Room Temperature Uniaxial Magnetic Anisotropy Induced By Fe‐Islands in the InSe Semiconductor Van Der Waals Crystal

Abstract The controlled manipulation of the spin and charge of electrons in a semiconductor has the potential to create new routes to digital electronics beyond Moore's law, spintronics, and quantum detection and imaging for sensing applications. These technologies require a shift from traditional semiconducting and magnetic nanostructured materials. Here, a new material system is reported, which comprises the InSe semiconductor van der Waals crystal that embeds ferromagnetic Fe‐islands. In contrast to many traditional semiconductors, the electronic properties of InSe are largely preserved after the incorporation of Fe. Also, this system exhibits ferromagnetic resonances and a large uniaxial magnetic anisotropy at room temperature, offering opportunities for the development of functional devices that integrate magnetic and semiconducting properties within the same material system.

are held together by weak vdW interactions. The extended family of vdW crystals includes graphene, hexagonal boron nitride, transition metal dichalcogenides, and many others. Although a variety of semiconductor crystals and stacks has been demonstrated, the available structures are nonmagnetic, weakly magnetic, or magnetic only at low temperature. [13] Here, we show that InSe, which is nonmagnetic in its pristine form, becomes magnetic following the incorporation of Fe-atoms during the growth of InSe by the Bridgman method. We show that the Fe-atoms self-assemble into islands embedded within the InSe host-crystal. The islands are crystalline and have a triangular shape in the plane of the vdW layers. Our material tends to retain the electronic, optical, and vibrational properties of pristine InSe. However, the Fe-islands imprint the InSe crystal with a large uniaxial magnetic anisotropy at room temperature with the magnetization preferentially oriented in the direction perpendicular to the plane of the vdW layers. Thus, room temperature magnetism and semiconducting properties are achieved within the same material system, offering opportunities for further research developments and exploitation.
The γ-polytype InSe and the Fe-doped γ-InSe crystals were grown using the Bridgman method from a polycrystalline melt of In 1.03 Se 0.97 . Fe-dopants were incorporated during the growth at a nominal concentration of 1% and 10% (Experimental Section). The primitive unit cell of γ-InSe contains three layers each of which has a thickness of L = 8.320 Å and consists of four covalently bonded monoatomic sheets in the sequence Se-In-In-Se; along the c-axis, the primitive unit cell has a lattice constant c = 24.961 Å; within each a-b plane atoms form hexagons with lattice parameter a = 4.002 Å (Figure 1a). The lattice parameters are weakly modified following the incorporation of Fe, as probed by X-ray diffraction (Experimental Section and Section S1, Supporting Information). However, studies of the crystals by spatially resolved energy-dispersive X-ray (EDX) spectroscopy and electron diffraction reveal that the Fe-atoms self-assemble into crystalline islands that are randomly oriented in the ab-plane (Figure 1b). In the thicker films (>1 µm), these islands are elongated along the c-axis (Figure 1c). These nanostructures contain a high content of Fe (>95%) and have a triangular shape in the ab-plane, as shown in the high-resolution EDX image of Figure 2a. In this figure, the two equal sides of the triangular Fe-islands have length of 0.8 µm. Furthermore, the Fe-atoms are arranged into a body centered cubic (bcc) lattice with lattice constant a = 2.87 Å (Figure 2b), as for bulk γ-Fe. Thus, Fe-islands with a bcc lattice ( Figure 2b) coexist with the rhombohedral crystal structure of γ-InSe (Figure 2c). The low solubility of Fe in InSe and the large Fe-content create supersaturation conditions during the Bridgman growth of γ-InSe, thus leading to the formation of two distinct crystals within the same material system (Experimental Section).
Magnetic force microscopy (MFM) measurements were conducted in the thermally demagnetized state of the crystals using a two-pass method and a CoCr-tip magnetized along the tip axis. The first pass was conducted in a tapping mode to reveal the surface topography (e.g., atomic force microscopy, AFM); this was then followed by the second pass at an increased scan height h (25-50 nm) to probe long-range magnetic interactions. During the MFM imaging, the scan height h is maintained constant and the changes in the phase of the oscillating probe are recorded. [14] These changes originate from the long-range magnetic interactions between the probe and the sample.  Figure 2d,e show with greater detail the shape, orientation, and height of an individual Fe-island and its threefold multidomain state. By comparing the morphology and the domain structure of this island, we conclude that the domain structure is 3D. The largest domain on the right-hand side morphs around the upper-right-hand edge. The two other domains are similar in size, completing the domain closure in an anticlockwise fashion (see schematic representation in the left bottom corner of Figure 2e). These findings demonstrate that ferromagnetic Fe-islands with a bcc lattice are embedded within the rhombohedral γ-InSe crystal.
