Magnetoresistance In Superlattices Research Articles

Radiation Stability of Fe/Cr and СоFe/Cu Superlattices upon Irradiation with Argon Ions (Е = 10 keV)

Abstract—The magnetoresistive characteristics of magnetic metallic Fe/Cr and Co90Fe10/Cu superlattices subjected to irradiation with accelerated Аr+ ion beams with an energy of 10 keV at the fluences F = 1013–1016 cm–2 are studied in this work. It was found that, at F ≤ 6 × 1014 cm–2, the studied superlattices retain their functional characteristics. Causes for a decrease in the magnetoresistance of superlattices with a subsequent increase in the fluence are discussed. It was concluded that the radiation stability of the advanced modern Co90Fe10/Cu superlattices is higher than that of the model Fe/Cr superlattices.

Magnetoresistance of CoFeNi/Cu Superlattices Differing in the Ferromagnetic Alloy Composition

Abstract—The structure and magnetoresistive properties of Ta(5 nm)/NiFeCr(5 nm)/[FM(tFM)/Cu(2.2 nm)]n/ Ta(5 nm) superlattices prepared by magnetron sputtering are studied in this work. The ferromagnetic layer thickness tFM is varied in a 1.1–1.5 nm range. The Co90Fe10, Co85Fe12Ni3, Co77Fe17Ni6, and Co70Fe20Ni10 alloys with an fcc structure and close saturation magnetization values are used as the ferromagnetic (FM) materials. It is shown that, as the Ni and Fe contents in the CoFeNi alloy increase, the magnetoresistance of superlattices containing eight bilayers remains high and equal to 24–28%; in this case, the twofold decrease in the hysteresis occurs. As the number of bilayers in the superlattices, which contain CoFeNi alloys differing in composition increases to 12, the close values of magnetoresistance 30–31% are obtained. The correlation between the saturation magnetization of the ferromagnetic material of superlattice layers and maximum value of magnetoresistance is discussed.

Magnetooptical, optical, and magnetotransport properties of Co/Cu superlattices with ultrathin cobalt layers

We investigated the field dependences of the magnetization and magnetoresistance of superlattices [Co(t x, A)/Cu(9.6 A)]30 prepared by magnetron sputtering, differing in the thickness of cobalt layers (0.3 A ≤ t Co ≤ 15 A). The optical and magnetooptical properties of these objects were studied by ellipsometry in the spectral region of hω= 0.09–6.2 eV and with the help of the transverse Kerr effect (hω= 0.5–6.2 eV). In the curves of an off-diagonal component of the tensor of the optical conductivity of superlattices with t Co = 3–15 A, a structure of oscillatory type (“loop”) was detected in the ultraviolet region, resulting from the exchange splitting of the 3d band in the energy spectrum of the face-centered cubic structure of cobalt (fcc Co). Based on magnetic experiments and measurements of the transverse Kerr effect, we found the presence of a superparamagnetic phase in Co/Cu superlattices with a thickness of the cobalt layers of 3 and 2 A. The transition from superlattices with solid ferromagnetic layers to superparamagnetic cluster-layered nanostructures and further to the structures based on Co and Cu (t Co = 0.3–1 A) with a Kondo-like characteristics of the electrical resistivity at low temperatures is analyzed.

3000% high-field magnetoresistance in super-lattices of CoFe nanoparticles

We report on magnetotransport measurements on millimeter-large super-lattices of CoFe nanoparticles surrounded by an organic layer. Electrical properties are typical of Coulomb blockade in three-dimensional arrays of nanoparticles. A large high-field magnetoresistance, reaching up to 3000%, is measured between 1.8 and 10 K. This exceeds by two orders of magnitude magnetoresistance values generally measured in arrays of 3d transition metal ferromagnetic nanoparticles. The magnetoresistance amplitude scales with the magnetic field/temperature ratio and displays an unusual exponential dependency with the applied voltage. The magnetoresistance abruptly disappears below 1.8 K. We propose that the magnetoresistance is due to some individual paramagnetic moments localized between the metallic cores of the nanoparticles, the origin of which is discussed.

Giant magnetoresistance of iron-chromium superlattices at microwaves

Interaction between an rf electromagnetic field and the Fe/Cr superlattice placed in a rectangular waveguide so that a high-frequency current passes in the plane of superlattice layers is considered. The transmission coefficient versus the magnetic field strength is found at centimeter waves, and a correlation between this dependence and the field dependence of the dc magnetoresistance is established. It is shown that a change in the transmission coefficient may greatly exceed the giant magnetoresistance of the superlattice. The frequency dependence of the microwave measurements has an oscillatory character. The oscillation frequencies are analyzed in terms of wavelet transformation. Two types of oscillation periods are found to exist, one of which corresponds to the resonance of waves traveling in the superlattice along the direction parallel to the narrow wall of the waveguide.

