Nm-order FeAlSi epitaxial films with a partially D03-ordered structure were grown on MgO substrates, and ideal soft magnetic properties were obtained. We found that the sign of the magnetocrystalline anisotropy constant K1 changes with increasing annealing temperature for certain FeAlSi compositions. This is caused by a change in the volume balance of the ordered phases with the annealing process and the point at which K1 ∼ 0 shifts to the Al-rich concentration as the degree of D03-ordering decreases. K1 was precisely measured by ferromagnetic resonance under the optimal condition, and the value of 1.6 × 102 (erg/cc) was obtained, which is comparable to that of bulk. The uniaxial component of the magnetic anisotropy due to magnetostriction was small, and a fourfold symmetric component due to magnetocrystalline anisotropy was dominant.

Sendust alloy (Fe73.7Al9.7Si16.6 atm %; hereinafter “FeAlSi”) is a soft magnetic material invented in 1937 by Masumoto and Yamamoto.1 FeAlSi has an ideal soft magnetic property comparable to that of NiFe,2 a conventional soft magnetic material. By fine-tuning the composition of FeAlSi alloys, Masumoto and Yamamoto discovered a specific composition (Sendust central composition, Fe73.7Al9.7Si16.6 atm%) with very high magnetic permeability by preparing numerous samples with different compositions. The soft magnetic property of the Sendust central composition stems from the magnetocrystalline anisotropy constant K1, and the saturation magnetostriction constant λs becomes zero simultaneously.3–6 The crystal structure of FeAlSi is a D03-ordered structure, which is an ordered structure of bcc(A2).7 As the degree of D03 order decreases, it changes into a B27 or A2 structure and causes the soft magnetic property to degrade.8 FeAlSi has various applications (e.g., magnetic heads using bulk FeAlSi8–10), and the compatibility of the FeAlSi central composition and the D03-ordered structure is essential for bringing out its soft magnetic properties. Previous studies on μm-order FeAlSi films using sputtering and evaporation and prepared films revealed soft magnetic properties and D03-ordered structures similar to that of bulk.11–14 Although there have been subsequent studies on FeAlSi, none have examined FeAlSi films with nm-order thickness. Although high-quality sputtering techniques have been established and the thin-film properties of various materials have been investigated in detail, research on FeAlSi thin films has been relatively insufficient. This should be addressed because the bcc soft magnetic material, FeAlSi, may be suitable for application to tunnel magneto-resistive (TMR) sensors, which have been attracting attention in the field of spintronics.15 TMR sensors consist of three thin-film layers: a pinned layer, an insulator, and a free layer. The sensitivity of the TMR sensor is expressed as TMR ratio/2Hk,15 where Hk is the magnetic anisotropy field of the free layer. The bcc structure is expected to have a high TMR ratio due to the coherent tunneling of the Δ1 electron,16 which makes FeAlSi a promising free layer material for attaining both a high TMR ratio and a low Hk. As such, it is crucial to investigate the magnetic properties of FeAlSi thin films in detail. In our previous work,17 we fabricated nm-order FeAlSi polycrystalline sputtered films for application to TMR sensors and found that the magnetic tunnel junctions (MTJs) with FeAlSi electrodes exhibited a TMR effect with ideal soft magnetic properties. We also fabricated FeAlSi epitaxial films, but their soft magnetic properties were inferior to that of bulk due to insufficient composition adjustment in sputtered FeAlSi films.18 The purpose of our present study is to obtain soft magnetic properties comparable to those of bulk by finely controlling the composition and atomic ordering of the FeAlSi films.

