gamma-Fe2O3/CoO core/shell奈米粒子磁性質與介面微結構關係之探討

Investigation of relations between the interface microstructures and the magnetism of gamma-Fe2O3/CoO core/shell nanoparticles

吉宗驥(Chong-Chi Chi)

指導教授 ： 歐陽浩

國立清華大學/工學院/材料科學工程學系/碩士(2014年)

摘要

加拿大J. van Lierop教授團隊以無水化學合成法製備出磁性core/shell -Fe2O3/CoO奈米粒子後，使用SQUID和Mössbauer光譜測量磁性質。由SQUID發現有接shell的奈米粒子會產生交換偏壓現象，且矯頑場也會變大。而觀察Mössbauer光譜則是發現core/shell奈米粒子對抗熱擾動的能力變得非常強，加拿大J. van Lierop教授團隊臆測主要原因為core/shell介面的intermixing造成。
所以本實驗主要是使用電子顯微鏡和EDS來觀察分析奈米粒子core/shell介面。EDS顯示在介面處主要是Co滲入-Fe2O3八面體空位的行為而不是鐵滲入CoO。分析HRTEM影像可以發現-Fe2O3在core/shell邊界處有Co滲入，且Co滲入會導致材料繞射點訊號改變。用此當作依據來判斷擴散深度並分析兩種製程溫度(150°C和235°C)對擴散深度的影響。發現當製備溫度150°C時Co的平均擴散深度為0.47nm。若製備溫度升高到235°C則Co的平均擴散深度為0.67nm。
使用VASP第一原理計算對Co擴散到不同位置做能量計算，得到Co擴散到-Fe2O3八面體空位能量最低代表最可能的情況。接著我們建立出模擬結構模擬intermixing行為並使用VASP第一原理計算來探討intermixing對磁性質的影響。我們發現Co擴散到-Fe2O3八面體空位會使整體磁化量變高，使材料更偏向鐵磁性。同時也發現Co擴散深度會影響磁化量，擴散深度深磁化量反而會下降，和 J. van Lierop教授觀測到的結果相符。
又在core/shell介面因為有氧離子存在，所以我們也建立了模擬結構來探討超交換性行為對磁性質的影響。

關鍵字

磁性奈米粒子 ； 介面混合

並列摘要

Prof. J. van Lierop et al and we have showed that the magnetism of core-shell nanoparticles (made of maghemite, -Fe2O3, cores and transition-metal and metal-oxide shells) is altered substantially by the interface, which is a doped iron-oxide layer formed naturally during the seed-mediated synthesis process, a route used typically to produce core-shell nanoparticles. Characteristics fundamental to useful applications, such as the anisotropy and superparamagnetic blocking temperature, were altered substantially with Cu, CoO, MnO, and NiO shells. To ascertain the origin of this behavior, the prototype -Fe2O3/CoO core-shell nanoparticles are described in detail. The magnetism originates essentially from an interfacial doped iron-oxide layer formed via migration of shell ions,e.g., Co2+, into octahedral site vacancies in the surface layers of the γ -Fe2O3 core. For this system, an overall Fe
morb/mspin = 0.15 ± 0.03 is measured (morb ∼ 0 for the Fe-oxides) and an enhanced Co morb/mspin = 0.65 ± 0.03 elucidates the origin of the unexpectedly high overall anisotropy of the nanoparticle. This interfacial layer isresponsible for the overall (e.g., bulk) magnetism and provides a perspective on how the magnetism of core-shell
nanoparticles manifests from the selected core and shell materials.

Within this work, TEM and first-principles calculations to prove the core/shell intermixing and magnetic property were performed in our group. By analyzing the HRTEM and EDS, the intermixing was confirmed, mainly by the doping of Co into the octahedral site vacancies of -Fe2O3. The average Co doping depths in different processing temperatures (150°C and 235°C) were 0.47nm and 0.67nm, respectively. The error of this measurement is within 5 percent through a simulation.

By first-principles calculations, the intermixing phase of -Fe2O3 with Co doping is ferromagnetic, with even higher magnetization as compared to that of pure -Fe2O3. Besides, Co doping (same numbers) into different octahedral sites can cause different magnetizations.

並列關鍵字

magnetic nanoparticle ； interfacial intermixing ； gamma Fe2O3 ； CoO

參考文獻

[11]. P.J. van der Zaag, Y. Ijiri, J.A. Borchers, L.F. Feiner, R.M. Wolf, J.M. Gaines, R.W. 　Erwin, M.A. Verheijen, Phys. Rev. Lett. 84 (2000) 6102.

[1]. MAGNETIC MATERIALS Fundamentals and Applications Second edition NICOLA A. SPALDIN

[2]. B.D.Cullity., Introduction to magnetic magerials(1972)

[16]. W. H. Meiklejohn, C. P. Bean, Phys. Rev. 102 (1956) 1413

[24]. J. AM. CHEM. SOC. 2010, 132, 12480–12486