A pr 2 01 8 Magnetism and high – magnetic field magnetization in alkali superoxide CsO 2

Alkali superoxide CsO2 is one of candidates for the spin–2 one–dimensional (1D) antiferromagnet, which may be sequentially caused by an ordering of the π–orbital of O−2 molecule below TS ∼ 70 K. Here, we report on the magnetism in powder CsO2 and high–magnetic field magnetization measurements in pulsed-magnetic fields of up to 60 T. We obtained the low temperature phase diagram around the antiferromagnetic ordering temperature TN = 9.6 K under the magnetic field. At 1.3 K, remarkable up–turn curvature in the magnetization around a saturation field of ∼ 60 T is found, indicating the low–dimensional nature of the spin system. The saturated magnetization is also estimated to be ∼ 1μB, which corresponds to the spin– 1 2 . We will compare it with the theoretical calculation. O2 is a magnetic molecule with a spin–1, originating with two unpaired electrons on antibonding π orbitals. The magnetism of three structural phases of solid O2 has been studied long ago [1, 2]. The magnetic exchange interaction between O2 molecules depends strongly on their distance as well as relative displacement [1, 2]. O2 molecules adsorbed in a porous metal complexes have been investigated by a magnetic susceptibility and high magnetic field magnetization [3, 4]. The O2–array is confined in the nano–channel region and exhibits a meta– magnetic transition under high–magnetic field, which is considered to be due to a configurational transition of O2 dimer. Other candidate for O2 molecular based magnet is achieved by a encapsulation of O2 molecules in single–walled carbon nanotubes. From the magnetic susceptibility and high–magnetic field magnetization, it is proposed as a spin–1 Haldane state [5]. Since then, the magnetism of O2 molecule has been attracted much attention. Magnetism of alkali superoxide, AO2 (A = Na, K, Rb and Cs), originates in unpaired π–electron on the O−2 molecular anion, where one electron transfers from the alkali metal to O2 making a spin– 2 state. The transferred electron has a freedom which orbital (π∗x or π ∗ y) to select and the orbital ordering would be expected to realize the three–dimensional magnetic exchange pathways. At room temperature, it is proposed that KO2, RbO2 and CsO2 have the same tetragonal crystal structure (I4/mmm) while NaO2 has a cubic crystal structure (Fm3̄m). Recently, CsO2 has been attracted considerable attention because of the one–dimensional (1D) antiferromagnetic (AF) nature [6]. Below the structural phase transition temperature TS = 70 K, the magnetic susceptibility follows a well–known Bonner– Fisher curve. It is proposed that the structural phase transition is accompanied