Antiparallel Magnetic Moments Research Articles

Emergence of a potential charge-disproportionated insulating state in SrCrO3

We use a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT) to investigate the potential emergence of a charge-disproportionated insulating phase in SrCrO3, whereby the Cr cations disproportionate according to 3Cr4+→2Cr3++Cr6+ and arrange in ordered planes perpendicular to the cubic [111] direction. We show that the charge disproportionation couples to a structural distortion where the oxygen octahedra around the nominal Cr6+ sites contract, while the octahedra surrounding the Cr3+ sites expand and distort. Our results indicate that the charge-disproportionated phase can be stabilized for realistic values of the Hubbard U and Hund's J parameters, but this requires a scaling of the DFT+DMFT double-counting correction by at least 35%. The disproportionated state can also be stabilized in DFT+U calculations for a specific magnetic configuration with antiparallel magnetic moments on adjacent Cr3+ sites and no magnetic moment on the Cr6+ sites. While this phase is higher in energy than other magnetic states that can arise in SrCrO3, the presence of an energy minimum suggests that the charge-disproportionated state represents a metastable state of SrCrO3. Published by the American Physical Society 2024

Nearly zero Poisson’s ratio and spin-state-controlled magnetoelastic response in an RKKY-mediated two-dimensional magnetic metal–organic framework

By means of first-principles calculations, the two-dimensional magnetic metal–organic framework, manganese phthalocyanine (MnPc), is proposed to exhibit nearly zero Poisson’s ratio within the elastic regime. Negative Poisson’s ratio can be reached by further increasing the uniaxial tensile strain beyond the yield point. Such an unusual mechanical property is found to rely on the spin-state of Mn atoms. Without strain, the ground state is ferrimagnetic and half-metallic. Interestingly, as the interatomic distance between Mn atoms increases under biaxial tensile strain, the alignment of magnetic moments of neighboring Mn atoms changes from antiparallel to parallel, associated with oscillatory exchange coupling strength. This not only reveals the strain-induced magnetic phase transition, but also indicates the magnetic interaction is mediated by the Ruderman–Kittel–Kasuya–Yosida exchange coupling and thus Kondo effect is likely to exist. Rather than the reported RKKY exchange coupling between supramolecular MnPc adsorbed on substrates, the predicted RKKY exchange coupling here exists in the pristine two-dimensional MnPc lattice. It would be more suitable for device applications because of the reduced size. Besides, it is noted that neighboring Mn atoms with antiparallel magnetic moments are responsible for the conducting behaviors, while those with parallel magnetic moments obstruct the flow of electrons. This property provides possibility of molding the flow of fully spin-polarized electrons through the manipulation of local magnetization by an external magnetic field, which is an attractive magnetoelectric response: the magnetic-field-controlled electron flow.

Nanocrystalline Iron Oxides with Various Average Crystallite Size Investigated Using Magnetic Resonance Method

A series of nanocrystalline iron oxide samples (M1–M5) which differ from each other in average crystallite size (from 26 to 37 nm) was studied. The raw material was nanocrystalline iron with an average crystallite size equal to 21 nm promoted with hardly reducible oxides: Al2O3, CaO, K2O (in total, max. 10 wt%). Nanocrystalline iron was subjected to oxidation with water vapor to achieve different oxidation degrees (α = 0.16–1.00). Metallic iron remaining in the samples after the oxidizing step was removed by etching. Magnetic resonance spectra of all samples were obtained at room temperature. All resonance lines were asymmetric and intense. These spectra were fitted by Lorentzian and Gaussian functions. All spectral parameters depend on the preparation method of the nanoparticles. We suppose that the Lorentz fit gives us a spectrum from larger agglomerated sizes whereas the Gaussian fit comes from much smaller magnetic centers. For the nanocrystalline samples with the largest size of iron oxide nanocrystallites, the highest value of total integrated intensity was obtained, indicating that at smaller sizes, they are more mobile in reorientation processes resulting in more settings of anti-parallel magnetic moments. The magnetic anisotropy should also increase with the increase in size of nanocrystallites.

