El Temperatures Research Articles

Electronic structure of the magnetic halide double perovskites Cs2(Ag,Na)FeCl6 from first principles

A family of magnetic halide double perovskites (HDPs) have recently attracted attention due to their potential to broaden application areas of halide double perovskites into, e.g., spintronics. Up to date the theoretical modeling of these systems have relied on primitive approximations to the density functional theory (DFT). In this paper, we study structural, electronic and magnetic properties of the ${\mathrm{Fe}}^{3+}$-containing HDPs ${\mathrm{Cs}}_{2}{\mathrm{AgFeCl}}_{6}$ and ${\mathrm{Cs}}_{2}{\mathrm{NaFeCl}}_{6}$ using a combination of more advanced DFT-based methods, including $\mathrm{DFT}+U$, hybrid-DFT, and treatments of various magnetic states. We examine the effect of varying the effective Hubbard parameter, ${U}_{\mathrm{eff}}$, in $\mathrm{DFT}+U$ and the mixing-parameter, $\ensuremath{\alpha}$, in hybrid DFT on the electronic structure and structural properties. Our results reveal a set of localized $\mathrm{Fe}(d)$ states that are highly sensitive to these parameters. ${\mathrm{Cs}}_{2}{\mathrm{AgFeCl}}_{6}$ and ${\mathrm{Cs}}_{2}{\mathrm{NaFeCl}}_{6}$ are both antiferromagnets with Ne\'el temperatures well below room temperature and are thus in their paramagnetic (PM) state at the external conditions relevant to most applications. Therefore, we have examined the effect of disordered magnetism on the electronic structure of these systems and find that while ${\mathrm{Cs}}_{2}{\mathrm{NaFeCl}}_{6}$ is largely unaffected, ${\mathrm{Cs}}_{2}{\mathrm{AgFeCl}}_{6}$ shows significant renormalization of its electronic band structure.

Microstructural and magnetic properties of Mn2FeSi and Mn2FeAl alloys prepared in bulk form

Microstructural and magnetic properties of the Mn2FeSi and Mn2FeAl alloys prepared in the bulk form have been investigated. Cylinder-shaped ingots produced by induction melting technique were analyzed in as-quenched state and additionally annealed at 773 K for 5 days in the protective argon atmosphere. The results show that Si and Al have different effects on the microstructural and magnetic properties of the alloys. The Mn2FeAl ingots are single-phase both before and after annealing, and their diffractograms surprisingly correspond not to the Heusler (L21 or XA) but to the primitive cubic β-Mn structure. Conversely, Mn2FeSi alloys show a two-phase behavior in the as-quenched state. From results of X-ray diffraction it was not possible to judge whether Mn2FeSi alloy has the inverse-Heusler (XA) or full Heusler (L21) structure. Annealing of Mn2FeSi leads to the formation of multiple phases. The EDX chemical area analyses resulted in only slight deviations of compositions compared to the nominal ones. The lattice parameters of 0.5672 nm and 0.6339 nm were estimated for the Mn2FeSi and Mn2FeAl samples from X-ray diffraction measurements. From the magnetic viewpoint, all samples are paramagnetic at room temperature and transform into antiferromagnetic state at the N é el temperatures about 50 K and 36 K for Mn2FeSi and Mn2FeAl, respectively. The negative Curie temperatures determined at all samples by Curie-Weiss law indicate an antiferromagnetic ordering of spins. Positron annihilation investigations revealed that Mn2FeSi contains a high concentration of vacancies. The local chemical environment of vacancies characterized by coincidence Doppler broadening is compatible with L21 rather than with XA structure. In contrast vacancy concentration in Mn2FeAl is very low and almost all positrons are annihilated in the free state.

