arXiv subjects

P. G. Freeman

Publications and source records attributed to P. G. Freeman.

31 records · Page 2Linked to original sources

arXiv2010

Ferromagnetic Excitations in La$_{0.82}$Sr$_{0.18}$CoO$_{3}$ Observed Using Neutron Inelastic Scattering

Polarized neutron inelastic scattering has been used to measure spin excitations in ferromagnetic La$_{0.82}$Sr$_{0.18}$CoO$_{3}$. The magnon spectrum of these spin excitations is well defined at low energies but becomes heavily damped at higher energies, and can be modeled using a quadratic dispersion. We determined a spin wave stiffness constant of $D=94\pm 3$\,meV\,Å$^{2}$. Assuming a nearest-neighbor Heisenberg model we find reasonable agreement between the exchange determined from D and the bulk Curie temperature. Several possible mechanisms to account for the observed spin-wave damping are discussed.

cond-mat.str-el↗

arXiv2009

Inward dispersion of the spin excitation spectrum of stripe-ordered La2NiO4+d

Polarized- and unpolarized-neutron scattering measurements of the spin excitation spectrum in the stripe-ordered phase of La2NiO4+d (d = 0.11) are presented. At low energies, the magnetic spectral weight is found to shift anomalously towards the two-dimensional antiferromagnetic wave vector, similar to the low-energy dispersions observed in cuprate superconductors. While spin-wave spectra in stripe phases can exhibit an apparent inward dispersion, we find that the peak shifts measured here cannot be accounted for by this effect. Possible extensions of the model are discussed.

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arXiv2009

Magnetic order and dynamics of the charge-ordered antiferromagnet La1.5Sr0.5CoO4

We describe neutron scattering experiments performed to investigate the magnetic order and dynamics of half-doped La1.5Sr0.5CoO4. This layered perovskite exhibits a near-ideal checkerboard pattern of Co2+/Co3+ charge order at temperatures below ~ 800 K. Magnetic correlations are observed at temperatures below ~ 60 K but static magnetic order only becomes established at 31 K, a temperature at which a kink is observed in the susceptibility. On warming above 31 K we observed a change in the magnetic correlations which we attribute either to a spin canting or to a change in the proportion of inequivalent magnetic domains. The magnetic excitation spectrum is dominated by an intense band extending above a gap of approximately 3 meV up to a maximum energy of 16 meV. A weaker band exists in the energy range 20-30 meV. We show that the excitation spectrum is in excellent quantitative agreement with the predictions of a spin-wave theory generalized to include the full magnetic degrees of freedom of high-spin Co2+ ions in an axially distorted crystal field, coupled by Heisenberg exchange interactions. The magnetic order is found to be stabilized by dominant antiferromagnetic Co2+ -- Co2+ interactions acting in a straight line through Co3+. No evidence is found for magnetic scattering from the Co3+ ions, supporting the view that Co3+ is in the S = 0 state in this material.

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arXiv2009

The Temperature Evolution of the Out-of-Plane Correlation Lengths of Charge-Stripe Ordered La(1.725)Sr(0.275)NiO(4)

The temperature dependence of the magnetic order of stripe-ordered La(1.725)Sr(0.275)NiO(4) is investigated by neutron diffraction. Upon cooling, the widths if the magnetic Bragg peaks are observed to broaden. The degree of broadening is found to be very different for l = odd-integer and l = even-integer magnetic peaks. We argue that the observed behaviour is a result of competition between magnetic and charge order.

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arXiv2005

Magnetization of La(2-x)Sr(x)NiO(4+ delta) (0 < x < 0.5) and observation of novel memory effects

We have studied the magnetization of a series of spin-charge ordered La(2-x)Sr(x)NiO(4+delta) single crystals with 0 < x < 0.5. For fields applied parallel to the ab plane there is a large irreversibility below a temperature T(F1) ~ 50 K and a smaller irreversibility that persists up to near the charge ordering temperature. We observed a novel memory effect in the thermo-remnant magnetization across the entire doping range. We found that these materials retain a memory of the temperature at which an external field was removed, and that there is a pronounced increase in the thermo-remnant magnetization when the system is warmed through a spin reorientation transition.

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arXiv2004

Spin Dynamics of Half-Doped La3/2Sr1/2NiO4

We report polarized- and unpolarized- neutron inelastic scattering measurements of the magnetic excitation spectrum in the spin-charge ordered phase of La3/2Sr1/2NiO4. Up to energies of ~30 meV we observe broad magnetic modes characteristic of a near checkerboard ordering. A linear spin-wave model for an ideal checkerboard ordering with a single antiferromagnetic exchange interaction J' = 5.8 +/- 0.5 meV between next-nearest-neighbour spins on Ni2+ sites, together with a small XY-like single-ion anisotropy, provides a reasonable description of the measured dispersion. Above 30 meV the excitations are not fully consistent with the linear spin-wave model, with modes near the two-dimensional reciprocal space wavevector (0.5,0.5) having an anomalously large intensity. Furthermore, two additional dispersive modes not predicted by spin wave theory were observed, both of which are probably magnetic. One disperses away from (0.5,0.5) in the energy range between 50-56 meV, and the other appears around (h,k) type positions (h,k = integer) in the energy range 31-39 meV. We propose a model in which these anomalous features are explained by the existence of discommensurations in the checkerboard ordering. At low energies there is additional diffuse scattering centred on the magnetic ordering wavevector. We associate this diffuse scattering with dynamic antiferromagnetic correlations between spins attached to the doped holes.

