We report different growth modes and corresponding magnetic properties of thin EuSe films grown by molecular beam epitaxy on BaF2, Pb1−xEuxSe, GaAs, and Bi2Se3 substrates. We show that EuSe grows predominantly in the (001) orientation on GaAs(111) and Bi2Se3, but along the (111) crystallographic direction on BaF2 (111) and Pb1−xEuxSe (111). High resolution transmission electron microscopy measurements reveal a sharp and highly crystalline interface for both (001) and (111) EuSe films. In agreement with previous studies, ordered magnetic phases include antiferromagnetic, ferrimagnetic, and ferromagnetic phases. In contrast to previous studies, we found a strong hysteresis of the antiferromagnetic–ferrimagnetic transition. The ability to grow epitaxial films of EuSe on Bi2Se3 and of Bi2Se3 on EuSe enables further investigation of interfacial exchange interactions between various phases of an insulating metamagnetic material and a topological insulator.

A renewed interest in magnetic insulators (MIs), such as europium chalcogenides, is motivated by a possibility to induce strong spin splitting in heterostructured materials via proximity exchange interactions.1 One of the most interesting applications of strong interfacial Zeeman fields is a possibility to locally control topological surface states (TSS) in topological insulators (TIs).2,3 For example, selective gapping of top and bottom TSS is needed to realize the quantum anomalous Hall effect4,5 and gapping of side TSS, and, thus, electrically disconnecting top and bottom TSS will benefit future spintronic devices,6 while local gapping of TSS is required to localize non-Abelian excitations in TI-based topological superconductors.7 Strong interfacial exchange requires the formation of clean epitaxial interfaces with controllable crystallographic orientation and atomic arrangement, which motivates growth and characterization of MI/TI heterostructures reported in this paper.

Opening of the exchange gap in the Dirac spectrum of TSS has been reported in Bi2Se3/EuS heterostructures.8 Polarized neutron reflectometry detected interfacial magnetization in these heterostructures up to room temperature, 20 times higher than the Curie temperature Tc=17K of the bulk EuS.9 In Cr2Te3/Bi2Te3/Cr2Te3 ferromagnet–TI heterostructures, one of the interfaces demonstrated magnetic textures and non-uniform gapping of TSS.10 Gapping of TSS by exchange-coupling to an antiferromagnetic insulator has an advantage of reducing stray magnetic fields from the bulk of the magnetic material and has been demonstrated in magnetically doped TI interfacing Cr2O311 and CrSb.12,13 At present, there are no reports of both top and bottom TSS modulations in non-magnetic TIs.

EuSe is an insulating metamagnetic material that exhibits a variety of magnetic phases in a narrow range of magnetic fields.14 Almost perfect cancellation of the nearest and next-nearest neighbor interactions between localized magnetic moments on the half-filled 4f levels of Eu2+ ions leads to a phase diagram that includes several antiferromagnetic (AF-I ↑↓↑↓, AF-II ↑↑↓↓), ferrimagnetic (FIM ↑↑↓), and ferromagnetic (FM) phases in the bulk, where magnetization vectors are confined to {111} planes of the NaCl-type crystal structure (as shown in Fig. 1). The magnetic moments of Eu atoms are ferromagnetically aligned within the {111} planes. AF sheets are stacked in a collinear AF order.15 There are two types of AF domain walls: T (twin)-domain and S (spin)-domain. Each of the four possible [111] stacking directions determines the T-domain. A single T-domain crystal has a triclinic symmetry with an easy axis along one of three [211] derived directions, [2¯11], [12¯1], and [112¯] with spins laying within a {111} plane.16–18 Each of the three [112¯] spin directions per T-domain defines an S-domain.19 It has been shown that T-domains can be eliminated by applying momentary pressure along the (111) direction or a large (>0.2 T) external magnetic field.20 Thus, it is of great interest to control the orientation of EuSe during molecular beam epitaxy (MBE) growth to enable proximity to FM (111) and FM or AF (100) surfaces. The relative strength of the magnetic phases and the positions of the phase boundaries are also highly sensitive to strain and can be tuned during epitaxial growth.21,22 The magnetic properties of bulk EuSe23,24 and thin (111) layers21 have been studied in the past, and exchange-enhancement of the Zeeman splitting in Al has been reported in Al/EuSe/Ag tunnel junctions.25 EuSe itself has optical absorption properties that are very sensitive to magnetic fields.14,26

FIG. 1.

Crystal structure of EuSe. The magnetization vector lies in one of four {111} planes (T-domains), which are marked by green and cyan planes; red arrows indicate three possible magnetization easy axes in the {111} planes for each T-domain (S-domains).

