A study of magnetic hardening in Pt1-xNixFe

Alloys of composition Pt
1-x
Ni
x
Fe with
x = 0.2, 0.3
, and 0.4 can be quenched from high temperature to retain a face centred cubic disordered phase. On aging, at lower temperatures, alloys with
x = 0.2 and 0.3
become magnetically hard with coercivities up to 2000 Oe at room temperature. In the hardened state the alloys have an ordered tetragonal structure with
c/a \sim 0.96
and a uniaxial magnetocrystalline anisotropy of
\sim10^{7}
erg cm
^-3
. Electron microscopy, of magnetically hard fully tetragonal samples, reveals a {110} lamellar structure. The
c
axis of each lamella is at 90° to the
c
axis of the locally dominant tetragonal matrix phase in which it is embedded. Lorentz microscopy reveals domain walls, parallel to the dominant
c
axis which are "kinked" where they cross lamellae. The coercive forces increase on cooling below room temperature. This increase can be attributed to the decreased probability of the thermal activation of domain wall segments, over barriers, as the temperature falls. An alloy with
x = 0.4
formed a cubic ordered phase on aging but did not develop high coercivity or high remanent magnetization.