Extremely low equivalent magnetic noise in a Metglas/piezofiber magnetoelectric (ME) magetic‐field sensor, realized through a combination of a giant ME effect and a reduction in constituent internal noise sources, is demonstrated. The ME coefficient is 52 V cm−1 Oe−1 at low frequency, the 1 Hz equivalent magnetic noise is 5.1 pT Hz−1/2, and the magnetic field sensitivity is 10 nT.

The kinetics of the A1 to L10 transformation are given in the form of time-temperature-transformation diagrams. These diagrams are calculated using experimentally measured data for 1μm thick FePt and FeCuPt thin films and three variations of the Johnson-Mehl-Avrami-Kolmogorov kinetic theory. For the FePt alloys, an increase in Pt content from 45.6 to 52.5 at.% results in a >300 times increase in the annealing time needed to reach a fraction transformed of 0.95 at 300°C. Extension of the calculations for 1μm thick films to 10nm thick films using geometric corrections shows not only that thinner films result in slower kinetics for all temperatures, but also that film composition has a significant impact on the transformation behavior. Transformation in nanoparticles is highly dependent on the nucleation of the L10 phase.

Differential scanning calorimetry is used to investigate the A1 to L10 ordering transformation in binary FePt films with compositions in the range of 39.3−55.3 at. % Pt and ternary FePt-based films with additions of Cu up to 20 at. % and Ni up to 25 at. % for a range of Fe/Pt ratios. For FePt and FeCuPt, these are extensions of previously studied ranges [K. Barmak, J. Kim, D. C. Berry, W. N. Hanani, K. Wierman, E. B. Svedberg, and J. K. Howard, J. Appl. Phys. 97, 024902 (2005); D. C. Berry, J. Kim, K. Barmak, K. Wierman, E. B. Svedberg, and J. K. Howard, Scr. Mater. 53, 423 (2005)]. Parameters including the A1 and L10 phase Curie temperatures, the kinetic ordering temperature, the activation energy, and the transformation enthalpy are reported. For FePt, the L10 Curie temperature reaches a maximum of 457 °C near 45 at. % Pt. In contrast, the A1 Curie temperature has an essentially constant value of 300 °C across the full composition. Both the kinetic ordering temperature and activation energy show a minimum near 46 at. % Pt, while the magnitude of the transformation enthalpy has its maximum near 50 at. % Pt. Ni additions to FePt in general lower the L10 Curie temperature, but they increase the A1 Curie temperature. The kinetic ordering temperature and the activation energy increase for all Ni additions, whereas the transformation enthalpy decreases measurably only for Ni additions over ∼10 at. %. Cu additions to FePt act as magnetic diluents regarding both the A1 and L10 phase Curie temperatures, with increased Cu content resulting in decreased Curie temperature. Conversely, Cu additions have no significant effect on the kinetic ordering temperature, activation energy, or transformation enthalpy when compared to equivalent binary alloys. The relevance of our findings to alloy development for ultrahigh density magnetic recording media is discussed, with specific focus on the potential for heat-assisted magnetic recording.

The effect of lamination process on the performance of Metglas/Pb(Zr,Ti)O3‐based composite magnetoelectric (ME) sensors has been studied. Improved sensor processing can lead to higher coupling between the magnetostrictive and piezoelectric phases. A higher coupling factor results in higher magnetic field sensitivity and lower equivalent magnetic noise floors. Spin‐coat/vacuum‐bag application and cure of interfacial epoxy, in addition to the use of Metglas foils with enhanced magnetostriction (λ = 42 ppm), has led to an increase in the ME coefficient from 6 to 21.6 V/cm·Oe, and a decrease in the 1 Hz equivalent magnetic noise from 2 × 10−10 to 4 × 10−11 T/√Hz.

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