Strong Ferromagnetic Exchange Interactions Research Articles

Large magnetocaloric refrigeration performance near room temperature in monolayer transition metal dihalides

Magnetocaloric effect (MCE) exhibits highly efficient and ecological cooling abilities for solid-state refrigeration in contrast to traditional vapor-compression refrigeration. Successive emerging two-dimensional (2D) magnetic materials provide a fertile platform for exploring low-dimensional MCE systems. Here, we focus on a series of 2D transition metal dihalides MX2 (M = Fe, Ru, Os; X = Cl, Br) to explore the maximum isothermal magnetic entropy change (−ΔSmagmax) and adiabatic temperature change (ΔTadmax) under external magnetic field. It is found that FeCl2, FeBr2, and RuCl2 have intrinsically sizable −ΔSmagmax, ΔTadmax, and high thermal conductivity near room temperature, demonstrating superior comprehensive refrigeration performance in comparison with other 2D magnets. It is revealed that strong nearest-neighbor ferromagnetic exchange interaction plays a decisive role in −ΔSmagmax, and the high lattice thermal conductivities of FeCl2 and RuCl2 are attributed to the longer phonon lifetime and larger group velocity of low-frequency acoustic branch. Moreover, moderate strain and carriers doping are able to effectively regulate Curie temperature and magnetocrystalline anisotropy energy and correspondingly enhance −ΔSmagmax. The present work provides important insights for the exploration of 2D magnets for magnetocaloric refrigeration near room temperature.

Enhancing ferromagnetism in the Sm(Co, Mn)5 system: Impact on phase stability and magnetic properties

This work demonstrates that Mn can be a beneficial candidate for the performance modulation of permanent magnetic materials like SmCo5. Theoretical calculations reveal that Mn has a large positive contribution to the magnetization of SmCo5, owing to the strong ferromagnetic exchange interaction between Co and Mn atoms. It is also found that Mn could occupy both 2c and 3g crystal sites because of the minute energy difference between them can be easily overcome through thermal activation. Co atoms on the same site with Mn have increasing moments with the increase in Mn content, whereas those on the other sites show decreasing moments. The experimental substitution of Mn for Co leads to a contraction of the SmCo5 single-phase region, and thus pure Sm(Co, Mn)5 phase could only be obtained through subtle control of Sm content by annealing. Significantly, a 13 % increase in saturation magnetization of SmCo5 is achieved through Mn substitution. A slight rise in the Curie temperature is also obtained, suggesting that Mn substitution does not strongly disturb the exchange interactions of the Co sublattice. This study offers a new option for enhancing magnetic properties in Sm–Co systems and introduces novel physical phenomena resulting from Mn substitution, which is worthy of further investigation.

Synthesis and photoinduced behavior of DPP-anchored nitronyl nitroxides: a multifaceted approach.

Understanding and controlling spin dynamics in organic dyes is of significant scientific and technological interest. The investigation of 2,5-dihydropyrrolo[4,3-c]pyrrolo-1,4-dione derivatives (DPPs), one of the most widely used dyes in many fields, has so far been limited to closed-shell molecules. We present a comprehensive joint experimental and computational study of DPP derivatives covalently linked to two nitronyl nitroxide radicals (DPPTh-NN2). Synthesis, single crystal X-ray diffraction study, photophysical properties, magnetic properties established using steady-state and pulse EPR, fast spin dynamics, and computational modelling using density functional theory and ab initio methods of electronic structure and spectroscopic properties of DPPTh-NN2 are presented. The single-crystal X-ray diffraction analysis of DPPTh-NN2 and computational modeling of its electronic structure suggest that effective conjugation along the backbone leads to noticeable spin-polarization transfer. Calculations using ab initio methods predict a weak exchange interaction of radical centers through a singlet ground state of DPPTh with a small singlet-triplet splitting (ΔEST) of about 25 cm-1 (∼0.07 kcal mol-1). In turn, a strong ferromagnetic exchange interaction between the triplet state of DPPTh chromophore and nitronyl nitroxides (with J ∼ 250 cm-1) was predicted.

