Broad-energy oxygen ion implantation controlled magnetization dynamics in CoFeTaZr

Abstract

In this paper, a novel pulsed broad energy spectrum ion-implantation technique, using the dense plasma focus device (DPF), for uniform oxygen-ion doping along the thickness of a ~250 nm thick magnetic CoFeTaZr layer is investigated. A new operational regime of the dense plasma focus – the off-focus mode – is explored to avoid the surface damage of the exposed sample by the high energy plasma streams/jets and instability accelerated ions, typically observed in conventional efficient-focus mode operation. The faraday cup measurements shows the increase in ion fluence from 3.83 × 10¹³ ion/cm² for efficient-focus mode to 8.76 × 10¹³ ion/cm² for off-focused mode operation in the broad-ion-energy range of 1–100 keV. The x-ray photoelectron spectroscopy (XPS) of the unexposed sample shows the presence of Co in Co⁰, Co²⁺ and Co³⁺, Fe in Fe⁰, Fe²⁺ and Fe³⁺, and Ta in Ta⁰ and Ta²⁺ oxidation states while Zr was observed with only metallic Zr binding energy peaks indicating the surface oxidation of the unexposed sample. The exposure to oxygen plasma in DPF device led to the increase in the higher oxidation states of Co, Fe and Ta with reduction in metallic binding energy peak and the deconvolution of oxygen XPS spectrum confirmed the bonding of oxygen to Co, Fe and Ta. The magnetization dynamics of unexposed and oxygen-ion doped samples was studied using magnetoimpedance measurements in the 1–2.5 GHz frequency range. Gilbert’s damping factor, in-plane anisotropy and effective magnetization of the magnetic substrate were calculated and it is found that these properties can be modulated with a lighter ion dosage using this novel pulsed broad-energy-ion implantation technique. It is concluded that the off-focus mode DPF operation can provide the ions of required energy and fluence to implant oxygen ions across the thickness of the CoFeTaZr magnetic thin film to modulate its magnetic properties.