Single-molecule magnet
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2008-05-28T07:25:37Z
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{{expert-portal|Science}}
A '''single-molecule magnet''' or '''SMM''' is an object that is composed of [[molecule]]s each of which behaves as an individual [[superparamagnetism|superparamagnet]]. This is distinct from a [[molecule-based magnet]], in which a group of molecules behave collectively as a magnet.
In 2004 it was said that: "Single-molecule magnets (SMMs) are large molecules consisting of several [[transition metal]] ions coupled through oxygens and surrounded by various [[ligand]]s."<ref> http://flux.aps.org/meetings/YR04/MAR04/baps/abs/S650.html - Session B25 - Single-Molecule Magnets.
ORAL session, Monday midday, March 22
514AB, Palais des Congres - ''Second-order transverse magnetic anisotropy induced by disorders in the single-molecule magnet Mn12-acetate''</ref>
As of 2008 there are many discovered types and potential uses. "Single molecule magnets (SMM) are a class of molecules exhibiting magnetic properties similar to those observed in conventional bulk magnets, but of molecular origin. SMMs have been proposed as potential candidates for several technological applications that require highly controlled thin films and patterns."<ref> [http://www.rsc.org/Publishing/Journals/CP/article.asp?doi=b711677b Phys. Chem. Chem. Phys., 2008, 10, 784 - 793, DOI: 10.1039/b711677b] article ''Single molecule magnets: from thin films to nano-patterns''</ref> "The ability of a single molecule to behave like a tiny magnet (single molecular magnets, SMMs) has seen a rapid growth in research over the last few years. SMMs represent the smallest possible magnetic devices and are a controllable, bottom-up approach to nanoscale magnetism. Potential applications of SMMs include [[Quantum computer|quantum computing]], [[Computer data storage|high-density information storage]] and [[magnetic refrigeration]]."<ref> [http://www.rsc.org/Publishing/Journals/dt/News/b716355jpersp.asp Royal Society of Chemistry - Dalton Transactions] article ''Ground state spin-switching via targeted structural distortion: twisted single-molecule magnets from derivatised salicylaldoximes'' by Constantinos J. Milios, Stergios Piligkos and Euan K. Brechin, - Dalton Trans., 2008, 1809 - DOI: 10.1039/b716355j quote from summary titled ''Beautiful new single molecule magnets'' published 26 March 2008</ref>
[[Image:Hard disk.jpg|thumb|250px|One possible use of SMMs is superior [[magnetic]] [[thin film]]s to coat [[hard disk]]s.]]
==Properties and uses==
A single molecule magnet is "a molecule that shows slow relaxation of the magnetization of purely molecular origin."<ref name=tutorial> [http://obelix.physik.uni-bielefeld.de/~schnack/molmag/material/123.pdf Introduction to Molecular Magnetism by Dr. Joris van Slageren]</ref> "It is a molecule that can be magnetized in a magnetic field, and that will remain magnetized even after switching off the [[magnetic field]]. This is a property of the molecule itself. No interaction between the molecules is necessary for this phenomenon to occur.This makes single molecule magnets fundamentally different from traditional bulk [[magnet]]s. You can dissolve a single molecule magnet in a [[solvent]] or put it in some other matrix, like a [[polymer]], and it will still show this property."<ref name=tutorial/>
The requisites for such a system are:
* a high [[spin (physics)|spin]] [[ground state]],
* a high [[Zero field splitting|zero-field-splitting]] (due to high magnetic [[anisotropy]]), and
* negligible magnetic interaction between molecules.
