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Soft magnetic and structural properties of (FeCo)–(AlSi) alloy thin films
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Soft magnetic and structural properties of (FeCo)–(AlSi) alloy thin films a,b,⁎
b,c
a
b,c
Takuma Nakano , Bhuwan Nepal , Yoshitomo Tanaka , Shuang Wu , Kyotaro Abe Gary Mankeyb,c, Tim Mewesb,c, Claudia Mewesb,c, Takao Suzukib,d,e
T
a,b
,
a
Materials Development Center, TDK Corporation, Narita 286-0805, Japan Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, AL 35487, USA Department of Physics and Astronomy, The University of Alabama, Tuscaloosa, AL 35487, USA d Department of Electrical and Computer Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA e Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Soft magnetic properties DO3 structure Effective damping parameter Coercivity Order parameter
The dependences of static and dynamic magnetic properties on both substrate deposition temperature (TS) and film-thickness (d) for (FeCo)-(AlSi) alloy thin films are discussed in conjunction with structure. The samples of a multi layered type (FeCo)-(AlSi) alloy thin films were fabricated at Ts by DC sputter-deposition onto (1 0 0) MgO substrates by using two targets of Fe75Co25 and Al50Si50. For the first time, the DO3 phase in the quaternary alloy films are realized for TS at around 400 ℃ with d larger than 25 nm. The order parameter (S) was estimated by taking into account crystal structure factor, absorption factor, multiplicity factor, temperature factor, and Lorentz polarization factor. It is found that S increases with d, becoming nearly 1 for d larger than 50 nm. Observations by high resolution transmission electron microscopy support this result. The coercivity (Hc) and effective damping parameter (αeff) decrease with increasing d and remain nearly constant, approximately 4 Oe and 5 × 10−3, respectively. The result that HC decreases with increasing S suggests that (FeCo)-(AlSi) thin films with the DO3 phase are an attractive candidate for future high frequency device applications.
1. Introduction
2. Experimental
Among many material candidates to choose for future high frequency device applications that requires in particular high saturation magnetization (MS), low coercivity (HC) and low damping parameter, FeCo-based alloys systems are the promising candidate as they provide opportunity to improve further the magnetic property. Regarding the correlation between soft magnetic properties and structure, Masumoto et al. [7] reported high permeability and low HC within a narrow composition range around the DO3 phase of the FeSiAl alloy system. Although this finding is very important, there has been very few works found in literature about the detailed correlation between them. Recently, iron based alloy thin films, such as FeCo [1], FeAl [2]. FeCoAl [3,4] and FeCoSi [5,6] have been reported to exhibit magnetic properties and effective damping parameter (αeff), which are attractive for future high frequency device applications. It is noted that the very low damping parameter of 0.0004 was reported in [3,4]. It is of interest to explore further the soft magnetic properties of (FeCo)-(AlSi) thin films in conjunction with the DO3 structure [8]. This work presents a systematic study of the static and dynamic magnetic properties and structure of (FeCo)-(AlSi) alloy thin films.
Multilayers of [Fe75Co25 (0.7 nm)/Al50Si50 (0.4 nm)] × N (N = 5–500) were sputter-deposited at a substrate deposition temperature (TS) over a range from ambient (approximately 40 °C) to 410 °C onto MgO(1 0 0) substrates using DC magnetron sputtering in 4 mTorr Ar atmosphere. During deposition, in order to induce a uniaxial magnetic anisotropy in the film-plane, an external magnetic field of 50 Oe was applied along 〈1 0 0〉MgO in the substrate plane. A 5 nm thick Ru layer was over-coated to prevent samples from oxidation. Two targets of Fe75Co25 and Al50Si50 were used, for both of which the deposition rates were 0.04 nm/s. Crystalline phase and quality were estimated by X-ray diffraction (XRD) using a X’pert, PHILIPS with Cu Kα. Film structure and composition analyses were performed by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) using Titan G2, FEI operated at 300 kV. In order to obtain EDS mapping, background subtraction from net count map and peak resolution for each elements were processed. The film thicknesses were estimated by X-ray reflectivity (XRR). Measurements of magnetic properties were carried out at room temperature by using a vibrating sample magnetometer (VSM).
⁎
Corresponding author at: Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, AL 35487, USA. E-mail address: [email protected] (T. Nakano).
https://doi.org/10.1016/j.jmmm.2020.166852 Received 14 January 2020; Received in revised form 25 February 2020; Accepted 30 March 2020 Available online 31 March 2020 0304-8853/ © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
The effective damping parameter αeff was measured using broadband ferromagnetic resonance (FMR) over a frequency range from 12 to 66 GHz at room temperature along 〈1 1 0〉bcc and 〈1 0 0〉MgO direction.
3. Results and discussion 3.1. Substrate deposition temperature dependence Fig. 1(a) shows the XRD patterns for the 50 nm thick Fe57Co21Al13Si9 alloy thin films deposited at the various TS. For the sample deposited at TS = 40 ℃, the diffraction peaks of (FeCo)-(AlSi) alloys are absent, and only those of the MgO substrate are present, indicating an amorphous structure of the films. With increasing Ts, only the (0 0 2)bcc peak appears and its intensity increases with TS beyond 110 °C, indicating the epitaxial growth of the 〈1 0 0〉bcc along the film normal. It is noted that the film deposited at TS = 410 ℃ does exhibit a diffraction peak at around 31° which is identified to be the super lattice peak (0 0 1) of the DO3 phase. This finding of the DO3 phase is important since this is the first time this ordered phase in the quaternary alloy system has been realized. It is also noted that the epitaxially grown films have the orientation correlation of 〈0 0 1〉bcc//〈1 1 0〉MgO found by low angle XRD. The dependence of lattice constant (c) estimated from the (0 0 2) bcc diffraction peak on TS is also shown in Fig. 1(b). The values of c are nearly constant at 0.284 Å over the entire range of TS, and is slightly smaller than the bulk lattice constant of Fe75Co25 [9]. The coherence length (L), which is defined as the distance over which the X-ray diffraction coherently takes place to form the (0 0 2) peak, is estimated from the full-width-half-maximum of the (0 0 2)bcc diffraction peak as shown in Fig. 1(c). The L is found to increase from 5 to 12 nm with TS. Fig. 2 shows (a) the selected area electron diffraction pattern, (b) and (c) TEM cross-sectional images of low and high magnification,
Fig. 1. (a) The XRD patterns with various TS and (b) the dependence of c and (c) L on TS for Fe57Co21Al13Si9 alloy thin films onto MgO (1 0 0).
