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Tailoring and control of electromagnetic properties of synthetic magnetic structures is advantageous for high frequency applications, such as shielding materials in magnetic recording, where characterisation in the gigahertz frequency range is necessary to determine parameters for simulations and aid component design. In such applications it is desirable to have a high saturation magnetization, low coercivity, high permeability and close to zero magnetostriction.&lt;br/&gt;Meeting almost all of these requirements, NiFe is the material of choice for many high frequency applications. We present a systematic study of the effects of process settings, dopants and composition on the ferromagnetic resonance (FMR) and spin wave properties of Ni&lt;sub&gt;x&lt;/sub&gt;Fe&lt;sub&gt;100-x&lt;/sub&gt; alloy thin films fabricated by magnetron sputtering and investigated using a coplanar waveguide, vector network analyser (VNA) [1], vibrating sample magnetometer (VSM) and x-ray diffraction (XRD), with the aim to determine any relation between microstructure and the static and dynamic magnetic properties.&lt;br/&gt;It has been found that changes in the NiFe crystal structure and deposition can have significant impact on the magnetic properties, examples of which can be seen in Fig. 1 and 2. This work extends into compositionally modulated superlattice structures to explore further possibilities with respect to material design and optimization [2]. VAMPIRE [3] atomistic simulations have been employed in conjunction with these experiments and to provide parameters for FMR simulations using the micromagnetic model, OOMMF [4].

**References:**

[1] Y. Ding et al. J. Appl. Phys. 96. (2004)&lt;br/&gt;[2] C.B. Hill et al. Meas. Sci. Technol. 24. (2013)&lt;br/&gt;[3] R.F.L. Evans et al. J. Phys.: Condens. Matter 26, 103202 (2014)&lt;br/&gt;[4] M. J. Donahue and D. G. Porter, “OOMMF User’s Guide, Version 1.0,” NISTIR 6376, National Institute of Standards and Technology, Gaithersburg, MD (Sept 1999).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ca3c03d9e435df9d90ed1b8a3bcadece.jpg)
Fig. 1 : FMR spectra at an applied field of 253Oe showing the broadening of the FMR and therefore increased damping with increasing Fe content in Ni&lt;sub&gt;x&lt;/sub&gt;Fe&lt;sub&gt;100-x&lt;/sub&gt; thin films. Note particularly the most significant changes occur at x=40 which marks the fcc/bcc transition.

![](https://assets.underline.io/uploads/markdown_image/1/image/f564dafe40ca24a0b86a77ae748d9a43.jpg)
Fig. 2 : FMR spectra at an applied field of 253Oe illustrating the effect of increasing substrate temperature during deposition on Ni&lt;sub&gt;80&lt;/sub&gt;Fe&lt;sub&gt;20&lt;/sub&gt;, where a supression of the FMR response is seen at 300°C and above.

INTERMAG 2021

 Control of Static and Dynamic Magnetic Properties of NixFe100-x Alloy Thin Films

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This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

Tailoring and control of electromagnetic properties of synthetic magnetic structures is advantageous for high frequency applications, such as shielding materials in magnetic recording, where characterisation in the gigahertz frequency range is necessary to determine parameters for simulations and aid component design. In such applications it is desirable to have a high saturation magnetization, low coercivity, high permeability and close to zero magnetostriction. Meeting almost all of these requirements, NiFe is the material of choice for many high frequency applications. We present a systematic study of the effects of process settings, dopants and composition on the ferromagnetic resonance (FMR) and spin wave properties of NixFe100-x alloy thin films fabricated by magnetron sputtering and investigated using a coplanar waveguide, vector network analyser (VNA) [1], vibrating sample magnetometer (VSM) and x-ray diffraction (XRD), with the aim to determine any relation between microstructure and the static and dynamic magnetic properties. It has been found that changes in the NiFe crystal structure and deposition can have significant impact on the magnetic properties, examples of which can be seen in Fig. 1 and 2. This work extends into compositionally modulated superlattice structures to explore further possibilities with respect to material design and optimization [2]. VAMPIRE [3] atomistic simulations have been employed in conjunction with these experiments and to provide parameters for FMR simulations using the micromagnetic model, OOMMF [4].

**References:**

[1] Y. Ding et al. J. Appl. Phys. 96. (2004) [2] C.B. Hill et al. Meas. Sci. Technol. 24. (2013) [3] R.F.L. Evans et al. J. Phys.: Condens. Matter 26, 103202 (2014) [4] M. J. Donahue and D. G. Porter, “OOMMF User’s Guide, Version 1.0,” NISTIR 6376, National Institute of Standards and Technology, Gaithersburg, MD (Sept 1999).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ca3c03d9e435df9d90ed1b8a3bcadece.jpg)
Fig. 1 : FMR spectra at an applied field of 253Oe showing the broadening of the FMR and therefore increased damping with increasing Fe content in NixFe100-x thin films. Note particularly the most significant changes occur at x=40 which marks the fcc/bcc transition.

![](https://assets.underline.io/uploads/markdown_image/1/image/f564dafe40ca24a0b86a77ae748d9a43.jpg)
Fig. 2 : FMR spectra at an applied field of 253Oe illustrating the effect of increasing substrate temperature during deposition on Ni80Fe20, where a supression of the FMR response is seen at 300°C and above.

