氧化物薄膜的磁性及磁电耦合研究
摘要
在现代科技中,氧化物材料展现出大量的功能性质,包括电性、磁性、光学反应,因而具有巨大的应用潜力。各种各样的材料,比如绝缘体、介电体、半导体、铁磁体、铁电体、巨磁阻抗材料、超导材料,都可以用氧化物来得到。一些特殊的氧化物可以同时展现出独特的多功能性质,这些材料通常称为多功能材料,比如,过渡金属掺杂的ZnO, TiO2和In2O3可以同时显示出铁磁性和半导体性质。此外,像Ni3V2O8、BiFeO3、BiMnO3和DyMnO3这些氧化物可以同时显示出铁电性和铁磁性,Pr1-xCaxMnO3这样的磁致伸缩材料展示出金属绝缘体相变和磁有序的耦合作用。因为多功能材料通常可以展示出独特的性质,所以从科研和技术应用的角度看,研究这些体系是很重要的。把这些材料融入到器件设计中去有利于新型应用器件的发展,比如把巨磁阻抗(GMR)材料应用到现代存储器件中去,已经很大程度提高了存储容量。
SnO2是一种很重要的宽禁带n型半导体,禁带宽度在300K时为3.6eV,具有广阔的应用范围,包括固态气体传感器、液晶显示器、光伏电池、和透明导电电极。目前已有大量的制备方法被用来制备SnO2薄膜,包括溶胶凝胶、化学气象沉积、磁控溅射、热蒸发、脉冲激光沉积、分子束外延。近几年,大量的研究集中在研究磁性元素掺杂SnO2薄膜或者纳米颗粒的磁性上,以便开发它的稀磁半导体特性,另一方面,在各种主体中掺杂稀土元素主要是由于它独特的荧光特性,稳定性,和高发射量子产率,还有一点很重要就是在各种氧化物主体总掺杂稀土离子更加的容易,这样能带激发可能导致有效的能量转移,产生有趣的光学和磁学性质,因此从技术应用的观点考虑,很有必要研究一下氧化物半导体中掺杂稀土元素实现光学和磁学性质共存的可能性,如果存在,那么对于磁光应用器件的研究很有帮助。
我们利用脉冲激光沉积PLD技术成功地在(001)蓝宝石衬底上生长了(200)方向的Sm掺杂SnO2薄膜,衬底温度为600℃,氧气压强为1.1×10-2Pa,X光衍射图谱(XRD)显示了很好的结晶性,并且在室温下观察到了铁磁性,饱和磁矩Ms为10.01emu/cm3,矫顽力为7.222Oe。通过第一性原理LDA+USIC算法对SnO2(200)面计算发现,当只有Sm掺杂时,体系的稳定态为反铁磁,而当Sm掺杂和O空位共存的时候,体系的磁矩没有太大变化,但是铁磁态变得比反铁磁态更加稳定,所以我们制备的Sm掺杂Sn02薄膜的室温铁磁性来自于Sm掺杂和O空位共存。
近年来,多铁材料由于独特的物理特性和应用潜力收到广泛的关注,特别是磁电耦合效应,也就是ME效应,通过电场来调控磁性或者通过磁场来调控电极化。近年来研究的多铁材料主要是Bi基钙钛矿化合物、Tb基锰化物、六方稀土锰化物和BaMF4家族(M代表二价过渡金属离子),然后由于磁性一般来自于过渡金属中的d电子,而d电子却降低了形成铁电必须的晶胞中非对称中心的形成趋势,所以单相的多铁材料很少见,而且大部分现有的单相多铁材料要么是室温反铁磁,比如BiFeO3,要么是低温铁磁性,比如BiMnO3,室温铁磁性极其少见。为了得到室温单相多铁材料,一种可行的方法就是通过本征缺陷的引入,比如阳离子或者阴离子缺陷,使得室温铁电材料中出现室温铁磁性,从而实验室温铁电和铁磁的共存,恰好PLD技术可以通过沉积过程中各种参数的控制实现薄膜中不同本征缺陷的形成。
(K,Na)NbO3(KNN)是一种很有应用前景的无铅压电陶瓷材料,而且铁电居里温度很高超过400度,BaNb2O6为基础的化合物作为一种新型的钨铜矿型铌酸盐铁电材料受到大量的关注,非化学计量比的BaNbO3-x在低温可以显示超导特性,LiNbO3和LiTaO3都是很重要的铁电材料,在光学和激光灯高科技领域有很重要的应用,这里我们对PLD技术制备了一系列薄膜样品进行了各种表征,研究了室温铁磁性和磁电耦合效应,通过第一性原理计算研究了磁性的可能来源,结论如下：
1.对于导电Si衬底上生长的KNN纳米晶薄膜,XRD结果显示对于KNN薄膜生长来说,氧气环境是必不可少的,否则沉积的KNN薄膜是非晶态的,扫描电镜图谱显示高的衬底温度有利于纳米颗粒的融合生长,磁性测量显示KNN薄膜中的室温铁磁性很有可能与K,Na阳离子缺陷有关,而不是由氧缺陷决定的,铁电性测量显示沉积过程中阳离子缺陷的引入会导致漏电流的增加,使样品的铁电性受到很大的损伤,不能得到饱和的电滞回线图,室温磁控电容MC效应测量显示室温铁磁性越强,磁性和电容之间的相互作用增强,形成的MC效应越强,磁电祸合研究发现样品的平面内饱和磁矩随着外加电场强度的增加而增大,电场和磁场的正交处理样品的平面内饱和磁矩会得到巨大的提高,是一个新的现象,可以看作一种新的ME效应。
2.对于LaAlO3衬底上制备了BaNb2O6薄膜,XRD结果表明：薄膜的相形成由衬底的选择决定,衬底温度和背景气氛对结构有一定影响,但是对薄膜的相形成没有决定作用；磁性测量结果表明：薄膜越薄应力效应越明显,导致磁性越强；BaNb2O6磁性很可能与氧空位有关,并且氮原子替换BaNb2O6晶格中的氧原子会使BaNb2O6薄膜的磁性变弱；真空环境下600℃制备的薄膜具有最大的饱和磁矩；X光电子能谱(XPS)结果表明：样品的磁性变化规律与O1s的不对称性变化规律一致,也就说明了样品的磁性与氧缺陷直接有关,样品中的氧缺陷浓度越大,O1s的不对称性越强,样品的磁性越强,饱和磁矩越大；第一性原理计算结果表明：不含有缺陷的BaNb2O6超晶格和含有一个Ba空位的时候,系统是无磁性的,而Nb空位和O空位由于O2p电子和Nb s电子的自旋极化使系统具有磁性,当系统中两个Nb空位处于第三近邻的时候铁磁态更稳定,系统的能量最低,相对稳定性最强；当两个O空位处于次近邻及第三紧邻的时候,铁磁态更稳定,当两个O空位处于第三近邻的时候系统的能量最低,系统的相对稳定性最高。终上所述,制备的BaNb2O6薄膜中的室温铁磁性主要是由样品中氧缺陷引起的,Nb空位也可能有一定的贡献。
3.对于Nb:SrTiO3衬底上生长的BaNbO3薄膜,衬底温度为600℃,氧气环境下制备的样品不具有室温铁磁性,但是当施加的电场强度大于10V/cm时,样品出现各向异性的铁磁性,并且随着外加电场强度的增大而增强,真空中制备的薄膜具有室温铁磁性,电场处理后铁磁性同样增强,只是电场强度阈值更大一些,这些现象来自于衬底和薄膜之间的电子转移,说明了BaNb03薄膜中的磁性可以通过极化电子密度来调控。
