Introduction

EuTiO3 (ETO) is quite unique material because magnetic Eu2+ and dielectric Ti4+ ions coexist in the same material. ETO with simple cubic perovskite structure is an incipient ferroelectric and exhibits quantum paraelectric behavior. Eu spins show G-type antiferromagnetic (AFM) ordering at Neel temperature (TN) of 5-5.5 K and dielectric constant (Er) drops sharply below TN. When an external magnetic field is applied to ETO, a ferromagnetic (FM) state appears, and Er below TN simultaneously increases. This coupling between magnetic and dielectric properties is known as magnetodielectric (MD) coupling. Recent first principle calculations have predicted that magnetic and dielectric properties of ETO are quite sensitive to cell parameters. It is expected that hydrostatic expansion of cell volume switches from AFM to FM ground state, and that biaxial strain may divergently enhance MD coupling. Furthermore, multiferroic behavior is also expected in strained ETO. For studying magnetic and dielectric properties of ETO as a function of cell parameters, epitaxial thin films are suitable. Strain can be introduced through lattice mismatch between the film and the substrate in a controlled manner. In addition, thin films, in general, have different cell volume from bulk, even if they are fabricated on a substrate without lattice mismatch. Such variation of cell volume would significantly affect the physical properties, particularly magnetic properties of ETO through anisotropic exchange interaction, which also may have influence on MD coupling. However, there have been only a few reports of the epitaxial growth of ETO films, especially on dielectric properties. This is mainly because it is difficult to fabricate high quality ETO films. Divalent Eu is stabilized relatively reducing condition, while tetravalent Ti in relatively oxidizing condition. In my doctor thesis, I have investigated magnetic and dielectric properties of ETO as influenced by cell parameters, using high quality ETO epitaxial films.

Experimental

ETO thin films were grown on Nb (0.05wt%)-doped SrTiO3 (NSTO) (100) substrates, which have no lattice mismatch with ETO, by pulsed laser deposition (PLD) at substrate temperature (Ts) of 650 - 1250 oC and oxygen pressures (Po2) in a range of 10-5- 10-8 Torr. Laser pulses with a fluence of 700 -1000 mJ / (cm2 shot) and a frequency ( fl ) in the range of 0.3 - 10 Hz were supplied by a KrF excimer laser (=248 nm). As a PLD target, a pyrochlore Eu2Ti2O7 polycrystalline pellet were employed. Crystallinity and surface morphology of the films were characterized by in situ reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), and atomic force microscopy. Magnetic and dielectric properties were measured using a SQUID susceptometer and a LCR meter, respectively. In LCR measurements, probe voltage E of 250mV was applied perpendicular to the film surfaces. As a top electrode, Ag was deposited by resistive heating, and Nb-doped (0.05wt%) STO substrates were used as a bottom electrode.

Epitaxial growth of ETO thin films

To find an optimized growth condition for high quality ETO films, I first constructed a growth phase diagram of EuTiOx film as shown in Fig. 1, where Ts was fixed at 650oC and Po2 and laser frequency were varied. The diagram clearly reveals a tendency that lower Po2 and higher fl conditions, that is, relatively reducing conditions are necessary for stabilizing the perovskite ETO phase, which consist of divalent Eu. In these conditions, ETO film growth proceeds in layer-by-layer mode, as seen from RHEED intensity oscillation in Fig.2(a), and atomically flat surface can be obtained. XRD measurement revealed that in-plane lattice constant of ETO films is completely locked to STO substrate (a = b = 3.905A), while the out-of-plane lattice constant is longer than that of bulk ETO (3.933 A ≦ c≦ 3.988A, 1.007≦ c/a ≦ 1.021). In higher Po2 and lower fl conditions in Fig.1, that is, in relatively oxidizing conditions, layered perovskite Eu2Ti2O7 with trivalent Eu, and amorphous phase appeared.

On increasing Ts beyond 920 oC, growth mode of ETO film changed from layer-by-layer mode to step-flow mode. Fig. 2(b) depicts a RHEED intensity profile monitored during step-flow growth (Ts = 950oC, Po2=1x10-7 and fl = 0.3 Hz). In this condition where step-flow growth mode is dominant, ETO films with a = c = 3.905A (c/a = 1) were obtained. At such higher Ts, surface morphology of ETO films was quite sensitive to Po2, partly due to the formation of layered perovskite Eu2Ti2O7 phase. At higher Ts, oxygen atoms easily defuse into perovskite ETO through the film surfaces, resulting in generation of trivalent Eu. As a consequence, I have determined Ts = 1250 oC, Po2 = 3x10-8 Torr, and fl = 2 Hz as an optimized film growth condition. To compensate oxygen vacancies generated in such a strongly reducing growth condition, conductive NSTO capping layer, which protects ETO form surface oxidation, were deposited on ETO films and annealing in air at 400oC for 24hours were performed. By careful tuning of growth condition as is mentioned above, high quality ETO films with systematically different cell parameters (1 ≦ c/a≦ 1.021 ) were successfully fabricated.

