Anit K. Giri

Coercivity of Fe‐SiO2 nanocomposite materials prepared by ball milling

Samples containing Fe nanoparticles dispersed in SiO2 were prepared by high energy ball milling. We have studied the variation with the milling time of both the saturation magnetization and the coercive force of these samples. The rapid increase of coercivity with the decrease of temperature observed in the low temperature range suggested the presence of superparamagnetic particles in the samples. Nevertheless, our experimental data were in poor agreement with the well‐known T1/2 law describing the coercive force of superparamagnetic particles. Coercivities as high as 540 and 850 Oe were obtained for samples with x=0.3 at room temperature and at 1.7 K, respectively. From the analysis of the temperature dependence of the saturation magnetization the spin wave stiffness constant was obtained. This quantity evidenced the enhancement of the thermal demagnetization associated with the reduction in size.

Time dependent properties of iron nanoparticles

The magnetic relaxation and AC response of dilute dispersions of monodisperse Fe nanoparticles are reported. Magnetic relaxation shows the effect of magnetic alignment on the energy barrier distribution.

Crystallization by ball milling: a way to produce soft magnetic materials in powdered form

We have produced by high energy ball milling a nanocrystalline material in powdered form. The starting material was amorphous melt spun Fe/sub 64.5/Co/sub 18/Si/sub 1/B/sub 16/C/sub 0.5/. From the results of a systematic study of the dependence of the saturation coercive force on the annealing time and temperature it was evidenced the possibility of achieving coercivities as low as 0.150/spl plusmn/0.004 Oe and therefore comparable with those measured in the as-quenched highly magnetostrictive melt spun samples of similar composition.

Soft magnetic properties of Fe-B prepared by mechanical alloying

Fe{sub 100{minus}x}B{sub x} (with x = 20, 25 and 50) samples were prepared by high energy ball milling. The phase distribution, thermal stability, saturation magnetization and coercive force of these samples were examined by means of X-ray diffraction, differential scanning calorimetry and vibrating sample and extraction magnetometries, respectively. An excellent soft magnetic behavior, characterized by coercive forces of the order to 2.5 {times} 10{sup {minus}5} T, was detected in the case of the Fe{sub 50}B{sub 50} samples milled during times from 150 up to 200 hours. This soft magnetic behavior was correlated to the presence of an amorphous phase in the as-milled sample with composition close to the nominal one.

Magnetic properties of mechanically alloyed amorphous Fe50B50

Abstract Samples with nominal composition Fe 50 B 50 were prepared by high energy ball milling. For milling times in the range from 40 to 140 h the milling product consisted of FeB and Fe 2 B (in addition to α-Fe) whereas a majority amorphous phase was formed in the samples milled for 200 h which displayed remarkably low coercive force values.

Experimental Comparison and Analysis of Coercivity Models

Results corresponding to the temperature dependence of the coercive force of Ba hexaferrite samples are discussed in the framework of two different coercivity models based, respectively, on considerations about the coupling between the magnetization and the microstructure (micromagnetic model) and on the conditions of propagation of a reversed magnetization nucleus (global model). We conclude that, in the case of our samples, upon modifying the global model to account for the temperature evolution of the shape of the reversed nucleus, equivalent information about the local anisotropy at the site from which magnetization reversal proceeds can be obtained from both models.

On the relationships between the temperature dependence of the magnetization and the average grain size in nanostructured samples

We report on the low temperature thermal demagnetization behavior observed in samples of the Vitrovac 7600TM commercial amorphous alloy which were submitted to different crystallization treatments, all of them resulting in the induction of nanostructures. Our experimental data were analyzed in terms of the predictions of different models of ferromagnetism allowing us to conclude that our samples behaved as weak ferromagnets with nanostructure dependent spin wave spectra and a significant contribution to the demagnetization processes from single particle excitations.

Low‐temperature Mössbauer study of the mechanically alloyed FexMn0.7−xAl0.3 (0.4≤x≤0.5) series

Results are presented concerning the magnetic properties at low temperatures of mechanically alloyed samples of the Fe x Mn0.7−x Al0.3 (0.4≤x≤0.5) series. According to these results the samples were characterized as having a broad spectrum of relaxation times related to the occurrence of clustering. Also, in the particular case of the x=0.40 sample, we were able to detect a reentrant spin glass freezing transition.

Thermally activated demagnetization in Fe-SiO2 granular solids

(SiO{sub 2}){sub 1{minus}x}Fe{sub x} samples, with 0.2{le}x{le}0.4, were prepared by high energy ball milling. The authors studies in these samples the temperature dependence of the magnetization, coercive force and thermally activated demagnetization parameters. From all these results it was concluded the occurrence, in all the examined samples, of two different maxima in the blocking temperature distributions, identified as corresponding to the extremes of the particle size distribution.

Coercivity relaxation in mechanically alloyed amorphous Fe/sub 50/B/sub 50/ samples

Samples of composition Fe/sub 50/B/sub 50/ have been prepared by high energy ball milling. The phase distribution of the as-milled material included a majority amorphous phase, although that distribution depended on the degree of mechanical hardness of the balls used during the preparation process (crystalline phases were detected in the samples alloyed using regular, non-hardened balls). The hysteretic properties depended also on the type of balls used, the as-milled coercive force being smaller in the case of the samples prepared with harder balls. The measurement of this parameter in samples prepared using non-hardened balls and submitted to thermal treatments evidenced the possibility of reducing significantly the as-milled coercive force. The observation of the same phase distribution (and average grain size of the crystalline phase) present in both the as-milled and the annealed samples led us to conclude that a combination of stress relaxation and of improvement of the interphase coupling could be responsible for the enhancement of the soft properties of the samples resulting from the treatments.