Fig. 2. (a) Diffraction pattern, TEM cross-sectional images of (b) low and (c) high magnification and (d) EDS mapping for Fe57Co21Al13Si9 alloy thin films onto MgO (1 0 0) with the various TS. 2
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
Fig. 5. (a) The relation between FMR frequency (fres) and FMR linewidth (ΔH) and (b) the dependence ofαeff on TS for Fe57Co21Al13Si9 alloy thin films onto MgO (1 0 0). Fig. 3. The M-H curves for Fe57Co21Al13Si9 alloy thin films onto MgO (1 0 0) with TS = 40, 310 and 410 °C.
Fig. 4. The dependences of (a) MS and (b) HC forFe57Co21Al13Si9 alloy thin films onto MgO (1 0 0) on TS.
respectively, and (d) EDS mapping for the films with the various TS. For TS = 40 ℃, a halo pattern is seen, in addition to a very weak (1 1 0)bcc peak. The high resolution TEM image does not show any sign of crystallinity, indicating an amorphous structure. It is noted in Fig. 2(b) for TS = 40 ℃ that the two fuzzy horizontal white lines with about a few
Fig. 6. In-plane angular dependence of the resonance field of FeCoAlSi films with TS = 40, 310, 410 ℃. 3
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
applied field in the film plane, where 0 deg. is along 〈1 1 0〉bcc, 〈1 0 0〉MgO, i.e. the external magnetic field direction during deposition. For TS = 40 °C, the easy axis for magnetization is found to be along the external magnetic field during deposition and 〈1 0 0〉MgO. This result indicates that the uniaxial in-plane magnetic anisotropy is induced. With increasing TS, the easy axis for magnetization changes its direction from 0 to 45 deg. This change of the easy axis direction is believed to be due to the evolution and enhancement of magneto-crystalline anisotropy due to the crystal grain growth of the films, which are confirmed by XRD and TEM results. The results from angular dependence of resonance field, shown in later part, also indicate same changing of easy axis with TS. The dependences of saturation magnetization (MS), and HC on TS are shown in Fig. 4(a) and (b), respectively. The MS values are nearly constant, approximately 700 emu/cm3 over an entire range of TS, although the MS with amorphous phase (40℃) might seems to be lower compere to one other. It is not understood for this constant saturation magnetization. However, it is plausible that as one would expect an error margin of about 10% in the magnetization values measured by VSM, which is mainly due to the film volume and system calibration, one would not see any change in Ms between amorphous and crystalline phases if the change is within the error bar. The HC are approximately 4 Oe at 40 ℃ and remain nearly constant over an entire range of TS. This result clearly suggests that even though the crystallographical feature changes from amorphous to DO3 ordered phases with increasing Ts, there is no significant change in the coercivity mechanism. Fig. 5 shows (a) the relation between FMR frequency (fres) and linewidth (ΔH) for the various Ts for Fe57Co21Al13Si9 alloy thin films onto MgO (1 0 0). For all the samples, ΔH increases with fres and the values of ΔH0 , which is the inhomogeneous broadening[10] are close to zero for all the samples. The values of the effective damping parameter αeff were estimated based on the linear relationship between ΔH and fres over a frequency range from about 12 to 66 GHz along to 〈1 1 0〉bcc and 〈1 0 0〉MgO direction (0 deg.), as shown by following equation [2–6].
Fig. 7. (a) The XRD patterns, (b) the thickness dependences of a, and (c) L for Fe57Co21Al12Si10 alloy thin films deposited onto MgO (1 0 0).
ΔH = ΔH0 +
4π α eff fres . 3γ
(1)
Fig. 5(b) shows the dependence of αeff on Ts. The αeff decreases and then slightly increases with TS. The minimum value of αeff is 2.2 × 10−3. The change of αeff with TS at low temperatures appears to be correlated with the evolution of crystallinity. The increasing crystallinity through grain growth homogenizes the crystalline anisotropy, resulting in a reduced inhomogeneous broadening of the magnetic resonance and a decreasing effective damping αeff. The increase of αeff. for Ts above 300C is marginal and may be due to increased intermixing at the interfaces. The in-plane angular dependencies of resonance field with TS = 40, 310 and 410 ℃ are shown in Fig. 6. The resonance field with TS = 40 ℃ shows a two-fold symmetry, reflecting an induced uniaxial anisotropy with an easy axis along 〈1 1 0〉 of FeCoAlSi. The angular dependencies of the resonance field with TS = 310 and 410 ℃ clearly show a four-fold symmetry with an easy axis along 45°, i.e. the 〈1 0 0〉 of FeCoAlSi, which is consistent with crystalline anisotropy. The angles with minimum values of the resonance field indicate the easy axis, and are 0, 45, 45°for TS = 20, 310, 410 ℃ respectively. These FMR results are consistent with the VSM results.
Fig. 8. XRD phi scan profiles for (blue) the (2 0 0) plane of 207 nm thick FeCoAlSi film and (red) the (2 0 0) plane of MgO (1 0 0) substrate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nm width are seen, which seems to correspond to the deficiency in iron concentration as shown in Fig. 2(d). Fig. 2(b) and (c) show that with increasing TS, the entire crystallinity is enhanced and there is seen little evidence of a multilayered structure. For TS = 410 ℃ the diffraction pattern indicates clearly the presence of the DO3 phase, which will be discussed later. In high magnification images (Fig. 2(c)), the lattice image can be observed. The grain size estimated by bright field TEM images increases with increasing TS up to approximately 10 nm, which is consistent with the result of the coherence length estimated by XRD. The film-compositions for Ts from 40 ℃ to 410 ℃ were found by EDS to be Fe57Co21Al13Si9 and nearly constant within about 1% variation. Fig. 3 shows the M-H curves for Fe57Co21Al13Si9 thin films deposited at TS = 40 °C, 310 ℃ and 410 ℃ measured for the various angles of the
3.2. Thickness dependence As mentioned above, the 50 nm thick Fe57Co21Al13Si9 thin films deposited at TS = 410 ℃ was found to be of the DO3 ordered phase. Therefore, it is of great interest to study the thickness dependence of magnetic properties and structure. It is noted that when the thicknesses were varied, the film-compositions were found to change within a few at%, where the average composition is Fe57Co21Al12Si10. The XRD patterns in Fig. 7(a) shows the (0 0 2) bcc diffraction peaks for all the 4
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
Fig. 9. TEM cross-sectional images of (a) the low and (b) high magnifications, and (c) the corresponding diffraction patterns for the Fe57Co21Al12Si10 sample with d = 207 nm.
