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.     

Magnetocrystalline anisotropy of Ni and Mn substituted Nd2Fe14B compounds

Measurements of the effects of Ni and Mn substitutions for Fe on the magnetic properties of Nd₂Fe₁₄B compounds are reported. The Curie temperature is slightly increased with Ni substitution whilst in the case of replacement of Fe by Mn it is reduced drastically. A monotonic decrease of both the lattice parameters a and c is observed. The saturation magnetization is decreased by both Ni and especially Mn substitutions. The composition dependence of both the reorientation spin transition temperature and the cone angle has been measured. The influence of the 3d metal substitution on the Nd anisotropy has been measured and discussed. The composition dependence of the room temperature anisotropy field values, which is an important figure of merit for permanent magnet applications, decreases slightly in the case of Ni and drastically for Mn substitution. A comparison with the case of Co substitution has been made.     

Research on the origin of magneto-crystalline anisotropy in R2Fe14B compounds

A model for the calculation of the magneto-crystalline anisotropy in R₂Fe₁₄B (R is a rare earth) compounds is presented. According to the model, a combined effect on the R-ion, coming from both the crystal field of ligands and the electric field of itinerant electrons, results in the anisotropy of the compounds. An approach of calculating the interaction between the itinerant electrons and the central R-ion is given. Taking account of the combined effect, we calculated the temperature dependences of the anisotropy of all the five R₂Fe₁₄B-type (R = Pr, Nd, Tb, Dy and Ho) compounds with uniaxial anisotropy. The results are in good agreement with the experiments. The calculation has shown that the effect of the itinerant electrons plays an important role for the anisotropy of the compounds.     

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.     

Magnetocrystalline anisotropy of Er2Fe14B

A rotation-magnetic-alignment method was used to align fine-powdered (< 20 μm) Er₂Fe₁₄B at room temperature while the easy magnetization direction of Er₂Fe₁₄B lies in the basal plane. X-ray diffraction was used to check the magnetic alignment. For the first time, the temperature dependence of the anisotropy field of Er₂Fe₁₄B was measured in a wide temperature range from about 170 to 530 K. The anisotropy field was determined using the SPD technique in a pulsed-field magnetometer from 170 to 320 K (T_SI = 323 K) on a magnetically aligned sample and from 330 to 530 K (T_C = 550 K) on a bulk polycrystal.     

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 .     

Magnetic Characteristics of Ball-Milled Nd2Fe14B - Gd

Mössbauer measurements using ⁵⁷Fe and ¹⁵⁵Gd have been made on samples of ball-milled Nd₂Fe₁₄B in a gadolinium matrix in order to determine the magnetisation of each phase separately. Although magnetometer measurements did not show any coercivity, both measurements showed the phases were in intimate magnetic contact. The spectra of each phase differed considerably from those of the respective original materials and provide information on the conduction electron polarisation through the transferred hyperfine fields.     

Uniaxial magnetocrystalline anisotropy of metal/semiconductor interfaces: Fe/ZnSe(001)

A theoretical study of the magnetic moments and the in-plane magnetic anisotropy of an interface between a cubic ferromagnet and a cubic semiconductor, Fe/ZnSe(001), is presented. Theory confirms the observed, much debated, uniaxial anisotropy of the iron film. This result is important since the calculations are for perfect interfaces with squarelike environments, proving that the fourfolded symmetry of the interface Fe atoms is broken beyond the nearest neighboring semiconducting layer, effects that are usually assumed small. It is demonstrated how the uniaxial anisotropy is produced by the directional covalent bonds at the interface, even without atomic relaxations.     

Magnetization and magnetocrystalline anisotropy in YCo4B compound

Easy and hard magnetization curves of YCo₄B compound have been measured in the temperature range from 1.5 to 300 K. It was found that the uniaxial magnetic anisotropy field decreases with decreasing temperature, and the magnetic anisotropy changes from the easy c-axis to the easy cone at approximately 150 K. The easy and hard magnetization curves did not cross up to 6 T. High field susceptibility of the compound for magnetic field parallel to the alignment direction seems different from that for a field perpendicular to the alignment direction. A jump was observed along the easy magnetization curve at 1.5 and 77 K. The critical field of the jump is about 1.5 T at 1.5 K and 1.2 T at 77 K. The jump was shown to be reversible at 1.5 K by down hill measurement.     

