Structural and mechanical stability of rare-earth diborides

Structural and mechanical properties of several rare-earth diborides were systematically investigated by first principles calculations. Specifically, we studied XB2 , where X=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Lu in the hexagonal AlB2 , ReB2 , and orthorhombic OsB2 -type structures. The lattice parameters, bulk modulus, bond distances, second order elastic constants, and related polycrystalline elastic moduli (e.g., shear modulus, Young’s modulus, Poisson’s ratio, Debye temperature, sound velocities) were calculated. Our results indicate that these compounds are mechanically stable in the considered structures, and according to "Chen’s method", the predicted Vickers hardness shows that they are hard materials in AlB2 - and OsB2 -type structures.     

Interplay of iron and rare-earth magnetic order in rare-earth iron pnictide superconductors under magnetic field

The magnetic properties of iron pnictide superconductors with magnetic rare-earth ions under strong magnetic field are investigated based on the cluster self-consistent field method. Starting from an effective Heisenberg model, we present the evolution of magnetic structures on magnetic field in RFeAsO(R = Ce, Pr, Nd, Sm, Gd, and Tb) and RFe_2As_2(R =Eu) compounds. It is found that spin-flop transition occurs in both rare-earth and iron layers under magnetic field, in good agreement with the experimental results. The interplay between rare-earth and iron spins plays a key role in the magneticfield-driven magnetic phase transition, which suggests that the rare-earth layers can modulate the magnetic behaviors of iron layers. In addition, the factors that affect the critical magnetic field for spin-flop transition are also discussed.     

Electronic and magnetic properties of the rare earth intermetallic compounds RRu4Sn6 (R=Nd, Sm, Gd, Tb, Dy and Ho)

A number of compounds of structural formula RRu₄Sn₆ (R=rare-earth element) have previously been reported to form in the tetragonal crystal structure with space group I4¯2m. In this structure the R atoms are well isolated from each other. We embarked on this study to investigate the physical properties and to compare with earlier results obtained on the strongly correlated, low charge-carrier density compound CeRu₄Sn₆. Here we report our results of crystallographic, electrical resistivity, and magnetic studies on this family of compounds. In contrast to the behaviour in CeRu₄Sn₆, magnetic ordering is evident at low temperatures in the compounds with R=Sm, Gd, and Dy, as is evidenced by well-resolved anomalies in the temperature dependence of the electrical resistivity and static magnetic susceptibility.     

AlB+n(n=2~10) 团簇结构和红外振动光谱研究

用密度泛函理论的B3LYP方法在6-311G(d)水平上对AlB+n（n=2~10）团簇几何结构、稳定性、电子结构和成键特性进行了系统理论研究,得到了AlB+n（n=2~10）团簇的最稳定结构.结果表明,硼原子间容易聚集,铝原子处于整个硼原子集团的外围.与相应中性AlBn团簇相比,Al-B键作用变弱,使正价团簇（n=6和10除外）结构变化较大|对AlB+n（n=2~10）和相应中性团簇能隙的计算分析表明,AlB+n 团簇的稳定性有所增强,其中AlB+3、AlB+5和AlB+8团簇尤为显著|通过对最稳定构型红外振动光谱的研究分析表明,硼原子间对称或非对称振动、铝原子不动的振动模式更容易出现较强谱峰,即硼原子间更容易成键.     

Low-temperature magnetic behavior of the rare-earth cobaltites GdCoO<Subscript>3</Subscript> and SmCoO<Subscript>3</Subscript>

The temperature and magnetic field dependences of the static magnetization of the polycrystalline rare-earth cobaltites GdCoO₃ and SmCoO₃ have been measured. It is shown that, below room temperature, the magnetization of both compounds derives primarily from the rare-earth ion paramagnetism. The GdCoO₃ and SmCoO₃ compounds have been found to differ substantially in magnetic behavior, which can be traced to differences in their electronic shell structures. The magnetic behavior of GdCoO₃ is close to that of an array of free Gd³⁺ ions, whereas in SmCoO₃ the deviation from the free-ion properties is very large because of the Sm³⁺ ground state being crystal-field split. Van Vleck magnetic susceptibility measurements of SmCoO₃ suggest that the splitting is ～10 K.     

