Change In Magnetic Interaction Research Articles

Magnetic Supraparticles Capable of Recording High‐Temperature Events

AbstractSuperparamagnetic iron oxide nanoparticles (SPIONs) are prone to oxidation at elevated temperatures (>300 °C) and lose their magnetizability upon transition from magnetite/maghemite (γ‐Fe2O3 / Fe3O4) to hematite (α‐Fe2O3). Silica (SiO2) shells can effectively prevent this undesired effect up to ≈1000 °C. Herein, the study shows how to utilize SPIONs with varying SiO2 shell thickness and thus, different oxidation susceptibility, and how to combine them in micrometers sized assemblies – so‐called supraparticles (SPs), to create a structurally emerging magnetic temperature recording functionality. The desired oxidation of non or weakly‐protected SPIONs within SPs upon temperature events reduces dipole–dipole interactions of well‐protected SPIONs in the confined SP entity. The resulting change of magnetic interactions therefore contains information on the thermal history of the SP, which can be spectrally read out via magnetic particle spectroscopy within seconds. Their working range can be tuned from 400 to 1000 °C on two independent structural hierarchy levels, namely the SiO2 shell thickness and the freely selectable ratios of different building blocks in the SP. The application of such SPs as particulate additives for magnetic recording of high‐temperature events, especially relevant in metal, alloy, and ceramic processing, representing a yet unexplored and optically‐independent option for bulk temperature recording is proposed.

Magnetic in situ determination of surface coordination motifs by utilizing the degree of particle agglomeration

Most analytical techniques used to study the surface chemical properties of superparamagnetic iron oxide nanoparticles (SPIONs) are barely suitable for in situ investigations in liquids, where SPIONs are mostly applied for hyperthermia therapy, diagnostic biosensing, magnetic particle imaging or water purification. Magnetic particle spectroscopy (MPS) can resolve changes in magnetic interactions of SPIONs within seconds at ambient conditions. Herein, we show that by adding mono- and divalent cations to citric acid capped SPIONs, the degree of agglomeration can be utilized to study the selectivity of cations towards surface coordination motifs via MPS. A favored chelate agent, like ethylenediaminetetraacetic acid (EDTA) for divalent cations, removes cations from coordination sites on the SPION surface and causes redispersion of agglomerates. The magnetic determination thereof represents what we call a “magnetically indicated complexometric titration”. The relevance of agglomerate sizes for the MPS signal response is studied on a model system of SPIONs and the surfactant cetrimonium bromide (CTAB). Analytical ultracentrifugation (AUC) and cryogenic transmission electron microscopy (cryo-TEM) reveal that large micron-sized agglomerates are required to significantly change the MPS signal response. With this work, a fast and easy-to-use characterization method to determine surface coordination motifs of magnetic nanoparticles in optically dense media is demonstrated.

Recording Temperature with Magnetic Supraparticles.

Small-sized temperature indicator additives autonomously record temperature events via a gradual irreversible signal change. This permits, for instance, the indication of possible cold-chain breaches or failure of electronics but also curing of glues. Thus, information about the materials' thermal history can be obtained upon signal detection at every point of interest. In this work, maximum-temperature indicators with magnetic readout based on micrometer-sized supraparticles (SPs) are introduced. The magnetic signal transduction is, by nature, independent of the materials' optical properties. This facilitates the determination of valuable temperature information from the inside, that is, the bulk, even of dark and opaque macroscopic objects, which might differ from their surface. Compared to state-of-the-art optical temperature indicators, complementary magnetic readout characteristics ultimately expand their applicability. The conceptualized SPs are hierarchically structured assemblies of environmentally friendly, inexpensive iron oxide nanoparticles and thermoplastic polymer. Irreversible structural changes, induced by polymer softening, yield magnetic interaction changes within and between the hierarchic sub-structures, which are distinguishable and define the temperature indication mechanism. The fundamental understanding of the SPs' working principle enables customization of the particles' working range, response time, and sensitivity, using a toolbox-like manufacturing approach. The magnetic signal change is detected self-referenced, fast, and contactless.

