Solid State Communications 152 (2012) 1945–1950

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Solid State Communications

0038-10

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journal homepage: www.elsevier.com/locate/ssc

Comparative study of magnetic behaviour in three classic molecular magnets

Tanmoy Chakraborty, Tamal K. Sen, Harkirat Singh, Diptaranjan Das,Swadhin K. Mandal, Chiranjib Mitra n

Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur Campus, PO: BCKV Campus Main Office, Mohanpur 741252, Nadia, West Bengal, India

a r t i c l e i n f o

Article history:

Received 17 April 2012

Received in revised form

6 July 2012

Accepted 12 August 2012

by E.V. Sampathkumaranmagnetic susceptibility measurements and isothermal magnetization measurements (as a function of

Available online 30 August 2012

Keywords:

A. Molecular magnets

D. Antiferromagnetism

D. Paramagnetism

98/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.ssc.2012.08.009

esponding author.

ail address: chiranjib@iiserkol.ac.in (C. Mitra)

a b s t r a c t

We have studied the magnetic properties of three phenalenyl based organic neutral radicals. The first

one is a Heisenberg chain antiferromagnet with one unpaired spin per molecule; second one is a

diamagnetic, exhibiting a diamagnetic to paramagnetic phase transition at high temperature; the third

one comprises of free neutral radicals and shows paramagnetic behaviour. Temperature dependent

magnetic field) were performed on all the three systems. In the case of the antiferromagnetic system,

temperature dependent susceptibility and magnetization isotherms were fitted to the Bonner Fisher

model. In the case of second system the diamagnetic to paramagnetic phase transition is investigated

by performing isothermal magnetization measurements in the two different phases. The diamagnetic to

paramagnetic phase transition seems to be of first order in nature.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The study of magnetism is mostly concentrated on the materi-als which are inorganic in nature and atom-based. But someexciting and interesting properties lie in the study of physics andchemistry of molecular magnets which are chemically a lot morecomplicated than inorganic magnets [1]. These relatively newclass of magnets provide a wide range of applications in manyareas of science and technology. Molecular magnets are primarilyorganic in nature. Magnetism in a molecular magnet deals withisolated molecules or assemblies of molecules with one or moremagnetic centre in a single molecule. As a result, the magneticbuilding block for a molecular magnet is molecules, instead of anatom. Intramolecular forces in these systems dominate over theintermolecular forces. Because of the fact that the intermolecularforces are non-covalent (hydrogen bondings, Van der Wallsinteractions, donor–acceptor charge transfer etc.) in nature. Con-sequently the crystals are relatively softer than the inorganic oneswhere ionic cores dominate. These weak intermolecular forceslead to sometimes special optical and magnetic properties. Thephysical properties of molecular magnets are determined by itscrystallographic and electronic structure [2]. One importantreason of studying molecular magnet is its extraordinary tun-ability. The flexibility of Tailor made chemical structures allowsone to modulate and fine tune its solid state packing interactionstructure and hence its physical properties. This way one can

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design one’s own model system with desired magnetic properties.The fascinating features of these of systems have drawn theattention of many researchers.

In this work we have chosen three molecular magnets whichare composed of phenalenyl-based neutral radicals [3–5]. Threebenzene rings fuse in a triangular fashion to form one phenalenylbasis. Presently we have focussed on different electronic spinstructures and diversity in magnetic properties of these threeradicals with same phenalenyl basis. Due to interesting spinstructure, tunability and higher spin-relaxation time, these delo-calised ply-based neutral radicals have tremendous impact incontext of quantum information processing and molecular spin-tronics [6]. These molecular magnetic system could have verypromising applications as entangling channel, which can be usedas quantum networks connecting to quantum gates. These neutralorganic molecules form molecular crystals with single molecularspecies that function like a mono-atomic metal [7,8]. This meansthat radicals (a radical is a molecule with unpaired electron)arranged in arrays in a molecular crystal is equivalent to theatoms in a metal. That is how a lattice is formed where spins arearranged in a periodic manner. These unpaired electrons serve asthe charge carriers and orbital overlap between the radicals giverise to quarter-filled energy bands, which results in the systems tobehave like a Mott insulator.