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The control of magnetization dynamics is an important issue for the development of high-frequency devices based on spintronics. There are several ways to excite magnetization dynamics. In our previous study, we reported the amplification of magnetization dynamics derived from the interaction between local magnetization dynamics and spin waves by using a slot line waveguide [1]. In this paper, we report the study on the evaluation of the interaction by measuring ST-FMR. The spin orbit torque is caused by the spin current induced by the spin hall effect in ferromagnetic and non-ferromagnetic bilayer system. The ST-FMR is originally used for the estimation of the spin orbit torque via the measurement of the spin hall magneto resistance. We found that the ST-FMR signals for the samples described below delivered information on the interaction between local magnetization dynamics and spin waves. Fig.1 shows the sample structure and measurement set up for the ST-FMR. We used yttrium iron garnet (YIG) as a ferromagnetic layer. Coplanar waveguides were prepared on the YIG film. The signal line consisted of Cu layer (200 nm) , and was partially replaced by 10nm thick Pt layer. In the separated Pt areas, additional local magnetization dynamics was excited by the spin orbit torque, emitting spin waves. The distance between Pt areas was systematically varied, leading the change of the strength of the interaction between the local magnetization dynamics and spin waves at the Pt areas. Fig.2 shows the distance between Pt areas dependence of ST-FMR signal measured at 7.0 GHz. The sample with one Pt shows symmetrical peaks. On the other hand, the samples with two Pt areas indicate that the asymmetry of the peaks changes with the distance of Pt areas. The symmetry and asymmetry of the peaks reflect the contribution of torques to the magnetization dynamics from the spin orbit torque and RF magnetic field respectively at Pt areas [2]. It should be noted that the strength of RF magnetic field was fixed for all the samples, meaning that the increase in the asymmetry was caused by an additional torque to the magnetization. Another factor is speculated from the interaction between local magnetization dynamics and spin waves because the direction of the torque for the interaction is considered the same as the direction for the RF Oersted field, leading the asymmetry of the peak for the ST-FMR measurement. The asymmetry of the curve showed strong dependence on the distance between Pt areas, reflecting the strength of the interaction. Those experimental results give a way to design the artificially controlled magnetization dynamics effectively. We thank GRANOPT Co,Ltd. for their support of YIG single crystal thin films. This work was supported in part by JSPS KAKENHI Grant Number JP18K14114. This work was supported in part by CSRN, Tohoku University. Part of this work was carried out under the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University. A part of this work was carried out at Nano-Fabrication Support Laboratory, Yamaguchi University supported by "Nanotechnology Platform Project" of the Ministry of Education，Culture，Sports，Science and Technology (MEXT)，Japan.

**References:**

[1] Tetsunori Koda, Sho Muroga, and Yasushi Endo, “Synchronized excitation of magnetization dynamics via spin waves in Bi-YIG thin film by slot line waveguide” Appl. Phys. Lett. 116, 102403 (2020). [2] Luqiao Liu, Takahiro Moriyama, D. C. Ralph, and R. A. Buhrman, “Spin-Torque Ferromagnetic Resonance Induced by the Spin Hall Effect” Phys. Rev. Lett., 106, 036601 (2011).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/163119439d61e9013a174ed3d596d259.jpg)
Schematic view of measurement set up for ST-FMR measurement. A part of the signal line consists of Pt layer for inducing the spin orbit torque.

![](https://assets.underline.io/uploads/markdown_image/1/image/72e2a7835f5ae71297581d9613fbf769.jpg)
Gap of Pt areas dependence of ST-FMR signal measured at 7.0 GHz.

 Evaluation of interaction between local magnetization dynamics and spin waves measured by ST-FMR

Spin current transmission in antiferromagnetic materials is one of the intriguing topics in antiferromagnetic spintronics. The antiferromagnetic order can mediate the transmission of the spin current in the form of propagating magnons. Several groups have reported this phenomenon in varieties of antiferromagnetic materials, such as NiO, CoO, Fe2O3, FeMn, and IrMn. Most of the investigations have so far been conducted at frequencies much lower than the resonant frequency of the antiferromagnetic materials. In order to explore the physics of the phenomenon, it is crucial to extend the investigation to the THz range, at which antiferromagnets typically have a good susceptibility to electromagnetic field, because this frequency domain embraces the antiferromagnetic resonant frequencies. A suitable method for investigating antiferromagnetic dynamics could be a relatively recently developed technique, where THz radiation is generated with an ultrashort laser pulse in a heavy metal (HM) / ferromagnetic metal (FM) bilayer structure [1,2]. In this work [3], we investigated laser stimulated THz emission from Pt/CoO/FeCoB and explored the sub-picosecond pulsed spin current transmission through the antiferromagnetic CoO interlayer. We also particularly look into the polarization of the THz electromagnetic wave and reveal that the latter is influenced by the Néel vector in the antiferromagnets. Figure 1 shows the schematic illustration of the measurement set up. Magnetic field of 100 mT was applied to &lt;110&gt; direction and the CoO Néel vector initially remains in one of the [100] directions, which is an easy axis of CoO. The field cooling process then invokes a twisting of the CoO magnetic moment. The THz electromagnetic wave with x and y components and the polarization angle φ was detected by using two wire grid polarizers. The polarization of the emitted THz wave is parallel to the electric field Ec induced in the Pt layer. The polarization of the THz wave essentially tells us the polarization direction of the spin current injected into the HM layer through the antiferromagnetic layer. Figures 2(a) and 2(b) show the y component of the THz wave, Eypeak, and φ as a function of temperature. Eypeak decreases with decreasing temperature regardless of CoO layer thickness dCoO, which is associated with the dependence of the sheet conductivity of the films and the inverse spin Hall conductivity of the Pt layer [4]. On the other hand, one can notice that φ apparently increases at lower temperature and has an onset at around 200K and 300K for dCoO=2.0 and 5.0 nm, respectively, which coincide with the blocking temperature for these samples. Assuming the polarity of spin density of the ultrafast spin current induced in the FM layer is same direction as magnetization in FeCoB layer, the variations of φ for dCoO=2.0 and 5.0 nm below the blocking temperature indicates that the spin current carried by the THz magnon experiences a non-uniform scattering background, which preferentially scatters the spin orthogonal to the Néel vector gradually rotating away from the axis of magnetization in FeCoB layer. This observation is consistent with previous reports on the spin current experiments in exchange biased systems where the spin current impedance was modified by an accommodation of Néel vector twisting [5]. In summary, we found that sub-picosecond pulsed spin current induced by the femtosecond laser pulse can transmit through antiferromagnetic CoO layers. Our results not only demonstrate the picosecond magnon spin current transmission, but also the picosecond interaction of the THz magnons with the Néel vector in the antiferromagnet. This work was partially supported by JSPS KAKENHI Grant (Nos. 17H04924, 17H05181, H1803787, 19K21972, and 26103004), the Center for Spintronics Research Network (CSRN), and the Center for Science and innovation in Spintronics (CSIS). This work was also partly promoted by the Collaborative Research Program of the Institute for Chemical Research, Kyoto University. One of the authors (YS) acknowledges the Graduate Program in Spintronics (GP-Spin) at Tohoku University.

