Effective Exchange Coupling Research Articles

Synthesis and Characterization of peri-Heptacene on a Metallic Surface.

The 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.

Effects of Shape Anisotropy on Hard-Soft Exchange-Coupled Permanent Magnets.

Exchange-coupled magnets are promising candidates for a new generation of permanent magnets. Here, we investigated the effect of soft magnetic shell thickness and the aspect ratio of the hard magnetic core on the magnetic properties for isolated core/shell cylinder exchange-coupled magnets, as well as the packing effect of the cylindrical array via a micromagnetic simulation method. It was found that the shape anisotropy contributions to the magnetic properties in the cylindrical core/shell exchange-coupled magnets are closely related to the thickness of the soft magnetic shell. When the soft magnetic shell is thin, the magnetic properties are dominated by the hard–soft exchange coupling effects, and the contributions of shape anisotropy are quite limited. When the soft magnetic shell is relatively thick, utilizing shape anisotropy would be an effective method to improve the magnetic performance of hard–soft exchange-coupled magnets. The present work provides an in-depth fundamental understanding of the underlying magnetization reversal mechanism. This work could be useful for designing high-performance permanent magnets and avoiding pitfalls.

Phase precipitation and magnetic properties of melt-spun ternary Gd2Fe14B alloy and advantages of gadolinium substitution in Y2Fe14B alloy

To take the advantage of gadolinium (Gd) in developing and manufacturing RE-permanent magnets, the magnetic properties and phase precipitation behavior of Gd2Fe14B alloys prepared by melt spinning were investigated in this work. The results show that optimally direct quenched nanocrystalline Gd2Fe14B alloy exhibits the magnetic properties with remanence Jr of 0.51 T, coercivity Hc of 187 kA/m, and maximum energy product (BH)max of 33.1 kJ/m3. It also shows excellent thermal stability with a small negative temperature coefficient of Jr (α = −0.066%/K) and a positive temperature coefficient of Hc (β = 0.171%/K). The phase precipitations of as spun Gd2Fe14B alloy during melt spinning and over-quenched amorphous Gd2Fe14B alloy during heat treatment were clarified. The magnetic properties of the crystallized alloys annealed at 800 °C are close to those of the optimized directly quenched as-spun alloys, indicating good performance stability under various processes. In addition, the role of Gd substitution in (Y1–xGdx)2Fe14B alloys was systematically studied. Gd substitution can significantly enhance the thermal stability and decrease the irreversible remanence loss of Y2Fe14B alloy. With increasing Gd content, Gd-doped Y2Fe14B alloys show continuous enhancement of Hc at elevated temperatures. The highest β value of 0.22%/K was obtained in (Y0.2Gd0.8)2Fe14B alloy at 300–400 K. Gd substitution is not only effective in improving the exchange coupling effect and corrosion resistance of Y2Fe14B alloy but also superior to Nd in terms of improving the oxidation resistance. The present work has certain instructions for designing and developing new low cost RE permanent magnets with improved thermal and environment stability.

Improved coercivity of W- and M-type composite hexaferrites using two different synthesis routes

Two series of W- and M-type composite hexaferrites were synthetized using the conventional ceramic method in two different synthesis routes. In the first synthesis route, W- and M-type composite hexaferrites were formed by reaction in the pre-sintering stage. In the second synthesis route, W- and M-type composite hexaferrites were obtained by physically mixing W- and M-type hexaferrites in the second ball milling stage. The influence of M-type phase on the structure and magnetic properties of W-type hexaferrites synthesized in both synthesis routes are investigated in detail. For both synthesis routes, X-ray diffraction (XRD) patterns of all samples indicated crystallization of W- and M-type hexaferrites phase and scanning electron microscopy (SEM) measurements revealed hexagonal platelet-like grains. In the pre-sintering stage, the saturation magnetization (4πMs) of all samples ranged between 3.30 kG and 4.25 kG. The remanence ratio (Mr/Ms) changed from 0.77 to 0.88 with the increase of pre-sintering temperature. The coercivity (Hc) of all samples ranged between 456 Oe and 1846 Oe. The ferromagnetic resonance (FMR) linewidth (ΔH) ranged between 466 Oe and 748 Oe. In the second ball milling stage, 4πMs and Mr/Ms maintained a relatively stable value of ~4.5 kG and ~0.89, respectively. Hc increased from 519 Oe to 620 Oe with the increase of M-type hexaferrites. ΔH ranged between 317 Oe and 573 Oe. We note that in both synthesis routes only the composite hexaferrites synthesized in the pre-sintering stage have a significantly improved coercivity from 456 Oe to 1846 Oe. This enhancement in coercivity is attributed mainly to the effect of exchange coupling among ferrite grains. In addition, the ΔH decreases with the addition of M-type hexaferrites.

