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  • 1
    Electronic Resource
    Electronic Resource
    Magnetic exchange and charge transfer in mixed-valence manganites and cuprates : The 40th annual conference on magnetism and magnetic materials (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 5172-5174 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Metallic resistivity occurs at cryogenic temperatures in insulators with small carrier trap energies, e.g., superconducting cuprates. A similar metallic regime has been reported for the lanthanide (RE) manganites (RE3+1−xA2+x)MnO3. To interpret the anomalous resistivity ρ as a function of temperature and magnetic field in these compounds, a model constructed from the relation for mobility activated semiconduction and the Brillouin–Weiss theory of ferromagnetism has been developed. The resistivity maximum occurs at the susceptibility peak slightly above the Curie temperature TC and its magnitude is related to the hopping electron trap energy Ehop by exp(Ehop/kTC). Where T〈TC, ρ is metallic because Ehop is small due to the collinear polarization of spins. For T≥TC, Ehop increases to a value ∼0.1 eV equal to the decrease in stabilization energy of the transfer electrons caused by the transition from spin alignment to disorder. The magnetoresistance sensitivity dρ/dH at T=TC is controlled by TC through (1/TC)exp(Ehop/kTC). The relative sensitivity (1/ρ)dρ/dH, however, is proportional to 1/T2C. These results also reinforce the concept that metallic resistivity in the superconducting cuprates occurs because of the frustration of antiferromagnetism. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361543
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  • 2
    Electronic Resource
    Electronic Resource
    Spectral origins of giant Faraday rotation and ellipticity in Bi-substituted magnetic garnets : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6127-6129 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electrical permittivity theory has been applied to the interpretation of data describing the anomalously high Faraday rotation and ellipticity in the Y3−xBixFe5O12 ferrimagnetic system. By the use of exact forms of the basic phenomenological equations for the off-diagonal tensor element ε1 = ε'1 + iε‘1, the measured Bi3+ influence on the separate component ε'1 and ε‘1 spectra from 1 to 5 eV has been closely fitted to theory by the superposition of three Fe3+ (diamagnetic) electric dipole transitions occurring at 2.6, 3.15, and 3.9 eV, respectively. The strong transition at 3.15 eV that is believed to originate from Fe-O6 molecular complexes of the octahedral sublattice may also have a smaller companion peak in the vicinity of 5 eV. Transition bandwidths and excited-state splittings determined from the matching of theory to experiment both indicate that strong Bi covalent interactions exist with the Fe-O6 and Fe-O4 complexes. These results also confirm that the enhanced magneto-optical effects in the longer-wavelength region (λ∼1 μm) are dominated by the tail of the ε'1 diamagnetic peak at 3.15 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352723
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  • 3
    Electronic Resource
    Electronic Resource
    Application of permittivity tensor for accurate interpretation of magneto-optical spectra : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6130-6132 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The inaccuracies of widely used approximations to the theory of the off-diagonal elements ε1= ε'1 + iε‘1 of the permittivity tensor are examined. Errors occurring in the "diamagnetic'' case can become important when curve fitting is employed to extract parameter values from Bi-substituted magnetic garnets. A discrepancy between the exact and approximate forms of the ε'1 relation (which is responsible for Faraday rotation and Kerr ellipticity at lower energies) as a function of transition energy is found in the low-energy (wavelength λ∼1 μm) tails of the lines and originates from the dropping of a nonresonant term from the complete theory. Errors also occur in the line shapes near resonance where the ratio of the excited-state splitting to the Lorentzian linewidth approaches unity (Δ/Γ→1). A family of curves with Δ/Γ as a parameter is presented, and an exact expression relating Δ/Γ to the resonance value of ε'1/ε‘0 (where ε‘0 is the imaginary part of the diagonal element ε0) is derived.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352724
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  • 4
    Electronic Resource
    Electronic Resource
