Understanding the unceasing evolution of Co(II) based single-ion magnets

Abstract

Single-molecule magnets (SMMs) are molecules that exhibit energy barriers to spin inversion. The slow relaxation of magnetization is of molecular origin and arises from the presence of a bistable magnetic ground state with a thermal barrier to inversion of the magnetization. Many such transition metal-based single-ion magnets (SIMs) have been known to show hysteretic behavior and hence, find potential applications in high-density nanoscale information storage, quantum computing, molecular spintronics, spin qubits, etc. To obtain practically applicable SIMs, two major requirements are high blocking temperature, TB (the temperature up to which the magnetization is completely retained), and high effective energy for the reversal of magnetization (Ueff). But due to the presence of several through barrier relaxation processes especially quantum tunneling of magnetization (QTM), the Ueff, as well as the TB, gets reduced. A viable strategy to reduce this QTM is the enhancement of magnetic anisotropy in the system. In this regard, Co(II) ion is a superior choice for the most explored SIMs among transition metals. The Co(II), being a Kramers ion (non-integer spin), exhibits high magnetic anisotropy thus inherently offering the minimization of QTM. In this review, we have discussed the superiority of the Co(II) ion over the other transition metal ions in designing SIMs and analyzed the pros and cons of such complexes having various coordination numbers with a vision of achieving higher magnetic anisotropy and Ueff value.