High Magnetoresistance Ratio Research Articles

Structural, electrical and magnetic properties of two-dimensional La1.2(Sr1.8−xCax)Mn2O7 manganites

The effects of structural, electrical and magnetic properties with the isovalent chemical substitution of Ca2+ into the Sr2+ sites in La1.2(Sr1.8−xCax)Mn2O7 (x=0–0.8) are investigated. The highest magnetoresistance ratio [ρ(0)/ρ(H)] of 208% (H=1.5 T) at a temperature of 102 K was observed for the x=0.4 sample. The Curie temperatures decreased from 135 to 102 K for x=0–0.4, respectively. Moreover, the Weiss constants θ were varied from the positive to negative value with increasing Ca concentration. The antiferromagnetic behavior with Néel temperature around 30 K was found in the x=0.8 sample. The magnetization measurements show that the hysteresis phenomenon appeared at the temperatures below the Curie or Néel temperatures.

High magnetoresistance ratio and high coercivity observed from epitaxial Co/Cu/Co/NiO spin valve sandwiches on MgO grown by molecular beam epitaxy

Bottom and symmetrical Co/Cu/Co/NiO sandwiches are fabricated on single crystal substrates including MgO(100), MgO(110), and MgO(111) by molecular beam epitaxy without introducing a bias field. Although most of samples only show very weak exchange anisotropy, high magnetoresistance (MR) ratios (5%–8%) accompanied by high coercivities (200–2500 Oe) are observed. Strong angular dependence on Hc and MR has been also observed, which represents a uniaxial symmetry. The minor loops on the MR measurements have shown clear local MR maxima (minima) indicating that some intermediate antiparallel (parallel) spin states may exist.

Compositional dependence of the giant magnetoresistance in FexRh1−x thin films

In this article we report on the magnetic and transport properties of FexRh1−x thin films, prepared by evaporation in high vacuum, in the composition range 0.41<xFe<0.59. Upon annealing (at a temperature of 870 K or higher) a certain volume fraction transforms to the ordered CsCl-type (α′) FeRh phase. Close to room temperature this phase shows a hysteretic transition between the antiferromagnetic (AF) and the ferromagnetic (F) state for samples with xFe<0.5, which gives rise to a magnetoresistance (MR) effect. Although the magnetic transition was never found to be complete, it is shown that the full MR ratio can be obtained by extrapolation of the measured MR ratio as a function of the relative change of the magnetization at the transition. The AF→F transition is only observed for films with xFe<0.505±0.015, for which the α′ phase with this (fixed) composition is present together with a nonmagnetic Rh-rich fcc-type phase, as is shown from a combination of x-ray diffraction, Mössbauer spectroscopy, and magnetization studies. This observation, which was not expected from the phase diagrams available from the literature, can explain our finding that the full MR ratio observed for films in this compositional range is independent of the Fe content. The full MR ratio is 85±6% at room temperature, which is very close to the MR ratio observed for bulk FeRh, implying a high structural quality of the films grown. X-ray diffraction and transmission electron microscopy were used to quantify microstructural aspects such as the grain diameter and strain. The highest MR ratio actually observed is 58%, in a 4400 kA/m field at 275 K. The full MR increases to approximately 150% at 250 K, and, as estimated from the resistivity of F and predominantly AF samples with almost the same composition, to 680±100% at 4.2 K.

Structural and magnetic properties of Fe ion implanted Al 2O 3

The structural and magnetic properties of Fe–Al 2O 3 films, which have been prepared by implantation of 100 keV 56Fe and/or 57Fe ions into single crystals of α-Al 2O 3 to a dose of 1 × 10 17 ions/cm 2, have been investigated by conversion electron Mössbauer spectroscopy at room temperature (RT). The magnetoresistance (MR) is also measured by a two-terminal method at RT and the MR ratios are obtained from 7% to 8.5%, which are about two times larger than the MR ratios of films prepared by sputtering. The MR ratio increases on annealing. Magnetization curves measured with a vibrating sample magnetometer show the nonhysteresis and unsaturation, namely the superparamagnetic nature. We have prepared granular Fe–Al 2O 3 films having the high MR ratio have been prepared for the first time by Fe ion implantation technique.