Optical Readout of the Néel Vector in the Metallic Antiferromagnet Mn2Au

Metallic antiferromagnets with broken inversion symmetry on the two sublattices, strong spin-orbit coupling and high N\'{e}el temperatures offer new opportunities for applications in spintronics. Especially Mn$_{2}$Au, with high N\'{e}el temperature and conductivity, is particularly interesting for real-world applications. Here, manipulation of the orientation of the staggered magnetization,\textit{\ i.e.} the N\'{e}el vector, by current pulses has been recently demonstrated, with the read-out limited to studies of anisotropic magnetoresistance or X-ray magnetic linear dichroism. Here, we report on the in-plane reflectivity anisotropy of Mn$_{2}$Au (001) films, which were N\'{e}el vector aligned in pulsed magnetic fields. In the near-infrared, the anisotropy is $\approx$ 0.6\%, with higher reflectivity for the light polarized along the N\'{e}el vector. The observed magnetic linear dichroism is about four times larger than the anisotropic magnetoresistance. This suggests the dichroism in Mn$_{2}$Au is a result of the strong spin-orbit interactions giving rise to anisotropy of interband optical transitions, in-line with recent studies of electronic band-structure. The considerable magnetic linear dichroism in the near-infrared could be used for ultrafast optical read-out of the N\'{e}el vector in Mn$_{2}$Au.

High-Pressure Synthesis of Magnetic Neodymium Polyhydrides

Ongoing search for room-temperature superconductivity is inspired by the unique properties of the electron-phonon interaction in metal superhydrides. Encouraged by the recently found highest-TC superconductor fcc-LaH10, here we discover several superhydrides of another lanthanoid, neodymium. We identify three novel metallic Nd-H phases at pressures ranging from 85 to 135 GPa: I4/mmm-NdH4, C2/c-NdH7, and P63/mmc-NdH9, synthesized by laser-heating metal samples in NH3BH3 media for in situ generation of hydrogen. A lower trihydride Fm3̅m-NdH3 is found at pressures from 2 to 52 GPa. I4/mmm-NdH4 and C2/c-NdH7 are stable from 135 to 85 GPa, and P63/mmc-NdH9 is stable from 110 to 130 GPa. Measurements of the electrical resistance of NdH9 demonstrate a possible superconducting transition at ∼4.5 K in P63/mmc-NdH9. Our theoretical calculations predict that all of the neodymium hydrides have antiferromagnetic order at pressures below 150 GPa and represent one of the first discovered examples of strongly correlated superhydrides with large exchange spin-splitting in the electronic band structure (>450 meV). The critical Néel temperatures for new neodymium hydrides are estimated using the mean-field approximation to be about 4 K (NdH4), 251 K (NdH7), and 136 K (NdH9).

Unique interfacial reaction and so-induced change in mechanical performance of Sn–3.0Ag–0.5Cu/Cu solder joints formed during undercooled and eutectic liquid soldering processes

A novel technique is proposed to prepare Sn–3.0Ag–0.5Cu/Cu joints at three different processing temperatures, which are called undercooled liquid (UL, 213.0 °C), eutectic liquid (EL, 217.0 °C) and normal liquid (NL, 231.0 °C) soldering temperatures correspondingly, and each of them is lower than, equal to and higher than Sn–3.0Ag–0.5Cu solder’s melting point (217.0 °C), respectively. The interfacial reaction, intermetallic compound growth and mechanical performance of Sn–3.0Ag–0.5Cu/Cu joints formed during UL, EL and NL soldering processes were investigated systematically. Results show that for Sn–3.0Ag–0.5Cu solder as the undercooled liquid melt or eutectic liquid melt or normal liquid melt in the formation of joints, the thickness of interfacial intermetallic compound (IMC) layer in the joints increases with prolonging dwelling time. For three different liquid soldering processes, the change tendency of the interfacial IMC layer thickness is distinct. For joints formed at UL and EL temperatures, the excessive growth of primary Cu6Sn5 and interfacial IMC (mainly Cu6Sn5) can be suppressed during isothermally dwelling in the liquid state for more than 5 min. Ball shear test results show that solder joints formed at UL and EL temperatures have lower value of the maximum shear force (MSF) than those formed during the NL soldering process. For UL and EL types of joints, the fracture occurs either by shear slide fracture totally along the shear tool tip movement plane (i.e., in-plane shear slide fracture) or by shear slide fracture initially along the shear tool tip movement plane and afterwards shear deformation fracture in the solder matrix under the plane (i.e., out-of-plane shear slide and deformation mixed-mode fracture), while the fracture takes place in NL joints only by in-plane shear slide fracture mode.

