Enhancement Of Magnetoresistance Research Articles

Enhancement of magnetoresistance in Mn substituted La 2/3Sr 1/3 Mn 1−xZr xO 3 granular system

Granular metal/insulator (La 2/3Sr 1/3MnO 3/Zr-contained compounds) composites were prepared by doping Zr in La 2/3Sr 1/3MnO 3. The effects of Zr doping on structure and grain size, as well as magnetic and electric properties have been investigated. The result shows that only a very limited amount of Zr ( x<0.05) can be doped into the lattice of La 2/3Sr 1/3MnO 3 and most Zr ions segregate as the secondary compounds (SrZrO 3, La 2Zr 2O 7 and ZrO 2). The grains in the undoped sample are relatively homogeneous with single average size of about 3 μm. For the low Zr-content samples ( x=0.05 and 0.1), however, two distinct particles of large and small sizes coexist, and these large (or small) particles have a well-defined average size of their own. For x⩾0.3, not only the grains become homogeneous again, but also their average size is significantly smaller than that of the undoped sample. It has also been found that with x increasing the paramagnetic insulator–ferromagnetic metal transition temperature T C (or T P) first decreases gradually ( x⩽0.3) and then stays nearly constant ( x>0.3), while the resistance and low- and room-temperature magnetoresistance (MR) increase substantially. Interestingly, a large and nearly temperature-independent MR has been found in a broad temperature range (200–300 K) for the samples with x>0. 3.

Enhancement of tunnel magnetoresistance with oxidized AlHf-alloy for a tunnel barrier

We investigated the tunnel magnetoresistance (TMR) with oxidized AlHf-Alloy barrier layer. As a result, at the range of Hf content (C/sub Hf/) from 0 at.% to 70 at.% the enhancement of the tunnel magnetoresistance ratio with the AlHf-oxide barrier was observed, especially, the TMR ratio was greatly increased from 9.6% to 21.4% when the C/sub Hf/ increased from 0 at.% to 17 at.%. Furthermore, we found the enhanced TMR ratio with AlHf layer at thin barrier thickness less than 0.55 nm at which the TMR ratio with Al layer was not observed. And the breakdown voltage was improved with the AlHf-oxide barrier. These results are explained by the consideration that a uniformity of the oxidation in the AlHf layer composed with an amorphous or a nanocrystallized state was better than that in the Al layer composed with the large grain boundaries such as fcc-Al.

Intergranular tunneling magnetoresistance of mechanically alloyed (Cr-M)O/sub 2/ powder compacts

The (Cr-M)O/sub 2/ (M=V, Mn, Fe, Co) powders were synthesized by the mechanochemical method and the magnetoresistance (MR) effect of their powder compacts was investigated as a function of the type and content of the dopant. The findings are summarized as follows. 1) Substituting limit of M for Cr in CrO/sub 2/ was 30 at% for V, Mn, and Co, and 5 at% for Fe. 2) Doping with V effectively decreases the T/sub C/, while Fe significantly elevates the same (420 K with 5 at% Fe). Furthermore, the changes observed in the cases of Co and Mn was marginal. 3) MR ratio was significantly enhanced with the doping of 5 at% of Fe, however, decreased in the cases of V and Co doping. 4) The reason for the MR enhancement in (Cr-Fe)O/sub 2/ powder compacts is due to the change in the barrier characteristics.

Study of grain boundary contribution and enhancement of magnetoresistance in La0.67Ca0.33MnO3/V2O5 composites

Electrical transport (90–300 K) and magnetic (5–375 K) properties of the La0.67Ca0.33MnO3 + x% V2O5 (x = 0, 2, 5, 15 and 30) type magnetic–nonmagnetic composite system show appreciable grain boundary contribution. These composites, except for the one with x = 30%, show metal–insulator transitions at T = Tp which decrease with increasing V2O5 concentration. The high temperature (T > Tp) semiconducting part of the resistivity (ρ) data follow a small polaron hopping conduction mechanism. The magnetoresistance (MR) in this system increases with increasing V2O5 content, showing a maximum value (25% at 93 K and field H = 1.15 T) for the sample with x = 15%. Spin polarization is considered to be responsible for such an increase of MR. Neither thermoelectric power (between 90 and 300 K) nor magnetization data indicate any noticeable change of the Curie temperature (TC) with change of x, suggesting that V2O5 remains at the grain boundary. The complex impedance (Z*) of the composite system, measured over a frequency range of 0.01 kHz–13 MHz, at 300 K shows strong frequency dependence and the impedance plane plots (Z′–Z″) are semicircular for different V2O5 content. An equivalent resistor–capacitor circuit (Rg (Rgb Cgb)) has been proposed to explain the impedance results in this composite system.

