Effective Exchange Coupling Research Articles

SrFe12O19@ZnFe2O4 core–shell composite fiber with enhanced exchange coupling interaction

In this paper, SrFe12O19@ZnFe2O4(Sr@Zn) core–shell composite fibers with molar ratios of 1:1,1:2,1:3,1:4 were prepared by coaxial electrostatic spinning. XRD revealed the coexistence of hexagonal ferrite and spinel phases in different proportions in the prepared composites. SEM and TEM showed that all samples showed the obvious fiber structure with uniform diameter distribution and length up to several microns. With the change of the molar ratio of soft magnetic phase, the magnetic properties of the composite first increase and then decrease, and the M2 composite exhibits the highest Ms(55 emu/g), Hc(4745 Oe) and Mr/Ms(0.51). In addition, the single-phase magnetization behavior of the hysteresis loop and the highest single peak in the SFD curve also confirm that the exchange coupling interaction of M2 composite is the highest. By adjusting the molar ratio of soft phase, the magnetic properties and exchange coupling effects of the composites are improved, which provides evidence for the development of composites with higher magnetic properties.

Epitaxial growth, magnetoresistance, and electronic band structure of GdSb magnetic semimetal films

Motivated by observations of extreme magnetoresistance (XMR) in bulk crystals of rare-earth monopnictide (RE-V) compounds and emerging applications in novel spintronic and plasmonic devices based on thin-film semimetals, we have investigated the electronic band structure and transport behavior of epitaxial GdSb thin films grown on III-V semiconductor surfaces. The ${\mathrm{Gd}}^{3+}$ ion in GdSb has a high spin $S=7/2$ and no orbital angular momentum, serving as a model system for studying the effects of antiferromagnetic order and strong exchange coupling on the resulting Fermi surface and magnetotransport properties of RE-Vs. We present a surface and structural characterization study mapping the optimal synthesis window of thin epitaxial GdSb films grown on III-V lattice-matched buffer layers via molecular-beam epitaxy. To determine the factors limiting XMR in RE-V thin films and provide a benchmark for band-structure predictions of topological phases of RE-Vs, the electronic band structure of GdSb thin films is studied, comparing carrier densities extracted from magnetotransport, angle-resolved photoemission spectroscopy (ARPES), and density-functional theory (DFT) calculations. ARPES shows a hole-carrier rich, topologically trivial, semimetallic band structure close to complete electron-hole compensation, with quantum confinement effects in the thin films observed through the presence of quantum-well states. DFT-predicted Fermi wave vectors are in excellent agreement with values obtained from quantum oscillations observed in magnetic field-dependent resistivity measurements. An electron-rich Hall coefficient is measured despite the higher hole-carrier density, attributed to the higher electron Hall mobility. The carrier mobilities are limited by surface and interface scattering, resulting in lower magnetoresistance than that measured for bulk crystals.

Correlation between interlayer exchange coupling and domain wall velocity in multilayered magnetic wires

The effect of interlayer exchange coupling (IEC) in {Tb/Co}4/Rh/{Co/Tb}4/Pt multilayered wires on the domain wall (DW) structure and DW velocity was investigated. It was found that the stability of the Néel-type DWs was enhanced and the velocity of the current-induced DW movement increased with an increase in the antiferromagnetic IEC strength. The micromagnetic simulation based on the Landau–Lifshitz equation clarifies that the direction of the magnetic moment in the DWs depends on the antiferromagnetic IEC strength, and the stability of the Néel-type DW increases with an increase in the antiferromagnetic IEC strength. As a consequence of this stability, the spin-orbital torque affects effectively on the magnetic moment of the DWs and the DW velocity increases. These results imply that the DW velocity in magnetic domain-based memories can be controlled by the antiferromagnetic IEC strength.

Magnetic interactions in SrFe12O19/CoFe2O4 composite: Influence of ball milling frequency and annealing temperature

SrFe12O19/CoFe2O4 magnetic composites were fabricated by mixer ball milling technique (with the oscillating frequency of 10, 15, 20 and 25 Hz) using hexagonal ferrite (80 wt%) and cubic ferrite (20 wt%) powders as precursors. The composites were divided into three groups for thermal treatments: i) 700 °C, 3 h; ii) 900 °C, 3 h; iii) 900 °C, 5 h. X - ray powder diffraction was carried out for structural analysis. Transmission electron microscopy was applied to investigate the morphology of the final products. Magnetic properties were studied at room temperature by measurements of magnetization versus applied external magnetic field, direct current demagnetization and isothermal remanence magnetization. A relationship was established between magnetic properties and the structural parameters of the composites and correlated to the oscillating frequency of ball milling. From all the groups, the samples processed at 10 Hz -frequency stand out in terms of magnetic properties. Annealing at 900 °C for 3 h allowed obtaining the composite with the energy product (BH)max value about 20 % larger than its parent compound. The existence of the exchange coupling effect was derived from constructing Henkel plots and can explain the improved magnetic characteristics of the standing-out composite.

