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Magnon valve effect (MVE) and magnon junction effect (MJE) which show the spin-dependent magnon transmission through ferromagnetic insulator (FMI) /spacer [S = nonmagnetic metal or antiferromagnetic insulator (AFMI=AFI)]/ferromagnetic insulator (FMI) sandwiches have been experimentally reported by our group [1-3]. It opens a door for promising applications in magnonics. However，the MJE is still not well interpreted, such as its physical origin and the optimized structure of AFMI spacer. It is intuitively regarded that the AFMI spacer would effectively modulate the magnon transmission and a high switching on-off ratio of magnon flow can be expected after a wise choice of the spacer, which would be proven in this work. Here, we study in theory magnon transmission through sandwich structure of FMI|AFI|FMI by atomistic spin model simulation [4]. We reproduced the MJE observed in experiments and proposed a magnon selection rule induced magnon blocking effect (MBE) to explain it. More importantly, following the proposed mechanism, different kinds of AFI spacers were evaluated. It showed that magnon transmission could be remarkably changed by the AFI spacers with various FMI configurations. This works on the MBE can provide significant and meaningful information for both the fundamental physics and the appealing applications of magnon junctions.

**References:**

[1] H. Wu, and X. F. Han* et al., &lt;i&gt;Phys. Rev. Lett.&lt;/i&gt; 120, 097205 (2018).&lt;br/&gt;[2] C. Y. Guo, C. H. Wan, and X. F. Han* et al., &lt;i&gt;Phys. Rev. B&lt;/i&gt; 98, 134426 (2018).&lt;br/&gt;[3] C. Y. Guo, C. H. Wan, and X. F. Han* et al. &lt;i&gt;Nat. Electron. &lt;/i&gt;3, 304–308 (2020).&lt;br/&gt;[4] Z. R. Yan, C. H. Wan and X. F. Han*, &lt;i&gt;Phys. Rev. Appl&lt;/i&gt;. 14, 044053 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/623bd53887b1d15f4775ef1bf710eeb4.jpg)
Fig. 1. Magnon junction structure and space dependence of magnon potential. (a) Magnon potential (|M&lt;sub&gt;z&lt;/sub&gt;|) as function of layer (&lt;i&gt;x&lt;/i&gt;) of P state (solid line), AP state (short dot) and FMI1|AFI system (green line). (b) Separated channel dependent |M&lt;sub&gt;z&lt;/sub&gt;| from (a): ↑|↑|↑ (black line), ↑|↓|↑ (red line), ↑|↑|↓ (blue line) and ↑|↓|↓ (green line). (c and d) Schematics of spin dependent magnon blocking of P and AP state respectively.  Spin-up (spin-down) lattices can only accommodate right- (left-) circularly polarized magnons. While only right-circularly polarized magnons are favored in FMI with upward  magnetization, both left- and right-circular polarizations are permitted in AFMI owing to two spin-opposite lattices. This selection rule thus makes the MBE occur when magnons try to diffuse into a spin lattice, which does not support their polarization.



INTERMAG 2021

 Magnon Blocking Effect in Antiferromagnet Spacered Magnon Junction

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Magnon valve effect (MVE) and magnon junction effect (MJE) which show the spin-dependent magnon transmission through ferromagnetic insulator (FMI) /spacer [S = nonmagnetic metal or antiferromagnetic insulator (AFMI=AFI)]/ferromagnetic insulator (FMI) sandwiches have been experimentally reported by our group [1-3]. It opens a door for promising applications in magnonics. However，the MJE is still not well interpreted, such as its physical origin and the optimized structure of AFMI spacer. It is intuitively regarded that the AFMI spacer would effectively modulate the magnon transmission and a high switching on-off ratio of magnon flow can be expected after a wise choice of the spacer, which would be proven in this work. Here, we study in theory magnon transmission through sandwich structure of FMI|AFI|FMI by atomistic spin model simulation [4]. We reproduced the MJE observed in experiments and proposed a magnon selection rule induced magnon blocking effect (MBE) to explain it. More importantly, following the proposed mechanism, different kinds of AFI spacers were evaluated. It showed that magnon transmission could be remarkably changed by the AFI spacers with various FMI configurations. This works on the MBE can provide significant and meaningful information for both the fundamental physics and the appealing applications of magnon junctions.

**References:**

[1] H. Wu, and X. F. Han* et al., Phys. Rev. Lett. 120, 097205 (2018). [2] C. Y. Guo, C. H. Wan, and X. F. Han* et al., Phys. Rev. B 98, 134426 (2018). [3] C. Y. Guo, C. H. Wan, and X. F. Han* et al. Nat. Electron. 3, 304–308 (2020). [4] Z. R. Yan, C. H. Wan and X. F. Han*, Phys. Rev. Appl. 14, 044053 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/623bd53887b1d15f4775ef1bf710eeb4.jpg)
Fig. 1. Magnon junction structure and space dependence of magnon potential. (a) Magnon potential (|Mz|) as function of layer (x) of P state (solid line), AP state (short dot) and FMI1|AFI system (green line). (b) Separated channel dependent |Mz| from (a): ↑|↑|↑ (black line), ↑|↓|↑ (red line), ↑|↑|↓ (blue line) and ↑|↓|↓ (green line). (c and d) Schematics of spin dependent magnon blocking of P and AP state respectively. Spin-up (spin-down) lattices can only accommodate right- (left-) circularly polarized magnons. While only right-circularly polarized magnons are favored in FMI with upward magnetization, both left- and right-circular polarizations are permitted in AFMI owing to two spin-opposite lattices. This selection rule thus makes the MBE occur when magnons try to diffuse into a spin lattice, which does not support their polarization.

poster

It is now well established that the propagation of spin waves can be controlled by using approaches similar to those used in optics. In recent years particular attention was given to the possibility to controllably focus propagating spin waves. Such focusing can be achieved relatively easily in confined geometries. In the case of extended magnetic films, the implementation of spin-wave focusing appears to be less straightforward. Several approaches have been suggested. However, all of them are rather complex in terms of practical implementation, particularly on the nanoscale. Here, we experimentally demonstrate tight focusing of a spin-wave beam excited in extended nanometer-thick films of YIG by a simple microscopic antenna functioning as a single-slit near-field lens. We show that the focal distance and the minimum transverse width of the focal spot can be controlled in a broad range by varying the frequency/wavelength of spin waves and the antenna geometry. The experimental data are in good agreement with the results of numerical simulations. Our findings provide a simple solution for the implementation of magnonic nanodevices requiring a local concentration of the spin-wave energy.

