Magnetochemistry, Volume 7, Issue 3 (March 2021) – 14 articles

A compact superconducting magnetic energy storage system (SMES) produced by Si micro fabrication technologies has been proposed to improve electricity storage volume density, w, in the sub-Wh/L range of conventional SMESs and to produce them at a low cost by mass production. In parallel with the experimental development reported previously, a series of trials was performed to estimate a feasible value of w based on the calculation of the magnetic field generated by the compact SMES by improving the calculation models step by step. In this work, the experimentally obtained magnetic flux density dependence of superconductive critical current density was taken into consideration for the first time in this series of trials, together with the additional improvement of the calculation models. The results of the estimation indicated that a compact SMES produced by the proposed concept can attain a w in the Wh/L range or more, ranking with or surpassing that of presently used capacitors. Full article

We have recently shown that a hybrid magnetic thin film with orthogonal anisotropies presenting weak stripe domains can achieve a high degree of controllability of its ferromagnetic resonance. This work explores the origin of the reconfigurability through micromagnetic simulations. The static domain structures which control the thin film resonance can be found under a deterministic applied field protocol. In contrast to similar systems reported, our effect can be obtained under low magnetic fields. We have also found through simulations that the spin wave propagation in the hybrid is nonreciprocal: two adjacent regions emit antiparallel spin waves along the stripe domains. Both properties convert the hybrid in a candidate for future magnonic devices at the nanoscale. Full article

Intermetallic compounds of Dy₂Fe₁₆Ga_1−xNb_x (x = 0.0 to 1.00) were synthesized by arc melting. Samples were investigated for structural, magnetic, and hyperfine properties using X-ray diffraction, vibration sample magnetometer, and Mossbauer spectrometer, respectively. The Rietveld analysis of room temperature X-ray diffraction data shows that all the samples were crystallized in Th₂Fe₁₇ structure. The unit cell volume of alloys increased linearly with an increase in Nb content. The maximum Curie temperature Tc ~523 K for x = 0.6 sample is higher than Tc = 153 K of Dy₂Fe₁₇. The saturation magnetization decreased linearly with increasing Nb content from 61.57 emu/g for x = 0.0 to 42.46 emu/g for x = 1.0. The Mössbauer spectra and Rietveld analysis showed a small amount of DyFe₃ and NbFe₂ secondary phases at x = 1.0. The hyperfine field of Dy₂Fe₁₆Ga_1−xNb_x decreased while the isomer shift values increased with the Nb content. The observed increase in isomer shift may have resulted from the decrease in s electron density due to the unit cell volume expansion. The substantial increase in Tc of thus prepared intermetallic compounds is expected to have implications in magnets used for high-temperature applications. Full article

A family of metal–organic frameworks with general formula {[Nd₂(ant)₂((NH₂)₂-bdc)(DMF)₄]·2DMF}_n (1) and {[Ln₂(ant)₂((NH₂)₂-bdc)(DMF)₄]·2DMF·2H₂O}_n (Ln = Tb (2), Ho (3), and Er (4)) has been obtained from reactions between 9,10-anthracenedicarboxylic (H₂ant) and 2,5-diaminoterephthalic ((NH₂)₂-H₂bdc) acids, and lanthanide ions in dimethylformamide (DMF). These lanthanide–organic frameworks (LnOFs) have been characterized, and their crystal structures have been elucidated by single crystal and powder X-ray diffraction methods (on the basis of a comparative refinement with similar structures), respectively for 1 and 2–4. All LnOFs present three-dimensional structures composed of dinuclear [Ln₂(µ-CO₂)₄] entities linked through both carboxylate ligands that yield open frameworks in which DMF and water molecules are located in the channels. Magnetic studies of these LnOFs have revealed slow relaxation of the magnetization for the Nd-based counterpart. The compounds also acknowledge relevant photoluminescence (PL) emissions in the visible (for the Tb-based homologue) and near-infrared (for the Nd- and Er-based compounds) regions. The strong green emission yielded by compound 2 at room temperature allows its study for photoluminescence (PL) sensing of various solvent molecules, finding a particular discrimination for acetone. Full article

Two series of Co/Co-oxide nanostructures have been synthesized by the co-precipitation method followed by different reduction and oxidation processes in an attempt to optimize their exchange bias (EB) properties. The samples are characterized by X-ray diffraction, scanning and transmission electron microscopy, and SQUID (superconducting quantum interference device) magnetometry. The two series differ with respect to their average Co core grain sizes: in one (the l-series), the size is ≈100 nm, and in the other (the s-series, obtained using lower synthesis temperatures than the l-series), it is ≈10 nm. In the l-series, progressive oxidation yields an increase in the EB field together with a reduction in Co core size. In contrast, progressive oxidation in the s-series results in growth of the Co-oxide fraction at the expense of the Co core upon oxidation, which is accompanied by a decrease in the EB effect that is attributed to an ordering of the ferromagnetic–antiferromagnetic interface and therefore a reduction of uncompensated spins density. These results illustrate how the interface details become relevant only for small enough ferromagnetic cores. Full article

