Multiferroic hexagonal HoMnO_1tn3 films [Elektronische Ressource] / von Jong-Woo Kim

Multiferroic hexagonal HoMnO_1tn3 films [Elektronische Ressource] / von Jong-Woo Kim

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Multiferroic hexagonal HoMnO lms3Von der Fakult at MaschinenwesenderTechnischen Universit at DresdenzurErlangung des akademischen GradesDoktoringenieur (Dr.-Ing.)angenommene DissertationvonM.Sc. Jong-Woo Kimgeboren am 21. Mai 1977in Chun-Cheon, Sud-Korea20091. Gutachter: Prof. Dr. Ludwig Schultz (TU Dresden)2. Gutachter: Prof. Dr. Manfred Fiebig (Universit at Bonn)Tag der Einreichung: 29.07.2009Tag der Verteidigung: 22.12.2009AbstractThe fundamental properties of hexagonal multiferric HoMnO lms have been thor-3oughly investigated. The lms are grown by pulsed laser deposition on Y:ZrO (111)2substrates. High quality epitaxial HoMnO lms of 25 { 1000 nm thickness were3successfully prepared. The lm properties are compared to those of single-crystals.The magnetization measurements revealed that the lms show a deviating magneticbehavior from the single-crystals in several ways. For instance, the lms have a3+weakened antiferromagnetic Ho order con rmed from magnetic susceptibility. Thedi erences are likely to be related to the modi ed (mostly larger) lattice parametersof lms. An approximate phase diagram in comparison with the single-crystal’sone is constructed. For multiferroicity investigations, Second Harmonic Generation(SHG; in collaboration with the group of M. Fiebig) has been employed. By SHG,the ferroelectric polar order of the lms is obviously con rmed.

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Multiferroic hexagonal HoMnO lms3
Von der Fakult at Maschinenwesen
der
Technischen Universit at Dresden
zur
Erlangung des akademischen Grades
Doktoringenieur (Dr.-Ing.)
angenommene Dissertation
von
M.Sc. Jong-Woo Kim
geboren am 21. Mai 1977
in Chun-Cheon, Sud-Korea
20091. Gutachter: Prof. Dr. Ludwig Schultz (TU Dresden)
2. Gutachter: Prof. Dr. Manfred Fiebig (Universit at Bonn)
Tag der Einreichung: 29.07.2009
Tag der Verteidigung: 22.12.2009Abstract
The fundamental properties of hexagonal multiferric HoMnO lms have been thor-3
oughly investigated. The lms are grown by pulsed laser deposition on Y:ZrO (111)2
substrates. High quality epitaxial HoMnO lms of 25 { 1000 nm thickness were3
successfully prepared. The lm properties are compared to those of single-crystals.
The magnetization measurements revealed that the lms show a deviating magnetic
behavior from the single-crystals in several ways. For instance, the lms have a
3+weakened antiferromagnetic Ho order con rmed from magnetic susceptibility. The
di erences are likely to be related to the modi ed (mostly larger) lattice parameters
of lms. An approximate phase diagram in comparison with the single-crystal’s
one is constructed. For multiferroicity investigations, Second Harmonic Generation
(SHG; in collaboration with the group of M. Fiebig) has been employed. By SHG,
the ferroelectric polar order of the lms is obviously con rmed. The ferroelectric
switching at room temperature could be clearly demonstrated, whereas leakage of
lms requires generally a more sophisticated approach.