4.我们利用GGA算法对VLi0,VNb/Ta0,VO0,(Nb/Ta)Li0,(Nb/Ta)Li4+,(Nb/Ta)Li4++4VLi-缺陷团和Lii0在LiNbO3和LiTaO3中产生磁性的可能性进行了研究,结果发现三种缺陷情况下,氧缺陷的形成能都是最低的,(Nb/Ta)Li4+的形成能是所有缺陷中最低的,(Nb/Ta)Li4++4VLi-比(Nb/Ta)Li0和VLi0更稳定,等化学计量比以及含有(Nb/Ta)Li4+,(Nb/Ta)Li4++44VLi-,和Lii0的LiTaO3和LiNbO3是非磁性的,而VLi0和VNb/Ta0可以产生磁性,来自于O2p电子的自旋极化,在LiNbO3中由于Nb s电子的贡献在费米能级和导带之间出现了一个杂质能带,导致了LiTaO3和LiNbO3中(Nb/Ta)Li0和VO0产生磁矩的不同,因为带有TaLi0的LiTaO3磁性主要来自于导带底Ta d电子的自旋极化,而带有NbLi0和VO0的LiNbO3磁性主要来自于杂质能带中Nb s电子的自旋极化。对于带有VO0空位缺陷对的LiNbO3来说,当VO0空位缺陷对处于第三近邻的时候,铁磁耦合比反铁磁耦合稳定,而VO0占又是三种缺陷中形成能最低,磁性态相对稳定性最高的,所以推测实验上在LiNbO3中通过氧缺陷的引入可以产生铁磁性。
所以我们从实验上和理论上证明了带有本征缺陷的氧化物铁电薄膜材料中铁磁性存在的可能性,并且实验上在(K0.5Na0.5)NbO3和BaNbO3中观察到了新的磁电耦合效应,我们的研究为室温多铁材料和磁电耦合效应的研究开辟了一条新的道路。
Oxide materials possess enormous potential for applications in modern technology. They exhibit arrange of functional properties including electrical, magnetic and optical response. A wide variety of materials such as insulators, dielectrics, semiconductors, ferromagnets, ferroelectrics, colossal magnetoresistive materials and superconductors have been produced from oxides. Certain classes of oxides exhibit multiple simultaneous distinct functional properties; such compounds are normally referred to as "multifunctional" materials. For example, oxides such as transition metal doped ZnO, TiO2,InO3etc. can exhibit both ferromagnetic and semiconducting properties. Additionally, oxides such as Ni3V2O8, BiMnO3, BiFeO3and DyMnO3, exhibit simultaneous ferromagnetic and ferroelectric properties. Magnetoresistive materials such as Pr1-xCaxMnO3exhibit a coupled metal-insulator transition and magnetic ordering. Because multifunctional materials can often exhibit new properties, it is important to understand these systems from both scientific and technological point of view. The incorporation of these materials in device technology will lead to the development to new device applications. For example, incorporation of giant magnetorsistive (GMR) materials into modem memory devices such as read heads has highly enhanced their storage capacity.
Tin oxides SnO2is an important n-type semiconductor with a wide band gap (Eg=3.6eV at300K), which has a wide range of applications, including solid gas sensors, liquid crystal displays, photovoltaic cells, and transparent conductive electrodes. A variety of methods such as sol-gel, chemical vapor deposition, magnetron sputtering, thermal evaporation, pulsed laser deposition and molecular beam epitaxy have been used to prepare SnO2film. In recent years, considerable efforts have been focused on the synthesis of magnetic impurity doped SnO2thin films in order to explore DMS properties. On the other side, rare earth ion doping in various hosts has been investigated most frequently due to their unique fluorescence properties, stability, and high emission quantum