Anisotropy Of Ultrathin Films Research Articles

Determining the Characteristics of Ultrathin Polymer Films: A Spectroscopic Ellipsometry Approach.

Ellipsometry is a powerful and convenient technique that is widely used to determine the thickness and optical characteristics of polymer thin films. The determination is accomplished by modeling the measured change in the polarization of an electromagnetic wave upon interacting with the thin film. However, due to the strong statistical correlations between the fit parameters in the model, simultaneous determination of the thickness and the refractive indices of optically anisotropic ultrathin films using ellipsometry remains a challenge. Here, we propose an approach that can be used to obtain reliable values of both the thickness and the optical anisotropy of ultrathin polymer films. The approach was developed by performing spectroscopic ellipsometry measurements on thin films of hydrophobic polystyrene and hydrophilic chitosan of thickness between a few tens to a few hundred nm and whose absolute value of the birefringence differed by approximately an order of magnitude. Careful consideration of the characteristics of the root mean squared error of the fits obtained by modeling the ellipsometry data and the statistical correlations between the fit parameters formed the basis of the proposed approach.

Aging effects in critical behavior of Heisenberg anisotropic ultrathin films

We present the results of Monte-Carlo studies of the non-equilibrium properties of ferromagnetic Heisenberg films. Aging effects were observed in non-equilibrium critical behavior. The calculations were carried out for both high-temperature and low-temperature initial states. The characteristic correlation time, which diverges at the transition temperature in the thermodynamic limit, was obtained as a function of system size and waiting time.

Renormalization group analysis of the finite two-dimensional XY model with fourfold anisotropy: Application to the magnetic susceptibility of a ferromagnetic ultrathin film

The topological phase transition introduced by Kosterlitz and Thouless has become a central concept in condensed matter physics. With the increasing relevance of topological phases to applications in spintronics, it is important to extend the description of the Kosterlitz-Thouless transition in an infinite, isotropic two-dimensional XY ferromagnet to experimentally realizable systems such as a finite, anisotropic ultrathin film with fourfold in-plane anisotropy. Using a renormalization group analysis, this description is used to calculate the magnetic susceptibility for comparison to experiment.

Interface-engineered oxygen octahedral coupling in manganite heterostructures

Control of the oxygen octahedral coupling (OOC) provides a large degree of freedom to manipulate physical phenomena in complex oxide heterostructures. Recently, local tuning of the tilt angle has been found to control the magnetic anisotropy in ultrathin films of manganites and ruthenates, while symmetry control can manipulate the metal insulator transition in nickelate thin films. The required connectivity of the octahedra across the heterostructure interface enforces a geometric constraint to the 3-dimensional octahedral network in epitaxial films. Such geometric constraint will either change the tilt angle to retain the connectivity of the corner shared oxygen octahedral network or guide the formation of a specific symmetry throughout the epitaxial film. Here, we will discuss the control of OOC in manganite heterostructures by interface-engineering. OOC driven magnetic and transport anisotropies have been realized in LSMO/NGO heterostructures. Competition between the interfacial OOC and the strain further away from the interface leads to a thickness driven sharp transition of the anisotropic properties. Furthermore, octahedral relaxation leading to a change of p-d hybridization driven by interfacial OOC appears to be the strongest factor in thickness related variations of magnetic and transport properties in epitaxial LSMO films on NGO substrates. The results unequivocally link the atomic structure near the interfaces to the macroscopic properties. The strong correlation between a controllable oxygen network and the functionalities will have significant impact on both fundamental research and technological application of correlated perovskite heterostructures. By controlling the interfacial OOC, it is possible to pattern in 3 dimensions the magnetization to achieve non-collinear magnetization in both in-plane and out of plane directions, thus making the heterostructures promising for application in orthogonal spin transfer devices, spin oscillators, and low field sensors.

In situmonitoring of X-ray strain pole figures of a biaxially deformed ultra-thin film on a flexible substrate

X-ray strain pole figures (SPFs) have been capturedin situduring biaxial deformation of a gold ultra-thin film (thickness = 40 nm) deposited on a polymer substrate. An area detector was used to extract one line in the reciprocal space while the strained sample was rotated azimuthally step by step to produce the SPF. Such SPFs have been obtained for a textured anisotropic ultra-thin film under controlled non-equibiaxial loading using the SOLEIL synchrotron DIFFABS tensile device. The experimental setup allows the pole figure measurements of {111} and {200} reflections to be performed simultaneously. Interestingly, those two crystallographic directions are related to the two-extreme elastic mechanical behaviour. The full directional lattice strain dependence (SPF) is obtained within 15 min and can be monitored step by step upon loading. This procedure gives an insight into ultra-thin film mechanical response under complex biaxial loading.

