The phase transformation and the exchange coupling in (Ndo095Lao005)9.5FebaICOsNb 2BI05 have been investigated. Nanocomposites were obtained by treating amorphous precursors at temperatures ranging from 650TC to 9500C for 10 minutes. The magnetic properties were characterized via the vibrating sample magnetometer (VSM). X-ray diffraction (XRD), thermomagnetic analysis (TMA), and transmission electron microscopy (TEM) were used to perform phase identification, measure grain size, and analyze phase distribution. The strength of the exchange coupling between the magnetically hard and soft phases in the corresponding nanocomposite was analyzed via the AM-versus-H plot. It was found that the remanence (Br), coercivity (Hci), and maximum energy product (BHmax) obtained were affected by the magnetic phases present as well as the grain size of constituent phases and their distribution. The optimal magnetic performance, BHm, occurred between 700°C to 750°C, where the crystallization has completed without excessive grain growth. TMA and TEM indicated that the system was composed of three phases at this point, Nd2(Fe Co) 14B, ca-Fe, and Fe3B. The exchange coupling interaction among these phases was consistently described via the AM-versus-H plot up to 750°C. The Br, Hci, and BHmax degraded severely when the thermal treatment temperature increased from 750°C. This degradation may be attributed to the grain growth of the main phases, from 45 to 68nm, and the development of precipitates, which grew from 5nm at 750°C to 12nm at 850°C. Moreover, the amount of the precipitates was found to increase with the thermal treatment temperatures. The precipitates, presumably borides, may cause a decrease in the amount of the a-Fe and Fe 3B and result in a redistribution of the Co in the nanocomposites. The increase of the Co content in the Nd 2(Fe Co) 14B may explain the increase of its Curie temperature with the thermal treatment temperatures. In this paper, we examine the impacts of these factors on the magnetic properties of (Ndo 95Lao 05)9 5FebaICosNb2B10.5 nanocomposite.

The production of remanence enhanced powders, using a method of mechanical alloying of free iron to melt spun ribbon, has been studied. The final composition is Nd8. 1Fe77.1Co10.9B3.9 and both hard and soft materials possess an initial fine grain size; approximately 30nm for the Nd2Fe14B phase in the melt spun ribbon and 1µm for the α-Fe. The magnetic properties of resin bonded magnets fabricated using such material have been measured and found to exhibit exchangespring behaviour. The experiments performed in this work are described with respect to the volume fraction of the soft phase and subsequent annealing temperature of the as-milled material. Remanence enhancement has been observed and this can be attributed to the exchange interactions between the ferromagnetic soft and hard phases. For a 30% by volume addition of α-Fe, typical values of remanence were found to be ∼880mT with a coercivity of ∼250 kA/m. The reversible and irreversible components of the magnetisation have been determined, to demonstrate the exchange coupling and coercivity behaviours respectively.

Investigation of the interactions in two magnetic composite systems composed of ∼8 vol% isolated Sm2Co17 nanoparticles of different size ranges in a CaO matrix has been made by comparing the isothermal remanent magnetization curves (IRM) with the dc demagnetization curves (DCD) using Wohlfarth remanence relation. The presence of a weak demagnetizing field was evidenced in the sample with smaller magnetic particles (7-90nm), indicating the magnetostatic interaction between the magnetic particles was dominant in this sample. The large positive deviation from Wolhfarth relation in the sample with larger particles (10-250nm) suggests the presence of multigrain particles in which neighboring grains interact strongly with each other. The hysteresis curves of the two composite systems were measured at temperatures between 5 and 350K. A nearly linear temperature dependence of the coercivity was observed in both samples up to 350 K. From these results it is shown that although the sizes of the Sm2Co17 particles are well below the critical size for single domain particle, there is no evidence that the magnetization reversals in these particles are coherent rotation.