Perovskite-based two-dimensional ferromagnet Sr<sub>2</sub>RuO<sub>4</sub> monolayer

At present, the research on two-dimensional (2D) ferromagnets is mainly concentrated in the field of van der Waals materials, while the successful preparation of strain-free freestanding 2D perovskite films provides a great opportunity for the design of 2D ferromagnets beyond van der Waals materials. Perovskite oxide SrRuO<sub>3</sub>, as a typical perovskite itinerant ferromagnet, has broad application prospects in many fields. In this paper, the lattice dynamics, ground-state structure, electronic and magnetic properties of its perovskite monolayer with formula Sr<sub>2</sub>RuO<sub>4</sub>, as well as the effect of external electric field, are studied by combining first-principles calculation, symmetry analysis and Monte Carlo simulation. The influence of the Hubbard parameter <i>U</i> is also revealed. The results indicate that the ground-state structure under all <i>U</i> values is the structural phase (space group <inline-formula><tex-math id="M2">\begin{document}$P4/mbm$\end{document}</tex-math></inline-formula>) generated by octahedral rotation distortion. Similar to the SrRuO<sub>3</sub> bulk, the ground-state phase of the Sr<sub>2</sub>RuO<sub>4</sub> monolayer exhibits ferromagnetism, which is independent of the <i>U</i> value and thus robust. Density functional theory calculation with Hubbard parameter <i>U</i> predicts the ground-state phase of the monolayer to be a ferromagnetic half metal with out-of-plane easy-magnetization axis, while excluding the <i>U</i> parameter predicts the ground-state phase to be a ferromagnetic metallic state. The ferromagnetism mainly originates from the strong ferromagnetic exchange interaction between the nearest neighbor spin pairs. The simulated Curie temperature of the Sr<sub>2</sub>RuO<sub>4</sub> monolayer is 177 K, which is close to the value (150 K) of its bulk phase. The out-of-plane electric field does not change the ground-state structure and ferromagnetism of the Sr<sub>2</sub>RuO<sub>4</sub> monolayer, but can significantly modulate its electronic and magnetic properties. When an external electric field exceeding 0.3 V/Å is applied, the system undergoes a transition from a ferromagnetic half-metal state to a ferromagnetic metallic state. This work indicates the potential application of Sr<sub>2</sub>RuO<sub>4</sub> monolayer in low-dimensional spintrnic devices, and provides a reference for the development of perovskite-based 2D ferromagnets and the realization of controlling 2D magnetism by electric field.

Close-packed nitronyl nitroxide radicals by Au-S self-assembly: strong ferromagnetic coupling.

The study of the magnetism of tightly arranged nitronyl nitroxide (NN) radicals via Au-S self-assembly is interesting. In this study, a series of radicals (S-NN, D-NN, BS-NN, BD-NN) along with two types of nanomaterials (S-NPs, D-NPs) were synthesized. NN was chosen for the magnetic units. Their structures have been successfully synthesized and analyzed. The spin magnetic properties were characterized by electron paramagnetic resonance (EPR) and superconducting quantum interference device (SQUID) measurement. The analysis revealed that the self-assembled NN formed via Au-S bonds exhibits high packing density. Furthermore, it was gratifying to observe that the AuNPs exhibit ferromagnetism after the surface modification by NN. This results in strong ferromagnetic exchange interactions of S-NPs and D-NPs : J S-NPs = +279.715 K and J D-NPs = +254.913 K, respectively.

A DFT study of electronic structure, magnetic properties and cyclization reaction of [5]helicene derivatives

A series of 1,14-dimethyl-[5]helicene derivatives comprising radicals ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl, 1,2,3,5-dithiadiazolyl, 1,5-dimethyl-6-oxo-verdazyl and 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide) was investigated by quantum-chemical modeling of their electronic structure, exchange interactions and cyclization reaction. The performed DFT study of 1,14-dimethyl-[5]helicene, supported by DFT/CASSCF and DFT/NEVPT2 calculations, have shown that cyclic form of this compound represents a singlet biradical. The energy barriers of transition between open and cyclic forms of 1,14-dimethyl-[5]helicene radical derivatives are comparable to the values characteristic of enantiomerization reaction, which indicates a high probability of formation of cyclic isomers under thermal conditions. The TD DFT calculations of electronic transitions predicted the presence of non-overlapping bands in the studied isomers that favors switching between them by irradiation. The variation in a number of paramagnetic centers as a result of interconversion of open and cyclic forms, and the presence of structures bearing strong ferromagnetic exchange interactions, make the considered [5]helicene derivatives a suitable basis for molecular electronics devices.

