L. Withanawasam

Nanocomposite R2Fe14B/Fe exchange coupled magnets

We have studied the crystallization, crystal structure, microstructure and magnetic properties of R‐Fe‐B (R=Nd,Pr,Dy,Tb) based melt‐spun ribbons consisting of a mixture of R2Fe14B and α‐Fe phases. All the samples crystallize first to α‐Fe and a metastable phase (Y3Fe62B14 for R=Nd,Pr,Dy and TbCu7 for R=Tb) before they finally transform to 2:14:1 and α‐Fe. The highest values of coercivity and reduced remanence, 4.5 and 0.63 kOe, respectively, were obtained in a Nd3.85Tb2(Fe‐Nb‐B)94.15 sample. These properties are the result of a fine grain microstructure consisting of a mixture of α‐Fe and 2:14:1 having an average grain size of 30 nm.

Enhanced remanence in isotropic Fe‐rich melt‐spun Nd‐Fe‐B ribbons

High isotropic remanences and energy products have been obtained in melt‐spun Nd‐Fe‐B samples with a fine grained mixture of α‐Fe and Nd2Fe14B1 containing more than 50 wt % of α‐Fe. Reduced remanences as high as 0.78 with high saturation magnetization (190 emu/g) led to (BH)max of 14 MGOe. The coercivities varied from 2–4 kOe for samples with 70–50 wt % α‐Fe, respectively. Reversible demagnetization curves were observed consistent with the ‘‘exchange spring’’ mechanism.

Nanocomposite R/sub 2/Fe/sub 14/B//spl alpha/-Fe permanent magnets

We have studied the crystallization, crystal structure, structure morphology, and magnetic properties of R/sub 6/Fe/sub 87/Nb/sub 1/B/sub 6/ with R=Nd, Pr, Dy, Tb and Nd/sub 3.5/Fe/sub 91/Nb/sub 2/B/sub 3.5/ melt-spun ribbons with a microstructure consisting of a mixture of exchange-coupled magnetically hard (R/sub 2/Fe/sub 14/B/sub 1/) and soft (/spl alpha/-Fe) phases. The as-spun ribbons of R-rich composition crystallize in two steps; at first the Y/sub 3/Fe/sub 62/B/sub 14/-type+/spl alpha/-Fe phases are formed for R=Nd, Pr, and Dy and subsequently they transform to 2:14:1 and /spl alpha/-Fe upon heating above 700/spl deg/C. The intermediate phase in the case of Tb/sub 6/Fe/sub 87/Nb/sub 1/B/sub 6/ is of the TbCu/sub 7/-type. Very high remanences up to 145 emu/g with reduced remanences, M/sub r//M/sub s/, up to 0.8 were observed. The coercivity of the samples was found to vary with the R element and the content of the hard phase. The highest room temperature coercivity of 4.5 kOe was obtained in a Nd/sub 4/Tb/sub 2/Fe/sub 36/Nb/sub 2/B/sub 6/ sample. Electron microscopy reveals a grain size of 30-50 nm, which is much larger than the size (10 nm) predicted for optimum coupling.

'Exchange spring' behavior in nanocomposite hard magnetic materials

Abstract The magnetic properties of nanocrystalline melt-spun single-phase (Nd 2 Fe 14 B-type) and composite (Nd 2 Fe 14 B + α-Fe or Nd 2 Fe 14 B + Fe 3 B + α-Fe) magnets have been studied systematically in an attempt to understand better their ‘exchange spring’ behavior. The reversibility of the recoil demagnetization curves has been found to increase with increasing content of the soft phase giving rise to the characteristic exchange spring behavior only in nanocomposite samples. δ M plots indicate positive interactions of the exchange type for small fields; for fields higher than the remanence coercivity, magnetostatic interactions become dominant. The relative strength of the magnetostatic interactions is increased in samples with higher soft phase contents.

Magnetic hardening of melt‐spun nanocomposite Nd2Fe14B/Fe magnets

Coercivity optimization studies were done on melt‐spun nanocomposite Nd4R2Fe87−xNbTxB6 (R=Nd,Y,Dy; T=Ag,Cu) isotropic ribbon samples. The maximum attainable coercivities, after adjusting the annealing time, were found to be very sensitive to the annealing temperatures. The optimum magnetic properties [HC=3.9 kOe, (BH)max=10 MGOe] were obtained by annealing at 750–775 °C for a few minutes. Optimization by flash annealing gave similar results. Microstructural studies show that the grain size is greater than the theoretically predicted grain size for optimum coupling between the hard and the soft phase. With the annealing conditions used, Nd4Dy2Fe87NbB6 samples gave moderate coercivities and in Nd4Y2Fe87NbB6 samples the coercivity was reduced more than the expected reduction in the anisotropy field due to the presence of Y.