yields. It is also important to note that rare earth ions can be doped into oxides with relative ease. It was envisaged that if rare earth ions can doped into oxide based semiconductors, then band gap excitation may result in efficient energy transfer that would result in interesting optical as well as magnetic properties. It is therefore interesting from a technical point of view to dope oxide semiconductors with rare earth ions to see the possibility of coexistence of optical and magnetic properties. If found, such a coexistence will be of great importance for magneto-optical device applications.
(200)oriented Sm-doped SnO2thin film has been successfully on (001)sapphire substrate at the substrate temperature of600℃, in oxygen atmosphere of1.1×10-2Pa. XRD pattern shows good crystallinity, and room temperature ferromagnetism has been observed with saturated magnetization of10.01emu/cm3and coercive field of7.222Oe. Ab inito calculations on the SnO2(200) plane with LDA+USIC method shows that, antiferromagnetic state is more stable when the system is merely doped with Sm, whereas ferromagnetic state becomes more stable than anti ferromagnetic state when Sm dopant and oxygen vacancy coexist in the system, without much change in the magnetic moment, therefore, the observed room temperature ferromagnetism in our sample should originate from the coexistence of Sm dopant and oxygen vacancy.
In recent years, considerable attention has been devoted to the investigation of multiferroic materials due to their fascinating physical properties and potential applications. Especially, the magnetoelectric (ME) effect, i.e. the manipulation of magnetization by application of electric field and vice versa, has been studied the most in multiferroic materials, such as Bi-based perovskite-type compounds, Tb-based manganites, hexagonal rare-earth manganites, and the family of BaMF4(where M is a divalent transition metal ion). However, single phase multiferroic materials are generally rare because transition-metal d electrons reduce the tendency of an off-centering ferroelectric distortion. Moreover, most of the existing single-phase ME multiferroics are either antiferromagntic above room temperature, such as BiFeO3, or ferromagnetic at low temperatures, such as BiMnO3. In order to achieve room temperature single phase multiferroic material, an alternative way is to introduce room temperature ferromagnetism (RTFM) into room temperature ferroelectric oxide materials through intrinsic defects, such as cation or oxygen vacancy, to realize the coexistence of ferromagnetism and ferroelectricity at room temperatre, while various intrinsic defects can be formed in the sample prepared by PLD through the adjustment of deposition parameters.