**References:**

B. Divinskiy, N. Thiery, L. Vila, O. Klein, N. Beaulieu, J. Ben Youssef, S. O. Demokritov, and V. E. Demidov, Appl. Phys. Lett. 116, 062401 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/678744363a23cd07e63934a39dba2cab.jpg)
Fig.1 Schematic of the experiment.

![](https://assets.underline.io/uploads/markdown_image/1/image/224225623364fe1dd737aeb316342533.jpg)
Fig. 2 Measured intensity map of spin waves radiated by the antenna.

 Sub-Micrometer Near-Field Focusing of Spin Waves in Ultrathin YIG Films

Magnonics utilizing spin waves for information transport, storage, and processing offers an alternative for CMOS-based technology free of detrimental Joule heating at high operation frequency [1]. The realization of magnonic devices requires active control of spin waves and low transmission losses. Magnonic crystals, i.e., metamaterials with periodically modulated magnetic properties, made of yttrium iron garnet (YIG) are promising candidates for performing such tasks because of ultralow magnetic damping [2]. Thus far, several types of micrometer-thick YIG magnonic crystals have been explored including crystals made of periodic shallow airgrooves, metallic stripe arrays on top of a YIG film, and width-modulated YIG structures [2]. On the other hand, programmable control of spin-wave transmission in micrometer-thick YIG films has been demonstrated using current-carrying meander structures [3], optical absorbers [4], and strain coupling to a piezoelectric layer [5]. However, micrometer-thick YIG magnonic crystals with a large number of crystal units are not compatible with miniaturized on-chip device integration. In addition, magnonic crystals based on thick YIG films exhibit small bandgaps in the range 5 MHz – 50 MHz, which limits the manipulation of propagating spin waves. Low-loss nanomagnonics requires smaller YIG magnonic crystals with larger tunable bandgaps [6]. Here, we present the experimental realization of magnonic crystals made of only 2 - 4 discrete 260-nm-thick YIG stripes with robust bandgaps up to 200 MHz for Damon-Eshbach spin waves [7]. The YIG stripes of the magnonic crystal are separated by airgrooves or grooves filled by CoFeB. Compared to micrometer-thick YIG films, the opening of larger bandgaps with less crystal units in arrays of thin YIG stripes is explained by strong Bragg reflection on the individual scatterers. Moreover, efficient dipolar coupling between the YIG stripes via in-plane dynamic fields facilitates low-loss transmission in the allowed bands of the magnonic crystal. The bandgaps of the nanometer-thick YIG magnonic crystals are tunable through a variation of the groove depth, lattice constant, and film thickness [8]. For instance, the bandgaps of a 260-nm-thick YIG crystal widen upon an increase of the groove depth from half to full film thickness. Importantly, low-loss spin-wave transmission in the allowed bands of the magnonic crystal is hardly affected by the patterning of fully discrete YIG stripes. Downscaling of the YIG film thickness to 35 nm decreases the bandgap size through a flattening of the spin-wave dispersion relation. We show that a reduction in the lattice constant effectively compensates for this trend. For 65-nm-thick YIG crystals, we realized 100–200 MHz bandgaps by decreasing the lattice constant from 50 µm to 10 µm. Finally, we show that robust bandgaps up to 220 MHz in combination with low-loss spin-wave transmission at allowed frequencies are attained in a 45-nm-thick YIG crystal with a lattice constant of 2 µm (see Fig. 1). We anticipate that further downscaling of YIG magnonic crystals to the nanoscale is possible through an optimization of the fabrication process and efficient spin-wave excitation over a broad range of wave vectors.

**References:**

[1] A.V. Chumak et al., Nat. Phys. 11, 453 (2015) [2] A.V. Chumak et al., J. Phys. D: Appl. Phys. 50 244001 (2017) [3] A.V. Chumak et al., Nat. Commun. 1, 141 (2010) [4] M. Vogel et al., Nat. Phys. 11, 487 (2015) [5] A.V. Sadovnikov et al., Phys. Rev. Lett. 120, 257203 (2018) [6] M. Krawczyk et al., J. Phys. Condens. Matter 26 (2014) [7] H.J. Qin et al., Nat. Commun. 9, 5445 (2018) [8] H.J. Qin et al., Appl. Phys. Lett. 116, 202403 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/8e63382b69aa64034d71dd49d355d495.jpg)
Fig. 1: (a) Contour plot of spin-wave transmission spectra (amplitude of S12) for a YIG magnonic crystal as a function of external magnetic field. The crystal consists of four 0.5-µm-wide airgrooves with a separation of 2 µm. (b) Line profile of spin-wave transmission (blue curve) for an external magnetic field of 5 mT and a reference spectrum (orange curve) measured on an unpatterned part of the same YIG film. (c) Dispersion relation of Damon-Eshbach spin waves in YIG for a magnetic field of 5 mT. The horizontal and vertical dashed lines indicate the bandgap center frequencies and corresponding Bragg scattering wave vectors. (d) Measured gap size of the first bandgap as a function of external magnetic field.

