Site‐selective substitution and resulting magnetism in arc‐melted perovskite ATiO₃₋δ (A = Ca, Sr, Ba)

Magnetic properties in perovskite titanates ATiO₃₋δ (A = Ca, Sr, Ba) were investigated before and after arc melting. Crystal structure analysis was conducted by powder synchrotron X‐ray diffraction with Rietveld refinements. Quantitative chemical element analysis was carried out by X‐ray photoelectron spectroscopy. Magnetic measurements were conducted by vibrating sample magnetometer and X‐ray magnetic circular dichroism (XMCD). The magnetic properties are found to be affected by impurities of 3d elements such as Fe, Co, and Ni. Depending on the composition and crystal structure, the occupation of the magnetic ions in perovskite titanates is selectively varied, which is interpreted to be the origin of the different magnetic behaviors in arc‐melted perovskite titanates ATiO₃₋δ (A = Ca, Sr, Ba). In addition, both formation of oxygen vacancies and the reduction of Ti⁴⁺ to Ti³⁺ during arc‐melting also play a role as proven by XMCD. Nevertheless, preferential site occupation of magnetic impurities is dominant in the magnetic properties of arc‐melted perovskite ATiO₃₋δ (A = Ca, Sr, Ba)

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The magnetocaloric effect (MCE), which is the reversible temperature change of magnetic materials due to an applied magnetic field, occurs largely in the vicinity of the magnetic phase transition temperature. This phenomenon can be used to induce magnetic refrigeration, a viable, energy-efficient solid-state cooling technology. Recently, Metal-organic frameworks (MOFs), due to their structural diversity of tunable crystalline pore structure and chemical functionality, have been studied as good candidates for magnetic refrigeration materials in the cryogenic region. In cryogenic cooling applications, MCE using MOF can have great potential, and is even considered comparable to conventional lanthanum alloys and magnetic nanoparticles. Owing to the presence of large internal pores, however, MOF also exhibits the drawback of low magnetic density. To overcome this problem, therefore, recent reports in literature that achieve high magnetic entropy change using a dense structure formation and ligand tuning are introduced

Heterometallic Gd-Dy Formate Frameworks for Enhanced Magnetocaloric Properties

Lanthanide-based metal-organic frameworks (MOFs) have great potential as magnetic refrigerants under cryogenic conditions and are comparable to conventional alloys and magnetic nanoparticles. In particular, MOFs with Gd3+ ions behave as excellent magnetic refrigerants because of their large spin ground states. However, the major drawback of Gd3+-based MOFs is that they are not affected by the ligand material owing to the excessively large spin-only magnetic moment; therefore, their application is limited to the cryogenic region in the magnetic cooling field. In this study, we report the magnetic properties and magnetocaloric effect (MCE) resulting from heterogenized MOFs obtained from the reaction of Gd3+ and Dy3+ ions and their varied molar composition with the formate ligand. For GdxDy1-x-(HCOO)3, where 0 < x < 1, the isothermal magnetic entropy change (Delta Sm) increased with the increase in the fraction of Gd in the heterogenized MOFs. Meanwhile, with increasing Dy contents, the maximum peak temperature of Delta Sm is shifted to a higher temperature while preserving a relatively high Delta Sm value of 22.35 J center dot kg-1 K-1 at T = 7 K for an applied field change (Delta H) of 7 T despite the anisotropy and crystalline electric field effects. Furthermore, it was confirmed that the samples with a Dy content of 75% or more maintained the Delta Sm operating temperature longer. Therefore, the current approach of including Dy3+ ions in lanthanide compounds provides the possibility of further extending the operating temperature of magnetic cooling materials from cryogenic temperatures

Magnetic Behavior of Single-Chain Magnets in Metal Organic Frameworks CPO-27-Co

Nanoscale molecular magnets of highly porous and crystalline Metal-organic frameworks (MOFs) have attracted increasing interest in recent years because of their potential application in nano-magnetic device or matrices for encapsulating of a large variety of substances. For that, a fundamental understanding of its origin of magnetism in MOFs would be essential and will provide useful insight for intelligent design of their electromagnetic properties (e.g., single-chain magnets, single-ion magnets or single-molecule magnets). Herein, we experimentally investigate the magnetic property of hexagonal 1-D channel MOFs composed of metal(cobalt) cluster connected with organic linker (so-called CPO-27-Co). Through a fundamental physisorption analysis, in-depth magnetic studies by use of superconducting quantum interference device (SQUID) magnetometry, and theoretically by the application of an isotropic Heisenberg Hamiltonian, we found a magnetic configuration of CPO-27-Co exhibiting both a weak ferro-and antiferromagnetism with cobalt magne-tocrystalline anisotropy. Interestingly, when a magnetic configuration of cobalt cluster (instead of ionic cobalt inside cluster of CPO-27) are considered, the inter Co cluster interaction exhibits superparamagnetic behavior while inside of Co cluster (ionic Co-Co interaction) is dominated by a weak ferromagnetic component

A metal-doped flexible porous carbon cloth for enhanced CO2/CH4 separation

From a practical perspective, carbon adsorbents are mostly prepared to form functional components from carbon powder by using binders or hot pressing, which results in the degradation of its textural properties. Thus, the flexible activated porous carbon cloths have attracted increasing interest in recent years because these are easier to handle than powder and granular materials when constructing more conformable and efficient membrane device designs. In this work, a metal-doped flexible porous carbon cloth with outstanding CO2/CH4 selectivity was developed. Carbon cloth deposited with spherical porous carbon nanoparticles and doped with nitrogen was prepared from viscose rayon cloth decorated with polypyrrole nanoparticles. Then, this flexible carbon cloth was doped with nickel. Two samples with moderate Ni content (7 and 10 wt%) were used to investigate the fundamental magnetic properties and CO2/CH4 separation performance. These two samples have both antiferromagnetic and ferromagnetic properties, and it is originated from the different ratios of NiO and pure Ni. The Ni (O) contents in the flexible carbon cloth influenced the CO2 and CH4 sorption properties and separation performance as well. Based on the ideal adsorbed solution theory (IAST) model, the developed metal-decorated flexible activated carbon cloth shows 13 times better separation performance than Ni-free sample and achieved CO2/CH4 (1:1) selectivity of 369.5 and 70.7 at 20 and 1 bar, respectively, at 298 K, which is among the highest selectivity values reported for porous carbon materials so far

Metal Organic Frameworks as Tunable Linear Magnets

Due to the tunable magnetic exchange interactions between nearest neighbor moment carriers by exchange linker length or metal ions, metal-organic frameworks (MOFs) in general, and MOF-74 in particular, have promising magnetic properties for novel industrial applications such as magnetic switchers, sensors, and multifunction devices in molecular spintronics. Hence, MOF-74-M [M-2(C8H2O6)] (M = Fe, Co, Ni) possessing a 1D channel is prepared and experimentally examined to describe its diverse magnetic properties, depending on the metal cluster used. By exchanging the metal ion (Fe, Co, and Ni) in the metal cluster, the magnetic state of MOF-74 is changed from ferromagnetic or antiferromagnetic to superparamagnetic. This phenomenon can be also confirmed by alternating current (AC)/direct current (DC) magnetic susceptibility and field-dependent magnetization. Therefore, the intrinsic magnetic properties of MOF-74-M such as blocking temperature, Neel temperature, and interaction energy between metals in magnetic susceptibility tests can be elucidated. Moreover, the superparamagnetic behavior in this system is also observed inM-Hcurves and magnetic dynamics as single-chain magnets. It is believed that MOF-74-M can be utilized for applications in which superconducting materials are required (e.g., memory-storage devices)