P. G. Freeman

Publications (40) View all

• Article: Stripe disorder and dynamics in the hole-doped antiferromagnetic insulator La5/3Sr1/3CoO4

  T. Lancaster, S. R. Giblin, G. Allodi, S. Bordignon, M. Mazzani, R. De Renzi, P. G. Freeman, P. J. Baker, F. L. Pratt, P. Babkevich, S. J. Blundell, A. T. Boothroyd, J. S. Moeller, D. Prabhakaran
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  ABSTRACT: We report on an investigation into the dynamics of the stripe phase of La5/3Sr1/3CoO4, a material recently shown to have an hour-glass magnetic excitation spectrum. A combination of magnetic susceptibility, muon-spin relaxation and nuclear magnetic resonance measurements strongly suggest that the physics is determined by a disordered configuration of charge and spin stripes whose frustrated magnetic degrees of freedom are strongly dynamic at high temperature and which freeze out in a glassy manner as the temperature is lowered. Our results broadly confirm a recent theoretical prediction, but show that the charge quenching remains incomplete well below the charge ordering temperature and reveal, in detail, the manner in which the magnetic degrees of freedom are frozen.
  01/2013;
• Article: Development of the Magnetic Excitations of Charge-Stripe Ordered La(2-x)Sr(x)NiO(4) on Doping Towards Checkerboard Charge Order

  P. G. Freeman, S. R. Giblin, K. Hradil, R. A. Mole, D. Prabhakaran
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  ABSTRACT: The magnetic excitation spectrums of charge stripe ordered La(2-x)Sr(x)NiO(4) x = 0.45 and x = 0.4 were studied by inelastic neutron scattering. We found the magnetic excitation spectrum of x = 0.45 from the ordered Ni^2+ S = 1 spins to match that of checkerboard charge ordered La(1.5)Sr(0.5)NiO(4). The distinctive asymmetry in the magnetic excitations above 40 meV was observed for both doping levels, but an additional ferromagnetic mode was observed in x = 0.45 and not in the x = 0.4. We discuss the origin of crossover in the excitation spectrum between x = 0.45 and x = 0.4 with respect to discommensurations in the charge stripe structure.
  01/2013;
• Article: Zn-induced spin dynamics in overdoped La2−xSrxCu1−yZnyO4

  Stephen D. Wilson, Z. Yamani, Chetan Dhital, B. Freelon, P. G. Freeman, J. A. Fernandez-Baca, K. Yamada, S. Wakimoto, W. J. L. Buyers, R. J. Birgeneau
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  ABSTRACT: Spin fluctuations and the local spin susceptibility in isovalently Zn-substituted La2−xSrxCu1−yZnyO4 (x=0.25, y≈0.01) are measured via inelastic neutron-scattering techniques. As Zn2+ is substituted onto the Cu2+-sites, an anomalous enhancement of the local spin susceptibility χ′′(ω) appears due to the emergence of a commensurate antiferromagnetic excitation centered at wave vector Q =(π,π,0) that coexists with the known incommensurate spin-density wave excitations at QHK=(π±δ,π),(π,π±δ). Our results support a picture of Zn-induced antiferromagnetic (AF) fluctuations appearing through a local staggered polarization of Cu2+ spins, and the simultaneous suppression of Tc as AF fluctuations are slowed in proximity to Zn impurities suggests the continued importance of high-energy AF fluctuations at the far overdoped edge of superconductivity in the cuprates.
  Physical Review B 01/2012; 85:014507. · 3.69 Impact Factor
• Source

  Available from: Chetan Dhital

  Article: Zn-induced spin dynamics in overdoped La_ {2− x} Sr_ {x} Cu_ {1− y} Zn_ {y} O_ {4}

  Stephen D. Wilson, Z. Yamani, Chetan Dhital, B. Freelon, P. G. Freeman, J. A. Fernandez-Baca, K. Yamada, S. Wakimoto, W. J. L. Buyers, R. J. Birgeneau
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  ABSTRACT: Spin fluctuations and the local spin susceptibility in isovalently Zn-substituted La2−xSrxCu1−yZnyO4 (x=0.25, y≈0.01) are measured via inelastic neutron-scattering techniques. As Zn2+ is substituted onto the Cu2+-sites, an anomalous enhancement of the local spin susceptibility χ′′(ω) appears due to the emergence of a commensurate antiferromagnetic excitation centered at wave vector Q =(π,π,0) that coexists with the known incommensurate spin-density wave excitations at QHK=(π±δ,π),(π,π±δ). Our results support a picture of Zn-induced antiferromagnetic (AF) fluctuations appearing through a local staggered polarization of Cu2+ spins, and the simultaneous suppression of Tc as AF fluctuations are slowed in proximity to Zn impurities suggests the continued importance of high-energy AF fluctuations at the far overdoped edge of superconductivity in the cuprates.
  Phys. Rev. B. 12/2011; 85(1).
• Source

  Available from: P. G. Freeman

  Article: Evidence of Magnetic Correlations Among the Charge Stripe Electrons of Charge-Stripe Ordered La2NiO4.11

  P. G. Freeman, S M Hayden, R. A. Mole, M. Enderle, D. Prabhakaran, A. T. Boothroyd
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  ABSTRACT: The magnetic excitations of charge-stripe ordered La2NiO4.11 where investigated using polarized- and unpolarized-neutron scattering to determine the magnetic excitations of the charge stripe electrons. We observed a magnetic excitation mode consistent with the gapped quasi-one-dimensional antiferromagnetic correlations of the charge stripe electrons previously observed in La(2-x)Sr(x)NiO(4) x = 1/3 and x = 0.275.
  11/2011;