Effective Exchange Coupling Research Articles

Insight into the Property Enhancement Mechanism of Chemically Prepared Multi-Main-Phase (Nd,Ce)2Fe14B.

Nd2Fe14B has attracted intensive attention because of its excellent magnetic properties since 1980s. However, large demands for the expensive rare earth (mainly refers to Nd/Pr/Dy) limit its wider applications. Investigations of Ce-doped Nd2Fe14B have been attempted recently and multi-main-phase (MMP) (Nd,Ce)2Fe14B provides a promising way for the preparation of high-performance Ce-doped permanent magnets even though the inner mechanism has not been absolutely understood. We synthesized Ce-doped Nd2Fe14B nanostructures by the chemical method and successfully realized the obvious property enhancement of the MMP sample compared with that of the single-main-phase one. The coercivity of the MMP nanostructures is nearly 4.5 kOe with a remanence ratio of 0.36 before magnetic orientation, which is much larger than that of the SMP sample (1.7 kOe and 0.21), respectively. The property enhancement mechanism of the MMP sample analyzed mainly by first-order reversal curves could be concluded in three aspects: first, the content of α-Fe will be decreased; hence, the difficulty of the magnetic nucleation is increased. Second, the exchange coupling effect between the adjacent magnetic structures will be strengthened significantly. Last, the grain boundary phases with various magnetic features are formed, enhancing the magnetic pinning effect and specially tuning the inner interactions. This work is helpful for the deeper understanding of the property enhancement mechanism in MMP nanomagnets and provides an instructive way for the effective design and preparation of high-performance MMP Ce-doped Nd2Fe14B nanomagnets.

Broad microwave absorption bandwidth achieved by exchange coupling interaction between hard and soft magnetic materials

To gain broad microwave absorption bandwidth over 2–18 GHz, hard/soft magnetic composites Ba(Zr–Ni)0.6Fe10·8O19/Fe3O4 have been synthesized in this work. The results show that the raising heat treatment temperature would cause more Fe3O4 particles attach to Ba(Zr–Ni)0.6Fe10·8O19 particles, increasing the number of interfaces between Ba(Zr–Ni)0.6Fe10·8O19 and Fe3O4 to enhance the hard/soft exchange coupling effect. Finally, the sample heat treated at 400 °C possesses the widest magnetic loss range (μ″>0.2) of 12.4 GHz, which contributes to a broad effective absorbing bandwidth of 6.68 GHz at 1.9 mm. The performance is superior than most of the state-of-the-art microwave absorbers.

Diradical Organic One-Dimensional Polymers Synthesized on a Metallic Surface.

We report on the synthesis and characterization of atomically precise one‐dimensional diradical peripentacene polymers on a Au(111) surface. By means of high‐resolution scanning probe microscopy complemented by theoretical simulations, we provide evidence of their magnetic properties, which arise from the presence of two unpaired spins at their termini. Additionally, we probe a transition of their magnetic properties related to the length of the polymer. Peripentacene dimers exhibit an antiferromagnetic (S=0) singlet ground state. They are characterized by singlet–triplet spin‐flip inelastic excitations with an effective exchange coupling (J eff) of 2.5 meV, whereas trimers and longer peripentacene polymers reveal a paramagnetic nature and feature Kondo fingerprints at each terminus due to the unpaired spin. Our work provides access to the precise fabrication of polymers featuring diradical character which are potentially useful in carbon‐based optoelectronics and spintronics.

Diradical Organic One‐Dimensional Polymers Synthesized on a Metallic Surface

AbstractWe report on the synthesis and characterization of atomically precise one‐dimensional diradical peripentacene polymers on a Au(111) surface. By means of high‐resolution scanning probe microscopy complemented by theoretical simulations, we provide evidence of their magnetic properties, which arise from the presence of two unpaired spins at their termini. Additionally, we probe a transition of their magnetic properties related to the length of the polymer. Peripentacene dimers exhibit an antiferromagnetic (S=0) singlet ground state. They are characterized by singlet–triplet spin‐flip inelastic excitations with an effective exchange coupling (Jeff) of 2.5 meV, whereas trimers and longer peripentacene polymers reveal a paramagnetic nature and feature Kondo fingerprints at each terminus due to the unpaired spin. Our work provides access to the precise fabrication of polymers featuring diradical character which are potentially useful in carbon‐based optoelectronics and spintronics.

