We present a magnetotransport study of the low--carrier density ferromagnet
EuB_6. This semimetallic compound, which undergoes two ferromagnetic
transitions at T_l = 15.3 K and T_c = 12.5 K, exhibits close to T_l a colossal
magnetoresistivity (CMR). We quantitatively compare our data to recent
theoretical work, which however fails to explain our observations. We attribute
this disagreement with theory to the unique type of magnetic polaron formation
in EuB_6.Comment: Conference contribution MMM'99, San Jos

Aronson, M. C.

Bogdanovich, S.

Fisk, Z.

Prasad, I.

Sarrao, J. L.

Suellow, S.

English

arXiv

We present a magnetotransport study of the low-carrier density ferromagnet EuB6. This semimetallic compound, which undergoes two ferromagnetic transitions at T1 = 15.3 K and Tc = 12.5 K, exhibits close to T1 a colossal magnetoresistivity (CMR). We quantitatively compare our data to recent theoretical work, which, however, fails to explain our observations. We attribute this disagreement with theory to the unique type of magnetic polaron formation in EuB6. © 2000 American Institute of Physics

Süllow, S

Prasad, I

Bogdanovich, S

Aronson, MC

Sarrao, JL

Fisk, Z

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitleMagnetotransport in the low carrier density ferromagnet EuB6Permalinkhttps://escholarship.org/uc/item/5j122488JournalJournal of Applied Physics, 87(9)ISSN0021-8979AuthorsSüllow, SPrasad, IBogdanovich, Set al.Publication Date2000-05-01DOI10.1063/1.372460Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, availalbe at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaJOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 9 1 MAY 2000Magnetotransport in the low carrier density ferromagnet EuB 6S. Süllowa)Department of Physics, 2071 Randall Laboratory, University of Michigan, Ann Arbor, Michigan 48109-1120 and Institut fu¨r Metallphysik. TU Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig,GermanyI. Prasad,b) S. Bogdanovich,c) and M. C. AronsonDepartment of Physics, 2071 Randall Laboratory, University of Michigan, Ann Arbor,Michigan 48109-1120J. L. Sarraod) and Z. FiskNational High Magnetic Field Laboratory, 1800 E. Paul Dirac Drive, Florida State University,Tallahassee, Florida 32310We present a magnetotransport study of the low-carrier density ferromagnet EuB6. Thissemimetallic compound, which undergoes two ferromagnetic transitions atTl515.3 K andTc512.5 K, exhibits close toTl a colossal magnetoresistivity~CMR!. We quantitatively compare ourdata to recent theoretical work, which, however, fails to explain our observations. We attribute thisdisagreement with theory to the unique type of magnetic polaron formation in EuB6. © 2000American Institute of Physics.@S0021-8979~00!62208-9#bs-e.ec-oresodisldg-snobiy.tiity,anostde-lic,ea-e onngwe-heetshee-ldfRecently, critical fluctuations have been proposedMajumdar and Littlewood1 as mechanism causing the colosal magnetoresistivity~CMR! in a number of nonmanganitmaterials, like in the pyrochlores or chalcogenide spinels2,3The authors of Ref. 1 argued that, because in a ferromagnmetal close to its critical point the dominant magnetic flutuations are those with a wave vectorq→0, the contributionsfrom these fluctuations to the resistivityr should grow asFermi numberkf and the carrier densityn decrease. For sufficiently small n, like in ferromagnetic semimetals, a majpart of the zero-field resistivity close toTc would be causedby magnetic fluctuations. Suppressing these in magnfields should generate the CMR in such materials.The resistivity of a low-carrier density system might albe affected by magnetic polarons. But magnetic polaronsappear~i.e., delocalize! if the magnetically correlated regionoverlap, implying that critical magnetic scattering shoudominate the resistivity forkfj(T)@1 @j(T): Magnetic cor-relation length#. In this regime of dominant critical scatterinand in the clean limit, i.e.,kfl@1, the low-field magnetoresistivity is quantitatively predicted to (r T,B)2r0)/r05Dr/r5C(M /M sat)2, with C'(kfj0)22 ~l: Mean freepath;M: Magnetization;j0 : Magnetic lattice spacing!. Then,for a free electron gas a relationship between magnetoretive coefficient