The combination of these properties can lead to an [[energy barrier]] so that, at [[low temperature]]s, the system can be trapped in one of the high-spin energy wells.<ref name=tutorial/>
"These molecules contain a finite number of interacting spin centers (e.g. [[paramagnetic]] ions) and thus provide ideal opportunities to study basic concepts of [[magnetism]]. Some of them possess magnetic ground states and give rise to [[hysteresis]] effects and metastable magnetic phases. They may show [[quantum tunneling]] of the magnetization which raises the question of coherent dynamics in such systems. Other types of molecules exhibit pronounced [http://www.ifw-dresden.de/institutes/iff/research/TMO/frustrated-magnets frustration effects], whereas so-called spin crossover substances can switch their magnetic ground state and related properties such as color under irradiation of laser light, pressure or heat. Scientists from various fields - chemistry, physics; theory and experiment - have joined the research on molecular magnetism in order to explore the unprecedented properties of these new compounds."<ref> [http://obelix.physik.uni-bielefeld.de/~schnack/molmag/introduction.html Molecular Magnetism Web] Introduction page</ref>
"Single-molecule magnets (SMMs) have many important advantages over conventional nanoscale magnetic particles composed of [[metal]]s, [[metal alloy]]s or metal oxides. These advantages include uniform size, solubility in organic solvents, and readily alterable peripheral [[ligand]]s, among others."<ref> [http://www.sciencedaily.com/releases/2000/03/000327084104.htm ScienceDaily (Mar. 27, 2000)] article ''Several New Single-Molecule Magnets Discovered''</ref>
"'As far as applications go, some academics are working to deposit Mn12 clusters on surfaces, but that too is not very advanced,' Christou says. 'We have been avoiding putting Mn12 on surfaces in our lab because two dimensions might not be the future of information storage,' he notes. 'A lot of us believe the future of SMMs and information storage is going to be three-dimensional. And Mn12 is probably not going to be the future of SMMs either. It's the best at the moment, but we need better compounds.'"<ref> [http://pubs.acs.org/cen/science/8250/8250sci1.html Chemical & Engineering News] article ''SINGLE-MOLECULE MAGNETS EVOLVE'' from December 13, 2004 - Volume 82, Number 50 pp. 29-32</ref>
"A single molecule magnet is an example of a macroscopic [[quantum system]]. [...] If we could detect [[spin flip]]s in a single atom or molecule, we could use the spin to store information. This would enable us to increase the storage capacity of computer [[hard disk]]s. [...] A good starting point for trying to detect spin flips is to find a molecule with a spin of several [[Bohr magneton]]s. [An electron has an intrinsic magnetic dipole moment of approximately one Bohr magneton.] There is a very well studied molecular magnet, Mn12-acetate, which has a spin S = 10 (Figure 3). This molecule is a disc-shaped [[organic molecule]] in which twelve Mn ions are embedded. Eight of these form a ring, each having a charge of +3 and a spin S = 2. The other four form a [[tetrahedron]], each having a charge of +4 and a spin S = 3/2. The exchange interactions within the molecule are such that the spins of the ring align themselves in opposition to the spins of the tetrahedron, giving the molecule a total net spin S = 10."<ref> [http://www.npl.co.uk/server.php?show=ConWebDoc.1175 National Physical Laboratory (UK)] Home > Science + Technology > Quantum Phenomena > Nanophysics > Research - article ''Molecular Magnets''</ref>
==Types==
[[Image:Ferritin.png|thumb|Ferritin]]
The archetype of single-molecule magnets is called "Mn12". It is a [[polymetallic]] [[manganese]] (Mn) complex having the formula [Mn12O12(OAc)16(H2O)4]. It has the remarkable property of showing an extremely slow relaxation of their magnetization below a blocking temperature.<ref> [http://www-ipcms.u-strasbg.fr/spip.php?article1341 IPCMS (Institut de Physique et Chimie des Matériaux de Strasbourg)] ARTICLE ''Liquid-crystalline Single Molecule Magnets'' - "For more details : Angew. Chem. Int. Ed., 2008, 47, 3, 490-495"</ref> [Mn12O12(OAc)16(H2O)4]·4H2O·2AcOH which is called "Mn12-acetate" is a common form of this used in research.
"Mn4" is another researched type single-molecule magnet. Three of these are:<ref name=mn4> [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TH8-48H2RV9-4&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=f50b054180dc3b2d314ce7999f9a8c25 Proceedings of the 8th International Conference on Molecule-Based Magnets (ICMM 2002)] article ''Mn4 single-molecule magnets with a planar diamond core and S=9'' - Volume 22, Issues 14-17, 15 July 2003, Pages 1857-1863
</ref>
* [Mn4(hmp)6(NO3)2(MeCN)2](ClO4)2·2MeCN (3),
* [Mn4(hmp)6(NO3)4]·(MeCN) (4), and
* [Mn4(hmp)4(acac)2(MeO)2](ClO4)2·2MeOH (5).