Enhancement of ``intrinsic'' magnetoresistance ratio and activation energy of La 0.67 Ca 0.33 MnO 3 single crystals by Fe doping

In order to avoid the complicated influence of grain boundaries on the resistivity in the investigation of the ``intrinsic'' magnetoresistance (MR) effect, the magnetic and magnetotransport properties of La0.67Ca0.33MnO3 and La0.67Ca0.33Mn0.96Fe0.04O3 single crystals were investigated. Owing to the absence of grain boundaries in the single crystals, MR ratio under 40×105A/m is about two orders of magnitude larger than that in polycrystalline counterparts. A further enhancement of MR ratio from 3400% to 17600% was achieved by a few percent of Fe doping. On the basis of the small polaron model, the activation energy derived from fitting the resistivity above Tc was found to increase upon introducing Fe. A connection between the strong localization of electrons arising from magnetic polarons and the enhancement of MR was observed in the single crystals.

Effect of La doping on magnetotransport and magnetic properties of double perovskite Sr 2FeMoO 6 system

We have investigated the magnetotransport and magnetic properties on polycrystalline samples of Sr 2− x La x FeMoO 6 ( x = 0 , 0.2, 0.4, 0.6, 0.8 and 1.0). The magnitude of intergrain tunneling magnetoresistance with low magnetic field of 0.88 T for x = 0.2 and 0.4 samples are as large as 5% and 7% at room temperature and 13% and 10% at 10 K, respectively. The increase of coercivity ( H c ), ratio of remanent magnetization with respect to saturation magnetization ( M r / M s ), high saturation fields, and reduction of the saturation magnetization indicate that random disorder of spin orientation is mainly responsible for enhancement of the low-field magnetoresistance for samples with x ⩽ 0.4 . Whereas rapid drop of H c , M r / M s , M r , and saturation fields for samples with x > 0.4 signifies the growth of antiphase boundary, which gives rise to lower values of low-field MR.

Current induced magnetic switching in Ni80Fe20, Ni, Fe, and Co wires

The current induced switching mechanism and domain wall magnetoresistance are studied in ferromagnetic wires of Ni80Fe20, Ni, Fe, and Co patterned with a nanocontact. A discontinuous resistance change is observed in the current–voltage measurements of Ni80Fe20 and Ni, 0.13% and 0.124%, respectively, at a critical current of ∼107A∕cm2. The magnetoresistance measurements of the Ni80Fe20 and Ni samples have shown a magnetoresistance enhancement within ±20Oe of the zero point when measured at a current below the critical current and of the same percentage change as for the current–voltage measurements. For the Fe and Co samples no resistance change which could be attributed to domain wall scattering was found. The differences in the current induced switching mechanism and domain wall magnetoresistance of the different materials studied are contrasted and analyzed.

Enhancement of magnetoresistance with low interlayer coupling by insertion of a nano-oxide layer into a free magnetic layer

We studied the interlayer coupling strength (Hin) and GMR ratio of a spin-valve with the top free layer, separated by a nano-oxide layer (NOL). With the total thickness of the top free layer being fixed at 60Å, the physical properties of the NOL spin-valve were studied with the thickness (tf) of the free layer under the inserted NOL and compared with those of the normal spin-valve with the same thickness as tf. It was found that the spin-valve with NOL has a higher GMR ratio than that of the normal spin-valve at the optimal condition (tf=40Å) after thermal annealing at T=250°C. The NOL spin-valve also shows a lower Hin than that of the optimal normal spin-valve with tf=40Å, which is comparable to that of the normal spin-valve with tf=60Å. This indicates that the enhancement of GMR, while keeping the Hin to be low, can be achieved by inserting a NOL into the top free layer.

Large enhancement of room temperature magnetoresistance in Ag-added La 0.67(Ca 0.65Ba 0.35) 0.33MnO 3

The electrical and magnetoresistant properties of La 0.67(Ca 0.65Ba 0.35) 0.33MnO 3/Ag x (abbreviated by LCBMO/Ag x ) have been studied. The results show that Ag addition causes a decrease of resistivity dramatically and especially induces a large enhancement of room temperature magnetoresistance (MR). The room temperature MR ratio for x=0.27 sample in 10 kOe magnetic field is 41%, almost 20 times larger than that for x=0 sample. This enhancement is related to that the Curie temperature ( T c) of the sample is near room temperature, as well as the significant reduction of resistivity. The good fits of experimental results for x=0.27 sample to Brillouin function indicate that the MR behavior in the Ag added LCBMO is induced by the spin-dependent hopping of the electrons between the spin clusters, which is an intrinsic property of the CMR materials.