FIG. 1.

Crystal structure of EuSe. The magnetization vector lies in one of four {111} planes (T-domains), which are marked by green and cyan planes; red arrows indicate three possible magnetization easy axes in the {111} planes for each T-domain (S-domains).

Close modal

In this work, we report the growth and characterization of thin EuSe with various crystalline orientations obtained on a variety of substrates including Bi2Se3. Using the adapted growth conditions, we achieve the growth of a (001)-oriented EuSe layer on GaAs(111)B and Bi2Se3. We also synthesized (111) oriented EuSe films on lattice-matched BaF2 (111) and a (Pb,Eu)Se pseudosubstrate for comparison. We performed superconducting quantum interference device (SQUID) magnetometry and constructed magnetic phase diagrams for the (001) and (111) EuSe films. The (111)-oriented layers yield a phase diagram similar to previous studies, albeit we observe large hysteresis for the FIM–AF transition. Magnetization in the (001) layers shows a strong contribution from the hard axis since both in-plane and out-of-plane directions do not coincide with the easy axes oriented in the (111) plane (Fig. 1). Knowledge of this phase diagram is important for interpreting proximity effect experiments with compensated (001) EuSe surfaces.

The EuSe films are grown by MBE on three types of substrates and buffer layers: (SA) lattice-mismatched GaAs, (SB) nearly lattice-matched BaF2 (111), (SC) Pb1−xEuxSe/BaF2 (111) pseudo-substrates, and (SD1) van der Waals TI Bi2Se3 grown on sapphire. Table I summarizes parameters of the samples. High-purity PbSe compound flux and Eu, Bi, and Se elemental fluxes are obtained from standard effusion cells. The growth is monitored by in situ reflection high energy electron diffraction (RHEED). The growth is carried out under Se-rich conditions. The Se flux is kept constant at the beam equivalent pressure (BEP) ratio of Se:Eu of 420. Under these conditions, the growth rate of EuSe (12nm/min) is controlled by the Eu flux. During the growth process, the substrate temperature is maintained close to 300 °C. Note that the actual surface temperature can depend on the thermal conductivity of each specific substrate. However, in past studies, growth temperatures between 260 °C (Ref. 27) and 420 °C (Ref. 28) have successfully been used. Various substrate treatments are carried out prior to the deposition of EuSe.

TABLE I.

Properties of EuSe films studied in this work. Film thickness is determined from MBE growth calibration and confirmed for several films using TEM imaging. The root mean square (RMS) surface roughness Ra is extracted from atomic force microscopy (AFM) images. The dominant crystal orientation is determined by XRD. Rare (111) inclusions are detected by TEM but not by XRD. The Néel temperature is extracted from the temperature dependence of magnetic susceptibility. Out-of-plane strain is calculated by comparing the out-of-plane lattice constant measured by XRD to the bulk lattice constant. AFM images used to extract Ra are shown in the supplementary material.

Growth directionGrowth sequence (sample ID)Thickness (nm)Ra(nm)TN (K)StrainNote on growth
(001) GaAs(111)B/EuSe (SA1) 120 2.96 5.1 0% Se:Eu = 10, pseudo 2D 
Mixed GaAs(111)B/EuSe (SA2) 120  5.4 +0.09% Se:Eu = 4 
(001) Sapphire/Bi2Se3/EuSe (SD1) 16.6/20 0.90 4.4 −0.08%  
(001) GaAs(111)B/Bi2Se3/EuSe (SD2) 18/16 1.74 4.4 +0.1  
(111) BaF2(111)/EuSe (SB) 90 2.58 4.9 −0.09% Rough surface initially 
(111) BaF2(111)/pseudo-substratea/EuSe/Bi2Se3 (SC) 164/20/28 0.98 5.5 +0.65% pseudosubstrate reestablishes 2D growth 
Trilayer EuSe/Bi2Se3/EuSe (SF)  See the supplementary material 
Growth directionGrowth sequence (sample ID)Thickness (nm)Ra(nm)TN (K)StrainNote on growth
(001) GaAs(111)B/EuSe (SA1) 120 2.96 5.1 0% Se:Eu = 10, pseudo 2D 
Mixed GaAs(111)B/EuSe (SA2) 120  5.4 +0.09% Se:Eu = 4 
(001) Sapphire/Bi2Se3/EuSe (SD1) 16.6/20 0.90 4.4 −0.08%  
(001) GaAs(111)B/Bi2Se3/EuSe (SD2) 18/16 1.74 4.4 +0.1  
(111) BaF2(111)/EuSe (SB) 90 2.58 4.9 −0.09% Rough surface initially 
(111) BaF2(111)/pseudo-substratea/EuSe/Bi2Se3 (SC) 164/20/28 0.98 5.5 +0.65% pseudosubstrate reestablishes 2D growth 
Trilayer EuSe/Bi2Se3/EuSe (SF)  See the supplementary material 
a

Pseudosubstrate: PbSe (64 nm)/Pb0.8Eu0.2Se(57 nm)/Pb0.58Eu0.42Se(43 nm).