 Low-loss magnonic crystals based on nanometer-thick YIG films

Brillouin Light Scattering (BLS) [1, 2] experiment has been used to determine the spin waves in Co-rich cobalt phosphorous (CoP) exchange spring [3, 4] thin films prepared by the electrodeposition techniques [5, 6] at room temperature. The static magnetic measurement using hysteresis loop tracer (ShB Instruments, USA) shown in FIG. 1 (a)-(b) reveals the hysteresis loop structures for the electrodeposited thin films – the staircase like exchange spring nature with the presence of two coercivities in the hysteresis loops – composed by the soft and hard magnetic phases in a single material. Contrasted with the conventional single-phase magnetic material having a comparatively low coercivity – such kind of exchange spring nanomagnetic frameworks are promising for cutting edge, forefront magnetic applications due to their relatively higher energy product – the blend of magnetization and permanent magnetic field [7]. The differential of the magnetization shown in FIG. 1 (c)-(d) confirms the presence of two coercivities in the 2.8 μm, 5.7 μm thick thin films, respectively. Utilizing a (3+3)-pass tandem Fabry-Perot interferometer, and considering the backscattering geometry, the BLS spectra of the thermally excited SWs were measured. The BLS Spectra of the 2.8 µm thick CoP thin film shows that the peaks are very broad. The field dependent BLS spectra of 5.7 µm thick CoP thin film electrodeposited for 60 min is shown in FIG. 2(a). The BLS study yields two main modes present in the BLS spectrum: the lowest frequency one is a band of bulk modes (B) while the one at the higher frequency (sharp peak) is the so-called Damon-Eshbach surface spin wave (S) [8]. On the Stokes-side of the spectra, two well resolved peaks (S and B, as mentioned in the figure) are observed while the anti-Stokes side of the spectra shows only one peak (B). The S mode is observed only on one side of the spectrum, depending on the direction of the external magnetic field. This is the consequence of the nonreciprocal character of the S wave and of the micrometer thickness of the investigated films. The external magnetic field dependent BLS spectra shows an almost linear dependence of the mode frequencies vs the magnetic field intensity. The BLS spectra shown in FIG. 2(b) reveals the effect of magnetization reversal at constant polarization which unveils the predicted side change of Damon Eshbach (DE) surface mode. The change in Stokes/anti-Stokes ratio has also been affirmed because of the associated transfer of the S and B mode interaction with this magnetization reversal [8].

**References:**

[1] G. Gubbiotti, X. Zhou, A. O. Adeyeye, Phys. Rev. B 101, 224431 (2020). [2] S. Tacchi, G. Gubbiotti, M. Madami, J. Phys. Condens. Matter 29, 73001 (2016). [3] E. E. Fullerton, J. S. Jiang, M. Grimsditch, Phys. Rev. B 58, 12193 (1998). [4] F. Magnus, M. E. Brooks-Bartlett, R. Moubah, Nat. Commun. 7, ncomms11931 (2016). [5] P. McCloskey, B. Jamieson, T. O’Donnell, J. Magn. Magn. Mater. 320, 2509 (2008). [6] S. Roy, A. Connell, M. Ludwig, J. Magn. Magn. Mater. 290–291, 1524 (2005). [7] H. Zeng, J. Li, J. P. Liu, Nature 420, 395 (2002). [8] J. R. Sandercock and W. Wettling, J. Appl. Phys. 50, 7784 (1979).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/824d97b4d904e77f940a3d8f4d27e808.jpg)
FIG. 1: Hysteresis loop of the electrodeposited CoP thin films measured ShB Hysteresis loop tracer in XY plane, and here, dir-1+90°=dir-2. The thickness of the films is (a) 2.8 μm, (b) 5.7 μm, for the electrodeposited time (a) 30 min, (b) 60 min. (c)-(d) The differential of the magnetization of the (a) 2.8 μm, (b) 5.7 μm thick thin films, respectively.

![](https://assets.underline.io/uploads/markdown_image/1/image/5584003a2099c29290540314fe3a0987.jpg)
FIG. 2 (a) BLS spectra of the 5.7 thick CoP thin film (electrodeposited for 60 min) at different in-plane bias magnetic fields measured from 1.50 kOe to 0 Oe with the backscattering geometry. (b) BLS Spectra of 5.7 in a bias magnetic field of +1.0 and -1.0 kOe revealing the effect of the reversal of the magnetization on the frequency position of the Damon-Eshbach wave.

