Inverse Magnetoresistance Research Articles

Observation of inverse magnetoresistance in perovskite oxide tunnel junctions

AbstractWe have observed a distinct inverse magnetoresistance in a hybrid tunnel junction. Two types of perovskite oxides, La0.67Sr0.33MnO3 and SrRuO3, were used as ferromagnetic electrodes with an SrTiO3 barrier in between them. Micron‐scale junctions were fabricated from 90° off‐axis sputtered trilayers, using standard optical lithography. Homo‐junctions that incorporate either La0.67Sr0.33MnO3 or SrRuO3 as both electrodes showed only normal positive magnetoresistances. In contrast, a clear inverse magnetic switching was found at 10 K for the hybrid junction. The spin polarization of SrRuO3 is about –9% as calculated based upon Julliere model, which is in quantitative agreement with the value (–10%) determined by the earlier tunneling experiment on SrRuO3/SrTiO3/Al structures. This is not only the manifestation of the existence of a negatively polarized material at the Fermi level, but also demonstrates that the widely‐cited Julliere model is applicable even to a negative polarization case. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin transport in a lateral spin-injection device with an FM/Si/FM junction

The spin injection and detection have been investigated in a lateral spin-injection device with an FeCo/Si/FeCo junction structure. Magnetoresistance (MR) signals were found to appear at low magnetic fields in the range 4– 300 K , revealing that the spin-polarized electrons are injected from the first contact and, after propagating through the bulk Si, are collected by the second contact. The inverse MR was found to be independent of temperature. Our results demonstrate spin-polarized current injection and detection in the FM/Si/FM structure at room temperature.

Magnetic properties of phase-separated (Ni/sub 80/Fe/sub 20/)/sub 1-x/Mn/sub x/ thin films

(Ni/sub 80/Fe/sub 20/)/sub 1-x/Mn/sub x/ alloy thin films were produced by codepositing Ni/sub 80/Fe/sub 20/ and Mn and annealed at 300/spl deg/C-600/spl deg/C. In spite of the structural stability predicted by the bulk phase diagram and the interdiffusion evidences observed in the magnetic exchange couples, the (Ni/sub 80/Fe/sub 20/)/sub 1-x/Mn/sub x/ alloy films decomposed into Ni/sub 80/Fe/sub 20/ and Mn phases when annealed above 400/spl deg/C. The phase separated structure closely resembles those of magnetic granular alloys with a strong crystallographic texture. The phase-separated alloys exhibit an inverse giant magnetoresistance as high as 0.5%.

Inverse magnetoresistance in (FeCoB)-(Al2O3) magnetic granular composites

The magnetoresistance, magnetization, and microstructure of granular composites with the general formula (Fe40Co40B20)x(Al2O3)100−x were studied for contents of the amorphous metallic component both above and below the percolation threshold (x≈43). The low-temperature transverse magnetoresistance of the composites is negative at x=41 and practically zero for x=49. For metal contents below the percolation threshold (x=31), a noticeable (7–8%) positive magnetoresistance, reached in magnetic fields of about 17 kOe, was observed. Possible mechanisms of the generation of inverse (positive) magnetoresistance are discussed.

Observation of inverse magnetoresistance in epitaxial magnetite/manganite junctions

We have fabricated and characterized epitaxial Fe3O4/CoCr2O4/La0.7Sr0.3MnO3 magnetic trilayer junctions (MTJs) grown on (110) and (100) SrTiO3 substrates. Large inverse junction magnetoresistance (JMR) as high as −25% has been observed at a field of 4 kOe in (110) oriented MTJs. The improvement of the JMR over that observed in previous epitaxial magnetite junctions is attributed to the choice of the spinel structure CoCr2O4 barrier, which minimizes structural disorder at the barrier/Fe3O4 interface. As a comparison, studies on (100) MTJs elucidate the effect of the magnetic domain state of the ferromagnetic electrodes on the JMR.

Negative spin polarization of Fe3O4 in magnetite/manganite-based junctions.

Epitaxial oxide trilayer junctions composed of magnetite (Fe3O4) and doped manganite (La0.7Sr0.3MnO3) exhibit inverse magnetoresistance as large as -25% in fields of 4 kOe. The inverse magnetoresistance confirms the theoretically predicted negative spin polarization of Fe3O4. Transport through the barrier can be understood in terms of hopping transport through localized states that preserve electron spin information. The junction magnetoresistance versus temperature curve exhibits a peak around 60 K that is explained in terms of the paramagnetic to ferrimagnetic transition of the CoCr2O4 barrier.

