radiation damage, self-assembled structures, and the use of nanotemplates. Then the magnetic properties of these nanostructures are reviewed, including properties of single dots, magnetic interactions in arrays, dynamic effects, magnetic behavior of nanostructured lines and wires, giant magnetoresistance effect, and properties of films with arrays of holes. Finally, the physical properties in hybrid systems, where the magnetic arrays interact with superconducting and semiconducting layers, are summarized.

Structure of the single‐phase high‐temperature superconductor YBa2Cu3O7−δ

We have determined the crystal structure of the single‐phase stoichiometric high‐temperature superconductor in the Y‐Ba‐Cu‐O system using high‐resolution neutron powder diffraction. This compound has an orthorhombic structure with space group Pmmm and lattice constants a=3.8231 A, b=3.8864 A, and c=11.6807 A. The structure consists of ‘‘dimpled’’ CuO2 layers in the a‐b planes loosely bonded by one‐dimensional fencelike square‐planar CuO3 chains in the b‐c planes.

Thermalization of sputtered atoms

We have calculated the energy distributions of sputtered Nb and Cu atoms ejected from amorphous targets under low‐energy Ar bombardment. A formula based on elementary kinetic gas theory is used to calculate the subsequent energy loss of the ejected atoms due to collisions in the sputtering gas. The energy distributions of the sputtered atoms arriving at the substrate is compared with the distributions obtained using thermal evaporation techniques. This comparison indicates that the preparation of epitaxial metallic films, such as Layered Ultrathin Coherent Structures using sputtering techniques may have fundamental advantages over thermal evaporation.

Phase diagram and superconductivity in the Y‐Ba‐Cu‐O system

We have determined the phase diagram of the Y‐Ba‐Cu‐O system through structural, superconducting critical temperature and critical current density characterization. Our results show that a single‐phase compound with a stoichiometry YBa2Cu3Oy is responsible for the high‐temperature superconductivity (92.5 K) in this system.

Reliability of normal-state current-voltage characteristics as an indicator of tunnel-junction barrier quality

We demonstrate that one of the most commonly used criteria to ascertain that tunneling is the dominant conduction mechanism in magnetic tunnel junctions—fits of current‐voltage (I ‐V) data—is far from reliable. Using a superconducting electrode and measuring the differential conductance below T c , we divide samples into junctions with an integral barrier and junctions having metallic shorts through the barrier. Despite the clear difference in barrier quality, equally reasonable fits to the I ‐V data are obtained above Tc . Our results further suggest that the temperature dependence of the zero-bias resistance is a more solid criterion, which could therefore be used to rule out possible pinholes in the barrier.

LARGE EXCHANGE BIAS AND ITS CONNECTION TO INTERFACE STRUCTURE IN FEF2-FE BILAYERS

Large exchange bias effects (ΔE∼1.1 erg/cm2) were observed in antiferromagnetic (FeF2)–ferromagnetic (Fe) bilayers grown on MgO. The FeF2 grows along the spin‐compensated (110) direction. The FeF2–Fe interface roughness was characterized using specular and diffuse x‐ray diffraction and atomic force microscopy. The magnitude of the exchange bias field HE increases as the interface roughness decreases. These results imply that magnetic domain creation in the antiferromagnet plays an important role.

Electronic and magnetic properties of rare-earth ions in REBa2Cu3O7-x (RE=Dy, Ho, Er)☆

Abstract Heat capacity, resistivity, and magnetic susceptability data have been obtained for the compounds REBa2Cu3O7-x, where RE = Dy, Ho or Er. Neutron diffraction data on the Ho compound show a structure identical to that of YBa2Cu3O7-x. Magnetic transitions are observed at Tm=0.95, 0.17 and 0.59 K for Dy, Ho and Er compounds, respectively. It is argued that these are due predominantly to dipolar interactions. Resistivity data show that the magnetic state is coexistent with superconductivity in all cases. From the heat capacity data, the degeneracies of the crystal field ground states are determined, and estimates are given for the magnetic moment in the ground state and the energy separation of the first excited crystal field state.

Perpendicular coupling at Fe–FeF2 interfaces

We have studied the exchange anisotropy of ferromagnetic Fe films grown on antiferromagnetic FeF2 single crystals. The behavior of the hysteresis loops of the Fe above and below the Neel temperature TN of FeF2 indicates a 90° rotation of the ferromagnetic easy axis due to the antiferromagnetic ordering. By examining the Fe hysteresis loops together with the FeF2 susceptibility behavior we infer that below TN the ferromagnetic and antiferromagnetic spins are coupled perpendicular to each other. This behavior can be explained by recent micromagnetic calculations on exchange bias systems, or by magnetoelastic effects.

Fabrication and thermal stability of arrays of Fe nanodots

We have fabricated arrays of 60-nm-size magnetic Fe nanodots over a 1-cm2-size area using nanoporous alumina membranes as shadow masks. The size and size distribution of the nanodots correlate very well with that of the membrane pores. By placing an antiferromagnetic FeF2 layer underneath the Fe nanodots, an exchange anisotropy can be introduced into the Fe/FeF2 system. We have observed an increase in the magnetic hysteresis loop squareness in biased nanodots, suggesting that exchange bias may be used as a tunable source of anisotropy to stabilize the magnetization in such nanodots.

Magnetic superlattices and multilayers

We briefly review the active areas of current research in magnetic superlattices, emphasizing later years. With recent widening use of advanced technologies, more emphasis has been made on quantitative atomic level chemical and structural characterization. Examples where the multilayer structure has been controlled, characterized and correlated with the physical properties are discussed. The physical properties are categorized according to the complexity of a structure needed to observe a particular effect. We outline a number of general important unsolved problems, which could considerably benefit from theoretical and experimental input. An extensive list of magnetic multilayer materials is provided, with references to recent publications.