Indirect Exchange Research Articles

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.

Impact of terbium (Tb) on ferromagnetism and thermoelectric behaviour of spinels MgTb2(S/Se)4 for spintronic

The ferromagnetism due to rare earth elements-based spinel chalcogenides has become emerging aspirant for spintronic devices. Therefore, theoretically computed electronic properties of MgTb2(S/Se)4 spinels are analyzed to comprehend the structural, magnetic, and thermoelectric characteristics. Thermodynamic stability is elucidated using the computed enthalpy of formation (ΔH), phonons dispersion band structures. Using structural optimizations, difference in the energy states has revealed the ferromagnetic (FM) state stability. The computed band structures (BS) and density of states (DOS) both depict a half-metallic ferromagnetic state exhibiting a net spin polarization at the Fermi energy level. The underlying ferromagnetic state has been suggested due to carrier mediated indirect RKKY exchange mechanism. The thermoelectric properties are calculated to evaluate their potential for wasted-heat to useful-electricity conversion applications. The small thermal conductivity, and high-power factor exhibited by MgTb2S4 at room temperature suggest it significant for applications in thermoelectric generators.

Model' spinovogo stekla dlya kvantovykh yam GaAs/AlGaAs, legirovannykh nemagnitnymi primesyami, vblizi perekhoda metall–izolyator

In a previous report, we presented experiments which suggested that ferromagnetic ordering of the spins of localized holes in GaAs/AlGaAs quantum wells could be observed when doped with shallow (Be) acceptors at impurity concentrations near the metal-insulator transition. The compensating impurity (Si) was introduced into a narrow region at the center of the barriers [4]. In this paper, we present results from magnetotransport experiments performed on similar structures, but without the compensating impurity (Si). In these samples, the compensation degree is expected to be controlled by the background defects located at the edges of the quantum wells and within the barriers. At low temperatures T ≤ 10 K, we observed isotropic, linear magnetoresistance, anomalous behavior of the Hall effect as a function of the magnetic field, and slow relaxation of resistance after the application of a magnetic field. We explain this anomalous magnetotransport as the manifestation of a ferromagnetic transition or spin glass, originating from indirect spin exchange between localized holes on impurities near the metal-insulator transition. However, we note that perfect disorder, including signs of interspin interactions, leads to unstable configurations. In what follows, we present a model in which we start with this perfect disorder, but apply a procedure to obtain a stable configuration. We show that the resulting spin structure, a “closely packed” structure of “droplets,” can reproduce the features observed in the experiment, particularly isotropic, linear magnetoresistance.

Nonperturbative indirect exchange in spin valley coupled two-dimensional crystals

Nonperturbative indirect exchange in spin valley coupled two-dimensional crystals

Low-temperature magnetic ordering in Co core/CoO shell nanoparticles on the copper surface

Cobalt/cobalt oxide nanoparticles electrochemically deposited onto a copper surface revealed an unusual behavior of the coercivity and exchange bias at liquid helium temperature [V.G. Bayev et al., Appl. Surf. Sci. 440 (2018) 1252–1260], pointing out the necessity of careful consideration of magnetic properties at these temperatures. Here, it is shown that the approach based on the random anisotropy model and analysis of law of the approach to magnetic saturation adopted for the two-dimensional (2D) magnetic system allows obtaining the correlation function of the magnetic anisotropy axes in real space, as well as the exchange, random and coherent anisotropy magnetic fields. The long-range correlation in the magnetic anisotropy axes and the oscillating character of the correlation function is the characteristic feature, suggesting the important role of the interplay between the exchange bias and random anisotropy on the one hand and indirect exchange coupling via the conducting electrons of copper and magnetic dipole interaction, on the other.

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.

