E. M. Kirkpatrick

Room-temperature ageing effects on the magnetic properties of mechanically milled SmCo5

Abstract Mechanical milling of ferromagnetic SmCo5 produces dramatic increases in coercivity or short (15 minutes to 2 hours) milling times. The milled nanostructured material is highly metastable and undergoes room-temperature recovery during the first two weeks after fabrication. Although changes in the coercivity with ageing time are significant, the diffracting crystallite size determined from x-ray diffraction remains constant. We conclude that disorder is a primary determinant of the coercivity of mechanically milled alloys.

Structural and magnetic properties of mechanically milled SmCo5:C

Mechanically milling SmCo5 powder significantly increases coercivity and remanence ratio by introducing defects; however, these defects can be removed by room-temperature aging, with a resultant decrease in coercivity. A series of (SmCo5)x:C1−x (0.15⩽x⩽1) samples has been fabricated to investigate the effect of C on oxidation protection and magnetic properties. SmCo5 was premilled for 1 h, then added to C powder and milled for times ranging from 15 min to 7 h. X-ray diffraction indicates the presence of crystalline graphite and SmCo5 for milling times ⩽6 h and also shows the presence of fcc Co for milling times >7 h. The magnetic properties are very weakly dependent on milling time after the C addition, which is attributed to the lack of further grain refinement. The saturation magnetization scales linearly with the wt % of SmCo5. Remanence ratios are approximately 0.7 and independent of volume fraction. The maximum coercivity of 16.5 kOe is comparable to the maximum obtained by milling SmCo5 without C. S...

Magnetic and structural properties of Mg–Co nanostructures fabricated by chemical synthesis

Reductive chemical synthesis is a versatile tool for fabricating elemental nanostructures; however, less work has been completed on understanding and controlling alloy formation. Magnetic Mg–Co and Mg–Co–C nanocomposites have been fabricated using a reductive chemical synthesis designed to produce highly active metals. The as-synthesized powder was annealed at temperatures from 150 to 650 °C. Samples were investigated using x-ray diffraction, alternating gradient force magnetometry, and superconducting quantum interference device magnetometry. X-ray diffraction indicates that the resulting structures are multiphase with MgCo2, MgCo3C0.5, fcc Co, Mg, MgO, and Li2CO3 present depending on annealing temperature. The temperature-dependent magnetization of the as-synthesized sample indicates ferromagnetic and antiferromagnetic or ferrimagnetic contributions. Increases in coercivity and remanance ratio with increasing annealing temperature are consistent with the formation and growth of small Co grains.

The Role of Disorder in the Magnetic Properties of Mechanically Milled Nanostructured Alloys

Abstract : Mechanical milling provides a unique means of studying the influence of grain size and disorder on the magnetic properties of nanostructured alloys. This paper compares the role of milling in the nanostructure evolution of two ferromagnets - SmCo5 and GdAl2 - and the subsequent impact of nanostructure on magnetic properties and phase transitions. The ferromagnetic properties of SmCo5 are enhanced by short (< 2 hours) milling times, producing up to an eight-fold increase in coercivity and high remanence ratios. The coercivity increase is attributed to defect formation and strain. Additional milling increases the disorder and produces a mix of ferromagnetic and antiferromagnetic interactions that form a magnetically glassy phase. GdAl2, which changes from ferromagnetic in its crystalline form to spin-glass-like in its amorphous form, is a model system for studying the dependence of magnetically glassy behavior on grain size and disorder. Nanostructured GdAl2 with a mean grain size of 8 nm shows a combination of ferromagnetic and magnetically glassy behavior, in contrast to previous studies of nanostructured GdAl2 with a grain size of 20 nm that show only spin-glass-like behavior.