Exchange Coupling Interaction Research Articles

Experimental and Theoretical Investigation of the Synthesis, Electronic and Magnetic Properties of MnFe2O4 Spinel Ferrite

MnFe2O4 ferrite nanoparticle was synthesized via the sol–gel method, and structural, morphology and magnetic characteristics were investigated. X-ray diffraction analysis showed that the synthesized sample was in a single phase with a spinel-ferrite-like structure (space group Fd-3m). The scanning electron microscopy displayed homogenous spherical grains with an agglomeration of the particles. The chemical composition determined by energy-dispersive spectroscopy shows the absence of any impurities. To understand the role of magnetic interaction in MnFe2O4 spinel ferrites, the structural and magnetic properties of MnFe2O4 have been explored theoretically. Based on the first-principles methods via density functional theory and Monte Carlo simulations, the magnetic hysteresis cycle has been plotted. Using the generalized gradient and GGA-PBE approximation in the full-potential linearized augmented plane wave (FP-LAPW) method, the exchange coupling interactions between magnetic elements and local magnetic moment were evaluated. Furthermore, the theoretical magnetic properties of MnFe2O4 were found to match the experimental ones. They both revealed that MnFe2O4 is a soft ferromagnetic material. The theoretical curve of magnetization versus temperature indicates that the transition occurred at Tc = 580.0 K. This was also in good agreement with the experimental Curie temperature.

Structural and magnetic properties of SrFe12O19/CoFe2O4 composites with exchange coupling interaction

SrFe12O19 and CoFe2O4 powders obtained via a chemical coprecipitation method were used to prepare SrFe12O19/CoFe2O4 composites after sintering at 1200 °C, and the structure, the magnetic properties and the effects of aligning magnetic field during molding are studied. Only SrFe12O19 and CoFe2O4 without any other impurity phase were found in all the composites, and the exchange coupling interaction was proved to exist by using the Henkel plots and the magnetic recoil loops. The exchange coupling interaction resulted in the improvement of saturation magnetization, but the reduction of coercivity in composite specimens. It was also found that for SrFe12O19/CoFe2O4 composites, the aligning magnetic field helped to improve the saturation magnetization and the coercivity, but did not affect the phase composition, the morphology and the exchange coupling interaction obviously.

Ru-Doping-Induced Spin Frustration and Enhancement of the Room-Temperature Anomalous Hall Effect in La2/3 Sr1/3 MnO3 Films.

In transition-metal-oxide heterostructures, the anomalous Hall effect (AHE) is a powerful tool for detecting the magnetic state and revealing intriguing interfacial magnetic orderings. However, achieving a larger AHE at room temperature in oxide heterostructures is still challenging due to the dilemma of mutually strong spin-orbit coupling and magnetic exchange interactions. Here, Ru-doping-enhanced AHE in La2/3 Sr1/3 Mn1-x Rux O3 epitaxial films is exploited. As the B-site Ru doping level increases up to 20%, the anomalous Hall resistivity at room temperature can be enhanced from nΩ cm to µΩcm scale. Ru doping leads to strong competition between the ferromagnetic double-exchange interaction and the antiferromagnetic superexchange interaction. The resultant spin frustration and spin-glass state facilitate a strong skew-scattering process, thus significantly enhancing the extrinsic AHE. The findings can pave a feasible approach for boosting the controllability and reliability of oxide-based spintronic devices.

Influence of exchange coupling interaction on coercivity in 2:17-type SmCo-based high temperature permanent magnets

The intergranular exchange coupling interaction in Sm(CobalFe0.10Cu0.10Zr0.03)7.2 high temperature permanent magnets with different isothermal aging time was studied. The magnet isothermally aging for 24 h with higher coercivity at room temperature exhibits a weaker exchange coupling, while that aging for 4 h with a relatively small temperature coefficient of coercivity shows a stronger exchange coupling. Microstructure and microchemistry analysis indicated that Cu gradually enriches in the cell boundaries with the prolongation of isothermal aging time, decreasing the exchange coupling strength. Theoretical analysis results revealed that with the increase of temperature, the cells gradually get exchange decoupled and demagnetized independently, and the exchange coupling strength between cells decreases, which slows down the decline of coercivity. It is proposed that an optimized temperature coefficient of coercivity requires a stronger exchange coupling at room temperature, conducing to a more significant compensation effect with the increase of temperature.

