Cu掺杂对Fe/Mo双钙钛矿居里温度的影响

研究了双钙钛矿结构化合物Sr2FeMoO6中Fe位的Cu2+离子替代效应,样品中Cu2+离子替代Fe3+离子没有引起结构变化,但导致样品B位离子有序度降低.并且随Cu2+离子的掺入,室温下样品的磁化强度迅速下降.而居里温度TC随Cu2+的掺入有所上升,因此认为Cu2+离子的掺入破坏了Fe3+-Mo5+反铁磁耦合,抑制了样品的原有铁磁性,促使Mo5+向Mo6+的转变和Fe3+向Fe2+的转变引起Fe/Mo反位无序的增加,形成了Fe3+-Fe2+反铁磁耦合团簇.     

Al掺杂NdFeO_3体系的晶体结构精修与磁特性研究

利用固相反应法制备了Al掺杂的NdFe1-xAlxO3系列化合物。通过X射线粉末衍射和结构精修分析发现,随着Al掺杂量的增加,样品晶胞参数变小,晶胞内Fe—O键长、Fe—O—Fe键角也发生了很大程度的改变,并且在a、b、c方向上存在不同的变化规律。Al掺杂导致样品结构上的改变对其磁学性质具有重要影响。磁测量结果表明,随着Al掺杂量的增加,由dzyaloshinsky-moriya(D-M)相互作用导致的宏观磁性逐渐减弱;由Nd离子磁晶格与Fe离子磁晶格耦合作用导致的体系自旋重取向相变温度向低温方向移动,样品的饱和磁化强度随着Al掺入浓度的增加呈减小趋势。表明Fe位非磁性元素Al的掺入,使得样品内部由D-M相互作用所引起的倾角反铁磁结构受到了明显的调制。     

Ca掺杂双钙钛矿锰氧化物La2NiMnO6的物性研究

通过磁化强度随温度变化曲线(M-T)和等温磁化曲线(M-H)可判定随着Ca的掺杂,系统的铁磁性减弱、反铁磁性增强;两样品的磁滞回线和2 K下磁滞回线的放大图可以说明Ca的掺杂对系统反位无序度的影响不大,但会导致系统的反相边界增加,反铁磁耦合增强;磁化率倒数随温度变化曲线χ~(-1)-T向上背离C-W定律,掺Ca后向上背离更明显,说明Ca掺杂增强了长程铁磁有序;最后我们讨论了系统磁熵和电性变化,同时阐述了Ca的掺杂使系统经历了弱一级相变。     

La0.67-xNdxSr0.33MnO3体系的磁性和电性

通过M-T曲线,ESR曲线,红外光谱,拉曼光谱,P-T曲线和MR-T曲线的测量,研究了La0.67-xNdxSr0.33MnO3(x=0.10,0.20,0.30,0.40,0.50)样品的磁电性质.实验结果表明:随着Nd掺杂的增加,样品的磁结构从长程铁磁有序向自旋团簇玻璃态、反铁磁态转变;当x=0.30、0.40时发生相分离,且x=0.40样品的ZFC曲线随温度升高在160K左右突然跌落,出现少见的"肩膀";样品的电行为随着Nd掺杂的增加发生变化.样品的磁电行为和CMR效应来源于Nd掺杂引起的次晶格不同耦合和与自旋相关的界面隧穿效应.     

Co0.525Fe0.475MnP化合物的室温磁热效应和磁电阻效应

采用多步骤固态烧结方法合成了具有单一Co₂P相的Co_0.525Fe_0.475MnP化合物,其反铁磁有序温度在室温附近。在升温过程中,这种化合物经历两个连续的磁转变：在285 K发生反铁磁到铁磁的一级相变,在375 K发生由铁磁到顺磁的二级相变。在0~5 T的外磁场中,两个相变点温度对应的最大磁熵变分别为1.1 J/(kg·K)(303 K)和-2.0 J/(kg·K)(383 K)。外磁场为零时,随着温度的降低电阻率曲线在铁磁到反铁磁转变温度附近出现极小值,是铁磁有序与反铁磁有序的竞争所致。在35 K再次出现的电阻率极小值,可归因于由Fe替代Co引起的自旋无序所导致的金属-绝缘体转变。在5 T磁场中磁电阻率的最大值对应温度为200 K时的-2.5%,在反铁磁温度以上磁电阻率迅速减小。这表明,这种化合物的磁电阻效应源于外磁场对反铁磁有序的影响。     

