THE DESIGN STRATEGIES ROUND TABLE DISCUSSION

Olivier KAHNa, David N. HENDRICKSONb, Hiizu IWAMURAc and JaumeVECIANAo.a: Laboratoire de Chimie Inorganique, URA nO 420, Universite de Paris-Sud, 91405Orsay.b : Department of Chemistry, University ofCalifornia at San Diego, La Jolla, California,U.S.A.c : Department ofChemistry, University ofTokyo, Hongo, Tokyo 113, Japan.d: Centro de Investigation y Desarrollo, C. Jordi Girona, 18-26,08034 Barcelona, Spain.

The first part of the discussion has been devoted to the various strategieswhich have already been explored to design high-spin molecules and molecular-basedferromagnets. Those strategies may be sumed up as follows:

The to.pological degeneracy. This approach proved to be very efficient, inparticular in polycarbene chemistry. A S = 5 pentacarbene has been reported during theConference. The main strength of this strategy is that it does not require symmetryconditions which are difficult to control. However, so far it did not lead to bulkferromagnetism. The chemical stabilization of the polycarbenes also remains a difficultproblem. This topological degenaracy approach could be extended to non-alternantsystems as well as to heterocycles.

TheMcConnell mechanism I (1961) based on the interaction between a highpositive spin density within a molecule and a weak negative spin density within aneighboring molecule. Several participants suggested that this approach might well be themost efficient to get intermolecular ferromagnetic interactions, i.e. to coupleferromagnetically high-spin molecules within the crystal lattice. This approach has beensuccessfully applied in double-decked dicarbenes and in lattices consisting ofantiferromagnetically coupledMn(ll)Cu(II) heterobinuclear units.

The McConnell mechanism II (1967) based on the interaction between theground state and an high-spin charge transfer excited state, for instance in donor-acceptorstacked compounds. This mechanism has been invoked for rationalizing theferromagnetism observed in Fe(Me5C5)2(TCNE) and Mn(C5MeS)2(TCNQ). Otherparticipants expressed their doubts aoout tois mechanism. The quesbon clearly remainsopen and more theoretical and experimental studies will be necessary to settle thisproblem.

The ortho~onality of the magnetic orbitals. This approach has been rathersuccessful at the scale of discrete units both in organic and inorganic chemistry, but isdifficult to extend along the three directions of a crystal lattice; this kind of orthogonalitymay be distroyed by small structural changes. In other respects, it has been stressed that afew organic diradicals did not exhibit a triplet ground state in spite of the orthogonality ofthe orbitals.

The polaronic awroach. This rather exotic strategy deserves to be tested. Forthat a prototype has to be prepared and studied.

The ferrimaenetic approach. In the last few years this strategy has led to

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several molecular-based compounds exhibiting a spontaneous magnetization in thetemperature range 4 - 30 K. The interacting magnetic centers were eitherMn(ll) and Cu(ll)ions, or Mn(ll) ions and nitronyl nitroxide organic radicals. So far, this approach has beenused with one-dimensional compounds. This is certainly a limitation. Indeed, the criticaltemperature is related to both intra- and interehain interactions. The latter ones are small bydefinition in chain compounds.

The double exchan&e, or the synergistic effect of electron transfer andmagnetic interaction. There is no report yet ofbulk molecular ferromagnetism attributed todouble exchange. However, this strategy seems to be appealing, in particular because thestabilization of the ferromagnetic state is expected to be one order of magnitude morepronounced than for the ferromagnetic interaction between localized electrons.

The buildin&-block awroach consisting to assemble high-spin moleculeswithin the erystallattice in such a way that all the molecular spins align in a parallel way.The first step is the design of molecules exhibiting a very high spin in their ground state.Molecules with S = 14 have been reported during the workshop. Future work probablyshould be more focused on preparing high-spin molecules developing connectivity in twoor three directions in order to obtain magnetic orderings at relatively high temperatures.High-spin molecules are also intrinsically interesting. They will prove to have fascinatingelectronic structures. Maybe also their reactivity might be appealing.

