Effective Exchange Coupling Research Articles

Bimagnetic Core/Shell Nanoprobes with Tunable Exchange Coupling for High Resolution and Sensitive Magnetic Particle Imaging

Magnetic particle imaging (MPI) has demonstrated versatile applications in biomedicine, including tumor imaging, cell tracking, and image‐guided hyperthermia. Despite these advancements, the prevalent use of clinically approved tracers has posed limitations on MPI's resolution and sensitivity. In this study, we engineered a bimagnetic core/shell nanocrystals (BMCS) tailored for MPI by optimizing the heterostructure and modulating the exchange coupling effect between the two magnetic components. The resulting BMCS exhibited remarkably heightened susceptibility and magnetization while maintaining low coercivity, thereby substantially improved both MPI resolution and sensitivity compared to conventional tracers such as VivoTrax. At an equivalent mass concentration, BMCS demonstrated a notable 5.08‐fold increase in signal intensity and achieved an unprecedentedly high resolution down to 1 mm. The excellent MPI performance contributes to high resolution MPI and the sensitive detection of orthotopic colorectal cancer in mice. The design strategy employed in BMCS, centered on the exchange coupling effect, introduces an efficacious approach for the development of high performance MPI tracers.

Bimagnetic Core/Shell Nanoprobes with Tunable Exchange Coupling for High Resolution and Sensitive Magnetic Particle Imaging.

Magnetic particle imaging (MPI) has demonstrated versatile applications in biomedicine, including tumor imaging, cell tracking, and image-guided hyperthermia. Despite these advancements, the prevalent use of clinically approved tracers has posed limitations on MPI's resolution and sensitivity. In this study, we engineered a bimagnetic core/shell nanocrystals (BMCS) tailored for MPI by optimizing the heterostructure and modulating the exchange coupling effect between the two magnetic components. The resulting BMCS exhibited remarkably heightened susceptibility and magnetization while maintaining low coercivity, thereby substantially improved both MPI resolution and sensitivity compared to conventional tracers such as VivoTrax. At an equivalent mass concentration, BMCS demonstrated a notable 5.08-fold increase in signal intensity and achieved an unprecedentedly high resolution down to 1 mm. The excellent MPI performance contributes to high resolution MPI and the sensitive detection of orthotopic colorectal cancer in mice. The design strategy employed in BMCS, centered on the exchange coupling effect, introduces an efficacious approach for the development of high performance MPI tracers.

Enhanced anisotropy of Pr-Nd-Fe-B HDDR magnetic powders by regulating the hydrogen decrepitated process

The evolution of microstructures for different hydrogen decrepitated routes was investigated in hydrogenation-disproportionation-desorption-recombination (HDDR) magnetic powders, with particular focus on the fracture of the SC heat-treated alloy during two-step hydrogen decrepitation (HD) and the influence of the first step temperature on magnetic properties. As the temperature increases, the properties of the magnetic powders increase and then decrease, the optimized performance of HDDR magnetic powders was obtained with Br=1.45 T, (BH)max=393 kJ·m−3, DOA=0.79 when the first HD temperature is 65°C. The microscopic analysis shows that there are two problems in the one-step HD route: one is that the particle size is larger ∼150 μm and there are incomplete fractured cracks, the particle surface is more PrNd-rich phase, which leads to different reaction rates between the surface and center in HDDR particles, and the texture orientation of the fine grains at the edge of surface and crack is disordered. The second issue is that HD grains have significant angular difference with the c-axis orientation in incomplete fractured grain clusters, which is directly inherited by the HDDR magnetic powder, resulting in a misalignment texture. In comparison, the two-step HD process effectively addresses the issues derived from one-step HD process. Not only is the grain size of HDDR magnetic powders uniform and the width of the thin grain boundary phases (GBs) is less than the exchange coupling length, but also (Fe+Co) content exceeding 30 at% in GBs with weak magnetism. This is beneficial for enhancing exchange coupling effect between adjacent grains. Moreover, magnetic powder particle exhibits single-domain characteristics, which improves the grain texture orientation. This research provides a new insight for the development of high-orientation and high-performance HDDR magnetic powders.

