09-03 magnonic illustration

[Image above] Who needs spintronics when you could have magnonics? Magnonics, an emerging field of magnetism, could provide an alternative method of data storage to silicon electronics. Credit: @tsarcyanide, MIPT Press Office

In the past couple weeks, CTT featured articles on magnetic tapes and memory phase change materials, two alternative methods of data storage in contrast to conventional silicon electronics. Today, we feature another alternative—magnonics.

Magnonics refers to an emerging field of magnetism that is similar to, but slightly different from, spintronics. While spintronics makes use of an electron’s electric charge and spin moment properties to encode data, magnonics makes use of the amplitude and phase of spin-wave signals in magnetic materials.

“Basically, spintronics still requires electric currents but usually restricts these currents to consisting only of spin-up or spin-down electrons, thus providing an additional degree of freedom to process or encode information,” Tobias Fischer, a doctoral student at the University of Kaiserslautern, explains in an IEEE Spectrum article on magnonics. “However, magnonics can operate without any electric currents by only relying on the propagation of spin waves in a magnetic material as a carrier of information.”

Absence of electric currents provides magnonics with some advantages—for example, losses from Joule (resistive) heating can be considerably reduced. Additionally, spin waves featuring wavelengths in the micrometer and nanometer ranges and gigahertz frequencies, controlled by an external magnetic field, allow creation of compact microdevices.

To build a magnonic device, the fundamental building blocks are magnonic crystals (MCs).

“MCs are magnetic metamaterials with periodic modulation of any magnetic parameter that is relevant to the dispersion of spin waves: external magnetic field, saturation magnetization, exchange properties, magnetic anisotropy, film thickness, mechanical stress, etc.,” researchers write in a recent open-access paper. These properties give MCs a wide range of potential applications, including filters, waveguides, grating couplers, and data processing devices.

The researchers of the open-access study come from several universities in Russia, the Netherlands, and Germany, and they note there are a variety of approaches to create 1D and 2D MCs. However, in their study, the researchers wanted to test a new approach—creating MCs using a ferromagnet/superconductor hybrid system.

Ferromagnetism and superconductivity are antagonistic phenomena—while electron pairs in a superconductor align in opposite directions, all spins in ferromagnets align in the same direction.

Traditionally, scientists have used ferromagnetism to influence a material’s superconducting properties (for example, the field of superconducting spintronics) because ferromagnetism is considered “stronger” than superconductivity.

“The energy that is required to break the exchange interaction [in ferromagnetic spin alignment] is proportional to the Curie critical temperature T, which is usually several hundreds of Kelvin or even about 1,000 K,” Igor Golovchanskiy, coauthor of the open-access study and researcher at the Moscow Institute of Physics (MIPT), explains in an email. In contrast, “The energy that is required to break [superconducting electron] pairs is proportional to the superconducting critical temperature Tc, which is usually below 100 K.”

Additionally, superconductivity can be destroyed by an external magnetic field and does not exist at room temperature. For these reasons, research on influencing ferromagnetism using superconductivity is uncommon.

Recent studies at cryogenic temperatures, though, have increased interest in controlling ferromagnetism with superconductivity.

“In previous research (including Ref. 32 and a number of papers in previous years) scientists did study the influence of superconductivity on spin waves in continuous media but not in a periodic media,” Golovchanskiy says. “In general, our research as well as Ref. 33 are the first attempts to achieve the magnonic [periodic] band system by comprising superconducting and ferromagnetic materials.”

To create a superconducting/ferromagnetic magnonic band system, the researchers placed a superconducting periodic microstructure made of niobium on top of a ferromagnetic nickel-iron (Ni80Fe20) permalloy (Py) thin film; an aluminum oxide insulating layer was deposited between the superconducting and ferromagnetic layers to avoid the superconducting proximity effect.

The system was then placed in a cryostat to conduct ferromagnetic resonance (FMR) absorption spectroscopy measurements.

“Studying the FMR spectrum of the hybrid, we have defined the actual contribution of the superconducting periodic subsystem to magnetization dynamics, that is the diamagnetic response of the superconductor,” the researchers write in the paper.

The FMR spectrum showed two lines, indicating the periodic structure consisted of two bound areas with alternating ferromagnetic resonance conditions. An MIPT press release explains the “ferromagnetic properties were modulated by means of the superconducting structure.”

Schematic illustration of the investigated test chip (not to scale). A 50-nm-thick permalloy film (shown in orange) is placed on top of the central conducting line of niobium coplanar waveguide (shown in blue). Niobium stripes of thickness 0.7 µm and width 3 µm are placed regularly on top of Py thin film with the period 4 µm. b) Scanning electron microscopy image of the fabricated structure. Credit: Golovchanskiy et al.Advanced Science (CC BY 4.0)

Following FMR measurements, the researchers performed “micromagnetic simulations” to recreate the magnonic band structure and further test how changes in superconductivity control the ferromagnetism. “In general, the proposed metamaterials offer a simple tunability of their dispersions by adjusting geometrical parameters of the superconducting periodic structure, or the orientation of the spin wave propagation,” they write in the paper.

The researchers note that although the results may eventually find use in microwave electronics and magnonics, currently the range of potential applications is limited because the system does not work at room temperature. Yet there are still plenty of experiments that can be conducted under this limitation.

“Currently we are studying nonlinear phenomena in hybrid magnonic systems,” Golovchanskiy says.

The open-access paper, published in Advanced Science, is “Ferromagnet/superconductor hybrid magnonic metamaterials” (DOI: 10.1002/advs.201900435).