[Paper Review] Multiple ferromagnetic transitions and structural distortion in the van-der-Waals ferromagnet VI$_3$ at ambient and finite pressures

Elena Gati, Yuji Inagaki+5 more|Iowa State University Digital Repository (Iowa State University)|Jul 19, 2019

Organic and Molecular Conductors Research4 citations

TL;DR

This study reveals that the van der Waals ferromagnet VI₃ exhibits two distinct ferromagnetic transitions at ambient pressure—T_FM1 ≈ 50 K and T_FM2 ≈ 36 K—alongside a structural transition at T_s ≈ 78 K. Under pressure, T_FM2 shifts strongly and merges with T_FM1 at ~0.6 GPa, and further pressure suppresses the structural transition, indicating strong magneto-elastic coupling and complex magnetic ordering involving two magnetically ordered V sites below T_FM2.

ABSTRACT

We present a combined study of zero-field $^{51}$V and $^{127}$I NMR at ambient pressure and specific heat and magnetization measurements under pressure up to 2.08 GPa on bulk single crystals of the van-der-Waals ferromagnet VI$_3$. At ambient pressure, our results consistently demonstrate that VI$_3$ undergoes a structural transition at $T_s \approx $78 K, followed by two subsequent ferromagnetic transitions at $T_{FM1} \approx $50 K and $T_{FM2} \approx $36 K upon cooling. At lowest temperature ($T < T_{FM2}$), two magnetically-ordered V sites exist, whereas only one magnetically-ordered V site is observed for $T_{FM1} < T\,< T_{FM2}$. Whereas $T_{FM1}$ is almost unaffected by external pressure, $T_{FM2}$ is highly responsive to pressure and merges with the $T_{FM1}$ line at $p \approx 0.6 $GPa. At even higher pressures ($p \approx $1.25\,GPa), the $T_{FM2}$ line merges with the structural transition at $T_s$ which becomes moderately suppressed with $p$ for $p < 1.25$ GPa. Taken together, our data point towards a complex magnetic structure and an interesting interplay of magnetic and structural degrees of freedom in VI$_3$.

Motivation & Objective

• To understand the interplay between magnetic and structural degrees of freedom in the van der Waals ferromagnet VI₃.
• To resolve conflicting reports on the magnetic and structural transitions in VI₃ by combining local and thermodynamic probes.
• To investigate the pressure dependence of magnetic transitions and their connection to structural changes in VI₃.
• To clarify the nature of magnetic ordering, including site-specific magnetism, in different temperature regimes.

Proposed method

• Zero-field ⁵¹V and ¹²⁷I NMR measurements at ambient pressure to probe local magnetic environments and transitions.
• Specific heat and magnetization measurements under hydrostatic pressure up to 2.08 GPa to track phase transitions.
• Analysis of NMR spectra to identify distinct magnetically ordered vanadium sites and their occupancy ratios.
• Extrapolation of transition lines (T_FM1, T_FM2, T_s) under pressure to map the phase diagram.
• Field-dependent magnetization measurements at 5 K, 30 K, and 41.5 K to confirm ferromagnetic character and hysteresis.
• Comparison of pressure responses of T_FM1 and T_FM2 to infer the dominant mechanism in magnetic stabilization.

Experimental results

Research questions

• RQ1 Does VI₃ exhibit more than one ferromagnetic transition at ambient pressure, and what is the nature of the magnetic ordering in each phase?
• RQ2 How do the magnetic transitions T_FM1 and T_FM2 respond to hydrostatic pressure, and what causes their differing pressure sensitivities?
• RQ3 What is the relationship between the structural transition at T_s and the magnetic transitions under pressure?
• RQ4 Why is T_FM2 more sensitive to pressure than T_FM1, and what does this imply about the underlying magnetic interactions?
• RQ5 Is there evidence of magneto-elastic coupling in VI₃, and how does it influence the phase diagram?

Key findings

• At ambient pressure, VI₃ undergoes a structural transition at T_s ≈ 78 K, followed by two ferromagnetic transitions at T_FM1 ≈ 50 K and T_FM2 ≈ 36 K.
• Below T_FM2, two distinct magnetically ordered vanadium sites exist with an approximate 2:1 occupancy ratio; above T_FM2 but below T_FM1, only one ordered V site is present.
• T_FM1 is nearly insensitive to pressure, with dT_FM1/dp ≈ +1.1 ± 0.3 K/GPa, while T_FM2 shows a much stronger pressure dependence of dT_FM2/dp ≈ +11.0 ± 1.0 K/GPa.
• At p ≈ 0.6 GPa, the T_FM2 and T_FM1 lines merge into a single ferromagnetic transition line.
• At p ≈ 1.25 GPa, the T_FM2 line merges with the structural transition line, which is moderately suppressed with increasing pressure.
• The merging of multiple transition lines under pressure indicates a simultaneous magneto-structural transition for p > 1.25 GPa, strongly supporting magneto-elastic coupling in VI₃.