Magnetic field modulated ultrafast spin dynamics at Ni80Fe20/Neodymium interface.

Ultrafast spin dynamics is crucial for the next-generation spintronic devices towards high-speed data processing. Here, we investigate the ultrafast spin dynamics of Neodymium/Ni80Fe20 (Nd/Py) bilayers by the time-resolved magneto-optical Kerr effect. The effective modulation of spin dynamics at Nd/Py interfaces is realized by an external magnetic field. The effective magnetic damping of Py increases with increasing Nd thickness, and a large spin mixing conductance (∼19.35×1015 cm-2) at Nd/Py interface is obtained, representing the robust spin pumping effect by Nd/Py interface. The tuning effects are suppressed at a high magnetic field due to the reduced antiparallel magnetic moments at Nd/Py interface. Our results contribute to understanding ultrafast spin dynamics and spin transport behavior in high-speed spintronic devices.

Bilayer hexagonal structure MnN2 nanosheets with room-temperature ferromagnetic half-metal behavior and a tunable electronic structure.

Two-dimensional (2D) layered materials have atomically thin thickness and outstanding physical properties, attracting intensive research in the past year. As one of these materials, a 2D magnet is an ideal platform for fundamental physics research and magnetic device development. Recently, a non-MoS2-type geometry was found to be more favorable in 2D transition-metal dinitrides. In this work, driven by this new configuration, we perform a comprehensive first-principles study on the bilayer hexagonal structure of 2D manganese dinitrides. Our results show that 2D MnN2 is a ferromagnetic half-metal at its ground state with 100% spin-polarization ratio at the Fermi energy level. The phonon spectrum calculation and ab initio molecular dynamics simulation show that the 2D MnN2 crystal has a high thermodynamic stability and its 2D lattice can be retained at room-temperature. Monte Carlo simulations based on the Heisenberg model predict a Curie temperature of over 563 K and its electronic properties can be regulated by biaxial strain. The half-metallic states are mainly contributed by Mn d orbitals, and the magnetic exchange of the system mainly comes from the Mn-N-Mn super-exchange. The p-d orbital hybridization will provide a small antiparallel magnetic moment of N atoms, and the p-orbital dangling bond can be eliminated by oxidation to enhance the total magnetic moment of the system. The study of magnetic anisotropy energy indicates that the easy magnetization axis is in-plane and hybridization between Mn dyz and dz2 orbitals gives the largest magnetic anisotropy contribution. In view of these results, we consider that novel 2D MnN2 is one of the most promising two-dimensional materials for nano-spintronic applications.

The impact of disorder on the 4O-martensite of Ni–Mn–Sn Heusler alloy

We have performed a quantum-mechanical study of thermodynamic, elastic, magnetic and structural properties of four different ferrimagnetic states in Ni1.9375Mn1.5625Sn0.5 martensite. They are modeled by the four-layer modulated 4O structures with Mn-excess atoms randomly distributed in Ni and Sn sublattices. The Mn atoms at the Ni sublattice turn out to be decisive for both thermodynamic and magnetic properties. A reversal of the orientation of their local magnetic moments has a huge impact on the properties of the whole system. The lowest-energy configuration exhibits anti-parallel local magnetic moments of these Mn atoms with respect to the orientation of the total magnetic moment. By testing both elastic properties and phonon modes we conclude that the lowest-energy state is mechanically stable. Vibrational properties of individual atoms are found to be very sensitive to the chemical disorder.

Pressure-Induced Increase of the Total Magnetic Moment in Ferrimagnetic Ni<sub>1.9375</sub>Mn<sub>1.5625</sub>Sn<sub>0.5</sub> Martensite: A Quantum-Mechanical Study

We have performed an ab initio study of disordered ferrimagnetic Ni1.9375Mn1.5625Sn0.5 martensite. Employing the supercell approach combined with the special quasi-random structure concept for modeling of disordered states we have determined thermodynamic, magnetic, structural, elastic and vibrational properties of the studied material. Its atomic and magnetic configuration is found to exhibit a pressure-induced increase of the total magnetic moment, i.e. the total magnetic moment increases with decreasing volume. This peculiar trend is revealed despite of the fact that the magnitudes of local magnetic moments of atoms decrease (or stay constant) with decreasing volume. The origin of the identified phenomena may be related to (i) the ferrimagnetic nature of the magnetic state when the parallel and antiparallel magnetic moments nearly compensate each other and (ii) chemical disorder that leads to different local atomic environments and, consequently, also to different local magnetic moments and their different response to hydrostatic pressures (the antiparallel moments are more sensitive). The studied state is mechanically and dynamically stable (no imaginary-frequency phonons) but, regarding its thermodynamic stability, it is an excited state.

