Tuning magnetic order in the van der Waals metal ${\mathrm{Fe}}_{5}{\mathrm{GeTe}}_{2}$ by cobalt substitution

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Tuning magnetic order in the van der Waals metal ${Fe}_{5} {GeTe}_{2}$ by cobalt substitution

Andrew F. May, Mao-Hua Du, Valentino R. Cooper, and Michael A. McGuire

Phys. Rev. Materials 4, 074008 – Published 21 July 2020

An article within the collection: Two-Dimensional Materials and Devices

Abstract

This paper is a contribution to the joint Physical Review Applied and Physical Review Materials collection titled Two-Dimensional Materials and Devices.

${Fe}_{5 - x} {GeTe}_{2}$ is a van der Waals material with one of the highest reported bulk Curie temperatures, $T_{C} \approx 310 K$ . In this study, theoretical calculations and experiments are utilized to demonstrate that the magnetic ground state is highly sensitive to local atomic arrangements and the interlayer stacking. Cobalt substitution is found to be an effective way to manipulate the magnetic properties while also increasing the ordering temperature. In particular, cobalt substitution up to $\approx 30 %$ enhances $T_{C}$ and changes the magnetic anisotropy, while $\approx 50 %$ cobalt substitution yields an antiferromagnetic state. Single crystal x-ray diffraction evidences a structural change upon increasing the cobalt concentration, with a rhombohedral cell observed in the parent material and a primitive cell observed for $\approx 46 %$ cobalt content relative to iron. First-principles calculations demonstrate that it is a combination of high cobalt content and the concomitant change to primitive layer stacking that produces antiferromagnetic order. These results illustrate the sensitivity of magnetism in ${Fe}_{5 - x} {GeTe}_{2}$ to composition and structure, and emphasize the important role of local structural order-disorder and layer stacking in cleavable magnetic materials.

Received 30 March 2020
Accepted 16 June 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.074008

Physics Subject Headings (PhySH)

Magnetism

Condensed Matter, Materials & Applied Physics

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This article appears in the following collection:

Two-Dimensional Materials and Devices

Physical Review Applied and Physical Review Materials are pleased to present the Collection on Two-dimensional Materials and Devices, highlighting one of the most interesting fields in Applied Physics and Materials Research. Papers belonging to this collection will be published throughout 2020. The invited articles, and an editorial by the Guest Editor, David Tománek, are linked below.

Authors & Affiliations

Andrew F. May ^*, Mao-Hua Du, Valentino R. Cooper , and Michael A. McGuire

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

^*mayaf@ornl.gov

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Vol. 4, Iss. 7 — July 2020

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Figure 1
(a) Schematic of ${Fe}_{5} {GeTe}_{2}$ layer with atomic types and positions labeled. Fe1a and Fe1b represent a split site that allows for local atomic order and disorder, and the associated split site of Ge is not illustrated for simplicity. (b) Change in lattice type (rhombohedral/primitive) with cobalt doping (gold spheres) and corresponding evolution of magnetic order and anisotropy (black arrows). Preferred cobalt locations are shown for ${Fe}_{4} {CoGeTe}_{2}$ and ${Fe}_{2} {Co}_{3} {GeTe}_{2}$ based on first-principles calculations; all Fe1 sites are filled for ease of viewing. Rhombohedral centering with ABC layer stacking is shown for ${Fe}_{5} {GeTe}_{2}$ and ${Fe}_{4} {CoGeTe}_{2}$ , while primitive centering with AAA layer stacking is shown for ${Fe}_{2} {Co}_{3} {GeTe}_{2}$ . In real crystals, the ${Fe}_{4} {CoGeTe}_{2}$ composition is expected to possess stacking disorder associated with the competition between these two stacking sequences.
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Figure 2
(a) Experimental versus nominal cobalt concentration with shaded regions where ferromagnetic (FM) and antiferromagnetic (AFM) behavior are observed. (b) X-ray diffraction data from crystal facets; these $00 l$ reflections (red) were fit (black lines) to obtain the layer spacings $d$ for select compositions.
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Figure 3
Anisotropic $M$ for parent ${Fe}_{4.87 (2)} {GeTe}_{2}$ in (a) and (d) and for the ferromagnetic cobalt-containing samples in (b), (c), (e), and (f). The data demonstrate that cobalt substitution for iron results in a slight enhancement in the Curie temperature, along with a change in magnetic anisotropy that is best observed in (d)–(f).
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Figure 4
Magnetization data for a crystal with 46% cobalt. (a), (b) Temperature-dependent $M$ demonstrating cusp associated with AFM order. (c), (d) Isothermal magnetization data showing evolution of anisotropy with temperature. In (d) a spin-flop transition is observed near 1.5 T at 2 K for $H ∥ c$ , which demonstrates that the moments align predominantly along [001] in zero field.
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Figure 5
Summary of magnetic behavior as a function of cobalt content with layer stacking indicated along the upper horizontal axis. (a) Curie and Néel temperatures, (b) magnetization induced along [001] at 300 K, and (c) saturation magnetization (2 K, $H ∥ c = 60$ kOe). The dashed vertical lines indicate the approximate region where the magnetic anisotropy inverts for the ferromagnetic compositions; the solid lines in the FM region are to facilitate viewing. The AFM region is likely characterized by FM planes that are coupled antiferromagnetically along [001].
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Physical Review Materials

Tuning magnetic order in the van der Waals metal Fe5GeTe2 by cobalt substitution

Andrew F. May, Mao-Hua Du, Valentino R. Cooper, and Michael A. McGuire

Phys. Rev. Materials 4, 074008 – Published 21 July 2020

An article within the collection: Two-Dimensional Materials and Devices

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Tuning magnetic order in the van der Waals metal ${Fe}_{5} {GeTe}_{2}$ by cobalt substitution