MAGNETIZATION BEHAVIOUR FOR TEXTURED BULK YBaCuO POLYCRYSTALLINE SUPERCONDUCTORS

In present paper.study of magnetization behaviour was carried out for textured bulk YBaCuO polycrystalline superconductors synthesized by MTG process.Themagnetization behaviour apparently different from that of sintered samples was observed.Magnetization hysteresis measurements were performed for the texturedYBaCuO superconductors at 77K in ± 10kOe magnetic field range.Based on the results ofmagnetization hysteresis.the magnetic field dependence of critical current density(J)at77K was evaluated for the textured YBaCuO samples.The magnetic flux penetration intextured bulk YBaCuO polycrystalline superconductors was investigated at 77K throughthe magnetization hysteresis measurements under various maximum applied magnetic fields(H_M)and reasonable discussion for the phenomena was conducted in terms of Bean’s criti-cal state model.The enhancement of magnetic properties for textured bulk YBaCuOpolycrystalline superconductors is presumably attributed to the improvement ofmicrostructure in these samples.     

Magnetic structure and phase composition of microinhomogeneous PdMn x Fe1 − x alloys

Mössbauer analysis of ternary PdMn _x Fe_{1 ? x} alloys in the range of concentrational transition from a ferromagnetic PdFe (F ₁) (T _C = 725 K) to an antiferromagnetic PdMn (A) (T _N = 815 K) phase has been carried out at temperatures T ～ 80 and 295 K. An estimation of the relative volumes of the ferromagnetic and antiferromagnetic ordered phases in the molecular-field approximation was performed via calculation of the magnetic susceptibility of the alloy with x = 0.5; an analysis of the microstructure of the alloy with x = 0.45 was performed using the Bitter-figures technique. For the alloys with extreme concentrations, Mössbauer spectra have been obtained in two different forms, i.e., a doublet with H _hf < 50 kOe for the antiferromagnetic alloys with x > 0.8 and a sextet with H _hf ～ 350 kOe for the ferromagnetic alloys with x < 0.2. In the intermediate concentration range, a wide spectrum of hyperfine fields 0 < H _hf < 350 kOe has been observed, which is due to a microinhomogeneous multiphase state.     

Magnetoresistance in Granular Metal–Dielectric Composites

Magnetoresistance in granular composites over a broad concentration range (25 < x < 60%) was investigated. In the studied composites both-positive and negative magnetoresistance were observed. Positive magnetoresistance at x ≤ 45 reaches 20% in magnetic fields of 11–12 kOe. This result can be explained by assuming that in the analyzed composites clusters and granules of isolated magnetic particles co-exist. This is why, they are characterized by different values of magnetic anisotropy and the presence of a dipole-dipole interaction between the clusters and the nearest-neighbor particles.     

Mössbauer effect measurement evidence for magnetic transition in ordered Fe-doped NiAl

Mössbauer effect measurements at room and cryogenic temperatures on powdered Fe-doped NiAl materials (designated as Ni–40Al–9Fe and Ni–50Al–9Fe) show that both were paramagnetic down to17 K, displaying one Fe site in the host. At 4.2 K, Ni–40Al–9Fe remained paramagnetic, while Ni–50Al–9Fe showed a magnetic transition, with a clearly resolved Fe environment. The determined internal magnetic field was 185 ± 8 kOe in comparison to 330 kOe for α-Fe (bcc) reference at room temperature. This shows that development and evolution of electronic and magnetic interactions associated with the dopant Fe in the ordered NiAl depend on its site preference tendencies. The temperature dependence of magnetic state of the dopant Fe, in conjunction with its site preference tendencies from Mössbauer effect measurements, confirms the effectiveness of this technique in the study of the hardening/softening behaviors of Fe-doped NiAl.     

Magnetic phase diagram of CrTe1−xSbx (0.0≤x≤1.0)

Measurements of the high field magnetization of CrTe_1−xSb_x (0.0≤x≤1.0) were carried out at 4.2 K in pulsed magnetic fields up to 300 kOe. The temperature dependence of the magnetization of CrTe_1−xSb_x was measured in the temperature range from 4.2 K to 800 K. The magnetic phase diagram of CrTe_1−xSb_x (0.0≤x≤1.0) was determined, which is similar to the typical one for the mixed crystals of the layered antiferromagnetic and ferromagnetic compounds proposed by de Gennes.     

