Mössbauer spectroscopical investigation of amorphous Fe–Y alloy ribbons prepared by melt spinning

Fe–Y amorphous alloy ribbons were prepared by the melt spinning method and characterized by X-ray diffraction, Mössbauer spectroscopy and inelastic neutron scattering. X-ray diffraction demonstrates that the Fe_0.7Y_0.3 ribbons are completely amorphous, whereas the Fe_0.3Y_0.7 ribbons contain a small fraction of crystalline Y precipitates in the amorphous Fe–Y matrix. Mössbauer spectroscopy between 4.2 to 300 K reveals the amorphous nature of the Fe–Y matrix and the Fe_0.7Y_0.3 ribbons. The preliminary neutron scattering results S(Q, ω) show excess low energy vibrational modes which gives rise to the so called “boson peak” in this amorphous material.     

Microstructures of (Fe0.88Co0.12)82La7Si11 prepared by arc-melting/melt spinning and subsequent annealing

In this study, we carried out a systematic investigation of the microstructures of (Fe_0.88Co_0.12)₈₂La₇Si₁₁ prepared by arc melting/melt spinning and subsequent annealing. The arc-melted sample contains 36 wt. % of the desired La(Fe,Co,Si)₁₃ phase (denoted by 1:13 in the following). Annealing of the arc-melted ingot at 1273 K for 200 h leads to a single 1:13 phase. Melt spinning enhances formation of the 1:13 phase. As the wheel speed reaches or is greater than 15 m/s, over 50 wt. % of 1:13 is directly formed in the melt-spun ribbons. A subsequent annealing of these melt-spun samples at 1273 K for 20 min results in a nearly single phase 1:13 structure. The samples prepared under different conditions were systematically studied by transmission electron microscopy in order to elucidate possible origins of the enhanced formation of the 1:13 phase when using the new technique. The presence of a high density of La-Fe-Co-Si clusters in the undercooled liquid is believed to contribute to the formation of the 1:13 phase in melt-spun ribbons. The enhanced atomic diffusivity as a result of structural refinement in melt-spun samples accounts for the subsequent fast transformation of the 1:13 phase during the annealing process. PACS 71.20.Eh; 81.07.Bc; 68.37.Lp     

Enhanced coercivity of melt-spun Sm(Co,Fe,Cu,Zr)z ribbons annealed by improved process

The effects of conventional thermal annealing (CTA), rapid thermal annealing (RTA) and rapid recurrent thermal annealing (RRTA) on coercivity of the melt-spun Sm(Co_0.6Fe_0.27Cu_0.1Zr_0.03)_7.5 ribbons were systematically studied. The results show that the annealing parameters greatly affect the coercivity of the ribbons. The optimum coercivity is 9.8, 8.9 and 10.2 kOe for the CTA-treated, RTA-treated and RRTA-treated ribbons, respectively, indicating that the coercivity is not enhanced only by elevating the heating rate. Nevertheless, the coercivity increases to 15.1 kOe for the RRTA-treated ribbons when the cooling rate decreases to 1 °C/min.     

Mössbauer study of Nd2Fe14B and Nd2(Fe1−xCox)14B compounds

The Mössbauer spectra of Nd₂Fe₁₄B and Nd₂(Fe_1?xCo_x)₁₄B compounds (x=0.0.05, 0.10 and 0.16) have been investigated at room temperature and at 77 K. Taking advantage of combined ME and NMR investigations of the Nd₂Fe₁₄B compund, the hyperfine field values and their assignment to the six Fe sites have been determined to be the following sequence: 378(j₂), 346(k₂), 334(j₁), 325(k₁), 322(c), 306(e) kOe. The substitution of Co for Fe decreases the hyperfine fields at all Fe sites. The intensity variations of the subspectra with Co content show that Co atoms have a strong preference to occupy the k₂ site, but have a rather less tendency to enter the j₂ site, which is preferred by Fe atoms.     

Atomic origin of magnetocrystalline anisotropy in Nd(2)Fe(14)B

The magnetic moment reversal at each of the two inequivalent Nd sites in a single crystal of ferromagnetic Nd(2)Fe(14)B is probed by dichroic resonant diffraction of circularly polarized x rays. The results, supported by theory, show that the c-axis intrinsic magnetic stability of this superior permanent magnetic material arises predominantly at one of the Nd sites (g). The other site (f) undermines magnetic stability by favoring a magnetic moment orientation in the basal plane.     

