D. Kaptás

Microscopic study of the magnetic coupling in a nanocrystalline soft magnet

The magnetic behaviour of nanosize ferromagnetic bcc granules embedded in an amorphous tissue (i.e., partially crystallized amorphous alloy) was studied by Mossbauer spectroscopy. The results are compared with the bulk counterparts: bcc-Fe and amorphous . Size dependent enhancement of the Curie point of the nanosize amorphous phase was not observed. At temperatures well above the Curie point of the amorphous phase superparamagnetic relaxation of the bcc crystallites is observed opening new possibilities to study the anisotropy energy of nanosize ferromagnetic grains.

Magnetic properties of FeZr metastable phases

Abstract Amorphous Fe 100− x Zr x (7 x 3 Zr were found in the fully crystallized samples; the Fe 2 Zr intermetallic phase of the ingots was not present.

Structure and magnetic properties of nanocrystalline (Fe1−xCox)90Zr7B2Cu1 (0⩽x⩽0.6)

Nanocrystalline (Fe1−xCox)90Zr7B2Cu1 (0⩽x⩽0.6) alloys were investigated by x-ray diffraction, 57Fe Mossbauer spectroscopy, and magnetic measurements. The grain size did not change significantly with composition. Co was preferentially partitioned to the residual amorphous phase, and the bcc grains were accordingly enriched by Fe. The room-temperature coercive field increased with the Co addition, which is attributed to the increasing magnetostriction constant.

Magnetic and transport properties of Fe-Ag granular multilayers

Results of magnetization, magnetotransport and Mossbauer spectroscopy measurements of sequentially evaporated Fe-Ag granular composites are presented. The strong magnetic scattering of the conduction electrons is reflected in the sublinear temperature dependence of the resistance and in the large negative magnetoresistance. The simultaneous analysis of the magnetic properties and the transport behavior suggests a bimodal grain size distribution. A detailed quantitative description of the unusual features observed in the transport properties is given.

Dipole–dipole interaction in superparamagnetic nanocrystalline Fe63.5Cr10Si13.5B9Cu1Nb3

Cr-substituted Finemet-type nanocrystalline alloy (Fe63.5Cr10Si13.5B9Cu1Nb3) has been studied by differential scanning calorimetry, x-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. The Curie temperature of the remaining amorphous phase decreases as the crystalline volume fraction increases, reaching values below room temperature. This feature makes the alloy adequate for studying the magnetic decoupling of the (Fe,Si) nanocrystals at moderated temperatures and, in particular, the superparamagnetic relaxation in broad temperature and crystalline fraction ranges. It was shown that the anomalous dependence of the coercive field on the annealing temperature can be satisfactorily explained assuming a dipolar-type interaction between the crystallites.

Anomalous magnetic properties of the nano-size residual amorphous phase in nanocrystals

The composition dependence of the Curie temperature and the magnetic moment of the nano-size residual amorphous phase in partially crystallized Fe92-xZr7BxCu1 (2 x 23) amorphous alloys were determined by 57Fe M?ssbauer spectroscopy. Both quantities show broad minima and for increasing relative Zr content (i.e. for decreasing B concentration) surpass the usual monotonic decrease observed for the bulk counterparts. The deviation does not scale with the characteristic size of the residual amorphous regions, which was found to be constant, ruling out explanations based on interphase exchange interaction. A magnetovolume origin of the observed anomalous composition dependence and the improved soft-magnetic characteristics of these nanocrystals is proposed.

Atomic and Magnetic Structure of the Interface in Multilayers

Temperature dependence of the magnetic properties of Fe/Ag vacuum evaporated multilayers was studied in a wide range of layer thickness. For Fe thickness larger than 1 nm continuous magnetic layers can be found, but its hyperfine field is significantly lower than that of pure α-Fe at elevated temperatures. It is attributed to a decrease of the Curie temperature due to Ag impurities in the Fe layer. Below 1 nm Fe thickness magnetic relaxation and the formation of a granular alloy with 35 T average hyperfine field was observed. Magnetoresistance results indicate the presence of Fe clusters in the Ag matrix, as well.

Tailoring Fe∕Ag superparamagnetic composites by multilayer deposition

Fe∕Ag granular multilayers were examined by magnetization and Mossbauer spectroscopy measurements. Very-thin (0.2 nm) discontinuous Fe layers show superparamagnetic properties that can be tailored by the thickness of both the magnetic and the spacer layers. Novel heterostructures, superparamagnetic and ferromagnetic layers stacked in different sequences, were prepared and the specific contribution of the ferromagnetic layers to the low-field magnetic susceptibility was identified.

Inter-grain coupling in nanocrystalline soft magnets

The magnetic coupling between nanosize ferromagnetic bcc crystalline grains embedded in a residual amorphous matrix was studied by Mossbauer spectroscopy in nanocrystalline Fe - Zr - B - Cu alloys studied above the Curie temperature of the amorphous phase. It is shown that the ferromagnetic coupling is mediated via an about two atomic layer thick interface region between the nanocrystallites.

Interface magnetoresistance of Fe/Ag multilayers

Magnetoresistance and magnetic behaviour of a vacuum evaporated multilayer consisting of 1.4 nm Fe and 2 nm Ag layers were studied. The magnetisation of the multilayer shows normal ferromagnetic behaviour while the resistance decreases both in parallel and transversal geometry up to 12 T magnetic field and can be well described by squared Langevin functions. The giant magnetoresistance (GMR) is attributed to a small amount of Fe clusters and/or single impurities in the Ag matrix. This explanation is supported by similar results on a 8 nm Ag/25 nm Fe/8 nm Ag trilayer. The concentration distribution due to interface mixing was studied by Rutherford backscattering spectrometry (RBS) measurements.