Magnetic Properties and Compensation Temperature of a Mixed Monolayer Coronene-Like Nanostructure: Monte Carlo Study

In this study, the ground-state phase diagrams and magnetic properties of mixed spin ([Formula: see text]-1, [Formula: see text]-7/2) of the monolayer coronene-like nanostructure have been investigated using Monte Carlo simulations (MCS) in the frame of the Blume Capel model. In the first part, ground-state phase diagrams have been analyzed in order to show the more stable configurations of spins corresponding to various planes. In addition, MCS is applied to study the behavior of the compensation and blocking temperatures by varying the exchange coupling interaction values. Finally, the hysteresis cycle behaviors have been investigated for selected values of exchange coupling parameters, temperature and crystal field. The theoretical results obtained in this study can enrich the magnetic properties of nanosystems and pave the way for researchers to experimentally study monolayer coronene-like nanostructure for future applications in nanotechnology.

In-situ investigation of exchange bias property in FePt (L10)/FeCo bilayer with increasing FeCo thickness

Magnetization reversal of soft ferromagnetic (FM) FeCo layer coupled to hard FM FePt (L10) layer in FePt (L10)/FeCo) bilayer has been studied in-situ with increasing thickness of FeCo layer. During FePt(L10) phase formation, an in-plane magnetic saturation field (HMAX) is applied to keep the hard FePt(L10) layer in its remanent state. Magneto-optical Kerr effect measurements reveal the absence of hysteresis loops initially up to 4.8 nm thickness of FeCo layer while loops with significant shift, i.e. exchange bias (EB) opposite to the HMAX direction, were observed on further deposition. The EB is found to decrease with the increasing thickness of the FeCo layer. It is due to the combined effect of both magnetostatic coupling and interfacial exchange interaction, where the contribution of magnetostatic interaction to EB is almost constant, and interfacial exchange coupling exhibits an inverse relation. Furthermore, different nucleation mechanisms during magnetization reversal in the descending and ascending branches of hysteresis loops lead to asymmetry in magnetization reversal at the lower thickness of the FeCo layer. This asymmetry decreases with the decrease in the interfacial coupling as the thickness increases. Azimuthal angle dependence of exchange shift with respect to the direction of HMAX further revealed the unidirectional character of the EB.

Influence of Cu distribution on the coercivity of Sm(CobalFe0.25Cu0.06Zr0.02)7.5 magnets during microwave aging process

In this study, we investigated the magnetic properties, microstructures, and Cu distribution on the mechanism of coercivity enhancement of Sm(CobalFe0.25Cu0.06Zr0.02)7.5 magnets during two-step and multistage microwave aging treatments. The intrinsic coercivity, Hcj, and maximum energy product, (BH)max, increase from 15.9 to 24.4 kOe and from 24.3 to 26.5 MGOe, respectively, upon transitioning from two-step to multistage aging. The aged alloys possess a cellular microstructure comprising rhombohedral 2:17 R cell phase and hexagonal 1:5 cell boundary phase after a short microwave aging time of 3 h. The multistage aged alloy exhibits a larger cell size, thicker cell walls, and higher Cu concentration in the 1:5 phase than the two-step aged sample. Elemental mapping reveals Cu atom enrichment at the cell boundary centers and at the 2:17 R/1:5 triple junction of the cell boundary phase, but depletion at the cell boundary edges during multistage aging. First-principles calculations indicate that Cu atoms occupy the Co(2c) sites of a SmCo5 unit cell and weaken not only the strong Co(2c)–Co(3g) direct exchange-coupling interaction, but also the exchange-coupling interaction between Sm 4f and Co 3d electrons, giving rise to low 1:5 phase exchange constant values. Furthermore, the enrichment of Cu atoms at the center of the cell wall and at the 2:17 R/1:5 triple junction of the cell boundary phase produces a gradual increase in the domain wall energy difference as a function of the edge-to-center distance in the cell boundary phase. This enrichment enhances the domain wall pinning strength and increases coercivity. A high concentration and optimal distribution of Cu are critical for achieving a high Hcj. The results of this study provide theoretical guidance to future studies on the mechanism controlling the coercivity of Sm2Co17-based permanent magnets.

