Magnetism in organic materials

Magnetism in organic materials

ELSEVIER Synthetic Metals 71 (1995) 1781-1784 Magnetism in organic materials Elmar Dormann Physikalisches Institut, Universit&t Karlsruhe (T.H.), D-...

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ELSEVIER

Synthetic Metals 71 (1995) 1781-1784

Magnetism in organic materials Elmar Dormann Physikalisches Institut, Universit&t Karlsruhe (T.H.), D-76 128 Karlsruhe, Germany Abstract Neutral mono-, di- and tri-radical solids and radical cation salts were used to study the variation of intra- and inter-molecular exchange interaction with molecular packing and systematic molecular modifications. Cooperative magnetic behaviour related to localized manganese moments was characterized for manganese oxides as a composite in organic matrix or charge transfer salts of tetrasubstituted porphinato manganese. Magnetic order was also observed in 1,3-dithienyl-5-phenyl-verdazyl electropolymerized with hexafluorophosphate as counterion.

1. INTRODUCTION

(2a)

Tailoring magnetic interactions of organic solids is currently an area of intense research activity. The cooperative efforts of chemists and physicists resulted in the preparation and analysis of a variety of new materials. Important steps forward have been achieved in recent years both in the chemical preparation of new promising materials and in elaborating the theoretical models for possible routes to ferromagnetic interactions [ 1, 21. Progress is speeded up by the imagination of applying the new materials in areas like magnetic shielding, electronic components or storage media. Every research team tries to present new results in the most promising way, naturally. The choice of the appropriate units plays a critical role in this respect. Therefore the relevant relations and units are compiled in section 2. In section 3 some recent results are presented, of necessity in a somewhat arbitrary selection.

2. RELEVANT RELATIONS AND UNITS In spite of all opposing efforts, the units most generally in use for the characterization of the properties of magnetic materials are still the cgs-Gaussian units or emu. Here the magnetic flux density or magnetic induction B (in units of Gauss, G) is given as B=H+4nM

(1)

where M is the magnetization (in units of Gauss), and the magnetic field strength H is frequently also given in “Gauss” instead of the correct Oersted (Oe) units [3]. Since this unit system is non-rational, the factor 4n in eq. 1 has to be taken into account. This has to be considered, if the magnetic susceptibility x is discussed, because x=MIH

(2)

is only a dimensionless number, thus the unit system is not discernible. This is important, because, if instead the SI units are adopted, the magnetic susceptibility x 0379-677!U95/$09.50 SSDI

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is again a dimensionless quantity, but its value is larger by a factor of 47c, because the magnetization M or the magnetic polarization J are introduced as BS’=p,(H+M)=p,H+J

0)

H and M have the SI-units A m-l, J and B are measured in Tesla (T) and ~1,= 47~.10m7T/(Am-‘) is the vacuum permeability. In order to prevent mistakes. the units emu/cm3 for the magnetic susceptibility 2 are occasionally given in the cgsGaussian unit system, which is correct, because the electromagnetic unit for susceptibilities is cm3, and which is helpful, because it identifies the unit system in use For the magnetic flux density or induction I3 the conversion between both unit systems is easy, because 1 T in SI units corresponds to 10 kG in cgs-Gaussian units. The conversion of the magnetic field strength H is less convenient, because 1 Oe corresponds to 103/4n A m.‘. The magnetization M represents the vectorial sum of the individual dipole moments pk per unit volume

MT1

Y c V k(V)

(3)

The magnetic dipole moment m of a sample is usually the quantity that is measured, i.e. m=M.V

(4)

with G,cm3 as cgs-Gaussian units or, again, emu. It should be considered that the volume-related quantity M or x is the relevant characterization for applications, but the mass- or mol-related quantities M, and Mm, or x, and x,, are more conveniently accessible in experiments on powder samples. Here the cgs-Gaussian units emu/g = G,cm3/g and emu/m01 = G.cm3/mol for the magnetizations or emu/g = cm3/g and emu/m01 = cm3/mol for the susceptibilities have to be used.

E. Dormann I Synthetic Metals 71 (1995) 1781-1784

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For magnetically ordered materials, the hysteresis loop is of special importance. A typical hysteresis loop is therefore presented in figure 1 in order to recall the definitions of the relevant quantities [3,4].

-J

1 i -J /

Figure 1. Typical hysteresis loop including virgin curve (a) and the derived quantities energy product 1B.H 1 (b) and differential susceptibility x (c) for polycrystalline magnetic materials. Coercitive field H,, remanent (B,) and saturation or spontaneous magnetic induction (B,), initial (xi) and maximal (x,,) differential susceptibility x are defined in the figure.

