W. Morgan

Spin reorientation in single-crystal CoFe2O4 thin films

Reorientation of magnetic anisotropy has been observed in single-crystal CoFe2O4 thin films deposited on (100)MgO substrate by pulsed laser deposition (PLD). The as-grown film exhibits a perpendicular anisotropy whereas after annealing the magnetization easy axis switches to be parallel to the film plane. The origin of such spin reorientation is explained in terms of competition between stress and magnetocrystalline anisotropies. The as-prepared film is under tensile stress, which induces a huge perpendicular uniaxial anisotropy dominating the in-plane magnetocrystalline component. Annealing releases the stress by relaxing the film lattice. Consequently, the perpendicular stress anisotropy is considerably reduced andmagnetocrystalline anisotropy prevails, leading to an in-plane alignment of the easy axis.

Mössbauer studies on LaNi4.7Sn0.3 and its hydride

Mossbauer measurements were made on LaNi4.7Sn0.3 at room and liquid nitrogen temperatures. Experimental data yielded a singlet at room temperature and a doublet at liquid nitrogen temperature. Spectra of the hydrided sample yielded a doublet at liquid nitrogen temperature with an increase in the quadrupole splitting compared to the unhydrided spectra, but no change in the isomer shift. These data indicate that there is no significant interaction between the tin and the hydrogen. The magnetic character remained the same down to liquid nitrogen temperature as evidenced by the Mossbauer data. X‐ray diffraction measurements on the hydride showed an expanded lattice with the same structure as found for the unhydrided sample. A decrease in particle size was observed upon hydriding.

The power of torque magnetometry: defect induced switching in hexaferrite nano-structures

The presence of magnetic heterogeneities in ferromagnetic systems can reduce their performances in applications such as data storage media and permanent magnets. The detection and the full characterization of these heterogeneities is difficult especially when they are present in a very low concentration within ferromagnetic systems. Here, torque magnetometry is applied to investigate the magnetism of heterogeneities in two-dimensional hexaferrite structures. The study was conducted on a two-dimensional BaFe₁₂O₁₉ structure with decoupled nano-platelets grown on oxidized silicon buffered with ZnO. Measured torque curves reveal anomalous effects manifested by the presence of reversible and hysteretic kinks at large and low magnetic fields respectively. These kinks represent the contribution of magnetic heterogeneities to the global anisotropy of the film in addition to the two-fold symmetry of the major perpendicular anisotropy component. The heterogeneities consist of two types of nano-metric crystallites with the same magnitude of anisotropy as the major magnetic phase, but tilted about -74° and 74° from the normal to the film plane. These results are supported by various types of material characterization (SEM, XPS, XRD, MFM and VSM) and are well reproduced with a simple theoretical model that replicates the symmetry, switching and the easy axes alignment of the heterogeneities magnetization.

Magnetization reversal in epitaxial highly anisotropic CoFe2O4 hetero-structures

Magnetization reversal in epitaxial cobalt ferrite films grown on (110) MgO substrate has been investigated through angular studies of hysteresis loop and DCD remanence curve. The angular dependences of the coercivity and the switching field of these films strongly deviate from those typically known for coherent and incoherent rotation modes. However, the best fit for these angular curves suggests that domain wall nucleation is the appropriate mode for the magnetization reversal in these hetero-structures.

Irreversible magnetic processes under biaxial and uniaxial magnetic anisotropies

Irreversible magnetic processes have been investigated in magnetic systems with two different anisotropy symmetries (uniaxial and biaxial) through angular measurement of the switching field, the irreversible susceptibility and the magnetic viscosity. These two systems consist of two-dimensional cobalt ferrite hetero-structures epitaxially grown on (100) and (110) MgO substrate. It is found that for uniaxial anisotropy the irreversible characteristics of the magnetization are large and display a strong angular dependence, which exhibits its maximum at the easy axis and drops quickly to vanish at the hard axis. However, for biaxial anisotropy the magnetization irreversible characteristics are considerably reduced and are less sensitive to the field angle.

Static magnetism and thermal switching in randomly oriented L10 FePt thin films

Static magnetism and thermally activated magnetic relaxation were investigated in granular FePt films (20 nm-200 nm thick) with random magnetic anisotropy through hysteresis loop, torque curve and magnetization time dependence measurements. While the magnetism of thicker film (200 nm thick) is dominated by a single switching of the ordered L10 phase, thinner film (20 nm) displays a double switching, which is indicative of the presence of the disordered cubic phase. The pronounced behavior of double switching in thinner film suggests that the film grain boundary is composed of soft cubic magnetic phase. The magnetic relaxation study reveals that magnetic viscosity S of the films is strongly dependent on the external applied field and exhibits a maximum value (12 kAm) around the switching field and a vanishing behavior at low (1 kOe) and large (12 kOe) fields. The activation volume of the thermal switching was found to be much smaller than the physical volume of the granular structure due to the incoherent ...

Magnetic and structural properties of MMNi4.15Fe0.85 and its hydride (abstract)

Mischmetal (MM) has been considered as a possible substitute for the rare‐earth component in both permanent magnets1,2 and hydrogen storage intermetallics. We have investigated the magnetic and structural properties of a mischmetal‐nickel‐iron intermetallic and its hydride using magnetization, Mossbauer, x‐ray diffraction, and SEM measurements. Magnetic susceptibility measurements were made between room and liquid‐nitrogen temperatures. There was a change in the magnetic character of the unhydrided sample at 140 K. Hydriding produced a large increase in the susceptibility with a corresponding change in the magnetization at low temperatures. The 57Fe Mossbauer spectra of the intermetallic yielded a doublet at room temperature and demonstrated a change in magnetic behavior similar to the magnetization results at low temperatures. Experimental data were used to interpret the hydrogen absorption mechanism acting in this intermetallic and the results are discussed with respect to previously proposed models for...