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1、 1新進教師學術研究計劃成果報告 新進教師學術研究計劃成果報告 新穎複鐵式(multiferroic)材料的合成, 測試及應用開發 新穎複鐵式(multiferroic)材料的合成, 測試及應用開發 計畫主持人：齊孝定 執行單位：國立成功大學材料科學與工程系(所) 中 華 民 國 98 年 8 月 3 日 Synthesis, characterization and application of multiferroic materials Abstract Multiferroic BiFeO3 (BFO) films have been deposited on a number of
2、substrates by RF magnetron sputtering. Two routes were adopted in order to obtain the films of high phase purity and accurate stoichiometry. The first was to sputter films at room temperature and then the BFO phase was formed after sintering at various temperatures under controlled ambient atmospher
3、e. The second was to grow BFO in-situ at high temperature during sputtering. Although the sintered films grown on SrTiO3 (STO) substrates were epitaxial and showed better texture than the in-situ films, they had much poorer ferroelectric properties, due to the residual traces of intermediate phases
4、formed during heating. Under right growth parameters, the in-situ films grown on the LaNiO3-x (LNO) buffered STO showed fully saturated ferroelectric hysteresis loops with large remanent polarisation of 64 C/cm2. Piezoresponse force microscopy (PFM) was used to examine the ferroelectric domain struc
5、tures. When scanned without DC bias, fine spontaneous domains were observed. Under +/-10V DC bias, PFM confirmed that the domains could be poled and switched. Dielectric measurements have also been carried out, which revealed a low-frequency contribution from both DC conductivity and interfacial pol
6、arisation, in addition to the high-frequency intrinsic dielectric loss of the epitaxial films. For polycrystalline films on Pt/Si, the dominant contribution to dielectric loss was space charge polarisation at grain boundaries. On the magnetic properties, vibration sample magnetometry detected a smal
7、l moment (0.09 B/Fe) for the epitaxial BFO films, which was consistent with the canted antiferromagnetic spin arrangement reported previously. An oxide ferromagnetic film of Zn0.7Ni0.3Fe2O4 has been grown epitaxially on top of the BFO layer in attempt to achieve exchange bias for the applications in
8、 all-oxide multiferroic spin-valves. In addition, searching for novel multiferroics has also been carried out, which included doping of BFO and substitution of tungsten-bronze structured ferroelectrics. A part of the results have been written in three SCI papers published in J. Phys. D: Appl. Phys.,
9、 Thin Solid Films, and J. Eur. Ceram. Soc., respectively. 1新穎複鐵式(multiferroic)材的合成, 測試及應用開發 (計畫編號 NSC 96-2628-E-006 -010 -MY3) 摘要 複鐵式材料鐵酸鉍(BFO)薄膜藉由射頻磁控濺鍍法生長在多種基板上。我們嘗試了兩種不同製程, 以便獲得純相且具有準確化學計量比的BFO. 第一種方法是在室溫濺鍍, 而後在不同溫度與氣氛下燒結形成BFO相. 第二種方法是在高溫下直接濺鍍成相. 雖然藉由濺鍍後燒結,成長在鈦酸鍶(STO)基板上的BFO膜是磊晶, 且具有比直接濺鍍成相的BFO膜有
10、更好的織構, 但是鐵電性質差很多. 這是因為燒結成相的膜中殘留有加熱過程中形成的中間相. 在正確生長參數下, 長在鍍有鎳酸鑭(LNO)電極之STO基板上的BFO膜, 具有完全飽合的電滯曲線, 殘余極化為64C/cm2. 以壓電響應力顯微術(PFM)觀測鐵電疇結構, 在沒有外加直流偏壓下, 所看到的是極細小的自發性電疇. 在外加+/-10V直流偏壓下, PFM結果顯示電疇可以被極化和反轉. 介電量測顯示, 磊晶膜除了在高頻段有BFO膜本身的介電損耗外, 在低頻段還存在著來源於直流漏電流以及薄膜與電極間界面的介電損耗. 對於鍍在鉑/矽基板上的多晶膜, 主要的介電損耗來源於晶界間的空間電荷極化. 在
11、磁性方面, 震動樣品磁度儀(VSM)測得純相BFO磊晶膜僅有微小的磁矩(0.09B/Fe), 符合之前的報導, 即BFO具有傾斜的反鐵磁自旋排列. 為了達到制做複鐵式自旋閥(spin-valve)所需的偏磁(exchange-bias)效果, 我們成功地將一氧化物鐵磁薄膜(鐵酸鋅鎳)長在BFO表面, 並形成磊晶. 除以上成果外, 我們也積極展開尋找新穎複鐵式材料的研究, 包括對BFO和鎢-青銅結構的鐵電材料進行參雜或置換. 本計劃的部份成果已寫成三篇SCI論文分別在J. Phys. D: Appl. Phys., Thin Solid Films和J. Eur. Ceram. Soc.上發表.
12、 21. INTRODUCTION During last a few years, magnetoelectric multiferroics, which are simultaneously ferroelectric and (anti-)ferromagnetic, have been attracting a great interest worldwide 1-11. In addition to the importance in the understanding of fundamental Physics, the coexistence of ferroelectric
13、 and magnetic domains in a single material provides a brand-new platform for the designing of next-generation devices beyond the reach of currently available materials. The potential applications are wide ranging, including sensors, digital memories, spin filters, electrically switchable spin-valves
14、, etc. As far as the device applications concerned, BFO is not doubt one of the best choices, because both its ferroelectric and magnetic transition points are well above room temperature. However, high-quality BFO samples with good ferroelectric properties have been extremely difficult to grow. Bul
15、k single crystals often showed a small remanent polarisation of about 3.5 C/cm2 12, which is inconsistent with its high Tc. A much higher remanence (50 C/cm2) was first demonstrated in the thin film samples and the great improvement was attributed to the heteroepitaxy-induced structural distortions
16、9. However, recent advance in the growth techniques has made higher-quality single crystals available, which also showed a same magnitude of remanence 13. Therefore, large remanent polarisation is intrinsic to BFO, but often poor quality of the samples has prevented it from achieving such a value. More researches on the film growth are required in order to optimise ferroelectric properties and to improve the reproducibility of good samples
