Fe Cr Superlattices Research Articles

Kinetic Monte Carlo simulation of phase-precipitation versus instability behavior in short period FeCr superlattices

The structural evolution of FeCr superlattices has been studied using a quasi-atomistic Object Kinetic Monte Carlo model. Superlattices with different spatial periods have been simulated for anneal durations from few hours to several months at 500°C. Relatively-long period superlattices stabilize into Fe-rich and Cr-rich layers with compositions close to those of bulk α and α′ phases. In contrast, superlattices with very short periods (4, 5, 6nm) are observed to undergo instability and, for long annealing times, evolve into three-dimensionally decomposed regions, in qualitative agreement to recent experimental observations. The instability onset is delayed as the spatial period increases, and it occurs via interface roughness. This evolution can be explained as a minimization of the free-energy associated to the α/α′ interfaces. A comprehensive description of the evolution dynamics of FeCr-based structures is obtained with our model.

Comment on “Scaling of the Interface Roughness in Fe-Cr Superlattices: Self-Affine versus Non-Self-Affine”

A Comment on the Letter by J. Santamaria et al., Phys. Rev. Lett. 89, 190601 (2002). The authors of the Letter offer a Reply.Received 17 December 2002DOI:https://doi.org/10.1103/PhysRevLett.91.119601©2003 American Physical Society

The Role of Intermixing in the Phase Determination of the Interlayer Exchange Coupling in Multilayers: Application to Fe-Cr Superlattices

The phase of the short-range interlayer exchange coupling oscillations in Fe n Cr m superlattices is expressed in terms of the intermixing at the Fe-Cr as well as at the Cr-Fe interfaces. The interdiffusion is modeled through stochastic algorithms, which presupposes floating of the atoms on the surface of the sample during the epitaxial growth. It automatically leads to the different chemical and magnetic structure of Fe on Cr and Cr on Fe interfaces. Self-consistent calculations of the magnetic moments are performed on the basis of the periodic Anderson model. Although short-range (2 monolayers (ML)) oscillations of exchange coupling were detected for all considered structures, its amplitude and phase strongly depend on the interface alloying. Introduction of the same intermixing at both interfaces does not change the phase as compared to the ideal superlattices with sharp interfaces. However different interdiffusion leads to the ~ -phase shift in accordance with experimental results for the Fe-Cr-Fe trilayers grown on an Fe whiskers. Distribution of magnetic moments on Fe atoms contains several distinct peaks but their position and relative area weakly depend on the alloying and interlayer exchange coupling in the superlattice. On the contrary, magnetic moments on Cr atoms are very sensitive to the intermixing and their behavior determines the strength and the phase of exchange coupling oscillations.

Quantitative roughness of sputtered Fe-Cr superlattices

We obtained quantitative values of all significant parameters describing the roughness of Fe-Cr superlattices, both in the lateral and growth directions, by statistical analysis of energy-filtered transmis- sion electron microscopy images using cross section samples. These results are in good agreement with the complementary low-angle X-ray scattering measurements. The interface roughness of sputtered Fe-Cr superlattices was changed systematically by varying Ar-pressure during the growth and the number of the bilayers. By scaling local window size we obtained the dependence of the saturated roughness and its correlation lengths in both the lateral and growth directions. The roughness and its correlation lengths (lateral and perpendicular) increase with pressure. However correlation length in the lateral is constant with bilayer index for low-pressure sputtered samples

Scaling of the interface roughness in Fe-Cr superlattices: self-affine versus non-self-affine.

We have analyzed kinetic roughening in Fe-Cr superlattices by energy-filtered transmission electron microscopy. The direct access to individual interfaces provides both static and dynamic roughness exponents. We find an anomalous non-self-affine scaling of the interface roughness with a time dependent local roughness at short length scales. While the deposition conditions affect strongly the long-range dynamics, the anomalous short-range exponent remains unchanged. The different short- and long-range dynamics outline the importance of long-range interactions in kinetic roughening.

In-plane magnetisation anisotropy of FeCr superlattices with biquadratic exchange coupling

Magnetisation anisotropy in non-collinear ordered (0 0 1)FeCr multilayers has been studied. It was found that the interplay between strong biquadratic exchange coupling and fourfold anisotropy causes change in magnetisation ‘easy’ and ‘hard’ axes during the magnetisation process. A simple phenomenological approach for extracting anisotropy and exchange parameters from magnetisation curves has been suggested.

Magnetic structure of nonideal [formula omitted] interface

Different kinds of space defects on the background of ideal smooth interface in FeCr multilayers, such as steps, embedded clusters, pinholes, random interface roughness, are investigated within model Hamiltonian approach. Calculations of magnetic structure are performed in the periodic Anderson model (PAM) in mean field approximation by recursion method in the real space.The distribution of magnetic moments obtained for two sets of interfaces (with different roughness) shows strong sensitivity of magnetic structure to the interface roughness and interdiffusion. Application of the theory for interpretation of Mössbauer spectra of FeCr multilayers is discussed. Spectra obtained for three perpendicular orientations of the sample relatively to the incident beam allowed to avoid the problem of nonrandom distribution of magnetic moment direction in the multilayers.Information about the angular dependence of hyperfine fields on the Fe atoms in FeCr superlattices obtained by means of Mössbauer spectroscopy technique is discussed within the framework of the theory of noncollinear magnetic structure.

