ALBERT

Notes: The change in magnetic properties corresponding to the structural changes that occur during aging at temperatures in the range 500–900 °C have been studied in melt-spun Fe-Mo alloys using magnetometry, x-ray diffraction (XRD), and transmission electron microscopy (TEM). The as-spun ribbons were found to be magnetically soft with a coercivity smaller than 180 Oe. After a heat treatment at temperatures in the range of 600–670 °C, the coercivity was found to increase to 600 Oe and the magnetization decreased. It is suggested that Mo clustering takes place with heat treatment and this may explain the decreased magnetization and increased coercivity of the samples. Mo-rich clusters can act as domain-wall pinning centers and therefore can lead to an increased coercivity.

Notes: Large coercive fields have been obtained in melt-spun Co-Zr(Hf)-B based alloys. The highest coercivities have been observed in Co76 Zr18 B3 Si3 and Co78 Hf16 B3 Si3 samples with values 6.7 and 6.5 kOe, respectively. The saturation magnetization for both samples is estimated to be around 60 emu/g. Thermomagnetic data showed the presence of two phases with the major phase having a Curie temperature at around 450 °C. TEM and SEM studies showed a microstructure consisting of a mixture of two phases with the grain size in the range of 1000–5000 A(ring). The origin of the high coercivity is believed to be due to the fine grain microstructure leading to a single-domain particle-type behavior.

Notes: The magnetic behavior of Co and Fe granular films was studied relative to their host matrix environment (BN, SiO2). The crystal structure of Co and Fe in the as-deposited samples is α-Co (hcp) and α-Fe (bcc) respectively, with particle sizes ranging between 3 and 9 nm. The coercivities measured ranged from a few tens to a few hundreds Oe, with the higher values observed for particles embedded in the oxygen based matrix.

Notes: Giant magnetoresistance (GMR) values were measured in thin films of Fe and Co in Ag. The best GMR results were observed in Ag-rich specimens, with maxima of 25% (30 K) and 14% (20 K) observed in Co20Ag80 and Fe25Ag75, respectively. Magnetic data indicate a spin glasslike behavior in the as-deposited Ag-rich films. The as-made samples have a nanostructure, with a face centered cubic structure. Annealing of the samples over the temperature range of 200 °C to 700 °C led to grain growth and subsequent phase separation of the constituent metals. A summary of the magnetic and electrical transport properties is presented, in relation to the crystal structure and microstructure of the TM-Ag films (TM=Fe,Co).

Notes: The magnetic properties and microstructure of sputtered Co80Ni20 thin films with a thickness in the range of 400–3800 A(ring) have been studied using SQUID magnetometry, x-ray diffraction and transmission electron microscopy (TEM). Both the electron and x-ray diffraction results show that all the films have an hcp structure. As the film thickness decreases the grain size decreases and the c axis is inclined to the film plane. The intrinsic coercivity decreases as the thickness increases when the applied field is parallel to the film plane. The dependence of coercivity on the angle θ between the field direction and the normal to the film plane indicates that the coercivity is due to domain wall pinning. The domain wall structure and pinning force play a major role in the thickness dependence of coercivity for films with thickness D〈1000 A(ring).

Notes: The interparticle interactions of hcp-Co particles embedded in silica (SiO2) and boron nitride (BN) matrices have been studied via isothermal remanence curve measurements. The films were prepared by a combination of dc/rf magnetron sputtering. The strength of the interaction fields was varied as a function of metal content (xv) and temperature. The average particle size was kept below 10 nm for all the samples investigated. The volumetric fraction of Co was varied between 0.035 and 0.66. Henkel plots (normalized Mr vs Md plots) showed that in hcp granular Co the interactions are negative, i.e., they have the tendency to destabilize the saturation state. Switching field distribution (SFD) plots showed a decreased width with increasing xv and temperature. In the low metal volume fraction specimens, magnetizing and demagnetizing SFD curves indicate that the dominant interactions are dipolar in origin due to the fact that they nearly overlap. However, as xv increases, the presence of exchange interactions is also evident. A detailed discussion of these studies will be presented in an effort to determine the interaction mechanisms controlling the magnetic behavior of these films as compared to existing theoretical models.