Cation speciation, compactness, and structure‐property correlations in manganese aluminosilicate glasses

AbstractWe report on the elastic properties of xMnO‐Al2O3‐(6‐x)SiO2 manganese aluminosilicate glasses, in which pronounced exchange coupling and antiferromagnetic interaction occurs among the paramagnetic Mn2+ ions. X‐ray absorption reveals a Mn‐O bond length of ∼2.1 Å, and a distance of ∼3 Å between Mn2+ ions in sixfold coordination, further corroborated by electron paramagnetic resonance and dynamic photoluminescence spectroscopy. This indicates the presence of edge‐sharing MnO6 units, in resemblance to edge‐sharing MnO8 in the crystalline Spessartine garnet Mn3Al2[SiO4]3. We argue that these cluster species have a profound effect on material stiffness: the compact structure of manganese aluminosilicate glasses enables a Young's modulus in the 100 GPa range and a Poisson's ratio of about 0.3. This exceeds the compositional trend with cation field strength observed for other aluminosilicate glasses.

Impact of sonication time on the structural and magnetic features of CoFe2O4/Ni0.8Cu0.1Zn0.1Fe2O4 hard-soft nanocomposites

Hard/soft (H/S) CoFe2O4/Ni0.8Cu0.1Zn0.1Fe2O4 nanocomposites (NCs) were prepared via ultrasonic irradiation method under different ultrasonication durations (10, 20, 30, 40, 50, and 60 min). The structure and morphology of these nanocomposites were analyzed through XRD, FT-IR, SEM, TEM, and EDX techniques, which confirmed the successful preparation of the desired nanocomposites. The crystallites/particles size was found to reduce with increasing ultrasonication duration. The magnetic properties of H/S CoFe2O4/Ni0.8Cu0.1Zn0.1Fe2O4 NCs prepared via ultrasonication method under different ultrasonication durations (10, 20, 30, 40, 50, and 60 min) were investigated through measurements of magnetization against the magnetic field (M-H) at 10 and 300 K. The soft character of the prepared NCs at room temperature depicted their superparamagnetic behaviors. At extremely low temperatures (T = 10 K), all H/S ferrite NCs revealed opened M-H hysteresis loops, indicating their ferrimagnetic behavior at low temperatures. All H/S NCs do not reveal any kind of anomalies in their M-H curves. Additionally, the SFD (switching field distribution) analysis pointed out a single peak in dM/dH against H curves. It was found that the intergranular exchange coupling interactions are more dominant than the intergranular dipolar interactions. Such a result indicates that the current synthesis method is useful to produce good exchange-coupled hard/soft ferrite NCs. It was also observed that the Ms and Mr values at both 10 and 300 K are decreasing with rising the ultrasonication duration. However, the Hc value was found to increase with rising ultrasonication duration, reaching a maximum value under an ultrasonication duration of 30 min, then, the Hc value decreases with further rising the ultrasonication duration. The plausible reasons for the variations in magnetic parameters were discussed.

Enhanced magnetic properties of Ce17Fe76.5Co1Zr0.5B6 alloys by magnetic field heat treatment

In the present work, Ce17Fe76.5Co1Zr0.5B6 ribbons were prepared by a direct melt spinning method. The effects of chamber pressure and magnetic field annealing temperature on the magnetic properties and microstructures of the alloys were investigated. The grain size and content of Ce2Fe14B phase can be changed by adjusting the chamber pressure, and the optimal magnetic performance is obtained at 0.04 MPa. The magnetic properties can be influenced under magnetic field heat treatment. When the annealing temperature is lower than the Curie temperature, the refinement and a uniform distribution of the grains is obtained. The irreversible magnetic susceptibility curve reveals that magnetic field heat treatment enhances the exchange coupling interaction between grains of the Ce2Fe14B phase. When the magnetic field annealing temperature is 438 K, the alloy displays the optimal magnetic properties. Compared with the as-spun sample, the values of intrinsic coercivity (Hci), remanence (Br) and maximum energy product ((BH)max) increase by 3.4%, 9.8% and 18.7%, respectively. This work provides an effective approach by which to enhance the magnetic properties of Ce–Fe–B alloys.

