Indirect Exchange Interaction Research Articles

Two-Dimensional Transition Metal Boron Cluster Compounds (MBnenes) with Strain-Independent Room-Temperature Magnetism.

Two-dimensional (2D) metal borides (MBenes) with unique electronic structures and physicochemical properties hold great promise for various applications. Given the abundance of boron clusters, we proposed employing them as structural motifs to design 2D transition metal boron cluster compounds (MBnenes), an extension of MBenes. Herein, we have designed three stable MBnenes (M4(B12)2, M = Mn, Fe, Co) based on B12 clusters and investigated their electronic and magnetic properties using first-principles calculations. Mn4(B12)2 and Co4(B12)2 are semiconductors, while Fe4(B12)2 exhibits metallic behavior. The unique structure in MBnenes allows the coexistence of direct exchange interactions between adjacent metal atoms and indirect exchange interactions mediated by the clusters, endowing them with a Néel temperature (TN) up to 772 K. Moreover, both Mn4(B12)2 and Fe4(B12)2 showcase strain-independent room-temperature magnetism, making them potential candidates for spintronics applications. The MBnenes family provides a fresh avenue for the design of 2D materials featuring unique structures and excellent physicochemical properties.

ThCr2Si2C: An Antiferromagnetic Metal with a Cr2C Square Lattice.

A quaternary compound, ThCr2Si2C, was synthesized by using the arc-melting technique. The compound adopts a tetragonal CeCr2Si2C-type crystal structure. The electronic resistivity and specific heat data exhibit metallic behavior, while the magnetic susceptibility displays a pronounced broad peak at around 370 K, indicating the antiferromagnetic phase transition. The first-principles calculations suggest A-type antiferromagnetic ordering of the Cr sublattice, which is confirmed by neutron diffraction experiments. By comparing the crystal structure of ThCr2Si2C with the isostructural Cr-based compounds, the magnetic state of Cr 3d orbital is discussed in terms of the band-filling effects and indirect spin exchange interaction.

Anion substitution effect on the crystal structure and properties of high-coercive Fe0.25TaX2 (X = S, Se)

Using the solid-phase method, a series of layered compounds Fe0.25TaS2–ySey has been synthesized and studied using X-ray diffraction, magnetization, electrical resistivity and magnetoresistance measurements to reveal the evolution of the crystal structure and properties with the replacement of sulfur by selenium. The crystal structure of Fe0.25TaS2–ySey consists of chains along the c axis, in which tantalum and iron atoms alternate in trigonal-prismatic and trigonal-antiprismatic coordination, respectively. The replacement of sulfur by selenium is accompanied by an anisotropic expansion of the crystal and a relative elongation of the structure in the direction perpendicular to the plane of the layers. All the Fe0.25TaS2–ySey compounds exhibit ferromagnetic behavior with a huge coercive field (Hc ∼ 40–60 kOe at low temperatures) which decreases exponentially with increasing temperature. It is assumed that an almost twofold decrease in the value of TС (from 120 K to ∼ 60 K) when sulfur is replaced by selenium occurs mainly due to a decrease in the polarization of the 5d electrons of tantalum, through which an indirect exchange interaction between the 3d electrons of Fe occurs. The change of the low-temperature (2 K) coercive field with the Se for S substitution is found to correlate with the concentration dependence of the Curie temperature, which is indicative of the Ising spin state of Fe ions in these compounds.

Electrical Effects of Ionic Deficient Cubic Ferrite Spinels: A Case Study on MgFe2O4

AbstractElectron transport via indirect exchange interactions between octahedral sites in spinels is affected by cationic or anionic vacancies. Thus, the deviation of the electron hopping mechanism is investigated, from a constant energy barrier toward a variable range energy barrier in iron‐deficient and oxygen‐deficient cubic magnesium ferrite spinels. Variable range electron hopping activation energy at high temperatures is consistent with the modified cation–anion interactions in the distorted structure of an ionic deficient cubic magnesium ferrite spinel. For these observations, an instrumental method is developed to evaluate the electron exchange interactions and general redox kinetics on surfaces, managed by exposure to the flow of a wide variety of desired gases. The applications of this instrument are far beyond the example studies reported here.

Temperature activated chiral induced spin selectivity.

Recent experiments performed on chiral molecules, comprising transition metal or rare earth elements, indicate temperature reinforced chiral induced spin selectivity. In these compounds, spin selectivity is suppressed in the low temperature regime but grows by one to several orders of magnitude as the temperature is increased to room temperature. By relating temperature to nuclear motion, it is proposed that nuclear displacements acting on the local spin moments, through indirect exchange interactions, generate an anisotropic magnetic environment that is enhanced with temperature. The induced local anisotropy field serves as the origin of a strongly increased spin selectivity at elevated temperature.

