Optimization of Target Composition in Nd-Fe-B Film Magnets Prepared by High Laser Energy Density

Miniaturized electronic devices comprising anisotropic Nd-Fe-B film magnets have been reported by a lot of researchers [1] [2]. Recently, our group reported isotropic Nd-Fe-B+a-Fe dispersed nano-composite film magnets prepared by a PLD method with laser energy density (LED) > 10 J/cm
²
in order to apply the films to a multi-polarly magnetized rotor for a miniaturized cylindrical motor [3]. Although the targets had Nd-rich compositions compared to a stoichiometric one such as Nd2.4Fe14B or Nd2.6Fe14B, the above-mentioned nano-composite structure together with good magnetic properties of Br > 0.9 T, Hc > 300 kA/m, and 90 kJ/m
³
, respectively, could be obtained. This contribution reports the influence of structure, surface morphology and magnetic properties on target compositions for the isotropic Nd-Fe-B film magnets prepared at LED > 10 J/cm
²
. An Nd-Fe-B film was deposited on a Ta substrate by ablating each rotated target with eight compositions of Nd
_x
Fe
₁₄
B (X = 1.8, 2.0. 2.2, 2.4, 2.6. 2.8, 3.2, 3.5) in the vacuum atmosphere of 10
^-5
Pa. An Nd-YAG pulse laser with LED above 10 J/cm
²
was could be obtained by a laser beam with the diameter less than 1 mm. All the as-deposited films thicker than 10 μm were crystallized by a pulse annealing (PA) method in the vacuum atmosphere. The observation of X-ray diffraction patterns indicated that the all as-deposited films had amorphous structure with α-Fe grains. The result suggests that a usage of the small laser beam of high LED enabled us to precipitate α-Fe phase even if we used an Nd-Fe-B target with large amount of a Nd element such as Nd3.2Fe14B and Nd3.5Fe14B. Moreover, the number and/or size of droplets with Nd-rich composition drastically increased as X of an Nd
_x
Fe
₁₄
B target became larger than 2.6 and the residual magnetic polarization decreased. On the other hand, the coercivity of films prepared by Nd
_x
Fe
₁₄
B (X=1.8, 2.0, 2.2) targets showed less than 300 kA/m. Resultantly, the X range from 2.4 to 2.6 in a Nd
_x
Fe
₁₄
B target is optimum in order to achieve good magnetic properties in Nd-Fe-B films prepared by using a PLD with LED > 10 J/cm
²
.