Out of these three materials the first system we considered isspiro-bis (1,9-disubstituted-phenalenyl) boron neutral radical(no. 1 in Fig. 1(a)), which henceforth will be referred to as thefirst system. Its molecular structure is shown in Fig. 1(a) and isbased on the diaminophenalenyl system [3]. It is known from themagnetic susceptibility measurements that these radicals exist as

Fig. 1. (a) Molecular drawings of (1) first (2) second and (3) third systems drawn by the software CHEMDRAW. (b) Temperature dependent magnetic susceptibility of the

first system. Open circles represent the experimental data and the fitted curve (Bonner–Fisher model) is shown by the solid red line. (c) Experimental data of

magnetization collected at T¼17.5 K for the first system (open circles) as a function of magnetic field. It also shows the fit to the theoretical curve (solid red line) derived

using the Heisenberg linear chain model. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

T. Chakraborty et al. / Solid State Communications 152 (2012) 1945–19501946

isolated free radicals with one spin per molecule. Earlier study hasshown [3] that for this particular system the unpaired spinsinteract with other neighbouring spins along one particulardirection by isotropic Heisenberg interaction. These spins arearranged in a chain like structure where intrachain overlapdominates over the interchain interactions. Therefore these sys-tems behave like a spin chain and can be modelled by aHeisenberg Hamiltonian with nearest neighbour coupling. Theinteraction between one spin and its next nearest neighbour is soweak that it can be neglected. The exchange interaction in thissystem is antiferromagnetic. The unpaired electron present in themolecule mostly resides in the nonbonding orbital of the phena-lenyl (the nonbonding orbital of the phenalenyl becomes theLUMO of the molecule before chemical reduction which actuallyaccepts the free electron during the chemical reduction) carriesthe free electron and two such adjacent molecules can interact

antiferromagnetically [9]. The general form of spin chain Hamil-tonian with nearest neighbour Heisenberg interaction can bewritten as

H¼ 2JX

i

½aSzi Sz

iþ1þbðSxi Sx

iþ1þSyi Sy

iþ1Þ� ð1Þ

where J is the exchange integral and Sx, Sy, Sz are the componentsof the site spin S along x, y and z direction, respectively. Whena¼b¼1 the Hamiltonian takes the form of isotropic Heisenbergmodel. We have studied temperature (T) dependent magneticsusceptibility (w) and have fitted the w vs. T data with Bonner andFisher model [10] (see Fig. 1(b)). The value of exchange integralobtained is �16.6 K which is close to the value reported in theprevious literature [3]. We have also obtained magnetizationisotherms at various temperatures where we studied the varia-tion of magnetization as a function of magnetic field. Bonner and

T. Chakraborty et al. / Solid State Communications 152 (2012) 1945–1950 1947

Fisher carried out calculation on isotropically interacting spinchain systems containing from 3 to 11 spins and extrapolated itfor infinite length chain with a good agreement [10]. We havegenerated numerical data for magnetization with varying mag-netic field for 10 spin chain using Matlab. The theoretical curvesare in very good agreement with the experimental data withinexperimental error.

The second system, that we have studied is butyl substitutedN,O-donor based spiro-biphenalenyl radical (no. 2 in Fig. 1(a)).In this system one can clearly see a phase transition from adiamagnetic to a paramagnetic phase in magnetic susceptibilityvs. temperature data [4]. This transition temperature is alsoassociated with the change in conductivity of the system. Wehave shown magnetization isotherms in these two differentphases to bring out the distinct nature of the paramagnetic anddiamagnetic phases. This system has already shown magneto-optical-electronic bistability with variation of temperature [9].This interesting property of this material promises a potentialapplication in molecular spintronics devices. The structure of thesystem is shown in Fig. 1(a).

