Magnetoresistance In Thin Films Research Articles

Influence of crystallographic orientation on electronic phase separation in manganite thin films

Thin film epitaxy is a powerful method to control the strain and crystallographic orientation of the thin films using epitaxial relationship with the substrate. For colossal magnetoresistive (CMR) manganites thin films, the strong correlation between spin, charge, orbital and lattice degrees of freedom make their physical properties depend sensitively on the choice of substrate. This is especially true when considering the fact that the physical behaviors of manganites are often governed by electronic phase separation (EPS) phenomena, whose physical origin is strongly tied to lattice strain distribution. While many studies have been conducted on the substrate lattice mismatch induced strain effect, research on the substrate crystallographic orientation effect on EPS has received less attention. In this work, using (La2/3Pr1/3)5/8Ca3/8MnO3 (LPCMO) films grown on SrTiO3 (STO) substrate as a protype system, we show that EPS and its associated physical properties are strongly affected by the substrate crystallographic orientations. Specifically, we observed a clear boost of ferromagnetism in (110)-oriented LPCMO thin films with elongated EPS domains. First principles calculations show that the magnetic phase transitions of LPCMO are dominantly influenced by the spin-lattice coupling.

A high-sensitive anisotropic magnetoresistive sensor based on hybrid Ta/NiFe/Ta/Al multilayer structure

High sensitivity is crucial for anisotropic magnetoresistive (AMR) sensors in industrial applications. In this paper, a high- sensitive AMR sensor based on magnetoresistive thin films with Ta/NiFe/Ta/Al four-layer structure is proposed and fabricated. Firstly, the structural parameters were optimized by finite element analysis. Secondly, thin film samples and AMR sensors were prepared. Through the analysis and characterization of reluctance change rate, hysteresis loop, x-ray diffraction and surface morphology and structure, the process parameters were optimized. Finally, the sensor was connected to the designed external circuit, and its technical parameters were tested in a magnetic field test system. The results show that the prepared AMR sensor performs well. It has a high sensitivity of 1.27 mV/V/Oe, a low bridge offset voltage of ±1.64 mV V−1, and a low temperature coefficient of sensitivity of −0.102%/°C. The results contribute to the future development of AMR magnetic field sensor chips.

A nonuniform demagnetizing field model for both static and dynamic responses of anisotropic magnetoresistive thin film sensors

In this paper, an anisotropic magnetoresistive (AMR) thin film sensor which can be used for magnetic scale has been prepared, and its output voltage is about 4.7–4.9 mV V−1. On the basis of the Stoner–Wohlfarth model and with considering the non-uniformity of the demagnetizing field along the width direction of the strips, both the static and dynamic responses of the AMR sensors have been calculated. The results have shown that the calculated results are in agreement with the experimental data. The magnetization rotation in the magnetic sensor strongly depends on the nonuniform demagnetizing field along the width direction. The magnetization at the center is easily rotated into the field direction, and the magnetization at the edge is difficult to be rotated. The smaller the width of the magnetoresistive strip is, the larger both the demagnetizing field at the edge and the saturation field of the magnetic sensor are. The results are helpful for understanding the magnetization rotation of magnetic sensors and developing the magnetic sensors with high performance.

Magnetic anisotropy instability due to magnetic field annealing in Ta/MgO/NiFe/MgO/Ta anisotropic magnetoresistive sensors

Ta/MgO/NiFe/MgO/Ta anisotropic magnetoresistive thin film sensors, which can be used for magnetic scales, were prepared. To improve the magnetoresistance ratio of the sensors, these sensors were vacuum-annealed with the magnetic field applied, and then, both the static and dynamic magnetoresistive responses of the sensors were obtained. The experimental results have shown that the instability of magnetic anisotropy occurs in the sensors after vacuum-annealing, leading to the significant hysteresis and abnormal output voltage peaks. To investigate the underlying physics, the distribution of the non-uniform demagnetizing field along the width of an anisotropic magnetoresistive (AMR) strip is considered, and both the static and dynamic responses of the AMR elements have been calculated on the basis of the Stoner–Wohlfarth model. The results have shown that the calculated results are in good agreement with the experimental data. The calculated results have revealed that the angles between the anisotropic field and the external magnetic field are different during the field cyclings of Hmax (maximum magnetic field) to −Hmax and −Hmax to Hmax. The angle difference is up to 6°, leading to different magnetoresistive responses. This paper is helpful for the understanding about the magnetization rotations in magnetic sensors and the manufacturing of sensors.