Mechanism of praseodymium yttrium regulating the absorption properties of M-type strontium ferrite: From single phase to multi phase mixed system

When Pr and Y ions are added to M-type strontium ferrite, a portion of the ions will enter the main phase lattice, while the excess ions will aggregate at the grain boundaries to form a secondary phase. The occupancy of rare earth ions in the main phase lattice and the formation of secondary phases are two key factors affecting the material's absorbing properties. The difference in ultimate solid solubility between rare earth ions Pr and Y in M-type strontium ferrite is significant, and they also form different secondary phases at grain boundaries. Consequently, the synthesized SrFe12-2xPrxYxO19 (0 < x ≤ 1.5) demonstrates its potential as a microwave absorbing material across a wide range of substitutions. Moreover, the performance changes have their own characteristics at different substitution levels. XRD results show that the samples include pure M phase and the coexistence of M phase and heterogeneous phases, with the heterogeneous phases being perovskite-type and garnet-type. SEM and EDS results indicate that Pr and Y co-substitution can refine grain size and reduce oxygen content. XPS results suggest that the valence change of Pr ions leads to the generation of Fe2+ ions and oxygen vacancies (Od). PPMS results reveal that saturation magnetization and coercivity are mainly affected by anisotropy field and particle size. Co-substitution of Pr and Y increases the dielectric and magnetic losses of the M phase, and the generated heterogeneous phases also enhance the dielectric and magnetic losses through interface polarization and magnetic exchange coupling effects, both cooperatively improving the wave-absorbing performance, with the minimum reflection loss (RLmin) reaching −51.57 dB (x = 0.1, d1 = 4.0 mm, EAB = 2.57 GHz).

Exploring the effect of exchange coupling in (SrFe9.8Al2La0.2O19)1-x/(Co0.7Zn0.3Fe2O4)x nanocomposites synthesised by sol–gel auto combustion method

Extensive research has been conducted over the last decade to develop permanent magnets utilizing less rare earth materials. Exchange-coupled nanocomposite magnets are a fascinating development in the field of permanent magnets. They involve the combination of hard and soft ferrites, resulting in a unique and evolving magnet. Here we present an in-depth structural, morphological and magnetic characterization of ferrite-based nanostructures obtained through a bottom-up sol−gel approach. The combination of high coercivity of a hard phase SrFe9.8Al2La0.2O19 (SFALO) and high saturation magnetization of a soft phase, Co0.7Zn0.3Fe2O4 (CZFO), allowed us to develop exchange-coupled bi-magnetic nanocomposites by varying the mass percentage (x = 0, 0.1, 0.2, 0.3, 0.4, 1). Thermogravimetric analysis (TGA) up to 1200 °C was used to investigate the material’s thermal properties with the phase formation temperature. Detailed atomic/nano-scale structural characterizations of these nanocomposites were performed by X-ray diffraction and Transmission Electron Microscopy. The average particle size was calculated from the SEM analysis and it was observed that most of the composites with ratio x = 0.1, 0.3 & 0.4 was 145 nm and x = 0.2 was 165 nm. The Switching Field Distribution (SFD) curve revealed the exchange coupling between soft and hard magnetic particles. The findings indicate that the inclusion of a soft magnetic phase has a considerable impact on the exchange coupling between the hard and soft ferrite phases. As the spinel concentration increased, in the composite the saturation magnetization increased from 18 emu/g to 48 emu/g. From the stroner-wholfrathmodel, the decrease in the remanence ratio below 0.5 concludes the crystallites are randomly oriented. The nanocomposite with the ratio x = 0.1 was observed with the high magneto crystalline anisotropy of 2183.23 × 102 (J/m3) and a high energy product (BH)max of 5.5 kJ/m3. The aforementioned features of nanocomposites demonstrate their potential for use in permanent magnet applications.