**References:**

[1] T. Seifert, et al., Nature Photon. 10, 483 (2016). [2] T. Seifert, et al., Nature Communications 9, 2899 (2018). [3] Y. Sasaki, et al., Appl. Phys. Lett. 117, 192403 (2020). [4] M. Matthiesen, et al., Appl. Phys. Lett. 116, 212405 (2020). [5] T. Moriyama, et al., Phys. Rev. Lett. 119, 267204 (2017).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/eb0b73916188b733cb3165bdf5db5783.jpg)
Figure 1. Schematic illustration of the measurement setup.

![](https://assets.underline.io/uploads/markdown_image/1/image/effab4554ad2e2d60020bae6c0b8fbbd.jpg)
Figure 2. Temperature dependence of (a) the peak intensity of Ey and (b)THz polarization angle φ. Open circle, triangle, and square show the data for films with dCoO=0, 2.0 and 5.0 nm, respectively.

 Laser pulse induced ultrafast spin current through the antiferromagnetic insulator in Pt/CoO/FeCoB

Elliptically or circularly polarized terahertz (THz) radiation plays increasingly important roles in both scientific research and practical applications, such as wireless communication, composition analysis and undamaged imaging1. Recently, the realization of highly efficient THz radiation in ferromagnetic multilayer via spin-charge conversion (SCC) brings new hope for the emerging spin THz emitter at room temperature2. However, achieving an easily-tunned THz emitter with polarization and ellipticity manipulable still be challenging. Topological insulator (TI), with strong spin-orbital coupling and spin-momentum-locked surface states, has been regarded as the most effective spin-charge convertor in spin THz emitter3. In addition, the broken inversion symmetry at surface provides topological insulator numerous fascinating nonlinear optical phenomena, the dominant one is linear photogalvanic effect (LPGE), which could yield a polarization-dependent terahertz radiation in topological insulator4. By contrast, the terahertz wave induced by SCC is polarization-independent. This difference in polarization-dependence of two terahertz components makes it possible to control them separately, inspiring a new approach in chirality-tunned terahertz emission by synthesizing the SCC-induced and LPGE-induced terahertz waves only by tailoring the polarization of incident pumping laser. Here we prepared a Bi2Te3 (10nm)/Fe (3nm) heterostructure by molecular beam epitaxy (MBE) and measure the terahertz response after illumination by a linear polarized femtosecond laser. The terahertz wave was probed by a terahertz time domain spectroscopy (THz-TDS). As shown in Figure 1a, the amplitude of the terahertz generated in Bi2Te3/Fe heterostructure is higher than single Bi2Te3 or Fe thin film, almost keeps in the same level with W/CoFeB/Pt trilayer5. The latter is usually regarded as a typical spin terahertz emitter and the result proves the high efficiency of Bi2Te3/Fe heterostructure in terahertz emission. After further careful examination, we divided the overall terahertz radiation into two components. The first is SCC-induced spin terahertz. Generated from the spin-charge conversion in the Bi2Te3/Fe interface, this component is only magnetization-dependent. Meanwhile, an ultrafast photocurrent can also originate in Bi2Te3 film due to LPGE in the inversion symmetry breaking surface, leading to a terahertz wave. Notably, the direction of the LPGE-induced terahertz is related with the polarization of incident laser, which provides a simple method to construct orthogonal terahertz components by adjusting the incident polarization. More importantly, a phase difference between these two terahertz components could be observe because of the delayed time between SCC and LPGE. After deliberately tunning the incident laser to promise both terahertz components in the same magnitude, an elliptical polarized terahertz wave appears in Bi2Te3/Fe heterostructure. Furthermore, a terahertz wave with arbitrarily tailored chirality and ellipticity could be got by tailoring the pumping laser polarization. Overall, we have demonstrated a highly efficient terahertz emission in Bi2Te3/Fe heterostructure. Compared with conventional spintronic terahertz emitter, Bi2Te3/Fe can realize terahertz wave with arbitrarily tailored chirality only by adjusting the incident polarization. The engineering of topological insulator and ferromagnet heterostructure may open up a great opportunity to study high-performance TI-based spintronic terahertz devices with easy controllability.

**References:**

(1) D. M. Mittleman, Opt. Express., Vol. 26, p.9417 (2018) (2) T. Kampfrath; M. Battiato; P. Maldonado, et al., Nat. Nanotechnol., Vol. 8, p.256 (2013) (3) X. Wang; L. Cheng; D. Zhu, et al., Adv.Mater., Vol. 30, p.1 (2018) (4) C. M. Tu; Y. C. Chen; P. Huang, et al., Phys. Rev. B., Vol. 96, p.1 (2017) (5) Y. Wu; M. Elyasi; X. Qiu, et al., Adv.Mater., Vol. 29, p.1 (2017)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/08f5430c71e14fba98452d73a3a96d90.jpg)
Fig. 1. (a) Typical THz temporal waveforms generated from Fe (3 nm), TI (10 nm), TI (10 nm)/Fe (3 nm) and W (1.8 nm)/CoFeB (1.8 nm)/Pt (1.8 nm), respectively. (b, c) Generation right and left-handed elliptical polarized THz waves by tuning incident laser polarization angle.