Irrespectively of the Fe-content, we find that for all our InSe bulk crystals the energy peak position of the room temperature (T = 292 K) photoluminescence (PL) emission is centered at an energy hv = 1.25 eV (Figure 3a) and the Raman peaks are at 115.7, 179.2, 201.2, 212.4, and 228.0 cm −1 , as observed for pristine bulk InSe ( Figure S2, Supporting Information). With increasing Fe-content, the intensity of the optical signals tends to decrease and spatial maps of the PL intensity reveal an increasing nonhomogeneity over length scales of a few micrometers (Figure 3b). Correspondingly, the room temperature conductivity in the layer plane decreases due to a reduction of the electron mobility from µ ≈ 10 3 cm 2 V −1 s −1 in pristine InSe to µ ≤ 10 2 cm 2 V −1 s −1 in the InSe crystals containing Fe. Thus, despite the incorporation of Fe-islands in InSe, the crystal preserves many of the functional properties of pristine InSe. Furthermore, the crystals can be exfoliated into thin layers and the PL emission peak undergoes a strong blueshift ΔE with decreasing layer thickness L ( Figure 3a). The measured energy shift is in agreement with that observed and calculated for pristine InSe (Figure 3a). In our quantum well model, the energy shift is described as ΔE = h 2 /8L 2 µ (continuous line in Figure 3a), where µ = 0.054 m e , is the exciton mass and m e is the electron mass in vacuum; a similar energy shift is calculated using density functional theory. [15] Although the semiconducting behavior of our samples is preserved, the magnetic properties of InSe are modified after the incorporation of Fe. Figure 4a shows typical room temperature (T = 292 K) electron spin resonance (ESR) spectra measured at Q-band (frequency v = 34.229 GHz) for bulk InSe containing Fe-islands. The experiment is conducted in perpendicular mode, that is, the external magnetic field B is perpendicular to the microwave field; also, B is at angle ϑ B relative to the c-axis (out-of-plane geometry, inset of Figure 4a). For B parallel to the c-axis (i.e., ϑ B = 0°), the ESR spectrum reveals two strong ferromagnetic resonances at B = 0.199 and 0.326 T, corresponding to effective g-values of g 1 = 12.3 and g 2 = 7.5, respectively. The position, linewidth, and intensity of the resonances depend on the orientation of B with a periodic modulation and turning points that occur when B is aligned close to main crystallographic directions, that is, parallel to the c-axis (ϑ B = 0°, 180°, and 360°) or to the ab-plane (ϑ B = 90° and 270°). For example, the intensity of the main ESR line (g 2 ) has minima at angles close to ϑ B = 90°and 270° ( Figure 4b); correspondingly, the resonance field B res increases to values of up to ≈1.5 T (Figure 4c) and the resonance linewidth ΔB increases by more than a factor of 5 ( Figure 4d). This strong magnetic anisotropy is supported by superconducting quantum interference device (SQUID) studies showing a saturation of the magnetization at lower magnetic fields for B parallel to the c-axis than for B in the ab-plane ( Figure S3, Supporting Information). We note that ESR spectra of pristine InSe do not reveal any signal.
The ESR resonances are observed for a wide range of temperatures from T = 5 to 292 K. Figure 5a shows the T-dependence of the ESR spectra for B parallel to the c-axis and the corresponding T-dependent peak-to-peak intensity, resonance field, and linewidth for resonance g 2 . The ESR intensity decreases steeply from a broad maximum centered at T ≈ 260 K to approximately zero for T < 50 K (Figure 5b). In the same range of temperatures, B res shifts to lower values ( Figure 5c) and the ESR linewidth ΔB broadens (Figure 5d). Furthermore, for T < 100 K, a weak ESR line emerges at B = 0.537 T corresponding to g 3 ≈ 4.6 (see also Figure S4 in the Supporting Information). The T-dependent ESR spectra were also acquired for B in the ab-plane ( Figure S5  orientation of B, g 1 and g 2 cannot be clearly resolved; however, a narrow ESR line is observed at B = 0.680T (g 4 = 3.6). This has weak dependence on the orientation of B in the ab-plane and its T-dependence is similar to that of g 1 and g 2 ( Figure S6, Supporting Information).
