I. Maartense

AC susceptibility and resistivity of the ferromagnetic phase of PdMn

AC susceptibility and electrical resistivity measurements have been carried out on a series of dilute PdMn alloys containing between 0.5 and 2.5 at.% Mn. The AC susceptibility taken in various collinear biasing fields exhibits a maximum in the vicinity of the Curie temperature as predicted by the scaling law equation of state. Further, the field dependence of these maxima is uniquely governed by the critical index delta , and estimates for this latter index have been made along the index gamma and the Curie temperature Tc. The electrical resistivity data, taken on the same samples, are in good overall agreement with the results obtained from the susceptibility study.

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.

Mixed ordering in PdMn alloys

The AC susceptibility and electrical resistivity of five PdMn alloys containing between 3.0 and 5.0at%Mn have been measured down to 1.4K. The AC susceptibility, taken in various collinear biasing fields, exhibits a complicated behaviour; at 3 at%Mn the general features of such data resemble those obtained on more dilute alloys of the ferromagnetic phase of this system, but a detailed comparison leads the authors to suggest that the true ferromagnetic phase terminates in the vicinity of 2.5at%Mn. The 4 and 4.5 at% Mn exhibit a double-peaked structure, while the 5 at%Mn sample behaves in a manner reminiscent of a spin glass. The electrical resistivity data can be understood in a similar manner, with the relatively sharp change in slope of the incremental resistivity of the 3 at% Mn alloy close to the ordering temperature evolving into a featureless behaviour at 5 at% Mn, the latter being consistent with a gradual freezing of impurity spins characteristic of spin glasses.

Bubble film characterization by ac susceptibility measurements

The use of low frequency magnetic susceptibility measurements as a means of characterizing bubble materials is illustrated. The parameters derived by this method agree to within a few percent with those obtained independently from optical measurements.

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.

Magnetic ordering in57Fe-doped high-T c superconductors

The high-temperature superconductor, Tl2CaBa2(Cu1−xFex)2O8+δ (the 2122 compound), has been investigated by a number of techniques, including X-ray diffraction, resistance and ac susceptibility measurements, and Mössbauer spectroscopy. The procedures followed to make close to single-phase samples are described. The decrease in the critical temperature for superconductivity,Tc, is less than for the iron-doped 123 compounds. The Mössbauer spectra at 77 K and above consist of an asymmetric doublet. Below about 10 K magnetic hyperfine splitting occurs; relaxation effects are still present at 2.3 K. The spectra can be fitted with two overlapping patterns. Their origin is discussed: comparisons are made with other high-Tc superconductors.

ac susceptibility of intermediate concentration PdMn alloys

We have measured the ac susceptibility and electrical resistivity of a large number of PdMn alloys containing between 0.5 and 10.45 at. % Mn, from which a detailed magnetic phase diagram has been constructed. This phase diagram appears to suggest the possibility of a double transition, i.e., a paramagnetic to ferromagnetic transition followed by a ferromagnetic spin‐glass transition at low temperature, for Mn concentrations between 3 and 5 at. %. A detailed analysis of the ac susceptibility of alloys in this concentration range does not support such implied behavior in this system.

Magnetic characterization of thin bubble films

Standard procedures for evaluating magnetic bubble materials are found to yield (∼9.25 micrometer) films. Several alternative procedures are described which yield values of magnetic parameters to 1% accuracy for thick (≳3 micrometer) bubble films. Application of these new procedures to thin (<3 micrometer) bubble films decreases the accuracy somewhat, but magnetic parameters are consistent within about 2%. The new procedure is described and results are presented for a number of thin films for comparison to the results achieved by the standard procedure.

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.