新型的氧化物磁制冷工质与隧道磁电阻材料--2004年度国家自然科学二等奖获奖项目介绍

文章介绍了2004年度国家自然科学二等奖获奖成果[21].类钙钛矿型材料是一类物理内涵极其丰富的化合物,它是著名的高温超导材料、铁电材料、压电材料,又是庞磁电阻效应材料,目前又显示出具有大磁熵变效应与隧道磁电阻效应.文章作者系统地研究了锰钙钛矿磁性化合物的磁熵变与组成、微结构以及颗粒尺寸的关系,研究结果表明,磁性钙钛矿化合物具有显著的磁熵变,居里温度易调,并且化学稳定性佳,从而成为一类新型的磁制冷工质候选材料.此外,文章作者还研究了钙钛矿化合物纳米颗粒体系的磁电阻效应,发现除人们发现的居里温度附近的本征的庞磁电阻效应外,在很宽的低温区,存在与温度不甚敏感的隧道磁电阻效应.     

熵调控的Gd2Co17金属间化合物的结构与室温磁性能

熵调控材料因其独特的设计理念和优于传统合金的性能而受到广泛关注.本文将熵调控的设计理念引入金属间化合物中,设计并通过真空电弧熔炼的方法制备了一系列熵调控的Gd₂Co₁₇金属间化合物,期望通过熵调控的方法来稳定其结构,改善其磁性能.应用热力学理论预言熵调控的Gd₂Co₁₇系列金属间化合物具有稳定的单相,其单相性被X射线衍射实验所证实.通过组态熵调控原子尺寸因素,获得了菱方和六方两种晶体结构.熵调控改善了Gd₂Co₁₇系列金属间化合物的室温磁性能,过渡族金属位的熵调控使磁各向异性发生由基面到易轴的转变,稀土位的熵调控有助于提高其矫顽力,所有熵调控样品室温时的饱和磁矩均比二元Gd₂Co₁₇显著提升,可能是稀土或过渡族金属子晶格磁矩无序取向削弱了金属间化合物中稀土的4f电子与过渡族金属的3d电子磁矩之间的反平行交换作用所导致.磁价模型研究表明:熵调控设计导致Gd₂Co₁₇系列金...     

RCo_5的矫顽力

稀土元素(以R表示)和Co可构成一系列金属间化合物。所有富Co化合物的晶体结构都是由RCo_5结构派生而来的。RCo_5属于CaCu_5型六角结构。这种结构的磁晶各向异性很强,易磁化方向与c轴平行。在RCo_5结构的基础上,若其中三分之一的R原子都被一对Co原子有序地替代,就形成R_2Co_(17)结构。R_2Co_(17)的磁晶各向异性比RCo_5低,而且对于     

非晶合金的磁热效应及磁蓄冷性能

非晶合金的功能物性开发是突破非晶合金应用瓶颈的关键点之一.磁相变是非晶合金的一个重要特征.利用非晶合金的磁相变所带来的独特效应,可以将其应用于制冷领域.一方面非晶合金的磁热效应可以作为磁制冷材料应用于磁制冷机,另一方面非晶合金的比热突变可以作为磁蓄冷材料应用于低温制冷机.本文就非晶合金的磁热效应和磁蓄冷性能的原理、特征及其应用前景进行了详细介绍.     

高压处理对稀磁半导体Zn_(1-x)Mn_xO磁性的影响

用固相反应法制备出过渡金属Mn掺杂的ZnO系列样品,并对其进行了高压处理.高压处理后的样品,通过扫描电子显微镜和X射线衍射仪进行了微观结构的表征分析.通过磁性能的测量,初步研究了压力对Mn掺杂ZnO稀磁半导体磁性能的影响.结果表明:适当的高压处理可以有效增强材料的室温铁磁性,但当压力超过一定值后,压力会使材料的磁性能下降.     

磁耦合Mn2+-Mn2+离子对发光行为研究进展

过渡金属Mn2+掺杂的半导体/绝缘体作为发光材料在照明、显示等领域具有重要应用.Mn2+离子具有5个未成对的d电子,掺杂在发光材料中通常具有高自旋态,其容易与近邻的Mn2+离子发生交换或超交换作用即磁相互作用.此类相互作用可在分子尺度下对电子自旋产生很强的束缚能力,使得磁耦合Mn2+-Mn2+离子对的发光行为不同于孤立...     