Mixed-valence manganites

Mixed-valence manganese oxides (R₁-_χA_χ)MnO₃ (R=rare-earth cation, A=alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO₃, exhibit a rich variety of crystallographic, electronic and magnetic phases. Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin films has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. This review gives an account of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, the role of cation disorder and the Jahn-Teller effect. Sample preparation, and the properties of related ferromagnetic oxides are also discussed.     

Electronic structure and magnetic properties of RCo5−xMx (R=Y,Pr and M=Al,Si) system

Detailed theoretical and experimental investigations of the electronic and magnetic properties of the RCo_5?xM_x compounds (R=Y, Pr and M=Si, Al) have been performed. All theoretical investigations of the electronic and magnetic properties of the system have been done using the Korringa–Kohn–Rostoker (KKR) band structure method. The Si for Co substitution in RCo₅ does not change the magnetic ordering: the RCo_5?xSi_x with R=Y, Nd and Pr is ferromagnetic, whilst the heavy rare-earth containing compounds are ferrimagnetic. The important modifications induced by this substitution concerns the magnetic properties of the system: the Curie temperature and the magnetic moments of Co decrease with Si content, indicating the weakening of the Co–Co exchange interaction. The band structure calculations evidence the hybridization between the 3d electronic states of Co and the 3p states of Si as possible reason for the diminishing of Co–Co exchange interaction. Also, the volume effect on the magnetic properties of the YCo₄Si was investigated using theoretical methods. The results are compared with the experimental measurements in order to distinguish the origin of magnetization reduction in YCo₄Si compared with YCo₄Al.     

Magnetic properties of RCr2Si2 compounds (R=Tb,Er)

We report on magnetic properties of RCr₂Si₂ compounds with R=Tb, Er. The polycrystalline samples were characterized by powder X-ray diffraction and found to be isostructurally crystallizing in the ThCr₂Si₂-type tetragonal structure. The samples were further investigated by specific heat, AC-susceptibility and magnetization methods in the temperature range 2–900 K and in magnetic fields up to 9 T. The magnetic measurements revealed the magnetic ordering of the rare-earth moments below about 2 K, whilst no high-temperature ordering of the Cr moments was observed. The evidence of for Cr magnetism is corroborated by results of first-principles calculations based on the density functional theory (DFT).     

A crystal field investigation of the properties of RCuAs2 (R=Pr, Nd, Sm, Tb, Dy, Ho, Er and Yb)

A crystal field (CF) investigation of the magnetic properties and heat capacities of RCuAs₂ (R=Pr, Nd, Sm, Tb, Dy, Ho, Er and Yb) has been carried out using the observed average magnetic susceptibilities (1.8-300 K) of the title compounds. The CF parameters proposed for the systems show a systematic variation throughout the rare-earth series. Other physical properties dependent on the CF are also computed and compared with available experimental data. The experimental heat capacity data reported for a limited range of temperature agree well with computed heat capacity for all the compounds (except SmCuAs₂ and YbCuAs₂). CF J mixing was found to be appreciable for all the samples except YbCuAs₂.     

Magnetic and magnetoelectric properties of terbium aluminum borate

The magnetic and magnetoelectric properties of terbium aluminum borate are studied. It is established that temperature dependences of the magnetoelectric effects of terbium aluminum borate are analogous to those of terbium ferroborate, despite the difference in effective magnetic fields acting on rare-earth ions in these compounds. This confirms the general assumption on the decisive role of the rare-earth element in the magnetoelectric properties of borates.     

Hartmagnetische Legierungen der Seltenen Erdmetalle mit 3d-Metallen

The phase relations obtained from alloys of the rare-earth metals (RE) with 3d-metals are initially discussed, as well as the crystal structures and the general physical properties of the corresponding intermetallic compounds. The magnetic properties of RE-3d-compounds and the essential data of a hard-magnetic material are then summarized. The different steps in the production of RE-Co-permanent magnets are reviewed in relation to the underlying basic physical mechanisms. Finally, other possible hard-magnetic RE-3d-alloys and some general trends of physical and metallurgical investigations, necessary for the improvement of the RECo₅-magnets, are outlined.

     

Ferromagnetic Kondo lattice CeRuSi2 with non-Fermi-liquid behavior

The structure, electronic, thermodynamic, and magnetic properties of the CeRuSi₂ Kondo lattice with ferromagnetic ordering characterized by a small moment of the ground state are investigated. Anomalies in the temperature dependences of heat capacity and resistivity (unusual power or logarithmic behavior) observed in the low-temperature range indicate a non-Fermi-liquid behavior. The results are compared with those for other Ce_lRu _n X _m compounds and anomalous systems based on rare-earth elements and actinides that had been studied earlier.     

