Exchange-coupled Magnets Research Articles

Magnetic properties of exchange-coupled permanent magnets based on barium hexaferrite and cobalt ferrite nanocomposites

The strategy of blending hard and soft magnetic materials to synthesize nanocomposites offers new possibilities for magnetic materials. We focus on synthesizing and optimizing BaFe12O19-CoFe2O4 nanocomposites using a co-precipitation method to enhance the magnetic properties through exchange coupling. The synthesis conditions including pH, annealing temperature, and blending ratio were systematically investigated. The precursor synthesized at pH 13 produced uniform precipitates, and upon annealing at 1000 °C, barium hexaferrite (BH) and cobalt ferrite (CF) with hexagonal and spinel structure were each formed. The nanocomposite with a blending ratio of 0.75BH-0.25CF exhibited enhanced saturation magnetization and the highest remanent magnetization compared to the individual components. The crystal structure, morphology, and magnetic properties were characterized using X-ray diffraction, field-emission scanning electron microscopy, and vibrating sample magnetometry.

Micromagnetic study of magnetization reversal in inhomogeneous permanent magnets

Macroscopic magnetic properties of magnets strongly depend on the magnetization process and the microstructure of the magnets. Complex materials such as hard-soft exchange-coupled magnets or just real technical materials with impurities and inhomogeneities exhibit complex magnetization behavior. Here we investigate the effects of size, volume fraction, and surroundings of inhomogeneities on the magnetic properties of an inhomogeneous magnetic material via micromagnetic simulations. The underlying magnetization reversal and coercivity mechanisms are revealed. Three different demagnetization characteristics corresponding to the exchange coupling phase, semi-coupled phase, and decoupled phase are found, depending on the size of inhomogeneities. In addition, the increase in the size of inhomogeneities leads to a transition of the coercivity mechanism from nucleation to pinning. This work could be useful for optimizing the magnetic properties of both exchange-coupled nanomagnets and inhomogeneous single-phase magnets.

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.

Single-molecule magnets bridged by a bismuth Zintl ion

In this issue of <i>Chem</i>, Demir and colleagues report on the synthesis, <i>ab initio</i> calculations, and magnetic behaviors of two organometallic lanthanide bismuth clusters involving an unprecedented Bi₆⁶⁻ Zintl ion. The serendipitous finding sheds light on the design strategies of exchange-coupled single-molecule magnets.

Magnetic spin exchange interaction in SmCo5/Co nanocomposite magnet for large energy product

Magnetic spin exchange-coupled magnets have been investigated for obtaining an enhanced energy product, however, approaches at the nanoscale have been greatly restricted because of the lack of consideration of the relationships among the individual components. Here, we suggest a facile strategy for fabricating exchange-coupled nanomagnets with a large energy product. As a bottom-up approach, this work introduces a combined thermal decomposition and reduction/diffusion process to obtain a magnetic spin exchange coupled SmCo5/Co nanocomposite magnet. The SmCo5/Co nanocomposite magnet was fabricated through a three-step approach: (1) chemical synthesis of Co@SmOx nanoparticles and Co nanoparticles as hard and soft magnetic phases, respectively, (2) 3-dimensional alternating arrangement of both magnetic phases and (3) a reduction/diffusion process for the magnetic spin exchange interaction. Our results demonstrate that an effective magnetic spin exchange interaction strongly depends on the dimension and arrangement of the hard and soft phases, which were synthetically tuned to be within the magnetic domain wall size.

Geometric effects in cylindrical core/shell hard–soft exchange-coupled magnetic nanostructures

We explore the optimal condition for cylindrical core/shell hard-soft exchange-coupled magnetic nanostructures by obtaining full hysteresis loops for various geometries by obtaining full hysteresis loops for various geometrical variables, including the dimensional scale and soft/hard-magnetic phase volume ratio through micromagnetic simulations. For achieving maximum energy product (BH), it is essential to increase the demagnetizing field by increasing the volume fraction of the soft magnet while maintaining a positive nucleation field and, which can be possible by the scaling-down. To scale up the nanostructure to a bulk magnet having high BH can be achieved by forming an array of needle-shaped exchange-coupled cylinders. These findings could lead to the flexible design and scalable fabrication of exchange-coupled permanent magnets.

