We numerically studied the spin transfer torque oscillation (STO) in a magnetic orthogonal configuration by introducing a strong biquadratic magnetic coupling [1]. The advantage of the orthogonal configuration is the high efficiency of spin transfer torque leading the high STO frequency, but it is difficult to maintain the STO in a wide range of electric current. By introducing a biquadratic magnetic coupling into the orthogonal structure, it became possible to expand the electric current region realizing the stable STO, resulting that a high STO frequency. In this report, we systematically compared STO properties between the orthogonal magnetization disks with and without the biquadratic magnetic coupling, that is, B12=0.0 and -0.6.
In our model, the orthogonal configuration is FePt 2 nm/spacer 2 nm/Co90Fe10, or Ni80Fe20, or Ni 2 nm. The time-domain magnetization precessions of the top layer Ni under current density J=0.5×107 A/cm2, 3.0×107 A/cm2, and 6.0×107 A/cm2 are shown in Fig.1 (a), (b) and (c). Only B12 was changed and A12 is set 0 for simplicity. In the case of the biquadratic magnetic coupling B12=0.0, the STO was observed only when J=0.5×107 A/cm2. Compared to this, in the case of B12=-0.6, the STO can be obtained in the wide range of J. This means that by introducing biquadratic magnetic coupling, we can expand the electric current region realizing the stable STO and increase the frequency. In Fig. 2, we show the STO frequency and the intensity as a function of the current density. First, even when the top layer is changed from Ni, to Ni80Fe20, or Co90Fe10, the current density region realizing the stable STO is widened by introducing the biquadratic magnetic coupling. In addition, we increased STO frequency and the intensity, meaning the stable STO. 
**References** [1] C. Liu, et al. IEEE Trans. Mag., 58 (2022) 172504.
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/82acec9c9943e8cd4a49cbcce0fe639e.png)

High Frequency Spin Torque Oscillation in Orthogonal Magnetization Disks with Strong Biquadratic Magnetic Coupling

Spin current control has attracted increasing attention from the viewpoint of fundamental research and application owing to their attractive performances; zero Joule heating, nano scale devices, and expansion of dimension. As typical phenomena we investigated the spin Seebeck effect (SSE) and the spin Hall magnetoresistance (SMR) in the bilayer consisting of a magnetic insulator Y3Fe5O12 (YIG) and a heavy metal (Pt). Since the spin current is originated from the diffusion phenomenon, the signals, that is, spin Seebeck coefficient and MRxx of SMR, are generally small. Therefore, it is important to increase the efficiencies of spin current generation and propagation for practical use. We observed significant enhancement of SSE and SMR values by inserting 0.3nm or 0.6 nm layers of magnetic materials with different composition (Cr, Ni80Fe20, Co90Fe10, Fe50Co50, and Fe) at the YIG/Pt interface [1]. The SSE and SMR showed the mostly same dependences on the insertion layer composition and thickness as shown in Fig. 1, indicating that there is the common reason of SSE and SMR enhancement. To find the reason, we investigated the magnetism of Pt on the insertion layers and the insertion layers themselves. It is known that the magnetization is induced in the Pt layer when Pt contacts the ferromagnetic metals including Fe or Co. However, it has not been clarified whether Pt on the sub-nm-thick magnetic layer is magnetized. In order to investigate the Pt magnetic property, we carried out X-ray magnetic circular dichroism (XMCD) at the BL39XU of SPring-8. The Pt thickness was decreased to 1 nm in order to emphasize the signal from Pt near the interface with the insertion layers. As a result, it was confirmed that there is merely negligible moment in Pt for all samples with and without insertion layers by XMCD. Next, we investigated the magnetism of the sub-nm insertion layers by generalized magneto-optical ellipsometry (GME). Although the magnetic materials with sub-nm thickness does not show magnetization at room temperature in general, there is the possibility that magnetization is induced when ultra-thin magnetic layer is deposited on the ferrimagnetic YIG. However, it is difficult to measure the magnetization of sub-nm insertion layers deposited on the YIG by using macroscopic measurement such as VSM or SQUID because the magnetization signal of the insertion layers is hidden by that of YIG. Compared to this, the surface magnetic information can be extracted by GME analysis. It was found that the magnetic insertion layers of 0.3 nm and 0.6 nm did not exhibit a measurable magnetization. On the contrary, when the thickness of insertion layers is increased to 1 nm, the magnetization becomes obvious and SSE and SMR were decreased. Taking it into consideration, the state of losing magnetic order in the insertion layers seems to increase the spin current propagation from YIG to Pt. In summary, the SSE and SMR was enhanced by the insertion of thin magnetic material layers, which did not result from a net interface magnetization in Pt and the insertion layers. On the contrary, the paramagnetic state is considered to have an important role to enhance the spin current related phenomena.

