ALBERT

Notes: Determination of the quantitative relationship between crystallographic texture and magnetic properties in advanced permanent magnets may be hampered by complex microstructures, which complicate methods that rely on diffraction, or by interparticulate interactions, which adversely affect methods based on magnetic remanence measurements. To this end, new techniques in the determination of texture of bulk permanent magnets are being explored to overcome these inherent experimental difficulties. The analysis of inverse paramagnetic susceptibility measurements constitutes a new method to investigate crystallographic texture. Such measurements also provide Curie temperature data, which are sensitive to chemical changes that may have occurred in the magnetic phase during processing. The mathematical formalism underlying the analysis of inverse susceptibility measurements is outlined, and is used to evaluate magnetic measurements taken from a series of Nd2Fe14B magnets that have been processed by different means, and thus contain different degrees of texture. While this method does provide qualitative information concerning the relative crystallographic alignment of magnet samples, it needs calibration to obtain an explicit value for a texture order parameter. © 1997 American Institute of Physics.

Notes: Quantitative evidence of a ferromagnetic Fe-rich minor phase present in both melt-quenched/die-upset and sintered magnets based on the Nd2Fe14B composition is presented. Full hysteresis loops were obtained at elevated temperatures (650 K≤T≤800 K) and were subsequently decomposed to obtain the saturation magnetization Ms of the minor phase as well as the paramagnetic slope of the 2-14-1 major phase as a function of temperature. Assuming a composition of pure iron, the calculated volume % of the impurity phase in the die-upset magnets is consistent with that obtained from previous observations of the widths and geometry of the intergranular phase found in the same magnets. The magnetization of the ferromagnetic impurity phase in the melt-quenched magnets decreases more rapidly than that of pure iron; extrapolation indicates a Curie temperature in the range 925 K≤T≤975 K. The paramagnetic susceptibility of the Nd2Fe14B main phases exhibits Curie–Weiss behavior with the same paramagnetic Curie temperature for both sintered and die-upset samples. The Curie constants differ, however, probably due to the different degrees of crystallographic alignment. © 1996 American Institute of Physics.

Notes: Melt-quenched and thermomechanically deformed samples with the nominal compositions Pr18Co82, Pr18Co81C, and Pr18Co76C6 were examined with optical and electron microscopy, differential thermal analysis, and superconducting quantum interference device magnetometry that was performed in the temperature range 20 K≤T≤300 K. At room temperature, Pr18Co82 exhibits poor coercivity and remanence, 600 Oe and 2.9 kG, respectively. Pr18Co81C exhibits relatively superior remanence, 7.7 kG, but poor coercivity, 2.7 kOe, while Pr18Co76C6 exhibits the opposite trend, BR=4.5 kG and Hci=12 kOe. The main phase present in all samples, PrCo5, has basically the same character and morphology for all three samples and shows no evidence of intragranular carbon, which is demonstrated to reside in the impurity phases. The superior coercivity found in Pr18Co76C6 is attributed to a previously unknown triclinic phase, Pr3Co4Cx (x≈3–4) that appears to undergo a magnetic–nonmagnetic transition with increasing temperature around T=80 K. The variety of magnetic properties exhibited by each sample is due to the variety of minor phases present in each sample, which may be a product of the effect that carbon has on the solidification rate of the parent alloy. © 1996 American Institute of Physics.

Notes: It is known that the addition of alloying elements to the Nd2Fe14B-based alloys significantly inhibits the onset of the crystallization in particles produced by the inert gas atomization (IGA) process. Transmission electron microscopy (TEM) was performed on a selection of as-atomized particles synthesized by IGA methods with the addition of 3 wt % TiC. TEM samples were successfully prepared using the wedge technique. Large particles (〉10 μm) tend to consist of well-defined grains of Nd2Fe14B while the submicron size particles are generally amorphous and particles with diameter d in the size range, 1 μm〈d〈10 μm, contain nanocrystallites dispersed evenly throughout the amorphous matrix. The selected area electron diffraction pattern of the nanocrystallites in powder particles with diameters below 10 μm generally show three or more diffused diffraction rings that are indicative of α-Fe, with a particulate size of several tens of angstroms. These observations demonstrate that while the alloying addition of Ti and C in the IGA process significantly suppresses the formation of properitectic α-Fe in the Nd2Fe14B alloy, they do not completely prevent it. © 1997 American Institute of Physics.

Notes: The Jiles–Atherton theory is based on considerations of the dependence of energy dissipation within a magnetic material resulting from changes in its magnetization. The algorithm based on the theory yields five computed model parameters, MS, a, α, k, and c, which represent the saturation magnetization, the effective domain density, the mean exchange coupling between the effective domains, the flexibility of domain walls and energy-dissipative features in the microstructure, respectively. Model parameters were calculated from the algorithm and linked with the physical attributes of a set of three related melt-quenched permanent magnets based on the Nd2Fe14B composition. Measured magnetic parameters were used as inputs into the model to reproduce the experimental hysteresis curves. The results show that two of the calculated parameters, the saturation magnetization MS and the effective coercivity k, agree well with their directly determined analogs. The calculated a and α parameters provide support for the concept of increased intergranular exchange coupling upon die upsetting, and decreased intergranular exchange coupling with the addition of gallium. © 1996 American Institute of Physics.

Notes: In order to investigate factors affecting coercivity a series of two-phase Nd2Fe14B-based nanocomposite alloys with different excess iron concentrations were produced by melt-spinning methods. The constituent grain size was estimated by diffraction methods to be in the range of 150–500 Å, and room-temperature demagnetization curves verify that the alloys studied exhibit a modest remanence enhancement. Isothermal remanence magnetization (IRM) and dc demagnetization (DCD) measurements performed at temperatures in the range 275 K≤T≤350 K illustrate that the coercivity and irreversible magnetization develop in a bimodal, incoherent manner from a demagnetized state but upon demagnetization from a saturated state the system evinces collective, exchange-coupled behavior as illustrated by the reversible magnetization Mrev. The temperature dependencies and values of the irreversible susceptibility χirr (DCD) suggest that a moderating phase with a magnetic anisotropy intermediate to the two constituent main phases may be present in the alloys. © 1997 American Institute of Physics.