Magnetic exchange coupled composite behavior in the doped manganite nanoparticles: A proposed phenomenological model

In this paper, we comprehensively investigate the isothermal magnetization behavior of doped perovskite manganite nanoparticles. The focus is on understanding the impact of variation of particle sizes on the soft and hard magnetic phases with respect to the changes in the coercive field and remanent magnetization, both theoretically and experimentally. The study seeks to correlate experimental findings with the proposed phenomenological model to gain deeper insights into the underlying mechanisms governing exchange coupling and anisotropy effects in the nanocrystalline composites. The proposed phenomenological model beautifully demonstrates how the values of saturation magnetization and coercive field changes with changing the particle size in the nanocrystalline La0.48Ca0.52MnO3 (LCMO48) and La0.46Ca0.54MnO3 (LCMO46) compounds. In addition, the model provide an insights into the limitations of critical radius, size and shape of the nanocrystalline particle. This investigation looks into how the size of particles affects their magnetic properties, specifically coercive field and remanent magnetization.

A Monte Carlo study of Magnetic and thermodynamic behaviors of L[formula omitted] structure like quadrangular frustum pyramid nanoisland

Based on the Monte Carlo simulation, research has been conducted on the magnetic and thermodynamic behaviors of a ferrimagnetic mixed-spin (5/2, 1/2) L10 nanoisland with a like quadrangular frustum pyramid structure. We studied the effects of the magneto-crystal anisotropy and exchange coupling on the magnetization, susceptibility, internal energy, and hysteresis loops. The ferrimagnetic system exhibits more abundant magnetic and thermodynamic behavior than the cubic L10 nanoisland. We found eight saturation values of magnetization, five-loop hysteresis loops, and abrupt transitions in the low-temperature region. In addition, the exchange coupling effect of ferrimagnetism also promotes magnetic properties. Interestingly, in this system, the ferromagnetic exchange coupling (Jaa) and external magnetic field (h) linearly depend on the blocking temperature TB. The hysteresis loops of the system have more magnetization steps, which reflects that it has more ground state spin configurations.

Influence of the Substrate on the Exchange Coupling of NiO/FeCo Bilayers

Antiferromagnetic/ferromagnetic (AF/F) systems have been extensively investigated due to the importance that interfacial exchange coupling effects have in the development of magnetic storage technologies. Recently, these systems have garnered interest for the potential they have to imprint the magnetic moments of the AF into an F layer, offering the possibility of using it as a read-out mechanism in antiferromagnetic spintronics. In this study, we explored the importance of crystalline orientation and strains induced by the substrate in the exchange coupling properties of NiO/FeCo AF/F bilayers. For that, we have grown NiO/FeCo bilayers on MgO (001) and Al2O3 (0001) substrates varying the FeCo layer thickness. In addition, we have analyzed both deposited samples and those with induced interfacial unidirectional anisotropy. For inducing such interfacial anisotropy, we used a field cooling procedure, heating the bilayers to 650 K and subsequently cooling down to room temperature under the presence of an external magnetic field of 300 mT. We have investigated the effect of the substrate in terms of crystalline orientation and lattice mismatching on the AF/F exchange coupling as well as the dependence of the coercivity and exchange bias on the inverse F layer thickness that is consistent with the interfacial origin of the AF/F exchange coupling. Moreover, the angular dependence of the magnetic properties was explored by using vectorial Kerr magnetometry, confirming the presence of both magnetocrystalline anisotropy, arising from the epitaxial character of the growing process mainly when the bilayer is grown on MgO (001) substrates, and the field cooling (FC)-induced unidirectional anisotropy.

Magnetic behaviour of a mixed-spin ising model on a two-dimensional square-octagon lattice in a dynamic magnetic field

The magnetic behaviours of a two-dimensional square-octagon lattice with mixed spin 5/2 and spin 1 magnetic atoms are investigated in a dynamic magnetic field using effective field theory with correlations. We studied the effects of exchange coupling, anisotropy, oscillating magnetic fields, and temperature on the dynamic magnetic and thermodynamic properties of the square-octagon lattice. The dynamic magnetisation, order parameters, susceptibility, internal energy, and phase diagram of the system were obtained. These results significantly contribute to our understanding of the dynamic magnetic properties of two-dimensional materials. The ability to manipulate and control magnetic behaviour in two dimensions holds promise for the creation of more efficient and versatile devices.