Optimization of large magnetoresistance of polycrystalline Bi film

The oriented growth and morphology of polycrystalline bismuth (Bi) films deposited on insulating substrates is investigated based on the molecular beam epitaxy method. The film thickness effect on the resistance-temperature characteristics and magnetoresistance properties is analyzed. The Bi film resistivity with thickness range [25, 65] nm is investigated using the four-probe method at 2–300 K. The magnetic field contributes to opening the energy gap of Bi thin films and a more obvious opening of the energy gap is observed in the thinner films. The Bi film has the weak antilocalization (WAL) effect and the thickness increase affects the WAL effect. The large magnetic field is beneficial to the observation of the WAL effect. The temperature-dependent resistivity of Bi thin films is described based on the Ziman-Faber theory. The competition between the metal surface state and the bulk state explains the semimetal-semiconductor transition and the magnetoresistance effect in the thickness range [25, 65] nm. In addition, the thickness effect on the magnetoresistance property is researched, which provides a new approach to specify the Bi thin film magnetoresistance.

GMR Biosensor Based on Spin-Valve Thin Films for Green-Synthesized Magnetite (Fe<sub>3</sub>O<sub>4</sub>) Nanoparticles Label Detection

The giant magnetoresistance (GMR) thin film with spin valve (SV) structure of Ta (2 nm)/Ir20Mn80(10 nm)/Co90Fe10(3 nm)/Cu (2.2 nm)/Co84Fe10B4(10 nm)/Ta (5 nm)] fabricated by RF magnetron sputtering method with a magnetoresistance (MR) of 6% was used in this work. Green synthesis of Fe3O4 magnetic nanoparticles (MNPs) using Moringa Oleifera (MO) leaf extract have been successfully conducted using the coprecipitation method. Fe3O4 MNPs demonstrated the inverse cubic spinel structure with the average crystallite size of 13.8 nm and decreased to 11.8 nm for Fe3O4/PEG. Fe3O4, as a magnetic label, integrated with a Wheatstone bridge-GMR sensor provides access to GMR-based biosensors. The induced-field increase leads the signal (ΔV) to increase with increasing nanoparticle concentration. It was discovered that a sensor can distinguish different types of magnetic labels. The sensitivity for Fe3O4 and MO-green synthesized Fe3O4 magnetic label was 0.04 and 0.1 mV/g/L, respectively. The GMR sensor performed the highest sensitivity on the MO-green synthesized Fe3O4 label. Thus, the SV thin film as a sensor and the green-synthesized Fe3O4 nanoparticles as a superior magnetic label are an excellent combination for biosensor application.

Reversal of the Pinning Direction in the Synthetic Spin Valve with a NiFeCr Seed Layer.

The effect of the seed layers on the magnetic properties of the giant magnetoresistance thin films has received a lot of attention. Here, a synthetic spin valve film stack with a wedge-shaped NiFeCr seed layer is deposited and annealed following a zero-field cooling procedure. The film crystallinity and magnetic properties are studied as a function of the NiFeCr seed layer thickness. It is found that the exchange coupling field from the IrMn/CoFe interface and the antiferromagnetic coupling field in the synthetic antiferromagnet both increase as the seed layer thickness increases, indicating the perfection of film texture. In this film, the critical thickness of the NiFeCr seed layer for the formation of the ordered IrMn3 texture is about 9.3 nm. Meanwhile, a reversal of the pinning direction in the film is observed at this critical thickness of NiFeCr. This phenomenon can be explained in a free energy model by the competition effect between the exchange coupling and the interlayer coupling during the annealing process.

Detecting topological phase transitions in cadmium arsenide films via the transverse magnetoresistance

Topological protection against localization causes electrical transport phenomena in disordered topological materials to differ from those in topologically trivial systems. For example, a transition between a regime of weak localization to one of weak antilocalization can occur in systems such as topological insulators and topological semimetals when an external potential is applied across the system. Here, we report on the transverse magnetoresistance of thin films of cadmium arsenide, a topologically nontrivial, as we tune the electronic states and the Fermi level. We show that the appearance of weak localization and weak antilocalization sensitively reflects the relative contributions of multiple transport channels involving both gapless (massless) and gapped (massive) Dirac fermion states present in these films. The data are consistent with expectations of the different topological states of these films. Weak (anti-)localization phenomena can, therefore, serve as a probe of the types of Dirac fermions present in topological semimetals.