 Brillouin Light Scattering Study of Spin Waves in CoP Exchange Spring Thin Films

Spin waves (SWs) are emerging as a potential alternative to electrical signals for computing and signal processing. Because SWs have relatively short wavelengths at microwave frequencies, they have the potential for applications in high-speed chip-scale electronics [1]. This work seeks to design and fabricate a spin-wave-based real-time spectrum analyzer that exploits SW interference in analogy to an optical spectrometer [2]. Here, microwave electrical signals are converted into SWs. All processing is then done by the diffraction and interference of SWs as they propagate in a magnetic thin film. After the interference pattern forms, SWs are then converted back into electrical signals. This type of device requires SWs having an isotropic dispersion relation, which, in turn, requires an out-of-plane DC bias field. These spin waves are forward-volume spin waves (FVSW). The focus of the work presented here is to explore the design space of a microwave-to-spin-wave transducer for FVSWs, where the microwave signal is introduced by a coplanar waveguide (CPW). The SW excitation spectrum of an external RF magnetic field acting on a magnetic thin film is the result of the interference between SWs launched from all points of the film that are excited by the magnetic field. For a magnetic field from a CPW, this interference results in a comb-like spatial-frequency response, which is equivalent to the Fourier transform (FT) of the spatial distribution of the magnetic field that acts on the magnetic film [3]. However, because the device described above requires a relatively constant transfer function over the full bandwidth, i.e., without nulls, the comb-like excitation spectrum from a CPW would not be suitable for our applications. Launching spin waves from the edge of the film can be used to improve the spin-wave excitation spectrum for a couple of reasons. First, it has been shown that the edge of a film can be an efficient broadband source of spin-waves [4]. And second, because only part of the CPW’s field acts on the magnetic film, the FT of only that part describes the excitation spectrum; the FT of the spatial distribution is more delta-function-like and therefore is more broadband than the previous case. To design the edge-launched SW transducer, micromagnetic simulations were done in Mumax3 [5] using the magnetic field from CPWs simulated in Ansys HFSS. Our simulations (Fig. 1) show the frequency response of this simulated structure with the edge 2 μm away from the CPW. These simulations verify that the transducer’s excitation spectrum is improved, i.e., made more uniform, by placing the CPW beside the edge of the film rather than on top of the film. This improved bandwidth comes at the cost of transducer efficiency, since only the field to the side of the CPW acts on the magnetic film. Because the magnetic field decreases rapidly with distance from the CPW, the edge of the film should be as close to the CPW as possible to increase coupling between the CPW and the film. Representative waveforms given in Fig. 2 show that because the small out-of-plane component of the RF magnetic field is parallel to the large DC-bias magnetic field, it contributes very little to the excitation of a SW compared to the in-plane components of the field. The results of the simulations show that for the same magnetic field, raising the bottom of the CPW increases the in-plane component of the field and increases the amplitude of the SW. Yet another configuration would be to place the CPW on the magnetic film and create an edge-launcher by etching a gap in the film to the side of the CPW. Excessively large fields can produce unstable spin-waves with additional unwanted undesirable frequency components [6]. With a gap in the magnetic material, the internal magnetic field crosses the gap at a reduced field strength, removing the unwanted structure and resulting in SWs at the desired wavelength, as well as a smoother frequency response (Fig. 1.). The structure of the previous paragraph was simulated with the properties of an YIG thin film having different thicknesses (30 nm and 243 nm). It is found that increasing the distance between the gap and the CPW increases the amplitude of the resulting SW with diminishing returns, as shown in Fig. 2. With a gap that is 2 μm wide and 6 μm away from the CPW, we find that for the thinner film this structure produces a SW with a smaller amplitude than when the material underneath the CPW is nonmagnetic and the edge is flush with CPW. However, with the thicker film (Fig. 2), the structure with the magnetic film under the CPW produces a larger SW than the structure without the magnetic material under the CPW.

**References:**

[1] G. Csaba, Á. Papp and W. Porod, Physics Letters A, Vol. 381, no. 17, pp. 1471-1476 (2017) [2] Á. Papp, et al., Scientific Reports, vol. 7, no. 1, pp.9245-9245 (2017) [3] S. Maendl, I. Stasinopoulos and D. Grundler, Applied Physics Letters, vol. 111, no. 1, p. 012403 (2017) [4] F. Mushenok, et al., Applied Physics Letters, Vol. 111, p. 042404 (2017) [5] A. Vansteenkiste, et al., AIP Advances, vol. 4, no. 10, p. 107133 (2014) [6] H. Aquino, et al., poster presented at the 78th Device Research Conference

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/09cf474cf41a0ae00817c2d3cf2617d2.jpg)
Fig. 1. The spatial frequency response of simulated microwave to SW transducers with the CPW placed to the side of the film with and without magnetic material underneath the CPW. The same magnetic field was used for both transducers. The FT of the field is shown to illustrate the comb-like response of a CPW on the film.

![](https://assets.underline.io/uploads/markdown_image/1/image/67403ddf0bb41726426c09e479eb943c.jpg)
Fig. 2. Simulation results with SWs (in-plane magnetization, Mx) launched from the edge of a 243 nm thick YIG film. All plots begin at the edge of the magnetic film. 1) The edge is flush to the CPW and the bottom of the 300 nm thick CPW is in the same plane as the bottom of the magnetic film; 2) the CPW is on the magnetic film with a 2 μm gap etched to the side of the CPW; 3) the CPW is raised on nonmagnetic material such that the bottom of the CPW is in the same plane as the top of the magnetic film; and 4) the CPW is on the magnetic film with the gap 6 μm to the side.