Inverse relationships for ellipsometry of uniaxially anisotropic nanoscale dielectric films on isotropic materials

The possibilities of determining the optical parameters of uniaxially anisotropic non-absorbing ultrathin films on the basis of ellipsometric parameters are analyzed in the framework of a long-wavelength approximation. It is shown that the special convenience of this analytical approach lies in the fact that it enables to find out the situations where it is possible to decouple the optical constants and the thickness (to provide correlation-free measurements) of an anisotropic ultrathin film. The accuracy of the obtained formulas for determining the parameters of ultrathin films is estimated by computer simulation of the reflection problem on the basis of the exact electromagnetic theory for anisotropic layered systems.

Correlation-free reflection diagnostics of biaxially anisotropic nanoscale films on transparent isotropic materials

The possibilities of determining the parameters of biaxially anisotropic ultrathin dielectric films on transparent isotropic substrates by differential reflectance and ellipsometric measurements are analyzed. The analysis is based on analytical reflection formulas obtained in the framework of a long-wavelength approximation. It is shown that it is possible to simultaneously determine the thickness and all six parameters of anisotropy for such films. The accuracy of the obtained analytical formulas for determining the parameters of anisotropic ultrathin films is estimated by computer simulations where the reflection problem was solved numerically on the basis of the rigorous electromagnetic theory for anisotropic layered systems.

Reflection equivalence of inhomogeneous and uniaxially anisotropic dielectric films withoptical axis normal to the film surface

A comparison between light reflection from inhomogeneous and uniaxially anisotropicdielectric films on isotropic and homogeneous substrates is carried out. The reflection frominhomogeneous and uniaxially anisotropic ultrathin films is analyzed using the equationsobtained in a long-wavelength approximation. Numerical calculations based onrigorous electromagnetic reflection theory for inhomogeneous and anisotropicinterference films are also realized. It is shown that in the case of the absorbingsubstrate the reflection properties of an ultrathin inhomogeneous dielectric film, therefractive index of which varies only in the direction perpendicular to the layer,are equivalent to the reflection properties of a uniaxially anisotropic ultrathinfilm, the optical axis of which is normal to the film surface. For interference filmson absorbing materials such equivalency takes place only in the case of weaklyinhomogeneous films. For dielectric films on transparent substrates it is generallylacking. Solely ellipsometric angles for inhomogeneous and uniaxially anisotropicultrathin films are equal in the first order in the ratio of film thickness to wavelength.

Reflection Characterization of Nanoscopically Stratified Surface Structures and Optical Probing of Anisotropic Nanolayers

The reflection of linearly polarized light from a layered nanoscopic system of anisotropic dielectric films on homogeneous isotropic substrate is investigated in the long-wavelength limit. The expressions for the reflectances and ellipsometric angles of an N-layer system of anisotropic ultrathin films are derived. For absorbing substrate mathematical relationships take a relatively simple additive form. In comparison with absorbing substrate the fully transparent system is more complicated and in this case an ultrathin film in multilayer system is sensitive to the presence of other ultrathin films (an interaction between them emerges) and the contribution of each layer to the reflection coefficient is not expressed in a purely additive form. The analytical results are supported by computer-aided analysis made on the basis of general wave propagation theory for anisotropic layered media. The most useful feature of obtained approximate expressions is that they are simply invertible, allowing a direct calculation of the parameters of ultrathin layers. [DOI: 10.1380/ejssnt.2010.167]

Ion sculpting: A tool for tuning magnetic anisotropy in ultrathin films

The possibility of modifying the magnetic properties of ultrathin films by means of ion sculpting has been verified for Co/Cu(001) and Fe/Ag(001). For both these systems, ion sputtering at grazing incidence performed in a proper, material-dependent, temperature range induces the formation of ripples at the surface of the films. Following ripple formation, the magnetic anisotropy in the plane of the films undergoes dramatic changes brought about by the out-of-equilibrium distribution of defect sites induced by the ripple structure itself.