Tuning the size of skyrmion by strain at the Co/Pt3 interfaces

SummaryBased on density functional theory calculations, we elucidated the tunability of the atomic structures and magnetic interactions of Co/Pt3 interface (one layer of hcp(0001) Co and three layers of fcc(111) Pt) and thus the skyrmion sizes using strain. The dispersion relations of the spin spiral in the opposite directions, E(q) and E(-q), were evaluated based on generalized Bloch equations. Effective exchange coupling (EC) and Dzyaloshinsky-Moriya interaction (DMI) parameters between different neighbors Ji and di at different lattice constants were derived by fitting the resulting spin spiral dispersion E(q) to EC model with DMI and E(q)-E(-q) formula, respectively. We observed an increase in DMI and a significant decrease in EC with an increase in strain. Hence, the size of Néel-type skyrmions determined by the ratio of EC/DMI can be controlled by applying strain, leading to an effective approach to tailor the formation of skyrmion lattices by inducing slight structural modifications on the magnetic thin films.

Three-axis torque investigation of interfacial exchange coupling in a NiFe/CoO bilayer micromagnetic disk

Micrometer diameter bilayers of NiFe (permalloy, Py) and cobalt oxide (CoO) deposited on nanomechanical resonators were used to investigate exchange bias effects. The mechanical compliances of two resonator axes were enhanced by severing one torsion arm, resulting in a unique three-axis resonator that responds resonantly to torques generated by a three-axis RF field. Our technique permits simultaneous measurement of three orthogonal torque components. Measurements of the anisotropies associated with interfacial exchange coupling effects have been made. At cryogenic temperatures, observations of shifted linear hysteresis loops confirmed the presence of exchange bias from the Py/CoO interface. An in-plane rotating DC bias field was used to probe in-plane anisotropies through the out-of-plane torque. Training effects in the rotational hysteresis data were observed and showed that features due to interfacial coupling did not diminish irrespective of substantial training of the unidirectional anisotropy. The data from the rotational hysteresis loops were fit with parameters from a macrospin solution to the Landau-Lifshitz-Gilbert equation. Each parameter of the exchange bias model accounts for specific features of the rotational loop.

Exchange coupled CoFe2O4/CoFe composites for enhanced microwave absorption properties by in-situ hydrothermal reduction

Exchange coupling magnetic composites have been proven to help improve the microwave absorption performance of absorbents. However, composite systems reported showed unsatisfactory exchange coupling effect due to poor particle dispersion and interface contact. Herein, exchange-coupled CoFe2O4/CoFe composites were prepared by in-situ hydrothermal reduction. The influence of surfactant on the microstructure and properties were studied in detail. The results showed that the CoFe2O4/CoFe samples with hexadecyltrimethylammonium bromide (CTAB) as the surfactant had more two-phase interfaces and excellent dispersion, the grain size of CoFe2O4 and CoFe were 45 nm and 38.4 nm. Close contact interface and small grain size caused suitable exchange coupling effect, which enhanced the microwave absorption performance of CoFe2O4/CoFe composites. The CoFe2O4/CoFe sample displayed maximum reflection loss (RLmax) of −54.8 dB at 3.5 mm thickness and effective absorption bandwidth (EAB) of 3.16 GHz, revealing promising prospect in the electromagnetic wave absorption field.

