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Feature Articles: Creating Novel Functional Materials

Creation of Novel Material Sr3OsO6 with the Highest Ferromagnetic Transition Temperature among Insulators

Yuki K. Wakabayashi, Yoshiharu Krockenberger, Yoshitaka Taniyasu, and Hideki Yamamoto

Abstract

We have synthesized a novel material, Sr3OsO6 (Sr: strontium, Os: osmium, O: oxygen) using a unique oxide thin-film growth technique that has been developed over many years at NTT Basic Research Laboratories. The Curie temperature (TC) value of this material, estimated from the magnetic measurements, is above 780°C, surpassing the TC record among insulators for the first time in 88 years by more than 100°C. As Sr3OsO6 has been synthesized in a single-crystalline thin film form, this brand-new material is expected to be readily implemented in high-performance magnetic device applications such as magnetoresistive random access memories and magnetic sensors that work above room temperature.

Keywords: molecular beam epitaxy, magnetic oxide, spin-orbit coupling

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1. Introduction

Ferromagnetism is a magnetic state in a material that gives it the properties of a magnet. In the ferromagnetic state, the net magnetic moment is large since the magnetic moments of the constituent atoms are aligned (Fig. 1). A ferromagnetic insulator is a magnet in which electric current cannot flow due to its high resistivity. Ferromagnetic insulators include maghemite, the first magnet that humans discovered and used as a compass. Today, ferromagnetic insulators are widely used as permanent magnets and in the microwave devices incorporated into, for instance, smartphones, cars, and computers—and such technology could not have been developed without ferromagnetic insulators. Spintronic devices, in which both the electrical and magnetic properties of electrons are utilized simultaneously, are now being extensively investigated to achieve high-speed devices with low power consumption. Ferromagnetic insulators will also serve as essential constituents that make such spintronic devices viable.


Fig. 1. Schematic diagram of ferromagnetism and paramagnetism.

In conjunction with trends in computerization, there has been a steadily growing demand for practical devices with higher performance. In terms of temperature, stable operation even above 200°C is required. However, the record Curie temperature (TC), which is the crucial factor determining the temperature range in which any ferri/ferromagnetic system remains stable, has stood in insulators ever since ferrite magnets*1 were first developed over eight decades ago in the 1930s. Therefore, researchers have sought to develop the next generation of ferromagnetic insulators with high TC values as well as to establish guiding principles to search for such materials.

*1 Ferrite magnets: Ferrite magnets are ferromagnetic insulators developed in the 1930s in Japan. They have been the most widely used magnets in the world. The major components are iron oxides, and many ferrite magnets also contain Co (cobalt), Ni (nickel), Mn (manganese), and other elements.

2. Preparation of high-quality Sr3OsO6 thin films

Solids in which atoms are periodically and orderly arranged and thus form lattices are called crystals. Crystals that have only a single atomic arrangement over an entire volume are known as single crystals. Samples whose thicknesses range from one atomic layer to about several tens of micrometers (1 μm = 1 × 10–6 m) are called thin films. Single-crystalline thin films are synthesized on single-crystalline substrates. For microfabrication of high-performance devices, it is necessary to prepare samples in the form of single-crystalline thin films with submicro­meter thicknesses. In this study, single-crystalline Sr3OsO6 thin films with a thickness of 300 nm (1 nm = 1 × 10–9 m) were synthesized on single-crystalline SrTiO3 (strontium titanate) substrates [1].

A schematic diagram of the double perovskite structure of Sr3OsO6 is shown in Fig. 2(a). The yellow, red, and blue spheres respectively indicate Sr (strontium), Os (osmium), and O (oxygen) atoms. Sr3OsO6 is a novel material synthesized in this study for the first time. In crystals, atoms are regularly ordered and form lattices. There are many kinds of atomic arrangements (crystal structures), and representative ones have specific names. One such arrangement is known as double perovskite, in which the lattice is twice as large as the perovskite structure. Many complex oxides are known to have the perovskite structure. Iodides and chlorides that have the perovskite structure have also been extensively studied recently to develop the next generation of solar cells [2].

