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  • Magnetism  (3)
  • 75.40.−S  (2)
  • 1
    ISSN: 1434-6036
    Keywords: 75.10.Jm ; 75.25.tZ ; 75.40.−S ; 75.30.Ds
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The static and dynamic spin fluctuations in the spinS=1, two-dimensional (2D) square-lattice antiferromagnet La2NiO4 have been studied over a wide temperature range using neutron scattering techniques. The spin correlations in La2NiO4 exhibit a crossover from two- to three-dimensional (3D) behavior as the Néel temperature is approached from above. Critical slowing down of the low-energy spin fluctuations is also observed just aboveT N . The correlation length, ξ(T), and the static structure factor,S(0), have been measured and are compared with recent theoretical calculations for the quantum 2D Heisenberg antiferromagnet using microscopic parameters determined from previous spin-wave measurements. Good agreement for ξ(T) is found with the exact low-temperature result of Hasenfratz and Niedermeyer provided that 2π p s is renormalized by ≈20% from the spin-wave value.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1434-6036
    Keywords: 75.10.Jm ; 75.25.+Z ; 75.40.−S
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have carried out a neutron scattering investigation of the static structure factorS(q 2D ) (q 2D is the in-plane wave vector) in the two-dimensional spinS=1/2 square-lattice Heisenberg antiferromagnet Sr2CuO2Cl2. For the spin correlation length ξ we find quantitative agreement with Monte Carlo results over a wide range of temperature. The combined Sr2CuO2Cl2-Monte Carlo data, which cover the length scale from ≈1 to 200 lattice constants, are predicted without adjustable parameteres by renormalized classical theory for the quantum nonlinear sigma model. For the structure factor peakS(0), on the other hand, we findS(0)∼ξ 2 for the reduced temperature range 0.16〈T/2πρ s 〈0.36, whereas current theories predict that at low temperaturesS(0)∼T 2 ξ 2. This discrepancy has important implications for the interpretation of many derivative quantities such as NMR relaxation rates. In the ordered phase, we have measured the temperature dependence of the out-of-plane spin-wave gap. Its low-temperature value of 5.0 meV corresponds to an XY anisotropyJ XY /J=1.4×10−4. From measurements of the sublattice mangetization we obtain β=0.22±0.01 for the order parameter exponent. This may either reflect tricricality as in La2CuO4, or it may indicate finite-size two-dimensional XY behavior as suggested by Bramwell and Holdsworth. As in theS=1 system K2NiF4, the gap energy in Sr2CuO2Cl2 scales linearly with the order parameter up to the Néel temperature. We also reanalyze static structure factor data for K2NiF4 using the exact low temperature result for the correlation length of Hasenfratz and Niedermayer and including the Ising anisotropy explicitly. Excellent agreement between experiment and theory is obtained for the correlation length, albeit with the spin-stiffnessρ s reduced by ≈20% from the spin-wave value. As in Sr2CuO2Cl2 we find thatS(0)∼ξ 2 for the reduced temperature range 0.22〈T/2πρ s 〈0.47.
    Type of Medium: Electronic Resource
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  • 3
    Publication Date: 2016-09-07
    Description: Author(s): L. J. Bannenberg, K. Kakurai, F. Qian, E. Lelièvre-Berna, C. D. Dewhurst, Y. Onose, Y. Endoh, Y. Tokura, and C. Pappas Small angle neutron scattering measurements on a bulk single crystal of the doped chiral magnet Fe 1 − x Co x Si with x = 0.3 reveal a pronounced effect of the magnetic history and cooling rates on the magnetic phase diagram. The extracted phase diagrams are qualitatively different for zero and field coolin… [Phys. Rev. B 94, 104406] Published Tue Sep 06, 2016
    Keywords: Magnetism
    Print ISSN: 1098-0121
    Electronic ISSN: 1095-3795
    Topics: Physics
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  • 4
    Publication Date: 2016-11-01
    Description: Author(s): L. J. Bannenberg, A. J. E. Lefering, K. Kakurai, Y. Onose, Y. Endoh, Y. Tokura, and C. Pappas We present a systematic study of the ac susceptibility of the chiral magnet Fe 1 − x Co x Si with x = 0.30 covering four orders of magnitude in frequencies from 0.1 Hz to 1 kHz, with particular emphasis to the pronounced history dependence. Characteristic relaxation times ranging from a few milliseconds to … [Phys. Rev. B 94, 134433] Published Mon Oct 31, 2016
    Keywords: Magnetism
    Print ISSN: 1098-0121
    Electronic ISSN: 1095-3795
    Topics: Physics
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  • 5
    Publication Date: 2017-04-29
    Description: Author(s): L. J. Bannenberg, K. Kakurai, P. Falus, E. Lelièvre-Berna, R. Dalgliesh, C. D. Dewhurst, F. Qian, Y. Onose, Y. Endoh, Y. Tokura, and C. Pappas Chiral magnetism plays an increasingly important role in condensed matter investigations driven by the discovery of exotic spin textures, such as the topologically protected chiral skyrmions that can form a lattice under magnetic fields. The nature and universality of the phase diagram in cubic chiral magnets has been the subject of numerous experimental and theoretical investigations, including the transition to the helimagnetic state. In MnSi, the archetypal system in this family, this transition is of first order and involves a precursor phase, where strong chiral fluctuating correlations build up. This work presents an experimental investigation of the structural and dynamical aspects of the phase transition in Fe 1 − x Co x Si, a system that belongs to the same family as MnSi but with the additional possibility to tune important physical interactions and parameters through variation of the Fe and Co concentration. In this system, the combination of small-angle neutron scattering and neutron spin echo spectroscopy uncovers that the scenario of the transition is qualitatively very different from that in MnSi. This goes beyond what can be expected from a comparison of the relevant length scales and thus challenges the validity of a universal approach to the helimagnetic transition in cubic chiral magnets. [Phys. Rev. B 95, 144433] Published Fri Apr 28, 2017
    Keywords: Magnetism
    Print ISSN: 1098-0121
    Electronic ISSN: 1095-3795
    Topics: Physics
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