(K,Na)NbO3is one kind of potential candidate for practical lead-free piezoelectric ceramics, and (K0.5Na0.5)Nb03is ferroelectric with a high ferroelectric Curie temperature exceeding400℃; BaNb2O-based compounds are receiving great attention as a new ferroelectric tungsten bronze niobate material; nonstoichiometric BaNbO3-x could exhibit superconductivity with a Tc as high as22K, LiNbO3and LiTaO3are ferroelectric materials that have found important applications in many areas of advanced technology where optics and lasers dominate, therefore, it is of considerable importance to investigate the possibility of intrinsic defect-induced ferromagnetism and magnetoelectric effect in these systems. Here a series of PLD film samples were characterized by various techniques, and room temperature magnetic properties and magnetoelectric effect were investigated. The origin of magnetism is checked by ab inito calculations, and the results are listed below:
1. For KNN nanocrystalline film on conductive Si substrate, XRD result indicates that oxygen atmosphere is prerequisite for the growth o f KNN, or the as-deposited film would be in amorphous state. FE-SEM images shows that higher substrate temperature facilitates the coalescence of grains. Magnetic measurement by AGM demonstrates the existence of ferromagnetism at room temperature, which is supposed to originate from cation (K, Na) vacancies, rather than oxygen vacancy. Ferroelectric measurement indicates that ferroelectricity is damaged and saturated P-E hysteresis loop cannot be obtained due to the increase of leakage current by the introduction of cation vacancies during deposition. MC effect measurement at room temperature shows that stronger ferromagnetism results in stronger interaction between magnetism and capacitance, and thus stronger MC effect. ME effect investigation displays that the in-plane Ms increases with increasing electric field, and the phenomenon that the orthometric application of electric field and magnetic field on the surface of film and subsequent removal leads to an enormous enhancement of in-plane saturation magnetization is a new phenomenon, which can be treated as a new kind of ME effect.
2. For BaNb2O6film on LaAlO3substrate, XRD result shows that the phase of the as-deposited film is determined by the choice of substrate, with certain influence by substrate temperature and background atmosphere. Magnetic measurement by AGM demonstrates that thinner film results in stronger magnetism due to larger strain effect, the magnetism in BaNb2O6is related to oxygen vacancy, while the substitution of O atom in BaNb2O6by N atom could lead to the decrease of magnetism, and the largest Ms is obtained in vacuum at600℃. XPS measurement indicates that the change tendency of magnetism in different samples is in accordance with the asymmetric tendency of O1s spectrum, which means the magnetism in BaNb2O6is directly related to oxygen vacancy. Ab inito calculations indicate that stoichiometric BaNb2O6and that with one Ba vacancy is nonmagnetic, while Nb and O vacancy can lead to magnetism due to the spin-polarization of O2p electron and Nb s electron, respectively. For BaNb2O6supercell with double Nb/O vacancies, ferromagnetic coupling is energetically more favorable when the two Nb/O vacancies are located third-nearest-neighbored. Therefore, we can conclude that the observed room temperature FM in BaNb2O6films is predominantly induced by oxygen vacancies introduced during vacuum deposition, with certain contribution by Nb vacancies.
3. For BaNbO3films deposited on Nb:SrTiO3substrate at600℃, the one deposited in oxygen atmosphere is not ferromagnetic at room temperature, but anisotropic ferromagnetism emerges after the application of an electric filed, and becomes stronger with increasing electric filed intensity. On the other side, the one deposited in vacuum shows ferromagnetism at room temperature, which is also enhanced after the application of an electric filed, but with higher critical electric field. These phenomena are supposed to originate from the electron transfer between the substrate and film at the interface, indicating that the magnetic properties of BaNbO3film can be adjusted by the density of polarized electron.
4. We have performed ab initio calculations for the possibility of defect-induced magnetism in LiTaO3and LiNbO3by introducing intrinsic defects, namely, VLi0, VNb0/Ta,VO0,(Nb/Ta)Li0,(Nb/Ta)Li4+,(Nb/Ta)Li4++4VLi-cluster, and Lii0. Calculated results show that VO0is most stable among three vacancy cases, and (Nb/Ta)LiLi4++4VLi-has the lowest formation energy among all the defects. Stoichiometric LiTaO3and LiNbO3and that with (Nb/Ta)Li4+,(Nb/Ta)Li4++4VLLi-, and Lii0are non-magnetic, while VLi0and VNb0/Ta can induce magnetism due to the spin-polarization of O2p electrons, and the magnetism in LiTaO3with TaLi0originates from the spin-polarization of Ta d electron at the bottom of the conduction band, while the magnetism in LiNbO3with NbLi0and VO0comes from the spin-polarization of Nb s electron in the impurity band. For LiNbO3with VO0vacancy pair, ferromagnetic coupling is more favorable than and ferromagnetic coupling when VO0vacancy pair are located third-nearest neighboured. Therefore, ferromagnetism is supposed to be achieved in LiNbO3through the introduction of oxygen vavancy.
