The effect of x-ray illumination on magnetic domain memory in [Co/Pd] / IrMn multilayers

Abstract

Thin ferromagnetic films with perpendicular anisotropy exhibit nano-sized magnetic domains forming various patterns. When an external magnetic field is applied and cycled, the domains pattern is usually lost. However, if the ferromagnetic layer is coupled with an underlying antiferromagnetic layer, it may show magnetic domain memory (MDM). [1-3] MDM is the ability of the magnetic domains to retain their exact same domain topology upon field cycling. We are studying the effect that illumination by coherent resonant x-rays may have on MDM in a [Co / Pd] / IrMn multilayers. Our x-ray resonant magnetic scattering (XRMS) data suggests that under high dose of x-ray illumination, the material may lose its existing MDM. To investigate this possible effect we have used magneto-transport measurements [4,5] to track the exchange bias while the sample is illuminated by x-rays, using synchrotron radiation (see Fig.1a). Magneto-transport is here used to measure the magnetization loop from which we can extract a bias field. A loss of exchange bias would indicate that the x-rays illumination dose may alter the strength of the exchange couplings and ultimately the amount of MDM. Knowing if a loss of exchange bias has occurred requires collecting magneto-transport data (Fig.1b) as well as XRMS data (Fig.1c) and correlating the observed changes under various dose of x-ray illumination.These measurements include Hall effect signal and as well as magnetoresistance signal, collected both on inner contacts and on outer contacts. For the outer contacts, the spacing between wire bonds is ~1cm, whereas for the inner contacts, the spacing between wire bonds is ~100 . These measurements help us understand the shape of the magneto-transport data collected under x-ray illumination and determine if x-ray illumination has an effect on exchange couplings, and ultimately MDM.  a) Instrument used for our x-ray resonant magnetic scattering (XRMS) experiment at the APS, showing the vacuum scattering chamber and the octupolar magnet to apply a field in any direction. b) Magneto-transport measurement taken while [Co/Pd] / IrMn multilayers was illuminated with x-rays. c) XRMS speckle pattern from the [Co/Pd] / IrMn film, measured at the Co L3 edge.