01

M,,/kGi

1

ztE! ,,,r-

10 I

10' /

i

r*

20r soft 1 lC

:

j

magnettc materlals

‘4

**

I

Ll 102

I

III 103

IO‘ HJOe

J :o53

Figure 2. Classification of magnetic materials. Left and lower axes cgs-Gaussian units, upper and right axes SI units.

2.1. Classification of magnetic materials Magnetic materials can be classified with the help of figure 2. The relevant quantities have been defined in equations 1 to 3 and in figure 1. Materials representative for the right or left border or the upper right or left comers of the property-field (figure 2) have still to be found [3,4]. For “permanent magnet” applications in an electromotor or generator, for magnetic bearings or other applications in mechanics magnetically hard materials are required, that should have as large as possible values of the coercitive field Hc and remanent magnetization M, (or induction B,). An essential criterion for such applications is the maximal value of the

product (B.H 1 that can be realized with the hysteresis loop in the upper let? quadrant (figure la and b). A material residing on the upper right comer in figure 2 would be most appropriate, but was not yet realized. Typical data for Br I’H, / 1B.H 1may are 0.21 T / 1.4~10~ A/m / 6.4 kJ,m-3 for Ba-ferrite, 0.84 T / 9.2~10~ A/m / 26 kJ.m” for ALNICO, an alloy of lron with Aluminum, Nickel and Cobalt, and 0.75 T / 7.2~10~ A/m / 110 kJ.m-3 for RECOMA, a Rare-Earth-Cobalt compound [4] (all in SI units). Cheaper Neodymium-Iron-Bor compounds are partly replacing the rather expensive Rare-Earth-Cobalt magnets. For magnetic shielding, transformers or electromagnets magnetically soft materials are needed. The material should combine as small as possible a coercitive field strength with as large as possible values of B,. Furthermore, the area enclosed by the hysteresis loop should be as small as possible, minimizing losses accompaniing magnetization inversion (Warburg’s law). The magnetic susceptibility (figure lc) should be as large as possible, the material would appear on the upper left comer of figure 2, which is again not yet reached. Typical data (SI) are x, = 104, x,, = 105, Hc = 0.4 A/m and Bs = 0.8 T for Supermalloy [4]. For ac and especially hf applications, eddy current losses have to be small, requiring isolated sheets (ac) or insulators. The known soft-magnetic materials are usually rich in Nickel, mechanically sensitive, occasionally amorphous (glass like, e.g. Metglas, xmax = 106), and their properties are deteriorated by mechanical deformation. Semi-hard magnetic materials with properties intermediate between soft and hard, are also useful for many applications, like switches or relays, where the magnetization has to be invertible but otherwise stable.

2.2. The pros and cons of organic magnets What are realistic goals for organic magnets considering the classification scheme of figure 2? Let us start with the evident limitations. It is unrealistic to expect record values of the saturation or remanent magnetization M. This follows from eq. 3. On a Dysprosium ion a dipole moment p = 10 /.tn/Dy (cl,: Bohr magneton) and on a Fe atom in ferromagnetic iron a moment p = 2.22 pn/Fe resides in the ionic or atomic volume of about 10 - 25 A3 [3], whereas - considering an other extreme - a moment of p(S=l/2) = 1 pB is spread over the huge volume of 410 A3 in a stable organic free radical like Triphenylverdazyl [S]. The larger orbital degeneracy of 7 or 5 of the atomic 4f- or 3d-orbitals compared to the p-orbitals of organic molecules (and the respective action of Hund’s rule) is one of the origins of this difference. In order to prevent chemical reactions of the unpaired electrons, the radical electron density and thus the spin density of eq. 3 - can not be raised to the high values of inner shells of transition elements, This restricts the achievable saturation magnetization for purely organic magnets probably to values of 0.1 T or below (or the order of 100 G, respectively). The mdar magnetization M,, is not reduced to the same extent, evidently, for an organic magnet, but - even if being of undenied fundamental importance - it is not the relevant quantity for applications. Furthermore, it is unrealistic to expect especially large values of the coercitive field Hc for organic magnets. To get large values of H,, besides the shape anisotropy, which is generally useable, the contribution of the crystal anisotropy is required that can be optimized e.g. by incorporation of Cobalt and Rare Earth ions in alloys or intermetallic compounds. The main ingredient is the strength of spin-orbit coupling, which is

E. Dormann / Synthetic Metals 71 (1995) 1781-l 784

strong for heavy and weak for light atoms. If special tricks in molecular architecture are not used (like introduction of heavy atoms), only coercitive fields up to some lo4 A/m can be realized for organic magnets. But this restriction excludes not yet anyone of the three types of applications mentioned in figure 2. The weak spin-orbit coupling of the magnetic moments in organic ferromagnets is, in contrast, a decisive advantage for the realization of soft-magnetic material properties. The spinorbit interaction energy of a single atomic electron is proportional to Z4-n3 l2 (Z: nuclear charge, n: main quantum number, 1: orbital quantum number). This guarantees for organic materials built up from light elements extremely low values of the coercitive field H, and thus materials that can reach the farleft side of the property-field in figure 2. Accordingly large values of the magnetic susceptibility may be found. Especially for the application of high-permeability materials to shielding of quasi-static magnetic fields, the reduced MS-value of organic magnets is no serious limitation. Thus there is only one serious problem with organic ferromagnets: appropriate materials still have to be synthesized and characterized!