**References:**

[1] T. Niimura et al., Phys. Rev. B 102 094411 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/5d801144e5ba2cac759e99351e9a6aa2.jpg)
Spin Seebeck coefficient SSSE (a) and MRxx of SMR (b) for YIG/insertion layer/Pt where the insertion layer material is Cr, Ni80Fe20, Co90Fe10, Fe50Co50, or Fe.

 Spin Current Enhancement by Inserting Ultra-thin Magnetic Layer at Interface between YIG and Pt INVITED

Geometrically confined standing spin wave resonance (SSWR) has attracted intense research interests as a fundamental physics and practical applications [1][2]. The magnetostatic surface wave (MSSW) mode has been reported owning larger signal intensity than magnetostatic backward volume wave (MSBVW) mode[3]. These two modes are controlled by bias field continuously applied to magnetic strip, where the bias field is applied in the vertical or parallel directions according to MSSW or MSBVW modes, respectively. In this study, we successfully deposited Co2FeAl films with 4-fold in-plane anisotropy and studied SSWR properties of the fabricated magnetic strips. Due to the 4-fold anisotropy, we succeeded in generating MSSW and MSBVW modes under zero bias field and confirmed absorption intensity difference between these two modes. Considering MSSW mode as ‘ON’ state and MSBVW mode as ‘OFF’ state , the absorption intensity difference demonstrated a spin wave switch or sensor device. Co2FeAl films were deposited on MgO (001) substrate at 400°C by sputtering method. The angular dependence of the squareness are plotted in Fig. 1(a), which showed strong 4-fold in-plane anisotropy (an easy axis corresponds to MgO[001]) when film thickness was 60 nm. Co2FeAl (400) peak at 65 degree of the corresponding XRD translated the 60-nm-thick film was well crystallized. The magnetic strip was fabricated by lithography techniques, where the lateral coordinates were defined to coincide with the anisotropy directions. 100-nm-thick SiO2 insulating layer was deposited on the strips and Grand-Signal-Grand coplanar waveguide (CPW) was fabricated on the SiO2 layer (Fig. 1(b)). The transmission characteristics of S21 was measured by VNA under the bias field, Hb, ranged from 600 Oe to 0. TThe S21 of MSSW and MSBVW at different Hb are shown in Fig. 2(a). The broad absorption dip includes multiple MSSW resonance modes, which are shown by the Lorenzian fitting. The absorption intensity of MSSW mode at various Hb were greater than 0.5 dB, while the absorption intensity of MSBVW mode were less than 0.2 dB. The absorption intensity ratio of MSSW to MSBVW exceed 20 dB when Hb = 600 Oe, which is large enough to distinguish the ON/OFF states. The resonance frequency estimated from the Lorenzian fitting were shown in Fig. 2(b). Micromagnetic simulation explains that the discontinuous Hb dependence of SSWR frequency is reasoned by the absorption intensity balance of multiple modes. The absorption intensity of each order was different and the dominant order changes with the bias field. The dominant order changed from 7th order to 5th order at strong bias field, and from 5th order to 3rd order at weak bias field.

**References:**

[1] T. Schneider, A. A. Serga, B. Leven, and B. Hillebrands, Appl. Phys. Lett., vol. 92, p. 022505-1-3 (2008) [2] X. Ya, H. Chen, S. Oyabu, B. Peng, H. Otsuki, T. Tanaka, and K. Matsuyama, J. Appl. Phys., vol. 117, p. 17A719-1-3 (2015). [3] Kazuto Yamanoi, Satoshi Yakata, Takashi Kimura, and Takashi Manago, JJAP, vol 52, p. 083001 (2013)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/9f8588c516a813a13f4ad7b26d48d415.jpg)
Fig. 1 (a) Squareness for the CoFeAl film deposited at various film thickness. (b) schematic of a CPW on magnetic strip.

![](https://assets.underline.io/uploads/markdown_image/1/image/c1f07eb8d3cdf0324e9fb175770a2c61.jpg)
Fig. 2 (a)S21 parameter at different bias field, (b) dependence of resonance frequency on the bias field.

