Giant magnetoresistance in ferromagnet/superconductor superlattices

We show magnetoresistance in excess of 1000% in trilayers containing highly spin-polarized La0.7Ca0.3MnO3 and high-Tc superconducting YBa2Cu3O7. This large magnetoresistance is reminiscent of the giant magnetoresistance (GMR) in metallic superlattices but with much larger values, and originates at spin imbalance due to the injection of spin-polarized carriers. Furthermore, in contrast to ordinary GMR, the magnetoresistance is intimately related to the superconductivity in the YBa2Cu3O7 layer and vanishes in the normal state. This result, aside from its fundamental importance, may be of interest for the design of novel spintronic devices based on ferromagnet/superconductor structures.     

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.     

Electronic structures and magnetoresistance of Co/Cu/Ni/Cu superlattices with different Orientations

Electronic structures and magnetoresistance (MR) of pseudo-spin-valves structure (PSV) Co₃/Cu₃/Ni₃/Cu₃(1 0 0), Co₃/Cu₃/Ni₃/Cu₃(1 1 0), and Co₃/Cu₃/Ni₃/Cu₃(1 1 1) are studied using the full potential linearized augmented plane wave (FLAPW) method with generalized-gradient approximation (GGA) for the exchange correlation interactions. It has been found that the spin-dependence scatterings at interfaces are insignificant compared with the ones of the interior layers for the three orientation superlattices. Also, we evaluate the MR ratio by means of the two-current model, and find that the MR ratio of Co₃/Cu₃/Ni₃/Cu₃(1 0 0) (73.72%) is larger than those of structures Co₃/Cu₃/Ni₃/Cu₃(1 1 0) (68.38%), and Co₃/Cu₃/Ni₃/Cu₃(1 1 1) (67.09%).     

Radiofrequency magnetoresistance of Fe/Cr superlattices

The rf magnetoresistance of Fe/Cr superlattices is studied for two orientations of the current: parallel and across the superlattice layers. A mutually single-valued correspondence is established between the relative magnetoresistance measured at dc current and the change in the transmission coefficient of electromagnetic waves in the magnetic field. When rf currents flow across the layers, the relative change in the signal amplitude is proportional to twice the change in the electrical resistance of the superlattice and is of opposite sign. It is shown that the rf losses are determined by the surface resistance which is proportional to the superlattice thickness and inversely proportional to its conductivity. An equation is derived for the rf electric field distribution in the superlattice. It is established that when the thickness of the superlattice is small compared with the skin layer depth, field and current components which penetrate through the entire superlattice exist.     

Giant magnetoresistance of Co/ITO multilayers

The electrical and magnetic properties of Co/ITO multilayers with various ITO layer thickness are studied. Negative giant magnetoresistance with a maximum of −1.9% at room temperature and −2.57% at 15 K are observed. The magnitudes of GMR oscillate with a period of about 1 nm when varying the thickness of ITO layer.     

Tunneling magnetoresistance in Ag/Co nanoparticle composites

Nanocomposite materials, consisting of ensembles of Ag and Co nanoparticles, have been successfully fabricated, with various compositions and packing densities. The transport and magnetic characteristics of the compounds were studied. In particular, a crossover from a positive magnetoresistance (MR) at low applied magnetic fields to a negative magnetoresistance at high applied magnetic fields was observed. The behaviors could be understood by the spin-dependent tunneling mechanism, known as tunneling magnetoresistance.     

Electronic transport in Mo/Ni superlattices

An experimental relationship between superconductivity, magnetism and localization is explored in short-wavelength (14Å ? ? ? 40Å) sputtered Mo/Ni superlattices. A crossover to a superconducting state is observed for ? < 9Å consistent with the observed paramagnetic behavior when the Ni strata are four atomic layers thick, or less. All samples show localization effects at helium temperatures and non-superconducting samples develop an unusual resistance plateau below T ? 0.5K.     

Giant magnetoresistance effect in hybrid ferromagnetic/semiconductor nanosystems

We theoretically investigate the giant magnetoresistance (GMR) effect in general magnetically modulated semiconductor nanosystems, which can be realized experimentally by depositing two parallel ferromagnetic strips on the top of a heterostructure. Here the exact magnetic profiles and arbitrary magnetization direction of ferromagnetic strips are emphasized. It is shown that a considerable GMR effect can be achieved in such nanosystems due to the significant transmission difference for electrons tunneling through parallel and antiparallel magnetization configurations. It is also shown that the magnetoresistance ratio is strongly influenced by the magnetization direction of ferromagnetic strips in nanosystems, thus possibly leading to tunable GMR devices.     