Critical behavior of the second-order magnetic transition in LaFe11.7-xCoxSi1.3C0.15 alloys

The critical behavior of NaZn13-type LaFe11.7-xCoxSi1.3C0.15 (x = 0.7, 0.8, and 0.9) alloys is investigated in this study. For LaFe11.7-xCoxSi1.3C0.15 (x = 0.7, 0.8, and 0.9) alloys, the values of the maximum magnetic entropy change, Δ|SM|max, are proportional to the magnetic field, H2/3. The reliable critical exponents, β, γ and δ are calculated. The value of γ for LaFe11.7-xCoxSi1.3C0.15 obtained by using the modified Arrott plot method is 1.013, 1.023, and 0.980 for x = 0.7, 0.8, and 0.9, respectively. With an increase in the Co content, the LaFe11.7-xCoxSi1.3C0.15 (x = 0.7, 0.8, 0.9) alloys become far away from the first-order phase transition and the magnetic interaction changes from short-range to long-range below the Curie temperature.

Magnetic field controlled behavior of magnetic gels studied using particle-based simulations

Abstract This contribution provides an overview of the study of soft magnetic materials using particle-based simulation models. We focus in particular on systems where thermal fluctuations are important. As a basis for further discussion, we first describe two-dimensional models which demonstrate two deformation mechanisms of magnetic gels in a homogeneous field. One is based on the change of magnetic interactions between magnetic particles as a response to an external field; the other is the result of magnetically blocked particles acting as cross-linkers. Based on the qualitative behavior directly observable in the two-dimensional models, we extend our description to three-dimensions. We begin with particle-cross-linked gels, as for those, our three-dimensional model also includes explicitly resolved polymer chains. Here, the polymer chains are represented by entropic springs, and the deformation of the gel is the result of the interaction between magnetic particles. We use this model to examine the influence of the magnetic spatial configuration of magnetic particles (uniaxial or isotropic) on the gel’s magnetomechanical behavior. A further part of the article will be dedicated to scale-bridging approaches such as systematic coarse-graining and models located at the boundary between particle-based and continuum modeling. We will conclude our article with a discussion of recent results for modeling time-dependent phenomena in magnetic-polymer composites. The discussion will be focused on a simulation model suitable for obtaining AC-susceptibility spectra for dilute ferrofluids including hydrodynamic interactions. This model will be the basis for studying the signature of particle–polymer coupling in magnetic hybrid materials. In the long run, we aim to compare material properties probed locally via the AC-susceptibility spectra to elastic moduli obtained for the system at a global level.

Crossover from Antiferromagnetic to Ferromagnetic Exchange Coupling in a New Family of Bis-(μ-phenoxido)dicopper(II) Complexes: A Comprehensive Magneto-Structural Correlation by Experimental and Theoretical Study.

Five neutral bis(μ-phenoxido)dicopper(II) complexes, [Cu2(LMe,Me,Me)2] (1), [Cu2(LMe,Me,Et)2]·CH2Cl2 (2), [Cu2(Li-Pr,i-Pr,i-Pr)2]·2H2O (3), [Cu2(Lt-Bu,Me,i-Pr)2] (4), and [Cu2(Lt-Bu,t-Bu,i-Pr)2]·H2O (5) have been synthesized and characterized by single-crystal X-ray diffraction analyses, magnetic studies, and density functional theory (DFT) calculations, in which the ligands [H2LMe,Me,Me = N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N′,N′-dimethylethylene-1,2-diamine, H2LMe,Me,Et = N,N-bis(2-hydroxy-3,5-dimethylbenzyl)-N′,N′-dimethylethylene-1,2-diamine, H2Li-Pr,i-Pr,i-Pr = N,N-bis(2-hydroxy-3,5-diisopropylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine, H2Lt-Bu,Me,i-Pr = N,N-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine, and H2Lt-Bu,t-Bu,i-Pr = N,N-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N′,N′-diisopropylethylene-1,2-diamine] contain the same [O,N,N,O]-donor atoms combination but differ in substituents at phenol rings and at an amino nitrogen atom. The effect of these remote substituents on the nature of exchange coupling interactions (ferromagnetic vs antiferromagnetic) between the copper(II) ions has been investigated. The average Cu–O–Cu angle, Cu–O–Cu–O torsion angle, and Cu···Cu separation in 1–5 are varied systematically by these remote ligand substituents in the range 98.6–83.3°, 26.0–46.5°, and 2.982–2.633 Å, respectively. As a result, the intramolecular spin–spin coupling in these complexes are changing gradually from a strong antiferromagnetic (J = −395 cm–1, where Ĥ = −JŜ1Ŝ2) to a moderate ferromagnetic (J = +53.2 cm–1) regime. The crossover angle at which the magnetic interaction changes from antiferromagnetic to ferromagnetic (J = 0) is determined to be ca. 87° for this series of dicopper(II) complexes. DFT calculations support the experimentally determined crossover angle and disclose various magneto–structural correlations in the series 1–5.