Fig. 9 shows the cross sectional TEM photographs of (a) the low and (b) high magnifications, and (c) the corresponding diffraction patterns for the sample with d = 207 nm. It is clearly seen that the superlattice diffraction spots are found, together with the lattice images (Fig. 9(c)). The order parameter (S) of DO3 phase was estimated using the measured XRD integrated intensity as follows. The value of S is based on the theoretically calculated intensity (I*) and the measured integrated intensity (I) of (0 0 1) and (0 0 2), using Eq. (2) [11]
Table 1 Each factors to estimate DO3 order parameter. (h k l)
(0 0 1)
(0 0 2)
2θ [deg.]
31.45
65.65
Crystal Structure factor
Fhkl
41.09 1688
276.58 76,494
Multiplicity factor Absorption factor Temperature factor Lorentz polarization factor Peak intensity
|Fhkl |2 P A (θ) D (θ) L(θ) I*
6.00 0.065 0.940 3.312 2041.37
6.00 0.033 0.781 1.284 15140.61
S=
Is × I ∗ f If × I ∗s
(2)
Measured integrated intensity, I is calculated using peak area after background subtraction. Here, I* for the (hkl) plane is expressed as:
samples, although the 5 nm thick one does show a very weak intensity. Fig. 8 shows XRD phi scan profiles for the (2 0 0) plane of 207 nm thick FeCoAlSi film and the (2 0 0) plane of MgO(1 0 0) substrate. It is clearly seen from the figure that both FeCoAlSi film and MgO substrate show the four-fold symmetry consistent with crystal structure, and are rotated by 45° respect to one other. These results indicates FeCoAlSi film is growth with epitaxialy onto MgO substrate. The dependences of c and L on the film thickness d estimated by the diffraction peaks of (0 0 2)bcc are shown in Fig. 7(b) and (c), respectively. The lattice constant c for the thinner samples is close to that for Fe75Co25 bulk and decreases with increasing d, and then remains nearly constant, at about 0.284 nm. With increasing d, the crystallinity is enhanced, and the films grow epitaxially. The coherence length L increases with d, and then remains nearly constant to be about 15 nm for d larger than about 50 nm. For the thickest sample (d = 537 nm), the L decreases slightly to about 10 nm. All the samples, except for the 11 nm thick one, exhibit the superlattice diffraction peak of (0 0 1) of the DO3 structure at around 2θ = 32 deg. The intensity of these superlattice diffraction peaks increases with d, and then decreases at d = 534 nm.
I ∗ = |F|2 × P × A (θ) × D (θ) × L (θ),
(3)
where F is the structure factor, P the multiplicity factor, θ the Bragg diffraction angle, A(θ) the absorption factor, D(θ) temperature factor and L(θ) Lorentz polarization factor. The expressions for A(θ), D(θ), L(θ) and F for the (hkl) plane are given as follows:
A (θ) = 1 − e−2μt /sin θ D (θ) = e (sin θ / λ) L (θ) =
2
1 + cos 2 2θ sin2θ
(4) (5)
(6)
Here, A is the approximation for a thin film [11]. For the DO3 structure, one unit cell contains 8 bcc unit cells as shown in Fig. 8(a). Therefore, the 16 (8 × 2) atoms were taken into account. The structure factor F of (2 0 0) and (1 0 0) planes can be described as: 5
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
Fig. 12. The correlation between HC and S for Fe57Co21Al12Si10 alloy thin films onto MgO (1 0 0).
Fig. 10. (a) Schematic figure of DO3 structure used for calculation of order parameter and (b) the thickness dependences of S for Fe57Co21Al12Si10 alloy thin films onto MgO (1 0 0).
Fig. 13. (a) The relation between FMR frequency (fres) and FMR linewidth (ΔH) and (b) the thickness dependences of αeff to Fe57Co21Al12Si10 alloy thin films onto MgO (1 0 0). The results of Fe73Co25Al2 and Fe69Co26Si5 alloy are also shown for comparison.
Fig. 11. The thickness dependences of (a) MS and (b) HC for Fe57Co21Al12Si10 alloy thin films onto MgO (1 0 0). The results of Fe73Co25Al2 and Fe69Co26Si5 alloy are also shown for comparison.
F200 = 4fFeCo − 4
where fFe , fCo , fAl and fSi are the atomic scattering factor of Fe, Co, Al
(CFeCo − 0.75) fFeCo + CAlSi fAlSi ((CFeCo − 0.75) + CAlSi )
and Si.
(7)
CAl f Al + CSI fSI
F100 = 12fFeCo + 4
CFe fFe + CCo fCo CFe + CCo
and
, respectively. CFe , CCo , CAl and CSi are the atomic concentraCAl + CSi tion of Fe, Co, Al and Si in FeCoAlSi alloy. In this case, they are 57, 21,
(CFeCo − 0.75) fFeCo + CAlSi fAlSi ((CFeCo − 0.75) + CAlSi )
fFeCo andfAlSi , which are described as
(8) 6
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
ones. This tendency is also found in the results of Fe73Co25Al2 and Fe69Co26Si5 alloys [1,2].
12, and 10 at%, respectively. All the physical quantities used to estimate DO3 order parameter for the 50 nm thick (FeCo)-(AlSi) thin films are listed in Table1. Based on this the order parameter for the DO3 (FeCo)-(AlSi) thin films can be estimated as follows.