UFO-shaped heteronuclear 3d–4f metal clusters: Synthesis, structure, and magnetic properties

Coordination molecule clusters assembled from a large number of metal ions and organic ligands are structurally and functionally challenging to characterize. Here, a series of new hetero-metallic nano-clusters {Mn6Ln2} (Ln = La 1, Ce 2, Sm 3, Gd 4) were obtained via solvothermal method. These complexes possess unique crystalline structures with vertex-sharing UFO-shaped [Mn6Ln2] cores. Interestingly, adjacent cluster molecules can further form a three-dimensional framework structure through π-π stacking interaction between acenaphthenequinone dioxime ligands. Magnetic tests show strong ferromagnetic exchange interactions within the {Mn6Ln2} clusters.

Spontaneous Valley Polarization in a Ferromagnetic Fe(OH)2 Monolayer

At present, creating sizable spontaneous valley polarization is at the center of the study of valleytronics, which, however, is still a huge challenge. In this work, we determined that the ferromagnetic Fe(OH)2 monolayer of the hexagonal lattice is a highly appealing candidate for valleytronics by using first-principles calculations in conjunction with tight-binding model analysis. In light of the simultaneous inversion symmetry breaking and time-reversal symmetry breaking, we illustrated that the strong spin-orbit coupling and robust ferromagnetic exchange interaction cause a spontaneous valley polarization as large as 67 meV for Fe(OH)2, indicative of room-temperature application. In addition, the physics of valley-selective circular dichroism, spin/valley Hall effects, and topological phase transition were also discussed.

One-dimensional ferromagnetic semiconductor CrSbSe3 with high Curie temperature and large magnetic anisotropy

In recent years, low-dimensional ferromagnetic semiconductors have attracted tremendous interest due to their great significance for both fundamental physics and device applications. In this work, a one dimensional (1D) ferromagnetic semiconductor $\mathrm{CrSb}{\mathrm{Se}}_{3}$ with high Curie temperature $({T}_{\mathrm{C}})$ and sizable magnetic anisotropy is proposed based on the first-principle calculations and theoretical model. Quasi-1D chain structure with a moderate cleavage energy of $0.49\phantom{\rule{0.16em}{0ex}}\mathrm{J}/{\mathrm{m}}^{2}$ suggests the possibility to exfoliate a 1D $\mathrm{CrSb}{\mathrm{Se}}_{3}$ ladder (a ladder structure formed by two rows of 1D monoatomic Cr chain) from the bulk. Monte Carlo simulation based on anisotropic Heisenberg model predicts ${T}_{\mathrm{C}}$ to be up to 170 K, which is much higher than that of the bulk. The high ${T}_{\mathrm{C}}$ of 1D $\mathrm{CrSb}{\mathrm{Se}}_{3}$ ladder originates from the combined effect of the strong intraladder ferromagnetic exchange interaction and large magnetic anisotropy, while the lower ${T}_{\mathrm{C}}$ of the bulk results from the weak interladder indirect exchange interaction. In addition, we find that the splitting of top valence bands depends on the direction of magnetization due to the reduced structural symmetry and spin-orbit coupling, indicating a strong magnetoband structure effect. Our theoretical prediction not only provides a material platform for understanding the fundamental physics of 1D magnetism, but also provides a strategy for the search for the 1D ferromagnetic semiconductor with high Curie temperature.

Auxiliary Atomic Relay Center Facilitates Enhanced Magnetic Couplings in Blatter's Radical.

The recent accomplishments in obtaining strong ferromagnetic exchange interactions in organic diradicals have made the field quite fascinating and even more promising toward its technological applications. In this context, herein, we report a unique combination of remarkably strong ferromagnetic exchange interactions coupled with molecular rigidity, utilizing superstable Blatter's radical as a spin source. The planar analogues of the parent Blatter's radical obtained by annulation with a chalcogen coupled to nitronyl nitroxide (NN) are investigated using density functional theory along with the wave function-based multiconfigurational self-consistent field methods, for example, complete active space self-consistent field (CASSCF)-N-electron valence state perturbation theory (NEVPT2). The calculations reveal phenomenal modulation in exchange couplings upon annulation such that remarkably strong ferromagnetic interactions are realized especially for a certain class of the Blatter-NN diradicals. The modulation of spin-spin interactions is rationalized by variation in spin density distribution and molecular torsional angles. We demonstrate that annulation in OMMs opens an additional coupling pathway via auxiliary X-atom acting as the atomic relay center which strongly manipulates the magnitude of exchange couplings.

