1 / 15

Abstract

Tabata lab. M1 Kouji Tsuruta 2005/5/9. (2002). Abstract.

toby
Download Presentation

Abstract

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Tabata lab. M1 Kouji Tsuruta 2005/5/9 (2002) Abstract In order to investigate the Verwey transition, they measured the resistivity on two single crystalline samples with distinct oxygen composition of Fe3O4.002 and Fe3O4.007 at temperatures from 300K down to 3K under a pressure up to 9GPa. They confirmed that a metallic ground state is realized in magnetite under pressure above Pc . This result is contradiction to that the Verwey transition still survived at finite temperature under a pressure extending over 16GPa reported By Rozenberg (1996).

  2. 1. Introduction In 1939, Verwey observed a discontinuous change in resistivity (i.e. a metal-insulator transition ) at 117K. Despite intensive studies for over 60 years, the mechanism of the verwey transition has remained elusive. knowledge cf ) spinel structure Change structure below Tv Tv above Tv inverse spinel structure monoclinic cubic Tv : Verwey transition temperature

  3. many models to explain the Verwey transition Verwey and Haayman (1942) order-disorder transition of Fe2+ and Fe3+ Neutron scattering studies denied it . (1968,1975) Mott (1974) the possibility of a Wigner crystallization Ihle and Lorenz (1985) ( the conductivity above Tv ) superposition of a small polaron band and hopping conductivities Mishra (1994) the possibility of a Verwey-like order the framework of a three-band spinless model

  4. Recently resonant X-ray scattering (2000) and NMR measurements (2001) no charge ordering of Fe2+ and Fe3+ ions in B site occurs even in the insulating phase below Tv We should reconsider the mechanism of conductivity in magnetite.

  5. 2. Experiment Sample A. Fe3O4.002 ( Tv=123K ) • floating zone melting method • annealed at 1200℃ at CO and CO2 B. Fe3O4.007 ( Tv=116K ) • melt at 1600℃ in a Pt-Rh crucible at Ar • annealed at 750℃ in an evacuated silica tube for 5 days • annealed again at 1300℃ at CO and CO2 measurement cubic-anvil device combined with cryostat for electrical measurements T : 2K ~ 300K P : ~ 10GPa

  6. 3. Results and discussion Fig. 1 The temperature-dependent resistivity of A and B in logarithmic scale at various pressures. • In the high-temperature region above Tv, resistivity of both the samples also changes in its temperature dependence from semiconducting to metallic with increasing pressure in the same way.

  7. Fig. 2 The resistivity of A just above(□) and below() Tv as a function of Tv and pressure • The resistivity just above Tv(□) changes more steeplythan just below Tv() only in higher region (Tv> 90K) . • The resistivity jump (− □) at Tvincreases with increasing pressure and with decreasing Tv.

  8. What is understood from these results • the semiconducting behavior above Tv is not intrinsic property of Verwey transition . In this sense, the polaron model proposed by Ihre and Lorenz could notgive a reasonable explanation of the Verwey transition . reason the semiconducting behavior appears Although it is not intrinsic, there remains a short-range order and local strain throughout the sample .

  9. Detailed account Fig. 1 (a) • Though the resistivity jump at Tv increases with pressure below 7.15GPa, it rapidly decreases at pressure beyond 7.2GPa. • A hysteresis of resistivity appears between 7.2GPa and 7.9GPa when the temperature is increased and decreased . reason of these results a mixture of the insulating and metallic phases in the narrow pressure range . at T < Tv 7.2GPa 7.9GPa P metal insulator metal and insulator jump jump hysteresis

  10. Fig. 3 Fe3O4.002, Fe3O4.007 and Rosenberg experiments Tv as a function of pressure • Application of pressure suppresses Tv . • The Tv seems to drop discontinuously to absolutezerotemperature at a critical pressure Pc . Pc Fe3O4.002 : 7.5GPa Fe3O4.007 : 6 GPa This result is different from that reported by Rozenberg . They observed no metallic ground state up to 16GPa .

  11. Reason of difference from Rozenberg This difference is recognized from a difference in the method of pressure generation . Rozenberg : clamped-diamond anvil device with solid pressure transmitting materials not hydrostatics conditions but rather in a highly stressed field below Tv above Tv (insulating) (metallic) monoclinic cubic elongated a little along <111> In conclusion, A highly stressed field may stabilize the insulating phase.

  12. On the other hand Mori : cubic-anvil device with the Teflon cell technique a relatively homogeneous pressure judging from the resistivity data accumulated for many single crystalline What is evident in comparing the results of Rozenberg and Mori • The physical parameters which control the Verwey transition in magnetite are extremely sensitive to strain .

  13. 4. Summary • They confirmed that the Verwey transition is not a semiconductor to insulator transition but intrinsically it is a metal to insulator transition . • A metallic ground state is realized in both samples of Fe3O4.002 and Fe3O4.007 at pressures above ≈ 7.5GPa and 6GPa respectively . As prospects Considering the noticing that Fe atoms B sites have the structure of a quasi-one dimension chain along <110> axes in the cubic phase, a Mott transition or Peierls transition may take place below Tv .

  14. Our study optical charge transfer {Ref. D.M.Sherman Phys. Chem. Minerals 14 (1987) } hν2.2eV Fe2+ + Ti4+Fe3+ + Ti3+ Fe2+ : magnetic anisotropy increasing magnetization （ｄ ６： Jahn-Teller effect） Fe3+ : no magnetic anisotropy photo controlled magnetization （ｄ ５：equal electrical distribution ） In my 4th grade research A localized electric state is necessary in order to occur photo-controlled magnetization .

  15. making a high speed photo-controlled magnetization cluster glass not appearance of a high speed in photon-mode local spin behavior like Fe3O4 higher temperature of localized state applied device cf) Fe3+ ,Mn2+ ,Co4+ d 5 same electric condition lattice mismatch stress

More Related