Determination of magnetic anisotropy due to spin-orbit coupling in Fe (II) spin crossover thin film

Spin crossover molecules such as [Fe(H₂B(pz)₂)₂(bipy)] (pz=pyrazol−1−yl, bipy=2,2’−bipyridine) are candidates for molecular spintronics and promise ultrafast and low-power devices for data storage and magnetic sensing. Most recently, this molecule has been coupled to a ferroelectric and shown to undergo a non-volatile, isothermal, reversible spin-state transition at room temperature. The spin state transitions may also be controlled by an external magnetic field, which calls for better understanding of the transition mechanism. Recently conducted field dependent, temperature dependent, and orientation dependent X-ray magnetic circular dichroism experiments on a 60 nm [Fe(H₂B(pz)₂)₂(bipy)] thin film deposited on a SiO₂ substrate show that the orbital moment of the molecule is soft to the magnetic field, and the hard axis of the magnetic moment is along the thin film surface normal direction. These results can be interpreted within the context of a 3D Ising model Hamiltonian with a magnetic field term, where an anisotropic energy barrier is observed in the high spin state of the molecule.