[Paper Review] Inhomogeneous magnetic phases: a LOFF-like phase in Sr3Ru2O7

A. M. Berridge, A. G. Green+2 more|ArXiv.org|Oct 12, 2008

Advanced Condensed Matter Physics1 references3 citations

TL;DR

The paper proposes that the anomalous phase in Sr3Ru2O7 is a magnetic analogue of the LOFF state, featuring spatially modulated transverse magnetization due to a van Hove singularity in the electronic density of states. Using a Ginzburg-Landau expansion of a Stoner model with band dispersion, it explains the observed bifurcation of the metamagnetic transition, enhanced and anisotropic resistivity, and the phase diagram's 'roof' structure, providing a unified explanation for experimental anomalies in clean samples.

ABSTRACT

The phase diagram of Sr3Ru2O7 contains a metamagnetic transition that bifurcates to enclose an anomalous phase with intriguing properties - a large resistivity with anisotropy that breaks the crystal-lattice symmetry. We propose that this is a magnetic analogue of the spatially inhomogeneous superconducting Fulde-Ferrel-Larkin-Ovchinnikov state. We show - through a Ginzburg-Landau expansion where the magnetisation transverse to the applied field can become spatially inhomogeneous - that a Stoner model with electronic band dispersion can reproduce this phase diagram and transport behaviour.

Motivation & Objective

• To explain the anomalous phase in Sr3Ru2O7, characterized by a resistivity peak and anisotropy, as a spatially inhomogeneous magnetic state.
• To establish a magnetic analogue of the LOFF superconducting state in itinerant magnets using a Stoner model with band dispersion.
• To account for the bifurcation of the metamagnetic transition line and the emergence of a 'roof'-shaped phase region in the phase diagram.
• To link the observed anisotropic resistivity to the breaking of lattice symmetry by inhomogeneous magnetization under in-plane fields.
• To provide a microscopic mechanism sensitive to disorder, consistent with the phase's appearance only in ultra-pure samples.

Proposed method

• A Ginzburg-Landau expansion of the microscopic Hamiltonian is used to model spatial modulation of transverse magnetization in the Stoner model.
• The theory incorporates band dispersion with van Hove singularities to stabilize inhomogeneous magnetic phases analogous to the LOFF state.
• The model identifies a critical point where the stiffness to spatial modulation of transverse magnetization vanishes, triggering phase reconstruction.
• Theoretical phase diagrams are rotated into experimental coordinates using analytic mappings of R, H, and K⊥ to temperature, angle, and field strength.
• The mechanism explains resistivity anisotropy via symmetry breaking: in-plane fields align magnetic inhomogeneities, breaking lattice symmetry.
• The model distinguishes itself from alternatives by excluding Dzyaloshinskii-Moriya interactions and quantum fluctuation corrections as primary drivers.

Experimental results

Research questions

• RQ1 Can the anomalous phase in Sr3Ru2O7, marked by a resistivity peak and anisotropy, be explained by a spatially modulated magnetic state?
• RQ2 Does the bifurcation of the metamagnetic transition line in Sr3Ru2O7 arise from a phase with spatially modulated magnetization?
• RQ3 How does the in-plane magnetic field orientation induce anisotropic resistivity in the anomalous phase?
• RQ4 What is the role of van Hove singularities in stabilizing inhomogeneous magnetic order in this system?
• RQ5 Why is the anomalous phase observed only in ultra-pure samples, and how does disorder affect the spatial modulation?

Key findings

• The anomalous phase in Sr3Ru2O7 is identified as a magnetic analogue of the LOFF state, with spatially modulated transverse magnetization stabilized by van Hove singularities in the electronic density of states.
• The Ginzburg-Landau theory reproduces the experimentally observed 'roof'-shaped phase diagram, including the bifurcation of the metamagnetic transition line.
• The resistivity peak and its anisotropy arise from enhanced scattering due to spatially modulated magnetization that breaks crystal symmetry under in-plane fields.
• The phase transition is driven by a vanishing stiffness to spatial modulation of transverse magnetization, identified as a critical point in the phase diagram.
• The model explains the phase's sensitivity to disorder, as disorder smooths out the density of states peaks that stabilize the inhomogeneous state.
• The theory predicts that elastic neutron scattering should show Bragg peaks in the anomalous region, though such data are currently unavailable.