Simultaneous enhancement in coercivity and remanence of Nd2Fe14B permanent magnet by grain boundary diffusion process using NdHx

We have successfully developed a Dy-free grain boundary diffusion process with neodymium hydride (NdH_x) alloy to the permanent magnet Nd₂Fe₁₄B powders using hydrogenation – disproportionation – desorption – recombination (HDDR) method. All the diffusion treatments were performed at 700–800 °C for various annealing time under the high vacuum with rotating diffusion method that effectively control the abnormal grain growth. The coercivities of Dy-treated Nd₂Fe₁₄B powders were varied from 9.5 kOe to 13.2 kOe but the remanence was decreased to 8.1 kG (10% reduction) depending on dysprosium hydride (DyH_x) content and diffusion treated time. However, the coercivity and remanence of Dy-free diffusion treated powder have been increased to 12.2 kOe (28.5% enhancement) and 11.1 kG (22% enhancement) at the optimal diffusion treatment (800 °C for 3 h), respectively. This unique simultaneous enhancement is to isolate the magnetic coupling between Nd₂Fe₁₄B grains by creating non-magnetic Nd grain boundaries and enhance the alignment of the Nd₂Fe₁4B hard magnetic phase, fabricated by optimal diffusion conditions.     

Magnetic and structural properties of the Nd2(Fe100−x Nb x )14B system prepared by arc melting

In this work the magnetic and structural properties are investigated by Mössbauer spectrometry, Vibrating Sample Magnetometry and X-ray diffraction of Nd₂(Fe_100?x Nb _x )₁₄B powdered alloys with x?=?0, 2 and 4 prepared by arc melting. The Mössbauer spectra of the samples were fitted with several contributions from: Nd₂Fe₁₄B, α-Fe and a paramagnetic phase associated with Nd_1.1Fe₄B₄ for x?=?0 and additionally from NbFeB and Nd₂Fe₁₇ for x?=?2 and x?=?4. The relative fractions of α-Fe and Nd₂Fe₁₄B are smaller for x?=?4 than for x?=?0, indicating that the amount of these two phases is reduced with increasing Nb content, while the relative fraction of Nd₂Fe₁₇ increases. The α-Fe grain size slightly decreases while that of the Nd₂Fe₁₄B phase is increasing, when the Nb content increases. The hysteresis loops indicate that these samples behave as hard ferromagnets, with a coercive field which decreases when the Nb content increases, but with rather low remanent magnetization.     

Magnetic properties of fcc (Co95Fe5)1-xAlx ribbons

Rapidly quenched (Co₉₅Fe₅)_1-xAl_x ribbons are investigated by X‐ray diffraction, magnetization, and Mössbauer effect measurements. A single fcc phase is obtained for all ribbons x ? 10 at.%. The lattice constant increases linearly with x and is discussed in connection with magnetic moment. The influence of Al substitution on both magnetization and Fe‐atom hyperfine field (H) is studied. At 296 K, the magnetization decreases linearly while H drops nonlinearly as x increases. Al substitution leads to substantial differences in iron hyperfine fields in bcc and fcc systems. Fe moment is perturbed differently by Al substitution in fcc (Co₉₅Fe₅)_1-xAl_x and bcc Fe–Al systems.     

Magnetic characteristics of R2(Fe, Co)14B systems (R = Y, Nd and Gd)

R₂(Fe, Co)₁₄B compounds (R = Y, Nd and Gd) were prepared in high purity. The magnetic behavior of R₂(Fe, Co)₁₄B compounds is reported over the temperature range 4 to 300 K. The effects of Fe substitution by Co on the saturation magnetization, Curie temperature and anisotropy are presented. The spin-reorientation temperature is lowered as Co replaces Fe. This also results in a reduced cone angle.

The R₂Fe_14−xCo_xB alloys crystallize in the tetragonal structure over the entire concentration range of 0 x 14. When Fe is substituted by Co, the Curie temperature increases significantly, the saturation magnetization increases to a maximum value around x = 2, and the anisotropy becomes planar for R = Y and Gd. The Nd₂(Fe, Co)₁₄B systems all exhibit uniaxial anisotropy at room temperature and Nd₂Co₁₄B is strongly uniaxial at 77 K. The Nd₂(Fe, Co)₁₄B systems are conical at 77 K.     