 Design of a Coplanar-Waveguide-Based Microwave-to-Spin-Wave Transducer

Signal processing, conditioning, filtering and are becoming critical parts of RF communications. We have designed a magnonic Bragg filter with a tailored frequency response. A systematic procedure for arriving at the period of Bragg grating, excitation, probe region etc., has been formulated to simulate the expected spectral response. We simulate and analyze a 1D magnonic Bragg narrow band rejection filter at 28 GHz, the frequency allotted to India, in the 5G spectrum. The magnonic filter with 10 grooves has a pass band from 12 GHz to 50 GHz and a tuneable band gap with attenuation as high as 36 dB. Modulating the depth and the duty cycle of the magnonic crystal, we demonstrate that we could tune parameters like, the bandgap, rejection ratio and group velocity of the spin waves. The rejection efficiency of the filter at the frequency of 28 GHz is 90% higher compared to a signal of 40 GHz, in the pass band. INTRODUCTION 5G mobile telecommunications operate above 24 GHz band and these signals suffer much higher path loss [1]. Magnons, or quanta of spin waves (SW) have been explored as information carriers and for computing [2] with the help of magnonic crystals (MCs). Experimental demonstrations show tuning of bandgaps in MCs, by varying parameters such as the applied magnetic field (HDC), width, thickness [3, 4] etc. With extensive experimental investigations being pursued, we arrive at an optimized set of design procedures to estimate different simulation parameters. A magnonic Bragg filter (MBF) is designed to operate at 28 GHz and simulated using the micromagnetic simulation tool, MuMax3. The tunability of bandgaps, rejection and group velocity with modulations of duty cycle and depth of the gratings are studied extensively to finally realize an optimized and highly efficient 1D MBF. DESIGN PROCEDURE The nonlinear dispersion relation between frequency(ω) and wavenumber(kSW) introduces a constraint in the filter design. We must invert ω(kSW), find kSW for a given excitation geometry, and then pick the Bragg wavelength(Λ). We must also use a broadband excitation to capture the full spectrum. Micromagnetics thus become computationally intensive, and a simple MBF simulation occupies terabytes of storage. Hence, we developed custom Go routines to extract the relevant spatially averaged and temporally sampled data from MuMax3 simulations [5]. For our purposes, we assume a strip of permalloy, with saturation magnetization 800 kA/m, exchange constant 1.3×10−11J/m and damping constant α= 0.006 [6]. The backward volume spin waves configuration excites SWs along x, parallel to HDC, as in Fig.1(a). At ω=28 GHz, and with HDC=54.9 kA/m (a convenient field value chosen for our future experiments), we obtained kSW = 0.145 rad/nm from the dispersion relation [7, Eqn(18)]. This corresponds to a wavelength λSW = 43.33 nm, and we choose Λ=21.6 nm to match this at normal incidence. A broadband small signal excitation of hy(x,t) with cut off values of kc = ±0.3 rad/nm and fc = ±50 GHz is applied using sinc pulses in space and time. This yields sinc pulses with a peak centered at 100 ps and spread over 108 nm. In the probe region, the magnetization component my is collected for post processing. We chose a probe region 6×λSW long, and locate it sufficiently far away from gratings to minimize transient interferences. Absorbing boundary regions are defined at either boundaries with a parabolic variation of α from 1 to 0.006 to absorb the reflections from edges[8]. With all these estimated parameters, the dimensions of the MC with 10 grooves on an 8 nm thick permalloy film is 1843.2×8×100 nm3. DUTY CYCLE & DEPTH MODULATIONS While the grating period decides the reflected kSW, the duty cycle (DC) of the grating affects the bandgap. We simulated gratings with depths of 2 nm and DCs of 50%, 33.3% and 16.7%. 1D spectral plots in Fig.1(b) show sharp dips of the order of 20 dB consistently, at kSW= 0.145 rad/nm for all MCs except the plain film, indicating that this wavevector is rejected irrespective of the DC variation. We also study the effect of depth modulation for gratings of depths 1 nm, 2 nm and 4 nm, all with a DC of 50%. The 1D-FFT plot in Fig.1(c), shows the occurrence of bandgaps in the designed frequency range. The effective thickness of the film allows propagation of both exchange and dipolar coupled SWs. From Fig.2(a), we observe a narrower bandgap, a reduced group velocity of the SWs, and an increase in frequency rejection ratio with increasing groove depth thickness. Based on these studies, the MBF is optimized to have 2 nm depth and a DC of 50%. Finally, to confirm the performance, a single frequency sinusoidal excitation at 28 GHz (in the bandgap), is compared with that of 40 GHz (not in bandgap) and observed that the rejection efficiency is about 90% higher at 28 GHz, as in Fig.2(b). CONCLUSION For a given frequency response of a magnonic device, we developed a systematic set of procedures to plan out various simulation parameters. The magnonic Bragg filter is designed and realized to reject frequency of 28 GHz efficiently. This study highlights the effect of variations, in duty cycle and depth, of MCs with sub-micron features, and assists in the fabrication process. We also expect that the outlined design procedure, and the computational routines, will be helpful to the magnonic community.

**References:**

 [1] Jack Browne, https://www.mwrf.com/technologies/systems/article/21120263/surveying-the-status-of-5g-technology, (2020) [2] A. Mahmoud, F. Ciubotaru and F. Vanderveken, J. Appl. Phys., Vol. 128,p. 161101 (2020) [3] P. Frey, A. A. Nikitin and D. A. Bozhko, Commn. Phys., Vol. 3, p.17(2020) [4] A. V. Chumak A. A. Serga and S. Wolff, Appl. Phys. Lett, Vol. 94,p.172511 (2009) [5] https://github.com/LekhaRam/Mumax310AveragedSampling (2020) [6] Y. Zhao, Q. Song and S. H. Yang, Sci. Rep., Vol. 6, p.22890 (2016) [7] B. A. Kalinikos, IEE Proc.H.,Vol. 127, p.4 (1980) [8] G. Venkat and A. Prabhakar, J. Magn. Magn. Mater., Vol. 450, p.34 (2018)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/5fbbbf88907c50bb875d36f1018d0bb3.jpg)
(a) Structure of MBF with dimensions in nm (b) Variation of my in k space with DC (c) Spectrum of depth modulated SW

![](https://assets.underline.io/uploads/markdown_image/1/image/67db7f859951c9d5171facd2890a6a12.jpg)
(a) Results of depth modulated MC (b) Sinusoidal excitation at 28 GHz & 40 GHz for the optimized 1D MBF

