Topological and Planar Hall Effect in Monoclinic van der Waals Ferromagnet NbFeTe$_2$

This study reports the first observation of the topological and planar Hall effects in the metallic monoclinic van der Waals ferromagnet $\text{NbFeTe}_2$, identifying it as a promising platform for spintronics due to its nontrivial electronic band structure and perpendicular magnetic anisotropy.

The Gist

The Magnetic "Dance" of NbFeTe2: A Simple Guide

Imagine you are trying to direct a massive, synchronized dance troupe in a dark stadium. If every dancer moves in perfect unison, the crowd can easily follow the pattern. But if some dancers start spinning wildly, others move in circles, and some move in zig-zags, the "flow" of the dance becomes incredibly complex and hard to predict.

Scientists have just discovered a new material, NbFeTe2, that acts like a very special, high-tech dance floor for electrons (the tiny particles that carry electricity). This paper describes how this material behaves when we apply magnetic fields to it.

Here is the breakdown of their discovery using everyday concepts:

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1. The "Soft" Magnet (The Easy-to-Move Dancers)

Most magnets, like the ones on your fridge, are "hard"—they want to stay pointed in one direction. NbFeTe2 is a "soft" ferromagnet.

The Analogy: Imagine a room full of compass needles. In a "hard" magnet, the needles are glued in place. In NbFeTe2, the needles are more like dancers on ice; they are magnetic, but they are very sensitive and can be easily nudged or redirected by an outside force. This makes the material "tunable," meaning we can control it easily for future gadgets.

2. The Topological Hall Effect (The "Hidden Obstacle Course")

When electricity flows through a normal metal, it moves like water through a straight pipe. But in this material, the scientists found something called the Topological Hall Effect (THE).

The Analogy: Imagine you are running a race on a straight track. Suddenly, you realize the track isn't flat; it has invisible, swirling whirlpools built into the ground. As you run, these whirlpools push you to the left or right, even though you’re trying to go straight.

In NbFeTe2, the magnetic "dancers" (the atoms) aren't just pointing up or down; they are arranged in complex, swirling patterns (called noncoplanar spin textures). When electrons try to flow through these swirls, they get pushed sideways. This "sideways push" is the Topological Hall Effect. It’s a smoking gun that tells scientists the magnetic structure is incredibly complex and "topological" (meaning it has a specific, twisted shape).

3. The Planar Hall Effect (The "Ghostly" Signal)

The researchers also found the Planar Hall Effect (PHE). Usually, this effect happens only when the material is strongly magnetic. However, in NbFeTe2, this signal stays strong even when the temperature rises above the point where the material stops being a magnet!

The Analogy: Imagine a group of people dancing in a circle. Even after the music stops and the dancers stop moving in a circle, you can still see the "ghost" of the dance pattern in the way they are standing.

Because this signal persists even when the magnetism "fades," the scientists suspect that the material has a very special electronic band structure. This means the "pipes" that the electricity flows through are shaped in a way that creates this effect naturally, regardless of whether the magnetism is fully active.

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Why does this matter? (The "So What?")

Why spend years studying tiny crystals and swirling electrons? Because this material is a candidate for the next generation of technology:

• Spintronics: Current computers use the charge of an electron (on or off) to process information. "Spintronics" uses the spin (the direction the electron is pointing). Because NbFeTe2 is easy to control and has these complex "swirls," it could lead to computers that are much faster and use much less power.
• Ultra-fast Sensors: The way this material reacts to magnetic fields could make it useful for creating incredibly sensitive sensors for high-speed communication (like 6G and beyond).

In short: The scientists found a new "playground" for electrons that is full of twists, turns, and swirls, offering a brand-new way to build the tiny, powerful machines of the future.

Technical Summary

Technical Summary: Topological and Planar Hall Effect in Monoclinic van der Waals Ferromagnet $\text{NbFeTe}_2$

1. Problem and Motivation

Two-dimensional (2D) van der Waals (vdW) ferromagnets are essential for the development of next-generation, low-power spintronic devices. A key frontier in this field is the discovery of materials that host topologically nontrivial spin textures (such as skyrmions or noncoplanar spin structures). These textures can be identified through the Topological Hall Effect (THE), where conduction electrons acquire a real-space Berry phase.