S = 2.72
I001 I002
4. Summary The dependences of soft magnetic and structural properties on both TS and d for (FeCo)-(AlSi) alloy thin films are discussed. The epitaxially grown thin films are obtained onto MgO(1 0 0) substrates held at Ts above about 110 °C, with the relation of 〈1 0 0〉bcc//〈1 1 0〉MgO in the film plane. For the first time, the DO3 phase in a quaternary alloy thin films is observed for TS = 400 ℃ and for films thicker than 25 nm. The ordering parameter S increases with film thickness, becoming nearly 1 for thicknesses larger than 50 nm. The Ms values of Fe57Co21Al13Si9 films are approximately 700 emu/cm3 and remain nearly constant throughout the entire range of d. Both the Hc and αeff decrease with d and remain nearly constant, approximately 4 Oe and 5 × 10−3, respectively. The result that HC decreases with increasing S suggests that (FeCo)-(AlSi) thin films with the DO3 phase are an attractive candidate for future high frequency device applications.
(9)
The result of order parameter S thus obtained is shown in Fig. 10(b). The order parameter S increases with d and then remains nearly 1. The error in estimating S is mainly due to the background subtraction and smoothing processes of the integrated intensity. It is of interest to note that the samples with d larger than about 50 nm are fully ordered. The dependences of MS, and HC on the film thickness d for Fe57Co21Al12Si10 thin films are shown in Fig. 11, together with the results previously reported of Fe73Co25Al2 [3] and Fe69Co26Si5 [5]. The saturation magnetization MS is approximately constant over the entire thickness range at 700 emu/cm3, lower than those of Fe73Co25Al2 and Fe69Co26Si5, since the composition of Fe57Co21Al12Si10 alloy investigated here contains more aluminum and silicon compared to Fe73Co25Al2 and Fe69Co26Si5 alloys. The coercivity HC decreases with increasing film thickness d, and then remains constant at approximately 4 Oe. The values of HC are lower than ones of Fe73Co25Al2 and Fe69Co26Si5 alloys, because of high aluminum and silicon contents that decreases crystalline magnetic anisotropy [12]. Fig. 12 shows the correlation between Hc and S. It appears that the lower coercivity of Fe57Co21Al12Si10 thin films is correlated with the DO3 structure. In general, as S is reflected by all crystalline factor, such as crystalline defects, stress and crystalline size, one possible explanation may be that the samples with higher S will likely have a lower crystalline defect density and crystalline defects, which can inhibit domain wall mobility. Therefore, one of the reasons of the relation of S and HC can be crystalline defects. In addition to this, ordering atom position of aluminum and silicon may change crystalline magnetic anisotropy as is the case for Sendust alloys as discussed in the results of TS part. This phenomenon needs to study further in order clarify it. Fig. 13 shows (a) fres andΔH and (b) the dependences of αeff on the film thickness d for Fe57Co21Al12Si10 thin films. The relations between fres andΔH for 11 and 534 nm thick samples shows nonlinearly characteristics. Inhomogeneous anisotropy may effects with the phenomena. One of the reason of nonlinear characteristics may be an inhomogeneous distribution of the anisotropy in the films. For the thinnest sample (11 nm), stress from substrate can be exist in the film and crystallinity is still low. Two-magnon scattering could be the other reason for the non-linearity as interfacial two-magnon scattering scales with the inverse square of the film thickness, i.e. is expected to be dominant for thin films with significant interfacial roughness. For the 534 nm film, multiple resonances were observed in the FMR measurements. The non-linearity for the thickest film therefore could be due to the intermixing of the different resonance modes. The values of αeff also decrease with increasing d, and remain between 3 ~ 9 × 10−3. The αeff contains both intrinsic and extrinsic contributions. The increase of αeff for thinner thicknesses may be due to two-magnon scattering and spin pumping, as already discussed elsewhere [6]. Two-magnon scattering is caused by inhomogeneity of magnetic anisotropy, stress and interfacial constitution due to roughness. Also eddy current damping increases with thickness, because of the low resistivity of metal alloy. Therefore, the values of αeff for the thickest sample is much larger than the thinner
CRediT authorship contribution statement Takuma Nakano: Conceptualization, Investigation, Writing - original draft. Bhuwan Nepal: Investigation, Formal analysis. Yoshitomo Tanaka: Investigation. Shuang. Wu: Investigation, Formal analysis. Kyotaro Abe: Investigation. Gary Mankey: Supervision, Writing - review & editing. Tim Mewes: Supervision, Writing - review & editing. Claudia Mewes: Supervision. Takao Suzuki: Supervision, Project administration, Writing - review & editing, Conceptualization. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The present work was supported in part by the MINT/UA-TDK collaboration program. References [1] M.A.W. Schoen, D. Thonig, M.L. Schneider, T.J. Silva, H.T. Nembach, O. Eriksson, O. Karis, J.M. Shaw, Nat. Phys. 12 (2016) 839. [2] I. Kanada, A. Cruce, T. Mewes, S. Wu, C. Mewes, G. Mankey, T. Suzuki, AIP Adv. 7 (2017) 056105. [3] Y. Ariake, I. Kanada, T. Mewes, G. Mankey, Y. Tanaka, S. Wu, C. Mewes, T. Suzuki, I.E.E.E. Trans, Mag. 53 (2017) 2003404. [4] Y. Ariake, S. Wu, I. Kanada, T. Mewes, Y. Tanaka, G. Mankey, C. Mewes, T. Suzuki, AIP Adv. 8 (2018) 056119. [5] K. Abe, S. Wu, Y. Tanaka, Y. Ariake, I. Kanada, T. Mewes, G. Mankey, C. Mewes, T. Suzuki, AIP Advances 9 (2019) 035139. [6] S. Wu, K. Abe, T. Nakano, T. Mewes, C. Mewes, G. Mankey, T. Suzuki, Phys. Rev. B 99 (2019) 144416. [7] H. Masumoto, H. Saito, Sci. Rep. RITU A4 (1952) 321. [8] M. Hayakawa, K. Hayashi, W. Ishikawa, Y. Ochiai, H. Matsuda, Y. Iwasaki, K. Aso, I.E.E.E. Trans, Magn. 23 (1987) 5. [9] R. M. Bozorth: Ferromagnetism D.Van Nostrand, New York, 192 (1951). [10] B. Heinrich, J.F. Cochran, R. Hasegawa, J. Appl. Phys. 57 (1985) 3690. [11] E. Yang, D.E. Laughlin, J.G. Zhu, IEEE Trans. Mag. 48 (2012) 7. [12] R.C. Hall, J. Appl. Phys. 31 (1960) 1037.