Magnetic properties of epitaxial ETO thin films

Fig. 3(a) and (b) show magnetization vs. temperature (M-T) curves of an ETO films with different cell parameter (c/a) under different magnetic field. In fig. 3(a), where external magnetic field perpendicular to the film surface (H⊥) was applied, all the films with different c/a showed characteristic cusp structure at 4.2-5.4 K, which correspond to TN and indicate that all the films undergo antiferromagnetic (AFM) transition. On the other hand, in fig. 3(b), where parallel magnetic field (H//) applied, cusp structure clearly depend on c/a. Fig. 4 depict the correlation between magnetization at 2 K under H// and c/a and cell volume (c2/a). This implies that larger c/a or cell volume lead to destabilize AFM ordering, or in other word, prefer FM ordering. These results demonstrate that the antiferromagnetism in ETO is quite sensitive to cell parameter, which is in good agreement with the recent band calculation.

Dielectric properties and coupling between magnetic and dielectric properties

Fig. 5 showsEr vs temperature (Er -T) curves measured under various magnetic fields (H⊥). Notably, the ETO film with c/a=1.011 exhibits quantum paraelectric behavior and sharp drop ofEr at TN under zero magnetic field. Fig. 6 (a) is a close-up view of the low temperature part (2-10 K) of Fig. 5. Fig. 6 (b) plots magnetization in the same temperature region. With increasing external magnetic field, the cusp in M-T curve in Fig. 6(b) tends to be suppressed and completely disappears at 5 T, corresponding to the evolution of FM ordering. In synchronization with the FM ordering,Er increases. This is the first observation of MD coupling in ETO thin films. For observation of quantum paraelectricity and magnetodielectric effect in thin film samples, dielectric loss factor (D), which determines leakage current, must be as low as possible. The typical D value of our samples is evaluated to be 10-3 at 2 K. Such low D is a consequence of high quality of the present ETO samples.

The shape of εr - T curves was found to be insensitive to the direction of magnetic field (H⊥ and H//,) and cell parameters (c/a). On the contrary, εr - H curves shows clear dependence on the direction of magnetic field and c/a. Fig.7 shows Δεr(H) vs H curves of ETO films with different c/a, where Δεr(H) represents [εr(H)- εr(0)]/ εr(0) and corresponds the degree of MD response. Evidently, under both H⊥ and H// larger c/a weakens MD coupling. At the same time, each sample exhibits anisotropic MD coupling depending on magnetic field direction. To clarify this anisotropy, Δεr(H⊥)/Δ4.(H//) vs H curve were plotted in Fig.8. Surprisingly, all the samples with different c/a show the same value, Δεr(H1)/ Δεr (H//) = 1.2 at higher H. In bulk ETO, it has been reported that εr as a function of temperature and magnetic field can be expressed by an empirical formula εr(T,H) = ε0(7)[1+α<Si・Sj>H], where <Si・Sj>H is the Eu spin pair correlation, and α is the spin-phonon coupling constant, related to the derivative of exchange interaction (ref.1,2). The anisotropic behavior of Δεr(H), as shown in Fig. 8, suggests that the a parameter is dependent on the direction of spin pairs. Here I further assume that a is the product of c/a dependent term, ada, and spin-pair-direction-dependent term, al. Then, the MD coupling in ETO films is given as Δεr(H⊥)/Δεr(H//) = [α(c/a)α(ij)(5T,H⊥)+α(c/a)α(ij)(0T)]/[α(c/a)α(ij)(,TSH//)+α(c/a)α(ij)(0T)] = [α(ij)(5T,H⊥)+α(ij) (0T)]/[α(ij)(5T,H//)+α(ij) (0T)] = 1.2, where <Si.Sj>ST,H⊥ = <Si・Sj>ST,H//= - <Si・Sj>0T in the mean field approximation at 2 K(ref.1). For better understanding of microscopic mechanism of MD coupling, the spin-pair-direction-dependence of a should be considered.

Summary

I have investigated magnetic and dielectric properties of ETO films as functions of cell parameters. As a result, I observed that the increase of cell volume destabilizes the AFM ordering, being consistent with the result of band calculation. I also found that anisotropy in MD coupling, i.e., spin-pair-direction-dependence of△Er(H), which seems to be a key phenomenon for better understanding of microscopic mechanism of MD coupling.

