Anisotropic Nd $_{2}$Fe $_{14}$B Submicron Flakes by Non-Surfactant-Assisted High Energy Ball Milling

This paper reports on the formation mechanism, structure, crystallographic anisotropy, magnetic properties and postannealing of single-crystal and [001] textured poly-nanocrystalline Nd
₂
Fe
₁₄
B flakes with a micron, submicron or nanosize thickness prepared by a non-surfactant-assisted high energy ball milling (HEBM) technique. Nd
_15.5
Fe
_78.5
B
₆
ingot micro-powders were the precursor while ethanol was the only milling medium. Similar to the structural evolution process in the surfactant-assisted HEBM, single-crystal flakes with a micron then submicron thickness were first formed. With further milling, [001] textured poly-nanocrystalline flakes with a submicron then nanosize thickness were formed. The formation of anisotropic flakes was mainly related to the polar nature of ethanol which would form a thin coating layer on the flakes through carboxylate bonds to prevent cold welding and agglomeration during the HEBM. The Nd
₂
Fe
₁₄
B flakes prepared by the HEBM in ethanol for 5 h have a [001]-in-plane texture, a thickness in the range of 100-450 nm and a width in the range of 0.7-18 μ m, a coercivity
_i
H
_c
of 2.3 kOe, and a I
₀₀₆
/I
₁₀₅
value of Nd
₂
Fe
₁₄
B phase of 5.3. Postannealing at 400-550
^°
C for 0.5 h had little effect on the flake dimensions while resulting in a reduced [001] texture and reduced remanence values besides coarser grain sizes. With increasing annealing temperature,
_i
H
_c
first increased, reached its maximum value of 2.8 kOe at 450
^°
C, then decreased. An anisotropic magnetic behavior was found in all of the as-milled and annealed flakes.