Strength Of Exchange Interaction Research Articles

Three-dimensional critical behavior in Mn1.5Cr1.5O4: A material for magnetocaloric energy conversion

The multifunctional and tunable properties of spinel oxides bring their applicability in various sustainable applications. In this line, the current work systematically examines the magnetic and magnetocaloric properties of Mn1.5Cr1.5O4. The presence of Mn2+ ions in the tetragonal site leads to complex magnetic interactions. It undergoes a ferrimagnetic to paramagnetic transition at 58 K. The critical exponents obtained from the modified Arrot plot, β = 0.346 and γ = 1.051, suggests that the system does not fall into any of the universal class. The exchange interaction strength reveals the coexistence of short- and long-range ordering. Mn1.5Cr1.5O4 displays a magnetic entropy change of -4.11 J kg−1K−1 and a relative cooling power of 70.68 J kg−1 at 58 K for a field change of 70 kOe. Thus, the major results of the study recommend the applicability of Mn1.5Cr1.5O4 towards gas (fuel, H2) liquefaction based on solid-state active magnetic refrigeration technology.

Electronic transport of an extended Ising chain with competing thermal fluctuation and magnetic ordering

In this study, we investigate the behavior of spin-dependent electronic transport in a magnetic nanowire with thermal random oriented moments, connected to two similar (anti-) ferromagnetic leads. The model is based on Green’s function technique within the nearest neighbor tight-binding approach in the framework of canonical ensemble. We analyze the electronic transmission and its variance in different ordered and disordered regimes at a finite temperature in an external magnetic field. The results indicate that at nonzero temperatures, thermal fluctuation of moment orientations decreases the system’s electronic conductance, leading to a smooth conductance-energy relationship. Depending on temperature and external magnetic field values as well as Ising exchange interaction strength, the system can transit to an ordered phase. Additionally, the larger length of the interacting part affects the transmission coefficient and amplifies its variance. This study sheds light on the intricate interplay between temperature, magnetic field, and moment orientations in the context of spin-dependent electronic transport through magnetic nanowires, offering valuable insights into the usage of such systems.

Room-temperature-persistent magnetic interaction between coordination complexes and nanoparticles in maghemite-based nanohybrids.

Maghemite nanoparticles functionalised with Co(II) coordination complexes at their surface show a significant increase of their magnetic anisotropy, leading to a doubling of the blocking temperature and a sixfold increase of the coercive field. Magnetometric studies suggest an enhancement that is not related to surface disordering, and point to a molecular effect involving magnetic exchange interactions mediated by the oxygen atoms at the interface as its source. Field- and temperature-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) studies show that the magnetic anisotropy enhancement is not limited to surface atoms and involves the core of the nanoparticle. These studies also point to a mechanism driven by anisotropic exchange and confirm the strength of the magnetic exchange interactions. The coupling between the complex and the nanoparticle persists at room temperature. Simulations based on the XMCD data give an effective exchange field value through the oxido coordination bridge between the Co(II) complex and the nanoparticle that is comparable to the exchange field between iron ions in bulk maghemite. Further evidence of the effectiveness of the oxido coordination bridge in mediating the magnetic interaction at the interface is given with the Ni(II) analog to the Co(II) surface-functionalised nanoparticles. A substrate-induced magnetic response is observed for the Ni(II) complexes, up to room temperature.