The last but not the least approach is serendipity, or the fisherman approach.Whatever the theory says, it may be worth trying. The area is quite new and unknownphenomena as well as unusual compounds may be expected.

In a second time, a short discussion took place concerning the new featureswhich may be found for molecular-based magnetic materials. The various ideas whichhave been put forward are the following :

The QPtical switchin&. Laser light could be used to reversibly change part ofthe molecular structure and this could impact on themagnetic properties of the material.

The resonance exchange. In analogy with long-distance exchangeinteractions propagated by electronic bands in many classical materials, molecular-basedsystems could be designed with electronically delocalized linkages.

The onset of dynamics. For molecular-based materials it can be anticipatedthat at various temperatures there could be the onset of dynamics associated with parts ofthe molecules. The onset of tumbling of the anion or cation, or of motion within theligands could influence the ordering temperature.

The spin transition for some of the metal ions involved in the magneticsystem. The spin transition may dramatically affect the ordering temperature. A newphysics may be associated with the synergy between spin transition and magneticinteraction, and dynamical molecular devices may be designed.

As a conclusion of this discussion, one of the participants emphasized thatthe difficulty in this field arose from the fact that it was necessary to control not only the

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primary StruCture, but also the secondary and the ternary ones. The molecular engineeringis still a difficult art; the crystal engineering in molecular chemistry is its first infancy. Tillnow, the chemists involved in the field have been more skilled to design one- than two- orthree-dimensional compounds. Since the ferromagnetism is a three-dimensional property,one cannot expect high ordering temperatures with chain compounds. Maybe, the mainefforts in the near future should be made along the synthesis of three-dimensionalmolecular magnetic compounds.

ROUND TABLE REPORT

PHYSICAL INVESTIGATION ON MAGNETIC MOLECULARMATERIALS

F. Palacio (zaragoza), R.L. Carlin (Chicago), A.J. van Duyneveldt (Leiden),AJ. Epstein (Columbus), H. Giidel (Bern), C.P. Landee (Worcester) and

J. Schweizer (Grenoble).

Introduction

All the attendants at the Workshop were invited to participate in the Round Table. In orderto have a fruitful discussion on the large variety of physical investigations that can bemadeon magnetic molecular materials (mmm's) it was considered very important to have first agood definition of which mmm's can be considered of scientific interest. In addition, itwas considered of great interest to comment on the problem of characterizingunambiguously a ferromagnet. Therefore, three major topics were proposed to the

audience for discussion:

Are ferromagnets the only mmm's worth looking at?

• Which properties should be investigated to unambiguously characterize a materialas a ferromagnet?What techniques are most useful? Useful caveats.

In the course of the discussion a fourth important topic emerged, giving rise to a very

lively debate:

Should any new physics be expected to come out ofmmm's?

In the following we shall report separately on the conclusions achieved on each of

these topics. At the end of this report we include a short indicative list of references whichwe consider may be useful to orient the newcomer in the jargon and pitfalls ofmagnetism.

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Topic 1: Are ferromagnets the only magnetic molecular materials worthlooking at?

This question was answered unanimously in the negative, but with two different points ofview expressed. Those concerned with potential applications voiced the opinion thatuseful materials are characterized by spontaneous net magnetic moments. Whether thesemoments arise from ferromagnetism, ferrimagnetism, weak ferromagnetism orsuperparamagnetism is of little consequence. On the other hand, from the view of basicscience, any mmm is of scientific interest, since

It is of general interest, both basic and technological, to learn better how to makecompounds with expected magnetic properties.Magnetically ordered molecular systems require extended interactions linking themolecules within the crystalline lattice. Since the interactions depend on thestructural packing of the molecules, the process by which they occur is still notwell understood. The engineering of supramolecular structures by assemblingmolecules in a crystal lattice remains more of an art than a well establishedtechnique.

• A better understanding of the superexchange interaction and its propagatingmechanisms in molecular solids is still required.

Topic 2: Should any new physics be expected to come out from magneticmolecular materials?

The fundamental laws of physics (such as thermodynamics) are not expected to undergorevision as a result of the development of mmm's. However, under the principle that oneshould expect the unexpected, we anticipate the discovery of new properties, newmechanisms and new phenomena. These may stimulate the formulation of new theoreticalmodels and yield a better understanding of already known properties and phenomena.

