MnBi forms peritectically at ∼450 °C. Preparation of MnBi employing conventional techniques such as arc melting and induction melting results in the segregation of manganese. In order to avoid this segregation, we followed the procedure recommended by Guo et al. [X. Guo, A. Zaluska, Z. Altounian, and J. O. Strom-Olsen, J. Mater. Res. 5, 2646 (1990)] and prepared a low-temperature phase of MnBi by melt spinning, followed by heat treatment. Fine powder of MnBi was prepared by ball milling the melt-spun ribbons for various lengths of time. Magnetic properties of these powders were determined. In particular, the temperature dependent coercivity was studied from room temperature to 360 °C for the powders ball milled for 2 and 10 h. The coercivity is found to increase with the increase in temperature reaching a maximum of 25.8 kOe at 280 °C and then decrease as the temperature is increased further. We also found that a peak in coercivity is observed for the samples milled for 10 h. MnBi shows a first-order transition to a paramagnetic phase at 360 °C. In an attempt to increase this transition temperature, an alloy of composition Mn0.75Ni0.25Bi0.5Sb0.5 was made by induction melting. The transition temperature increases from 360 °C for MnBi to 400 °C for Mn0.75Ni0.25Bi0.5Sb0.5.

MnBi crystallizes in a NiAs-type hexagonal crystal structure, exhibits a high uniaxial anisotropy, and is potentially useful as a permanent magnet material. We have examined the effect of partial substitution of Bi with rare earth elements on the magnetic properties of MnBi. MnBi1−xRx (R=Nd, Dy) were prepared by mechanically alloying powders of the constituent elements at liquid nitrogen temperature followed by heat treatment. X-ray diffraction and magnetic measurements were performed on powder samples to characterize the samples. We found that in MnBi1−xNdx, coercivity (at room temperature) increases from 0.7 kOe to 6.6 kOe for x=0.0 and 0.3, respectively. In MnBi1−xDyx the coercivity increases from 0.7 kOe to 7.9 kOe for x=0.0 and 0.3. The increase in coercivity may be in part due to the increase in the crystal field anisotropy as Nd or Dy is introduced and in part due to the finer particle size. A magnet made from MnBi shows coercivity of ∼17 kOe. A very fine particle size is considered to be the reason for this high coercivity.

Alloys with nominal composition LaCo 7 ( = Zr, or Ti and = 0-0.6) were synthesized and characterized in the temperature range of 10-1273 K in fields up to 5 T. The experimental results show that the effects of Zr or Ti doping on the structure, phases present and magnetic properties of the LaCo 7 alloys are different. In the case of Zr, for as-cast alloys, besides the 1-5 (CaCu 5 ) and 1-13 (NaZn 13 ) phases, a new phase with = 1073 1173 K was detected. This new phase is probably in a hexagonal structure and shows a uniaxial anisotropy. After annealing at 1273 K, the 1-5 phase increases and almost dominates the materials when = 0 3 0 5. It shows a strong uniaxial anisotropy with = 95 135 kOe at both 300 K and 10 K. The 's are 838 K, which is almost the same as that of LaCo 5 . In the case of Ti, besides the 1-5 and 1-13 phases, another new phase with Th 2 Zn 17 structure was formed by Ti doping and almost dominates the materials when = 0 3 0 6. This phase exhibits a uniaxial anisotropy with 25 kOe at both 300 K and 10 K. decreases from 998 K to 901 K when the Ti content increases from = 0 25 to = 0.6. Index Terms-CaCu 5 , Hard magnetic properties, La-Co-Zr-Ti alloys, Th 2 Zn 17 structure.

Magnetic and mechanical properties of Fe60Pt40, Fe60.5Pt39.5 and (Fe1−xCox)60.5Pt39.5 bulk alloys prepared by a number of processing techniques have been examined. Processing techniques include induction melting, mechanical milling (at ∼77 K), hot and cold work, and melt extraction. Magnetic properties were determined in the temperature range from 300 to 1100 K using a vibrating sample magnetometer. Melt extracted Fe60.5P39.5 sample appeared to be fully dense and the magnetic properties found to be 4πMs (at 1.5 T)∼1.08 T, Hc∼270.6 kA/m, and (BH)max∼55.7 kJ/m3. Freezer milled Fe60.5Pt39.5 sample (loose powder) showed a saturation induction of 1.33 T, and coercivity of 270.6 kA/m at room temperature. Curie temperature for this sample is found to be 450 °C. For the Fe45.37Co15.13Pt39.5 (loose powder) sample, coercivity increases to 318 kA/m and the Curie temperature increases to 540 °C. Tensile strength was measured for selected samples. It is found that Fe–Pt and (Fe,Co)–Pt magnets are about 5–10 times mechanically stronger than the rare earth based permanent magnets. Preliminary examination of the structural and magnetic properties of these alloys indicates that the (Fe,Co)–Pt bulk alloys are an excellent system to explore exchange coupling mechanism in permanent magnets.

FePt (for 40–60 at. % Fe) exhibits an order–disorder transformation. The disordered phase is face centered cubic and magnetically soft while the ordered phase is tetragonal and shows high magnetic anisotropy. Since the changes in volume between the two phases are small, it is easy for the soft and hard phases to coexist in a uniform manner. Thus, we have an ideal system with which to investigate the basic features of exchange coupled magnets. Bulk Fe0.6Pt0.4 exhibits reasonably large permanent magnetic properties with a maximum energy product of ∼15 MG Oe (120 kJ/m3) without the need for special processes to promote grain alignment. The high energy product is partially a result of the high ratio of remanence to the saturation induction which amounts to 0.68 as opposed to the ratio of 0.5 for an assembly of randomly oriented uniaxial magnets. This enhanced remanence is predicted by the exchange-spring magnet model for a mixture of cubic and uniaxial phases. In order to verify that the high remanence of the FePt magnets is indeed caused by this exchange mechanism and not by a fortuitous formation of magnetic texture during sample preparation, we have prepared cubic samples of Fe0.6Pt0.4 and measured the B–H loops along the three principal directions of the cube using a vibrating sample magnetometer. The hysteresis loops are identical in all three directions, indicating the absence of strong texture in this material. After correction of the demagnetizing factor, the ratio of remanence to saturation induction is 0.66–0.68. This is consistent with that in the exchange coupling model. The highest energy product in our study was found for the as-quenched samples. Further annealing, with the intention of promoting the formation of hard magnetic phases, reduced the coercivity and remanence. This suggests that improved results may be achieved by initial suppression of the formation of the hard phases by modification of the quenching process and alloying. An ideal exchange coupled system could be designed based on such a starting material.