Dinuclear and polymer complexes of 4-phenylpyridine (4-phpy), pyazine (pyz), and 4,4′-bipyridine (4,4′-bpy) were prepared by using cis-[Rh₂(4-Me-pf)₂(O₂CR)₂] (4-Me-pf^- =N,N’-bis(4-methylphenyl)formamidinate anion; R = CF₃ and CMe₃) as precursor dinuclear units. The dinuclear structures of cis-[Rh₂^II,II(4-Me-pf)₂(O₂CR)₂(4-phpy)₂] and cis-[Rh₂^II,III(4-Me-pf)₂(O₂CCMe₃)₂(4-phpy)₂]BF₄ and polymer structures of [Rh₂^II,II(4-Me-pf)₂(O₂CR)₂(L)]_n (L = pyz and 4,4′-bpy) were confirmed by X-ray crystal structure analyses. In these complexes, the lantern-type dinuclear core structures with cis-(2:2) arrangement of formamidinato (4-Me-pf^-) and carboxylato ligands are preserved with Rh–Rh distances of 2.44–2.47 Å, regardless of the difference in the axial ligand and oxidation state Rh₂^II,II or Rh₂^II,III. In the cyclic voltammograms (CVs) in CH₂Cl₂, the redox potentials for Rh₂^II,III/Rh₂^II,II were estimated as E_1/2 = 0.07 V and −0.28 V (vs. Fc⁺/Fc) for cis-[Rh₂(4-Me-pf)₂(O₂CCF₃)₂(4-phpy)₂] and cis-[Rh₂(4-Me-pf)₂(O₂CCMe₃)₂(4-phpy)₂], respectively, negatively shifted by 0.16 and 0.12 V compared with those of corresponding parent dinuclear complexes. The results were interpreted that the axial interaction with 4-phpy ligands makes the Rh₂^II,II core oxidized easily. The oxidized complex cis-[Rh₂(4-Me-pf)₂(O₂CCMe₃)₂(4-phpy)₂]BF₄ is paramagnetic, which was confirmed by effective magnetic moment value μ_eff = 1.90 μ_B at 300 K per Rh₂^II,III unit (S = 1/2). The polymer complexes [Rh₂(4-Me-pf)₂(O₂CR)₂(L)]_n (L = pyz and 4,4′-bpy) showed Type II gas-adsorption properties for N₂. Full article

In this study, [Co/Ni]₂/PtMn thin films with different PtMn thicknesses (2.7 to 32.4 nm) were prepared on Si/SiO₂ substrates. The post-deposition perpendicular magnetic field annealing (MFA) processes were carried out to modify the structures and magnetic properties. The MFA process also induced strong interlayer diffusion, rendering a less sharp interface between Co and Ni and PtMn layers. The transmission electron microscopy (TEM) lattice image analysis has shown that the films consisted of face-centered tetragonal (fct) PtMn (ordered by MFA), body-centered cubic (bcc) NiMn (due to intermixing), in addition to face-centered cubic (fcc) Co, Ni, and PtMn phases. The peak shift (2-theta from 39.9° to 40.3°) in X-ray diffraction spectra also confirmed the structural transition from fcc PtMn to fct PtMn after MFA, in agreement with those obtained by lattice images in TEM. The interdiffusion induced by MFA was also evidenced by the depth profile of X-ray photoelectron spectroscopy (XPS). Further, the magnetic properties measured by vibrating sample magnetometry (VSM) have shown an increased coercivity in MFA-treated samples. This is attributed to the presence of ordered fct PtMn, and NiMn phases exchange coupled to the ferromagnetic [Co/Ni]₂ layers. The vertical shift (M_shift = −0.03 memu) of the hysteresis loops is ascribed to the pinned spins resulting from perpendicular MFA processes. Full article

Nonvolatile, molecular multiferroic devices have now been demonstrated, but it is worth giving some consideration to the issue of whether such devices could be a competitive alternative for solid-state nonvolatile memory. For the Fe (II) spin crossover complex [Fe{H₂B(pz)₂}₂(bipy)], where pz = tris(pyrazol-1-yl)-borohydride and bipy = 2,2′-bipyridine, voltage-controlled isothermal changes in the electronic structure and spin state have been demonstrated and are accompanied by changes in conductance. Higher conductance is seen with [Fe{H₂B(pz)₂}₂(bipy)] in the high spin state, while lower conductance occurs for the low spin state. Plausibly, there is the potential here for low-cost molecular solid-state memory because the essential molecular thin films are easily fabricated. However, successful device fabrication does not mean a device that has a practical value. Here, we discuss the progress and challenges yet facing the fabrication of molecular multiferroic devices, which could be considered competitive to silicon. Full article

