Tuning 2D magnetism in Fe3+XGeTe2 films by element doping

Abstract

Two-dimensional (2D) ferromagnetic materials have been discovered with tunable magnetism and orbital-driven nodal-line features. Controlling the 2D magnetism in exfoliated nanoflakes via electric/magnetic fields enables a boosted Curie temperature (T _C) or phase transitions. One of the challenges, however, is the realization of high T _C 2D magnets that are tunable, robust and suitable for large scale fabrication. Here, we report molecular-beam epitaxy growth of wafer-scale Fe_3+XGeTe₂ films with T _C above room temperature. By controlling the Fe composition in Fe_3+XGeTe₂, a continuously modulated T _C in a broad range of 185-320 K has been achieved. This widely tunable T _C is attributed to the doped interlayer Fe that provides a 40% enhancement around the optimal composition X = 2. We further fabricated magnetic tunneling junction device arrays that exhibit clear tunneling signals. Our results show an effective and reliable approach, i.e. element doping, to producing robust and tunable ferromagnetism beyond room temperature in a large-scale 2D Fe_3+XGeTe₂ fashion.

Keywords: 2D ferromagnetic material; Fe3+XGeTe2 film; TC tunability; above room temperature; element doping.

Figure 1.

2D layered structure in Fe_3+XGeTe₂ thin films. (a) Fe₃GeTe₂ structure geometry. (b) XRD spectrum from Fe_3+0.18GeTe₂, with the peaks ascribed to (0002), (0004), (0006), (00010), (00012) and (00014) according to PDF# 75-5620. Inset, an RHEED pattern. (c) A cross section HAADF image of Fe_3+1.06GeTe₂. Layered structure with an interlayer distance of 0.8 nm is well-preserved in such Fe-rich films. The scale bar is 1 nm. (d) EDS for Fe_3+1.06GeTe₂. Left inset, a photograph of a 2-inch Fe_3+1.06GeTe₂ film. Right inset, an atomic force microscopy image taken from a 10 μm × 10 μm surface, showing the average surface roughness of 0.32 nm. The scale bar is 3 μm.

Figure 2.

Out-of-plane ferromagnetic anisotropy of Fe_3+1.80GeTe₂ film with T_C of ∼320 K. (a) Temperature-dependent AHE under the perpendicular measurement geometry. Top inset, a schematic configuration of the perpendicular geometry between the sample surface and the magnetic field. Bottom inset, coercive field tracked from AHE. Up to 320 K, visible hysteresis can be distinguished, and vanishes at 330 K. T_C can be determined to be ∼320 K. (b) Angle-dependent AHE at 2.5 K. Because H_C increases with θ tilting from 0° to 90°, the easy axis is determined to be out-of-plane. Inset, the schematic geometry that defines the angle θ. (c) Zero-field-cooled (ZFC) and field-cooled (FC) susceptibility curves under a magnetic field of 200 Oe. T_C is determined to be 316.1 ± 2.6 K by the Curie-Weiss law as shown in the inset. The detailed estimation process is described in Supplementary Note S3. (d) Temperature-dependent polar RMCD curves. H_C and remanent magnetization decrease as the temperature increases, while ferromagnetic order still exists at 287 K.

Figure 3.

XAS spectra and XMCD signals of an Fe_3+1.80GeTe₂ sample at Fe L_2,3 edges. (a) Room-temperature XAS and XMCD spectra of Fe L_2,3 edges at the field of 5T. The agreement with the XAS of Fe₃GeTe₂ bulks [43] in the spectra shape further confirms the intrinsic room-temperature ferromagnetism. The two peaks at the Fe L₃ edge suggest two sites of Fe, with the XMCD percentages calculated to be (10.90 ± 1.0)% and (1.47 ± 0.1)%, respectively. Inset, schematic of the XMCD experiments. (b) Temperature-dependent XMCD of Fe L_2,3 edges where the spectra at different temperatures have vertical offsets for clarity. The magnetic field is fixed at 5T. (c) XMCD percentage versus temperature. As the temperature rises, the XMCD percentage decreases continuously. The dashed lines represent the XMCD percentages fitting to the empirical equation . T_C values are determined to be 313.3 ± 9.5 K, which further confirms the above-room-temperature ferromagnetism in Fe_3+1.80GeTe₂. (d) Field-dependent XMCD percentage, showing a large remanent XMCD percentage of 26.7% at zero-field.

Figure 4.

T _C modulation in Fe_3+XGeTe₂ film via Fe composition and DFT calculations. (a) T_C versus X ratio, reaching a peak value of 320 K at X = 1.80. Inset: an optical image of MTJ device arrays. The scale bar is 2 μm. (b) Schematic diagrams for the four defined magnetic states, the orange arrows illustrating the spin direction of each Fe1, Fe2 and Fe3 atom. (c) Relative total energies map of an extra Fe atom in the different interlayer positions of Fe₃GeTe₂ calculated by LSDA + U. There are three most stable sites at (0,0), (1/3,2/3) and (2/3,1/3). (d) Local structure of an extra Fe at (0,0) or (1/3,2/3) in bulk Fe₃GeTe₂.