Insight into the structural and magnetotransport properties of epitaxial heterostructures {\alpha} - Fe2O3-Pt(111): Role of the reversed layer sequence

We report on the chemical structure and spin Hall magnetoresistance (SMR) in epitaxial {\alpha}-Fe2O3(hematite)(0001)/Pt(111) bilayers with hematite thicknesses of 6 nm and 15 nm grown by molecular beam epitaxy on a MgO(111) substrate. Unlike previous studies that involved Pt overlayers on hematite, the present hematite films were grown on a stable Pt buffer layer and displayed structural changes as a function of thickness. These structural differences (the presence of a ferrimagnetic phase in the thinner film) significantly affected the magnetotransport properties of the bilayers. We observed a sign change of the SMR from positive to negative when the thickness of hematite increased from 6 nm to 15 nm. For {\alpha}-Fe2O3(15 nm)/Pt, we demonstrated room-temperature switching of the N\'eel order with rectangular, nondecaying switching characteristics. Such structures open the way to extending magnetotransport studies to more complex systems with double asymmetric metal/hematite/Pt interfaces.

can lead to the formation of FePt alloy and induction of magnetization in Pt, which affects the magnetotransport properties of the system at low temperature. [24] To dispel doubts about the stability of the Pt/α-Fe2O3 interface and its changes induced by annealing, a reversed α-Fe2O3/Pt structure can be considered. The use of the Pt layer as the substrate enables its deposition at elevated temperature and/or postdeposition annealing prior to the growth of α-Fe2O3, which provides the formation of a stable α-Fe2O3/Pt interface.
In our studies, we investigated the chemical and magnetotransport properties in the epitaxial α-Fe2O3/Pt structure, where hematite α-Fe2O3(0001) thin films with thicknesses of (6±0.3) nm and (15±0.5) nm were grown on a 7-nm thick epitaxial Pt(111) layer on MgO(111). Whereas surface-sensitive methods indicated only the hematite phase, Mössbauer spectroscopy measurements showed that the bulk-like hematite phase contributed 96% and 66% of the total spectral intensity for the thicker and thinner oxide layers, respectively.
Although the thinner film contains almost a one-third contribution from a spinel phase, for simplicity, we refer to the thinner and thicker samples as α-Fe2O3(6)/Pt and α-Fe2O3(15)/Pt, respectively. An SMR study revealed that the chemical structure determines the magnetotransport properties of the α-Fe2O3/Pt bilayer.
We noted a sign change of the SMR from positive to negative when the thickness of the oxide was increased, which was discussed in terms of the residual ferrimagnetic phase present in the thinner layer. Finally, for α-Fe2O3(15)/Pt, we demonstrated electrical switching. For the as-grown sample, we registered steplike, nondecaying switching between three antiferromagnetic order states, which demonstrates the stability of the interface in our samples.

