Magnetoelastic Effects in Epitaxial Co 80 Pt 20 Films

Transcription

1 Magnetoelastic Effects in Epitaxial Co 8 Pt Films Bin Xu, Timothy J. Klemmer*, and William D. Doyle MINT Center and Department of Physics and Astronomy The University of Alabama * eagate Research, Pittsburgh This project was funded in part by grants from the NF [DMR-9894] and eagate Research. MINT Fall Review, November.

2 Introduction Epitaxial H-i()/Ag()(nm)/Cr(W)()(nm)/Co 8 Pt ( ) (t)/ Cr(W)() (5nm) films prepared by sputtering Mismatch for Co 8 Pt : with Cr, -.5% along [ ], -.8% along []; with W underlayer, 5.8% along [ ], 7.7% along [] Epitaxial strains relaxation and magnetoelastic anisotropy in Co 8 Pt films Magnetic anisotropy of very thin Co 8 Pt films x z ε [ ] ϕ θ M v [ ] ε ε y Magnetoelastic anisotropy energy of oriented films: () f = in θ me = [ Bε + Bε + B( ε + ε) ] [(B + B ) ε + B ε + B ε ] = K () me in θ in () where K me = (B + B) ε + Bε + Bε is anisotropy constant, ε s are strains ( ) θ

3 In-plane Hard Axis Loops of Co 8 Pt Films with a Cr Underlayer t= nm t= nm t= nm.8.4 t= nm t= nm t= nm M/Ms M/M H (koe) Measured using VM along CoPt Hard axis loops fit to M M H = HM 8K [ ] + HM 8K H (koe) direction, t is the thickness of CoPt K + 6K ( from ) MH MH MH MH = K 4K, M + M M + HM 8K Anisotropy constants do not change down to t = nm For t < nm, K decreases sharply and K slightly increases HM 8K K + 6K

4 In-plane Hard Axis Loops of Co 8 Pt Films with a W Underlayer t= nm t= nm t= nm.8.4 t= nm t= nm t= nm M/Ms M/Ms H (koe) Measured using VM along CoPt [ ] H (koe) direction, t is the thickness of CoPt For 4nm < t < nm, K decreases with decreasing thickness For t < 4nm and t >nm, K relatively constant K relatively constant for all thickness but smaller than K of CoPt films with a Cr underlayer

5 X-ray Grazing Incidence Diffraction (GID) Diffracted X-rays Incident X-rays γ Projection of X-rays in the film plane γ θ γ θ Diffracting planes Projection of X-rays in the film plane Penetrating X-rays Film plane () Peak position vs x-ray incidence angle γ γ θ d(å) * d/d X-ray diffraction from planes nearly perpendicular to film plane. Peak positions independent of x-ray incidence angle. mall amount of fcc phase found

6 Intensity (counts/s) Intensity (counts/s) X-ray pectrum for Co 8 Pt Films with a Cr Underlayer CoPt() i() CoPt() Cr() Ag() film plane CoPt() Cr() i() i() CoPt() θ (degree) bulk Co8Pt (4) t=nm bulk Cr() t=4nm θ (degree) Intensity (counts/s) Intensity (counts/s) bulk Co8Pt () t=nm t=4nm θ (degree) Cr-CoPt () t=4nm bulk Co8Pt () t=nm θ (degree) θ-θ scan t=nm () peak () peak () peak t is the thickness of Co 8 Pt films

7 Epitaxial trains vs Co 8 Pt Film Thickness 4 4 ε() ε() train (%) ε() ε() train (%) ε() ε() t (nm) trains of films with a Cr underlayer - t (nm) trains of films with a W underlayer trains much lower than mismatch even for films as thin as nm train relaxation not as well defined as /t / in MBE films [] Out-of-plane strain likely released by fcc stacking fault (found in pole figure) For the same t, films with Cr underlayer have smaller rocking curve peak width than that of films with a W underlayer (better crystal quality with Cr).

8 Anisotropy Constants as a Function of Co 8 Pt Thickness Anisotropy constant ( 6 erg/cc) K K KE+K(bulk) t (nm) Anisotropy constants of films with a Cr underlayer Anisotropy constant ( 6 erg/cc) K K KE+K(bulk) t (nm) Anisotropy constants of films with a W underlayer K E calculated using measured strains and B s of bulk pure Co Magnetoelastic effect alone can t explain the change in anisotropy constants K possibly lowered by diffusion of W (small amount of W reduces K [])

9 Conclusions Lattice strains much smaller than the mismatches Magnetoelastic effect alone can t explain the change in anisotropy constant Very thin films (<nm) have much smaller anisotropy Dislocation incorporation (indicated by wide rocking curve) and stacking fault may reduce the anisotropy Reference [] N. Kikuchi, O. Kitakami,. Okamoto, Y. himada, A. akuma, Y. Otani and K. Fukamichi, J. Phys.: Condens. Matter., L485 (999) [] K. Ha, M. Ciria, R. C. O Handley, P. W. tephens and. Pagola, Phys. Rev. B 6, 78 (999)