Quantum Bi-stability and Robust Room-Temperature Spin Excitation in a Single-Ion Magnet

The search for single-ion magnetism (SIM) has been largely focused on hybrid metal-organic systems. Here, we experimentally demonstrate the spin-relaxation mechanism of SIM in a pure inorganic transition-metal oxide, Sr2Ca2Mn2CoO9, an Ising-chain magnet, employing neutron diffraction and inelastic neutron scattering (INS), complemented by SpinW simulations and a machine-learning framework. Interestingly, SIM mechanism persists even in the presence of long-range magnetic ordering, a phenomenon that is rarely observed. This pioneering investigation shows that bistability between the two quantum states Ms=±3/2 is maintained at zero magnetic field, driven by a dominant Orbach spin-relaxation mechanism with an effective energy barrier, U=4.4 meV. This behaviour arises from strong spin-phonon coupling in the presence of negative axial anisotropy (D approx -2.2meV) and the high-spin state S=3/2 of the Co(II) ion. Furthermore, we observe that magnon excitations persist up to room temperature, reflecting low-dimensional magnetic interactions and extended magnetic correlations within the oxide lattice. This interplay between single-ion magnetism and room-temperature spin excitations underscores the chemical tunability of magnetic anisotropy in oxide lattices.