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  • Magnetic order, spin-induced ferroelectricityand structural studies in multiferroic

    Mn1xCoxWO4by

    IRENE URCELAY OLABARRIA

    THESIS SUBMITTED TO THEDEPARTAMENT DE FSICA OF THE FACULTAT DE CINCES OF THE

    UNIVERSITAT AUTNOMA DE BARCELONAFOR THE DEGREE OF

    DOCTOR OF PHILOSOPHY

    c

    a

    -4 -3 -2 -1 0 1 2 3 4

    -1

    0

    1

    -4-2

    024

    l

    k

    h

    T = 2 K

    (t)

    c

    aP

    (t)

    -4 -3 -2 -1 0 1 2 3 4

    -1

    0

    -4-2

    024

    l

    kkkkkk

    h

    T = 2 K1

    T = 2 KKKKKK111

    mmllj =kkkkkkkkkkkkkkkkkkkkkkkkkkkkk

    SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkjjjjjjjjjjjjjjjjjjjeeeeeeeeeeeeeeeeeeeeeeeeeeeee2iiiiiiiiiikkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRlllllllllllllllllllll

    00+HHR

    r)=)) =) =) =) =) =) =) =) =) =) =) =) =) =) =) =)) ==) ==) ====) =) =))))))))))))) =))))))))))) ===)) ==)))))detdet

    det((((RRRRRRRRRRRRRRRRRRRRRRRRRR

    )))))))))))))))))))))))))))))RRRRRRRRRRRRRRRRMMMM

    ((((((3xxxxxxx33 4444444

    )))))44)

    ++H) = pppfpfpfffpfpffpfff00fff(((((((((QQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQ))))))))))))))))))

    jjjjjjjjjjjjj

    SSSSSSSSSSSSSSSSkkkkkkkkkk,jjjeeeeeeeeeeeeeiiiiiiiiiiiiiiiiiiiiQQQQQQQQQQQQQQQQQQrrrrjjjjeeWWjW

    =CCCCCCCCCCCCC

    T PPPPPPPP

    acacaaacacacacaacac ============== cos (s (s (s (((((tttt) +) +) +) +) +) +

    FFFFFFFFFFFFFFFFFMMMMMMMMMMMMMMMMM(((((((((((((QQQQQQQQQQQQQQQQQ ========== kkkkkkkkkkk

    tttt ++++++1.2

    1.4

    1.6TN

    TN2

    ac

    (10-

    4 C

    GS

    ) b

    H = 1 kOe

    10 12 141.45

    1.50

    1.55c axis

    TN2

    TN

    ssssis n (

    P rijjjjjjjijij ((((((((((((((((((((SSSSSSSSSSSSSSSSSSSSSSSSSSSSiiiiiiiiiiiiiiiiiiiii

    SSSSSSSSSjjjjjjjjjSSSSS ))))))))))

    n (

    MMMMMMMMMMMMMMMMMMMMMM((((((((((((((((((RRRRRRRR

    IIIIxxx4444

    +0

    ) +) +) +++ sssissss n(((ttttt) +) +) +) +++) +) + 1.2

    0 5 10 15 200

    25

    50

    75

    100

    Pb

    Pc

    Pa

    P (

    C/m

    2 )

    Temperature (K)

    5 10 151.00

    1.01

    1.02

    1.03

    cx10

    a

    Temperature (K)

    (T

    )/(5

    K)

    SUPERVISORS:DR. VASSIL SKUMRYEVDR. ERIC RESSOUCHE

    DR. JOSE LUIS GARCA MUNOZ

    December, 2012

  • SUPERVISORS:DR. VASSIL SKUMRYEVDR. ERIC RESSOUCHE

    DR. JOSE LUIS GARCA MUNOZ

    December, 2012

    Magnetic order, spin-induced ferroelectricityand structural studies in multiferroic

    Mn1xCoxWO4by

    IRENE URCELAY OLABARRIA

    THESIS SUBMITTED TO THEDEPARTAMENT DE FSICA OF THE FACULTAT DE CINCES OF THE

    UNIVERSITAT AUTNOMA DE BARCELONAFOR THE DEGREE OF

    DOCTOR OF PHILOSOPHY

  • Dr. Vassil Skumryev, Dr. Eric Ressouche andDr. Jos Luis Garca Muoz

    CERTIFY

    that the work reported in the present by Irene Urcelay Olabarria, entitled"Magnetic order, spin-induced ferroelectricity and structural studies in multifer-roic Mn1xCoxWO4", has been done at the Institut Laue Langevin in Grenoble un-der their supervision. This work constitutes the Doctoral Thesis Memory submit-ted by the interested person to the Facultat de Cincias de la Universidad Autnomade Barcelona to apply for the degree Doctor of Philosophy.

    Bellaterra, 19th of December, 2012.

    Signature:

    Dr. Jos Luis Garca MuozResearch Professor of the Insti-

    tuto de Ciencia de Materiales

    de Barcelona, Consejo Superior

    de Investigaciones Cientficas.

    Signature:

    Dr. Eric RessoucheResearch Director,

    CEA/Grenoble,

    INAC/SPSMS.

    Signature:

    Dr. Vassil SkumryevProfessor of the Institut Catal

    de Recerca i Estudis Avanats

    (ICREA) in the Dep. de Fsica

    of the Universitat Autnoma

    de Barcelona.

  • Ama ta aitandako

  • Acknowledgements

    Time flies. Actually very fast. It is already more than three years since this storybegan. I have found many people in the path who, in some way or another, helpedand supported me. I would like to thank them all, because without them reachingthis point of the adventure would have been tougher than what it has been, oreven impossible.