Enhancement of the maximum energy product in Ba0.5Sr0.5Fe12O19/Y3Fe5O12 nanocomposites synthesized by the co-precipitation method

Ba0.5Sr0.5Fe12O19/Y3Fe5O12 (BSFO/YIG) nanocomposite ferrite with hard (BSFO) and soft (YIG) magnetic phases, were prepared by the two-step co-precipitation method. The soft magnetic phase was introduced in different weight ratios (x= 0.0, 0.1, 0.2, 0.3, 0.4 and 1) in the (1−x)Ba0.5Sr0.5Fe12O19/(x)Y3Fe5O12 nanocomposite ferrites. The structural, morphological and magnetic properties of nanocomposite ferrites were analyzed by x-ray diffraction (XRD), high-resolution transmission electron microscope (HR-TEM) and room temperature vibrational sample magnetometer (VSM). The presence of the hard and soft phase have been confirmed without any secondary phase from the XRD analysis, indicating the formation of nanocomposite ferrite. The crystallite size is found to be in the range of 45–55 nm calculated by Scherrer’s formula. The HR-TEM revealed hexagonal platelets of BSFO with YIG particles with an average particle size of 90 nm formed at the surface of the (0.9)BSFO/(0.1)YIG nanocomposite. The room temperature magnetic properties of the nanocomposite, such as saturation magnetization (Ms), squareness ratio (Mr/Ms), coercivity (Hc) and nucleation field (Hn) were evaluated by employing VSM. The magnetic measurements have displayed an enhancement in coercivity and magnetization for (0.9)BSFO/(0.1)YIG. Compared with pure BSFO, the optimized (0.9)BSFO/(0.1)YIG nanocomposite showed 57% enhancement in energy product (BH)max, indicating that the nanocomposite possessed excellent exchange coupling. To investigate the exchange coupling between the hard and soft magnetic phases, dM/dH values were plotted using the demagnetization curves which indicated the effective exchange coupling effect between the hard and soft phases.

Time-dependent spintronic anisotropy in magnetic molecules

We theoretically study the quench dynamics of induced anisotropy of a large-spin magnetic molecule coupled to spin-polarized ferromagnetic leads. The real-time evolution is calculated by means of the time-dependent density-matrix numerical renormalization group method implemented within the matrix product states framework, which takes into account all correlations in very accurate manner. We determine the system's response to a quench in the spin-dependent coupling to ferromagnetic leads. In particular, we focus on the transient dynamics associated with crossing from the weak to the strong coupling regime, where the Kondo correlations become important. The dynamics is examined by calculating the time-dependent expectation values of the spin-quadrupole moment and the associated spin operators. We identify the relevant time scales describing the quench dynamics and determine the influence of the molecule's effective exchange coupling and leads spin-polarization on the dynamical behavior of the system. Furthermore, the generalization of our predictions for large values of molecule's spin is considered. Finally, we analyze the effect of finite temperature and show that it gives rise to a reduction of magnetic anisotropy by strong suppression of the time-dependent spin-quadrupole moment due to thermal fluctuations.

Interlayer exchange coupling and spin Hall effect through an Ir-doped Cu nonmagnetic layer

