(Fe0.6Co0.4)5GeTe2: A Van der Waals Antiferromagnet

• FCGT is a layered van der Waals itinerant antiferromagnet where cobalt substitution tailors stacking and drives A-type magnetic order.
• It exhibits robust room-temperature antiferromagnetism with strong out-of-plane anisotropy and interfacial uncompensated moments key to exchange bias.
• The material supports chiral spin textures and Néel-type skyrmions, enabling reconfigurable spintronic heterostructures with parity-dependent behavior.

Searching arXiv for papers on Fe–Co–Ge–Te / FCGT to ground the article in current literature. (Fe$_{0.6}$Co$_{0.4}$)$_5$GeTe$_2$ (FCGT) is a cobalt-substituted member of the Fe–Ge–Te van der Waals magnetic family whose properties place it at the intersection of itinerant antiferromagnetism, interfacial spin transport, and chiral magnetism. In the compositional regime near $x \approx 0.4$–$0.5$, Co substitution transforms the magnetic and structural ground states of Fe$_5$GeTe$_2$, producing A-type collinear antiferromagnetism with strong out-of-plane anisotropy in some stacking variants, while polar stacking near $x \approx 0.5$ enables intrinsic Dzyaloshinskii–Moriya interaction (DMI) and room-temperature Néel-type skyrmions (Chen et al., 2022, Abuawwad et al., 23 Apr 2026). Closely related experiments on $$(\mathrm{Fe}_{0.56}\mathrm{Co}_{0.44})_5\mathrm{GeTe}_2$$ and on FCGT itself further show that this material platform supports preset-controlled exchange bias in all-van der Waals heterostructures, parity-dependent antiferromagnetic tunnel junction behavior, and thickness-dependent magnetic phase evolution down to the monolayer limit (Wang et al., 7 May 2025, Zhao et al., 17 Jul 2025, Lu et al., 2023).

1. Composition, crystal chemistry, and stacking

FCGT has nominal stoichiometry $_{0.4}$0 and belongs to the cobalt-substituted Fe$_{0.4}$1GeTe$_{0.4}$2 family, commonly written as $_{0.4}$3 or Fe$_{0.4}$4Co$_{0.4}$5GeTe$_{0.4}$6 (Zhao et al., 17 Jul 2025, Yan et al., 2023). It is a layered van der Waals metal composed of slabs coupled by van der Waals forces (Zhao et al., 17 Jul 2025). In the broader Co-substituted series, Co preferentially occupies Fe sites, with first-principles calculations indicating a preference for Fe(1) before Fe(2), while local Fe(1)a/b order-disorder remains important for the magnetic ground state (May et al., 2020, Yan et al., 2023).

The crystal structure is highly sensitive to cobalt content. Undoped Fe$_{0.4}$7GeTe$_{0.4}$8 adopts rhombohedral ABC stacking with space group $_{0.4}$9, whereas Co substitution beyond $_5$0 drives a transition toward AA stacking with hexagonal $_5$1 symmetry (Chen et al., 2022). For a crystal at $_5$2, the refined lattice parameters were reported as $_5$3 Å and $_5$4 Å (Chen et al., 2022). Earlier work further showed that a sample with approximately $_5$5 Co exhibits a primitive trigonal cell with AAA stacking, while the compositional interval near $_5$6–$_5$7 Co is difficult to synthesize and likely hosts stacking competition and disorder (May et al., 2020).

A distinct structural branch emerges near $_5$8: an AA$_5$9-stacked polar phase. In the Fe–Ge–Te family, this polar stacking breaks inversion symmetry and is associated with intrinsic, bulk-like DMI and skyrmion stabilization (Abuawwad et al., 23 Apr 2026). The review on chiral spin textures identifies this AA$_2$0 polar phase as the essential symmetry setting for skyrmion formation in Co-substituted Fe$_2$1GeTe$_2$2, and states that FCGT with $_2$3 is expected to preserve the same polar symmetry and intrinsic DMI as the $_2$4 material, although detailed composition-dependent tabulation was not provided (Abuawwad et al., 23 Apr 2026). This suggests that, within the FCGT compositional window, stacking is not a secondary structural detail but a primary control parameter for whether the system realizes a centrosymmetric A-type antiferromagnet or an intrinsically chiral magnet.

