Effect of disorder on various physical properties ofCo2CrAlHeusler alloy films: Experiment and theory

Abstract

We have investigated the effects of atomic disorder on the electronic structure and some physical properties of ${\text{Co}}_{2}\text{CrAl}$ Heusler alloy (HA) films. Flash evaporation onto glass substrates at different temperatures (from 150 to 750 K) and postannealing at various temperatures were employed to manipulate the structural order in ${\text{Co}}_{2}\text{CrAl}$ HA films. The $L{2}_{1}$-ordered ${\text{Co}}_{2}\text{CrAl}$ HA films exhibit the magnetic, the transport, and the optical properties close to those of the bulk ordered sample. Increase in the structural disorder ($L{2}_{1}\ensuremath{\rightarrow}B2\ensuremath{\rightarrow}A2\ensuremath{\rightarrow}\text{amorphous}$ state) causes the reduction in the saturation magnetization and the Curie temperature $({T}_{C})$ down to the paramagnetic state for amorphous films. The recrystallization of amorphous ${\text{Co}}_{2}\text{CrAl}$ HA films is accompanied by an increase in resistivity by about 10%, which is interpreted to be related to the energy gap formation at the Fermi level for the minority bands in the $L{2}_{1}/B2$-ordered state of ${\text{Co}}_{2}\text{CrAl}$ HA. The energy-band structures of $L{2}_{1}$-ordered ${\text{Co}}_{2}\text{CrAl}$ HA have been calculated by using an all-electron full-potential linearized-augmented-plane-wave method. The optical and the magneto-optical properties of $L{2}_{1}$-ordered ${\text{Co}}_{2}\text{CrAl}$ HA have been experimentally investigated and explained in terms of the band structures. It was experimentally shown that the optical properties of crystalline $L{2}_{1}$ (or $B2$)-type ordered alloy at a temperature much higher than ${T}_{C}$ do not significantly change. Structural disorder in ${\text{Co}}_{2}\text{CrAl}$ HA from the crystalline to the amorphous state also does not radically alter the states responsible for the interband-absorption-peak formation.