Microwave magnetoresistance of Fe/Cr multilayers in current-perpendicular-to-plane geometry

The microwave giant magnetoresistance effect in the current-perpendicular-to-plane geometry is measured in Fe/Cr superlattices. The magnetic field dependence of microwave absorption evidently correlates with the current-in-plane DC magnetoresistance of superlattices.

Magnetoresistance and magnetization of Fe/Cr(001) superlattices with noncollinear magnetic ordering.

We study the magnetization and magnetoresistance of superlattices with biquadratic exchange. The equilibrium states are analyzed on the base of the free energy expression that includes all terms up to the fourth order in components of magnetizations of magnetic sublattices. The correlation between the magnetization curve and the magnetoresistance for various orientation of magnetic field relative to the film plane is established. The experimental studies are made on the samples of molecular-beam epitaxy grown Fe/Cr superlattices. The positive magnetoresistance was found for the perpendicular-to-plane magnetic field. It is shown that this effect as well as the characteristic features of the magnetization curves are connected with the noncollinear magnetic alignment, which exists in our samples, and the fourth order magnetic anisotropy of unfamiliar type. \textcopyright{} 1996 The American Physical Society.

Anomalous pressure dependence of giant magnetoresistance in magnetic superlattice

The magnetoresistance (MR) of Fe/Cr, Co/Cu and Fe/Cu magnetic superlattices (SL) has been measured at 4.2 K under high pressure up to 2 GPa. It is found that MR of magnetic SL decreases with increasing pressure but different behaviour is observed between Fe/Cr and Fe/Cu or Co/Cu SL. Such a difference is also found in the pressure dependence of the saturation field Hs. The origin of the difference is discussed briefly.

Interfacial-roughness effects on giant magnetoresistance and interlayer coupling in Co/Cu superlattices.

Giant magnetoresistance and interlayer exchange coupling in Co/Cu superlattices grown by e-beam evaporation have been investigated by using the combined three-axis magneto-optical Kerr effect, magnetotransport measurements, x-ray diffraction, Rutherford backscattering, transmission electron microscopy (TEM), and domain observation with scanning transmission electron microscope electron holography. Large-angle x-ray scattering data and TEM images indicate that the superlattices have fcc (111) orientation with polycrystalline grain sizes ranging from 10--14 nm. Small angle x-ray scattering and cross-section TEM images show that the superlattices have a well-defined layer structure, coherent grain-to-grain epitaxial growth, and interfacial roughness of \ifmmode\pm\else\textpm\fi{}0.2--0.4 nm. The correlations between the 90\ifmmode^\circ\else\textdegree\fi{} magnetization and magnetoresistance oscillation curves suggest that the giant magnetoresistance in superlattices with imperfect interfaces results from the 90\ifmmode^\circ\else\textdegree\fi{} orientation of domains within adjacent Co layers.

90° domains in Co/Cu giant magnetoresistance superlattices

The magnetic properties of Co/Cu superlattices grown in a dual-beam electron evaporation chamber have been investigated with the three-axis magneto-optical Kerr effect and magnetoresistance measurements. The magnetoresistance oscillates with a period of about 1 nm as a function of the Cu layer thickness. The magnitude of the component of the magnetization perpendicular to the field direction oscillates with the same period. The total magnetization, calculated from the two in-plane hysteresis loops, is used to determine that 90° domains are formed between ferromagnetic Co layers across the Cu spacer layers in the superlattice. The correlations between the 90° magnetization curves and magnetoresistance measurements suggest that the giant magnetoresistance in superlattices with imperfect interfaces results from the 90° orientation of domains within adjacent Co layers.

Unified Semi-Classical Theory of Parallel and Perpendicular Giant Magnetoresistance in Superlattices

ABSTRACTThe giant magnetoresistance in magnetic superlattices for the current perpendicular to and in the layer planes is studied within a unified semi-classical approach that is based on the Boltzman equation with exact boundary conditions for the spin-dependent distribution functions of conduction electrons. We show that the main differences between the in-plane and perpendicular-to-plane magnetoresistance result from the fact that they originate from different interface processes responsible for spin-dependent scattering. A correlation between the giant magnetoresistance and the superlattice magnetization is also discussed and it is shown that its study has much potential for yielding information about properties of spin-dependent scattering in magnetic superlattices.

Observation of optical-phonon-scattering inhibited region in longitudinal magnetoresistance in superlattices

We have measured the longitudinal magnetoresistance in GaAs/AlGaAs superlattices in order to investigate the transport properties of one-dimensional coupled quantum boxes, and observed strong magnetophonon resonance whose peak width is dependent on the subband width. In samples for sufficiently narrow subband width, a plateau has been observed between the two adjacent resonant peaks corresponding to the inhibition of optical phonon scattering.