For GaAs (111)B substrates, a specific treatment with Se is carried out prior to the growth. First, a GaAs (111)B substrate is heated to ∼570 °C for removing native oxide. Next, the substrate is annealed under Se flux (2×106Torr) at 600 °C for 20 min to obtain streaky (1 × 1) RHEED pattern. Se passivation smoothens the surface and terminates it with Se bonds. EuSe grown on the GaAs (111)B substrate has two initiation processes. Growth on GaAs without a buffer layer often yields a three-dimensional growth mode with a rough surface, as seen in Fig. 2(a). Alternatively, a specific atomic-layer-epitaxy (ALE) growth process is performed to initiate epitaxial layer-by-layer growth mode: the growth is initiated by deposition of six periods of Se monolayers followed by a monolayer of Eu at lower temperature (200 °C). Then, the substrate is gradually heated to 300 °C, and a nice streaky RHEED pattern [not shown, similar to Fig. 2(b)] appears prior to the MBE growth of the EuSe film. A latter process with a higher Se:Eu flux ratio yields uniform EuSe in the (001) growth direction with a 12-fold symmetry RHEED pattern, which suggests that the four EuSe {100} planes are parallel (aligned) to the three {110} GaAs (111) planes, similar to what was recently reported for the growth of PbSe on GaAs29 [see also Fig. S2(b) in the supplementary material].

FIG. 2.

RHEED patterns of EuSe films grown on different substrates: (a) GaAs (111)B, (b)Bi2Se3, (c) BaF2 (111), and (d) Pb1−xEuxSe pseudosubstrate. The [110] plane spacing on the RHEED patterns is indicated by a white arrow. The direction of the beam is always perpendicular to the [110] direction of EuSe.

FIG. 2.

RHEED patterns of EuSe films grown on different substrates: (a) GaAs (111)B, (b)Bi2Se3, (c) BaF2 (111), and (d) Pb1−xEuxSe pseudosubstrate. The [110] plane spacing on the RHEED patterns is indicated by a white arrow. The direction of the beam is always perpendicular to the [110] direction of EuSe.

Close modal

We also performed EuSe growth on Bi2Se3 grown on sapphire c-plane (sample SD1) or GaAs (111)B (SD2) substrates. 15–20 nm thick Bi2Se3 were grown at 300 °C under an Se-rich condition as in Refs. 30–32, followed by the EuSe growth. Under the Se-rich condition (Se:Eu > 10), the RHEED pattern is blurry but streaky, indicating a layer-by-layer growth mode [see Fig. 2(b)].

The BaF2 and sapphire substrates are annealed in situ at 650–700 °C for at least 1 h for thermal cleaning. We find that EuSe on BaF2 without a buffer layer is quasi-two-dimensional and yields a blurred and spotty RHEED pattern, as seen in Fig. 2(c), and a rough surface. An introduction of a graded Pb1−xEuxSe buffer layer (from bottom to top x=0,x=0.2,andx=0.425) restores 2D epitaxial growth, as indicated by the streaky RHEED pattern in Fig. 2(d). The growth of this buffer layer provides a surface that is isostructural with EuSe and is found to be ideal to initiate the growth of a thin layer of this material. An EuSe film grown on a gradient buffer is compressively strained, whereas all other growths yield relaxed layers.

Structurally, the films are characterized by high resolution x-ray diffraction (HRXRD) on a six-angle diffractometer equipped with a 1.5406 Å Cu_Kα1 source. Transmission electron microscopy (TEM) measurements are also carried out and shown in the supplementary material. Figures 3 and 4 show the x-ray diffraction (XRD) characterizations revealing the growth direction and strain of EuSe thin films grown on various substrates. The growth of EuSe on GaAs (111)B yields two different types of samples: samples with a mixed phase with both EuSe (111) and EuSe (001), and a single phase with EuSe (001) shown in Figs. 3(a) and 3(b) for two different samples. The growth direction depends on the Se:Eu ratio: for Se:Eu > 10, a (001) growth is preferred (SA1), while for Se:Eu ∼ 4, a (001) growth with (111) inclusions is observed (SA2), as can be seen from both XRD spectrum and TEM images. The structural properties of EuSe grown on GaAs (111)B and the corresponding interface alignments are discussed in the supplementary material (Figs. S2 and S3).