In conclusion, we have proved the existence of ferromagnetism in ferroelectric oxide film with intrinsic defects both experimentally and theoretically, and new kind of magnetoelectric effect has been observed in (K0.5Na0.5)NbO3and BaNbO3film. Our research opens up a new path for the research of multiferroics and magnetoelectric effect at room temperature
SnO2是一种很重要的宽禁带n型半导体,禁带宽度在300K时为3.6eV,具有广阔的应用范围,包括固态气体传感器、液晶显示器、光伏电池、和透明导电电极。目前已有大量的制备方法被用来制备SnO2薄膜,包括溶胶凝胶、化学气象沉积、磁控溅射、热蒸发、脉冲激光沉积、分子束外延。近几年,大量的研究集中在研究磁性元素掺杂SnO2薄膜或者纳米颗粒的磁性上,以便开发它的稀磁半导体特性,另一方面,在各种主体中掺杂稀土元素主要是由于它独特的荧光特性,稳定性,和高发射量子产率,还有一点很重要就是在各种氧化物主体总掺杂稀土离子更加的容易,这样能带激发可能导致有效的能量转移,产生有趣的光学和磁学性质,因此从技术应用的观点考虑,很有必要研究一下氧化物半导体中掺杂稀土元素实现光学和磁学性质共存的可能性,如果存在,那么对于磁光应用器件的研究很有帮助。
我们利用脉冲激光沉积PLD技术成功地在(001)蓝宝石衬底上生长了(200)方向的Sm掺杂SnO2薄膜,衬底温度为600℃,氧气压强为1.1×10-2Pa,X光衍射图谱(XRD)显示了很好的结晶性,并且在室温下观察到了铁磁性,饱和磁矩Ms为10.01emu/cm3,矫顽力为7.222Oe。通过第一性原理LDA+USIC算法对SnO2(200)面计算发现,当只有Sm掺杂时,体系的稳定态为反铁磁,而当Sm掺杂和O空位共存的时候,体系的磁矩没有太大变化,但是铁磁态变得比反铁磁态更加稳定,所以我们制备的Sm掺杂Sn02薄膜的室温铁磁性来自于Sm掺杂和O空位共存。
近年来,多铁材料由于独特的物理特性和应用潜力收到广泛的关注,特别是磁电耦合效应,也就是ME效应,通过电场来调控磁性或者通过磁场来调控电极化。近年来研究的多铁材料主要是Bi基钙钛矿化合物、Tb基锰化物、六方稀土锰化物和BaMF4家族(M代表二价过渡金属离子),然后由于磁性一般来自于过渡金属中的d电子,而d电子却降低了形成铁电必须的晶胞中非对称中心的形成趋势,所以单相的多铁材料很少见,而且大部分现有的单相多铁材料要么是室温反铁磁,比如BiFeO3,要么是低温铁磁性,比如BiMnO3,室温铁磁性极其少见。为了得到室温单相多铁材料,一种可行的方法就是通过本征缺陷的引入,比如阳离子或者阴离子缺陷,使得室温铁电材料中出现室温铁磁性,从而实验室温铁电和铁磁的共存,恰好PLD技术可以通过沉积过程中各种参数的控制实现薄膜中不同本征缺陷的形成。
(K,Na)NbO3(KNN)是一种很有应用前景的无铅压电陶瓷材料,而且铁电居里温度很高超过400度,BaNb2O6为基础的化合物作为一种新型的钨铜矿型铌酸盐铁电材料受到大量的关注,非化学计量比的BaNbO3-x在低温可以显示超导特性,LiNbO3和LiTaO3都是很重要的铁电材料,在光学和激光灯高科技领域有很重要的应用,这里我们对PLD技术制备了一系列薄膜样品进行了各种表征,研究了室温铁磁性和磁电耦合效应,通过第一性原理计算研究了磁性的可能来源,结论如下：
1.对于导电Si衬底上生长的KNN纳米晶薄膜,XRD结果显示对于KNN薄膜生长来说,氧气环境是必不可少的,否则沉积的KNN薄膜是非晶态的,扫描电镜图谱显示高的衬底温度有利于纳米颗粒的融合生长,磁性测量显示KNN薄膜中的室温铁磁性很有可能与K,Na阳离子缺陷有关,而不是由氧缺陷决定的,铁电性测量显示沉积过程中阳离子缺陷的引入会导致漏电流的增加,使样品的铁电性受到很大的损伤,不能得到饱和的电滞回线图,室温磁控电容MC效应测量显示室温铁磁性越强,磁性和电容之间的相互作用增强,形成的MC效应越强,磁电祸合研究发现样品的平面内饱和磁矩随着外加电场强度的增加而增大,电场和磁场的正交处理样品的平面内饱和磁矩会得到巨大的提高,是一个新的现象,可以看作一种新的ME效应。
2.对于LaAlO3衬底上制备了BaNb2O6薄膜,XRD结果表明：薄膜的相形成由衬底的选择决定,衬底温度和背景气氛对结构有一定影响,但是对薄膜的相形成没有决定作用；磁性测量结果表明：薄膜越薄应力效应越明显,导致磁性越强；BaNb2O6磁性很可能与氧空位有关,并且氮原子替换BaNb2O6晶格中的氧原子会使BaNb2O6薄膜的磁性变弱；真空环境下600℃制备的薄膜具有最大的饱和磁矩；X光电子能谱(XPS)结果表明：样品的磁性变化规律与O1s的不对称性变化规律一致,也就说明了样品的磁性与氧缺陷直接有关,样品中的氧缺陷浓度越大,O1s的不对称性越强,样品的磁性越强,饱和磁矩越大；第一性原理计算结果表明：不含有缺陷的BaNb2O6超晶格和含有一个Ba空位的时候,系统是无磁性的,而Nb空位和O空位由于O2p电子和Nb s电子的自旋极化使系统具有磁性,当系统中两个Nb空位处于第三近邻的时候铁磁态更稳定,系统的能量最低,相对稳定性最强；当两个O空位处于次近邻及第三紧邻的时候,铁磁态更稳定,当两个O空位处于第三近邻的时候系统的能量最低,系统的相对稳定性最高。终上所述,制备的BaNb2O6薄膜中的室温铁磁性主要是由样品中氧缺陷引起的,Nb空位也可能有一定的贡献。