Macroscopic phenomenon of exchange coupling and microscopic effect on magnetization reversal in sintered Nd–Fe–B magnets

Permanent magnets of Nd13.5Fe79.18M1.52B5.8 and Nd13.5Fe79.76M0.94B5.8 were prepared, respectively, via strip casting, jet milling and sintering followed by annealing. By adding the non-ferromagnetic elements M (Al, Cu, Ga and Zr) into the magnets, it could not only modify the microstructure, but also regulate the exchange coupling effect in the sintered magnets. From the macroscopic point of view, the recoil loops exhibit spring behavior in Nd13.5Fe79.76M0.94B5.8, indicating that the energy barrier can be overcome by the intergranular exchange coupling. From the microcosmic point of view, the exchange coupling can increase the domain wall size by suppressing the nucleation of reversed domains, and so the activation volume increases with thermal activation. In Nd13.5Fe79.76M0.94B5.8 the exchange coupling effect is stronger, and both the coercivity of 15.0 kOe and the remanence of 14.3 kGs are a little higher than those of Nd13.5Fe79.18M1.52B5.8 magnets in which the content of non-ferromagnetic elements is a little higher and the exchange coupling effect is weaker. Thus, the exchange coupling does not decrease the coercivity due to the exchange coupling suppressing the nucleation of reversed domains, though the microstructure is inhomogeneous in the sintered magnets of Nd13.5Fe79.76M0.94B5.8. Reducing the defect size and decreasing the defect concentration should be a practical way to improve the coercivity in Nd–Fe–B permanent magnets.

Enhancement of spin to charge conversion efficiency at the topological surface state by inserting normal metal spacer layer in the topological insulator based heterostructure

In this study, we report efficient spin to charge conversion (SCC) in the topological insulator (TI) based heterostructure (BiSbTe1.5Se1.5/Cu/Ni80Fe20) by using spin-pumping technique, where BiSbTe1.5Se1.5 is the TI and Ni80Fe20 is the ferromagnetic (FM) layer. The SCC, characterized by inverse Edelstein effect length (λIEE) in the TI material, gets altered with an intervening Copper (Cu) layer, and it depends on the interlayer thickness. The introduction of Cu layer at the interface of TI and FM metal provides a new degree of freedom for tuning the SCC efficiency of the topological surface states. The significant enhancement of the measured spin-pumping voltage and the increased linewidth of ferromagnetic resonance absorption spectra due to the insertion of Cu layer at the interface indicate a reduction in spin memory loss at the interface that resulted from the presence of exchange coupling between the surface states of TI and the local moments of FM metal. The temperature dependence (from 8 to 300 K) of the evaluated λIEE data for all the trilayer systems, TI/Cu/FM with different Cu thicknesses, confirms the effect of exchange coupling between the TI and FM layer on the SCC efficiency of the topological surface state. This study offers promising ways for designing more efficient spin-charge conversion devices of the TI-based heterostructure by controlling the interface effects.