Ab initio calculations of structural, electronic, optical, and magnetic properties of delafossite SMoO2 (S = Na, K, Rb, Cs) for spintronics

Half metallic ferromagnetic (HMF) are key materials for quantum computing information storage devices and green energy. So, the structural, electronic, optical, and magnetic properties of SMoO2 (S = Na, K, Rb, Cs) are studied by quantum simulations based on density functional theory (DFT) within WIEN2K code. The Perdew − Burke − Ernzerhof (PBE + GGA) was used for the exchange correlation potential to study the said compounds. The volume optimization properties indicated that these compounds have minimum ground state energy in monoclinic crystal structure with space group 12 C2/m. Moreover, electronic band gap properties and density of states (DOS) graphs depicted the metallic nature for spin up a channel and semiconductor nature for spin down channel. This confirms the half-metallic ferromagnetism (HMF), and Heisenberg's classical model confirms the 100% spin polarization. Various frequency dependent optical parameters explain the optical properties. The double exchange process, crystal field energy ∆Ecry, direct Δx(d), and indirect exchange energies Δx(pd), is demonstrated by the quantum ferromagnetic behavior. The negative value of Δx(pd) and magnetic exchange constants confirm the ferromagnetism due to the quantum exchange mechanism of electrons. In addition to the above, it has been noted from the density of states that Mo d-states are responsible for the response of half metallic nature of these compounds. The magnetic parameter values of SMoO2 (S = Na, K, Rb, and Cs) show higher magnetic moment and major contribution coming from the Mo atom. Hence the studied results show that these compounds are valuable materials for spintronic applications.

Effect of annealing on magnetic and magnetocaloric properties of SmCo3B2 thin films

In this study, SmCo3B2 ternary thin films of manifold thickness were fabricated via e-beam evaporation technique. All the as-deposited SmCo3B2 thin films were subjected to electron beam rapid thermal annealing (ERTA). The X-ray diffraction studies showed amorphous nature for all the as deposited thin films and a highly crystalline hexagonal structure for all ERTA subjected thin films. The elemental composition, purity and surface morphology of the samples were analyzed using energy-dispersive x-ray spectroscopy (EDS) and High-resolution Scanning Electron Microscopy (HR-SEM). Furthermore, the study investigates the magnetocaloric properties of SmCo3B2 thin films, with significant changes depending on the thickness and annealing time. The thin films exhibited a magnetic phase transition around the Curie temperature (Tc) within the range of 40–46 K, attributed to the indirect exchange coupling of Sm–Sm sublattice interactions. Moreover, a significant shift in Tc towards higher temperatures with respect to thickness and annealing were observed in both as deposited and ERTA subjected thin films. The Arrot plots in the positive quadrant suggested the second order magnetic phase transition (SOPT) for all the films. The maximum magnetic entropy change (−ΔSm) of 1.172 Jkg−1K−1 accompanied by a relative cooling power of 5.33 Jkg-1 was achieved for 240 nm ERTA subjected thin film under a change in magnetic field of 5T. The study reveals the effect of ERTA and thickness dependent magnetic and magnetocaloric properties of SmCo3B2 thin films.

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.

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.

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.

Half-metallic ferromagnetism and thermoelectric effect in spinel chalcogenides SrX2S4 (X = Mn, Fe, Co) for spintronics and energy harvesting

Spintronics is an emerging technology in advanced quantum electronics, which has multidirectional applications. In this study, the electronic, magnetic, and thermoelectric characteristics of SrX2S4 (X = Mn, Fe, Co) are computed systematically. The stability in ferromagnetic states is verified by energy released from optimized structures and enthalpy of formation. The Heisenberg model is applied for Curie temperature. The band structures are plotted for both spin ↑ and ↓ configurations, which ensure 100% spin polarization and integer magnetic moments. Furthermore, the metallic behavior in spin ↑ and insulating behavior in spin ↓ confirm half-metallic ferromagnetism. The crystal field energy, direct exchange energy, indirect exchange energy, and exchange constants indicate that ferromagnetism is due to spin of electrons rather than any clusters of transition metals. The effect of thermoelectric parameters on spin functionality of electrons is addressed to enhance the device reliability. The thermoelectric performance is reported by power factor in the temperature span 100–500 K, which is important for energy harvesting.

