Absence of magnetic order and magnetic fluctuations in RuO$_{2}$

TL;DR

This work addresses whether bulk RuO$_2$ hosts altermagnetic order by employing zero-field nuclear quadrupole resonance (NQR) on RuO$_2$ powder to detect static and dynamic magnetism without external field effects. The measurements reveal no internal magnetic field $H_{ m int}$ (constrained to $<6.4$ mT) and Ru moments $m_{ m Ru}\lesssim3\times10^{-4}\,mu_B$, along with a temperature-independent spin-lattice relaxation rate $1/T_1T$ down to $3.3$ K, indicating the absence of magnetic fluctuations. The large EFG asymmetry $\\eta\approx0.77$ and the isotope-relaxation ratio $(^{99}\gamma/^{101}\gamma)^2\approx0.8$ point to a conventional metal with weak electron correlations and no magnetic order in bulk RuO$_2$. Collectively, the results demonstrate that RuO$_2$ lacks the magnetic characteristics essential for altermagnetism in its bulk form, guiding future exploration of spintronic properties in films where spin-splitting phenomena may arise.

Abstract

A novel magnetic class blending ferromagnetism and antiferromagnetism, termed altermagnetism, has gained significant attention for its staggered order in coordinate and momentum spaces, time-reversal symmetry-breaking phenomena, and promising applications in spintronics. Ruthenium dioxide (RuO$_{2}$) has been considered a candidate material for altermagnetism, yet the presence of magnetic moments on Ru atoms remains a subject of debate. In this study, we systematically investigated the magnetic properties of RuO$_{2}$ powder using nuclear quadrupole resonance (NQR) measurements. The NQR spectra show that there is no internal magnetic field. Furthermore, the temperature independence of spin-lattice relaxation rate, $1/T_1T$, proves that there are no magnetic fluctuations. Our results unambiguously demonstrate that Ru atoms in RuO$_{2}$ possess neither static magnetic moments nor fluctuating magnetic moments, and thus RuO$_{2}$ does not possess the magnetic characteristics essential for altermagnetism.

Figures (4)

• Figure 1: (a) X-ray diffraction data obtained at room temperature for commercial RuO$_2$ powder. The pattern indicates a phase-pure sample without any detectable impurity peaks. (b) Temperature dependence of the magnetic susceptibility measured under a magnetic field of 0.5 T. (c) Magnetization as a function of magnetic field up to 7 T at temperatures of 2 K and 50 K.
• Figure 2: (a) The NQR spectrum of RuO$_2$ at $10$ K. The two low-frequency peaks are attributed to the $\pm 3/2\rightarrow\pm 1/2$ and $\pm 5/2 \rightarrow\pm 3/2$ transitions for $^{99}$Ru, while the two high-frequency peaks are attributed to $\pm 3/2\rightarrow\pm 1/2$ and $\pm 5/2 \rightarrow\pm 3/2$ transitions for $^{101}$Ru. The NQR spectra at various temperatures of (b) $^{99}$Ru and (c) $^{101}$Ru. Baselines have been vertically offset for visual clarity.
• Figure 3: The temperature dependence of (a) peak positions of $\pm 3/2 \rightarrow \pm 1/2$ (solid) and $\pm 5/2 \rightarrow \pm 3/2$ (open) transitions, (b) quadrupole resonance frequency $\nu_\mathrm{Q}$, and (c) asymmetry parameter $\eta$ for $^{101}\mathrm{Ru}$ and $^{99}\mathrm{Ru}$, respectively.
• Figure 4: Temperature dependence of $1/T_1T$ measured at the $\pm 5/2\rightarrow \pm 3/2$ peak for $^{101}$Ru and $^{99}$Ru, respectively. The lines are guides to the eye. The ratio of $^{101}T_{1}/^{99}T_{1} = (^{99}\gamma / ^{101}\gamma)^2 \sim 0.8$.