 Standing spin wave resonance properties of multiple spin wave modes on Co2FeAl magnetic strip under zero bias field

The orthogonal magnetization multilayer has the in-plane and perpendicular anisotropy magnetic layers, which has been proposed for the fast magnetization switching by spin transfer torque [1]. On the other hand, it is difficult to obtain the stable spin transfer torque oscillation (STO) in the orthogonal magnetization since the switching occurs too easily. In this study, we introduced the 90 degree magnetic coupling to obtain the STO in the orthogonal configuration. In typical, the 90 degree coupling between two magnetic layers is described by the bilinear coefficient A12 and biquadratic coefficient B12, and obtained under the conditions of A12&lt;|B12| and B12&lt;0 by using suitable layer as a spacer. [2] In our model, the multilayer consists of three layers with thickness of 2 nm; a top layer with the in-plane magnetic anisotropy, a nonmagnetic spacer, and a bottom layer with the perpendicular magnetic anisotropy sequentially. Figure 1 shows the calculated magnetization map in the top layer (a) and the side views of the magnetization configuration of the top and bottom layers (b) when the top layer is CoFe with Ms of 1450 emu/cc, Hk of 35 Oe, and a damping constant of 0.01, and the bottom layer is Fe/Pt with Ms of 800 emu/cc and Hk of 15000 Oe. The top and bottom layer has the vortex magnetization and the perpendicular magnetization, respectively, when a current density is zero. By increasing the current density, the magnetization of the top layer oscillates with frequency 35 GHz while the bottom layer maintains the perpendicular magnetization. This stable STO is due to the 90 degree coupling. Figure 2 shows the STO frequency as a function of the current density for various conditions. First, let us focus on the relation between the STO frequency and magnitude of 90 degree coupling. The STO frequency of the CoFe top layer was increased by decreasing B12 from −0.3 to −0.6, that is, increasing the 90 degree coupling. Next, we compared the STO performances by changing the top layer material from CoFe to NiFe and Ni with small Ms and small damping constant. As a result, the frequency achieved 45 GHz and the necessary current density was reduced to 6x107 A/cm2. In summary, it was found that the STO was stably realized in the orthogonal configuration by introducing 90 degree magnetic coupling, and the STO frequency was increased by enhancement of 90 degree coupling. Additionally, our calculation indicates that it is possible to increase the frequency and necessary current density by material selection.

**References:**

[1] H. Liu et al., Appl. Phys. Lett. 97, 242510 (2010). [2] G. Nagashima., et al, J. Appl. Phys. 126, 093901 (2019)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/0ab8e84ec749f770d055d306dfad3181.jpg)
Fig. 1 (a) Magnetization configuration in the top layer. (b) Side views of magnetization configuration in the top and bottom layers. The shown magnetization corresponds to the enclosed area in (a).

![](https://assets.underline.io/uploads/markdown_image/1/image/0d156a12ee9515515270d71c3b90fa7a.jpg)
Fig. 2 STO frequency as a function of the current density for various conditions; B12=−0.3 and B12=−0.6 for CoFe top layer, and B12=−0.6 for CoFe, NiFe, and Ni top layers.

 Spin Transfer Torque Oscillation in Orthogonal Magnetization Disks

The anomalous Nernst effect (ANE), one of the long-studied thermomagnetic effects, has been regarded as a promising next-generation energy harvesting technology for large-area and flexible devices. However, since the ANE power is too short for practical use, it is necessary to enhance ANE and many ferromagnetic materials have been widely investigated [1]. We experimentally investigated the anomalous Nernst effect (ANE) in an Fe3Si film whose ANE was predicted to be large, based on the topological band structure [2]. We also measured the Seebeck and anomalous Hall effects to separate the ANE term from the Hall effect of the carrier flow induced by the Seebeck effect. The sample structure and measurement system are shown in Fig. 1. In addition to Fe3Si, Fe2Si, FeSi, and FeSi2 50nm films were fabricated on SiO2 substrate by co-sputtering and annealed in vacuum at 1123K for 1 hour. Figure 2 shows the ANE coefficient as a function of Si composition in Fe-Si compounds. The ANE coefficients were estimated as 1.0 and 0.5 µVK-1 for the Fe3Si and Fe2Si samples, respectively. These values are much larger than the ANE coefficient of film Fe, whereas Fe3Si and Fe2Si both have a smaller saturation magnetization than Fe. In general, the Nernst coefficient can be decomposed into two terms: Syx= ρxxαyx - tanθAHESxx. Here, the first term S1 exhibits the intrinsic ANE term that converts heat flow into a transverse voltage, and the second term S2 reflects the voltage originating from the Hall effect of the carrier flow induced by the Seebeck effect during the anomalous Nernst effect. From Fig. 2, it was found that the first term S1 was dominant in the ANE improvement of Fe3Si, suggesting that the ANE of Fe3Si is enhanced because of its band structure, as expected from the theoretical calculation. Fig.1 Schematics of the sample structure and ANE measurement configuration. Fig.2 Negative value of the ANE coefficient Sxy, the first term S1 , and the second term S2 as a function of Si composition in Fe-Si compounds.

