Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 52 (2019)

Magnetic properties of Alkali-metal Superoxide, NaO2
F. Astuti,∗1,∗2 M. Miyajima,∗3 T. Kambe,∗3 T. Nakano,∗4 and I. Watanabe∗1,∗2
Magnetism in the p-electron system has attracted
attention for the possibility of new types of magnetic
informative materials. Alkali-metal superoxides, AO2(A = Na, K, Rb, Cs), present an interesting example of magnetic materials on the basis of unpaired pelectrons. The magnetic ordering of KO2 , RbO2 , andCsO2 have been observed at temperatures 7 K, 15 K,
and 9.6 K, respectively using specific heat.1) However,
the magnetic ground state in NaO2 is not yet clarified.
Clear anomalies from the measurement of magnetic
susceptibility were observed at a temperature between
230 and 200 K in NaO2 . These anomalies were consistent with the expected structural phase transition, asdiscussed in the previous study.1) The magnetic susceptibility, χ, drops sharply below 40 K, as shownin Fig. 2 (a). We checked that the sample quality
was good and the anomalies in magnetic susceptibility could be reproduced in all NaO2 sample batches(see sample condition in Fig. 1). The sudden changeof susceptibility value toward zero is an indication ofspin gap state as observed in other spin gap systems,
NaTiSi2 O6 and TiOCl.2,3) Mahanti et al. reported thatNaO2 has some similarities with one-dimensional spinsystem.4)
There was no emergence of muon-spin precession
from the previous zero-field (ZF) µSR experiment in
NaO2 down to 0.3 K measured at the RIKEN-RAL
Muon Facility and DOLLY PSI, which indicated theabsence of magnetic ordering. To further study themagnetic properties in NaO2 , we measured the depolarization rate in ZF close to anomaly ∼ 40 K (observed by magnetic susceptibility) using CHRONUS
spectrometer at the RIKEN-RAL Muon Facility.
The temperature dependence of exponential relaxation rate, λ, measured using ZF- µSR showed ananomaly at a temperature below 40 K that was significantly consistent with the anomaly observed bymagnetic susceptibility measurement, as shown inFig. 2 (a). This indicated the possibility of magnetictransition around this temperature.5) The red line inFig. 2 (b) is the fitting line that utilizes the following
function: λ(T) = λ0 /[1 + A exp( −2∆T )], in which A isthe constant and ∆ is the spin-gap value.6) The obtained result of ∆ is estimated to be ∼ 35 K. The possibility of the formation of spin gap in NaO2 must be
further explored using other experimental techniques.
∗1∗2∗3∗4Fig. 1. Some sample batches of NaO2 used for µSR experiment. Labels indicate the number of sample batches.
Bright and dense yellow color imply that sample quality
is good.

Fig. 2. (a) Magnetic susceptibility of NaO2 and (b) Temperature dependence of exponential relaxation rate, λ,measured using ZF-µSR at various temperature aboveand below 40 K.

References
1) A. Zumsteg et al., Phys. Cond. Matter 17, 267–291(1974).
2) M. Isobe et al., J. Phys. Soc. Jpn, 71, 1423 (2002).
3) A. Seidel et al., Phys. Rev. B 67, 020405(R) (2002).
4) S. D. Mahanti et al., Solid State Commun. 18, 159–162(1976).
5) S. J. Blundell et al., J. Phys. Condens. Matt. 9, 119(1997).
6) P. J. Baker et al., Phys. Rev. B 75, 094404 (2007).

Department of Condensed Matter Physics, Hokkaido University
RIKEN Nishina Center
Department of Physics, Okayama University
Graduate School of Science and Engineering, Ibaraki University- 20 -