It seems worthwhile citing the example of recent developments in superconductivity.Not long ago only the BCS mechanism was known to induce superconductivity and itserved to explain all known materials. However. in the past decade, three distinct newclasses of superconductors have appeared: the heavy fermion family, organicsuperconductors and the superconducting oxides. While the mechanisms leading tosuperconductivity are still not known, they are certainly not BCS in nature. These newmaterials contain new mechanisms and will required new theoretical models to explain

their properties.Molecular systems contain specific peculiarities which may affect their magnetic

properties. Indeed, properties such as electronic delocalization, spin density andpolarizability, to mention only a few. are expected to affect the mechanism ofsuperexchange magnetic interaction or to favour new types of propagating magnetic

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interactions not possible to find in classicmagnetic materials.

Topic 3: Which properties should be investigated to unambiguouslycharacterize a material as a ferromagnet?

Ferromagnets, as well as weak ferromagnets and ferrimagnets, are magnetically orderedmaterials which at a critical temperature Tc undergo a ttansition to a state with a

spontaneous magnetization. A key property to identify materials exhibiting spontaneousmagnetization is the raising of the initial magnetic susceptibility up to a constant valuewhich depends only of the geometry of the sample. Since all three classes of materialsmentioned above share this behavior, additional experiments are necessary to distinguishbetween them. A measurement of the field dependent magnetization, carried out at atemperature well below Tc' serves this purpose. A great deal of relevant information canbe obtained obtained from ac-susceptibility experiments. Measurements made at zeroexternal magnetic field and, if possible, at several frequencies of the alternating magneticfield are important.The ultimate experiment in the characterization of any magnetic material is the

determination of the magnetic sttucture by neutton diffraction. However, this is acomplicated and expensive experiment which is often beyond the reach of the syntheticchemist since it requires rather large single crystals or deuterated powder samples. Evenassuming the samples are available, a through knowledge of the basic magnetic propertiesof the substance is required before proposing such an experiment.

Topic 4: What techniques are the most useful (with caveats)?

Cooperation between chemists and physicists is usually very fruitful and highlyrecommended in this area of scientific research. It is noted that there is a distinct culturaldifference between the approaches of experimental physicists and synthetic chemistsregarding magnetic characterization. The physicists are prepared to study many propertiesof a given substance (using ac susceptibility, magnetization, specific heat, etc.), while thechemists normally use just one technique as an analytical tool. From either perspective,the following rule of thumb always applies: When working with any instrument, be sureto understand the physical properties being measured.

It is, therefore, important to develop a common language of scientific understandingbetween chemists and physicists and to know how to interest to each other in a specificproblem. Minimum requirements for a material to have physical interest are an as good as

possible sttuctural characterization, reproducibility and, at least, XYs. T and M Ys. HIfexperimental data showing that some interesting phenomena occurs.As the initial analytical tool, magnetometers (Faraday, SQUID, vibrating sample) are

ubiquitous and recommended. They are the simplest magnetic insttuments and provideinformation both about the dc susceptibility and magnetization. When working with

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mmm's possessing spontaneous moments. however. it is worth a reminder that suchmaterials have magnetizations that are non-linear with the magnetic field. Susceptibility

measurements (limM/H as H~ 0) are only valid in the low field limit This limit must bedetermined experimentally and can restrict the applied fields to only a few Oersteds. In allcases. the temperature scale of the instrument should be checked yearly with a reliablecalibrated sensor.The ac susceptometers form an altemative to the magnetometers and tend to be favored

by physicists. Their advantages are their ability to study the response ofmaterials in zeromagnetic field and as a function of frequency. However. both the use of the equipmentand the interpretation of the experimental data are more complicated and require moreexpertise.The ac and dc susceptibility techniques give the highest return of information for the

effort. Other valuable experimental techniques include resonance studies (EPR.ferromagnetic resonance). specific heat measurements (either by adiabatic techniques orthrough the use ofoptical birefringence) and optical and Mlissbauer spectroscopy.Neutron scattering (NS). both elastic (neutron diffraction) and inelastic, has

traditionally been used for the study of magnetic materials. As a general rule. suitablematerials for powder measurements must be hydrogen free or fUlly deuterated; partialdeuteration is not recommended. However, deuteration is not required for neutrondiffraction experiments on single crystals. NS studies require relatively large amounts of

material; thus, about 4-5 cm3of powder or a single crystal of about 1-3 mm3are necessaryfor a typical elastic NS experiment and still larger amounts of material are required forinelastic experiments. Measurements down to 1.5 K are routine, in contrast to X-raydiffraction.