FIG. 3.

Structural characterization of EuSe growth on various substrates. HRXRD of EuSe films grown on (a) GaAs (111)B, (b) Bi2Se3, (c) BaF2 (111), and (d) Pb1−xEuxSe/BaF2 (111) substrates.

FIG. 3.

Structural characterization of EuSe growth on various substrates. HRXRD of EuSe films grown on (a) GaAs (111)B, (b) Bi2Se3, (c) BaF2 (111), and (d) Pb1−xEuxSe/BaF2 (111) substrates.

Close modal
FIG. 4.

(a) Polar plot of a φ-scan of the (224) EuSe Bragg peak with (001) growth direction on Bi2Se3, and ϕ-scan of the {0 1 22} Bi2Se3 Bragg peak for sample SD1. (b) Alignment of the [100] direction of EuSe with a {110} direction of Bi2Se3. (c) Alignment of the [110] direction of EuSe with a {110} direction of Bi2Se3. (d) TEM of EuSe on Bi2Se3 showing grains with various in-plane orientations, consistent with the ϕ-scan.

FIG. 4.

(a) Polar plot of a φ-scan of the (224) EuSe Bragg peak with (001) growth direction on Bi2Se3, and ϕ-scan of the {0 1 22} Bi2Se3 Bragg peak for sample SD1. (b) Alignment of the [100] direction of EuSe with a {110} direction of Bi2Se3. (c) Alignment of the [110] direction of EuSe with a {110} direction of Bi2Se3. (d) TEM of EuSe on Bi2Se3 showing grains with various in-plane orientations, consistent with the ϕ-scan.

Close modal

Figure 3(b) shows the XRD patterns of EuSe grown on Bi2Se3. A strong (002) EuSe peak is observed, indicating that EuSe is predominantly (001) oriented. A wide-angle scan is included in the supplementary material, ruling out the formation of any phases in this layer other than EuSe. The lattice constant extracted from the (002) peak (6.186 Å) suggests that EuSe grows along the (001) direction and is fully relaxed in the c-plane of Bi2Se3 (001). The Bi2Se3 (006) peak also appears in the data and yields a lattice constant (28.511 Å) close to the lattice constant of bulk Bi2Se3.

The XRD pattern of EuSe grown on BaF2 (111) is shown in Fig. 3(c). In this case, an EuSe layer with the (111) orientation is obtained, which is negligibly strained (<0.09%). The XRD pattern from EuSe grown on the Pb1−xEuxSe buffer layer is shown in Fig. 3(d). This layer is also (111) oriented but slightly compressively strained in-plane and tensile strained in the (111) direction (0.6%).

To further understand the planar arrangement of the (001) EuSe with the C4 symmetry of the surface on a Bi2Se3 surface with the C3 symmetry, we carried out a φ-scan of {224} planes of the (001) EuSe. It exhibits a 24-fold periodic structure shown in Fig. 4(a). The origin of this structure can be understood as follows. The four EuSe {100} planes align with the three {110} planes of the Bi2Se3 (001) surface, forming a 12-fold periodic structure [Fig. 4(b)]. The four EuSe {110} planes can also align with the three {110} planes of the Bi2Se3 (001) surface, yielding an additional 12-fold periodic structure shifted by 15° [Fig. 4(c)]. This 24-periodic structure is contrasted with the threefold periodic pattern obtained from Bi2Se3. The type of alignment observed here is similar to the alignment found for the rock salt PbSe on zinc blende GaAs(111)B.29 Thus, while the out-of-plane orientation of EuSe occurs predominantly along the (001) direction, two types of in-plane alignments are recovered. Rotated domains can also be resolved in the TEM image of EuSe(001) grown on Bi2Se3, shown in Fig. 4(d) confirming this conclusion.

Overall, XRD measurements clearly demonstrate growth in the (111) orientation on BaF2 (111) and Pb1−xEuxSe (111) with a nearly matched lattice, as well as a mixed phase or a pure (001)-oriented growth on GaAs(111)B and the Bi2Se3 surfaces with a large lattice mismatch. This flexibility of crystallographic orientation can provide additional control of the magnetic exchange-coupling at the EuSe/Bi2Se3 interface and an additional knob to alter the properties of the TSS.

