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  • 1
    Electronic Resource
    Electronic Resource
    Oxidation kinetics of thin and ultrathin cobalt films (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 5199-5204 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have prepared polycrystalline Co films on epitaxially oxidized Si(111) surfaces and studied their oxidation kinetics under atmospheric conditions using the fact that metallic cobalt is a ferromagnet but CoO is an antiferromagnet and therefore practically nonmagnetic. As a consequence, oxidation is associated with a loss in magnetism. Results show that all samples with an initial thickness of di (approximately-greater-than) 5 nm oxidize practically instantaneously, whereby a constant amount of 2.5 nm of metal is transformed into oxide. For di 〈 5 nm the time constant for oxidation increases considerably and follows an approximately linear dependence with decreasing film thickness, reaching an extrapolated value of τ=190 days for di → 0. This increasing time constant let all samples with di 〈 2.5 nm appear ferromagnetic within the course of this study due to a nonoxidized metallic rest. Auger electron spectroscopy analyses revealed that the main oxidation product is in fact CoO, especially near to the metal interface, but that other compounds are also likely to be formed summing up to a total thickness of 6 nm including adsorbates. A second point of interest was the unidirectional magnetic anisotropy which the oxide imposes to the metal due to an exchange interaction at the interface. This anisotropy shows a sharp onset at a CoO thickness as small as ∼0.25 nm from which an anisotropy constant of ∼4.5×106 J/m3 can be estimated for ultrathin CoO layers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351378
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  • 2
    Electronic Resource
    Electronic Resource
    Upper bounds for the complexity of sparse and tally descriptions (1996)
    Springer
    Theory of computing systems 29 (1996), S. 63-94 
    Details
    ISSN: 1433-0490
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science
    Notes: Abstract We investigate the complexity of computing small descriptions for sets in various reduction classes to sparse sets. For example, we show that if a setA and its complement conjunctively reduce to some sparse set, then they also are conjunctively reducible to a P(A ⊕ SAT)-printable tally set. As a consequence, the class IC[log,poly] of sets with low instance complexity is contained in theEL 1 Σ -level of the extended low hierarchy. By refining our techniques, we also show that all word-decreasing self-reducible sets in IC[log, poly] are in NP ∩ co-NP and therefore low for NP. We derive similar results for sets inR d p (SPARSE) andR hd p (R c p (SPARSE)), as well as in some nondeterministic reduction classes to sparse sets.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01201814
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  • 3
    Electronic Resource
    Electronic Resource
    Graph isomorphism is low for PP (1992)
    Springer
    Computational complexity 2 (1992), S. 301-330 
    Details
    ISSN: 1420-8954
    Keywords: graph isomorphism ; complexity classes ; lowness ; counting properties ; 68Q15 ; 68Q25 ; 05C60 ; 68R10
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science
    Notes: Abstract We show that the graph isomorphism problem, is low for PP and for C=P, i.e., it does not provide a PP or C=P computation with any additional power when used as an oracle. Furthermore, we show that graph isomorphism belongs to the class LWPP (see Fenner, Fortnow, Kurtz [12]). A similar result holds for the (apparently more difficult) problem Group Factorization. The problem of determining whether a given graph has a nontrivial automorphism, Graph Automorphism, is shown to be in SPP, and is therefore low for PP, C=P, and Mod k P,k≥2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01200427
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  • 4
    Electronic Resource
    Electronic Resource
    Effective magnetic moments of the europium-monochalcogenides (1975)
    Springer
    The European physical journal 20 (1975), S. 391-397 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have measured the paramagnetic susceptibility and the saturation magnetic moment of a number of polycrystalline samples of all four europium monochalcogenides. Many of them exhibit a negative susceptibility at 2 K and magnetic fields of 70 kOe, rather than the expected saturation behaviour. These anomalies are believed to be due to lattice defects. Reproducible saturation moments have been obtained after subtraction of this linear high field magnetization. Since all materials were nonstoichiometric to within a few percent, the ratio between the Curie-Weiss constantC and the saturation momentM 0 has been evaluated in order to eliminate the unreliably known numberN of the Eu+ + ions. The quantityR = g J (J+1) extracted in this way should have the valueR 0 = 9.00 forg = 2 andJ ≡S = 7/2. The measured values, 9.72, 9.33, 9.32 and 9.50 for EuO, EuS, EuSe and EuTe, respectively, can be correlated directly with the magnetic hyperfine fields, which are − 305, − 330, − 328 and − 315 kOe in the ferromagnetic aligned states. Both results together lead to the assumption that the magnetic moment per Eu-ion is enhanced over the8 S 7/2-value by spin contributions of outer 5d or 6s electrons.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01313209