To grow high-quality Sr3OsO6 thin films, precise control of the flux rate of each constituent cation (Sr, Os) is mandatory. Generally, controlling the flux of Os is a challenge because of its high melting point (3033°C). Nevertheless, we have succeeded in precisely controlling both the Sr and Os flux rates. We accomplished this by monitoring the flux rates with an atomic emission spectrometer and feeding them back to the evaporation source power supplies in real time, which enabled the synthesis of Sr3OsO6 thin films with the Sr and Os atoms arranged in a highly ordered structure. An atomic resolution microscopy (scanning transmission electron microscopy) image of an Sr3OsO6 film viewed along the [110] direction is shown in Fig. 2(b). We can clearly see the atomic ordering depicted in Fig. 2(a).


Fig. 2. Crystal structure of Sr3OsO6 (double perovskite).

3. Ferromagnetism above 780°C in Sr3OsO6

As described above, we have synthesized the novel material Sr3OsO6 having the highest TC among insulators by using a unique oxide thin-film growth technique that we have developed over many years. First, we measured the electrical and optical properties of the Sr3OsO6 thin films. The resistivity at room temperature was 75 Ω·cm, which is about 109 times as large as typical metals such as Au (gold) and Cu (copper). Also, resistivity increased exponentially as the temperature decreased. Furthermore, the optical band gap of Sr3OsO6 was found to be about 2.65 eV. All these results indicate that Sr3OsO6 is an insulator.

Next, we examined magnetic properties. The magnetization versus applied magnetic field curves of an Sr3OsO6 film is shown in Fig. 3(a). It shows ferromagnetic behavior (Fig. 3(b)) with a finite magnetization even at the high temperature of 727°C. The magnetization versus temperature curve of an Sr3OsO6 film is shown in Fig. 3(c). The applied magnetic field was 2000 Oe. The gradual change in the magnetization up to about 400°C is suitable for high-performance magnetic devices that can be stably operated at high temperatures (above room temperature). The TC value, at which ferromagnetism disappears, is above 780°C, surpassing the TC record among insulators for the first time in 88 years by more than 100°C.


Fig. 3. Magnetic properties of Sr3OsO6.

In addition to the experiments, density functional theory*2 calculations were carried out by the Tsuneyuki Research Group at the University of Tokyo. These calculations revealed that the ferromagnetic insulating state of Sr3OsO6 originates from the large spin-orbit coupling of the 5d element Os. The spin-orbit coupling consists of interactions between the spin magnetic moment coming from the axial rotation of electrons and the orbital magnetic moment coming from the revolution of the charged particles (electrons) around the nucleus (Fig. 4). The elements in the lower rows of the periodic table are heavier than those in the higher rows, and they have a larger spin-orbit coupling. Thus, the spin-orbit coupling in Os is larger than in Fe (iron) and Co (cobalt), which are used in typical magnets. This insight into the mechanism of the emergent high-temperature ferromagnetism will open a new avenue for developing functional materials in which elements having large spin-orbit coupling play a role.


Fig. 4. Spin-orbit coupling.

This novel material Sr3OsO6 has been synthesized in the form of single-crystalline thin films, which have high compatibility with device fabrication processes. This is in marked contrast to typical new oxides often synthesized in a powder or sintered polycrystalline form. Thus, Sr3OsO6 is expected to be readily implemented in high-performance magnetic device applications such as magnetoresistive random access memories (MRAMs) and magnetic sensors that work above room temperature.

*2 Density functional theory: This theory states that the energies of electrons in solids can be determined by using spatially dependent electron density n(r). The word functional, which means the function of another function, is used since the energy is calculated as a function of n(r), which is already a function of r. The electronic states of materials can be calculated and predicted from the fundamental equation that electrons follow based on this theory, without experimental data.

4. Future outlook

In our quest to better understand the fundamentals of ferromagnetism, we will further investigate the electronic structures of Sr3OsO6 using advanced spectroscopy techniques provided by synchrotron radiation facilities.*3 As part of efforts to develop high-performance magnetic devices that can be operated at high temperatures, we are working on fabricating some test devices using Sr3OsO6 to examine the tunnel magnetoresistance effect. This effect occurs when the tunnel resistance of an insulator sandwiched between two ferromagnets changes depending on the magnetic configuration of the ferromagnets (parallel or antiparallel) (Fig. 5(a)). The tunnel magnetoresistance effect has been widely utilized in commercial devices such as hard disc drives (Fig. 5(b)), MRAMs, and magnetic sensors. Therefore, demonstration of the tunnel magnetoresistance effect in Sr3OsO6 will lead to progress in developing high-performance magnetic devices that work at high temperatures.


