FORMATION AND MAGNETIC PROPERTIES OF NEW COMPOUNDS R3Fe29-xCrx(R=Y,Ce,Nd,Sm,Gd,Tb,and Dy)

A systematic investigation of structure and magnetic properties of the new R₃Fe_29-xCr_x compounds(R=Y,Ce,Nd,Sm,Gd, Tb,and Dy)has been performed. The Curie temperature of R₃Fe_29-xCr_x increased with increasing atomic number fromR=Ce to Gd and de creased from Gd to Dy. The saturation magnetization of R₃Fe_29-xCr_x at 4.2 K decreased gradually with increasing atomic number from R=Y to Dy,except for Ce. The spin reorientations of the easy magnetization d irection were observed at around 230 K for Nd₃Fe_24.5Cr_4.5 and 180 K for Tb₃Fe_28.0Cr_1.0,and the magnetohistory effects were obser ved for Nd₃Fe_24.5Cr_4.5 and Sm₃Fe_24.0Cr_5.0 in a low field of about 0.04 T. First order magnetization process occurs in magnetic field of around 2.3 T at room temperature for Tb₃Fe_28.0Cr_1.0. The saturation magnetization of Y₃Fe_27.2Cr_1.8 at 4.2 K is 52.2μ_B/f.u., which corresponds to an average magnetic moment of 1.92μ_B per each Fe atom.     

Mössbauer studies of CoxFe1−xCr2S4

The mixed spinel systems Co_xFe_1−xCr₂S₄ has been studied using the Mössbauer effect. Spectra have been collected over the temperature range 83–300 K. The isomer shifts indicate that the charge states of Fe ions are ferrous in character throughout the series. Absence of quadrupole splitting above the magnetic ordering temperature T_c suggests that iron ions occupy only tetrahedral sites. It is notable that as the temperature decreases below T_c, both quadrupole shift and asymmetrical line-broadening appear and increase with decreasing temperature, suggesting the presence of electric field gradient and accompanying relaxation effects.     

High-resolution electron microscopy study of Ni81Fe19 film with Co33Cr67 buffer layer

The anisotropic magnetoresistance (AMR) in permalloy Ni₈₁Fe₁₉ film deposited on a 1.2 nm Co₃₃Cr₆₇ buffer layer was significantly enhanced. The high-resolution electron microscopy was used to study the microstructure of Ni₈₁Fe₁₉ film with and without Co₃₃Cr₆₇ buffer layer. It was found that Co₃₃Cr₆₇ buffer layer can induce good (1 1 1) texture, while without Co₃₃Cr₆₇ buffer layer, Ni₈₁Fe₁₉ film show randomly oriented grain structure. The Δρ/ρ enhancement is attributed to the decrease in the resistivity ρ of the Ni₈₁Fe₁₉ film due to the formation of the large (1 1 1) textured grains in Ni₈₁Fe₁₉ film with Co₃₃Cr₆₇ buffer layer. However, the surface roughness of substrate may limit the (1 1 1) textured grain size and induce additional grain boundaries in Ni₈₁Fe₁₉ film with Co₃₃Cr₆₇ buffer layer, limit the enhancement of the AMR effect.     

Crystal and magnetic structure of the sulfur spinels Cu0.45Co0.55Cr2S4 - xSex

The magnetic and crystal structures of the metallic sulfospinels Cu_0.45Co_0.55Cr₂S_{4 - x}Se_x have been investigated for x = 0, 0.42 and 1.0 by neutron powder diffraction techniques. The data have been analyzed by the Rietveld method. All three compositions show ferrimagnetism at low temperatures with a chromium moment of (2.7±0.1)μ_B and a cobalt moment of (2.8±0.1)μ_B. The Curie temperature varies from 293 to 253 K.     

MAGNETIC PROPERTIES OF Pr2Co17-x Mx ( M＝Ga,Si) COMPOUNDS

The influence of Ga or Si substitution for Co on the structural and magnetic properties of Pr₂Co₁₇ compounds is investigated. All samples studied here are single phase and have the rhombohedral Th₂Zn₁₇-type structure. The unit-cell volume is found to increase linearly by the substitution of Ga for Co,but reduce by the substitution of Si for Co in Pr₂Co₁₇ compounds. In Pr₂Co_17-x M_x, the Curie temperature decreases monoto nically with increasing at an approximate rate of 153K per Ga atom and 175K per Si atom. The saturation magnetic moment of Pr₂Co_17-x M_x ( M=Ga,Si) decreases with increasing x. The rates of the decrease are larger than that expected as a simple dilution. For Pr₂Co_17-xSi_x ,the spin reorientation transition is observed above room temperature. The spin-reorientation temperature Tsrfirst decreases with increasing Si content and then increases at higher x values (x>2). The spin reorientat ion behavior is interpreted by the competition between the Pr and Co sublattice anisotropies. The easy magnetization direction in Pr₂Co_17-xGa_x compounds is perpendicular to the c-axis, and no spin reorientation transition is observed.     

