A.S. Murthy

Recent developments in rare-earth nitrides and carbides

Abstract In this article, we review our recent studies on new rare-earth intermetallic compounds including the Ga, Si substituted 2:17-type compounds, their nitrides and carbides, and the Sm 3 (Fe,Ti) 29 N 5 compounds. Much of our recent work has been focused predominantly in the Sm 2 (Fe,Ga) 17 C x alloys, where we used melt-spinning and subsequent annealing to obtain a high coercivity. The highest coercivity obtained so far was in Sm 2 Fe 14 Ga 3 C 2.5 , with a value of 12.8 kOe at room temperature. The off-stoichiometric Sm 2 Fe 14− x Co x Si 2 N y nitrides maintain the Th 2 Zn 17 -type structure but with a unit-cell expansion ΔV V up to 5% compared to the host materials. The Sm 2 Fe 14− x Co x Si 2 C z carbides maintain the Th 2 Zn 17 -type structure when z = 1 and transform to BaCd 11 -type structure when z = 2. A very large anisotropy field with H a value of 227 kOe for Sm 2 Fe 14 Si 2 N 2.6 and 276 kOe for Sm 2 Fe 10 Co 4 Si 2 N 2.3 is observed at low temperature (1.5 K). The Sm 3 (Fe,Ti) 29 N 5 compound and its nitrides show very interesting magnetic properties. Both of these compounds exhibit ferromagnetic ordering with T c of 486 and 750 K, respectively. The room-temperature saturation magnetization is 119 e.m.u. g −1 for the parent compounds and 145 e.m.u. g −1 for the nitrides. The easy-magnetization direction changes from planar to uniaxial upon nitrogenation. The anisotropy field for the nitrides is 12 T at room temperature and 25 T at 4.2 K.

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.

MAGNETIC PROPERTIES OF EXCHANGE-COUPLED FE/FEO BILAYERS

The magnetic, structural and microstructural properties of sputtered Fe thin films and Fe\Fe–O bilayers were studied as a function of the Fe layer thickness, the type of the Fe oxide and the substrate used. Two different ways to prepare the oxide layers were used; postdeposition oxidation and reactive sputtering. Postdeposition oxidation produced films with mixed Fe–oxides (FeO, Fe3O4, Fe2O3); however reactive sputtering led to bilayers with controlled stoichiometry, Fe\FeO, Fe\Fe3O4, and Fe\FE2O3, respectively. The coercivity of both the Fe films and fE\Fe–O bilayers, deposited on substrates with or without Cr buffer layer, was found to increase with decreasing Fe film thickness. The coercivity of the samples deposited on a Ag buffer layer was much lower and did not change substantially with the Fe film thickness. The presence of the Fe–oxide layer led to a large increase of coercivity. This is attributed to the higher anisotropy of the oxide and to exchange coupling of Fe–oxide with the softer Fe layer.

Magnetoresistance in (Fe–Co)/Ag films

The structural and magnetotransport properties of (FeyCo1−y)100−xAgx films were studied as a function of composition. Giant magnetoresistance (GMR) values were measured in these granular films, with the best GMR obtained for the composition (Fe0.33Co0.67)27Ag73, with values of 29% at 30 K and 11.7% at 300 K. XRD and TEM results have shown a fcc crystal structure with a relatively homogeneous microstructure. Magnetic data for the samples with the best GMR indicate a superparamagnetic behavior. The narrow peak in thermomagnetic data and low blocking temperature suggest a small and uniform size distribution of magnetic granules. A summary of the electrical transport properties is presented, in relation to the structural, microstructural, and magnetic properties.

Melt-spun SmFeGaC alloys with high coercivity

Abstract The coercivity and microstructure of melt-spun Sm 2 Fe 17− x Ga x C y ( x = 2,3 and y = 1, 1.5, 2, 2.5) carbides have been studied as a function of composition and annealing temperature. The highest coercivity was obtained in Sm 2 Fe 14 Ga 3 C x ( x = 2, 2.5) samples when annealed at 800°C for 20 min with a value of 12.8 kOe at room temperature. Microstructure studies showed the coexistence of α-Fe and 2:17-type phases in all the samples studied. Smooth hysteresis curves characteristic of exchange coupled magnets have been obtained only in samples with uniformly distributed phases having a fine grain size around 25 nm.

Magnetic properties and microstructure of melt-spun Nd-Fe-Ga-B alloys

The magnetic and microstructural properties of melt-spun Nd/sub 18/Fe/sub 76-x/Ga/sub x/B/sub 6/ alloys have been studied as a function of Ga content. Coercivities of 17 kOe were obtained in underquenched samples having a large grain size of 500 nm. Samples prepared at wheel speeds of 15-19 m/sec, as well as overquenched (40 m/sec) and subsequently annealed, gave coercivities of around 20 kOe with no significant improvement of H/sub c/ upon Ga substitution. In Ga-free samples prepared by annealing highly overquenched ribbons (at 60 m/sec) only moderate coercive fields could be achieved (14 kOe). These samples had grain sizes of 70-100 nm. However Ga substituted samples prepared under the same conditions have a fine grain structure consisting of unequiaxed grains with size around 20 nm and show coercivities up to 23 kOe.

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

TEM studies in melt-spun Cu-Mn-Al system exhibiting giant magnetoresistance

Giant magnetoresistance (GMR) has recently been observed in a new system, melt-spun ribbons of Cu-Mn-Al, with /spl Delta/R/R(0)=15% at 30 K. The GMR behavior in this system has been extensively studied for a range of compositions. It is suggested that the onset of GMR may be attributed to the appearance of tweed microstructure in the ferromagnetic Cu/sub 2/MnAl phase. It has also been observed that GMR existed in all samples consisting of a mixture of varying amounts of Cu/sub 2/MnAl and Cu/sub 9/Al/sub 4/ phases. Extensive transmission electron microscopy studies have been carried out to evaluate the microstructural evolution of the tweed and the observed phases, and to correlate the microstructure with the observed GMR behavior.

MAGNETIC AND PHASE TRANSFORMATION STUDIES IN ND15DYFE75(C,B)9 CAST ALLOYS

Previous studies have shown that high coercivities can be achieved in cast Nd–Fe–C alloys through an annealing heat treatment at 900 °C, which causes a peritectoidal transformation from Nd2Fe17Cx to Nd2Fe14Cx. We have studied the characteristics of this transformation in cast Nd15DyFe75(C,B)9 alloys. Optimum coercivities were obtained by annealing for 6 h at 900 °C. A highly faulted carbide phase was identified in samples annealed for 5 min at 900 °C. From electron diffraction investigations, this phase was identified to be hexagonal with lattice parameters, a=0.48 nm and c=0.81 nm. The development of coercivity in the various stages of transformation is related to the crystal structure and microstructure developed in the transformation. The presence of α‐Fe in samples with increased Fe content leads to reduced coercivities.