3. EXAMPLES 3.1. Substituted triphenylverdazyls The well-known free radical 1,3,5-Triphenylverdazyl (TPV) [6] has been substituted in order to modify molecular packing and thus the strength and sign of intra-stack and interstack exchange interaction [5, 7-101. About 80% of the radical electron’s spin density resides on the four nitrogen atoms of the central heterocyclic ring (4 N, 2 C atoms), but spin density distribution on the phenyl substituents and their overlap with the verdazyl units of neighbouring TPV molecules is of decisive importance for the intermolecular exchange integrals. The influence of the molecular modifications on the spin density distribution was monitored by electron spin resonance (ESR) (14N (I = 1) hype&e structure [9]) and ‘H nuclear magnetic resonance (NMR) (n-electron-proton coupling constants [S]) in dilute solution. 1,3,5,6_Tetraphenylverdazyl (Ph-TPV) grown from methanol solution and crystallized in the orthorhombic space group Pnma was found to exhibit especially strong antiferromagnetic quasi-onedimensional (1 d) exchange interaction. The magnetic properties were characterized by static magnetic susceptibility measurements (e.g. spin concentration, paramagnetic Curie temperature, susceptibility maximum T,, = 20 K), solid state ESR (e.g. temperature dependence of line width, intensity, anisotropy of resonance field), specific heat measurement (e.g. TN) and model calculations [5, 9, lo]. The additional phenyl ring of Ph-TPV results in an increase by a factor of three for the antiferromagnetic intrastack exchange integral compared to TPV (J,,/k, = -16 K). The PhTPV molecule with four non-coplanar phenyl rings, only light atoms and a largely delocalized spin density represents a model system for exchange coupled spins S = 112 with very weak spinorbit coupling. Accordingly the single crystals of Ph-TPV behave like typical quasi-l d Heisenberg antiferromagnets: the temperature dependence of the magnetic susceptibility follows the Banner-Fisher model [ 1 l] accurately. Three-dimensional (3d) exchange interaction is weak, as can be concluded from the low Neel temperature of TN = 0.92 K determined from specific heat measurements, JInterstack= 3x10” .I,, [IO]. One-dimensionality of exchange interaction is thus more pronounced for Ph-TPV than in known inorganic Copper Heisenberg antiferromagnets.

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3.2. Triradical systems The covalent bonds of di- and tri-radical compounds guarantee a better-defined arrangement of the individual radical units than the sole action of van der Waals forces. If use is made of the alternating n-electron spin densities along conjugated bonds of the molecular framework, high-spin ground states can be expected for appropriate undistorted high symmetry arrangements of multiradicals. 1,3,5-tri-6’( 1,3,5-triphenylverdazyl)-benzene (tri-TPVbenzene) was synthesized and characterized in order to determine the influence of the molecular packing in the solid state on the molecular magnetic properties - especially the doublet-quartet separation [ 12, 131. The spin density distribution on the TPV substituents, derived from the isotropic hype&e coupling constants of the protons (‘H NMR in solution) is similar to that of isolated TPV molecules. The spin density on the central benzene unit is negligible according to the NMR analysis. This corresponds to the weak zero-field splitting (]D/gpn] = 45 G) and small doublet-quartet splitting (AE&kn = 48 K) derived from ESR of tri-TPV-benzene in liquid or frozen dilute solution. In solution as well as in the crystalline state tri-TPVbenzene has a doublet ground state and a low-lying quartet state that is thermally populated. The linewidth of about 1 G indicates efficient exchange narrowing of the ESR line in the concentrated solid, but no magnetic order was observed above 2 K. Interestingly enough, the activation energy is smaller by a factor of two in the crystalline state compared to the frozen solution (i.e. AEa_&n = 20 K) as was derived from static susceptibility measurements. It is important to note that the small distortions accompaniing molecular packing in the crystalline state result in easily measurable differences in intramolecular doublet-quartet splitting [12]. This points to one of the difficulties encountered in the attempt of systematic tailoring of magnetic properties of organic materials

3.3. Perylene radical cation salts Depending on molecular packing, Pauli-paramagnetic quasi-onedimensional organic conductors or magnetic semiconductors with high concentration of localized magnetic moments can be realized for radical cation salts based on aromatic hydrocarbons like perylene and diamagnetic inorganic anions. The small ESR linewidth of only about 1 G observed despite the comparatively high spin concentration of one spin S = l/2 per formula unit in single crystals of hexaperylene hexafluorophosphate proves the substantial exchange interaction of the radical spins [ 141. Nevertheless, the asymptotic Curie temperature determined by static susceptibility measurements resides below 1 K. In order to achieve larger magnetization and ordering temperature, paramagnetic counter ions and molecular units incorporating heteroatoms have to be used.