Microstructure and magnetic properties of Sm-Co-based amorphous nanocrystals controlled by B addition

The crystalline Sm-Co alloys show excellent hard magnetic properties, while the amorphous Sm-Co alloys have a poor performance. In this paper, Sm12Co76-xFe4Zr7Cu1Bx (x = 0, 2, 4, 5, 5.2) ribbons were melt-spun at 40 m/s, and the effects of B content on the phase composition, microstructure, and magnetic properties of the ribbons were studied. The results show that the main phase of all ribbons with different B content is Sm(Co, M)7. With an increase in B content, the content of amorphous and fine soft magnetic phases increases. Besides, nano-scale crystalline clusters are embedded in the amorphous matrix, forming different forms of exchange coupling microstructures between the soft and hard magnetic phases. The x = 5 ribbons have the finest grains, the most uniform grain distribution, and the strongest exchange coupling effect. Without heat treatment, their coercivity reaches 19.5 ± 0.9 kOe, and the other magnetic properties are also excellent, Mr = 24.8 ± 1.7 emu/g, M2T = 29.0 ± 1.7 emu/g, and Mr/M2T = 0.858 ± 0.015.

Influence of Cr interlayer with different thickness on transition of magnetoresistance effect of Gd/FeCo thin films

As one of the most representative features characterizing the spin valve structure, magnetoresistance is an important method to study the interlayer coupling in multilayers. Considering the induced magnetism of rare earth at room temperature due to the coupling and magnetic proximity effect in the structure of rare earth/magnetic transition metal, an intermediate nonmagnetic metal can be inserted to form the spin valve structure to regulate the interlayer coupling, which expands the scope of applications of rare earth in spintronics. In this work, the interlayer exchange coupling and interfacial effects of Gd (4 nm)/Cr (<i>t</i><sub>Cr</sub>)/FeCo (5 nm) trilayers with different Cr layer thickness (<i>t</i><sub>Cr</sub>) are studied by means of in plane magnetoresistance. Compared with FeCo film, Gd/FeCo film obtains more obvious anisotropic magnetoresistance. While the magnetoresistance value obtained for the configuration of <i>I</i>⊥<i>H</i> shows a minimum value at the peak due to the insertion of Cr layer, and this minimum value becomes more pronounced with the increase of <i>t</i><sub>Cr</sub>. When<i> t</i><sub>Cr</sub> = 3 nm, the negative spin valve effect almost totally overcomes the anisotropic-magnetoresistance effect. Different spin asymmetries of scattering that are formed in FeCo layer and Cr/Gd layers are mainly responsible for creating the negative spin valve magnetoresistance, in which the resistance becomes smaller near the coercive, while the resistance becomes larger at high field parallel to magnetic moment. The oscillation of magnetoresistance with <i>t</i><sub>Cr</sub> at <i>I</i> // <i>H</i> and the hysteresis loops at 5 K further confirm the existence of interlayer coupling both at room temperature and 5 K.

Macroeffect of magnons and thermal fluctiation on magnetization reversal

The magnetization reversal resulting from the thermal fluctuation is irreversible for overcoming the energy barrier, ant it is called the thermally assisted tunneling. In this paper the relaxation in magnetizaition reversal resulting from the thermal fluctuation is observed in Pr-Fe-B permanent magnet. The dependence of magnetic moment on the time natural logarithm is the same as that on the energy barrier in the thermally assisted tunneling. So the relaxation in magnetization reversal originates from the macroeffect of magnons which follow Bose distribution law. The critical size in the irreversible magnetization reversal obtained by the fluctuation field is on a nanometer scale and close to the theoretical domain wall size, indicating that the thermally assisted magnetization reversal undergoes the nucleation and de-pinning of domain wall. The increase of coupling volume will reduce the possibility of magnons tunneling in magnetization reversal due to the weakening effect of thermal fluctuation. The variation of fluctuation field with the field verifies the effect of exchange coupling in Pr-Fe-B magnets, and the calculated value of fluctuation field is consistent with the aftereffect of thermal activation. With the increase of temperature the thermal fluctuation energy increases, and though the aftereffect of thermal fluctuation weakens due to the exchange coupling, the ratio of thermal fluctuation aftereffect to coercivity increases in Pr-Fe-B magnet.