and carrier density,C}(n2/3j02)21, emergesas central result of Ref. 1, with a proportionality consta'1/38.To test this prediction we performed a detailed studythe magnetoresistive properties of the divalent cuhexaboride EuB6.4 This semimetal, with a carrier densitdetermined from quantum oscillation experiments of 831023 electrons per unit cell, undergoes two ferromagnea!Electronic mail: s.suellow@tu-bs.deb!Present address: Harvard University, Cambridge, MA 02138.c!Present address: NHMFL, Tallahassee, FL 32310.d!Present address: LANL, Los Alamos, NM 87545.5590021-8979/2000/87(9)/5591/3/$17.00ytic-tics-is-tfc8ctransitions atTc512.5 K andTl515.3 K,5 derived here fromthe maxima in the temperature derivative of the resistivdr/dT. The effective carrier masses are slightly smaller ththe free electron mass, and the Fermi surface is almspherical. Hence, a free electron model appropriatelyscribes this compound. The zero-field resistivity is metaland in free electron approximation we findkfl@1 up toroom temperature.6 Further, with j05j300 K54.185 Å, inOernstein–Zernicke approximation,x(0)}j2(T), and withthe experimentally determined dc-susceptibilityx0 from Ref.5 the conditionkfj(T)@1 is fulfilled below 17 K. Hence,EuB6 fulfills all requirements of the model of Ref. 1.Here, we present resistivity and magnetoresistivity msurements employing a standard four-probe ac-techniquthe crystal studied in Ref. 5, with the current applied alothe @100# and the field along the@010# of the crystalline unitcell. For a quantitative analysis of the magnetoresistivityuse the magnetization from Ref. 5.In Figs. 1~a! and 1~b! we plot our raw data: The temperature~T! dependent resistivityr of EuB6 in fieldsB up to5 T and the normalized magnetoresistivityDr(B)/r between5.5 and 20 K, corrected for demagnetization effects. Tfield dependence ofr reveals two different magnetoresistivregimes: For smallB a rapid decrease ofr(B) close toTloccurs, while hardly any effect onr is observable below.10 K. The suppression ofr close toTl , Dr/r'20.9 in 2T, is comparable in size to other CMR compounds.2,3 Incontrast, for large fieldsr(B) increases withB, this in par-ticular at low T. The positive magnetoresistivity representhe normal metallic contributionrmet to r(B).We extract the magnetic scattering contribution from ttotal magnetoresistivity by subtracting the metallic magntoresistivity rmet. To do that we parametrize the high-fiemagnetoresistivity withrmet5r01aBx, x'2 and derive themagnetic partrmag5r(B)2rmet. The field dependence o1 © 2000 American Institute of Physicsignatroor.e.lids-pf3,sthend.onfveer-heRef.ure,yci-53.tchastion5592 J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Süllow et al.rmet thus established for the data at 13 K is included in F1~b! as dashed line.At any given temperature the minimum value ofr as afunction ofB, rmin , constitutes an upper limit for the phonocontribution tor. We have included the valuesrmin as afunction of T in Fig. 1~a! as shaded area, illustrating thatand aboveTc phonons contribute less than 15% to the zefield resistivity.To examine the dependence of the magnetic magnetsistivity Drmag/r on the normalized magnetizationM /M satwe plot the two quantities at different temperatures in Figin a log–log plot, with the magnetic field as implicit variablIn order to compare with the model of Ref. 1, which is vaonly in the paramagnetic phase, we restrict our analysitemperaturesT>Tl . As is illustrated in Fig. 2, at these temperatures all data sets collapse on a universal curve. InFIG. 1. ~a! The temperature~T! dependent resistivityr of EuB6 in zero field~—! and in fields of 0.021 T~h!, 0.029 T~s!, 0.05 T ~n!, 0.076~,!, 0.1T ~L!, 0.2 T ~1!, 0.5 T ~3!, 1 T ~d!, and 5 T~- - -!. The gray shaded aredenotes an upper limit of the phonon contribution tor. ~b! The normalizedmagnetoresistivity (r(B)2r0)/r05Dr/r at 10 K~h!, 13 K ~s!, 15.5 K~–d –!, 17.5 K ~,!, and 20 K~L!. The dashed line~- - -! visualizes the fielddependence of the metallic magnetoresistivityrmet5rB501aBx for the dataat 13 K, which is used to extract the magnetic contributionrmag5r(B)2rmet.FIG. 2. The magnetic magnetoresistivityDrmag/r of EuB6 vs the normal-ized magnetizationM /M satat 15.5 K~h!, 16 K ~s!, 17 K ~n!, 18 K ~,!, 19K ~L!, and 20 K ~1! in a log–log plot. The solid line representDrmag/r575•(M /M sat)2..