Demonstration of toroidal metasurfaces through near-field coupling of bright-mode resonators

Bright-mode resonances are not well-acknowledged for inducing mode hybridizations. However, we demonstrate that multiple bright resonators coupled through electromagnetic fields can induce resonance mode hybridizations. Although one of the hybridized modes shows parallel magnetic moments, the other demonstrates anti-parallel magnetic moments leading to magnetic toroidal resonances. The excitation of toroidal modes usually demands complex structures and/or bright–dark-mode interactions. However, in this study, we employ only bright resonators to excite toroidal modes. Unlike bright–dark-mode coupling, exclusive bright-mode resonance coupling enables larger free-space energy to merge into the metasystem, which leads to stronger energy confinement in the metasurface array.

Interfacial magnetic coupling and orbital hybridization for D022-Mn3Ga/Fe films

Interfacial magnetic coupling interactions between D022-Mn3Ga and ultrathin Fe films were studied using first principles calculations. Based on the calculations of surface energy and interface energy, it is demonstrated that Mn-Ga/Fe is the most stable interfacial structure. As a result, four possible coupling states between D022-Mn3Ga and Fe films may be present by considering the relative direction of magnetic moment for the interfacial and inner-layer Mn atoms. Their corresponding energies were calculated by varying the Fe atomic layer thicknesses from 1 to 6 monolayers. It is found that the antiferromagnetic coupling energy in D022-Mn3Ga/Fe films with an anti-parallel magnetic moment of the interfacial and inner-layer Mn atoms is the smallest one, regardless of the Fe layer numbers. The possible mechanism about the antiferromagnetic interactions between D022-Mn3Ga and ultrathin Fe films was analyzed by the orbital-resolved electronic density of states.

Breakdown of Hund's rule for CuFeAs

The ground-state properties of CuFeAs were investigated by applying density functional theory calculations within generalized gradient approximation (GGA) and GGA+U. We find that the bicollinear antiferromagnetic state with antiparallel orbital magnetic moments on each iron which violates the Hund's rule is favored by the on-site Coulomb interaction, which is further stabilized by Cu vacancy. The magnetic ground state can be used to understand weak antiferromagnetism in CuFeAs observed experimentally. We argue that breakdown of the Hund's rule may be the possible origin for reduced magnetism in iron pnictides, rather than magnetic fluctuations induced by electronic correlations.

Quantum theory of femtosecond optomagnetic effects for rare-earth ions in DyFeO3

Our theoretical analysis shows that a femtosecond laser pulse can efficiently launch magnetization dynamics of ${\mathrm{Dy}}^{3+}$ ions in ${\mathrm{DyFeO}}_{3}$ and ${\mathrm{DyAlO}}_{3}$. Excitation of electrons from the ground state to the low-lying electronic level of ${\mathrm{Dy}}^{3+}$ ions by circularly or linearly polarized light can be seen as a result of an effective magnetic field acting on the magnetic moments of the rare-earth ions. It is shown that the launched magnetization dynamics can be expressed as a combination of coherent oscillations of mutually parallel and mutually antiparallel magnetic moments of ${\mathrm{Dy}}^{3+}$ ions, respectively. While the antiparallel magnetic moments lie in the plane perpendicular to the wave vector of light in the medium $\mathbit{k}$, the parallel magnetic moments are aligned along $\mathbit{k}$. The magnetization dynamics depend strongly on the duration and the shape of the pumping laser pulse, as well as on the anisotropy in properties of the rare-earth ion.

