Torque anisotropy in λ-(BEDT-TSF)2FeCl4

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Torque Anisotropy in h-(BEDT-TSF)zFeCld J.I. Oh’, M.J. Naughton ‘* , T . Courcet’, I. Malfant2, P. Cassoux2, M. Tokumoto3, H. Akutsu4, H. Kobayashi4, A. Kobayash? ‘Department

of Physics, State University of New York, Buffalo, New York 14260 USA ‘LCC/CNRS, Toulouse, France 3Electrotechnical Laboratory Tsukuba, Japan ‘Institute for Molecular Science, Okazaki, Japan “The Universig of Tokyo, Japan

Abstract The title compoundwasrecently shownto exhibit a spin-floptransition(SF) from an antiferromagnetic(AF) to a cantedAF (CAF) phase.Here,we reporton the evolutionof the magneticanisotropyin the AF and CAF phasesasmeasured by cantilevermagnetometry. New curvaturewasfound in the AF-CAF phaseboundarybelow2K, providing evidencethat the T=Ogroundstatemay be CAF rather thanAF, andthat the CAF stateisreentrantat low temperature. Keywords: organicconductors,magneticmeasurementi 1. Jntroduction The molecular organic conductor h-(BEDT-TSF)2FeC14 (BEDT-TSF=bis(ethylenedithio)tetraselenafulvalene, alsoknown asBETS) is an interestingcompounddueto its magneticground stateandsimilarity to its GaC14superconducting cousin. Based on informationreportedin Refs. 1, 2, 3 and4, we have drawna schematicdiagramof the crystal structureof h-(BETS)2FeC14 as shownin Fig. l(a). BETS moleculesare fourfold-quasi-stacked alongthe a-axisand constituteconductingplanesparallelto the a-c plane.The conductinglayersalternatealongthe b-axiswith layers containinglinear chainsof FeC14-magneticanions,The BETS moleculesand FeC14-anionscontributeto quasi-twoand quasi-onedimensional conductions,respectively. The h-(BETQFeCL, crystal hasbeen reportedto exhibit a sharpmetal-insulator(M-I) transitionat TM.,- 8.5K at ambient pressure[2]. The origin of the M-I transition obviously comes from the condensationof the conduction electronsinto an insulatingstate.Goze,et al. reportedthat suppression of the M-I transitionoccursbelow TM.~in h-(BETS)2FeC14 whenmagnetic fields are applied [3,5]. Before observingthis magnetic-fieldinducedsuppression of the M-I transition,the pressure-induced suppression of the transitionhasbeenobserved,andexplainedby the fact that the dimensionalityof the electronicsystemshouldbe increasedby applying pressure.The field-inducedrestoration, theq was considereda striking result sincea magneticfield is generally expected to decreasethe dimensionality of the electronicsystem.Their observationhasgiven riseto studieson the natureof the groundstateof the h-(BETS)2FeC14 crystal,and attractedmanyphysicists’interestin the temperature-magnetic *Supported by the US National Science Foundation, Grant No’s, DMR9258579 and DMR-9701597; A& Sept., 1998: Department of Physics, Boston College, Chestnut Hill, Mass 02 1G7; email: [email protected]