Some caveats.

Small 9 values in a Curie-Weiss fitting may be meaningless unless you have areliable T and H calibration.If anything unusual arises along an experiment. have it reproducible before callingit interesting.Never use metallic spatulae while handling small samples of low magnetic signal.Do not mill using iron balls!.If you reflux in glass. do not be surprised if you find a small amount offerromagnetic material in your sample.Emphasize collaboration between chemists and physicists. It is usually very

useful to send temporarily chemists students to physics laboratories and viceversa.

Orientative references.

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• Physics ofMagnetism, S. Chikazumi, John Wiley & Sons (1964).The Physical Principles ofMagnetism, A.H. Morrish, John Wiley & Sons, NewYork (1965).Magnetism in Solids, D.H. Martin, MIT Press, Cambridge, Mass. (1967).

• Theorie du Magnetisme, A. Herpine, Presses Universitaires de France, Paris(1968).

• Experiments on Simple Magnetic Model Systems, L.J. de Jongh and A.R.Miedema, Taylor and Francis, London (1974).Magneto-Structural Correlations in Exchange Coupled Systems, Ed. by R.D.Willett, D. Gatteschi and O. Kahn, D. Reidel PubI. Co., Dordrecht, C140(1985).Magnetochemisrry, R.L. Carlin, Springer-Verlag (1986).Electron Spin Resonance: Elementary Theory and Practical Applications, J.E.Wertz and J.R. Bolton, McGraw-Hill (1972).EPR of Exchange Coupled Systems, A. Bencini and D. Gatteschi, Springer­Verlag (1990).Neutron Scattering in Chemistry, G.E. Bacon, Butterworths, London (1977).Introduction to the Theory of Thermal Neutron Scattering, G.L. Squires,Cambridge University Press (1978).

MOLECULAR MAGNETIC MATERIALS· APPLICATIONSDISCUSSION

CHRISTOPHER P. LANDEE,a DAVID MElVILlEb AND JOEL S. MILLERCa Department of Physics, Clark University, Worchester, MA 01610-1477U. S. A.; b Vice Rector, Lancashire Polytechnic, Preston, PR1 2TO U. K;and C Central Research and Development, Du Pont, ExperimentalStation-E328, Wilmington, DE 19880-0328 U. S. A.

Abstract. Fundamental scientific insight is the primary goal of theinternational quest for molecular magnets. These materials may also providethe basis for new or improved magnetic applications or devices. Assuming thatthe critical temperature for an organic/molecular/polymeric magnet can beincreased to at least 125 °C. several feasible applications may be considered.They include a bulk magnet, magnetic recording, and magneto-optic recordingas well as other applications. Additionally. new phenomena may expandapplications for magnetic materials.

A discussion aimed at identifying feasible applications for molecularmagnetic materials in a brain-storming ambiance formed the basis of the final'round-table' study group at the NATO ARW. This round-table discussion dealtentirely with enumerating potential uses for a magnet comprised of molecules,as opposed to conventional atom-based magnetic materials. To set the scale ofpotential commercial impact, the present U. S. magnetic materials industry hasannual sales larger than the much more celebrated semiconductor industry.Due to the newness of the field, attempts were not made to prioritize, and moreimportantly, to assess critically the compatibility of the embryonic ideas with thetechnological and business/economic needs. We hope that readers of this'round-table' report will accept this in that spirit. They would be seriouslymisguided if they assumed otherwise.