Microwave-assisted switching in granular media with continuous magnetic overlayers for interaction control

Optimizing switching behavior under a microwave field is a key issue for microwave-assisted magnetic recording. We study the effect of inter-grain exchange coupling induced by a continuous magnetic layer on microwave-assisted switching of CoCrPt-SiO2 granular media. The microwave assistance effect is improved in the low-frequency region by introducing the inter-grain exchange coupling, but the coupling reduces the critical frequency. The maximum assistance effect is almost independent of the film structure since the dual effect cancels each other. Micromagnetic simulations also show similar results on exchange coupling between grains. These results indicate that the inter-grain exchange coupling can be used to optimize the critical frequency of the media in practical applications.

Nonequilibrium dynamics in a spin valve with noncollinear magnetization

We utilize a hybrid quantum-classical equation of motion approach to investigate the spin dynamics and spin-transfer torque in a spin valve under bias voltage. We show that the interplay between localized classical magnetic moments and conduction electrons induces a complex effective exchange coupling between the magnetic layers. This leads to a declination of magnetizations from layers anisotropy axes even in equilibrium. Introducing a finite bias voltage triggers spin currents and related spin-transfer torques which further tilt the magnetizations and govern the relaxation processes of the spin dynamics. Analyzing different scenarios of the applied bias voltage, we show that symmetric and asymmetric voltage drops can lead to relaxation times of the spin dynamics that differ by several orders of magnitude at comparable charge currents. In both cases we observe resonant features, where the relaxation is boosted whenever the chemical potential of the leads matches the maxima in the density of the states of the spin-valve electrons.

Broken-symmetry self-consistent GW approach: Degree of spin contamination and evaluation of effective exchange couplings in solid antiferromagnets

We adopt a broken-symmetry strategy for evaluating effective magnetic constants J within the fully self-consistent GW method. To understand the degree of spin contamination present in broken-symmetry periodic solutions, we propose several extensive quantities demonstrating that the unrestricted self-consistent GW preserves the broken-symmetry character of the unrestricted Hartree-Fock solutions. The extracted J are close to the ones obtained from multireference wave-function calculations. In this paper, we establish a robust computational procedure for finding magnetic coupling constants from self-consistent GW calculations and apply it to solid antiferromagnetic nickel and manganese oxides.

Antiferromagnetic–Ferromagnetic Heterostructure‐Based Field‐Free Terahertz Emitters (Adv. Mater. 42/2022)

Terahertz Emitters In article number 2204373, Xiaojun Wu, Caihua Wan, Jimin Zhao, Sai Chen, and co-workers demonstrate efficient terahertz generation from a unique antiferromagnetic–ferromagnetic (IrMn3|CoFeB) heterostructure without an external magnetic field, using femtosecond laser pulse excitation. This can be attributed to the exchange bias or interfacial exchange coupling effect. Such field-free THz spintronics emitters can be more easily integrated with optical devices, shining light on the development of spintronic THz optoelectronic devices.

Strategy of refining the microstructure of Nd-Fe-Co-Ga-B ribbons by high-speed melt-spinning and annealing

• Increasing the melt-spinning speed to 50 m/s refines the grain size. • Changing the crystallization process is the mechanism of grain refinement. • The size of equiaxed grains in the annealed ribbons is reduced to 50–200 nm. • The intense exchange coupling effect contributes to the higher remanence. • The microstructure evolution models are proposed. Given the uneven grain size distribution in the ribbons melt-spun at 25 m/s, the strategy of improving the microstructure and magnetic properties of the ribbons by increasing the melt-spinning speed is discussed. The experimental results show that the grain size of Nd 13.65 Fe 73.55 B 5.6 Co 6.6 Ga 0.6 ribbons annealed at 700 °C is reduced to 50–200 nm by melt-spinning at 50 m/s. The change in the crystallization process is the primary mechanism for grain refinement. At high quenching speed, the amorphous matrix is formed first in the as-spun ribbons, and then uniform crystallization can be achieved through annealing. Compared to the annealed ribbons melt-spun at 25 m/s, the Mr and M 2T increased by ∼8 % due to the intense exchange coupling effect and the refined grains.

