The frequency difference between two oppositely propagating spin waves can be used to probe several interesting magnetic properties, such as the Dzyaloshinkii-Moriya interaction (DMI). Propagating spin wave spectroscopy is a technique that is very sensitive to this frequency difference. Here we show several elements that are important to optimize devices for such a measurement. We demonstrate that for wide magnetic strips there is a need for de-embedding. Additionally, for these wide strips there is a large parasitic antenna-antenna coupling that obfuscates any spin wave transmission signal, which is remedied by moving to smaller strips. The conventional antenna design excites spin waves with two different wave vectors. As the magnetic layers become thinner, the resulting resonances move closer together and become very difficult to disentangle. In the last part we therefore propose and verify a new antenna design that excites spin waves with only one wave vector. We suggest to use this antenna design to measure the DMI in thin magnetic layers.

The frequency difference between two oppositely propagating spin waves can be used to probe several interesting magnetic properties, such as the Dzyaloshinskii-Moriya interaction (DMI). Propagating spin wave spectroscopy is a technique that is very sensitive to this frequency difference. Here, we show several elements that are important to optimize devices for such a measurement. We demonstrate that for wide magnetic strips, there is a need for de-embedding. Additionally, for these wide strips, there is a large parasitic antenna-antenna coupling that obfuscates any spin wave transmission signal, which is remedied by moving to smaller strips. The conventional antenna design excites spin waves with two different wave vectors. As the magnetic layers become thinner, the resulting resonances move closer together and become very difficult to disentangle. In the last part, we therefore propose and verify an alternative antenna design that excites spin waves with only one wave vector. We suggest to use this antenna design to quantify the DMI in thin magnetic layers.

We performed time-domain propagating spin-wave spectroscopy for forward spin waves in a ferromagnetic metallic waveguide in the presence of electric current. The forward spin waves exhibit a large propagation delay and a 35% amplitude change with a smaller current injection of 6 × 1010 A/m2, showing a 2 times larger modulation than backward volume spin waves. By measuring electric-current effects in 4 distinct experimental configurations, we found that a current-induced Doppler shift coincided with an Oersted field frequency shift under our experimental conditions. By separating the two phenomena, we evaluated each contribution on the propagating forward spin-wave packet.

Many recent papers report on the interest of spin waves for applications. This paper revisits the propagating spin wave spectroscopy when using inductive transceivers connected to a network analyzer. The spin wave conduit can be made of a non-reciprocal material. The formalism offers a method to understand, design and optimize devices harnessing propagating spin waves, including when a unidirectional energy flow is desired. The concept of the mismatch of helicity between the spin wave and the magnetic field radiated by antennas is first clarified. Owing to the form of the susceptibility tensor reflecting the precession ellipticity, there exists specific orientations of the wavevector for which a perfect helicity mismatch is reached. The spin waves with this orientation and this direction of wavevector are "dark" in the sense that they do not couple with the inductive antenna. This leads to single-sided wavevector generation, that should not to be confused with a unidirectional emission of energy. A method to calculate the antenna-to-antenna transmission parameter is then provided. Analytical approximations are then applied on situations that illustrate the respective role of the direction of the spin wave wavevector versus that of the group velocity. The often-encountered cases of spin waves possessing either a V-shaped or a flat dispersion relation are revisited. These reciprocal dispersion relations lead to amplitude non-reciprocity because of the helicity mismatch phenomenon. Conversely, for spin waves with a line-shaped dispersion relation, a quasi-unidirectional emission of spin waves occurs. This situation can be obtained when using the acoustical spin waves of synthetic antiferromagnets when the wavevector is close to parallel to the applied field. We finally show that this configuration can be harnessed to design reconfigurable frequency filters.

Magnonics is seen nowadays as a candidate technology for energy-efficient data processing in classical and quantum systems. Pronounced nonlinearity, anisotropy of dispersion relations and phase degree of freedom of spin waves require advanced methodology for probing spin waves at room as well as at mK temperatures. Yet, the use of the established optical techniques like Brillouin light scattering (BLS) or magneto optical Kerr effect (MOKE) at ultra-low temperatures is forbiddingly complicated. By contrast, microwave spectroscopy can be used at all temperatures but is usually lacking spatial and wavenumber resolution. Here, we develop a variable-gap propagating spin-wave spectroscopy (VG-PSWS) method for the deduction of the dispersion relation of spin waves in wide frequency and wavenumber range. The method is based on the phase-resolved analysis of the spin-wave transmission between two antennas with variable spacing, in conjunction with theoretical data treatment. We validate the method for the in-plane magnetized CoFeB and YIG thin films in $k\perp B$ and $k\parallel B$ geometries by deducing the full set of material and spin-wave parameters, including spin-wave dispersion, hybridization of the fundamental mode with the higher-order perpendicular standing spin-wave modes and surface spin pinning. The compatibility of microwaves with low temperatures makes this approach attractive for cryogenic magnonics at the nanoscale.

