【作者】 张卿；

【导师】 宋波；

【作者基本信息】 哈尔滨工业大学 ， 物理学， 2016， 硕士

【摘要】 钙钛矿(perovskite)氧化物自从被发现以来就以丰富的物理性质和广泛的应用前景得到人们的关注,例如铁电性质、庞磁电阻和高温超导等。近年来,与钙钛矿具有相似晶体结构的反钙钛矿(antiperovskite)化合物也得到广泛的研究,因为此类化合物也具有一些特殊的物理现象,如负热膨胀、磁卡效应、巨磁阻效应和近零电阻温度系数等。本论文对反钙钛矿化合物Mn3Ag1-xNbxN的合成制备及其负热膨胀、磁性以及电输运等物理性质进行了研究,并分析了它们的物理机制和相互的关联性。研究表明Nb原子在Ag位的取代会对Mn3AgN的负热膨胀性质产生影响。Mn3Ag1-xNbxN的负热膨胀温度区间随Nb掺杂量的增加逐渐展宽并向低温方向移动,其线性热膨胀系数也逐渐减小。Mn3AgN母体在270.1至288.7 K之间(ΔT=18.6 K)的线热膨胀系数为-22.43 ppm/K,而Mn3Ag0.9Nb0.1N的负热膨胀温度区间为260.1至284.8 K(ΔT=24.7 K),线性热膨胀系数减小至-18.2 ppm/K,这表明Nb原子的取代不仅可以有效地展宽负热膨胀温区,还能使负热膨胀速率减缓。磁性测量表明Mn3AgN在278.6 K处发生反铁磁-顺磁相变。当Nb原子取代部分Ag原子后,Mn3Ag1-xNbxN的奈尔温度随Nb含量的增加而逐渐降低,当x=0.04时奈尔温度降低至271.3 K。此外,Mn3Ag1-xNbxN自旋玻璃态的冻结温度随Nb掺杂量增加向高温区移动,从Mn3AgN母体的74 K升高至Mn3Ag0.96Nb0.04N的129 K,这说明晶胞内Mn原子之间的磁交换作用随Nb掺杂量的增加而逐渐增强。电输运测试表明Mn3Ag1-xNbxN的电阻率在磁相变温度以上具有近零电阻温度系数即电阻率几乎不随温度变化。样品的近零电阻温度系数随Nb掺杂量增加更接近于0,从Mn3AgN的7.7×10-4/k减小至Mn3Ag0.9Nb0.1N的3.996×10-4/k。原因是由于Nb原子取代部分Ag原子引起晶格常数减小并改变材料的电子结构,使得费米能级处的电子态密度发生重构所致。通过对比负热膨胀温度区间、奈尔温度以及近零电阻温度系数的起始温度可以发现,Mn3Ag1-xNbxN的晶格、自旋和电荷之间存在一种内在关联。Nb原子取代不仅使晶格常数发生了明显的变化,同时还引起晶格内磁交换机制变化,而材料的电输运性质是电子结构、晶格散射、磁散射等多种物理机制共同作用的结果。以上实验结果说明可以通过改变反钙钛矿化合物晶胞顶角位置原子的种类来调控母体的物理性质。更多还原

【Abstract】 Perovskite oxides have attracted numerous attentions since their discovery because of the various physical properties and promising applications, such as ferroelectric, colossal magnetoresistance, and superconductivity, etc. In recent years, antiperovskite compounds, with the similar crystal structure to that of perovskites, have been intensively investigated owing to their novel properties including negative thermal expansion, magnetocaloric effect, giant magnetoresistance, and near zero temperature coefficient of resistance. In this work, we synthesized the antiperovskite Mn3Ag1-xNbxN and studied its negative thermal expansion, magnetic, and transport properties, further, the intrinsic correlation between crystal structure and thermal expansion were also discussed.It is shown that Nb substitution of Ag could alter the negative thermal expansion in Mn3Ag1-xNbxN. Nb doping resulted in the broadening of temperature window of negative thermal expansion, in which the linear thermal expansion coefficient decreased, while the onset of negative thermal expansion shifts to lower temperature. Specifically, pure Mn3 AgN exhibits negative thermal expansion from 270.1 K to 288.7 K(ΔT = 18.6 K) with-22.43 ppm/K, while the linear thermal expansion coefficient of Mn3Ag0.9Nb0.1N decrease to-18.2 ppm/K in temperature section from 260.1 K to 284.8 K.The magnetic measurement shows that parent material Mn3 AgN has an antiferromagnetic-to-paramagnetic transition at 278.6 K, and the introduction of Nb into host Mn3AgN(Mn3Ag0.96Nb0.04 N sample) can realize a lower Neel temperature of 271.3 K. Notably, with the temperature decreasing, Mn3Ag1-xNbxN exhibits a spin – glass behavior, with the freezing temperature shift from 74 K(undoped Mn3AgN) to 129 K(Mn3Ag0.96Nb0.04N), implying an enhanced magnetic exchange interaction of Mn-Mn induced by Nb doping.The transport property demonstrates that near zero temperature coefficient of resistance exists in above magnetic transition temperature of Nb doped Mn3 AgN, in which the temperature coefficient of resistance decreased gradually with increasing the concentration of Nb in system. Partial Nb doping at Ag site induced the contraction of lattice and variation of electric structure, therefore the density of state of electrons reconstructed at Fermi level.Comparing the negative thermal expansion, Neel temperature, and onset of near zero temperature coefficient of resistance, it can be found that Mn3Ag1-xNbxN is a strongly internal correlated system in lattice, spin, and charge. Nb doping in host material leads to an obvious variation in lattice and induces enhancement of magnetic exchange mechanism simultaneously. In addition, transport property is affected by various factors such as electric structure, lattice scattering and magnetic scattering. The above experimental results show that physical properties of antiperovskites could be ameliorated by altering the species of the atoms located in corner of unit cell.更多还原