Chemically controlled properties of layered perovskites

The magnetic and magnetotransport properties of have been investigated. In the low doping region x < 0.5, the samples show similar properties to (x = 0), but magnetic transition temperature , insulator-metal transition temperature and the temperature of magnetoresistance (MR) maximum all decrease with x increasing. When the samples resemble (x = 1.2), without spontaneous magnetization and being insulating. The MR maximum of increases with Nd doping, and the highest MR ratio, , is observed in the transition region x = 0.5 and 0.6. It is shown that the properties of change gradually with Nd doping and can be chemically controlled by tuning the La/Nd ratio.

Ferromagnetic tunnel junctions with plasma-oxidized Al barriers and their annealing effects

Cross-geometrical Ni–Fe/Co/Al–AlO/Co/Ni–Fe/Fe–Mn/Ni–Fe tunnel junctions were fabricated by magnetron sputtering. To form the insulating layer, an Al layer was reverse sputtered in an atmosphere of either oxygen or oxygen–argon mixture at low power after deposition. The oxidization time necessary to form an AlO barrier was much shorter than that by natural oxidization, lasting for only a half to a few minutes. By adding argon to oxygen, the oxidization was slowed down and high MR ratios were obtained for a wide range of time. A magnetoresistance (MR) ratio of 16% was observed in the as-deposited junction when the barrier was oxidized in oxygen plasma for 35 s. In addition, the MR ratio increased to 24% by annealing at 300 °C. In as-deposited junctions, the tunnel resistances were increased by increasing the plasma oxidization time, but the MR ratios gradually decreased. The estimated tunnel barrier width increased and the barrier height decreased with the plasma oxidization time. After annealing, the MR ratio increased only for those junctions oxidized for short times. This suggests that the remaining Al between the AlO and the Co surface plays an important role in the effects of annealing.

Oscillating magnetoresistance of Co/Cu(1 1 1) films

For two-layer Co(134 Å)/Cu(1 1 1) films on mica, obtained by e-beam evaporation, room temperature giant oscillations of resistance and the magnetoresistance (MR) (highest MR ratio – 8.9%) were discovered as functions of Cu film thickness ( t Cu) in the range t Cu⩽230 Å, where the Cu film had an island-type structure. The high MR ratio (⩾1.5%) was preserved up to t Cu⩽850 Å. The separation distances of the resistance and MR extrema are multiples of the length 6.25 Å, which is practically equal to the spatial [1 1 1] diagonal of Cu unit cell. The resistance and MR oscillations are caused by the ordered variations of the geometric characteristics of island Cu films, induced by changing of the contact resistance of Cu–Co interfaces.

Exchange-biased spin valves combining a high magnetoresistance ratio with soft-magnetic behavior

We report on the preparation of bottom spin valves combining a high giant magnetoresistance effect (above 10%) with a soft-magnetic behavior (coercivities of a few hundreds of A/m). By optimization of film composition and preparation conditions, it is possible to obtain materials having magnetoresistance values around 16%, which display minor loop coercivities of 0.2 kA/m, and sensitivities of 11%/kA/m.

Soft a-CoZr-based spin valves (abstract)

The materials options to combine high giant magnetoresistance (GMR) ratios with soft magnetic properties have been relatively few, with NiFe providing low-field switching in the great majority of studies. These NiFe layers tend to be hardened appreciably when directly exchange coupled to Co to provide higher GMR ratios, as in (Ta)/NiFe/Co/Cu/Co/NiFe/FeMn spin valves. We propose magnetically soft, amorphous CoZr as a replacement couple to Co in these and other structures. While the high resistivity of amorphous films discourages their use as a single switching element, it favors their use as the drive layer for Co in a spin valve. Our transport simulations, using the Camley–Barnas model modified by Dieny,1 indicate that GMR ratios fall off much less steeply with increasing a-CoZr drive layer thickness than with NiFe drive layer thickness. Since lower coercivities are expected for higher fractions of the drive material, CoZr-based spin valves show promise in combining soft properties and high GMR ratios. We have demonstrated the combination of magnetic softness and high GMR ratio in our own CoZr-based spin valves. A free layer coercivity of 7.2 Oe and GMR ratio of 6.4% were achieved in a nominal structure of NiO(400)/Co(50)/Cu(20)/Co(25)/CoZr(200AA). Differential coercivity structures were deposited as well, using Cr underlayers to induce hardness of the bottom Co. An identical “active layer” structure of the form SiO2/Cr/Co(50)/Cu(20)/Co(25)/CoZr(200AA) demonstrates GMR ratios as high as 4.9%; removal of the interfacial Co layer reduces the GMR ratio to 2.8%, in accordance with our expectations. Discussion of optimization and detailed structural analysis will be presented in the full paper.