II. STRUCTURAL AND MAGNETIC CHARACTERIZATION
Hematite (α-Fe2O3), which is the most stable phase of iron oxide, crystallizes in the trigonal corundum structure with a hexagonal unit cell. Above the Morin transition temperature (TM=250 K) [28] and below the Néel temperature (TN=953 K), [28]  plane. [29] Below TM, hematite undergoes a transition to an easy-axis AFM with spins antiferromagnetically aligned along the c axis. [30] In thin films, with the decrease in hematite thickness, a decrease in TM or complete disappearance of the transition was observed. The transition was also shown to be restorable by doping. [31], [32], [33] In our study, α-Fe2O3/Pt bilayers were prepared in a multichamber UHV system. A one-side polished     Table I). Both spectra are dominated by six line components (magenta shaded regions) with QS = -0.11 mm/s, IS = 0.37 mm/s and an average hyperfine magnetic field BHF of 50.9 T and 51.7 T for the thinner and thicker layers, respectively, which unambiguously identifies regular Fe 3+ sites in the corundum structure of hematite. [26] These components possess a bimodal BHF distribution, where the low intensity broader subcomponents with lower BHF (darker shaded regions in Fig. 2) represent surface hematite layers. [25] Additionally, the more complex spectrum for the 6 nm film exhibits a 30% contribution of sites (green shaded component in Fig.   2(a)) whose hyperfine parameters, in particular IS = 0.26 mm/s and QS = 0.03 mm/sec, are close to those of a spinel phase, namely, maghemite (γ-Fe2O3). The spinel phase (maghemite or nonstoichiometric magnetite, whose Mössbauer sextets are indistinguishable [36]) is a natural residue of the precursor phase for hematite that is obtained via oxidation of magnetite. The epitaxially stabilized spinel phase is more likely to occur in the thinner 6 nm layer than in the 15 nm film, as the stabilization of the spinel/hematite bilayer for the latter case would incur a much higher strain energy. A similar tendency was also found during direct growth of iron oxide films on Pt (111) Notably, down to the temperature of 110 K, we did not observe the Morin transition for either film (which, in the bulk, occurs at a temperature of approximately 250 K). The absence of the Morin transition in thin hematite films was previously reported for α-Fe2O3 grown on Pt(111) [26] and Al2O3(001) [37] substrates.
To summarize, whereas the 15 nm film is fully composed of antiferromagnetic hematite, in the 6 nm film, a 33% contribution of a ferrimagnetic spinel phase must be considered. Comparison of the volume-sensitive methods (XRD and CEMS) with LEED indicates that the spinel phase is localized in deeper layers. The presence of this non-hematite phase can be substantial for the magnetotransport properties of the α-Fe2O3/Pt bilayer, as discussed below. For the magnetotransport experiments, we patterned Hall bar mesa structures on our sample with the use of photolithography and ion beam milling. The width and length of the Hall bars were 30 μm and 80 μm, respectively. Room-temperature magnetoresistance measurements were performed by rotating the external magnetic field Hα relative to the [1 ̅ 100] axis of α-Fe2O3 within the (0001) plane (Fig. 3a). The SMR measurements were performed as a function of magnetic field strength. Longitudinal resistances (Rxx) were determined from 4-wire resistance measurements in which a dc current density of 9.2 x 10 10 A/m 2 (Jc) was applied along the [1 ̅ 100] crystal axis of α-Fe2O3 (x axis) (see Supplemental Materials for details of the measurements). Figure 3 shows the Rxx (α) dependence obtained for the α-Fe2O3/Pt bilayers with hematite thicknesses of 15 nm (Fig. 3(b)) and 6 nm (Fig. 3(c)). For both hematite thicknesses, we noted SMR oscillations with a period of 180°. Interestingly, we observed a phase shift of 90° in the Rxx(α) dependences measured for the dh of the 6 nm and 15 nm bilayers. Whereas for the thinner film, we noted Rxx maxima for

III. SMR MEASUREMENTS
Hα parallel/antiparallel to the x direction and longitudinal resistance minima for Hα perpendicular to x (parallel/antiparallel to the y direction), the Rxx maxima and minima for dh = 15 nm appear for Hα ║±y and Hα ║±x, respectively. The Rxx(α) characteristic observed for the thicker hematite layer ( Fig. 3(b)) results from the perpendicular alignment of the Néel vector with respect to the external magnetic field and is a fingerprint of AFM SMR (so-called "negative SMR"). [16] The amplitude of the AFM SMR obtained in our study for α-Fe2O3(15)/Pt (7) saturates at magnetic field of 2T (Fig. S3, Supplemental Materials), similarly to the previous studies obtained for reversed Pt/α-Fe2O3 structure [17]. At 2T the amplitude of SMR is approximately 1x10 -4 . This value is higher than the SMR amplitude noted for the Pt(3.5)/NiO(120) bilayer registered at magnetic field of 15T. [16] For our thinner film, we noted a 90° phase shift in the SMR signal ( Fig. 3(c)), i.e., the SMR exhibits the cos 2 α dependence expected for FM/HM bilayers (positive SMR), in which the FM magnetization aligns parallel to Hα. [2] A positive SMR has been recently reported for AFMs, and its origin was attributed to the presence of either a ferromagnetic component at the AFM/HM interface or a canted AFM spin structure. [13], [38], [39], [40], [41] For bulk hematite, the small net magnetization induced by the DMI was shown to not contribute to the electrical response, which is dominated by the AFM Néel vector. [42] Thus, a negative SMR is expected for α-Fe2O3/Pt bilayers, as we found for the thicker hematite layer. In our study, the appearance of a positive SMR for the α-Fe2O3(6)/Pt bilayer is associated with a notable contribution of a spinel-like component detected in the Mössbauer spectrum, which we localize at the Pt/iron oxide interface. Both Fe3O4 and γ-Fe2O3 are ferrimagnets and can contribute to the positive SMR. The amplitude of the positive SMR obtained in our study for α-Fe2O3(6)/Pt is three times greater than that obtained for Pt(7)/Fe3O4(20)/MgO(001) [5] and twice as large as that obtained for the γ-Fe2O3/Pt bilayer. [43] The SMR signal appears in thinner α-Fe2O3 sample for much weaker magnetic fields, i.e. we noted a clear oscillations of resistance for α-Fe2O3(6) at 0.05T while at the same magnetic field no SMR was noted for α-Fe2O3(15) (compare orange in Figure 3(b) and (c)). This confirms higher sensitivity of ferrimagnetic moment of α-Fe2O3(6) to the external magnetic field. Amplitudes of SMR at considerable magnetic fields (see Fig. S3 in Supplemental Materials) are comparable. This suggests that the effectiveness of the spin transfer from Pt layer to the α-Fe2O3(6) and α-Fe2O3(15) layer is similar. This result may not be surprising if we consider that both materials consist of Fe 3+ insulating oxide.
To exclude proximity-induced anisotropic magnetoresistance (AMR) contribution to the magnetoresistance in our samples we performed additional magnetoresistance measurements in which we rotated the external magnetic field within plane perpendicular to y direction (xz plane). [5] For both α-Fe2O3(6)/Pt and α-Fe2O3(15)/Pt heterostructures we did not observe oscillation of magnetoresistance within xz plane characteristic for AMR (Fig. S4 in Supplemental Materials) [5]. Thus, proximity-induced AMR do not contribute to the magnetoresistance in our study.