Competing chemistry and structural anisotropy-driven magnetic ground state of GdPt5Al2 as compared to GdPd5Al2

Abstract Magnetic susceptibility measurements on RT5Al2 (R = Gd, Y; T = Pd, Pt) show that while GdPd5Al2 orders antiferromagnetically at T N = 7 K, GdPt5Al2 does not order down to 2 K. This result is in confirmation of earlier published reports suggesting that although RPd5Al2 exhibit antiferromagnetic transitions, no magnetic ordering has been found down to 2 K for isoelectronic RPt5Al2 compounds. To understand the evolution of magnetic ordering from RPd5Al2 to RPt5Al2, band structure analysis has been carried out in Gd and Y based analogues of these two series. Band structure calculations show enhanced two dimensionality of the conduction electrons, triggered by the enhanced anisotropic behavior of the Pt-5d electrons in GdPt 5 Al 2 . More extended nature of Pt-5d electrons, compared to the Pd-4d electrons, might be the reason behind a change in the magnetic interaction from Pd to Pt-based compounds. We also note that tetragonal anisotropy and spin-orbit coupling do not have primary roles in such a change in magnetic interaction.

Radical-Doped Metal-Organic Framework: Route to Nanoscale Defects and Magnetostructural Functionalities.

Nanosized structural defects in metal-organic frameworks (MOFs) attract growing attention and often remarkably enhance functional properties of these materials for various applications. In this work, a series of MOFs [Cu2(TPTA)1- x(BDPBTR) x] (H4TPTA, [1,1':3',1″-terphenyl]-3,3'',5,5''-tetracarboxylic acid; H4BDPBTR, 1,3-bis(3,5-dicarboxyphenyl)-1,2,4-benzotriazin-4-yl radical)) with a new stable radical linker doped into the structure has been synthesized and investigated using Electron Paramagnetic Resonance (EPR). Mixed linkers H4TPTA and H4BDPBTR were used to bridge copper(II) paddle-wheel units into a porous framework, where H4BDPBTR is the close structural analogue of H4TPTA. MOFs with various x = 0-0.4 were investigated. EPR studies indicated that the radical linker binds to the copper(II) units differently compared to diamagnetic linker, resulting in the formation of nanosized structural defects. Moreover, remarkable kinetic phenomena were observed upon cooling of this MOF, where slow structural rearrangements and concomitant changes of magnetic interactions were induced. Thus, our findings demonstrate that doping of structurally mimicking radical linkers into MOFs represents an efficient approach for designing target nanosized defects and introducing new magnetostructural functionalities for a variety of applications.

Temperature blocking and magnetization of magnetoactive elastomers

Abstract The magnetization of a magnetoactive elastomer (MAE) with microparticles of soft magnetic carbonyl iron embedded in a highly elastic matrix has been studied. It is shown that at high temperatures its magnetization curve has the form of a specific hysteresis loop. This hysteresis is attributed to the influence of displacement of magnetized particles in the elastically soft elastomer matrix under the effect of magnetic forces, leading to the change of magnetic interaction between the particles. In this case, there is a maximum in the field dependence of the magnetic susceptibility, the occurrence of which has been associated with the competition between re-arrangement of particles, when they are displaced in a magnetic field, and saturation of particles’ magnetization. When the MAE is cooled below approximately 225 K, both the magnetic hysteresis and the maximum in the field dependence of the magnetic susceptibility disappear. When the MAE material is cooled below the solidification temperature of the elastomer matrix, the displacements of the magnetic particles during magnetization are blocked by the rigid matrix. The magnetization reversal of the MAE is reversible. This means that the shape of subsequent magnetization loops remains constant and the sample returns into the initial non-magnetized state after the magnetic field is turned off.

