Fabrication and characterization of graded magnetocrystalline anisotropy iron-nickel-platinum alloy thin films

Abstract

The further increase of magnetic storage density is limited by superparamagnetism: the size of the magnetic grains has reached a scale (several nanometers) at which thermal fluctuations can erase the stored information. Attempts to increase the thermal stability by making the grains `magnetically harder' have failed because they have also made them impossible to write with available write fields. One approach to overcome this perplexing dilemma is to grade the uniaxial magnetocrystalline anisotropy, Ku, such that one end is `magnetically soft' to switch while the other end is `magnetically hard' to anchor the switch from intrinsic thermal stability issues. This change in Ku can be accomplished by changing the composition of the magnetic material along the magnetic easy-axis direction. However, there has been a lack of experimental studies on the fabrication of the gradients in the [001] orientation in FePt based structures coupled with isolated magnetic pillars to verify domain wall switching advantages in gradients. In this dissertation, a highly ordered [001] oriented FexNi0.48-xPt0.52 (0<x<0.48) compositional gradient thin film was deposited onto Pt (12 nm)/Cr (4 nm) underlayers on an MgO [001] substrate. The influence of the Pt/Cr underlayers on the (001) growth texture and L10 chemical ordering of the single compositional FexNi0.48-xPt0.52 thin films and the gradient FexNi0.48-xPt0.52 (0<x<0.48) films were studied using X-ray diffraction (XRD) and in situ stress measurements. This gradient thin film was fabricated into nano-dots with a diameter approximately 80 nm and 200 nm pitch-to-pitch spacing in hexagonal arrays using electron beam lithography and reactive ion etching for magnetic characterization. The magnetic properties of a patterned single composition film, exchange-coupled-composites (ECC) multilayer film, and a continuous graded film were studied. A Victoria figure of merit which compares the switching strength and thermal stability for all three films was low. This has been mainly contributed to the nano-dots diameters being larger than single switching volume domains. Normalizing the figure of merit by the single composition value did show that the ECC and gradient films had significant switching and thermal stability advantages over single, magnetically hard compositions. This dissertation demonstrates experimentally that aligned, patterned and graded magnetocrystalline anisotropic energy films can provide further decreases in grain sizes for ultrahigh density magnetic media while still using available write head technologies. A formable challenge of reducing nano-dots sizes over large patterning areas, to increase the absolute value of the figure of merit, provides direction for future studies.