[Paper Review] Highly tunable magnetic phases in transition metal dichalcogenide Fe$_{1/3+δ}$NbS$_2$

Shan Wu, Zhijun Xu+9 more|arXiv (Cornell University)|Jun 2, 2021

Paleontology and Stratigraphy of Fossils4 citations

TL;DR

This study demonstrates that magnetic defects—specifically Fe vacancies and interstitials—in Fe₁/₃₊δNbS₂ induce highly tunable antiferromagnetic ground states, with distinct stripe (k₁ = (0.5, 0, 0)) and zig-zag (k₂ = (0.25, 0.5, 0)) orderings in under- and over-intercalated samples, respectively. The tunability arises from competing next-nearest-neighbor exchange and oscillatory RKKY interactions, enabling control over magnetic phases in bulk crystals for potential antiferromagnetic spintronic devices.

ABSTRACT

Layered transition metal dichalcogenides (TMDCs) host a plethora of interesting physical phenomena ranging from charge order to superconductivity. By introducing magnetic ions into 2H-NbS$_2$, the material forms a family of magnetic intercalated TMDCs T$_x$NbS$_2$ (T = 3d transition metal). Recently, Fe$_{1/3+δ}$NbS$_2$ has been found to possess intriguing resistance switching and magnetic memory effects coupled to the Néel temperature of T$_N \sim 45$ K [1,2]. We present comprehensive single crystal neutron diffraction measurements on under-intercalated ($δ\sim -0.01$), stoichiometric, and over-intercalated ($δ\sim 0.01$) samples. Magnetic defects are usually considered to suppress magnetic correlations and, concomitantly, transition temperatures. Instead, we observe highly tunable magnetic long-ranged states as the Fe concentration is varied from under-intercalated to over-intercalated, that is from Fe vacancies to Fe interstitials. The under- and over- intercalated samples reveal distinct antiferromagnetic stripe and zig-zag orders, associated with wave vectors $k_1$ = (0.5, 0, 0) and $k_2$ = (0.25, 0.5, 0), respectively. The stoichiometric sample shows two successive magnetic phase transitions for these two wave vectors with an unusual rise-and-fall feature in the intensities connected to $k_1$. We ascribe this sensitive tunability to the competing next nearest neighbor exchange interactions and the oscillatory nature of the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism. We discuss experimental observations that relate to the observed intriguing switching resistance behaviors. Our discovery of a magnetic defect tuning of the magnetic structure in bulk crystals Fe$_{1/3+δ}$NbS$_2$ provides a possible new avenue to implement controllable antiferromagnetic spintronic devices.

Motivation & Objective

• To investigate how magnetic defects (Fe vacancies and interstitials) tune magnetic ground states in bulk Fe₁/₃₊δNbS₂.
• To determine the magnetic order parameters and wave vectors in under-, stoichiometric, and over-intercalated single crystals.
• To understand the origin of the observed magnetic phase tunability in terms of competing exchange interactions and RKKY mechanisms.
• To correlate the magnetic structure with the previously reported resistance switching and magnetic memory effects near T_N ~ 45 K.
• To explore the potential for controlling antiferromagnetic states in van der Waals materials via defect engineering for spintronic applications.

Proposed method

• Single crystal neutron diffraction was performed on Fe₁/₃₊δNbS₂ samples with δ ≈ -0.01 (under-intercalated), δ ≈ 0 (stoichiometric), and δ ≈ 0.01 (over-intercalated).
• Magnetic structure factors were calculated using irreducible representations and basis vectors for wave vectors k₁ = (0.5, 0, 0) and k₂ = (0.25, 0.5, 0), with domain averaging and normalization to nuclear peaks.
• The selection rules δ₂ₕδₖ and δ₂ₕδₕ₊₂ₖ were applied to determine magnetic peak intensities based on symmetry and wave vector transfer.
• Magnetic moment sizes were extracted by comparing calculated and measured neutron diffraction intensities using normalization via unit cell volumes and peak areas.
• Magnetization measurements (ZFC/FC) were used to identify phase transitions and magnetic anisotropy, with Curie-Weiss analysis applied to paramagnetic regions.
• Theoretical analysis linked the tunable magnetic phases to competing next-nearest-neighbor exchange interactions and the oscillatory RKKY mechanism.

Experimental results

Research questions

• RQ1 How do Fe vacancies and interstitials in Fe₁/₃₊δNbS₂ affect the long-range magnetic order and wave vector selection?
• RQ2 What is the origin of the two successive magnetic transitions observed in the stoichiometric sample, and why does the intensity of the k₁ = (0.5, 0, 0) peak exhibit a rise-and-fall behavior?
• RQ3 How do competing exchange interactions and the RKKY mechanism mediate the tunability of magnetic phases in this system?
• RQ4 To what extent do magnetic defects enhance rather than suppress magnetic correlations in this 2D-like van der Waals system?
• RQ5 Can the observed magnetic phase transitions be correlated with the previously reported resistance switching and magnetic memory effects in Fe₁/₃₊δNbS₂?

Key findings

• Under-intercalated Fe₁/₃₋₀.₀₁NbS₂ exhibits a long-range antiferromagnetic stripe order with wave vector k₁ = (0.5, 0, 0), confirmed by neutron diffraction and intensity analysis.
• Over-intercalated Fe₁/₃₊₀.₀₁NbS₂ displays a zig-zag antiferromagnetic order with wave vector k₂ = (0.25, 0.5, 0), distinct from the stripe phase.
• The stoichiometric sample (δ ≈ 0) shows two successive magnetic transitions at T_N1 ≈ 32 K and T_N2 ≈ 45 K, with a non-monotonic intensity evolution for the k₁ = (0.5, 0, 0) peak.
• The ordered magnetic moment size was determined to be 1.5(2) μB per Fe atom in the under-intercalated sample and 1.3(2) μB per Fe atom in the over-intercalated sample.
• Magnetization measurements revealed strong uniaxial anisotropy, with θ_cw = -50(2) K and μ_eff = 5.0(3) μB along the c-axis for x > 1/3, and θ_cw = -24(1) K and μ_eff = 5.0(3) μB along c for x < 1/3.
• The rise-and-fall intensity feature for k₁ is attributed to the interplay of competing exchange interactions and the oscillatory RKKY mechanism, which modulates the effective exchange coupling.