Mechanistic insights into the interaction between energetic oxygen ions and nanosized ZnFe2O4: XAS-XMCD investigations.

The interactions of energetic ions with multi-cation compounds and their consequences in terms of changes in the local electronic structure, which may facilitate intriguing hybridization between O 2p and metal d orbitals and magnetic ordering, are the subject of debate and require a deep understanding of energy transfer processes and magnetic exchange mechanisms. In this study, nanocrystals of ZnFe2O4 were exposed to O7+ ions with an energy of 100 MeV to understand, qualitatively and quantitatively, the metal-ligand field interactions, cation migration and magnetic exchange interactions by employing X-ray absorption fine structure measurements and X-ray magnetic circular dichroism to get deeper mechanistic insights. Nanosized zinc ferrite nanoparticles (NPs) with a size of ∼16 nm synthesized in the cubic spinel phase exhibited deterioration of the crystalline phase when 100 MeV O7+ ions passed through them. However, the size of these NPs remained almost the same. The behaviour of crystal deterioration is associated with the confinement of heat in this interaction. The energy confined inside the nanoparticles promotes cation redistribution as well as the modification of the local electronic structure. Prior to this interaction, almost 42% of Zn2+ ions occupied AO4 tetrahedra; however, this value increased to 63% after the interaction. An inverse effect was observed for metal ion occupancies in BO6 octahedra. The L-edge spectra of Fe and Zn reveal that the spin and valence states of the metal ions were not affected by this interaction. This effect is also supported by K-edge measurements for Fe and Zn. The t2g/eg intensity ratio in the O K-edge spectra decreased after this interaction, which is associated with detachment of Zn2+ ions from the lattice. The extent of hybridization, as estimated from the ratio of the post-edge to the pre-edge region of the O K-edge spectra, decreased after this interaction. The metal-oxygen and metal-metal bond lengths were modified as a result of this interaction, as determined from extended X-ray absorption fine structure measurements. These measurements further support the observation of cation migration from AO4 tetrahedra to AO6 octahedra and vice versa. The Fe L-edge magnetic circular dichroism spectra indicate that Fe3+ ions occupying sites in AO4 tetrahedra and BO6 octahedra exhibited antiferromagnetic-like ordering prior to this interaction. The NPs that interacted with energetic O ions displayed a different kind of magnetic ordering.

Adsorbate-Induced Changes in Magnetic Interactions in Fe2(dobdc) with Adsorbed Hydrocarbon Molecules

Metal–organic frameworks, which are a special case of coordination polymers, form a class of materials with numerous applications due to their high porosities and large internal surface areas and in some cases also due to paramagnetic metal ions. A family of such materials of general formula M2(dobdc) (where M is a divalent metal ion, and dobdc4– is 2,5-dioxido-1,4-benzenedicarboxylate) has attracted considerable attention for gas separation, catalysis, and magnetism. In this work, we explore the magnetic properties of a member of this family, Fe2(dobdc), both in the activated form (bare MOF after solvent removal) and when hydrocarbons are bound to the open coordination sites of the metal. We report quantum mechanical electronic structure calculations using both cluster models and periodic models, and we compare our results to previously reported theoretical studies. We find that hydrocarbon adsorption only mildly affects the isotropic couplings but that the isotropic magnetic couplings obtained with hybrid exchange–correlation functionals for the cluster models were found to be in good agreement with the experimental values, and the ones obtained using local exchange–correlation functionals with empirical Coulomb and exchange integrals for the periodic models were also found to agree well with experiments. Furthermore, local density functionals with empirical Coulomb and exchange integrals for periodic models are found to give good agreement with the experimental result that the adsorption of ethylene changes the magnetic ordering. We also used second-order n-electron valence state perturbation theory and contracted spin–orbit configuration interaction to study the role of adsorption on the crystal-field splitting of the quintet manifold and on the single-ion anisotropy of the iron center.