Probing chiral discrimination in biological systems using atomic force microscopy: The role of van der Waals and exchange interactions

We analyze from a theoretical perspective recent experiments where chiral discrimination in biological systems was established using Atomic Force Microscopy (AFM). Even though intermolecular forces involved in AFM measurements have different origins, i.e., electrostatic, bonding, exchange, and multipole interactions, the key molecular forces involved in enantiospecific biorecognition are electronic spin exchange and van der Waals (vdW) dispersion forces, which are sensitive to spin–orbit interaction (SOI) and space-inversion symmetry breaking in chiral molecules. The vdW contribution to chiral discrimination emerges from the inclusion of SOI and spin fluctuations due to the chiral-induced selectivity effect, a result we have recently demonstrated theoretically. Considering these two enantiospecific contributions, we show that the AFM results regarding chiral recognition can be understood in terms of a simple physical model that describes the different adhesion forces associated with different electron spin polarization generated in the (DD), (LL), and (DL) enantiomeric pairs, as arising from the spin part of the exchange and vdW contributions. The model can successfully produce physically reasonable parameters accounting for the vdW and exchange interaction strength, accounting for the chiral discrimination effect. This fact has profound implications in biorecognition where the relevant intermolecular interactions in the intermediate-distance regime are clearly connected to vdW forces.

Hexagonal MnTe with Antiferromagnetic Spin Splitting and Hidden Rashba–Dresselhaus Interaction for Antiferromagnetic Spintronics

AbstractHexagonal MnTe emerges as a critical component in designing magnetic quantum heterostructures, calling for a detailed study. After finding a suitable combination of exchange–correlation functional and corrections, this study within ab initio density functional theory uncovers an insulating state with a preferred antiferromagnetic (AFM) order. The exchange interaction strengths are computed to estimate the AFM ordering temperature via Monte Carlo calculations. These calculations and symmetry analysis reveal a large spin splitting in the system due to the AFM order without considering spin–orbit interaction, except in the kx‐ky plane. Critically examining the band dispersion and spin textures obtained from these calculations and comparing them with an insightful symmetry analysis and analytical model, a combined Rashba–Dresselhaus interaction in the kx‐ky plane, around the K point of the system, is confirmed. These results and insights would help design heterostructures of MnTe for technological applications.

Electron Spin Decoherence Dynamics in Magnetic Manganese Hybrid Organic-Inorganic Crystals: The Effect of Lattice Dimensionality.

Organic-inorganic metal hybrids with their tailorable lattice dimensionality and intrinsic spin-splitting properties are interesting material platforms for spintronic applications. While the spin decoherence process is extensively studied in lead- and tin-based hybrids, these systems generally show short spin decoherence lifetimes, and their correlation with the lattice framework is still not well-understood. Herein, we synthesized magnetic manganese hybrid single crystals of (4-fluorobenzylamine)2MnCl4, ((R)-3-fluoropyrrolidinium)MnCl3, and (pyrrolidinium)2MnCl4, which represent a change in lattice dimensionality from 2D and 1D to 0D, and studied their spin decoherence processes using continuous-wave electron spin resonance spectroscopy. All manganese hybrids exhibit nanosecond-scale spin decoherence time τ2 dominated by the symmetry-directed spin exchange interaction strengths of Mn2+-Mn2+ pairs, which is much longer than lead- and tin-based metal hybrids. In contrast to the similar temperature variation laws of τ2 in 2D and 0D structures, which first increase and gradually drop afterward, the 1D structure presents a monotonous rise of τ2 with the temperatures, indicating the strong correlation of spin decoherence with the lattice rigidity of the inorganic framework. This is also rationalized on the basis that the spin decoherence is governed by the competitive contributions from motional narrowing (prolonging the τ2) and electron-phonon coupling interaction (shortening the τ2), both of which are thermally activated, with the difference that the former is more pronounced in rigid crystalline lattices.

The Role of Polaronic States on the Spin Dynamics in Solution-Processed Two-Dimensional Layered Perovskite with Different Layer Thickness.

2D lead halide perovskites (LHPs) show strong excitonic and spin-orbit coupling effects, generating a facile spin injection. Besides, they possess a polaron character due to the soft crystal lattice, which can prolong the spin lifetime, making them favorable materials for spintronic applications. Here, the spin dynamics of 2D PEA2 PbI4 (MAPbI3 )n -l thin films with different layers by temperature- and pump fluence-dependent circularly polarization-resolved transient absorption (TA) measurements is studied. These results indicate that the spin depolarization mechanism is gradually converted from the Maialle-Silva-Sham (MSS) mechanism to the polaronic states protection mechanism with the layer number increasing from <n> = 1 to 3, which is determined by the interplay between the strength of Coulomb exchange interaction and the strength of polaronic effect. While for <n> ≥ 4, the Elliot-Yafet (EY) impurities mechanism is proposed, in which the formed polaronic states with free charge carriers no longer play the protective role.