Contrasting hysteresis behavior between the magneto-optical Kerr rotation and ellipticity in NiFe/FeMn/Co trilayer films

We report an experimental observation of strikingly contrasting hysteresis behaviors between the longitudinal magneto-optical Kerr rotation, θK, and ellipticity, εK, in NiFe/FeMn/Co films. For example, one is likely to reproduce a typical magnetic hysteresis loop obtainable with vibrating sample magnetometry while the other one does reproduce magnetoresistance (MR)-like curves in some cases. Interestingly, θK/εK loops form inverse MR curves. The contrasting hysteresis behavior could be explained by the discriminate sensitivity of θK and εK to the ferromagnetic NiFe as well as Co layers. Using a simple model, we determine the sensitivity ratios of the Co to NiFe layer for both signals, which are far from unity as much as θKCo/θKNiFe=−1.4 and εKCo/εKNiFe=8.5. These values may allow one to understand the anomaly of the remarkably contrasting loops observed in NiFe/FeMn/Co trilayer films.

Biquadratic coupling effect on magnetoresistance response curves of spin valves with a synthetic antiferromagnet

Both normal and inverse magnetoresistance (MR) response curves were observed for synthetic spin valves with the structure of Si(100)/Ru/Co(t1)/Ru(0.7 nm)/Co(t2)/Cu(3 nm)/Co(t3)/Ru. Under the assumption of a coherent rotation of the magnetization in the three Co layers, the hysteresis loops, magnetization response, and MR response curves were calculated as a function of the parameters of the system. The parameters include antiferromagnetic coupling of Co(t1) and Co(t2) through Ru layer, a weak ferromagnetic coupling of Co(t2) and Co(t3) through Cu spacer, giant magnetoresistance of the Co/Cu/Co and Co/Ru/Co systems, and AMR contribution of each Co layer. The uniaxial anisotropy of each Co layer and a distribution of the coupling strength of Co(t1)/Ru/Co(t2) were also included. To fit the experimental data well, it was necessary to include a biquadratic coupling in the Co(t1)/Ru/Co(t2) trilayer.

Inverse magnetoresistance in chromium-dioxide-based magnetic tunnel junctions

Magnetic tunnel junctions have been fabricated using half-metallic chromium-dioxide (CrO2) epitaxial film with a Co counterelectrode. The native insulating layer formed on the surface of CrO2 after air exposure is used as the tunneling barrier. These junctions exhibit nonlinear current–voltage characteristics, and the changes in junction resistance with applied field correspond to the coercivities of the two magnetic layers. The maximum observed magnetoresistance (MR) is about 8% at 4.2 K and has a negative sign, i.e., the resistance of the junction with parallel alignment of the electrodes is higher than with antiparallel alignment. This is opposite of what is normally observed with transition-metal electrodes and an Al2O3 barrier. Possible reasons for the inverse MR are discussed based on previous results on manganite/Co junctions.

Inverse TMR Effect in Co0.9Fe0.1/SrTiO3/La0.7Sr0.3MnO3 Tunnel Junctions

ABSTRACTMagnetic tunnel junctions of Co0.9Fe0.1/SrTiO3 (STO)/ La0.7Sr0.3MnO3 (LSMO) with a spin-valve structure having an antiferromagnetic MnIr layer have been fabricated by sputtering. Junction magnetoresistance (MR) behavior and its dependence on the bias voltage are examined for junctions with epitaxial STO barrier formed under different sputtering conditions. Spin dependent transport measurements show that these junctions exhibit spin-valve type MR loops with an inverse (positive) MR of the ratio of 14-22 % at 4.2 K. The inverse MR observed is asymmetric with respect to the bias voltage direction. Stoichiometric STO barrier, as characterized by Rutherford backscattering (RBS) analysis, is found to result in very large asymmetric bias dependence, while the junctions with nonstoichiometric STO barrier exhibit the symmetric bias dependence. Our results suggest that the nonstoichiometry of STO barrier modifies the electronic structures of electrode/barrier interfaces, and thereby reducing the asymmetry of bias voltage dependence of junction MR.