112型铁基化合物EuFeAs2的单晶生长与表征

铁基超导体中含有一类特殊的112型结构化合物,其层状结构中含有一层锯齿形的As链构型.本文报道了用CsCl助熔剂法生长新型铁基112型EuFeAs₂母体单晶的具体方法,以及对该单晶的结构和物性的详细表征.通过能量色散X射线能谱扫描对单晶样品进行的化学成分分析,以及单晶X射线衍射的结构解析,确定该单晶样品属于EuFeAs₂相,结构精修得到EuFeAs₂具有空间群为Imm2（No.44）的正交晶体结构,晶格常数分别是a=21.285（9）Å,b=3.9082（10）Å,c=3.9752（9）Å.通过低温电阻测量,发现在110 K附近和46 K附近存在两个异常电阻跳变.进一步分析表明,110 K附近存在两个邻近的相变,这两个相变与铁基母体材料中常见的结构相变和Fe²⁺的反铁磁相变相符合.结合磁化率测量分析,可知46 K附近的相变属于Eu²⁺的反铁磁相变.     

Microstructures and magnetic domain configurations of NdFe<Subscript>11</Subscript>Ti and Nd<Subscript>2</Subscript>(Fe,Ti)<Subscript>17</Subscript> aggregates

Nd:11Fe:Ti alloys produced by arc melting and splat quenching have been investigated by scanning electron microscopy, transmission electron microscopy, magnetic force microscopy and magnetic measurements. Both the as-cast and splat-quenched materials crystallized adopting essentially the ThMn₁₂-type structure. However, the RFe₁₁M (where R is a rare-earth element and M is an early 3d element or other non-magnetic element) stoichiometry was not strictly followed and α-Fe(Ti) and Nd₂(Fe,Ti)₁₇ could be detected in the microstructures. Magnetic force microscopy showed that the critical size for single-domain behavior in the splat-quenched aggregates was close to 100 nm. Annealing treatments induced a NdFe₁₁Ti → Nd₂(Fe,Ti)₁₇+α-Fe(Ti) + Fe₂Ti reaction. Transmission electron microscopy investigations showed that this solid-state transformation produced a fine intergrowth of the two ternary rare-earth compounds with a specific crystallographic orientation relation. The lower saturation magnetization of the splat/annealed condition as compared to the splat-quenched condition may reflect the disorderly intergrown Nd₂(Fe,Ti)₁₇ phase in NdFe₁₁Ti. On the other hand the higher coercivity obtained for the splat-quenched condition seems related to the smaller scale of the microstructure due to a possible, albeit weak, pinning effect by the grain boundaries.     

Structural and optical properties of sputtered gadolinium nitride films

Rare-earth (RE) materials have high magnetic moments and form a wide range of magnetic structures. There has been speculation in the literature that rare-earth nitrides may form half-metallic ferromagnetics. This is surprising because, based on a simple ionic model, trivalent rare-earth nitrides would be expected to be insulators with a similar electronic structure to the divalent rare-earth chalcogenides. However if it is the case that they are half-metallic or narrow gap insulators, then they have potential applications in spin-filtering devices. In the present investigation, We have deposited GdN films on glass substrate at room temperature by Ar/N₂ mixed gas plasma-radio frequency (rf) sputtering method. The structure and the complex optical properties as well as the energy gap of GdN thin films as a function of N₂ partial pressure are determined.     

Crystal structures,phase relationships,and magnetic phase transitions of R_5M_4 compounds (R = rare earths,M = Si,Ge)

Our recent studies of the crystal structures, phase transitions, and magnetic properties of intermetallic compounds R5M4 (R = rare earths; M = Si, Ge) are reviewed briefly. First, crystal structures, phase relationships, and magnetic properties of several 5:4 compounds, including Nd5Si4xGex , Pr5Si4xGex, Gd5xLaxGe4,La5Si4, and Gd5Sn4 , are presented. In particular, the canted spin structures as well as the magnetic phase transitions in Pr5Si2Ge2 and Pr 5 Ge 4 investigated by neutron powder diffractions and small-angle neutron scattering are reviewed. Second, the crystal structures and magnetic properties of the most studied compounds Gd5(Si,Ge)4 are summarized. The focus is on the parent compound Gd5Ge4 , which is an amazing material exhibiting magnetic anisotropy, angular dependent spin-flop transition, metastable magnetic response, Griffiths-like phase, thermal effect under pulsed fields, antiferromagnetic and ferromagnetic resonances, pronounced effects of impurities, and high-field induced magnetic transitions.     