The magnetic characterization of EuSe films is performed with a Quantum Design MPMS-3 SQUID magnetometer. More information on sample preparation and data analysis can be found in the supplementary material. This section is focused on the comparison of samples grown on BaF2 and Bi2Se3. The temperature dependence of the magnetic susceptibility χ for BaF2/EuSe and Bi2Se3/EuSe is plotted in Figs. 5(a) and 5(b), respectively. The Néel temperature TN=4.44.9K for strain-free EuSe thin films SB and SD1 is found to be close to TN in bulk crystals (4.6 K) and in thick epitaxial films (4.75±0.25K).21 For EuSe (111), we also find that the in-plane χ||0 for T0, while the out-of-plane χ remains finite [Fig. 5(a)]. This confirms that the magnetization at low fields lays in the (111) plane perpendicular to the growth axis. For (001) EuSe growth, both χ|| and χ are finite in weak magnetic fields when T0, as shown by dashed lines [Fig. 5(b)]. This is consistent with magnetic field pointing at an angle to the magnetization axis for both directions of the field and indicates that the magnetization does not lie along the growth axis but remains in the (111) plane.

FIG. 5.

Temperature dependence of magnetic susceptibilities of EuSe with in-plane H and out-of-plane H magnetic fields for (a) EuSe (111) and (b) EuSe (001): Both samples show the evolution of magnetic properties from AF to FM when increasing the applied magnetic field. The dashed lines indicate a comparison of zero temperature susceptibilities at small H and H for both samples.

FIG. 5.

Temperature dependence of magnetic susceptibilities of EuSe with in-plane H and out-of-plane H magnetic fields for (a) EuSe (111) and (b) EuSe (001): Both samples show the evolution of magnetic properties from AF to FM when increasing the applied magnetic field. The dashed lines indicate a comparison of zero temperature susceptibilities at small H and H for both samples.

Close modal

In Figure 6, we plot the magnetization loops M(H) normalized to fully magnetized Eu2+ (6.94 μB) for sample SD1. Magnetization loops are shown for different temperatures and for a field applied parallel (H) and perpendicular (H) to the {001} planes. Magnetic transitions are more pronounced in the derivative plot (dM/dH), as shown in Fig. 7. The four peaks correspond to the expected phase transitions FM–FIM (A) and FIM–AF (B). In agreement with previous reports, the FM–FIM transition is observed at H(A)=1.52.5kOe and is not hysteretic. The sharpness of this transition does not depend on the temperature, as can be seen from the almost T-independent height of the A peaks. It was unexpectedly found that the AF–FIM transition (peaks B) is strongly hysteretic; the hysteresis is emphasized in Fig. 7(a), where the upward sweep (dashed line) is overlapped with the downward sweep curves. The temperature dependence of peak B indicates that the AF–FIM transition broadens with the temperature decrease. The presence of a peak around H=0 confirms that in thin (001) EuSe films, magnetization is retained within the {111} planes. For H, the differential magnetic susceptibility shows a larger peak centered at H=0, which indicates a stronger out-of-plane canting of the spins. The magnetic susceptibility of EuSe (001) films is a combination of the in-plane and out-of-plane responses of the EuSe (111) film (see the supplementary material). Therefore, the observation of peak A (and peak B), which corresponds to the FM–FIM (and AF–FIM) transitions, is almost identical for both H and H.

FIG. 6.

Normalized magnetic moment per Eu2+ ion as a function of magnetic field at different temperatures for (a) H and (b) H for EuSe (001) on van der Waals TI Bi2Se3 grown on sapphire. The saturation values of FM and FIM are indicated by dashed lines. Inset: hysteresis shown on an enlarged scale.

FIG. 6.

Normalized magnetic moment per Eu2+ ion as a function of magnetic field at different temperatures for (a) H and (b) H for EuSe (001) on van der Waals TI Bi2Se3 grown on sapphire. The saturation values of FM and FIM are indicated by dashed lines. Inset: hysteresis shown on an enlarged scale.

Close modal
FIG. 7.

Magnetic properties of EuSe (001) on Bi2Se3: Differential magnetic susceptibility (dM/dH) as a function of magnetic field with two opposite field sweep directions at different temperatures for in-plane H [(a) and (b)] and out-of-plane H [(d) and (e)]: FM–FIM transitions are labeled by A+ and A and guided by solid lines, AF–FIM transitions are labeled by B+ and B and guided by dotted lines, and arrows indicate the field sweep direction corresponding to the dM/dH curves; (c) An amplitude of peaks A and B is plotted as a function of temperature; (f) EuSe (001) crystal structure with a magnetization plane (cyan color) and spin rotation (blue arrows).