Metallic superlattices where the magnetization vectors in the adjacent ferromagnetic layers are antiferromagnetically coupled by the interlayer exchange coupling through nonmagnetic spacer layers are systems available for the systematic study on antiferromagnetic (AF) spintronics. As a candidate of nonmagnetic spacer layer material exhibiting remarkable spin Hall effect, which is essential to achieve spin-orbit torque switching, we selected the Ir-doped Cu in this study. The AF-coupling for the Co / Cu$_{95}$Ir$_{5}$ / Co was investigated, and was compared with those for the Co / Cu / Co and Co / Ir / Co. The maximum magnitude of AF-coupling strength was obtained to be 0.39 mJ/m$^{2}$ at the Cu$_{95}$Ir$_{5}$ thickness of about 0.75 nm. Furthermore, we found a large spin Hall angle of Cu$_{95}$Ir$_{5}$ in Co / Cu$_{95}$Ir$_{5}$ bilayers by carrying out spin Hall magnetoresistance and harmonic Hall voltage measurements, which are estimated to be 3 ~ 4 %. Our experimental results clearly indicate that Cu$_{95}$Ir$_{5}$ is a nonmagnetic spacer layer allowing us to achieve moderately strong AF-coupling and to generate appreciable spin-orbit torque via the spin Hall effect.

High energy product FexPt100-x thin films (x = 60–66) prepared by rapid thermal annealing

Magnetic properties of sputtered FexPt100-x (x = 60–66) thin films via rapid thermal annealing (RTA) are studied. The structural results show that FexPt100-x films through RTA at Ta above 400 °C exhibit (00L) texture. (00L) texture is considered to result from L10 phase and L12 phase. All studied FexPt100-x films through RTA at Ta = 300–600 °C exhibit strong in-plane magnetic anisotropy with well-squared M-H curve and good permanent magnetic properties. Fine microstructure with small size of grain here gives arise to high Mr and good squareness of demagnetization curve. Magnetic force microscopy shows a fine interaction domain. Strong exchange coupling effect between grains due to fine microstructure results in high remanence and therefore (BH)max. As a result, high coercivity of 5.7 kOe and a maximum (BH)max of 26.0 MGOe are achieved. The results of this study reveal that Fe-rich FexPt100-x thin films (x = 60–66) through RTA is a cost-effective and simple method to obtain high energy product, which could be useful for related applications.

Magnetic and thermodynamic characteristics of a rectangle Ising nanoribbon

Using Monte Carlo simulation, we investigated magnetic and thermodynamic characteristics of the rectangle core–shell ferrimagnetic mixed-spin (3/2, 2) Ising nanoribbon under the effects of sublattice crystal field, exchange coupling and temperature. Typical temperature dependence of the total magnetization, the core and shell magnetization, the susceptibility and the internal energy for several fixed values of Hamiltonian parameters were discussed in detail. The phase diagrams of the critical and compensation temperatures were presented in various Hamiltonian parameter planes. In addition, particular emphasis was given to the interesting multi-cycle hysteresis behaviors of the total system and sublattices. Our results show reasonable consistency compared with many previous theoretical and experimental studies.

Structure, magnetic properties, and exchange-coupling effect of Co0.6Mg0.15NdxFe2.25–xO4/Co

Hard/soft Co0.6Mg0.15NdxFe2.25–xO4/Co composites have been synthesized by reducing Co0.6Mg0.15NdxFe2.25–xO4 (CMNFO) at 550 °C in 5% H2–Ar atmosphere. The results of X-ray diffraction measurements confirm that CMNFO samples, calcined at 900 °C, consist of the main cubic MgFe2O4 phase in combination of trace amounts of Fe2O3 and FeNdO3 phases. The surface particles in the CMNFO are transformed into soft magnetic Co phase, and Fe2O3 phase disappears after being reduced at 550 °C for 30 min. The magnetic measurement shows that specific saturation magnetization of the CMNFO decreases with the increase in Nd3+ content. By contrast, specific saturation magnetization and coercivity of CMNFO/Co composites obviously increase. Co0.6Mg0.15Nd0.04Fe2.21O4/Co composite exhibits a maximum specific saturation magnetization value (73.56 emu/g) at a reduction time of 20 min. Co0.6Mg0.15Nd0.04Fe2.21O4/Co composite still behaves a single-phase hysteresis loop, attributed to coherent magnetization rotation and strong exchange-coupling between hard magnetic CMNFO and soft magnetic Co particles.