Microstructure and Performance of Mn-Ga/Fe-Cr-Co Magnetic Composites Fabricated by Mechanical Alloying

Mn-Ga and Fe-Cr-Co have potential use as components in exchange-coupled composite magnets. Herein, we prepared Mn-Ga composites through mechanical alloying, which was followed by sintering. During high-energy ball milling, Fe-Cr-Co powder was added into the Mn-Ga powder to form a composite. The main phases in the composites existed as pure Mn and pure Ga in the as-milled state; subsequently, these phases changed into intermetallic compounds, such as Mn3Ga and Mn0.85Ga0.15, after sintering at 385°C for 6 h. When the Fe-Cr-Co fraction increased from 0% to 20%, the coercivity (Hc) of the Mn-Ga/Fe-Cr-Co composites decreased monotonically from 8.04 kOe to 2.28 kOe; furthermore, their remanence (Mr) increased from 8.52 emu/g to 13.33 emu/g, and the maximum energy product (BH)max increased from 0.15 MGOe to 0.26 MGOe. The results obtained in this study facilitate the improvement of the magnetic properties of Mn-Ga composites for utilization in permanent magnet applications.

Magnetic field enhanced coercivity of Fe nanoparticles embedded in antiferromagnetic MnN films

The exchange coupling effect in nanocomposite samples with ferromagnetic (FM) body-centred tetragonal (bct) Fe nanoparticles (NPs) (~20 nm) embedded in an MnN antiferromagnetic (AFM) matrix is investigated. Both the bct Fe NPs control sample and the MnN (×nm)\\bct Fe NPs\\MnN (20–35 nm)\\Ta (5 nm) nanocomposite samples are synthesized by a gas-phase condensation method. Both the coercivity and the remanence ratio of the nanocomposite samples are significantly enhanced compared with the bct Fe NPs sample. The coercivity and the remanence ratio increase with MnN thickness up to 30 nm and then decrease. Additionally, the exchange coupling strength between the bct Fe NPs and AFM MnN matrix is enhanced by magnetic field training. The magnetic field training leads to a higher coercivity and remanence ratio after one cycle of field-cooled hysteresis loop measurement. The coercivity of the composite sample increases by 80%, while the remanence ratio increases by around 30% compared with the bct Fe NPs sample. This work may provide an alternative approach to the design and manufacturing of FM-AFM exchange-coupled permanent magnets.

A novel analytical model for hysteresis loops of exchange-coupled hard-soft magnets

Hard/soft permanent magnets have attracted a lot of attention because of their rich magnetic properties and their potential for realizing giant energy products. However, energy products obtained by scientists in experiments are much smaller than the theoretical values, which has been studied by various analytical and numerical methods. The famous Stoner-Wohlfarth model (S-W model) is too simple to give the hysteresis loops whereas the intensively used variational method is too complicated to reveal the underlying mechanism in a simple form. The analytical model proposed in this paper maintains a balance between simplicity and precision, where the spins in the soft layer rotate fast and coherently with the applied field while those in the hard layer response to the applied field much slower but also coherent. An exchange coupling is provided to maintain the exchange spring which drags the spins in the hard layer to follow those in the soft layer. Similar to the more sophisticated model, the calculated hysteresis loops display three typical magnetic phases, i.e., the rigid composite magnet, the exchange spring and decoupled magnet, whereas the simple SW model can only give one single phase, i.e., the rigid composite one. In addition to the hysteresis loop, the energy product and the nucleation fields have been calculated and compared with those calculated by other methods, which justifies our model. Careful comparisons show that our calculations are in good agreement with the experimental results and other theoretical results, especially for the important coercivity value and the related mechanism.

