Unravelling the nature of spin reorientation transition in quasi-2D vdW magnetic material, Fe4GeTe2

Fe4GeTe2, a recently discovered van der Waals magnet, hosts rather complex magnetic phases with two distinct transitions: paramagnetic to ferromagnetic at around TC ∼ 266 K and another spin reorientation transition (SRT) at around TSRT ∼ 100 K, unlike the other family member (Fe3GeTe2). Understanding the magnetic phases, nature of phase transitions and magnetic anisotropy have both fundamental and technological importance. Here, we report a detailed investigation of magnetization (M (T, H)), magneto-caloric effect (MCE), and heat capacity (Cp) in Fe4GeTe2 single crystal prepared via chemical vapor transport (CVT) method to shed light on the nature the magnetic transitions and the magnetic anisotropy, in particular, near the spin reorientation transition. M (T) measurements exhibit a prominent thermal hysteresis in proximity to TSRT, implying the first-order nature of SRT. A change in magnetic easy axis around SRT observed from M (T) and M (H) concludes a magnetic anisotropy is present at around TSRT. SRT is further observed in Cp (T) measurement, in terms of a prominent anomaly around SRT, suggesting the transition as real thermo-magnetic. Not only that we observe a higher MCE value near TSRT in comparison to near TC (near TC, -ΔSMmax = 1.95 and 1.99 J Kg−1K−1 and near TSRT, -ΔSMmax= 3.9 and 2.4 J Kg−1K−1 along H∥ab and H∥c, respectively, at 50 kOe magnetic field). The scaling analysis of MCE at TC, shows that the rescaled ΔSM(T,H) follows a universal curve confirming the second-order character of the ferromagnetic transition, while the same analysis breaks down at TSRT, suggesting that SRT is indeed not a second order phase transition. Thus, our experimental findings not only confirm the first-order nature of SRT at round 100 K in Fe4GeTe2, but provide additional insight into the complex spin texture with the possibility of canted or ferrimagnetic ordering.