The microstructure and the room-temperature hysteretic magnetic properties of sputtered, 10 nm thin films of equiatomic binary alloys of CoPt and FePt were characterized using transmission electron microscopy (TEM) and a superconducting quantum interference device (SQUID) magnetometer. A transformation from an atomically disordered, face-centered-cubic structure to the L10 ordered structure occurred during postdeposition annealing and was characterized using digital analysis of dark-field 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 the SQUID magnetometer. In contrast to the relationship most commonly described in the literature, 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 for both CoPt and FePt. 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.

No abstract

The influence of Cu, Ag, and Au additives on the L1 0 ordering, texture, and grain size of FePt thin films has been examined. Lattice parameter data indicated that Au and Ag additives tended to segregate from FePt, but Cu alloyed with FePt. FePt films with Au or Ag additive showed 1-2 kOe higher coercivity values compared to a pure FePt film after annealing at 450°C and above for 10 min. The addition of at least 20 vol. % Cu to FePt boosted average coercivity values and increased ͑001͒/͑002͒ x-ray peak intensity ratios, suggesting an accelerated L1 0 ordering process for annealing temperatures exceeding 350°C. Decreasing the film thickness promoted ͑001͒ film texture in FePtϩ20% Cu films, but higher annealing temperatures were required to achieve large coercivity. Au and Ag limited the average grain size compared to a pure FePt film. Cu additive increased the average grain size and film roughness.

We show that an in situ Kerr rotation measurement is a very effective technique for the study of antiferromagnetic (AF) ferromagnetic (F) film couples. Magnetic signals can be obtained even in the case where the (AF) is the top layer up to at least 200 A of AF thickness. We have used this in situ approach combined with ion milling to study the thickness dependence of the magnetic properties of MnsoFesoiNisoFe2o systems. We observe that the exchange bias field has a surprisingly sharp onset at a critical thickness of AF -50 A. We show that this is consistent with a simple model and that the magnetic anisotropy of MnFe can be estimated from the observed critical thickness to be -1.35 X 105 erg/cm3. The exchange field showed the predicted proportionality to the inverse of the F thickness from -50 to 400 A. Auger spectroscopy and spin polarized secondary electron emission have been used to rule out gross artifacts due to ion milling.

Dc magnetron sputtered Ni-Mn and Ni-Mn-Cr films are demonstrated to exhibit strong and thermally stable antiferromagnetism, as well as high corrosion resistance. For a 25.2 nm thick 53.3 Ni-46.7 Mn (in atomic percent) film deposited on top of a 28.5 nm thick 81 Ni-19 Fe film, a unidirectional anisotropy field (HUA) of 120.6 Oe is obtained at room temperature after annealing in vacuum. The equivalent interfacial exchange coupling energy (JK) is 0.27 erg/cm2, three times higher than that of bilayer Ni-Fe/50Fe-50Mn films. This strong exchange coupling appears correlated with the presence of an antiferromagnetic θ (NiMn) phase with a CuAu-I-type ordered face-centered-tetragonal structure. The blocking temperature, at which the exchange coupling disappears, is higher than 400 °C. The Cr addition to the Ni-Mn film dilutes the exchange coupling, but the JK for the Cr content ≤10.7 at. %, is still higher than that of the Ni-Fe/Fe-Mn films. Both Ni-Mn and Ni-Mn-Cr films exhibit corrosion behaviors much better than the Fe-Mn film and comparable to the Ni-Fe film. The films are proposed as longitudinal bias layers for the stabilization of magnetoresistive read sensors.