    Molecular-orbital analysis of magneto-optical Bi-O-Fe hybrid excited states : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 6372-6374 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Molecular-orbital estimates of the excited-state spin-orbit coupling multiplet splitting are presented to support the theory that enhanced magneto-optical effects in Y3−xBixFe5O12 originate from covalent interactions between Bi3+ and Fe3+ ions. A self-consistent approximation is applied to a two-level bonding-antibonding hybrid formed from the excited 4P term of Fe3+ and the excited 3P of Bi3+. For term energy values chosen from spectroscopic data, overlap integrals s〈0.05 are sufficient to account for bonding-state depressions that match those reported for the tetrahedral and octahedral site-enhanced transitions at 2.6 and 3.15 eV. The hybrid eigenfunctions of the upper states can contain more than 30% of the Bi3+ 3P orbital term. Multiplet splittings that are enhanced by the large Bi3+ spin-orbit coupling constant (λ≈2 eV) are found to be on the order of 0.5 eV with a full complement of Bi surrounding Fe in the garnet lattice, in agreement with estimates based on experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355353
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetic hysteresis properties of BaFe12−xInxO19 ceramic ferrites with c-axis oriented grains : The 5th Joint MMM−Intermag Conference (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 6140-6142 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: To study the effects of reduced magnetic anisotropy on hysteresis loops of hard magnets for possible use in self-biased microwave devices, a new family of magnetically oriented (c-axis) In-substituted Ba ferrite was prepared by conventional ceramic techniques. Earlier studies of BaFe12−x(In,Sc)xO19 series in single-crystal form have shown that magnetization 4πM and anisotropy field Hk decrease sharply with increasing x. Since hysteresis loops with optimum energy products require uniformly small grains (1 to 3 μm), sintering temperatures were carefully controlled over a range from 1180 to 1075 °C for 2 h, depending on In content. For nominal values of x ranging from 0 to 1.5, square hysteresis loops with Hc values decreasing from 3500 to 150 Oe were recorded with a high-field hysteresisgraph. The effective Hk values were measured and compared with single-crystal data to estimate the degree of grain alignment for each composition.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.350022
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  • 6
    Electronic Resource
    Electronic Resource
    Magnetic frustration in high-Tc superconductors : 35th annual conference on magnetism and magnetic materials San Diego, California (USA) (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 5194-5196 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The breakdown of antiferromagnetic order and its coincidence with the onset of the superconducting state (the TN, Tc→0 condition) is explained by the itinerancy of polarons that are created by "impurity'' cations or by oxygen vacancies. In both La2−xSrxCuO4and YBa2Cu3Oy systems, the electrostatic balance is maintained in a traditional sense by the formation of Cu3+(d8) ions, here in low-spin (S=0) electron configurations because of tetragonal crystal fields at the Cu sites. Since the spins of the host Cu2+(d9) ions (S=1/2) surrounding the diamagnetic Cu3+ p-type polaron ion will undergo canting that results from the magnetic dilution effects, a dynamic region of frustration will exist within the confines of the polaron cell boundaries. As cells begin to merge through increased polaron concentration, percolation of cells will destroy magnetic order at a threshold determined by the product of cell radius and carrier concentration. Analysis of the cell radii indicate that frustration should set in at a Cu3+ concentration (x≈0.07) that is consistent with the TN, Tc→0 condition observed in both of the above superconductor families. From these concepts, a rationale emerges for a superconductor phase diagram that links the magnetic, insulating, superconducting, and normal metallic regions as functions of temperature and composition.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348096
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  • 7
    Electronic Resource
    Electronic Resource
    Spin states and electronic conduction in Ni oxides (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 4561-4563 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetic and electronic properties of the mixed-valence semiconductor LixNi2+1−2xNi3+xO are reinterpreted in terms of low-spin states for both Ni ions. Anomalous decreases in hopping electron activation energies are discussed on the basis of (i) breakdown in antiferromagnetic ordering through spin canting of the Ni sublattices through exchange isolation caused by diamagnetic Li1+ ions that group with the low-spin Ni3+ (S= (1)/(2) ) to form polarons, and (ii) enhanced disruption of magnetic superexchange that results from a combination of Li1+ dilutants and S=0 states of surrounding Ni2+ ions induced at low temperatures by static Jahn-Teller tetragonal distortions of the oxygen octahedra around the Ni3+ polarons. Reported magnetic ordering and conduction anomalies in La2−xSrxNiO4 are then compared to the behavior of Cu in LixCu1−xO, and in the high-Tc superconducting La2−xSrxCuO4 system. Spontaneous conduction through molecular-orbital states involving zero-spin Ni and Cu ions is discussed, together with the role of S=0 polarons in other oxide superconductors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.344870
