Anisotropic Magnetoresistance Value Research Articles

Combining annealing temperature and interface engineering for improving anisotropic magnetoresistance in Ta/NiFe/Ta films

Anisotropic magnetoresistance (AMR) sensors are pivotal in various applications due to their low power consumption, scalability, and cost-effectiveness due to the simple sensor structure, comprising one NiFe film, usually encased in a buffer and cap layer. In this work, we explore the effects of inserting MgO and Pt dusting layers between the NiFe sensing layer and adjacent capping and buffer layers, on the electric, magnetic and structural properties of AMR sensors. We describe results on sensors based on Ta/NiFe/Ta, with an as-processed AMR value of 2.0 %. The insertion of Pt thin films had a positive impact, with AMR values increasing to 2.2 %, contrary to the observed with MgO dusting films. Magnetic annealing up to 370 °C caused an increase of the resistivity and reduction in AMR (with Pt dusting layers), on the contrary, MgO dusting layers improved the sensor performance upon annealing, with AMR increasing to 2.5 % (5 h at 370 °C). In light of the findings, the incorporation of Pt and MgO dusting layers enables tailoring the grain size and resistance of Ta/NiFe/Ta films, while combined with proper annealing, which is relevant for applications where Ta and NiFe are available for AMR sensor fabrication.

Field-induced metal-to-insulator transition and colossal anisotropic magnetoresistance in a nearly Dirac material EuMnSb2

Realizing applicably appreciated spintronic functionalities basing on the coupling between charge and spin degrees of freedom is still a challenge. For example, the anisotropic magnetoresistance (AMR) effect can be utilized to read out the information stored in magnetic structures. However, the application of AMR in antiferromagnet-based spintronics is usually hindered by the small AMR value. Here, we discover a colossal AMR with its value reaching 1.84 × 106% at 2 K, which stems from the field-induced metal-to-insulator transition (MIT), in a nearly Dirac material EuMnSb2. Density functional theory calculations identify a Dirac-like band around the Y point that depends strongly on the spin–orbit coupling and dominates the electrical transport. The indirect band gap at the Fermi level evolves with magnetic structure of Eu2+ moments, consequently giving rise to the field-induced MIT and the colossal AMR. Our results suggest that the antiferromagnetic topological materials can serve as a fertile ground for spintronics applications.

The correlation between anisotropic magnetoresistance and phase separation in La0.4Pr0.3Ca0.3MnO3/NGO films

In this paper, the relationship between anisotropic magnetoresistance (AMR) and phase separation in La0.4Pr0.3Ca0.3MnO3 (LPCMO)/NGO films has been investigated. The films were grown on orthorhombic NGO (001) single crystal substrates by the pulsed laser deposition (PLD) method. The as-grown films were ex-situ annealed at 780 °C for 30 min in 400 mTorr oxygen pressures to clarify the effects of annealing on magnetoresistance (MR) and AMR. The X-ray diffraction (XRD) pattern confirmed that the substrate distance from the target is a key parameter for strong preferred crystallographic orientation growth. The temperature-dependent resistivity of the as-grown films revealed the coexistence of ferromagnetic and antiferromagnetic phases and the frozen glassy state at low temperatures. In the film with significant phase coexistence, the considerably large AMR and MR values of about 140% and 100% are achieved under the 5 kOe magnetic field. By tuning the target-substrate distance, different crystalline orientations of LPCMO can be obtained, and as a result, the metal-insulator transition (MIT) temperature, MR, and AMR can be modified.

Silver addition in polycrystalline La0.7Ca0.3MnO3: Large magnetoresistance and anisotropic magnetoresistance for manganite sensors

Perovskite manganite La1−yCayMnO3 has giant and tunable magnetoresistance (MR) and anisotropic magnetoresistance (AMR), still the following two challenges are of great concern: (i) large magnetic field is required for achieving excellent electromagnetic performance; and (ii) metal–insulator (M–I) transition temperature (TMI) significantly different from room temperature; both these factors limit its practical applications. In this study, polycrystalline La0.7Ca0.3MnO3:Agx composites were fabricated by sol–gel and solid–phase addition methods, and large MR and AMR (84.1% and 32%, respectively) were obtained at near room temperature in 1 T magnetic field when x = 0.2. Evidently, it was found that the grain boundary scattering effects were weakened due to Ag−addition, which resulted in an enhancement of MR. The tremendously high AMR value was attributed to the anisotropic spin–orbit coupling (SOC) effect. The electron–scattering and small–polaron hopping models were used to successfully fit the resistivity data, which support the SOC effect under different magnetic fields that enhance the AMR. These results support further development of the perovskite manganese oxide with potential applications in manganite devices.