2. Magnetic order and anisotropy

A central property of FCGT is A-type collinear antiferromagnetism. In this ordering pattern, each individual van der Waals layer is ferromagnetic, while adjacent layers are antiferromagnetically coupled (Zhao et al., 17 Jul 2025, Lu et al., 2023). The RMXS study on $_2$5 established an A-type, Ising-like antiferromagnetic ground state with ordered moments along the crystallographic $_2$6 axis and propagation vector $_2$7, evidenced by the magnetic Bragg reflection at $_2$8 (Chen et al., 2022). The Néel temperature was reported as $_2$9 K from magnetization and $x \approx 0.4$0 K from RMXS, summarized conservatively as $x \approx 0.4$1 K (Chen et al., 2022). A related heterostructure study on $x \approx 0.4$2 reported $x \approx 0.4$3–$x \approx 0.4$4 K, again above room temperature (Wang et al., 7 May 2025).

This antiferromagnetic state is not magnetically trivial. In the A-type structure, a surface terminates on a ferromagnetic monolayer, so a single surface is uncompensated and carries a net interfacial moment even when the bulk remains antiferromagnetic (Zhao et al., 17 Jul 2025). This uncompensated termination is fundamental to both exchange bias and antiferromagnetic tunneling in FCGT-based devices. Even-layer flakes have nearly zero net moment overall, whereas odd-layer flakes retain a residual net magnetization due to layer parity (Zhao et al., 17 Jul 2025).

Magnetic anisotropy is strongly composition- and thickness-dependent across the Fe–Co–Ge–Te family. For the $x \approx 0.4$5 AA-stacked antiferromagnet, the easy axis is along $x \approx 0.4$6, and in-plane magnetic fields suppress the antiferromagnetic Bragg intensity continuously to zero without inducing incommensurate peaks (Chen et al., 2022). In contrast, the atomically thin $x \approx 0.4$7 flakes studied in transport exhibit perpendicular magnetic anisotropy with out-of-plane easy axis, with experimental uniaxial anisotropy density $x \approx 0.4$8 MJ/m$x \approx 0.4$9 at $0.5$0 K and $0.5$1 MJ/m$0.5$2 at $0.5$3 K, and monolayer DFT magnetic anisotropy energy of approximately $0.5$4 meV per unit cell (Lu et al., 2023). The antiferromagnetic tunnel-junction study likewise describes FCGT as an A-type AFM with perpendicular magnetic anisotropy and preserved bulk $0.5$5 symmetry in the AFM state (Zhao et al., 17 Jul 2025).

These results indicate that FCGT is best understood as a layered itinerant antiferromagnet with robust out-of-plane anisotropy, uncompensated interfaces, and strong sensitivity to stacking registry. Earlier work on the broader series emphasized that the magnetic ground state near $0.5$6 Co lies close to an FM–AFM boundary and is acutely sensitive to primitive versus rhombohedral stacking (May et al., 2020). Later studies narrowed this picture by establishing that near $0.5$7–$0.5$8, the A-type AFM state is robust and room-temperature-stable (Chen et al., 2022, Lu et al., 2023).

3. Field response, thickness dependence, and parity effects

The field evolution of FCGT is a defining feature of its metamagnetism. In the heterostructure study on $0.5$9, out-of-plane $_5$0–$_5$1 at $_5$2 K showed four regimes: small-field ferromagnetic-like saturation near $_5$3 mT due to uncompensated moments or defects, an AFM ground state at intermediate fields, a spin-flop transition at $_5$4 T, and a spin-flip at $_5$5 T (Wang et al., 7 May 2025). The same work reported a broad superparamagnetic blocking signature centered near $_5$6–$_5$7 K and spin-freezing below approximately $_5$8 K (Wang et al., 7 May 2025).

