2020年11月6日

Date: 16:00 – 18:00, Friday, November 6, 2020

日時: 2020年11月6日(金)16:00 – 18:00

講演者:青木 勝敏

Speaker: Katsutoshi Aoki

Title: Neutron Diffraction Study of Structural and Magnetic Phase Boundaries of Iron

Iron (Fe) is a typical ferromagnetic metal. In the ferromagnetic state, the electron spins are collectively ordered and aligned in a certain direction. Why the spins of electrons, which freely travel through the iron lattice, are aligned? The origin of ferromagnetism and the mechanism of magnetic transition have long been the major subject of physics. Besides the magnetic transition, iron shows complex structural transition under high temperature and high pressure: bcc a-phase stable at ambient conditions, high-temperature fcc g-phase, and high-pressure hcp e-phase. Only the a-phase is ferromagnetic and its bcc lattice is stabilized by magnetic ordering. The a-g phase boundary develops from 1185 K at ambient pressure to 678 K at 8.2 GPa, where the boundary meets the g-e phase boundary. Observation of the degree of magnetic ordering or the magnetic moment of Fe atoms, which should vary depending temperature and pressure along the a-g phase boundary, is essential to elucidate the interplay between the magnetic and structural transitions.

Iron is the most abundant metallic element in the universe and a major component of cores of planets. On a planet with a mild temperature and pressure environment, the core likely consists of ferromagnetic a-Fe. Psyche is considered to be an iron-rich asteroid formed by stripping away a rocky mantle with violent impacts. Targeted to launch in August of 2022, the Psyche spacecraft would arrive at the asteroid in early 2026 to map the magnetic field around it. The temperature and pressure variation of the magnetic moment of Fe are indispensable experimental data for analyzing the magnetic field to be measured. We have conducted in situ neutron diffraction measurements on iron at high temperatures and high pressures at MLF, J-PARC. Neutron beam is scattered from local magnetic fields as well as atomic nuclei, and hence penetrating probe of ordered magnetic structure. The variations of the magnetic moment with temperature were measured at 2 and 6 GPa, allowing estimation of ferromagnetic–paramagnetic transition temperatures (Curie temperatures). Curie temperatures thus obtained for compressed iron provided some aspects different from those reported in early studies and highlighted characteristics to the magnetism of itinerant electron system.

講演者:高萩 航

Speaker: Wataru Takahagi

Title: Metal availability controlled by hydrothermal activity in the early Archean seafloor

生命誕生前の初期海洋におけるNiは水素と二酸化炭素からアセチルCoAを合成する経路 (Wood-ljungdahl pathway)に似た反応を非酵素的に駆動でき、生命前駆進化の観点から初期熱水噴出域における局所的なNiの存在度を制約することは重要である。深海の熱水噴出域では熱水中の水素分子などの還元性成分の酸化により電子が生み出され、熱水と海水の電位差に沿って導電性を持つ硫化物などの鉱物を通って輸送される熱水発電と呼ばれる現象が恒常的にみられる。熱水と海水の電位差が顕著であった生命誕生前の冥王代から前期太古代にかけては、現存する熱水噴出域よりも低い電位が達成されていた可能性が高い。この電位差により、熱水噴出域で熱水中のS2-と海水中のNi2+の混合により沈殿したNiSはNi3S(Heazlewoodite)というより安定な鉱物に相転移することで、溶解度は環境電位に依存する (3Ni2+ + H2S + 2e ↔ Ni3S2 + 4H+)。これにより、熱水噴出域のNiの存在度はKonhauserらによって報告された原始海洋中の400 nMという値に制約を受けず、熱水の活動度 (pHや温度に依存する)によって熱水噴出域の局所的なNiの存在度が規定される可能性が高い。本発表では、水素解離の熱力学ポテンシャル計算に基づく地球初期の熱水系における発電現象の定量化と、電位に制約を受けるNiの局所的な存在度の予測結果を紹介する。計算結果に基づくと、熱力学的非平衡状態にあるアルカリ高温熱水と弱酸性海水の間には恒常的に-0.6~-0.8Vの電位差が存在し、この電位域でのNi3S2の溶解度はNiSより2桁以上低く、熱水噴出域に30nM程度の硫化水素が存在していれば海水中のNiを熱水系に濃集できる可能性が示唆された。