Effect of Ta Underlayer on Thickness-Dependent Magnetic Properties of Ni–Fe Films

We report the effect of Ta underlayer on the structural and the magnetic properties of Permalloy (Ni₈₁Fe₁₉) thin films over a wide range of film thicknesses (tNi-Fe = 10-200 nm). For this purpose, Ta (5 nm)/Ni-Fe (tNi-Fe nm)/Ta (2 nm) films were grown on thermally oxidized Si substrates under the presence and the absence of biasing magnetic field (1000 Oe). The Ni-Fe films grown under the presence and the absence of biasing magnetic fields are referred to Ta/Ni-Fe(M) and Ta/Ni-Fe(NM), respectively. X-ray diffraction studies revealed that the Ta underlayer plays an important role in the formation of fcc-(111) Ni-Fe texture in both the Ta/Ni-Fe(M) and Ta/Ni-Fe(NM) films. In addition, the values of average crystallite size depend on the film thickness, biasing field, and Ta underlayer. Thickness-dependent magnetic hysteresis (M-H) loops demonstrate a strong in-plane uniaxial anisotropy in both the Ta/Ni-Fe(NM) and Ta/Ni-Fe(M) films. The uniaxial anisotropy constant (K_u) depends not only on the film thickness, but also strongly on the biasing magnetic field. Coercivity decreases with increasing film thickness for both the Ta/Ni-Fe(NM) and Ta/Ni-Fe(M) films, and approaches to a minimum of ~0.4 Oe even at tNi-Fe = 200 nm. Furthermore, the observed magnetic domain patterns of Ni-Fe films show their dependence on the Ta underlayer, biasing field, and Ni-Fe thickness. The effects of the Ta underlayer, the Ni-Fe film thickness, and the biasing-field driven uniaxial anisotropy on the structural and the magnetic properties are discussed on the basis of reduction in strain at the Ta/Ni-Fe interface.