 Magnonic Bragg Filter Design for India's 5G Spectrum

In contemporary developing world demand for small and less energy-consuming devices rapidly increases. To meet this requirement a new generation of technology has to be developed which overcome problems known from the usage of electronics such as frequency limit and heat dissipation. One possible answer promises spin waves (SW) which have a short wavelength and do not suffer from Joule heating as electrons [1]. To successfully use SWs in widespread devices there is a need to find methods that allow handling this type of low energy consuming information carrier. The usage of metasurfaces consisting of arrays of nano-resonators was an innovative concept in modern optics, as made it possible to design a new class of optical systems with properties unheard in classical optics as for example flat lenses [2]. Such devices become available because metasurfaces enable complete control of electromagnetic wave’s phase and amplitude at subwavelength distances. However, such a concept has not been yet widespread in the realm of magnonics, although it would be extremely useful in the design of SW devices [3]. Especially because SW propagation is significantly limited by damping, so requires the usage of technology that works well in the constrain of ultra-short distances. Here, we present extensive theoretical research on the influence of the position and geometry of a ferromagnetic stripe on the reflection and transmission of the SWs from the edge and middle, respectively, of a permalloy film (Fig. 1) [4]. We have focused on how properties of SW change with the resonator width and separation between both elements. Interactions between the stripe and the layer, which depend on the value of separation between the elements and dimensions of the stripe, affect the propagation and reflection of SW. To check the scope of the interactions we use three numerical methods. Namely, finite element method in frequency domain, micromagnetic simulations [5], and finite element modal method. Employing three different methods allowed us to cross-check their results to confirm their correctness. We show that the phase shift as a function of the stripe’s width is characterized by intervals of small and steady growth which are separated by narrow areas of resonances in which phase increases linearly (Fig. 1 inset a). In the areas of resonances there is a visible increase of amplitude and resonances appear periodically in function of a stripe width with a period of about 160 nm. Simulations with increasing separation show (Fig.1 inset b) that resonances also occur with changes of separation but then the function is not periodic. Additionally above certain separation resonances do not appear anymore. The behavior of phase changes is explained with analytical model in which the reflection coefficient consists of two terms of different origins. The first one is responsible for a slow increase of phase with increasing width of the resonator and is a result of phase accumulated in the reflection of the long-wavelength mode. The second term has origin in constructive interference of the short-wavelength mode in the bilayer part of the system, playing a role of the resonator. This term gives significant input only at certain widths of the resonator, which can be predicted from Fabry-Pérot resonance conditions [6]. Intervals of rapid phase changes (Fig. 1 inset a) are caused by the presence of the second term. Because of the properties of the second term, the system used in the simulations can be considered as a realization of the Gires-Tournois type magnonic interferometer [7]. Furthermore, we extend the idea of resonance control of SW phase to the transmission mode. Acquired knowledge allows designing a new class of SW devices based on the Gires-Tournois interferometer concept and manipulations of SW with the usage of magnetic elements smaller than the wavelength of SW. Control of the SW phase can be used for the creation of SW-optical elements such as flat lenses and phase shifters. Especially interesting idea is utilization of proposed interferometer in the system to detect magnetic particles in the vicinity of the bilayer part. The research leading to these results has received funding from the Polish National Science Centre projects No. UMO-2015/17/B/ST3/00118, UMO-2019/33/B/ST5/02013, and UMO-2019/35/D/ST3/03729. The simulations were partially performed at the Poznan Supercomputing and Networking Center (Grant No. 398).

**References:**

 [1] M. Krawczyk and D. Grundler, J. Phys.: Condens. Matter 26, 123202 (2014) [2] N. Yu, et al., Nat. Mater. 13, 139–150 (2014) [3] M. Zelent, et al., Nanoscale 11, 9743-9748 (2019) [4] K. Sobucki, et al., arXiv preprint arXiv:2007.15226 (2020) [5] A. Vansteenkiste et al., AIP Advances 4, 107133 (2014) [6] F. Fabry, A. Perot, Ann. Chim. Phys. 16, 115 (1899) [7] F. Gires, P. Tournois, R. Acad. Sci. Paris. 258, 6112–6115 (1964)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c03f5d01e1f59b26517f8a0224a9f829.jpg)
Fig. 1 Geometry of the system used in the simulations. Parameters w and s were manipulated in the simulations, where d1 = 5 0nm, d2 = 40 nm and μH0 = 0.1 T were kept constant. In simulations frequency of 11 GHz was assumed. Between coordinates x = 0 and x = w the bilayered part is regarded as resonator. Inset (a) Phase shift as a function of stripe’s width. Resonances appear periodically with increasing stripe’s width with period around 160 nm. Inset (b) Phase shift as a function of stripe-film separation.

 Subwavelength resonant control of the spin-wave phase in thin ferromagnetic films

