Single molecule magnet

Molecular structure of a Mn ₁₂ nanomagnet.

A single molecule magnet , referred to in English as "Single-Molecule Magnet" (SMM), is a molecular complex of one or more metal ions and organic ligands with a total spin, the orientation of the total spin at sufficiently low temperatures below the "blocking temperature" over a certain , temperature-dependent time is stable. The "blocking temperature" is defined as the temperature above which no overall magnetization can be observed within a time window. Corresponding magnetic systems are called superparamagnets . ${\displaystyle T_{\rm {B}}}$

Ferromagnets are materials with local spins with a fixed, preferred orientation (anisotropy) . The alignment or magnetization is achieved by long-range coupling below a critical temperature . In contrast, in superparamagnetic materials, the long-range couplings are broken, e.g. B. by the particle size (classic superparamagnets). Due to the anisotropy, there is still a preferred alignment, but it can now switch over time. One speaks of a magnetic relaxation . The magnetic relaxation takes place via a spontaneous change in the alignment of the overall spin. The anisotropy corresponds to the energy barrier that has to be overcome during the switching process. The energy barrier is defined as having the anisotropy energy density and the volume. For classic superparamagnets, the Boltzmann statistics result in the Néel lifetime of the total spin, with the Boltzmann constant and the test time (from the Néel relaxation model) . ${\displaystyle T_{\rm {C}}}$ ${\displaystyle U}$ ${\displaystyle U}$ ${\displaystyle U=KV}$ ${\displaystyle K}$ ${\displaystyle V}$ ${\displaystyle \tau _{N}}$ ${\displaystyle \tau _{N}=\tau _{0}\exp \left({\tfrac {U}{k_{\mathrm {B} }T}}\right)}$ ${\displaystyle k_{\rm {B}}}$ ${\displaystyle \tau _{0}}$

A single-molecule magnet is a spin-bearing molecule, the total spin of which , due to the structural geometry of the molecule (see also crystal field and ligand field theory ), has a preferred orientation (anisotropy) and, due to its size, which excludes ferromagnetism, shows superparamagnetism. The energy barrier can now be written as having a material-specific and experimentally determinable zero field splitting. This means that this molecule can be magnetized in a magnetic field and retains this magnetization for a certain time after the field is switched off . In single-molecule magnets, superparamagnetism is the property of the single molecule and is independent of its environment. Correspondingly, measurements of the magnetic hysteresis are not only dependent on the temperature, but also on the measuring speed. This means that single-molecule magnets can be embedded in other materials, for example plastics, and the effect of superparamagnetism is retained. ${\displaystyle S}$ ${\displaystyle U}$ ${\displaystyle U=S^{2}|D|}$ ${\displaystyle D}$ ${\displaystyle \tau _{N}}$

In the experimental investigation of the Néel service life as a function of temperature, however, it was found that the known relationship between service life and temperature is no longer generally valid for single-molecule magnets at low temperatures. In addition, another cause of spontaneous relaxation was identified, which is referred to as quantum tunneling of magnetization in the literature. The reason for the deviation is that the spin axis is not a real quantization axis, but rather the axis of the total angular momentum quantum number due to the coupling of the spin to the orbital moment. It follows from this that states with reversed total spin have an overlap of the electron wave functions, and one speaks of a transverse anisotropy . Due to the overlapping of the wave functions, switching of the magnetization can take place not only through thermally excited switching across the energy barrier, but also through temperature-independent tunneling through the barrier.

A characteristic of single-molecule magnets is the presence of a spin center, which is embedded in an organic ligand structure that shields the spin center from external influences. The spin center can be a single metal ion, a well-known example is a terbium ion in Tb phthalocyanine ₂ (TbPc ₂ ) or it can also consist of many coupled spins of spatially separated spin centers, as in Mn ₁₂ acetate. In principle, there are no limits to the choice of spin-bearing centers, the nature of the ligands or the size. However, it is necessary that the spin-carrying electrons communicate only weakly with those of the ligands, as this contributes to the quantum tunneling of the magnetization. In this respect, all known single-molecule magnets have metallic ions as spin centers with strongly localized electrons such as 4f electrons in TbPc ₂ , which show little interaction with the organic ligands. This also results in a limitation with regard to the number of coupled spin centers. The coupling of several spin centers, which advantageously increases the energy barrier for thermally excited switching, requires a spin-spin interaction via organic components, which in turn also increases the rate of the disadvantageous quantum tunneling of the magnetization. Current research in this area is focused on synthesizing molecular structures in which the energy barrier is maximized without increasing the rate of quantum tunneling.