The third system we have attempted is N,O-donor based hexylsubstituted spiro-biphenalenyl radical (no. 3 in Fig. 1(a)). Thissystem consists of monomeric radicals with each moleculecarrying a spin. Therefore they behave like free spins to exhibitparamagnetic Curie behaviour [5]. We have measured the mag-netic susceptibility as a function of temperature. Our magneticsusceptibility data also captured the paramagnetic behaviourwith some difference [11] from the existing literature [5] and isreported in next section.

2. Materials and methods

We have synthesised and crystallised the first, second and thethird samples in a single crystalline form as mentioned in thereference [3], [4], and [5], respectively. The generation of radicaland crystallization were carried out inside a N2 filled Gloveboxusing dry solvents. Subsequently we have measured magneticproperties of each of the three samples in a Quantum DesignSQUID magnetometer.

For the first sample we have studied magnetic susceptibility ina temperature range of 2 K to 300 K. We have studied magnetiza-tion isotherms with varying magnetic field (H) from 0 to 7 T. Thismeasurement was carried out at different temperatures from 2 Kto 30 K. For the second sample, w vs. T data were taken from 2 K to360 K as we wanted to capture the diamagnetic to paramagnetictransition around 322 K and M vs. H data was taken at 300 K and335 K. For the third sample only susceptibility measurement wasdone from 6 K to 60 K.

3. Results and discussion

We have performed zero field magnetic susceptibility measure-ment on the first system in a temperature range of 2 K to 300 K (seeFig. 1(b)). Earlier it has been shown that for high temperaturesgreater than 35 K, the magnetic susceptibility data can be describedby Curie–Weiss behaviour w¼C/(T�Y) with Weiss constantY¼�14 K and Curie constant C¼and 0.34 K emu/mol [3]. TheCurie constants are close to 0.375 K emu/mol, the value expectedfor a neutral radical with one unpaired spin per molecule. This isalso supported by the fact that the molecule does not pack withinvan der Waals radii of carbon atoms indicating non-interacting spinsat least at higher temperature. However, the low-temperature datashow significant deviations from Curie–Weiss behaviour due toantiferromagnetic coupling of the unpaired spins, with a maxima in

the w(T) curves at TMAX¼23 K. For spin systems with isotropicHeisenberg coupling, Bonner and Fisher have derived zero fieldantiferromagnetic susceptibility for 3 to 11 spins and extrapolatedfor an infinite number of spins with a good agreement [10]. Hall [12]efficiently fitted the numerically evaluated data obtained byBonner–Fisher to the following equation

w�Ng2m2

B

KBT

0:25þ0:14995Xþ0:30094X2

1:0þ1:9862Xþ0:68854X2þ6:0626X3

ð2Þ

where X¼ J/KBT and the other symbols have their usual meaning. Wehave used this equation to get a reasonably good fit with our zerofield magnetic susceptibility data. In this fitting we have used theintrachain exchange coupling J as a fitting parameter. We obtained J

value of 16.6 K in units of KB. The fit is shown in Fig. 1(b) where thesolid line depicts the fitted curve and the open circles representingthe experimental data.

We have investigated the behaviour of magnetization iso-therms (of the first system) as a function of magnetic field forvarious temperatures. For our physical system there is oneunpaired spin per molecular site which interacts with otherneighbouring spins along one particular direction. As mentionedearlier this spin system behaves as a 1D spin chain. Thereforephysically it has infinite number of interacting spins in one chain.For fitting the experimental magnetization curve one needs toderive an expression of magnetization for infinite number ofspins. Since practically it is not possible to deal with infinitedimensional Hilbert space, Bonner and Fisher have numericallyevaluated field dependent magnetization for N¼10 spins andassured the convergence for N tending to infinity [10]. Based onthis we have written a code in MATLAB for 10 spins to calculatemagnetization isotherms as a function of field and fitted to ourexperimental data. Here we have considered the data set forT¼17.5 K which is below the antiferromagnetic ordering tem-perature. This fitting is shown in Fig. 1(c) and it appears to be agood fit. The open circles represent experimental data and thefitted line is shown by the solid red line. The plot shown inFig. 2(a) is a 3D plot which captures the variation of magnetiza-tion as a function of field and temperature.