Multilayer Epitaxial Heterostructures with Multi-Component III-V:Fe Magnetic Semiconductors.

Three-layer structures based on various multi-component films of III-V semiconductors heavily doped with Fe were grown using the pulsed laser sputtering of InSb, GaSb, InAs, GaAs and Fe solid targets. The structures comprising these InAsSb:Fe, InGaSb:Fe and InSb:Fe layers with Fe concentrations up to 24 at. % and separated by GaAs spacers were deposited on (001) i-GaAs substrates at 200 °C. Transmission electron microscopy showed that the structures have a rather high crystalline quality and do not contain secondary-phase inclusions. X-ray photoelectron spectroscopy investigations revealed a significant diffusion of Ga atoms from the GaAs regions into the InAsSb:Fe layers, which has led to the formation of an InGaAsSb:Fe compound with a Ga content up to 20 at. %. It has been found that the ferromagnetic properties of the InAsSb:Fe magnetic semiconductor improve with an increasing Sb:As ratio. It has been concluded that the indirect ferromagnetic exchange interaction between Fe atoms occurs predominantly via Sb atoms.

RKKY exchange interactions in two-dimensional Mn-doped ZnO dilute magnetic semiconductor

In this paper, we have theoretically studied the RKKY indirect exchange interactions in a two-dimensional model of Zn1-xMnxO dilute magnetic semiconductor. Under the RKKY interaction framework, we have calculated the exchange coupling constants for four different doped samples of host ZnO, which are 5%, 10%, 15%, and 20%. The carrier density for the respective configurations is taken as 0.1, 0.5, 0.75, and 1 hole per dopant, considering long-range interactions controlled by the cutoff scale in the RKKY range function. The variation of magnetic coupling strength with the carrier density is also examined, and the overall magnetic behavior of the proposed 2-D system is estimated.

Indirect Exchange Interaction Leads to Large Lattice Contribution to Magnetocaloric Entropy Change.

Materials with a large magnetocaloric response are highly desirable for magnetic cooling applications. It is suggested that a strong spin-lattice coupling tends to generate a large magnetocaloric effect, but no microscopic mechanism has been proposed. In this Letter, we use spin-lattice dynamics simulation to examine the lattice contribution to the magnetocaloric entropy change in bcc iron (Fe) and hcp gadolinium (Gd) with exchange interaction parameters determined from abinitio calculations. We find that indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction in hcp Gd leads to longer-range spin-lattice coupling and more strongly influences the low-frequency long-wavelength phonons. This results in a higher lattice contribution toward the total magnetocaloric entropy change as compared to bcc Fe with short-range direct exchange interactions. Our analysis provides a framework for understanding the magnetocaloric effect in magnetic materials with strong spin-lattice couplings. Our finding suggests that long-range indirect RKKY-type exchange gives rise to a larger lattice contribution to the magnetocaloric entropy change and is, thus, beneficial for magnetocaloric materials.

Interplay between Kondo and magnetic interactions in Pr0.75Gd0.25ScGeH

Combined experimental and density functional theory (DFT) study of Pr0.75Gd0.25ScGe and its hydride (Pr0.75Gd0.25ScGeH) reveals intricacies of composition-structure-property relationships in those distinctly layered compounds. Hydrogenation of the intermetallic parent, crystalizing in a tetragonal CeScSi-type structure, leads to an anisotropic volume expansion, that is, a(=b) lattice parameter decreases while the lattice expands along the c direction, yielding a net increase of cell volume. DFT calculations predict an antiparallel coupling of localized Gd and Pr magnetic moments in both materials at the ground state. While experiments corroborate this for the parent compound, there is no conclusive experimental proof for the hydride, where Pr moments do not order down to 3 K. DFT results also reveal that rare-earth – hydrogen interactions reduce spin-polarization of the Pr and Gd 5d and Sc 3d states at the Fermi energy, disrupt indirect exchange interactions mediated by conduction electrons, dramatically reduce the magnetic ordering temperature, and open a pseudo-gap in the majority-spin channel. Both experiments and theory show evidence of Kondo-like behavior in the hydride in the absence of an applied magnetic field, whereas increasing the field promotes magnetic ordering and suppresses Kondo-like behavior.