![](https://assets.underline.io/uploads/markdown_image/1/image/1a38d99e3e16bb20f0ad9211f86d83f3.jpg)

Fig.1 Schematics of the sample structure and ANE measurement configuration.

![](https://assets.underline.io/uploads/markdown_image/1/image/2ec8641a0197aa716c6bbd15067d45b9.jpg)

Fig.2 Negative value of the ANE coefficient Sxy, the first term S1 , and the second term S2 as a function of Si composition in Fe-Si compounds.

Anomalous Nernst effect in Fe-Si alloy films

Since it is important to find materials with large spin Hall angles for utilizing spintronic devices, various materials have been reported, e. g. Ir–O, Pt-Cu, Ta50W50, and BiSb [1-4]. We investigated the composition effect on spin Seebeck coefficient in YIG/Pt-based alloy systematically, where Pt-based alloy is Pt1−XRuX, Pt1−XCuX, and Pt1−X(Cu0.5Ru0.5)X. The additive elements of Cu and Ru have equally small spin Hall angles in the single state, whereas they have the different atom sizes. In all systems the composition of around 50 at% showed the maximum spin Seebeck coefficient, from which we estimated spin Hall angles [5, 6]. As a result, the spin Hall angle was increased from 0.1 to 0.19, 0.25, 0.34 by changing Pt to Pt-Ru, Pt-RuCu, and Pt-Cu. It means that additive Cu is more effective on spin Hall angle improvement than additive Ru although their own original spin Hall angles are comparably small. X-ray diffraction analysis revealed that Cu and/or Ru doping in Pt host decreased the unit cell size, and there is an obvious relation between the lattice constant and the spin Hall angle as shown in Fig. 1(a). Since the resistivity dependence on lattice constant exhibit the similar tendency in Fig. 1(b), it suggests that the unit cell size modification in heavy metal Pt layer increases the electron scattering including spin-dependent scattering, leading the extrinsic spin Hall angle enhancement.References: [1] K. Fujiwara, Y. Fukuma, J. Matsuno, H. Idzuchi, Y. Niimi, Y. Otani, and H. Takagi, Nat. Commun. 4, 1 (2013). [2] R. Ramaswamy, Y. Wang, M. Elyasi, M. Motapothula, T. Venkatesan, X. Qiu, and H. Yang, Phys. Rev. Appl. 8, 024034 (2017). [3] H. Yuasa, F. Nakata, R.Nakamura and Y. Kurokawa, J. Phys. D: Appl. Phys. 51, 134002 (2018). [4] N. H. D. Khang, Y. Ueda and P. N. Hai, Nat. Mater. 17, 808 (2018). [5] K. Tian and A. Tiwari, Sci. Rep. 9, 1 (2019). [6] H. Li, Y. Kurokawa, T. Niimura, T. Yamauchi, and H. Yuasa, Jpn. J. Appl. Phys. 59, 073001 (2020).

Spin Hall Angle Control by Doping Element into Pt Host

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Presentations

High Frequency Spin Torque Oscillation in Orthogonal Magnetization Disks with Strong Biquadratic Magnetic Coupling

Spin Current Enhancement by Inserting Ultra-thin Magnetic Layer at Interface between YIG and Pt INVITED

Standing spin wave resonance properties of multiple spin wave modes on Co2FeAl magnetic strip under zero bias field

Spin Transfer Torque Oscillation in Orthogonal Magnetization Disks

Anomalous Nernst effect in Fe-Si alloy films

Spin Hall Angle Control by Doping Element into Pt Host

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