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.     

STRUCTURE AND MAGNETIC PROPERTIES OF NITRIDES R3Fe29-xCrxN4

A systematic investigation of nitrides R₃Fe_29-xCr_xN₄(R=Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed. The nitrogen concentration in the nitride R₃Fe_29-xCr_xN_y was determined to be y=4. Nitrogenation leads to a relative volume expansion of about 5.3%. The lattice constants and unit cell volume decrease with in creasing rare earth atomic number from Nd to Dy, reflecting the lanthanide contraction. In average, the increase of Curie temperature upon nitrogenation is about 200 K, compared with its parent compound. The nitrogenation also results in a remarkable improvement in the saturation magnetization and anisotropy fields for R₃Fe_29-xCr_xN₄ at 4.2 K and room temperature, comp ared with their parent compounds. A spin reorientation of Nd₃Fe_24.5 Cr_4.5N₄ occurs at around 368 K, which is 138 K higher than that of Nd₃Fe_24.5Cr_4.5.Magnetohistory effects of R₃Fe_29-xCr_xN₄(R=Nd and Sm) are observed in a low field of 0.04 T. First order magneti zation process occurs in Sm₃Fe_24.0Cr_5.0N₄ in magnetic fields of around 3.0 T at 4.2 K. After nitrogenation the easy magnetization direction of Sm₃Fe_24.0Cr_5.0 is changed from the easy cone structure to the uniaxial. The excellent intrinsic magnetic properties of Sm₃Fe_24.0Cr_5.0N₄ make this compound a hopeful candidate for new high performance permanent magnets.     

The magnetic structure of chalcogenid spinel CoRh2S4

Neutron diffraction study on CoRh₂S₄, having the unexpectedly high Néel temperature T_N, shows the layered antiferromagnetic structure. The magnetic moment is (3.0±0.8)μ_B/Co atom. The T_N is confirmed to be 418 K from the temperature dependence of the intensity of the (200) reflection.     

A low temperature route to nanocrystalline Co9S8

Nanocrystalline Co₉S₈ has been successfully prepared by a hydrothermal-reduction method at 120°C. The reducing agents were produced from disproportionation of white phosphorus in alkaline medium. X-ray diffraction pattern reveals that the product is single phase of Co₉S₈. Transmission electron microscopy shows that the average particle size of Co₉S₈ is about 20 nm. The possible mechanism for the formation of Co₉S₈ and the influence of experimental conditions are discussed.     

Specific heat measurements of (Ho, Y)Co2 and (Dy, Y)Co2

We report on concentration- and temperature-dependent specific heat measurements of (Dy_xY_1−x)Co₂ in the temperature range from 1.5 to 60 K in comparison with recent results of (Ho_xY_1−x)Co₂. The magnetic contribution to the specific heat, C_m, is analysed with regard to the appearance of spin-fluctuations and in terms of spin-glass phenomena. The loss of magnetic entropy in the dilute RE-concentration range indicates presumably an instability of the RE-moments.     

Electronic structures of vacancies in Co_3Sn_2S_2

Co_3Sn_2S_2 has attracted a lot of attention for its multiple novel physical properties, including topological nontrivial surface states, anomalous Hall effect, and anomalous Nernst effect. Vacancies, which play important roles in functional materials, have attracted increasing research attention. In this paper, by using density functional theory calculations, we first obtain band structures and magnetic moments of Co_3Sn_2S_2 with exchange–correlation functionals at different levels. It is found that the generalized gradient approximation gives the positions of Weyl points consistent with experiments in bulk Co_3Sn_2S_2. We then investigate the electronic structures of defects on surfaces with S and Sn terminations which have been observed in experiments. The results show that the single sulfur vacancy on the S-terminated surface introduces localized bond states inside the bandgap near the Fermi level. For di-and tri-sulfur vacancies, the localized defect states hybridize with neighboring ones, forming bonding states as well as anti-bonding states. The Sn vacancy on the Sn-terminated surface also introduces localized bond states, which are merged with the valence bands. These results provide a reference for future experimental investigations of vacancies in Co_3Sn_2S_2.     