Fig1. Growth phase diagramof EuTiOx at Ts = 650oC

Fig2. RHEED specular spotintensity during ETO filmdeposition

Fig.3. M - T curve of the ETO film under (a) perpendicular magnetic field (H⊥) and (b) parallel magnetic field (H//)

Fig.4. Magnetization at 2K under H⊥ vs. c/a and cell volume (V = c2/a) curve

Fig.5Er vs temperature curve under various magnetic field

Fig.6 temperature dependenceof Er and magnetization

Fig.7. △Er(H)- H curve of the ETO film under H⊥ and H// at 2K

Fig.8 △Er(H⊥)/△Er(H//) - Hcurve at 2K

Ti4+をBサイトとするペロブスカイト型酸化物A2+Ti4+O3は数々の興味深い誘電物性を示すことが知られているが、中でも、大きな磁気モーメントを有するEu2+をAサイトとするEuTiO3(ETO)は、外部磁場により誘電率が増大する磁気誘電効果を示し、磁性と誘電性との間に強い結合が存在する点で特異的である。本研究では、ETOエピタキシャル薄膜の磁性-誘電性結合に関して取り扱っている。

本論文は以下の6章より構成されている。

第1章は序論であり、本論文の背景および目的が述べられている。本論文で取り扱うEuTiO3(ETO)の諸物性を、典型的2価Eu化合物であるEuOおよび4価Ti化合物であるSrTiO3との対比を通じて概観している。また、ETOの磁性、誘電性、およびそれらの間の結合(磁気誘電効果)が格子定数に対し多彩な応答を示すという、近年の第一原理計算による理論予測に触れている。格子定数を変調させるにはエピタキシャル薄膜が適しているが、従来のETO薄膜の研究は、主にバルク同様の磁気特性が得られるかどうかに主眼が置かれており、格子定数依存性や、特に誘電性に関する研究は、高品質工ピタキシャル薄膜試料の作製の困難さゆえ、ほとんど報告例がないことを指摘している。

第2章は実験手法の説明である。薄膜作製方法であるパルスレーザー蒸着法、および薄膜結晶構造解析手法である反射高速電子線回折(RHEED)、X線回折(XRD)、原子間力顕微鏡(AFM)について、各測定手法の原理とそこから得られる情報について解説している。さらに、超伝導磁束量子干渉計(SQUID)による磁気物性測定法、およびLCRメータを用いた低温・磁場下における誘電物性測定法について詳説している。

第3章はSrTiO3(001)単結晶基板上に作製したETO(001)薄膜のエピタキシャル成長に関して述べている。薄膜成長を支配するパラメータとして、基板温度(Ts)、酸素分圧(Po2)、レーザー発振周波数(f1)の3つに着目し、ETOの結晶成長相図を作製することで、ETOが比較的還元雰囲気でのみ成長することを明らかにしている。そして薄膜成長機構はTsに依存し、高Tsではlayer-by-layer成長からstep一flow成長へと切り替わることを述べている。また、薄膜面内の格子定数(a=b)は基板と完全にコヒーレントに成長している一方、レーザーフルーエンスを調整することで、面直方向の格子定数(c)を1≦c/a≦1.021(c/aは格子歪みの程度を表す)の範囲で系統的に変調させうることを示している。本研究で得られたETO薄膜は全て原子レベルの平坦性を有し、高品質であることを確認している。

第4章は第3章で得られたETOエピタキシャル薄膜の磁性について述べている。1≦c/a≦1.021の範囲ではすべてネール温度4.2-5.4Kの反強磁性であるものの、c/aが増大するにつれて反強磁性秩序が壊れて強磁性的秩序をとりやすくなることを示している。これは第一原理計算で予測された傾向と一致することを指摘している。さらにclaが1からずれることで反強磁性スピン対がc軸方向に向きやすくなることを新たに見出している。

第5章はETOエピタキシャル薄膜の誘電性と磁気誘電効果について述べている。本研究以前のETO薄膜試料では報告されていない低温での誘電率の飽和(量子常誘電性)、および磁場印加による誘電率の増加(磁気誘電カップリング)の観測に初めて成功している。さらに、磁気誘電効果はc/aの増大とともに弱くなり、磁場印加方向に対して異方的な挙動を示すことを新たに見出している。この磁場印加方向に対する依存性は、従来の磁気誘電効果のメカニズムでは説明できないことから、スピン-フォノン結合定数がc/a依存項とスピン対の方向依存項の積であらわされるような新しいモデルを提唱した。

第6章は結論と総括である。

以上のように、本研究はEuTiO3の磁性および誘電性が格子定数により変調できることを実験的に明らかにしており、磁気誘電効果に関しても、格子定数依存性を検証するとともに、これまで考えられていなかったスピン対の方向依存性を見出し、新たなモデルを提案するに至っている。これらの研究は理学の発展に大きく寄与する成果であり、博士(理学)に値する。なお本論文は複数の研究者との共同研究であるが、論文提出者が主体となって行ったものであり、論文提出者の寄与は十分であると判断する。

したがって、博士(理学)の学位を授与できると認める。