Spin waves (SWs) and their quanta magnons in the magnetic structures offer data transmission and data encoding capability in both amplitude and phase, providing an additional degree of freedom compared to their electronic counterparts. Most of the works in this field are focused on developing novel and energy efficient control mechanisms of SW by different means such as voltage [1], current [2], light [3], heat [4] etc., to use it in logic applications. However, this wave-based computation has the tremendous potential of parallel data processing. A multifrequency magnon network would allow parallel data processing within a single element, which is not possible in conventional transistor-based electronic logic. There are few reports about SW channeling in multiple paths in metallic ferromagnet [5][6]. In this work, we experimentally demonstrated a novel multifrequency SW device that can operate at multiple frequencies simultaneously using the shape anisotropy of the magnonic film. We have used yttrium iron garnet Y3Fe5O12 (YIG) as the magnonic material due to its extraordinarily low damping constant. A 145 nm-thick YIG film was grown on a (111)-oriented single crystalline Gd3Ga5O12 (GGG) substrate by pulsed laser deposition technique. Two patterned YIG waveguide with width w1 and w2 were fabricated by hot orthophosphoric etching at 160 °C temperature after photolithography as shown in Figure 1(a). w1 was maintained at 100 mm, whereas w2 was varied in the steps of 10, 20, 40, 60, 80, and 100 μm to implement a multichannel spin wave device. Subsequently, a system of two co-planner waveguide (CPW) made of 90 nm thick Au was integrated into the YIG film using dc magnetron-sputtering. The separation between the center lines of the exciting and detecting antenna (S) was designed as 200 μm. Another series of single channel SW device as shown as Figure 1(b) was fabricated by the similar procedure as Figure 1(a), where the width w of the devices was varied in the step of 10, 15, 25, 35, 45 60, 80, and 100 μm to investigate the contribution of shape anisotropy. For device characterization, we employed the microwave technique using the vector network analyzer (VNA). A dc-biased magnetic field ( μ0Hext = 25mT ) was applied in the in-plane direction and perpendicular to the SW propagation path, where magnetostatic surface SW (MSSW) was excited in the YIG film. To investigate SW propagation, we followed a similar approach as in the reference [7] The transmitted SW, scattering parameter S21 for a multichannel device (Figure 1(a)) has been shown in Figure 2(a)-(f). Figure 2(a) to (d) shows a single mode SW transmission with maximum propagation at f1= 2.15 GHz. However, Figure 2(e) and (f) shows the appearance of an additional SW propagation mode at f2= 2.06 GHz and f2= 1.92 GHz, respectively, along with the first mode at 2.15 GHz. Thus, it indicates that additional mode appears only when the width of the second waveguide has been shrunk beyond 20 μm, and it happens at a lower frequency than the first mode. To investigate the origin of the emergence of the second low frequency SW mode we have performed another set of the experiment where we have done the same measurement for the single channel SW device shown in Figure 1(b). Transmitted SW signals for these devices are shown in 2(g) to (n). We have found that the center frequency (indicated by arrow) of the transmission spectra has been moved from 2.25 GHz to 2.00 GHz when waveguide width has been changed from 100 mm to 10 mm. This frequency shifting can be attributed to the reduction effective magnetic field due to shape anisotropy induced demagnetizing field (Hd). When a magnonic film is micro structured as a bar, an Hd is developed inside the micro structured waveguide [8]. We have calculated this Hd as 3.2, 4.0, 5.3, 8.0, 15 and 32 mT for 100, 80, 60, 40, 20 and 10 μm waveguide respectively from Hd =t*4πMs/w, where t, Ms and w are the thickness, saturation magnetization and width of the waveguide, respectively. We have used Ms = 140 kA/m, which is the standard value for YIG film. This demagnetizing field reduces the effective magnetization based on the relation: Heff=Hext-Hd . According to the Kittle equation, the reduction of will result in the reduction of SW frequency, fFMR : fFMR=γ√Heff(Heff+Meff)/2π, where γ is the gyromagnetic ratio. As the width of the waveguide gets smaller, the demagnetizing field becomes larger, causing more reduction of Heff and SW frequency. These results provide us with a guideline that even by using same magnetic material operating under the same magnetic bias field, the resonant frequencies of the individual micron size magnets on a single chip can be tuned by varying the width of the magnet. In this work, combining such two different magnonic channel with the distinguishable anisotropic demagnetizing field has made it possible to make a multifrequency SW device for parallel data processing.

**References:**

[1] S. Choudhury, H. Takahashi, A. Barman, Sci. Adv. 2020; 6:eaba5457 [2] S. Sarker, H. Yamahara, H. Tabata, Appl. Phys. Lett. 117, 152403 (2020) [3] M. Vogel, A. V. Chumak, B. Hillebrands, Nat. Phys. 11, 487-491 (2015) [4] V. Castle, G.E.W. Bauer, Phys. Rev. B 96, 064407, (2017). [5] F. Heussner, B. Hillebrands, P. Pirro, Phys. Status Solidi RRL 2020, 14, 1900695 [6] K. Vogt, F.Y. Fradin, H. Schultheiss, Nat. Comm, 5:3727 (2014) [7] S. Sarker, H. Yamahara, H. Tabata, AIP Advances 10, 015015 (2020) [8] T. Manago, S. Yakata, T. Kimura, JJAP 52 (2013) 053001

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/27cafc7606015defe46e2dd9e20bfb9c.jpg)
(a) Optical image of multichannel SW device (b) Microscopic image of single channel SW device

![](https://assets.underline.io/uploads/markdown_image/1/image/41d800e831da93a8464b8ee58408f1f8.jpg)
Transmitted SW signal for in multichannel device for w1 = 100 μm and (a) w2 = 100 μm (b) w2 = 80 μm (c) w2 = 60 μm (d) w2 = 40 μm (e) w2 = 20 μm (f) w2 = 10 μm. Transmitted SW for single channel device for (g) w = 10 μm (h) w = 15 μm (i) w = 25 μm (j) w = 35 μm (k) w = 45 μm (l) w = 60 μm (m) w = 80 μm (n) w = 100 μm

 Multifrequency Spin Wave Device for Parallel Data Processing using Micro Structured Yttrium Iron Garnet Thin Films

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

Nonreciprocity is a vital, unique, and fundamental signal transmission characteristic widely required in a large number of RF, microwave and millimeter-wave circuit, and module applications, which include communication systems, radar, and measurement equipment. Without any doubts, the nonreciprocal properties are indispensable for the development of emerging circuits and systems such as the highly anticipated in-band full-duplex transceiver systems The recent development of nanotechnology in combination with the increased interest in the properties of artificial crystals brought back the topic of spin-wave (SW) nonreciprocity and its potential application in functional devices. The surface spin waves (SSWs) propagating in a ferromagnetic film along the direction perpendicular to a tangentially applied magnetic field H0 (Damon-Eshbach geometry) [1] possess nonreciprocal properties. Recently, of great interest is the study of the properties of spin waves during their propagation in magnetic waveguide structures of micro and nano scale. In this case, the main attention is paid to the use of spin waves (STs) as carriers of information signals, since it is possible to implement a number of signal processing devices based on the principles of magnonics [2], taking into account the possibility of coding the signal using both amplitude and spin phase the waves. As is known, the properties of SW spin waves are determined by the dipole and exchange interactions in magnetic media and can change significantly with changing medium parameters, for example, the magnitude and direction of the magnetization field [3]. In this paper, we consider structures consisting of laterally coupled YIG micro- and nanowaveguides that can be used as basic elements for creating various devices of functional magnonics: waveguides, filters, couplers. It also explores the possibility of controlling the propagation modes of spin waves by adding a metal layer. The structure of a directional coupler with metal layer on it is schematically shown in Fig. 1. Two parallel waveguides with length Lc = 8 mm, width w1 = w2 = 200 μm and thickness t 10 μm are placed laterally parallel with a gap d0 = 15 μm. Metal layer placed above it on distance d. The numerical modelling of this structure is performed using the micromagnetic simulation and finite element method with the following parameters of an Yttrium Iron Garnet (YIG) film: saturation magnetization Ms = 1.4*105 A/m, exchange constant A = 3.5 pJ/m and Gilbert damping alpha = 2*10-4. A external magnetic field Bext = 1200 Oe is applied along the long axis of the waveguides (x-direction in Fig. 2). The results of numerical simulation by the finite element method are presented on Fig.2. Figure 2(a) shows the dependence of the coupling length of the MSSW on the frequency in the case of the positive and negative directions of wave propagation along the structure. The wavelength depends on the frequency of the wave, which propagates in the negative direction. The lines show the normalized value of the pumping lengths when the distance between the metal and the surface of the microwave ovens changes from 10 μm to 90 μm. It can be seen that at a distance of d = 30 μm at a frequency of 5.6 GHz, the largest difference is observed, about 9 times, between the pumping length of waves propagating oppositely to each other in the structure under study. At the same time, it can be said that at a distance of more than 90 μm, the influence of the metal has practically no force over the entire frequency range of the MSSW wave, which can propagate in this structure under the considered parameters. This work was supported by the Russian Science Foundation (20-79-10191)

