Temperature- and magnetic field-induced magnetic structural changes in the Fe3Si/FeSi2 superlattice

Artificial lattices with semiconductor spacers are expected to exhibit changes in their magnetic structure owing to the control of their electronic states. The temperature (T) and magnetic-field (H ext) dependence of the in-plane magnetic structure of an [Fe3Si/FeSi2]20 superlattice with a nonmagnetic and semiconducting FeSi2 spacer layer is investigated using magnetization and polarized neutron reflectivity measurements. When H ext = 5 mT, nearly collinear antiferromagnetic (AF) structures are observed from 4 to 298 K. When H ext = 1 T, field-induced fan-like, noncollinear AF structures showing ferromagnetic components along H ext and transverse AF components are observed at low T.

Because temperature (T) and optical irradiation can modify the electronic state of semiconducting spacers, the resultant change in the interlayer coupling has received considerable attention.The relationship between electric conductivity and magnetism of the spacer has been investigated for Fe/Fe 1-x Si x (0.4 ⩽ x ⩽ 1.0). 11)At x = 1.0, this is a pure Si spacer, and an analysis of the temperature-dependent magnetization using Bruno's quantum interference model 12) shows that the spacer has properties intermediate between metallic and insulating.The remanence ratio M r /M s and saturation field H s increase rapidly with decreasing temperature.In contrast, at x = 0.63, the spacer is purely metallic and M r /M s is independent of temperature and nearly equal to zero.Furthermore, the bilinear and biquadratic interlayer couplings of metallic spacers were found to be smaller than those of insulating spacers.The Fe 3 Si/FeSi 2 artificial lattice consists of magnetic layers (Fe 3 Si) and nonmagnetic semiconducting spacers (FeSi 2 ), and similar magnetoresistance is obtained in its trilayered form 13) and the superlattice. 14)It has recently attracted attention in spintronics applications because of its numerous merits: (i) Fe and Si are rich resources characterized with low geographical risk and cost, good sustainability and environmental safety; (ii) the electric conductivity mismatch in this combination is within an order of magnitude; 15) (iii) FeSi 2 has an extremely large optical absorption coefficient, [16][17][18] which is two orders of magnitude greater than that of Si, and thus a response with a high sensitivity for irradiation is expected; (iv) the d electrons contribute to the electric conduction in both layers; 19) and (v) Fe 3 Si is suitable for practical use because it has a Curie temperature of 840 K and the half saturation magnetization of Fe.The Fe 3 Si/FeSi 2 artificial lattice is a promising candidate for spin-valve devices, and thus a pure spin current in spintronics.20) Thin films of two-dimensional (2D) materials are strongly influenced by band structure and magnetic domains.On the other hand, nanowires (1D) 21) and nanodots (0D) 22) have attracted considerable attention in recent years.In the case of Fe 3 Si, the saturation magnetization of the nanodots is approximately 30% of that of the thin film; this is believed to be due to the strong influence of dead layers on the particle surface.22) In this study, a 2D superlattice, which can be described by a simple magnetic structure model, was chosen to evaluate the interlayer coupling.A previous study on the [Fe 3 Si/FeSi 2 ] 20 superlattice involving the thickness dependence of the spacer revealed that the ferromagnetic (F) interlayer coupling undergoes antiferromagnetic (AF) coupling and then becomes F or noncoupled.15) Furthermore, studies on the T dependence of the interlayer coupling and carrier concentration indicated that changes in the electronic state of nonmagnetic FeSi 2 induce switching of the interlayer coupling.23) Sakai et al. performed preliminary polarized neutron reflectivity (PNR) experiments 24) to observe the magnetic structure of a superlattice, and found the F structure at room temperature and 1 T and the AF structure at 5 mT.However, the detailed magnetic structure of the Fe 3 Si/FeSi 2 superlattice has not yet been reported. Theenergy of interlayer couplings can be obtained from the magnetic structure under a certain T and magnetic field (H ext ) anisotropy.Herein, we clarify the detailed magnetic structure of the [Fe 3 Si/FeSi 2 ] 20 superlattice under various T and H ext using PNR.The [Fe 3 Si/FeSi 2 ] 20 superlattice consisted of a 0.79 nm thick semiconducting nonmagnetic FeSi 2 spacer layer with maximum AF interlayer coupling and a 2.50 nm thick metallic magnetic Fe 3 Si layer.The superlattices, the deposition of which has been described in detail in Refs.15, 23, 24, were deposited on n-type Si(111) substrates with specific resistances of 1000-4000 Ω cm.The deposition was carried out at a substrate temperature of 300 °C by facing targets direct-current sputtering using Fe 3 Si and FeSi 2 alloy targets with Fe/Si atomic ratios of 3:1 and 1:2, respectively.It has been confirmed that B2-type Fe 3 Si layers are epitaxially grown not only on Si(111) substrate but also up to the top layer across the nanocrystalline FeSi 2 layers using electron diffraction and X-ray diffraction.15,25) Vibrating-sample magnetometry was used to measure typical magnetic hysteresis loops at various T.After fieldcooling, the magnetization curves were measured from +9 to −9 T at each T investigated.
PNR investigations of the [Fe 3 Si/FeSi 2 ] 20 superlattice were performed without and with neutron spin polarization analysis under an H ext generated by electromagnets using the polarized neutron reflectometer SHARAKU (BL17). 26)tructure-refinement fitting was performed using original software written by Masayasu Takeda of the JAEA and GenX. 27)wo-channel PNR (2ch-PNR) measures two types of reflectivity, R + and R − , with the neutron spin polarization of the incident beam in the up and down states, respectively.A H ext of 5 mT or 1.0 T was applied vertical to the scattering plane.Figure 1 shows the scattering geometry of PNR.