Dynamical resilience to disorder: The dilute Hubbard model on the Lieb lattice

In itinerant systems, electron-electron interactions may lead to the formation of local magnetic moments and their effective exchange coupling, which in turn gives rise to long-range magnetic order. Therefore, when moment formation is weakened, such as in the single-band Hubbard model on a square lattice with the on-site repulsion being randomly switched off on a fraction $x$ of sites, magnetic order is suppressed beyond some critical $x_c$, which was found to lie below the classical percolation threshold, $x_c^\text{(perc,sq)}$. Here we study dilute magnetism in flat band systems, namely in the Hubbard model on a `Lieb' lattice. Interestingly, we show that magnetic order persists to $x$ almost twice as large as the classical percolation threshold for the lattice, thus emphasizing the central role of electron itinerancy to the magnetic response. The analysis of the orbital-resolved order parameters reveals that the contribution of the four-fold coordinated `d' sites to magnetism is dramatically affected by dilution, while the localized `p' states of the flat band provide the dominant contribution to long-range correlations. We also examine the transport properties, which suggest the existence of an insulator-to-metal transition in the same range of the critical magnetic dilution.

Fabrication of the novel Fe2+αO3+α–CoFe2O4 composite fibers and their magnetic properties

Soft-hard magnetic composites have attracted much attention as magnetic materials for permanent magnets due to high coercivity and saturation magnetization in various fields such as motor, storage device and magnetic actuators. Hard magnetic materials with low saturation magnetization improve magnetic properties through exchange coupling effect with soft magnetic materials. Rare earth metals, which are used as magnetic materials in permanent magnetic, have issued such as high cost due to resource scarcity. Transition metals have been used for fabricating magnetic material with high magnetic properties due to low cost and abundant in the resource. Spinel magnetic materials with transition metals have excellent chemical, thermal stability, large magnetocrystalline anisotropy and magnetic properties. Composite magnetic materials in one dimensional (1D) morphology are effective in improving coercivity. In this work, soft-hard composite fiber has been fabricated by electrospinning method using iron oxide as soft magnetic material and cobalt ferrite as hard magnetic material. The change in the oxidation state of iron oxide can be controlled with sintering atmosphere and influences on the magnetic properties. In comparing with simple CoFe2O4, the fabricated Fe3O4–CoFe2O4 exhibits excellent saturation magnetization of 133.63 emu g−1 and enhanced coercivity of 2746.9 Oe.

Comparative Study of Structure, FMR and Magnetic Properties of Nanocomposite and Mixture Ferrite.

This investigation presents design of 20% M-type BaFe12O19 hexaferrite as a core coated by 80% MgFe2O4 spinel ferrite as a shell (〖Mg〗_80-H_20) in addition to, a mixture of 80% MgFe2O4 spinel ferrite and 20% M-type BaFe12O19 hexa-ferrite powders. The spinel and nanocompsite ferrite materials were synthesized by the wet mechano-chemical co-precipitation route and all prepared samples; spinel, nanocomposite and mixture are annealed at 1200 oC for 6 hours. The results of X-ray diffraction patterns, High Resolution Transmission electron microscope (HRTEM) images and Fourier-transform infrared spectroscopy (FT-IR) revealed the formation of two different composite materials. Comparing the FMR behavior of the nanocomposite and the mixed materials, the resonance field H_r of nanocomposite is lower than that of the mixture. This may be attributed to the effect of spring exchange coupling in the nanocomposite sample. The hysteresis loop of the 〖Mg〗_80-H_20 nanocomposite structure is similar to that of cubic spinel ferrite and exhibits the same Ms (18 emu/g) despite of its different values of Mr and Hc. Enhancing the remnant magnetization for the 〖Mg〗_80-H_20 nanocomposite and mixed materials is mainly attributed to the overcoming of magnetic exchange interaction and high magneto-crystalline anisotropy of hard phase in competition with dipolar interaction of soft phase. Comparing the FMR behavior of the nanocomposite and the mixed materials, the resonance field H_r of nanocomposite is lower than that of the mixture. This may be attributed to the effect of spring exchange coupling in the nanocomposite sample.