In case of the second system we performed temperaturedependent magnetic susceptibility measurement in the tempera-ture range of 2 K to 360 K and is shown in Fig. 2(b). At lowtemperature regime (o100 K) this system exhibits Curie beha-viour while at a higher temperature (approximately 320–340 K)the system undergoes a phase transition from diamagnetic toparamagnetic phase. Though the system exhibits paramagneticbehaviour in the temperature range of 2–300 K this is in principlea diamagnet. The paramagnetic behaviour arises owing to thepresence of some impurities [4]. Above 200 K it shows tempera-ture independent diamagnetic behaviour up to 340 K. At 340 Kthe susceptibility shows a random jump and then exhibits aparamagnetic behaviour above this temperature. Upon cooling itcontinues in the paramagnetic phase down to 320 K, whereuponthe susceptibility suddenly drops and continues in the diamag-netic phase as we further cool the system. The heating andcooling runs are indicated by arrows in the inset of Fig. 2(b).One can clearly see a hysteresis in susceptibility as a function oftemperature. This hysteresis accompanied by the sudden jump at340 K in the heating curve and a sudden drop in susceptibility inthe cooling curve at 320 K is indicative of a first order phasetransition. This magnetic phase transition is associated with astructural change in the present system [4]. At high temperature(e.g., 335 K) the unpaired electrons are localised on the phenaly-nyl units which are not parts of the p-dimer stack and the systemremains in a paramagnetic state. However, after the phasetransition at low temperature (e.g., 300 K), the unpaired electronschange their positions to the stacked phenalynyl units, which are

Fig. 2. (a) 3D plot showing magnetization, magnetic field and temperature along the three axes for the first system. (b) Magnetic susceptibility as a function of

temperature for the second system. Inset shows the expanded region to give emphasis on the hysteresis (see text).

T. Chakraborty et al. / Solid State Communications 152 (2012) 1945–19501948

engaged in the p-dimer stack and pair-up to create a diamagneticstate [9,13]. Electrical and optical measurements on the samesystem have also shown this bistability when the temperature isvaried [9].To demonstrate that the phase below 320 K is indeed adiamagnetic phase, we have carried out magnetization measure-ments at 300 K, which is just before the transition occurs. This isshown in Fig. 3(a). We have also taken magnetization isothermsat 335 K, which is above the transition temperature, shown inFig. 3(b). The former clearly shows a diamagnetic behaviour,whereas the latter magnetization isotherm depicts a paramag-netic behaviour. We have fitted the magnetization curve shown inFig. 3(a) to a straight line. This corroborates the diamagneticbehaviour. In the isotherm taken at 300 K, the small increase inmagnetization at low field value is due to the paramagneticimpurities mentioned above. To fit the magnetization isothermtaken at 335 K fits, we excluded some low field data points toavoid the contribution from the impurities. We tried to fit theBrillouin function (Eq. (3)) to the data (Fig. 3(b)), but it was not asatisfactory fit.

BJðyÞ ¼2Jþ1

2Jcoth

2Jþ1

2Jy

� ��

1

2Jcoth

y

2Jð3Þ

where y¼ gmBJB=KBT and the other symbols have their usualmeaning. However, when this data was fitted to a straight line, weobtained a very good fit, which suggests that the system is a Pauliparamagnet [14]. The occurrence of the Pauli paramagnetic statein this temperature regime (above 320 K) suggests that there arefree electrons in the system, whereas in the regime below 320 K,the system behaves like a diamagnet, suggestive of an insulatingphase. The contribution to diamagnetism comes from ring cur-rents arising from closed shells. This is a common feature ininsulating systems. The presence of free electrons above 320 Kindicates metallic behaviour, whereas below this temperature itexhibits insulating behaviour. Thus the system undergoes ametal-insulator transition around 320 K. This behaviour has beencaptured earlier through conductivity measurements [9], whichresults from a structural phase transition around 320 K. Weintend to explore this in more details in future.