FIG. 7.

Magnetic properties of EuSe (001) on Bi2Se3: Differential magnetic susceptibility (dM/dH) as a function of magnetic field with two opposite field sweep directions at different temperatures for in-plane H [(a) and (b)] and out-of-plane H [(d) and (e)]: FM–FIM transitions are labeled by A+ and A and guided by solid lines, AF–FIM transitions are labeled by B+ and B and guided by dotted lines, and arrows indicate the field sweep direction corresponding to the dM/dH curves; (c) An amplitude of peaks A and B is plotted as a function of temperature; (f) EuSe (001) crystal structure with a magnetization plane (cyan color) and spin rotation (blue arrows).

Close modal

Using the field dependence of peaks A and B, we construct and compare the magnetic phase diagrams of EuSe (001) and EuSe (111) thin films in Fig. 8. Both thin films show similar phase diagrams. For EuSe (111) [see Figs. 8(c) and 8(d)], we observe an increase in the field values by 20% for FM–FIM compared to the purple lines, which show the phase boundaries in bulk EuSe (111), extracted from Ref.21. In bulk samples, two distinct AF regions with different magnetic periodicities are observed above 3.6 and below 2.8 K, and the AF phase is not formed in the temperature range of 2.8–3.6 K. In thin films, we found that the AF–FIM transition is present for all T<TN, and the transition field H||(B) for field sweeping down increases uniformly with decreasing T. The value H||(B)0.4H||(A) of the FM–FIM transition and has a similar dependence on T. On the contrary, H||(B+) for the FIM–AF transition decreases with decreasing T; H||(B+)0 for T<3.2K. A small hysteresis in the bandgap energy as a function of magnetic field has been reported in μm-thick films;33 the large hysteresis in the current work (1kOe) can be attributed to the strongly reduced film thickness compared to the previous work. The strong dependence of the dynamics of the phase transition on the film thickness and the presence of surfaces has been extensively studied for the AF–FM transition, which occurs near room temperature in FeRh alloys.34–36 Such a strong modification of the AF–FIM transition in thin EuSe films can also affect the strength of the EuSe/TI magnetic exchange.

FIG. 8.

Phase diagram of EuSe (001) on Bi2Se3 for (a) field sweeping down and (b) field sweeping up. In-plane phase diagram of EuSe (111) on nearly lattice-matched BaF2 (111) for (c) field sweeping down and (d) field sweeping up. Dots and triangles are the positions of peaks A and B in Fig. 7. Black and red symbols indicate H and H, respectively. The red, green, blue, and yellow areas represent the AF, FIM, FM, and paramagnetic (PM) regions, respectively. The arrows indicate the directions of the field sweep. For a comparison, the purple lines are phase boundaries for a bulk μm-thick EuSe (111) (data from Ref. 21) and orange lines are phase boundaries for a single crystal EuSe (001) (data from Ref. 23).

FIG. 8.

Phase diagram of EuSe (001) on Bi2Se3 for (a) field sweeping down and (b) field sweeping up. In-plane phase diagram of EuSe (111) on nearly lattice-matched BaF2 (111) for (c) field sweeping down and (d) field sweeping up. Dots and triangles are the positions of peaks A and B in Fig. 7. Black and red symbols indicate H and H, respectively. The red, green, blue, and yellow areas represent the AF, FIM, FM, and paramagnetic (PM) regions, respectively. The arrows indicate the directions of the field sweep. For a comparison, the purple lines are phase boundaries for a bulk μm-thick EuSe (111) (data from Ref. 21) and orange lines are phase boundaries for a single crystal EuSe (001) (data from Ref. 23).

Close modal

In summary, we have studied the structural and magnetic properties of thin EuSe epilayers grown on various substrates and buffer layers. EuSe grows in the (001) direction on both GaAs(111)B and Bi2Se3 substrates with good morphology. On BaF2 (111), EuSe grows in the (111) direction in the quasi-2D growth mode, which leads to the formation of a rough surface. The introduction of Pb1−xEuxSe buffer layers improves the surface morphology. The magnetic phase diagram for thin films has AF, FIM, and FM phases similar to bulk crystals. The field value for the FIM–FM transitions is slightly enhanced compared to the bulk one, while the AF–FIM transition is highly hysteretic and has a T-dependence that is different from bulk crystals. Controlling the dynamics of the phase transition is the key for the successful realization of selective gaping of TSS in magnetic material/TI heterostructures. The observed enhancement of the AF–FIM transition and strong hysteresis may provide a useful means to control magnetic order and phase transition by tuning the film thickness. The growth of (001) EuSe on Bi2Se3 and GaAs (111)B makes it possible to realize an AF interface with a TSS.

