Article pubs.acs.org/IC
An Antiferro-to-Ferromagnetic Transition in EuTiO3−xHx Induced by Hydride Substitution Takafumi Yamamoto,† Ryuta Yoshii,† Guillaume Bouilly,† Yoji Kobayashi,† Koji Fujita,‡ Yoshiro Kususe,‡ Yoshitaka Matsushita,§ Katsuhisa Tanaka,‡ and Hiroshi Kageyama*,†,∥ †
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, and ‡Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan § Materials Analysis Station, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ∥ CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: We have prepared the oxyhydride perovskite EuTiO3−xHx (x ≤ 0.3) by a low temperature CaH2 reduction of pyrochlore Eu2Ti2O7 and perovskite EuTiO3. The reduced EuTiO3−xHx crystallizes in the ideal cubic perovskite (Pm3̅m), where O/H anions are randomly distributed. As a result of electron doping by the aliovalent anion exchange, the resistivity of EuTiO3−xHx shows metallic temperature dependence. Moreover, an antiferromagnetic-to-ferromagnetic transition is observed even when a small amount of hydride (x ∼ 0.07) is introduced. The Curie temperature TC of 12 K is higher than those of any other EuTiO3-derived ferromagnets. The ferromagnetism can be explained by the Ruderman−Kittel−Kasuya− Yosida (RKKY) interaction between the Eu2+ spins mediated by the itinerant Ti 3d electrons. The present study shows that controlling the oxide/hydride ratio is a versatile method to tune magnetic and transport properties. Eu2+ ions, seems more suitable to investigate the relation between carrier concentration and the TC. However, the TC of EuTiO2.86 thin film (5 K) is much less than the maximum TC for the Eu-site substituted case,6 which might result from greater chemical randomness induced by oxygen defects. Furthermore, the lack of EuTiO3−δ samples other than δ ∼ 0.14, which is related to synthetic difficulties, does not permit systematic study on physical properties as a function of δ. Since 2002, several transition metal oxyhydrides, such as LaSrCoO3H0.7 and Srn+1VnO2n+1Hn, have been prepared by a topochemical reaction using CaH2 as a reducing agent.11,12 We have recently reported a perovskite oxyhydride BaTiO3−xHx synthesized by the same method from BaTiO3 as a precurcor.13 The hydride concentration can be varied in the range of 0 ≤ x ≤ 0.6. This aliovalent H/O exchange supplies electrons to the conduction band derived from the Ti 3d t2g orbitals, and as a result the insulating AETiO3 (AE = Ba, Sr, Ca) changes to a Pauli paramagnetic metal, with a room temperature (rt) resistivity of 3 × 10−4 Ω cm for SrTiO2.75H0.25.14 Contrary to the oxygen deficient BaTiO3−x film, itinerant electrons can be introduced to a greater extent and in a controllable fashion, allowing an extensive and systematic study as a function of carrier concentration. In addition, the chemical disorder induced by the H/O substitution may be smaller than that
1. INTRODUCTION Europium titanate perovskite EuTiO3 is an antiferromagnetic (AFM) insulator with the Néel temperature TN of 5.5 K.1,2 This AFM state originates from AFM superexchange interactions between the Eu2+ 4f spins via the nonmagnetic Ti4+ 3d orbitals.3 Interestingly, by applying a tensile lattice strain to an epitaxial EuTiO3 thin film on a DyScO3 substrate, competing ferromagnetic (FM) exchange interactions via the Eu2+ 5d states become dominant,3 leading to a FM transition together with a paraelectric-to-ferroelectric transition.4,5 The FM state is also induced by supplying conduction electrons to the otherwise empty Ti 3d orbitals. This can be achieved by aliovalent substitution at the Eu site (Eu1−xLaxTiO3)6−8 or by creation of oxygen defects (EuTiO3−δ).9 This ferromagnetism is ascribed to magnetic interactions between localized Eu2+ 4f spins mediated by itinerant Ti 3d electrons, which is termed the Ruderman−Kittel−Kasuya−Yosida (RKKY) interaction.6 From a quantitative point of view, however, the observed behaviors in the FM metal are not fully understood yet. For example, a complex relation between carrier density and FM transition temperature (TC) is observed. In Eu1−xLnxTiO3 (Ln = La and Gd),6 the TC decreases with increasing x (e.g., TC = 8 K at x = 0.1 of La and TC = 4 K at x = 0.5 of Gd). Katsufuji and Tokura9 pointed out that such reduction in TC might be originated from the RKKY oscillation10 or the electroncorrelation effect in the Ti 3d state. Carrier doping by oxygen defects, which does not break a magnetic network composed of © XXXX American Chemical Society
Received: October 10, 2014
A
DOI: 10.1021/ic502486e Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry induced by oxygen “vacancy” because of the similarity in ionic radii between H− and O2− ions.15,16 In this paper, we have prepared polycrystalline samples of EuTiO3−xHx (x ≤ 0.3) from perovskite EuTiO3 and pyrochlore Eu2Ti2O7 by CaH2 reduction. An epitaxial thin film of the corresponding oxyhydride is also prepared for a transport measurement. It is found that EuTiO3−xHx (x ≥ 0.07) is metallic and undergoes a transition from the AFM to the FM state, the first oxyhydride to exhibit ferromagnetism. The highest TC of 12 K is attained among EuTiO3-based compounds, which is discussed in terms of the suppressed chemical disorder.