In order to obtain directly powders of nanocomposite a Fe/NdFeB alloys with the composition of NdxFebaICO8NbyB, where x: 8.0-8.5, y: 1.5-2.5, z: 6.0-6.5 in at%, have been prepared using the two-step rapid solidification apparatus, which is composed of a gas atomizing system at the upper side and a collision-and-solidificating rotor system at the lower side. It is found that the coercivity (HcJ) of flat-shaped powders obtained depends on powder sizes. The best HcJ in each sieved powder is 525 kA/m (6.60 kOe) for a size more than 500 p m, 449 kA/m (5.64 kOe) for 150-500 pm, and 391 kA/m (4.91 kOe) for 50-150 p m. These values are not so high compared to those of powders of ribbons prepared using a single-roller melt spinning apparatus. This should be attributed to the powder size distribution that causes fluctuation of the quenching rate. Thickness of the flat powder is found to be controllable up to 70/g m, which is useful to get compression molded bonded magnets with a high density and thus a high remanence (Br). An epoxy-bonded magnet with (BH)max: 61.8 kJ/m 3 (7.76 MGOe), Br: 0.732 T (7.32 kG), HcJ: 427 kA/m (5.36 kOe), and density of 6.20 Mg/m 3 is obtained.

We have used Magnetic Force Microscopy (MFM) to study the magnetic domain structures of melt-spun Nd-Fe-B ribbons. The ribbons are commercial products (Magnequench International, Inc. MQP-B and MQP-B+) with a thickness of approximately 20 microns. These materials have identical composition, Nd12.18B5.36Fe76.99Co5.46, but differ in quenching conditions. In order to study the distribution of domain sizes through the ribbon thickness, we have prepared cross-sectional samples in epoxy mounts. In order to avoid artifacts due to tip-sample interactions, we have used high coercivity CoPt coated MFM tips. Our studies show domain sizes typically ranging from 50-200 nm in diameter. This is in agreement with studies of similar materials in which domains were investigated in the plane of the ribbon. We also find that these products differ substantially in mean domain size and in the uniformity of the domain sizes as measured across the ribbon. While the B+ material shows nearly uniform domain sizes throughout the cross section, the B material shows considerably larger domains on one surface, followed by a region in which the domains are smaller than average. This structure is presumably due to the differing quench conditions. The region of coarse domains varies in thickness, disappearing in some areas, and reaching a maximum thickness of 2.75 µm in others. We also describe bulk magnetic measurements, and suggest that.

The coercive field, He, in rapidly quenched Nd-Fe-B was found to increase for samples with excess of Nd. In addition, Hc, strongly depends on the Fe/B ratio, increasing from 1.67 T for Fe/B = 1.07 to 2.05 T for Fe/B = 14.6 in Nd30(Fe,B)70. The increase in Hc seems to correlate with an increase of the γ-Nd phase in the sample. The initial magnetization curves of Nd30(Fe,B)70 show that the domain wall pinning plays a more important role as the Fe/B ratio increases. Thus, γ-Nd may play an important role in promoting a pinning mechanism. The excess of Nd in rapidly quenched Nd-Fe-B samples was also found to promote growth of Nd2Fe14B grains elongated in shape.

Two changes have gone hand in hand during the 20th century. First, applications were conceived that required new or greatly improved materials. Need being the mother of invention, materials were then developed to satisfy the application needs. Permanent magnets are today used in devices that could only have been dreamed about 25 years ago. Who could have foreseen, for example, the advances in data storage that have occurred over the last 15 years? Hard disk drives of under 10 megabytes in size and with random access times of over 65 milliseconds were the norm in 1984. Today, the standard drive is over 8 gigabytes, is smaller in size and costs less than a fourth of the old drives, with an access time of 10 milliseconds and more than 33 mbps burst throughput. Novel has two interpretations: first it may refer to the magnetic material or it may refer to the application of a material. Several examples of each are presented.

A variety of promising permanent magnet structures based upon the magic cylinder and magic mangle concepts are investigated with regard to their suitability for application to electrical generators and motors. Such structures feature high magnetic flux densities in large volumes relative to their masses and bulks as well as minimal stray fields and net magnetic moments. Magic mangles also afford small moments of inertia of their rotating elements thereby resulting in improved rotational responsiveness. One computer calculation showed that a small conventional electric generator can be replaced by a magic mangle with 70% more flux amplitude but with only one half the mass and bulk.

