# Presentation: 527988

## We have quite literally disassembled a molecule-based nanomagnet, then reassembled it, with a view to gaining important new insights into the interactions that govern its fascinating low-temperature quantum properties. This study focuses on a simple Ni4O4 cube, which exhibits magnetic quantum tunneling below 1 K. By preparing crystals of a non-magnetic Zn4O4 analog, doped with a small quantity of Ni, we have been able to determine (by EPR) the magnetic properties of the individual NiII ions which make up the Ni4O4 molecule. This enables us to then piece the molecule back together using a fairly simple numerical model, and to compare the results with EPR data obtained for the real molecule. We find an unexpected cause of the quantum tunneling in Ni4O4, which we believe to be relevant to many other single-molecule magnets (SMMs). This work exposes limitations of widely used models for describing SMMs.

- We have quite literally disassembled a molecule-based nanomagnet, then reassembled it, with a view to gaining important new insights into the interactions that govern its fascinating low-temperature quantum properties. This study focuses on a simple Ni4O4 cube, which exhibits magnetic quantum tunneling below 1 K. By preparing crystals of a non-magnetic Zn4O4 analog, doped with a small quantity of Ni, we have been able to determine (by EPR) the magnetic properties of the individual NiII ions which make up the Ni4O4 molecule. This enables us to then piece the molecule back together using a fairly simple numerical model, and to compare the results with EPR data obtained for the real molecule. We find an unexpected cause of the quantum tunneling in Ni4O4, which we believe to be relevant to many other single-molecule magnets (SMMs). This work exposes limitations of widely used models for describing SMMs.

- See: E.-C. Yang et al., Inorg. Chem. **44**, 3827 (2005); A. Wilson et al., cond-mat/0606252 (2006).

- Calculated energy levels for 4 coupled *S* = 1 NiII ions. Standard approaches to SMMs consider only the lowest 9 levels (in red). Our work highlights that one cannot neglect the remaining 72 levels.

**Notes:**
The Ni4O4 molecule is ideally suited for examining the validity of models that are widely used to describe the quantum properties of single-molecule nanomagnets. This molecule possesses a reasonably large ground state spin of S=4, together with a significant uniaxial anisotropy, giving rise to magnetic bistability and hysteresis below about 1K. The beauty of this molecule is its simplicity. It has a high symmetry and its magnetic moment arises from simple ferromagnetic coupling between four S=1 Ni ions. However, in spite of its high symmetry, it exhibits fast magnetic quantum tunneling at low temperatures. The origin of this fast tunneling was a mystery until recently. Because the Ni ions have spin S=1, they contribute to the magnetic anisotropy only in 2nd order. However, 2nd order transverse (tunneling) anisotropy is forbidden for the Ni4O4 molecule due to its four-fold symmetry. Thus, if one assumes a strong exchange limit, tunneling should be forbidden for this system due to its high symmetry, yet it tunnels very fast. The reason for the tunneling is a breakdown of the strong exchange limit, which leads to a significant 4th order contribution to the tunneling which is symmetry allowed. It is only because of the simplicity of this molecule that one can clearly identify the mechanisms which give rise to higher order processes that lead to quantum tunneling. We firmly believe that this is true for essentially all single-molecule magnets, including the most widely studied Mn12-acetate. However, a study of this kind would be impossible for such a complex molecule.

## NHMFL

**Education: **Undergraduate Emmitt Thompson presented work conducted under this award at an international conference. Emmitt is pictured at his poster (top) and together with another UF undergraduate (lower) who attended the meeting.

**Broader Impact:**
- The PI is serving as chair of internal scientific advisory committee for the Electron Magnetic Resonance (EMR) group at the NHMFL. In this capacity, the PI has been involved in developing a vision for the EMR program as part of the NHMFL five year renewal proposal.
- The PI’s research group participates in regular WebCast seminars with the EMR group at the NHMFL, as well as hosting joint visitors.
- The PI’s research group have developed a unique high-frequency EPR spectrometer operating to frequencies up to 670 GHz (24 T for g = 2) that is now available to users of the NHMFL.

**NHMFL**

**EMR**