 Tunable Terahertz Wave via Synthesis of Spin and Charge Induced Radiations in Topological Insulator

The ability to generate and control pulses of terahertz (THz) radiation has revolutionised a multitude of critical technologies, from medical imaging [1] to the detection of explosives [2]. Furthermore, sub-picosecond pulses of THz radiation have future application in the ultrafast control of electron spin states [3] and picosecond magnetisation switching in ferromagnets [4], ferrimagnets [5] and antiferromagnets [6]. To exploit the full potential of THz technology, broadband emitters are required which can generate radiation over a spectral range of 1-10 THz [7]. Spintronic emitters, consisting of ferromagnetic (FM)/non-magnetic (NM) heavy metal thin films, have been shown to produce THz pulses with large electric field amplitudes and gapless bandwidths of up to 30 THz [8]. This THz emission is produced when the spintronic structure is excited by femtosecond laser pulses, demonstrating their potential as low-cost sources of broadband radiation. Whilst it has been well established that particular combinations of FM/NM materials, such as Co20Fe60B20/Pt, produce pulses of THz radiation with high electric field amplitudes (300 kV/cm) [8], a detailed understanding of the role that material properties contribute to the generation process of THz radiation has yet to be fully explored in order to inform the future design of optimum performance spintronic emitters. In particular, post deposition annealing of CoFeB based structures has been shown to enhance the emission of THz radiation by up to a factor of 3 [9,10] further increasing the potential utility of these emitters. This enhanced electrical field amplitude has been attributed to two possible effects: 1) diffusion of Boron into the heavy metal layer, leading to an increased number of scattering centres and a reduced hot electron relaxation length, λNM [9]; or 2) local crystallisation of CoFeB leading to an increase in mean free path of hot electrons travelling through the FM layer [9,10]. In order to explore the electric field enhancement, we present a systematic study which investigates the effects of inducing defects in the NM layer and crystallisation of the FM/NM interface upon the emission of THz radiation from Co20Fe60B20 (2.5 nm)/Pt (3 nm) spintronic thin films. The films have been grown onto fused silica substrates using magnetron sputtering where the base pressure was better than 5 x10-8 Torr. We present results from two series of films. In the first series, the dc sputtering power for Pt was varied between 25 W and 100 W in order to control defects in the Pt layer [11]. In the second series, the films were annealed post deposition between 200°C and 400°C to induce crystallisation at the CoFeB/Pt interface. For all films, the Co20Fe60B20 layer was sputtered at 100 W dc power to enable direct comparison. The structural properties of Co20Fe60B20/Pt films were characterised via X-ray diffraction (Fig. 1a) and atomic force microscopy (Fig. 1b-e), from which the crystalline ordering and the surface roughness of the Pt were observed to change with varying sputtering power (series 1) and annealing temperature (series 2). X-ray reflectivity analysis of Co20Fe60B20/Pt films (series 2) shows a decrease in density of the Pt layer and increase in interfacial roughness of the CoFeB layer, indicating diffusion at the CoFeB/Pt interface with post deposition annealing above 200°C. We have used terahertz time-domain spectroscopy to measure the electric field amplitude of the emitted THz pulses from Co20Fe60B20/Pt films (see Fig. 2). Initial analysis shows that interfacial interdiffusion has a significant influence on the THz electric field amplitude. A full analysis of our results will be presented at the conference.

**References:**

[1] R. M. Woodward et. al., J.Biol.Phys. 29, 257 (2003); [2] K. Kawase et. al., Opt. Express 11, 2549 (2003); [3] S. Baierl et. al., Nat. Photonics 10, 715 (2016); [4] D. Polley et. al., J. Phys. D: Appl. Phys. 51, 084001 (2018); [5] S. Wienholdt et. al. Phys. Rev. Let. 108, 247207 (2012); [6] T. Kampfrath et. al., Nat. Photonics 5, 31 (2001); [7] M. T. Hibberd et. al., Appl. Phys. Let. 114, 031101 (2019); [8] T. Seifert et. al., Nat. Photonics 10, 483 (2016); [9] Y. Sasaki et. al., J. Phys. Soc. Jpn. 111, 102401 (2017); [10] Y. Gao et. al., IRMMW-THz Paris, France, 1-3 (2019) DOI: 10.1109/IRMMW-THz.2019.8874416; [11] S. Mahieu et. al. Thin Solid Films 515, 1229 (2006).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e1893bb8b173d49ae6661079e92079f8.jpg)
Fig. 1: Structural analysis of Co20Fe60B20 (2.5 nm)/Pt (3 nm) thin films deposited onto fused silica substrates. (a) X-ray diffraction spectra for Co20Fe60B20/Pt films with increasing Pt sputtering power (series 1 = pink - 25 W, and green - 50 W) and annealing temperature (series 2 = black - unannealed, red - Tann = 300°C and blue - Tann = 400°C). (b)-(e) Atomic force microscopy images of (series 2) b) unannealed Co20Fe60B20/Pt film, and films annealed at c) Tann = 200°C, d) Tann = 300°C and e) Tann = 400°C. RMS roughness observed to increase with annealing temperature from 0.81 nm (unannealed) to 2.06 nm (Tann = 400°C).

![](https://assets.underline.io/uploads/markdown_image/1/image/848c3f049ddc56bf9bbf0df330871380.jpg)
Fig. 2: Example of THz pulses generated from FS-CoFeB/Pt and FS-Pt/CoFeB thin films (capped with MgO), measured using terahertz time-domain spectroscopy. Inset shows the corresponding frequency spectrum for the FS-CoFeB/Pt thin film.

 Emission of THz Radiation from Co20Fe60B20/Pt Spintronic Thin Films with Varying Microstructural Properties

The development of small and simple room-temperature devices capable of generating and/or receiving resonant signals in a "THz-gap" still stands a significant challenge. However antiferromagnetic spintronics promises to be the right direction for the research and development of THz technology. Recently [1], [2], it has been theoretically proposed to use active AFM generators for the detection of external THz-frequency signals via the mechanism of injection-locking of such a signal to the oscillations generated by a DC-current-driven AFM/HM THz generator. An alternative way to develop quasi-passive AFM/HM-based detectors [3] is to use the fact that resonance eigenfrequencies of the AFM dynamic modes (standing AFMR modes) lie in the THz frequency range. It has been shown theoretically in [3], that a dielectric AFM having bi-axial anisotropy, such as NiO, can be used for the resonance quadratic rectification of a linearly-polarized AC spin current of THz-frequency, and could have a sensitivity in the range of 102-103 V/W. In this work [4], we propose the concept of an electrically tunable resonance detector of THz-frequency signals based on an antiferromagnetic/heavy metal bilayer. The conversion of a THz-frequency input signal into DC voltage is done using the inverse spin Hall effect in a bilayer. An additional bias DC current in the HM layer can be used to vary the effective anisotropy of the AFM, and, therefore, to tune the AFMR frequency. The proposed hetero-structure works as a resonance-type quadratic detector which can be tuned by the bias current in the range of at least 10% of the AFMR frequency. In addition, we discuss experimental results obtained during spin-pumping experiments with antiferromagnetic/heavy metal bilayer (NiO/Pt). The results were obtained in the HFML-FELIX laboratory where we have access to intense THz radiation free-electron laser source as well as high DC magnetic fields up to 38T.