Gd‐based molecular coolants: Aggregating for better magnetocaloric effect

AbstractTwo series of 3d‐Gd mixed‐metal phosphonate complexes with either only two gadolinium centers such as {Gd2}, {Ni2Gd2}, {Co4Gd2}, {Co8Gd2}, {Fe6Gd2}, and {Fe17Gd2} or more than two gadoliniums such as {Co8Gd4}, {Mn8Gd4}, {Co4Gd6}, {Mn4Gd6}, {Co6Gd8}, {Ni5Gd8}, {Ni6Gd6}, {Co8Gd8}, and {Mn9Gd9} have been solvothermally prepared and magnetothermally studied. The nearly identical environments of the Gd(III) dimer in the first series allow us to qualitatively analyze the effect of magnetic exchange coupling on the magnetocaloric effect (MCE). By doubling, tripling, or quadrupling of the Gd(III) centers, the second series of 3d‐Gd mixed‐metal complexes was built to further test the other effects of exchange couplings on MCE in more complicated circumstances. For the antiferromagnetic coupling cases, the results are nearly identical but diversify when topological spin frustrations are created, whose massive low‐lying excited spin states help enhance MCE. For presumably ferromagnetically coupled ones, albeit are rare in phosphonate complexes, they do exhibit excellent MCE. Meanwhile, the complexes with weakly coupled metal centers serve as excellent examples for studying the effect of molecular mass on MCE when its magnitude is expressed in the unit of Joule per kilogram, from which we can see the values are directly proportional to the percentage of the Gd(III) ions in molecular weight.

Pseudo Temperature Independent Paramagnetism in Co2Si5N8 Nitridosilicate

Herein, the synthesis of Co2Si5N8 nitridosilicate from α‐Ca2Si5N8 by ion exchange is described. The monoclinic structure (space group Cc) is compared with those of the related nitridosilicates Ca2Si5N8 and Fe2Si5N8 its magnetic properties are profoundly investigated. Most interestingly, polycrystalline Co2Si5N8 exhibits a quasi‐temperature‐independent paramagnetic susceptibility of about over a wide temperature range from 100 to 600 K. Two different magnetic models based on a phenomenological Hamiltonian that takes into account magnetic exchange coupling, crystal‐field interactions, and Zeeman effects within the framework of an angular momentum basis set are refined to the experimental susceptibility datasets. The results of the refinements point to the realization of a pseudo temperature independent paramagnetism for Co2Si5N8 that results from a near‐perfect cancellation of temperature‐dependent contributions to the susceptibility depending delicately on the mixing of ion levels in the electronic states, their relative energies, and the magnetic coupling between them. Additionally, performed direct current field scan, alternating current susceptibility, and heat capacity measurements complete the magnetic characterization and facilitate the identification of impurity phase contributions.

Electrostatic Gating of Spin Dynamics of a Quasi-2D Kagome Magnet.

Electrostatic gating has emerged as a powerful technique for tailoring the magnetic properties of two-dimensional (2D) magnets, offering exciting prospects including enhancement of magnetic anisotropy, boosting Curie temperature, and strengthening exchange coupling effects. Here, we focus on electrical control of the ferromagnetic resonance of the quasi-2D Kagome magnet Cu(1,3-bdc). By harnessing an electrostatic field through ionic liquid gating, significant shifts are observed in the ferromagnetic resonance field in both out-of-plane and in-plane measurements. Moreover, the effective magnetization and gyromagnetic ratios display voltage-dependent variations. A closer examination reveals that the voltage-induced changes can modulate magnetocrystalline anisotropy by several hundred gauss, while the impact on orbital magnetization remains relatively subtle. Density functional theory (DFT) calculations reveal varying d-orbital hybridizations at different voltages. This research unveils intricate physics within the Kagome lattice magnet and further underscores the potential of electrostatic manipulation in steering magnetism with promising implications for the development of spintronic devices.

Structural and magnetic properties of hexagonal ferrite composites with planar and perpendicular anisotropy

In the present work, composites of (x)SrFe12O19(SrM)/(1-x) Ba2Co2Fe12O22(Co2Y) [where x = 0.1, 0.2, 0.3, 0.4] were prepared by physical mixing method. The effect of exchange coupling between SrM and Co2Y phases on composites structural, morphological, and magnetic properties has been investigated. XRD patterns confirmed the co-existence of SrM and Co2Y phases with the presence of a secondary Co2Z phase. Smooth hysteresis loops without kink confirmed that SrM and Co2Y phases in composites are exchange-coupled and signify cooperative magnetic switching among M and Y phase spins. A linear increase in magnetization was observed with an increase in M-phase. On the other hand, a decrease in coercivity was observed due to strong intervening coupling between the phases. A single peak in the switching field distribution curve (SFD) of composites also confirm strong coupling between M and Y phases.