Positive muon spectroscopy of Nd2Fe14B and Pr2Fe14B

We have performed positive muon spin rotation measurements on polycrystalline samples of Nd₂Fe₁₄B and Pr₂Fe₁₄B in zero applied field. In both samples a single sharp μSR line was observed which was unexpected in this complicated structure. The temperature dependence of the muon frequency for Nd₂Fe₁₄B clearly reflects the spin reorientation below 150 K and can be explained qualitatively by assuming that only the c-axis component of a magnetization is sampled by the muon. A smooth decrease of the muon frequency with increasing temperature is observed for Pr₂Fe₁₄B.     

Effect of composition on texture and magnetic the crystallographic properties of the Sm（Co,Fe,Cu,Zr）z ribbons prepared by simple processing

Sm(Co_balFe_yCu_xZr_w)_z ribbons have been prepared by melt spinning at a low wheel velocity followed by short-time aging and slow cooling the as-spun ribbons from 850 to 400℃. It is found that the composition can significantly influence the degree of crystallographic texture of the ribbons. The 1:7 phase of the as-spun ribbons is segregated into 1:5 and 2:17 phases by the simple processing procedure. However, the crystallographic texture is still preserved in the ribbons after precipitation hardening. (BH)_{\rm max} about 86kJ/m^{3} can be obtained in the Sm(Co, Fe, Cu, Zr)_{z} ribbons by the adjustment of composition.     

Study of the spin reorientation phenomenon in Nd2(Fe, Si)14BHy

We have studied the influence of the Si for Fe substitution and of the H insertion on the spin reorientation phenomenon in the Nd₂Fe₁₄B phases. To determine the temperature of spin reorientation and the tilt angle between the c-axis and the easy magnetisation direction, we have measured the angular dependence of the components of the magnetisation vector. Our measurements are based on powder samples that have been previously aligned under an external magnetic field. The Si for Fe substitution induces a decrease of the spin reorientation temperature and of the tilt angle. Meanwhile, we have found that the tilt angles are almost the same for Nd₂Fe₁₃SiB and Nd₂Fe₁₂Si₂B. These features are analysed in terms of changes in the crystal electric field and the unit cell volume induced by the Si for Fe substitution. The insertion of hydrogen in the Si-containing samples leads to an additive decrease of the spin reorientation temperature. This decrease is not linked to a change in the tilt angle at 4 K but to a different thermal behaviour in the H-containing samples. The magnitude of the effects of H insertion on the crystal electric field in comparison with the Si for Fe substitution is discussed together with the role of the lattice expansion.     

Spin reorientations in R2Fe14-xCoxB systems (R = Pr,Nd and Er)

The effect of cobalt substitution on spin reorientation phenomena in R₂Fe_14-xCo_xB systems (R = Pr, Nd, Er) was studied by means of bulk magnetometry in the temperature range 4.2–1100 K. It was established that in the Nd-based system the introduction of cobalt resulted not only in a shift of the low temperature spin reorientation (cone to axis) but also triggered, for x ⩾ 10, an appearance of a second spin reorientation (axis to plane) at high temperature. In the Pr-based system, for x ⩾ 9.5, a high temperature spin reorientation (axis to plane) was also observed. Its dependence on Co content was determined. For the Er-based system, an increase of the spin reorientation temperature (plane to axis) was observed as more cobalt was introduced into that system. It was also established that the tetragonal single-phase materials in this system exist only up to x = 5. Temperature-composition diagrams are presented, indicating types of spin arrangements observed in the investigated systems.     

钕铁硼的冲击压缩特性

 利用二级轻气炮对恒磁体钕铁硼进行冲击压缩,采用阻抗匹配法进行测量,获得了平均初始密度为7.346 g/cm³的钕铁硼Hugoniot关系数据。实验结果表明,该种钕铁硼在19~78 GPa范围内,其D-u_p满足线性变化关系,即：C₀、λ分别为3.686 km/s、1.059,是一种稳定的压缩过程,其间没有相变产生。而较小的λ值表明该种钕铁硼材料偏向于疏松体结构,且容易被压缩。同时实验结果也为其状态方程和脉冲功率源等方面的研究工作提供了可资参考的实验参数。     

