Quantum Tunneling of the Magnetization in Systems with Anisotropic 4f Ion Pairs: Rates from Low-Temperature Zero-Field Relaxation

Anisotropic open shell 4f ions have magnetic moments that can be read and written as atomic bits. If it comes to quantum applications where the phase of the wave function has to be written, controlled, and read, it is advantageous to rely on more than one atom that carries the quantum information on the system because states with different susceptibilities may be addressed. Such systems are realized for pairs of lanthanides in single-molecule magnets, where four pseudospin states are found and mixed in quantum tunneling processes. For the case of endohedral fullerenes like Dy₂S@C₈₂ or Tb₂ScN@C₈₀, the quantum tunneling of the magnetization is imprinted in the magnetization lifetimes at sub-Kelvin temperatures. A (4 × 4) Hamiltonian that includes quantum tunneling of the magnetization and the dipole and exchange interaction between the two lanthanide magnetic moments predicts the lifting of the zero-field ground state degeneracy and nonlinear coupling to magnetic fields in such systems.