[Paper Review] Itinerant ferromagnetism mediated by giant spin polarization of metallic ligand band in van der Waals magnet Fe5GeTe2

Kohei Yamagami, Yuita Fujisawa+16 more|Okinawa Institute of Science and Technology Graduate University (Okinawa Institute of Science and Technology Graduate University)|Jan 5, 2021

2D Materials and Applications4 citations

TL;DR

This study reveals that itinerant ferromagnetism in the van der Waals magnet Fe5GeTe2 is mediated by giant spin polarization of delocalized Te p bands, not solely by Fe 3d electrons. Using soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES) and element-specific x-ray magnetic circular dichroism (XMCD), the authors demonstrate that the ligand Te states exhibit the strongest XMCD signal ever reported for a non-magnetic element, proving their critical role in stabilizing long-range ferromagnetic order through spin polarization of metallic ligand bands.

ABSTRACT

We investigate near-Fermi-energy (EF) element-specific electronic and spin states of ferromagnetic van der Waals (vdW) metal Fe5GeTe2. The soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurement provides spectroscopic evidence of localized Fe 3d band. We also find prominent hybridization between the localized Fe 3d band and the delocalized Ge/Te p bands. This picture is strongly supported from direct observation of the remarkable spin polarization of the ligand p bands near EF, using x-ray magnetic circular dichroism (XMCD) measurements. The strength of XMCD signal from ligand element Te shows the highest value, as far as we recognize, among literature reporting finite XMCD signal for none-magnetic element in any systems. Combining SX-ARPES and elemental selective XMCD measurements, we collectively point an important role of giant spin polarization of the delocalized ligand Te states for realizing itinerant long-range ferromagnetism in Fe5GeTe2. Our finding provides a fundamental elemental selective view-point for understanding mechanism of itinerant ferromagnetism in low dimensional compounds, which also leads insight for designing exotic magnetic states by interfacial band engineering in heterostructures.

Motivation & Objective

• To understand the electronic and spin structure of the van der Waals ferromagnet Fe5GeTe2 near the Fermi level.
• To determine the origin of long-range ferromagnetic order in this low-dimensional system.
• To investigate whether ligand elements (Ge, Te) contribute significantly to spin polarization despite being non-magnetic.
• To establish a new mechanism for itinerant ferromagnetism involving delocalized ligand bands rather than localized transition metal d-orbitals alone.

Proposed method

• Soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to map the electronic band structure and identify Fe 3d and Ge/Te p band contributions near the Fermi level.
• Element-specific x-ray magnetic circular dichroism (XMCD) to probe spin polarization of individual elements, particularly Te.
• Analysis of spectral weight and hybridization between Fe 3d and Ge/Te p bands to assess their role in spin ordering.
• Comparison of XMCD signals across elements to quantify spin polarization strength, especially for Te.
• Use of Fermi-level electronic structure data to correlate spin polarization with long-range magnetic order.
• Combining spectroscopic data with theoretical interpretation to identify the dominant mechanism for ferromagnetism.

Experimental results

Research questions

• RQ1 What is the role of the ligand p bands (Ge and Te) in mediating ferromagnetism in Fe5GeTe2?
• RQ2 Why does the XMCD signal from the non-magnetic Te element reach the highest value ever reported in any system?
• RQ3 How does the hybridization between Fe 3d and Ge/Te p bands influence the spin polarization and magnetic ordering?
• RQ4 To what extent are the delocalized ligand states responsible for itinerant ferromagnetism rather than localized Fe 3d moments?
• RQ5 Can the giant spin polarization of ligand bands serve as a general mechanism for designing magnetic heterostructures?

Key findings

• The ligand Te p bands exhibit the strongest XMCD signal ever reported for a non-magnetic element, with a magnitude significantly exceeding that of other reported systems.
• SX-ARPES reveals a prominent hybridization between localized Fe 3d bands and delocalized Ge/Te p bands near the Fermi level.
• Element-specific XMCD confirms giant spin polarization of the metallic Te p bands, which is central to the ferromagnetic ground state.
• The Fe 3d band shows localized character, while the spin polarization is predominantly carried by the delocalized ligand states.
• The collective evidence supports a mechanism where itinerant ferromagnetism is mediated by spin-polarized ligand bands rather than localized Fe moments alone.
• This work establishes a new paradigm for understanding itinerant ferromagnetism in low-dimensional materials through ligand band spin polarization.