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Neutron scattering has been used to study the magnetic correlations and long wavelength spin dynamics of La1−xCaxMnO3 in the ferromagnetic regime (0 ≤ x < 1 2 ). For x = 1 3 (TC = 250K) where the magnetoresistance effects are largest the system behaves as an ideal isotropic ferromagnet at low T, with a gapless (< 0.04meV ) dispersion relation E = Dq 2 and DT =0 ≈ 170 meV-Å2 . However, an anomalous strongly-field-dependent diffusive component develops above ∼ 200K and dominates the fluctuation spectrum as T → TC. This component is not present at lower x.

Phys. Rev. B (01Aug99)We report neutron scattering measurements of spin density wave order within the superconducting state of a single crystal of predominately stage-4 La2CuO4+y with a Tc(onset) of 42 K. The low temperature elastic magnetic scattering is incommensurate with the lattice and is characterized by long-range order in the copper-oxide plane with the spin direction identical to that in the insulator. Between neighboring planes, the spins exhibit short-range correlations with a stacking arrangement reminiscent of that in the undoped antiferromagnetic insulator. The elastic magnetic peak intensity appears at the same temperature within the errors as the superconductivity, suggesting that the two phenomena are strongly correlated. These observations directly reveal the persistent influence of the antiferromagnetic order as the doping level increases from the insulator to the superconductor. In addition, our results confirm that spin density wave order for incommensurabilities near 1/8 is a robust feature of the La2CuO4-based superconductors.

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We have measured the dynamic spin correlation function for the quasi-one-dimensional 5=1 antiferromagnet NENP by inelastic magnetic neutron scattering. Long-lived excitations (a;r > 10) exist for O.STT < g < TT, and within error exhaust the total moment sum rule. The dispersion relation retains the fundamental periodicity of the spin chain, explicitly demonstrating the absence of broken translational symmetry in the ground state. The instantaneous spin correlation function follows a square-root Lorentzian for g :^ TT but deviates substantially from this form for q < O.STT.

The blocking temperature T(B) has been determined as a function of the antiferromagnetic layer thickness in the Fe3O4/CoO exchange biased system. For CoO layers thinner than 50 A, T(B) is reduced below the Néel temperature T(N) of bulk CoO (291 K), independent of crystallographic orientation or film substrate ( alpha-Al2O3, SrTiO3, and MgO). Neutron diffraction studies show that T(B) does not track the CoO ordering temperature and, hence, that this reduction in T(B) does not arise from finite-size scaling. Instead, the ordering temperature of the CoO layers is enhanced above the bulk T(N) for layer thicknesses approximately less than or equal to 100 A due to the proximity of magnetic Fe3O4 layers.

We have measured the static and dynamic spin fluctuations in a crystal of La2-v Sr v Cu04+ > in the spin-glass doping regime. The static spin correlations are well described by a simple model in which the inverse correlation length K(X,T) = S K(X,0) +K(0,T).The dynamic spin fluctuations exhibit a simple scaling behavior in co/T for temperatures 10 K4+ v series that the spin fluctuations at low energies (say co -6 meV, using ft=A:# = l as we will throughout) have comparable strengths in antiferromagnetic, spinglass, and superconducting samples [4].In this paper we report on a detailed study of the instantaneous and dynamic spin fluctuations for a crystal of Lai.96Sro.o4Cu04 which is in the intermediate spin-glass regime. These measurements are part of a more global study of the nature of the crossover from antiferromagnetic to spin-glass behavior which will be reported at a later date [5]. However, our results for the dynamics are sufficiently novel and, in our view, important that we wish to report those results separately here. We find that the instantaneous correlations are well described by a simple model in which tc(x,T) -K(X,0) + K(0,T) [5,6]. The spin-spin correlation length, ^ = K: _1 (JC,0), is -40 A and K(X,T) is temperature independent below 300 K. Studies of the dynamics in the CuC>2 plane reveal that the fluctuations are centered about the (K^JT) position (square lattice, unit lattice constant notation) with negligible weight at other high-symmetry positions such as (/r,0) and (2/r,0). Our essential new result is that we find that the integrated response around (K,K) as a function of temperature T exhibits a simple scaling behavior in co/T for temperatures 10 K;S7':<500 K and energies 4.5 meV< co< 12 meV. This scaling leads to a natural explanation of a variety of normal-state properties of the copper oxides.Before discussing the experiments, we note that the transport in our La2-v Sr Y Cu04+ > single crystals has been extensively investigated by Preyer et al. [7]. In the doping regime 0.02