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  • 5
    Electronic Resource
    Electronic Resource
    Optical spectra of Eu3+ ion pairs in Y2O3 (1971)
    Springer
    The European physical journal 247 (1971), S. 289-303 
    Details
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract All the optical transitions of Eu3+ ions substituted on Y3+ lattice sites of point symmetryC 2 orC 3i in Y2O3 show weak extra lines (satellites) extending over a range of few wavenumbers on both sides of the main line. It is proved by a study of the concentration dependence of the intensities of these satellites that they arise from pairs of two identical Eu3+ ions, interacting with each other when their separation is less then ∼9 Å. By means of absorption and fluorescence excitation spectra of the transitions of Eu3+ at lattice sites of point symmetryC 3i , first, second and third nearestC 3i -C 3i neighbour pairs could be identified. The mean pair interaction is of the order of 5 cm−1 and is assumed to be due to superexchange via the oxygen ions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01400931
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  • 6
    Electronic Resource
    Electronic Resource
    On the magnetic anisotropy of chromium (1985)
    Springer
    The European physical journal 61 (1985), S. 257-262 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Magnetization and susceptibility measurements have been performed on a single crystal of chromium for field orientations parallel to the symmetry directions [100], [110] and [111] of the cubic unit cell. Particular emphasis is given to the weak temperature-orientation-and field-dependent contributions to the longitudinal (field parallel) susceptibility. For the high-temperature antiferromagnetic phase with transversally polarized spin-density wave (TSDW) the magnetic easy direction is along [100] and the magnetic hard one along [110]. In the low-temperature phase with longitudinal spin density waves (LSDW) the significance of these directions has reversed. The magnetization is not strictly a linear function of the field and $$\chi = \frac{J}{{B_0 }}$$ shows a weak minimum atB 0∼1.8 T in the TSDW phase. This minimum does shift discontinuously at the spin-flip transition (T sf=132 K) to a value of ∼4.6 T in the LSDW phase. TheJ/B 0-minimum is considered as due to the field induced change from the orientationally degenerated multidomain state into a state with only polarization directions perpendicular to the magnetic field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01317792
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  • 7
    Electronic Resource
    Electronic Resource
    On the T Bloch law in magnets with fourth-order exchange interaction (1999)
    Springer
    The European physical journal 8 (1999), S. 207-216 
    Details
    ISSN: 1434-6036
    Keywords: PACS. 75.30.Et Exchange and superexchange interactions - 75.30.Kz Magnetic phase boundaries (including magnetic transitions, metamagnetism, etc.) - 75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: For the ferromagnets EuS and GdMg, in which fourth-order exchange interactions (i.e. biquadratic, three-spin and four-spin interactions) have been identified, the deviation of the spontaneous magnetization with respect to the T =0 value is shown to follow a T2 law instead of the famous T3/2 law expected for a Heisenberg ferromagnet. Moreover, the observed T2 law holds for temperatures as large as 0.8TC and the extrapolated magnetization value for does not conform to ferromagnetic saturation. This is because the fourth-order exchange interactions generate a second order-parameter which is assumed to govern the order of the transverse moment components. These moment components have a finite expectation value for at the expense of the Heisenberg order parameter. Like the spontaneous magnetization, the critical field curves B c ( T ) of the metamagnet EuSe and the antiferromagnet EuTe also start decreasing with a T2 term for . It is argued that the T2 law is a consequence of the fourth-order exchange interactions. This is shown experimentally by a study of the critical field curves [0pt] pertinent to the longitudinal (Heisenberg) order-parameter in the diamagnetically diluted antiferromagnets EuxSr1-xTe. In this solid solution series a particular composition of x c =0.85 exists at which the different fourth-order interaction processes compensate each other in the high temperature average. As a consequence, an EuxSr1-xTe sample with x =0.85 meets the requirements of a Heisenberg antiferromagnet at least if a quantity is considered for which the high-temperature average over all fourth-order interactions is decisive. This seems to be the case for the critical field curve [0pt] which gives the phase boundary to the paramagnetic phase. In fact, a crossover from a T2 to a T3/2 law is observed for [0pt] on approaching xc. This, we believe, shows the frequently observed T2 law is caused by the fourth-order interactions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100510050683
