High resolution electron energy loss spectroscopy

2013-09-04T19:30:54Z

128.135.100.112: /* Overview of HREELS */

'''Single-molecule magnets''' or '''SMMs''' are a class of [[Metalorganics|metalorganic compounds]], that show [[superparamagnetism|superparamagnetic]] behavior below a certain [[Superparamagnetism#Blocking temperature|blocking temperature]] at the molecular scale. In this temperature range, SMMs exhibit [[magnetic hysteresis]] 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> Contrary to conventional bulk [[magnets]] and [[molecule-based magnets]], collective long-range magnetic ordering of [[magnetic moment]]s is not necessary.<ref name=tutorial/>

==Intramolecular coupling==
{{over-quotation|date=August 2013}}
The magnetic coupling between the spins of the metal ions is mediated via [[superexchange]] interactions and can be described by the following isotropic [[Heisenberg model (quantum)|Heisenberg Hamiltonian]]:

:<math>\mathcal{\hat{H}}_{HB} = \sum_{i<j}J_{i,j}\mathbf{S}_{i} \cdot \mathbf{S}_{j}</math>

where <math>J_{i,j}</math> is the coupling constant between spin i (operator <math>\mathbf{S}_{i}</math>) and spin j (operator <math>\mathbf{S}_{j}</math>). For positive J the coupling is called ferromagnetic (parallel alignment of spins) and for negative J the coupling is called antiferromagnetic (antiparallel alignment of spins).
* 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 frustration effects,<ref>[http://www.ifw-dresden.de/institutes/iff/research/TMO/frustrated-magnets Frustrated Magnets], Leibniz Institute for Solid State and Materials Research, Dresden, Germany</ref> 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>

"A single molecule magnet is an example of a macroscopic [[quantum system]]. [...] If we could detect spin flips 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>

==Blocking temperature==
Measurements take 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>

==Future applications==
{{over-quotation|date=August 2013}}
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>{{cite journal|last1=Cavallini|first1=Massimiliano|last2=Facchini|first2=Massimo|last3=Albonetti|first3=Cristiano|last4=Biscarini|first4=Fabio|title=Single molecule magnets: from thin films to nano-patterns|journal=Physical Chemistry Chemical Physics|volume=10|pages=784|year=2008|doi=10.1039/b711677b|bibcode = 2008PCCP...10..784C|issue=6|pmid=18231680 }}</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 Beautiful new single molecule magnets''], 26 March 2008 – summary of the article {{cite journal|last1=Milios|first1=Constantinos J.|last2=Piligkos|first2=Stergios|last3=Brechin|first3=Euan K.|title=Ground state spin-switching via targeted structural distortion: twisted single-molecule magnets from derivatised salicylaldoximes|journal=Dalton Transactions|pages=1809|year=2008|doi=10.1039/b716355j|issue=14}}</ref>

[[File:Hard disk.jpg|thumb|One possible use of SMMs is superior [[magnetic]] [[thin film]]s to coat [[hard disk]]s.]]
"A single molecule magnet is an example of a macroscopic [[quantum system]]. [...] If we could detect spin flips 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==
{{over-quotation|date=August 2013}}
[[File: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], where OAc stands for [[acetate]]. 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 Liquid-crystalline Single Molecule Magnets] – summary of the article {{cite journal|last1=Terazzi|first1=Emmanuel|last2=Bourgogne|first2=Cyril|last3=Welter|first3=Richard|last4=Gallani|first4=Jean-Louis|last5=Guillon|first5=Daniel|last6=Rogez|first6=Guillaume|last7=Donnio|first7=Bertrand|title=Single-Molecule Magnets with Mesomorphic Lamellar Ordering|doi=10.1002/anie.200704460|journal=Angew. Chem. Int. Ed.|year= 2008|volume=47|issue=3|pages=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>{{cite journal|doi=10.1016/S0277-5387(03)00173-6|last1=Yang|year=2003|first1=E|pages=1857|volume=22|journal=Polyhedron |title=Mn4 single-molecule magnets with a planar diamond core and S=9|issue=14–17}}
</ref>
* [Mn4(hmp)6(NO3)2(MeCN)2](ClO4)2·2MeCN
* [Mn4(hmp)6(NO3)4]·(MeCN)
* [Mn4(hmp)4(acac)2(MeO)2](ClO4)2·2MeOH

In each of these Mn4 complexes "there is a planar diamond core of MnIII2MnII2 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 MnIII2MnII2 complexes."<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 [Fe8O2(OH)12(tacn)6]8+ with tacn representing [[1,4,7-triazacyclononane]].