Giant magnetoresistance and magnetothermopower in Co/Cu multilayers

The thermopower (Seebeck coefficient, S) has been measured on a series of Co/Cu multilayers that exhibit giant negative magnetoresistance (GMR). Negative in zero applied field, S(H) increases in magnitude as the field is increased, approaching that of bulk Co. The change in S is inversely proportional to the resistance of the sample which is, in turn, proportional to the square of the magnetization. A model is presented that yields both the GMR and S(H) from the spin-split density of states of the Co without the need of a spin-dependent scattering potential at the interfaces.     

Modulated room temperature magnetoresistance in manganite superlattices

In this study, the spacer-thickness-dependent room-temperature magnetoresistance (RTMR) is investigated in ferromagnetic/antiferromagnetic (FM/AFM) superlattices composed of 10 repetitions of La_0.7Sr_0.3MnO₃/Nd_0.6Ca_0.4MnO₃ [LSMO/NCMO]₁₀. A series of superlattice samples with the thickness of the LSMO layer being fixed at 5 nm while that of the NCMO layer varied from 0 to 5 nm were fabricated using pulsed laser deposition. RTMR is measured at the fields from zero to 10 kOe with two different configurations, in plane (IP) and out-of-plane (OOP). At 10 kOe, RTMR varies with increasing spacer thickness of NCMO at both IP and OOP configurations. A maximum IP MR ratio of 16% at 10 kOe is found in the sample of [LSMO(5 nm)/NCMO(0.45 nm)]₁₀, which is three times that for the pure LSMO film. Therefore, it is concluded that the inserting of the NCMO layer between LSMO layers can effectively tune the RTMR ratio, which might be related to the magnetic coupling between FM and AFM domains.     

Linear magnetoresistance effect in lateral magnetic superlattices

Two-dimensional electron gas systems modulated by a lateral magnetic superlattice are proposed and the related magnetoresistance effect is described in this work. It is found that the magnetoresistance (MR) ratio of the given structures depends strongly on the uniform magnetic field, and the peaks of the MR ratio depressed linearly with the increase of the uniform magnetic field. This feature can be utilized in practical linear magnetoresistance (LMR) devices.     

Step-induced canting of magnetization in Fe/Ag superlattices

We have investigated the reorientation of the easy axis of magnetization in (0 0 1) Fe/Ag superlattices using vibrating sample magnetometry, Mössbauer spectroscopy and nuclear resonance scattering of synchrotron radiation. Clear evidence is found that the Fe-layer magnetization can be oriented considerably out of the plane of the sample at room temperature, even for Fe-layer thicker than 6 ML at which the spin-reorientation transition usually occurs in Fe/Ag. The spin canting is attributed to frustration and a strong contribution of a step-induced anisotropy.     

Mini-gaps and novel giant negative magnetoresistance in InAs/GaSb semimetallic superlattices

We have studied the temperature dependence of the magnetoresistance of semimetallic InAs/GaSb structures with magnetic field applied parallel to the layers. We present the first unambiguous evidence for the presence of a mini-gap at the crossing point between the electron and hole dispersion relations. The resistivity is found to change from semiconductor-like behaviour with a strong temperature dependence at low parallel magnetic fields to that of a semimetal with a weak temperature dependence at high field. Furthermore, the magnetoresistance, for intrinsic samples, is found to decrease with field by as much as 70% at low temperatures. As the parallel magnetic field is increased the centres of the electron and hole dispersions are shifted apart in k-space and at sufficiently high field the mini-gap is destroyed and the bands overlap fully. Finally, a theoretical model allows us to estimate that the mini-gap is of order 7 meV.     

The giant magnetoresistance of Co/Cu superlattices grown by MBE

We report the first observation of a giant magnetoresistance (MR) in Co/Cu superlattices grown by MBE. The maximum value of the MR is - 26% and this is found in a specimen for which the copper layers are about 7 Å thick and which includes a thin 10 Å layer of gold in the buffer region between the substrate and the superlattice. From RHEED and X-ray diffraction it is shown that the metallic layers in the specimens grown in this way are extremely flat and that the orientation of the copper is (111).