At the level of few-layer flakes, the magnetic behavior becomes parity dependent. In FCGT flakes studied at $_5$9 K, odd-layer samples display a single spin-flop, while even-layer flakes with $_2$0 exhibit two-step spin-flop transitions (Lu et al., 2023). A linear-chain macrospin Hamiltonian was used:

$_2$1

with antiferromagnetic interlayer exchange $_2$2 and perpendicular anisotropy $_2$3 (Lu et al., 2023). This model reproduces the qualitative difference between odd and even thicknesses, including an intermediate ferrimagnetic $_2$4 state in even-layer flakes when

$_2$5

(Lu et al., 2023)

The parity effect also appears in Hall transport. In the AFMTJ study, even-layer FCGT electrodes showed nearly flat Hall resistance in the AFM state, whereas odd-layer electrodes displayed a finite zero-field anomalous Hall effect arising from residual uncompensated termination (Zhao et al., 17 Jul 2025). At $_2$6 K, standalone Hall measurements identified the AFM state up to about $_2$7 T, an AFM-to-FM transition between approximately $_2$8 T and $_2$9 T, and a fully ferromagnetic state beyond about $x \approx 0.5$0 T (Zhao et al., 17 Jul 2025).

Low-temperature even-layer ferromagnetism adds another layer of complexity. In $x \approx 0.5$1, even-layer flakes that would be compensated in the ideal A-type AFM show robust ferromagnetic hysteresis at $x \approx 0.5$2 K, with a sharp upturn in remanent Hall signal at $x \approx 0.5$3 K (Lu et al., 2023). The authors attribute this to spin-polarized defects and local magnetic polarons, which break compensation in even layers below $x \approx 0.5$4 (Lu et al., 2023). A plausible implication is that native defect populations can materially affect the low-temperature net moment and spin-flop phenomenology of nominally compensated FCGT flakes.

4. Chiral spin textures and room-temperature skyrmions

In the context of chiral magnetism, FCGT is notable because skyrmions need not rely exclusively on interfacial heavy-metal engineering. The review on chiral spin textures in van der Waals heterostructures identifies Co substitution in Fe$x \approx 0.5$5GeTe$x \approx 0.5$6 as a route to a polar AA$x \approx 0.5$7-stacked phase that breaks inversion symmetry and produces intrinsic DMI throughout the film (Abuawwad et al., 23 Apr 2026). In that framework, strong spin–orbit coupling, perpendicular magnetic anisotropy, DMI, exchange, and dipolar interactions collectively stabilize Néel-type skyrmions.

For $x \approx 0.5$8, the review reports a Néel-type skyrmion lattice persisting from roughly $x \approx 0.5$9 K to $$(\mathrm{Fe}_{0.56}\mathrm{Co}_{0.44})_5\mathrm{GeTe}_2$$0 K under moderate perpendicular fields of approximately $$(\mathrm{Fe}_{0.56}\mathrm{Co}_{0.44})_5\mathrm{GeTe}_2$$1 T (Abuawwad et al., 23 Apr 2026). For FCGT with $$(\mathrm{Fe}_{0.56}\mathrm{Co}_{0.44})_5\mathrm{GeTe}_2$$2, the review states that the same polar symmetry and intrinsic DMI are expected, implying a similar parameter window, although explicit $$(\mathrm{Fe}_{0.56}\mathrm{Co}_{0.44})_5\mathrm{GeTe}_2$$3 versus $$(\mathrm{Fe}_{0.56}\mathrm{Co}_{0.44})_5\mathrm{GeTe}_2$$4 values were not tabulated (Abuawwad et al., 23 Apr 2026). It also states that FCGT exhibits Néel-type skyrmion lattices at room temperature, with helicity fixed by the sign of the intrinsic DMI from polar AA$$(\mathrm{Fe}_{0.56}\mathrm{Co}_{0.44})_5\mathrm{GeTe}_2$$5 stacking (Abuawwad et al., 23 Apr 2026).

