Magnetic ordering in the ammoniated alkali fullendes (NH₃)K_3-xRb_xC₆₀ (x = 2, 3)

In this work, we present a detailed study of the ammoniated füllendes (NH₃)KRb₂C₆₀ and (NH₃)Rb₃C₆₀ using theμ⁺ spin relaxation/rotation (μ⁺SR) technique in its zero-field (ZF), longitudinal-field (LF) and transverse-field (TF) variants as a function of temperature and cooling rate. The ZF and TF μ⁺SR spectra of the investigated ammoniated alkali füllendes clearly reveal the onset of long-range antiferromagnetic (AF) order at low temperatures. In the AF state, the local field exhibits large spatial inhomogeneities, while its quasi-static nature is confirmed by the complete recovery of the asymmetry in an LF of 200-500 Oe. A large fraction of paramagnetic domains survives well into the AF state. In addition, the cooling rates of the samples affect sensitively the coherent ordering of the magnetic moments, while the Néel temperature 7N remains essentially unchanged. Finally, the anomalous evolution of the Néel temperatures with x across the (NH₃)K_3-xRb_xC₆₀ (0≤ x ≤3) series is ascribed to the different low-temperature orientationally ordered superstructures appearing between x = 2 and 3.