Stable antiferromagnetic property and tunable electronic structure of two-dimensional MnPX3 (X = S and Se) from pristine structure to Janus phase

Transition-metal phosphorus trichalcogenides have been considered as very promising two-dimensional (2D) magnetic candidates up-to-date. We performed a systematical first-principles study on the electronic structures and magnetic properties of pristine MnPX3 (X = S and Se) and Janus Mn2P2S3Se3 monolayers. All monolayers behave as a direct-band-gap semiconductor in antiferromagnetic ground state which is caused by strong direct and indirect exchange interactions. It is found that the electronic structures and magnetic properties can be manipulated by Janus phase. The calculated band gap is 2.44 eV, 1.80 eV and 1.86 eV for MnPS3, MnPSe3 and Mn2P2S3Se3 with a valley polarization with consideration of spin–orbital coupling (SOC), respectively. In particular, significant energy-splittings emerge in the SOC-band structures of Janus Mn2P2S3Se3 due to its broken-inversion-symmetry. Estimated by Monte Carlo simulations, the Néel temperature is 96 K, 71 K and 79 K based on Ising model while halved down to 41 K, 33 K and 36 K on the basis of XY model for MnPS3, MnPSe3 and Mn2P2S3Se3, respectively, indicating the XY model should be more reliable to describe the spin dynamics. Our research offers an insight into the magnetic mechanism and paves a feasible path to modulate the magnetism for 2D magnets in realistic applications on spintronics.

Spin-induced valley polarization in heterobilayer Janus transition-metal dichalcogenides

Inspired by potential application prospects of spintronics and valleytronics, a novel heterobilayer Janus structure is designed by replacing the chalcogenide atomic layers in the original bilayer MoS2. Based on first-principles calculations, it is found that the SMoS/SeMoS structure exhibits a direct band-gap semiconductor and a typical type-II band alignment with longer carrier lifetime. The transition metal (TM) atom represented by V/Cr/Mn can be stably adsorbed on the heterobilayer Janus SMoS/SeMoS sheet and effectively introduce magnetic moments (m). The calculation results demonstrate that the most stable adsorption site of the TM atom is CX(A), and the TM (V/Cr/Mn) adatom modified SMoS/SeMoS system is converted into metal (V-) or half-metal (Cr/Mn-), respectively. Under the coupling of different indirect exchange interactions, the structure exhibits stable intrinsic anti-ferromagnetic interactions for V-SMoS/SeMoS and ferromagnetic ground state for Cr/Mn-SMoS/SeMoS, respectively, and the magnetic transition temperature (T c) reaches a high temperature or even room temperature. Moreover, the robust out-of-plane magnetocrystalline anisotropy energy ensures stable long-range magnetic order. Specifically, the combination of spin injection and strong spin–orbit coupling interaction effectively breaks the time-reversal symmetry, which leads to valley polarization of the system. Based on this, the biaxial strain can effectively regulate the electronic structure, magnetic properties and valley polarization of TM-SMoS/SeMoS nanosheets with double breaking of spatial-inversion and time-reversal symmetry.

Pseudobrookite Fe2-2xCoxTi1-½xO5: Structural, magnetic phase transformation and reflection loss characteristic

Single-phase pseudobrookite Fe2-2xCoxTi1-½xO5 (x = 0, 0.1, 0.2, and 0.4) with an orthorhombic structure has been successfully synthesized using a solid reaction and sintering at 1000 °C. Pseudobrookite Fe2-2xCoxTi1-½xO5 is an element that is abundant on earth and is attracting rapidly growing interest as a very promising candidate for the application of microwave absorbing materials. This paper studies the performance of exchange coupling in addition to the structural properties and microwave absorption characteristics of pseudobrookite Fe2-2xCoxTi1-½xO5. Qualitative and quantitative analysis of the X-ray diffraction (XRD) pattern show that all samples were single-phase pseudobrookite Fe2TiO5 with a space group of C m c m. The presence of Co causes the expansion of the lattice parameter with an indicator of an increase in the value of the unit cell volume as the composition of Co in the structure increases. Particle morphology and elemental composition were also observed using a scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS). The presence of Co2+ dopant ions does not affect the shape and size of the particles, while the crystallite size increased with the increase in the content of Co2+ dopant ions in the structure and Co elements of 1.82, 2.81, and 5.69 at% have been detected, which are contained in the composition x = 0.1, 0.2, and 0.4, respectively. The presence of the Co2+ dopant ion it stimulates the rearrangement of the cationic distribution in both tetrahedral and octahedral sites which will have an impact on changes in the magnetic properties of this pseudobrookite. Magnetic properties were analyzed by measuring magnetization (M) against a magnetic field (H) at 300 K (RT). The spontaneous magnetization at room temperature in pseudobrookite Fe2-xCoxTi1-½xO5 is ∼2.55, 7.41 and 8.67 emu/g for compositions of x = 0.1, 0.2 and 0.4, respectively. The coercive field is ∼424, 356 and 284 Oe for compositions of x = 0.1, 0.2 and 0.4, respectively. The presence of Co in the structure causes a magnetic phase transformation from paramagnetic to ferromagnetic behavior at room temperature. This ferromagnetic behavior can be explained due to the strengthening of indirect exchange interactions of Ti/Co – O – Fe/Co in addition to the interactions of Fe/Co – O – Fe/Co and Ti/Co – O – Ti/Co, as well as changes in the degree of inversion of this pseudobrookite structure, due to the presence of magnetic ion dopants. Magnetic parameters both Ms, Mr, and Hc show an increase with increasing Co2+ ion content at room temperature. This magnetic parameter is very important at room temperature to determine the absorption characteristics of microwaves through the resonant frequency. Microwave absorption characteristics are measured in the 8–12 GHz frequency range. The best reflection loss (RL) peaks were obtained at composition x = 0.1 of -24 dB at 10.6 GHz frequency. The presence of Co2+ dopant ions can produce a very significant increase in RL almost 3 times at the same absorption frequency. This increase in reflection loss can be attributed to the effect of the magnetic phase transformation of this material which has been described accordance of the magnetic coupling exchange interaction, which in turn changes the magnitude of the relative permeability value of this material complex.