Giant anisotropic magnetoresistance in oxygen-vacancy-ordered epitaxial La0.5Sr0.5CoO3−δ films

Anisotropic Magnetoresistance (AMR) is a property of all conductive ferromagnets, where the electrical resistivity depends on the relative orientation of the current and magnetization. In this work it is shown that the perovskite cobaltite La1-xSrxCoO3-d displays a giant form of AMR in epitaxial thin films, 20 times enhanced over bulk, and 10-100 times typical transition metals. The effect is ascribed to symmetry lowering due to spatial ordering of oxygen vacancies, pointing to a new route to enhance properties of complex oxides. The AMR values (up to 40%) are among the largest reported in the 160-year history of the effect.

Application of an extended van der Pauw method to anisotropic magnetoresistance measurements of ferromagnetic films

We demonstrate anisotropic resistivity measurements using the extended van der Pauw (vdP) method in ferromagnetic Ni80Fe20 (Py) films. We apply it to measure anisotropic magnetoresistance (AMR) and compare the results of the vdP method with the more conventional Hall-bar method along the hard and easy axis of the film and show that the vdP method gives more reliable AMR result. For instance the AMR result along the hard and easy axis of the film are in close agreement. Further, we applied the vdP method to study AMR in a series of Py films with thicknesses ranging between 10–250 nm. The films were grown by sputtering deposition at an angle with respect to the substrate normal and with an in situ magnetic field, both conditions assisting in the definition of in-plane uniaxial anisotropy. The microstructure of Py films was characterized using x-ray reflectivity, diffraction and polar mapping of (1 1 1) planes. We detected no off-normal texture and negligible surface roughness, which indicates that self-shadowing is not dominating in our growth. Yet the films have well defined uniaxial anisotropy. Abrupt changes in the average resistivity versus film thickness were observed, which cannot be explained by the models accounting for the thickness and grain size but strongly correlate with the changes in (1 1 1) texture in the films. We compared our results with the literature and show that independent of growth method, substrate and deposition temperature, the AMR value presents a saturation behavior with thickness at about 100 nm.

Interplay between magnetic domain patterning and anisotropic magnetoresistance probed by magneto-optics

We study the correlation between the magnetic reversal and the anisotropic magnetoresistance (AMR) response in magnetic hybrid structures that were created by local modification of magnetic properties induced by ion implantation. The stripe pattern has been investigated simultaneously by dual-wavelength Kerr microscopy and magnetoresistance measurements. We observe that the switching of the stripe pattern introduces an additional AMR maximum. The domain wall in between the stripes provides a positive resistance contribution, whereas domains at the stripe edges lead to an asymmetric AMR response. A method for calculating the AMR response from the quantitative Kerr micrographs is demonstrated that allows the reconstruction of the AMR value within a region of interest only.

Effect of a CoFeB layer on the anisotropic magnetoresistance of Ta/CoFeB/MgO/NiFe/MgO/CoFeB/Ta films

The anisotropic magnetoresistance (AMR) and magnetic properties of NiFe films can be remarkably enhanced via CoFeB layer. In the case of an ultrathin NiFe film having a Ta/CoFeB/MgO/NiFe/MgO/CoFeB/Ta structure, the CoFeB/MgO layers suppressed the formation of magnetic dead layers and the interdiffusions and interface reactions between the NiFe and Ta layers. The AMR reached a maximum value of 3.56% at 450°C. More importantly, a single NiFe (111) peak can be formed resulting in higher AMR values for films having CoFeB layer. This enhanced AMR also originated from the significant specular reflection of electrons owing to the crystalline MgO layer, together with the sharp interfaces with the NiFe layer. These factors together resulted in higher AMR and improved magnetic properties.