This group of substances is interesting because they are real nanomaterials with a size of a few nanometers and effects such as B. magnetism, behave on the one hand strictly quantum mechanically , on the other hand according to the laws of classical physics. That means, these molecules move in the border area between these two areas of physics.

Blocking temperature

The measurements take place at very low temperatures. In this context, the so-called "blocking temperature" is of interest, below which the effect of relaxation becomes slow compared to the examination method. For example, a molecule that was magnetized at 2 K still shows 40% of the magnetization after 60 days. If the temperature is lowered to 1.5 K, the same value would only be reached after 40 years.

history

The first reports of an unusual magnetic behavior of the complex Mn ₁₂ O ₁₂ (MeCO ₂ ) ₁₆ (H ₂ O) ₄ ( abbreviated in the technical literature as Mn ₁₂ acetate and the synthesis of which was first reported in 1980) come from European countries in 1991 Researchers in Florence. In 1996, the effect of quantum tunneling of magnetization was first demonstrated on the same substance. The blocking temperature was below 4 K.

The term "single-molecule magnet" was coined by David Hendrickson, a chemist from the University of California, San Diego, and George Christou (Indiana University) in 1996.

Based on this initial work, a number of other single molecule magnets have been found. The highest blocking temperatures are reported for TbPc ₂ , with a spin lifetime of up to 1 ms at 40 K.

Research on these substances is part of nanophysics .

Applications

Potential applications are conceivable in the field of quantum computers and nanoscopic information storage. These materials are of interest in this area, because such a molecule can be imagined as a bit and thus extremely high data densities could be achieved. Data densities of up to 100 Tbit / in² (150 Gbit / mm²) could be achieved, which is three to four orders of magnitude higher than what is currently possible.

Web links

• Molecular magnets, University of Stuttgart
• Molecular Magnetism Community, Popular Articles, Scientists Worldwide
• European Institute of Molecular Magnetism EIMM
• MAGMANet Molecular Approach to Nanomagnets and Multifunctional Materials

swell

1. ↑ Introduction to Molecular Magnetism by Dr. Joris van Slageren (PDF file; 1.9 MB).
2. ↑ ^{a b} Dante Gatteschi, Roberta Sessoli and Andrea Cornia: Single-molecule magnets based on iron (III) oxo clusters. In: Chem. Commun. , 2000 , pp. 725-732, doi: 10.1039 / a908254i .
3. ^ T. Lis, Acta Crystallogr. B 1980, 36, 2042, doi: 10.1107 / S0567740880007893 .
4. ↑ Andrea Caneschi, Dante Gatteschi, Roberta Sessoli, Anne Laure Barra, Louis Claude Brunel, Maurice Guillot: Alternating current susceptibility, high field magnetization, and millimeter band EPR evidence for a ground S = 10 state in [Mn12O12 (Ch3COO) 16 (H2O ) 4]. 2CH3COOH.4H2O In: Journal of the American Chemical Society 1991 , Volume 113 , 5873-5874 doi: 10.1021 / ja00015a057 .
5. ↑ Thomas, L .; Lionti, F .; Ballou, R .; Gatteschi, D .; Sessoli, R. & Barbara, B .: Macroscopic quantum tunneling of magnetization in a single crystal of nanomagnets In: Nature 1996 , Volume 383 , 145-147.
6. ↑ Sheila MJ Aubin, Michael W. Wemple, David M. Adams, Hui-Lien Tsai, George Christou and David N. Hendrickson: Distorted Mn ^IV Mn ^III 3 Cubane Complexes as Single-Molecule Magnets In: Journal of the American Chemical Society 1996 , Volume 118 , 7746–7754, doi: 10.1021 / ja960970f .
7. ↑ Naoto Ishikawa, Miki Sugita, Tadahiko Ishikawa, Shin-ya Koshihara and Youkoh Kaizu: Lanthanide Double-Decker Complexes Functioning as Magnets at the Single-Molecular Level In: Journal of the American Chemical Society 2003 , Volume 125 , 8694–8695 doi: 10.1021 / ja029629n .
8. ↑ General introduction to molecular magnetism.  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. University of Stuttgart.  

literature

• Dante Gatteschi, Roberta Sessoli, Jacques Villain : Molecular Nanomagnets (Mesoscopic Physics and Nanotechnology; Vol. 5). Oxford University Press, Oxford 2006, ISBN 978-0-19-856753-0 .