Chemical synthesis and coercivity enhancement of Nd2Fe14B nanostructures mediated by non-magnetic layer

High-performance Nd2Fe14B magnets have been widely required in various fields recently due to the lightweight and miniaturization of devices. In this work, we synthesize Nd2Fe14B nanostructures with tunable magnetic properties through surfactant-assisted high energy ball milling (SAHEBM) process, achieving prominently enhanced coercivity by forming non-magnetic layers as grain boundary phase. When the reduction annealing process was carried out as pellet with Ca, the coercivity increased from 0.8 kOe to ver 3 kOe as Nd2Fe14B powder, which is proved to be the contribution of the chemical diffusion of Nd elements and the formation f Nd-rich layer as magnetic insulating medium. In addition, two-dimensional graphene oxide (GO) was employed to build extra grain boundary, by which the coercivity of the core@dual-shell structure can achieve up to 8 kOe, tenfold of the original sample. The intrinsic mechanism indicated that the Nd-diffusion induced Nd-rich phase along with the reduced GO in the system could form non-magnetic layer as grain boundary and magnetically isolate the adjacent grains, significantly enhancing the exchange coupling effect. This work markedly opens up an effective approach for the chemical preparation of high-performance Nd2Fe14B nanostructured magnets, especially after post treatment, and gives an insight on the interactions at nanoscale.

Composition-dependent magnetic properties of exchange-coupled hard/soft Co0.55Ni0.4NdxFe2.05–xO4/Co composites

The cubic Co0.55Ni0.4NdxFe2.05–xO4 (0 ≤ x ≤ 0.12) ferrites are synthesized by thermal decomposition of metal oxalates at 900 °C, and subsequently reduced at 550 °C in 5% H2–Ar ambient to obtain the Co0.55Ni0.4NdxFe2.05–xO4/Co composites. X-ray diffraction patterns of Co0.55Ni0.4NdxFe2.05–xO4 samples confirm the co-existence of the main spinel CoFe2O4 and trace amounts of impurity NdFeO3 and Fe2O3 phases. After being reduced, impurity NdFeO3 and Fe2O3 phases disappear, a new reduction product cobalt (Co) phase appears. All Co0.55Ni0.4NdxFe2.05–xO4/Co composites exhibit a typical single-phase magnetic behavior, suggesting the existence of exchange-coupling. Compared with Co0.55Ni0.4NdxFe2.05–xO4, the specific saturation magnetization of composites increases markedly from 55.05 (Co0.55Ni0.4Nd0.04Fe2.01O4 calcined at 900 °C) to 70.04 emu/g (Co0.55Ni0.4Nd0.04Fe2.01O4/Co reduced for 30 min), accompanied with a relatively small decrease of coercivity from 622.98 to 579.94 Oe. Exchange-coupling effect in Co0.55Ni0.4NdxFe2.05–xO4/Co composites is confirmed by the switching field distribution curves further.

Effects of Sm content on the phase structure, microstructure and magnetic properties of the SmxZr0.2(Fe0·8Co0.2)11.5Ti0.5 (x=0.8–1.4) alloys

To introduce a paramagnetic intergranular phase into the ThMn12-type permanent materials, SmFe12-based alloys with the composition of SmxZr0.2(Fe0.8Co0.2)11.5Ti0.5 (x = 0.8–1.4, at.%) are designed and prepared by arc-melting and melt-spinning. Roles of extra Sm on the phase constitution and magnetic properties are systematically investigated. First of all, within the range of x = 0.8–1.2, the as-cast ingots mainly constitute of 1:12, 1:10, 1:2 and α-(Fe,Co) phase. With increasing Sm content, both 1:12 and α-(Fe,Co) phases decrease whereas 1:2 and 1:10 phases increase. Similarly, for the heat-treated ribbons with x = 0.8–1.2, α-(Fe,Co) phase also decreases with the increasing Sm content. Consequently, the Js and Jr of the alloys exhibit a decreasing trend while the Hc and (BH)max show an increasing trend. As a result, excellent magnetic properties of Jr = 1.04 T for x = 0.8 alloy, (BH)max = 61 kJ/m3 and Hc = 3.0 kOe for x = 1.2 alloys are acquired. Henkel curves of the annealed ribbons prove the existence of an exchange coupling effect in all alloys, which has contributed to the remanence enhancement. Interestingly, the observed kink in the demagnetization curve of x = 1.2 annealed alloy suggests the ineffective exchange coupling between soft and hard grains. An amorphous grain boundary phase is observed in the annealed x = 1.2 alloy while the refined 57Fe Mössbauer spectrum confirms its paramagnetic behavior. The present work may provide a dependable guideline for designing the composition and microstructure of 1:12-based permanent material.

