Valquíria Villas-Boas

Reversible processes and magnetic viscosity of nanocrystalline permanent magnets

The accurate determination of the magnetic viscosity parameter Sv requires the calculation of a new parameter η=(∂Mrev/∂Mirr)Hi which describes the dependence of reversible magnetization on irreversible magnetization. Measurements of the magnetic viscosity were performed at 300 K on the major demagnetization curves of two different nanocrystalline permanent magnetic materials: (i) melt-spun and flash-annealed Pr4Fe78B18 and (ii) mechanically alloyed and heat-treated Sm18Fe33Co49. We show that η is proportional to the intrinsic reversible susceptibility χrevi, in good agreement with predictions of the moving Preisach model. Measurements of the reversible susceptibility were made in order to determine η. Experimental values of η are positive on the major demagnetization curve in both materials, resulting in substantial corrections to Sv near Hc.

(Pr,Nd)–Fe–B nanocrystalline magnetic materials with TiC additions

The effect of titanium carbide (TiC) addition and Pr substitution in nanocrystalline Nd-Fe-B based magnetic materials has been investigated. Rapidly solidified (Pr,Nd)-Fe-B alloys were prepared by melt spinning. The compositions studied were (PrxNd1−x)9.5Fe84.5B6 (x=0, 0.1, 0.25, 0.5, 0.75, and 1) with the addition of 3at.% TiC. Varying the wheel speed from 15to30m∕s, the best magnetic properties were found for samples prepared at 20m∕s. All samples were nanocrystalline in the as-cast state. The average coercive field ranged from 10.0to12.6kOe and energy product from 7.8to9.0MGOe. The improvement in the coercive field compared to similar Nd based alloys was about 30%. Rietveld analysis of x-ray data for the Pr9.5Fe84.5B6 sample revealed that this material consists of approximately 70wt.% of 2:14:1 phase and 30wt.% of α‐Fe. This result was confirmed by Mossbauer data analysis and is in agreement with compositions expected from the ternary phase diagram.

Effect of Ti-C and Cr Additions on Magnetic Properties of Nanocrystalline (Pr,Nd)-Fe-B Alloys

The addition of both Ti-C and Cr as grain refiners in Nd-Fe-B nanocomposites substantially increases the coercive field Hc. This motived our investigation of the effect of Ti-C and Cr on Pr-Fe-B nanocomposites. Melt-spun ribbons of composition (Pr_9.5 Fe_84.5 B₆ )_0.97 _- _x Cr_x (TiC)_0.03(x = 0; 0.25; 0.5; 0.75; 1) and (Nd _9.5Fe _84.5B ₆) _0.97-x Cr_x(TiC) _0.03( x = 0.5 and 1) were produced for study. For a Pr nanocomposite with 1% Cr, Hc = 12.5 kOe. However, the energy product was limited to 13.6 MGOe by the remanence value. Rietveld analysis of X-ray spectra showed the ribbons to consist of predominantly hard ( ~ 70 wt%) R ₂Fe ₁₄B, the soft phase being ( ~ 30 wt%) alpha-Fe. Mossbauer measurements at 300 K are consistent with a reduced hyperfine field for the hard magnetic phase due to the Cr addition. Analysis of transmission electron microscopy images showed the Pr nanocomposite with 1% Cr to have an increased average grain size.