Single-layer antiferromagnetic semiconductor CoS2 with pentagonal structure

Structure-property relationships have always been guiding principles in discovering new materials. Here we explore the relationships to discover novel two-dimensional (2D) materials with the goal of identifying 2D magnetic semiconductors for spintronics applications. In particular, we report a density functional theory + $U$ study of single-layer antiferromagnetic (AFM) semiconductor CoS$_2$ with the pentagonal structure forming the so-called Cairo Tessellation. We find that this single-layer magnet exhibits an indirect bandgap of 1.06 eV with light electron and hole effective masses of 0.03 and 0.10 $m_0$, respectively, which may lead to high carrier mobilities. The hybrid density functional theory calculations correct the bandgap to 2.24 eV. We also compute the magnetocrystalline anisotropy energy (MAE), showing that the easy axis of the AFM ordering is out of plane with a sizable MAE of 153 $\mu$eV per Co ion. We further calculate the magnon frequencies at different spin-spiral vectors, based on which we estimate the N$\acute{e}$el temperatures to be 20.4 and 13.3 K using the mean field and random phase approximations, respectively. We then apply biaxial strains to tune the bandgap of single-layer pentagonal CoS$_2$. We find that the energy difference between the ferromagnetic and AFM structures strongly depends on the biaxial strain, but the ground state remains the AFM ordering. Although the low critical temperature prohibits the magnetic applications of single-layer pentagonal CoS$_2$ at room temperature, the excellent electrical properties may find this novel single-layer semiconductor applications in optoelectronic nanodevices.

Temperature dependent carrier localization in AlGaInN near-ultraviolet light-emitting diodes.

We investigate the carrier localization effect in the low-indium ultraviolet AlGaInN light-emitting diodes with a 365 nm peak and a wide yellow luminous band over the visible range. Temperature-dependent electroluminescence spectra (EL) are measured under a wide range of temperature. We found that carrier localization effect relies on the carrier mobility and manifests itself by altering several macroscopic quantities, such as ELs and electrical resistance of the device. Under moderate injection densities, plots of EL peak energy vs. temperatures exhibits S-shapes. At low temperatures, line-width broadening in EL spectra and irregular humps in I-V curves were observed at similar level of injection densities. Both phenomena diminish as temperature increases and eventually disappear at room temperature. All the results stem from carrier localization and following delocalization effect. It suggests that the carrier mobility determine the degree of carrier localization effect - inactive carriers tend to be localized at low temperature but escape at high temperature from bindings of localization centers. As a result, carrier localization is intense only at low temperature for low-indium devices.

Strain-induced magnetic phase transition inSrCoO3−δthin films

It has been well established that both in bulk at ambient pressure and for films under modest strains, cubic SrCoO$_{3-\delta}$ ($\delta < 0.2$) is a ferromagnetic metal. Recent theoretical work, however, indicates that a magnetic phase transition to an antiferromagnetic structure could occur under large strain accompanied by a metal-insulator transition. We have observed a strain-induced ferromagnetic to antiferromagnetic phase transition in SrCoO$_{3-\delta}$ films grown on DyScO$_3$ substrates, which provide a large tensile epitaxial strain, as compared to ferromagnetic films under lower tensile strain on SrTiO$_3$ substrates. Magnetometry results demonstrate the existence of antiferromagnetic spin correlations and neutron diffraction experiments provide a direct evidence for a G-type antiferromagnetic structure with Ne\'el temperatures between $T_N \sim 135\,\pm\,10\,K$ and $\sim 325\,\pm\,10\,K$ depending on the oxygen content of the samples. Therefore, our data experimentally confirm the predicted strain-induced magnetic phase transition to an antiferromagnetic state for SrCoO$_{3-\delta}$ thin films under large epitaxial strain.

