Epitaxial Magnetic Thin Films Research Articles

Tailoring the topology of an artificial magnetic skyrmion.

Despite theoretical predictions, it remains an experimental challenge to realize an artificial magnetic skyrmion whose topology can be well controlled and tailored so that its topological effect can be revealed explicitly in a deformation of the spin textures. Here we report epitaxial magnetic thin films in which an artificial skyrmion is created by embedding a magnetic vortex into an out-of-plane aligned spin environment. By changing the relative orientation between the central vortex core polarity and the surrounding out-of-plane spins, we are able to control and tailor the system between two skyrmion topological states. An in-plane magnetic field is used to annihilate the skyrmion core by converting the central vortex state into a single domain state. Our result shows distinct annihilation behaviour of the skyrmion core for the two different skyrmion states, suggesting a topological effect of the magnetic skyrmions in the core annihilation process.

Effect of strain on the magnetic anisotropy in the epitaxial magnetic thin films

A phenomenological thermodynamic theory of magnetic thin films epitaxially grown on cubic substrates is developed by considering the thermodynamic potential, which corresponds to the actual mechanical boundary conditions. For clamped magnetic thin films with two-dimension strains, the study reveals that the magnitudes of remanent magnetization M r and coercive field H c are dependent on the strain induced by the lattice mismatch. The tensile strain can significantly enhance the out-of-plane magnetization with a larger coercivity, while the compressive strain makes correspondingly thin films exhibit an opposite in-plane magnetic anisotropy. Our analysis provides a profound understanding of magnetic anisotropy mechanism and the relationship with lattice strain in the epitaxial magnetic thin film.

Symmetry breaking effects in epitaxial magnetic thin films: Nonsymmetric reversal and butterfly remanence behavior

A consistent picture which explains the effects of magnetic anisotropy symmetry breaking on the magnetic properties of epitaxial magnetic thin films is provided, including experimental results in ${\ensuremath{\gamma}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{Fe}}_{4}\mathrm{N}(100)$ thin films and numerical simulations. As expected for a thin film with fourfold crystal symmetry, the results show the existence of two easy and two hard magnetization axes. However, in this case, the easy axes are not orthogonal, the hard axes are not equivalent, the magnetization reversal behavior around the two easy axes is not symmetric, and the reversal behaviors of the two hard axes are not alike. As a consequence, the polar plot of the remanence displays a butterfly shape behavior. These effects depend on additional terms of the magnetic anisotropy. Our results are extended to other symmetry breaking epitaxial magnetic systems.

Wideband detection of transient solid-state dynamics using ultrafast fiber lasers and asynchronous optical sampling

We demonstrate optical time-domain spectroscopy from femtoseconds to nanoseconds using an ultrafast dual-fiber-laser system with kilohertz continuous scanning rates. Utilizing different wavelengths for the pump and probe beams, we exploit this system's broad range of timescales for quantitative studies of thermal transport and the detection of coherent spin and lattice excitations in epitaxial magnetic thin films. The extraordinary temporal dynamic range provides a way to connect the fast and slow timescales in the observation of dissipation and decoherence processes.

Nonlinear optical spectroscopy of epitaxial magnetic garnet films

Second and third harmonic optical spectra are studied in epitaxial magnetic thin films in the spectral ranges 1.7–3.2 eV and 2.4–4.2 eV, respectively. No significant increase of the intensity of the nonlinear spectra is found above the bandgap near 3.2 eV, where the linear absorption increases by two orders of magnitude. Large magnetic contributions to the second harmonic spectra and magnetic contrast as high as 100% are observed at selected photon energies. Contrary to that, no magnetic contribution to the third harmonic spectra is found.

Magnetic characterization of (001) and (111) Ni films epitaxially grown on MgO

ABSTRACTOne interesting application of epitaxial magnetic thin films is to use them as one of the electrodes in a spin-dependent tunneling junction, in order to use the magnetocrystalline anisotropy to define the required two states of the magnetization. [1] In our preliminary work, we prepared epitaxial magnetic films on copper buffer layers grown on silicon substrates. [2] The single crystalline quality of the films was particularly evident in the magnetization hysteresis loops, showing a sharp reversal at fairly high fields (120 Oe), when the samples were magnetized along the crystallographic easy axis. One technological disadvantage in this type of samples is the chemical interaction between the metallic layers and the silicon substrate.In order to explore the possibility of epitaxial magnetic films on less reactive substrates, we studied the growth on MgO substrates. We have shown that it is possible to obtain epitaxial (001) and (111) Ni films grown on MgO substrates. [3] In particular we observed that the crystalline quality of the films improved considerably after 10 nm of film growth. We will now present our studies on the magnetic properties of these films, particularly the azimuthal dependence of the magnetization reversal using MOKRE, correlating our finding with the structural characterization obtained with RHEED, STM and XRD.

Magnetization Reversal by Rotation in Magnetic Thin Films Deposited on Step Bunched Si(111) Substrates

ABSTRACTMagnetic and magnetoresistive properties of magnetic thin films deposited on periodically structured substrates have been studied. By using a thermodynamical heat treatment, we activate the step bunching phenomenon on vicinal Si(111) substrates misoriented towards [112] which results in a laterally modulated surface with a period around 0.1 μm. Epitaxial magnetic thin films deposited on these substrates exhibit an in-plane uniaxial anisotropy and a specific magnetoresistive behavior which characterizes the magnetization reversal process. In this contribution, we demonstrate that these properties can also be observed in non-epitaxial structures grown either by molecular beam epitaxy or by sputtering. In particular, magnetotransport measurements show that by varying parameters such as the nature of the magnetic metal, the layer thickness or the strength of the induced anisotropy, we can modulate the interplay between the anisotropy and the exchange stiffness, and, consequently, observe gradual magnetization reversal behaviors between a Stoner-Wohlfarth uniform mode and a non-uniform mode.

Magnetic domain structure in ultrathin Cu/Ni/Cu/Si(001) films (invited)

We present a series of magnetic force microscope (MFM) images of epitaxial magnetic thin films. The films studied, Ni/Cu/Si(001) capped by 2 nm of Cu, exhibit perpendicular anisotropy over an exceptionally broad thickness range, 2 nm<h<14 nm. The magnetic domain structure of the as-grown films shows a sharp transition to a finer length scale above a finite critical thickness of 12 nm. Micromagnetic theory provides the first quantitative description for these general but previously unexplained phenomena. Further we discuss MFM data obtained on films with a thickness larger than 14 nm. These films show a pronounced in-plane anisotropy.

Magnetic Domain Structure in Ultrathin Films.

The first magnetic force microscope (MFM) images of a series of epitaxial magnetic thin films is presented. The films studied, Ni $/$Cu $/$Si(001) capped by 2 nm of Cu, exhibit perpendicular magnetization over an exceptionally broad Ni thickness range $2lhl14$ nm. The Ni domain structure shows a sharp transition to a finer length scale above a finite critical thickness of order 9 nm. The average force measured by the MFM tip reflects this refinement in domain structure. Micromagnetic theory, combined with our measurements of ${K}^{\mathrm{eff}}\left(h\right)$, provides the first quantitative description for these general but previously unexplained phenomena.