Gd_5Ge_4中磁场温度诱导的磁结构相变研究

最近的研究表明,在低温低场时,Gd_5Ge_4的一级磁结构相变被阻止而进入一种磁玻璃态,即在反铁磁母体中随机分布着铁磁团簇.结合这一最新研究结果,通过对Gd_5Ge_4样品不同低温的磁场诱导的磁化强度进行两次循环测量,研究发现在不同条件下Gd_5Ge_4的磁结构相变存在可逆性与不可逆性,并结合磁玻璃态进行了分析讨论.验证了Gd_5Ge_4的等温磁化行为和结构变化的一致性,这为证明在磁和晶格之间存在耦合作用提供了直接的实验证据,揭示了一级磁结构相变对巨磁热效应的产生起了重要作用.     

RPdAl(R=Gd-Er)的磁性和磁热效应

低温退火的LTM-RPdAl和高温退火的HTM-RPdAl分别具有正交TiNiSi型和六角ZrNiAl型结构.综述了磁场和温度变化对LTM-RPdAl和HTM-RPdAl化合物磁性和磁热效应的影响.实验结果表明,LTM-RPdAl化合物均是反铁磁性的,当R=Gd,Tb,Dy,Ho,Er时,奈尔温度TN分别为31,45,21,10和10 K.对于HTM-RPdA1化合物,当R=Tb,Ho,Er时是反铁磁性的,TN分别为43,12和5K,而当R=Gd,Dy时是铁磁性的,其居里温度分别为Tc=49,25 K.反铁磁RPdAl化合物均呈现磁场诱导的反铁磁-铁磁变磁转变.对HTM-RPdAl(R=Ho,Er)化合物,由于弱的反铁磁耦合,在低磁场下呈现出高的饱和磁化强度,从而产生大的磁热效应,在0～5 T磁场变化下的最大磁熵变值和制冷能力分别高达20.6,24.3 J/kg·K和386,299 J/kg,是优异的低温磁制冷材料.     

几种磁制冷材料的室温磁热效应

用高频悬浮炉(冷坩埚)熔炼了Gd5Si2Ge2、Gd4Sb1.5Bi1.5、Gd3Al2、Mn5Ge3等金属化合物.在磁场H=1.3T下,用XHY磁热效应测量仪直接测量它们的绝热温变△Tad,得到了这几种材料的居里点,并和金属Gd进行了比较.     

Gd5SixSn4-x合金的结构与磁热性能

利用XRD和振动样品磁强计研究了Gd5SixSn4-x(x=2.4、2.6、2.8)合金的结构与磁热性能.结果表明,在x=2.8时合金具有Gd5Si4形正交结构,x=2.4和2.6时则为Gd5Si4和Gd5Si2Gi2型两种结构,此外均存在少量其它杂相.随着Si含量的增大,居里温度分别为276、290.5和301.5K,逐渐升高,1.8T外加磁场变化时的磁熵变分别为1.88、2.26和1.69J/(kg·K),x=2.6时最大.低温XRD分析表明合金发生磁相变的同时没有结构相变发生.     

具有大磁熵变的Gd基四元合金

研究了以纯度为99.94%的Gd为原料配制的Gd5Si1.9Ge2Sn0.1四元合金的磁热性能.磁性测量表明,该四元合金在260～300K温度区间内,出现了两个居里温度;合金的最大磁熵变为-15.2J·kg-1·K-1(0～5T),并且具有较宽熵变峰值平台.通过合金的粉末衍射曲线对比分析表明,Gd5Si1.9Ge2Sn0.1合金和Gd5Si2Ge2合金具有相同相结构.     