High-Sensitivity Tunnel Magnetoresistance Sensors Based on Double Indirect and Direct Exchange Coupling Effect* Supported by the Framework Project of SGCC (Grant No. 5700-202058381A-0-0-00), and the National Key Research and Development Program of China (Grant No. 2017YFA0206200).

Detection of ultralow magnetic field requires magnetic sensors with high sensitivity and low noise level, especially for low operating frequency applications. We investigated the transport properties of tunnel magnetoresistance (TMR) sensors based on the double indirect exchange coupling effect. The TMR ratio of about 150% was obtained in the magnetic tunnel junctions and linear response to an in-plane magnetic field was successfully achieved. A high sensitivity of 1.85%/Oe was achieved due to a designed soft pinned sensing layer of CoFeB/NiFe/Ru/IrMn. Furthermore, the voltage output sensitivity and the noise level of 10.7 mV/V/Oe, 10 nT/Hz1/2 at 1 Hz and 3.3 nT/Hz1/2 at 10 Hz were achieved in Full Wheatstone Bridge configuration. This kind of magnetic sensors can be used in the field of smart grid for current detection and sensing.

Correlation of the heat treatment feature and magnetic properties of the SrFe12O19@ZnFe2O4 core-shell nanofibers

Nanocomposite SrFe12O19@ZnFe2O4 with core-shell structure has been synthesized in one step via coaxial electrospinning followed by heat treatment in air. The samples comprise two phases, the structure change from fiber coated particles to core-shell fibers, and finally form a scattered sheet structure. The inner diameter of the nanofiber with core-shell structure is 100 nm and the outer diameter is 170 nm, two-phase coupling interface is obvious, and the length of the nanofiber can reach 1.8 μm. The magnetic properties of the composites show a decreasing trend after upward, reaching the highest Ms (43 emu/g) and Hc(4524 Oe) at 900 °C, which is better than for other materials with various morphologies such as sheet and particle. The hysteresis loop at 900 °C shows a single-phase-like magnetic characteristic, indicating the existence of effective exchange coupling. This study provides a basis for the preparation of hard-soft nanocomposite ferrite fibers with controlled core-shell structure, excellent exchange coupling and outstanding magnetic properties.

Preparation of high-performance Nd–Fe–B magnetic powder by hydrothermal method assisted via ball milling

In recent years, there is an urgent need to improve the preparation process of Nd–Fe–B magnets to obtain magnets with high coercivity and magnetic energy products. In this study, we introduced the mechanical ball-milling process into the chemical preparation process of the magnetic powder, and the as-synthesized magnetic powder has a (BH)max of 5.5 MGOe and a Hc of 4.4 ​kOe. X-ray diffraction and X-ray photoelectron spectroscopy analysis have shown that the milling process does not introduce new impurity phases while also making the raw material powder more refined, which promotes the formation of the main phase Nd2Fe14B. Furthermore, the Henkel plots indicate that the presence of α-Fe is responsible for the increased exchange coupling effect and improved magnetic properties.