Dimensionality driven exchange coupling effect in cuprate-manganite superlattices

The coupling and competition between various degrees of freedom at the interface of transition metal oxide heterointerfaces greatly enrich their physical properties and expand their relevant application scope. It has been reported that dimensionality is an effective method to regulate the properties of oxide heterostructure. The structure of SCO film exhibits a planar-type-to-chain-type transformation with the change of thickness. In this work, the high-quality SCO/LCMO superlattices are deposited by a pulsed laser deposition system. And the interfacial exchange coupling effect is effectively manipulated by controlling the dimensionality of SCO layer. X-ray absorption spectrum (XAS) measurement shows that the charge transfer occurs at the heterointerface. When the SCO layer is thin, the interfacial superexchange coupling supported by charge transfer generates a weak magnetic moment to pin the ferromagnetic LCMO layer. As the SCO layer thickens, the charge transfer will decrease. Meanwhile, the long-range antiferromagnetic order in thicken SCO layer can interact with LCMO layer, resulting in the exchange bias effect. This experiment confirms the important role of dimensionality in modulating the properties in multifunctional oxide heterostructure.

Fourfold anisotropic magnetic damping induced by anisotropic spin absorption in amorphous CoFeB/epitaxial IrMn3 bilayers

Magnetic damping rooted in various relaxation processes of spin angular momentum plays a crucial role in determining the energy consumption and the operating speed of emerging spintronic devices. Here, we reported a fourfold anisotropic magnetic damping extrinsically caused by spin current absorption in amorphous ferromagnetic/antiferromagnetic bilayers. The angular dependent broadband ferromagnetic resonance measurements were employed to investigate the effect of interfacial exchange coupling on magnetization dynamics of the CoFeB/IrMn3 bilayers. In addition to the conventional exchange bias, the amorphous CoFeB layer exhibits an induced fourfold magnetic anisotropy along the IrMn3⟨100⟩ axes because of the exchange coupling with the uncompensated IrMn3 moments along IrMn3⟨100⟩ caused by the 3Q antiferromagnetic structure. The magnetic damping of the CoFeB/IrMn3 bilayer also exhibits an obvious fourfold symmetry, which is ascribed to the anisotropic spin absorption caused by the uncompensated IrMn3 moments. The ratio of the fourfold anisotropic magnetic damping decreases dramatically with reducing the interfacial exchange coupling. When the interfacial exchange coupling is isolated by a Cu spacer layer, both the induced magnetic anisotropy and the magnetic damping exhibit a uniaxial symmetry.

Magnetic anisotropy engineering in onion-structured metal oxide nanoparticles combining dual exchange coupling and proximity effects.

A series of exchange-coupled magnetic nanoparticles combining several magnetic phases in an onion-type structure were synthesized by performing a three-step seed-mediated growth process. Iron and cobalt precursors were alternatively decomposed in high-boiling-temperature solvents (288-310 °C) to successively grow CoO and Fe3-δO4 shells (the latter in three stages) on the surface of Fe3-δO4 seeds. The structure and chemical composition of these nanoparticles were investigated in depth by combining a wide panel of advanced techniques, such as scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy-spectrum imaging (EELS-SI), 57Fe Mössbauer spectrometry, and X-ray circular magnetic dichroism (XMCD) techniques. The size of the nanoparticles increased progressively after each thermal decomposition step, but the crystal structure of core-shell nanoparticles was significantly modified during the growth of the second shell. Indeed, the antiferromagnetic CoO phase was progressively replaced by the CoFe2O4 ferrimagnet due to the concomitant processes of partial solubilization/crystallization and the interfacial cationic diffusion of iron. A much more complex chemical structure than that suggested by a simple size variation of the nanoparticles is thus proposed, namely Fe3-δO4@CoO-CoFe2O4@Fe3-δO4, where an intermediate Co-based layer was shown to progressively become a single, hybrid magnetic phase (attributed to proximity effects) with a reduction in the CoO amount. In turn, the dual exchange-coupling of this hybrid Co-based intermediate layer (with high anisotropy and ordering temperature) with the surrounding ferrite (core and outer shells) stabilized the particle moment well above room temperature. These effects allow for the production of Fe oxide-based magnetic nanoparticles with high effective anisotropy, thus revealing the potential of this strategy to design rare-earth-free permanent nanomagnets at room temperature.