Anomalous fluctuations of 2D-electron recombination radiation intensity in the quantum Hall regime

Anomalous intensity fluctuations are observed in the spectrum of radiative ecombination of quasi-two-dimensional (2D) electrons with photoexcited holes in a single quantum well. The fluctuations are observed exclusively under the conditions of the quantum Hall effect (QHE). It is shown that, if the QHE conditions are not fulfilled, the radiation intensity fluctuates strictly following the Poisson distribution 〈δN 2〉/〈N〉= 1), whereas in the QHE regime the fluctuation amplitude increases by several orders of magnitude (〈δ N 2〉/〈N〉∼102). It is demonstrated that the maxima of the emission noise amplitude coincide with the maxima of inverse magnetoresistance of 2D electrons in the QHE regime and correspond to establishing an anomalously high uniformity of the system.

Unified interfacial inverse magnetoresistance in UHV evaporated sandwiches

Several series of Co(3 nm)/Ru( x nm)/Co 1− x X x (3 nm) sandwiches (with x=0 and 0.08, and X= Au, Cu and Ru) have been prepared by ultra-high vacuum (UHV) evaporation onto mica substrate in order to study the effect of non-magnetic dopants, in the second magnetic layers, on the magnetoresistance (MR). MR measurements performed at room temperature on these series have clearly evidenced the presence of inverse giant magnetoresistance (GMR) for non-doped and doped (Cu, Au) series in a thickness range corresponding to the ferromagnetic region. Moreover, for the series with Ru as dopant, the inverse GMR has been observed for all Ru spacer thicknesses. This inverse GMR is mainly due to the interface spin-dependent scattering which results from the chemical asymmetry between the Co/Ru interfaces. The analysis shows that the doping of the Co with non-miscible metals (Cu and Au) has a small effect on the bulk and interface spin-dependent scattering contributions to the GMR, while the doping with miscible metal (Ru) induces a strong reduction of the bulk spin-dependent scattering contribution to the GMR.

Inverse giant magnetoresistance at room temperature in antiparallel biased spin valves and application to bridge sensors

We report a spin-engineering scheme whereby the sign of the giant magnetoresistance in an antiparallel biased spin valve may be selected by varying only layer thicknesses within the structure. The antiferromagnetic coupling in the biased layers leads to either positive exchange bias or positive magnetoresistance in the response of the structure. Hence, spin valves may be fabricated with either positive or negative sensitivities to applied fields in the same sense. When two spin valves of each type are connected in the appropriate Wheatstone bridge configuration, a sensor with four active legs may be constructed. The bias may be set by either the growth field, or by application of uniform field to the entire sensor structure in a single postprocessing anneal step.

Inverse magnetoresistance in Fe/Si ion beam sputtered sandwiches

A series of Fe/Si sandwiches have been prepared by ion beam sputtering at room temperature onto glass substrate with the following nomenclature: Glass/Si20 nm/Fe5/6 nm/Fe5 nm/Ru2 nm. Magnetization measurements have been performed at 300 K and show no evidence of antiferromagnetic exchange coupling. However, the magnetoresistance curves recorded at 300 K are very interesting and show an inverse magnetoresistance for sandwiches with Si spacer layer thicknesses between 1.2 and 1.5 nm. The resistance is smaller at zero field than at saturation. This inverse magnetoresistance is due to the superparamagnetic interfaces and finds its origin in the difference of the electronic nature of the Fe/Si interfaces and Fe/Ru interfaces. Fe1−xSix iron silicide at Fe/Si interfaces has a scattering spin asymmetry ratio (α=ρ↓/ρ↑) larger than one, whereas, Fe with Ru impurities at the Fe/Ru interfaces presents a scattering spin asymmetry ratio smaller than one.

Inverse magnetoresistance in Co/Ru/Co and dopedCo/Ru/Co0.92Ru0.08sandwiches

The investigation of the transport properties of $\mathrm{C}\mathrm{o}(3.2\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{R}\mathrm{u}{(t}_{\mathrm{Ru}})/\mathrm{C}\mathrm{o}(3.2\mathrm{}\mathrm{n}\mathrm{m})$ and the doped $\mathrm{C}\mathrm{o}(3.2\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{R}\mathrm{u}{(t}_{\mathrm{Ru}})/{\mathrm{Co}}_{92}{\mathrm{Ru}}_{8}(3.2\mathrm{}\mathrm{n}\mathrm{m})$ sandwiches series is reported, where ${t}_{\mathrm{Ru}}$ is the thickness of the Ru layer. Inverse magnetoresistance (MR) for all spacer layer thicknesses in the doped series and for sandwiches which present no antiferromagnetic coupling in the nondoped series is observed. This behavior is the consequence of the competition between bulk and interface spin-dependent scattering which contribute to negative and positive (inverse) MR, respectively. Detailed analysis of the results indicates that the main effect of the doping is to reduce the bulk spin-dependent scattering whose contribution becomes smaller than the interface spin-dependent scattering one. This gives rise to an inverse MR. This is further supported by ab initio calculations of the density of states at the Fermi level for pure Co and CoRu alloys as well as experimental temperature-dependent magnetization and MR measurements performed for the two series at room and low temperature.