Low field colossal anisotropic magnetoresistance in spatially confined electronically phase separated La0.3Pr0.4Ca0.3MnO3 microbridges

Colossal in-plane anisotropic magnetoresistance (AMR) of >16 000% has been engineered in spatially confined La0.3Pr0.4Ca0.3MnO3 films. Recalling that typical AMR values in films are only a few percent, these results mark an astonishing increase that might potentially lead to fabrication of manganite-based switching and sensor devices. The unique colossal behavior is discussed within the context of anisotropic domain growth.

Anisotropic magnetoresistance of individual CoFeB and Ni nanotubes with values of up to 1.4% at room temperature

Magnetic nanotubes (NTs) are interesting for magnetic memory and magnonic applications. We report magnetotransport experiments on individual 10 to 20 μm long Ni and CoFeB NTs with outer diameters ranging from 160 to 390 nm and film thicknesses of 20 to 40 nm. The anisotropic magnetoresistance (AMR) effect studied from 2 K to room temperature (RT) amounted to 1.4% and 0.1% for Ni and CoFeB NTs, respectively, at RT. We evaluated magnetometric demagnetization factors of about 0.7 for Ni and CoFeB NTs having considerably different saturation magnetization. The relatively large AMR value of the Ni nanotubes is promising for RT spintronic applications. The large saturation magnetization of CoFeB is useful in different fields such as magnonics and scanning probe microscopy using nanotubes as magnetic tips.

Effects of ZnO layer on anisotropy magnetoresistance of Ni81Fe19 films

A series of Ta (4 nm)/ ZnO (t nm )/ Ni 81 Fe 19 (20 nm)/ ZnO (t nm )/ Ta (3 nm) magnetic thin films were prepared on lower experimental conditions by magnetron sputtering method. Effects of ZnO layer thickness and substrate temperature on anisotropic magnetoresistance and magnetic properties of these Ni 81 Fe 19 films have been investigated. The experiment results show that the anisotropic magnetoresistance value of the Ni 81 Fe 19 film is enhanced with the increasing of the inserted ZnO layer thickness. When the ZnO thickness is 2 nm, the anisotropic magnetoresistance value achieves the maximum. In addition, the anisotropic magnetoresistance of the Ni 81 Fe 19 film is also enhanced with the increasing of substrate temperature, and when the temperature is 450°C, the anisotropic magnetoresistance reaches the maximum. The anisotropic magnetoresistance value of 20 nm Ni 81 Fe 19 films with 2 nm ZnO layer can achieve 3.63% at 450°C which is enhanced 11.6% compare with the films without ZnO layer.

Strain induced tunable anisotropic magnetoresistance in La0.67Ca0.33MnO3/BaTiO3 heterostructures

In this paper, we investigated the influence of strain on anisotropic magnetoresistance (AMR) in La0.67Ca0.33MnO3 (LCMO) films epitaxially grown on BaTiO3(001). For 250-nm-thick LCMO film, the AMR shows a peak near the metal-insulator transition (MIT) temperature, which is similar to that in bulk LCMO. When the thickness of LCMO is decreased to 150 nm, the AMR value achieves a maximum at low temperature. For 80-nm-thick LCMO film, in addition to the appearance of the maximum AMR at low temperature, the symmetry and sign of AMR are also changed, associated with interface strain in the different phases of BaTiO3. In comparison, the AMR for the reference LCMO films grown on SrTiO3(001) shows a maximum value near the MIT temperature regardless of the thickness of film. Our experiment results suggest that not only the strain value but also the distortion type can considerably tune the AMR of LCMO films.

Enhancement of anisotropic magnetoresistance in MgO/NiFe/MgO trilayers via NiFe nanoparticles in MgO layers

MgO/NiFe/MgO trilayers, the new development in highly sensitive anisotropic magnetoresistance (AMR) sensor film materials, exhibit severely reduced magnetoresistance ratios at small NiFe thicknesses. By inserting ultrathin NiFe(І) layers into the top and bottom MgO layers of MgO/NiFe/MgO trilayers, films with a structure of MgO/NiFe(І)/MgO/NiFe/MgO/NiFe(І)/MgO were designed and synthesized. The AMR value can be significantly enhanced for thin NiFe films due to the improved specular reflections of electrons at both NiFe/MgO interfaces. For a thin NiFe film with the structure of MgO/NiFe(І)(1.5 nm)/MgO/NiFe(5 nm)/MgO/NiFe(І)(1.5 nm)/MgO, the AMR value was greatly enhanced to as high as 2.71%, an increase of 37% over MgO/NiFe(5 nm)/MgO film.