All films were deposited on MgO[001] substrates by DC/RF magnetron sputtering (Pbase < 2 × 10−6 Pa). The stacking structure was MgO-sub/MgO(20)/FeAlSi(30)/Ta(5) (thickness in nm). The calculated lattice mismatch between FeAlSi and the MgO buffer layer was a relatively low value of 4.7%, so epitaxial growth of FeAlSi films on MgO was expected. The structure of the FeAlSi layers was measured with x-ray diffraction (XRD) of out of the plane 2θθ scan, (111)-plane ϕ scan, and in-plane reciprocal space mapping (RSM). The magnetic properties were investigated using a vibrating sample magnetometer (VSM), ferromagnetic resonance (FMR), and the optical lever method.19 FMR was measured to determine the exact magnetocrystalline anisotropy constant K1. The rotating magnetic field was applied in the in-plane direction of the film, and K1 and Ku were calculated from the angular dependence of the resonant field by the following equation:

ωγ=1MsMsHres+K121cos 4ϕ2MsHres+2Ku cos ϕ+2K1 cos 4ϕ,
(1)

where K1 is a cubic magnetic anisotropy constant, Ku is a uniaxial magnetic anisotropy constant, ω is the resonance frequency, and γ is a gyromagnetic ratio constant. K1 and Ku represent the fourfold and twofold in-plane components of the magnetocrystalline anisotropy constant, respectively. The optical lever method was used to determine the magnetostriction constant of the FeAlSi films. We applied a rotating magnetic field in the in-plane direction of the films and observed the change in the reflection position of the laser beam due to the distortion of the sample caused by the inverse magnetostriction effect. The shift between the sample distortion and the laser reflection position is expressed by the following equation:

δll=dlaserEsub.tsub.218pLtFeAlSiEfilm,
(2)

where δl/l is the film distortion, dlaser is the laser reflection position, E is Young's modulus, t is the thickness, p is the spot position, and L is the sensor position.

Film strain was measured along the FeAlSi [110] direction, and then λ111 was calculated using the following equation:20 

δll110=34λ111sin2ϕ+14λ100+aϕ,
(3)

where aϕ is an offset term.

All films were annealed at TFeAlSi = 300–600 °C for 1 h without applying a magnetic field to improve the degree of D03 order and soft magnetic properties. As mentioned above, the soft magnetic properties of FeAlSi are very sensitive to the composition of the film, but the composition of sputtered films generally deviates from the target composition.21 In this study, five samples (1–5) with varying film compositions were used to obtain FeAlSi films with optimal compositions. The compositions of the as-deposited FeAlSi films were measured with inductively coupled plasma (ICP) spectrometry. Table I summarizes the composition (at. %) of each target and film, where the samples are arranged in order of the Al concentration and categorized into Al rich or Al poor. As shown, a composition deviation between the films and targets was observed, as reported previously.21 The thin-film composition of samples 1–3 differed despite having the same target composition; this was likely due to individual differences in the target fabrication process.

TABLE I.

Composition of five sputtering targets and films deposited from respective targets (Sendust center composition for bulk: Fe73.7Al9.7Si16.6 at. %).

TargetFilm
FeAlSiFeAlSi
Al rich Sample 1 68.7 13.4 17.9 71.6 12.8 15.6 
Sample 2 68.7 13.4 17.9 73.7 11.7 14.6 
Sample 3 68.7 13.4 17.9 74.8 11.1 14.1 
Al poor Sample 4 69.9 11.6 18.5 75.3 9.2 15.5 
Sample 5 71.6 11.9 16.5 76.2 8.6 15.2 
TargetFilm
FeAlSiFeAlSi
Al rich Sample 1 68.7 13.4 17.9 71.6 12.8 15.6 
Sample 2 68.7 13.4 17.9 73.7 11.7 14.6 
Sample 3 68.7 13.4 17.9 74.8 11.1 14.1 
Al poor Sample 4 69.9 11.6 18.5 75.3 9.2 15.5 
Sample 5 71.6 11.9 16.5 76.2 8.6 15.2 