7
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Soft magnetic and structural properties of (FeCo)–(AlSi) alloy thin films a,b,⁎
b,c
a
b,c
Takuma Nakano , Bhuwan Nepal , Yoshitomo Tanaka , Shuang Wu , Kyotaro Abe Gary Mankeyb,c, Tim Mewesb,c, Claudia Mewesb,c, Takao Suzukib,d,e
T
a,b
,
a
Materials Development Center, TDK Corporation, Narita 286-0805, Japan Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, AL 35487, USA Department of Physics and Astronomy, The University of Alabama, Tuscaloosa, AL 35487, USA d Department of Electrical and Computer Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA e Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Soft magnetic properties DO3 structure Effective damping parameter Coercivity Order parameter
The dependences of static and dynamic magnetic properties on both substrate deposition temperature (TS) and film-thickness (d) for (FeCo)-(AlSi) alloy thin films are discussed in conjunction with structure. The samples of a multi layered type (FeCo)-(AlSi) alloy thin films were fabricated at Ts by DC sputter-deposition onto (1 0 0) MgO substrates by using two targets of Fe75Co25 and Al50Si50. For the first time, the DO3 phase in the quaternary alloy films are realized for TS at around 400 ℃ with d larger than 25 nm. The order parameter (S) was estimated by taking into account crystal structure factor, absorption factor, multiplicity factor, temperature factor, and Lorentz polarization factor. It is found that S increases with d, becoming nearly 1 for d larger than 50 nm. Observations by high resolution transmission electron microscopy support this result. The coercivity (Hc) and effective damping parameter (αeff) decrease with increasing d and remain nearly constant, approximately 4 Oe and 5 × 10−3, respectively. The result that HC decreases with increasing S suggests that (FeCo)-(AlSi) thin films with the DO3 phase are an attractive candidate for future high frequency device applications.
1. Introduction
2. Experimental
Among many material candidates to choose for future high frequency device applications that requires in particular high saturation magnetization (MS), low coercivity (HC) and low damping parameter, FeCo-based alloys systems are the promising candidate as they provide opportunity to improve further the magnetic property. Regarding the correlation between soft magnetic properties and structure, Masumoto et al. [7] reported high permeability and low HC within a narrow composition range around the DO3 phase of the FeSiAl alloy system. Although this finding is very important, there has been very few works found in literature about the detailed correlation between them. Recently, iron based alloy thin films, such as FeCo [1], FeAl [2]. FeCoAl [3,4] and FeCoSi [5,6] have been reported to exhibit magnetic properties and effective damping parameter (αeff), which are attractive for future high frequency device applications. It is noted that the very low damping parameter of 0.0004 was reported in [3,4]. It is of interest to explore further the soft magnetic properties of (FeCo)-(AlSi) thin films in conjunction with the DO3 structure [8]. This work presents a systematic study of the static and dynamic magnetic properties and structure of (FeCo)-(AlSi) alloy thin films.
Multilayers of [Fe75Co25 (0.7 nm)/Al50Si50 (0.4 nm)] × N (N = 5–500) were sputter-deposited at a substrate deposition temperature (TS) over a range from ambient (approximately 40 °C) to 410 °C onto MgO(1 0 0) substrates using DC magnetron sputtering in 4 mTorr Ar atmosphere. During deposition, in order to induce a uniaxial magnetic anisotropy in the film-plane, an external magnetic field of 50 Oe was applied along 〈1 0 0〉MgO in the substrate plane. A 5 nm thick Ru layer was over-coated to prevent samples from oxidation. Two targets of Fe75Co25 and Al50Si50 were used, for both of which the deposition rates were 0.04 nm/s. Crystalline phase and quality were estimated by X-ray diffraction (XRD) using a X’pert, PHILIPS with Cu Kα. Film structure and composition analyses were performed by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) using Titan G2, FEI operated at 300 kV. In order to obtain EDS mapping, background subtraction from net count map and peak resolution for each elements were processed. The film thicknesses were estimated by X-ray reflectivity (XRR). Measurements of magnetic properties were carried out at room temperature by using a vibrating sample magnetometer (VSM).
⁎
Corresponding author at: Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, AL 35487, USA. E-mail address: [email protected] (T. Nakano).
https://doi.org/10.1016/j.jmmm.2020.166852 Received 14 January 2020; Received in revised form 25 February 2020; Accepted 30 March 2020 Available online 31 March 2020 0304-8853/ © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
The effective damping parameter αeff was measured using broadband ferromagnetic resonance (FMR) over a frequency range from 12 to 66 GHz at room temperature along 〈1 1 0〉bcc and 〈1 0 0〉MgO direction.
3. Results and discussion 3.1. Substrate deposition temperature dependence Fig. 1(a) shows the XRD patterns for the 50 nm thick Fe57Co21Al13Si9 alloy thin films deposited at the various TS. For the sample deposited at TS = 40 ℃, the diffraction peaks of (FeCo)-(AlSi) alloys are absent, and only those of the MgO substrate are present, indicating an amorphous structure of the films. With increasing Ts, only the (0 0 2)bcc peak appears and its intensity increases with TS beyond 110 °C, indicating the epitaxial growth of the 〈1 0 0〉bcc along the film normal. It is noted that the film deposited at TS = 410 ℃ does exhibit a diffraction peak at around 31° which is identified to be the super lattice peak (0 0 1) of the DO3 phase. This finding of the DO3 phase is important since this is the first time this ordered phase in the quaternary alloy system has been realized. It is also noted that the epitaxially grown films have the orientation correlation of 〈0 0 1〉bcc//〈1 1 0〉MgO found by low angle XRD. The dependence of lattice constant (c) estimated from the (0 0 2) bcc diffraction peak on TS is also shown in Fig. 1(b). The values of c are nearly constant at 0.284 Å over the entire range of TS, and is slightly smaller than the bulk lattice constant of Fe75Co25 [9]. The coherence length (L), which is defined as the distance over which the X-ray diffraction coherently takes place to form the (0 0 2) peak, is estimated from the full-width-half-maximum of the (0 0 2)bcc diffraction peak as shown in Fig. 1(c). The L is found to increase from 5 to 12 nm with TS. Fig. 2 shows (a) the selected area electron diffraction pattern, (b) and (c) TEM cross-sectional images of low and high magnification,
Fig. 1. (a) The XRD patterns with various TS and (b) the dependence of c and (c) L on TS for Fe57Co21Al13Si9 alloy thin films onto MgO (1 0 0).
