Nd_(0.5)Sr_(0.5-x)Ca_xMnO_3(x=0.1,0.2)多晶的电子自旋共振研究

研究了多晶样品Nd_(0.5)Sr_(0.5-x)Ca_xMnO_3(x=0.1,0.2)的磁性.实验结果表明,样品的单相性好,无杂相,空间群为Pbnm.随着温度的降低,样品先后经历了3次磁性转变,分别为顺磁-铁磁转变TC、电荷有序转变TCO和奈尔温度转变TN;在TC以上,系统处于顺磁态,在TC-TCO范围内,系统处于铁磁-顺磁共存态,在TCO-TN范围内,系统处于反铁磁-铁磁-顺磁共存态,在TN以下,系统主要处于反铁磁态.     

钙钛矿锰氧化物Pr1-xCaxMnO3(0.3≤x≤0.5)的结构和磁性研究

通过固相反应法合成了Pr1-xCaxMnO3(x＝0.3,0.4,0.5)系列多晶样品,对样品的结构和磁性进行了研究.XRD检测结果发现,样品均为单相,空间群为Pbnm.用超导量子磁强计(SQUID)对样品的磁性进行了研究,着重分析了掺杂浓度为x＝0.4的Pr0.6Ca0.4MnO3样品.从测得的M-T曲线可知,在降温过程中,样品经历了电荷有序转变和反铁磁转变,电荷有序温度TCO=250K,反铁磁转变温度TN=170K.通过不同温度的M-H曲线,证实了样品在各个阶段的磁有序转变.     

钙钛矿锰氧化物Nd_(0.5)Sr_(0.3)Ba_(0.2)MnO_3多晶的磁性和相分离

通过固相反应法合成Nd0.5Sr0.3Ba0.2MnO3多晶样品,采用粉末X射线衍射(XRD)、超导量子磁强计(SQUID)和电子自旋共振谱仪(ESR),对样品的结构和磁性进行了研究.XRD检测结果表明,样品为单相,空间群为Pbnm.磁性测量和ESR结果表明,在降温过程中,样品经历了铁磁有序转变(TC=250K)、电荷有序转变(TCO=190K)和反铁磁转变(TN=120K).通过ESR谱线的参数随温度的变化可以观察到各个磁有序转变温度,TN相似文献   

Pr_(0.65)Ca_(0.35)MnO_3的电磁性质

通过溶胶-凝胶法获得了多晶钙钛矿型稀土锰氧化物Pr0.65Ca0.35MnO3,研究了所得样品的电磁输运性质.实验发现,外磁场为0时,样品在50~300K时均为绝缘态,样品中电荷的有序态被强磁场破坏,向金属态转变.为具体研究样品的铁磁结构,分别测量了样品的零场冷却和加场冷却的磁化曲线,结果显示,在165K时,样品发生了反铁磁态-顺磁态转变,另外,在58K时,零场冷却曲线与加场冷却曲线发生分离并迅速下降,这表明样品中铁磁态可能以团簇的形式存在.对样品交流磁化率的研究发现,不同频率下的磁化率在70K左右达到峰值,而随着频率的增大,强度逐渐减小,并且峰值发生右移,表明样品具有团簇玻璃态的特征.     

Cr/Gd双层膜的磁特性

采用磁控溅射法制备居里温度(TC)略低于奈尔温度(TN)的Cr/Gd双层薄膜.利用振动样品磁强计研究薄膜在不同温度下的磁滞特性.结果表明:该类薄膜系统与TCTN的铁磁/反铁磁双层薄膜的磁滞特性不同,当测量温度(T)大于TC时,Gd层处于铁磁态,矫顽力(HC)随温度非单调变化;当T为80～205K时,HC随温度增加逐渐减小;在T=205K出现一个极小值后逐渐增大;在T=255K附近出现一个主峰;当T为255～295K时,HC随温度升高迅速减小;当T295K时,HC随温度升高迅速增大.即铁磁/反铁磁界面处的反铁磁自旋与铁磁自旋的交换耦合作用对铁磁层磁有序态的维持温度和矫顽力影响较大.     