Neutron diffraction study of magnetic ordering in ErMn2Si2, ErMn2Ge2 and ErFe2Si2

Neutron diffraction study of polycrystalline compounds ErMn₂Si₂, ErMn₂Ge₂ and ErFe₂Si₂ was performed in the temperature range between 1.8 and 293 K. All compounds have tetragonal, ThCr₂Si₂-type crystal structure. The antiferromagnetic collinear structure of ErMn₂Si₂ and ErMn₂Ge₂ at both RT and LNT, consists of a sequence + - + - of ferromagnetic layers of Mn atoms. The magnetic moment of an Mn atom (≈2μ_B) is parallel to the c-axist. At low temperatures (LHT and lower), the ferromagnetic ordering within the Er sublattice is observed. The magnetic moment (μ_Er ≈ 9μ_B) is perpendicular to the c-axis. From the temperature dependence of the intensities of the magnetic peaks, the following values for the Curie temperatures were obtained: (10±5) K for ErMn₂Si₂ and (8.5±3) K for ErMn₂Ge₂. For ErFe₂Si₂ a collinear antiferromagnetic structure of the + - - + type was found, the magnetic unit cell consisting of the chemical one, doubled along the c-axis.     

含新结构单元[Co3(SS)3(PR3)3]的钴化合物的磁性研究

通过含新结构单元钴化合物:Co₃(bdt)₃(PBu₃)₃(Ⅰ),Co₃(tdt)₃(PBu₃)₃(Ⅱ),[Co₃(bdt)₃(PPh₃)₃][CoBr₃(dmf)](Ⅲ)和[Co₃(edt)₃(PEtB)₃]₂[Co₂Cl₄(Et₂SO₂)₂](Ⅳ)(bdt=1,2-S₂C₆H₄^2-,tdt=4-Me-1,2-S₂C₆H₃^2-,edt=SCH₂CH₂S^2-)的磁极矩和¹H NMR等研究表明:它们均具顺磁性,钴原子之间存在着反铁磁偶合作用,同时还讨论了它们的磁学性质和分子结构之间关系。     

准一维Cr基超导体RbCr3As3的超导能隙

RbCr₃As₃是具有[（Cr₃As₃）^-]_∞线性链的准一维超导体,超导转变温度约为6.6 K.对RbCr₃As₃单晶进行了电输运和极低温热输运性质的研究.低温下,拟合了RbCr₃As₃正常态电阻率随温度的变化,发现其满足费米液体行为.通过拟合超导转变温度随磁场的关系,得到RbCr₃As₃单晶的上临界场约为25.6 T.对RbCr₃As₃进行了零场下的极低温热导率测量,得到其剩余线性项为7.5 μW·K^-2·cm^-1,占正常态热导率值的24%.测量不同磁场下RbCr₃As₃的热导率,发现与单带s波超导体相比较,RbCr₃As₃剩余线性项随磁场增加相对较快.这些结果表明RbCr₃As₃单晶很可能是有节点的非常规超导体.     

三元Co_(79)Zr_(18)Cr_3合金中高矫顽力

通过在Co_(82)Zr_(18)合金中添加过渡族元素Cr的方法,利用快淬工艺,制备出了Co_(82-x)Zr_(18)Cr_x(x=0,2,3,4)快淬合金薄带,利用磁性测量、X光衍射、热磁分析、扫描电子显微镜,对其磁性能、相组成、微结构进行了研究,实验结果表明,在Co_(82)Zr_(18)合金中添加少量的Cr可以使其矫顽力(iHc)显著提高,其中,Co_(79)Zr_(18)Cr_3快淬薄带经600℃退火处理后iHc=6.5 kOe.相分析发现,600℃退火后的Co_(79)Zr_(18)Cr_3快淬薄带由单一Co_(11)Zr_2相组成,Cr原子进入到了Co_(11)Zr_2相的晶格之中替换了原子半径相对较小的Co原子,这导致了Co_(11)Zr_2居里温度(T_C)的降低却使其磁晶各向异性场(Ha)显著提高;另一方面,通过微结构研究发现,未退火的Co_(79)Zr_(18)Cr_3快淬薄带由50—80 nm的等轴晶粒组成,经600℃退火后,其晶粒形态并未发生改变然而晶粒尺寸却增长到400—500nm。     

