Physica C: Superconductivity and its Applications
Anisotropic spin freezing in lightly-doped La2−xSrxCuO4
Introduction
The interplay between magnetism and conductivity is crucial to understand the mechanism of superconductivity in high- cuprate superconductors. The mother compound La2CuO4 shows a Néel ordering below ∼320 K. When hole carriers are doped, the magnetic ordering is quickly destroyed at . The short-ranged magnetic correlations remain even above . Extensive neutron-scattering studies have shown that the spin correlations of the short-ranged magnetic phase are incommensurate. Furthermore, the static spin modulation in La2−xSrxCuO4 changes from being diagonal to parallel at , coincident with the insulator-to-superconductor transition [1], [2]. This shows an intimate relation between magnetism and conductivity. One possibility to explain the diagonal incommensurate spin correlations is the stripe model. On the other hand, the diagonal incommensurate spin correlations can be explained using the spiral model, originating from magnetic frustration caused by the localized hole spins. In order to understand the nature of the incommensurate spin correlations in this region, it is important to clarify the magnetic anisotropy (Heisenberg, XY, or Ising) and determine the spin Hamiltonian.
Inelastic neutron scattering experiments were performed to observe the zone center gap originating from the magnetic anisotropy in the diagonal incommensurate phase [3]. Since the magnetic excitations become broadened, it is not clear whether the gap disappears or the magnetic excitations are just smeared out so that the gap is difficult to be identified.
Polarized neutron scattering is an important tool to clarify the magnetic anisotropy in the magnetic materials. Using three-dimensional (3D) polarization analysis [4], we clarified the magnetic anisotropy in La1.982Sr0.018CuO4 and determined that the XY model is the most suitable model to describe the low energy spin fluctuations.
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Experimental
The single crystal of La1.982Sr0.018CuO4 was grown by the traveling solvent floating zone (TSFZ) method. The crystal was annealed in an Ar atmosphere at 900 °C for 24 h. The magnetic properties of this crystal, measured with unpolarized neutrons, were reported in Ref. [5].
Unpolarized neutron scattering measurements were performed on the thermal triple-axis spectrometer TAS-2 at JAEA. The fixed initial neutron energy was 14.7 meV with an instrumental energy resolution of . The horizontal
Results and discussion
It was previously reported that La1.982Sr0.018CuO4 exhibits a Néel ordering and spin-glass behavior at low temperatures [5]. The spin-glass phase shows short-ranged antiferromagnetic ordering with diagonal incommensurate correlations. Since the magnetic correlation perpendicular to the CuO2 plane is finite, a modulation of scattering intensity along L direction is observed, as shown in Fig. 1. The correlation length along the c axis was estimated to be 11 Å [5], which almost corresponds to c.
Summary
We performed polarized neutron scattering experiments in lightly-doped La1.982Sr0.018CuO4. 3D neutron polarization analysis was performed on diffuse magnetic peaks to investigate the magnetic anisotropy in the spin-glass phase. It was found that the spins lie almost in the CuO2 plane although a finite out-of-plane component was observed. It was also found that there exists a small anisotropy in the CuO2 plane and the spin component is slightly larger along the b axis, suggesting the remnant of
Acknowledgements
This study was supported in part by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science and Technology.
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