Extreme sensitivity of the magnetic ground state to halide composition in FeCl3−xBrx

Mixed halide chemistry has recently been utilized to tune the intrinsic magnetic properties of transition-metal halides---one of the largest families of magnetic van der Waals materials. Prior studies have shown that the strength of exchange interactions, hence the critical temperature, can be tuned smoothly with halide composition for a given ground state. Here we show that the ground state itself can be altered by a small change of halide composition in ${\mathrm{FeCl}}_{3\ensuremath{-}x}{\mathrm{Br}}_{x}$. Specifically, we find a threefold jump in the N\'eel temperature and a sign change in the Weiss temperature at $x=0.08$ corresponding to only $3%$ bromine doping. Using neutron scattering, we reveal a change of the ground state from spiral order in ${\mathrm{FeCl}}_{3}$ to $A$-type antiferromagnetic order in ${\mathrm{FeBr}}_{3}$. From first-principles calculations, we show that a delicate balance between nearest and next-nearest neighbor interactions is responsible for such a transition. These results demonstrate how varying the halide composition can tune the competing interactions and change the ground state of a spiral spin liquid system.

BaMn2P2 : Highest magnetic ordering temperature 122-pnictide compound

We report the growth of high-quality single crystals of ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type tetragonal ${\mathrm{BaMn}}_{2}{\mathrm{P}}_{2}$ and investigation of its structural, electrical transport, thermal, and magnetic properties. Our results of basal plane electrical resistivity and heat capacity measurements show that the compound has an insulating ground state with a small band gap. Anisotropic susceptibility ${\ensuremath{\chi}}_{ab,c}(T)$ data infer a collinear local-moment N\'eel-type antiferromagnetic (AFM) ground state below the ordering temperature ${T}_{\mathrm{N}}=795(15)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, which is highest among all the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$- and ${\mathrm{CaAl}}_{2}{\mathrm{Si}}_{2}$-type 122-pnictide compounds reported so far suggesting that the strength of magnetic exchange interactions is strongest in this material. The magnetic transition temperatures of ${\mathrm{BaMn}}_{2}P{n}_{2}$ ($Pn=\mathrm{P}$, As, Sb, Bi) compounds exhibit a monotonic decrease with the increase of tetragonal unit cell parameters $a$ and $c$, suggesting a strong dependence of the strength of the decisive magnetic exchange interactions on the separation between the localized spins residing on the Mn ions. The observed monotonic increase of both ${\ensuremath{\chi}}_{ab}$ and ${\ensuremath{\chi}}_{c}$ for $T&gt;{T}_{\mathrm{N}}$ suggests that short-range dynamic quasi-two-dimensional AFM correlations persist above the ${T}_{\mathrm{N}}$ up to the highest temperature of the measurements. The large ${T}_{\mathrm{N}}$ of ${\mathrm{BaMn}}_{2}{\mathrm{P}}_{2}$ demands for systematic hole-doping studies on this material as similar investigations on related ${\mathrm{BaMn}}_{2}{\mathrm{As}}_{2}$ with ${T}_{\mathrm{N}}=618\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ have led to the discovery of an outstanding ground state where AFM of localized Mn spins and itinerant half-metallic ferromagnetism with ${T}_{\mathrm{c}}\ensuremath{\approx}100\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ originating from the doped holes coexist together.

