Magnetic Order and Its Interplay with Structure Phase Transition in van der Waals Ferromagnet VI₃

Abstract

Van der Waals magnet VI3 demonstrates intriguing magnetic properties that render it great for use in various applications. However, its microscopic magnetic structure has not been determined yet. Here, we report neutron diffraction and susceptibility measurements in VI3 that revealed a ferromagnetic order with the moment direction tilted from the 𝑐-axis by ∼36 ∘ at 4 K. A spin reorientation accompanied by a structure distortion within the honeycomb plane is observed, before the magnetic order completely disappears at 𝑇C = 50 K. The refined magnetic moment of ∼1.3𝜇B at 4 K is much lower than the fully ordered spin moment of 2𝜇B/V 3+ , suggesting the presence of a considerable orbital moment antiparallel to the spin moment and strong spin-orbit coupling in VI3. This results in strong magnetoelastic interactions that make the magnetic properties of VI3 easily tunable via strain and pressure.

Refined magnetic structure of VI$_3$

“…Previous XRD, magnetic susceptibility and heat capacity studies [7][8][9] have suggested a structural transition to occur around 32 K. This distortion resulted into a spin reorientation of the FM order [18]. However, transitions or anomalies are not observed around these temperatures in the presented data (Fig.…”

“…2(b)) are expected to change. Moreover, the obtained value of f 0 (the precession frequency at base temperature) is inconsistent with the reported magnetic structure by neutron diffraction [9,18]. These points are further discussed in Sec.…”

“…The precession frequencies (f FMi ) show an order param-eter like behaviour. A fit with the mean field expression, f (T ) = f 0 (1 − T /T C ) α , close to the critical temperature yields f 0 = 25.4(6), T C = 50.012 (7) and α = 0.232 (12) for the f FM1 and f 0 = 53.8(1.7), T C = 50.04(3) and α = 0.255 (18) for f FM2 . Since α and T C should be muon site independent, the average value of the two sites defines T C = 50.03(1) and α = 0.244 (11).…”

“…In this case however, the structural distortion was accompanied by a magnetic structure reorientation. Powder neutron diffraction measurements reported two different ferromagnetic structure below and above 30 K [18]. However, the difference in the magnetic order between the two (apart from the small structure distortion) consists of a rotation within the ab plane by 90 • .…”

“…Neutron diffraction (ND) is the most straight forward and suitable technique to determine the magnetic structure. However, the strong incoherent scattering from V and absorption from I make such determination difficult for VI 3 [9,18]. Instead, we propose to clarify the magnetic structure in this material via zero field (ZF) µ + SR combined with DFT calculations.…”