Fig. 2. (a) Diffraction pattern, TEM cross-sectional images of (b) low and (c) high magnification and (d) EDS mapping for Fe57Co21Al13Si9 alloy thin films onto MgO (1 0 0) with the various TS. 2
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
Fig. 5. (a) The relation between FMR frequency (fres) and FMR linewidth (ΔH) and (b) the dependence ofαeff on TS for Fe57Co21Al13Si9 alloy thin films onto MgO (1 0 0). Fig. 3. The M-H curves for Fe57Co21Al13Si9 alloy thin films onto MgO (1 0 0) with TS = 40, 310 and 410 °C.
Fig. 4. The dependences of (a) MS and (b) HC forFe57Co21Al13Si9 alloy thin films onto MgO (1 0 0) on TS.
respectively, and (d) EDS mapping for the films with the various TS. For TS = 40 ℃, a halo pattern is seen, in addition to a very weak (1 1 0)bcc peak. The high resolution TEM image does not show any sign of crystallinity, indicating an amorphous structure. It is noted in Fig. 2(b) for TS = 40 ℃ that the two fuzzy horizontal white lines with about a few
Fig. 6. In-plane angular dependence of the resonance field of FeCoAlSi films with TS = 40, 310, 410 ℃. 3
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
applied field in the film plane, where 0 deg. is along 〈1 1 0〉bcc, 〈1 0 0〉MgO, i.e. the external magnetic field direction during deposition. For TS = 40 °C, the easy axis for magnetization is found to be along the external magnetic field during deposition and 〈1 0 0〉MgO. This result indicates that the uniaxial in-plane magnetic anisotropy is induced. With increasing TS, the easy axis for magnetization changes its direction from 0 to 45 deg. This change of the easy axis direction is believed to be due to the evolution and enhancement of magneto-crystalline anisotropy due to the crystal grain growth of the films, which are confirmed by XRD and TEM results. The results from angular dependence of resonance field, shown in later part, also indicate same changing of easy axis with TS. The dependences of saturation magnetization (MS), and HC on TS are shown in Fig. 4(a) and (b), respectively. The MS values are nearly constant, approximately 700 emu/cm3 over an entire range of TS, although the MS with amorphous phase (40℃) might seems to be lower compere to one other. It is not understood for this constant saturation magnetization. However, it is plausible that as one would expect an error margin of about 10% in the magnetization values measured by VSM, which is mainly due to the film volume and system calibration, one would not see any change in Ms between amorphous and crystalline phases if the change is within the error bar. The HC are approximately 4 Oe at 40 ℃ and remain nearly constant over an entire range of TS. This result clearly suggests that even though the crystallographical feature changes from amorphous to DO3 ordered phases with increasing Ts, there is no significant change in the coercivity mechanism. Fig. 5 shows (a) the relation between FMR frequency (fres) and linewidth (ΔH) for the various Ts for Fe57Co21Al13Si9 alloy thin films onto MgO (1 0 0). For all the samples, ΔH increases with fres and the values of ΔH0 , which is the inhomogeneous broadening[10] are close to zero for all the samples. The values of the effective damping parameter αeff were estimated based on the linear relationship between ΔH and fres over a frequency range from about 12 to 66 GHz along to 〈1 1 0〉bcc and 〈1 0 0〉MgO direction (0 deg.), as shown by following equation [2–6].
Fig. 7. (a) The XRD patterns, (b) the thickness dependences of a, and (c) L for Fe57Co21Al12Si10 alloy thin films deposited onto MgO (1 0 0).
ΔH = ΔH0 +
4π α eff fres . 3γ
(1)
Fig. 5(b) shows the dependence of αeff on Ts. The αeff decreases and then slightly increases with TS. The minimum value of αeff is 2.2 × 10−3. The change of αeff with TS at low temperatures appears to be correlated with the evolution of crystallinity. The increasing crystallinity through grain growth homogenizes the crystalline anisotropy, resulting in a reduced inhomogeneous broadening of the magnetic resonance and a decreasing effective damping αeff. The increase of αeff. for Ts above 300C is marginal and may be due to increased intermixing at the interfaces. The in-plane angular dependencies of resonance field with TS = 40, 310 and 410 ℃ are shown in Fig. 6. The resonance field with TS = 40 ℃ shows a two-fold symmetry, reflecting an induced uniaxial anisotropy with an easy axis along 〈1 1 0〉 of FeCoAlSi. The angular dependencies of the resonance field with TS = 310 and 410 ℃ clearly show a four-fold symmetry with an easy axis along 45°, i.e. the 〈1 0 0〉 of FeCoAlSi, which is consistent with crystalline anisotropy. The angles with minimum values of the resonance field indicate the easy axis, and are 0, 45, 45°for TS = 20, 310, 410 ℃ respectively. These FMR results are consistent with the VSM results.
Fig. 8. XRD phi scan profiles for (blue) the (2 0 0) plane of 207 nm thick FeCoAlSi film and (red) the (2 0 0) plane of MgO (1 0 0) substrate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nm width are seen, which seems to correspond to the deficiency in iron concentration as shown in Fig. 2(d). Fig. 2(b) and (c) show that with increasing TS, the entire crystallinity is enhanced and there is seen little evidence of a multilayered structure. For TS = 410 ℃ the diffraction pattern indicates clearly the presence of the DO3 phase, which will be discussed later. In high magnification images (Fig. 2(c)), the lattice image can be observed. The grain size estimated by bright field TEM images increases with increasing TS up to approximately 10 nm, which is consistent with the result of the coherence length estimated by XRD. The film-compositions for Ts from 40 ℃ to 410 ℃ were found by EDS to be Fe57Co21Al13Si9 and nearly constant within about 1% variation. Fig. 3 shows the M-H curves for Fe57Co21Al13Si9 thin films deposited at TS = 40 °C, 310 ℃ and 410 ℃ measured for the various angles of the
3.2. Thickness dependence As mentioned above, the 50 nm thick Fe57Co21Al13Si9 thin films deposited at TS = 410 ℃ was found to be of the DO3 ordered phase. Therefore, it is of great interest to study the thickness dependence of magnetic properties and structure. It is noted that when the thicknesses were varied, the film-compositions were found to change within a few at%, where the average composition is Fe57Co21Al12Si10. The XRD patterns in Fig. 7(a) shows the (0 0 2) bcc diffraction peaks for all the 4
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
Fig. 9. TEM cross-sectional images of (a) the low and (b) high magnifications, and (c) the corresponding diffraction patterns for the Fe57Co21Al12Si10 sample with d = 207 nm.
