Nonlinearity and harmonics of the magnetic susceptibility of a Co$^{2+}$ single-ion magnet in the paramagnetic region above the magnetic ordering temperature

In single-ion complexes based on Co$^{2+}$ ions, the second and third harmonics of the magnetic susceptibility were found at temperatures of 2–4 K, which exceed the Neel temperature. The maxima of the second and third harmonics of the magnetic susceptibility are observed at a frequency of $\sim$ 1 Hz, at which the maximum of the first harmonic is observed in a field of 3.2 kOe. An analysis of the dependences of the second and third harmonics of the magnetic susceptibility on the field and temperature showed that the nonlinearity arises as a result of the formation of a spin glass state at temperatures slightly higher than the Neel temperature. In this state, there is no long-range spin order, but there are spin clusters in the spin glass state. The spin-glass state in a compound with a Co$^{2+}$ ion with a high magnetic anisotropy is unusual in that the exchange interaction is much smaller than the single-ion anisotropy energy.