Magnetic properties of R3Co29Si4B10 (R=La, Ce, Pr, Nd, Sm, Gd and Dy) borides

The crystal structure and magnetic properties of quaternary rare-earth intermetallic borides R₃Co₂₉Si₄B₁₀ with R=La, Ce, Pr, Nd, Sm, Gd and Dy have been studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in a tetragonal crystal structure with the space group P4/nmm. Compounds with R=La, Ce, Pr, Nd and Sm are ferromagnets, while ferrimagnetic behavior is observed for R=Gd and Dy. The Curie temperatures vary between 149 K and 210 K. The Curie temperatures in R₃Co₂₉Si₄B₁₀ (R=Ce, Pr, Nd, Sm, Gd, Dy) compounds are roughly proportional to the de Gennes factors.     

Magnetoelectric and magnetoelastic properties of easy-plane ferroborates with a small ionic radius

The magnetic, magnetoelectric, and magnetoelastic properties of RFe₃(BO₃)₄ ferroborates are studied. The measurement of the field dependences of the magnetoelectric polarization along the a axis in holmium ferroborate and in the mixed composition Ho_0.5Sm_0.5Fe₃(BO₃)₄ revealed the following dependences for easy-plane ferroborates: (a) the longitudinal and transverse magnetoelectric effects have the opposite signs and (b) magnetically induced polarization changes its sign in a field close to the field of exchange between rare-earth and iron ions. These dependences agree well with theoretical predictions based on the symmetry of the compounds. The relatively low f-d exchange field in holmium ferroborate (about 20 kOe), which magnetizes the rare-earth subsystem, causes smaller polarization jumps (about 30 ??C/m²) in fields lower than 10 kOe as compared to the jumps in other easy-plane ferroborates (R = Sm, Nd). The increase in the electric polarization induced in HoFe₃(BO₃)₄ in magnetic fields higher than 100 kOe (200?C300 ??C/m²) is found to be significantly smaller than in neodymium ferroborate, which indicates a substantial dependence of the magnetoelectric effects on the electronic structure of a rare-earth ion.     

Magnetic properties of rare-earth transition metal aluminides R6T4Al43 with Ho6Mo4Al43-type structure

The magnetic properties of 53 aluminium-rich intermetallic compounds R₆T₄Al₄₃ with R=rare-earth elements and T=Ti, V, Nb, Ta, Cr, Mo, W were investigated using polycrystalline samples and a SQUID magnetometer in the temperature range from 2 to 300 K with magnetic flux densities up to 5.5 T. The yttrium and lutetium compounds are Pauli paramagnetic, indicating that the transition metal atoms do not carry magnetic moments. The samarium compounds show van Vleck behavior and antiferromagnetic order with Néel temperatures of less than 12 K. Of these Sm₆Ti₄Al₄₃ becomes metamagnetic. The ytterbium compounds show a mixed or intermediate valent behavior and no magnetic order down to 2 K. All other compounds obey the Curie–Weiss law above 30 K. Their effective magnetic moments correspond to the theoretical moments of the rare-earth ions. They show ferromagnetic or metamagnetic behavior with ordering temperatures all below 20 K. The magnetization curves of most compounds (recorded up to 5.5 T) reach about 50% of the theoretical magnetization already at 0.5 T. The gadolinium compounds are exceptional in that they reach at 0.5 T only about 10% of their theoretical magnetization. The crystal structures of the isotypic compounds Yb₆V₄Al₄₃ and Yb₆Ta₄Al₄₃ were refined from single-crystal X-ray data.     

Tolerance Factor for Huntite-Family Compounds

Physics of the Solid State - 85 RM3(BO3)4 (R is the rare-earth element (Y, La–Lu) and M = Al, Sc, Cr, Fe, or Ga) compounds with the huntite structure have been analyzed. The analysis of the...