Temperature dependence of the electrical resistivity and the anisotropic magnetoresistance (AMR) of electrodeposited Ni-Co alloys

The electrical resistivity and the anisotropic magnetoresistance (AMR) was investigated for Ni-Co alloys at and below room temperature. The Ni-Co alloy layers having a thickness of about 2 μm were prepared by electrodeposition on Si wafers with evaporated Cr and Cu underlayers. The alloy composition was varied in the whole concentration range by varying the ratio of Ni-sulfate and Co-sulfate in the electrolyte. The Ni-Co alloy deposits were investigated first in the as-deposited state on the substrates and then, by mechanically stripping them from the substrates, as self-supporting layers both without and after annealing. According to an X-ray diffraction study, a strongly textured face-centered cubic (fcc) structure was formed in the as-deposited state with an average grain size of about 10 nm. Upon annealing, the crystal structure was retained whereas the grain size increased by a factor of 3 to 5, depending on alloy composition. The zero-field resistivity decreased strongly by annealing due to the increased grain size. The annealing hardly changed the AMR below 50 at.% Co but strongly decreased it above this concentration. The composition dependence of the resistivity and the AMR of the annealed Ni-Co alloy deposits was in good quantitative agreement with the available literature data both at 13 K and at room temperature. Both transport parameters were found to exhibit a pronounced maximum in the composition range between 20 and 30 at.% Co and the data of the Ni-Co alloys fitted well to the limiting values of the pure component metals (fcc-Ni and fcc-Co). The only theoretical calculation reported formerly on fcc Ni-Co alloys yielded at T = 0 K a resistivity value smaller by a factor of 5 and an AMR value larger by a factor of about 2 than the corresponding low-temperature experimental data, although the theoretical study properly reproduced the composition dependence of both quantities.

Enhancement of the magnetic field sensitivity in Al2O3 encapsulated NiFe films with anisotropic magnetoresistance

The anisotropic magnetoresistance value (ΔR/R) and magnetic field sensitivity (Sv) of NiFe films can be remarkably enhanced by using Al2O3 encapsulation, and the Sv is comparable to that of a spin valve. For an ultrathin NiFe film with the structure of Ta/Al2O3/NiFe/Al2O3/Ta, the Al2O3 layers suppress the formation of the magnetic dead layers and the interdiffusions between the NiFe layer and Ta layers, and decrease the current shunting of the Ta layers. More importantly, the flatter Al2O3/NiFe and NiFe/Al2O3 interfaces can significantly enhance the specular reflection of conduction electrons and lead to a higher ΔR/R. In addition, the formations of the NiFe (111) texture and the columnar grains by annealing can also increase the ΔR/R.

Domain wall anisotropic magnetoresistance in planar nanowires

The anisotropic magnetoresistance (AMR) of individual magnetic domain walls in planar nanowires has been systematically investigated as a function of the micromagnetic wall structure, width, and thickness of the nanowire. Experimentally derived thickness dependence of both the thin film resistivity and the anisotropic magnetoresistance were incorporated into the calculations. We found that the AMR value can be used to identify the wall structure and that the wall resistance is sensitive to wire width for widths less than 300 nm. Furthermore, in comparison with the detailed analysis here, a previous simplified domain wall model significantly underestimates domain wall resistance in narrower wires.