**References:**

1. R. W. Damon and J. R. Eshbach, J. Phys. Chem. Solids 19, 308 (1961). 2. S.A. Nikitov, A.R. Safin, D.V. Kalyabin, A.V. Sadovnikov, E.N. Beginin, M.V. Logunov, M.A. Morozova, S.A. Odintsov, S.A. Osokin, A.Yu.Sharaevskaya, Yu.P. Sharaevsky, A.I. Kirilyuk Phys. Usp. 63 945–974 (2020) 3. A. V. Sadovnikov, E. N. Beginin, S. A. Odincov, S. E. Sheshukova, Y. P. Sharaevskii, A. I. Stognij, S. A. Nikitov, Applied Physics Letters 108 (2016).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a929c59507eb2da2039e24ebb5da6935.jpg)
(a)Schematic view of lateral magnonic stripes 
with metal layer; (b)view on stucture along x-axis 
(along wave propogation)

![](https://assets.underline.io/uploads/markdown_image/1/image/761c698d6d8beeaa6c1766edccc9c3fd.jpg)
(a)Frequency dependence of the coupling length of the MSSW, with opposite directions of wave propagation along x-axis; (b) dependance of normalized 
coupling length of gap between waveguides and metal layer

 Nonreciprocal spin-wave propagation in lateral magnonic stripes

The study of physical principles that determine the possibility of using spin waves (SW) to create information signal processing devices based on magnonics principles is of great interest [1]. Microstructures based on the iron-yttrium garnet (YIG) can be used in the processing of spin-wave signals due to the low attenuation. To control the properties of SW propagating, the method of structuring YIG films and creating irregular micro- and nanoscale waveguides, including structures with broken translational symmetry, can be used [2]. In this work, the numerical simulations and Brillouin light scattering technique results of the unidirectional spin wave propagation in latelal nonidentic magnonic crystals was shown(MC) [3,4]. In Figure 1 present the structure was made up of two MC waveguides (MC1 and MC2) with different width, oriented along the direction of the y axis. Each MC waveguide represents a structure with grooves 1 microns deep, repeating with a period of D = 200 microns. The length of sections of microwave structures with a periodic system of grooves is 5 mm. The width of magnon crystals MC1 and MC2 along the z axis were 500 μm and 200 μm, respectively. The value of lateral gap d between MC1 and MC2 was 20 μm. The structure was placed in an external magnetic field with the magnitude H0 = 1300 Oe directed along the z-axis in order to excite a surface magnetostatic spin wave (SMSW). By the means of Brillouin light scattering technique, we obtained 2-D intensity maps of the spin wave propagating in the structure for case spin wave excitation on MC1 . The possibility of unidirectional coupling of SW propagating in MCs near the frequency boundary of the transmission zone is demonstrated. Based on the results of micromagnetic modeling, the mechanism that accompanies the observed mode of intermode coupling in the lateral system of nonidentical MCs is determined, and the spatial-frequency signal separation modes are studied. The proposed device can act as spin-wave unidirectional coupler for planar interconnecting element for magnonic conduits operating in a regime of single mode propagation in one of the MCs. As results we demonstrate the perspective system of nonidentical magnonic crystals as a building block for integrated magnonic circuits for the concept of spin-wave signal processing units for logic gates such as demultiplexer, directional coupler and signal splitter. Experimental strucrure and was performed by the Russian Foundation for Basic Research (project no. 20-37-90020). Micromagnetic simulation was performed by Grant of the President of RF (No. MK-1870.2020.9).

**References:**

[1] V. V. Kruglyak, S. O. Demokritov, D. Grundler, Journal of Physics D: Applied Physics Vol. 43, P.260301 (2010) [2] A. V. Sadovnikov, C. S. Davies, V. V. Kruglyak, at al., PHYSICAL REVIEW B. Vol. 96, P. 060401(R) (2017) [3] A. V. Sadovnikov, V.A. Gubanov, S.E. Sheshukova, at.al.,Physical Review Applied, 9(5) (2018) [4] A. V. Sadovnikov, A. A. Grachev, A. A. Serdobintsev, at al., IEEE Magnetics Letters, 1–1 (2019)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a23efc808f3bb4570c013365c71273b1.jpg)
Fig.1 Scheme of the uniditectional spin-wave coupler.

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Sub-Micrometer Near-Field Focusing of Spin Waves in Ultrathin YIG Films

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