The features of the prototype with NdFeB-magnets will be presented and also the sensorless control strategy will be outlined. Measured and calculated data show good agreement. Focus will be on the magnetic circuit and the thermal behaviour and not on the control aspects of the motor. Specific parameters demonstrate the good quality of the drive system. A thermal analysis was also done in order to meet the requirements given by the medical specialists.

A new series of Sm(CowFevCuxZry)z magnet products with 1Hc up to 13 kOe at 400°C has been developed. These magnets have low temperature coefficients of iHc and a straight line B vs. H (extrinsic) demagnetization curve up to 550°C. A straight line extrinsic curve provides greater design flexibility and facilitates reduced size and weight of magnetic circuits. The straight line extrinsic demagnetization B curves are required for magnets used in dynamic applications. The maximum use temperature, defined here as TM, can be determined by finding the maximum temperature at which the B curve is a straight line. The results of this work show that increasing the Co content in the magnet enhances high temperature performance. A higher Co content provides a lower temperature coefficient β of iHe. Magnets with Co content w = 0.65 have β = 0.26%C from 25°C to 300°C. Magnets with w = 0.82 have β = 0.116%C from 25°C to 300°C. The lower β results in a higher (BH)max at high temperature, and a higher TM. The TM is 240°C for w = 0.65,330°C for w = 0.68, 400°C for w = 0.76, 490°C for w = 0.78, and 550°C for w = 0.82. By increasing the Co content and adjusting the content of other elements, along with appropriate changes of processing parameters, a new series of magnets has been developed for use at temperatures from 400°C to 550°C. Magnetic properties vs. temperature for the new series of magnets, in comparison to those of the best commercial magnets, are shown in this paper to illustrate the improvements which have been made. Magnetic properties at different temperatures vs. Co content for the series are also reported in this paper.

The importance of newly developed permanent magnetic materials in many electro-, magnetomechanical and electronic applications can be attributed to the drastic improvement of the magnetic energy density product and coercive field. A systematic study has been undertaken in order to determine the influence of oxygen content on microstructure and coercivity of high remanence Nd2Fe14B based sintered magnets. The energy density product >400 kJ/m3 (50 MGOe) and the coercive field of 800 kA/m were obtained after a combination of rubber isostatic and transverse die pressing methods. Magnets of the composition Nd151−xFe78+xB6Cu0.03A10.7 [x= 0-2.5] were prepared using strip cast materials. The high oxygen content of the magnets was gradually decreased from values of 4000-6000 ppm to a value < 1000 ppm. Abnormal grain growth (AGG) of the 2:14:1 grains occurred preferentially in magnets with low oxygen content, thus the squareness of the demagnetisation curve drastically decreased. The oxygen content strongly affects the AGG and the magnets with higher oxygen content have the higher critical temperatures at which the AGG occurs. On the other hand, isotropic magnets tend to have the lower critical temperatures than anisotropic magnets by 10-20°C. In the second part of the paper examples of the influence of intergranular phases, grain size and grain shape observed by TEM on coercivity of sintered and melt-spun Nd-Fe-B magnets are compared.

