报告题目：Giant Negative Thermal Expansions Induced by Electronic Phase Transitions

报 告 人：Prof. Masaki Azuma, Institute of Integrated Research, Institute of Science Tokyo
报告时间：2026年8月27日 (星期四) 14:30

报告地点：M楼249会议室

 

Abstract

Negative thermal expansion (NTE) materials which shrink on heating attracts the keen attention because these can compensate for the thermal expansion of structural materials by making composites and solve the critical problems caused by the thermal expansion. We utilize 6s² lone pair activity of Pb²⁺ and Bi³⁺ and valence skipping nature of these ions for exploration of NTE materials [1]. PbVO₃ is a PbTiO₃-type compound with an enhanced polar tetragonal structure (c/a = 1.23) owing to dxy orbital ordering of V⁴⁺ (d¹). Hole doping by Bi³⁺ substitution for Pb²⁺ decreases the polar distortion and enables temperature induced polar-nonpolar transition accompanied by ~ 9 % volume shrinkage [2, 3]. Similarly ferroelectric transition temperature of BiFeO₃ can be reduced by A- and B-site substitutions and NTE has been achieved [4]. BiNiO₃ is a perovskite compound stabilized by high-pressure (HP) synthesis at 6 GPa. It has a characteristic valence distribution of Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃ and undergoes a pressure induced intermetallic charge transfer transition resulting in Bi³⁺Ni³⁺O₃ HP phase above 4 GPa. This transfer causes Ni’s valence to change from Ni²⁺ to Ni³⁺, leading to the Ni–O bond contracting and unit cell volume de-creasing by 2.5% [5]. In the case of BiNi_1-xFe_xO₃, the charge transfer transition between Bi⁵⁺ and Ni²⁺ can be induced by heating at ambient pressure (AP), leading to an NTE [6, 7]. Similarly, PbCrO₃ exists in a Pb²⁺_0.5Pb⁴⁺_0.5Cr³⁺O₃ valence distribution at AP and exhibits a 9.8% pressure-induced volume collapse [8]. We investigated the phase relation of PbCrO₃ in the pressure-temperature space and found that, contrary to BiNiO₃, PbCrO₃ returns to the ambient pressure phase when the temperature is increased under pressure. The slope of the phase boundary in the P-T phase diagram of BiNiO₃ is negative because the metallic Bi³⁺Ni³⁺O₃ HP-LT phase has higher entropy than Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃. On the other hand, glassy distribution of Pb²⁺ and Pb⁴⁺ enhances the entropy of the Pb²⁺_0.5Pb⁴⁺_0.5Cr³⁺O₃ phase and the phase boundary has a positive slope [9]. Large thermal expansion rather than NTE is expected in PbCrO₃ if the high-pressure phase is stabilized by chemical substitutions and indeed, Pb_0.7Ca_0.3CrO₃ exhibits approximately 12% unit cell volume expansion on heating between 300 – 400 K [9].

 

References

[1] M. Azuma et al., Annu. Rev. Mater. Res. 51, 329 (2021).

[2] H. Yamamoto et al., Angew. Chem. Int. Ed. 57, 8170 (2018).

[3] T. Nishikubo et al., Chem. Mater. 35, 870 (2023).

[4] K. Hatayama et al., J. Am. Chem. Soc., 147, 44845 (2025).

[5] M. Azuma et al., Nat. Commun. 2, 347 (2011).

[6] K. Nabetani et al., Appl. Phys. Lett. 106, 061912 (2015).

[7] T. Nishikubo et al., J. Am. Chem. Soc. 141 19397 (2019).

[8] R. Yu et al., J. Am. Chem. Soc. 137, 12719 (2015).

[9] Q Liu et al., Chem. Mater., 37, 3305 (2025).

 

Biography

Professor Masaki Azuma obtained his Ph.D. from Kyoto University, Japan in 1995. From 2004 to 2010, he served as an associate professor at the Institute for Chemical Research, Kyoto University. Currently, he is a professor at Institute of Science Tokyo, Japan. He is the chairman of the World Research  Hub Initiative at Institute of Science Tokyo, and a council  member of the High Pressure Science and Technology Advanced Research Institute (HPSTAR) in Japan. He is a senior researcher in the field of solid-state physics and chemistry. He has discovered various functional new materials such as spin ladder compounds, ferromagnetic ferroelectrics, lead-free piezoelectric materials, and negative thermal expansion materials by high-pressure synthesis, and clarified their functional manifestation mechanisms by synchrotron radiation X-ray diffraction and spectroscopy.

 

报告时间：2026年8月27日14:30

报告地点：M楼249会议室

邀 请 人：龙有文

联 系 人：潘  昭（82649750）

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报告题目：Domain Structure and Magnetic Reversal by Electric Field in Cobalt-substituted Bismuth Ferrite Thin Films and Nanodots

报 告 人：Dr. Kei Shigematsu, Institute of Integrated Research, Institute of Science Tokyo