Figure 1(a) shows the XRD patterns (2θθ scan) of FeAlSi films annealed at Ta = 300–600 °C. Because there was no significant difference in the XRD results between the compositions, only the results for sample 3 are shown as a typical result. (The results for other composition samples are shown in the supplementary material Figs. S3 and S4.) FeAlSi(004) A2 peaks were observed for all annealed films, indicating the crystallization of FeAlSi. In addition, FeAlSi(002) B2 peaks were observed at annealing temperatures above 400 °C, indicating a change in the ordered structure above B2. Figure 1(b) shows the XRD patterns (φ scan) in which we observed FeAlSi(111) D03 peaks and verified a D03-ordered structure. Figure 1(c) shows the degree of order for D03 and B2 (SD03, SB2). SD03 and SB2 were calculated by using the intensity ratio between superlattice peaks (I111-D03 and I222-B2) and fundamental peaks (I444-A2) observed in the (111)-plane ϕ scan XRD patterns, with the following formulas as a Ref. 22:

S=Isuperlatticeobs./Ifundamentalobs.Isuperlatticecal./Ifundamentalcal.,
(4)
Ihklcal=LPψFhkl2exp2Bsin2θλ2.
(5)
FIG. 1.

XRD patterns of (a) 2θθ scan and (b) (111)-plane ϕ scan for sample 3 FeAlSi film annealed at 300–600 °C. (c) Ta dependence of degree of D03 and B2 ordering.

FIG. 1.

XRD patterns of (a) 2θθ scan and (b) (111)-plane ϕ scan for sample 3 FeAlSi film annealed at 300–600 °C. (c) Ta dependence of degree of D03 and B2 ordering.

Close modal

In Eq. (5), LP is the Lorentz-polarization factor of single crystals,23ψ is the powder ring distribution factor,24 and Fhkl is the structure factor of the D03-FeAlSi unit cell for (hkl) diffraction. The factor expressed as an exponential function is a Debye–Waller factor. The degree of D03 order increased monotonically as annealing temperature increased, while the degree of B2 order did not change much. The increase in the degree of D03 order is presumably due to the enhancement of atomic order by annealing. In other words, the overall trend is for the crystal structure to become more ordered as annealing temperature increases, approaching a perfect D03-ordered structure similar to that of bulk.

Figure 2(a) depicts the dependence of the magnetization curves measured by VSM on the annealing temperature, with the results of sample 3 shown as a typical example. (The results for other composition samples are shown in the supplementary material Fig. S5.) The two-step behavior of the magnetization curves for the as deposited samples is considered to be a phase separation, and the films are homogenized by annealing. Fluctuations in composition or degree of order may have caused phase separation and will need to be further investigated in the future. We added smaller scales of the magnetization curves to the inset only for those with small anisotropy. It is important to note that the magnetic easy axis changes as the annealing temperature increases. Specifically, below 400 °C, the [100] direction is the easy axis, while above 500 °C, the [110] direction is the easy axis. This indicates that the sign of the magnetocrystalline anisotropy constant K1 has changed from positive to negative, and K1 approaches zero at around 450 °C. The sign reversal of K1 with annealing was also observed for samples 1 and 2 with Al-rich composition, which suggests that Al composition is important for attaining K1 ∼ 0 in FeAlSi films. The annealing temperature at which K1 approached zero varied slightly between 400 and 450 °C in the Al-rich samples, showing that the sign of K1 and the temperature at which K1 changes both depend on the film composition. Figure 2(b) shows the annealing temperature dependence of Hk for all samples with varying compositions estimated in magnetization curves, where we can see that Hk is minimal at around 400–450 °C for samples 1, 2, and 3. In contrast, samples 4 and 5 (films with poor Al) did not show K1 ∼ 0. The lowest Hk value was below 1 Oe for sample 3, which is a comparable value to that of other soft magnetic materials such as NiFe or CoFeSiB.

FIG. 2.

(a) Magnetization curves for the sample 3 FeAlSi film annealed at 300–600 °C. (b) Annealing temperature dependence of Hk.

FIG. 2.

(a) Magnetization curves for the sample 3 FeAlSi film annealed at 300–600 °C. (b) Annealing temperature dependence of Hk.