Inverse of magnetic susceptibility vs. temperature data for thethird system is shown in Fig. 3(c) in the temperature range 6 K to60 K. This curve shows the signature of existence of paramagneticstate in this temperature range. Temperature dependence ofmagnetic susceptibility of paramagnetic materials is describedby the Curie function C/T. Where C is the Curie constant and T is

Fig. 3. (a) Field dependent magnetization of the second system for a fixed temperature T¼300 K. The data points are shown by circles and the fitted curve (see text) is

shown by the solid blue line. (b) Field dependent Magnetization of the second system for a fixed temperature T¼335 K. The data points are shown by circles. The red line is

fit to the experimental data using Brillouin function and the blue line is linear fit to the data which represents Pauli paramagnetic behaviour. (c) Inverse of magnetic

susceptibility of the third system. Open circles represent the experimental data and the solid blue line is the linear fit to the experimental data.

T. Chakraborty et al. / Solid State Communications 152 (2012) 1945–1950 1949

temperature. Consequently, the Inverse of magnetic susceptibilitywill have a linear relationship with temperature. Hence, theexperimental data is fitted to a straight line. The fit is shownin Fig. 3(c) and it appears to be a good fit. The data is shown bythe open circles and the fitted curve by the blue solid line. Fromthis fitting the value of Curie constant (C) is obtained0.234 K emu mol�1. Therefore the radicals in this system existas free radicals with one spin per molecule to show free spinparamagnetism.

4. Conclusion

This study was a part of understanding these molecularmagnetic systems from the perspective of organic spintronicswhere one can have multiple applications of these systems eitheras an entangling channel for quantum computation or as mole-cular switches which can also be used as quantum gates. The firstsystem in this sample exhibited antiferromagnetic ordering at

23 K, which can have significant application in quantum net-works, since any isotropic antiferromagnetic spin half system willexhibit entanglement well below the ordering temperature. Andthis system being one dimensional in nature can be used asquantum wires, which will teleport quantum informationbetween two quantum gates. The second system which exhibitsa temperature dependent transition between insulating diamag-netic state to a metallic paramagnetic state can be used as amolecular switch, where the system can be locally heated by aninfrared laser thereby inducing the system from one state toanother. This can have potential application in terms of fabricat-ing both classical and quantum gates. The magnetization iso-therms in the two regimes – paramagnetic metallic anddiamagnetic insulating – show strikingly linear feature, showingthat the paramagnetic regime is a rare Pauli paramagnetic one,quite unprecedented in organic systems. If one can switch thesystem’s magnetic state under the application of a high magneticfield (say 5 T), then there would be an enormous jump in themagnetization as the two regimes show linear dependence as a

T. Chakraborty et al. / Solid State Communications 152 (2012) 1945–19501950

function of field but with slopes of different signs. This can help infabrication in magnetic switches. The third system did not showany striking feature and was a paramagnet in the entire tempera-ture range. This result disagrees with previously reported dataand we suspect that the antiferromagnetic phase reported in theprevious case could result owing to the ordering of someimpurities present in the system.

Acknowledgment

The authors would like to thank the Ministry of HumanResource Development, Government of India, for funding.

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[10] J.C. Bonner, M.C. Fisher, Phys Rev. 135 (3A) (1964) A640–A658M.[11] In Our Case We Haven’t Got Any Peak at Low Temperature in the Magnetic

Susceptibility vs. Temperature Curve Even after Doing Repeated Synthesisand Measurement. Although in Literature (Reference No. [5]) This SystemHas Shown Antiferromagnetic Coupling at Low Temperature. This Might BeDue to Some Impurity Present in Their Sample.

[12] J.W. Hall, Ph.D. Dissertation, University of North Carolina, Chapel Hill, NC27514, 1977.

[13] Robin G. Hicks, Nature 3 (2011) 189.[14] Charles Kittel, Introduction to Solid State Physics, Wiley India Pvt. Ltd., 2009,

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