Kurzfassung
Die fundamentalen Eigenschaften von hexagonalen multiferroischen HoMnO Schi-3
chten werden eingehend untersucht. Die dunnen Schichten wurden mittels gepulster
Laserdeposition auf Y:ZrO (111)-Substraten gewachsen. Hochwertige epitaktische2
HoMnO -Dunnsc hichten von 25 { 1000 nm Dicke wurden erfolgreich hergestellt. Die3
Dunnsc hichteigenschaften werden mit denen von Einkristallen verglichen. Die Mag-
nitisierungsmessungen ergeben, dass die dunnen Schichten ein von den Einkristallen
in verschiedener Weise abweichendes magnetischen Verhalten zeigen. Zum Beispiel
3+haben die dunnen Schichten eine abgeschw achte antiferromagntetische Ho Ord-
nung, die durch die magnetische Suszeptibilit at best atigt wird. Die Unterschiede
sind wahrscheinlich auf die ver anderten (meistens gr osseren) Gitterparameter der
dunnen Schichten zuruc kzufuhren. Ein Phasendiagramm wird zum Vergleich mit
Einkristallen konstruiert. Durch Second Harmonic Generation (SHG; in Zusamme-
narbeit mit der Gruppe von M. Fiebig) wird die ferroelektrische Ordnung der dunnen
Schichten eindeutig best atigt. Das ferroelektrische Umschalten bei Raumtemperatur
kann eindeutig nachgewiesen werden, wobei durch den Leckstrom der dunnen Schi-
chten allgemein eine detailliertere Vorgehensweise ben otigt wird.Table of contents
Introduction 1
1 Fundamentals 5
1.1 Ferroic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Magnetoelectric e ect . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3 Symmetry considerations . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4 Curie-Weiss extrapolation . . . . . . . . . . . . . . . . . . . . . . . . 11
1.5 Nonlinear optics: Second Harmonic Generation . . . . . . . . . . . . 11
2 Hexagonal rare-earth manganites, HoMnO 153
2.1 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3+2.1.1 In uence of RE radius on crystal structure . . . . . . . . . . 16
2.1.2 Hexagonal HoMnO . . . . . . . . . . . . . . . . . . . . . . . 173
2.1.3 Ferroelectricity in hexagonal REMnO . . . . . . . . . . . . . 183
2.2 Magnetic structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.1 Spin arrangements . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.2 Magnetic interactions . . . . . . . . . . . . . . . . . . . . . . . 22
2.3 Phase diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.4 Magnetoelectric coupling . . . . . . . . . . . . . . . . . . . . . . . . . 26
3 Experimental methods 29
3.1 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2 characterization . . . . . . . . . . . . . . . . . . . . . . . . . 32
4 Epitaxially grown HoMnO thin films and capacitor layers 393
4.1 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.2 Lattice parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.3 HoMnO /YMnO superlattices . . . . . . . . . . . . . . . . . . . . . 433 3
iii Table of contents
4.4 Capacitor trilayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5 Magnetic properties and phase diagram of films and single-crystals 49
5.1 Temperature dependence of magnetization . . . . . . . . . . . . . . . 49
5.1.1 Curie-Weiss law . . . . . . . . . . . . . . . . . . . . . . . . . . 49
5.1.2 Spin reorientation temperature, T . . . . . . . . . . . . . . 51SR
5.1.3 Low temperature anomalies . . . . . . . . . . . . . . . . . . . 53
5.2 Magnetic eld dependence of the magnetization . . . . . . . . . . . . 60
5.2.1 Magnetic eld-induced phase transition . . . . . . . . . . . . . 60
5.2.2 High- eld transition of single-crystals . . . . . . . . . . . . . . 64
5.3 The HoMnO phase diagram . . . . . . . . . . . . . . . . . . . . . . . 653
5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6 Characterization by SHG 69
6.1 Electric polar order . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
6.2 Magnetic order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
7 Electrical properties 79
7.1 Ferroelectric polarization . . . . . . . . . . . . . . . . . . . . . . . . . 79
7.2 Experimental di culties with leakage current . . . . . . . . . . . . . 80
7.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
8 Conclusions and outlook 83
References 86Introduction
Multiferroics, the term introduced by Schmid [Schmid 94a], exhibit two or more
primary ferroic properties simultaneously in the same phase. Occasionally, if some
coupling exists among these ferroic order parameters, a new fascinating eld comes
into sight, i.e. the order parameters can be controlled in a joined way. This peculiar
phenomenon has got much attention because of both its intrinsic physical nature
and the potential applications [Spaldin 05, Eerenstein 06].
