Two-dimensional ferromagnetic superlattices

Abstract

Mechanically exfoliated two-dimensional ferromagnetic materials (2D FMs) possess long-range ferromagnetic order and topologically nontrivial skyrmions in few layers. However, because of the dimensionality effect, such few-layer systems usually exhibit much lower Curie temperature (T _C) compared to their bulk counterparts. It is therefore of great interest to explore effective approaches to enhance their T _C, particularly in wafer-scale for practical applications. Here, we report an interfacial proximity-induced high-T _C 2D FM Fe₃GeTe₂ (FGT) via A-type antiferromagnetic material CrSb (CS) which strongly couples to FGT. A superlattice structure of (FGT/CS)_n, where n stands for the period of FGT/CS heterostructure, has been successfully produced with sharp interfaces by molecular-beam epitaxy on 2-inch wafers. By performing elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally discovered that T _C of 4-layer Fe₃GeTe₂ can be significantly enhanced from 140 K to 230 K because of the interfacial ferromagnetic coupling. Meanwhile, an inverse proximity effect occurs in the FGT/CS interface, driving the interfacial antiferromagnetic CrSb into a ferrimagnetic state as evidenced by double-switching behavior in hysteresis loops and the XMCD spectra. Density functional theory calculations show that the Fe-Te/Cr-Sb interface is strongly FM coupled and doping of the spin-polarized electrons by the interfacial Cr layer gives rise to the T _C enhancement of the Fe₃GeTe₂ films, in accordance with our XMCD measurements. Strikingly, by introducing rich Fe in a 4-layer FGT/CS superlattice, T _C can be further enhanced to near room temperature. Our results provide a feasible approach for enhancing the magnetic order of few-layer 2D FMs in wafer-scale and render opportunities for realizing realistic ultra-thin spintronic devices.

Keywords: (Fe3GeTe2/CrSb)n superlattice; 2-inch Fe3GeTe2 film wafers; 2D ferromagnetic material; proximity effect; room temperature.

Figure 1.

Thin-film growth and characterizations. (a) RHEED oscillations of 2D ferromagnetic Fe₃GeTe₂ films. The layer-by-layer epitaxial mode can be verified by the periodic RHEED intensity oscillations, from which the growth period is determined to be ∼167 ± 8 s per layer. The left inset is a streaky RHEED pattern, suggesting the smooth surface of Fe₃GeTe₂. (b) An XRD spectrum of (FGT/CS)₆ superlattice. Peaks from Fe₃GeTe₂ and CrSb are marked in red and blue, respectively. The epitaxial orientations of Fe₃GeTe₂ and CrSb are ascribed to be along 〈002〉 and 〈002〉, respectively. (c) A schematic geometry of FGT/CS superlattice. Ideally, the c-axis of Fe₃GeTe₂ and CrSb should be along the same direction; however, experimentally, it has a slight deviation. (d) A cross-section HAADF image of a (FGT/CS)₃ superlattice, where the thickness of CrSb is estimated to be ∼1.6 nm and Fe₃GeTe₂ is 7 layers (∼5.6 nm). (e) A zoom-in HAADF picture. Sharp interfaces between Fe₃GeTe₂ and CrSb layers can be clearly distinguished. Note that the Pt layer is deposited during the TEM sample preparation process. (f) Thickness-dependent T_C. As the films become thinner, T_C has a dramatic drop from 220 K (bulk) to 138.4 K (bilayer) as a result of a strong dimensionality effect. The dashed line is a theoretical fit to the finite-size scaling law.

Figure 2.

FM/AF interaction induced double-switching behavior in AHE/M-H curves and the enhanced T_C in the (FGT/CS)₃ superlattice. (a) A schematic structure of one period FGT/CS superlattice that is made up of ∼3.2 nm Fe₃GeTe₂ (4-layer) and ∼1.6 nm CrSb. (b) Angle-dependent AHE at 2.5 K with the measurement geometry defined in the inset. The easy axis is determined to be out-of-plane, the same as that of pure Fe₃GeTe₂. (c) Temperature-dependent AHE under perpendicular geometry. Inset shows the experimental setup. At 2.5 K, another small switching behavior appears at ∼±0.18 T besides the sharp resistance jump at ∼±0.96 T. Small hysteresis exists when scanning the magnetic field back and forth in a small field region, denoted as minor loops, as displayed in (d). The interaction between Fe₃GeTe₂ and CrSb interface is manifested to be ferromagnetic coupling, evidenced by the negative exchange field H_EX in minor loop. (e) Major and minor M-H loops at 2.5 K. (f) ZFC-FC data under 200 Oe for (FGT/CS)₃ superlattice. T_C is roughly determined to be ∼201 K, comparable to 206.3 ± 1.6 K as deduced by the Arrott-plots in the inset.