The ferrous cube complex Fe4C40H52N4O12 (commonly called [Fe4(sae)4(MeOH)4]) was the first example of a single-molecule magnet involving an Fe(II) cluster, and the core of this complex is a slightly distorted cube with Fe and O atoms on alternating corners.<ref name = "JACS2000">{{cite journal | author1 = Oshio, H. | author2 = Hoshino, N. | author3 = Ito, T. | title = Superparamagnetic Behavior in an Alkoxo-Bridged Iron(II) Cube | journal = J. Am. Chem. Soc. | year = 2000 | volume = 122 | pages = 12602–12603 | doi = 10.1021/ja002889p | issue = 50}}</ref> Remarkably, this single molecule magnet exhibits non-collinear magnetism in which the atomic spin moments of the four Fe atoms point in opposite directions along two nearly perpendicular axes.<ref name = "JACS2004">{{cite journal | author1 = Oshio, H. | author2 = Hoshino, N. | author3 = Ito, T. | author4 = Nakano, M. | title = Single-Molecule Magnets of Ferrous Cubes: Structurally Controlled Magnetic Anisotropy | journal = J. Am. Chem. Soc. | year = 2004 | volume = 126 | pages = 8805–8812 | doi = 10.1021/ja0487933 | issue = 28}}</ref> Theoretical computations showed approximately two magnetic electrons are localized on each Fe atom with the other atoms being nearly nonmagnetic, and the spin-orbit coupling potential energy surface has three local energy minima with a magnetic anisotropy barrier just below 3 meV.<ref name = "JCTC2011">{{cite journal | author1 = Manz, T. A. | author2 = Sholl, D. S. | journal = J. Chem. Theory Comput. | year = 2011 | volume = 7 | pages = 4146–4164 | doi = 10.1021/ct200539n | issue = 12}}</ref>

==History==
Although the term "single-molecule magnet" was first employed in 1996,<ref>{{cite journal|last1=Aubin|first1=Sheila M. J.|last2=Wemple|first2=Michael W.|last3=Adams|first3=David M.|last4=Tsai|first4=Hui-Lien|last5=Christou|first5=George|last6=Hendrickson|first6=David N.|title=Distorted MnIVMnIII3Cubane Complexes as Single-Molecule Magnets|journal=Journal of the American Chemical Society|volume=118|pages=7746|year=1996|doi=10.1021/ja960970f|issue=33}}</ref> the first single-molecule magnet was reported in 1991.<ref>{{cite journal|last1=Caneschi|first1=Andrea|last2=Gatteschi|first2=Dante|last3=Sessoli|first3=Roberta|last4=Barra|first4=Anne Laure|last5=Brunel|first5=Louis Claude|last6=Guillot|first6=Maurice|title=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|journal=Journal of the American Chemical Society|volume=113|pages=5873|year=1991|doi=10.1021/ja00015a057|issue=15}}</ref> The complex Mn12O12(MeCO2)16(H2O)4 complex (Mn12Ac16), first described in 1980,<ref>{{cite journal|last1=Lis|first1=T.|title=Preparation, structure, and magnetic properties of a dodecanuclear mixed-valence manganese carboxylate|journal=Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry|volume=36|pages=2042|year=1980|doi=10.1107/S0567740880007893|issue=9}}</ref> exhibits slow relaxation of the magnetization at low temperatures. This [[manganese]] [[oxide]] compound features a central Mn(IV)4O4 cube surrounded by a ring of 8 Mn(III) units connected through bridging oxo [[ligand]]s.<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>{{cite journal|last1=Milios|first1=Constantinos J.|last2=Vinslava|first2=Alina|last3=Wood|first3=Peter A.|last4=Parsons|first4=Simon|last5=Wernsdorfer|first5=Wolfgang|last6=Christou|first6=George|last7=Perlepes|first7=Spyros P.|last8=Brechin|first8=Euan K.|title=A Single-Molecule Magnet with a "Twist"|journal=Journal of the American Chemical Society|volume=129|pages=8|year=2007|doi=10.1021/ja0666755|pmid=17199262|issue=1}}</ref>