Fig. 9 shows the cross sectional TEM photographs of (a) the low and (b) high magnifications, and (c) the corresponding diffraction patterns for the sample with d = 207 nm. It is clearly seen that the superlattice diffraction spots are found, together with the lattice images (Fig. 9(c)). The order parameter (S) of DO3 phase was estimated using the measured XRD integrated intensity as follows. The value of S is based on the theoretically calculated intensity (I*) and the measured integrated intensity (I) of (0 0 1) and (0 0 2), using Eq. (2) [11]
Table 1 Each factors to estimate DO3 order parameter. (h k l)
(0 0 1)
(0 0 2)
2θ [deg.]
31.45
65.65
Crystal Structure factor
Fhkl
41.09 1688
276.58 76,494
Multiplicity factor Absorption factor Temperature factor Lorentz polarization factor Peak intensity
|Fhkl |2 P A (θ) D (θ) L(θ) I*
6.00 0.065 0.940 3.312 2041.37
6.00 0.033 0.781 1.284 15140.61
S=
Is × I ∗ f If × I ∗s
(2)
Measured integrated intensity, I is calculated using peak area after background subtraction. Here, I* for the (hkl) plane is expressed as:
samples, although the 5 nm thick one does show a very weak intensity. Fig. 8 shows XRD phi scan profiles for the (2 0 0) plane of 207 nm thick FeCoAlSi film and the (2 0 0) plane of MgO(1 0 0) substrate. It is clearly seen from the figure that both FeCoAlSi film and MgO substrate show the four-fold symmetry consistent with crystal structure, and are rotated by 45° respect to one other. These results indicates FeCoAlSi film is growth with epitaxialy onto MgO substrate. The dependences of c and L on the film thickness d estimated by the diffraction peaks of (0 0 2)bcc are shown in Fig. 7(b) and (c), respectively. The lattice constant c for the thinner samples is close to that for Fe75Co25 bulk and decreases with increasing d, and then remains nearly constant, at about 0.284 nm. With increasing d, the crystallinity is enhanced, and the films grow epitaxially. The coherence length L increases with d, and then remains nearly constant to be about 15 nm for d larger than about 50 nm. For the thickest sample (d = 537 nm), the L decreases slightly to about 10 nm. All the samples, except for the 11 nm thick one, exhibit the superlattice diffraction peak of (0 0 1) of the DO3 structure at around 2θ = 32 deg. The intensity of these superlattice diffraction peaks increases with d, and then decreases at d = 534 nm.
I ∗ = |F|2 × P × A (θ) × D (θ) × L (θ),
(3)
where F is the structure factor, P the multiplicity factor, θ the Bragg diffraction angle, A(θ) the absorption factor, D(θ) temperature factor and L(θ) Lorentz polarization factor. The expressions for A(θ), D(θ), L(θ) and F for the (hkl) plane are given as follows:
A (θ) = 1 − e−2μt /sin θ D (θ) = e (sin θ / λ) L (θ) =
2
1 + cos 2 2θ sin2θ
(4) (5)
(6)
Here, A is the approximation for a thin film [11]. For the DO3 structure, one unit cell contains 8 bcc unit cells as shown in Fig. 8(a). Therefore, the 16 (8 × 2) atoms were taken into account. The structure factor F of (2 0 0) and (1 0 0) planes can be described as: 5
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
Fig. 12. The correlation between HC and S for Fe57Co21Al12Si10 alloy thin films onto MgO (1 0 0).
Fig. 10. (a) Schematic figure of DO3 structure used for calculation of order parameter and (b) the thickness dependences of S for Fe57Co21Al12Si10 alloy thin films onto MgO (1 0 0).
Fig. 13. (a) The relation between FMR frequency (fres) and FMR linewidth (ΔH) and (b) the thickness dependences of αeff to Fe57Co21Al12Si10 alloy thin films onto MgO (1 0 0). The results of Fe73Co25Al2 and Fe69Co26Si5 alloy are also shown for comparison.
Fig. 11. The thickness dependences of (a) MS and (b) HC for Fe57Co21Al12Si10 alloy thin films onto MgO (1 0 0). The results of Fe73Co25Al2 and Fe69Co26Si5 alloy are also shown for comparison.
F200 = 4fFeCo − 4
where fFe , fCo , fAl and fSi are the atomic scattering factor of Fe, Co, Al
(CFeCo − 0.75) fFeCo + CAlSi fAlSi ((CFeCo − 0.75) + CAlSi )
and Si.
(7)
CAl f Al + CSI fSI
F100 = 12fFeCo + 4
CFe fFe + CCo fCo CFe + CCo
and
, respectively. CFe , CCo , CAl and CSi are the atomic concentraCAl + CSi tion of Fe, Co, Al and Si in FeCoAlSi alloy. In this case, they are 57, 21,
(CFeCo − 0.75) fFeCo + CAlSi fAlSi ((CFeCo − 0.75) + CAlSi )
fFeCo andfAlSi , which are described as
(8) 6
Journal of Magnetism and Magnetic Materials 507 (2020) 166852
T. Nakano, et al.
ones. This tendency is also found in the results of Fe73Co25Al2 and Fe69Co26Si5 alloys [1,2].
12, and 10 at%, respectively. All the physical quantities used to estimate DO3 order parameter for the 50 nm thick (FeCo)-(AlSi) thin films are listed in Table1. Based on this the order parameter for the DO3 (FeCo)-(AlSi) thin films can be estimated as follows.