Knowledge of the spin-wave dispersion relation is a prerequisite for the explanation of many magnonic phenomena as well as for the practical design of magnonic devices. Spin-wave dispersion measurement by established optical techniques such as Brillouin light scattering or the magneto-optical Kerr effect at ultralow temperatures is often forbiddingly complicated. By contrast, microwave spectroscopy can be used at all temperatures but it usually lacks spatial and wave-number resolution. Here we develop a variable-gap-propagating-spin-wave-spectroscopy (VGPSWS) method for the deduction of the dispersion relation of spin waves in a wide frequency and wave-number range. The method is based on the phase-resolved analysis of the spin-wave transmission between two antennas with variable spacing, in conjunction with theoretical data treatment. We validate the method for in-plane magnetized $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$ and yttrium iron garnet thin films in $\mathbf{k}\ensuremath{\perp}\mathbf{B}$ and $\mathbf{k}\ensuremath{\parallel}\mathbf{B}$ geometries by deducing the full set of material and spin-wave parameters, including spin-wave dispersion, hybridization of the fundamental mode with the higher-order perpendicular standing spin-wave modes, and surface spin pinning. The compatibility of microwaves with low temperatures makes this approach attractive for cryogenic magnonics at the nanoscale.

In this paper, we investigate the characteristics of antenna for propagating-spin-wave-spectroscopy (PSWS) experiment in ferromagnetic thin films. Firstly, we simulate the amplitude and phase distribution of the high-frequency magnetic field around antenna by high frequency structure simulator (HFSS). And then k distribution of the antenna is obtained by fast Fourier transformation (FFT). Furthermore, three kinds of antenna designs, i.e. micro-strip line, coplanar waveguide (CPW), loop, are studied and compared. How the dimension parameter of antenna influence the corresponding high-frequency magnetic field amplitude and k distribution are investigated in details.

In this paper, near-field antenna probe is designed for propagating spin wave spectroscopy (PSWS) measurement. We compare the meander-line and straight-line antenna probes. Effect of the dimension parameters of antenna probe on scattering parameter is investigated. Near-field distribution and radiation pattern of these two kinds of antenna probe are discussed. Also, the placement of antenna probe is optimized for low mutual coupling. It is found that the meander-line antenna probe has better symmetric field distribution than the straight-line one.

Performing propagating spin-wave spectroscopy of thin films at millikelvin temperatures is the next step towards the realisation of large-scale integrated magnonic circuits for quantum applications. Here we demonstrate spin-wave propagation in a $100\,\mathrm{nm}$-thick yttrium-iron-garnet film at the temperatures down to $45 \,\mathrm{mK}$, using stripline nanoantennas deposited on YIG surface for the electrical excitation and detection. The clear transmission characteristics over the distance of $10\,\mu \mathrm{m}$ are measured and the subtracted spin-wave group velocity and the YIG saturation magnetisation agree well with the theoretical values. We show that the gadolinium-gallium-garnet substrate influences the spin-wave propagation characteristics only for the applied magnetic fields beyond $75\,\mathrm{mT}$, originating from a GGG magnetisation up to $47 \,\mathrm{kA/m}$ at $45 \,\mathrm{mK}$. Our results show that the developed fabrication and measurement methodologies enable the realisation of integrated magnonic quantum nanotechnologies at millikelvin temperatures.

Performing propagating spin-wave spectroscopy of thin films at millikelvin temperatures is the next step toward the realization of large-scale integrated magnonic circuits for quantum applications. Here, we demonstrate spin-wave propagation in a 100 nm-thick yttrium-iron-garnet (YIG) film at temperatures down to 45 mK, using stripline nanoantennas deposited on YIG surface for electrical excitation and detection. The clear transmission characteristics over the distance of 10 μ m are measured and the extracted spin-wave group velocity and the YIG saturation magnetization agree well with the theoretical values. We show that the gadolinium-gallium-garnet (GGG) substrate influences the spin-wave propagation characteristics only for the applied magnetic fields beyond 75 mT, originating from a GGG magnetization up to 62 kA / m at 45 mK. Our results show that the developed fabrication and measurement methodologies enable the realization of integrated magnonic quantum nanotechnologies at millikelvin temperatures.