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To provide a basis for discussion, the anticipated properties of molecularmagnetic materials were identified, Table 1. It was noted that many of theproperties (transparency, solubility, and insulating behavior) are a directconsequence of the nonmetallic nature of the materials. With this background avariety of applications were discussed. Foremost was the direct replacement ofexisting magnets, either as a bulk magnet or for magnetic or magneto-opticrecording. It is important to note that in any conceivable application newmaterials must be chemically stable with time and the critical temperature mustbe sufficiently high (ca 125 OC) to be of practical use. A critical temperature aslow as 77 K might suffice if other properties are such as to justify the magnetsuse at this temperature.

The initial question considered was whether new molecular magnetscould be expected to directly replace present materials, either as bulk magnetsor in magnetic or magneto-optical recording applications. While the saturationmagnetizations, Ms, for molecular/organic based magnets are comparable tometallic magnets on a mole basis, their inherent large molecular weights andlow densities result in smaller saturation magnetizations on either a volume ormass basis. Furthermore, since bulk magnets (such as the Nd2Fe14B orSmCos families of magnets) are sold on an energy product per volume basis, i.e., (BH)maxN (or 4n2Ms2N), basis, Table 2, and it is clear that high energyproducts are unlikely to be achieved, molecular magnets are unlikely tocompare well with existing magnets. Hence, all contributors to the 'round-table'stUdy group concurred that bulk magnets would be an unlikely opportunity formolecular/organic based magnets.

TABLE 1. Anticipated Properties of Molecular Magnetic Materials

Insulating ProcessibilityLow Density SolubilityLow Magnetic Anisotropy Large Polarizabilities of Molecules or IonsTransparent Low Environmental ContaminationOptical Changes Potential for BiocompatibilityLow elastic modulus Photomagnetic EffectModulatelTuning of Properties via Organic Chemistry

Materials with magnetic moments parallel to the plane of a film or a diskform the basis for magnetic tapes and disks. Increased data density, however.requires larger demagnetization fields; thus, materials with larger coercive fieldsare necessary. Alternatively, perpendicular magnetic recording media whereby

397

the magnetic moments lie perpendicular to the plane of the film or disk also canincrease the data density, but materials with the moment in this direction arerequired. Relatively high coercive fields are noted for the few molecularmagnetic materials reported in Table 2 and suggest that molecular/organicbased magnets may be useful as magnetic recording media. More importantly,it may be feasible to design materials with the direction of the magnetic momentperpendicular to the plane of the substrate, creating perpendicular magneticrecording media.

Optical disks with their high data density will to be an increasinglyimportant data storage technology. Such systems (reversible read/writesystems) rely on the magneto-optic effect. Either the Kerr (reflection) or Faraday(transmission) effects can be used to determine the change of rotation of apolarized beam of light. A rotation of polarized light is required and the effect isgreatest when the magnetic moments are perpendicular to the plane of thesubstrate. Magneto-optic effects have yet to be studied for an organic/molecularmagnet, but may lead to improved active recording media for optical disks. Itmight be particularly beneficial to develop a material with a strong magneto­optic response light emitted from a next generation blue laser and therebyincrease the data density.

It was pointed out that a ultra-high density optical storage device with adata density of 1018 bitlcm3 based on the 2-D addressing of 1 Jlm3 bits with lightcould be envisioned. Use of blue light would substantially increase this datadensity.

Unlike most conventional magnets, molecular magnetic materials areunlikely to be metallic. As a consequence of their insulating properties a varietyof optical properties may be expected; some combinations of which may proveto be useful. Examples may be photomagnetic switches and polarized lightmanipulation in integrated optical devices.

New phenomena and mechanisms for cooperative magnetic phenomenamay emerge in addition to the magnetic/magneto-optical recording mediadiscussed above. Potentially feasible areas include new colloidal dispersionsof highly magnetic materials ('ferrofluids' and magnetic inks), thin film andlayered (2-D) langmuir-Blodgett (lB) based thin film or multilayer magnets,magnetostrictive sensors, microwave materials, magnetic bubbles, and softmagnetic materials with low coercive fields for ac motors, generators, andtransformers. Finally, biocompatibility may lead to several potential applicationsthat include magnetic imaging and transducers for medical implants.

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