Interlayer couplings and magnetoresistance effects of Fe/Cr/Fe/Ni-Fe/NiO sandwiches

Interlayer coupling between Fe layers and magnetoresistance effect have been investigated in Fe/Cr/Fe/Ni-Fe/NiO sandwiches. The interlayer coupling between two Fe layers oscillates as the thickness of Cr layer changes. The strongest antiferromagnetic coupling is observed when the thickness of the Cr layer is 1.2 nm. The highest magnetoresistance ratio due to spin-dependent scattering is only 0.13%. The low magnetoresistance ratio is thought to be caused by high resistivity of the Cr layer.

Magnetoresistance in (Co xFe 1 − x) 20Cu 80 granular alloys produced by mechanical alloying

The supersaturated solid solution (Co x Fe 1 − x ) 20Cu 80 alloys ( x = 0–1) were prepared by mechanical alloying. The magnetoresistance (MR), magnetic properties and crystal structure of these alloys were investigated. The MR ratio for the (Co x Fe 1 − x ) 20Cu 80 alloys shows a maximum of 6.4% in the Co 20Cu 80 alloy ( x = 1), and decreases with decreasing Co concentration. The b.c.c.-Fe and h.c.p.-Co phases disappear after milling for 50 h, and the f.c.c. phase only is observed. The b.c.c. phase is observed for the alloys with a small MR ratio ( x ≤ 0.8), and is the alloy phase of Fe and Co. The b.c.c. phase is not observed for the alloys with a high MR ratio ( x > 0.9).

Magnetotransport and Magnetic Properties of Mechanically Alloyed CoxCu100-x

Co–Cu binary alloys, whose Co concentrations range from 10 to 30 at.%, are produced by mechanical alloying. The structure, magnetoresistance (MR), and magnetism have been investigated for the as-milled and annealed alloys. After milling for 50 h, a supersaturated solid solution of fcc-CoCu is formed at Co concentrations of 10 to 30 at.%. The Cox Cu100-x (x = 10 – 30) alloys annealed at 700° C for 1 h separate into two phases of fcc-Co and fcc-Cu. The maximum MR ratio, under 15 kOe at room temperature, is 6.4% for the sample with x = 20 annealed at 450° C for 1 h. The samples annealed at temperatures less than 450° C exhibit high MR ratio at x = 25. The magnetism of the sample with x = 20, which shows a maximum MR ratio, exhibits superparamagnetic behavior, but a slight ferromagnetic behavior have also been observed. Co–Cu alloys produced by mechanical alloying have a wide distribution of Co particle size with blocking temperature from low temperature to room temperature.

Magnetoresistance Effects of Fe-Mn/Ni-Fe-Co/(Au, Ag, Al)/Ni-Fe-Co Sandwiches

Magnetoresistance effects and film structures have been investigated in [Hf/Fe-Mn/Ni-Fe-Co/(Au, Ag, Al)/Ni-Fe-Co/Hf/Si] sandwiches. The sandwiches show high magnetoresistance ratios when the Au and Ag spacers are thicker than 2.0 and 4.0 nm, respectively. The resistivity changes are almost equal to those of [Hf/Fe-Mn/Ni-Fe-Co/Cu/Ni-Fe-Co/Hf/Si] sandwiches. The sandwiches with Al spacers show magnetoresistance ratios of almost 0%, although the configuration of magnetisations of the two Ni-Fe-Co layers changes between parallel and antiparallel. Each sandwich has a strong (111) texture. The thickness of the Ag spacer is not homogeneous. The sandwiches show about the same thermal stabilities, whether the nonmagnetic spacer is Cu, Ag or Au.