Study on the microstructure and properties of Nd-Fe-B regenerated magnet by adding Ho69.3Ni30.7 alloy at grain boundary

The Nd-Fe-B regenerated magnets are prepared by adding low melting point Ho69.3Ni30.7 alloy at the grain boundary, and the microstructure, structure and overall performance of the Nd-Fe-B regenerated magnets are studied. The results show that the addition of Ho69.3Ni30.7 alloy to grain boundary can optimize the distribution of grain boundary phase, accompanied by the formation of clear and continuous grain boundary phase, which weakens the exchange coupling effect between the adjacent main phases. Meanwhile, the coercivity of the regenerated magnet increases with the increase of Ho69.3Ni30.7 alloy content. The optimal coercivity (16.35 kOe) was obtained at 2 wt% Ho69.3Ni30.7 additions, and the coercivity increased by 1.24 kOe (8.2%) compared with the initial magnet (15.11 kOe). In addition, with the addition of Ho69.3Ni30.7 alloy, the curie temperature of Nd-Fe-B regenerated magnet increased from 303.2℃ to 305.6℃. The reflection loss of regenerated magnet reached −26.42 dB when the amount of 2 wt% Ho69.3Ni30.7 was added, which greatly improved the absorbing performance of the regenerated magnet. Finally, the secondary utilization of waste Nd-Fe-B magnets was realized.

Magnetic and thermodynamic characteristics of edge-modified graphyne

By using Monte Carlo simulation, the magnetic and thermodynamic characteristics of the edge-modified graphyne structure with mixed-spin (3/2, 2) configuration have been studied. The magnetization, susceptibility, internal energy and specific heat are discussed in detail with various parameters. In particular, various interesting magnetization plateaus under the effects of different exchange coupling and anisotropy have been investigated. Moreover, we also found interesting double-peak phenomenon on the specific heat curve with the decrease of anisotropy D4. The results may provide to test prediction of magnetism in experiment and potential applications in spintronic devices.

MoP3SiO11 : A 4d3 honeycomb antiferromagnet with disconnected octahedra

We report the crystal structure and magnetic behavior of the $4d^3$ spin-$\frac32$ silicophosphate MoP$_3$SiO$_{11}$ studied by high-resolution synchrotron x-ray diffraction, neutron diffraction, thermodynamic measurements, and ab initio band-structure calculations. Our data revise the crystallographic symmetry of this compound and establish its rhombohedral space group ($R\bar 3c$) along with the geometrically perfect honeycomb lattice of the Mo$^{3+}$ ions residing in disconnected MoO$_6$ octahedra. Long-range antiferromagnetic order with the propagation vector $\mathbf k=0$ observed below $T_N=6.8$ K is a combined effect of the nearest-neighbor in-plane exchange coupling $J\simeq 2.6$ K, easy-plane single-ion anisotropy $D\simeq 2.2 $ K, and a weak interlayer coupling $J_c\simeq 0.8$ K. The 12% reduction in the ordered magnetic moment of the Mo$^{3+}$ ions and the magnon gap of $\Delta\simeq 7$ K induced by the single-ion anisotropy further illustrate the impact of spin-orbit coupling on the magnetism. Our analysis puts forward single-ion anisotropy as an important ingredient of $4d^3$ honeycomb antiferromagnets despite their nominally quenched orbital moment.

Antiferromagnetic interlayer exchange coupling and large spin Hall effect in multilayer systems with Pt/Ir/Pt and Pt/Ir layers

We investigated Pt/Ir/Pt and Pt/Ir multilayers as candidates of nonmagnetic spacer layers in synthetic antiferromagnetic (AF) layers, which are available for the systematic study on AF spintronics. In these systems, we observed (i) AF interlayer exchange coupling in Pt/Ir/Pt and Pt/Ir nonmagnetic spacer layers sandwiched by Co layers and (ii) large spin Hall conductivity in Pt/Ir multilayer heavy-metal systems which is essential to achieve low power consumption spin-orbit torque switching. We found that total nonmagnetic spacer layer thickness $[{t}_{\mathrm{total}}={t}_{\mathrm{Pt}}(\mathrm{Pt}\phantom{\rule{0.16em}{0ex}}\mathrm{thickness})+{t}_{\mathrm{Ir}}(\mathrm{Ir}\phantom{\rule{0.16em}{0ex}}\mathrm{thickness})]$ range in which AF interlayer exchange coupling is observed is wide in Co/nonmagnetic spacer layer/Co with Pt/Ir/Pt and Pt/Ir nonmagnetic spacer layers. Moreover, the large spin Hall angle of ${\ensuremath{\theta}}_{\mathrm{SH}}=10.3%$ and low resistivity of ${\ensuremath{\rho}}_{xx}=35\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ in Pt/Ir multilayer heavy metal are observed. These results indicate that Pt/Ir/Pt and Pt/Ir are nonmagnetic spacer layers allowing us to achieve the AF interlayer exchange coupling and generation of large spin-orbit torque via spin Hall effect in synthetic AF coupling layer system.