High coercivity powders based on Sm-Fe-Ta-N prepared by a novel technique

Nitrided Sm₂Fe₁₇-based materials possess excellent intrinsic magnetic properties. In this study, we investigate two compositions, Sm_13.7Fe_86.3 and Sm_13.8Fe_82.2Ta_4.0. The stoichiometry of each phase was determined and the SmFeTa material was found to include Ta₃Fe₇, in addition to the Sm₂Fe₁₇, SmFe₂, and SmFe₃ phases observed in the SmFe alloy, but without the α-iron dendrites characteristic of the binary material. SEM and TEM studies revealed that in the cast structure, approximately 2.0% Ta is initially dissolved in the Sm₂(FeTa)₁₇ phase; however, HDDR processing causes the formation of Ta-based precipitates, leaving a 2: 17 phase with much less dissolved Ta. The HDDR process, with subsequent nitrogenation, was used to prepare coercive powders. The coercivities of these powders were found to be very dependent on the HDDR conditions and Ta addition. The highest coercivity achieved was 1280 kA/m for the composition with Ta.     

Spontaneous magnetostriction in R2Fe14B (R=Y,Nd, Gd,Tb, Er)

Thermal expansion anomalies of R₂Fe₁₄B (R=Y, Nd, Gd, Tb, Er) stoichiometric compounds were studied by X-ray diffraction with high-energy synchrotron radiation using a Debye–Scherrer geometry in temperature range of ∼10–1000 K. A large invar effect with a corresponding large temperature dependence of lattice parameters ∼10–15 K above their Curie temperatures (T_c) are observed. The a-axes show a larger invar effect than the c-axes in all compounds. The spontaneous magnetostrain of the lattices and bonds are calculated. The iron sublattice is shown to dominate the volumetric spontaneous magnetostriction of the compounds and the contribution from the rare-earth sublattice is roughly proportional to the spin magnetic moment of the rare earths. The bond-length changes are consistent with the theoretical spin-density calculation. The average bonds magnetostrain around Fe sites is approximately proportional to their magnetic moments.     

取向Pr2Fe14B与Nd2Fe14B多晶磁化曲线的计算

基于单离子晶场模型 ,提出了计算稀土 Fe(Co)金属间化合物取向多晶样品磁化曲线的方法 .用此方法计算了取向Pr2 Fe14 B和Nd2 Fe14 B多晶的高场磁化曲线 ,计算中使用了拟合化合物单晶磁化曲线得到的交换场与晶场参数 .计算曲线与实验曲线相符合 .     

Field-induced magnetic transitions in Pr2(Co,Fe)17

A study of the different magnetic field-induced transitions has been performed for Pr₂Co_17−xFe_x compounds in the temperature range 78–300 K. By using the singular point detection technique, it has been shown that all compounds with x>0 exhibit at least one field-induced transition, which has been identified as a first-order magnetization process (FOMP). Different types of FOMP (A1, A2, P1) are observed, and the change of the type can be induced by varying either temperature or composition. Intermediate Co–Fe compositions show a P1-type FOMP which persists even above room temperature, while a double transition has been found in iron-rich compounds with x⩾13.6. It has also been noticed that, for the particular values of x when a temperature-induced spin reorientation is present, the change from A1 to P1 type FOMP (or vice versa) exactly occurs at the spin reorientation temperature, with a vanishing critical field value. A comparison with the behavior of Y₂Co_17−xFe_x compounds leads to the conclusion that the contributions of the Pr sublattice are essential for the development of these field-induced transitions (some of which are still present at room temperature), even if it is inferred that the overall anisotropy is mainly determined by the 3d sublattice.     

Preparation for exchange-coupled permanent magnetic composite between α-Fe (soft) and Nd2Fe14B (hard)

Soft magnetic α-Fe nanoparticles were prepared by a coprecipitation route and hard magnetic Nd₁₅Fe₇₇B₈ nanoparticles were prepared by ball milling for 20 h by using a shaker mill. A mechanical ball-mill technique was applied to build up exchange-coupled nanoparticles. A mixture of Nd₂Fe₁₄B and α-Fe nanoparticles in a stainless steel boat was milled for 2 h and annealed in a vacuum furnace under vacuum (∼10⁻⁵ Torr) at 650 °C for 30 min. The crystal structure of the nanoparticles was confirmed by using X-ray powder diffraction (XRD). The surface morphology was identified by FE-SEM. The magnetization curve was measured with a vibrating-sample magnetometer (VSM). Thermogravimetry using a microbalance with magnetic field gradient positioned below the sample was used for the measurement of a thermomagnetic analysis (TMA) curve showing the downward magnetic force versus temperature.