Magnetoelectric (ME) effect is a result of the interplay between magnetism and electric field and now, it is regarded as a principle that can be applied to the technique of controlling the antiferromagnetic (AFM) domain state. The ME-controlled AFM domain state can be read out by the magnetization of the adjacent ferromagnetic layer coupled with the ME AFM layer via exchange bias. In this technique, the reduction in the ME layer thickness is an ongoing challenge. In this paper, we demonstrate the ME-induced switching of exchange bias polarity using the 30-nm thick ME Cr₂O₃ thin film. Two typical switching processes, the ME field cooling (MEFC) and isothermal modes, are both explored. The required ME field for the switching in the MEFC mode suggests that the ME susceptibility (α₃₃) is not deteriorated at 30 nm thickness regime. The isothermal change of the exchange bias shows the hysteresis with respect to the electric field, and there is an asymmetry of the switching field depending on the switching direction. The quantitative analysis of this asymmetry yields α₃₃ at 273 K of 3.7 ± 0.5 ps/m, which is comparable to the reported value for the bulk Cr₂O_3. Full article

Copper(II) complexes with 2,3,4-trimethoxybenzoic acid (H234-tmbz) and 2,4,6-trimethoxybenzoic acid (H246-tmbz), [Cu₂(234-tmbz)₄(H₂O)₂] (6) and [Cu(246-tmbz)₂(µ-H₂O)₂(H₂O)₂]_n (7), were synthesized and characterized by elemental analysis, infrared and UV-vis spectra and temperature dependence of magnetic susceptibilities (1.9–300 K). The X-ray crystal structures revealed that the former 6 is a dinuclear cluster having syn-syn-bridged Cu₂(µ-234-tmbz)₄ core with Cu···Cu separation of 2.6009(7) Å, while the latter 7 is a µ-aqua-bridged chain molecule consisting of Cu(246-tmb)₂(µ-H₂O)₂(H₂O)₂ units with Cu···Cu separation of 4.1420(5) Å. Temperature dependence of magnetic susceptibilities showed that an antiferromagnetic interaction with 2J = −272 cm⁻¹ for 6 and a weak antiferromagnetic interaction with J = −0.21 cm⁻¹ for 7, between the two copper(II) ions. The adsorption isotherm of 6 showed Types I behavior having a 125.4 m²g⁻¹ of specific surface area. Full article

We present a new formulation of the dynamical matrix method for computing the magnetic normal modes of a large system, resulting in a highly scalable approach. The motion equation, which takes into account external field, dipolar and ferromagnetic exchange interactions, is rewritten in the form of a generalized eigenvalue problem without any additional approximation. For its numerical implementation several solvers have been explored, along with preconditioning methods. This reformulation was conceived to extend the study of magnetization dynamics to a broader class of finer-mesh systems, such as three-dimensional, irregular or defective structures, which in recent times raised the interest among researchers. To test its effectiveness, we applied the method to investigate the magnetization dynamics of a hexagonal artificial spin-ice as a function of a geometric distortion parameter following the Fibonacci sequence. We found several important features characterizing the low frequency spin modes as the geometric distortion is gradually increased. Full article

The magnetic suspension wind tunnel balance (MSBS) is an entirely new device for aerodynamic measurement, and it makes the best of the electromagnetic force to suspend the aircraft model in the wind tunnel without contact. Compared with conventional wind tunnel balance, it absolutely abandons the model support and airflow interference. Therefore, the aerodynamic measurement environment is more authentic and the aerodynamic measurement results are more accurate. The electromagnetic field in MSBS plays a major role in bearing the force of wind. The numerical computation and finite element numerical analysis are performed to investigate key factors of electromagnetic force under different conditions. The calculation results based on finite element method (FEM) have revealed that the diameter and the spacing of of the axial coil, the number of segments and the pitch angle of the suspension model are key factors of electromagnetic force. Based on the above key factors, the structure of the magnetic suspension balance is optimized to maximize the electromagnetic force under multiple constraints. Full article

The main objective of this research was to investigate the influence of curing temperature on the phase transformation, mechanical properties, and microstructure of the as-cured and sintered kaolin-ground granulated blast furnace slag (GGBS) geopolymer. The curing temperature was varied, giving four different conditions; namely: Room temperature, 40, 60, and 80 °C. The kaolin-GGBS geopolymer was prepared, with a mixture of NaOH (8 M) and sodium silicate. The samples were cured for 14 days and sintered afterwards using the same sintering profile for all of the samples. The sintered kaolin-GGBS geopolymer that underwent the curing process at the temperature of 60 °C featured the highest strength value: 8.90 MPa, and a densified microstructure, compared with the other samples. The contribution of the Na₂O in the geopolymerization process was as a self-fluxing agent for the production of the geopolymer ceramic at low temperatures. Full article

Nickel/carbon composites were synthesized by the carbonization method with the use of the steam-gas activation technique. Beet pulp and corn stems were used as initial raw materials for composites synthesis. The obtained substances were analyzed by means of scanning electron microscopy, X-ray diffraction analysis, and magnetic properties investigation. The presence of nickel in synthesized composites was estimated during analysis, and the average particle size of nickel was determined as well. The specific surface area of the obtained samples was measured by the technique of Methylene blue sorption from the aqueous solution. The results of the performed investigation demonstrate clearly the promising application of synthesized composites as magneto-active sorbents, which could be easily separated from water solutions. Full article