Anisotropy Of Epitaxial Films Research Articles

Harnessing ion beam erosion engineering for controlled self-assembly and tunable magnetic anisotropy in epitaxial films

The engineering of the surface morphology and the structure of the thin film is one of the essential technological assets for regulating the physical properties and functionalities of thin film-based devices. This study presents an easy and handy approach to tailor the surface structure of epitaxial thin films utilizing low-energy ion beam. Here, we investigate the evolution of the surface structure and magnetic anisotropy (MA) in epitaxial Fe/MgO (001) model systems subjected to multiple cycles of ion beam erosion (IBE) after thin film growth. The growth of Fe film occurs in the form of three–dimensional islands and exhibits intrinsic biaxial MA. Following a few cycles of IBE, an induced uniaxial magnetic anisotropy leads to a split in the hysteresis loop, and the film displays almost uniaxial magnetic switching behavior. More distinctly, we present a clear and conclusive evidence of (2 × 2) reconstruction of the Fe surface due to the atomic rearrangement by IBE. Furthermore, 57Fe isotope sensitive nuclear resonance scattering measurement provides insight into the depth-resolved magnetic information due to the modified surface topography. We also demonstrate that thermal annealing can reversibly tune the surface reconstruction and induced UMA. The feasibility of the IBE technique by adequately selecting IBE parameters for surface structure modification has been highlighted apart from conventional tailoring of the morphology for the tuning of UMA and introduces a new dimension to our understanding of self-assembled surface morphology evolution by IBE.

Plastic relaxation anisotropy in epitaxial films grown on miscut Si(001) substrates

A way of calculating the structural parameters of semiconductor epitaxial layers grown on miscut (0 0 1) substrates is developed. It is shown that the difference between the tensor of film elastic strains and the spherical tensor of strains translating the film’s crystal lattice into the substrate’s lattice is a tensor whose components are proportional to X-ray strains. The technique is used to analyze GaAs and Ge0.2Si0.8 films grown on (1 1 13) Si substrates. The anisotropy of the degree of plastic relaxation is established for lateral directions perpendicular and parallel to the interfacial steps in the GeSi film. It is proposed that noninteger Miller indices be used to denote the direction and value of the rotation of the film’s crystal lattice with respect to the substrate lattice.

Evidence of high-spin Ru and universal magnetic anisotropy in SrRuO3thin films

We have investigated the origins of enhanced saturated magnetic moment and strong strain-dependent magnetic anisotropy in epitaxial films of the transition-metal oxide SrRuO${}_{3}$. We find evidence for an enhancement of the carrier concentration and the stabilization of high-spin Ru${}^{4+}$. Through x-ray magnetic circular dichroism measurements, we also observe a strain-dependent enhancement of the out-of-plane orbital magnetic moment. Such an enhancement is consistent with the universal out-of-plane anisotropy axis shown to occur in films in a variety of different crystallographic orientations. We explain all these effects in terms of an anisotropic reduction in the crystal field resulting from anisotropically reduced orbital overlap in distorted films.

Magnetocrystalline volume and interface anisotropies in epitaxial films: Universal relation and Néel’s model (invited)

The thickness dependence of the fourfold in-plane magnetic anisotropy was first observed in epitaxial Fe(001) films and described by the volume anisotropy of bcc Fe with a positive anisotropy constant (K1vol>0) superimposed by a negative interface term (K1int<0). This results in a spin reorientation of the easy axis from [100] in thick films to [110] for thicknesses tFe<tcrit=6 ML. To clarify the origin of the interface term Fe and bcc Fe1−xCox alloy films (x<0.8) epitaxially grown on different substrates—Ag(001), Au(001), GaAs(001)—were studied. As a result, magnetoelastic interactions can be ruled out because exactly the same anisotropies are found for tensile strain [Fe on Ag(001) and Au(001)] and for compressive strain [Fe on GaAs(001)]. Also, K1int is not affected by the particular interface material or overlayer, e.g., vacuum or Au. A universal correlation was found between the volume and interface anisotropy constants: −K1int/K1vol=tcrit=6 ML. It is shown that this is indeed expected within Néel's phenomenological theory of magnetic anisotropy if contributions from nearest and next-nearest neighbors are taken into account. Electronic hybridization effects in this case seem to play a minor role compared to the local symmetry. It is suggested that observed effects of overlayers on perpendicular interface anisotropies might be related to surface relaxations which have not been sufficiently studied so far and which, on the other hand, would not affect the fourth-order in-plane anisotropy according to Néel's model in agreement with present experimental results.