**References:**

[1] O. Gomonay, T. Jungwirth, and J. Sinova. Physical Review B 98.10 (2018) [2] R. Khymyn, V. Tiberkevich, and A. Slavin, Book of abstracts “Intermag – 2018”, p. 795, (2018) [3] R. Khymyn, V.Tiberkevich, and A.Slavin, AIP Advances 7.5 (2017) [4] A. Safin, V. Puliafito, M. Carpentieri, G. Finocchio, S. Nikitov, P. Stremoukhov, A. Kirilyuk, V. Tyberkevych, and A. Slavin Appl. Phys. Lett. 117, 222411 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8912bbefc6eefba483c89d307904df9b.jpg)
Schematic view of the THz-frequency resonance detector based on the AFM-Pt structure.

 Electrically Tunable Detector of THz-Frequency Signals Based on an Antiferromagnet

In recent years, magnon-phonon coupling has attracted renewed interest[1] both from the physics and technological perspectives owing to its efficient conversion of energy between magnon and phonon. In particular, the magneto-acoustic resonance, has become a focal point, partly owing to the possibility for rapid switching, which is significant for applications in cloud storage, advanced spin memory, logic, and other spintronic devices. An ultrafast optical approach, specifically femtosecond laser pulses, is a powerful tool to initiate the coupling and its successive detection. The laser pulse simultaneously generates the phonon and magnon, through thermal expansion and fast demagnetization. The thermal expansion produces a local strain or phonon, whose frequency is tunable by both the material composition and geometry. At the same time, the fast demagnetization excites the magnetization out of equilibrium and creates magnons. The femtosecond time scale of the laser pulse benefits the detection of both magnon and phonon, as their dynamics endure longer than the pulse time scale. However, research reported in the literature mostly launch strain electrically in piezoelectric materials, which leads to a different phonon propagation direction. In addition, the literature study of the coupling is confined to materials unsuitable for spintronic applications. These materials, limit the resonance frequency or require low temperature for the magnetoacoustic resonance. To date, direct demonstration of magnon-phonon coupling remains elusive in materials with high perpendicular magnetic anisotropy, which are capable of achieving ultrahigh frequency resonance at room temperature. Here, we report an extremely high frequency magnetoacoustic resonance up to 60 GHz, originating from magnon-phonon coupling in [Co/Pd]n multilayer with perpendicular magnetic anisotropy. Our theory uncovers the physics of resonance in a coupled system, and is based on the equation of motion for both magnon and phonon, with coupling introduced in the Hamiltonian through the magnetostrictive energy. Owing to the magnon and phonon possessing the same symmetry, the physics at resonance presents an enhanced wave envelope in the time domain, an anticrossing in the frequency domain, and significant hybridization of both magnons and phonons. When the frequencies of magnon and phonon approach each other in the resonance regime, these two modes hybridize into a quasiparticle contributed by both magnon and phonon. This hybridization generates an anticrossing in the dispersion with a frequency gap Δf in Fig. (a), which is quantitatively determined by the coupling coefficient in our model and consistent with our experiment. The amplitudes of both modes increase in the hybridization regime in Fig. (b): the original phonon mode is now highly visible owing to the admixed magnon, the original magnon mode is enhanced owing to the pumping from phonons. In addition, the resonance in the time domain (an enhanced wave envelope) indicates the strain substantially influences the spin dynamics at an ultrafast picosecond time scale. All these physical features are demonstrated in both ultrafast optical measurements and micromagnetic simulations. The ultrafast optical measurements [3] of the time-resolved magneto-optical Kerr effect (TR-MOKE) and time-domain thermoreflectance (TDTR) directly observe the dynamics of magnon and phonon. The frequency and the amplitude of TR-MOKE and TDTR signals are consistent with the modeling, which indicates the transition of the system from hybridization to de-hybridization, controlled by the external applied field. The simulations apply the properties of the [Co/Pd]n multilayers, whose structures are optimized to achieve a maximum resonance frequency allowable with current experimental settings. All the [Co/Pd]n multilayers samples were prepared in an ultra-high vacuum magnetron sputtering system. With an appropriate strain, the simulation predicts that the system is able to undergo a large-angle magnetization precession and achieve a rapid switching. In summary, our work demonstrates an efficient energy transfer among magnon and phonon at an ultrafast picosecond scale, which paves a potential pathway for enabling an ultrahigh speed strain-assisted magnetization switching in a technologically relevant magnetic system.

**References:**

[1] A. Kirilyuk, A. V. Kimel, T. Rasing, Ultrafast optical manipulation of magnetic order. Rev. Mod. Phys. 82, 2731–2784 (2010). [2] D.-L. Zhang, J. Zhu, T. Qu, D. M. Lattery, R. H. Victora, X. Wang, J.-P. Wang, High frequency magnetoacoustic resonance through strain-spin coupling in perpendicular magnetic multilayers. Sci. Adv. 6, eabb4607 (2020) [3] D. B. Huang, D. Lattery, X. Wang, Materials Engineering Enabled by Time-Resolved Magneto-Optical Kerr Effect for Spintronic Applications, ACS Appl. Electron. Mater., 2020 (appears online).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/da1d7a3d7e4d08d14d19d9c77f1710f1.jpg)
The frequency of the [Co(0.8 nm)/Pd(1.5 nm)]11 multilayers as a function of Hext. Two frequencies of spin precession (mode 1, open black circles; mode 2, open red diamonds) are derived by fitting the experimental data of TR-MOKE. The figure also includes the frequency of acoustic waves measured from TDTR (blue stars). The anticrossing point of mode 1 and mode 2 occur at the resonance field (Hext ≈ 21 kOe), where the frequencies of modes 1 and 2 split and open a gap Δf. We assign the strain mode as the one with field-independent frequencies that are nearly identical to the acoustic wave frequencies from TDTR. The frequency of the FMR mode increases linearly with Hext.

