MAGNETIC DOMAIN FORMATION WITHIN PSEUDO-SPIN-VALVE NANOPILLARS

 

Kathryn Krycka, Brian Maranville, Julie Borchers

NIST Center for Neutron Research, Gaithersburg, MD, USA

 

Fernando Castano, Brian Ng, Joy Perkinson, Caroline Ross

Massachusetts Institute of Technology, Cambridge, MA, USA

 

Magnetic thin-film multilayers patterned with nanometer-scale lateral dimensions are the basis of magnetic storage media technology, including random access memory and magnetoresistive read heads.  These devices exploit the differences in resistance resulting from whether their constituent ferromagnetic layers are aligned in parallel or anti-parallel.  To be technologically useful the structural and magnetic characteristics of these patterned structures must be scaleable and reproducible down to the nanoscale.  Any variability among the size, shape or microstructure of the nanopillars can adversely affect the magnetic stability and leads to issues such as distribution of the magnetic switching field. Additionally, the cross-sectional size and shape of the ferromagnetically active region is key to the power consumption and stability. Therefore, detailed understanding of how magnetic domains form, evolve, and finally re-orient during field switching is vital.

 

Neutron reflectivity is an ideal technique for the study of ferromagnetic domain formation because of its high sensitivity to the vector magnetization in buried layers [1]. It is non-destructive and simultaneously samples the average behavior from an ensemble of patterned devices, which is a strong advantage since performance can vary significantly from one pillar to another.  In-plane and depth-dependent spatial information regarding the magnetic structure can be obtained from analysis of the difference in diffuse neutron scattering [2] as a function of applied magnetic field.  Our sample consists of an array of 350 nm by 550 nm ellipses patterned using interference lithography over a region more than a cm2, with periodicity of 450 nm and 900 nm, respectively. These pillars are constructed from polycrystalline multilayers containing 4 nm NiFe | 4 nm Cu | 4 nm Co.  Strikingly, the shape (though not magnitude) of the extracted magnetic scattering profile was found to be invariant of applied field up to 0.65 Tesla.  This suggests that the magnetic moments within each domain rotate collectively.  To extract additional information regarding the average in-plane domain size and shape we have modeled the magnetic scattering by calculating the scattering profile expected from magnetic domains arranged in various configurations on our structural ellipse template.  A ferromagnetic ellipse 75 nm smaller in radius, or 200 by 400 nm in area, models the data very well.  This implies that there may be an unexpectedly large fraction of magnetically dissimilar, and possibly magnetically inactive, material on the order of 75 nm around each ellipse boundary.

[1] B.J. Kirby, J.A. Borchers, J.J. Rhyne, S.G.E. Te Velthuis, A. Hoffman, K.V. O'Donovan, T. Wojtowicz, X. Liu, W.L. Lim, and J.K. Furdyna, Phys. Rev. B 69, 081307R (2004)

[2] J.A. Borchers, J.A. Dura, J. Unguris, D. Tulchinsky, M.H. Kelley, C.F. Majkrzak, S.Y. Hsu, R. Loloee, W.P. Pratt Jr., and J. Bass, Phys. Rev. Lett. 82, 2796 (1999)

 

Category: Materials

Author: Kathryn Krycka

Mentor:  Julie Borchers

Division, Laboratory: NIST Center for Neutron Research (610)

Laboratory:  NIST Center for Neutron Research

Room E130-1, Building 235, Mail Stop 6102

Phone: (301) 975-8685

Fax: (301) 921-9847

Email: kathryn.krycka@nist.gov

Not Sigma Xi member