Key Takeaways & Executive Findings
- •• • Cr doping at x = 1/3 stabilizes the 1T phase of NbTe2, enabling a ferromagnetic ordering with TC = 254 K, which exceeds the TC of most Cr-based vdW ferromagnets (e.g., CrI3 TC = 61 K, CrGeTe3 TC = 61 K). This near-room-temperature operation is critical for practical spintronic devices that require thermal stability. • • The 1T Nb2/3Cr1/3Te2 single crystal exhibits a positive magnetoresistance of 32.1% at 2 K under 9 T, a metric that indicates potential for magnetic field sensing and non-volatile memory applications where large MR ratios are essential. • • The Kondo effect observed in the metallic state suggests strong spin scattering, which could be exploited for spin injection and detection, but also implies that device performance may degrade at cryogenic temperatures due to enhanced electron scattering. • • The phase transition from 1T′ to 1T is induced by Cr doping, as confirmed by DFT calculations showing thermodynamic stability of the 1T phase at x = 1/3. This provides a design rule for stabilizing metastable phases in other TMD systems, potentially unlocking topological spintronic properties.
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Abstract
Magnetic transition metal dichalcogenides (TMDs) offer intrinsic spin polarization for spintronic devices, yet ferromagnetic TMDs remain scarce. The theoretically predicted nontrivial topological 1T NbTe2 is thermodynamically unstable relative to the 1T′ phase under ambient conditions, impeding its spintronic application. Heteroatom doping can stabilize the 1T phase and introduce magnetism. We synthesized Nb1−xCrxTe2 (x = 0, 0.1, 0.2, 1/3, 0.4) crystals and discovered the 1T Nb2/3Cr1/3Te2 phase. Cr doping induces a 1T′-to-1T structural transition in NbTe2. Density functional theory confirms the thermodynamic stability of 1T Nb2/3Cr1/3Te2. Magnetic measurements reveal a transition from diamagnetic to ferromagnetic behavior with increasing Cr content. The ferromagnetism in 1T Nb2/3Cr1/3Te2 originates primarily from localized Cr 3d electrons, achieving a Curie temperature (TC) of 254 K, surpassing most Cr-based van der Waals ferromagnets. The compound exhibits metallic behavior coexisting with the Kondo effect and a positive magnetoresistance of 32.1% at 2 K under μ0H = 9 T. This work unveils a doping-induced phase transition mechanism and provides a new layered ferromagnetic material for spintronic devices.
1. Introduction
The development of ferromagnetic van der Waals (vdW) materials for spintronic devices has been hindered by the scarcity of compounds that combine robust magnetism with air stability and high Curie temperatures. While Cr-based TMDs such as CrI3 and CrGeTe3 exhibit intrinsic ferromagnetism, their Curie temperatures remain well below room temperature (TC < 70 K), limiting their practical deployment in energy-efficient spin-based logic and memory. Furthermore, the theoretically predicted nontrivial topological properties of 1T NbTe2—a promising candidate for quantum spin Hall and anomalous Hall effects—are inaccessible because the 1T phase is thermodynamically unstable relative to the distorted 1T′ phase under ambient conditions. This structural instability has stalled experimental exploration of 1T NbTe2 and its derivatives for spintronic applications.
Heteroatom doping has emerged as a viable strategy to simultaneously stabilize metastable phases and introduce magnetic ordering. In this work, we synthesized a series of Nb1−xCrxTe2 (x = 0, 0.1, 0.2, 1/3, 0.4) single crystals and discovered that Cr doping drives a 1T′-to-1T phase transition, resulting in the first realization of the 1T Nb2/3Cr1/3Te2 phase. Density functional theory calculations confirm the thermodynamic stability of this phase, and magnetic measurements reveal a transition from diamagnetic to ferromagnetic behavior with increasing Cr content. The optimized composition achieves a Curie temperature of 254 K, surpassing most Cr-based vdW ferromagnets, and exhibits metallic transport with a positive magnetoresistance of 32.1% at 2 K under 9 T. This doping-induced phase transition mechanism not only stabilizes the 1T phase but also provides a new platform for near-room-temperature spintronic devices.
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LI Kunqi, ZHOU Yuxuan, YANG Ke, HAN Yutao, YUAN Yakun, TU Xueyang, WANG Yiyang, SUN Xuzhou, BI Hui, FANG Yuqiang, WU Hua, HUANG Fuqiang (2025). Near-Room-Temperature Ferromagnetic 1T Nb1−xCrxTe2 from Doping-Induced Phase Transition of 1T′ NbTe2. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3395-4
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Frequently Asked Questions
What is the thermal stability of the 1T Nb2/3Cr1/3Te2 phase at device operating temperatures, and does it degrade over time?
The 1T phase is thermodynamically stable at room temperature, as confirmed by DFT calculations and experimental synthesis. However, long-term stability under ambient conditions and at elevated temperatures (e.g., 254 K near TC) requires further investigation. The compound is metallic and air-stable, but oxidation at grain boundaries or defects could degrade magnetic properties. Accelerated aging tests at 300 K for 1000 hours are recommended to assess degradation rates.
How does the magnetoresistance of 32.1% at 2 K compare to commercial magnetic sensors, and what is the temperature dependence?
The 32.1% MR at 2 K and 9 T is competitive with giant magnetoresistance (GMR) materials (e.g., Co/Cu multilayers show ~20% at 4.2 K), but it decreases significantly at higher temperatures. At 254 K (TC), the MR is expected to be much lower due to thermal fluctuations. For practical applications, MR at room temperature is essential; thus, further optimization of doping or heterostructures is needed to enhance high-temperature MR.
What are the scalability and cost challenges for synthesizing Nb1−xCrxTe2 single crystals?
The current synthesis uses chemical vapor transport (CVT), which is a slow, high-temperature process (typically 800–1000°C for several days) and yields small crystals (millimeter-scale). Scalability to wafer-scale thin films would require alternative methods like molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD), which are capital-intensive. The cost of high-purity Nb, Cr, and Te precursors is moderate, but the complex phase control (x = 1/3) demands precise stoichiometry, increasing manufacturing complexity.
Does the Kondo effect interfere with ferromagnetic ordering, and how does it affect spin transport?
The Kondo effect arises from the interaction between localized Cr 3d electrons and conduction electrons, leading to increased resistivity at low temperatures. While it coexists with ferromagnetism, it can reduce spin polarization and increase scattering, potentially degrading spin injection efficiency. However, the Kondo temperature (TK) is likely below TC, so at 254 K, the effect is negligible. For spintronic devices operating near TC, the Kondo effect may not be a major limitation, but at cryogenic temperatures, it could reduce magnetoresistance and spin lifetime.
What is the origin of the positive magnetoresistance, and can it be tuned for memory applications?
The positive MR likely originates from the Lorentz force acting on charge carriers in the metallic state, combined with spin-dependent scattering. The MR value of 32.1% at 2 K and 9 T is moderate; for memory applications, a larger MR at lower fields is desirable. Tuning Cr content or applying strain could modify the electronic structure and enhance MR. However, the required magnetic field (9 T) is too high for practical devices, necessitating research into alternative mechanisms like tunneling MR in heterostructures.
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