Here, the magnetic Fe 3 Si layers in the superlattice are denoted as (i) layers (i = 1, 2, …, 20) and the FeSi 2 spacer layer is omitted.The moments of each Fe 3 Si layer, H ext , and neutron direction are indicated by red, blue and black arrows, respectively.The tilt angle between the moment and H ext is γ.The relative angle θ refers to the angle between the moments of the (i) and (i + 1) layers.Time-of-flight spectra were measured at incident angles of 0.4°, 1.2°and 3.6°to cover the momentum transfer (Q) range 0.1-3.0nm −1 with an irradiated surface area of 5 mm × 5 mm.Data reduction was conducted with a momentum transfer resolution (δQ/Q) of 5.4%.
In four-channel PNR (4ch-PNR), R ++ and R −− are nonspin-flip (NSF) profiles in which the polarization state does not change at the sample, whereas R +− and R −+ are spin-flip (SF) profiles in which the polarization state changes. 28)olarized neutron beams with an irradiated surface area of 16 mm × 16 mm compensate for the analyzer transmission loss of the neutron intensity, and the irradiation area and momentum transfer resolution were increased to 16 mm × 16 mm and δQ/Q = 11.2%,respectively.
Figures 2(a) and 2(b) show the macroscopic magnetic hysteresis loops of the superlattice layers at each T investigated under various H ext and at each H ext investigated under various T, respectively.The volumes of Fe 3 Si and FeSi 2 were calculated from the parameters obtained from the PNR analysis.The inset in Fig. 2(a) shows the magnetization curves in the low-field region.Remanent magnetization was observed at T below 50 K.In Fig. 2(b), the nonmonotonic T dependence of the magnetization was observed under a constant H ext , which suggested a change in noncollinear magnetic structure with respect to H ext .
The origin of magnetization was investigated via magnetic structure analysis using PNR measurements.Typical T (4 K and 298 K) and H ext (5 mT and 1.0 T) conditions were selected, and magnetic structure determination was performed using the NSF (R ++ and R −− ) and SF (R +− and R −+ ) profiles.The T-dependent magnetic structures were analyzed in detail using 2ch-PNR with the same parameters used for the superlattice structure.bilayer SF peak is intrinsic, it is considered an F structure in this analysis because γ i < −2°, which is smaller than the error.At 4 K and 5 mT [Fig.3(c)], the Bragg peaks for both the single and double bilayers consist of NSF and SF components.This magnetic structure was determined as the canted-AF structure between Fe 3 Si magnetic layers [Fig.4(c)].The single-bilayer Bragg peak intensities differ between measurements and fittings, but the qualitative relationship is consistent.SA is sensitive to the F structure, and the measurements and fittings closely agree and could be largely explained by the proposed model structure.The discussion in the next paragraph suggests domain formation.Magnetic structures that are line-symmetric with respect to the direction of H ext and indistinguishable were also observed.The canted-AF structure near the sample surface was slightly oriented toward H ext .The remanent magnetization observed in the magnetization measurements confirms this result.
At 298 K and 5 mT [Fig.3(d)], the R + and R − profiles are nearly identical, indicating no SA.4ch-PNR indicated that the single-bilayer Bragg peaks consist of pure NSF components, whereas the double-bilayer peaks consist of NSF and SF components.This result reveals that the (i) and (i + 1) layers are AF aligned and that the moments are tilted with respect to H ext without a net F component.When the moment exists, SA ≠ 0, which is a contradiction.This discrepancy can be resolved by assuming that magnetic domains exist within the layer and that the various reflectivity profiles overlap according to the magnetic domain distribution.These reflectivity results can be explained by the AF structure within the magnetic domain with respect to the H ext direction, as shown in Fig. 4(d).This structure model is static, but it could also be dynamic, that is, the moment can fluctuate with increasing T.However, the PNR data do not provide further information.
Table I shows the T dependence of the magnetic scattering length density (SLD M ), γ, θ and the Fe moment in the magnetic Fe 3 Si layer from 4 to 298 K.The γ relative to H ext and other parameters are described as (i) layer/(i + 1) layer.When the values are the same for the (i) and (i + 1) layers only the (i) layer is described.When H ext = 5 mT, the structure is nearly AF between magnetic layers and multidomains are formed.
In summary, macroscopic and microscopic measurements of the Fe 3 Si/FeSi 2 superlattice with semiconducting spacers showed that canted-AF noncollinear structures with a small F component changed continuously with T and H ext .The almost collinear AF structure at 298 K and 5 mT indicates AF coupling in the superlattice and the formation of magnetic domains owing to the moment fluctuations in space or time at high T.The former can be observed by polarized off-specular scattering and the latter by inelastic scattering such as spinecho reflectometry.035002-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd

Fig. 2 . 2 ©
Fig. 2. (a) Macroscopic magnetic hysteresis loops of the superlattice layers at each T investigated under various H ext .Inset: magnified view of the low-field region.(b) T dependence at each H ext .035002-2

Figure 4 (
a) shows a schematic of the magnetic structure of the superlattice.At 298 K and 1.0 T [Fig.3(b)], just the single-bilayer Bragg peaks consisting of only NSF components are detected by 4ch-PNR, indicating an F order between each single [Fe 3 Si/FeSi 2 ] bilayer.Figure 4(b) shows the F structure.A small SF peak appears near the double-bilayer position, but the maximum peak is shifted toward the high-Q side, indicating the presence of inhomogeneities along the thickness of the superlattice at 4 K and 1 T.Even if the double-