Direct observation of the magnetic anisotropy of an Fe(II) spin crossover molecular thin film

Abstract

Spin crossover molecules are a promising candidate for molecular spintronics that aim for ultrafast and low-power devices for data storage and magnetic and information sensing. The rapid spin state transition is best controlled by non-thermal methods including magnetic field. Unfortunately, the magnetic field normally required to switch the spin state is normally high (~30 T), which calls for better understanding of the fundamental mechanism. In this work, we provide clear evidence of magnetic anisotropy in the local orbital moment of a molecular thin film based on spin crossover complex [Fe(H2B(pz)2)2(bipy)] (pz = pyrazol−1−yl, bipy = 2,2’−bipyridine). Field dependent X-ray magnetic circular dichroism measurements indicate that the magnetic easy axis for the orbital moment is along the surface normal direction. Along with the presence of a critical field, our observation points to the existence of an anisotropic energy barrier in the high-spin state. The estimated nonzero coupling constant of ~2.47 × 10-5 eV molecule-1 indicates that the observed magnetocrystalline anisotropy is mostly due to spin-orbit coupling. The spin- and orbital-component anisotropies are determined to be 30.9 and 5.04 meV molecule-1, respectively. Furthermore, the estimated g factor in the range of 2.2-2.45 is consistent with the expected values. This work has paved the way for an understanding of the spin-switching mechanism in the presence of magnetic perturbations.