While materials like $\text{Fe}_3\text{GeTe}_2$ and $\text{Cr}_x\text{Te}_2$ have shown such phenomena, the monoclinic phase of $\text{NbFeTe}_2$ has remained under-explored. Previous studies suggested $\text{NbFeTe}_2$ might behave as an Anderson insulator or a spin-glass; however, recent synthesis advances allow for a metallic ferromagnetic (FM) phase. This study aims to characterize the magnetotransport properties of this metallic monoclinic phase to determine if it can host topological spin textures.

2. Methodology

• Synthesis: Single crystals of $\text{NbFeTe}_2$ were grown using the Chemical Vapor Transport (CVT) method with iodine as the transport agent in a two-zone gradient furnace ($800^\circ\text{C}/700^\circ\text{C}$ for 10 days).
• Structural Characterization: The researchers used X-ray diffraction (XRD) to confirm the monoclinic phase (space group $P2_1/c$) and High-Resolution Transmission Electron Microscopy (HRTEM) with Selected Area Electron Diffraction (SAED) to verify high crystalline quality.
• Magnetic Measurements: Magnetization ($M$ vs. $T$ and $M$ vs. $H$) was measured using a SQUID vibrating sample magnetometer (MPMS 3) from 2 K to 300 K. AC magnetic susceptibility ($\chi'$ and $\chi''$) was used to determine the Curie temperature ($T_C$) and magnetic ground state.
• Magnetotransport Measurements: Electrical resistivity and Hall effects were investigated using a Physical Property Measurement System (PPMS) via a standard four-probe configuration. The researchers measured:
  • Longitudinal Magnetoresistance (LMR) and Transverse Magnetoresistance (TMR).
  • Anomalous Hall Effect (AHE) and Topological Hall Effect (THE) by analyzing the Hall resistivity ($\rho_{yz}$) components.
  • Planar Hall Effect (PHE) by rotating the magnetic field within the crystal plane.

3. Key Results

• Magnetic Properties: The material exhibits FM order with a $T_C \approx 80\text{ K}$. It possesses a strong out-of-plane (a-axis) easy axis of magnetization. The researchers identified a possible low-temperature spin-glass state coexisting with FM ordering, evidenced by a downturn in zero-field-cooled (ZFC) magnetization and shoulder-like anomalies in AC susceptibility.
• Topological Hall Effect (THE): For the first time in metallic $\text{NbFeTe}_2$, a THE signature was observed. By subtracting the ordinary and anomalous Hall contributions from the total Hall resistivity, a finite $\rho_{yz}^T$ was extracted, persisting up to 45 K. The magnitude of the THE is comparable to the AHE, suggesting the presence of noncoplanar spin structures rather than mesoscopic skyrmions.
• Planar Hall Effect (PHE): A robust PHE was observed, notably persisting well above $T_C$ (up to 120 K).
• Electronic Structure Evidence: The combination of a negative longitudinal magnetoresistance (LMR) and the persistent PHE suggests a nontrivial electronic band structure. The researchers noted that the PHE does not saturate with the magnetization, which argues against a simple spin-dependent scattering origin and points toward more complex mechanisms like anisotropic magnetoresistance (AMR) or the chiral anomaly.

4. Key Contributions and Significance

• First Observation of THE/PHE in Metallic $\text{NbFeTe}_2$: This work distinguishes the metallic monoclinic phase from the previously reported insulating/spin-glass phases, establishing it as a high-quality FM metal.
• Identification of Nontrivial Spin Textures: The detection of THE provides strong evidence for complex, noncoplanar magnetic arrangements driven by the competition between exchange interactions and uniaxial anisotropy.
• Spintronic Potential: The coexistence of perpendicular magnetic anisotropy (PMA) and a substantial THE makes $\text{NbFeTe}_2$ a highly promising platform for future topological spintronics.
• Material Engineering Insights: The study highlights how synthesis temperature and structural symmetry (monoclinic vs. orthorhombic) are critical in tuning the magnetic and topological properties of vdW materials.