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Heavy-ions for controlled modification of magnetic thin films and multilayers

Ajay Gupta

UGC-DAE Consortum for Scientific Research, University Campus, Khandwa road,Indore-452017

Magnetic thin films and multilayers are an important class of nanostructured

materials with wide ranging applications in magnetic memory devices, sensors, high

frequency cores for inductors etc. Heavy ion irradiation has been an effective tool to

modify the properties of such nanostructures in a controlled manner. Swift heavyions have several advantages over low energy ions in terms of depth of modification

and low ion fluences, etc. Various effects induced by heavy ion irradiation include,

stress relaxation, atomic ordering/disordering, interface mixing. Specific examples ofboth soft-magnetic (Fe-nitride) as well as hard-magnetic (FePt, CoPt) films will be

taken in order to demonstrate the use of heavy ions in controlled modification of their

magnetic properties.

Understanding of the atomic level processes responsible for these effects is important

from both basic as well as application points of view. Results will be presented on

the use of some novel x-ray based techniques towards understanding the heavy ioninduced effects in thin films and multilayers. i) nuclear resonance reflectivity in

isotopic multilayers [1] has been used to study the irradiation induced atomic

diffusion of Fe in compositionally homogeneous films of Fe, Fe-nitride and FePt.The results are being interpreted in terms of possible mechanism of irradiation

induced atomic motion. ii) In case of multilayers, depth resolved information about

various phases formed across the interfaces has been obtained using x-rayfluorescence and XAFS under standing wave conditions [2]. This information in tern

would be useful in understanding the irradiation induced changes in the magnetic

properties, as well as the mechanism of the induced changes. A better understandingof the atomic level mechanisms involved in ion beam induced modification will help

in a better harnessing of the technique.

[1] Ajay Gupta, Mukul Gupta, Sujoy Chakravarty, Rudolf Rffer, Hans-ChristianWille, Olaf Leupold, Phys. Rev. B 72 (2005) 014207.

[2] Ajay Gupta, Parasmani Rajput, and Carlo Meneghini, Phys. Rev. B 76, 195401

(2007); Ajay Gupta, Dileep Kumar, and Carlo Meneghini, Phys. Rev. B 75 (2007)

064424.

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- stress relaxation

o improvement in soft magnetic properties - FeN

- structural transformation

o amorphization

o order-disordertransformation FePt

- Interface modification

o Variation in interface roughness Fe/Cr

o Interface mixing Fe/Si

- non-equilibrium phase formationo miscible systems

o immissible systems Fe/Ag

- Atomic level mechanism of ion-induced modification

o Irradiation induced atomic motions

Nuclear resonance reflectivity

o Depth resolved structural studies across the interfaces

X-ray standing waves