The supplementary material includes atomic microscopy images of devices listed in Table I, XRD ω2Θ characterization of EuSe films grown on GaAs(111)B substrates, wide XRD scans of EuSe/Bi2Se3 films, growth and properties of trilayer EuSe/Bi2Se3/EuSe films, details of magnetic measurements and analysis, and phase diagram for a (111) EuSe/BaF2 film.

The authors acknowledge support from the NSF DMR [Nos. 1836758 (Y.W.), 2005092 (L.P.R.), and 1905277 (X.L., S.-K.B., J.K.F., J.W., and B.A.A.)]. M.Z. and T.O. acknowledge the use of facilities for High Resolution Electron Microscopy at the University of Notre Dame. V.L. acknowledges the use of resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory.

The authors have no conflicts to disclose.

Y.W. and X.L. contributed equally to this work.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

1.
Y.
Tokura
,
K.
Yasuda
, and
A.
Tsukazaki
, “
Magnetic topological insulators
,”
Nat. Rev. Phys.
1
,
126
143
(
2019
).
2.
P.
Chen
 et al, “
Tailoring the hybrid anomalous Hall response in engineered magnetic topological insulator heterostructures
,”
Nano Lett.
20
,
1731
1737
(
2020
).
3.
S.-J.
Chang
 et al, “
Heterostructured ferromagnet-topological insulator with dual-phase magnetic properties
,”
RSC Adv.
8
,
7785
7791
(
2018
).
4.
R.
Yu
 et al, “
Quantized anomalous Hall effect in magnetic topological insulators
,”
Science
329
,
61
64
(
2010
).
5.
M.
Mogi
 et al, “
Magnetic modulation doping in topological insulators toward higher-temperature quantum anomalous Hall effect
,”
Appl. Phys. Lett.
107
,
182401
(
2015
).
6.
D.
Xiao
 et al, “
Realization of the axion insulator state in quantum anomalous Hall sandwich heterostructures
,”
Phys. Rev. Lett.
120
,
056801
(
2018
).
7.
L.
Fu
and
C. L.
Kane
, “
Superconducting proximity effect and Majorana fermions at the surface of a topological insulator
,”
Phys. Rev. Lett.
100
,
096407
(
2008
).
8.
P.
Wei
 et al, “
Exchange-coupling-induced symmetry breaking in topological insulators
,”
Phys. Rev. Lett.
110
,
186807
(
2013
).
9.
F.
Katmis
 et al, “
A high-temperature ferromagnetic topological insulating phase by proximity coupling
,”
Nature
533
,
513
516
(
2016
).
10.
J.
Chen
 et al, “
Evidence for magnetic skyrmions at the interface of ferromagnet/topological-insulator heterostructures
,”
Nano Lett.
19
,
6144
6151
(
2019
).
11.
F.
Wang
 et al, “
Observation of interfacial antiferromagnetic coupling between magnetic topological insulator and antiferromagnetic insulator
,”
Nano Lett.
19
,
2945
2952
(
2019
).
12.
Q. L.
He
 et al, “
Tailoring exchange couplings in magnetic topological-insulator/antiferromagnet heterostructures
,”
Nat. Mater.
16
,
94
100
(
2017
).
13.
C.
Lei
,
H.
Chen
, and
A. H.
MacDonald
, arXiv:2107.02307 (2021).
14.
A.
Mauger
and
C.
Godart
, “
The magnetic, optical, and transport properties of representatives of a class of magnetic semiconductors: The europium chalcogenides
,”
Phys. Rep.
141
,
51
176
(
1986
).
15.
W. L.
Roth
, “
Neutron and optical studies of domains in NiO
,”
J. Appl. Phys.
31
,
2000
2011
(
1960
).
16.
H.
Kondoh
and
T.
Takeda
, “
Observation of antiferromagnetic domains in nickel oxide
,”
J. Phys. Soc. Jpn.
19
,
2041
2051
(
1964
).
17.
T.
Yamada
, “
Magnetic anisotropy, magnetostriction, and magnetic domain walls in NiO. I. Theory
,”