Significant Magnetoresistance Enhancement due to a Cotunneling Process in a Double Tunnel Junction with Single Discontinuous Ferromagnetic Layer Insertion

We fabricate CoFe/AlOx/CoFe/AlOx/CoFe ferromagnetic double tunnel junctions and observe spin-dependent tunneling phenomena. A middle CoFe layer becomes discontinuous by forming CoFe particles two dimensionally, of which the average diameter is evaluated to be 2.0-4.5 nm from cross-sectional transmission electron microscopy images. Below 50 K, a Coulomb gap is observed in current-voltage curves, and both magnetoresistance ratios and resistances are found to increase significantly with decreasing temperature. This indicates that a cotunneling process is dominant within the gap, which agrees very well with theoretical prediction [Phys. Rev. Lett. 80, 1758 (1998)].

Electron Holography Characterization of Potential Barrier in a Spin Valve Structure with Nano-Oxide Layers

The potential barrier at the metal/oxide junction in a specular spin valve structure with nano-oxide layers has been mapped by off-axis electron holography in a field emission gun transmission electron microscope. A potential jump of ~3V across the metal/oxide junction was detected. Presence of the potential barrier confirms formation of metal/insulator/metal structure, which contributes to confinement of conductance electrons with spin polarity characteristic in the key SV structure by the specular reflection of the spin-polarized electrons at the metal/oxide junction and leads to nearly double enhancement of magnetoresistance (MR) ratio from 8% to ~16%.

Enhancements of magnetoresistance in Bi- or Ti-doped La 0.67Ba 0.33MnO 3

Magnetic and transport properties for the series of (La 0.67Ba 0.33MnO 3)/(Bi 2O 3) x/2 and (La 0.67Ba 0.33MnO 3)/(TiO 2) x have been studied. The doping Bi almost has no influence on the magnetization M and Curie temperature T c, but remarkably improves the low-field magnetoresistance at low temperature. On the contrary, doping Ti depresses ( M) and lowers T c and peak resistivity temperature T p, accompanying the rapid increase in the resistivity, and increases the high-field magnetoresistance at low temperature obviously. Different physical mechanisms are discussed for these different behaviors. The surface state of the particles is a key factor influencing on the magnetoresistance effects. Small amount addition of Bi or Ti can improve the room temperature magnetoresistance obviously. These results are favorable for the application studies.

Enhanced ordinary magnetoresistance in Co∕Si systems

The enhancement of the ordinary magnetoresistance (OMR) in Si has been attempted by constructing two type of Co∕Si systems; one is the sputter-deposited current perpendicular to plane-type Co∕Si multilayers and the other is the sputtered-Co∕anodized-porous-Si system. The Co∕Si multilayers show the sharp OMR at a low field and the negative granular-type giant magnetoresistance at higher fields. At 50K, however, only the OMR appears with the ratio of about 3.0% for an in-plane field of 10kOe. In the Co∕anodized-Si system, a very large OMR of about 60% is observed at 50K for perpendicular field of 10kOe.

Enhancement of low-field magnetoresistance in polycrystalline Sr2FeMoO6 with Al doping

Polycrystalline Sr2Fe1−xAlxMoO6 compounds with x=0, 0.05, 0.10, 0.15, and 0.30 were fabricated and their low-field magnetoresistance (LFMR) performance studied. The LFMR was greatly enhanced as x increased from nil to 0.15, and its origin was found to be intragranular spin-dependent scattering. The replacement of Fe by Al weakened the antiferromagnetic exchange in the antiphase boundary arising from the antisite defect and acted as a barrier for electron transport along the Mo–O–Fe–O–Mo–O–Fe chain in the ferromagnetic segregation and weakened the ferromagnetic exchange. When the external magnetic field was applied, the spin in the antiphase boundary appeared to align more easily to the external field after Al doping and the transport of the electron was easier due to the weakening of the double exchange barrier.

Electrical Transport andMagnetoresistance in the Sol-Gel PreparedLa2/3Ca1/3Mn1−xTExO3 (x=0 and 4%, TE=Cuand Zn) Polycrystalline Samples

A comparative study of electrical transport and magnetoresistance (MR)is performed for sol-gel prepared samples of nominal compositionLa2/3Ca1/3MnO3,La2/3Ca1/3Mn1−xCuxO3 andLa2/3Ca1/3Mn1−xZnxO3 with x = 4%. Comparedwith La2/3Ca1/3MnO3, the introduction of Cu or Zn causesa significant downward shift of insulator–metal transition and a sharpMR peak near the transition. The sharp MR peak becomes observable evenupon the application of low magnetic fields. It is also shown thatalthough the same behaviour of transport and MR is observed, the samplecontaining Cu stands out clearly from the sample containing Zn for asubstantial enhancement in MR near the transition.