Ferroelectric control of the magnetocrystalline anisotropy of the Fe/BaTiO(3)(001) interface

Density-functional calculations are employed to investigate the effect of ferroelectric polarization of BaTiO(3) on the magnetocrystalline anisotropy of the Fe /BaTiO(3)(001) interface. It is found that the interface magnetocrystalline anisotropy energy changes from 1.33 to 1.02 erg cm (-2) when the ferroelectric polarization is reversed. This strong magnetoelectric coupling is explained in terms of the changing population of the Fe 3d orbitals at the Fe/BaTiO(3) interface driven by polarization reversal. Our results indicate that the electronically assisted magnetoelectric effects at the ferromagnetic/ferroelectric interfaces may be a viable alternative to the strain mediated coupling in related heterostructures and the electric field-induced effects on the interface magnetic anisotropy in ferromagnet/dielectric structures.     

R2Fe14B型永磁材料中第二磁晶各向异性常数对反磁化过程的影响

针对低温下各向同性Pr2Fe14B永磁材料的最小形核场问题,用数值计算法和近似解析解研究了第二磁晶各向异性常数K2对最小形核场的影响.研究发现,尽管对于Nd2Fe14B永磁材料一级近似的解析解与数值计算结果很接近,但是对于低温下各向同性Pr2Fe14B永磁材料则至少要用二级近似下的解析解才能与数值计算结果相接近.用有关最小形核场的计算结果很好地解释了低温时各向同性Pr2Fe14B永磁材料的矫顽力与最小形核场的关系.     

Fabrication and magnetocrystalline anisotropy of NiCo(002) films

A series of 30-nm-thick epitaxial NixCo1-x(002) alloy films are fabricated by DC magnetron sputtering. MgO(002)and SrTiO3(002) single substrates are used for x 0.5 and x 0.5, respectively. The magnetocrystalline anisotropy of Ni x Co1-x(002) alloy films is studied in the entire composition region for 0 ≤ x ≤ 1.0. When x decreases, the cubic magnetic anisotropy constant K1 changes sign from negative to positive at x = 0.96 and becomes negative again at x = 0.79.It becomes more negative as x decreases from 0.79 to 0. The uniaxial anisotropy K u is smaller than the K1 by a factor of two orders.     

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.     

Mössbauer study of the permanent magnetic material (Nd1−xYx)2Fe14B

Samples of (Nd_1?xY_x)₂Fe₁₄B, 0≦x≦0.80, with good phase homogeneity have been made by heating under vacuum for 300 hours. The⁵⁷Fe Mössbauer parameters for all six different crystallographic sites have been determined at room temperature. Generally, the influence of the yttrium ions is greatest for those iron sites nearest the Nd-Y planes. The yttrium appears to be randomly distributed over the neodymium 4f and 4g sites.     

Mössbauer spectra of Nd2Fe14B and related compounds

Mössbauer spectra and ac susceptibility data on Nd₂Fe_{1 4} B with substitution of Pr or Sm for Nd indicate that the spinreorientation temperature is depressed by Pr substitution but increased in alloys containing Sm so that it occurs at roomtemperature for Nd_{1 2}Sm₅Fe_{7 6}B₇. In the Nd end-member, the onset of the transition appears about 210 K although the main reorientation is at 134 K. Nickel shows no marked site preference: its effects are to increase slightly the Curie temperature and reduce the iron moment.     

钕铁硼在高温超导磁悬浮技术实验中的应用

将新材料钕铁硼作为磁导轨引入到高温超导磁悬浮技术实验中,介绍了高温超导悬浮技术实验的基本原理、仪器及钕铁硼对实验的改进.     

Exchange interaction and magnetocrystalline anisotropy in GdMn2Ge2

Tetragonal GdMn₂Ge₂ is a ferrimagnet at low temperatures and changes to an antiferromagnet undergoing a first-order transition at 95 K. The temperature dependence of the magnetization has been analyzed in terms of the molecular-field approximation with interactions between Gd-Gd, Gd-Mn and Mn-Mn atoms. The Gd-Mn interaction is antiferromagnetic, and this permits the appearance of the ferrimagnetic structure at low temperatures. The uniaxial anisotropy (the easy axis of magnetization is the c-axis for both Gd and Mn moments) plays an important role in the first-order ferrimagnetic to antiferromagnetic transition.