Journal of the Mineralogical Society of Korea (한국광물학회지)

The Mineralogical Society of Korea (한국광물학회)
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Magnetic Properties of Magnetites at Low Temperatures

자철석의 저온 자화특성

  • Hong, Hoa-Bin (Department of Geology and Earth Environmental Sciences, Chungnam National University) ;
  • Yu, Yong-Jae (Department of Geology and Earth Environmental Sciences, Chungnam National University)
  • 홍화빈 (충남대학교 지질환경과학과) ;
  • 유용재 (충남대학교 지질환경과학과)
  • Received : 2011.03.09
  • Accepted : 2011.03.23
  • Published : 2011.03.31
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Abstract

Magnetic properties at low-temperatures can diagnose the presence of certain magnetic minerals in rocks. At the Verwey transition temperature ($T_v$, ~105~120 K), magnetite transforms from monoclinic to cubic structure as the temperature increases. At the isotropic point ($T_i$, ~135 K), magnetocrystalline anisotropic constant of magnetite passes through zero (from negative to positive) as the temperature decreases so that its optimal remanence acquisition axis changes from [111] to [001]. A sharp remanence drop was observed at $T_v$ during warming of LTSIRM (low-temperature saturation isothermal remanent magnetization). For cooling of RTSIRM (room-temperature saturation isothermal remanent magnetization), the remanence decreased on passing $T_i$ and $T_v$. On warming of RTSIRM, remanence recovery becomes more prominent as the average grain size of magnetite increases. In summary, the SIRM memory decreases with increasing grain size of magnetite. A similar, but rather gradual, remanence transition occurs for natural samples due to contribution of cations other than Fe. As a non-destructive tool, low-temperature magnetic behavior is sensitive to unravel the magnetic remanence carriers in terrestrial rocks or meteorites.

지구의 암권에서 가장 중요한 자성광물은 자철석이다. 암석 내에 존재하는 자철석의 성분과 입자 크기에 관한 정보를 알아내는데 저온 자화특성을 이용하려 한다. 물리적 방법의 하나인 저온 자화특성 실험은 비파괴적이며 운석과 같이 연구 대상 시료의 수량이 제한된 경우 특히 유용하게 사용된다. 금번 연구에서는 세 종류의 합성 자철석 시료와 세 종류의 자철석 함유 암석 시료에 대해 저온포화잔류자화의 가열실험과 실온포화잔류자화의 냉각/가열 실험을 수행하였다. 실험 결과 저온포화잔류자화는 가열함에 따라 자철석의 버웨이변환온도(~105~120 K)를 지나며 급격히 감소한다. 자철석의 버웨이변환온도와 등방점(135 K)를 모두 거치는 실온포화잔류자화의 냉각과 가열 실험 결과에서는 저온포화잔류자화의 가열 실험 결과와 비교하였을 때 자화회복력이 뛰어나고, 자화상실이 상대적으로 완만히 진행된다. 저온포화잔류자화와 실온포화잔류자화 모두 자철석의 입자 크기가 증가할수록 소실되는 포화잔류자화의 비가 증가한다. 결국 저온포화잔류자화기억도와 실온포화잔류자화기억도 모두 자철석의 입자가 커질수록 감소한다. 따라서 저온 자화특성을 이용하면 암석 내 자철석 입자의 크기를 비파괴적인 방법으로 유추할 수 있다.

Keywords

References

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