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The understanding of a coercivity mechanism in high performance Nd–Fe–B permanent magnets relies on the analysis of magnetic properties of all phases present in magnets. By adding Cu in such compounds, a new Nd6Fe13Cu grain boundary phase is formed; however, the magnetic properties of this phase and its role in the magnetic decoupling of matrix Nd2Fe14B grains are still insufficiently studied. In this work, we have grown Nd6Fe13Cu single crystals by the reactive flux method and studied their magnetic properties in detail. It is observed that below the Néel temperature (TN = 410 K), Nd6Fe13Cu is antiferromagnetic in zero magnetic field; whereas when a magnetic field is applied along the a-axis, a spin-flop transition occurs at approximately 6 T, indicating a strong competition between antiferromagnetic and ferromagnetic interactions in two Nd layers below and above the Cu layers. Our atomistic spin dynamics simulation confirms that an increase in the temperature and/or magnetic field can significantly change the antiferromagnetic coupling between the two Nd layers below and above the Cu layers, which, in turn, is the reason for the observed spin-flop transition. These results suggest that the role of an antiferromagnetic Nd6Fe13Cu grain boundary phase in the coercivity enhancement of Nd–Fe–B–Cu magnets is more complex than previously thought, mainly due to the competition between its antiferro- and ferromagnetic exchange interactions. [ABSTRACT FROM AUTHOR]

Applications of magnetostrictive materials, such as sensors and energy-harvesting devices, generally involve the use of the dynamic deformation, i.e., the piezomagnetic effect, induced by an exciting magnetic field Hac. However, this effect is rarely measured directly but commonly approximated by the strain derivative ∂λ/∂H, which is deduced from the quasi-static magnetostrictive curve. In this article, we propose an investigation of the piezomagnetic effect in nickel ferrite by directly measuring the dynamic magnetostriction under various exciting fields using a dynamic strain-gauge setup. These values are compared with the strain derivative in order to disclose the limitations thereof. In addition, the magnetoelectric effect is measured in a ferrite/PZT bilayer and compared with the calculated magnetoelectric coefficient based on an analytical model. When the dynamic magnetostrictive coefficients are integrated into the calculation, good accuracy is found with experimental values, whereas it is not the case for calculated coefficients based on the strain derivatives. The findings of the present study highlight the importance of accurately characterizing the piezomagnetic effect in magnetostrictive materials in order to optimize their performances in dynamic applications. [ABSTRACT FROM AUTHOR]

Twin boundaries impact negatively the magnetic properties in conventionally-fabricated L1 0 -type and RETM 12 -type permanent magnets. Herein, we propose a strategy to suppress twins' formation by decreasing the grain and/or particle sizes below a critical size of D t ∼300 nm in L1 0 -MnAl permanent magnets, thereby relieving twinning-inducing stress by shifting the balance between volume and surface energy. Twin-free monocrystalline nanoparticles smaller than D t were used for preparing field-aligned polymer-bonded magnets with high texture. Generalizing our research findings, we propose here a roadmap for selecting the best route for manufacturing high-performance MnAl magnets, depending on their grain / particle size. The dependence of coercivity on grain/particle sizes shows that optimal coercivity can be reached within the range of 50–200 nm, which also shows a pathway to achieve the twin-free microstructure allowing the achievement of high texture degree in polymer-bounded or sintered magnets textured in a magnetic field. The results reported here can be applied to other twin-containing permanent magnet compounds, offering opportunities to drastically increase their texture and maximal energy products. [Display omitted] [ABSTRACT FROM AUTHOR]