The micromagnetic condition emphasized for skyrmion-lattice formation is that the domain-wall energy approaches zero:

$$(\mathrm{Fe}_{0.56}\mathrm{Co}_{0.44})_5\mathrm{GeTe}_2$$6

where $$(\mathrm{Fe}_{0.56}\mathrm{Co}_{0.44})_5\mathrm{GeTe}_2$$7 is exchange stiffness, $$(\mathrm{Fe}_{0.56}\mathrm{Co}_{0.44})_5\mathrm{GeTe}_2$$8 is effective anisotropy, and $$(\mathrm{Fe}_{0.56}\mathrm{Co}_{0.44})_5\mathrm{GeTe}_2$$9 is the DMI constant (Abuawwad et al., 23 Apr 2026). Numerical values of $_{0.4}$00, $_{0.4}$01, and $_{0.4}$02 were not provided for FCGT; instead, the review emphasizes that Co substitution near $_{0.4}$03–$_{0.4}$04 increases inversion asymmetry and strengthens intrinsic $_{0.4}$05, pushing the system toward $_{0.4}$06 (Abuawwad et al., 23 Apr 2026).

Thickness dependence is especially important. The skyrmion diameter follows Kittel scaling, $_{0.4}$07, and the most stable lattices were observed for thicknesses of approximately $_{0.4}$08–$_{0.4}$09 nm (Abuawwad et al., 23 Apr 2026). The review does not give a single diameter value for FCGT, but states that micromagnetic maps indicate comparable tens-to-hundreds of nanometers scales (Abuawwad et al., 23 Apr 2026). Transport signatures include a topological Hall resistivity that decreases above current density $_{0.4}$10 A cm$_{0.4}$11, signaling depinning and motion (Abuawwad et al., 23 Apr 2026).

The topological charge of a Néel skyrmion is expressed as

$_{0.4}$12

with $_{0.4}$13 for the Néel skyrmions discussed in the review (Abuawwad et al., 23 Apr 2026). Tilted LTEM and induction maps showing radial spin rotation and contrast reversal were identified as fingerprints of these Néel textures (Abuawwad et al., 23 Apr 2026).

5. Spin transport, Hall response, and antiferromagnetic tunnel junctions

FCGT supports several distinct transport modalities. In atomically thin flakes of the closely related $_{0.4}$14, monolayer anomalous Hall conductivity of approximately $_{0.4}$15 S/cm at low temperature was reported, together with square out-of-plane hysteresis and Curie temperature from AHE Arrott analysis of about $_{0.4}$16 K (Lu et al., 2023). Few-layer flakes retained positive Arrott intercepts at $_{0.4}$17 K, implying $_{0.4}$18 K for $_{0.4}$19–$_{0.4}$20 layers (Lu et al., 2023). These observations indicate robust itinerant ferromagnetic behavior in ultrathin limits despite the bulk antiferromagnetic host.

The most distinctive transport result for FCGT itself is the realization of all-collinear antiferromagnetic tunnel junctions using FCGT electrodes and WSe$_{0.4}$21 barriers (Zhao et al., 17 Jul 2025). In these devices, tunneling magnetoresistance arises entirely within the AFM state, rather than through an AFM-to-FM transition (Zhao et al., 17 Jul 2025). The reported TMR ratio reaches approximately $_{0.4}$22 at $_{0.4}$23 K in an even-even device, with two prominent peaks at $_{0.4}$24 T, highest resistance $_{0.4}$25 k$_{0.4}$26 at $_{0.4}$27 T, and lowest resistance $_{0.4}$28 k$_{0.4}$29 at $_{0.4}$30 T (Zhao et al., 17 Jul 2025). The corresponding switching fields were $_{0.4}$31 T and $_{0.4}$32 T at $_{0.4}$33 K (Zhao et al., 17 Jul 2025).