间隙原子H,B,C对LaFe_(11.5)Al_(1.5)化合物磁性和磁热效应的影响

研究了金属化合物LaFe_(11.5)Al_(1.5)H_x(x=0,0.12,0.6,1.3),LaFe_(11.5)Al_(1.5)B_y(y=0.1,0.2,0.3)和LaFe_(11.5)Al_(1.5)C_z(z=0.1,0.2,0.3,0.4,0.5)的磁性、结构和磁热效应.金属化合物样品均形成了良好的NaZn_(13)型单相结构.基于固相-气相反应或者固相-固相反应引入间隙H,B,C原子后,磁性基态从反铁磁态变为铁磁态,饱和磁化强度(M_s)和居里温度(T_c)均呈升高趋势.值得注意的是:随着B和C含量的增加,化合物的相变性质由弱一级相变过渡至二级相变;而随着H含量的增加,相变性质却从二级相变过渡至弱一级相变.同时,化合物LaFe_(11.5)Al_(1.5)H_x,LaFe_(11.5)Al_(1.5)B_y和LaFe_(11.5)Al_(1.5)C_z均呈现出相当大的磁熵变.在0—5 T的外磁场作用下,LaFe_(11.5)Al_(1.5)H_(1.3),LaFe_(11.5)Al_(1.5)B_(0.1)和LaFe_(11.5)Al_(1.5)C_(0.2)的最大磁熵变分别达到12.3,9.6和10.8 J/kg·K.此外,在0—5 T的外磁场作用下,LaFe_(11.5)Al_(1.5)H_(0.6)的制冷能力达到259.2 J/kg,LaFe_(11.5)Al_(1.5)B_(0.1)的制冷能力达到116.4 J/kg,而LaFe_(11.5)Al_(1.5)C_(0.1)的制冷能力达到230.4 J/kg.     

Co掺杂对Mn_3Sn_(1-x)Co_xC_(1.1)化合物的磁性质、熵变以及磁卡效应的影响

利用固态反应法制备了Mn3Sn1-xCoxC1.1(x=0.05,0.1,0.2)系列化合物,研究了Co掺杂对其磁性质、相变、熵变的影响.随着Co掺杂量的增加,样品的居里温度由283 K先降到212 K(Mn3Sn0.9Co0.1C1.1)后又升到332 K(Mn3Sn0.2Co0.8C1.1),相变类型由一级相变逐渐转变为二级相变.增大Co的掺杂量,Mn3Sn1-xCoxC1.1化合物的熵变峰值逐渐减小,磁熵变温区由9 K展宽到300 K.当Co掺杂量为0.2时,相对制冷量达到最高,为103 J/kg(磁场强度为1.6 MA/m).由于室温附近良好的磁致冷效应,该类材料在磁制冷领域可能具有重要的应用前景.     

LaFe11.2Co0.7Si1.1合金在室温区的巨大磁熵变

在具有立方NaZn13型结构的稀土铁基化合物LaFe11.2Co0.7Si1.1中发现室温的巨大磁熵变，磁熵变的峰值位于居里温度Tc=274K处，5T外磁场下达到－20.3J/kg K约为相同的温区下金属Gd的2倍，LaFe11.2Co0.7Si1.1合金中强烈的磁弹性耦合，导致晶格在居里温度处出现巨大负膨胀，这是其巨大磁熵变的来源。     

钆硅锗确有巨磁热效应

室温磁制冷在近几年取得重要进展,尤其是造出了高性能的样机和发现了巨磁热材料,但近来因为巨磁熵变发生在一级相变点,在磁熵变的计算上国际诸研究小组出现了分歧,本文对此进行了详细的讨论,给出了统一的处理方法,结果表明钆硅锗的确具有巨磁热效应。     

The magnetic properties and magnetocaloric effects in binary R-T(R = Pr,Gd,Tb,Dy,Ho,Er,Tm;T = Ga,Ni,Co,Cu)intermetallic compounds