Notes: The stray flux manifestations of surface magnetic domains found in as-grown Nd2Fe14B single crystals were observed, with a resolution in the range of 1 μm, using conventional scanning electron microscopy (SEM) without instrumental modifications. A modified image-distortion mode was applied to image the three-dimensional stray flux emanating from the sample. The simplicity of the technique and the ready adaptability of the SEM to such modifications as in situ current and magnetic field application suggest that the results of this study may be extended to investigations of other materials of technological interest, such as perpendicular media disks. © 1998 American Institute of Physics.

Notes: To elucidate the interphase interactions inherent to nanocomposite magnetic alloys, measurements of remanence Br, and coercivity Hci were made on a series of four meltspun, remanence-enhanced nanocomposite alloys consisting solely of various amounts of Nd2Fe14B and α-Fe. The phase constitution and microstructural scale of the alloys were characterized with synchrotron x-ray diffraction. Magnetic measurements were made using superconducting quantum interference device (SQUID) magnetometry on evacuated and encapsulated samples in the temperature range of 300 K≤T≤750 K, in order to characterize the α-Fe component independently of the Nd2Fe14B component. The high-temperature coercivities of the samples increase with the amount of α-Fe present in the samples, ranging from an average value of approximately 75 Oe for the sample with 14 wt % excess Fe to over 400 Oe at 700 K for the sample with 27 wt % excess Fe. The relatively high coercivities of the samples found at elevated temperatures imply that a tabular morphology of the α-Fe grains is conferring anisotropy to the phase; this conclusion is supported by transmission electron microscopy. It is concluded that while the significant coercivity of the α-Fe phase likely reduces the room-temperature remanence enhancement of the alloy below its theoretical ideal, the favorable interphase interface orientation promotes exchange coupling. © 1998 American Institute of Physics.

Notes: Quantifying the relationship between crystallographic texture and magnetic properties is highly desirable for engineering high-energy-product magnets. Such an undertaking is a challenge for melt-quenched magnets formed from Nd2Fe14B-based alloys because they possess a nanoscale microstructure complicated by the presence of multiple phases, both ferromagnetic and paramagnetic. Procedures for the evaluation of texture in permanent magnets often rely upon magnetic remanence measurements; however, such determinations are based on the use of the Stoner–Wohlfarth model, and are applicable only to assemblies of noninteracting particles, which nullifies their use in texture determinations of "exchange-spring" magnets. To overcome these inherent experimental difficulties, alternative techniques for the measurement of the bulk texture of permanent magnets are explored in this work. Crystallographic alignment was investigated in both neutron and hard x-ray diffraction experiments. Transmission synchrotron x-ray diffraction measurements of the rocking curve width as a function of both the vertical and horizontal positions were performed on a series of thermomechanically processed, melt-quenched magnets based on the Nd2Fe14B composition. The results of the hard x-ray diffraction experiments were verified by neutron diffraction work. A simple model based on a squared-Lorentzian distribution is used to obtain a quantitative estimation of the population of misoriented grains for a given rocking curve half-width. It is deduced that even the most deformed magnets still possess a significant degree of particle misalignment; over 50% of the constituent grains have a misorientation of at least 15° from the overall axial direction at the center of the magnet. The spatial distribution of the rocking curve widths indicates that the development of the c-axis texture is initiated in the center region of the magnet and progresses to the perimeter with increasing deformation. Samples with lower deformation levels possess more uneven texture distributions than do those with higher deformation levels, and the degree of orientation seems to develop rapidly near a deformation level of 50%. The results of this work are utilized to produce a phenomenological model, consistent with the findings of past researchers, of deformation based on mass transport under applied stress via a liquid grain-boundary phase. It is proposed that the micromagnetic structure consists primarily of relatively large clusters of exchange-coupled grains that result in lower-coercivity areas within the magnet. Recommendations are included for improving the production and processing of thermomechanically deformed melt-quenched magnets. © 1997 American Institute of Physics.

Notes: Magnetic nanocomposite alloys developed from amorphous precursors by annealing often exhibit metastable transition phases that ultimately control the kinetics of the nucleation and growth of the desired RE2Fe14B-type phase. It is thus important to understand the effect of starting composition on the formation of the metastable transition phases found in the nanocomposite system. The relationships between composition and the nature of metastable phase formation in the low-boron content melt-spun nanocomposite alloy Nd2[Co0.06(Fe1−xCrx)0.94]23.2B1.48 (0≤x〈0.09) were studied and compared with results of other authors. The crystallization sequence undertaken by the quenched samples during annealing was followed by differential thermal analysis and x-ray diffraction. Differences in the nominal starting boron:RE compositional ratio of the alloy may produce significant changes in the nature and stability of the metastable phases present in the system. In general, a high compositional ratio (B:RE(approximate)1–4) promotes the initial formation of Fe3B and complex boron-rich intermetallic phases such as the cubic Nd2Fe23B3, the hexagonal NdFe12B6 and the cubic Y3Fe62B14-type phases in the quenched material, while a low compositional ratio (B:RE〈0.5) is favors the formation of α-Fe and a 2-17-type phase. It is suggested that the focus of further studies done to optimize the performance of these nanocomposite alloys should be on the effect that alloying additions have upon the metastable phases, and not on the final equilibrium phases. © 1999 American Institute of Physics.