3.对于Nb:SrTiO3衬底上生长的BaNbO3薄膜,衬底温度为600℃,氧气环境下制备的样品不具有室温铁磁性,但是当施加的电场强度大于10V/cm时,样品出现各向异性的铁磁性,并且随着外加电场强度的增大而增强,真空中制备的薄膜具有室温铁磁性,电场处理后铁磁性同样增强,只是电场强度阈值更大一些,这些现象来自于衬底和薄膜之间的电子转移,说明了BaNb03薄膜中的磁性可以通过极化电子密度来调控。
4.我们利用GGA算法对VLi0,VNb/Ta0,VO0,(Nb/Ta)Li0,(Nb/Ta)Li4+,(Nb/Ta)Li4++4VLi-缺陷团和Lii0在LiNbO3和LiTaO3中产生磁性的可能性进行了研究,结果发现三种缺陷情况下,氧缺陷的形成能都是最低的,(Nb/Ta)Li4+的形成能是所有缺陷中最低的,(Nb/Ta)Li4++4VLi-比(Nb/Ta)Li0和VLi0更稳定,等化学计量比以及含有(Nb/Ta)Li4+,(Nb/Ta)Li4++44VLi-,和Lii0的LiTaO3和LiNbO3是非磁性的,而VLi0和VNb/Ta0可以产生磁性,来自于O2p电子的自旋极化,在LiNbO3中由于Nb s电子的贡献在费米能级和导带之间出现了一个杂质能带,导致了LiTaO3和LiNbO3中(Nb/Ta)Li0和VO0产生磁矩的不同,因为带有TaLi0的LiTaO3磁性主要来自于导带底Ta d电子的自旋极化,而带有NbLi0和VO0的LiNbO3磁性主要来自于杂质能带中Nb s电子的自旋极化。对于带有VO0空位缺陷对的LiNbO3来说,当VO0空位缺陷对处于第三近邻的时候,铁磁耦合比反铁磁耦合稳定,而VO0占又是三种缺陷中形成能最低,磁性态相对稳定性最高的,所以推测实验上在LiNbO3中通过氧缺陷的引入可以产生铁磁性。
所以我们从实验上和理论上证明了带有本征缺陷的氧化物铁电薄膜材料中铁磁性存在的可能性,并且实验上在(K0.5Na0.5)NbO3和BaNbO3中观察到了新的磁电耦合效应,我们的研究为室温多铁材料和磁电耦合效应的研究开辟了一条新的道路。
Oxide materials possess enormous potential for applications in modern technology. They exhibit arrange of functional properties including electrical, magnetic and optical response. A wide variety of materials such as insulators, dielectrics, semiconductors, ferromagnets, ferroelectrics, colossal magnetoresistive materials and superconductors have been produced from oxides. Certain classes of oxides exhibit multiple simultaneous distinct functional properties; such compounds are normally referred to as "multifunctional" materials. For example, oxides such as transition metal doped ZnO, TiO2,InO3etc. can exhibit both ferromagnetic and semiconducting properties. Additionally, oxides such as Ni3V2O8, BiMnO3, BiFeO3and DyMnO3, exhibit simultaneous ferromagnetic and ferroelectric properties. Magnetoresistive materials such as Pr1-xCaxMnO3exhibit a coupled metal-insulator transition and magnetic ordering. Because multifunctional materials can often exhibit new properties, it is important to understand these systems from both scientific and technological point of view. The incorporation of these materials in device technology will lead to the development to new device applications. For example, incorporation of giant magnetorsistive (GMR) materials into modem memory devices such as read heads has highly enhanced their storage capacity.