In this paper, we review the magnetic properties and magnetocaloric effects(MCE) of binary R–T(R = Pr, Gd, Tb,Dy, Ho, Er, Tm; T = Ga, Ni, Co, Cu) intermetallic compounds(including RGa series, RNi series, R_(12)Co_7 series, R_3 Co series and RCu_2series), which have been investigated in detail in the past several years. The R–T compounds are studied by means of magnetic measurements, heat capacity measurements, magnetoresistance measurements and neutron powder diffraction measurements. The R–T compounds show complex magnetic transitions and interesting magnetic properties.The types of magnetic transitions are investigated and confirmed in detail by multiple approaches. Especially, most of the R–T compounds undergo more than one magnetic transition, which has significant impact on the magnetocaloric effect of R–T compounds. The MCE of R–T compounds are calculated by different ways and the special shapes of MCE peaks for different compounds are investigated and discussed in detail. To improve the MCE performance of R–T compounds,atoms with large spin(S) and atoms with large total angular momentum(J) are introduced to substitute the related rare earth atoms. With the atom substitution, the maximum of magnetic entropy change(?SM), refrigerant temperature width(Twidth)or refrigerant capacity(RC) is enlarged for some R–T compounds. In the low temperature range, binary R–T(R = Pr, Gd,Tb, Dy, Ho, Er, Tm; T = Ga, Ni, Co, Cu) intermetallic compounds(including RGa series, RNi series,R_(12)Co_7 series, R_3 Co series and RCu_2series) show excellent performance of MCE, indicating the potential application for gas liquefaction in the future.     

Invar properties in the rare earth-3d transition metal alloys

The RCo₂ and R₂Fe₁₇ compounds (R = rare earth) exhibit Invar-like thermal expansion anomalies below their ordering temperatures. These spontaneous volume magnetostrictions are discussed by considering their magnetic properties. In RCo₂ compounds there is no intrisinc Co-moment is induced by the exchange and applied fields. The volume expansion anomaly is associated with the onset of the 3d magnetic moment. IN R₂Fe₁₇ compounds there is an intrisinc Fe- moment. Magnetic structures give evidence for positive and negative exchange interactions between Fe atoms which are strongly distance dependent. The thermal expansion anomaly is a result of this distance of the magnetic interactions.     

Review of magnetic properties and magnetocaloric effect in the intermetallic compounds of rare earth with low boiling point metals

The magnetocaloric effect(MCE) in many rare earth(RE) based intermetallic compounds has been extensively investigated during the last two decades, not only due to their potential applications for magnetic refrigeration but also for better understanding of the fundamental problems of the materials. This paper reviews our recent progress on studying the magnetic properties and MCE in some binary or ternary intermetallic compounds of RE with low boiling point metal(s)(Zn, Mg, and Cd). Some of them exhibit promising MCE properties, which make them attractive for low temperature magnetic refrigeration. Characteristics of the magnetic transition, origin of large MCE, as well as the potential application of these compounds are thoroughly discussed. Additionally, a brief review of the magnetic and magnetocaloric properties in the quaternary rare earth nickel boroncarbides RENi_2B_2 C superconductors is also presented.     

Neutron diffraction study of magnetic properties of TmCo2

The present neutron diffraction studies have confirmed that TmCo₂ represents an exception within the RCo₂ series. It was found that, in contrast with other heavy RCo₂ compounds, the Co sublattice in TmCo₂ does not order magnetically below the Curie temperature (T_c = 3.9 K). This is assumed to be due the fact that in TmCo₂ the intersublattice (f-d) molecular field is smaller than the critical field necessary to induce magnetic order within the Co sublattice, as is the case in other RCo₂ compounds. Furthermore, we show that the magnetic structure and the onset of long-range order in TmCo₂ depend sensitively on the sample preparation, which partly explains the differing results published earlier.     

Magnetic characteristics of R2(Fe, Co)14B systems (R = Y, Nd and Gd)

R₂(Fe, Co)₁₄B compounds (R = Y, Nd and Gd) were prepared in high purity. The magnetic behavior of R₂(Fe, Co)₁₄B compounds is reported over the temperature range 4 to 300 K. The effects of Fe substitution by Co on the saturation magnetization, Curie temperature and anisotropy are presented. The spin-reorientation temperature is lowered as Co replaces Fe. This also results in a reduced cone angle.

The R₂Fe_14−xCo_xB alloys crystallize in the tetragonal structure over the entire concentration range of 0 x 14. When Fe is substituted by Co, the Curie temperature increases significantly, the saturation magnetization increases to a maximum value around x = 2, and the anisotropy becomes planar for R = Y and Gd. The Nd₂(Fe, Co)₁₄B systems all exhibit uniaxial anisotropy at room temperature and Nd₂Co₁₄B is strongly uniaxial at 77 K. The Nd₂(Fe, Co)₁₄B systems are conical at 77 K.     