Co掺杂对Mn$lt;sub$gt;3$lt;/sub$gt;Sn$lt;sub$gt;1-$lt;em$gt;x$lt;/em$gt;$lt;/sub$gt;Co$lt;sub$gt;$lt;em$gt;x$lt;/em$gt;$lt;/sub$gt;C$lt;sub$gt;1.1$lt;/sub$gt;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Mn₃Sn_1-xCo_xC_1.1 （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 （Mn₃Sn_0.9Co_0.1C_1.1） 后又升到332 K （Mn₃Sn_0.2Co_0.8C_1.1）,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,Mn₃Sn_1-xCo_xC_1.1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. 关键词： 磁性质 相变 磁卡效应 相对制冷量     

Bandgap narrowing in the layered oxysulfide semiconductor Ba_3Fe_2O_5Cu_2S_2: Role of FeO_2 layer

A new layered Cu-based oxychalcogenide Ba_3Fe_2O_5Cu_2S_2 has been synthesized and its magnetic and electronic properties were revealed. Ba_3Fe_2O_5Cu_2S_2 is built up by alternatively stacking [Cu_2S_2]~(2-) layers and iron perovskite oxide[(FeO_2)(BaO)(FeO_2)]~(2-)layers along the c axis that are separated by barium ions with Fe~(3+) fivefold coordinated by a square-pyramidal arrangement of oxygen. From the bond valence arguments, we inferred that in layered CuC h-based(Ch =S, Se, Te) compounds the +3 cation in perovskite oxide sheet prefers a square pyramidal site, while the lower valence cation prefers the square planar sites. The studies on susceptibility, transport, and optical reflectivity indicate that Ba_3Fe_2O_5Cu_2S_2 is an antiferromagnetic semiconductor with a Ne′el temperature of 121 K and an optical bandgap of 1.03 eV. The measurement of heat capacity from 10 K to room temperature shows no anomaly at 121 K. The Debye temperature is determined to be 113 K. Theoretical calculations indicate that the conduction band minimum is predominantly contributed by O 2p and 3 d states of Fe ions that antiferromagnetically arranged in FeO_2 layers. The Fe 3d states are located at lower energy and result in a narrow bandgap in comparison with that of the isostructural Sr_3Sc_2O_5Cu_2S_2.     

Mn42Al50-xFe8+x合金的磁性和磁热效应

研究了Mn₄₂Al_50-xFe_8+x合金的结构、磁性和磁热效应. 通过成分调节, 居里温度T_C在室温附近一宽温区连续可调, 分别为270 K (Mn₄₂Al₄₂Fe₁₆), 341 K (Mn₄₂Al₄₀Fe₁₈)和370 K(Mn₄₂Al₃₈Fe₂₀). 磁化强度在相变温度处发生一陡降, 热磁曲线和等温磁化曲线均未观察到热和磁的滞后, 表明发生一可逆的二级相变. 在各自居里温度附近, 0-5 T的外磁场变化下磁熵变峰值分别为2.48, 2.52和2.40 J·kg^-1·K^-1. Mn_50-xAl_50-yFe_x+y合金的磁熵变峰值虽然与许多优良的磁制冷材料相比并不大, 但是制备该化合物的原材料价格非常低廉, 制备工艺简单, 加工成型也较容易, 化合物本身耐腐蚀性、延展性较好, 且在居里温度附近发生的是可逆的二级相变, 无晶格或结构的变化, 有利于制冷剂的多次循环使用. 关键词： 磁性 磁热效应 二级相变     

Magnetic properties and magneto-optical spectroscopy of Heusler alloys based on transition metals and Sn

The formation and magnetic properties of Heusler compounds of the general formula X₂YSn were studied, where X represents a 3d transition metal or Cu and where Y represents a second 3d transition metal or a metal of group IV A, VA and VI A of the periodic table. The lattice constant was determined for all Heusler compounds studied. The Co moment in the Co₂YSn compounds was not found to scale in a sample manner to the electronegativity difference E φ^* between Co and the Y components as was previously observed in the series Co₂YAl and Co₂YGa. All compounds of the type Ni₂YSn were found to be Pauli paramagnetic when Y is non-magnetic metal. Compounds having a Curie temperature above room temperature were investigated by means of magneto-optical polar Kerr-effect spectroscopy. Experimental indications were obtained for charge transfer transitions in the series Co₂TiSn, Co₂HfSn.     

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.