No abstract

The Laves phase compound YCo2 is a well-known exchange-enhanced Pauli paramagnet. We report here that, in the nanocrystalline form, this compound interestingly is an itinerant ferromagnet at room temperature with a low coercive field. The magnitude of the saturation moment (about 1μB per formula unit) is large enough to infer that the ferromagnetism is not a surface phenomenon in these nanocrystallites. Since these ferromagnetic nanocrystallites are easy to synthesize with a stable form in air, one can explore applications, particularly where the hysteresis is a disadvantage.

R 2 Fe 17 (R ¼ rare earth) intermetallic compounds constitute one of the most important classes of materials identified as high-energy permanent magnet materials. They crystallize either in the rhombohedral Th 2 Zn 17 structure (for light R) or in the hexagonal Th 2 Ni 17 structure (for heavy R). In this article, we discuss the variations in the lattice parameters (unit cell volume), site occupancies and Curie temperature when non-transition and transition metals are substituted for Fe in R 2 Fe 17 compounds.

The effect of conventional annealing and Joule annealing on magnetoimpedance (MI) of Co68Fe5Si12B15 ribbons was studied. The ribbons used for the MI measurements were 1 and 5 cm long. The as cast 5-cm-long ribbons exhibit a larger MI compared with the 1-cm-long ribbons, at all frequencies. The maximum MI of the as cast ribbons of 5 cm length is 17% at 600 kHz whereas for the 1-cm-long ribbons it is 3% at 500 kHz. Conventional annealing carried out at 200 and 300 °C for 1 h results in the decrease of the MI in both the 5 and 1-cm-long ribbons, respectively. The Joule annealing of the 5-cm-long ribbons employing a current of 500 mA for 5 min. causes the increase of the MI to 26% at 800 kHz. For the 1 cm Joule-annealed ribbon, employing a current of 500 mA for 5 min., the maximum MI observed is 6% at 500 kHz.

We investigated magnetization reversal mechanism in elliptical shaped nanomagnets made from single layer and pseudospin valve Co50Fe50 films. The structures were fabricated using deep ultraviolet lithography and the lift-off process. We observed that the magnetization reversal process of the single layer elements is strongly dependent on the film thickness. For thickness tCoFe=10 nm, the magnetization reversal process is dominated by a systematic coherent rotation, whereas for tCoFe=60 nm, the reversal process is mediated by vortex nucleation, displacement, and annihilation. By exploiting the thickness dependence of the magnetization reversal process, pseudospin valve nanomagnets from two Co50Fe50 thicknesses (10 and 60 nm) were fabricated. We also investigated the effect of interlayer exchange coupling in pseudospin valve structures by varying the Cu spacer layer (tCu). For tCu≤5 nm, the two ferromagnetic layers are found to be strongly coupled by exchange interaction. The strength of the coupling is significantly dependent on temperature. For tCu≥20 nm, the two Co50Fe50 layers are antiferromagnetically coupled at 300 K. As the temperature is reduced below 50 K, we observed a clear transition from antiferromagnetic to ferromagnetic coupling.