Tin oxides SnO2is an important n-type semiconductor with a wide band gap (Eg=3.6eV at300K), which has a wide range of applications, including solid gas sensors, liquid crystal displays, photovoltaic cells, and transparent conductive electrodes. A variety of methods such as sol-gel, chemical vapor deposition, magnetron sputtering, thermal evaporation, pulsed laser deposition and molecular beam epitaxy have been used to prepare SnO2film. In recent years, considerable efforts have been focused on the synthesis of magnetic impurity doped SnO2thin films in order to explore DMS properties. On the other side, rare earth ion doping in various hosts has been investigated most frequently due to their unique fluorescence properties, stability, and high emission quantum yields. It is also important to note that rare earth ions can be doped into oxides with relative ease. It was envisaged that if rare earth ions can doped into oxide based semiconductors, then band gap excitation may result in efficient energy transfer that would result in interesting optical as well as magnetic properties. It is therefore interesting from a technical point of view to dope oxide semiconductors with rare earth ions to see the possibility of coexistence of optical and magnetic properties. If found, such a coexistence will be of great importance for magneto-optical device applications.
(200)oriented Sm-doped SnO2thin film has been successfully on (001)sapphire substrate at the substrate temperature of600℃, in oxygen atmosphere of1.1×10-2Pa. XRD pattern shows good crystallinity, and room temperature ferromagnetism has been observed with saturated magnetization of10.01emu/cm3and coercive field of7.222Oe. Ab inito calculations on the SnO2(200) plane with LDA+USIC method shows that, antiferromagnetic state is more stable when the system is merely doped with Sm, whereas ferromagnetic state becomes more stable than anti ferromagnetic state when Sm dopant and oxygen vacancy coexist in the system, without much change in the magnetic moment, therefore, the observed room temperature ferromagnetism in our sample should originate from the coexistence of Sm dopant and oxygen vacancy.
In recent years, considerable attention has been devoted to the investigation of multiferroic materials due to their fascinating physical properties and potential applications. Especially, the magnetoelectric (ME) effect, i.e. the manipulation of magnetization by application of electric field and vice versa, has been studied the most in multiferroic materials, such as Bi-based perovskite-type compounds, Tb-based manganites, hexagonal rare-earth manganites, and the family of BaMF4(where M is a divalent transition metal ion). However, single phase multiferroic materials are generally rare because transition-metal d electrons reduce the tendency of an off-centering ferroelectric distortion. Moreover, most of the existing single-phase ME multiferroics are either antiferromagntic above room temperature, such as BiFeO3, or ferromagnetic at low temperatures, such as BiMnO3. In order to achieve room temperature single phase multiferroic material, an alternative way is to introduce room temperature ferromagnetism (RTFM) into room temperature ferroelectric oxide materials through intrinsic defects, such as cation or oxygen vacancy, to realize the coexistence of ferromagnetism and ferroelectricity at room temperatre, while various intrinsic defects can be formed in the sample prepared by PLD through the adjustment of deposition parameters.