Single Domain SmCo5@Co Exchange-coupled Magnets Prepared from Core/shell Sm[Co(CN)6]·4H2O@GO Particles: A Novel Chemical Approach

SmCo5 based magnets with smaller size and larger maximum energy product have been long desired in various fields such as renewable energy technology, electronic industry and aerospace science. However, conventional relatively rough synthetic strategies will lead to either diminished magnetic properties or irregular morphology, which hindered their wide applications. In this article, we present a facile chemical approach to prepare 200 nm single domain SmCo5@Co core/shell magnets with coercivity of 20.7 kOe and saturation magnetization of 82 emu/g. We found that the incorporation of GO sheets is responsible for the generation of the unique structure. The single domain SmCo5 core contributes to the large coercivity of the magnets and the exchange-coupled Co shell enhances the magnetization. This method can be further utilized in the synthesis other Sm-Co based exchange-coupled magnets.

Surface termination dependent structural and magnetic properties of (0001) SmCo5 slabs

We employ first principles calculations to understand the surface termination dependent structural and magnetic properties of (0001) SmCo5 surface slabs. For our study, three different sub-layer terminated surface slabs, namely Co3–(SmCo2–Co3)n, SmCo2–(Co3–SmCo2)n, and (Co3–SmCo2)n with thicknesses varying from n = 1 to n = 10, are considered. Our results revealed that the Co3 sub-layer terminated surface slab (first case) has higher structural stability, spin polarization, and work function when compared to the other two cases and such terminated surface slabs can be potentially used for fabricating exchange-coupled magnets.

Significant deterioration of energy products in exchange-coupled composite magnets

Hysteresis loops and energy products have been calculated reliably for a hard/soft/hard trilayer system with a deviation of easy axis β taken into account, which affects the coercivity significantly, and hence leads to much smaller energy products than those predicted by the previous theory. Such a deterioration is much sharper than the corresponding fall for a single-phased material and a 30° deviation of in-plane easy axis could result in a drop of the maximum energy product by more than 60%. Consequently, the advantage of the composite phase can be realized only when the material is well oriented, which offers a possible explanation to the large discrepancy between the experimental and theoretical energy products.

The mixing of Fe/Co and its effect on the exchange interaction in SmCo5/α-Fe nanocomposites: A first-principles study

SmCo 5 / α - Fe is an ideal model for an exchange-coupled magnet. Depending on preparation conditions, some Fe/Co mixing occurs at the interface between the SmCo5 and the α-Fe phases. The mixing behavior of Fe/Co and its effect on the magnetic properties have been studied by a first-principles density functional calculation. For a model system of SmCo5+4Fe (SmCo5Fe4), the calculated substitution energy of Fe in SmCo5 is positive and increases from 40 meV/atom to 61 meV/atom with increasing x from 1 to 3 in SmCo5-xFex, indicating that Fe destabilizes the 1:5 structure. However, the substitution energy of Co in α-Fe changes from negative to positive as x is above 0.68 in Fe1-xCox. The corresponding mixing energy of Fe/Co changes from negative to positive as x is above 2.3 in the virtual alloy of SmCo5-xFex+Fe4-xCox, implying the existence of a concentration limit for the mixing of Fe/Co. The exchange interaction calculations indicate that the inter-atomic exchange interaction of Fe-Co nearest neighbor pairs is stronger than that of Fe-Fe and Co-Co pairs in Sm(Co,Fe)5 and α-(Fe,Co), respectively. The results imply that the Fe/Co mixing enhances the interface exchange-coupling between SmCo5 and α-Fe, which improves the coercivity in the exchange-coupled nano-composite magnets.

Influence of the phenomena occurring at the soft/hard interface on the coercivity behavior in exchange-spring magnets

L 1 0 -ordered FePd thin films were grown onto MgO-(100) monocrystalline substrates at 450 and 650 °C by means of a molecular beam epitaxy system. Fe/FePd bi-layers were grown by covering the L 1 0 -FePd hard films with Fe layers of different thickness. For a soft/hard thickness ratio up to 1, these bi-layers show behavior typical of a single phase hard magnet while, for a higher thickness ratio, they behave as exchange-coupled magnets characterized by two critical fields. The bi-layers’ coercivity decreases as a function of the deposited Fe when it is controlled by the domain wall pinning mechanism, while it initially increases and then decreases, when the nucleation of reversed domains is the predominant coercivity mechanism. These different behaviors are due to the phenomena occurring at the soft/hard interface and in particular to the degree of Fe/FePd intermixing and to the composition of the thin interfacial layer.

