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  • 1
    Digitale Medien
    Digitale Medien
    Magnetic properties of epitaxial ferrite multilayer films : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 5923-5925 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We have fabricated and studied the structure and magnetic properties of high quality single crystalline (Mn,Zn)Fe2O4, NiFe2O4, and CoFe2O4 films. Although (Mn,Zn)Fe2O4 and NiFe2O4 films grown directly on SrTiO3 and MgAl2O4 show mediocre structural and magnetic properties, these same films grown on SrTiO3 and MgAl2O4 buffered with CoCr2O4 or NiMn2O4 exhibit excellent crystallinity and bulk saturation magnetization values, thus indicating the importance of lattice match and structural similarity between the film and the underlying layer. X ray, Rutherford backscattering spectroscopy, atomic force microscopy, and transmission electron microscopy analysis provide a consistent picture of the structural properties of these ferrite films. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.361844
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  • 2
    Digitale Medien
    Digitale Medien
    Temperature dependent exchange anisotropy in single-crystal ferrite thin films (abstract) : The 41st annual conference on magnetism and magnetic materials (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 4989-4989 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Single-crystal bilayers of (magnetically soft) (Mn,Zn)Fe2O4 and (magnetically hard) CoFe2O4 can exhibit nearly ideal exchange anisotropy. This is directly related to the extremely high structural quality that can be achieved by epitaxial growth on appropriate substrates. For example, we use (110)-oriented MgAl2O4 substrates with 100 nm CoCr2O4 buffer layers. The paramagnetic buffer layer provides a close lattice match for the ferrites, and is annealed at about 1000 °C to provide excellent crystallinity. Subsequent growth of the ferrite bilayer is executed at 400 °C, which is high enough to permit growth of highly crystalline epitaxial films, yet low enough to avoid substantial intermixing at the interfaces. Measurements at elevated temperatures (290 K〈T〈1000 K) permit characterization of the exchange anisotropy in regimes of low and moderate crystal anisotropy in the soft layer, as well as directly revealing the blocking temperature and the portion of the overall magnetization contributed by each layer. The results provide additional evidence that the exchange coupling energy across the interface is comparable to that within each layer, and in that sense is nearly ideal. The detailed shapes of the M–H loops are found to be in semiquantitative agreement with micromagnetic theory. ©1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.364962
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  • 3
    Digitale Medien
    Digitale Medien
    Interlayer exchange coupling in amorphous/crystalline NiFe2O4 thin-film bilayers : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 5926-5928 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Amorphous/crystalline bilayers of NiFe2O4 exhibit interlayer magnetic exchange coupling, which results from an interaction between a spin-glass material and a ferrimagnetic material. The observed effect is reminiscent of the well-known exchange coupling effect between an antiferromagnet and a ferromagnet, which is widely used in applications where field biasing of thin magnetic films is desirable. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.362173
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  • 4
    Digitale Medien
    Digitale Medien
    Deposition and properties of NiFe2O4 thin films (abstract) : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 6124-6124 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We have investigated the properties of NiFe2O4 thin films prepared by laser ablation of a stoichiometric NiFe2O4 target. Textured polycrystalline films were obtained on a-SiO2 as well as on various substrates with Au, Ag, Pt, and MgF2 buffer layers. Epitaxially oriented films were obtained on MgO, (11¯02)-oriented Al2O3, (112¯0)-oriented Al2O3, Y-stabilized ZrO2 (YSZ), and SrTiO3, although the crystalline quality of the films varied. Contamination by diffusion from the substrate and strains induced by both lattice constant mismatch and differential thermal expansion degraded the magnetic properties of the films, and in some cases decreased the electrical resistivity as well. By choosing the right substrate (YSZ), temperature (600 °C), and PO2 (0.01 mT), we are able to prepare epitaxial films with bulk saturation magnetization (Ms=270 G) and fairly low anisotropy (K∼105 erg/cm3) as inferred by torque magnetometry. These films and bilayers are expected to be useful in a variety of fundamental investigations as well as having the potential for technological applications.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.355481
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  • 5
    Digitale Medien
    Digitale Medien
    Anomalous composition dependence of anisotropy in amorphous Tb-Fe and Ho-Fe (abstract) : 35th annual conference on magnetism and magnetic materials (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 5466-5466 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The magnetic anisotropy Ku of magnetron cosputtered amorphous Tb-Fe has been found to drop precipitously when the composition is within several at. % of 22 at. % Tb, a composition which is the room-temperature compensation point. The samples showing this low value for Ku are magnetically saturated (fields up to 100 kOe were used) and torque curves have the expected uniaxial behavior. The unexpected decrease is correlated with a previous finding of anomalous, nonuniaxial anisotropy for films prepared with less than 22 at. % Tb when nonperpendicular incident angles are used for the deposition. We hypothesized that an undetected phase separation was occurring in these Fe-rich samples. The present study examines films prepared on rotating substrates, essentially eliminating the effect of incident angles. As previously predicted, with no incident angles the phase separation is unobservable by magnetic anisotropy, but presumably is still present and may account for the observed drop in Ku. Analogous results have been seen in amorphous Ho-Fe; the relevant composition is far from magnetic compensation, supporting the conclusion that the coincidence of compensation with the anomalies in Tb-Fe is accidental. This behavior of the anisotropy is inconsistent with simple models for the atomic structure and the source of Ku. The effect of deposition temperature will also be discussed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.348011
