In this study, we have compared magnetic and magnetostrictive properties of polycrystalline CoFe 2 O 4 pellets, produced by three different methods, focusing on the use of Spark Plasma Sintering (SPS). This technique allows a very short heat treatment stage while a uniaxial pressure is applied. SPS was utilized to sinter cobalt ferrite but also to make the reaction and the sintering (reactive sintering) of the same ceramic composition. Magnetic and magnetostrictive measurements show that the reactive sintering with SPS induces a uniaxial anisotropy, while it is not the case with a simple sintering process. The induced anisotropy is then expected to be a consequence of the reaction under uniaxial pressure. This anisotropy enhanced the magnetostrictive properties of the sample, where a maximum longitudinal magnetostriction of −229 ppm is obtained. This process can be a promising alternative to the magnetic-annealing because of the short processing time required (22 minutes).

Applications of magnetostrictive materials commonly involve the use of the dynamic deformation, i.e., the piezomagnetic effect. Usually, this effect is described by the strain derivative ∂λ=∂H, which is deduced from the quasistatic magnetostrictive curve. However, the strain derivative might not be accurate to describe dynamic deformation in semihard materials as cobalt ferrite (CFO). To highlight this issue, dynamic magnetostriction measurements of cobalt ferrite are performed and compared with the strain derivative. The experiment shows that measured piezomagnetic coefficients are much lower than the strain derivative. To point out the direct application of this effect, low-frequency magnetoelectric (ME) measurements are also conducted on bilayers CFO=PbðZr; TiÞO 3 . The experimental data are compared with calculated magnetoelectric coefficients which include a measured dynamic coefficient and result in very low relative error (<5%), highlighting the relevance of using a piezomagnetic coefficient derived from dynamic magnetostriction instead of a strain derivative coefficient to model ME composites. The magnetoelectric effect is then measured for several amplitudes of the alternating field H ac , and a nonlinear response is revealed. Based on these results, a trilayer CFO/PbðZr; TiÞO 3 /CFO is made exhibiting a high magnetoelectric coefficient of 578 mV=A (approximately 460 mV=cm Oe) in an ac field of 38.2 kA=m (about 48 mT) at low frequency, which is 3 times higher than the measured value at 0.8 kA=m (approximately 1 mT). We discuss the viability of using semihard materials like cobalt ferrite for dynamic magnetostrictive applications such as the magnetoelectric effect.

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In this paper, we investigate the demagnetizing effect in ferrite/PZT/ferrite magnetoelectric (ME) trilayer composites consisting of commercial PZT discs bonded by epoxy layers to Ni-Co-Zn ferrite discs made by a reactive Spark Plasma Sintering (SPS) technique. ME voltage coefficients (transversal mode) were measured on ferrite/PZT/ferrite trilayer ME samples with different thicknesses or phase volume ratio in order to highlight the influence of the magnetic field penetration governed by these geometrical parameters. Experimental ME coefficients and voltages were compared to analytical calculations using a quasi-static model. Theoretical demagnetizing factors of two magnetic discs that interact together in parallel magnetic structures were derived from an analytical calculation based on a superposition method. These factors were introduced in ME voltage calculations which take account of the demagnetizing effect. To fit the experimental results, a mechanical coupling factor was also introduced in the theoretical formula. This reflects the differential strain that exists in the ferrite and PZT layers due to shear effects near the edge of the ME samples and within the bonding epoxy layers. From this study, an optimization in magnitude of the ME voltage is obtained. Lastly, an analytical calculation of demagnetizing effect was conducted for layered ME composites containing higher numbers of alternated layers ( ≥ 5). The advantage of such a structure is then discussed. I. Introduction.Magnetoelectric (ME) composites using the product-property concept are particularly suitable for smart sensors fabrication (e.g. magnetic field or current sensors 1-5 ). The product-property effect is obtained when piezoelectric and magnetostrictive phases are mechanically coupled to each other. At the present date, layered ME composites have high interest because they produce the best ME performances. Bilayers of piezoelectric and magnetostrictive materials are the simplest layered composites but these structures exhibit low ME effects. In order to achieve high ME responses, some authors 6,7 have focused their studies on co-sintered ME samples containing a high number of alternated PZT/ferrite thin layers. Among the different structures of layered composites, the trilayer, consisting of a piezoelectric layer sandwiched between two magnetostrictive layers, achieves a good balance between ease of fabrication and performances 8,9 . In a recent paper 4 , we have shown that a piezoelectric layer stressed on its

The influence of quenching rate and nitrogenation in melt-spun Nd 1.2 Fe 10.6 Mo 1.4 has been investigated in terms of microstructure, phase formation and magnetic properties. Increasing the quenching rate leads to smaller grain size. However, it also implies a change in the crystallized phase structure. We obtained a pure ThMn 12 (1:12) structure at quenching rates up to 30 m/s, leading to an average grain size of 220 nm. Magnetic measurements of the as-spun ribbons revealed a reduction of the saturation magnetization for samples quenched above 30 m/s. This is attributed to the formation of a paramagnetic phase and/or magnetic phase with a Curie temperature (T C ) close to room temperature which is confirmed by 57 Fe Mössbauer spectroscopy. The analysis of the spectra rules out the presence of a ferromagnetic TbCu 7 (1:7) phase, which is usually reported in such system. The ribbons were nitrogenated in order to form the harder magnetic phase Nd 1.2 Fe 10.6 Mo 1.4 N x . The ribbon quenched at 30 m/s with the pure ThMn 12 nitride structure is the optimum sample for getting hard magnetic properties, with a coercivity of 0.6 T, saturation magnetization of 1.15 T and Curie temperature of 350 • C. Finally, we show the good stability of the later phase structure at elevated temperatures (≤ T C ), making this compound a good candidate for permanent magnet applications.

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Nd-xZrxFe10Si2 alloys have been prepared in the tetragonal ThMn12-type structure by arc-melting and melt spinning and then nitrogenated to improve their magnetic properties. For x = 0.4 and 0.6 the Curie temperature and magnetic anisotropy fields increases from 280-300 ºC to about 390 ºC and from 2.8-3 Tesla to 4.5-5 Tesla respectively. The saturation magnetization remains almost unchanged. The nitrogenated powders were processed by Spark Plasma Sintering (SPS) leading to compact pellets, which retain the full Nitrogen content and magnetic properties up to 600ºC, but segregated Fe-Si at elevated temperatures. Nitrogenation and SPS processing are, therefore, appropriate for sintering metastable materials such as (NdZr)Fe10Si2 into compact material without loosing functional properties. This opens a way towards a new family of permanent magnets, lean of critical raw materials.

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