The hysteresis loops of nanocrystalline (nc) permanent magnets (pms) produced by the melt-spin technique have been investigated for compositions based on the intermetallic compounds R2Fe14B (R = Nd, Pr) and the carbides Sm2Fe17−xGaxCy. The following three types of pms have been studied: 1) High-coercivity pins with exchange decoupled grains. 2) High-remanence exchange-spring pms. 3) High-coercive-high-remanence composite pins with exchange coupled soft and hard magnetic grains. The temperature dependence of the coercive field μ0Hc for all three types ofpms obeys a relation for a modified nucleation field, Hc = (2K1/Js)α - Neff Ms (K1 = first anisotropy constant, Ms = spontaneous magnetization). For an analysis of the characteristic differences between the microstructural parameters a and Neff as obtained for the three types of pms, computational micromagnetism on the basis of the Finite Element Technique is applied. This powerful method allows a quantitative analysis of the role of grain size, grain boundaries (gbs), texture of easy directions and of soft magnetic phases in composite materials. In order to obtain satisfactory results, a self-adapting algorithm has been developed where the mesh size is adapted to the gradients of the direction cosines of the spontaneous magnetization. It turns out that excellent magnetic properties of composite pms can only be obtained if the gbs are as ideal as possible. Remanence and coercive field are found to decrease linearly with a corresponding reduction of both, the crystal anisotropy and the exchange constant within the gbs. In composite pins the diameters of the soft magnetic grains should be smaller than twice the domain wall width, of the hard magnetic phase in order to obtain a remarkable remanence enhancement. From these model calculations general rules for the development of optimized nc pms with large remanences and large coercivities are derived.

The microstructure of V stabilized RE3(Fe,V)29 (RE=Nd,Tb) has been investigated by transmission electron microscopy (TEM). The investigated samples were prepared by arc melting and subsequent annealing above 910°C. X-ray diffraction confirms that the samples can be indexed based on a monoclinic Nd3(Fe,Ti)29-type structure (3:29) with A2/m space group. Our TEM investigation confirms that Nd3(Fe,V)29 contains usually grains with 3:29 structure and A 2/m space group. In addition grains with rhombohedral Th2Zn17 structure are observed regularly. The Tb3(FeV)29 alloy consists also of grains with monoclinic Nd3(Fe,Ti)29-type structure and contains a high density of planer defects like crystallographic twins or antiphase boundaries. Twinning occurs preferably on (402) of the monoclinic 3:29 structure. No tetragonal RE(Fe,V)12 phase or Fe is found.

Melt-quenched NdFeB is an important modem permanent magnet material. However there still remains doubt as to the magnetization reversal mechanism which controls coercivity in material prepared by this processing route. To investigate this problem a new technique based on measurements of reversible magnetization along recoil curves has been used. The technique identifies the presence of free domain walls during magnetic reversal. For this study samples of isotropic (MQI), hot pressed (MQII) and die upset (MQIII) melt-quenched NdFeB were examined. The results indicate that in MQI free domain walls are not present during reversal and the reversal mechanism is most likely incoherent rotation of some form. Free domain walls are also not present during reversal in the majority of grains of MQII, even though initial magnetization measurements indicate that the grain size is large enough to support them. In MQIII free domain walls are present during reversal. These results are attributed to the reduced domain wall nucleation field in MQII compared with MQII and the increased dipolar interactions in MQIII.

The effect of chemical order and defect structure on the magnetization process in Sm2Fe17 and Sm2Fe17Nx has been investigated. In Sm2Fe17, chemical disorder results in the development of random anisotropy and a small degree of magnetic hardness. The anisotropy is reduced as long-range order increases. The motion of domain walls in Sm2Fe17Nx is more dependent on the antiphase domain structure than on the amount of long-range order.

The magnetic properties of permanent magnetic nanocomposites are investigated both experimentally and by model calculations. Rare-earth transition-metal films and nanocomposite thin films containing a L10 hard phase (PtFe) have been prepared by plasma sputtering and heat treatment. By choosing suitable heat-treatment processes a nearly ideal nanostructure and high energy products of up to about 50 MGOe have been achieved for the Fe-Pt system. The behavior of the nanocomposites is discussed in terms of a model applicable to nearly ideal magnets, where there are no pronounced shoulders in the hysteresis loops. Analytic expressions for M(H) are obtained for a number of cases, and it is discussed how the loops depend on the micromagnetic parameters of the involved phases. A novel aspect of magnetic nanostructures is that very small magnetic grains exhibit very large energy products, even if they are intrinsically soft-magnetic.

