Magnetochemistry, Volume 8, Issue 4 (April 2022) – 11 articles

Ceramic–polymer paste-injection molding is demonstrated as a facile fabrication route for barium hexaferrite magnets. Interestingly, these low-density (1.90–2.35 g/cm³) magnets exhibit substantial coercivity of 3868–4002 Oe. When ceramic paste without polymeric additives is used, reduced coercivity and slightly increased magnetizations are obtained from a magnet with the density of 2.55 g/cm³. Their magnetizations are also higher than those obtained from compactions of sol–gel-derived powders. For compact magnets (3.46–3.77 g/cm³), the DI water addition results in a slightly higher magnetization but lower coercivity than dry-pressed magnets. Compactions into disk and bar magnets give rise to comparable magnetic properties. The morphological characterizations reveal smaller barium hexaferrite particles leading to larger coercivity, and the density and shape of magnets have a less pronounced effect. Full article

This is a simple but realistic microscopic theory of spontaneous spin reorientation in rare-earth perovskites, orthoferrites RFeO${}_3$ and orthochromites RCrO${}_3$, induced by the 4f-3d interaction, namely, the interaction of the well-isolated ground-state Kramers doublet or non-Kramers quasi-doublet of the 4f ion with an effective magnetic field induced by 3d sublattice. Both the temperature and the nature of the spin-reorientation transition are the result of competition between the second- and fourth-order spin anisotropy of the 3d sublattice, the crystal field for 4f ions, and 4f-3d interaction. Full article

A quantum-chemical analysis of the effect of a constant magnetic field on radical formation in the processes of chain oxidation of organic compounds by molecular oxygen is presented. The calculation of the total electronic energies and thermodynamic functions of the compounds involved in the reactions was performed by the density functional method with the hybrid exchange-correlation functional of Becke, Lee, Yang and Parr DFT B3LYP/6-311G** using the NWChem software package. The effect of the magnetic field on the individual stages of chain oxidation is associated with the evolution of radical pairs. It is assumed that the dipole–dipole interaction in a radical pair is not averaged by the diffusion of radicals and should be taken into account. To a large extent, the magnetic field effect (MFE) value is influenced by the ratio between the relaxation time of the oscillatory-excited state in the radical pair (t_vib) and the relaxation time of the inter-combination transitions (t_st). Although the developed technique refers to liquid-phase reactions, it can be used to study the MFE for oxidation of biologically significant compounds in multiphase systems, such as micelles, liposomes and membranes. Full article

Pulse dipolar electron paramagnetic resonance spectroscopy (PDS) is continuously furthering the understanding of chemical and biological assemblies through distance measurements in the nanometer range. New paramagnets and pulse sequences can provide structural insights not accessible through other techniques. In the pursuit of alternative spin centers for PDS, we synthesized a low-spin Co^II complex bearing a nitroxide (NO) moiety, where both the Co^II and NO have an electron spin S of 1/2. We measured Co^II-NO distances with the well-established double electron–electron resonance (DEER aka PELDOR) experiment, as well as with the five- and six-pulse relaxation-induced dipolar modulation enhancement (RIDME) spectroscopies at Q-band frequencies (34 GHz). We first identified challenges related to the stability of the complex in solution via DEER and X-ray crystallography and showed that even in cases where complex disproportionation is unavoidable, Co^II-NO PDS measurements are feasible and give good signal-to-noise (SNR) ratios. Specifically, DEER and five-pulse RIDME exhibited an SNR of ~100, and while the six-pulse RIDME exhibited compromised SNR, it helped us minimize unwanted signals from the RIDME traces. Last, we demonstrated RIDME at a 10 μM sample concentration. Our results demonstrate paramagnetic Co^II to be a feasible spin center in medium magnetic fields with opportunities for PDS studies involving Co^II ions. Full article

Magnetic force microscopy (MFM) enables to characterize magnetic properties with submicron (nanoscale) resolution and without much demand on sample surface preparation. MFM can operate in a wide range of temperatures and environmental conditions, that is, vacuum, liquid, or air, therefore this technique has already become the most common tool used to characterize variety of magnetic materials ranging from ferromagnetic thin films and 2D materials to biomedical and/or biological materials. The purpose of this review is to provide a summary of MFM basic fundamentals in the frame of other related methods and, correspondingly, a brief overview of physics and chiefly biomedical as well as biological applications of MFM. Full article