Low-temperature properties of single-crystalCrB2

We report the low-temperature properties of $^{11}$B-enriched single-crystal CrB$_{2}$ as prepared from high-purity Cr and B powder by a solid-state reaction and optical float zoning. The electrical resistivity, $\rho_{\rm xx}$, Hall effect, $\rho_{\rm xy}$, and specific heat, $C$, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap $\Delta \approx 220\,{\rm K}$ below $T_{\rm N}=88\,{\rm K}$. This observation is corroborated by the absence of a Curie dependence in the magnetization for $T\to0$ reported in the literature. Comparison of $C$ with $d\rho_{\rm xx}/dT$, where we infer lattice contributions from measurements of VB$_2$, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature $T_{\rm sf}\approx 257\,{\rm K}$ in the paramagnetic state, followed by a pronounced second-order mean-field transition at $T_{\rm N}$, and unusual excitations around $\approx T_{\rm N}/2$. The pronounced anisotropy of $\rho_{\rm xx}$ above $T_{\rm N}$ is characteristic of an easy-plane anisotropy of the spin fluctuations consistent with the magnetization. The ratio of the Curie-Weiss to the N$\acute{\rm{e}}$el temperatures, $f=-\Theta_{\rm CW}/T_{\rm N}\approx 8.5$, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to $14\,\,{\rm T}$, the highest field studied. In contrast to earlier suggestions of local-moment magnetism our study identifies CrB$_{2}$ as a weak itinerant antiferromagnet par excellence with strong geometric frustration.

화성 암권의 진화해석을 위한 예비연구: 3가철 산화물의 자화특성

화성의 암권 진화 연구에서 최근 각광받는 광물은 적철석으로 대표되는 3가철 산화물이다. 물리적 방법의 하나인 잔류자화기억도 실험은 비파괴적이고 지구 기원이 아닌 고체 시료의 자화특성 규명에 유용하게 사용된다. 금번 연구에서는 알루미늄 농도를 조절하며 열수반응과 탈수반응을 통해 총 8개 성분의 3 가철 산화물을 합성하였다. 이들 시료에 대해 잔류자화기억도와 자화상실온도(<TEX>$T_N$</TEX>)를 측정하였다. 3가철 산화물의 격자상수는 알루미늄의 3가철 함량이 증가하며 감소한다. 3 가철 산화물의 자화상실 온도 역시 알루미늄의 몰농도가 증가하며 감소한다. 알루미늄이 거의 첨가되지 않은 적철석의 <TEX>$T_N$</TEX>은 광물의 합성방법과 무관하게 대략 <TEX>$690^{\circ}C$</TEX>로 수렴한다. 탈수반응으로 합성된 3가철 산화물의 잔류자화기억도는 알루미늄의 함량에 거의 무관하게 매우 높지만, 열수반응으로 합성된 3가철 산화물의 잔류자화기억도는 알루미늄 함량이 증가하며 동반 상승한다. 상대적으로 쉽고 측정이 간단하며 비파괴적인 잔류자화기억도를 이용하면 추후 암석 내 3가철 산화물 입자의 성인 유추가 가능하며, 특히 화성의 암권 진화 규명에도 일조하리라 예상된다. Martian satellite missions indicate that Martian equatorial plains are covered by ferric iron oxide. As a non-destructive technique, low-temperature treatment of remanent magnetization is effective in identifying magnetic minerals in rocks. In the present study, four sets of ferric iron oxides were prepared by aqueous alteration of ferrihydrite at warm conditions and four others by dehydration of goethite. As the amount of aluminous trivalent cations increases, crystallographic lattice parameters and N<TEX>$\acute{e}$</TEX>el temperatures decrease. Such declines originate from lattice distortion as the smaller aluminous trivalent cations substitue the larger terric irons. Whilst high remanence memory was observed for aqueously produced ferric iron oxide, low remanence memory was observed for dehydrated ferric iron oxide. In the future. magnetic remanence memory would be powerful in diagnosing the origin of ferric iron oxide.