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arXiv2004

Stripe Order and Magnetic Transitions in La(2-x)Sr(x)NiO(4)

Magnetic order has been investigated in stripe-ordered La(2-x)Sr(x)NiO(4) (x = 0.275, 0.37, 0.4) by d.c. magnetization and by polarized- and unpolarized-neutron diffraction. In the magnetically ordered phase, all three compositions exhibit a magnetic transition consistent with a spin reorientation in the ab plane. For x = 0.37, the spin axes rotate from an angle of 37.7 +/- 0.3 degrees to the stripe direction at 71 K, to 52.3 +/- 0.2 degrees at 2 K. The x = 0.275 and 0.4 compounds were found to undergo a similar spin reorientation. A spin reorientation has now been observed to occur for five different doping levels in the range x = 0.275 - 0.5, suggesting that this spin transition is an intrinsic property of the stripe phase.

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arXiv2003

Magnetization of Charge-ordered la(2-x)sr(x)nio(4+delta)

We report magnetization measurements on La(2-x)Sr(x)NiO(4+ delta) single crystals, with 0 < x < 0.5. Glassy behaviour associated with the formation of spin-charge stripes, and a separate spin-glass phase at low temperatures were observed. We have also found a `memory effect' in the magnetic field -- temperature history, which is found to be suppressed in the low temperature spin state of the x = 0.33 crystal.

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arXiv2003

Spin correlations among the charge carriers in an ordered stripe phase

We have observed a diffuse component to the low-energy magnetic excitation spectrum of stripe-ordered La5/3Sr1/3NiO4 probed by neutron inelastic scattering. The diffuse scattering forms a square pattern with sides parallel and perpendicular to the stripe directions. The signal is dispersive, with a maximum energy of ~10 meV. Probed at 2 meV the scattering decreases in strength with increasing temperature, and is barely visible at 100 K. We argue that the signal originates from dynamic, quasi- one-dimensional, antiferromagnetic correlations among the stripe electrons.

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arXiv2003

Spin excitations in stripe-ordered La2-xSrxNiO4 (x=0.275 and 1/3)

We report neutron scattering measurements of the spectrum of magnetic excitations in the stripe-ordered phase of La2-xSrxNiO4 (x=0.275 and 1/3). The propagating spin excitations follow a similar dispersion relation for the two compositions, but the line widths are broader for x=0.275 than for x=1/3.

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arXiv2003

Magnetic Order and Dynamics in Stripe-Ordered La2-xSrxNiO4

We have studied magnetic correlations in several compositions of stripe-ordered La2-xSrxNiO4. In this paper we show how polarized-neutron scattering has helped uncover important features of the magnetic ordering and spin dynamics. In particular, polarization analysis has enabled us (1) to characterize a spin reorientation transition, (2) to identify anisotropy gaps in the spin excitation spectrum, and (3) to investigate an anomalous dip in the spin-wave intensity suggestive of coupling between collective spin and charge excitations.

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arXiv2002

Spin reorientation transition in the incommensurate stripe-ordered phase of La3/2Sr1/2NiO4

The spin ordering of La3/2Sr1/2NiO4 was investigated by magnetization measurements, and by unpolarized- and polarized-neutron diffraction. Spin ordering with an incommensurability epsilon ~ 0.445 is observed below T_so ~ 80 K. On cooling, a spin reorientation is observed at 57 +/- 1 K, with the spin axes rotating from 52 +/- 4 degrees to 78 +/- 3 degrees. This is the first time a spin reorientation has been observed in a La2-xSrxNiO4+delta compound having incommensurate stripe order.

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arXiv2002

Spin dynamics in stripe-ordered La5/3Sr1/3NiO4

Polarized and unpolarized neutron inelastic scattering has been used to measure the spin excitations in the spin-charge-ordered stripe phase of La5/3Sr1/3NiO4. At high energies, sharp magnetic modes are observed characteristic of a static stripe lattice. The energy spectrum is described well by a linear spin wave model with intra- and inter-stripe exchange interactions between neighbouring Ni spins given by J = 15 +/- 1.5 meV and J' = 7.5 +/- 1.5 meV respectively. A pronounced broadening of the magnetic fluctuations in a band between 10 meV and 25 meV is suggestive of coupling to collective motions of the stripe domain walls.

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