Close modal

These findings suggest that the K1 ∼ 0 point differs between bulk and thin films, and the composition of K1 ∼ 0 for films is Al-rich compared with that of bulk. This is presumably due to the volume balance of various ordered phases. As reported previously,25 the soft magnetic property of bulk Sendust is composed of a complex structure of D03-Fe3Si (K1 > 0) and D03-Fe3Al (K1 < 0). When these structures are mixed in a Sendust central composition ratio, the K1 of Sendust approaches zero. The degree of D03 order is almost 100% in the bulk, while in the film, it decreases, and disordered B2 and A2 structures are included, namely, D03-Fe3Si (K1 > 0), D03-Fe3Al (K1 < 0), B2-Fe3Si (K1 > 0), B2-Fe3Al (K1 > 0), and A2-FeAlSi (K1 > 0).25,26 Because D03-Fe3Al only contains negative K1, if the degree of order decreases from D03 to B2, a richer Al is needed to obtain K1 ∼ 0. For a more quantitative discussion, we simulated the dependence of K1 on the Al concentration and degree of order using the K1 values reported previously.25,26Figure 3 summarizes the experimental and simulation results of the dependence of K1 on the Al concentration for the compositions where K1 ∼ 0 was attained. The solid lines show the simulated results for the dependence of K1 on the Al concentration for the bulk and samples 1–3 films. The volume fraction of D03, B2, and A2 was assumed to be calculated by SD03, SB2-SD03, and 100-SB2, and the Fe composition was measured by the ICP method. The total K1 was calculated as the sum of the products of the volume fraction and the reported value of K1 for D03-Fe3Si, D03-Fe3Al, B2-Fe3Si, B2-Fe3Al, and A2-FeAlSi. The calculated results were roughly in agreement with the experimental results, and the points of K1 ∼ 0 shifted to the Al-rich composition as atomic ordering decreased. The slight deviation between the calculation and experiment was caused by errors in the ordering parameter evaluation from the XRD results, the composition analysis by the ICP method, and the estimated volume fraction calculated by the ordering parameter. According to the previous studies on bulk and μm-order films with a perfect D03-ordered structure, the Sendust central composition needs to obtain an ideal soft magnetic property. Meanwhile, for nm-order films with a disordered structure, we found that the volume balance of D03-Fe3Si, D03-Fe3Al, B2-Fe3Si, B2-Fe3Al, and A2-FeAlSi was important for attaining K1 ∼ 0. It is particularly important to control the volume fraction for D03-Fe3Al, as it only yields negative K1. In other words, in the case of nm-order FeAlSi films, K1 ∼ 0 can be easily attained by controlling the film composition and atomic ordering while varying the sputtering target composition and annealing temperature. Our findings should prove useful for the application of FeAlSi films with optimal soft magnetic properties to highly sensitive TMR sensors.

FIG. 3.

Experimental (plotted points) and simulated (solid lines) results of K1 dependence on the Al concentration for samples 1–3 films and bulk with K1 ∼ 0.

FIG. 3.

Experimental (plotted points) and simulated (solid lines) results of K1 dependence on the Al concentration for samples 1–3 films and bulk with K1 ∼ 0.

Close modal

We further clarified the magnetic properties of FeAlSi films prepared by sample 3 (Ta = 450 °C), which exhibited the most optimal soft magnetic properties in the VSM measurements. Figure 4 shows the angle dependence of the resonant field in FMR. We determined that the change in the resonant field was dominated by the fourfold component of the epitaxial thin film, and the contribution of the uniaxial component was very small. The fitted parameter values were K1 = 1.6 × 102 (erg/cc), Ku = 10 (erg/cc), and ω/γ = 88.5 (Oe). The observed K1 value was comparable to that of bulk-FeAlSi in the previous study,25 and we fabricated nm-order FeAlSi films with desirable soft magnetic properties. The exact value of Hk was quite small (0.43 Oe), indicating that the soft magnetic properties of the prepared FeAlSi film are more ideal than those of other typical soft magnetic materials such as NiFe and CoFeSiB.

FIG. 4.

Angle dependence of ferromagnetic resonance field for the sample 3 FeAlSi film.