Magnetic transitions and different valence states of manganese ions in solid solutions (Ni,Cu,Mn)-=SUB=-3-=/SUB=-BO-=SUB=-5-=/SUB=-

The content of metal ions and their valence state for three NixCuyMn3-x-yBO5 compositions were estimated by EXAFS and XANES spectroscopy methods. In all three compositions, the copper y content did not exceed 0.25; in the third composition, some manganese ions entered the compound not only in the trivalent, but also in the divalent state. A detailed study of the magnetic properties showed that two magnetic transitions are observed in all compositions: one in the region of 60-75 K, and the second in the region of 10 K. Based on the exchange interactions calculated within the framework of the empirical model, the magnetic ordering temperatures for two- and three-lattice magnets were determined and it was assumed that the magnetic transition in the region of 60-75 K is associated with the ordering of magnetic moments in positions 2 and 3. Keywords: ludwigites, magnetic phase transition, indirect exchange interactions.

Size effect in electron paramagnetic resonance spectra of impurity centers in diamond particles

The experiments have shown qualitative changes in electron paramagnetic resonance (EPR) spectra with decreasing average size of diamond particles from 100 μm to 18 nm. The characteristic hyperfine splitting in the EPR spectrum due to the interaction of an unpaired electron of a neutral impurity (nitrogen atom) in the diamond lattice with its nuclei disappears, and the resulting EPR spectrum is a superposition of two singlet Lorentzian lines. The reason for the observed effect is the indirect exchange interaction of electrons localized on bulk impurity centers through electrons localized on surface dangling bonds. Such an interaction of electrons localized on bulk centers becomes possible due to the resonant energy transfer between them. The transfer is result of the exchange interaction with electrons of surface centers. It is shown, that the energy of the indirect exchange interaction depends on the size of the diamond particle; therefore, the effect can be observed only in the case when particle size is smaller than a critical value R cr . The calculated value of the R cr based on the suggested model fitted well with experimental data.

Indirect exchange interaction in two-dimensional materials with quartic dispersion

We investigate the indirect magnetic exchange interaction between two magnetic moments in a two-dimensional semiconductor with quartic dispersion, featuring a singularity at the band edge. We obtain the Green's functions analytically to calculate the magnetic exchange interaction at zero temperature. We show that the singularity in the density of states (DOS) for quartic dispersion gives rise to an enhancement in the amplitude of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction as the Fermi energy is swept toward the band edge. Furthermore, a region of finite exchange interaction arises, with a range increasing as the Fermi energy approaches the band edge. The results lay the possibility of an electrical/chemical control over the exchange interactions.

Enhanced d0 ferromagnetism via carbon doping in rare-earth sesquioxides: DFT prediction

In this study, a new possible diluted magnetic semiconductor (DMS) with d0 ferromagnetism was predicted. RE2O3 compounds (RE = Sc, Y, and Lu) doped with a nonmagnetic impurity (carbon) are investigated using density functional theory. It was found that Sc2O2.875C0.125 and Lu2O2.875C0.125 are similar to the recently predicted room-temperature DMS in Y2O2.875C0.125 [Phys. Rev. B 97, 184,411 (2018)]; while, both show an enhanced Curie temperature. An RKKY-like interaction with indirect exchange interaction is assumed to induce ferromagnetic coupling in these materials. The calculations in this work also suggest that the Lu2O2.875C0.125 compound has an extended magnetism compared to other studied compounds because the carbon-cluster configuration (case 1) is less favorable. The findings of this study suggest that carbon-doped Lu2O3 would be the best candidate for spintronic devices.