Theoretical antiferromagnetism of ordered face-centered cubic Cr-Ni alloys

Contrary to prior calculations, the Ni-rich ordered structures of the Cr-Ni alloy system are found to be antiferromagnetic under semilocal density-functional theory. The optimization of local magnetic moments significantly increases the driving force for the formation of ${\mathrm{CrNi}}_{2}$, the only experimentally observed intermetallic phase. This structure's ab initio magnetism appears well described by a Heisenberg Hamiltonian with longitudinal spin fluctuations; itinerant Cr moments are induced only by the strength of exchange interactions. The role of magnetism at temperature is less clear and several scenarios are considered based on a review of experimental literature, specifically a failure of the theory, the existence of an overlooked magnetic phase transition, and the coupling of antiferromagnetism to chemical ordering. Implications for related commercial and high-entropy alloys are discussed for each case.

Exchange-stiffness measurement scheme based on domain-wall chirality transition

Herein, we propose an experimental scheme to determine the strength of the Heisenberg exchange interaction in ultrathin magnetic films. In this scheme, the chirality transition between the Bloch- and Néel-type domain walls is analyzed under an in-plane magnetic field. Subsequently, the exchange stiffness constant is estimated based on the strength of the magnetic field for chirality transitions, as proposed by an analytical theory pertaining to chirality transitions, and confirmed based on micromagnetic prediction. Using the magneto-optical Kerr effect with high sensitivity on surface atomic layers, the present scheme is applicable to ultrathin magnetic films down to a few atomic layers, whose sensitivity is well below the lowest sensitivity limit of conventional measurement schemes. Hence, the present scheme is useful for extending the experimental range to investigate the exchange stiffness of few-atomic-layer-thick magnetic films.

Varying critical behaviour of stacked triangular lattice Ising antiferromagnets in the presence of spin–lattice coupling

We perform Monte Carlo simulations on the multilayered triangular-lattice Ising antiferromagnet with six sublattices in the frustrated abab stacking and quantify the impact of spin–lattice coupling on its critical properties in the vicinity of the antiferromagnetic phase transition. By invoking the Einstein site spin–phonon model and by varying the strengths of both antiferromagnetic interlayer exchange interactions and spin–lattice coupling, we compute various thermodynamic observable that characterize how the geometrical frustration and the spin–phonon coupling affect the critical behaviour of the system. Using finite-size scaling, we determine the nature of phase transition and the critical exponents, the latter being found to vary with varying strength of couplings. We find that the transition temperature is more sensitive to the interlayer coupling strength rather than that of magnetoelastic coupling. The variation of critical exponents with the strength of spin–lattice coupling hints to the importance of magnetoelastic interaction in the vicinity of magnetic phase transition in strongly correlated magnetic systems, as already observed in several experiments.

Static and dynamic disorder in Aβ40 fibrils

Deposition of Aβ aggregates in the form of amyloid fibrils is a pathological hallmark of Alzheimer's disease. Understanding the structure and dynamics of Aβ fibrils is important for delineating the mechanism of Aβ aggregation and developing effective therapeutic strategies. Here we used site-directed spin labeling and EPR spectroscopy to study the Aβ40 fibril structure and dynamics. We obtained the EPR spectra of 40 spin-labeled Aβ40 fibril samples, with spin labeling coverage of the entire Aβ40 sequence. Analysis of the spin exchange interaction and spin label mobility using spectral simulations suggest that the strength of spin exchange interaction is primarily determined by static disorder in the Aβ40 fibrils. EPR data suggest that the entire Aβ40 sequence except residue D1 is highly ordered and the two hydrophobic regions at residues 17–20 and 31–36 show the lowest static disorder. Dynamic disorder is relatively constant across all reside positions, with residues 22 and 23 having the highest dynamic disorder. Comparison of the EPR data for Aβ40 and Aβ42 fibrils shows overall more ordered packing interactions in Aβ40 fibrils. Another noteworthy difference is the C-terminal residue, which has high static disorder in Aβ42 fibrils, but is ordered in Aβ40 fibrils. The higher static disorder in Aβ42 fibrils may lead to increased fragmentation, monomer dissociation, and structural defects, which may contribute to increased aggregation through secondary nucleation.