Intra- and Inter-Molecular Spin Coupling in Phenalenyl Dimeric Systems.

Phenalenyl is a triangular aromatic molecule made of three fused benzene rings, carrying an unpaired electron, and many of its derivatives show crystal structures with stacked radicals. Here, we investigate the inter-molecular binding in phenalenyl dimers by state-of-the-art computational methods and phenomenological models. Aside from being important for the supramolecular assembly of such radical molecules, the theoretical insight is relevant in methodological aspects, due to the interplay of long-range exchange coupling effects and van der Waals forces. We used comparative wave function-based and density functional theories. Drawing the potential energy surfaces as a function of inter-planar separation and mutual rotation of the monomer units, we found an interesting pattern which is not discovered in previous computational reports on the title systems. The dependence can be nicely interpreted by a transparent phenomenological model based on an orbital overlap paradigm of exchange coupling. We also brought forth a simplified phenomenological valence bond (VB) model of inter-molecular coupling, which is realized on the background of the VB spin model inside of the aromatic monomers and calibrated with the corresponding ab initio data. As the systems can be considered good candidates with potential applications in spintronics and organic magnetism, the theoretical rationalization opens up prospective ways to realize such promises.

Domain wall dynamics in ferromagnet/Ru/ferromagnet stacks with a wedged spacer

Synthetic antiferromagnetic spintronics is an important field of study in the area of spintronics. Here, we fabricate a wedged-Ru layer in a ferromagnet/Ru/ferromagnet stack with alternating antiferromagnetic and ferromagnetic exchange-couplings. We systematically investigate the effect of the interlayer exchange coupling J on the chiral domain wall dynamics. The domain wall moves along the wedge unidirectionally driven by an out-of-plane magnetic field in the antiferromagnetically exchange-coupled region. In the ferromagnetically exchange-coupled region, only field-induced maze domain walls can be observed. The boundary between the antiferromagnetically and ferromagnetically exchange-coupled regions can be identified according to the behavior of domain walls. The strongest antiferromagnetic exchange coupling point Jmax in our sample is determined. A simple phenomenological picture is proposed to explain our experimental results. Moreover, we demonstrate that the Néel-type domain wall near the boundary of J = 0 could be driven into motion by an applied electric current. A stripe domain wall can be observed at the boundary of J = 0 only with a negative electric current resulting from the domain wall chirality. Our results may provide a platform to investigate the domain wall chirality and pave a way to spintronic devices based on synthetic antiferromagnets.

Simulation on the openness in recoil loop of nanocomposite magnet

In this paper, the open recoil loop is simulated by establishing the simulation model of hard-phase grains (Sm2Co7) dispersed in soft-phase (α-Fe) matrix. The influence of inter-phase exchange coupling effect on the openness is simulated by adjusting the soft/hard phase interface integral exchange constant (Aint). The results show that the openness does not depend on the inter-phase exchange coupling effect, since the open recoil loops can be acquired even the Aint decrease from 20 to 0 pJ/m. The interaction energy density analysis also does not provide strong evidence to support the correlation between openness and exchange coupling effect. The magnetization configuration evolution shows the irreversible reversal of the hard-phase grains is root cause of the openness, and the adjacent soft phase enlarge the openness degree for the recoil loop of the nanocompiste magnet. Our results are helpful to deepen the understanding of the openness in recoil loops for nanostructural permanent magnet materials.