FIG. 4.

Angle dependence of ferromagnetic resonance field for the sample 3 FeAlSi film.

Close modal

Next, we discuss the uniaxial magnetic anisotropy constant Ku measured by FMR. The uniaxial strain in the in-plane direction of the film during the fabrication of the epitaxial film may have caused the uniaxial magnetic anisotropy as a result of the magnetostriction effect. To investigate this, we evaluated the magnetostriction constant and crystal strain by using the optical lever method and RSM, respectively, and the magnetic anisotropy constant due to magnetostriction was estimated using the following equation:

Ku=32EFeAlSiλσ,
(6)

where σ is the film strain.

Figure 5 shows the angle dependence of δl/l110 measured by the optical lever method. The results for sample 2 with Ta = 400 °C are shown as a typical example. The calculated λ111 was 3.53 × 10−7, which is a similar value to that of bulk (10−7).21 The supplementary material Fig. S1 shows the results of in-plane RSM for sample 3 with Ta = 400 °C. The epitaxial growth of the FeAlSi film was observed, and the in-plane strain, which was calculated from the position of the (100) and (010) peaks, was σ = 2.06 × 10−5. The value of Ku calculated by Eq. (6) was 20.3 (erg/cc), which differs slightly from the value of the uniaxial component of the magnetic anisotropy constant observed by FMR but is almost identical. This suggests that the uniaxial in-plane magnetic anisotropy in FeAlSi films is due to magnetostriction, and that its contribution to the total magnetic anisotropy is small. Thus, we conclude that the fourfold symmetric magnetocrystalline anisotropy constant K1 is dominant in the soft magnetic properties of FeAlSi films.

FIG. 5.

Angle dependence of magnetic strain for the sample 2 FeAlSi film measured by the optical lever method.

FIG. 5.

Angle dependence of magnetic strain for the sample 2 FeAlSi film measured by the optical lever method.

Close modal

In summary, we fabricated epitaxial FeAlSi films on MgO substrates and verified that the annealed FeAlSi films contained a mixture of A2, B2, and D03 ordered structures. VSM measurements showed that the sign of the magnetocrystalline anisotropy constant K1 changed as the annealing temperature increased in certain FeAlSi compositions. This is presumably due to the change in the volume balance of ordered phases caused by the annealing process and the point at which K1 ∼ 0 shifts to the Al-rich concentration, decreasing the degree of D03 ordering. The exact value of K1 measured by the FMR measurement was 159.3 (erg/cc), which is comparable to that of bulk, and the obtained Hk of 0.43 Oe was lower than that of other conventional soft magnetic materials. The uniaxial component of the magnetic anisotropy due to magnetostriction was negligible compared with magnetocrystalline anisotropy with fourfold symmetry. The developed FeAlSi films will be useful as a soft magnetic layer for highly sensitive TMR sensors. Although this study demonstrated the potential of FeAlSi epitaxial films for TMR sensors, further studies are needed to attain both high TMR ratio and soft magnetic properties in MTJ device structures.

See the supplementary material Fig. S1 for the RSM using an XRD pattern in-plane scan of (a) the entire reciprocal lattice space and (b) only the first quadrant, Fig. S2 for a schematic diagram of the FMR, and Figs. S3–S5 for the results of other composition samples (samples 1, 2, 4, and 5). In addition, we provided calculation details in a supplementary material to validate the calculations.

This work was partly carried out at the Fundamental Technology Center (Research Institute of Electrical Communication, Tohoku University) and supported by the Japan Society for the Promotion of Science (JSPS), the Japan Science and Technology Agency (JST) S-Innovation program, the Center for Science and Innovation in Spintronics (CSIS, Tohoku University), the Center for Spintronics Research Network (CSRN, Tohoku University), the Instrumental Analysis Group (Technical Division, School of Engineering, Tohoku University), the GP-Spin at Tohoku University, the ANRI Fellowship, and the Tohoku University (Professor Endo group).

The authors have no conflicts to disclose.

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

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Supplementary Material