Tuning of Cr-Cr Magnetic Exchange through Chalcogenide Linkers in Cr2 Molecular Dimers.

A set of three Cr-dimer compounds, Cr2Q2(en)4X2 (Q: S, Se; X: Br, Cl; en: ethylenediamine), with monoatomic chalcogenide bridges have been synthesized via a single-step solvothermal route. Chalcogenide linkers mediate magnetic exchange between Cr3+ centers, while bidentate ethylenediamine ligands complete the distorted octahedral coordination of Cr centers. Unlike the compounds previously reported, none of the chalcogenide atoms are connected to extra ligands. Magnetic susceptibility studies indicate antiferromagnetic coupling between Cr3+ centers, which are moderate in Cr2Se2(en)4X2 and stronger in Cr2S2(en)4Cl2. Fitting the magnetic data requires a biquadratic exchange term. High-frequency EPR spectra showing characteristic signals due to coupled S = 1 spin states could be interpreted in terms of the "giant spin" Hamiltonian. A fourth compound, Cr2Se8(en)4, has a single diatomic Se bridge connecting the two Cr3+ centers and shows weak ferromagnetic exchange interactions. This work demonstrates the tunability in strength and type of exchange interactions between metal centers by manipulating the interatomic distances and number of bridging chalcogenide linkers.

Multiferroic properties of GdFe0.9M0.1O3 (M\u2009=\u2009Ag1+, Co2+ and Cr3+) nanoparticles and evaluation of their antibacterial activity

Multifunctional nanoparticles NPs with material composition GdFe0.9M0.1O3; M = Ag, Co, and Cr have successfully been synthesized using the citrate auto-combustion technique. The single phase of the orthorhombic perovskite structure is ratified from the XRD data. The structural, magnetic, and thermoelectric power of the samples along with the results of antibacterial activities are reported in the present manuscript. The variation in the magnetization is argued in view of the strength and type of exchange interaction as well as buckling of the < BO6 > octahedron. The super exchange interaction between the Fe–O–Fe and the Cr–O–Cr and the randomness of Cr ions in the host lattice site are the main reasons behind the weak ferromagnetism obtained from GdFe0.9Cr0.1O3. Ferroelectricity and antiferromagnetism have a dissimilar origin and appear independently. The origin of antiferromagnetism is the spin canting of the B ions. However, the origin of the ferroelectric properties is the hybridization between B cations and O2− anion. The use of silver metal particles as antibacterial agents is noteworthy due to their advantages in terms of chemical stability, efficacy and long-term durability. These advantages can be extended by considering the relatively low toxicity of these particles to the human body compared to other inorganic metals.

Magnon polarons in spin Seebeck effect of easy axis antiferromagnets

The formation of magnon polarons, quanta of magnetoelastic waves, was found to be able to stimulate an enhancement or suppression in the magnetic field dependence of the spin Seebeck effect when the dispersion curve of the magnon becomes tangential to those of acoustic phonons. In the present work, we systematically analyzed the properties of the magnon spectrum in body-centered cubic easy-axis antiferromagnets with varying strength of exchange interaction and magnetic anisotropy, which allowed us to classify the antiferromagnets according to the number of solutions for the tangential condition between the dispersion curves of magnons and acoustic phonons. The anomaly features were found to occur only in the relatively weak magnetic field regime before spin flop transition. The manifestation of magnon–polaron-induced anomaly on the longitudinal spin Seebeck coefficient was also calculated directly from which a triple-peak feature, never observed or proposed before, was predicted. Our analysis also works for antiferromagnets with other magnetic lattices.