SrFe12O19/CoFe2O4 magnetic composites: Nanoparticle size and effect of annealing temperature on magnetic properties

Nanostructured magnetic composites of hexagonal SrFe12O19 and cubic CoFe2O4 powders were prepared by ball milling technique varying the mean size of CoFe2O4 grains and using 80/20 mass ratio. Subsequently, the composites were divided into two series of distinct annealing conditions: i) at 1000 °C and ii) at temperature at which the corresponding CoFe2O4 phase was synthesized. X-ray powder diffraction was carried out for phase identification and structural analysis. Transmission electron microscopy and/or scanning electron microscopy were applied for morphological monitoring. Energy dispersive X -ray analysis allowed elemental analysis. Magnetic properties were studied at room temperature by magnetization versus the applied external magnetic field, direct current demagnetization and isothermal remanence magnetization measurements. A relation was established between the structural parameters of the composite and the magnetic properties, putting emphasis on the exchange coupling effect. Although the stored energy product for all the composites studied is lower than that of hexagonal SrFe12O19, a visible gain of remanence magnetization of selected composites is observed. In the first series of composites, the one made from the CoFe2O4 grains of the largest mean size exhibits the highest energy product, (BH)max = 9.5 kJ/m3. In the second series of composites the one thermally treated at 900 °C is magnetically the hardest, Hc = 3920 Oe.

The effect of oscillatory interlayer exchange coupling on current-induced magnetization switching in pentalayer nanopillar alloys

The effect of oscillatory interlayer exchange coupling on current-induced magnetization switching in pentalayer nanopillar alloys

Effect of interlayer exchange coupling in spin-torque nano oscillator

The dynamics of the magnetization of the free layer in a spin-torque nano oscillator (STNO) influenced by a noncollinear alignment between the magnetizations of the free and pinned layers due to an interlayer exchange coupling has been investigated theoretically. The orientations of the magnetization of the free layer with that of the pinned layer have been computed through the macrospin model and they are found to match well with experimental results. The bilinear and biquadratic coupling strengths make the current to switch the magnetization between two states or oscillate steadily. The expressions for the critical currents between which oscillations are possible and the critical bilinear coupling strength below which oscillations are not possible are derived. The frequency of the oscillations is shown to be tuned and increased to or above 300 GHz by the current, which is the largest to date among nanopillar-shaped STNOs.

High performance and exchange coupling in magnetization reversal of sintered (Nd,Dy)-Fe-B magnets

Improving the coercivity in sintered Nd-Fe-B magnets would sacrifice the remanence and the energy product. In this work it develops a method of preparing the composite (Nd,Dy)-Fe-B magnets to enhance the coercivity with a slighter decrease of remanence. The high performance (Nd,Dy)-Fe-B magnets were prepared via adding Dy-Fe-B into Nd-Fe-B powders in blending, followed by compacting and sintering. The addition of Dy-Fe-B powders gave rise to the enhancement of coercivity in the sintered magnets. The investigation of thermal activation indicates that the fluctuation field in thermal activation doesn’t obey the law of increasing with the coercivity. The grains of Dy-Fe-B crystal phase coexist in (Nd,Dy)-Fe-B magnets, which could increase the domain wall size by exchange coupling and decrease the fluctuation field in magnetization reversal. Some of Dy atoms diffuse into the grain outer-layer of Nd2Fe14B crystal phase, the grain boundary structure is probably optimized, the anisotropy increases at grain shell, and so the domain wall size would be reduced, leading to an increase of the fluctuation field. Though their effects of the exchange coupling and the anisotropy increasing are opposite on the changes of the domain wall size and fluctuation field, they are beneficial to improve the nucleation field in magnetization reversal, and the coercivity increases significantly in the sintered magnets. Since Dy-Fe-B powders mainly evolve into the main phase of RE2Fe14B crystal instead of the intergranular phase, the mass fraction of ferromagnetic phase doesn’t decrease evidently. Owing to the exchange coupling effect, the core-shell morphology of grains and the no remarkable increase in the amount of intergranular phase, the high performance with high energy product of 46.6 MGOe and high coercivity of 20.5 kOe could be obtained in the sintered (Nd,Dy)-Fe-B composite magnets.

