Synthetic Metals 148 (2005) 119–121

Magnetic field effects in decamethyl ferrocenium tetracyanoethanide

Animesh Chakraborty

Ohio State University, 1179 University Drive, Newark, Ohio 43055-1797, USA

Received 17 April 2004; accepted 10 September 2004Available online 18 October 2004

Abstract

The specific heatC(H,T) of (DMeFc) (TCNE) in the temperature range of 2–12 K in the presence of applied magnetic fields is reported.Previous results had shown anomaly at 4.82 K corresponding to a transition atTc from a high temperature one-dimensional (1D) magnet(aboveTc) to a three-dimensional (3D) macroscopic ferromagnet (belowTc). The results show a dramatic behavior of the peak associatedwith the ferromagnetic transition. The peak height decreases with increasing magnetic field, i.e. magnetic fields smear the transition. Forsmall values of applied magnetic fields the peak associated with the ferromagnetic transition is completely smeared. The results are indicativeo by the 1Dc©

K

1

imiswotdorft(iag(

n-ee-

nionf theegen-rbitalf theasedem-av-rs inwise

n adi-mere

0d

f anisotropic chains with enhanced 1D coherence and frustration in interchain coupling. The spin-wave spectrum is dominatedhains belowTc.

2004 Elsevier B.V. All rights reserved.

eywords:Specific heat; Magnetization; Phase transition; Ferromagnet; Magnetic material

. Introduction

Magnets have fascinated man since the discovery that irons attracted to lodestone[1]. Sometime in the 12th century,

ariners in China and Europe made the discovery apparentlyndependently, that a piece of lodestone tends to align itselfo as to point in the direction of the polestar. This discoveryas presumably quickly followed by a second, that an ironr steel needle touched by a lodestone for long enough also

ends to align itself in a north-south direction. The last twoecades have seen a tremendous excitement in the physicsf low-dimensional magnetic systems[1–6]. Decamethylfer-ocenium tetracyanoethanide is the first reported molecularerromagnet. Above the transition temperatureTc the sys-em has been described as having primarily one-dimensional1D) ferromagnetic interactions along the spin S = 1/2 rad-cals along the chain axis. AtTc, magnetization, neutronnd specific heat studies showed that DMeFcTCNE under-oes a phase transition from a high temperature 1D magnetaboveTc) to a macroscopic three-dimensional (3D) ferro-

magnet (belowTc) [7]. (DMeFc)(TCNE) crystallizes in aorthorhombic structure space groupCmcawith stacks of alternating DMeFc TCNE radical ions parallel to the long ndle axis of the solution grown crystals. Both cation and ahave spin 1/2, with the highest occupied energy levels odonor being degenerate and those of the acceptor nonderate. The presence of the partly occupied degenerate oon the donor DMeFc has been proposed as the origin ointerstack exchange. A major advantage of molecule-bmagnets is their production via controllable molecular chistry allowing fine-tuning of structures and magnetic behiors. It is then possible to modulate structural parameteorder to elucidate the important factors governing othercomplex behavior[1,8].

2. Experiment

The specific heat measurements were performed in aabatic calorimeter[9,10]. The samples were in pellet forwith a mass of 0.5 gm. Polycrystalline powder samples w

E-mail address:Animesh500@aol.com. handled and pressed into pellets in an Argon environment.

379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2004.09.012

120 A. Chakraborty / Synthetic Metals 148 (2005) 119–121

The measurements between 2 and11 K were performed usinga drift method described earlier[8,9]. The experimental setupincluded a sample in the form of a pellet weighing approxi-mately 500 mg. A 220� carbon chip thermometer was madeby grinding the resistor and then attaching the thermometerto the sample pellet. A heater was wound around the samplepellet. The thermal links between the holders and the reser-voir were calibrated separately. The drift method consistedof relating the specific heat to the time (t)—temperature (T)decay of the sample temperature (dT/dt= 10–20 mK/s). Thecarbon chip sample thermometers were calibrated in situ. Asuperconducting magnet was used to vary the magnetic fieldfrom a few Gauss to a few Tesla.