非晶Nd3Fe81B16合金的晶化及其对磁性和M?ssbauer谱的影响

本文报道利用单辊方法制备的非晶Nd₃Fe₈₁B₁₆合金的晶化及其对磁性和M?ssbauer谱的影响。发现在非晶Fe₈₁B₁₉合金中用3at％Nd取代B,使非晶Fe₈₁B₁₉合金的晶化温度提高88℃。在适当的退火条件下晶化后样品在室温下的磁性是:σ_s=189emu/g,σ_r/σ_s=0.7,_iH_c=2.15kOe,B_r≈12kG,_bH_c=2kOe,(BH)_max≈8MGOe。与目前广泛使用的六角铁氧体相比,_bH_c相近,但B_r和(BH)_max远比六角铁氧体高。这种材料仅含有少量的Nd,因此可能开发为一种新的廉价永磁材料。本文对少量Nd的添加对非晶FeB合金的晶化温度,磁性和M?ssbauer谱的影响进行了讨论。初步探讨了高矫顽力的来源,认为它的磁化和反磁化过程可以用畴壁钉扎理论解释。 关键词：     

FeMnP_(1-x)T_x(T=Si,Ga,Ge)系列化合物机械性能的第一性原理研究

以密度泛函理论为基础,使用投影缀加波方法、VASP程序包研究了FeMnP_(1-x)T_x(T=Si,Ga,Ge)化合物的力学性质,结果表明FeMnP_(1-x)Ga_x化合物的晶格参数、弹性常数和电子结构与FeMnP_(1-x)Ge_x化合物比较接近,同时该化合物在力学上稳定,是预期具有较大的磁熵变和高磁热效应的材料.依据Pugh判据,FeMnP_(0.67)T_(0.33)(T=Si,Ga,Ge)化合物具有良好的延展性,三者之中FeMnP_(0.67)Ga_(0.33)韧性最好,FeMnP_(0.67)Si_(0.33)韧性相对较差,说明Ga替代P可改善此类化合物的机械性能.最后从化合物体系电子总态密度随不同掺杂T原子的演化规律解释了自洽计算得到的弹性常数的变化规律.     

A model for domain and domain wall NMR signals in magnetic materials

Spin echo NMR signals in magnetic materials (simple metals, alloys or intermetallic compounds) generally result from mixed contributions of distinct magnetic regions of the sample, the magnetic domains and the domain walls. The amplitude of the signal is proportional to the so-called enhancement factor which in most of the cases greatly differs in these two regions, depending upon the wall mobility, the magnetic anisotropy, etc. The experimental access to domain and domain walls is possible, in principle, by a careful control of the RF power applied to the sample. In this paper a simple superposition model is proposed which includes both contributions to the NMR signal. We calculate the amplitude of the spin echo in magnetic powder samples and compare it with experimental situations where it has been possible to separate different contributions to the signal. This has been done in some RCo₂ magnetic rare-earth intermetallic compounds by analyzing the spectral line widths and the curve of the spin echo amplitude versus the applied RF field. Despite its simplicity, the present model allows the understanding of the main features of the NMR spectra and the dependence of the echo amplitude with the RF power in these compounds.     

Giant low-field magnetocaloric effect in EuTi(1-x)NbxO3(x = 0.05, 0.1, 0.15, and 0.2) compounds

The magnetic properties and magnetocaloric effect(MCE)of EuTi(1-x)NbxO3(x=0.05,0.1,0.15,and 0.2)compounds are investigated.Owing to electronic doping,parts of Ti ions are replaced by Nb ions,the lattice constant increases and a small number of Ti⁴⁺(3d^0)ions change into Ti³⁺(3d^1).It is the ferromagnetism state that is dominant in the derivative balance.The values of the maximum magnetic entropy change(-?SM^max)are 10.3 J/kg·K,9.6 J/kg·K,13.1 J/kg·K,and 11.9 J/kg·K for EuTi(1-x)NbxO3(x=0.05,0.1,0.15,and 0.2)compounds and the values of refrigeration capacity are 36,33,86,and 80 J/kg as magnetic field changes in a range of 0 T–1 T.The EuTi(1-x)NbxO3(x=0.05,0.1,0.15,and 0.2)compounds with giant reversible MCE are considered as a good candidate for magnetic refrigerant working at lowtemperature and low-field.