[Paper Review] A 57Fe Moessbauer Spectroscopy Study of the 7 K Superconductor LaFePO

Marcus Tegel, Inga Schellenberg+2 more|ArXiv.org|May 8, 2008

Iron-based superconductors research1 references3 citations

TL;DR

This study investigates the electronic and magnetic properties of the 7 K superconductor LaFePO using 57Fe Mössbauer spectroscopy. The results reveal Pauli paramagnetism with a small transferred magnetic hyperfine field of 1.15(1) T at 4 K, indicating weak spin polarization, and demonstrate that superconductivity emerges after removal of ferromagnetic Fe2P impurities, confirming the intrinsic nature of superconductivity in LaFePO.

ABSTRACT

A polycrystalline sample of superconducting LaFePO was prepared in a tin flux at 1123 K. The structure was determined from single crystal data (ZrCuSiAs-type, P4/nmm, a = 3.9610(1), c = 8.5158(2) A, Z = 2) and the phase analysis was performed by the Rietveld method. LaFePO is Pauli-paramagnetic and becomes superconducting at 7 K after removing the ferromagnetic impurity phase Fe2P from the sample. 57Fe Moessbauer spectroscopy measurements at 298, 77, 4.2 and 4 K show single signals at isomer shifts around 0.35 mm/s, subject to weak quadrupole splitting. At 4 K, a symmetric line broadening appears, resulting from a small transferred magnetic hyperfine field of 1.15(1) T and accompanied by an angle of 54.7(5) between Bhf and Vzz, the main component of the electric field gradient tensor.

Motivation & Objective

• To characterize the local electronic environment of 57Fe in superconducting LaFePO using Mössbauer spectroscopy.
• To identify and quantify magnetic impurities, particularly Fe2P, that may mask intrinsic superconducting behavior.
• To determine the magnetic hyperfine interaction at low temperatures to assess spin polarization and superconducting pairing mechanisms.
• To confirm the intrinsic nature of superconductivity in LaFePO by eliminating extrinsic magnetic contributions.

Proposed method

• Polycrystalline LaFePO was synthesized via a tin flux method at 1123 K to achieve high purity.
• Single-crystal X-ray diffraction was used to determine the ZrCuSiAs-type structure (P4/nmm space group) with lattice parameters a = 3.9610(1) Å and c = 8.5158(2) Å.
• Rietveld refinement of powder X-ray diffraction data confirmed phase purity and identified residual Fe2P impurities.
• 57Fe Mössbauer spectroscopy was performed at 298 K, 77 K, 4.2 K, and 4 K to probe hyperfine interactions and magnetic behavior.
• Isomer shifts and quadrupole splitting were analyzed to assess the oxidation state and electric field gradient at the Fe site.
• Temperature-dependent line broadening and hyperfine field analysis were used to detect weak magnetic ordering or spin polarization.

Experimental results

Research questions

• RQ1 What is the local electronic environment of 57Fe in LaFePO, as revealed by isomer shift and quadrupole splitting?
• RQ2 Does the presence of magnetic impurities such as Fe2P significantly affect the superconducting transition temperature or electronic properties?
• RQ3 Is there evidence of a magnetic hyperfine field at 4 K, indicating spin polarization or local moments in the superconducting state?
• RQ4 What is the nature of the line broadening observed at 4 K, and does it originate from a small transferred magnetic field?
• RQ5 How does the orientation of the hyperfine field relative to the electric field gradient tensor influence the observed spectral features?

Key findings

• LaFePO exhibits a single 57Fe Mössbauer signal with an isomer shift of approximately 0.35 mm/s at all temperatures, indicating a Fe2+ oxidation state and Pauli paramagnetic behavior.
• A weak quadrupole splitting is observed, consistent with a small electric field gradient at the Fe site.
• At 4 K, a symmetric line broadening appears, indicating the presence of a small transferred magnetic hyperfine field of 1.15(1) T.
• The angle between the hyperfine field and the principal axis of the electric field gradient tensor is 54.7(5)°, suggesting a specific symmetry in the local electronic environment.
• The absence of a large magnetic hyperfine field or long-range magnetic order confirms that superconductivity in LaFePO is intrinsic and not mediated by magnetic impurities.
• After removal of the ferromagnetic Fe2P phase, LaFePO becomes superconducting at 7 K, confirming the material's intrinsic superconducting nature.