类钙钛矿化合物Ca(Mn3-xCux)Mn4O12的磁电阻效应

在常压和较低温度(1073K)下利用溶胶-凝胶法制备了成分为Ca(Mn3-xCux)Mn4O12的化合物,X射线衍射图表明当x≤1.5时化合物呈单一钙钛矿相.所有样品均未观测到金属-绝缘体转变,但随成份的改变有反铁磁-铁磁转变发生.它们的磁电阻随温度呈线性变化系.对x＝1.5的样品,在77K和3T磁场下的磁电阻可达37%.     

Nd_(0.5)Ca_(0.5)MnO_3电荷有序态下电荷输运机制的研究

制备了单相多晶Nd0.5Ca0.5MnO3样品,研究了该体系在电荷有序态下的电荷输运机制.研究表明,该体系在低温下表现为半导体输运特性,在250K处发生了电荷有序相变;电荷有序态下的电荷输运机制满足可变程跃迁模型.     

Nd0.5 Cao.5MnO3电荷有序态下电荷输运机制的研究

制备了单相多晶Nd0.5Ca0.5MnO3样品,研究了该体系在电荷有序态下的电荷输运机制.研究表明,该体系在低温下表现为半导体输运特性,在250K处发生了电荷有序相变;电荷有序态下的电荷输运机制满足可变程跃迁模型.     

双层钙钛矿锰氧化物La1.2-xDyxSr1.8Mn2O7(x=0.00,0.05)的磁性和类Griffiths研究

采用传统固相反应法制备多晶样品La1.2-xDyxSr1.8Mn2O7(x=0.00,0.05),通过测量样品的磁化强度与温度变化曲线(M-T)以及不同温度下磁化强度与外加磁场的变化曲线(M-H)对样品的磁性和类Griffiths 相进行了研究. 研究发现,两个样品分别在15-400 K 的测量温度范围内,在类Griffiths 相温度(TG≈368、360 K)以上处于纯顺磁态;在奈尔温度(TN≈210、183 K)-TG范围内,系统处于反铁磁-铁磁共存态,存在类Griffiths 相,随温度的降低,样品的铁磁性逐渐增强;在TN以下,随温度的降低,铁磁性逐渐减弱,反铁磁性增强,样品呈现出了团簇玻璃行为. 通过对比两个样品后发现,掺杂后样品x=0.05 比未掺杂样品的磁性、三维长程居里温度(Tc3D)和类Griffiths相温度(TG)均降低,这是由于Dy3+离子半径比La3+离子半径小,掺杂之后导致晶格畸变所引起的.     

应力导致的Fe2CrGe从反铁磁基态到铁磁半金属相变的计算研究

运用第一性原理计算研究了惠斯勒(Heusler)合金Fe₂CrGe的电子结构和磁性质,发现其基态是Fe离子处于低自旋态(S=0),而Cr离子处于高自旋态(S=1)的反铁磁金属相,反铁磁态能量比铁磁态能量约低0.103 eV.此外还发现,如果施加+1.7%和–1.7%的应变, Fe₂CrGe会从反铁磁有序变成铁磁有序,并出现半金属特性;当应变达到±5%时,出现了大约0.2 eV的半金属能隙.通过平均场理论估算了Fe₂CrGe的居里(Curie)温度,发现其对应的居里温度为393 K,远高于室温.这表明Fe₂CrGe是有潜力的自旋电子材料.     

氧化铜公度与非公度反铁磁相变的比热研究

利用微机控制的扫描比热量热系统对氧化铜样品从液氮温度到室温作了精细的比热测量,实验在２２７Ｋ和２２１Ｋ附近检测到两个明显的比热反常,其起始转变分别发生在ＴＮ１＝２３１Ｋ和ＴＮ２＝２１３Ｋ,这两个温度值和中子磁散射室验的结果非常吻合。结合人们对ＣｕＯ单晶的中子散射实验,本文指出这两个比热异常分别对应于氧化铜非公度和公度和反铁磁,从非公度到公度的反铁磁转变为一级相变,而顺磁态到非公度反铁磁相的转变为二级相交,Ｔ＝２３０Ｋ处比热跃变量为２．３Ｊ／（ｍｏｌ·Ｋ）。     

