Magnetic Thin Films and Nanostructures: Patterning New Properties

Exceptional advances in the control of material properties has been achieved, through careful manipulation of geometry on nano- and sub-nanometre length scales, in magnetoelectronics and nanomagnetism.¹ Advanced techniques now allow for the creation of structures patterned on sub-micron length scales in three dimensions. New phenomena has been discovered in patterned magnets that can be strongly controlled by ion bombardment, multilayering, and lithographic patterning.

Examples include: materials for microwave signal processing technologies, whose properties that can be tuned by magnetic and electric fields; high speed switching of magnetization in elements used for data storage and spin electronics; and manipulation of magnetic domains and domain walls in carefully crafted structures that serve as model experimental systems for studies of complex dynamics.

Figure 1. Array of magnetic dots patterned from a Py film.

Perhaps the most famous example of how geometry can control fundamental material properties is Bragg scattering of electrons in crystals. Most recently an analogy has been created for microwave excitations in two dimensional magnetic arrays, known as 'magnonic crystals'. These excitations can be diffracted by magnetic features with appropriate dimensions.

An array of magnetic wires was constructed from a 30 nm thick Ni₈₀Fe₂₀ film using deep ultraviolet lithography and lift-off, forming a diffraction array for magnetostatic spinwaves. The magnetic wires were 350 nm wide and spaced 55 nm apart A stop band was observed for propagation perpendicular to the stripe axes, demonstrating the possibility of engineering a magnonic band structure.²

Figure 2. Labyrinth array formed by magnetic domain walls in a thin Co film.

A completely different type of dynamics can be controlled using patterning: magnetic domain wall mobilities.^3,4 A Co film bilayer (each film 0.6 nm thick) was covered by an array of square Co dots created using ion beam etching.⁵ The function of the dots was to produce stray fields of sufficient strength in the bilayer to affect domain wall motion. Significant effects on wall mobility were observed, demonstrating for the first time that domain wall motion can be controlled using simple, field controllable, magnetic dot elements.

Some of the most exciting results in recent years have emerged from studies of how conduction currents interact with magnetization. One result is that conduction currents can cause magnetic domain boundary walls to move. The physics can be understood simply in terms of reflection and transmission of spins from the magnetic domain wall, which acts like a four point resistor in an effective circuit model.⁶ A number of exciting new applications are being explored for new logic schemes and data storage technologies.

References

1. R. E. Camley and R. L. Stamps, J. Phys.: Condensed Matter, 1993, 5, 3727
2. M. Kostylev, P. Schrader, R. L. Stamps, G. Gubbiotti, G. Carlotti, A. O. Adeyeye, S. Goolaup, N. Singh, Appl. Phys. Lett. 2008, 92, 32504
3. M. Bauer, A. Mougin, J. P. Jamet, V. Repain, J. Ferre, R. L. Stamps, H. Bernas, C. Chappert, Phys. Rev. Lett. 2005, 9420, 7211
4. P. J. Metaxas, J. P. Jamet, A. Mougin, M. Cormier, J. Ferre, V. Baltz, B. Rodmacq, B. Dieny, R, L. Stamps, Phys. Rev. Lett., 2007, 99, 217208
5. P. J. Metaxas, P.-J. Zermatten, J.-P. Jamet, J. Ferre, G. Gaudin, B. Rodmacq, A. Schuhl, R. L. Stamps, Appl. Phys. Lett., 2009, 94, 132504
6. P. E. Falloon, R. A. Jalabert, D. Weinmann, R. L. Stamps, Phys. Rev. B 2004, 70, 174424