Interfacial origin of inverse magnetoresistance in uncoupled Fe/Si/Fe/Ru sandwiches

Experimental evidence of inverse magnetoresistance for ferromagnetic layers separated by a Si layer is reported. A series of Fe/Si sandwiches have been prepared by ion-beam sputtering at room temperature onto a glass substrate with the following nomenclature: glass/Si20 nm/Fe5 nm/Six nm/Fe5 nm/Ru2 nm. Magnetization measurements have been performed at 300 K and show no evidence of antiferromagnetic exchange coupling. However, the magnetoresistance curves recorded at 300 K are very interesting and show a reversed magnetoresistance for sandwiches with Si spacer layer thicknesses between 1.2 and 1.5 nm. Indeed, the resistivity is smaller at zero field than at saturation. This reversed magnetoresistance is due to the superparamagnetic interfaces and finds its origin in the difference of the electronic nature of the Fe/Si interfaces and Fe/Ru interfaces. Indeed, iron silicide Fe1−ySiy at Fe/Si interfaces have scattering spin asymmetry ratios (α=ρ↓/ρ↑) larger than 1, whereas, Fe with Ru impurities at the Fe/Ru interfaces present scattering spin asymmetry ratios lower than 1.

Inverse Giant Magnetoresistance in Magnetic Multilayers

The inverse giant magnetoresistance (MR) effect for current in plane (CIP) and current perpendicular to plane (CPP) geometries is studied by using a semiclassical approach. Interestingly, we find that, besides that the inverse/normal MR can occur in both CIP and CPP cases, it is also possible that the inverse MR effect appears only in one of them. We attribute this phenomenon to the different roles of interface and bulk scatterings in these two cases: the interface scattering is more important for CIP-MR than for CPP-MR while the bulk scattering is the opposite.

Inverse giant magnetoresistance (invited)

Inverse giant magnetoresistance (GMR) is obtained in multilayers alternating two ferromagnetic layers F1 and F2 with different asymmetry of spin scattering, α1≳1 and α2<1. This is clearly demonstrated in the simple spin-valve system with perpendicular magnetization Fe1−xVx/Au/Co. With respect to Fe, the FeV alloys with x=0.18 and 0.29 exhibit an inversion of the spin scattering coefficients (αFe≳1, αFeV<1) due to the change of the densities of states at the Fermi level. The inverse MR of FeV/Au/Co is studied as a function of FeV layer thickness and temperature and compared to calculations based on the Camley–Barnas model. The data show that the inverse GMR is due to the bulk scattering within the FeV layer, which coexists with a substantial interface scattering favoring normal GMR.

Inverse magnetoresistance in the simple spin-valve system Fe1-xVx/Au/Co.

Magnetoresistance (MR) measurements have been performed on Fe/Au/Co and ${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{V}}_{\mathit{x}}$/Au/Co trilayers with perpendicular orientation of the magnetization of the thin ferromagnetic layers. While the MR is larger for an antiparallel arrangement of the magnetizations than for a parallel one in Fe/Au/Co (the usual spin-valve-type MR), an inverse spin-valve effect is observed in ${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{V}}_{\mathit{x}}$/Au/Co. This inverse MR is attributed to an increase of the density of states at the Fermi level for majority-spin electrons when Fe is alloyed with V.

Theory on the giant magnetoresistance in magnetic layered structures at finite temperatures

The temperature dependence of the giant magnetoresistance (GMR) in M 1/N/M 2 multilayers, consisting of magnetic M i ( i = 1, 2) and non-magnetic N layers, is discussed using the finite temperature band theory in which the effect of spin fluctuations is taken into account by the static functional integral method. When the temperature is increased, the magnetoresistance (MR) ratio ( ΔR R ) decreases in the normal MR, but increases in the inverse MR where the MR ratio is negative, although its absolute magnetude decreases. In a multilayer, where normal and inverse MRs coexist, ΔR R may show a maximum vs. temperature. In the compensated MR, the temperature coefficient of ΔR R becomes small because of the cancellation between the two contributions over a fairly wide temperature range; this could be beneficial for practical applications.