In each of these Mn4 complexes "there is a planar diamond core of MnIII 2MnII 2 ions. An analysis of the variable-temperature and variable-field magnetization data indicate that all three molecules have intramolecular ferromagnetic coupling and a S = 9 ground state. The presence of a frequency-dependent alternating current susceptibility signal indicates a significant energy barrier between the spin-up and spin-down states for each of these three MnIII 2MnII 2."<ref name=mn4/>
Single-molecule magnets are also based on [[iron]] clusters<ref name=Gatteschi/> because they potentially have large spin states. In addition the [[biomolecule]] [[ferritin]] is also considered a nanomagnet. In the cluster '''Fe8Br''' the [[cation]] Fe8 stands for [Fe<sub>8</sub>O<sub>2</sub>(OH)<sub>12</sub>(tacn)<sub>6</sub>]<sup>8+</sup> with tacn representing [[1,4,7-triazacyclononane]].
==History==
Although the term "single-molecule magnet" was first employed by David Hendrickson, a chemist at the University of California, San Diego and George Christou (Indiana University) in 1996,<ref>J. Am. Chem. Soc. 1996, 118, 7746-7754</ref> the first single-molecule magnet reported dates back to 1991.<ref>A. Caneschi et al. in J. Am. Chem. Soc. 1991, 113(15), 5873-5874</ref> The European researchers discovered that a Mn<sub>12</sub>O<sub>12</sub>(Me</sub>CO<sub>2</sub>)<sub>16</sub>(H<sub>2</sub>O)<sub>4</sub> complex ('''Mn12Ac16''') first synthesized in 1980<ref>[http://dx.doi.org/10.1107/S0567740880007893 T. Lis, Acta Crystallogr. B 1980, 36, 2042]</ref> exhibits slow relaxation of the magnetization at low temperatures. This [[manganese]] [[oxide]] compound is composed of a central Mn(IV)<sub>4</sub>O<sub>4</sub> cube surrounded by a ring of 8 Mn(III) units connected through bridging oxo [[ligand]]s. In addition, it has 16 [[acetate]] and 4 [[water]] ligands.<ref>''Chemistry of Nanostructured Materials''; Yang, P., Ed.; World Scientific Publishing: Hong Kong, '''2003'''.</ref>
It was known in 2006 that the "deliberate structural distortion of a Mn6 compound via the use of a bulky [[salicylaldoxime]] derivative switches the intra-triangular magnetic exchange from [[antiferromagnetic]] to [[ferromagnetic]] resulting in an [[Spin (physics)|S]] = [[Quantum number|12]] [[Stationary state|ground state]].<ref> [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/2007/129/i01/abs/ja0666755.html J. Am. Chem. Soc., 129 (1), 8 -9, 2007. 10.1021/ja0666755] article ''A Single-Molecule Magnet with a "Twist"'' - Received September 21, 2006 - Web Release Date: December 15, 2006</ref>
A record magnetization was reported in 2007 for a compound related to MnAc12 ([Mn(III)
<sub>6</sub>O<sub>2</sub>(sao)<sub>6</sub>(O2CPh)<sub>2</sub>(EtOH)<sub>4</sub>]) with S = 12, D = -0.43cm<sup>-1</sup> and hence U = 62 cm<sup>-1</sup> or 86 K<ref>''A Record Anisotropy Barrier for a Single-Molecule Magnet'' Constantinos J. Milios, Alina Vinslava, Wolfgang Wernsdorfer, Stephen Moggach, Simon Parsons, Spyros P. Perlepes, George Christou, and Euan K. Brechin [[J. Am. Chem. Soc.]]; '''2007'''; 129(10) pp 2754 - 2755; (Communication) {{DOI|10.1021/ja068961m}}</ref> at a blocking temperature of 4.3 K. This was accomplished by replacing acetate ligands by the bulkier [[salicylaldoxime]] thus distorting the manganese ligand sphere. It is prepared by mixing the [[perchlorate]] of manganese, the sodium salt of [[benzoic acid]], a [[salicylaldoxime]] derivate and [[tetramethylammonium hydroxide]] in water and collecting the filtrate.