Antiferromagnetic-Ferromagnetic Heterostructure-Based Field-Free Terahertz Emitters.

Recently, ferromagnetic-heterostructure spintronic terahertz (THz) emitters have been recognized as one of the most promising candidates for next-generation THz sources, owing to their peculiarities of high efficiency, high stability, low cost, ultrabroad bandwidth, controllable polarization, and high scalability. Despite the substantial efforts, they rely on external magnetic fields to initiate the spin-to-charge conversion, which hitherto greatly limits their proliferation as practical devices. Here, a unique antiferromagnetic-ferromagnetic (IrMn3 |Co20 Fe60 B20 ) heterostructure is innovated, and it is demonstrated that it can efficiently generate THz radiation without any external magnetic field. It is assigned to the exchange bias or interfacial exchange coupling effect and enhanced anisotropy. By precisely balancing the exchange bias effect and enhanced THz radiation efficiency, an optimized 5.6 nm-thick IrMn3 |Co20 Fe60 B20 |W trilayer heterostructure is successfully realized, yielding an intensity surpassing that of Pt|Co20 Fe60 B20 |W. Moreover, the intensity of THz emission is further boosted by togethering the trilayer sample and bilayer sample. Besides, the THz polarization may be flexibly controlled by rotating the sample azimuthal angle, manifesting sophisticated active THz field manipulation capability. The field-free coherent THz emission that is demonstrated here shines light on the development of spintronic THz optoelectronic devices.

Rapidly quenched heavy rare earth-based ternary Lu-Fe-B alloys: Phase evolution and hard magnetic properties

Aiming to clarify the fundamental properties of Lu-Fe-B alloys and facilitate the application of the Lu in rare earth (RE)-iron-boron permanent magnets, the phase constitutions, phase precipitation behavior, and hard magnetic properties of Lu12+xFe82−xB6 alloys (x = 0–4) prepared by melt spinning were systematically studied. The intrinsic coercivity Hcj= 313 kA/m, remanence Jr = 0.72 T, and maximum energy product (BH)max= 68.5 kJ/m3 were obtained in optimally quenched Lu12Fe82B6 alloy. Different from many other RE-Fe-B alloys, increasing Lu content leads to the formation of the soft magnetic amorphous phase and Lu6Fe23 phase, which eventually increases the exchange coupling effect and decreases Hcj and (BH)max of the Lu-rich alloys (x = 1–4). For the amorphous Lu12+xFe82−xB6 alloys, after annealing treatment, Lu2Fe14B and Fe2B phases precipitate in the alloys with x = 0 and 1, but the crystalline phases change into Lu6Fe23, Lu2Fe14B, and Fe2B phases in the alloys with x = 2–4. For the Lu-rich alloys, all crystalline phases tend to precipitate simultaneously from the amorphous matrix, which is beneficial to obtaining optimized microstructure under various processes. The alloy with x = 2 annealed at 750 °C for 10 min exhibits a good combination of magnetic properties with Jr = 0.69 T, Hcj= 234 kA/m, and (BH)max= 51.9 kJ/m3. The present results can serve as a useful reference for promoting the utilization of Lu in RE permanent magnets.

Aerodynamic correlation and flow pattern of high-rise building with side ratio of 3:1 under twisted wind profile: A computational study

This study numerically investigates twisted-wind effect on aerodynamic correlation and flow pattern of high-rise building with 3:1 side ratio. Spatial-temporal correlation of aerodynamic forces and flow filed under conventional wind profile (CWP) and twisted wind profile (TWP) are comprehensively compared. Proper orthogonal decomposition (POD) is further employed to extract wake flow pattern and mode correlation. The results show that twisted-wind effect on building with long afterbody is unique. Specifically, compared with CWP, TWP augments mean wind loads in three perpendicular directions (e.g., CMx under TWP30 is increased by 60.56% compared to CWP). The original two vortex structures (i.e., leading-edge vortex shedding (LEVS) and trailing-edge vortex shedding (TEVS)) appearing in CWP transit to relatively weaker alternate-edge vortex shedding (AEVS) in TWP, thus, the fluctuating lateral base moment and torsional force are reduced by 30.60% and 28.24% respectively under TWP30 compared to CWP. Correlation of wind load components and correlation of layer force are significantly magnified because TWP intensifies vertical moment exchange and force coupling effect, this possibly results in stronger wind-induced responses. POD analysis reveals essential mechanism of twisted-wind effect and shows that flow pattern (i.e., energy proportion, dominant frequency, mode shape and mode correlation) is largely changed by TWP.

