[Paper Review] Observation of Two-dimensional Spin Fluctuations in the Bilayer Ruthenate Sr3Ru2O7 by Inelastic Neutron Scattering

L. Capogna, E. M. Forgan+8 more|arXiv (Cornell University)|Jan 30, 2002

Advanced Condensed Matter Physics4 citations

TL;DR

This study reports the first observation of two-dimensional incommensurate spin fluctuations in the bilayer ruthenate Sr3Ru2O7 using inelastic neutron scattering. The fluctuations, strongest at wavevectors near Fermi surface nesting vectors and evolving from predominantly ferromagnetic at high temperatures to antiferromagnetic at low temperatures, reveal competing magnetic interactions and a low-energy scale, suggesting a link to the material's metamagnetic quantum critical behavior.

ABSTRACT

We report the first observation of two-dimensional incommensurate magnetic fluctuations in the layered metallic perovskite Sr3Ru2O7. The wavevectors where the magnetic fluctuations are strongest are different from those observed in the superconducting single layer ruthenate Sr2RuO4 and appear to be determined by Fermi surface nesting. No antiferromagnetic ordering is observed for temperatures down to 1.5K. For temperatures T<20K, the fluctuations become predominately ferromagnetic. Our inelastic neutron scattering measurements provide concrete evidence of the coexistence of competing interactions in Sr3Ru2O7 and of the low energy scale of the fluctuations.

Motivation & Objective

• To investigate the nature of magnetic correlations in the bilayer ruthenate Sr3Ru2O7, a close relative of the unconventional superconductor Sr2RuO4.
• To determine whether spin fluctuations in Sr3Ru2O7 are commensurate or incommensurate, and how they evolve with temperature.
• To probe the role of Fermi surface nesting in driving magnetic fluctuations in this correlated metal.
• To assess the interplay between competing ferromagnetic and antiferromagnetic interactions in a high-quality, stoichiometric oxide.

Proposed method

• Inelastic neutron scattering was performed on high-quality single crystals of Sr3Ru2O7 using the IN14 three-axis spectrometer at the Institut Laue-Langevin (ILL).
• Measurements were conducted at base temperature (1.5 K) and across a range of temperatures up to 150 K, with scattering in the (h,k,0) and (h,0,l) reciprocal space planes.
• The data were analyzed using Lorentzian fits to extract the energy and wavevector dependence of magnetic excitations.
• Background subtraction was applied using scattering data at wavevectors away from magnetic peaks, such as (1.48,0,0) and (0.55,0,0), to isolate magnetic contributions.
• The magnetic form factor of ruthenium was used to confirm the magnetic origin of the observed peaks.
• Data were normalized to absolute units using the intensity of a transverse acoustic phonon at 3.1 meV, with an accuracy of ~20%.

Experimental results

Research questions

• RQ1 What is the nature (commensurate or incommensurate) of the low-energy spin fluctuations in Sr3Ru2O7?
• RQ2 How do the wavevectors of the magnetic fluctuations in Sr3Ru2O7 compare to those in Sr2RuO4 and to Fermi surface nesting vectors?
• RQ3 What is the temperature evolution of the magnetic scattering, and does it indicate a crossover from ferromagnetic to antiferromagnetic character?
• RQ4 Are the observed fluctuations related to the metamagnetic quantum critical point in Sr3Ru2O7?
• RQ5 What is the role of electronic correlations and Fermi surface topology in driving the observed spin fluctuations?

Key findings

• Two incommensurate spin fluctuation peaks were observed at Q ≈ (1±0.25, 0, 0) and (1±0.09, 0, 0) at 1.5 K, with the latter being most intense and consistent with the Ru magnetic form factor.
• The magnetic fluctuations are strongest along the (h,0,0) direction, not along (h,h,0), distinguishing Sr3Ru2O7 from Sr2RuO4.
• At temperatures T ≳ 20 K, the fluctuations become predominantly ferromagnetic, with a broad peak emerging at (1,0,0), indicating a crossover in magnetic character.
• The incommensurate peaks vanish with increasing temperature, while the (1,0,0) peak grows, suggesting a transition from antiferromagnetic to ferromagnetic dominance.
• The temperature dependence of the neutron scattering response correlates with macroscopic measurements: a peak in static susceptibility and a sign change in longitudinal magnetoresistance at ~20 K.
• The data support a scenario where competing interactions—ferromagnetic and antiferromagnetic—coexist, with the low-energy scale of fluctuations pointing to proximity to a quantum critical point.