Theoretical and computational investigation of meta-phenylene as ferromagnetic coupler in nitronyl nitroxide diradicals

We predict the magnetic exchange coupling constant (J) for 27 m-phenylene-based nitronyl nitroxide (NN) diradicals with nine different substituents in three unique (common ortho, ortho–meta and common meta) positions on the coupler unit by using the broken-symmetry density functional methodology. For all investigated diradicals, J values are computed using B3LYP, B3LYP-D3 and M06-2X functionals with 6–311+G(d,p) basis set. The JM06-2X value is larger than JB3LYP and closer to the observed value for the unsubstituted species. Substitutions at common ortho position always produce a greater angle of twist between the spin source and the coupler units. When the twist angle is very large, the nature of intramolecular magnetic interaction changes from ferromagnetic to antiferromagnetic. In these cases, the coupler–NN bond order becomes small. Substitution at the common meta position of m-phenylene in the diradical has little steric and hydrogen-bonding effects. Electron-withdrawing groups reveal a specific trend for single-atom substitution. An ortho substitution generally decreases J and a meta substitution always increases J with a decreasing −I effect. Variation of J with planarity as well as Hammett constant is investigated. The nucleus-independent chemical shift value is found to decrease from the corresponding mono-substituted phenyl derivatives. The dependence of J on these factors is explored.

Influence of local structure on magnetic properties of layered cobaltites PrBaCo2O5+δ, δ > 0.5

The effect of local structure on the magnetic and transport properties of the layered perovskites has been investigated. The samples PrBaCo2O5+δ, (δ = 0.80 and 0.67) crystallize in the same 112 type tetragonal structure but have different magnetic ground states. Extended x-ray absorption fine structure (EXAFS) spectroscopy has been employed to explain the changes in magnetic interactions based on the rotation and tilting of CoO polyhedra in these oxygen-rich double perovskites.

Magnetic interactions in iron superconductors studied with a five-orbital model within the Hartree-Fock and Heisenberg approximations

We have analyzed the magnetic interactions of a five orbital model for iron superconductors treated both within Hartree-Fock and Heisenberg approximations. We have found that the exchange constants depend non-trivially on the Fe-As-Fe angle and on the charge and orbital filling. Within the localized picture, columnar ordering is found for intermediate Hund's coupling J_H. At smaller J_H, an unusual orbital reorganization stabilizes checkerboard ordering. Ferromagnetism appears at large J_H. Ferromagnetic correlations are enhanced with electron doping while large hole doping stabilizes checkerboard antiferromagnetism, explaining the change in magnetic interactions upon substitution of Fe by Co or Mn. For intermediate and large values of U, Hartree-Fock shows similar results as strong coupling though with a double stripe phase instead of ferromagnetism. Itinerancy enhances the stability of the columnar ordering. Comparison of the two approaches reveals a metallic region of the phase diagram where strong coupling physics is determinant.

Origin of magnetic properties and martensitic transformation of Ni-Mn-In magnetic shape memory alloys

The nature of unique magnetic properties and martensitic transformation of Ni2Mn1+xIn1−x shape memory alloy has been revealed by first-principles calculations. The uncommon magnetic properties occurring upon martensitic transformation are originated from the change of Mn-Mn interatomic distances. The critical value of Mn-Mn interatomic distance corresponding to the change of magnetic interactions has been determined. A decrease of the intensity of Ni 3d states at the Fermi level upon martensitic transformation has been observed for x = 0.5. Moreover, the Ni 3d-Mn 3d hybridization plays an important role in establishing the magnetic properties and driving the martensitic transformation.

