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Magnetic anisotropy in Dy-based Single Ion Magnets (SIM) determined using experimental electron densities

In recent years, we have focused in particular on achieving a more general experimental quantification of magnetic anisotropy by using diffraction methods. The very best SMM compounds are based on the chemical of dysprosium, and the so-called single-ion Magnets (SIM) are based on the use of only a single Dy atom in a coordination complex. With the help of simplified theoretical considerations, it has been shown that you can control the anisotropy and thus the energy barrier against spin-relaxation by selecting the distribution of ligand atoms around the lanthanid ion. The argument is that the 4f electrons in the lowest energy mode for a given electronic state of the Dy have an oblate form. The problem is that these simple theoretical arguments have never been experimentally demonstrated.

That is why we have now determined the precise and accurate distribution of 4f electrons around a Dy-ion in a SIM, and quantified exactly how oblate the density is, while also quantifying the composition of the wavefunction in terms of Mj-state contributions.

Linear 3d metal complexes electron density, and experimental d-orbital links to magnetic properties

Coordination complexes composed of many 3d metals were originally the most promising SMM compounds, but since then the interest was lost when the anisotropy could not be improved, and the focus shifted to lanthanides. Recently, however, there has been a recurring interest in the 3d metals. It is now better understood that one can achieve amazingly large anisotropy by making complexes with unusual geometry, and e.g. linear (two-coordinated) compounds are very promising. In short, it is about avoiding the  "quenching" of orbital angular torque(?), which otherwise often happens in 3d complexes since the ligand field is relatively very large. With a linear (axially) ligand field, the orbital conveyance(?)to magnetism is preserved and there is a possibility of having a SIM.

For Fe and Co, it has recently been proven and we have contributed to the research by determining the experimental D-orbital populations. The results have been published in Science and Nature Chemistry.

Magnetic anisotropy from single-crystal PND measurements

Measuring the electronic density with X-rays as described above is an indirect way of determining magnetic properties. An alternative is therefore to measure the response from the magnetization density, and it can be done with polarized neutrons. In the case of strong magnetic anisotropy, a method of using polarized neutron single-crystal diffraction has recently been developed to determine the complete anisotropy, in the form of a determination of the magnetic susceptibility tensor.

We have as the very first used this method to describe such tensor for two different Dy-SIM, and it can be displayed completely analogous to a thermal vibration-ellipsoide which is known from crystal structure determination.This work was published in 2018.

In the most recent research, we collaborate with researchers at the Laboratoire Leon-Brillouin (LLB), on developing a method to achieve the same results with powder PND measurements. During the last half of 2019, we expect to have first results published. The first experiments were done in June 2019.

Magnetic susceptibility from single-crystals

We also have the possibility to determine the magnetic anisotropy, through an experimental and direct determination of the response from a single-crystal to an external magnetic field. We measure this in an instrument called a PPMS, which is installed at the Dept. of Chemistry. There is a restriction on this method by measuring the crystal response, which is the sum of all the molecules' responses. Depending on the symmetry of the crystal, some elements of the magnetic susceptibility tensor are inaccessible. Nonetheless, it is a very strong method and we use it together with the above methods for the determination of anisotropy.

High-pressure crystallography

In collaboration with the CMC, we have developed equipment that enables us to measure structural properties under very high pressure. Basically, we do this by placing a crystal between two diamond tips and squeezing together the tips. By making(?) diffraction through the diamonds we get insight into structural changes as a function of the external pressure. It is even possible to induce completely new and unknown phases in this way. We are working towards being able to make so accurate measurements of diffraction data in a pressure cell that we can determine not only the position of the atoms and thus the molecular structure, but also the electron density.

Other research topics

In addition to the above-mentioned research topics, the group also works with other themes focusing on the use of accurate diffraction, especially single-crystal diffraction. Here are just some headlines, more info can be obtained from Jacob Overgaard: 

  • Chemical bonding, among other things for low-valent main-group metal complexes
  • Experimental determination of wave functions from diffraction
  • Polymorphism in drug design, coupling of intermolecular energies to macroscopic properties of pharmaceuticals
  • Development of extremely low-temperature diffraction