Co-existence of room temperature ferromagnetic and ferroelectric properties in Ba4SmFe0.5Nb9.5O30 ceramics

Ba4SmFe0.5Nb9.5O30 ceramics were prepared by solid-state reaction method and the sample crystallized into single phase tungsten bronze (TB) structure with space group P4bm. The absence of dielectric peak suggests that there is no long-range ferroelectric order exists above room temperature (RT). Therefore, the observed ferroelectric property should be related to the polar nanoregions that survived to Burns temperature TB. Obvious RT ferromagnetism is observed for the first time in Ba4SmFe0.5Nb9.5O30 ceramics and it could be interpreted as the effective exchange coupling interaction between oxygen vacancies and Fe ions 3d spins, while the isolated Fe ions create the paramagnetic characteristic. In addition, the RT ferromagnetism and ferroelectricity also confirms that introduce of Fe ions could help to change TB structural ferroelectric to RT multiferroic material effectively.

Effects of interlayer and bi-quadratic exchange coupling on layered triangular lattice antiferromagnets

The magnetic field evolution of ground spin states of the stacked planar triangular antiferromagnet with antiferromagnetic interlayer interaction Jc is explored using a minimal 3D classical Heisenberg model. A bi-quadratic coupling is also used to mimic the effect of spin fluctuations (Zhitomirsky 2015 J. Phys.: Conf. Ser. 592 012110) which are known to stabilize the magnetization plateau. A single ion anisotropy is included and states with a magnetic field applied in the ab plane and along the c axis are determined. For -plane, an additional new state, in contrast to 2D model (Zhitomirsky 2015 J. Phys.: Conf. Ser. 592 012110), is obtained with weak interlayer interaction, while the magnetization plateau vanishes at large Jc and other new states with z components of spins emerge. For -axis, an extra state, compared with 2D model, is obtained with a weak interlayer interaction. When Jc is large enough, only the state corresponding to the Umbrella phase in 2D model exits.

Enhancement of coercivity of self-assembled stacking of ferrimagnetic and antiferromagnetic nanocubes.

The coercivity of magnetic nanoparticles is enhanced by the exchange coupling effect at the interface of ferrimagnetic and antiferromagnetic self-assembled monolayers. Antiferromagnetic Co3O4 nanocubes were regularly stacked on an ordered monolayer of ferrimagnetic Fe3O4 nanocubes by layer-by-layer manipulation using evaporation-driven self-assembly. The ordered arrangements of the ferrimagnetic and antiferromagnetic nanocubes are effective for the enhancement of the ferromagnetic character.

Stabilization of coupled Dzyaloshinskii domain walls in fully compensated synthetic anti-ferromagnets

We examine the combined effects of interlayer exchange coupling (IEC) and the interfacial Dzyaloshinskii-Moriya Interaction (DMI) on the structure of magnetic domain walls in fully compensated synthetic anti-ferromagnets (SAFs). Ir-based SAFs with ferromagnetic (FM) layers based on [Pt/(Co/Ni)M]N were characterized by Lorentz transmission electron microscopy (LTEM). The multi-layer design of the individual ferromagnetic layers enables control of the interfacial Dzyaloshinskii-Moriya interaction (via ‘M’) and, in turn, the structure and chirality of domain walls (DWs). We compare the Fresnel-mode LTEM images in SAF designs with only a change in the purported strength of the DMI. The existence of anti-ferromagnetically coupled Dzyaloshinskii domain walls (DWs) in a high DMI SAF is confirmed through application of in-situ perpendicular magnetic field and sample tilt. This conclusion is based on a unique set of conditions required to observe contrast in Fresnel-mode LTEM, which we outline in this document.