Antiferromagnetic properties of a water-vapor-insertedYBa2Cu3O6.5compound studied by NMR, NQR, andμSR

We present a detailed NQR, NMR and $\mu $SR study of a magnetic phase obtained during a topotactic chemical reaction of YBa$_{2}$Cu$_{3}$O$_{6.5}$ high- temperature superconductor with low-pressure water vapor. Our studies give straightforward evidence that the ''empty'' Cu(1) chains play the role of an easy water insertion channel. It is shown that the NQR spectrum of the starting material transforms progressively under insertion of water, and completely disappears when one H$_{2}$O molecule is inserted per unit cell. Similarly, a Cu ZFNMR signal characteristic of this water inserted material appears and grows with increasing water content, which indicates that the products of the reaction are non-superconducting antiferromagnetic phases in which the bilayers are ordered. These antiferromagnetic phases are felt by proton NMR which reveals two sites with static internal fields of 150 and about 15 Gauss respectively. Two muon sites are also evidenced with similar local fields which vanish at $T\approx 400$ K. This indicates that the magnetic phases have similar N\'{e}el temperatures as the other bilayer undoped compounds. An analysis of the internal fields on different sites of the structure suggests that they can be all assigned to a single magnetic phase at large water content in which the Cu(1) electron spins order with those of the Cu(2). It appears that even samples packed in Stycast epoxy resin heated moderately at a temperature (200$^{0}$C) undergo a reaction with epoxy decomposition products which yield the formation of the same final compound. It is then quite clear that such effects should be considered quite seriously and avoided in experiments attempting to resolve tiny effects in such materials, as those performed in some recent neutron scattering experiments.

Temperature and doping-level dependence of magnetic order inLa2−xSrxNiO4+δstudied by muon spin rotation

We report muon-spin-rotation (I¼SR) measurements on a series of compounds with composition La2-xSrxNiO4+I´ with strontium doping level x between 0 and 1. A magnetic transition is found in all the samples studied, which occurs at a composition-dependent temperature, TM. Below TM clear precession signals are observed in zero applied magnetic field for all x, indicating the existence of at least short-range magnetic order on a time scale longer than 10-8 s. Above TM the magnetic correlation times decrease by several orders of magnitude. At xâ??0.33, we find peaks in both TM and the zero-temperature staggered magnetization, an observation which we attribute to the higher degree of localization of the holes at this doping level. The measurements of TM as a function of x extend the determination of NA©el temperatures by previous neutron diffraction and I¼SR measurements. © 1999 The American Physical Society.

Magnetic critical properties of the undoped cuprates in a quasi-two-dimensional nonlinear-sigma-model approach.

A quasi-two-dimensional (2D) nonlinear sigma model (NLSM) is used to model the antiferromagnetism of the insulating layered undoped cuprate materials. As Chakravarty, Halperin, and Nelson (CHN) did for their 2D NLSM, we make a renormalization-group analysis of the quasi-2D NLSM. In contrast with CHN's 2D model, the quasi-2D NLSM yields a nonzero Ne\ifmmode\acute\else\textasciiacute\fi{}el temperature so that our theoretical Ne\ifmmode\acute\else\textasciiacute\fi{}el temperatures can be fitted completely with the experimental ones. Our theoretical correlation lengths are fitted very well with the experimental ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ data of the materials within 220 K above ${\mathit{T}}_{\mathit{N}}$.

FLUORINE BOMB CALORIMETRY. III. THE HEAT OF FORMATION OF BORON TRIFLUORIDE1-3

The heat of formation of boron trifluoride was measured by direct combination of the elements in a bomb calorimeter. DELTA Hf f at 25 f was found to be -270.10 el temperatures 0.24 kcal. mole -1.

Rapid assay of tritium-labelled substances inside GM gas counting tubes

A relatively simple and rapid method is described for measuring the radioactivity of tritiated substances in the form of acetylene inside GM internal gas counting tubes. A single working line makes it possible to carry out reproducible determinations of relative specific activities of as many as l8 solid samples within 8 hours with an error less than el temperatures 3.5% when the standard deviation of counting is el temperatures 2%. The arrangement described permits routine determinations of specific activities to be performed in selfquenching counters down to l0/sup -5/ mu c/mmole C/sub 2/H/sub 2/. (auth)