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  • 8
    Electronic Resource
    Electronic Resource
    Magnetic design for low-field tunability of microwave ferrite resonators (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 4856-4858 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An advanced microstrip resonator design consisting of a ferrite substrate formed in a closed magnetic path (toroidal structure) is shown to produce substantially improved tunability over previous versions that operated in an external magnetic field. Magnetically tunable toroidal resonators with tunability approaching 10% in magnetic fields of 60 Oe have been demonstrated using conventional ferrite compositions. Calculations with a hysteresis-loop theory developed specifically for modeling the low-field magnetization of ferrites were successfully fitted to resonator frequencies measured as a function of magnetic field. Performance predictions for resonators with ferrites optimized for low-field magnetization properties suggest that tunability approaching 15% is possible at magnetic fields of less than 5 Oe. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370044
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  • 9
    Electronic Resource
    Electronic Resource
    Temperature dependence of spin-lattice relaxation in rare-earth iron garnets (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 4963-4965 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The spin-lattice relaxation rate in iron garnets containing ions of the rare-earth series features a peak in the temperature range from 30 to 100 K that is consistent throughout the group. This effect is important in microwave applications at liquid nitrogen temperatures because magnetic losses increase directly with the intrinsic ferrimagnetic resonance linewidth ΔH, which in turn is proportional to the relaxation rate τ−1. The decrease in X-band linewidth with lowering temperatures is accounted for by the Orbach exponential relation for τ−1 of individual paramagnetic rare-earth ions. To explain the decrease in ΔH above the peak, the Orbach function is multiplied by the magnetic exchange energy between the iron and rare-earth spins. Computations of the linewidth as a function of temperature for Dy3+, Ho3+, Er3+, and Yb3+ are made by utilizing the multiple sublattice molecular-field theory. By fitting theory to data, values are extracted for excited state energy splitting D that enters the exponent (−D/kT) of the Orbach function. Measurement data from high purity yttrium iron garnet also confirm that iron relaxation rates decrease monotonically with reducing temperatures, indicating that microwave magnetic loss at cryogenic temperatures can be reduced below room temperature values if offending rare-earth impurities are removed. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.373216
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  • 10
    Electronic Resource
    Electronic Resource
    Origin of exchange field reductions in diluted magnetic garnets (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 4627-4629 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: New theory on the source of magnetic frustration that causes canting of spin alignments in diluted ferrimagnetic systems is presented, providing a fundamental basis for the widely used semiempirical relations between molecular field coefficients and sublattice dilution fractions in the ferrimagnetic garnets. The theory explains the remarkable linearity and symmetry found in the original relations between the coefficients and the sublattice dilution fractions. It also distinguishes between the dilution and canting contributions to the weakening of the exchange fields, which has remained an unresolved issue. As part of the analysis, a probability is defined for each sublattice to characterize the canting effect on a particular spin caused by the removal of a neighboring magnetic ion from the opposing sublattice. The canting probabilities are found to be almost identically 0.07 for both the octahedral and tetrahedral garnet Fe3+ sublattices, thereby confirming the overall reciprocity of the garnet magnetic system. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370429
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