-e-2toar-ticular, for M /M sat<0.07 we findDrmag/r5C(M /M sat)2,with C575 ~solid line!.The value ofC is in striking contrast to the prediction oRef. 1. With the carrier densityn51.231024/Å3 for ourcrystal EuB6 we computeC'(38n2/3j02)2150.62 rather thanthe observed 75. More generally, following Ref. 1 we plotCvs nj03 for metallic ferromagnets and manganites in Fig.together with the data for EuB6 of our crystal and from pre-vious works.7,8 In the plot we include the predicted valueC5(38n2/3j02)21. The data for EuB6 deviate by an order ofmagnitude from those of the other materials, emphasizingvastly different magnetoresistive behavior of this compouWe believe that the unique type of magnetic polarformation feature in EuB6 causes the failure of the model oRef. 1 to account for the observed behavior. As we haproposed elsewhere,9 at Tl polaron metallization via polaronoverlap leads to a drop ofr(T). The polaron metallization isaccompanied by a filamentary type of ferromagnetic ording, which arises from internal structure of the polarons. Tbulk magnetic transition occurs atTc . The field dependenceof the resistivity close toTl then is mainly governed by theincrease of the polaron size with magnetic field~rather thanby the suppression of critical scattering, as suggested in1!, causing the metallization to occur at a higher temperatand leading to the reduction of the resistivity atTl in mag-netic fields.Work at the University of Michigan was supported bthe U.S. Department of Energy, Office of Basic Energy Sences, under Grant Nos. 94-ER-45526 and 97-ER-27Work at the TU Braunschweig was supported by the DuForschungsgemeinschaft~DFG!.1P. Majumdar and P. Littlewood, Phys. Rev. Lett.81, 1314~1998!; Nature~London! 395, 479 ~1998!.2A. P. Ramirez, J. Phys.: Condens. Matter9, 8171~1997!; A. J. Millis, B.I. Shraiman, and R. Mueller, Phys. Rev. Lett.77, 175 ~1996!.3Y. Shimikawa, Y. Kubo, and T. Manako, Nature~London! 379, 53 ~1996!;FIG. 3. Experimental valuesC vs scaled carrier densitynj03 for metallicferromagnets~solid symbols!, manganites~open symbols! ~from Ref. 1! andEuB6: this work; 2: Ref. 7, 3: Ref. 8. The dashed line denotes the predicC5(38n2/3j02)21, the solid line is a guide to the eye.5593J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Süllow et al.M. A. Subramanianet al., Science273, 81 ~1996!; A. P. Ramirez, R. J.Cava, and J. Krajewski, Nature~London! 386, 156 ~1997!.4L. Degiorgi et al., Phys. Rev. Lett.79, 5134~1997!; D. P. Younget al.,Nature~London! 397, 412 ~1999!.5S. Süllow et al., Phys. Rev. B57, 5860~1998!; M. C. Aronsonet al., ibid.59, 4720~1999!.6We calculate at room temperature, withr'700 mV cm, in free electronapproximationkfl5(3p2)2/3\/e2rn1/3'10.7C. N. Guyet al., Solid State Commun.33, 1055~1980!.8J. Cooleyet al., Phys. Rev. B56, 14541~1997!; J. Cooley, Ph.D. thesis,Ann Arbor ~1997!, unpublished.9S. Süllow et al. ~to be published!.

Magnetotransport in the low carrier density ferromagnet EuB6

We present a magnetotransport study of the low-carrier density ferromagnet EuB6.EuB6. This semimetallic compound, which undergoes two ferromagnetic transitions at Tl=15.3Tl=15.3 K and Tc=12.5Tc=12.5 K, exhibits close to TlTl a colossal magnetoresistivity (CMR). We quantitatively compare our data to recent theoretical work, which, however, fails to explain our observations. We attribute this disagreement with theory to the unique type of magnetic polaron formation in EuB6.EuB6. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69886/2/JAPIAU-87-9-5591-1.pd

Süllow, S.

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Magnetotransport in the low carrier density ferromagnet EuB6EuB6

S. Süllow

I. Prasad

S. Bogdanovich

M. C. Aronson

J. L. Sarrao

Z. Fisk

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http://deepblue.lib.umich.edu/bitstream/2027.42/69886/2/JAPIAU-87-9-5591-1.pdf

Magnetotransport in the low carrier density ferromagnet EuB_6

Magnetotransport in the low carrier density ferromagnet EuB_6

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