Magnetization reversal processes and hysteresis reflect the responses of magnetization to applied fields, and thus they are fundamental to understand reversal mechanisms in nanomagnetism. Epitaxial artificial structures represent model systems to study the influence of reduced dimensionality and symmetry on magnetic properties. For instance, the broken symmetry at surfaces and/or interfaces creates contributions to the magnetic anisotropy, leading to alter both magnetization easy-axes directions and reversal processes. We report novel features observed in epitaxial magnetic thin films with broken symmetry.
Thin films of iron nitride have been grown on Cu(100) single-crystals by molecular beam epitaxy of Fe in the presence of a beam of atomic N provided by a radio-frequency plasma source. The mechanisms of growth have been studied from the early stages up to 50 nm thick films by scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and Auger electron spectroscopy. Under the appropriate growth conditions, the films are epitaxial and single-phase γ'-Fe4N(100) [1]. The four-fold symmetry of the films is broken by a two-fold contribution, probably originating from stress relaxation of the film.
The effects of symmetry breaking of magnetic anisotropy on magnetization reversal behaviour have been investigated by high resolution vectorial Kerr magnetometry and numerical simulations. In general, the reversal depends on the applied magnetic field angle and on the anisotropies involved. As expected for a thin film with four-fold crystal symmetry, the results show the existence of two easy and two hard magnetization axes. But, in this case, the easy axes are not orthogonal, the hard axes are not equivalent, the magnetization reversal behaviour around the two easy axes is not symmetric, and the reversal behaviours of the two hard axes are not alike [2]. As a consequence, the polar plot of the remanence displays a "butterfly" shape behaviour. These effects depend on additional terms of the magnetic anisotropy. Our results are extended to other symmetry breaking epitaxial magnetic systems.
[1] J. M. Gallego et al, Phys. Rev. B 69, 121404(R) (2004); ibid, Phys. Rev. Lett. 95, 136102 (2005).
[2] Ecija et al. submitted to Phys. Rev. Lett.
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