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Spin glass dynamics in a structural glass of C$$_{4}$$mimFeCl$$_{4}$$

Kofu, Maiko  ; Watanuki, Ryuta*; Sakakibara, Toshiro*; Kawamura, Seiko  ; Nakajima, Kenji  ; Ueki, Takeshi*; Akutsu, Kazuhiro*; Yamamuro, Osamu*

Ionic liquids (ILs) have been in the spotlight due to their unique and interesting properties. It is remarkable that their physicochemical properties are controlled by varying cations and anions. Magnetic IL is an example. The first discovered magnetic IL C$$_{4}$$mimFeCl$$_{4}$$ is easily vitrified upon cooling and also crystallized by annealing. Interestingly, an antiferromagnetic transition occurs at 2.3 K in the crystalline state while spin-glass behavior is observed below 0.45 K (= $$T_{rm SG}$$) in the glassy state. Our inelastic neutron scattering experiments have demonstrated that the glassy C$$_{4}$$mimFeCl$$_{4}$$ exhibits a broad and non-dispersive excitation, while the crystal displays spin-wave excitations. The excitation spectrum in the glass state is scaled by the Bose population factor below $$T_{rm SG}$$, which is highly reminiscent of "boson peak" commonly observed in structural glasses. We guess that, since there is no periodicity in structural glasses, magnons hardly propagate through magnetic medium and are localized. The localized magnetic excitations are suggestive of the formation of spin clusters.



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