Thermal-activated escape of the bistable magnetic states in 2D Fe3GeTe2 near the critical point

Abstract

Great effort has been made recently to investigate the phase transitions in two-dimensional (2D) magnets while leaving subtle quantification unsolved. Here, we demonstrate the thermal-activated escape in 2D Fe3GeTe2 ferromagnets near the critical point with a quantum magnetometry based on nitrogen-vacancy centers. We observe random switching between the two spin states with auto-correlation time described by the Arrhenius law, where a change of temperature by 0.8 K induces a change of lifetime by three orders of magnitude. Moreover, a large energy difference between the two spin states about 51.3 meV is achieved by a weak out-of-plane magnetic field of 1 G, yielding occupation probability described by Boltzmann’s law. Using these data, we identify all the parameters in the Ginzburg-Landau model. This work provides quantitative description of the phase transition in 2D magnets, which paves the way for investigating the critical fluctuation and even non-equilibrium phase transitions in these 2D materials.

Imaging the Magnetic Anisotropy in Ultrathin Fe4GeTe2 with a Nitrogen-Vacancy Magnetometer

“…By quantitative measurements, we reveal the temperature dependent change in the zenith angle of the in-plane and out-of-plane magnetization. Second, when the temperature is slightly lower than T c , − we observe the thermal-fluctuation-induced random switching of the in-plane magnetizations. This behavior is well understood as a thermally activated escape process within the Ginzburg–Landau (GL) theory by taking into account the in-plane anisotropy effect.…”

“…To further check the in-plane anisotropy in Fe 4 GeTe 2 , we apply a weak in-plane magnetic field to tune the relative depth of the two potential minima 2δE m . 39 A magnetic field parallel or antiparallel to the spin direction will cause a change in the relative depth of the two potentials, which affects the occupation probability and dwell time in these two states (Figure 4d). The probabilities in states |0⟩ and |1⟩, P 0 and P 1 , satisfy the Boltzmann distribution law as…”

“…We next study the weak in-plane magnetic behavior in Fe 4 GeTe 2 near T c (∼264.4 K; see Figure b). It is well-known that thermal fluctuations at temperatures slightly below T c can lead to random switching of the magnetizations. − However, for the in-plane case, it has not been reported in the previous literature as being limited by the sensitivity of the magnetometer. Since the free energy of ferromagnets can be described by a GL model for continuous phase transformation in an infinite system, a general picture of the in-plane magnetization escape process is presented in Figure a.…”