相分离Gd_5Ge_4合金的低温磁化行为及温度依赖性研究

系统研究了Gd_5Ge_4合金的晶体结构和低温磁化行为.结果表明,Gd_5Ge_4具有相分离特征,低温下出现反铁磁(AFM)和铁磁(FM)共存现象.由于相分离的存在导致127K时发生奈尔反铁磁转变.在外磁场诱导下,在4.2K以下发生AFM-FM磁转变,并导致台阶式磁化现象发生,但仅发生在第一次外加磁场增加过程中,表现出磁不可逆性.随磁场升高,在10K以下体系存在类台阶式响应和不可逆的磁滞行为,并在5572kA/m下均达到饱和磁化.在温度50～60K温区,磁循环所出现的台阶式磁化转变则是完全可逆的,更高温区域则表现为部分铁磁直至室温下的顺磁特性.     

Fe掺杂多晶样品DyMn1-xFexO3的磁热效应和电性

采用传统的固相反应法制备DyMn_(1-x)Fe_xO_3多晶样品,通过测量了样品的磁化强度与温度的变化关系曲线(M-T)、磁化强度与温度的变化关系曲线(M-H)和电阻率与温度的变化曲线(ρ-T),对各组分下样品的磁性和电性进行了研究。研究结果表明,在低温区x=0和x=0.025样品表现为反铁磁态,而x=0.075样品在低温区ZFC曲线与FC曲线出现分叉,表现为铁磁反铁磁共存。分别在57,137和157 K以下观察到类Griffiths相,T_G温度以上样品都表现为顺磁特性。在外加磁场为7 T时磁熵变达到最大值,最大值分别为10,12,9 J/(kg·K)。最大制冷能力为320 J/kg(x=0.025)。综合磁熵变最大值及制冷能力数值来看,该材料可以作为磁制冷候选材料。通过对ρ-T曲线及ρ-T曲线的拟合曲线研究发现,系列样品均为半导体且加磁场后的电阻率高于零场下的电阻率,说明在低温处磁场有不利于电传导。系列样品在高温部分的导电方式均遵循小极化子的导电方式。     

Ba2Fe1+xMo1-xO6双钙钛矿的制备与磁卡效应

采用湿化学法制备了一系列Ba2Fe1+xMo1-xO6双钙钛矿材料,研究了Fe/Mo原子比对其磁性能尤其是磁卡效应的影响.实验结果表明,随着x增加,样品的磁化强度由x=0时的37.3A·m2/kg(3.49μB/f.u.)下降到x=0.3时30.4A·m2/kg(2.78μB/f.u.);当x=0时,样品的最大磁熵变值为1.54J/kg·K.随着x增加,样品的最大磁熵变值逐渐下降,磁熵变随温度变化曲线的峰形变宽.     

双钙钛矿氧化物Pr(2-x)Tbx CoMnO6(x=0.00,0.05,0.10)的磁性研究

利用传统高温固相反应法制备了钙钛矿钴氧化物Pr_(2-x)Tb_xCoMnO_6(x=0.00,0.05,0.10)系列多晶样品,通过样品的XRD谱线、磁化强度随温度、外加磁场的变化曲线(M-T、M-H)等对样品的晶体结构和磁性进行了研究.结果表明:系列样品Pr_(2-x)Tb_xCoMnO_6(x=0.00,0.05,0.10)呈现良好的单相性,空间点群为P2_1/n;由于Co~(2+)与Mn~(4+)之间的铁磁超交换相互作用和少量无序的Co~(3+)与Mn~(3+)之间的铁磁超交换相互作用,3个样品均出现两个铁磁转变点(T_(C1)和T_(C2)),观察到系列样品存在类Griffiths相,温度TT_G则为纯顺磁态;在T_(C2)-T_G之间属于顺磁-铁磁共存状态;在低温区(TT_N时),铁磁态与反铁磁态相互竞争,出现了自旋团簇玻璃行为;随着Tb的掺杂,T_(C1)、T_(C2)、T_N和T_G均下降。在测量温区内,随着温度的降低,3个样品均先后经历了纯顺磁态、顺磁-铁磁共存态、铁磁-反铁磁相互竞争的过程。     