Ab-initio and Monte Carlo Simulations of the New Half-Heusler Alloy NiCrGa

Abstract: In this work, we are investigating the electronic and magnetic properties of the new NiCrGa half-Heusler alloy (HHA), by using the ab-initio and Monte Carlo simulations (MCSs). The ab-initio method is performed under the pseudo-potential method and the generalized gradient approximation GGA. The density of states (DOS) and the band structure calculations show that the alloy NiCrGa reveals a nearly half-metallic (HM) behavior. In particular, the Slater-Pauling (SP) rule has been confirmed when using the magnetic moments of the individual constituents of the NiCrGa compound. The Monte Carlo simulations (MCSs) are accomplished using the Metropolis algorithm. In order to determine the transition temperature, we are based on the behavior of the total magnetization and susceptibility of this material. We also presented and discussed the hysteresis loops of the half-Heusler compound, for fixed values of temperature, exchange coupling interactions and crystal field. It is found that when increasing the crystal field, the surface of the loops increases and vice versa. Keywords: Half-Heusler alloy, NiCrGa, Slater pauling, DFT method, Monte Carlo study.

Room-temperature surface multiferroicity in Y2NiMnO6 nanorods

We report observation of surface-defect-induced room-temperature multiferroicity---surface ferromagnetism (${M}_{S}$ at 50 kOe $\ensuremath{\approx}0.005$ emu/g), ferroelectricity (${P}_{R}\ensuremath{\approx}2\phantom{\rule{0.16em}{0ex}}\mathrm{nC}/{\mathrm{cm}}^{2}$), and significantly large magnetoelectric coupling (decrease in ${P}_{R}$ by $\ensuremath{\approx}80%$ under $\ensuremath{\approx}15$ kOe field)---in nanorods (diameter $\ensuremath{\approx}100$ nm) of double perovskite ${\mathrm{Y}}_{2}{\mathrm{NiMnO}}_{6}$ compound. In bulk form, this system exhibits multiferroicity only below its magnetic transition temperature ${T}_{N}\ensuremath{\approx}$ 70 K. On the other hand, the oxygen vacancies, formed at the surface region (thickness $\ensuremath{\approx}10$ nm) of the nanorods, yield long-range magnetic order as well as ferroelectricity via Dzyloshinskii-Moriya exchange coupling interactions with strong Rashba spin-orbit coupling. Sharp drop in ${P}_{R}$ under magnetic field indicates strong cross coupling between magnetism and ferroelectricity as well. Observation of room-temperature magnetoelectric coupling in nanoscale for a compound which, in bulk form, exhibits multiferroicity only below 70 K underscores an alternative pathway for inducing magnetoelectric multiferroicity via surface defects and, thus, in line with magnetoelectric property observed, for example, in domain walls or boundaries or interfaces of heteroepitaxially grown thin films which do not exhibit such features in their bulk.

Remarkable Magnetic Exchange Coupling via Constructing Bi‐Magnetic Interface for Broadband Lower‐Frequency Microwave Absorption

AbstractSevere lower‐frequency (2–8 GHz) microwave pollution caused by the rapid development of 5th generation (5G) communication posts significance on cutting‐edge microwave absorbers. However, the intensely coupled wave‐impedance and microwave dissipating ability dramatically hinder their performance in the exact lower‐frequency range. The rationally designed heterostructure of hard/soft ferrite composite provides an efficient solution to address the issue. In this context, core‐shell structured hard/soft BaFe(12‐x)CoxO19@Fe3O4 with abundant heterointerface is created using facile spray‐drying and subsequent solvothermal approach, where hard magnetic BaFe(12‐x)CoxO19 serves as the core and soft magnetic Fe3O4 serves as the shell, respectively. The unique core‐shell integration contributes sufficient magnetic exchange coupling interaction for strong magnetic loss beyond Snoek's limitation, which considerably boosts a lower‐frequency microwave absorption. Accordingly, the minimum reflection loss (RLmin) of typical BaFe11.6Co0.4O19@Fe3O4 microcomposite reaches −48.9 dB at the thickness of 3.5 mm, its bandwidth of reflection loss < −10 dB can cover almost all the S and C bands (2.6–8 GHz). Generally, an easy and controllable pathway is conveyed in this work to encourage improved magnetic loss ability as well as decouple the wave‐impedance and microwave dissipating ability in magnetic composites, which widens the road to the development of advanced lower‐frequency magnetic absorbers.