Magnetoresistance Effects in Ni-Fe-Co/Cu/Co-Pt Trilayers

Magnetoresistance effects in Ni-Fe-Co/Cu/Co-Pt trilayers have been investigated with various thicknesses of each layer. The Co-Pt layers exhibited high coercivities when deposited directly on Si substrates to a thickness of more than 3.2 nm. An antiparallel alignment of the magnetizations in the Ni-Fe-Co and the Co-Pt layers with each other occurred under low magnetic fields because the Co-Pt layer had a higher coercivity than the Ni-Fe-Co layer; thus the trilayers have magnetoresistance ratios higher than 4.0% for Cu layer thicknesses between 1.5 nm and 3.0 nm. The thermal stability of these trilayers was also investigated. These trilayers maintained high magnetoresistance ratios after annealing up to 300° C.

Magnetostriction and giant magnetoresistance in annealed NiFe/Ag multilayers

Magnetostriction data are reported for NiFe/Ag multilayer thin films displaying giant magnetoresistance. Magnetostriction and magnetoresistance were measured as functions of annealing temperature for NiFe/Ag samples having different numbers of NiFe/Ag bilayers and Ag spacer thicknesses. They varied systematically with annealing temperature in a manner consistent with residual stress reduction and microstructural changes such as grain-boundary diffusion and grain growth. Zero magnetostriction concurrent with high magnetoresistance ratio (5%) and field sensitivity [7.5%/(kA/m) (0.6%/Oe)] was observed for an optimal multilayer configuration and annealing temperature. This combination of zero magnetostriction and high magnetoresistive response makes the NiFe/Ag multilayer system an attractive candidate for high-performance magnetic recording read-head sensors.

Magnetoresistance and Interlayer Exchange Coupling between Magnetic Layers in Fe–Mn/Ni–Fe–Co/Cu/Ni–Fe–Co Multilayers

Magnetoresistance effects and interlayer exchange coupling between magnetic layers have been investigated in [Hf/Fe–Mn/Ni–Fe–Co/Cu/Ni–Fe–Co/Hf] multilayers by varying the thicknesses of the Ni–Fe–Co, Fe–Mn, and Cu layers. When the Fe–Mn layer is thicker than 5.0 nm, it applies a strong exchange-bias field to the neighboring Ni–Fe–Co layer. The multilayers show high magnetoresistance ratios at Cu-layer thicknesses between 2.0 and 3.0 nm, because the interlayer exchange coupling is weak. The interlayer exchange coupling between the two Ni–Fe–Co layers is always ferromagnetic at Cu-layer thicknesses between 0.5 and 5.0 nm.

Magnetoresistance and Preferred Orientation in Fe–Mn/Ni–Fe/Cu/Ni–Fe Sandwiches with Various Buffer Layer Materials

Magnetoresistance effects and film structures have been investigated in [Fe–Mn/Ni–Fe/Cu/Ni–Fe/buffer layer/Si] sandwiches with various buffer layer materials. The sandwich with Nb, Ta, Ti, Zr or Hf buffer layer has strong (111) texture. Cu, Ag, Au and Cr buffer layers do not cause strong (111) texture. When the sandwich has strong (111) texture, the Ni–Fe layer neighboring the Fe–Mn antiferromagnetic layer is strongly exchange-biased by the Fe–Mn layer, and thus the sandwich shows a high magnetoresistance ratio.

Effects of Fe Buffer Layer Thickness on Magnetoresistance in Ni-Fe/Cu Multilayers

We present the magnetoresistance and film structure in Ni-Fe/Cu multilayers depend on the Fe buffer layer thickness. With 5-8 nm thick Fe buffer layers, the multilayers have <100> preferred orientations and show high magnetoresistance ratios. On the other hand, when the thickness is 0-4 or 9-10 nm, they have <111> preferred orientations and show low magnetoresistance ratios. The Fe buffer layer thickness affects the preferred orientation, furthermore the preferred orientation strongly correlates to the MR ratio.