Law of approach to saturation in highly anisotropic ferromagnets Application to Nd---Fe---B melt-spun ribbons

The magnetization curves of melt-spun ribbons of Nd---Fe---B with (BH)_max=16 MGOe have been measured in pulsed high magnetic fields up to 400 kOe to examine the applicability of the law of approach to ferromagnetic saturation to highly anisotropic uniaxial ferromagnets. The magnetization in the region M≥0.95 Ms can be described well with the law of approach to saturation. From the analysis of the magnetization curve, the spontaneous magnetization, effective anisotropy constant and high-field susceptibility are estimated at room temperature. These values are consistent with those evaluated from single-crystalline data of Nd₂Fe₁₄B.     

Structure and magnetic behaviors of Gd6FeBi2 compound

Gd-based compounds have attracted a great deal of interest as materials with large magnetocaloric effect and as materials for spintronic applications. Fe₂P-type Gd₆FeBi₂ alloy was synthesized successfully, and the compound shows good soft magnetic behavior at room temperature with a Curie temperature ∼350 K. The compound exhibits non Curie–Weiss behavior in a large temperature range above Curie temperature, and slightly enhanced Gd moment at low temperature. The elevated Curie temperature and enhanced Gd magnetic moment were discussed based on measured electronic structure and the results from another isostructural compound Gd₆CoTe₂. The magnetocaloric effect was also measured in terms of the maximum magnetic entropy change of −4.3 J kg⁻¹ K⁻¹ at 50 kOe and −2.3 J kg⁻¹ K⁻¹ at 20 kOe, respectively.     

Mn掺杂对快淬NdFeB永磁材料晶格与磁性能的影响

应用DTA、XRD、VSM，EXAFS对快淬Nd9Fe85-xMnxB6(x=0,0.5,1)纳米复合材料磁性能进行了研究。发现少量Mn的掺杂能够显著促进快淬样品的晶化并提高快淬样品的永磁性能，在合适的热处理条件下，得到的最佳矫顽力和剩磁比分别从339．6kA/m(4266.5Oe)和0．70提高到398．2kA/m(5002.5Oe)和0．72，最大磁能积（BH）max从84kJ/m^3(10.5MGOe)提高到88kJ/m^3(11MGOe)。认为永磁性能的提高是由于Mn的掺杂使快淬NdFeB具有更有序的晶体结构。     

Magnetic and electrical properties of the UCu2T3Al7 alloys

The UCu₂T₃Al₇-type compounds with T = Cr, Mn and Fe crystallize in the tetragonal ThMn₁₂-type structure (I4/mmm space group). Magnetic properties have been examined in magnetic fields up to 50 kOe in the temperature range 1.9–400 K using a SQUID magnetometer. The Cr based pseudoternary alloy is paramagnetic with the linear field dependence of the magnetization at 1.9 K. The temperature dependence of the magnetic susceptibility is described by a modified Curie–Weiss (MCW) law. The Mn containing compound is ferrimagnetic and the field dependence of the magnetization does not reach saturation while the magnetic susceptibility follows the MCW law at high temperature. The Fe alloy is ferromagnetic and at higher temperature the MCW law is fulfilled. The temperature dependence of the electrical resistivity of all materials was examined between 4.2 and 300 K showing metallic character. The Mn and Fe based compounds demonstrate some anomalies in the ρ(T) plot at low temperature. The results are discussed in relation to the anomalies observed in magnetic measurements.     

Magnetic properties of cobalt ferrite/SiO2 nanocomposite

Nanocrystalline cobalt ferrite dispersed in silica was prepared by mechanical alloying and subsequent heat-treatment. For Co-ferrite with 15 wt% silica, a high coercivity of up to 3.3 kOe and a magnetization of 68 emu/g were obtained after an optimized heat treatment (in air at 900°C). Experiments were conducted to study the optimum amount of silica required to obtain optimum magnetic properties. It was found that a small amount of 4.5 wt% silica (SiO₂) allows the composite to possess good magnetic properties (coercivity of 3 kOe and saturation magnetization of 72 emu/g).     

