Approaching the uniaxiality of magnetic anisotropy in single-molecule magnets

Single-molecule magnets (SMMs), which can exhibit slow magnetization relaxation and bulk-magnet-like hysteresis of purely molecular origin, are promising candidates for high-density information storage, molecular spintronics, and quantum computing. To realize their applications, it is crucial to improve the blocking temperature ( T B ) and the effective relaxation barrier ( U eff ). Three decades of multidisciplinary research have yielded distinct SMMs with a state-of-the-art U eff of up to 2,000 K and a T B of up to the liquid nitrogen region. Several strategies have been investigated and summarized, which revealed that enhancing the uniaxiality of magnetic anisotropy is critical for constructing high-performance SMMs. Therefore, magnetic anisotropy, a key property that connects the molecular structure symmetry and performance of SMMs, plays a fundamental role in dictating magneto-structural correlations. Understanding and employing magnetic anisotropy would be significantly beneficial for rationally designing high-performance SMMs. This review focuses on the magnetic anisotropy of SMMs. We illustrate the origin and manifestation of magnetic anisotropy in mononuclear 3d- and 4f-block metal complexes. We then introduce developed approaches to investigate magnetic anisotropy both theoretically and experimentally. Typical SMMs by optimizing uniaxial magnetic anisotropy through lanthanide metallocene, symmetry controlling, and low-coordination approaches are represented. Furthermore, the remaining challenges and opportunities in this field will be discussed.