Oe from 2 to 300 K. Magnetic hysteresis was measured on a vibrating sample magnetometer attached to a Physical Properties Measurement System (PPMS, Quantum Design) at 4 and 100 K, with applied field ranging from −5 to 5 T. An epitaxial EuTiO3 thin film with a thickness of 50 nm was deposited on a single crystalline substrate of (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) using the pulsed laser deposition (PLD) technique.25 Deposition was made using a KrF excimer laser at λ = 248 nm, with a deposition rate of 10 Hz and a laser fluence of 0.3 J/cm2. The substrate temperature was kept at 800 °C with an oxygen partial pressure of 1.0 × 10−4 Pa during the deposition. The EuTiO3 film was embedded with 0.2 g of CaH2 powder in a Pyrex tube and then sealed under vacuum ( 0) increased remarkably below 12 K, indicating the occurrence of a FM
Figure 3. (a) Temperature dependence of magnetic susceptibility for EuTiO3−xHx (x = 0, 0.07, 0.15, 0.30). The field of 50 Oe was applied after zero field cooling. The inset shows the dχ/dT versus T curve. The TC are determined from the inflection point, and all the compounds show TC = 12 K. (b) The inverse susceptibility. (c) Magnetic hysteresis of EuTiO3 and EuTiO2.70H0.30 at 4 K. The inset shows the M−H curve of EuTiO3 in a low field region at 2 K. The M− H curve of EuTiO3 is concave up, indicating the AFM ground state in zero field.
Table 4. Curie Constant and Weiss Temperature for EuTiO3 and its Hydride-Reacted Samples composition
C (emu K mol−1)
ΘW (K)
EuTiO3 EuTiO2.93H0.07 EuTiO2.85H0.15 EuTiO2.70H0.30
6.66(1) 7.33(1) 7.46(1) 7.01(1)
3.99(11) 8.51(2) 10.23(2) 10.73(2)
transition (Figure 3a). The TC’s of EuTiO3−xHx were determined from the inflection point in the dχ/dT−T curve D
DOI: 10.1021/ic502486e Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
progressive suppression of the anomaly around TC. Such negative magnetoresistance is typically observed in ferromagnetic metals.32−34 Hall coefficient measurements for the oxyhydride film revealed a carrier concentration of 3.9(4) × 1021 cm−3 at 300 K. Suppose the substituted hydride ions entirely provide itinerant electrons in a conduction band of Ti 3d t2g orbitals. Then, the composition of EuTiO2.79H0.21 gives 3.62 × 1021 cm−3, which is very close to that obtained from the Hall coefficient measurement. This means that all the donor electrons are in the conduction band. As mentioned in the Introduction, two mechanisms have been proposed as the origin of the AFM-to-FM transition in EuTiO3-based compounds. One mechanism is the enhanced FM interactions by the lattice strain.4,5 It is theoretically shown that +0.7% or −1.1% of biaxial strain4 or 3% of isotropic expansion3 (vs bulk EuTiO3) stabilizes the FM phase. In the present case, however, EuTiO3−xHx (x > 0) is of cubic symmetry with a cell parameter only slightly longer than the bulk (
