Fe Cr Multilayers Research Articles

Temperature dependence of the giant magnetoresistance in Fe–Cr multilayers—Intralayer and interlayer exchange energies

We present field and temperature dependence data on giant magnetoresistive (GMR) ion-beam sputtered Fe–Cr multilayers of varying Cr thickness. We show that the decrease in GMR with temperature is related to the decrease in sublattice magnetization due to thermal excitation of magnons in the antiferromagnetic configuration. The intralayer and the interlayer exchange energies thus obtained vary systematically as the Cr thickness increases. The corresponding decrease in the measured saturation field further supports our interpretation leading to a better understanding of the physics of GMR.

Role of heterostructures and multiple magnetic phases in the low-field magnetization of Fe–Cr giant magnetoresistive multilayers

Zero-field-cooled (ZFC) and field-cooled (FC) magnetizations along with ac magnetization versus temperature and M-H loop measurements are reported for two series of ion beam sputtered Fe–Cr giant magnetoresistive (GMR) (maximum of 33%) multilayers where the interface roughness is different. The ZFC and FC magnetization data follow different curves below an irreversibility temperature (Tirr). The FC data show a T3∕2 thermal demagnetization behavior at lower temperatures with a very small spin-wave stiffness constant but they go as 1∕T at higher temperatures (above Tirr). The ZFC data show maxima at Tm(H), from which we find the glass transition temperature Tg using the de Almeida and Thouless plot. ac magnetization versus T also shows broad peaks roughly near the same Tg. This behavior is interpreted in terms of the coexistence of spin-glass∕superparamagnetic, FM and AF phases. Most of the above exotic low-field features are absent in molecular beam epitaxy grown Fe–Cr multilayers (GMR >50%) with very smooth interfaces. The relatively rough interfaces in the present sputtered samples give rise to frustrations of Fe and Cr moments, leading to spin-glass-like behavior.

Evidence of spin-wave demagnetization in Fe–Cr giant magnetoresistance multilayers

The temperature dependence of the saturation magnetization of Xe ion-beam sputtered Fe–Cr multilayers has been measured between 5 and 300K in the presence of a field greater than the saturation field (Hsat). With the application of Hsat, the zero-field antiferromagnetically coupled ferromagnetic Fe layers are forced to align ferromagnetically. We find that the thermal demagnetization of such magnetically composite structure follows the Bloch formula for spin waves with anharmonic term in the magnon dispersion relation, viz., M(T)=M(0)[1+AT3∕2+BT5∕2]. Comparisons are presented between the multilayers and bulk crystalline Fe.

Magnetic scattering in Fe–Cr multilayers in the ferromagnetic state at low temperatures

We report here an interpretation of the electrical resistivity (ρ) below 15 K of two Fe–Cr multilayers (30 layers of 20 Å Fe/10 and 12 Å Cr) produced by Xe ion-beam sputter deposition. These multilayers have a negative giant magnetoresistance (GMR) of ∼21% at 4.2 K and Hsat=13 kOe. Very high-resolution resistance measurements (better than 3 ppm) were made down to 2.3 K at every (100±10) mK. Excellent results were obtained when the ρ(T) data in the ferromagnetic state (H=Hsat) were fitted to an equation containing the residual resistivity [ρ(0)], electron–phonon interband s–d scattering (Bloch–Wilson integral) and a magnetic BT2 term, where B is proportional to s–d interaction strength responsible for the electron–magnon s–s scattering. The value of B is found to be typically (9±2)×10−5 μΩ cm K−2 compared to the much smaller value of 1.5×10−5 μΩ cm K−2 for bulk crystalline Fe. The fits without the magnetic term are distinctly inferior as seen from their residuals plotted against T. However, the data in the antiferromagnetic state could be fitted very well with only the lattice term and ρ(0). We have restricted our analysis to T⩽15 K in order to avoid electron–magnon s–d scattering which dominates above 20 K and is very difficult to estimate. Thus magnetic scattering in 3d metals are best interpreted at low temperatures even for these magnetic multilayers.

Spin-dependent electrical transport in ion-beam sputter deposited Fe-Cr multilayers

The temperature dependence of the electrical resistivity and magnetoresistance of Xe-ion beam sputtered Fe-Cr multilayers has been investigated. The electrical resistivity between 5 and 300 K in the fully ferromagnetic state, obtained by applying a field beyond the saturation field (H_sat) necessary for the antiferromagnetic(AF)-ferromagnetic(FM) field-induced transition, shows evidence of spin-disorder resistivity as in crystalline Fe and an s-d scattering contribution (as in 3d metals and alloys). The sublattice magnetization m(T) in these multilayers has been calculated in terms of the planar and interlayer exchange energies. The additional spin-dependent scattering \Delta \rho (T) = \rho(T,H=0)_AF - \rho(T,H=H_sat)_FM in the AF state over a wide range of temperature is found to be proportional to the sublattice magnetization, both \Delta \rho(T) and m(T) reducing along with the antiferromagnetic fraction. At intermediate fields, the spin-dependent part of the electrical resistivity (\rho_s (T)) fits well to the power law \rho_s (T) = b - cT^\alpha where c is a constant and b and \alpha are functions of H. At low fields \alpha \approx 2 and the intercept b decreases with H much the same way as the decrease of \Delta \rho (T) with T. A phase diagram (T vs. H_sat) is obtained for the field- induced AF to FM transition. Comparisons are made between the present investigation and similar studies using dc magnetron sputtered and molecular beam epitaxy (MBE) grown Fe-Cr multilayers.

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.

Characterization of multilayers by means of EDXS in the analytical TEM

Referring to the characterization of nanoscale multilayers in cross section, the resolution limits of the EDXS method have been investigated. For that purpose EDXS line scan profiles of nanoscale Fe-Cr multilayers have been calculated assuming an increased specimen thickness and different tilts between electron beam and layer interface. The resolution limit seems to be greater than 2 nm layer thickness for regular multilayers. Experimentally a limit of 5 nm was reached.

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.

Magnetotransport and exchange coupling in [formula omitted] multilayers: effects of annealing and ion irradiation

A comparative study of the effects of heat treatment, as well as Xe + ion irradiation on the structure, magnetotransport and magnetic properties of Fe Cr multilayers exhibiting giant magnetoresistance (GMR) is presented. The experimental observations on the GMR are explained in terms of increasing roughness at the interfaces, and the consequent loss of antiferromagnetic (AF) coupling in the sample. We also discuss the role of interfacial as well as bulk scattering in the electronic transport of Fe Cr multilayers.

Atom Probe Studies of Interfaces in Metallic Multilayers

AbstractThe atom probe field-ion microscope has been used to study the diffusion at interfaces in metallic multilayers deposited directly onto field-ion specimens and to develop models for the solid state reactions occuring at the atomic-scale in multilayer systems. Results are presented for the low temperature annealing of a Co-Ni multilayer. Intermixing over about 2 atomic planes is found even in as-deposited samples, extending to mnm after heating at 300°C for 1 hour. Using atom probe results from bulk alloys, a Monte Carlo simulation has been developed for the Fe-Cr system, in which a miscibility gap exists, and is being used in an attempt to model the behaviour of interfaces in Fe-Cr multilayers. Preliminary results are presented, showing that interfaces which are initially mixed over 10 atomic planes become sharper by an ‘interface spinodal’ reaction.