The mechanism is explicitly interfacial. Bulk FCGT in the AFM state preserves $_{0.4}$34 symmetry and therefore has spin-degenerate bands, so bulk transport is spin-independent (Zhao et al., 17 Jul 2025). However, each surface terminates on an uncompensated ferromagnetic monolayer, and these interfaces generate spin-polarized tunneling. In the parallel interfacial configuration, the spin-resolved transmissions differ, whereas in the antiparallel configuration transmission is strongly suppressed and similar for both spins (Zhao et al., 17 Jul 2025). The conductance is described in Landauer form as

$_{0.4}$35

and the TMR ratio as

$_{0.4}$36

(Zhao et al., 17 Jul 2025)

Device behavior depends strongly on layer parity. Even-layer electrodes yield volatile TMR through Néel-vector switching of uncompensated interface moments, while odd-layer electrodes can show non-volatile TMR above approximately $_{0.4}$37 K due to exchange-bias-like self-pinning and an intermediate $_{0.4}$38 state (Zhao et al., 17 Jul 2025). The odd-even contrast is consistent with the general parity physics established independently in thin FCGT flakes (Lu et al., 2023).

FCGT also exhibits Hall signatures associated with spin texture dynamics. The review on chiral textures gives the Hall decomposition

$_{0.4}$39

with $_{0.4}$40 and $_{0.4}$41 (Abuawwad et al., 23 Apr 2026). In FCGT, a decrease in topological Hall resistivity above $_{0.4}$42 A cm$_{0.4}$43 is interpreted as evidence of skyrmion depinning and motion (Abuawwad et al., 23 Apr 2026). This places FCGT among the few van der Waals magnets where AHE, THE, and tunneling transport can all serve as complementary probes of magnetic state.

6. Heterostructures, exchange bias, and device implications

The best-developed heterostructure application of FCGT to date is room-temperature exchange bias in all-van der Waals stacks. In $_{0.4}$44/Fe$_{0.4}$45GaTe$_{0.4}$46 heterostructures, the sign and magnitude of the exchange bias field are controlled by switching the Néel order of FCGT with a preset magnetic field (Wang et al., 7 May 2025). The protocol is to zero-field cool to the target temperature, apply a preset field $_{0.4}$47, and then measure the anomalous Hall hysteresis of the Fe$_{0.4}$48GaTe$_{0.4}$49 layer. The exchange bias and coercivity are defined as

$_{0.4}$50

where $_{0.4}$51 and $_{0.4}$52 are the positive and negative switching fields of the ferromagnet (Wang et al., 7 May 2025).

The underlying mechanism is interfacial exchange coupling between the uncompensated moments of the FCGT antiferromagnet and the perpendicular-anisotropy ferromagnet Fe$_{0.4}$53GaTe$_{0.4}$54 (Wang et al., 7 May 2025). Robust, training-free exchange bias required preset fields above the flipping threshold of the interfacial AFM layer. Empirically, preset fields of $_{0.4}$55 T produced stable exchange bias, with the maximum reported $_{0.4}$56 mT at $_{0.4}$57 K under $_{0.4}$58 T preset (Wang et al., 7 May 2025). By contrast, $_{0.4}$59 T preset fields produced weak and unstable bias with strong training, although exchange bias was observable from $_{0.4}$60 K to $_{0.4}$61 K and reached approximately $_{0.4}$62 mT at $_{0.4}$63 K before vanishing by $_{0.4}$64 K (Wang et al., 7 May 2025).

The macrospin model used in that work represents the heterostructure as one ferromagnetic macrospin and three antiferromagnetic macrospins:

$_{0.4}$65

with parameter set $_{0.4}$66, $_{0.4}$67 for $_{0.4}$68, $_{0.4}$69, $_{0.4}$70 for $_{0.4}$71, $_{0.4}$72, and $_{0.4}$73 for $_{0.4}$74 (Wang et al., 7 May 2025). In this description, Néel-order switching corresponds to flipping the interfacial antiferromagnetic macrospin relative to deeper AFM layers (Wang et al., 7 May 2025).

The exchange-bias results directly inform expectations for $_{0.4}$75, because the studied composition $_{0.4}$76 lies very close in composition and shares the same A-type AFM phenomenology (Wang et al., 7 May 2025). The paper states that A-type AFM order with easy axis along $_{0.4}$77, high $_{0.4}$78, and field-driven spin-flop and spin-flip transitions should persist across $_{0.4}$79–$_{0.4}$80, although precise transition fields and anisotropy may shift modestly (Wang et al., 7 May 2025). A plausible implication is that FCGT can serve as a reconfigurable antiferromagnetic pinning layer in van der Waals MTJs and Hall devices without relying on conventional oxide-based exchange-bias architectures.

