Magnetic Materials for Quantum Magnonics

TL;DR

The paper addresses the challenge of identifying magnetic materials suitable for quantum magnonics, with a focus on achieving ultralong magnon lifetimes and coherent transport for single-magnon operations. It surveys material platforms (ferromagnets, Heuslers, antiferromagnets, altermagnets, 2D magnets, hexaferrites) and highlights YIG as the benchmark due to its ultra-low damping and long dipolar-exchange magnon lifetimes at cryogenic temperatures; substrate-induced losses on GGG are a major obstacle. The authors propose YIG films on lattice-matched diamagnetic substrates such as YSGAG as a practical route to bulk-like damping in thin films, enabling millikelvin magnons with lifetimes potentially exceeding $20\,\mu\text{s}$ for DESW in ultra-pure samples. They argue that combining ultra-pure YIG growth with YSGAG substrates can realize on-chip single-magnon propagation and magnonic networks interfacing with superconducting qubits.

Abstract

Quantum magnonics studies the quantum properties of magnons, the quanta of spin waves, and their application in quantum information processing. Progress in this field depends on identifying magnetic materials with characteristics tailored to the diverse requirements of magnonics and quantum magnonics. For single-magnon excitation, its control, hybrid coupling, and entanglement, the most critical property is the ability to support long magnon lifetimes. This perspective reviews established and emerging magnetic materials, including ferromagnetic metals, Heusler compounds, antiferromagnets, altermagnets, organic and 2D van der Waals magnets, hexaferrites, and in particular yttrium iron garnet (YIG), highlighting their key characteristics. YIG remains the benchmark, with bulk crystals supporting sub-microsecond Kittel-mode lifetimes and ultra-pure spheres achieving $\sim18\,μ$s for dipolar-exchange magnons at millikelvin temperatures. However, thin YIG films on gadolinium gallium garnet (GGG) substrates suffer from severe lifetime reduction due to substrate-induced losses. In contrast, YIG films on a new lattice matched, diamagnetic alternative, yttrium scandium gallium/aluminum garnet (YSGAG), overcomes these limitations and preserves low magnetic damping down to millikelvin temperatures. These advances provide a practical pathway toward ultralong-living magnons in thin films, enabling scalable quantum magnonics with coherent transport, strong magnon-photon, magnon-qubit coupling, and integrated quantum networks.

Figures (2)

• Figure 1: (a) Temperature-dependent magnetization of hematite. Inset shows the easy-axis crystal structure for $T<T_{\mathrm{M}}$ (taken from Chen2025). (b) Sketch of the antiferromagnetic (AFM) spin texture under microwave excitation. The magnetic field is applied along the $x$ direction. Inset shows a SEM image of the coplanar waveguide (CPW); scale bar $500~\mathrm{nm}$ (taken from Chen2025). (c) Altermagnet: $d$-wave–type spin-split bands (upper left) and perovskite schematic with sublattice-dependent, anisotropic spin-dependent hoppings (red/blue arrows) (taken from Naka2025). (d) Illustration of spin waves in a 2D van der Waals magnet. Magnon spintronics relies on manipulation and control of magnon spin transport from an injector to a detector. The electrical contacts for generating, controlling, and detecting magnons are shown as gold pads, while the van der Waals magnet is depicted by blue and yellow atoms (magnetic and non-magnetic, respectively). The spin is represented by a blue arrow whose orientation varies spatially to indicate the transported magnon (taken from MaasValero2025). (e) Atoms and spins in a bilayer of the van der Waals antiferromagnet CrPS$_4$. Red and blue arrows indicate the local magnetic moments of the Cr atoms (turquoise). The interlayer (intralayer) exchange coupling is ferromagnetic (antiferromagnetic) (taken from deWal2023). (f) Lifetime $\tau$ of secondary dipolar–exchange magnons (DESW) at $f_{\mathrm{FMR}}/2$ as a function of temperature $T$ (logarithmic $T$-axis) for three YIG spheres. Illustration shows a schematic of three-magnon splitting — the $k=0$ uniform mode ($f_{\mathrm{FMR}}=3.17~\mathrm{GHz}$) splits into two counter-propagating magnons at $1.59~\mathrm{GHz}$ with $|k|\approx 3~\mathrm{rad}\,\mu\mathrm{m}^{-1}$ in the dipolar-exchange spin-wave regime (taken and adapted from Serha2025b). (g) Experimental FMR linewidth $\Delta f$ as a function of the FMR frequency $f_{\mathrm{FMR}}$ for a $100~\mathrm{nm}$ YIG film on a GGG substrate. The half-solid points are measurement points up to $18~\mathrm{GHz}$ for temperatures down to $2~\mathrm{K}$ and up to $10~\mathrm{GHz}$ for temperatures below $2~\mathrm{K}$ (gray dashed lines mark these limits). The straight lines are Gilbert fits performed up to $18~\mathrm{GHz}$ and $10~\mathrm{GHz}$, respectively. Above these values, the linewidth shows non-Gilbert behavior versus $f_{\mathrm{FMR}}$. The linewidth broadening and deviation from Gilbert behavior are due to magnetization of the GGG substrate at low temperatures (inset), which creates an inhomogeneous stray field affecting the internal field of the YIG film (taken and adapted from Serha2025). (h) Crystal structure of diamagnetic YSGAG, an alternative substrate to paramagnetic GGG for YIG. The cubic lattice structure of YSGAG is similar to that of YIG, but with iron ions replaced at the octahedral sites by scandium, gallium, and aluminum, while tetrahedral sites are occupied by gallium and aluminum (taken from Serha2025d).
• Figure 2: FMR FWHM linewidth $\Delta B$ (measure for magnetic damping) as a function of temperature $T$ for an FMR frequency $f_{\mathrm{FMR}} \approx 8\,\mathrm{GHz}$, displayed on a double logarithmic scale. The graph compares measurements obtained in this work with selected results of sputtered and LPE grown YIG films from relevant literature, enabling a direct evaluation of temperature-dependent damping behavior across different samples, their substrates and studies Cole2023Guo2023Legrand2024Serha2025d (taken and adapted from Serha2025d).