Compensation and transition order temperature behavior of mixed spin-1 and spin-1/2 ising model on a centered honeycomb-hexagonal structure: two points of compensation

The magnetic properties and phase diagrams of the mixed spin-1/2 and spin-1 Ising model on a honeycomb inside a hexagonal structure have been studied using the Monte Carlo simulations based on the Metropolis update protocol. The effect of different exchange interactions (intralayer and interlayer Js') and single-ion anisotropy D on the magnetic properties, transition and compensation temperatures, and magnetic susceptibility have been investigated. The phase diagrams, T-J and T-D, for different exchange interactions and crystal field values have been examined. Compensation behavior exists for critical values of interlayer's exchange interactions. Our calculations reveal the existence of two points of compensation for a narrow range of negative D.Article highlightsA mixed spin Ising model on a centered honeycomb-hexagonal structure was proposedThe effects of crystal field and exchange coupling were studiedTwo points of compensation temperature were exist.

The evolution of microstructure, micromechanical and magnetic properties of FeCoNiSi alloys solidified under high pressure

A comprehensive study has been conducted on the effects of high-pressure solidification (HPS) on the evolution of microstructures, micromechanical properties, and magnetic properties of FeCoNiSi alloys. As solidification pressure increases(up to 7GPa), FeCoNiSi alloy's columnar grain changes into an equiaxed grain, and the grain size and Ni-rich precipitates are significantly refined. Furthermore, HPS facilitates the conversion of in-coherent matrix/precipitates interface of alloy solidified under atmospheric pressure into semi-coherent, which reduces the pinning effect of phase boundaries on the motion of the magnetic domains and enhances the exchange coupling between matrix and precipitates. As FeCoNiSi alloy solidifying under 7GPa, the maximum magnetic permeability (μ m ) is significantly increased (from 52,600 to 113,000), the inherent coercivity is decreased by 29.5%, and the saturation magnetic polarization( Js ) is increased by 21%. In addition, the high temperature magnetic properties of FeCoNiSi alloys are improved by HPS. Furthermore, HPS produces compressive stress in both the precipitates and matrix, which significantly improved the modulus of precipitates and matrix (i.e. from 171GPa to 215 GPa in matrix),as well as microhardness (i.e. from 5.46 GPa to 10.65 GPa in matrix). Based on the results of these studies, HPS is capable to improve the comprehensive properties of magnetic alloys significantly. • The J s and μ max (H MC ) of FeCoNiSi alloy are increased (decreased) by 21% and 114% (29.5%) after solidified under 7GPa. • HPS produces compressive stress in the precipitates and matrix and significantly improved the modulus and microhardness. • HPS refine the microstructure of FeCoNSi alloy and result in the semi-coherent interface. • The semi-coherent interface enhances the exchange coupling effect between precipitates and matrix.

Interphase exchange coupling and magnetocaloric effect in Co3Gd4/Co7Gd12 magnetic nanocomposites, obtained by mechanical milling

The effect of interphase exchange coupling on the Curie temperature of mechanically milled Co3Gd4/Co7Gd12 magnetic nanocomposites was studied. It was shown that the nanocomposites have a single critical temperature (195 K), approximately half way between those of the constituent alloys (160 K for Co7Gd12 and 220 K for Co3Gd4). The interphase exchange coupling within the nanocomposites was investigated, at 4 K, by the First Order Reversal Curve method.

Synthesis and Characterization of peri‐Heptacene on a Metallic Surface

AbstractThe synthesis of long n‐peri‐acenes (n‐PAs) is challenging as a result of their inherent open‐shell radical character, which arises from the presence of parallel zigzag edges beyond a certain n value. They are considered as π‐electron model systems to study magnetism in graphene nanostructures; being potential candidates in the fabrication of optoelectronic and spintronic devices. Here, we report the on‐surface formation of the largest pristine member of the n‐PA family, i.e. peri‐heptacene (n=7, 7‐PA), obtained on an Au(111) substrate under ultra‐high vacuum conditions. Our high‐resolution scanning tunneling microscopy investigations, complemented by theoretical simulations, provide insight into the chemical structure of this previously elusive compound. In addition, scanning tunneling spectroscopy reveals the antiferromagnetic open‐shell singlet ground state of 7‐PA, exhibiting singlet–triplet spin‐flip inelastic excitations with an effective exchange coupling (Jeff) of 49 meV.