More broadly, the chiral-texture review identifies several heterostructure strategies applicable to FCGT: capping with WTe$_{0.4}$81 or WSe$_{0.4}$82 to add interfacial DMI, adding Pt or W to enhance spin–orbit torque, h-BN encapsulation to preserve surface quality, and the use of twist, moiré stacking, strain, and gating to modulate $_{0.4}$83 and $_{0.4}$84 (Abuawwad et al., 23 Apr 2026). These strategies are presented as routes to smaller skyrmions, thinner devices, and improved current-driven control (Abuawwad et al., 23 Apr 2026).

7. Relation to the broader Fe–Co–Ge–Te family and open problems

FCGT occupies a particularly informative point in the Fe–Co–Ge–Te phase space. Lower Co compositions such as Fe$_{0.4}$85CoGeTe$_{0.4}$86, corresponding to $_{0.4}$87, were reported as nearly-room-temperature itinerant ferromagnets with in-plane easy axis and thickness-tunable intrinsic anomalous Hall effect (Yan et al., 2023). In that system, bulk $_{0.4}$88 K, thick flakes retain $_{0.4}$89–$_{0.4}$90 K, and bilayers still show $_{0.4}$91 K (Yan et al., 2023). By contrast, near $_{0.4}$92, antiferromagnetism emerges, with A-type AFM and $_{0.4}$93 K firmly established at $_{0.4}$94 (Chen et al., 2022).

This compositional evolution was anticipated in earlier work showing that Co substitution tunes both magnetic order and stacking, with ferromagnetism up to approximately $_{0.4}$95 Co, and predominantly antiferromagnetic order at approximately $_{0.4}$96–$_{0.4}$97 Co (May et al., 2020). Primitive stacking stabilizes AFM interlayer coupling, whereas rhombohedral stacking favors FM interlayer coupling at comparable Co levels (May et al., 2020). The subsequent literature makes clear that the $_{0.4}$98 regime is not merely a boundary but a functional materials platform where antiferromagnetism, uncompensated interfaces, parity effects, exchange bias, and interfacial tunneling all coexist (Wang et al., 7 May 2025, Zhao et al., 17 Jul 2025, Lu et al., 2023).

Several unresolved issues remain. The review on chiral textures explicitly notes that numerical values of $_{0.4}$99, $_5$00, and $_5$01 for FCGT were not provided and that quantitative extraction versus Co content requires DFT-derived spin models combined with micromagnetic fits (Abuawwad et al., 23 Apr 2026). The same review identifies the correlation of LTEM, NV, or Kerr imaging with Hall decomposition as essential for validating emergent-field magnitudes and skyrmion densities (Abuawwad et al., 23 Apr 2026). For device engineering, the antiferromagnetic tunnel-junction work notes that disorder in real FCGT, including split sites and dopant randomness, likely reduces experimental TMR relative to ideal calculations (Zhao et al., 17 Jul 2025). The exchange-bias study similarly shows that robust, training-free operation requires preset fields above the interfacial AFM flipping threshold and that thermal agitation strengthens training near room temperature (Wang et al., 7 May 2025).

Taken together, the literature presents FCGT as a van der Waals itinerant antiferromagnet whose significance derives from a rare combination of properties: room-temperature-scale A-type AFM order, uncompensated magnetic interfaces, strong perpendicular anisotropy, interfacial and possibly intrinsic chirality depending on stacking, and direct compatibility with all-van der Waals spintronic heterostructures (Chen et al., 2022, Wang et al., 7 May 2025, Zhao et al., 17 Jul 2025, Abuawwad et al., 23 Apr 2026). This combination makes FCGT a model system for studying how composition, stacking symmetry, and interface termination reorganize the conventional boundaries between ferromagnetic, antiferromagnetic, and topological spintronic functionality.