Our crystals combine the electronic properties of the nonmagnetic van der Waals crystal InSe with the magnetic properties of Fe. As shown by MFM (Figure 2e), magnetic domains are observed at room temperature and are localized within the Fe-islands. Furthermore, the ESR lines correspond to effective g-values (e.g., g 1 = 12.3, g 2 = 7.5, and g 3 = 4.6 for ϑ B = 0°), significantly larger than those expected for isolated Fe-ions, that is, g ≈ 4.3 and 2. [16] The large g-factors and the angular dependence of the ESR lines demonstrate strong ferromagnetic spin-spin interactions in the Fe-islands, leading to an internal magnetic field with easy axis parallel to the c-axis of InSe.
We model the measured angular dependence of the resonance field B res by Equation (1), commonly used for systems with uniaxial magnetic anisotropy in an out-of-plane configuration [17] Here γ is the gyromagnetic factor, and ϑ M and ϑ B are the angles between the c-axis and the magnetization, M, and external magnetic field, B, respectively (inset of Figure 4a). The term 4πM eff represents an effective demagnetizing field defined as 4πM eff = 4πM s -2K/M s , where M s is the saturation magnetization, B d = 4πM s is the demagnetizing field, B a = 2K/M s is the anisotropy magnetic field, and K is an anisotropy constant. The simulation of the angular dependence of B res in Figure 4c gives K = −9 × 10 3 J m −3 and an average anisotropy field B a oriented close to the c-axis with amplitude B a ≈ 1 T. We have assumed M s = 3 emu g −1 (B d = 2.7 × 10 −1 T), as obtained from our SQUID studies ( Figure S3, Supporting Information), and γ = 2πgµ B /h with g = 2.09. [18] We describe the angular anisotropy of the resonance linewidth ΔB (Figure 4d) as where ΔB 0 and αω/γ are the inhomogeneous and homogenous broadening, respectively, and α is the dimensionless Gilbert damping parameter. [19] The inhomogeneous linewidth broadening ΔB 0 is attributed to the nonhomogeneous internal magnetic fields arising from the random distribution of the Fe-islands (Figure 1). The Gilbert damping parameter accounts for the losses of spin angular momentum during the precession of the magnetization around an effective magnetic field B eff that includes the external, internal, and microwave field. When B eff is parallel to the c-axis, the external magnetic field and the magnetization direction are parallel and ΔB has a minimum; in contrast, in the out-of-plane rotation of B eff , ΔB first increases by about 10% for ϑ B ≈ 45° and then increases steeply for ϑ B approaching a value of ϑ B ≈ 90° (Figure 4d). The angular dependence of ΔB suggests a magnetic dragging effect due to the noncolllinearity of the external magnetic field and the magnetization direction. [19] The contribution to the linewidth broadening ΔB of the Gilbert parameter (Equation (2)) implies angular momentum losses of the magnetization precession in the magnetic Fe-islands into the nonmagnetic InSe matrix. This spin-pump mechanism [5] occurs at room temperature, offering prospects for the generation of a charge current in InSe via the inverse spin Hall effect. [20] The uniaxial magnetic anisotropy is observed at room temperature and it depends only weakly on temperature, as assessed by SQUID at low (T = 5 K) and room temperature (T = 300 K) ( Figure S3, Supporting Information). Due to the coexistence of mixed phase states within a system that consists of a diamagnetic InSe crystal and ferromagnetic Fe-islands (Figure 2), the temperature dependence of the magnetic susceptibility tends to be weak and different from that expected for ferromagnetic γ-Fe ( Figure S3, Supporting Information). Furthermore, the ESR spectra reveal a complex behavior. The weakening of the main ESR lines, g 1 and g 2 , with decreasing temperature indicates the emergence of an anisotropic antiferromagnetic (AF) order at a Néel temperature T N ≈ 260 K (Figure 5b). An AF order with a Néel temperature T N up to 100 K was reported for γ-Fe thin films with face-centered cubic (fcc) crystal symmetry obtained by epitaxial growth [21] or precipitation, [22] an AF order can emerge when bcc γ-Fe undergoes a crystal phase transition to fcc below a critical layer thickness [23] due to stronger magnetic interactions arising from a smaller lattice constant [24] and/or surface effects [25] ; AF and FM orders can also coexist within an Fe-cluster due to its composite crystal structure. [26] The coexistence of different magnetic phases  in our system may arise from nonequivalent Fe-atoms in the islands and strain effects at the interface with the diamagnetic InSe, which requires further investigation.
