Nanosecond Structural Transformation of Magnetic thin Films: PtMnSb, Structure and Magnetic Properties
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IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305
ABSTRACT
We report on the phase transformation of amorphous PtMnSb thin films induced by laser annealing in the nanosecond time regime. Structural and magnetic transformations are investigated by TEM, XRD, AFM and in-situ MOKE and VSM. We have established that a minimum laser fluence is required to crystallize the amorphous films and thus, to induce magnetic activity. The transformation kinetics vs number of irradiation pulses reveals that the magnetically active Clb phase is formed via an intermediate phase, namely, tetragonal-PtMn. We have also established that the thin film crystallization induced by the nanosecond laser annealing proceeds via nucleation rather than grain growth. Measurement of the lattice parameter of the Clb-PtMnSb produced by the laser quenching (LQ) indicates an essentially unstrained structure with a =6.17 A vs a =6.201 A reported for the bulk. Nevertheless, we observe the generation of large surface undulations upon laser annealing and suggest that this is the mechanism for stress relaxation concomitant with the large volumetric changes involved in the phase transformation. In addition, we observe decrements in saturation moments and Curie temperatures which are attributed to the nanocrystalline nature of the LQ specimens.
INTRODUCTION
The Heusler alloy, Clb-PtMnSb [1], is known to exhibit the largest Kerr rotation at room temperature of all known ferromagnetic materials. [2] Thus, there is considerable interest for optical storage applications and due to its unique band structure (metal for the majority spin electrons and semiconductor for minority spin electrons) [3], it is also a promising material for magneto-resistive applications. Previous reports on the generation of the Clb crystalline phase have utilized long term, high temperature annealing employing conventional isothermal annealing. Recently Carey et al [4], reported also the usage of rapid thermal annealing to produce the Clb phase. The use of nanosecond duration laser pulses to crystallize amorphous PtMnSb was first reported by Marinero. [5] In this work we report for the first time, details pertaining the crystallization kinetics of these alloys, the nature of the structural transformation and the development of magnetic properties intimately related to the nano scale of the crystalline film produced under this non-equilibrium thermal conditions.
33 Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
EXPERIMENTAL
PtMnSb thin films were deposited by DC magnetron sputtering (base pressure lxl0-7 Torr.) onto quartz substrates from elemental targets utilizing Argon sputter pressures around 3 mTorr. The Heusler alloy was grown between SiN layers as follows: quartz / SiN (20 nrm) / PtMnSb (150 nrm) / SiN (12.5 rnm). All layers were grown in the same pumpdown and the nitride was formed by reactive sputtering. To correlate film struct
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