As ferroelectrics and magnets have served as crucial materials in industry for
some decades, the coexistence of these two properties has attracted strong interest.
The term magnetoelectric (ME) e ect describes the dependence of the magnetization
on an electric eld or of the ferroelectric polarization on a magnetic eld. This is
obviously desirable for device miniaturization trends, because a single device can
perform more than one task, and magnetization could be controlled power-less.
Some possible applications of magnetoelectric materials have been proposed al-
ready more than three decades ago [Wood 74]. However, the attempts to design
multiferroics that combine ferromagnetism and ferroelectricity in the same phase,
in particular those working at room temperature, have been unexpectedly di cult.
The smallness of the ME e ect and the rareness of ME materials has stagnated the
investigations into this topic.
Recently, a urry of research about ME e ects has been triggered again, be-
cause of the improved characterization techniques and newly discovered compounds
which have a strong ME coupling [Fiebig 05a]. The search for multiferroic mag-
netoelectric materials is categorized into two elds. One is the designed com-
posite materials, such as multilayers. For instance, when ferroelectric and ferro-
magnetic layers are combined, a large magnetoelectric coe cient can be achieved
through the interplay of these two layers [Ryu 01, D orr 07, Ramesh 07]. The other
is searching for single-phase materials. Since fundamental restrictions for the coex-
istence of ferroelectricity and magnetism have been revealed by theoretical works
[Hill 00, Hill 02, Khomskii 06], further mechanisms for the ME e ect have been
12 Introduction
proposed [Ederer 04, Khomskii 06]. TbMnO [Kimura 03] and TbMn O [Hur 04]3 2 5
are examples of single-phase multiferroics. These new candidates o er very good
access to explore the fundamental nature of magnetoelectric interactions. The cur-
rent trend for single phase multiferroics is summarized precisely in recent reviews
[Prellier 05, Eerenstein 06, Cheong 07].
The hexagonal rare earth manganites with a chemical composition of REMnO3
3+(RE = Lu { Ho, Y) form one group of single-phase multiferroics. Among these,
HoMnO attracts much interest because of its unusual interplay between the fer-3
roelectric and magnetic properties. HoMnO turned out to be a promising single3
phase multiferroic due to its large magnetoelectric coupling. In this compound, it is
observed that the magnetic phase can be reversibly controlled by an external elec-
tric eld [Lottermoser 04b]. Furthermore, the dielectric permittivity is in uenced by
magnetic phase transitions [Lorenz 04a, Yen 05]. These observations reveal a strong
coupling between the ferroic orders.
Recently, considerable experimental progress has been made in understanding
the physical properties of hexagonal HoMnO in various ways, e.g. by neutron3
di raction [Munoz~ 01, Vajk 05], non-linear optics (Second Harmonic Generation;
SHG) [Fohlicr h 99, Fiebig 02, Fiebig 03], and magnetic and dielectric properties
[Sugie 02, Lorenz 05] investigations. Most of these experiments have been done
with single-crystal samples or bulk sintered polycrystalline samples.
The aim of this work is to prepare and to characterize multiferroic HoMnO3
thin lms. When this work had been started in 2005, nothing was published about
HoMnO lms. Thus, the fabrication of high quality epitaxial thin lms was the3
starting point. Investigations of the low temperature magnetic properties and mag-
netoelectric behavior of HoMnO thin lms in comparison with single-crystals are3
the main objectives of this thesis work.
Chapter 1 and chapter 2 are devoted to background knowledge and a literature
survey of HoMnO . In chapter 1, the fundamental concepts and de nitions for3
multiferroics and magnetoelectrics are introduced. The symmetry classi cation of
hexagonal HoMnO and the magnetic interactions are brie y addressed. Further,3
fundamentals of magnetic and magneto-optical properties studied in this work are
described.
In chapter 2, the literature on the structural and physical properties of rare-earth
manganites, especially HoMnO single-crystals, is examined. This chapter explains3
the hexagonal structure, the origin of ferroelectricity as well as magnetic orders. The
magnetic phase diagram and direct proofs of magnetoelectric coupling in HoMnO3