Influence of local structural distortion on the magnetism of Na2IrO3 compounds

First-principles calculations are conducted to investigate the magnetic properties of the ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ compounds. We reveal that the ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$'s local structural distortions are essential for an accurate description of the magnetism of such systems. They provide a feasible explanation for the experimentally observed antiferromagnetic zigzag magnetic ground state. We demonstrate that the underlying competition between the spin-orbit coupling and the crystal-field splitting rules the crystal structure and profoundly influences the strength of the magnetic exchange interactions. We unambiguously identify that the ${D}_{4h}$-type distortions, due to the Jahn-Teller effect, are disclosed in an elongation along the ${\mathrm{IrO}}_{6}$ polyhedra's apical axis in the $ac$ plane. On the other hand, the Ir atoms off-centering in the basal plane (perpendicular to the local ${D}_{4h}$ elongation axis) arises owing to an inhomogeneous cationic charge distribution of ${\mathrm{Ir}}^{4+}/{\mathrm{Na}}^{+}$ in the transition-metal layer resulting in both effects being decisive in controlling the magnetism of ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ together with the spin-orbit interactions.

Study of the effect of nitrogen and hydrogen on the structure and magnetic properties of (Sm, Er)2Fe17 alloys

Materials based on the Sm2Fe17 compound with nitrogen have great potential for the manufacture of highly efficient permanent magnets. The initial, nitrided, and hydrogenated alloys based on the Sm2Fe17 intermetallic compound with partial substitution of erbium atoms for samarium atoms Sm1.2Er0.8Fe17(H, N)x have been studied by X-ray phase analysis and scanning electron microscopy. Nanopowders of Sm1.2Er0.8Fe17N2 were obtained by mechanical grinding. The grinding time was varied from 0 to 60 minutes. The magnetic hysteresis properties of all powder samples were studied in static magnetic fields up to 7 T, as well as in pulsed magnetic fields up to 60 T. The strength of the intersublattice exchange interaction was estimated. The obtained values λ are valid only for the explored concentration of Sm/Er ions.

Heterobilayer moiré magnets: Moiré skyrmions and commensurate-incommensurate transitions

We study untwisted heterobilayers of ferromagnetic and antiferromagnetic van der Waals materials, with in particular a Dzyaloshinskii-Moriya interaction in the ferromagnetic layer. A continuum low energy field theory is utilized to study such systems. We develop a phase diagram as a function of the strength of inter-layer exchange and Dzyaloshinskii-Moriya interactions, combining perturbative and strong coupling analyses with numerical simulations using Landau-Lifshitz-Gilbert equations. Various moir\'e-periodic commensurate phases are found, and the commensurate-incommensurate transition is discussed. Among the commensurate phases, we observe an interesting skyrmion lattice phase wherein each moir\'e unit cell hosts one skyrmion.

Chloride Reduction of Mn3+ in Mild Hydrothermal Synthesis of a Charge Ordered Defect Pyrochlore, CsMn2+Mn3+F6, a Canted Antiferromagnet with a Hard Ferromagnetic Component.

Geometrically frustrated systems play an important role in studying new physical phenomena and unconventional thermodynamics. Charge ordered defect pyrochlores AM2+M3+F6 offer a convenient platform for probing the interplay between electron distribution over M2+ and M3+ sites and structural distortions; however, they are limited to compounds with M2+/3+ = V, Fe, Ni, and Cu due to difficulties in the simultaneous stabilization of other 3d elements in the +2 and +3 oxidation states. Herein, we employ Cl- anions under hydrothermal conditions for the mild reduction of Mn2O3 in concentrated HF to obtain the CsMn2+Mn3+F6 composition as a phase pure sample and study its properties. The magnetism of CsMn2F6 was characterized by measuring the magnetic susceptibility and isothermal magnetization data, and a magnetic transition to a canted antiferromagnet state was found at 24.1 K. We determined the magnetic structure of CsMn2F6 using powder neutron diffraction, which revealed successive long-range ordering of the Mn2+ and Mn3+ sites that is accompanied by a second transition. The role and strength of magnetic exchange interactions were characterized using DFT calculations.