Impurity states and indirect exchange interaction in irradiated graphene

We theoretically investigate the impurity levels and exchange interaction between magnetic impurities in graphene driven by an off-resonant circularly polarized light field. Our analysis captures the non-perturbative effects resulting from scattering with magnetic impurities with a strong onsite potential. Under irradiation, a dynamical band gap opens up at the Dirac point, allowing impurity levels to exist inside the gap. These impurity levels are shown to give rise to a resonance feature in the exchange energy for impurities located either at the same or different sublattices. The exchange interaction also shows a wider spatial range of antiferromagnetic behavior due to irradiation. Our work demonstrates that the exchange energy of magnetic impurities in graphene is extensively tunable by light irradiation in the presence of strong potential scattering.

Observation of Thickness-Dependent Exchange Interaction in EuO Ultrathin Films

The electronic structure of single-crystalline EuO (100) ultrathin films, together with their exchange splitting energy (ΔEEX) and ferromagnetic phase transition temperature (TC), was investigated by temperature- and thickness-dependent angle-resolved photoemission spectroscopy. Both ΔEEX and TC decreased monotonically with decreasing film thickness. The band shift showed an opposite thickness dependence at the Γ and X points, reflecting the balance of the hybridization between the Eu 4f and O 2p states (super-exchange interaction) and between the Eu 4f, O 2p, and Eu 5d states (indirect-exchange interaction). The observed transition from an indirect energy gap in the bulk to a direct gap in the ultrathin films of the ferromagnetic semiconductor EuO could be potential in future spintronic devices.

Tuning valley splitting and magnetic anisotropy of multiferroic CuMP2X6 (M=Cr,V;X=S,Se) monolayer

Monolayer $\mathrm{Cu}M{\mathrm{P}}_{2}{X}_{6}$ $(M=\mathrm{Cr},\mathrm{V};X=\mathrm{S},\mathrm{Se})$ has been attracting increasing attention due to the ferromagnetism derived from the indirect exchange interaction between Cr (V) atoms and the ferroelectricity originating from the spontaneous displacement of Cu atoms. Using first-principles calculations, we predicted the intrinsic valley splittings and anomalous valley Hall effect in $\mathrm{Cu}M{\mathrm{P}}_{2}{X}_{6}$ monolayers. In addition, the valley splittings and Berry curvature are independent of the orientation of out of plane (OOP) ferroelectric polarization, which makes it a promising storage device with dual degrees of freedom in ferroelectric polarization and ferrovalleys. Due to the variations in the energy of the spin-dependent valence-band maximum (VBM) and orbital compositions of the valleys, the magnitude of valley splitting at VBM can be effectively tuned by the biaxial strain and external electric field. Large OOP magnetic anisotropy can be obtained by changing the energy and occupation (unoccupation) states of $d$ orbital compositions through strain and charge doping, respectively, and the magnetic anisotropy of $\mathrm{Cu}M{\mathrm{P}}_{2}{X}_{6}$ increases with increasing tensile biaxial strain from \ensuremath{-}5% to 5%. Our research results are expected to tune the valley splittings and magnetic anisotropy of $\mathrm{Cu}M{\mathrm{P}}_{2}{X}_{6}$ and to help understand the physical mechanism.

Synthesis, structural and magnetic properties of ludwigite Mn-=SUB=-1.32-=/SUB=-Ni-=SUB=-0.85-=/SUB=-Cu-=SUB=-0.83-=/SUB=-BO-=SUB=-5-=/SUB=-

Single crystals of Mn1.32Ni0.85Cu0.83BO5 with the ludwigite structure were obtained by the solution-melt method during spontaneous crystallization. The concentration of copper, manganese and nickel ions was determined by transmission microscopy. Manganese ions are included in compounds in the divalent and trivalent states. The study of temperature and field dependences showed that we have several magnetic transitions. The first transition, associated with the ferrimagnetic ordering of the magnetic moments of parts of the ions, observed in the region of 50 K. In the region of 25 K, a second magnetic transition is observed. Substitution of nickel ions for copper and divalent manganese of better disorder and competition of exchange interactions, which leads to the division of the magnetic system into several magnetic subsystems that are ordered at different temperatures. Keywords: ludwigites, magnetic interface transition, indirect exchange interactions.