Probing of micromagnetic configuration in manganite channels by transport measurements

We explore the possibility of tracking magnetization orientation in artificial structures patterned in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ epitaxial films by magnetotransport measurements, exploiting the anisotropic magnetoresistance effect. We perform resistance measurements as a function of temperature, magnetic field, and angle between the applied in-plane magnetic field and the channel or crystalline axes in micrometric channels of different widths. We analyze quantitatively our results and extract information about magnetization easy axes, crystalline anisotropy, domain wall resistance, anisotropic magnetoresistance, energy of magnetic domain pinning, and magnetic reversal mechanisms. For channel widths larger than a few micrometers, the magnetization direction at low field $(\ensuremath{\leqslant}200\phantom{\rule{0.3em}{0ex}}\mathrm{Oe})$ is determined by magnetocrystalline easy axes, whereas for channel widths of $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ or smaller, the shape anisotropy forces the magnetization to align along the channel axis. This gives a precise indication on how artificial patterning can be used to force the magnetization direction in manganite based spintronic devices. Values of magnetocrystalline constant up to $8000\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{3}$ and anisotropic magnetoresistance values between 0.1% and 0.6% are found. Our data also indicate that, in the low field $(\ensuremath{\leqslant}200\phantom{\rule{0.3em}{0ex}}\mathrm{Oe})$ hysteretic regime, magnetization reversal occurs by thermal activated hopping of domain walls, with a characteristic hopping distance of $4--5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.

Ultrathin high anisotropic magnetoresistance Ni0.81Fe0.19 films

We have fabricated an ultrathin high anisotropic magnetoresistance (AMR) Ni0.81Fe0.19 (10.0 nm) film grown on a (Ni0.81Fe0.19)0.63Cr0.37 (5.5 nm) underlayer with dc magnetron sputtering. The AMR value (ΔR/R) and the coercivity of the film attain 1.61(6)% and 178 A m−1 (i.e. 2.2 Oe), respectively. The ΔR/R of (Ni0.81Fe0.19)0.63 Cr0.37(5.5 nm)/ Ni0.81Fe0.19(10.0 nm) film has increased by ∼34% over that of Ta(5.4 nm)/Ni0.81Fe0.19(10.0 nm) film (ΔR/R = 1.20(6)%) prepared using the same magnetron sputtering. The results of x-ray diffraction and atomic force microscopy show that the underlayer of (Ni0.81Fe0.19)0.63 Cr0.37 makes it easier to promote the formation of Ni0.81Fe0.19(111) texture and dramatically improves the crystalline grain size in the columnar direction and the average surface grain size of the Ni0.81Fe0.19film. The result of x-ray photoelectron spectroscopy shows that the (Ni0.81Fe0.19)0.63Cr0.37 underlayer, unlike the traditional Ta, does not react with the Ni0.81Fe0.19 film at the interface.

Effect of magnetic coupling on the magnetoresistive properties in La0.67Sr0.33MnO3/BaFe11.3(ZnSn)0.7O19 composites

The La0.67Sr0.33MnO3/BaFe11.3(ZnSn)0.7O19 (LSMO/BaM) composites have been prepared by cosintering the LSMO and the BaM powders. The microstructural, magnetic, and magnetoresistive (MR) properties of those systems were systematically studied. Due to the magnetic coupling between the LSMO (a soft magnet metal) and BaM (a hard magnet insulator) grains, the low-field MR sensitivity was reduced and the high-field MR slope was enhanced for the composites. In addition, an abnormal MR hysteresis (resistance reaches its maximum before field reversal) was identified for the composite when the field is applied vertical to the sample plane. Its origin was attributed to a coplay of the LSMO/BaM spin coupling and the demagnetization effect. Finally, an anisotropic magnetoresistance (AMR) effect was observed in the composites. The AMR value decreased with increasing the BaM content, which can be explained by the changes of the local effective field on the grain boundaries.

Effect of bar width on magnetoresistance of nanoscale nickel and cobalt bars

The anisotropic magnetoresistance (AMR) of nickel and cobalt bars of 1 μm length, 35 nm thickness, and width varying from 50 to 500 nm is studied. As the bar width is reduced from 500 to 50 nm, longitudinal MR change decreases from 0.8% to 0.2% and transverse MR change increases from 1.3% to 1.8% for the Ni bars; longitudinal MR change is zero and transverse MR change is constant at 1.6% for the Co bars. The zero longitudinal MR is attributed to a single-domain magnetization. The AMR value indicates that the electron scattering from bar edges has little effect on AMR even in a 50 nm width. Furthermore, strong, repeatable Barkhausen noise is observed in the Co bars with a width larger than 200 nm, which can be explained by a micromagnetic model.