过渡金属(Mn2+、Ni2+、Fe3+、Cu2+)掺杂ZnO基稀磁半导体的制备及性质

利用溶液腐蚀法制备了Mn2+、Ni2+、Fe3+、Cu2+离子掺杂的ZnO基稀磁半导体。XRD表明掺杂后的ZnO仍然保持单一的纤锌矿结构,没有任何杂质相产生。由紫外-可见光反射谱可知掺杂后吸收边发生了红移。掺杂前ZnO的带隙为3.20eV,对样品分别掺入Mn、Ni、Fe和Cu后的带隙分别为3.19eV、3.15eV、3.08eV和3.17eV。掺杂后样品的室温PL谱除了紫外发射峰外,对于Mn掺杂的样品还在蓝光区域出现了2个分别位于424nm和443nm的发射峰,Fe掺杂的样品出现了一个位于468nm的微弱发射峰,Cu掺杂的样品出现了位于469nm及535nm的很宽的发射峰。室温磁滞回线显示掺杂后样品有明显的铁磁性,掺入Mn、Ni、Fe和Cu样品的剩余磁化强度(Ms)分别为0.3902×10-3emu/cm3、0.454emu/cm3、0.372emu/cm3和0.962×10-3emu/cm3,矫顽力分别为47Oe、115.92Oe、99.33Oe和23Oe。经分析室温铁磁性来源于缺陷调制的Mn2+-Mn2+长程铁磁交换相互作用。     

Structural and Magnetic Properties of Pr0.6Sr0.4Mn1−x Fe x O3 (0≤x≤0.3) Manganites Oxide Prepared by the Ball Milling Method

We present the structural and magnetic properties of Pr_0.6Sr_0.4Mn_1?x Fe _x O₃ (x=0, 0.1, 0.2, and 0.3) compounds. Samples have been prepared by the ball milling method. Rietveld refinements of the X-ray powder diffraction data show that all our synthesized samples are single phase and crystallize in the orthorhombic symmetry with the Pnma space group. The unit cell volume increases with increasing the Fe content. The infrared spectrum shows two active bands, which can be ascribed to the internal stretching and bending modes. The magnetization measurements versus temperature showed that Fe doping leads to a weakening of the ferromagnetic ordering at low temperature, the Curie temperature T _C decreases from 300 K for x=0.0 to 88 K for x=0.2. The magnetization versus applied magnetic measurements at low temperature lead to conclude that the substitution of Mn³⁺ ions by Fe³⁺ ions triggers antiferromagnetic interactions between the Fe³⁺ and Mn⁴⁺ spins, and also the magnetization versus applied magnetic measurements at room temperature shows a small hysteresis loop and a low coercive field, which indicates that the samples are superparamagnetic.     

Composition dependence of magnetocaloric effect in Pr0.6Ca0.4Mn(1-x)Cr(x)O3 (x = 0.02-0.08)

We report the effect of varying Cr content on magnetic and magnetocaloric properties of Pr0.6Ca0.4Mn(1-x)Cr(x)O3 samples (x = 0, 0.02, 0.04, 0.06 and 0.08). While the parent compound (x = 0) is a charge ordered and antiferromagnetic insulator, Cr doped compounds are ferromagnetic metals with nearly same Curie temperature (T(c) approximately 140 K). We find unusual field-induced meta-magnetic transition above T(c) in x = 0.02 and 0.04 which is absent in x = 0.06 and 0.08. It is suggested that the paramagnetic phase in these compounds is inhomogeneous with coexistence of nano-size ferromagnetic clusters and short range charge ordered clusters. Field induced growth of ferromagnetic nano-clusters and destruction of short-range charge ordering leads to the observed metamagnetic transition, which results in large magnetic entropy change of -deltaS(M) = 5.043, 6, 5.509 and 4.375 J/kg K under deltaH = 5 T, for x = 0.02, 0.04, 0.06 and 0.08, respectively. In addition, large relative cooling power (RCP) found in these materials (327.384, 286.36, 272.22 and 279.936 J/kg) makes it interesting for practical applications. Our study suggests that creation of ferromagnetic nano-clusters in the paramagnetic phase by Mn-site doping in charge ordered compounds provides an alternative approach to achieve high AS(M) and RCP values.     