0379-6779/99/$ - see front matter 0 1999 Elsevier HI: SO379-6779(98)00258-6

field (T-B) phasediagramof the crystal.A sharpdrop of the spin susceptibility observed below TM., has been consideredas evidenceof a non-magneticspin-Peierls-like groundstate.[1,6,7] On the other hand,Goze et al. proposedanotherinterpretation: that the condensationof the conduction electrons into an insulating state could also be induced by antiferromagnetic orderingof the localizedmagneticmomentsof the Fe3;ions[3]. Tokumoto,et al. have observeda spin-flop-like(SF) transition below T,.,, a phenomenonwhich commonly occurs in antiferromagnets (AF); suggestingthat the ground state of h(BETS)2FeC14 may be AF rather than spin-Peierls[4]. A SF transitionline subdividingthe insulatingregion into two states wasthen addedto the T-B phasediagramof Ref. 3, with TM., associated with a Nkeltemperature,TN. The T-B phasediagramhas beenstudiedin more detail and further verified by Brossardandcollaborators[8]. As a result,the phasediagramhasbeendivided into AF, cantedAF (CAF), and paramagnetic(PM) states,the latter being a metallic state possiblypartially oriented.However, sincethey didn’t examine the SF (i.e., the A!?+GlF) transition line below T - 2K, the natureof T=Ogroundstateof h-(BETS)2FeC14 is still a puzzling question.It is, therefore, natural to explore low temperature behaviorof the SFtransitionline. In thispaper,we reporttorque anisotropyresultsdownto 0.4K for the purposeof studyingthe low temperature behaviorof the SFtransitionline. 2. Experiment Torque measurementsusing a 3He refrigerator and micromachinedsiliconcantilevermagnetometry(SCM) [9] have beenperformedon a singleh-(BETS)2FeC14 crystal measuring -0.58x0.12x0.04 mm3. Ln someinstances,magnetoresistance wasmonitorsimultaneously. In orderto investigatethe spin-flop transitionline in the T-B phasediagrammoreprecisely,torque

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cryostat relative to a horizontal field magnet.

Some

measurements were also taken with the crystal oriented in a perpendicular orientationcomparedto that shownin Fig. l(b),

suchthat c*-axisis in therotationplane. 3.

Results and Discussion

3.a.Field SweepData-- Spin-Flop Fig. 2 exhibitsthe magneticfield dependence of the torque7 at 9 = 143” below 2K. The anisotropicmagnetization&=z/B has been derived from the capacitancechangeAC = C(B)-C(O) (whereAC - pxB = 7) measured while sweepingfield at different temperatures.The inset displaysthe raw torque signal. The devicewascalibratedin situ [93. We tentatively interpretthehill-

1 BETS F

q-2-D

*

q- 1-D

FeC14 -

Fig. l(a). A schematic diagram of h-(BETS)2FeCb

B ..~... v

5-

6-33 5 4?0 7V

3-

m & II

2-

‘- 7 0

l(b). The B-field orientation with respect to the sample on the microcantilever for the data in Figs. 2-4. A Znd orientation was used with the crystal

c*-axis

along the arm of the cantilever,

with the field

then rotated in the u-c* plane. The sample volume is -2.8~10’~ cm3. signals have been measured down Co 0.4K by sweeping magnetic fields of up to 6T. 7Xe fields have been oriented in a plane perpendicular to the needle or c*-axis as shownin Fig. l(b),

where (3 definesa field direction relative to the plane of the cantilever. At this point, the exact orientation of the crystala and b-axeswith respectto field is not known for a given9, pendingxray determination. Also, the angulardependence of torquesignal hasbeenexaminedat severalstaticfieldsby rotatingthe fieldsin the a-b plane. This was achievedex sihf by rotating the 3He

I

1.4K

t

Fig, 2. Anisotropic magnetic moment derived torque at 0.4K and 1.4K, showing an increase of the spin-flop field (arrows) with temperature. The inset shows the raw torque signal. The volume magnetization atZT is approximately

Fig.

I

T=

0.5 kNm,

or 0.5 emu/cm’.

like bumpsdepictedby arrowsto signify the spin-floptransition. Noticethat the bumpat 0.4 K occursat lower field thantheoneat 1.4K, which meansthe spin-flop transition line has positive curvature. This meansthis spin-flop line is reentrant at a particular IieId (-1T) and low temperature,an increaseof temperaturemovesonefrom the CA.Finto the AF phase.Upon further increaseof r, you re-enterthe CAF phasebeforefinally obtainingthe PM metalphasenw 7’,.1. This declarationof a reentrantportion of the T-B phasediagramrelies on the fact, perhapsnot yet fully established, that the metal- CAF andmetalAF phaseboundaries meeton@atB=O;that is, that no ftite field tri-critical pointexists. Turningbackto the tentative natureof our assignment of the peaksin Fig. 2 to the SF,5vcha metamagnetic transitionis usually represented by a more abrupt changein magnetization [IO]. In fact, for a fust ordertransition,=as the SF should&, we expect a discontinuityii~AI(H) or M(7). This is not observed here, nor was it observedin similar torque data in Ref. 8 (although static SQUIDdata there showedreasonablygood evidence for a SF transition). Tne lack of a magnetic discontinuity could be due to sample inhomogeneity(not considered very likely), thermalsmetig, or perhapsotherun-

