Two-Dimensional Itinerant Ising Ferromagnetism in Atomically thin Fe3GeTe2

Abstract: Recent discoveries of intrinsic two-dimensional (2D) ferromagnetism in insulating/semiconducting van der Waals (vdW) crystals open up new possibilities for studying fundamental 2D magnetism and devices employing localized spins. However, a vdW material that exhibits 2D itinerant magnetism remains elusive. In fact, the synthesis of such single-crystal ferromagnetic metals with strong perpendicular anisotropy at the atomically thin limit has been a long-standing challenge. Here, we demonstrate that monolayer Fe3GeTe2 is a robust 2D itinerant ferromagnet with strong out-of-plane anisotropy. Layer-dependent studies reveal a crossover from 3D to 2D Ising ferromagnetism for thicknesses less than 4 nm (five layers), accompanying a fast drop of the Curie temperature from 207 K down to 130 K in the monolayer. For Fe3GeTe2 flakes thicker than ~15 nm, a peculiar magnetic behavior emerges within an intermediate temperature range, which we show is due to the formation of labyrinthine domain patterns. Our work introduces a novel atomically thin ferromagnetic metal that could be useful for the study of controllable 2D itinerant Ising ferromagnetism and for engineering spintronic vdW heterostructures.

• The paper demonstrates that monolayer Fe3GeTe2 exhibits stable 2D itinerant Ising ferromagnetism with marked out-of-plane anisotropy.
• The paper uses mechanical exfoliation, magneto-transport, and magneto-optical techniques to reveal a significant Curie temperature reduction from bulk values to near 130K in monolayers.
• The paper identifies a dimensional crossover from 3D to 2D critical behavior, with a critical exponent β closely matching the 2D Ising model prediction.

Two-Dimensional Itinerant Ising Ferromagnetism in Atomically Thin Fe<sub\>3</sub>GeTe<sub\>2</sub> Flakes

This paper explores the synthesis and characterization of monolayers of the transition-metal compound Fe<sub\>3</sub>GeTe<sub\>2</sub> (FGT) for studying two-dimensional (2D) itinerant ferromagnetism. The research addresses a longstanding challenge of demonstrating robust ferromagnetic behavior in atomically thin vdW crystals, which typically exhibit strong perpendicular magnetic anisotropy and itinerant magnetic ordering.

Key Findings and Methodology

The study demonstrates that monolayer FGT is a stable 2D itinerant ferromagnet with strong out-of-plane anisotropy. Notably, it uncovers a dimensional crossover from 3D to 2D Ising ferromagnetism for FGT flakes of less than five layers, identified by a significant reduction in the Curie temperature (T_C). The T_C drops from approximately 207 K in a monolayer from a bulk value of around 220-230 K.

Key experimental methods include mechanical exfoliation of FGT to the monolayer limit, with devices characterized through magneto-transport and magneto-optical techniques, such as Kerr rotation and reflective magnetic circular dichroism (RMCD). These approaches revealed strong out-of-plane magnetic anisotropy and 2D Ising model-like critical behavior.

Numerical Results and Analysis

The experiments determined a T_C of around 130 K for FGT monolayers, which is significantly reduced from the bulk value. The dimensional crossover is further verified by a power-law fit yielding a critical exponent β of approximately 0.14 ± 0.02 in monolayers, consistent with the expected 0.125 for 2D Ising systems. For thicker flakes, β aligns with the 3D Ising value of about 0.33.

The researchers observed domain formation in the magnetic states of thicker FGT flakes, especially noticeable in samples exceeding 15 nm. The presence of labyrinthine domains indicated by magnetic force microscopy (MFM) suggests complex domain structures reminiscent of Co/Pt films with perpendicular anisotropy.

Implications and Future Directions

These findings highlight FGT's suitability as a testbed for exploring 2D itinerant magnetism, with potential applications in spintronics and nanoscale magnetic devices. The monolayer properties of FGT invite further exploration into electrically gating these materials to modulate their magnetic characteristics. Additionally, the ferromagnetic contacts of FGT monolayers could integrate into heterostructures, enabling the examination of emergent physical phenomena involving spin injection into 2D materials, such as topological insulators and superconductors.

The paper presents an important step in realizing atomically thin ferromagnetic metals, opening avenues for manipulating spintronic properties in 2D materials. Future research could focus on engineering vdW heterostructures with FGT to explore spin-dependent transport phenomena, as well as leveraging abilities to gate FGT layers to access different magnetic phases or motivation through external stimuli like strain or electric fields.