Magnetic Anisotropy Constants of Epitaxial (110) Fe/GaAs Films from 77k To 293k Studied by Magneto-Resistance

ABSTRACTA first-order phase transition in the magnetization of Fe films, driven by an applied magnetic field, was first reported by Hathaway and Prinz [1]. Further studies were performed on this phase transition using anisotropic magnetoresistance measurements by Riggs and Dahlberg [2]. Here we report the extension of these studies to include temperatures between 77K and 293K. Emphasis is on a determination of the fourth-order and uniaxial anisotropy constants (K1 and Ku). It is shown that the temperature dependence of the anisotropy energies in these films varies with thickness, which may be useful in sorting out the origin and magnitude of different contributions to the total effective anisotropy. The present study suggests that similar studies of (110) iron on other substrates might contribute to achieving a better understanding of in-plane anisotropies in epitaxial films.

Annealing behavior and temperature dependence of the growth-induced magnetic anisotropy in epitaxial Sm-YIGG

The annealing behavior and temperature dependence of the growth-induced magnetic anisotropy in epitaxial films of Y2.6Sm0.4Ga1.2Fe3.8O12 have been investigated. This garnet composition is presently being widely used for magnetic bubble domain device work. Decay times for the anisotropy are tens of hours at 1250°C and several hours at 1325°C. This rapid decrease in annealing time with increasing annealing temperature can be understood by postulating that oxygen vacancies play a significant role in determining the diffusion rate and that the oxygen vacancy concentration increases rapidly with increasing temperature. From these considerations, a lower limit of 2.8×107 yr is estimated for the decay time of the anisotropy at 100°C, based on an assumed activation energy of 1 eV. By using torque magnetometer measurements, the anisotropy has been found to vary by about an order of magnitude over the temperature range −10 to 110°C. Only two different kinds of temperature dependences were observed among films grown on four differently oriented substrates, in agreement with predictions of the site-preference model for growth-induced anisotropy. Also described is a new torque magnetometer technique for measuring the effective anisotropy field in magnetic materials which support bubble domains.

Growth-Induced Magnetic Anisotropy in Epitaxial Garnet Films

Using the models established for the growth-induced anistropy in flux-grown crystals and parameters determined from anisotropy measurements in bulk crystals, the anisotropy in epitaxial films is predicted. Comparison with experiment shows that the signs of the anisotropy are correctly predicted and that magnitudes are in fair agreement.

Mobile Cylindrical Magnetic Domains in Epitaxial Garnet Films

The properties of cylindrical domains in wafers of bulk single crystals of various magnetic oxides have been extensively studied and described in recent months. Mobile cylindrical domains have been produced in single-crystal thin films of gallium-substituted yttrium iron garnet. The deposits are formed by chemical vapor deposition onto nonmagnetic garnet substrates greater than 1 cm2 in area. A model of magnetic anisotropy in epitaxial films is proposed and shown to be in good agreement with experimental domain observations. In a typical sample, a domain diameter of 8 μ is observed in a 6-μ-thick film at a bias field of 25 Oe. From the domain translational velocity of 5600 cm/sec, a cylindrical domain mobility of 1500 cm/sec Oe has been calculated. A fractional change in cylindrical domain diameter of −0.07%/°K has been measured at 300°K. Annealing at 1570°K for several hours did not alter the Néel temperature or the optical transmission spectrum.