Fig. 5. Tunnel magnetoresistance and its application.

*3 Synchrotron radiation facilities: In synchrotron radiation facilities, we can use light with various wavelengths (synchrotron radiation) such as ultraviolet rays and X-rays. Synchrotron radiation is emitted from accelerated electrons that travel in a huge ring in an ultrahigh-vacuum environment. The versatile capabilities of such synchrotron facilities include emissions of intense and variable-wavelength light, and this enables us to perform many kinds of high-resolution spectroscopy to investigate the physical properties of specimens in detail. Therefore, synchrotron radiation facilities are also very useful for materials science research. In Japan, there are several synchrotron facilities in operation, including SPring-8 in Hyogo Prefecture and the Photon Factory in Ibaraki Prefecture.

References

[1] Y. K. Wakabayashi, Y. Krockenberger, N. Tsujimoto, T. Boykin, S. Tsuneyuki, Y. Taniyasu, and H. Yamamoto, “Ferromagnetism above 1000 K in a Highly Cation-ordered Double-perovskite Insulator Sr3OsO6,” Nat. Commun., Vol. 10, 535, 2019.
[2] M. Liu, M. B. Johnston, and H. J. Snaith, “Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition,” Nature, Vol. 501, pp. 395–398, 2013.
Yuki K. Wakabayashi
Researcher, NTT Basic Research Laboratories.
He received a B.S. in solid-state physics from Osaka University in 2012, and an M.S. and Ph.D. in electrical engineering from the University of Tokyo in 2014 and 2017. Since joining NTT in 2017, he has been conducting research on thin film growth of ferromagnetic oxides. He received the English Presentation Award by the Spintronics Committee of the Japan Society of Applied Physics (JSAP) at the 62nd Spring Meeting of JSAP in 2015, the Young Researcher Best Poster Award at the 9th International Conference on Physics and Applications of Spin-Related Phenomena in Solids in 2016, and the 45th JSAP Presentation Award at the 66th Spring Meeting of JSAP in 2019.
Yoshiharu Krockenberger
Senior Researcher, Low-Dimensional Nanomaterials Research Group, NTT Basic Research Laboratories.
He received a diploma in physics from the Technical University of Munich in 2002 and a Ph.D. in physics from the Darmstadt University of Technology, Germany, in 2006. His principal research areas are superconductivity, strongly correlated electron systems, and thin film growth. He has been working at NTT since 2010. He also worked as a research scientist at RIKEN, Advanced Science Institutes, in the cross-correlated materials research group of Prof. Y. Tokura (2006–2010).
Yoshitaka Taniyasu
Senior Researcher, Group Leader of Low-Dimensional Nanomaterials Research Group, NTT Basic Research Laboratories.
He received a B.E., M.S., and Dr. Eng. in electrical engineering from Chiba University in 1996, 1998, and 2001. He joined NTT Basic Research Laboratories in 2001. He has been engaged in wide-bandgap semiconductor research. He received the Young Scientist Award at the 2007 Semiconducting and Insulating Materials Conference, the Young Scientist Award of the International Symposium on Compound Semiconductors 2011, the Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology of Japan (the Young Scientists’ Prize) in 2011, and the Japan Society for the Promotion of Science Prize in 2019.
Hideki Yamamoto
Senior Research Scientist, Supervisor, Executive Manager of Materials Science Laboratory, NTT Basic Research Laboratories.
He received a B.S., M.S., and Ph.D. in chemistry from the University of Tokyo in 1990, 1992, and 1995. He joined NTT in 1995. His principal research fields are thin film growth, surface science, and superconductivity. He was a visiting scholar at the Geballe Laboratory for Advanced Materials, Stanford University, USA (2004–2005). He received the 2nd Young Scientist Presentation Award (1997) from JSAP and the 20th Superconductivity Science and Technology Award (2017) from the Forum of Superconductivity Science and Technology, the Society of Non-traditional Technology. He is a member of JSAP, the Physical Society of Japan, the Japan Society of Vacuum and Surface Science, the American Physical Society, and the Materials Research Society.

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