Notes: We report an unusual magnetic behavior in powders prepared by spark-eroding (in liquid Ar) alloy electrodes containing approximately equal weights of Fe and Nd2Fe14B in an effort to prepare composite permanent magnets. Magnetization exhibits reproducible thermal hysteresis, peaking in all applied fields near 520 °C when warming, but increasing monotonically when cooling to room temperature from 700 °C and above. Mössbauer spectroscopy was used to show that the behavior is due to the metastability of Fe1−xO produced in the powders by partial oxidation in Ar gas flow. This compound, paramagnetic at room temperature, decomposes only slowly below 570 °C into ferromagnetic Fe and ferrimagnetic Fe3O4. The reverse reaction occurs readily at higher temperatures. © 1996 American Institute of Physics.

Notes: At present Nd2Fe14B is the best permanent magnet because of its extremely high coercivity and energy product. Optimum properties of Nd2Fe14B magnets can be attained by producing single domain particles, and then aligning and compacting them. Due to the high reactivity of the Nd constituent, it is challenging to produce and handle a large amount of fine particles of this material. We have prepared fine particles of Nd2Fe14B by spark erosion with various dielectric media. Yield, size, size distribution, structure, and magnetic properties are discussed. The Nd2Fe14B particles were made by the shaker pot spark erosion method. Relaxation oscillators or a pulse generator were used to power the spark erosion. Commercial Neomax 35 was employed as the primary material. The dielectric media were liquid Ar, Ar gas, and hydrocarbons, which provided an oxygen free environment. Structure and size were studied by TEM, SEM, and x-ray diffraction. Magnetic properties were measured by VSM with temperatures in the range of 4.2–1200 K. The particles produced in these three different dielectric media had different microstructures and crystal structures. The particles made in Ar gas were pure Nd2Fe14B phase. The particles made in liquid Ar were a mixture of amorphous and crystalline Nd2Fe14B, because the liquid Ar provided a much higher quench rate than Ar gas, which produced some amorphous Nd2Fe14B. Upon annealing, the amorphous particles became crystalline. The fine particles produced in hydrocarbons, such as pentane and dodecane, had more complex mixed phases, since the rare earth reacted with the hydrocarbons during the sparking process. The phases were NdC2, α-Fe, and amorphous and crystalline Nd2Fe14B. The effects of power parameters, such as voltage and capacitance, on particle size were investigated. Particle sizes from 20 nm to 50 μm were obtained. We concentrated on Nd2Fe14B fine particles made in liquid Ar and Ar gas. Particles were classified by centrifuging and sizes were confirmed by TEM and x-ray diffraction. The size dependence of coercivity, anisotropy, and magnetization of these Nd2Fe14B particles will be discussed.

Notes: The microstructure of as-deposited granular Ag-rich (75%–90%) Ag-Fe and Ag-Co thin films was investigated by atomic resolution and nanochemical analysis electron microscopy. Our results suggested that the thin films are two separate phases. The poorly crystallized transition metal particles, with size of less than 1.5 nm in the thin films, were directly observed and were found to be separated by nanosized Ag crystalline particles

Notes: Giant magnetoresistance (GMR) has been observed in thin films of Fe and Co in Ag. The best results were obtained in the Ag-rich samples with a maximum value of 25% in Co20Ag80 at 30 K. The GMR values are slightly increased upon annealing. Magnetic data show a magnetic transition below room temperature which may be due to spin-glass type of behavior. These results are consistent with the microstructure studies which showed a single face centered cubic phase, with very fine grains. Upon annealing separation of the two components is achieved with substantial grain growth.