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  • 8
    Electronic Resource
    Electronic Resource
    Investigations of the sublattice magnetizations M (T) in antiferromagnets with fourth-order exchange interactions: Eu Sr Te (1999)
    Springer
    The European physical journal 8 (1999), S. 217-224 
    Details
    ISSN: 1434-6036
    Keywords: PACS. 75.30.Et Exchange and superexchange interactions - 75.30.Kz Magnetic phase boundaries (including magnetic transitions, metamagnetism, etc.) - 75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We present a neutron scattering study of the temperature and composition dependence of the MnO-type superstructure reflection intensities in the diamagnetically diluted antiferromagnetic compounds EuxSr1-xTe. In these materials antiferromagnetic biquadratic and ferromagnetic three-spin interactions have been identified recently. These fourth-order non-Heisenberg interactions are able to create their own order parameter which is believed to govern the order of the transverse moment components and which, hence, is directed perpendicular to the common Heisenberg order parameter. The observed MnO-type diffraction intensities originate in the sublattice magnetizations, , of both order parameters. Due to the different composition dependencies for biquadratic interaction processes and three-spin interaction processes , the ferromagnetic three-spin interactions dominate for x 〉 x c =0.85, while for x 〈0.85 the antiferromagnetic biquadratic interactions dominate. Associated with this sign change in the fourth-order interaction sum the transverse order parameter changes from the antiferromagnetic MnO type for x 〈0.85 to ferromagnetic for x 〉0.85. This is noticed as a sudden decrease of the low-temperature MnO scattering intensities at x c =0.85. Although susceptibility measurements reveal clearly a ferromagnetic component for x 〉0.85 no ferromagnetic Bragg intensities were observed in standard neutron scattering spectra using EuTe powder samples. We explain this by the competition of antiferromagnetic biquadratic and ferromagnetic three-spin interactions whereby a disturbed ferromagnetic superstructure may be generated which gives rise also to weak MnO-type diffraction lines. It is found that the resulting obeys a T2 law until a temperature as large as 0.75TN irrespective of the nature of the transverse order parameter. The T2 law must, hence, be common to both types of order parameter showing that the fourth-order interactions re-define the spin dynamics of both completely. From the linear composition dependence of the normalized T2 coefficient the existence of three-spin interactions is again confirmed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s100510050684
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  • 9
    Electronic Resource
    Electronic Resource
    Room temperature ferromagnetism of the Co monolayer on Cu(100) (1990)
    Springer
    The European physical journal 80 (1990), S. 1-2 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We provide experimental proof that the 1 ML Co on Cu(100) is ferromagnetically ordered at room temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01390644
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  • 10
    Electronic Resource
    Electronic Resource
    On the magnetic anisotropy of CuO (1991)
    Springer
    The European physical journal 82 (1991), S. 383-386 
    Details
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The anisotropy of the magnetic susceptibility of CuO has been measured. In both, the paramagnetic and the antiferromagnetic state the susceptibility can be described to a good approximation by $$\chi = \chi _\alpha \cdot \cos ^2 \alpha + \chi _b \cdot \cos ^2 \beta + \chi _c \cdot \cos ^2 \gamma $$ whereχ a ,χ b andχ c are the ξ-values along the crystallographica,b andc axes respectively and α, β, γ are the angles of the magnetic field, direction with respect to these axes. AtT N 2=212 K apositive jump in ξ is observed forB 0 parallel to the crystallographicb-axis and a negative jump (with increasing temperature) forB 0 parallel to thec-axis, but no effect at all is observed for fields along thea-axis. It is argued that CuO behaves like a low dimensional antiferromagnet due to strong magnetic interactions along thea-direction. ForT→O the parallel susceptibility e.g.χ b remains finite, indicating that the copper moments include appreciable orbital contributions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01357183
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