How Plausible Is Getting Ferromagnetic Interactions by Coupling Blatter's Radical via Its Fused Benzene Ring?

With ongoing efforts to synthesize super-stable Blatter's diradicals having strong ferromagnetic exchange interactions, all the 10 possible isomers of di-Blatter diradical coupled through the fused benzene rings are investigated. A variety of electronic structure theory such as broken-symmetry methods in density functional theory (DFT), spin-constraint DFT (CDFT), and wave function-based multi-configurational methods, e.g., CASSCF/NEVPT2 are applied to compute the magnetic exchange interactions. Surprisingly, anti-ferromagnetic interactions are revealed for all the stable isomers of di-Blatter diradicals. Indeed, it is commensurate with the experimental observations for the only available synthesized isomer. However, the other nine isomeric diradicals in the series are yet to be synthesized. Despite a good match between theory and experiment, the anti-ferromagnetic exchange interactions could not be explained based on the spin alternation rule due to unique spin distributions in the triazinyl ring. Thus, we propose the zonal spin-alternation rule, which explains the observed ground spin-state for the conjugated di-Blatter diradicals quite accurately. Further, the fractional spin-moment localization on the N-atoms activates multiple exchange pathways and the dominating exchange interactions render anti-ferromagnetic interactions in the conjugated isomers. The study further reveals that, due to strong steric hindrance in certain coupled isomers, the exchange interaction switches from anti-ferromagnetic to weak ferromagnetic interactions with the cost of stabilization energy of the radicals. Thus, it questions the possibility of synthesizing ferromagnetic di-Blatter diradicals.

Nickel(II) and Cobalt(III) bis(dioxolene) complexes with di(2-pyridyl)imine ligands: Synthesis and magnetic properties

Here the synthesis, the molecular structure and the magnetic propertiesof four novel six-coordinated bis-dioxolene nickel and cobalt complexes with various di(pyridyl)imines as ancillary N-donor ligands is discussed. Despite the analogous composition the obtained compounds show different charge distribution dependently on the nature of complexing metal. Bis-o-semiquinonato nickel(II) complexes demonstrate the strong ferromagnetic exchange interaction between unpaired electrons of the high-spin nickel ion and anion-radical ligands. Cobalt complex containing naphtyl-substituted ancillary ligand demonstrates the proceeding of solid-state redox-isomeric transformation at temperatures above 220 K.

Smart Ligands for Efficient 3d-, 4d- and 5d-Metal Single-Molecule Magnets and Single-Ion Magnets

There has been a renaissance in the interdisciplinary field of Molecular Magnetism since ~2000, due to the discovery of the impressive properties and potential applications of d- and f-metal Single-Molecule Magnets (SMMs) and Single-Ion Magnets (SIMs) or Monometallic Single-Molecule Magnets. One of the consequences of this discovery has been an explosive growth in synthetic molecular inorganic and organometallic chemistry. In SMM and SIM chemistry, inorganic and organic ligands play a decisive role, sometimes equally important to that of the magnetic metal ion(s). In SMM chemistry, bridging ligands that propagate strong ferromagnetic exchange interactions between the metal ions resulting in large spin ground states, well isolated from excited states, are preferable; however, antiferromagnetic coupling can also lead to SMM behavior. In SIM chemistry, ligands that create a strong axial crystal field are highly desirable for metal ions with oblate electron density, e.g., TbIII and DyIII, whereas equatorial crystal fields lead to SMM behavior in complexes based on metal ions with prolate electron density, e.g., ErIII. In this review, we have attempted to highlight the use of few, efficient ligands in the chemistry of transition-metal SMMs and SIMs, through selected examples. The content of the review is purely chemical and it is assumed that the reader has a good knowledge of synthetic, structural and physical inorganic chemistry, as well as of the properties of SIMs and SMMs and the techniques of their study. The ligands that will be discussed are the azide ion, the cyanido group, the tris(trimethylsilyl)methanide, the cyclopentanienido group, soft (based on the Hard-Soft Acid-Base model) ligands, metallacrowns combined with click chemistry, deprotonated aliphatic diols, and the family of 2-pyridyl ketoximes, including some of its elaborate derivatives. The rationale behind the selection of the ligands will be emphasized.

