鎳奈米微粒間交互作用對磁特性的影響

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题名:	鎳奈米微粒間交互作用對磁特性的影響
作者:	蕭詠銜;Yeong-Shyan Hsiao
贡献者:	物理研究所
关键词:	鎳;奈米微粒;磁;nanoparticle;magnetic;Ni
日期:	2006-01-09
上传时间:	2009-09-22 10:57:07 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	本實驗利用熱蒸鍍法製作鎳奈米微粒，並藉由對樣品壓合的方式來調控微粒間的間距，以觀察微粒間交互作用對磁特性的影響。自然聚合的鎳奈米微粒在外加磁場200 Oe時blocking temperature為200 K，並觀察到在4.5 K至300 K的溫度範圍內皆無磁滯現象。外加磁場0.15 T時自然聚合的鎳奈米微粒約在50 K有一反鐡磁性現象的轉折點，聚合密度增加，微粒間作用力增強，反鐡磁性的轉折點溫度愈高，且在此轉折點溫度以上無磁滯現象，以下有磁滯現象，顯示在轉折溫度以下因微粒間交互作用力較熱擾動大而顯現出來。此外亦發現經過壓合的樣品，其磁滯曲線並未對零場對稱，偏移的程度隨壓合密度增加而增大，在壓合密度達到60 %時偏移達到最大值約400Oe，此偏移可能為奈米鎳粒子表面效應所造成的自旋玻璃態。當外加磁場回到零點時，這些表面的磁矩排列的方向仍然維持在原外加磁場方向，因此奈米鎳微粒中鐵磁核心的部份仍然可以感受到這些來自表面磁矩所造成的磁場影響，而使磁滯偏移。此偏移量隨壓合密度增加而增加意味著表面的這層自旋玻層的厚度隨著壓力增加而變大，各個不同壓力的飽和磁矩隨著壓力增加而減少，壓合密度達60 %時，樣品的飽和磁矩僅剩下塊材的50 %。 Ni nanoparticles were fabricated by the thermal evaporation method. We control the distance among particles by pressing the loosely packed nanoparticle powder samples, and studying the influence of interparticle interaction on the magnetic characters of Ni nanoparticle. The blocking temperature of the loosely packed Ni nanoparticle powder sample in applied magnetic field 200 Oe is 200 K, we observe that there is not hysteresis at temperature range 4 K to 300 K. And at applied magnetic field 0.15 T, there is an antiferromagnetic transition at 50 K. The interaction among nanoparticles increases when the compacting density increases. The temperature of antiferromagnetic transition also increases, and there is not hysteresis above the antiferromagnetic transition temperature. Hysteresis is observed below the transition temperature. It means interaction of particles is lager than thermal energy below the transition temperature. Beside, we also observe the hysteresis curve is not symmetry at zero field in compacted samples. When compacting density increases, shift of hysteresis is increases. Compacting density is 60 %, the shift is about 400 Oe. This shift may form by spin glass of surface effect of Ni nanoparticles. As applied magnetic field gets back to zero field, the direction of surface magnetic moment is ordered by original applied magnetic field direction. Ferromagnetic central part of nickel nanoparticles can experience these come from surface magnetic moment effect, and make the hysteresis shift. The shift increases as compacting density increases, it means that surface spin glass layer become heavy as the pressure increases, and saturation magnetism of each different pressure reduces as the pressure increases. While compacting density is 60 %, the saturation magnetism of sample only 50 % of bulk left.
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