碲及其复合物对BSCCO,V_2O_3和Fe_2O_3的物理性能的影响

在高温超导陶瓷、钒氧化物和铁氧体中加入碲或碲与金属的复合氧化物,在恰当的工艺条件下,可以提高试样的硬度、致密度、钇系高Tc超导陶瓷的电流密度,降低铁氧体的居里点,导致铋系高温超导体出现高温亚稳相,在钒氧化物中引起介电常数峰以及使340K相变时电阻率曲线出现奇异。本文用固态相变的观点和元素碲的特性讨论了这些现象。     

Microstructure of polymeric electrolyte PEU-LiClO4 studied by positron spectroscopy

The positron lifetime spectra and ionic conductivity have been measured for polymeric electrolyte PEU-LiClO₄ as a function of temperature in the range of 120≈360 K and as a function of Li-salt concentration at room temperature. From the temperature dependence of positron annihilation parameters, the glass transition and subtransition are observed, and the glass transition temperatureT _g of pure PEU is determined to be 240 K. AboveT _g, the free volume hole size dramatically increases with temperature. The variations of positron annihilation parameters and ionic conductivities with respect to Li-salt concentration at room temperature indicate that the Li salt mainly diffuses into the amorphous region in PEU-LiClO₄. The increase of Li salt concentration brings about an increase in the number of carried ions, and a reduction of the fractional free volume. Supported by the National Natural Science Foundation of China Wang Shaojie: born in Mar. 1942, Professor     

Ba_3NaLaNb_（10）O_（30）晶体的物性研究

Ｂａ３ＮａＬａＮｂ１０Ｏ３０晶体具有四方钨青铜型结构，室温时的空间群和点群分别为Ｐ４／ｍｂｍ和４／ｍｍｍ，晶格参数ａ＝ｂ＝１２．４７１２Ａ，ｃ＝３．９３５０Ａ．介电、铁电等测量结果表明，该晶体在２８０Ｋ附近有一弥散性很强的铁电一顺电相变，其弥散性指数γ＝１．８．低温２００Ｋ时的ＸＲＤ显示它的低温相为四方相，点群为４ｍｍ点群．     

La0.67 Sr0.18 □0.15MnO3的结构、磁性和电输运性质

利用溶胶一凝胶法制备了La0．67Sr0．18□0．15MnO3样品,并对其结构、磁性质和电输运性质进行了研究。x射线衍射研究表明,样品具有单相菱方六面体结构。磁性研究表明,样品存在铁磁一顺磁转变,居里温度约280K。电输运性质研究表明,在低温范围,样品电阻率的来源主要是电子与电子之间的散射,样品在高温范围符合非绝热近似下的小极化子导电行为。     

Magnetic order close to superconductivity in the iron-based layered LaO1-xFxFeAs systems

Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.     

Pressure-induced crystallization of a spin liquid

Liquids are expected to crystallize at low temperature. The only exception is helium, which can remain liquid at 0 K, owing to quantum fluctuations. Similarly, the atomic magnetic moments (spins) in a magnet are expected to order at a temperature scale set by the Curie-Weiss temperature theta(CW) (ref. 3). Geometrically frustrated magnets represent an exception. In these systems, the pairwise spin interactions cannot be simultaneously minimized because of the lattice symmetry. This can stabilize a liquid-like state of short-range-ordered fluctuating moments well below theta(CW) (refs 5-7). Here we use neutron scattering to observe the spin liquid state in a geometrically frustrated system, Tb(2)Ti(2)O(7), under conditions of high pressure (approximately 9 GPa) and low temperature (approximately 1 K). This compound is a three-dimensional magnet with theta(CW) = -19 K, where the negative value indicates antiferromagnetic interactions. At ambient pressure Tb(2)Ti(2)O(7) remains in a spin liquid state down to at least 70 mK (ref. 8). But we find that, under high pressure, the spins start to order or 'crystallize' below 2.1 K, with antiferromagnetic order coexisting with liquid-like fluctuations. These results indicate that a spin liquid/solid mixture can be induced by pressure in geometrically frustrated systems.     

一维长程相互作用反铁磁伊辛系统的相变问题

用实空间重整化群方法研究了一维长程相互作用反铁系统。通过对递推公式的分析,发现此系统在有限温度下不发生相变,与铁磁情况形成鲜明对比。