Creation of novel material Sr3OsO6 with the highest ferromagnetic transition temperature among insulators - Breaking the world record for the first time in 88 years -

Researchers of Nippon Telegraph and Telephone Corporation (NTT) have synthesized Sr3OsO6 , a novel material that exhibits ferromagnetism*1 above 780℃, which is the highest temperature among insulators. In collaboration with the Tsuneyuki's Research Group at the University of Tokyo (UTokyo), they have also revealed the electronic state*2 of this material, which is the key to comprehending the origin of the emergent ferromagnetism.
Our discovery surpasses the long-standing Curie temperature*3 (Tc) record among insulators for the first time in 88 years and is thus epoch-making for the development of magnetic materials. It also provides fundamental knowledge about the mechanism of the emergent ferromagnetism at high temperatures. Unlike most conventional magnetic materials, our brand-new material is free from Fe (iron) and Co (cobalt) and hence paves a new way to the exploration and development of other novel magnetic materials. Furthermore, the Sr3OsO6 was synthesized in the form of single-crystalline thin films.*4 This suggests that the Sr3OsO6 films can be readily implemented in device fabrication and are thus promising for high-performance magnetic devices that can be stably operated at high temperatures (room temperature to 250℃). Examples of such devices include magnetic random access memories (MRAM) and magnetic sensors.
This research was reported in Nature Communications on February 12, 2019.

1.Background

Ferromagnetic insulators*5 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. Recently, spintronic devices, in which both the electrical and magnetic properties of electrons are utilized simultaneously, are being intensely investigated to realize high-speed devices with low power consumption. Ferromagnetic insulators are also thought to be essential constituents that will make such spintronic devices viable.
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℃ is required. However, the record 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*6 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's as well as establish guiding principles to search for such materials.

2.Achievements

Researchers of NTT Basic Research Laboratories (NTT-BRL) have synthesized a novel material, Sr3OsO6, (Fig. 1) using a unique oxide thin-film growth technique that they have developed over many years. The Tc value of this material, estimated from the magnetic measurements, is above 780℃ (Fig. 2), which surpasses the Tc record among insulators for the first time in 88 years by more than 100℃.
Density functional theory*7 calculations carried out by the UTokyo team revealed that the ferromagnetic insulating state of Sr3OsO6 originates from the large spin-orbit coupling*8 of the 5d element*9 Os. 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.
Sr3OsO6 was 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 (MRAM) and magnetic sensors that work above room temperature.

3.Technical Features

Preparation of the high-quality Sr3OsO6 thin films

We used the molecular beam epitaxy*10 method to synthesize the Sr3OsO6 thin films, which have a crystal structure called double perovskite*11 (Fig. 1a). To grow high-quality Sr3OsO6 thin films, precise control of the flux rate of each constituent cation (Os, Sr) is mandatory. Generally, controlling the flux of Os is a challenge because of its high melting point (3033℃). Nevertheless, we have succeeded in controlling both the Sr and Os flux rates precisely. 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 (Fig. 3), which enabled the synthesis of Sr3OsO6 thin films with the Sr and Os atoms arranged in a highly ordered structure (Fig. 1b).

4.Future plans

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*12. Toward the development of high-performance magnetic devices that can be operated at high temperatures, we are trying to fabricate some test devices comprising Sr3OsO6 to examine the tunnel magnetoresistance effect.*13

4.Future plans

>Fig. 1a: Schematic diagram of Sr₃OsO₆ (double perovskite). The yellow, red, and blue spheres indicate Sr, Os, and O atoms, respectively. Fig. 1b: Atomic scale microscopy (scanning transmission electron microscopy) image of a Sr₃OsO₆ film viewed along the [110] direction. We can clearly see the atomic ordering depicted in Fig. 1a.

Fig. 2a: The magnetization versus applied magnetic field curves of a Sr₃OsO₆ film, showing ferromagnetic behavior with a finite magnetization even at the high temperature of 727℃. Fig. 2b: Schematic diagram of the magnetization versus applied magnetic field curve for ferromagnetism and paramagnetism. Fig. 2c: The magnetization versus temperature curve of a Sr₃OsO₆ film. The applied magnetic field is 2000 Oe. The T_c value, at which ferromagnetism disappears, is above 780℃.

Fig. 3a: Schematic diagram of the molecular beam epitaxy system used in this study. We control the elemental fluxes of both Sr and Os by feeding the measured flux rates back to the power supply of the e-beam evaporators in real time. The flux rates are measured with a light sensor that detects specific wavelength lights emitted from the evaporant fluxes. Fig. 3b: Schematic illustration of the e-beam evaporation. Electron beam irradiation increases temperatures of the source materials. A larger e-beam current gives a larger flux rate.

Fig. 4a: Schematic diagram of ferromagnetism. Fig. 4b: Schematic diagram of paramagnetism. The purple arrows indicate the magnetization of each atom.

Fig. 5: The periodic table. The elements enclosed by the red frame are called 5d transition elements

Fig. 6: The spin magnetic moment coming from the rotation of the electron, and the orbital magnetic moment coming from the revolution of the charged particle (electron) around the nucleus.

Fig. 7a: Tunnel magnetoresistance effect. The resistivity of the insulator sandwiched between the two ferromagnets becomes low when the magnetic configuration of the ferromagnets is parallel and vice versa. Fig. 7b: The tunnel magnetoresistance effect has been used for the magnetic head in hard disc drives (HDDs).