Critical behavior in the layered organic-inorganic hybrid (CH3NH3)2CuCl4**Project supported by the National Natural Science Foundation of China (Grant No. 51725104) and the Beijing Natural Science Foundation, China (Grant No. Z180009).

The critical properties and the nature of the ferromagnetic–paramagnetic phase transition in the 2D organic-inorganic hybrid (CH3NH3)2CuCl4 single crystal have been investigated by dc magnetization in the vicinity of the magnetic transition. Different techniques were used to estimate the critical exponents near the ferromagnetic–paramagnetic phase transition such as modified Arrott plots, the Kouvel–Fisher method, and the scaling hypothesis. Values of β = 0.22, γ = 0.82, and δ = 4.4 were obtained. These critical exponents are in line with their corresponding values confirmed through the scaling hypothesis as well as the Widom scaling relation, supporting their reliability. It is concluded that this 2D hybrid compound possesses strong ferromagnetic intra-layer exchange interaction as well as weak interlayer ferromagnetic coupling that causes a crossover from 2D to 3D long-range interaction.

Electronic Structure and Magnetic Properties of o-Benzoquinone Iron Complexes with Tetraazamacrocyclic Ligands

Structure, energy characteristics, and magnetic properties of o-benzoquinone iron complexes with tetradentate macrocyclic nitrogen containing bases (pyridinophane and tris(2-pyridylmethyl)amine) are studied with a DFT (UTPSSh/6-311++G(d,p)) quantum chemical method. Electron withdrawing substituents in the redox-active ligand stabilize its dianionic form but do not significantly affect the energy difference between the electromers. Strong ferromagnetic exchange interactions are predicted in the isomers of complexes containing a high-spin divalent iron ion and the semiquinone form of o-benzoquinone. According to the calculations, all studied compounds can undergo thermally induced spin crossover transitions.

First-Principle Design of Blatter’s Diradicals with Strong Ferromagnetic Exchange Interactions

The stable organic diradicals that exhibit strong intramolecular ferromagnetic exchange interactions are suitable building blocks for organic magnetic materials (OMMs). Based on the ab initio calcu...

Enhancement of magnetic and microstructural properties in Fe-Cr-Co-Mo-V-Zr-Y permanent magnetic alloy

Abstract The 43-xFe-28Cr-23Co-3Mo-2V-1Zr-xY; x = 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 wt% permanent magnets were produced through casting, thermomagnetic annealing and aging treatments. Magnetic and microstructural properties were improved by Y addition and process optimization. The Y element modifies the morphology of ferromagnetic F-Co rich phase, introduces filed anisotropy and contributes in coercivity enhancement. XRD and EDS results showed that Y free 43Fe-28Cr-23Co-3Mo-2V-1Zr alloy is composed of Fe-Co-V (54.4%) and Cr-Mo-Zr (45.6%) phases while Y doped 41Fe-28Cr-23Co-3Mo-2V-1Zr-2Y alloy is consists of Fe-Co-V-Y (55.7%), Cr-Mo-Zr (29.3%) and Y1Co5 (15%) magnetic phases. Micrographs obtained by scanning transmission electron microscope elucidate nanostructural exchange coupled Fe-Co-V-Y (15–20 nm), Cr-Mo-Zr and Y1Co5 (10 nm) phases for 41Fe-28Cr-23Co-3Mo-2V-1Zr-2Y alloy. Pole figures and ODFs deduced sharp magnetic {1 1 0}〈0 0 1〉 textures or preferential orientation of Fe-Co rich rod like particles. Henkel plots deduced strong ferromagnetic exchange interactions between the magnetic phases. The 43Fe-28Cr-23Co-3Mo-2V-1Zr alloy yields Br of 0.71 T, Hc of 97 kA/m and (BH)max of 51.3 kJ/m3 which were remarkably improved to Br of 1.05 T, Hc of 130 kA/m and (BH)max of 61.6 kJ/m3 with 41Fe-28Cr-23Co-3Mo-2V-1Zr-2Y alloy.