Fabrication and magnetic properties of CoPd nanowire arrays

Highly ordered CoPd nanowire arrays from a co-ordination complex electrolyte solution have been fabricated by alternating current electrodeposition onto nanoporous anodic alumina. Structural and magnetic properties of the arrays were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), x-ray diffraction (XRD), and vibrating sample magnetometer (VSM). A coercivity of about 1 kOe and a fair loop squareness has been obtained in the out-of-plane direction, which is promising for perpendicular magnetic recording.     

Structure and magnetic properties of mechanically milled Nd2xFe100−3xBx (x = 2–6)

The effects of composition and heat treatment on the structure and magnetic properties of mechanically milled Nd_2xFe_100−3xB_x alloys (x = 2–6) have been studied. High remanences have been obtained in isotropic samples consisting of a nanocrystalline mixture of Nd₂Fe₁₄B and -Fe. For optimally heat treated samples, an increase in the volume fraction of -Fe from 14% to 73%, increased the reduced remanence (M_r/M_s) from 0.56 to 0.64. The corresponding coercivities decreased from about 8 kOe to only 1.6 kOe. The magnetic behaviour of these Nd₂Fe₁₄B--Fe alloys is associated with exchange coupling between the hard and soft magnetic phases.     

Magnetic and magnetocaloric properties in Cu-doped high Mn content Mn50Ni40−xCuxSn10 Heusler alloys

The effects of Cu substitution on the phase transitions and magnetocaloric effect of Mn₅₀Ni_40−xCu_xSn₁₀ Heusler alloys were investigated. With the increase of Cu content, the martensitic transformation (MT) temperature shifts substantially towards lower temperature, while the Curie temperature of austenite remains almost unchanged. The reverse MT temperature decreases from 180 to 171 K for Mn₅₀Ni₃₉Cu₁Sn₁₀ alloy as the magnetic field increases from 1 to 30 kOe. Under an applied magnetic field of 30 kOe, the maximum values of magnetic field induced entropy changes are 19.6, 28.9, and 14.2 J/kg K for x = 0, 1, and 2, respectively. The effective refrigerant capacities and hysteresis losses for these alloys were discussed in this paper.     

Magnetic anisotropy and magnetic properties of RTSi (R=Gd, Y; T=Mn, Fe) compounds

Magnetic properties of Gd_xY_1−xMnSi and GdMn_xFe_1−xSi compounds with CeFeSi-type structure have been studied by magnetization measurements in the temperature range 77–600 K in static magnetic fields up to 12 kOe. The measurements for some compounds have been carried out on single crystal samples. The easy magnetization direction of all single crystal samples was found to be the c axis. The value of the anisotropy constant K₁ for GdMnSi was estimated to be 2.0·10⁶ erg cm⁻³ at 77 K. The substitution in both rare earth and 3d sublattices leads to a sharp increase in magnetic anisotropy of these compounds. The concentration dependencies of magnetic ordering temperatures, effective magnetic moments and paramagnetic Curie temperatures have been determined. The obtained results can be explained by the modification of the band structure due to the change of interatomic distances and the filling of 3d band.     

Low-temperature properties of the orderable Cu<Subscript>3</Subscript>Pd alloy

The effect of atomic ordering on the transport properties of the Cu₃Pd alloy in the low-temperature range (T < 70K) in strong magnetic fields (H ≤ 150 kOe) has been studied. The temperature dependence of the resistivity during alloy disordering has been found to have a minimum at T ～ 10 K. The Hall effect has a negative sign. The magnetoresistance, on the contrary, is positive; the longitudinal effect equals ～50% of the transverse one.     