3. Results and discussion

Fig. 1 shows the specific heat of DMeFc TCNE in ap-plied magnetic field in the range of 2–11 K. The magneticcontribution to the specific heat was plotted by subtractingthe lattice term. For this purpose the specific heat of spinless(DMeCo) (C3 (CN)5) was measured to obtain an experimen-tal measurement of the background lattice contribution andhence the magnetic contribution to the specific heat (CM) in(DMeFc) (TCNE). The samples of (DMeFc)(TCNE) stud-i elowt gne-t eena as

10 G). For small applied fields the cusp is smeared and iscompletely gone for applied fields of 2.3 kG. This is sub-stantially smaller in energy compared to the ferromagneticordering temperature of 4.82 K. If one were to argue us-ing a mean field approach that the ordering temperature isgiven byKBT=Jalong the chain× Jperpendicular to the chain; using aTc = 4.82 K andJalong chain axis= 35 K one obtains a value forJperpendicular to chain axis= 1 K. The value ofJalong chain axisis ob-tained from the specific heat fit of the anisotropic Heisenbergferromagnet[7] to the experimental data as well as from thephenomenological Hamiltonian using theganisotropy alongthe chain and perpendicular to the chain axis. One wouldthen argue that a field of 1 T (10 kG) would be needed tocompletely smear the ferromagnetic ordering temperature.From the experimental data shown inFig. 1. the ferromag-netic ordering is disrupted for applied fields much smaller inmagnitude compared to the expected number. This is possi-bly due to the fact that there is frustration/glassiness amongthe linear ferromagnetic chains which causes the ferromag-netic macroscopic ordering to be destroyed at substantiallysmaller applied fields. The existence of a spin glass phaseimplies that there is some form of frustrated magnetic in-teraction present in the system. Possible causes include nextnearest neighbor interactions, random exchange, and randomanisotropy[8,11].

eticfi het ed as:

C

isa uresb thea m ofa stra-t rted

ed had saturation magnetization of 1.6 emu kG/mole bhe transition temperature of 4.82 K as measured by maization studies. A cusp in the specific heat is clearly st 4.82 K for small values of the applied field (as low

Fig. 1. Magnetospecific heat as a function of temperature.

The specific heat as a function of the applied magneld is shown inFig. 2. The data is plotted while holding temperature constant. The specific heat can be express

= Co + α(T − Tc)H

For temperatures aboveTc the magnetic specific heatn increasing function of the applied field. For temperatelow Tc the specific heat is a decreasing function ofpplied field. These results are consistent with a systenisotropic chains with enhanced 1D coherence and fru

ion in inter chain coupling. In summary, we have repo

Fig. 2. Specific heat as a function of the applied magnetic field.

A. Chakraborty / Synthetic Metals 148 (2005) 119–121 121

the specific heat of (DMeFc)(TCNE) in applied magneticfields. The nature of the magnetism is one-dimensional aboveTc, behaving as an anisotropic Heisenberg 1D ferromagnet.At Tc there is a crossover from a high temperature1Dstate (aboveTc) to a 3D ordered ferromagnet (belowTc).The spin-wave spectrum belowTc is dominated by 1Dchains. As the magnetic field is increased the peak is notonly smeared but is also moving up in temperature. Thisindicates that the transition is ferromagnetic. However, theferromagnetic order is smeared at magnetic fields whichare much lower than the number predicted by a meanfieldestimation.

Acknowledgements

A.C. acknowledges the support of Ohio State Universityin carrying out this work. A.C. expresses sincere thanks to histhesis advisor Dr. A. J. Epstein for guidance and support. A.C.thanks Dr. Joel Miller of the University of Utah for providing

the samples and Dr. Bill Lawless of Ceram Physics Inc. forhelp with the instrumentation.

References

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