S = 2.72
I001 I002
4. Summary The dependences of soft magnetic and structural properties on both TS and d for (FeCo)-(AlSi) alloy thin films are discussed. The epitaxially grown thin films are obtained onto MgO(1 0 0) substrates held at Ts above about 110 °C, with the relation of 〈1 0 0〉bcc//〈1 1 0〉MgO in the film plane. For the first time, the DO3 phase in a quaternary alloy thin films is observed for TS = 400 ℃ and for films thicker than 25 nm. The ordering parameter S increases with film thickness, becoming nearly 1 for thicknesses larger than 50 nm. The Ms values of Fe57Co21Al13Si9 films are approximately 700 emu/cm3 and remain nearly constant throughout the entire range of d. Both the Hc and αeff decrease with d and remain nearly constant, approximately 4 Oe and 5 × 10−3, respectively. The result that HC decreases with increasing S suggests that (FeCo)-(AlSi) thin films with the DO3 phase are an attractive candidate for future high frequency device applications.
(9)
The result of order parameter S thus obtained is shown in Fig. 10(b). The order parameter S increases with d and then remains nearly 1. The error in estimating S is mainly due to the background subtraction and smoothing processes of the integrated intensity. It is of interest to note that the samples with d larger than about 50 nm are fully ordered. The dependences of MS, and HC on the film thickness d for Fe57Co21Al12Si10 thin films are shown in Fig. 11, together with the results previously reported of Fe73Co25Al2 [3] and Fe69Co26Si5 [5]. The saturation magnetization MS is approximately constant over the entire thickness range at 700 emu/cm3, lower than those of Fe73Co25Al2 and Fe69Co26Si5, since the composition of Fe57Co21Al12Si10 alloy investigated here contains more aluminum and silicon compared to Fe73Co25Al2 and Fe69Co26Si5 alloys. The coercivity HC decreases with increasing film thickness d, and then remains constant at approximately 4 Oe. The values of HC are lower than ones of Fe73Co25Al2 and Fe69Co26Si5 alloys, because of high aluminum and silicon contents that decreases crystalline magnetic anisotropy [12]. Fig. 12 shows the correlation between Hc and S. It appears that the lower coercivity of Fe57Co21Al12Si10 thin films is correlated with the DO3 structure. In general, as S is reflected by all crystalline factor, such as crystalline defects, stress and crystalline size, one possible explanation may be that the samples with higher S will likely have a lower crystalline defect density and crystalline defects, which can inhibit domain wall mobility. Therefore, one of the reasons of the relation of S and HC can be crystalline defects. In addition to this, ordering atom position of aluminum and silicon may change crystalline magnetic anisotropy as is the case for Sendust alloys as discussed in the results of TS part. This phenomenon needs to study further in order clarify it. Fig. 13 shows (a) fres andΔH and (b) the dependences of αeff on the film thickness d for Fe57Co21Al12Si10 thin films. The relations between fres andΔH for 11 and 534 nm thick samples shows nonlinearly characteristics. Inhomogeneous anisotropy may effects with the phenomena. One of the reason of nonlinear characteristics may be an inhomogeneous distribution of the anisotropy in the films. For the thinnest sample (11 nm), stress from substrate can be exist in the film and crystallinity is still low. Two-magnon scattering could be the other reason for the non-linearity as interfacial two-magnon scattering scales with the inverse square of the film thickness, i.e. is expected to be dominant for thin films with significant interfacial roughness. For the 534 nm film, multiple resonances were observed in the FMR measurements. The non-linearity for the thickest film therefore could be due to the intermixing of the different resonance modes. The values of αeff also decrease with increasing d, and remain between 3 ~ 9 × 10−3. The αeff contains both intrinsic and extrinsic contributions. The increase of αeff for thinner thicknesses may be due to two-magnon scattering and spin pumping, as already discussed elsewhere [6]. Two-magnon scattering is caused by inhomogeneity of magnetic anisotropy, stress and interfacial constitution due to roughness. Also eddy current damping increases with thickness, because of the low resistivity of metal alloy. Therefore, the values of αeff for the thickest sample is much larger than the thinner
CRediT authorship contribution statement Takuma Nakano: Conceptualization, Investigation, Writing - original draft. Bhuwan Nepal: Investigation, Formal analysis. Yoshitomo Tanaka: Investigation. Shuang. Wu: Investigation, Formal analysis. Kyotaro Abe: Investigation. Gary Mankey: Supervision, Writing - review & editing. Tim Mewes: Supervision, Writing - review & editing. Claudia Mewes: Supervision. Takao Suzuki: Supervision, Project administration, Writing - review & editing, Conceptualization. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The present work was supported in part by the MINT/UA-TDK collaboration program. References [1] M.A.W. Schoen, D. Thonig, M.L. Schneider, T.J. Silva, H.T. Nembach, O. Eriksson, O. Karis, J.M. Shaw, Nat. Phys. 12 (2016) 839. [2] I. Kanada, A. Cruce, T. Mewes, S. Wu, C. Mewes, G. Mankey, T. Suzuki, AIP Adv. 7 (2017) 056105. [3] Y. Ariake, I. Kanada, T. Mewes, G. Mankey, Y. Tanaka, S. Wu, C. Mewes, T. Suzuki, I.E.E.E. Trans, Mag. 53 (2017) 2003404. [4] Y. Ariake, S. Wu, I. Kanada, T. Mewes, Y. Tanaka, G. Mankey, C. Mewes, T. Suzuki, AIP Adv. 8 (2018) 056119. [5] K. Abe, S. Wu, Y. Tanaka, Y. Ariake, I. Kanada, T. Mewes, G. Mankey, C. Mewes, T. Suzuki, AIP Advances 9 (2019) 035139. [6] S. Wu, K. Abe, T. Nakano, T. Mewes, C. Mewes, G. Mankey, T. Suzuki, Phys. Rev. B 99 (2019) 144416. [7] H. Masumoto, H. Saito, Sci. Rep. RITU A4 (1952) 321. [8] M. Hayakawa, K. Hayashi, W. Ishikawa, Y. Ochiai, H. Matsuda, Y. Iwasaki, K. Aso, I.E.E.E. Trans, Magn. 23 (1987) 5. [9] R. M. Bozorth: Ferromagnetism D.Van Nostrand, New York, 192 (1951). [10] B. Heinrich, J.F. Cochran, R. Hasegawa, J. Appl. Phys. 57 (1985) 3690. [11] E. Yang, D.E. Laughlin, J.G. Zhu, IEEE Trans. Mag. 48 (2012) 7. [12] R.C. Hall, J. Appl. Phys. 31 (1960) 1037.
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