[Paper Review] Quantum Oscillations and Magnetic Reconstruction in the Delafossite PdCrO$_2$

Clifford W. Hicks, Alexandra S. Gibbs+8 more|arXiv (Cornell University)|Apr 30, 2015

Copper-based nanomaterials and applications4 citations

TL;DR

This study reports quantum oscillation measurements in the metallic delafossite PdCrO2, demonstrating that its Fermi surface reconstructs into a six-site magnetic unit cell with √3×√3 in-plane and ×2 interplane periodicity. The results confirm that the observed frequencies match a reconstructed Fermi surface from nonmagnetic PdCoO2, indicating strong k_z-dependent magnetic coupling and a magnetic reconstruction driven by spin order, with evidence of a spin-flop transition under in-plane fields.

ABSTRACT

We report quantum oscillation data on the metallic triangular antiferromagnet PdCrO$_2$. We find that, to very high accuracy, the observed frequencies of PdCrO$_2$ can be reproduced by reconstruction of the (nonmagnetic) PdCoO$_2$ Fermi surface into a reduced zone. The reduced zone corresponds to a magnetic cell containing six chromium sites, giving a $\\sqrt{3} \ imes \\sqrt{3}$ in-plane reconstruction, and $\ imes 2$ interplane reconstruction. The interplane ordering represents a reduction in lattice symmetry, possibly to monoclinic, and an associated lattice distortion is expected. In addition, we report a magnetic transition under an applied in-plane field that is probably equivalent to the spin-flop transition reported for CuCrO$_2$, and present data on its field-angle dependence. We also report measurements of the resistivity of PdCrO$_2$ up to 500 K.

Motivation & Objective

• To investigate the electronic structure of the metallic delafossite PdCrO2, a system with conducting Pd layers and Mott-insulating CrO2 layers.
• To determine whether the observed quantum oscillation frequencies arise from a reconstructed Fermi surface due to magnetic order.
• To probe the nature of magnetic ordering in PdCrO2, particularly the symmetry reduction and potential lattice distortion associated with the magnetic cell.
• To examine the field-angle dependence of the spin-flop transition and its implications for spin anisotropy.
• To compare the electronic response of PdCrO2 with the nonmagnetic reference compound PdCoO2 to isolate magnetic contributions.

Proposed method

• Quantum oscillation measurements were performed on single-crystalline PdCrO2 at low temperatures and high magnetic fields to extract Fermi surface frequencies.
• The observed oscillation frequencies were compared with the reconstructed Fermi surface derived from the nonmagnetic PdCoO2 structure, using a reduced magnetic Brillouin zone.
• Angle-resolved measurements of the spin-flop transition were conducted to determine the field-angle dependence of the critical field.
• Resistivity measurements up to 500 K were performed to analyze the magnetic contribution to resistivity and its temperature evolution.
• The analysis focused on identifying k_z-dependent magnetic scattering vectors and symmetry breaking associated with the magnetic unit cell.
• Theoretical interpretation was based on symmetry considerations and comparison with known systems like CuCrO2 and AgCrO2, without relying on density functional theory.

Experimental results

Research questions

• RQ1 Do the quantum oscillation frequencies in PdCrO2 arise from a reconstructed Fermi surface due to magnetic order, and if so, what is the nature of the magnetic unit cell?
• RQ2 What is the symmetry of the magnetic reconstruction in PdCrO2, and does it involve a lattice distortion that breaks the original R̄3m symmetry?
• RQ3 How does the magnetic coupling in PdCrO2 depend on the wavevector component k_z, and what does this imply about the magnetic interaction range?
• RQ4 What is the origin of the observed spin-flop transition, and how does its critical field vary with the orientation of the in-plane magnetic field?
• RQ5 To what extent do the magnetic correlations in PdCrO2 persist above the Néel temperature, and how does this affect the resistivity?

Key findings

• The observed quantum oscillation frequencies in PdCrO2 are reproduced with high accuracy by reconstructing the nonmagnetic PdCoO2 Fermi surface into a six-site magnetic unit cell with √3×√3 in-plane and ×2 interplane periodicity.
• The magnetic reconstruction implies a reduction in lattice symmetry, possibly to monoclinic, with an associated small lattice distortion expected at T_N = 37.5 K, though not detected by neutron or X-ray diffraction.
• The magnetic coupling is k_z-dependent, being significantly weaker at k_z = ±π than at k_z = 0, indicating anisotropic spin interactions along the c-axis.
• A spin-flop transition is observed under in-plane magnetic fields, with a critical field that varies smoothly from ~15 T at θ ≈ +55° to nearly zero at θ ≈ -55°, suggesting adiabatic connection between high- and low-field states.
• The magnetic resistivity ρ_m in PdCrO2 continues to increase well above T_N, indicating persistent spin correlations up to at least 500 K, consistent with a Weiss temperature of ~-500 K.
• The resistivity of PdCrO2 is linear from ~200 K to 500 K, contrasting with the nonmagnetic PdCoO2, suggesting that magnetic correlations obscure or modify phonon contributions to resistivity.