![](https://assets.underline.io/uploads/markdown_image/1/image/33d75b34edaa03dce17f237aff203cd9.jpg)
The individual Mz amplitudes of modes 1 and 2 as a function of Hext for the [Co(0.8nm)/Pd(1.5nm)]11 multilayers. There exists an apparent amplification of both modes due to the coupling between these two modes near the anticrossing point. (Figure is taken from Ref. 2.)

 High-frequency magnetoacoustic resonance through strain-spin coupling in perpendicular magnetic multilayers INVITED

We study coupling of electromagnetic waves to an antiferromagnetic resonance at THz frequencies [1,2]. In ferromagnetic materials, the Purcell effect and magnon-polaritons were shown [3], allowing single magnon detection [4] or observation of level attraction due to dissipative coupling [5]. Antiferromagnetic materials, in contrast to ferromagnets, are interesting due to their high-frequency dynamics, while they are still characterized by low coupling of spins to the lattice. For applicational reasons their abundance, huge number of existing materials and lack of stray fields are also advantages. However, in the case of antiferromagnets, there are only a few examples of an indirect strong coupling of spin waves with electromagnetic waves [6, 7] or at GHz frequencies [8]. Here, we report a direct and strong magnon-phonon coupling in high-temperature antiferromagnet hematite α-Fe2O3. A cube of hematite (0.2 mm edge length) was placed inside a gold-plated tube (2 mm long, 0.58 mm internal diameter). This cavity has the lowest mode at about 0.24 THz, as predicted by electrodynamic simulations. The cavity was placed inside a double-cone holder that allowed focusing THz beam into the cavity and collecting the transmitted THz beam form it. We measured transmission spectra in the 0.2-0.3 THz frequency band using a continuous wave spectrometer based on extenders to a vector network analyser. This system is characterized by very high dynamic range and very high frequency resolution. We used temperature as a tuning parameter, because the frequency of the antiferromagnetic resonance in hematite rises monotically with it from room temperature until about 700 K. Our data (Fig. 1) show very clear avoided crossing of the cavity mode and the antiferromagnetic resonance with the minimum splitting of about 6 GHz and the cooperativity factor of about 40. The splitting predicted by a microscopic model [3] is about 15 GHz. The difference is caused by imperfect matching of the cavity magnetic field with the magnetic moment of the antiferromagnetic resonance in the cube, that is confirmed by elecrodynamic simulations. We notice that we can tune the coupling between the experimental setup and the cavity that results in different lineshapes of the cavity mode. We fitted observed spectra with a model based on the input-output theory [3]. We also observed coupling with higher order modes of the cavity that is not as clear as with the lowest mode. We acknowledged support by the Sino-Swiss Science and Technology Cooperation (SSSTC) grant no. EG-CN_02_032019, EPFL and the SNF R'Equip under Grant No. 206021_144983.

**References:**

[1] M. Bialek, J. Zhang, H. Yu et al, arXiv:2012.10910 [2] D. L. Mills and E. Burstein, Rep. Prog. Phys. 37, 817 (1974). [3] H. Huebl, Ch. W. Zollitsch, J. Lotze et al, Phys. Rev. Lett. 111, 127003 (2013). [4] D. Lachance-Quirion, S. P. Wolski, Y. Tabuchi et al, Science 367, 425 (2020). [5] M. Harder, Y. Yang, B. M. Yao et al, Phys. Rev. Lett. 121, 137203 (2018). [6] X. Li, M. Bamba, N. Yuan et al, Science 361, 794 (2018). [7] P. Sivarajah, A. Steinbacher, B. Dastrup et al, J. Appl. Phys. 125, 213103 (2019). [8] J. R. Everts, G. G. G. King, N. J. Lambert et al, Physical Review B 101, 214414 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/803040ba5ac649ea7e6e3374d83e2c49.jpg)
(a) Normalized transmission spectra at different temperatures. (b) Normalized transmission spectra in a form of map.

 Strong coupling of antiferromagnetic resonance with sub-THz cavity fields

The phenomena of interlayer exchange coupling (JIEC) between thin ferromagnetic (FM) layers allows materials with precisely tailored properties to be created, and is key to the future development of spintronics. Synthetic ferromagnets (SFMs) have the same FM/Non-magnetic(NM)/FM structure as the widely studied Synthetic Antiferromagnet (SAF) [1], but describe the case where the magnetisation of the two FM layers are aligned in parallel. The structure of a SFM and its resonant dynamics is shown in Fig.1a. It is known that the formation of a double resonance consisting of an Acoustic Mode (AM) and an Optic Mode (OM) requires a difference in layer magnetisation (MDiff) and a FM JIEC [2] [3]. However, to date, a comprehensive description of the resonant properties as a function of JIEC and difference in layer magnetisation has not been reported despite their widespread use in quasi-static devices [4]. Here, we investigate, both experimentally and via numerical simulations, the hitherto uncharted dependence of the observed double resonance on MDiff and JIEC [5]. The samples were fabricated using magnetron sputtering and characterized by X-ray Reflectivity (XRR) to measure layer thickness, vector Vibrating Sample Magnetometry (VSM) to verify AF coupling and a Vector Network Analyser – Ferromagnetic Resonance (VNA-FMR) setup which allows fundamental dynamic properties to be explored. These SFMs exhibited a JIEC consistent with coupling on the 1st FM RKKY coupling peak and have structure Ta(2 nm)/CoFeB(5 nm)/Ru(tRu)/CoFeB(5 nm)/Pt(4 nm) where tRu is the thickness of the ruthenium layer, acting as the non-magnetic spacer layer. Our simulations, performed using MuMax3, show that MDiff and JIEC have orthogonal dependencies on the two resonant modes. We further demonstrate that the conventional understanding of the phases of these modes, described as possessing an in-phase or out-of-phase nature for the AM and OM respectively, does not capture fully the intricacy of the resonant dynamics. Comparison between experimental and simulated data allows MDiff, JIEC and the resonant phases of each mode to be accurately determined. These findings are directly applicable to the creation of tailored SFMs for spintronic devices with applications in STT/SOT-MRAM where control of these parameters is paramount [3] [6].