Reflection of light from nanoscopically stratified anisotropic media and optical probing of dielectric nanofilms

The reflection of linearly polarized light from a layered nanoscopic system of anisotropic dielectric films on homogeneous isotropic substrate is investigated in the long-wavelength limit. The expressions for reflection characteristics of an N-layer system of arbitrarily anisotropic ultrathin films are derived. The analytical results are supported by computer-aided analysis made on the basis of general wave propagation theory for anisotropic layered media. We show that the accuracy of the long-wavelength approximation for nanoscopic anisotropic multilayers is quite satisfactory: if the thickness of a multilayer divided by the wavelength comprises a few hundredths, then the errors of approximate formulas do not exceed a few percent. The most useful feature of obtained approximate expressions is that they are simply invertible, allowing a direct calculation of the parameters of ultrathin layers.

Growth and magnetic anisotropy of Fe films deposited on Si(111) using an ultrathin iron silicide template

The growth and magnetic properties of thin Fe films deposited at room temperature on ultrathin iron silicide seed layer epitaxially grown on Si(111) single crystal were investigated as a function of Fe thickness ($0<{t}_{\mathrm{Fe}}<300$ monolayers). The growth mode and structure have been determined in situ by means of scanning tunneling microscopy, low energy electron diffraction, and x-ray photoelectron diffraction. The magnetic properties were characterized ex situ by conventional polar and longitudinal magneto-optical Kerr effect, transverse biased initial inverse susceptibility and torque (TBIIST) measurements, and superconducting quantum interference device magnetometry. Fe growth is of Volmer-Weber type (island growth) and the epitaxial film adopts the bcc $\ensuremath{\alpha}$-Fe structure. Onset of long-range ferromagnetic order occurs at 4.7 monolayers (ML), in the vicinity of the percolation threshold of the Fe islands. The Curie temperature increases continuously with Fe coverage, varying from 135 K for 4.7 ML to 260 K for 7.3 ML. Two different spin reorientation transitions have been observed versus Fe coverage. First, the magnetic easy axis rotates from normal to the film plane, for coverage below 6 ML, to in plane, for thickness above 7 ML. Then, in-plane magnetized films present tiny (anisotropy fields less than 4 Oe) uniaxial and sixfold magnetic anisotropies. From sixfold anisotropy the small higher order cubic anisotropy constant ${K}_{2}$ was measured precisely by TBIIST. It decreases monotonously with increasing coverage and changes its sign at approximately 20 ML, which in turn results in a switching of the sixfold anisotropy easy axis from ⟨1--10⟩ to ⟨1--21⟩ directions. It appears that TIBIIST magnetometry is a powerful method for a quantitative determination of the various contributions to in-plane magnetic anisotropies in ultrathin films.

Effects of step decoration by oxygen on ultrathin film magnetic anisotropy; p(1×1)Fe on vicinal W(100)

The mechanisms responsible for surface-step-induced magnetic anisotropy in ultrathin films on vicinal surfaces were explored by studying the effects resulting from surface oxygen adsorption. Magnetic hysteresis loops produced by ultrathin (1–4 ML) Fe films on a variable-step-density W(100) surface were measured using the magneto-optic Kerr effect. Auger electron spectroscopy was used to determine surface oxygen coverage. Significant reductions of the switching fields that characterize the uniaxial surface-step-induced anisotropy were observed. The oxygen-coverage dependencies were found to be consistent with step decoration (preferential chemisorption at steps) and support the highly localized nature of magnetic anisotropy energies associated with the Néel pair-bonding model of surface-step-induced magnetic anisotropy.

Metal–insulator transition and magnetotransport in III–V compound diluted magnetic semiconductors

Structural, magnetic and transport properties of diluted magnetic semiconductors, (Ga, Mn)As and (In, Mn)As, have been investigated. Manganese can be substitutionally doped into the group III site of the zincblend structure up to several percent. With Mn content of a few percent, these systems exhibit ferromagnetism at low temperatures. The highest Curie temperature so far achieved is ∼100 K for (Ga, Mn)As. The saturated magnetization values are consistent with S=5/2 local moment, suggesting divalent Mn which acts as an acceptor. The system becomes metallic with increasing Mn content, but a further increase of Mn content tends to decrease the hole density and increase disorder so that the system becomes nonmetallic again at higher Mn concentrations. Large negative magnetoresistance and highly anisotropic transport are observed in the semiconducting samples at low temperatures. The magnetic anisotropy in ultrathin films is found to be strongly affected by the lattice-mismatch-induced strain.