J. Phys. Soc. Jpn.
21
,
664
671
(
1966
).
18.
S.
Saito
,
M.
Miura
, and
K.
Kurosawa
, “
Optical observations of antiferromagnetic S domains in NiO (111) platelets
,”
J. Phys. C: Solid State Phys.
13
,
1513
1520
(
1980
).
19.
K.
Nakahigashi
,
N.
Fukuoka
, and
Y.
Shimomura
, “
Crystal structure of antiferromagnetic NiO determined by x-ray topography
,”
J. Phys. Soc. Jpn.
38
,
1634
1640
(
1975
).
20.
G. A.
Slack
, “
Crystallography and domain walls in antiferromagnetic NiO crystals
,”
J. Appl. Phys.
31
,
1571
1582
(
1960
).
21.
R. T.
Lechner
 et al, “
Strain induced changes in the magnetic phase diagram of metamagnetic heteroepitaxial EuSe/PbSe1-xTex multilayers
,”
Phys. Rev. Lett.
94
,
157201
(
2005
).
22.
R. T.
Lechner
 et al, “
Spin configurations in strained magnetic EuSe/PbSe1-xTex superlattices grown by molecular beam epitaxy
,”
Physica E
32
,
379
382
(
2006
).
23.
R.
Griessen
,
M.
Landolt
, and
H. R.
Ott
, “
A new antiferromagnetic phase in EuSe below 1.8 K
,”
Solid State Commun.
9
,
2219
2223
(
1971
).
24.
U.
Köbler
 et al, “
Biquadratic exchange interactions in the europium monochalcogenides
,”
Z. Phys. B Condens. Matter
100
,
497
506
(
1996
).
25.
J. S.
Moodera
,
R.
Meservey
, and
X.
Hao
, “
Variation of the electron-spin polarization in EuSe tunnel junctions from zero to near 100% in a magnetic field
,”
Phys. Rev. Lett.
70
,
853
856
(
1993
).
26.
A. B.
Henriques
 et al, “
Band-edge polarized optical absorption in europium chalcogenides
,”
Phys. Rev. B
72
,
155337
(
2005
).
27.
K.
Prokeš
 et al, “
Search for enhanced magnetism at the interface between Bi2Se3 and EuSe
,”
Phys. Rev. B
103
,
115438
(
2021
).
28.
B.
Díaz
 et al, “
Magnetic ordering and transitions of EuSe studied by x-ray diffraction
,”
Phys. Rev. B
81
,
184428
(
2010
).
29.
X.
Liu
,
J.
Wang
,
L.
Riney
,
S. K.
Bac
,
D. J.
Smith
,
M. R.
McCartney
,
I.
Khan
,
A. J.
Hoffman
,
M.
Dobrowolska
, and
J. B. A. A.
Furdyna
, “
Unraveling the structural and electronic properties of the strained PbSe on GaAs
,”
J. Cryst. Growth
570
,
126235
(
2021
).
30.
A.
Richardella
 et al, “
Coherent heteroepitaxy of Bi2Se3 on GaAs (111)B
,”
Appl. Phys. Lett.
97
,
262104
(
2010
).
31.
X.
Liu
 et al, “
Structural properties of Bi2Te3 and Bi2Se3 topological insulators grown by molecular beam epitaxy on GaAs(001) substrates
,”
Appl. Phys. Lett.
99
,
171903
(
2011
).
32.
S.
Vishwanath
 et al, “
Controllable growth of layered selenide and telluride heterostructures and superlattices using molecular beam epitaxy
,”
J. Mater. Res.
31
,
900
910
(
2016
).
33.
R.
Kirchschlager
,
W.
Heiss
,
R. T.
Lechner
,
G.
Bauer
, and
G.
Springholz
, “
Hysteresis loops of the energy band gap and effective g factor up to 18 000 for metamagnetic EuSe epilayers
,”
Appl. Phys. Lett.
85
,
67
(
2004
).
34.
I.
Suzuki
,
T.
Koike
,
M.
Itoh
,
T.
Taniyama
, and
T.
Sato
, “
Stability of ferromagnetic state of epitaxially grown ordered FeRh thin films
,”
J. Appl. Phys.
105
,
07E501
(
2009
).
35.
S. P.
Bennett
 et al, “
Direct evidence of anomalous interfacial magnetization in metamagnetic Pd doped FeRh thin films
,”
Sci. Rep.
5
,
9142
(
2015
).
36.
S. P.
Bennett
 et al, “
Giant controllable magnetization changes induced by structural phase transitions in a metamagnetic artificial multiferroic
,”
Sci. Rep.
6
,
22708
(
2016
).

Supplementary Material