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10

6

g t-

---(f-

e=

30"

d

e=

90”

--o-

e=143"

4

50

0.5

1.0

1.5

-90

2.0

I

I

0

90

T-B phase diagram for the A.F#CAF spin-flop transition derived Gem magnetization data at various tilt angles (see Fig. l(b)). The slope is seen to be positive below 4K, indicating a possible reentrant nature to the phase space.

known factors. The total torque signal is only -1% of the compliancelimit of the cantilever. At this point, we only havethe two setsof torquedatashown in Fig. 2 for the angle8 = 143”. Additional datawere obtained in other orientations,with resultsfrom theseshownin Fig. 3. Every point has beencollectedin the sameway as in Fig. 2, exceptthe datumpoint in zero field, which is takenfrom Ref. 8. Thesedataconfii the existenceof the positive slopedT/dB of the AF-CAF phasediagrambelow4K, anda negativeslopeat higher temperature.Thus, the phaseboundarystrongly suggests reentranceexists.It is notedthat the critical spin-flopfield hasan angledependence aswell as a temperaturedependence.This is probablya resultof the low dimensionalnatureof the AF state, similar to the spin flop seen in the quasi-l-D (TMTSF)zX materials[ll],

/

270

Fig. 4. Angular dependence of the torque at different B fields at 1.5K. The symmetry points of the traces begins to shift above l-2T, interpreted as resulting from a rotation in the magnetization vector associated with the spin-flop transition.

I

/

I

t

T

170

I 0

I

2

3.b. TorqueRotationData We have measuredthe torque and transversemagnetoresistanceat 1.5K and above,to examinethe anisotropyof the system. Figure 4 showstorque anisotropyresults at several magneticfields. Throughout,the signalis dominatedby sin(20) dependence,modulatedby a term related to the magnetic anisotropy,similarto that employedin reversiblemagnetization in layeredsuperconductors [ 121.The leadingterm can be easily explainedif the anisotropicsusceptibilityAx=xa-xt,, andthusthe perpendicularmagnetic moment b=n~,-nzb, varies with the cosineof the field tilt angle(a andb arecrystaldirections). Since torqueis [mxB~=m(B)L?sir& with m(8)-co&, the torquevariesas cos9xsinf3 - sin2(0). This impliesthat the dominantcontribution to the magneticmomentis emanates from a particulardirection, or crystal axis. Further analysisof the crystal, as mentioned previously,will be necessaryto resolvethe issue. Nonetheless, we can seefrom the insetto Fig. 4 that the magneticsymmetry axis remainsessentiallyconstantbelow- 2T, andthen smoothly shifts with increasingfield above this. Again, this magnetic vectorrotation appearsgradual,unlikewhat we had expectedfor a 1” ordertransition. This is seenmoreclearly in Fig. 5, where we plot the anglesof the torque maximaagainstfield. This variation of &,x with B canbe dueto a gradualrotation of the

,

180

8 (degree)

f3 CT) Fig. 3. The

,

I

6 B 0-J

4

Fig. 5. Dependence ofthe angle at which the torque in Fig. 4 reaches its maximal value, versus magnetic field. This angle is observed to remain essentially constant up to -2T (Bsr), then gradually shift above the spinflop field.

magnetizationvector 3.~.Resistance RotationData The resistance anisotropyin the samerangeof fields and Fig. 4, measured simultaneous to the torque,is shownbelowin Fig. 6, for an applied current of 20~4 parallelto the c* needleaxis. There is essentiallyno measurable anisotropybelow 5T, while R,,/R,, is only 1.026at 6T. The only value to compareto, of which we areaware,is a ratio of - 7.0 observedby Goze,et al. in the metallic state (at 1ST) [5]. However, if we make the assumption that the resistance reachesa minimumfor field in the sameorientationin both cases(high field metal and low field CAF states),we canassignthe a’ axis to O-30” for all our data. Referringto Fig. 4 above,we seethat thetorqueat 6T possesses a symmetrypoint (~=0)at this angle,aswouldthenbe expected.