==Blocking temperature==
Measurements takes place at very low temperatures. The so-called [[blocking temperature]] is defined as the temperature below which the relaxation of the magnetisation becomes slow compared to the time scale of a particular investigation technique.<ref name=Gatteschi/> A molecule magnetised at 2 K will keep 40% of its magnetisation after 2 months and by lowering the temperature to 1.5 K this will take 40 years.<ref name=Gatteschi>''Single-molecule magnets based on iron(III) oxo clusters'' Dante Gatteschi, Roberta Sessoli and Andrea Cornia [[Chem. Commun.]], '''2000''', 725 - 732, {{DOI|10.1039/a908254i}}</ref>
==Detailed behavior==
Molecular magnets exhibit an increasing product ([[magnetic susceptibility]] times [[temperature]]) with decreasing temperature, and can be characterized by a shift both in position and intensity of the a.c. magnetic susceptibility.
[[Image:EffetTunnel.gif|250px|left|thumb|Electron tunneling through barrier (most of the electron wave function does not make it through)]]
Single-molecule magnets represent a molecular approach to nanomagnets (nanoscale magnetic particles). In addition, single-molecule magnets have provided physicists with useful test-beds for the study of [[quantum mechanics]]. Macroscopic [[quantum tunneling]] of the magnetization was first observed in Mn<sub>12</sub>O<sub>12</sub>, characterized by evenly-spaced steps in the hysteresis curve. The periodic quenching of this tunneling rate in the compound Fe8 has been observed and explained with [[geometric phase]]s.
Due to the typically large, bi-stable spin [[anisotropy]], single-molecule magnets promise the realization of perhaps the smallest practical unit for [[magnetic memory]], and thus are possible building blocks for a [[quantum computer]]. Consequently, many groups have devoted great efforts into synthesis of additional single molecule magnets; however, the Mn<sub>12</sub>O<sub>12</sub> complex and analogous complexes remain the canonical single molecule magnet with a 50 cm<sup>-1</sup> spin anisotropy.
The spin anisotropy manifests itself as an energy barrier that spins must overcome when they switch from parallel alignment to antiparallel alignment. This barrier (U) is defined as:
<math>\ U = S^2|D|\,</math>
where S is the dimensionless total spin state and D the [[zero-field splitting parameter]] (in cm<sup>-1</sup>). D can be negative but only its [[absolute value]] is considered in the equation. The barrier U is generally reported in cm<sup>-1</sup> units or in units of [[Kelvin]] (see: [[electronvolt]]). The higher the barrier the longer a material remains magnetized and a high barrier is obtained when the molecule contains many unpaired electrons and when its zero field splitting value is large. For example the MnAc12 cluster the spin state is 10 (involving 20 unpaired electrons) and D = -0.5 cm<sup>-1</sup> resulting in a barrier of 50 cm<sup>-1</sup> (equivalent to 60 [[Kelvin]]).{{Fact|date=April 2008}}.
The effect is also observed by [[hysteresis]] experienced when magnetization is measured in a [[magnetic field]] sweep: on lowering the magnetic field again after reaching the maximum magnetization the magnetization remains at high levels and it requires a reversed field to bring magnetization back to zero.
Recently, it has been has been reported that the energy barrier, U, is slightly dependent on Mn12 crystal size/morphology, as well as the magnetization relaxation times, which varies as function of particle size and size distributions .<ref name=Munto> "Controlled crystallization of Mn12 single-molecule magnets by compressed CO2 and its influence on the magnetization relaxation" Maria Munto,Jordi Gomez-Segura, Javier Campo, Motohiro Nakano, Nora Ventosa,Daniel Ruiz-Molina and Jaume Veciana, [[J. Mat. Chem.]] '''2006''', 16,2612-2617, [[DOI:10.1039/b603497g]] </ref>
==See also==
*[[Ferromagnetism]]
*[[Single-molecule experiment]]
==Sources and notes==
{{reflist}}
==Further reading==
*[http://www.magmanet-eu.net/ magmanet-eu]
*[http://www.molmag.de/ molmag]
{{magnetic states}}
[[Category:Types of magnets]]
[[Category:Physics]]
[[Category:Condensed matter physics]]
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