Abstract
Magnetic properties of coordination polymers like single-chain magnets (SCMs) are based on magnetic domains, which are formed due to magnetic exchange between neighboring anisotropic spin centers. However, the computational restrictions imposed by the high level of theory needed for an adequate ab initio quantum mechanical treatment on the basis of multi-reference methods for these systems limit the feasibility of such calculations to mononuclear fragments as appropriate structural cutouts for the metal centers along the chains. Hence, results from such calculations describe single-ion properties and cannot be directly correlated with experimental data representing magnetic domains. We present a theoretical approach based on n-membered Ising-spin rings with n = 3-12, which allows us to simulate magnetic domains and to derive important magnetic properties for SCM compounds. Magnetic exchange, which is not provided by calculations of mononuclear fragments, is obtained by fitting the theoretical magnetic susceptibility against experimental data. The presented approach is tested for cobalt(ii)-based SCMs with three types of repeating sequences, which differ in nuclearity and symmetry. The magnetic parameters derived using the presented approach were found to be in good agreement with the experimental data. Moreover, the energy spectra obtained for the three test cases using the presented approach are characteristic of a deviation of the individual systems from the ideal Ising behavior. An extrapolation technique towards larger systems (n > 12) is presented which can provide information on the statistical mean length of the magnetic domains in the three investigated SCM compounds.
Highlights
All three single-chain magnets (SCMs) are based on cobalt(II) ions with an [N4S2] pseudooctahedral coordination sphere, with the individual spin centers linked by two thiocyanate bridges, which mediate the ferromagnetic coupling
We present an approach that allows us to directly relate the results derived from high-level ab initio quantum mechanical calculations on mononuclear fragments to the experimental magnetic data of 1D periodic compounds
In order to follow this approach, in addition to the electronic structure of the individual spin centers, the exchange coupling between them is required for a full ab initio simulation of the magnetic susceptibility for an n-membered spin ring
Summary
Magnetic compounds that show a slow relaxation of magnetization have received a considerable attention in recent years, since they are of great interest for future technologies.[1,2,3,4] The socalled single-chain magnets (SCMs) are one-dimensional (1D) coordination polymers and a promising class of nanomagnets.[5,6,7] Recent reports in the eld of SCMs discuss the in uence of single-ion anisotropy[8] and relaxation mechanisms,[9,10,11] and show interesting properties, e.g. the ability of a notable number of compounds with SCM behavior have been reported within the last few years, only a few have been investigated by ab initio quantum mechanical methods on the level of multi-reference methods.[23,24,43,44,45,46,47] From our point of view this situation is unsatisfactory, since we are rmly convinced that synergies evolve wherever experimental and theoretical methods can be combined. Coordination polymers need to be divided into smaller structural fragments of individual spin centers, which can be treated by ab initio computational methods.[23,24,48] This approach allows us to calculate single-ion properties for the spin centers of SCMs, such as the magnetic axes, corresponding g factors, and energies of spin–orbit coupled magnetic states. These single-ion parameters cannot be directly related to the experimental magnetic properties of 1D periodic compounds, as they are cooperative in nature and based on magnetic domains
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