Ligand Effects toward the Modulation of Magnetic Anisotropy and Design of Magnetic Systems with Desired Anisotropy Characteristics Tamal Goswami and Anirban Misra* Department of Chemistry, University of North Bengal, Siliguri, Darjeeling 734013, West Bengal, India * S Supporting Information ABSTRACT: Magnetic anisotropy of a set of octahedral Cr(III) complexes is studied theoretically. The magnetic anisotropy is quantified in terms of zero-field splitting (ZFS) parameter D, which appeared sensitive toward ligand substitution. The increased π- donation capacity of the ligand enhances the magnetic anisotropy of the complexes. The axial π-donor ligand of a complex is found to produce an easy-plane type (D > 0) magnetic anisotropy, while the replacement of the axial ligands with π-acceptors entails the inversion of magnetic anisotropy into the easy-axis type (D < 0). This observation enables one to fabricate a single molecule magnet for which easy-axis type magnetic anisotropy is an indispensable criterion. The equatorial ligands are also found to play a role in tuning the magnetic anisotropy. The magnetic anisotropy property is also correlated with the nonlinear optical (NLO) response. The value of the first hyperpolarizability varies proportionately with the magnitude of the ZFS parameter. Finally, it has also been shown that a rational design of simple octahedral complexes with desired anisotropy characteristics is possible through the proper ligand selection. ■ INTRODUCTION Magnetically interacting open-shell transition metal ion clusters have been a topic of thorough investigation in the past few decades, which has caused the divergent areas of chemistry and physics to meet. 1 Interesting catalytic, biochemical, and physical properties of paramagnetic metal complexes have drawn the attention of many researchers and material scientists. 2 Magnetic materials based on molecular lattices, rather than continuous lattices of classical magnets, have been designed and synthesized. 3 Recently, polynuclear clusters assembled from mononuclear coordination complexes have become a subject of increased interest since it is relevant for the study of “single- molecule magnets” (SMMs). 4 A phenomenon hindering spin inversion causes certain molecules to exhibit slow relaxation of the magnetization after removal of an applied magnetic field, thus showing SMM behavior. 5,6 The discovery that some metal coordination clusters may behave as SMMs 5,7,8 has provoked plentiful research in the direction of their potential applications in high-density information storage and quantum comput- ing. 9-11 The genesis of SMM behavior is a large easy-axis magnetic anisotropy and concomitant high energy barrier that needs to be overcome for the reversal of the magnetic moment. The barrier to reorient spin in magnetic molecules can be given by | D|S 2 for molecules with integer spins and |D|(S 2 - 1/4) for molecules with half integer spins, where D is the zero-field splitting (ZFS) parameter and S is the ground state spin. 12 Molecular systems containing a large number of paramagnetic centers with significant negative D are the most suitable candidates to be used as SMMs. 5 However, most of these species show either low negative or positive D value in spite of having high ground state spin. Recently, a few lanthanide complexes have been reported to show slow magnetic relaxation behavior. For example, phthalocyanine double- decker complexes with Tb(IV) and Er(III) encapsulated in a polyoxometalate framework exhibit an extremely high negative anisotropy barrier. 13,14 Several complexes of Fe(II), U(III) and Dy(III) also show similar characteristics. 15-17 Another novel class of nanomagnets called the single-chain magnets (SCMs), can be formed by combination of the SMMs. 18-23 A series of one-dimensional cyano-bridged coordination solids (DMF) 4 MReCl 4 (CN) 2 , with M = Mn, Fe, Co, Ni, have been reported to show a slow relaxation of magnetization. 24 Moreover, in the combination of SMMs in which the easy- axes of anisotropies are linked in a parallel manner, can lead to a large easy-axis type (D < 0) anisotropy in the long-chain range, and manifestation of a slow relaxation of magnetization can occur. 25 The dependence of the ZFS parameter (D) on the nature of ligands has long been a subject of enormous interest. 26 For example, the synthesis and characterization of a series of high- spin hexa-coordinated dihalide Mn(II) complexes [Mn(tpa)X 2 ] (tpa = tris-2-picolylamine; X = I, Br, and Cl) advocate for the presence of such ligand effects showing an increase in the D Received: January 19, 2012 Revised: May 6, 2012 Published: May 8, 2012 Article pubs.acs.org/JPCA © 2012 American Chemical Society 5207 dx.doi.org/10.1021/jp3006603 | J. Phys. Chem. A 2012, 116, 5207-5215