Figure 3.

Element-specific magnetic states and T_C modulation in (FGT/CS)₃ superlattice. (a) Typical XAS and XMCD spectra of the Fe L_2,3 edge obtained at 3 K. The ferromagnetic state of Fe₃GeTe₂ can persist up to 200 K. Dashed lines are the integrations of the spectra, which is used to analyze the moments of Fe (see details in Supplementary section 3). (b) Typical XAS and XMCD spectra of the Cr L_2,3 edge at 3 K. This ferrimagnetic order results from the interfacial CrSb that is converted from the intrinsic antiferromagnetic state, which induces the double-switching behavior in AHE. (c) Experimental setup of the XMCD measurement. Left (μ⁻) and right (μ⁺) circular polarized X-ray incident normally onto the sample surface and in parallel to the magnetic field. (d) Temperature-dependent XMCD percentage of the Fe L₃ and Cr L₃ edge. Here, XMCD percentage (β) is defined in the equation . T_C = 208.6 ± 7.5 K by fitting the temperature-dependent Fe XMCD percentage using the empirical equation [46,47], consistent with that obtained from the Arrott-plots (206.3±1.6 K). (e) T_C versus the period n, which increases ∼60% from 140.3 ± 2.7 K of the pure 4-layer FGT to 230.9 ± 1.3 K in (FGT/CS)₁₀ superlattice. Note that the thicknesses of Fe₃GeTe₂ and CrSb are ∼3.2 nm (4-layer) and ∼1.6 nm, respectively.

Figure 4.

DFT calculations for FGT/CS superlattice. (a–d) Spin density plots in the (110) plane of FGT/CS superlattice in four different magnetic states of FM, Layered AF1, Layered AF2 and Layered AF3, respectively. Red (blue) color stands for the Fe or Cr up (down) spin. The magnetic moments are marked for each atom. (e) Three kinds of interfaces in the FGT/CS superlattice: Fe-Te/Cr-Sb interface named interface I, Fe-Te/Sb-Cr interface named interface II and FGT van der Waals monolayer interface named interface III with the exchange constants for each corresponding interface defined as J₁, J₂ and J₃, respectively. The changes of atomic charge (Δq) and magnetic moments (Δm) of the FGT/CS superlattice against the FGT monolayer and CS bulk. Compared to interface II, larger (Δq, Δm) and the exchange constant J₁ = 39 meV can be determined at the FM coupled interface I, which result in significant T_C enhancement in the FGT layer.

Figure 5.

T _C enhancement in the (Fe_3+xGeTe₂/CrSb) superlattice. (a) Temperature-dependent AHE in (Fe_3+xGeTe₂/CrSb)₃. Up to 280 K, hysteresis can still be observed. The inset is the perpendicular geometry for the measurement. (b) ZFC-FC curves for (Fe_3+xGeTe₂/CrSb)₃. T_C can be roughly determined to be ∼280 K, complying with that of 286.7 ± 5.4 K calculated by the Arrott-plots (Supplementary Fig. 13). The inset is the ZFC-FC curve for the 4-layer Fe_3+xGeTe₂ with T_C at ∼220 K. (c) Period-dependent Curie temperature. As the period increases, T_C can be raised from 217.5 ± 2.6 K (n = 0, the pure Fe_3+xGeTe₂) to 286.7±5.4 K (n = 3, the superlattice). The definition of period n = 1 is a bilayer structure of Fe_3+xGeTe₂ and CrSb, the same as that depicted in Fig. 3d inset. The thickness of Fe_3+xGeTe₂ and CrSb is ∼3.2 nm and ∼1.6 nm, respectively.