**References:**

[1] H. J. Waring, N. A. B. Johansson, I. J. Vera-Marun et al., Zero-field Optic Mode beyond 20 GHz in a Synthetic Antiferromagnet, Phys. Rev. Appl., Vol. 13, p. 034035 (2020) [2] B. Heinrich and J. F. Cochran, Ultrathin metallic magnetic films: magnetic anisotropies and exchange interactions, Adv. Phys., Vol. 42, p. 523–639 (1993) [3] T. McKinnon, P. Omelchenko, B. Heinrich et al., FMR study of interlayer exchange coupling in FeCoB|Ta|FeCoB trilayers with in-plane anisotropy, J. Appl. Phys., Vol. 123, p.223903 (2018) [4] T. Devolder, E. Liu, J. Swerts et al., Ferromagnetic resonance study of composite Co/Ni - FeCoB free layers with perpendicular anisotropy, Appl. Phys. Lett., Vol. 109, p.142408 (2016) [5] H. J. Waring, Y. Li, I. J. Vera-Marun et al. Phys. Rev. Lett., Manuscript In Preparation. [6] S. Yakata, H. Kubota, T. Seki et al., Enhancement of thermal stability using ferromagnetically coupled synthetic free layers in MgO-based magnetic tunnel junctions, IEEE Trans. Magn., Vol. 46, p. 2232–2235 (2010)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/b5294be2ca537e2a7cb2e6695213d4ce.jpg)
Fig.1 a) The resonant dynamics of a synthetic ferromagnetic (SFM) system with a ferromagnetic (FM1)/non-magnetic (NM)/FM2 structure and a FM interlayer exchange coupling JIEC. An optic mode is seen in addition to the acoustic mode only in cases where the layer magnetization of FM1 (Ms,1) and FM2 (Ms,2) are different. b) 2D map of the resonant spectra of a SFM when the NM spacer layer, consisting of Ru, thickness tRu = 1.10nm.

 Characterizing Interlayer Coupling in Synthetic Ferromagnetic Thin Films

The Y3Fe5O12 (YIG) ferrimagnetic insulator is recognized as a model system for spintronic and magnonic phenomena, including spin pumping, spin Seebeck, proximity effects, and spin wave propagation. In these materials, the spin current is produced by spin-waves or magnons. Magnons can be electrically excited and sensed via the Spin Hall Effect (SHE) and Anomalous Spin Hall Effect (ASHE) or they can be thermally excited through the Spin Seebeck Effect (SSE). To detect the spin current, a thin non-magnetic layer such as Pt is deposited on top of the material of interest. This transforms the spin current into an observable thermoelectric voltage (VSSE) via the Inverse Spin Hall Effect (ISHE). Pt/YIG systems allow efficient spin-charge conversion and pure detection of spin-current effects, respectively, due to their unique magneto-optical and spin dynamic properties [1]. These properties allow a clean detection of the pure spin current thanks to the spin-orbit effects in the Pt/YIG microstructures [2]. Additionally, Rare Earth (RE) garnet films are magnetic and magnetoelastic, and their properties can be manipulated through choice of composition and substrate. When a proportion of bismuth (Bi) is added, the magneto-optical properties of YIG increase according to the concentration of bismuth (Bi) at the yttrium site [3]. But there are few studies on how the spin dynamic properties are modified due to this substitution. In this work we study different parameters in the manufacture of ferrimagnetic/non-magnetic bilayers to raise the thermoelectric spin voltage VSSE. Samples with different thicknesses of pure Y3Fe5O12 (YIG) films and a 5% Bi substitution, that is, thin films of BiY2Fe5O12 (BiYIG), were grown on Gd3Ga5O12 (111) and Gd3Sc2Ga3O12 (111) substrates by pulsed laser deposition (PLD). XRD, HR-TEM / STEM-HAADF and GPA measurements revealed high quality crystalline films as it can be seen in Figure 1. Ferromagnetic resonance (FMR) measurements were performed at room temperature for all heterostructures. A resonance field (Hres) and a resonance linewidth (FWHM) were extracted and are shown in Figure 2 as a function of frequency. These values allow the calculation of a Gilbert damping coefficient that is directly correlated with the SSE according to Chang et. Al [4]. In our case, the Gilbert damping coefficient is (5-14)x10-4 for the YIG//GGG samples and (18-19) x10-4 for the YIG//SGGG samples. The linewidth of BiYIG samples is much wider. After growth of an 8 nm-Pt coating layer by sputtering, the damping in all the samples increases due to the contribution due to spin pumping from the YIG or BiYIG to the metal film. The difference between the Heff result and the bulk value of 4πMS reported (1750 G [5]) demonstrates the existence of a perpendicular anisotropy. With a maximum 20K thermal difference across the entire structure, we measure longitudinal SSE voltages in both YIG and BiYIG samples with sputtered Pt. The YIG//SGGG combination reported to have the best voltage signal from the Inverse spin-Hall effect (VISHE) and Spin Seebeck Effect (VSSE), along with the co-occurrence of a positive perpendicular anisotropy. In the case of the BiYIG samples, the voltages induced in the BiYIG//SGGG bilayers were lower with respect to YIG//SGGG but higher with respect to YIG//GGG. The samples with the lowest perpendicular magnetic anisotropy showed the highest VSSE in both cases, YIG and BiYIG. This work is funded thanks to the SPICOLOST (GA 734187) project framed within “Horizon 2020 Funding” of the European Union with the MARIE SKLODOWSKA-CURIE - RISE Actions (Research and Innovation Staff Exchanges) program coordinated by the University of Zaragoza.