Higher-order contribution and temperature dependence of the magnetic anisotropy in ultrathin films (invited)

The pseudomorphic growth of ultrathin films produces artificial structures, and with it anomalously large magnetic anisotropy energies (MAEs). Second- and fourth-order MAEs K2,K4 can be measured and analyzed. To understand the continuous (steplike) rotation of M in a spin reorientation transition (SRT), K4<0(K4>0) is necessary. By means of ferromagnetic resonance measurements we show that for fct Ni/Cu(001) the SRT is of second order. It is important to measure the “surface” and “volume” contribution for all MAE contributions K2,K4,… as a function of temperature. These results are compared with T=0 K first principle electronic structure calculations.

Temperature-driven in-plane anisotropy reorientation transition in Fe(110) films

The competition between surface and volume magnetic anisotropy in ultra-thin films may induce a dramatic thickness and temperature dependence of the anisotropy. Whereas the former has been studied extensively, only a few reports have appeared on the latter for in-plane anisotropy. We have analysed such an in-plane reorientation in a W(110)/Fe(110)(60Å)/W(110) film, which occurs because of the differences in the temperature dependencies of the various contributions involved in the total anisotropy. The second-order interface anisotropy constant is deduced for this system (100 to 300 K) from total anisotropy measurements and subtraction of bulk contributions. The thermal variation of the interface anisotropy constant with the reduced spontaneous magnetization m s is found to follow an m s 13/2 law. Phase diagrams are derived.

The temperature dependence of magnetic anisotropy in ultra-thin films

Results for the reorientation of the magnetization in 7.6 ML Ni(001)/Cu(001) as a function of temperature T are presented. The magnetocrystalline anisotropy coefficients up to fourth order are determined by in situ UHV ferromagnetic resonance. The angle θ eq of the spontaneous magnetization measured from the film normal is found to change continuously from 90° at 200 K, i.e. in-plane, to 16° at 250 K, to 0° above 300 K. The unique reorientation of the magnetization to a perpendicular direction above a critical temperature T R is described quantitatively by the temperature dependence of the volume and interface anisotropy contributions K V 2 and K S 2 which were also measured. This is compared with the case of Gd(0001)/W(110) and Ni(111)/W(110). One finds that there is no universal behavior for the temperature dependence of K V 2 and K S 2. In the case of Ni, the behavior of K V 2 is well described by a magnetoelastic model which uses only the temperature dependence of the elastic and magnetostrictive constants of bulk Ni and the lattice mismatch to the substrate.

Magnetic dichroism in X-ray absorption spectroscopy

A theoretical treatment of magnetic circular and linear dichroism in X-ray absorption using nj symbols is presented. A practical example is given to determine the magnetocrystalline anisotropy in ultrathin films on magnetic substrates.

The statistical roughness and magnetic surface anisotropy

The magnetic surface anisotropy in ultrathin films with a distribution of surface roughness is calculated with a magnetostatic model. The surface roughness can give the famous 1/t dependence of shape anisotropy for thick films; however, the anisotropy deviates from 1/t dependence for very thin films. It can be concluded that a rougher film results in a larger surface anisotropy. The calculated results are reasonably consistent with the experiment. >

Magnetic anisotropy in ultrathin films grown on vicinal surfaces (abstract)

Growth of thin epitaxial magnetic films on terraced substrates, such as Co/Cu(1 1 13) vicinal to (001), leads to unexpected magnetic anisotropy associated with the surface steps. This anisotropy remains largely unexplained and is of potential technological importance. We have studied the anisotropy of fcc Co/Cu(1 1 13) films of 3–20 ML thicknesses using in situ MOKE. The presence of monoatomic steps on the Cu(1 1 13) surface induces a uniaxial anisotropy favoring magnetization parallel to the steps. The strength of this anisotropy decreases with increasing film thickness. In this paper we focus on the interpretation of this anisotropy in terms of the Néel model.1,2 Vicinal surfaces are characterized by (001) terraces separated by monoatomic steps. The Néel anisotropy energy for such a surface has the form EtotalN= Ebulk atoms+(Esurface atoms /t) + Estep-edge atoms /td), where t and d are film thickness and terrace width, respectively. The last term includes contributions from sites at both the top and bottom of the step and is responsible for the step-induced anisotropy. For 10 ML of Co on Cu(1 1 13), the magnitude of this term is approximately 0.3 erg/cm2, favoring M parallel to the steps, as we have observed. Fe/W vicinal to (001) films show a preference for in-plane magnetization perpendicular to the steps.3 The Néel model predicts this anisotropy for a bcc Fe film provided the magnetization lies in the film plane, not in the (001) plane.