Theory of superconductivity in hole-doped monolayer MoS2

We theoretically investigate the Cooper-pair symmetry to be realized in hole-doped monolayer MoS$_2$ by solving linearized BCS gap equations on the three-orbital attractive Hubbard-like model in the presence of the atomic spin-orbit coupling. In hole-doped monolayer MoS$_2$, both spin-orbit coupling and the multi-orbital effects are more prominent than those of electron-doped system. Near the valence band edge, the Fermi surfaces are composed of three different types of hole pockets, namely, one mainly consisting of the almost spin-degenerate $|d_{z^{2}}\rangle$ orbital near $\Gamma$ point, and the others of the spin-split upper and lower bands near ${\rm K}$ and ${\rm K}'$ points arising from the $|d_{x^{2}-y^{2}}\rangle$ and $|d_{xy}\rangle$ orbitals. The number of relevant Fermi pockets increases with increase of the doping. At very low doping, the upper split bands of $|d_{x^{2}-y^{2}}\rangle$ and $|d_{xy}\rangle$ are concerned, yielding extremely low $T_{\rm c}$ due to small density of states of the split bands. For further doping, the conventional spin-singlet state (SS) appears in the $\Gamma$ pocket, which has a mixture of the spin-triplet (orbital-singlet) (ST-OS) and spin-singlet (orbital-triplet) (SS-OT) states in the K and K$'$ pockets. The ratio of the mixture depends on the relative strength of the interactions, and the sign of the exchange interactions. Moderately strong ferromagnetic exchange interactions even lead to the pairing state with the dominant ST-OS state over the conventional SS one. With these observations, we expect that the fascinating pairing with relatively high $T_{\rm c}$ emerges at high doping that involves all the three Fermi pockets.

Strong ferromagnetic exchange interaction under ambient pressure in BaFe2S3

Inelastic neutron scattering measurements have been performed to investigate the spin waves of the quasi-one-dimensional antiferromagnetic ladder compound BaFe$_2$S$_3$, where a superconducting transition was observed under pressure [H. Takahashi {\it et al.}, Nat. Mater. 14, 1008-1012 (2015); T. Yamauchi {\it et al.}, Phys. Rev. Lett. 115, 246402 (2015)]. By fitting the spherically averaged experimental data collected on a powder sample to a Heisenberg Hamiltonian, we find that the one-dimensional antiferromagnetic ladder exhibits a strong nearest neighbor ferromagnetic exchange interaction ($SJ_R=-71\pm4$ meV) along the rung direction, an antiferromagnetic $SJ_L=49\pm3$ meV along the leg direction and a ferromagnetic $SJ_2=-15\pm2$ meV along the diagonal direction. Our data demonstrate that the antiferromagnetic spin excitations are a common characteristic for the iron-based superconductors, while specific relative values for the exchange interactions do not appear to be unique for the parent states of the superconducting materials.

Domain-wall profiles in Co/Irn /Pt(111) ultrathin films: Influence of the Dzyaloshinskii-Moriya interaction

We perform a study of domain walls in Co/Ir$_n$/Pt(111) (n = 0,1,...,6) films by a combined approach of first-principles calculations and spin dynamics simulations. We determine the tensorial exchange interactions and the magnetic anisotropies for the Co overlayer in both FCC and HCP geometries, depending on the number of Ir buffer layers. We find strong ferromagnetic nearest-neighbor isotropic exchange interactions between the Co atoms and an out-of-plane magnetic anisotropy for the films in FCC geometry. Our simulations show that the magnetic domain walls are of N\'eel type, and their rotational sense (chirality) is changed upon the insertion of an Ir buffer layer as compared to the pristine Co/Pt(111) system. Our spin dynamics simulations indicate a twisting of the spins with respect to the planar domain wall profile on the triangular lattice. We discuss this domain wall twisting using symmetry arguments and in terms of an appropriate micromagnetic continuum model considering extra energy terms compared to the available literature.