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  • 6
    Digitale Medien
    Digitale Medien
    Easy-axis transition in SmMn2Ge2 (abstract) : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 6603-6603 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: SmMn2Ge2 is a layered ferromagnet with unusual magnetic and electrical properties. In some regards it is a natural analog to artifically layered thin film materials, and its study can provide valuable insights. SmMn2Ge2 is ferromagnetic from 0 to 90 K, with easy axes parallel to the (110) directions (FM1 state). Between 90 and 150 K it is antiferromagnetic, and from 150 K to Tc (340 K) it is again ferromagnetic, with an easy axis parallel to the (001) direction (FM2 state). We have used a continuous recording torque magnetometer to monitor the energy surface of SmMn2Ge2 as a function of temperature. We have found that the moments are parallel to (110) in the AFM state, and that the anisotropy constant varies continuously, i.e., that K→0 at the easy-axis transition, which is found to be at essentially the same temperature as the zero-field AFM/FM2 transition. The FM1/AFM and AFM/FM2 transition temperatures can be varied over a wide range by substitution on the various sites. Substitution of Y or Gd on the Sm site increases TAFM/FM2 strongly, and is found to increase the temperature of the easy axis transition, but not by precisely the same factor. Substitution of a small amount of Cr on the Mn site decreases TAFM/FM2 but appears to have no effect on the easy-axis transition. M-H curves and M-T curves taken in a vibrating sample magnetometer and in a SQUID magnetometer provide corroborating evidence for these conclusions.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.356910
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  • 7
    Digitale Medien
    Digitale Medien
    In situ formation of YBa2Cu3Ox thin films by physical sputtering (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 1163-1165 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We have found that in situ superconducting Y-Ba-Cu-O thin films can be produced by sputtering without the aid of any external O2 source during deposition. The films were produced by rf magnetron sputtering from a composite target onto MgO(100) substrates heated at 650 °C. Films made in a pure Ar pressure of 24 mTorr and fast cooled in N2 ambient contain the tetragonal YBa2Cu3Ox phase with x estimated to be 6.1. Films made under the same sputtering conditions but cooled in 30 mTorr oxygen after deposition, show a superconducting transition Tc(R=0) at 71 K with an onset at 85 K. The Tc(R=0) rises to 80 K if 0.5 mTorr oxygen was added during sputtering. The experimental results provide conclusive evidence that sufficient oxygen can be incorporated into the films to form the tetragonal YBa2Cu3Ox phase simply by physical sputtering from an oxide target. The films absorb additional oxygen and transform into the orthogonal, superconducting phase when subsequently cooled in partial O2.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.101560
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  • 8
    Digitale Medien
    Digitale Medien
    Critical currents in proton-irradiated single-crystal Ba2YCu3O7−δ (1990)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 56 (1990), S. 2681-2683 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Irradiation with 3.5 MeV protons creates defects in Ba2YCu3O7−δ, which can act as strong pinning sites, comparable to those produced by neutron irradiation. Indeed protons as well as neutrons undergo momentum-transferring collisions that result in atomic displacements; the additional electronic proton-solid interaction does not appear to yield defects which contribute to pinning. Under optimized conditions we have obtained Jc(T→0, H=1 T)∼2×107 A/cm2 in single crystals. At 77 K the value is still Jc∼1.7×105 A/cm2, decreasing only threefold in a field of 6 T. The fluence and temperature dependence of Jc suggest a model, in which spatially uncorrelated defects pin vortex cores.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.103269
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  • 9
    Digitale Medien
    Digitale Medien
    Sharp angular sensitivity of pinning due to twin boundaries in Ba2YCu3O7 (1990)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 56 (1990), S. 2465-2467 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We have measured the pinning force due to twin boundaries by measuring the magnetic torque produced when a field is applied perpendicular to the c axis and rotated with respect to the ab axes. At 76 K, the torque increases roughly fourfold, when the field is within 2° of the parallel to the twin boundaries. This peak is absent at 27 K. Irradiation with 5×1016 cm−2 3.5 MeV protons increases the overall critical current and eliminates the peaks observed at 76 K.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.103257
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  • 10
    Digitale Medien
    Digitale Medien
    Cu valence and the formation of high Tc superconductor oxides studied by x-ray photoemission spectroscopy on 200 A(ring) Bi-Sr-Ca-Cu oxide thin films (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 377-379 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We have used x-ray photoemission spectroscopy to study the core level spectra of 200 A(ring) Bi-Sr-Ca-Cu oxide thin films, in the hope that the importance of the surface sensitivity of photoemission is minimized for these samples, whose thickness is only 6–7 unit cells. The samples were annealed in oxygen at temperatures ranging from 600 to 870 °C, over which temperature range the thin films are converted from an amorphous insulating phase to an ordered superconducting phase with Tc (R=0)=80 K. Regardless of the annealing temperature, the Cu 2p spectra of all films show satellite structure indicative of Cu2+ states. Even the spectrum of an as-deposited film has a satellite similar to those observed for the annealed films, including the superconducting film. The similarity of the satellite structure in the core level spectra for all films suggests that the Cu-O interactions, such as the Cu d-O p orbital charge transfer energies and the hopping integrals, remain similar when the oxides undergo the insulator-superconductor transition through high-temperature anneal. That is, the same local Cu-O interaction exists before and after the superconducting phase sets in. This implies that the Cu valence alone does not determine the properties of high Tc superconductors.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.100971
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