Magnetic Properties and Nanocrystalline Phases in Sn Containing SmCo5 Alloys

SmCo5 alloys with Sn additions (0.2-2.0 at.%) were prepared by mechanical milling of arc-melted samples. The nano-phase structures and magnetic properties of as-milled powders were investigated. The Sn additions resulted in development of nanocrystalline structures producing exchange-coupled magnets with better remanence magnetization to maximum magnetization ratios (Mr/Mmax), typically 0.92 at 9.9 kOe coercivity. In addition, it was observed that the Sn concentrations lead to higher Mr/Mmax ratios and maximum magnetization accompanying lower coercivity. X-ray diffraction revealed formation of 2:17 and 2:7 phases in 1:5 matrix, which were found to be dependent on Sn percentage. It appeared that higher Sn concentrations promoted 2:17 phase and helped in the formation of nano-sized phases.

Grain boundary contribution to recoil loop openness of exchange-coupled nanocrystalline magnets

It has been well known that recoil loop openness is related to soft-phase presence in exchange-coupled hard-soft nanocomposite magnets. Our study on recoil loop openness of exchange-coupled nanocrystalline magnets (both single-phase and composite) using a micromagnetic finite-element method has revealed that the recoil loop openness is also due to decreased grain size. Open recoil loops exist in single-phase magnets as well. Simulation of magnetization distribution in both nanocrystalline single-phase magnets and nanocomposite magnets shows that the openness of the recoil loops is correlated with unstable magnetization behavior in grain boundary and soft-phase regions, which is attributed to high energy state caused by exchange coupling in these regions. The simulation results are supported by experimental data.

Ground Spin State Changes and 3 D Networks of Exchange Coupled [MnIII3] Single‐Molecule Magnets

A biased spin doctor: Structural distortions change the intramolecular exchange from antiferromagnetic to ferromagnetic, producing a range of 3D networks of [MnIII3] exchange-coupled single-molecule magnets (SMMs, 1–6), which demonstrate that quantum tunnelling of magnetisation (QTM) can be controlled using exchange interactions. This opens up new perspectives in the use of supramolecular chemistry to modulate the quantum physics of molecular nanomagnets.

Nanocomposite exchange-coupled magnets with two hard phases PrCo5Cδ/Pr5Co19Cε

Nanocomposite exchange-coupled magnets with two hard phases PrCo5Cδ/Pr5Co19Cε have been fabricated by melt spinning. Adding carbon is found to favour the formation of the Pr5Co19Cε phase. The exchange-coupling effect between the PrCo5Cδ and Pr5Co19Cε phases leads to a high remanence ratio and single-phase magnetic behaviour in the ribbons. The coercivity of the PrCo5−xCx ribbons increases with increasing carbon concentration. The best overall properties including a coercivity of 6.1 kOe, a remanence ratio of 0.81 and a maximum energy product of 12.0 MGOe in the ribbons with x = 0.5, have been obtained at room temperature.

Origin of recoil hysteresis loops in Sm–Co∕Fe exchange-spring magnets

Open recoil loops are often interpreted as a consequence of a breakdown in exchange coupling and attributed to the decoupled soft phase in exchange-coupled permanent magnets. However, in element-specific recoil loop measurements on Sm–Co∕Fe exchange spring magnets, the authors found that the open recoil loops were present not only in the soft (Fe) layer but also in the hard (Sm–Co) layer, and were not a consequence of exchange coupling breakdown between the soft and hard layers. Comparison between the experimental results and micromagnetic calculations reveals that the observed open recoil loops originate from the anisotropy variations in the Sm–Co layer.

Coherent Rotation and Effective Anisotropy

It is shown in this paper that the concept of effective anisotropy is of sound physical sense only when the nucleation mode is coherent rotation. Comparison of coherent and incoherent nucleation fields demonstrates that the coherent rotation can take place at very small defect size. The critical size below which the coherent rotation is favorable is roughly the domain wall-width with 3-D soft defects and is smaller for oriented exchange-coupled permanent magnets with planar soft defects. The effect of misaligned grains has also been considered and it is illustrated that the effective anisotropy has to be used very carefully: it can only be used in limited cases and its expression varies as the microstructures change