(K,Na)NbO3is one kind of potential candidate for practical lead-free piezoelectric ceramics, and (K0.5Na0.5)Nb03is ferroelectric with a high ferroelectric Curie temperature exceeding400℃; BaNb2O-based compounds are receiving great attention as a new ferroelectric tungsten bronze niobate material; nonstoichiometric BaNbO3-x could exhibit superconductivity with a Tc as high as22K, LiNbO3and LiTaO3are ferroelectric materials that have found important applications in many areas of advanced technology where optics and lasers dominate, therefore, it is of considerable importance to investigate the possibility of intrinsic defect-induced ferromagnetism and magnetoelectric effect in these systems. Here a series of PLD film samples were characterized by various techniques, and room temperature magnetic properties and magnetoelectric effect were investigated. The origin of magnetism is checked by ab inito calculations, and the results are listed below:
1. For KNN nanocrystalline film on conductive Si substrate, XRD result indicates that oxygen atmosphere is prerequisite for the growth o f KNN, or the as-deposited film would be in amorphous state. FE-SEM images shows that higher substrate temperature facilitates the coalescence of grains. Magnetic measurement by AGM demonstrates the existence of ferromagnetism at room temperature, which is supposed to originate from cation (K, Na) vacancies, rather than oxygen vacancy. Ferroelectric measurement indicates that ferroelectricity is damaged and saturated P-E hysteresis loop cannot be obtained due to the increase of leakage current by the introduction of cation vacancies during deposition. MC effect measurement at room temperature shows that stronger ferromagnetism results in stronger interaction between magnetism and capacitance, and thus stronger MC effect. ME effect investigation displays that the in-plane Ms increases with increasing electric field, and the phenomenon that the orthometric application of electric field and magnetic field on the surface of film and subsequent removal leads to an enormous enhancement of in-plane saturation magnetization is a new phenomenon, which can be treated as a new kind of ME effect.
2. For BaNb2O6film on LaAlO3substrate, XRD result shows that the phase of the as-deposited film is determined by the choice of substrate, with certain influence by substrate temperature and background atmosphere. Magnetic measurement by AGM demonstrates that thinner film results in stronger magnetism due to larger strain effect, the magnetism in BaNb2O6is related to oxygen vacancy, while the substitution of O atom in BaNb2O6by N atom could lead to the decrease of magnetism, and the largest Ms is obtained in vacuum at600℃. XPS measurement indicates that the change tendency of magnetism in different samples is in accordance with the asymmetric tendency of O1s spectrum, which means the magnetism in BaNb2O6is directly related to oxygen vacancy. Ab inito calculations indicate that stoichiometric BaNb2O6and that with one Ba vacancy is nonmagnetic, while Nb and O vacancy can lead to magnetism due to the spin-polarization of O2p electron and Nb s electron, respectively. For BaNb2O6supercell with double Nb/O vacancies, ferromagnetic coupling is energetically more favorable when the two Nb/O vacancies are located third-nearest-neighbored. Therefore, we can conclude that the observed room temperature FM in BaNb2O6films is predominantly induced by oxygen vacancies introduced during vacuum deposition, with certain contribution by Nb vacancies.
3. For BaNbO3films deposited on Nb:SrTiO3substrate at600℃, the one deposited in oxygen atmosphere is not ferromagnetic at room temperature, but anisotropic ferromagnetism emerges after the application of an electric filed, and becomes stronger with increasing electric filed intensity. On the other side, the one deposited in vacuum shows ferromagnetism at room temperature, which is also enhanced after the application of an electric filed, but with higher critical electric field. These phenomena are supposed to originate from the electron transfer between the substrate and film at the interface, indicating that the magnetic properties of BaNbO3film can be adjusted by the density of polarized electron.
4. We have performed ab initio calculations for the possibility of defect-induced magnetism in LiTaO3and LiNbO3by introducing intrinsic defects, namely, VLi0, VNb0/Ta,VO0,(Nb/Ta)Li0,(Nb/Ta)Li4+,(Nb/Ta)Li4++4VLi-cluster, and Lii0. Calculated results show that VO0is most stable among three vacancy cases, and (Nb/Ta)LiLi4++4VLi-has the lowest formation energy among all the defects. Stoichiometric LiTaO3and LiNbO3and that with (Nb/Ta)Li4+,(Nb/Ta)Li4++4VLLi-, and Lii0are non-magnetic, while VLi0and VNb0/Ta can induce magnetism due to the spin-polarization of O2p electrons, and the magnetism in LiTaO3with TaLi0originates from the spin-polarization of Ta d electron at the bottom of the conduction band, while the magnetism in LiNbO3with NbLi0and VO0comes from the spin-polarization of Nb s electron in the impurity band. For LiNbO3with VO0vacancy pair, ferromagnetic coupling is more favorable than and ferromagnetic coupling when VO0vacancy pair are located third-nearest neighboured. Therefore, ferromagnetism is supposed to be achieved in LiNbO3through the introduction of oxygen vavancy.
In conclusion, we have proved the existence of ferromagnetism in ferroelectric oxide film with intrinsic defects both experimentally and theoretically, and new kind of magnetoelectric effect has been observed in (K0.5Na0.5)NbO3and BaNbO3film. Our research opens up a new path for the research of multiferroics and magnetoelectric effect at room temperature
引文
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