To conclude, recent advances in the science and technology of vdW crystals have demonstrated the potential of this class of materials for novel functional devices. Among these crystals, InSe has emerged as a semiconducting system with unique electronic and optical properties, including high electron mobility [11] and strong photosensitivity. [12] Here, we have shown that the formation of crystalline Fe-islands in InSe induces a uniaxial internal magnetic field (≈1 T) perpendicular to the InSe layers. Thus, this hybrid system, which consists of Feinclusions and a van der Waals crystal, enables the coexistence of magnetic and semiconducting properties within the same structure. Our findings will stimulate further research on magnetism in novel semiconductor materials beyond conventional Si [27] and GaAs. [28] Since vdW crystals are compatible with other vdW crystals, magnetic metals, and dielectrics, we envisage further developments and a new class of devices that exploit the magnetic properties of hybrid magnetic-semiconducting materials. In particular, losses of spin angular momentum during the precession of the magnetization in the ferromagnetic Feislands into the nonmagnetic InSe offer prospects for the generation of a charge current in InSe via the inverse spin Hall effect and its control by the magnetic anisotropy of the crystal. Further developments also include the homogenous incorporation of substitutional Fe-atoms in InSe, which has recently been proposed as a route to create a homogeneous ferromagnetic semiconductor. [29] Experimental Section  www.advancedscience.com ampoule and was cooled down slowly using a moving crucible inside the Bridgman furnace.
The crystal structure of all crystals was probed by X-ray diffraction using a DRON-3 X-ray diffractometer that used monochromatic Cu Kα radiation of wavelength λ = 1.5418 Å (Section S1, Supporting Information). Transmission electron microscopy (TEM) experiments were conducted on thin sections of the crystals prepared by focused ion beam scanning electron microscope, FIB-SEM (FEI Quanta 3D). The crystals were studied using a JEOL 2100F microscope operating at 200 kV, equipped with a Gatan Orius camera and Oxford Instruments X-Max 80 EDX detector. SEM EDX studies were performed on the crystals using an FEI Quanta 650 operating at 20 kV, equipped with an Oxford Instruments X-Max 150 detector. The flakes were prepared from the as-grown crystals by mechanical exfoliation using adhesive tape and then transferred onto a Si/SiO 2 substrate. Images of the flakes' topography were acquired using an Asylum Research MFP-3D AFM operated in tapping mode under ambient conditions.
Magnetic and Optical Studies: MFM measurements were performed on a Dimension Icon (Bruker) scanning probe microscope (SPM). The MFM probe (Nanosensor PPP MFMR) had a typical spring constant k = 2-3 N m −1 and curvature radius r < 30 nm. [30] The MFM imaging was carried out using a two-pass technique. During the first pass, the SPM was operated in the atomic force microscopy mode to determine the topography. During the second pass, the topography line (obtained during the first pass) was retraced while oscillating the probe at a frequency f = 69.56 kHz (free-space amplitude A f ≈ 200 nm), maintaining a set distance of h = 25 nm between the probe and sample, and recording the cantilever phase change due to probe-sample magnetic interactions. The scan across the sample was conducted at a rate of 0.7 Hz.
ESR measurements were recorded on commercial Bruker EMX and E580 spectrometers operated at Q-band (34 GHz). The microwave field was perpendicular to the external magnetic field B (perpendicular mode). Typical experimental conditions were as follows: modulation amplitude of 0.5 mT, modulation frequency of 50 kHz, and conversion time of 20 ms. The experimental setup for the micro-PL and Raman studies comprised a He-Ne laser (λ = 633 nm) or a frequency doubled Nd:YVO 4 laser (λ = 532 nm), an x-y-z motorized stage and an optical confocal microscope system equipped with a 0.5 meter long monochromator with a 150 and 1200 g mm −1 gratings. The laser beam was focused to a diameter d ≈ 1 µm using a 100× objective. Optical experiments were performed at low excitation power (P < 0.1 mW) to avoid excessive heating. The signal was detected by a Si chargecoupled device camera. Magnetometry was performed on a commercial Quantum Design MPMS 3 SQUID.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.