Gd2Co2Al合金的磁相变及磁热效应

电弧炉熔炼的Gd2Co2Al合金在铸态条件下即为W2Co2B型单相正交结构。变温磁化曲线表明,当外加磁场为0.01和0.1T时,可以在40、77和215K附近观察到磁相变;而外加磁场增加到1T以上时,40和215K温度处的磁相变消失。在排除第二相相变的前提下,推测215和77K处的相变对应Gd2Co2Al合金中Co和Gd次晶格的磁有序相变,而40K处的相变可能是由于自旋重取向产生。在0～5T磁场变化下,Gd2Co2Al合金在77K附近的最大磁熵变(-ΔSMmax)为10.7J/kgK,相对制冷量的值为5.4×102J/kg,表明该合金适合作为工作在液氮温区附近的磁致冷工质。     

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。     

C对Gd5Si4磁致冷材料结构和性能的影响

以商业蒸馏Gd为原料,采用非自耗电弧炉在氩气保护下熔炼了Gd5Si4Cx(x=0、0.24、0.35、0.5)系列合金,研究了C对Gd5Si4磁致冷合金组织结构与磁热性能的影响.粉末XRD结果表明,Gd5Si4Cx系列合金主相均为正交的Gd5Si4型结构,此外C(x=0.24、0.35、0.5)加入后合金中出现了少量的GdSi相.利用振动样品磁强计测量的合金的磁性能的结果表明,Gd5Si4Cx(x=0、0.24、0.35、0.5)系列合金的居里温度Tc在344～297K连续可调.在1.5T外加磁场变化时居里温度附近的最大磁熵变分别是2.79、2.47、2.03、1.65J/(kg·K).     

Ni0.75Fe0.25MnGe1.01合金的相变和磁热效应研究

通过研究Ni_(0.75)Fe_(0.25)MnGe_(1.01)合金的结构和磁性,发现Ni2In型结构的奥氏体相可以在室温下稳定存在,并且该合金的一阶马氏体相变与二阶铁磁相变温度相邻,从而扩宽了该合金的工作温度区域,使其在磁制冷方面有重要应用。同时在研究中发现,当磁场增大时,合金的相变温度降低。最后,在马氏体相变温度和居里温度附近各得到了正、负磁熵变。     

快淬低纯Gd5Si2Ge2合金的结构和磁熵变

采用纯度为99%的低纯稀土金属Gd经电弧炉熔炼获得Gd5Si2Ge2 母合金,然后通过快淬研究快淬工艺对低纯Gd5Si2Ge2 合金结构与磁熵变的影响。粉末衍射结构分析表明,铸态Gd5Si2Ge2 合金由Gd5Si2Ge2 相、Gd5 (Si、Ge)3 相和Gd(Si、Ge)相所组成;而甩带速度为25、40m/s的合金均为Gd5Si4 型正交相。用最小二乘法计算得两种不同快淬速度合金的晶格常数没有明显差异。甩带速度为40m/s 的Gd5Si2Ge2 合金的居里温度(Tc=305K)在室温区间,等温磁熵变为7.25J/kg·K(0~5T)。     

Diluted Magnetic Semiconductors in the Low Carrier Density Regime

This paper, based on a presentation at the Spintronics 2001 conference, provides a review of our studies on II–VI and III–V Mn-doped Diluted Magnetic Semiconductors. We use simple models appropriate for the low carrier density (insulating) regime, although we believe that some of the unusual features of the magnetization curves should qualitatively be present at larger dopings (metallic regime) as well. Positional disorder of the magnetic impurities inside the host semiconductor is shown to have observable consequences for the shape of the magnetization curve. Below the critical temperature the magnetization is spatially inhomogeneous, leading to very unusual temperature dependence of the average magnetization as well as specific heat. Disorder is also found to enhance the ferromagnetic transition temperature. Unusual spin and charge transport is implied.     

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。     

Effect of magnetic field on a spin-peierls cuprate, CuGeO3

We measured the static magnetization of polycrystalline CuGeO₃, which exhibits the spineierls transition around 14.0 K in the absence of the magnetic field. The field dependence of the magnetization below 14.0 K shows a characteristic nonlinear process associated with a phase transition, and phase boundaries between the dimerized phases and other phases were obtained. The magnetic phase diagram qualitatively agrees with that of an organic spin-Peierls material. The phase transition is of first order below 10.0 K, and of second order above 10.0 K. Above 10.0 K, the critical field decreases from 13 to 0 T with increasing temperature. On the other hand, it is nearly independent of temperature below 10.0 K.     