Superstructure and magnetic properties of R15X9C compounds (R = rare earth; X = Si and Ge)

We report the synthesis of the new compounds R₁₅Si₉C with R = Sm, Gd-Er, Y and R₁₅Ge₉C with R = Ce, Pr and Nd. These compounds crystallize in the hexagonal La₁₅Ge₉Fe structure type (hP50, P6₃mc, Z = 2) which is an ordered superstructure of La₅Ge₃ (Mn₅Si₃-type, hP16, P6₃/mcm, Z = 2). The superstructure is absent in a single-phase R₅Ge₃C_0.33 alloy with R = Sm and Gd. The addition of interstitial carbon modifies the magnetic properties significantly as regards the magnetic transition temperature and the nature of the magnetic ordering. Carbon doping induces mostly a ferromagnetic or ferrimagnetic configuration. The preponderance of ferromagnetic interactions in the superstructure compounds, with respect to the parent phases, can tentatively be attributed to a change in the sign of the RKKY interactions. The dense Kondo lattice state of Ce in ferrimagnetic Ce₅Ge₃ is preserved in Ce₁₅Ge₉C. Large coercive fields (∼4-5 kOe) at low temperatures are observed in most compounds, which exceed ∼50 kOe in the two Sm based compound.     

A Constitutive Equation for Magnetorheological Fluid Characterization

A microstructural model of the motion of particle pairs in MR fluids is proposed that accounts for both hydrodynamic and magnetic field forces. A fluid constitutive equation is derived, from the model that allows the prediction of velocity and particle structure fields. The analysis is similar to that of bead-spring models of polymeric liquids with replacement of the elastic connector force by a magnetic force. Results for simple shear flow are presented for the case when the two particles remain in close contact so they are hydrodynamically equivalent to an ellipsoid with an aspect ratio of two and only the component of the magnetic force normal to the connecting vector between the centers of the two particles affects motion. The model predicts oscillatory motion of the particle pairs at low magnetic fields. The fluid reaches a steady state at high magnetic fields. The time required to reach the steady state for a given shear rate reduces significantly as the field increases.     

真空退火对镀Al薄膜NdFeB磁体矫顽力和耐蚀性的影响

采用电弧离子镀技术在烧结NdFeB表面沉积一层Al薄膜,并进行真空退火热处理。利用扫描电镜、永磁材料测量系统、电化学工作站和盐雾试验箱等,研究了不同退火工艺对Al/NdFeB试样显微组织、磁性能和耐蚀性的影响。结果表明,随着退火温度的升高,矫顽力先升高后降低,550 ℃退火60 min时矫顽力最高,达到22.41 kOe,较镀态试样增幅7.6%,但耐蚀性有所降低;随着退火时间的增加,矫顽力先降低后升高,550 ℃退火5 min时矫顽力为22.16 kOe,且自腐蚀电流密度较基体NdFeB降低了1~2个数量级。550 ℃退火5 min试样具有较高的矫顽力和较好的耐蚀性能。     

Uneven Evolution of Microstructure,Magnetic Properties and Coercivity Mechanism of Mo-Substituted Nd-Ce-Fe-B Alloys

The purpose of this paper is to study the influence of Mo addition on the phase morphologies,microstructures and magnetic properties of the designated alloys.It is found out that the coercivity Hcj increases unevenly from 12.2 kOe for (Nd0.8Ce0.2)13Fe82B5 to the maximum value of 13.3 kOe for (Nd0.8Ce0.2)13Fe80B5Mo2.The transmission electron microscopy images demonstrate that the grain size decreases with the addition of Mo,which indicates that Mo has grain refinement effect.The correlative analysis gives rise to the conclusion that the coercivity mechanism of the investigated alloys is dominated by pinning type.All in all,the enhancement of the magnetic properties is mainly attributed to the synergistic impact of grain refinement,pinning effects and the microstructural homogenization.The research may shed light on the potential development and application of rare earth-based counterpart magnets.     

Glassy alloy composites for bit-patterned-media

With the aim of developing a novel perpendicular bit-patterned-media, combined process of thermal imprinting of glassy alloy thin film and embedding Co/Pd multilayer was developed. Nano hole array with a hole diameter of 30 nm was successfully formed onto Pd-based glassy alloy thin film by thermal imprinting. Co/Pd multilayer was overlaid on a textured Pd-based glassy alloy thin film. After finishing by sputter etching, prototype bit-patterned-media was prepared. Switching behavior examined under magnetic field of ±10 kOe reveals that the isolated magnetic dots began to switch at 10 kOe. These results suggest that the prototype composite has a strong possibility for the realization of next generation bit-patterned-media with high data density.