报告时间：2026年8月27日 (星期四) 15:30

报告地点：M楼249会议室

Abstract

Magnetization reversal induced by an electric field in multiferroic materials has been extensively investigated because it can be applied to ultra-low-power voltage-write magnetic-read-out memory devices. Co substitution in BiFeO₃ (BiFe_0.9Co_0.1O₃; BFCO) destabilizes the cycloidal spin modulation and generates a canted collinear spin state with a non-zero saturation magnetization [1]. We have examined the magnetic and ferroelectric domain structures of BFCO thin films before and after polarization switching using piezoresponse force microscopy (PFM) and magnetic force microscopy (MFM), and clarified how the magnetoelectric response is determined by the polarization switching pathways, i.e., 71°, 109°, and 180° [2]. In the case of (110)_pc-oriented BFCO/SrTiO₃ (110) films, we demonstrated that 109° polarization switching under an electric field along [1-10]_pc deterministically reversed the out-of-plane magnetization component. This provides an in-plane-write/out-of-plane-read configuration highly advantageous for device applications [3]. We have also examined the ferroelectric and magnetic domains of BFCO nanodots fabricated by deposition through an anodized porous alumina mask. PFM and MFM confirmed that 60-nm-diameter dots are single-domain in both states, whereas 190-nm dots are multidomain, and the comparison of the two domain patterns indicates strong magnetoelectric coupling [4]. More recently, scanning NV magnetometry, free from the tip stray field inherent to MFM, provided quantitative stray-field maps of individual 190-nm BFCO nanodots and revealed reversal of the net magnetization upon electric-field poling [5]. Such insights into ferroelectric and ferromagnetic domains are essential for the design of high record-density BFCO memory devices.

References

[1] H. Hojo et al., Adv. Mater., 29, 1603131 (2017).

[2] K. Shigematsu and M. Azuma, Appl. Phys. Express 19, 040101(2026).

[3] T. Itoh et al., Adv. Mater. 37, 2419580 (2025).

[4] K. Ozawa et al., ACS Appl. Mater. Interfaces 16, 20930 (2024).

[5] K. Lee et al., Sci. Adv. 12, eaec2861 (2026).

 

Biography

Dr. Kei Shigematsu obtained his Bachelor and Master degrees from The University of Tokyo, Japan, and obtained his Ph.D. from Tokyo Institute of Technology, Japan in 2015. From 2015 to 2016, he served as a researcher at the PIIF Kanagawa Academy of Science and Technology, Japan. From 2016 to 2018,  he is a specially appointed assistant professor at Laboratory for Materials and Structures, Institute of Science Tokyo, Japan. Currently, he is an assistant professor at Laboratory for Materials and Structures, Institute of Science Tokyo, Japan. His research interests manily focused on synthesis, fabrication, and nanofabrication of perovskite oxides and related compounds, and invesigating related magnetic, electric, electric conduction, dielectric, ferroelectric, and optical properties.

 

报告时间：2026年8月27日15:30

报告地点：M楼249会议室

邀 请 人：龙有文

联 系 人：潘  昭（82649750）

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报告题目：Application and Demonstration of Novel Properties in the Electronic Ferroelectric RFe₂O₄

报 告 人：Dr. Hongwu Yu, Institute of Integrated Research, Institute of Science Tokyo
报告时间：2026年8月27日 (星期四) 16:30

报告地点：M楼249会议室

Abstract

The electronic ferroelectric material RFe₂O₄ possesses a layered structure in which rare-earth/oxygen layers (R-layers) and iron/oxygen double layers (W-layers) alternate; it was predicted to exhibit ferroelectricity through the formation of a charge ordered within the W-layers formed by Fe²⁺ and Fe³⁺ ions[1]. This charge order arises from the effects of charge frustration, where adjacent ions share the same valence state, leading to erexchange interactions. Furthermore, since the spins of these identical ions are also aligned, the system—incorporating spin exchange interactions, adopts a highly degenerate energy state. Consequently, as the temperature rises, this polar charge order gains entropy associated with this degeneracy and become stable. It is through the complex interplay of thermal, magnetic, and electric factors that the material becomes a room-temperature ferroelectric [2]. Such ferroelectric polarization was expected to exhibit various interesting properties. For instance, since the polarization is associated with charge ordering, it’s reversal should be able to achieve through the movement of electrons., which is lighter than moving atoms, that can be anticipated to have a very low coercive field. Furthermore, given the presence of charge ordering coupled with magnetism, it is conceivable that this system exhibits multiferroic polarization capable of being reversed by a magnetic field. However, because this system exhibits high electrical conductivity, standard techniques for evaluating ferroelectricity, such as pyroelectric current measurements cannot be applied, making experimental verification challenging. Hence, we developed a serials methods utilizing pulsed lasers that avoids current generation. By this approach, we demonstrated that RFe₂O₄ possesses a coercive field of 15 V/cm, which is four orders of magnitude lower than that of conventional ferroelectrics. Furthermore, we also demonstrated that ferroelectric polarization of RFe₂O₄ can be reversed by a magnetic field as low as 800 G. These results indicate the material's potential for application as a novel semiconductor in the development of energy efficient devices. We also conducted experiments demonstrating polarization control via ultrafast optical electric fields and the generation of reversible terahertz electric fields [3]. Such result we plan to present at this event.

 

Reference

[1] Sumio Ishihara, J. Phys. Soc. Jpn. 79, 011010 (2010).

[2] K. Fujiwara, H. Yu et al., Sci. Rep., 11, 4277 (2021).

[3] H. Yu et al., Materials 16, 1989 (2023).

  

Biography

Dr. Hongwu Yu obtained his Bachelor and Master degrees from Tokyo Institute of Technology, Japan, and obtained his Ph.D. from Tokyo Institute of Technology, Japan in 2024. From 2024 to 2025, he served as an assistant professor in the department of physics, Okayama University, Japan.  Currently, he is an assistant professor at the insittute of integrated research, Institute of Science Tokyo, Japan.  His research interests manily focused on synthesis, fabrication, and invesigating related magnetic, electric, and optical properties of inorganic materials.

 

报告时间：2026年8月27日16:30

报告地点：M楼249会议室

邀 请 人：龙有文

联 系 人：潘  昭（82649750）