Ab initio comparative study of the magnetic, electronic and optical properties of AB2O4 (A, B= Mn, Fe) spinels

The comparison of the magnetic, electronic, and optical properties of the spinel transition-metal oxides AB2O4 (A, B = Fe, Mn) and their relationship with the structure and composition were studied within DFT-GGA + U approximation. The spinels were considered both in the normal and inverse structure. We have found that regardless of composition and structure, the studied spinels are ferrimagnetic with antiparallel magnetic moments on A- and B-site cations. Electronic and structural properties of spinels depend on the composition: FeMn2O4 has a tetragonal structure and half-metallic properties; however, in the inverse FeMn2O4, the bandgap opens for the spin-up channel. MnFe2O4 is a cubic insulator with a bandgap of about 1.5 eV, which decreases in the inverse structure. The superexchange constants estimate within the simple indirect coupling model and have values close to the experimental ones. The total magnetization of FeMn2O4 is drop-down to zero under hydrostatic pressure above 60 GPa due to the strong dependence of the magnetic moment of octahedral manganese ion on the pressure. The microscopic mechanisms of the relationship between the structure, composition and properties are studied.

Current-induced linear motion of antiferromagnetically coupled skyrmion-like bubble domains stabilized without Dzyaloshinskii–Moriya interaction

Magnetic domains are used to represent binary digits in magnetic memories. In domain-motion magnetic memories, the domain must move longitudinally along a ferromagnetic nanowire. Antiferromagnetically coupled magnetic skyrmions, which are domains meeting this requirement, can be stabilized via the Dzyaloshinskii–Moriya interaction (DMI) in an antiferromagnetically coupled bi-layered perpendicularly magnetized thin wire. We find that antiferromagnetically coupled skyrmion-like bubble domains (BDs) may be produced in such a wire without DMI. The stability and the dynamics of such domains are studied by micromagnetic simulations based on the Landau–Lifshitz–Gilbert equation. The BDs in the two layers have clockwise and anti-clockwise Bloch type domain walls, so the BD cores have anti-parallel magnetic moments and are antiferromagnetically coupled. Domain size decreases with decreasing wire width and/or increasing antiferromagnetic coupling strength of the wire. The minimum value of the BD radius is about 10 nm. When a spin-polarized current flows through the wire, the BD moves with the current.

Current-Induced In-Plane Magnetization Switching in a Biaxial Ferrimagnetic Insulator

Ferrimagnetic insulators (FiMI) have been intensively used in microwave and magneto-optical devices as well as spin caloritronics, where their magnetization direction plays a fundamental role on the device performance. The magnetization is generally switched by applying external magnetic fields. Here we investigate current-induced spin-orbit-torque switching of the magnetization in ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ ($\mathrm{YIG}$)/$\mathrm{Pt}$ bilayers with in-plane magnetic anisotropy, where the switching is detected by spin Hall magnetoresistance. Reversible switching is found at room temperature for a threshold current density of ${10}^{7}\phantom{\rule{0.1em}{0ex}}\mathrm{A}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The $\mathrm{YIG}$ sublattices with antiparallel and unequal magnetic moments are aligned parallel or antiparallel to the direction of current pulses, which is consistent to the N\'eel-order switching in an antiferromagnetic system. It is proposed that such a switching behavior may be triggered by the antidamping torque acting on the two antiparallel sublattices of FiMI. Our finding not only broadens the magnetization switching by electrical means and promotes the understanding of magnetization switching, but also paves the way for all-electrically modulated microwave devices and spin caloritronics with low power consumption.