Structure and magnetic investigation of hard/soft Ba0.5Sr0.5Fe12O19/x(Ni0.5Zn0.5)Fe2O4 nanocomposite

In this work, magnetic nanocomposites Ba0.5Sr0.5Fe12O19/x(Ni0.5Zn0.5)Fe2O4 with the compositions x = 10, 20, 30, and 50 wt%, arising from nickel-zinc ferrite added to barium strontium hexaferrite were prepared and characterized. The samples were synthesized using co-precipitation and ball milling methods. X-ray powder diffraction measurements validated the synthesis of nanocomposite with high purity and crystallinity. A transmission electron microscope was also used to analyze the morphology of the nanocomposites that demonstrated hexagonal platelet-like and spherical-like shapes for hard and soft phases, respectively. Also, multiple shapes of different grain sizes were formed, and their grain size decreased upon the addition of the soft phase. Besides, the elemental compositions and the oxidation states of (Ba2+, Sr2+, Ni2+, Ni3+, Zn2+, Fe2+, Fe3+and O2-) constituting the nanocomposites were investigated, using X-ray photoelectron spectroscopy. Additionally, the magnetic properties were examined using a vibrating sample magnetometer. According to Henkel plots, the existence of a weak exchange coupling interaction and the dominance of dipolar interactions were visible upon the addition of Ni0.5Zn0.5Fe2O4. Furthermore, the maximum energy product, (BH)max, is the energy density that a hard ferrite can store with low magnetic anisotropy. (BH)max of the nanocomposite containing 10 wt% of soft phase increased by 10% when compared to pure hard phase, reaching the highest value of (BH)max equal to 22.89 kJ/m3.

Competing long-range and short-range interactions in ferromagnetic semiconductors: Evidence from first principle calculations

Based on ab-initio calculations, we have investigated the structural, electronic, and magnetic properties of diluted magnetic semiconductors (Ba1−yKy)F(Zn1−xMnx)tAs with (K, Mn) substitution in the parent compounds, BaFZnAs and BaFMnAs. The coexistence of two types of magnetic exchange coupling interactions was detected via the calculation of the distance-resolved and the individual exchange parameters J(dMn0−Mni)≡Ji, where i runs over the ith nearest neighbor atom (iNN) of the reference one Mn0 distanced by dMn0−Mni. The short-range interactions (SRI), at J1 and J2, are strongly influenced by hole doping and substitutional disorder of both hole and spin co-doping. The long-range interaction (LRI), at distances beyond the 2NN, of a ferromagnetic nature, is less sensitive to hole doping, although these holes are the reason for their appearance. The results analysis showed that the dependence of the LRI on y/x doping ratio, in the co-doping system, is more systematic than on the y or x doping separately. In (BaK)F(ZnMn)As system, a peak appears in the LRI at around 1.5 of y/x doping ratio. Therefore, an estimation of the optimal hole doping for each spin doping concentration was given, where it is important to obtain the maximal ferromagnetic coupling, therefore the highest ferromagnetic ordering temperature, TC. Our findings are consistent with experiments, manifested, especially, in the saturation of the ordering temperature at large hole doping, as was obtained for the experimental TC of (BaK)(ZnMn)2As2 system. The results also show a doubling of the TC when the spin doping is doubled at high hole doping. Finally, we demonstrated via a proposed model that the SRI has the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction nature.