**References:**

[1] S. M. Rezende, R. L. Rodríguez-Suárez, and R. O. Cunha, Physical Review B, Vol. 89, p. 014416 (2014) [2] C. Du, H. Wang, and P. C. Hammel, Journal of Applied Physics, Vol. 117, p. 172603 (2015) [3] R. Kumar, B. Samantaray, and Z. Hossain, Journal of Physics: Condensed Matter, Vol. 31, p. 435802 (2019) [4] H. Chang, P. P. Janantha, and J. Ding, Science advances, Vol. 3, p. e1601614 (2017) [5] Bhoi B., Journal of Alloys and Compounds, Vol. 797, p. 523-528 (2019)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ea513c609599a29b0c52d36018d33073.jpg)
Figure 1: Structural characterization at room temperature of the Y3Fe5O12 film grown on Gd3Sc2Ga3O12 (111) crystal by PLD with an 8 nm-Pt coating layer grown by sputtering. Measurements were made by HRTEM / STEM-HAADF.

![](https://assets.underline.io/uploads/markdown_image/1/image/00ba5bba922754ea9aa452f7776f4cb0.jpg)
Figure 2: Room temperature ferromagnetic resonance measurements (FMR) of raw 66nm - Y3Fe5O12 film grown on Gd3Ga5O12 (111) crystal by PLD. The resulting α value is 5.8 ± 0.2. The difference between Heff (2335 G) and the bulk value of 4πMS reported (1790 G) demonstrates the existence of a perpendicular anisotropy.

 FMR and Thermal Spin Pumping in Garnets|Pt Bilayers

Voltage controlled magnetic anisotropy (VCMA) has attracted much attention, since it can dramatically reduce the power consumption during the writing in magnetic random access memory (MRAM) [1]. The voltage-induced anisotropy change triggers the magnetization precession, and a stable magnetization switching is confirmed by the application of appropriate voltage pulses [1]. It is also reported that the stable switching is significantly influenced by the relaxation time of the magnetization precession [2], and thus the magnetization dynamics under the application of the electric field is an important topic to be understood [2]. In this study, we report the electric-field modulation of perpendicular magnetic anisotropy (PMA) and the magnetization dynamics in MgO/Co/Pt trilayers by using time-resolved magneto-optical Kerr effect (TRMOKE). ITO(20) /MgO(10)/Co(1.2)/Pt(1.6)/Ta(10)/oxidized Si substrate (thickness in nanometer) was prepared by using a magnetron spattering system. The film was microfabricated into a Hall-bar structure and a gate electrode of ITO(15 nm) was deposited on the junction. Gate voltage was applied from -3 V to 3 V corresponding to the electric field from -0.3 V/nm to 0.3 V/nm. To investigate PMA, we measured anomalous Hall effect (AHE). The magnetization dynamics was measured by TRMOKE using high-power fiber laser with λ=1040 nm, a pulse width of 500 fs, and a repetition frequency of 100 kHz. During the measurement, an external magnetic field Hext in the range of 5-14 kOe was applied in the direction of 73 deg from the film normal. The measured precession was fitted with the damped oscillation function, exp(-t/τ)sin(ωt), where τ and ω denote relaxation time and angular frequency of the precession. By fitting the Hext dependence ofω, we estimated effective magnetic anisotropy field Hk and g-factor. Figure 1 shows the electric field E dependence of Hk estimated from AHE and TRMOKE. Hk decreases linearly with increasing the electric field in both measurements. The variation of PMA with the electric field η obtained from AHE and TRMOKE were -32 fJ/Vm and -47 fJ/Vm, respectively. Figure 2 (a) shows the electric field dependence of the inverse of relaxation time 1/τ obtained at Hext=14 kOe. 1/τ is known to consist of four contributions; effective Gilbert damping αeff, anisotropy axis distribution ΔθH, anisotropy distribution ΔHk and two-magnon scattering (TMS). The four contributions depend on Hext and E, since Hk depends on E, and we fitted Hext and E dependences of 1/τ by assuming material-dependent parameters which are independent of Hext and E. The extracted four contributions at Hext=14 kOe are also shown in Fig. 2 (a). The results indicate a large electric-field modification of 1/τ is caused by the two contributions: effective Gilbert damping and ΔθH, which have the same trend as the electric-field modification of 1/τ. Figure 2 (b) shows the electric field dependence of αeff. αeff is found to decrease with increasing electric field with a ratio of -13 % per 1 V/nm. Okada et al. reported a ferromagnetic resonance (FMR) study on MgO/CoFeB/Ta trilayers, and they showed PMA variation of η=-60 fJ/Vm and α modulation of -21 % per 1 V/nm [3]. These results are similar to our results, however their samples have MgO(001)/bcc-CoFe interface. Our study indicates similar PMA and α modulations are found in MgO/Co/Pt trilayers having MgO(111)/fcc-Co interface.

**References:**

[1] Y. Shiota, et al., Nat. Mater., 11, 39 (2011). [2] R. Matsumoto, et al., Appl. Phys. Express 12, 053003 (2019) [3] A. Okada, et al., Appl. Phys. Lett. 105, 052415 (2014).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/121663ae9205bffe840fa6e0e620c23c.jpg)
Fig.1 Electric field dependence of effective magnetic anisotropy field Hk of the MgO/Co/Pt trilayers.

![](https://assets.underline.io/uploads/markdown_image/1/image/03117393df9d4f663c79f64c0cda057f.jpg)
Fig.2 (a)Electric field dependence of the inverse of relaxation time 1/τ at an applied magnetic field Hext=14 kOe. Four contributions to 1/τ: effective Gilbert damping αeff, anisotropy axis distribution ΔθH, anisotropy distribution ΔHk and two-magnon scattering, are extracted by the fitting of Hext and E dependences of 1/τ, and are plotted in the same figure. (b)Electric field dependence of the effective damping constant αeff of the MgO/Co/Pt trilayers.

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