Investigation of Full-Heusler compound Mn2MgGe for magnetism, spintronics and thermoelectric applications: DFT study

First principle method based Density functional theory calculations are used to investigate electronic, magnetic, thermoelectric and lattice-dynamical properties of Full-Heusler compound Mn2MgGe. We considered the energetic, thermodynamical and lattice-dynamical stability criteria for Mn2MgGe by evaluating the total energy, formation energy, and phonon density of states respectively, which thereby suggest that the synthesis of Full-Heusler compound Mn2MgGe is possible. Half metallic spin-polarized nature from electronic band structure and total magnetic moment (2μB) of Hg2CuTi type structure of Mn2MgGe potentially facilitate its spintronics applications. The results of electronic structure and magnetism predicts that Hg2CuTi type structure of Mn2MgGe is an indirect band gap semiconductor with half-metallic ferrimagnetic behaviour. Antiparallel and equal magnetic moment of Mn atoms cancel out the magnetization in Cu2MnAl type structure of Mn2MgGe, making its total magnetic moment 0 μB/cell. The electronic and magnetic properties for Cu2MnAl type structure of Mn2MgGe has not been previously reported. Positive and negative value of Seebeck coefficient for spin up and spin down channel indicate the presence of P-type and N-type charge carriers in the compound. Mn2MgGe exhibits high Seebeck coefficient −38 μV/K and 180 μV/K in spin-up and spin-down channel at room temperature (300 K) along with huge power factor of 4.23×1011 w/mK2s at 600 K, predicting Mn2MgGe as a potential candidate for thermoelectric applications.

Tailoring magnetic interactions in atomic bilayers of Rh and Fe on Re(0001)

Using density functional theory, we investigate the interplay between the stacking order and sequence of bilayers composed of an Fe and a Rh layer on the Re(0001) and their magnetic properties. We find that fcc/ffc stacked bilayers are energetically very unfavorable, while all other combinations of hcp and fcc stacking are energetically close. The magnetic interactions are evaluated by mapping the DFT total energies onto a spin model, which contains Heisenberg exchange, Dzyaloshinskii-Moriya interaction, the magnetocrystalline anisotropy energy, and higher-order exchange interactions. We find that the stacking sequence of the bilayer significantly modifies the magnetic interactions. As a result, we find a DMI driven cycloidal spin spiral ground state with a period of 11~nm for hcp-Fe/hcp-Rh. For fcc-Fe/hcp-Rh and hcp-Fe/fcc-Rh, we obtain a ferromagnetic ground state. The spin spiral energy dispersion of hcp-Fe/hcp-Rh including spin-orbit coupling suggests that isolated skyrmions can be stabilized in the field-polarized ferromagnetic background at external magnetic fields. If the Fe layer is sandwiched between the Rh overlayer and the Re(0001) substrate, there is a competition between the ferromagnetic coupling preferred by the Rh-Fe hybridization and the antiferromagnetic coupling induced by the Fe-Re hybridization. Due to the Fe/Re interface the DMI can become very large. For fcc-Rh/hcp-Fe, we obtain a cycloidal spin spiral with a period of 1.7~nm which is induced by frustration of exchange interactions and further stabilized by the DMI. For hcp-Rh/hcp-Fe, we find a DMI driven cycloidal spin spiral with a period of 4~nm and locally nearly antiparallel magnetic moments due to antiferromagnetic nearest-neighbor exchange. The higher-order exchange constants can be significant in the considered films, however, they do not stabilize multi-$Q$ states.

Magnetic ground state in FeTe2,VS2 , and NiTe2 monolayers: Antiparallel magnetic moments at chalcogen atoms

Our analysis based on the results of hybrid and semilocal density-functional calculations with and without Hubbard $U$ correction for on-site Coulomb interactions reveals the true magnetic ground states of three transition-metal dichalcogenide monolayers, viz., ${\mathrm{FeTe}}_{2},{\mathrm{VS}}_{2}$, and ${\mathrm{NiTe}}_{2}$, which comprise inhomogeneous magnetic moment configurations. In contrast to earlier studies considering only the magnetic moments of transition-metal atoms, the chalcogen atoms by themselves have significant, antiparallel magnetic moments owing to the spin polarization through $p\text{\ensuremath{-}}d$ hybridization. The latter is found to be true for both $H$ and $T$ phases of ${\mathrm{FeTe}}_{2},{\mathrm{VS}}_{2}$, and ${\mathrm{NiTe}}_{2}$ monolayers. Our predictions show that the ${\mathrm{FeTe}}_{2}$ monolayer in its lowest-energy structure is a half metal, which prevails under both compressive and tensile strains. Half metallicity occurs also in the ${\mathrm{FeTe}}_{2}$ bilayer but disappears in thicker multilayers. The ${\mathrm{VS}}_{2}$ monolayer is a magnetic semiconductor; it has two different band gaps of different character and widths for different spin polarization. The ${\mathrm{NiTe}}_{2}$ monolayer, which used to be known as a nonmagnetic metal, is indeed a magnetic metal with a small magnetic moment. These monolayers with intriguing electronic and magnetic properties can attain new functionalities for spintronic applications.