3.4. Magnetic Manganese oxides in organic matrix Composite materials allow the combination of attractive properties of the respective components. Recently we showed that magnetic manganese oxides can be realized as composite in organic matrices [ 13, 1.51. Spontaneous magnetization reaching up to 10 emu/g, hysteresis loops with coercitive field strength of about 40 Oe at 5 K and ordering temperatures of about 40 K were realized [15]. These samples were characterized by combination of chemical elementary analysis, wide angle X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, selected area

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diffraction and electron energy loss spectroscopy. The magnetic properties were analysed by SQUID magnetometry, alternating current susceptibility, specific heat measurement and electron spin resonance. On the basis of this detailed analysis, it is tempting to speculate that several of the “organic magnets” with comparatively low volume- or weight-related magnetization reported in recent years reached their magnetic ordering with help of transition metal oxides or other impurity phases, too.

3.5. Charge transfer salts with TCNE and TCNQ Charge transfer (CT) salts based on metalloporphyrins like the tetracyanoethenide of meso-tetraphenylprophinato manganese (RI) ([MnTPP]-[TCNE].2PhMe) were first studied by Miller, Epstein and coworkers [16]. An ordering temperature of about 18 K was estimated for this system consisting of onedimensional chains with alternating (S = 2>donator and (S = l/2)-acceptor units [17]. The related and modified systems meso-(tetratolylprophinato) manganese (III) - tetracyanoquinodimethanide (abbr. [MnTTP]-[TCNQ]) and [MnTTP]-[TCNE] show different types of cooperative magnetic behaviour in the low-temperature range. The quantitative analysis of the magnetization data for polycrystalline samples of these CT salts indicates that in both systems ferrimagnetic correlations in the onedimensional chains are established before three-dimensional magnetic ordering takes place [ 181. While parallel coupling of the moments of the ferrimagnetic chains was concluded for the TCNE salts, antiferromagnetic ordering at 7 K with characteristic spin-flop behaviour in the dependence on external magnetic field (e.g. at 500 Oe for T = 5 K) was observed for the TCNQ salt, Thus again minor modifications result in drastic changes of the macroscopic magnetic properties.

3.6. Magnetic order in Poly-Dithienylphenylverdazyl The carbon and nitrogen p-like orbitals of TPV and its derivatives mentioned in section 3.1. give preferentially rise to a quasi-onedimensional intermolecular admixture of the wave functions and Id-antiferromagnetic exchange. Since it may be assumed, that the d-like wave functions accessible by the incorporation of sulphur into the molecular units support a two- or threedimensional overlap and exchange interaction, phenyl units of TPV were replaced by thiophene units. 1,3-dithienyl-Sphenylverdazyl (DTPV) was synthesized and electropolymerized with hexafluorophosphate as the counter anion. Magnetic characterization of the resulting material indicated a large concentration of free spins exceeding one spin S = l/2 per formula unit substantially. Magnetic order for portion of the sample corresponding to about 5% of the low-temperature magnetic moment was detected at 10 K and below via the respective hysteresis loop with Hc = 250 Oe at 5 K [ 191. The content of 3d transition element impurities, detected by ESR and static magnetic susceptibility analysis, could not explain this magnetically ordering portion of the samples. A systematic optimization of the ordering portion of the poly-DTPV samples seems promising.

3.7. Concluding remark It seems very important in the study of magnetism in organic materials, that the influence of - frequently ferro- or fenimagnetic - transition metal impurity phases is taken into

account to avoid the typical pitfalls frequently encountered in this field of research. Thus a close and critical cooperation between physicists and chemists seems a prerequisite of relevant contributions to the field. Since small variations in molecular packing, occasionally governed by the inclusion of solvent molecules in the solid state, have dramatic influence on the threedimensional magnetic interactions, the search for organic materials with interesting magnetic properties requires trial-and-error attempts and is time-consuming!

ACKNOWLEDGMENT The author thanks H. Naarmann, R. Gompper and their coworkers for their cooperation and B. Gotschy, B. Pilawa and H. Winter for their contributions. This work was financially supported by the Bundesminister fur Forschung und Technologie as part of project 03 M 4067 6.

REFERENCES

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