Temperature-dependent study of ion-channeling in [formula omitted] superlattices

Giant- Magneto- Resistance (GMR), as large as 150% at 4 K, occurs in Fe Cr superlattices as a result of antiferromagnetic interlayer coupling. We have successfully grown epitaxial single-crystal Fe Cr multilayers using magnetron sputtering. Ion channeling was employed to study the structural and vibrational properties of the sputter-deposited Fe Cr superlattices, and of a Cr thin film, between temperatures of 100 and 330 K. Channeling in the latter specimen was used to investigate the importance of depositing a Cr-buffer layer in order to obtain superlattices with large GMR values. Once the buffer layer exceeded a critical thickness, a high quality Cr film was observed. The epitaxial quality of the superlattices grown on such buffer layers by sputtering was found to be excellent. Minimum yields nearly equal to theoretical predictions were found for channeling along the 〈001〉 growth direction; slightly higher values were found along the 〈111〉 axis. Because of the high structural quality of the sputter-deposited films, it was possible to investigate changes in thermal vibration amplitudes, even though their magnitude is only of the order of a few pm (10 −12 m). No unusual structural changes of this magnitude were observed in angular channeling scans obtained while cooling the Fe Cr superlattice from 330 down to 100 K.

Interfacial roughness of Fe–Cr GMR superlattices (abstract)

Fe–Cr superlattices exhibit giant magnetoresistance effect which can be utilized in magnetic recording heads. There are many studies indicating the role of the interfacial structure on the magnetoresistance effect. To elucidate this point we have performed x-ray diffraction study of three [Fe(30 Å)/Cr(12 Å)]10 superlattices which had been sputtered in 4, 7, and 10 mTorr of Ar. We used position sensitive detector in horizontal setup to collect simultaneously specular scattering intensity (z component of the scattering vector Q perpendicular to the film) and off-specular intensity with y parallel to the plane of the sample. This technique allows to evaluate interfacial roughness at various length scales. Short range roughness may be due to interdiffusion, while some medium range roughness is also present, and could have effects on transport phenomena. In conventional low angle diffraction analysis these contributions are intermixed. From the Fourier transform of the scattered intensity distribution in Qx−Qy plane the interfacial height fluctuation function g(r)can be obtained (r is a lateral spacing). Figure 1 presents g(r) obtained for three samples of Fe–Cr. It is apparent that 4 mTorr sample is very different than 7 and 10 mTorr in terms of interfacial roughness. We will discuss in detail results of the diffraction studies together with the magnetization and magnetotransport data.

Relation between structural and physical properties in magnetic and superconducting superlattices

The physical properties of superlattices are strongly affected by the chemical and physical properties of the individual layers and by the superlattice structure. In this paper the relationship between structure and properties will be illustrated by the giant magnetoresistance (GMR) effect, as well as the dimensional transitions and pinning mechanisms in respectively magnetic and superconducting superlattices. In the first example we will analyse the effect of the sample structure and interface quality on the GMR of poly crystalline and epitaxial Fe Cr superlattices grown by molecular beam epitaxy. Secondly, the effect of thermal annealing and ion irradiation on the electrical and magnetic properties of Ag Fe multilayers is discussed. In a third example, the influence of a very thin Fe interlayer on the coupling phenomena in Fe Nb multilayers is analysed. Finally, it will be shown that an additional lateral modulation (lattice of submicron holes) in Pb Ge multilayers substantially changes the critical superconducting parameters.

Antiferromagnetic superexchange in interlayer magnetic coupling

Abstract The data on the interlayer coupling in Fe Cr superlattices can be interpreted as if there exists an antiferromagnetic bias at short distances. Although calculations based on the s-d Friedel-Anderson model yield this bias, it has not been found in ab-initio band structure calculations. We discuss possible reasons for this discrepancy.

Influence of nitrogen on 475°C embrittlement of high-chromium ferritic steels

AbstractThe hardening of iron-chromium alloys containing more than 13 wt-%Cr in the temperature range 550–800°C is well known and is ascribed to the formation of σ-FeCr. Severe embrittlement at lower temperatures, commonly referred to as ‘475°C embrittlement’, is less well understood and has been variously ascribed to σ-phase, ordered Fe-Cr superlattices, and coherent precipitation of Cr-rich ferrite. The microstructure and properties of Fe-22 wt-%Cr alloys resulting from controlled changes in nitrogen concentration are described. The variation of mechanical properties with ageing time at 500°C was followed using microhardness measurements, and corresponding changes of microstructure were observed by transmission electron microscopy. A pronounced change in microstructure accompanied by severe embrittlement occurs when a critical nitrogen concentration is exceeded. The alloy then contains a high density of coherent disc-shaped particles with high associated strain and is consistent with the proposed model ...