A second-order ferromagnetic transition in the martensitic state of Ni49.5Mn32.5Cu4Sn14: A critical behavior study

A second-order ferromagnetic transition is observed in the martensitic state of Ni49.5Mn32.5Cu4Sn14 alloy and the critical behavior around the transition is investigated by dc magnetization measurements. With the help of modified Arrott plots, Kouvel–Fisher method, and Widom scaling relation, the values of TcM (ferromagnetic transition temperature in the martensite), and critical exponents: β (associated with the spontaneous magnetization), γ (relevant to the initial susceptibility), and δ (associated with the critical magnetization isotherm) are obtained. The scaling plots show that the obtained values of the critical exponents are reliable. The values of the critical exponents of Ni49.5Mn32.5Cu4Sn14 are different from those predicted by several theoretical models, i.e., mean-field theory, three-dimensional Heisenberg model, and three-dimensional Ising model. The magnetic interactions exhibit two different behaviors: long-range magnetic interaction below TcM and local magnetic interaction above TcM. The change in Mn moments may be responsible for the change in magnetic interactions around TcM.

Modification of the spin state in Sm0.52Sr0.48MnO3 by external magnetic field

The effect of applied magnetic field (H) on the magnetic properties of Sm0.52Sr0.48MnO3 single crystal in the paramagnetic (PM) state has been investigated. We observe a field induced steplike jump in magnetization (M) above TC (110K). The temperature and magnetic field dependence of susceptibility reveal that the PM phase of this system is quite complicated due to the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) interactions. The nature of magnetic interaction changes abruptly from AFM to FM at around 3.5T, above which M ceases the steplike jump.

Novel orbital order in the A-site ordered manganite, TbBaMn2O6

The crystal structure and electromagnetic properties of the A-site ordered perovskite manganite, TbBaMn2O6, have been studied. TbBaMn2O6 has successive phase transitions; the structural transition at Tt=450 K, the charge/orbital order transition at TCO =430 K and the antiferromagnetic transitions on Mn ions at TN=210 K and on Tb ions at TN=12 K. At Tt, both a- and b-axis expand by 0.7%, and the c-axis shrinks by 1.3% in the low-temperature phase. The magnetic interaction changes from ferromagnetic (the high-temperature triclinic phase) to antiferromagnetic (the low-temperature monoclinic phase) across Tt. The electrical resistivity clearly shows anisotropic behaviors at Tt; an increase of resistivity along the c-axis and no change within the ab-plane. The obtained results strongly suggest the dx2−y2-type orbital order at Tt.

Paramagnetic properties of the skutterudites Ce 1−zYb zFe 4Sb 12

We report on the rare-earth valence and paramagnetic properties of Ce 1− z Yb z Fe 4Sb 12, Ba y Fe 4Sb 12 and La y Fe 4Sb 12. Yb valence decreases from 2.7 for z = 0.05 – 2.2 for z = 0.93 whereas Ce is always trivalent. The dominant character of the magnetic interaction changes from ferromagnetic with divalent filler to antiferromagnetic with trivalent filler. The paramagnetic properties are dominated by the delocalized electrons of [Fe 4Sb 12] with a constant contribution close to 3.2 μ B (f.u.) −1.

Critical behavior of disordered fccFe70Pt30alloy under high pressure

Employing the disordered local moment formalism in combination with a first-principles tight-binding linear muffin-tin orbital method we find that in disordered fcc ${\mathrm{Fe}}_{70}{\mathrm{Pt}}_{30}$ alloy the effective magnetic interaction changes from ferromagnetic to antiferromagnetic as the lattice constant is reduced from its equilibrium value at ambient pressure. This result explains recent experiments on this alloy that have claimed an appearance of the new magnetic spin-glass phase under high pressure. The value of the volume where the change in the magnetic interaction occurs is just slightly higher than the critical volume at which ${\mathrm{Fe}}_{70}{\mathrm{Pt}}_{30}$ becomes nonmagnetic, suggesting that under applied pressure the system exhibits two types of magnetic critical points. A comparison of our results with earlier theoretical studies of the fcc Fe-Ni Invar alloys reveals a similarity between the high-pressure magnetic state of ${\mathrm{Fe}}_{70}{\mathrm{Pt}}_{30}$ and the magnetic ground state of the Fe-Ni alloys at ambient pressure.