Critical magnetic behavior of the half Heusler alloy RhCrSi: Monte Carlo study

In this paper, we study the critical magnetic properties of the Half Heusler alloy RhCrSi, using Monte Carlo simulations (MCS) under the Metropolis algorithm. In fact, to study this alloy, we apply an Ising model using the MCS simulations, we concentrate only on the magnetic atoms: Rh and Cr. For this purpose, these magnetic atoms are modeled by the spin moments S = 5/2 for Rh atoms and σ = 2 for Cr atoms, respectively. In addition, we discuss the ground state phase diagrams in different planes corresponding to different physical parameters. On the other hand, for non-null temperature values, we perform the Monte Carlo simulations (MCS) to study the critical behavior of the compound RhCrSi, in the Ising approximation. Indeed, we present a detailed discussion of the obtained results for the magnetizations as a function of the temperature, the crystal field and the exchange coupling interactions. Additionally, we give the reliance of the basic temperature as an element of precious crystal field when fixing the exchange coupling interactions. To finish this work, we built up and examined the magnetic hysteresis cycles and the relating coercive fields as a part of the external magnetic field.

Exchange bias and exchange spring in CoFe2O4/FeO/CoFe nanocomposites

A series of CoFe2O4/FeO/CoFe nanocomposites was prepared by reducing cobalt ferrite nanoparticles under the mixed gas of Ar (95%) and H2 (5%) at 450 °C for different times, and the effects of exchange bias and exchange spring have been observed. X-ray diffraction and Mössbauer spectroscopy demonstrate CoFe2O4/FeO/CoFe nanocomposites are obtained when the reduction time exceeds 0.5 h, and the concentrations of CoFe2O4 and FeO decrease, accompanied by the increase of CoFe on extending the reduction time. The field emission scanning electron microscope and transmission electron microscope images show the spherical CoFe2O4, FeO, and CoFe nanoparticles with the average sizes about 17.0 nm are randomly distributed with a certain degree of agglomeration. Magnetic measurements indicate there is a strong exchange bias behavior between the CoFe2O4 and FeO nanoparticles in the sample reduced for 0.5 h with an exchange bias field being 1161.4 Oe at 50 K, and a good exchange spring effect between CoFe2O4 and CoFe phases in the sample reduced for 1.5 h with saturation magnetization, remnant magnetization, and coercivity at 300 K being 121.33 emu/g, 41.90 emu/g, and 802.4 Oe. These results are important not only for understanding the exchange-coupling interaction in the reduction process of cobalt ferrite but also for their application in spin-valve devices.

Theoretical Study of the Magnetic Properties of a Ferrimagnetic Graphene-Like Nanoribbon: Monte Carlo Treatment

Monte Carlo simulation was used to study the magnetic properties of the zigzag graphene-like nanoribbon predicated on the ferrimagnetic (5/2,3/2) combined Ising system. The magnetic and thermodynamic properties curves induced by varying exchange coupling interactions and ion anisotropies have all been well studied. For several physical parameters, phase diagrams were provided. Values of exchange couplings and single-ion anisotropy thresholds, indicating whether the system has two compensation temperature points, have been determined. In addition, the impacts of different Hamiltonian parameters on the behavior of multiple-loop hysteresis were explored.

Dielectric Properties of Carbon-Like Nanotube Structure: Monte Carlo Study

In this paper, we study the thermal and dielectric properties of a carbon-like nanotube structure. The ferrielectric mixed spin (1,3/2) Blume–Capel Ising model has been investigated, using Monte Carlo simulations under the Metropolis algorithm. We illustrate the thermal and dielectric properties of different physical parameters. Besides, we examine the effects of the exchange coupling interactions, the external longitudinal electric and the crystal fields on the transition temperature. It is found that the polarization decreases toward a transition temperature and undergoing the paraelectric phase, faster for low values of the external longitudinal electric field and the exchange coupling interactions. Besides, the total polarization versus the crystal field undergoes two principal transitions and exhibits a first-order and a second-order transitions.

Magnetic Behaviors of the Kesterite and the Stannite Nanostructures: Monte Carlo Study

In this work, the Monte Carlo simulations have been performed to investigate the magnetic properties of the kesterite and the stannite nanostructures with mixed spins. The Blume–Capel model has been adopted to study the Cu2ZnSnSe4 nanostructures. Furthermore, we have examined different transitions as a function of the temperature, the crystal field, and the exchange coupling interactions. Also, the behavior of the magnetizations and magnetic susceptibilities are investigated as a function of the different physical parameters. A comparison of the magnetic properties of each nanostructure has been studied and discussed.