【作者】 王楠；

【导师】 何巨龙； 柳忠元；

【作者基本信息】 燕山大学 ， 材料物理与化学， 2011， 博士

【摘要】 多铁性材料,由于其同时具有铁磁有序、铁电有序或铁弹有序而且它们之间在一定温度范围内存在耦合的特性,使得这种材料在信息存储、自旋电子设备以及传感器等方面都颇具发展潜力。在所有的多铁性材料中,具有亚铁磁性的ε-GaFeO₃由于具有较高的居里温度（Tc=200K）而引起各国科学家的广泛关注。通过固相烧结方法成功制备得到了一系列不同Fe含量的ε-Ga_2-xFe_xO₃（0.8≤x≤1.3）样品。讨论了ε-Ga_2-xFe_xO₃化合物的化学组成与不同阳离子位置上Fe含量的分布和高场磁化强度M以及居里温度Tc之间的关系。采用X射线Rietveld精修方法确定了不同Fe含量时样品中四个不同阳离子Fe1,Fe2,Ga1和Ga₂位置上的Fe离子分布情况。随着Fe含量的增加,晶格常数a,b和c值单调增加。居里温度Tc和高场磁化强度M与单晶样品结果非常一致。根据实验确定出的不同阳离子位置上Fe离子的分布与Fe含量x的依赖关系,Tc和M可以按照一个简单的统计模型被计算出来。计算得到的Tc和M值与实验值具有很好的一致性。由于Fe离子分布的无序性,存在一部分对亚铁磁性没有贡献的无效Fe离子,这些Fe离子表现出了不同的磁学行为。在亚铁磁背景下,它们引起了明显的峰强的频率依赖性,也就是所观察到的磁阻挫效应。据报道,ε-Ga_2-xFe_xO₃化合物的磁学性质以及Fe离子在四个阳离子位置上的分布非常依赖于样品的制备方法和热处理过程。使用熔盐法制备得到的样品则表现出了独特的性质。其中慢冷ε-Ga_1.1Fe_0.9O₃样品的交流磁化率的测量表明这种材料具有类自旋玻璃行为,而通过磁滞廻线的测量我们观察到了明显的交换偏置和磁锻炼效应。以ε-Ga_1.2Fe_0.8O₃为前驱物,通过高温高压技术在压力为2-5GPa、400-1300°C温度条件下,获得了一种菱方相α-Ga_1.2Fe_0.8O₃,其空间群为R（3|-）c ,晶格常数为a=5.0137（A|。） , c=13.5594（A|。）。拉曼结果的解析表明α-Ga_1.2Fe_0.8O₃具有与α-Fe2O₃和α-Ga₂O₃相同的拉曼激活模式。而在波数660cm^-1处出现的峰则被归结为是由样品中的晶格扭曲造成的。对α-Ga_1.2Fe_0.8O₃进行的晶格热膨胀性能测试表明,在室温-800oC温度范围内,α-Ga_1.2Fe_0.8O₃呈现各向异性的晶格热膨胀特性,得到的晶格膨胀系数分别为:α_a=9.96×10^-6K^-1,α_c=1.2×10^-5K^-1。α-Ga_1.2Fe_0.8O₃样品的FC和ZFC磁化强度曲线的分叉行为以及低温时磁滞廻线在高场下的不饱和行为都表明在α-Ga_1.2Fe_0.8O₃中存在阻挫性质,而交流磁化率的测量则确定了这种物质存在自旋玻璃的特征。在弛豫效应中,可以明显的观察到α-Ga_1.2Fe_0.8O₃样品的ZFC磁化强度对等待时间的依赖性。而升降温过程中FC磁化强度在停留温度处的台阶式行为表明了FC过程中存在记忆效应。对于ZFC磁化强度的记忆效应,我们发现等待与参考ZFC磁化强度曲线之差在等待温度附近给出了一个明显的低谷,但是两条曲线之间发生偏移的温度区间不仅仅局限于停留温度附近。在这种材料中,引起磁性玻璃现象的原因可能是由化学键各向异性引起的阻挫效应和Ga/Fe离子的随机分布引起的随机效应共同引起的。更多还原

【Abstract】 Multiferroic materials, that show ferroelectric order, （anti）ferromagnetic and/or ferroelastic order in the same phase and a coupling between them over a certain range of temperature, are extensively studied because of the potential application to provide a wide range of applications, including the field of spintronics, sensors, data-storage media, etc. Amonge these materials, the ferrimagnetic GaFeO₃ with higher Curie temperature （Tc=200K） has attracted much attaintion in recent years.In this thesis, a series ofε-Ga_2-xFe_xO₃ （0.8≤x≤1.3） samples have been successfully prepared by solid state reaction. The occupations of Fe ions at four different cation sites are determined by Rietveld refinement, and their effects on high-field magnetization M and Curie temperature Tc have been investigated. The results shows that the lattice parameters of a, b, and c monotonously rise with increasing Fe content. The values of Tc and M are consistent with those of single crystal, which can be calculated by a simple statistical model using experimental value of Fe content at four different cation sites. The calculated Tc and M are well fitted to experimental results. The non-effect Fe ions exist in the crystal due to the disordered distribution, which have no contribution to ferrimagnetism and show different magnetic behavior. Their presence in the ferrimagnetic ordering background gives rise to the observed frequency suppression of the peak height in the ac susceptibility, demonstrating the frustration effect.According to the previous reports, the Fe ions distribution and magnetic property ofε-Ga_2-xFe_xO₃ compounds are dependent on the sample preparation method and heat history. The samples synthesized by molten salt exhibit unique property. The spin glass behavior of slow-coolingε-Ga_1.1Fe_0.9O₃ sample is determined by ac susceptibility, and the exchange bias effect and traning effect are also observed.A rhombohedralα-Ga_1.2Fe_0.8O₃ has been successfully prepared by high pressure and high temperature technique usingε-Ga_2-xFe_xO₃ as precursor. It has been found thatα-Ga_1.2Fe_0.8O₃ crystallizes in a rhombohedral structure with space group R（3|-）c, and the lattice parameters area=5.0137（A|。） , c=13.5594（A|。）. The Raman active modes ofα-Ga_1.2Fe_0.8O₃ compound are corresponding to those ofα-Fe2O₃ andα-Ga₂O₃. The Raman active mode at 660 cm-1 indicates the formation of lattice distortion. The lattice thermal expansion coefficient of theα-Ga_1.2Fe_0.8O₃ in the range of room temperature-800oC have been found to beα_a=9.96×10^-6K^-1,α_c=1.2×10^-5K^-1, exhibiting anisotropy.The deviation between FC and ZFC magnetization and the unsaturated hysteresis loop even at 50kOe field forα-Ga_1.2Fe_0.8O₃ compound indicate the existence of frustration effect, and the spin glass behavior is unambiguously confirmed by ac susceptibility measurement. The dependence of ZFC magnetization on waiting time is observed. The step-like behavior around stop-temperature in the heating and cooling FC magnetization indicates that the system keeps a memory imprinted in the FC process. The difference between the reference ZFC magnetization and waiting ZFC magnetization shows a dip around the stop temperature, and the splitting between them is not restricted only to the vicinity of stop temperature. The glassy behavior of theα-Ga_1.2Fe_0.8O₃ compound can be ascribed to both the common frustration, from the bond anisotropy, and the randomness resulting from the random distribution of Ga/Fe ions.更多还原