A record magnetization was reported in 2007 for [Mn(III)
6O2(sao)6(O2CPh)2(EtOH)4], with S = 12, D = -0.43 cm−1 and hence U = 62 cm−1 or 86 K<ref>{{cite journal|last1=Milios|first1=Constantinos J.|last2=Vinslava|first2=Alina|last3=Wernsdorfer|first3=Wolfgang|last4=Moggach|first4=Stephen|last5=Parsons|first5=Simon|last6=Perlepes|first6=Spyros P.|last7=Christou|first7=George|last8=Brechin|first8=Euan K.|title=A Record Anisotropy Barrier for a Single-Molecule Magnet|journal=Journal of the American Chemical Society|volume=129|pages=2754|year=2007|doi=10.1021/ja068961m|issue=10|pmid=17309264}}</ref> at a blocking temperature of 4.3 K. This was accomplished by replacing acetate ligands (OAc) 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.

In 2011 it was reported by the [[University of Nottingham]] that a dinuclear complex of [[depleted uranium]] could be mistaken for a single molecule magnet by chemist [[Stephen Liddle]].<ref>{{cite journal|last=Czyzewski|first=Andrew|title=News: Compound is an attractive prospect|journal=[[The Engineer (magazine){{!}}The Engineer]]|url=http://www.highbeam.com/doc/1G1-264389423.html | date=May 9, 2011|accessdate=May 26, 2013 (from [[HighBeam Research]])|publisher=Centaur Communications}}</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.

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 Mn12O12, 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 Mn12O12 complex and analogous complexes remain the canonical single molecule magnet with a 50 cm−1 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−1); D can be negative but only its [[absolute value]] is considered in the equation. The barrier U is generally reported in cm−1 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 "Mn(OAc)12" cluster the spin state is 10 (involving 20 unpaired electrons) and D = -0.5 cm−1 resulting in a barrier of 50 cm−1 (equivalent to 60 [[Kelvin|K]]).<ref name=Barco>{{cite journal|last1=del Barco|first1=E.|last2=Kent|first2=A. D.|last3=Hill|first3=S.|last4=North|first4=J. M.|last5=Dalal|first5=N. S.|last6=Rumberger|first6=E. M.|last7=Hendrickson|first7=D. N.|last8=Chakov|first8=N.|last9=Christou|first9=G.|title=Magnetic Quantum Tunneling in the Single-Molecule Magnet Mn12-Acetate|journal=Journal of Low Temperature Physics|volume=140|issue=1/2|pages=119|year=2005|doi=10.1007/s10909-005-6016-3|bibcode = 2005JLTP..140..119B }}</ref>

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 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>{{cite journal|last1=Muntó|first1=María|last2=Gómez-Segura|first2=Jordi|last3=Campo|first3=Javier|last4=Nakano|first4=Motohiro|last5=Ventosa|first5=Nora|last6=Ruiz-Molina|first6=Daniel|last7=Veciana|first7=Jaume|title=Controlled crystallization of Mn12 single-molecule magnets by compressed CO2 and its influence on the magnetization relaxation|journal=Journal of Materials Chemistry|volume=16|pages=2612|year=2006|doi=10.1039/b603497g|issue=26}}</ref>

==See also==
* [[Ferromagnetism]]
* [[Single-molecule experiment]]
* [[magnetic anisotropy]]

==References==
{{Reflist|2}}

==External links==
* [http://www.eimm.eu/ European Institute of Molecular Magnetism EIMM]
* [http://www.unizar.es/magmanet/magmanet-eu/ MAGMANet (Molecular Approach to Nanomagnets and Multifunctional Materials)], a [[Network of centres of Excellence]], coordinated by the [[INSTM]] – Consorzio Interuniversitario Nazionale per la Scienza e la Tecnologia dei Materiali
* [http://www.molmag.de/ Molecular Magnetism Web], Jürgen Schnack

{{magnetic states}}

[[Category:Types of magnets]]
[[Category:Condensed matter physics]]
[[Category:Quantum magnetism]]

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High resolution electron energy loss spectroscopy