Development of Nanostructured CoFe-Based Alloys ...

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Journal of Nanoscience and Nanotechnology Vol. 9, 1–4, 2009

Development of Nanostructured CoFe-Based Alloys for High Temperature Magnetic Applications A. K. Panda∗ , O. Mohanta, M. Ghosh, and A. Mitra National Metallurgical Laboratory, Jamshedpur 831007, India The effect of substituting Fe by Co on the crystallization, structural and magnetic behaviour of Fe72−x Cox Si4 B20 Nb4 (X = 10 20 36 50 at%) and Co36 Fe36 Si4−Y AlY B20 Nb4 (Y = 0 1 at%) alloys prepared in the form of melt spun ribbons has been discussed. Alloys containing optimum content of cobalt = 36 at% showed consistent coercivity at elevated temperatures. This soft magnetic property was further improved with aluminium incorporation. Transmission electron microscopy (TEM) indicated that such enhancement in the properties was due to finer dispersions of (CoFe)SiAl nanoparticles in amorphous matrix. Nanocrystallisation also raised the Curie temperature of the aluminium contained alloy.

Keywords: Melt-Spun Ribbons, Amorphous, Nanocrystalline, Magnetic, Curie Temperature.

2. EXPERIMENTAL DETAILS

High temperature magnetic applications conventionally comprise of CoFe-based crystalline materials. These materials though revealed high Curie temperature but had fairly high coercivity values in the range of 80 to 160 A/m.1 With the advent of nanocrystalline Fe-based systems, the soft magnetic properties could be drastically improved. However, these systems revealed Curie temperature less than 700 K. Recent reports suggest the incorporation of the element Cobalt in these alloys systems so as to not only enhance the Curie temperature but also facilitate nanocrystallisation.2 Such effect of nanocrystallisation has been reported by us for the CoFeSiBNb alloy with Co/Fe in the stoichiometry of 50:50.3 Despite of such an advantageous effect of nanophase generation, the addition of Co has a tendency of magnetic hardening. Hence, there is a quest to modify the alloy chemistry further to compensate for such deterioration in magnetic properties. A justified modification could be varying the cobalt content in the system and also the incorporation of additional element like aluminium as it has been found to enhance the properties in classically developed alloys like Sendust (FeSiAl). The present work was aimed to optimize the composition of cobalt content in CoFeSiNbB-based alloy system and then investigate further enhancement in the properties by replacement of Si by Al.

Alloys with nominal compositions of Fe72−x Cox Si4 B20 Nb4 (X = 10 20 36 50 at%) and Co36 Fe36 Si4−Y AlY B20 Nb4 (Y = 0 1 at%) respectively were prepared in the form of amorphous ribbons by melt spinning technique. Crystallization and structural behaviour was evaluated using Differential Scanning calorimetery (Perkin-Elmer DSC-7) and X-ray analysis (Philips D-500 X-ray), Transmission Electron Microscope (Philips, CM-200). Magnetic hysteresis properties were evaluated at a quasi-dc magnetizing field. Magnetic coercivity was measured from hysteresis loop obtained in open-flux configuration at a quasi-dc magnetizing frequency of 50 mHz. The temperature variation of magnetisation was determined using a Vibrating sample magnetometer (VSM, Lake Shore: Model 7404)

∗

Author to whom correspondence should be addressed.

J. Nanosci. Nanotechnol. 2009, Vol. 9, No. xx

3. RESULTS AND DISCUSSION The as-quenched ribbons revealed exothermic transformations as observed from Differential Scanning calorimetry (DSC) plots shown in Figure 1 obtained at a scan rate of 20 K/min. Thermal scanning upto 975 K showed shallow exothermic peaks indicating amorphous to nanocrystalline transformation with primary onset of crystallization at TX1 . Prior to this nanocrystallisation, the plots also displayed endothermic onsets indicating glass transition at Tg . Such distinct glass transition has also been reported by us for the bulk amorphous Fe72 Si4 B20 Nb4 (at%) alloy.4 The

1533-4880/2009/9/001/004

doi:10.1166/jnn.2009.1134

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RESEARCH ARTICLE

1. INTRODUCTION

Development of Nanostructured CoFe-Based Alloys for High Temperature Magnetic Applications

Panda et al.

Fe72-XCoXSi4B20Nb4 (at%) 880

X : 10 at%

Primary Cryst.(TX1) Tg

Temperature (K)

860 TX1

Exothermic heat flow (AU)

X : 20 at%

TX1

Tg

840 820 800 780

Glass Transition (Tg)

760

X : 36 at%

Tg

10 TX1

20

TX2

30

40

50

Cobalt content (at%) Fig. 2. Variation in onset of primary crystallization (TX1 ) and glass transition (Tg ) with cobalt content in alloys.

700

TX2

TX1

Tg 750

800

850

900

950

Temperature (K)

RESEARCH ARTICLE

Fig. 1. Differential Scanning calorimetry plots of as- spun Fe72−x Cox Si4 B20 Nb4 (X = 10 20 36 50 at%) alloys. Scan rate 10 K/min.

characteristic temperatures obtained from the DSC plots are shown in Table I. The variation of primary crystallization (TX1 ) and glass transition (Tg ) with Cobalt content in the Fe72−x Cox Si4 B20 Nb4 (at%) alloys is shown in Figure 2. Both TX1 and Tg were found to decrease with increasing Cobalt content. Interesting to observe that the present alloys exhibited a fairly wide super-cooled region TX between the first crystallization onset TX1 and glass transition Tg which is in the range of 45 K to 55 K. A DSC exotherm indicating secondary onset of crystallization (TX2 ) also appeared when the cobalt content was above 36 at%. TX2 was found to decrease from 930 K to 860 K as the cobalt content increased from 36 at% to 50 at%. Therefore, the trend suggests that alloys with cobalt