Reduction of magnetostatic interactions in self-organized arrays of nickel nanowires using atomic layer deposition

Fabricating dense arrays of narrow and long magnetic nanowires is one bottleneck for implementing three-dimensional race-track memories [1]. Self-organized nanoporous alumina templates filled by electrodeposition [2] is an appealing route for this. However anodizing processes yield pores with diameter d not much smaller than the pitch of the array D. Thus magnetostatic interactions are strong, and nanowires cannot be addressed independently. We applied Atomic Layer Deposition (ALD) [3] to reduce d while leaving D unchanged before electrodeposition [4], significantly reducing interactions in agreement with an analytical model, and yielding arrays with 100% remanent hysteresis loops.
Figure 1 : Right : Left : SET/TEM images of the nanowires. Right : Minor magnetisation cycles for 100 cycles of ALD
Porous alumina membranes are prepared by a two-step aluminum anodization in oxalic acid, with d = 46 nm and D = 105 nm [5]. These are exposed to various numbers of ALD cycles with long exposure times (30s) to allow time for diffusion and conformal coating of the deep pores. This yields pore diameters and later electrodeposited Ni nanowires from 46 down to 18 nm (fig.1), uniform along the wires length. SEM and TEM characterizations were applied. Magnetization reversal processes of such arrays are studied by Vibrating Sample Magnetometry at 300 K. Coercivity, anisotropy and switching field distributions (SFD) were extracted. An analytical modeling of magnetostatic interactions with no adjustable parameter is proposed, adapted from [6] to fit our much longer wires.
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Figure 2 : Switching field distribution
Minor loops reveal negligible reversible components (fig.1). This first hints at magnetization reversal through nucleation/propagation in each wire [7], and second yields the SFD of the wires. While coercivity varies little with d/D, the SFD is dramatically reduced for reduced d, as expected from the strong reduction of porosity of the arrays (fig.2). The strength of inter-wires dipolar interactions (i.e. for an initially fully magnetized array) is defined as the width encompassing 50% of the experimental SFD. This width compares well with the analytical model. The loops display full remanence for wires of diameters below 30 nm, which opens the way towards the field or current manipulation of individual wires while still in the dense matrix.

Contributors :

  • Institut Néel (France) : S. Da Col, M. Darques, O. Fruchart, L. Cagnon

Our related publication

    Reduction of magnetostatic interactions in self-organized arrays of nickel nanowires using atomic layer deposition, S. Da Col, M. Darques, O. Fruchart, L. Cagnon, Applied Physics Letters 98, 112501 (2011)

Références Bibliographiques

  • [1] Parkin S S P, U.S. Patent No. 6,834,005, 2004
  • [2] Nielsch K et al. 2001 App. Phys. Lett. 79, 9
  • [3] Redaction underway
  • [4] Ott A W et al. 1997 Chem. Mat. 9, 3
  • [5] Masuda H et al. 1995 Science 268, 5216
  • [6] Wang T et al. 2008 App. Phys. Lett. 92, 192504
  • [7] Hertel R 2001 J. App. Phys. 90, 5752

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