Nb、K共掺对BiFeO3纳米晶体结构和磁性的影响

采用溶胶-凝胶法制备Bi_(1-x)K_xFe_(1-x)Nb_xO_3(0≤x≤0.05)纳米晶体,并研究Nb、K共掺杂对BiFeO_3样品晶格结构和磁学性质的影响。根据XRD图谱和Rietveld精修可知,所有样品保持R3c相,晶格常数a、c,晶胞体积V和Fe-O-Fe发生微弱的变化。XPS测量显示,少量的Nb、K共掺杂不引起样品中Fe~(3+)和Fe~(2+)比例的变化。磁测量表明,所有掺杂样品的磁性都得到了增强。当掺杂量x=0.01时,剩余磁化强度Mr达到最大值,Mr=0.1978emu/g,相比于纯BiFeO_3增大了18倍。低掺杂时剩余磁化强度Mr增大,可能是由于掺杂样品中存在束缚磁极化子引起的。     

掺杂Mg对纳米CoFe2O4/SiO2复合薄膜结构和磁性的影响

采用溶胶-凝胶旋涂法制备了纳米Co_1-xMg _xFe₂O₄/SiO₂(x = 0, 0.2, 0.4, 0.6, 0.8) 复合薄膜。利用XRD、SEM、原子力显微镜、振动样品磁强计对薄膜的结构、形貌和磁性进行了分析, 研究了Mg²⁺含量对样品结构和磁性的影响。结果表明, 样品中Co_1-xMg _xFe₂O₄具有尖晶石结构, 晶粒尺寸在38~46 nm之间。随着Mg²⁺含量的增加, Co_1-xMg _xFe₂O₄的晶格常数减小, 样品的饱和磁化强度减小, 矫顽力先增大后减小。样品Co_0.4Mg_0.6Fe₂O₄/SiO₂垂直和平行膜面的矫顽力分别为350.7 kA·m^-1和279.4 kA·m^-1, 剩磁比分别为67.2%和53.9%, Co_1-xMg _xFe₂O₄/SiO₂复合薄膜具有较明显的垂直磁各向异性。     

Iron doped hexagonal ErMnO3: structural, magnetic, and dielectric properties

The single phase ErFe(x)Mn1-xO3 (0 < or = x < or = 0.15) compounds were synthesized by the solid-state reaction method. The doping effects on the crystal structural, magnetic, thermal, and dielectric properties were systematically investigated. The XRD patterns show all samples crystallize in the hexagonal structure with P6(3)cm space group. The lattice parameters a and c first decrease with doping, which is followed by a subsequent increase at higher doping levels. Although both the Fe3+ and Mn3+ ions remain stable in high spin trivalent states in all samples, the magnetization is weakened with increasing Fe contents. The heat capacity data shows the antiferromagnetic transition slightly shifts from 77 K for ErMnO3 to 80 K for ErFe015Mn0.85O3, which can not be observed in the magnetic susceptibility data. The real part of complex impedance of these samples rises as the doping level increases, indicating the enhancement of insulativity of doped samples.     

Possible weak ferromagnetism in pure and M (Mn, Cu, Co, Fe and Tb) doped NiGa2O4 nanoparticles

We report on the structural and magnetic properties of nanoparticles of NiGa2O4 and 5 at.% M doped (M = Mn2+, Cu2+, Co2+, Fe3+ and Tb3+) at Ga site of NiGa2O4, synthesized by gel-combustion method. The particle size, as investigated by X-ray diffraction and transmission electron microscopy, could be fine tuned by a controlled annealing process. Weak ferromagnetism becomes significant, when the particles are in the nano regime (5-7 nm). The magnetization becomes insignificant at larger particle size ( 150 nm). Cu2+ and Tb3+ doped NiGa2O4 nanoparticles showed relatively large room temperature ferromagnetism compared to other doped (Fe, Mn and Co) and undoped NiGa2O4 samples. The weak ferromagnetism observed in the nanoparticles of NiGa2O4, which is antiferromagnetic in the bulk, is due to the surface disordered states with uncompensated spins.     