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phase boundary consistent with the magnetic Clausius-Clapeyron equation [13], ti g -A&4(&/&). This means that the magnetization change AM at the spin flop transition should be positive abovf -4K and negative below 4K. However, no magnetic diseontinuities have been observed for which to fully characterize this transition from a thermodynamic point of view (fast order versus second order). We observe the same “2”dorder”-type behavior at all temperatures. Additional measurements at yet lower temperatures may be required to determine the true ground state and the nature of the spin flop transition.

References

0.5T .---

1.31” -90

0

90

180

270

8 (degree) Fig. 6. Resistance rotations in the a*-b* plane in (BEDT-TSF)tFeC& in the canted antiferromagnetic state at 1.X, for various magnetic fields. Current (ZOnA) is applied, and voltage measured, along the needle c* axis. Anisotropy begins to become detectable above - 5T, amounting to 2.6% at 6T.

4. Conclusion We have investigated the a&ferromagnetic state in the molecular organic conductor (BEDT-TSF)*FeC& via torque magnetometry and resistivity. We find that a presumed spin-flop transition f?om the AF state to a canted AF state possesses a reentrant phase boundary in magnetic field - temperature space. lf the AF-CAF transition is of 1” order, we expect that the variation of the magnetization versus field and even temperature should undergo a change (increase or decrease) upon passing through the

[l ] H. Kobayashi, H. Ton& T. Naito, A Kobayashi, F. Sakai, T. Watanabe, P. Cassoux, J. Am, ChemSoc. 118,368 (1996). [2] A. Kobayashi, T. Udagawa, H. Tornita, T. Naifo, H Kobayashi, Chem. Lett. 2 179 (1993). [3] F. Goze, V. N. Laukhin, L. Brossard, A. Audouard, J. P. Uhnet, S. Askenazy, T. Naito, H. Kobayashi, A. Kobayashi, M. Tokumoto, P. Cassoux, Europhys. Lett. 28,427 (1994). [4] M. Tok.nunoto, T. N&o, H. Kobayashi, A. Kobayashi,V. N. Laukhin, L. Brossard, P. Cassouq Synth~Met. 86,2161 (1997). [5] F. Goze, V. N. Laukhin, L. Brossard, A. Audouard, J. P. Ulme< S. Askenazy, T. Naito, H, Kobayashi, A. Kobayashi, M Tokumoto, P. Cassoux, Physica 3 211,240 (1995). [6] H. Kobayashi, H. Tom&, T. Udagawa, T. Naito,- A. Kobayashi, Synth. Met. 70,867 (1995). [7] H. Kobayashi, T. Naito, A. Sato, K. Kawano, A. Kobayashi, H. Tanaka, T. Saito, M. Tokumoto, P. Cassoux, Mol. Cryst. Liq. Cry& 284,61 (1996) [S] L. Brossard, R. Clerac, C. Coulon, M. Tokwnoto, T. Ziman, D. K. Petrov, V. N. Laukhin M. J. Naughton, A. Audouard, F. Gaze, A. Kobayashi, H. Kobayashi, P. Cassoux, Eur. Phy. J.B 1, 439 (1998). [9] M.J. Naughton, M. Chaparala, A.P. Hope, J.P.Ul.met, N. Narjis and S. Askenazy, Rev. Sci. J&rum 68,406l (1997). [lOI S. Chikammi, Physics ofFerronmgneti.m, 2”‘Edition (Oxford Science Publications (1997). [l l] M. Miljak, J.R. Cooper and K. Bechgaard, J. Phys. (Paris) Colloq. C3 44, C3-893 (1983). {12] V.G. Kogan, Phys. Rev. B 387049 (I 988). [ 131B. Clapeyron, J. L!Ecole Polytechnique 14, 153 (1834).