Synergistic grain boundary modification and magnetic hardening for coercivity enhancement in Nd-Ce-Fe-B sintered magnets with Dy[sbnd]Cu addition

Obtaining high coercivity in bulk magnets is essential to broaden the prospects for large-scale applications of Nd-Ce-Fe-B magnets. In this work, the potential of low-cost Nd-Ce-Fe-B magnets is unlocked by improving their magnetic properties, and the coercivity enhancement mechanism is elucidated through systematic material characterization. First, by designing the substrate with RE-rich (31.2 wt%), coercivity up to 13.5 kOe was obtained in Nd-Ce-Fe-B magnets with Ce/RE = 30 wt% (RE: rare earth). Microstructural analysis revealed that the high coercivity was attributed to the formation of a continuous GB phase layer induced by the excess RE. Furthermore, by doping only 2.4 wt% DyCu, the coercivity was substantially enhanced to 19.5 kOe and the coercivity temperature coefficient (20–150 °C) achieved −0.529%/°C while the remanence was maintained at 12.0 kG. Elemental distribution analysis shows that Dy enters the matrix phase while Nd/Ce is expelled, and the additional RE further widens the GB thickness, which weakens the exchange coupling effect between adjacent grains. Moreover, Dy mainly replaces Ce rather than Nd on the grain surface, resulting in a Dy-rich shell, which greatly enhances the anisotropy field of the shell region. Magnetic domain analysis shows that GB modification and magnetic hardening effectively suppress the nucleation and cascade propagation of the reversal domains, and their synergistic enhancement of the coercivity is also verified by micromagnetic simulations.

High Efficiency and Flexible Modulation of Spintronic Terahertz Emitters in Synthetic Antiferromagnets.

Spintronic terahertz (THz) emitters based on synthetic antiferromagnets (SAFs) of FM1/Ru/FM2 (FM: ferromagnet) have shown great potential for achieving coherent superposition and significant THz power enhancement due to antiparallel magnetization alignment. However, key issues regarding the effects of interlayer exchange coupling and net magnetization on THz emissions remain unclear, which will inevitably hinder the performance improvement and practical application of THz devices. In this work, we have investigated the femtosecond laser-induced THz emission in Pt (3)/CoFe (3)/Ru (tRu = 0-3.5)/CoFe (tCoFe = 1.5-10)/Pt (3) (in units of nm) films with compensated and uncompensated magnetic moments. Antiferromagnetic (AF) coupling occurs in the Ru thickness ranges of 0.2-1.1 and 1.9-2.3 nm, with the first peak (tRu = 0.4 nm) of the AF coupling field (Hex) significantly higher than that of the second peak (2.0 nm). Rather high THz amplitude is found for the samples with strong AF coupling. Nevertheless, despite the same remanence ratio of zero, the THz amplitude for the symmetric SAF films declines significantly as the tRu decreases from 0.8 to 0.4 nm, which is mainly ascribed to the noncolinear magnetization vectors due to the increased biquadratic coupling term. Specifically, we demonstrate that an asymmetric SAF structure with a dominant FM layer is more favored than the completely compensated one, which could generate significantly enhanced THz electric field with well-controlled polarity and intensity. In addition, as the temperature decreases, the THz emission intensity increases for the SAF samples of tRu = 0.9 nm with negligible biquadratic coupling, which is contrary to the decreasing trend of the tRu = 0.4 nm sample and has been attributed to the greatly enhanced Hex.