C. W. Searle

Magnetic transition temperatures of ion‐implanted layers on bubble garnet films

By means of susceptibility measurements, it has been found that the magnetic ordering temperatures of ion‐implanted layers are depressed relative to those of the original garnet films. Implantation of 1015 Ne+ ions/cm2 at 100 keV brings the ordering temperature of the layer down to ∼30 °C, in the film composition which was investigated. Such behavior explains why implanted films can become ’’nonmagnetic’’ near room temperature, as is reported in the literature. Thus it appears unnecessary to invoke an overdamage mechanism which is qualitatively different than that of implanted layers normally used for hard bubble suppression. Annealing raises a layer’s transition temperature, as expected, but high‐temperature annealing results in an anomalous behavior which suggests that healing of the radiation damage does not restore a low‐damage structure.

Permanently stored echoes in powdered materials

A permanent echo following a two‐pulse write sequence which has been observed from piezoelectric powders is also observed in other types of powders. It is shown that a simple model in which the echo originates from the mechanical alignment of particles is consistent with all known echo properties.

Effects of dosage and annealing on the magnetic behavior of ion‐implanted garnet films

The temperature dependence of the anisotropy is obtained from ac susceptibility measurements of bubble garnet implanted with 100 keV Ne+ ions. The dose‐ and annealing dependent behavior is examined, specifically the effects resulting from shifts in the layer’s ordering temperature, such as early indications of overdamage and increases in the anisotropy after annealing.

Susceptibility measurements on the ion‐implanted layer of bubble garnet films

Measurements in the plane of bubble films show a sharp maximum in the ac susceptibility of implanted layers when the underlying film is saturated. The nature of this peak and its behavior as a function of bias field and etching carry information about the layer’s uniformity and anisotropy as well as the film’s domain collapse and stripe propagation fields and coercivity.

Temperature dependence of the coercive field of SmCo5 at high temperatures

The coercive fields of SmCo5 single crystals and sintered permanent magnets are compared with a simple model for the magnetization reversal process. It is found that a consideration of microscopic variations in the exchange energy is required to understand the magnetization reversal at high temperatures.

Characterization of domain‐wall nucleation sites in SmCo5 single crystals

Domain‐wall nucleation fields in several single crystals of SmCo5 have been measured. These results, together with a model for magnetization reversal, were used to determine the domain‐wall gradients and local demagnetizing factors at specific nucleation sites. Exposing the samples to hydrogen led to a degradation of permanent magnet properties.

Temperature and interface effects on the cubic anisotropy of ion‐implanted bubble garnets

The effective cubic anisotropy field of garnet films implanted with 100‐keV Ne+ ions has been obtained from ac susceptibility measurements. When normalized to the layer’s ordering temperature, the anisotropy field is constant for doses up to 2×1014 Ne+/cm2. An abrupt change in the magnetic state is inferred from the results for higher doses. The measured anisotropies are also strongly affected by the coupling‐in of the bulk parameter through exchange at the layer/bulk interface.

Domain‐wall transitions in magnetic bubble garnet films

In‐plane ac susceptibility measurements on bubble film garnets containing stripe domains reveal sharp events which are interpreted as the formation and annihilation of horizontal Bloch lines. By etching the film it is found that the critical field for these events varies inversely with film thickness, as expected. Metastable wall structure changes are caused by in‐plane fields above (6–8) Ms.

Magnetomechanically excited echoes in ferrites

Echoes following a two‐rf‐pulse excitation were observed from bulk and powder samples of polycrystalline ferrites. The echoes originate in refocusing of the elastic modes of the sample, excited by the magnetoelastic interaction.

Abstract: Cubic anisotropy measurements of ion‐implanted layers on magnetic bubble garnets

Effective cubic anisotropy fields of 100 keV Ne+ implanted layers on (111) garnet films can be directly measured by monitoring the in‐plane ac susceptibility or loss while a dc in‐plane field is varied along the projection of a cubic hard axis. The effective field is defined by the onset of irreversibility in the in‐plane rotation of the magnetization. In the presence of various perpendicular bias fields, the effective field boundary can be plotted for magnetization directions from ∼10° to 90° out‐of‐plane, with the 90° orientation defining the uniaxial anisotropy field, as reported previously. The effective fields have been measured as a function of temperature for (YSmLuCa)3(FeGe)5O12 films subjected to several implantation dosages and annealing temperatures. The measurement technique is described in an accompanying paper. In a uniform layer, the in‐plane effective cubic field should vary as sin ϑM cos2 ϑM, with the magnetization angle given approximately by sin ϑM = Hb/ Hku, where Hb is the applied bia...