[Paper Review] Hard ferromagnetism down to the thinnest limit of iron-intercalated tantalum disulfide

Samra Husremović, Katherine Inzani+12 more|arXiv (Cornell University)|Mar 10, 2022

2D Materials and ApplicationsMaterials Science87 references54 citations

TL;DR

This study demonstrates hard ferromagnetism in iron-intercalated bilayer 2H-TaS2 down to the two-dimensional limit, achieving giant coercive fields up to 3 T. Using chemical intercalation and advanced characterization, the authors show that strong magnetocrystalline anisotropy arises from high unquenched orbital angular momentum of Fe ions and spin-orbit coupling in the TaS2 host, enabling stable, switchable 2D magnetism with tunable properties via intercalation control.

ABSTRACT

Two-dimensional (2D) magnetic crystals hold promise for miniaturized and ultralow power electronic devices that exploit spin manipulation. In these materials, large, controllable magnetocrystalline anisotropy is a prerequisite for the stabilization and manipulation of long-range magnetic order. In known 2D magnetic crystals, relatively weak magnetocrystalline anisotropy results in typically soft ferromagnetism. Here, we demonstrate that ferromagnetic order persists down to the thinnest limit of Fe$_x$TaS$_2$ (Fe-intercalated bilayer 2H-TaS$_2$) with giant coercivities up to 3 tesla. We prepare Fe-intercalated TaS$_2$ by chemical intercalation of van der Waals layered 2H-TaS$_2$ crystals and perform variable-temperature quantum transport, transmission electron microscopy, and confocal Raman spectroscopy measurements to shed new light on the coupled effects of dimensionality, degree of intercalation, and intercalant order/disorder on the hard ferromagnetic behavior of Fe$_x$TaS$_2$. More generally, we show that chemical intercalation gives access to a rich synthetic parameter space for low-dimensional magnets, in which magnetic properties can be tailored by the choice of the host material and intercalant identity/amount, in addition to the manifold distinctive degrees of freedom available in atomically thin, van der Waals crystals.

Motivation & Objective

• To stabilize long-range ferromagnetic order in two-dimensional transition metal dichalcogenides with strong magnetocrystalline anisotropy.
• To overcome the limitation of soft ferromagnetism in known 2D magnets by engineering large magnetic anisotropy through chemical intercalation.
• To explore the role of intercalant order, stoichiometry, and dimensionality in tuning magnetic properties in 2D van der Waals magnets.
• To establish a synthetic platform for designing low-dimensional magnets with controllable magnetic anisotropy via host and intercalant engineering.

Proposed method

• Chemical intercalation of Fe(CO)5 into mechanically exfoliated 2H-TaS2 flakes under inert atmosphere to form FexTaS2 heterostructures.
• Use of hexagonal boron nitride (hBN) capping to protect the TaS2 basal plane during intercalation and annealing.
• High-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDS) for atomic-scale structural and compositional analysis.
• Differential phase contrast STEM (DPC-STEM) to image intercalated Fe atoms and confirm their occupancy in 6-coordinate interstitial sites.
• Variable-temperature quantum transport measurements to probe magnetic ordering and coercivity.
• Confocal Raman spectroscopy to assess lattice distortions and superlattice formation, including √3a × √3a periodicity.

Experimental results

Research questions

• RQ1 Can long-range ferromagnetic order be stabilized in the two-dimensional limit of Fe-intercalated TaS2?
• RQ2 What is the origin of the large magnetocrystalline anisotropy in FexTaS2, and how does it enable hard ferromagnetic behavior?
• RQ3 How do the degree of intercalation and intercalant order/disorder influence the magnetic and electronic properties of 2D FexTaS2?
• RQ4 Can chemical intercalation in van der Waals heterostructures serve as a general route to engineer tunable 2D magnets with strong magnetic anisotropy?

Key findings

• Ferromagnetic order persists down to the bilayer limit of Fe-intercalated 2H-TaS2, with long-range magnetic order confirmed via variable-temperature transport measurements.
• Giant coercive fields of up to 3 T were measured, indicating hard ferromagnetic behavior in the 2D limit.
• DPC-STEM imaging confirmed Fe intercalants occupy 6-coordinate interstitial sites in a √3a × √3a superlattice structure, consistent with Fe1/3TaS2.
• The strong magnetocrystalline anisotropy arises from high unquenched orbital angular momentum of Fe ions in trigonal antiprismatic coordination and spin-orbit coupling from the TaS2 host.
• Raman spectroscopy revealed distinct phonon modes and superlattice signals, confirming intercalation and structural ordering.
• The degree of intercalation and intercalant order significantly modulates the magnetoelectronic response, enabling tunability of magnetic properties.