Hysteresis behavior and microstructure of exchange coupled R/sub 2/Fe/sub 14/B/sub 1///spl alpha/-Fe magnets

The hysteresis behavior of nanocomposite R/sub 2/Fe/sub 14/B//spl alpha/-Fe magnets with R=Nd and Pr was investigated at low temperatures. The room temperature coercivity depends on annealing with a peak value at 700/spl deg/C. The low coercivity of samples annealed at temperatures below the optimum is due to incomplete transformation of R/sub 3/Fe/sub 62/B/sub 14/ to R/sub 2/Fe/sub 14/B phase. Hysteresis loops of Pr/sub 2/Fe/sub 14/B//spl alpha/-Fe samples change from a smooth to a constricted loop when temperature is varied from 300 to 10 K. This may be due to the decoupling of a fraction of the larger 2:14:1 grains which reverse independently of the rest of the sample.

Crystallization behavior of melt-spun Nd/sub x/Fe/sub 93-x/Nb/sub 1/B/sub 6/ alloys

The crystallization behavior of Nd/sub x/Fe/sub 93-x/Nb/sub 1/B/sub 6/ melt-spun ribbons was investigated using calorimetric techniques. The crystallization products were identified by X-ray diffraction and thermomagnetic measurements. Samples with x/spl ges/10 crystallize directly from amorphous to a mixture of Nd/sub 2/Fe/sub 14/B and /spl alpha/-Fe phases. For 10<x<7, the final mixture of 2:14:1 and /spl alpha/-Fe phases is obtained through a two stage crystallization where the intermediate metastable Nd/sub 2/Fe/sub 23/B/sub 3/ and Nd/sub 3/Fe/sub 62/B/sub 14/ phases are produced at the initial crystallization. In samples with 7<x<4 the intermediate phase Is Nd/sub 3/Fe/sub 62/B/sub 14/. No metastable phases were observed in samples containing less than 3 at% Nd although the calorimetric measurements indicated a two-step crystallization process.

Effect of Cu and Al Substitutions on the Microstructure of R-Fe-B Magnets

The coercivity of melt-spun Pr-Fe-B ribbons was found to increase with the addition of Cu and Al. The change in size and shape of grains with Cu and Al substitution were investigated by transmission eletron microscopy (TEM) and the grain boundary structure was further examined with high resolution electron microscopy (HREM). For small substitutions only “disturbed lattice” regions were observed at most of the grain boundaries. Secondary phases rich in the added elements were observed mostly at tripple grain boundaries and sometimes at grain boundaries in samples with larger amounts of substitution. The grain size in the substituted samples does not decrease much with further substitution. However, the shape of grains changes from polyhexagons to facets. The enhancement in coercivity can be explained by the grain size reduction and the modification of microstructure at the grain boundary regions.

Nanocomposite R2Fe14B/ α-Fe and Sm2 (Fe-Ga)17Cx/ α-Fe magnets

Abstract We have studied the crystallization, crystal structure, microstructure, and magnetic properties of melt-spun ribbons of R6(Fe-Nb)88B6 with RNd, Pr, Dy, Tb and Nd3.5(Fe-Nb)93B3.5 consisting of a mixture of exchange-coupled magnetically hard R2Fe14B and soft α-Fe phases. The as-spun ribbons of R-rich composition crystallize in two steps; for RNd, Pr, and Dy at first the Y3Fe62B14-type + α-Fe phases and subsequently they transform to 2:14:1 and α-Fe upon heating above 700 °C. The intermediate phase in the case of Tb6(Fe-Nb)88B6 is of the TbCu7-type. Very high remanences up to 145 emu/g with reduced remanences ranging from 0.6 to 0.7 were observed. The coercivity of the samples was found to vary with the R element and the hard phase content. The highest room temperature coercivity of 4.5 kOe was obtained in a Nd4Tb2(Fe-Nb)88B6 sample. The observed reversible demagnetization curves are characteristic of exchange-spring magnets. A two-phase microstructure was also obtained in Sm-Fe-Ga-C ribbons consisting of exchange-coupled 2:17 and α-Fe phases after annealing in the temperature range of 700–800 °C. The highest coercive force (12.8 kOe) was obtained in samples annealed at 800 °C. Annealing at 700 °C led to finer grains with lower coercivities (5kOe) but higher remanences.

Intermediate phases formed during crystallization of Fe-rich NdFeB alloys

Melt-spun R 6 (Fe-Nb) 88 B 6 ribbons with R = Nd, Pr, Dy, Tb have been studied via differential scanning calorimetry, differential thermal analysis, transmission electron microscopy, X-ray diffraction and magnetic measurements. The amorphous as-spun samples crystallize in two or more steps to a final microstructure consisting of a mixture of R 2 Fe 14 B 1 and α-Fe phases. The metastable phase in Nd, Pr, Dy samples was identified as Y 3 Fe 62 B 14 -type and in the Tb samples as TbCu 7 -type