In this study, we prepared a streptavidin magnetic bead based on graphene-coated iron nitride magnetic beads (G@FeN-MB) and tried to use it for the enrichment of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). The outer shell of our magnetic bead was wrapped with multiple graphene sheets, and there is no report on the application of graphene to the magnetic-bead-coating material. First, the graphene shell of G@FeN-MB was oxidized by a modified Hummer method so as to generate the carboxyl groups required for the coupling of streptavidin (SA) on the surface of the magnetic beads. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM) were used to characterize the oxidized G@FeN-MB (GO@FeN-MB). Streptavidin was then linked to the surface of the GO@FeN-MB by coupling the amino of the streptavidin with the carboxyl on the magnetic beads by carbodiimide method; thus, the streptavidin magnetic beads (SAMBs) were successfully prepared. To prove the practicality of the SAMBs, biotinylated SARS-CoV-2 S1 antibody was linked with it to respectively capture SARS-CoV-2 Spike-protein-coupled polystyrene beads (S-PS) and pseudovirus with S-protein expressed. Microplate reader and fluorescence microscope results show that the SAMBs can effectively enrich viruses. In conclusion, the preparation of SAMBs with G@FeN-MB is feasible and has potential for application in the field of virus enrichment. Full article

The corrosion of metals is a major problem of modern societies, demanding new technologies and studies to understand and minimize it. Here we evaluated the effect of a magnetic field (B) on the corrosion of copper in aqueous HCl solution under open circuit potential. The corrosion product, Cu²⁺, is a paramagnetic ion and its concentration in the solution was determined in real time in the corrosion cell by time-domain NMR relaxometry. The results show that the magnetic field (B = 0.23 T) of the time-domain NMR instrument reduces the corrosion rate by almost 50%, in comparison to when the corrosion reaction is performed in the absence of B. Atomic force microscopy and X-ray diffraction results of the analysis of the corroded surfaces reveal a detectable CuCl phase and an altered morphology when B is present. The protective effect of B was explained by magnetic forces that maintain the Cu²⁺ in the solution/metal interface for a longer time, hindering the arrival of the new corrosive agents, and leading to the formation of a CuCl phase, which may contribute to the rougher surface. The time-domain NMR method proved to be useful to study the effect of B in the corrosion of other metals or other corrosive liquid media when the reactions produce or consume paramagnetic ions. Full article

An experimental study of the rotational magnetocaloric effect in Ni(en)(H₂O)₄SO₄∙2H₂O (en = ethylenediamine) single crystal is presented. The study was carried out at temperatures above 2 K and was associated with adiabatic crystal rotation between the easy plane and hard axis in magnetic fields up to 7 T. The magnetocaloric properties of the studied system were investigated by isothermal magnetization measurement. The experimental observations were completed with ab initio calculations of the anisotropy parameters. A large rotational magnetic entropy change ≈12 Jkg⁻¹K⁻¹ and ≈16.9 Jkg⁻¹K⁻¹ was achieved in 5 T and 7 T, respectively. The present study suggests a possible application of this material in low-temperature refrigeration since the adiabatic rotation of the single crystal in 7 T led to a cooldown of the sample from the initial temperature of 4.2 K down to 0.34 K. Finally, theoretical calculations show that S = 1 Ni(II)-based systems with easy-plane anisotropy can have better rotational magnetocaloric properties than costly materials containing rare-earth elements in their chemical structures. Full article

Magnetic gold nanoparticles (mGNP) have become a great interest of research for nanomaterial scientists because of their significant magnetic and plasmonic properties applicable in biomedical applications. Various synthetic approaches and surface modification techniques have been used for mGNP including the most common being the coprecipitation, thermal decomposition, and microemulsion methods in addition to the Brust Schiffrin technique, which involves the reduction of metal precursors in a two-phase system (water and toluene) in the presence of alkanethiol. The hybrid magnetic–plasmonic nanoparticles based on iron core and gold shell are being considered as potential theranostic agents. In this critical review, in addition to future works, we have summarized recent developments for synthesis and surface modification of mGNP with their applications in modern biomedical science such as drug and gene delivery, bioimaging, biosensing, and neuro-regeneration, neuro-degenerative and arthritic disorders. This review includes techniques and biological applications of mGNP majorly based on research from the previous six years. Full article

Computational electron paramagnetic resonance (EPR) spectroscopy is an important field of applied quantum chemistry that contributes greatly to connecting spectroscopic observations with the fundamental description of electronic structure for open-shell molecules. However, not all EPR parameters can be predicted accurately and reliably for all chemical systems. Among transition metal ions, Cu(II) centers in inorganic chemistry and biology, and their associated EPR properties such as hyperfine coupling and g-tensors, pose exceptional difficulties for all levels of quantum chemistry. In the present work, we approach the problem of Cu(II) g-tensor calculations using double-hybrid density functional theory (DHDFT). Using a reference set of 18 structurally and spectroscopically characterized Cu(II) complexes, we evaluate a wide range of modern double-hybrid density functionals (DHDFs) that have not been applied previously to this problem. Our results suggest that the current generation of DHDFs consistently and systematically outperform other computational approaches. The B2GP-PLYP and PBE0-DH functionals are singled out as the best DHDFs on average for the prediction of Cu(II) g-tensors. The performance of the different functionals is discussed and suggestions are made for practical applications and future methodological developments. Full article