    First of all, I would like to thank the Institut Laue-Langevin for giving me athree-year fellowship to do the thesis. I would like to thank Prof. J. Rodriguez-Carvajal for kindly accepting me in the Diffraction Group, as well as to all themembers of this group.

    This work would not be possible without the hundreds of hours of beamtime used in distinct installations. Therefore, I am grateful to the Institut Laue-Langevin, to the CEA-Grenoble (CRG-D23), to the Spanish CRG-D15, LaboratoireLon Brillouin and European Synchrotron Radiation Facility for the provision ofbeam time.

    I acknowledge the financial support from MICINN (Spanish government)under Project No. MAT2009-09308.

    I would like to thank my supervisors, for their help and guide.

    I am very grateful to Prof. Alexander Mukhin and his team for providing uswith the samples and the electric measurements as well as for the useful discus-

    ix

  • sions we have had.

    I would like to thank Prof. Arsen Goukasov and Dr. Caroline Curfs for theirhelp in the experiments performed in the Laboratoire Lon Brillouin and EuropeanSynchrotron Radiation Facility, respectively.

    Thanks to Dr. N. Martin and Dr. L. P. Regnault we could perform a Larmor-diffraction experiment; Merci beaucoup!

    Dr. N. Qureshi for kindly cutting the samples, which made easier certaindiffraction experiments.

    Tambin quisiera agradecer al Prof. J. M. Perez-Mato por todo lo que me haenseado y por lo que me ha ayudado. El estudio de la supersimetra de las es-tructuras magnticas incommensurables no habra sido posible sin su dedicacin.

    Por supuesto, no quiero olvidarme de compaeros y amigos, por las (habit-ualmente largas) discusiones de distinta ndole (cientficas, polticas, lingsticas,muchas de ellas totalmente absurdas pero divertidsimas), cafs, cervezas, paseosen las montaas Grenobloises, excursiones . . . en definitiva por haber hecho que es-tos tres aos estn llenos de buenos recuerdos: scar, Laura C., Laura O., Alberto,Edurne, Carlos, Luiso, Ins, Pedro y otros muchos.

    Gracias Jessica por haberme ayudado en mis cortas estancias en Barcelona.

    Eskerrik asko Txoli! Nahiz eta urrin egon eta gutxitan ikusi gan, hor zarelakobeti!

    Maitetxu! Eskerrik asko hor egotearren. . . eta zelako azken hilabetiak! Ospatubiharra dago! Bixok be nahikua aguantau dou alkarri. Eskerrik asko!!

    Mersis Josu. Zure lanaren ondorio be badalako. Lehen aukera zuk emanzoztazulako. Zu barik ezinezkua izango zalako.

    Eskerrik bereziena familiari, lan hau zeuon esfortzuaren fruitu be badalako.

    Eta orain, uztak batzeko garaia da...

  • Preface

    The manuscript summarizes the work done during the last three years in the In-stitut Laue Langevin (Grenoble) in collaboration with the Instituto de Ciencia deMateriales de Barcelona, the Universitat Autnoma de Barcelona and the Commis-sariat lnergie atomique et aux nergies alternatives (Grenoble).

    Since ferroelectricity was found to be coupled to the magnetic order in TbMnO3,there has been a renewed interest in multiferroics, both for fundamental reasonsand for possible technological applications.

    About five year ago, it was discovered that the electric polarization thatemerges in MnWO4 is coupled to a cycloidal magnetic structure. This material hasthree magnetic states: AF3 (13.5 K T 12.5 K), moments order collinearly along adirection u within the ac plane with a sinusoidally modulated amplitude and an in-commensurate propagation vector k = (-0.214, 12 , 0.457); AF2 (12.5 K T 6.8 K),it presents an additional magnetic component along b, the propagation vector be-ing the same. Thus, the magnetic ordering is an elliptical cycloidal spin structurewithin the ub plane. In this phase the electric polarization arises along the b axisand therefore, this is the multiferroic phase. AF1 (T 6.8 K), the magnetic struc-ture is collinear (along u) with k = ( 14 , 12 , 12 ).

    The succession of magnetic structures is induced by a strong magnetic com-petition which turns into magnetic frustration. The application of external fieldsor chemical substitution could unbalance the subtle equilibrium between the mag-netic interactions. The work on the thesis was initiated by a publication in the

    xi

  • literature (on polycrystalline samples) which showed that a subtle substitution ofmanganese by cobalt induces changes and stabilizes the multiferroic structure atlow temperatures. Moreover, new magnetic structures have been determined fromneutron-powder experiments.

    The thesis presents a study of the crystal and magnetic structures, and elec-tric polarization of Mn1xCoxWO4 (x =0, 0.05, 0.10, 0.15 and 0.20) at zero and non-zero magnetic field. We have studied the interplay between the magnetic structureand the electric polarization. To achieve our purposes bulk magnetometry, py-roelectric measurements, neutron- and synchrotron-diffraction experiments havebeen carried out on single crystals.

    The manuscript is organized in the following way:

    (i) Part I, Introduction: multiferroics, magnetic structures, the studied materialsand the experimental techniques used are introduced. At the end the interestand objectives of the thesis are enumerated.

    (ii) Part II, Magnetic, ferroelectric and structural properties of Mn1xCoxWO4 underzero magnetic field.

    (iii) Part III, Magnetic-field-induced transitions.

    (iv) Part IV, Summary and Conclusions.

    (v) Part V, Appendices.

    Throughout the thesis I have participated in all neutron diffrac