Magnetoresistance of Ferromagnetic Point Junctions From Tunneling to Direct Contact Regimes

The behavior of magnetoresistance in NiFe (permalloy) point junctions was investigated using an STM set-up, where the junction resistance was continuously varied from 10/sup 8/ /spl Omega/-10/sup 3/ /spl Omega/. Our results reveal an enhancement of magnetoresistance of more than 80% at one conductance quantum, and the decaying magnetoresistance as one moves away from 12.9 k/spl Omega/ to either the diffusive regime or the tunneling regime, as supported by independent theories on spin-dependent transport. The suppression of magnetoresistance with incorporation of a 35-nm-thick Au barrier was observed, as well as the dependence of magnetoresistance on the relative orientations of the electrodes.

MR Ratio Enhancement by NOL Current-Confined-Path Structures in CPP Spin Valves

We have compared the magnetoresistance (MR) performance of current-confined-path (CCP) current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin valve films with a nano-oxide-layer (NOL), made between natural oxidation (NO) and ion-assisted oxidation (IAO). For the NO, an MR ratio was only 1.5% at an RA of 370 m/spl Omega//spl mu/m/sup 2/, whereas for the IAO, an MR ratio was greatly increased to 5.4% at an RA of 500 m/spl Omega//spl mu/m/sup 2/. Fitted data by the Valet-Fert model showing larger MR enhancement effect by the IAO is explained by the improvement of the metal-purity of the Cu inside the CCP structure. By further improvement of metal-purity of the Cu, a large MR ratio of more than 30% can be expected at a small RA of 300 m/spl Omega//spl mu/m/sup 2/. The CCP-CPP spin valve film is a promising candidate for realizing high-density recording heads for 200 to 400-Gbpsi recording.

Optimization of Nano-Oxide Layer in CIP Spin Valves

The influence of nano-oxide layer (NOL) on interlayer coupling (Hin) and magnetoresistance (MR) of bottom spin-valves (SV) were studied by inserting NOL at four positions: 1) before the Cu spacer; 2) in the middle of the Cu spacer; 3) after the Cu spacer; and 4) after the Cu cap layer. The NOL smoothens the surface of the interface at position 1), 3), and 4), except for 2). Surface flatten by NOL is associated with CoFeOx. Ten percent of MR enhancement can be achieved with NOL at the position 3) and 4). Weak antiferromagnetic interlayer coupling was observed with NOL at position 3). A 27.2% MR ratio has been reached in dual spin valve with two NOLs in two pinned layers.

Magnetoresistance and microstructure evolution upon rapid thermal annealing of giant magnetoresistive Co/Cu/Co/CoNbZr multilayers

The magnetoresistance (MR) variation of Co/Cu/Co/CoNbZr spin valves as a result of rapid thermal annealing has been investigated. MR ratio of 3.8% was obtained in the as-deposited sample and a considerable increase to 6.86% was observed in the 450°C×60 s treated sample. Microstructure studies show that the enhancement of MR ratio is a consequence of the nano-crystallization of amorphous CoNbZr soft layer. The nano-crystallized CoNbZr possess fine and dense microstructure and excellent electrical and soft magnetic properties which leads to the MR enhancement. With increasing annealing temperature or annealing time, interface roughness caused by rapid grain growth decrease the MR ratio rapidly. XRD studies imply that the interfusion of Cu atom into the Co layer is another possible degradation mechanism of Co/Cu/Co/CoNbZr spin valves at annealing temperature beyond 550°C.

Magnetoresistance and thermal stability enhancement in FeCr-based spin valves

In a synthetic spin valve (SV), a thin FeCr layer inserted in the pinned layer closer to the antiferromagnetic layer was found to improve the magnetoresistance, the pinning field strength, and thermal stability. Single SVs with 1-nm-thick FeCr inserted in the pinned layer shows a magnetoresistance ratio (MR) of 18% and a pinned field of about 1700Oe. This letter reports on the high MR in an all-metal single SV without specular scattering using a nano-oxide layer or spin filtering. This result, suggests that FeCr with high resistance and low spin polarization is effective in increasing the spin scattering and maximize the contribution of the pinned layer closer to spacer on the magnetotransport while keeping its antiferromagnetic exchange coupling high. The FeCr thin layer may also have an effect on minimizing Mn diffusion into the pinned layer and spacer as revealed by the increase of the pinning field.