This study relates the microstructure of equiatomic binary alloys of CoPt and FePt with their room-temperature hysteretic magnetic properties, particularly their high coercivity. A transformation from an atomically disordered, face-centered-cubic structure to the Li0 ordered structure occurred during post-deposition annealing and was characterized using digital analysis of dark-field transmission electron microscopy (TEM) images. The transformation was observed to follow first-order nucleation and growth kinetics, and the ordered volume fraction transformed was quantified at numerous points during the transformation. The ordered volume fraction was then compared to the magnetic coercivity data obtained from a superconducting quantum interference device (SQUID) magnetometer. Although the relationship most commonly described in the literature is that the highest coercivity corresponds to a two phase ordered/disordered mixture, the maximum value for coercivity in this study was found to correspond to the fully ordered state. Furthermore, in samples that were less than fully ordered, a direct relationship between ordered volume fraction and coercivity was observed. The proposed mechanism for the high coercivity in these films is an increasing density of magnetic domain wall pinning sites concurrent with an increasing fraction of ordered phase.

Thin film CoPt/Co bilayers have been prepared as a model system to investigate the relationship between microstructure and exchange coupling in two-phase hard/soft composite magnets. CoPt films, with a thickness of 25 nm, were sputter-deposited from a nearly equiatomic alloy target onto oxidized Si wafers. The films were subsequently annealed at 700°C and fully transformed from the FCC phase to the magnetically hard, ordered L1 0 phase. The coercivity of the films increased rapidly with annealing time until it reached a plateau at approximately 9.5 kOe. Fully-ordered CoPt films were then used as substrates for deposition of Co layers, with thicknesses in the range of 2.8-225 nm, in order to produce the hard/soft composite bilayers. As predicted by theory, the magnetic coherency between the soft Co phase and the hard, ordered CoPt phase decreased as the thickness of the soft phase increased. This decrease in coupling was clearly seen in the magnetic hysteresis loops of the bilayers. At small thicknesses of Co (a few nanometers), the shape of the loop was one of a uniform material showing no indication of the presence of two phases with extremely different coercivities. At larger Co thicknesses, constricted loops, i.e., ones showing the presence of a mixture of two ferromagnetic phases of different hardnesses, were obtained. The magnetic exchange present in the bilayer samples was qualitatively analyzed using magnetic recoil curves and the dependence of exchange coupling on the soft phase dimension in the bilayer hard/soft composite magnet films is discussed.

Quad-crystal Co/NiAl/Ag thin films were epitaxially grown on hydrofluoric acid etched Si(111) single crystal substrates by sputter deposition. The orientation relationship was studied by x-ray 0/20 diffraction and pole figure Φ-scan, and it was determined to be Co(1011) ‖ NiAl(110) ‖ Ag(111) ‖ Si(111). The Ag layer contains two twin-related orientations of grains. NiAl grows with three variants of grains on the Ag(111) layer. Since Co(1011) contains grains with four possible easy axis directions when grown on each NiAl variant, the quad-crystal media consist of grains with twelve easy axis directions and exhibit nearly isotropic in-plane magnetic properties. Meanwhile, uni-crystal Co(1010)/NiAl(112) films were also epitaxially grown on Ag(110)/HF-Si(110) templates. The orientation relationships determined from these epitaxial films were very useful in understanding the texture evolution of the Co alloys on NiAl. It indicates that the Co (1010) texture evolution on the (110) texture of NiAl on amorphous substrates, such as NiP/AlMg or glass, is due to a difficult to observe NiAI (112) texture on the top of the (110) texture.

We have applied the Neel's spin pair potential model to model the dependence of magnetocrystalline anisotropy on the long range order parameter. We have produced CoPt nanoscale thin films, with and without an immiscible second phase, by RF and DC magnetron sputtering. These thin films showed a shoulder in their magnetic hysteresis, indicative of a disordered soft magnetic phase for samples annealed above 600 °C while it was barely detectable in 500-600 °C annealed samples. A (111) texture is observed in as-deposited and annealed films is identified as one of the possible contributions to the lower coercivity squareness S* and perpendicular magnetic properties. Micromagnetic calculations confirmed a poor exchange interaction, identified rotational energy barriers and inferred that the ordered region was not fully ordered. The initial stage of ordering clearly showed a discontinuous type transformation.