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.

Spin moment cancelation of high Curie temperature Ir doped ZrO2 gradual atomic change

This paper presents the structural, magnetic properties, and spin moment cancelation phenomenon of the zirconium oxide with a gradual change of zirconium for Iridium. The density functional theory (DFT) is shown to be a key feature of magnetic properties of solid materials treatment. It is shown that a small adjustment of the spin moment (less than 6%) is allowed. The Zr1-xIrxO2 crystal structure deformation leads to a magnetic compensation that occurs at x=0.06. Spin and orbital moments behaviors are discussed. The stability of the alloy compounds is confirmed by energy calculations. The material presents ferromagnetic stability and an indirect exchange coupling with a hybridization effect that permitted the evolution from a non-magnetic to a host material with magnetic properties. The Ir orbitals are set at the Fermi level and are spin polarized which indicates a half metallic behavior then makes the material a good candidate for spintronics’ applications.

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.

Densely packed skyrmions stabilized at zero magnetic field by indirect exchange coupling in multilayers

Room-temperature stabilization of skyrmions in magnetic multilayered systems results from a fine balance between several magnetic interactions, namely, symmetric and antisymmetric exchange, dipolar interaction and perpendicular magnetic anisotropy as well as, in most cases, Zeeman through an applied external field. Such field-driven stabilization approach is, however, not compatible with most of the anticipated skyrmion based applications, e.g., skyrmion memories and logic or neuromorphic computing, which motivates a reduction or a cancellation of field requirements. Here, we present a method to stabilize at room-temperature and zero-field, a densely packed skyrmion phase in ferromagnetic multilayers with moderate number of repetitions. To this aim, we finely tune the multilayer parameters to stabilize a dense skyrmion phase. Then, relying on the interlayer electronic coupling to an adjacent bias magnetic layer with strong perpendicular magnetic anisotropy and uniform magnetization, we demonstrate the stabilization of sub-60 nm diameter skyrmions at zero-field with adjustable skyrmion density.

Evolution and Parametric Analysis of Concrete Temperature Field Induced by Electric Heating Curing in Winter

Electric heat treatment is a widely used concrete curing method during the winter. Through direct and indirect heat exchange, the electric heating system tracks and controls the temperature of the heating medium based on a positive temperature coefficient (PTC) effect. In this study, to standardize the application of this treatment in the winter curing of concrete, the thermal energy conversion of an electric heating system and the heat-transfer characteristics of concrete have been studied. Based on the theoretical derivation, a calculation model of the relationship between the thermal energy of the electric heating system and the temperature of the concrete is established. The model is verified using the concrete heating and curing test results. The numerical analysis program COMSOL is used to analyze the effects of various factors on the concrete temperature field, including the electric heating power (e.g., the surface temperature of the electric heating system), concrete casting temperature, thermal conductivity, and heat release coefficient. The results show that decreasing the surface exothermic coefficient and increasing the heating temperature will significantly increase the peak temperature of the concrete. When the heat source temperature increases by 20 °C, the peak temperature could increase by approximately 13 °C. When the heating stops, the concrete volume increases temporarily, particularly in the region where the heating cable is buried. Consequently, an excessive heating power increase may cause cracks on the concrete surface. Compared with the factors of thermal conductivity and surface exothermic coefficient, the ambient temperature has the most significant effect on the concrete cooling rate when the heating stops. When the ambient temperature decreases by −20 °C, the cooling rate of concrete increases by 0.72 °C/h. The role of concrete insulation materials needs to be strengthened to reduce cooling rates during power outages and form removal. The findings from the study provide industry practitioners with a comprehensive guide regarding the specific applications of the electric heating system in early-age concrete curing.

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.