IV. CURRENT-INDUCED SWITCHING OF THE NEEL ORDER IN α-Fe2O3(14.7)/Pt(7 nm)
To demonstrate the possibility of current-induced switching between the three easy axes of an AFM α-Fe2O3 layer, we patterned the α-Fe2O3(15)/Pt(7) bilayer into 8 terminal Hall crosses ( Fig. 4(a)) in addition to the Hall bar described in the previous section. Three ( Fig. 4(a), respectively), whereas the fourth path (5 μm wide) was used to measure the Hall voltage (horizontal path, y direction in Fig. 4(a)). During the measurements, we used the following switching protocol. A set of twenty 1-ms-long current pulses (Ip) was applied along one of the easy hematite directions. ( Fig. 4(c), green triangle) and L||[01 ̅ 10] (Fig. 4(c), red square), respectively. In agreement with the theory of a damping-like SOT, [44] together with an increase in the current density, we noted an increase in ΔRxy from 19 mΩ to 26 mΩ when j changed from 9.37 x 10 11 A/m 2 to 9.45 x 10 11 A/m 2 . The current densities required for reorientation of the Néel vector are similar to the values reported for magnetization switching in FM/HM multilayers. [45], [46] Importantly, in our experiment, we noted rectangular, nondecaying switching characteristics with no traces of sawtooth features for the as-grown sample. This result is distinct from switching experiments performed for the reversed Pt/α-Fe2O3 structure, for which sawtooth-shaped switching of ΔRxy was observed for the as-prepared sample and step-like switching appeared only after annealing of the sample with high current pulses. [18] We attribute this difference to the stable α-Fe2O3/Pt interface formed during the growth of our sample, which remained unaffected by the current pulse treatment.

V. SUMMARY AND CONCLUSIONS
We investigated the structure and magnetotransport properties of α-Fe2O3/Pt bilayers with two iron oxide thicknesses, 6 nm and 15 nm. Mössbauer spectroscopy measurements showed that the hematite phase dominates for both oxide thicknesses; however, for the thinner film, interfacial contributions of a residual spinel structure are notable. The structural difference affects the magnetotransport properties of the α-Fe2O3/Pt bilayers. Whereas for the thicker film, we observed a negative SMR, which is characteristic of AFM/HM bilayers, the sign of the SMR for the bilayer with the thinner oxide film was positive. We attributed the change in the SMR sign to the interfacial ferrimagnetic order within the spinel phase.
Finally, for α-Fe2O3(15)/Pt, we demonstrated room-temperature switching of the Néel vector. The steplike nondecaying switching observed for the as-grown sample reveals the formation of a stable AFM/HM interface in our samples. We also note that such structures, with hematite grown on Pt, open the way to extending magnetotransport studies to more complex systems with double asymmetric HM/hematite/Pt interfaces.