Extreme narrow magnetic domain walls in U ferromagnets: The UCoGa case

Surface magnetic domains of a UCoGa single crystal during magnetization/demagnetization processes in increasing/decreasing magnetic fields were investigated by means of magnetic-force-microscopy (MFM) images at low temperatures. The observed domain structure is typical for a ferromagnet with strong uniaxial anisotropy. The evolution of magnetic domains during cooling of the crystal below TC has also been manifested by MFM images. Analysis of the available data reveals that the high uniaxial magnetocrystalline energy in combination with the relatively small ferromagnetic exchange interaction in UCoGa gives rise to the formation of very narrow domain walls formed by the pairs of the nearest U neighbor ions with antiparallel magnetic moments within the basal plane. Since the very high anisotropy energy is a common feature of the majority of the uniaxial U ferromagnets, analogous domain-wall properties are expected for all these materials.

Spin-Dependent Electronic and Thermoelectric Transport Properties for a Sawtoothlike Graphene Nanoribbon Coupled to Two Ferromagnetic Leads

The spin-dependent electronic transport and thermoelectric properties for a sawtoothlike graphene nanoribbon between two ferromagnetic leads with parallel or antiparallel magnetic moments are investigated theoretically. Both Christmas-tree graphene nanoribbon and tree-saw graphene nanoribbon have been considered to exemplify the effect of the chirality. By using ab initio calculations combined with a non-equilibrium Green’s function method, we obtain the important result that these sawtoothlike graphene nanoribbon junctions, regardless of their chirality, can exhibit giant charge and spin Seebeck coefficients as well as high thermoelectric efficiency at room temperature by putting ferromagnetic stripes or magnetic fields on the nanoribbons. Specifically, large $$|S_\mathrm{C}|\,(\sim 5.8~\hbox {mV/K})$$, $$|S_\mathrm{S}|(\sim 1.8~\hbox {mV/K})$$, charge figure of merit $$ZT_\mathrm{C}$$ and spin figure of merit $$ZT_\mathrm{S}(\sim 20)$$ were found in these sawtoothlike graphene nanoribbon junctions. In addition, negative differential resistance and spin filtering were also realized in these sawtoothlike graphene nanoribbon junctions. The maximum of spin filtering efficiency can be up to 100%. Our studies suggest that sawtoothlike graphene nanoribbon junctions are suitable for use as highly efficient spin caloritronics at room temperature.

Anomalous ferromagnetism and magneto-optical Kerr effect in semiconducting double perovskite Ba2NiOsO6 and its (111) (Ba2NiOsO6)/(BaTiO3)10 superlattice

We carry out a first-principles investigation on magnetism, electronic structure, magneto-optical effects and topological property of newly grown cubic double perovskite Ba2NiOsO6 and its (111) (Ba2NiOsO6)/(BaTiO3)10 superlattice, based on the density functional theory with the generalized gradient approximation (GGA) plus onsite Couloumb interactions. Interestingly, we find that both structures are rare ferromagnetic (FM) semiconductors with estimated Curie temperatures of ~150 and 70 K, respectively. The calculated near-neighbor exchange coupling parameters reveal that the ferromagnetism is driven by exotic FM coupling between Ni and Os atoms, which is due to the FM superexchange interaction caused by the abnormally strong hybridization between the half-filled Nieg and unoccupied Os eg orbitals. The strong spin-orbit coupling (SOC) on the Os atom is found to not only open the semiconducting gap but also produce a large antiparallel orbital magnetic moment on the Os atom, thus reducing the total magnetization from 4.0 uB/f.u.,