Ca掺杂EuMnO3体系的磁性和磁卡效应研究

采用传统的高温固相反应法制备了钙钛矿氧化物Eu1-xCaxMnO3(x=0,0.1,0.2,0.3,0.4,0.5)多晶体系样品。通过样品的XRD谱线、磁化强度随温度、外加磁场的变化曲线(M-T、M-H)等数据研究了Ca不同掺杂量对钙钛矿氧化物EuMnO3系列多晶样品的居里温度、磁熵变、磁相变等的影响。结果表明,Eu1-xCaxMnO3(x=0,0.1,0.2,0.3,0.4,0.5)系列陶瓷样品呈现良好的单相性,空间点群为Pnma。低浓度掺杂的样品存在一个TC,高浓度掺杂的样品出现两个磁转变点,且由于双交换相互作用,随着Ca的掺杂,TC逐渐增大,当掺杂量达到0.4时,低温下的TC却明显减小。根据-ΔSM和RCP值可知,Eu0.7Ca0.3MnO3是该组样品中比较适合做磁制冷的材料。通过对Loop曲线的分析可知该组样品经历了由一级相变向二级相变转变的过程。     

Magnetic Properties of GdCoAsO

The magnetic properties of GdCoAsO have been investigated as a function of temperature and magnetic field. Complicated magnetism consists of antiferromagnetic, ferromagnetic, and paramagnetic has been observed. Most interestingly, magnetization measurements performed under low field, exhibit negative magnetization. Ferrimagnetic transition occurs at 137 K, which results from antiferromagnetic exchange interaction between Gd and Co magnetic moments, and then followed by transition to antiferromagnetic state. There exist three antiferromagnetic transitions at 12 K, 43 K, and 75 K, respectively. Antiferromagnetic transition at 75 K is independent of the magnetic field and at 43 K decreases with increasing field. Antiferromagnetic transition at 12 K corresponds to the antiferromagnetic ordering of Gd. A metamagnetic transition between two antiferromagnetic orders is observed just below 75 K. With increasing temperature, the metamagnetic transition field decreases. The magnetic phase diagram has been established.     

Ti^4+掺杂M型六角铁氧体BaFe12–xTixO19陶瓷的磁学和介电特性

六角铁氧体由于其具备高温下的低场磁电耦合特性,有望应用于新型多态存储器及磁电传感器等微电子器件。利用Ti^4+离子对M型六角铁氧体BaFe12O19进行B位掺杂,不仅可以调控材料的磁结构和磁学特性,同时,Ti离子在六角铁氧体B位的不等价掺杂还可以产生相关缺陷、载流子和变价Fe离子进而改变其电学特性。本研究采用固相烧结法制备了M型六角铁氧体BaFe12–xTixO19(x=0,0.5,1,1.5)陶瓷,并对其进行了性能表征和测试,研究了B位Ti^4+掺杂对材料结构、磁学和介电特性的影响。研究结果表明,BaFe12–xTixO19呈现上、下自旋反平行的亚铁磁序。当Ti^4+离子掺杂量较低时,更易取代位于上自旋格子的Fe3+离子,其磁化强度随Ti掺杂量的增加而减小;随着Ti4+离子掺杂量的进一步增加,位于下自旋格子的Fe^3+离子也会逐渐被取代,此时,饱和磁化强度随掺杂量的增加而增加。此外,Ti^4+离子的引入也会使晶粒内部呈现半导性,在晶粒/晶界处产生Maxwell-Wagner界面极化,故而M型六角铁氧体BaFe12–xTixO19陶瓷会出现明显的低频介电增强并伴随着Maxwell-Wagner介电弛豫。