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.     

Monte Carlo Study of the Interface Effects on the Magnetic Properties of the Magnetic/Nonmagnetic (M/NM) Layers

A Monte Carlo study of the thermal variation of multilayers magnetization is carried out. We find a number of characteristics behaviors of the interface magnetization with its roughness. The spontaneous magnetization and magnetic susceptibility are calculated. It is found that multilayers magnetization decreases and phase transition temperature from ferromagnetic to paramagnetic varies with increasing interface thickness and concentration of nonmagnetic atoms in interface. Moreover, the susceptibility exhibits peaks at the transition temperature and it is interface morphology dependent. The dependence to the spin-exchange coupling is reported.     

Tunable Ferromagnetism and Thermally Induced Spin Flip in Vanadium-Doped Tungsten Diselenide Monolayers at Room Temperature

The outstanding optoelectronic and valleytronic properties of transition metal dichalcogenides (TMDs) have triggered intense research efforts by the scientific community. An alternative to induce long-range ferromagnetism (FM) in TMDs is by introducing magnetic dopants to form a dilute magnetic semiconductor. Enhancing ferromagnetism in these semiconductors not only represents a key step toward modern TMD-based spintronics, but also enables exploration of new and exciting dimensionality-driven magnetic phenomena. To this end, tunable ferromagnetism at room temperature and a thermally induced spin flip (TISF) in monolayers of V-doped WSe₂ are shown. As vanadium concentration increases, the saturation magnetization increases, which is optimal at ≈4 at% vanadium; the highest doping level ever achieved for V-doped WSe₂ monolayers. The TISF occurs at ≈175 K and becomes more pronounced upon increasing the temperature toward room temperature. The TISF can be manipulated by changing the vanadium concentration. The TISF is attributed to the magnetic-field- and temperature-dependent flipping of the nearest W-site magnetic moments that are antiferromagnetically coupled to the V magnetic moments in the ground state. This is fully supported by a recent spin-polarized density functional theory study. The findings pave the way for the development of novel spintronic and valleytronic nanodevices and stimulate further research.     

La_(1-x)Pr_xFe_(11.206)Al_(1.794)(x=0、0.1、0.2、0.3)化合物的磁热效应研究

对La1-xPrx(Fe0.862Al0.138)13(x=0、0.1、0.2、0.3)化合物的磁性能进行了研究。该系列化合物在1223K下退火15天后,室温下表现为NaZn13型结构。升温过程中,该系列化合物经历从铁磁态到反铁磁态的一级磁性转变。随着Pr含量的逐渐增多,磁性转变温度逐渐升高,铁磁态下化合物的磁化强度增大。在2T外磁场下,La0.8Pr0.2Fe11.206Al1.794化合物升温过程中的最大磁熵变为-4.21J/kg·K,降温过程中的最大磁熵变为-4.39J/kg·K。     

Magnetization of EuS–PbS Multilayers with Antiferromagnetic Interlayer Coupling

SQUID magnetometry is applied to study the temperature and magnetic field dependence of magnetization M(T, H) of semiconductor EuS–PbS ferromagnetic multilayers grown on insulating KCl(100) and conducting n-PbS(100) monocrystalline substrates. For low external magnetic fields (of the order of 10 Oe) and PbS spacer layers thinner than about 2 nm, we observe in EuS–PbS–EuS trilayers the strongly nonmonotonic temperature dependence of magnetization with almost zero total magnetic moment below the Curie temperature. The application of the magnetic field of the order of 100 Oe restores the regular monotonic increase of magnetization with decreasing temperature. To explain this M(T,H) dependence we present a model that considers the competition of three (temperature dependent) contributions to the total magnetic energy of the trilayer: the antiferromagnetic interlayer interaction energy, the Zeeman energy, and the energy of in-plane magnetocrystalline anisotropy.