Electrical Joining of Paper-Based Multilayer Magneto-Resistive Sensor Stacks

Abstract With the increase in the new development of paper electronics, there is a great demand for paper-compatible and reliable electrical joining techniques. In this work, we particularly addressed the interconnecting of multilayers of paper-based magneto-electronics. We deployed three room-temperature electrical joining techniques: (i) through-paper via, (ii) mechanical caulking, and (iii) collapsible daisy chain to fabricate an anisotropic magnetoresistive five-layer thin film sensor stack for planar rotary motion sensing. We studied the interplay between the electrical joining technique and the sensor characteristics such as magnetoresistive sensitivity and asymmetries in the sensor signal within the magnetic field strength domain. Despite process complexity and the precision limitations of manual machining and positioning, deployed in this work, the sensor stacks prepared by the through-paper via technique exhibited the closest uniformity in the magnetization planes across the stack, and hence the highest cumulative magnetoresistive sensitivity and lowest unfavorable asymmetries. Last, using peeling tests, we verified the mechanical reliability of the interconnects prepared by the through-paper via and collapsible daisy chain techniques.

Non-conventional ferromagnetism and high bias magnetoresistance in [formula omitted]: A simple phenomenological approach

A simple phenomenological approach is used to describe magnetization and magnetoresistance in thin films of oxygen-deficient rutile titanium dioxide (TiO2). Magnetization curves were successfully simulated by considering the ferromagnetic (FM) contribution of magnetically interacting oxygen vacancies (VO’s) forming a single-domain using the Stoner-Wohlfarth (SW) model combined with paramagnetic (PM) contribution of isolated VO’s described by a Langevin function. Whereas approximately half of the VO’s behave as ferromagnetically coupled, rotating coherently with the applied magnetic field, the remaining half of the VO’s tend to align individually with the local magnetic field overcoming the thermal fluctuation deviations. The conduction mechanism is described by variable range hopping (VRH) regime with electron hopping between VO’s centers in the TiO2-x matrix. The magnetoresistance (MR) curves are consistently simulated through the scaling with square of PM contribution of magnetization, indicating that resistance depends on the relative orientation of the magnetic moments associated with non-interacting VO’s dispersed in TiO2 matrix at low temperatures. Density functional theory (DFT) calculations were performed to validate the parametrization of the phenomenological approach used to described the ferromagnetic groundstate of the VO network and spin polarization in the VRH regime in the TiO2-x matrix. Spin-resolved density of states (DOS) calculations consistently reveal that magnetoresistance can be ascribed to electron hopping from defect-states located immediately below Fermi level to the bottom of conduction band states with an opposed spin-polarization.

Interface defects induced vertical magnetic anisotropy in Sr2FeMoO6 thin films

Pulsed laser deposition was used to fabricate high quality Sr2FeMoO6 thin films on SrTiO3 and SrLaAlO4 single crystal substrates. The focus of our research has been on the magnetic properties of Sr2FeMoO6 thin films and we have observed an outstanding shift from parallel in-plane magnetic anisotropy to perpendicular out-of-plane anisotropy, induced by the substrate. Our results also provide the first experimental evidence of magnetic stripe domain pattern in Sr2FeMoO6 thin films. Compared with previous studies for magnetoresistive thin films, we propose a conspicuous mechanism behind the shift in magnetic anisotropy. Rather than being induced directly by the lattice mismatch, we find the possible contribution to perpendicular magnetic anisotropy from over-relaxation at the film substrate interface or from other crystalline defects such as low-angle grain boundaries.

Epitaxial growth and magnetic/transport properties of La0.7Sr0.3MnO3 thin films grown on SrTiO3 with optimized growth conditions

Epitaxial growth of colossal magnetoresistive thin films of La0.7Sr0.3MnO3 (LSMO) has been achieved on TiO2-terminated (001) SrTiO3 (STO) single-crystal substrates using PLD (pulsed laser deposition).

Asymmetric magnetoresistance in the graphite intercalation compounds with cobalt

ABSTRACTThe results of measurements of the magnetoresistance for graphite intercalation compounds with cobalt based on highly oriented pyrolytic graphite and fine crystalline pyrolytic graphite in the temperature range from 1.6 K to 293 K and in a magnetic field up to 16 T are presented. For the investigated intercalation compounds, the effect of asymmetry of the magnetoresistance relative to the magnetic field direction is revealed. It is shown that this effect can be satisfactorily explained within the Segal model of asymmetric magnetoresistance in thin films with large magnetic anisotropy.

The effects of oxygen pressure on disordering and magneto-transport properties of Ba2FeMoO6 thin films grown via pulsed laser deposition

Epitaxial Ba2FeMoO6 thin films were grown via pulsed laser deposition under low oxygen pressure and their structural, chemical, and magnetic properties were examined, focusing on the effects of oxygen pressure. The chemical disorder, off-stoichiometry in B site cations (Fe and Mo) increased with increasing oxygen pressure and thus magnetic properties were degraded. Interestingly, in contrast, negative magneto-resistance, which is the characteristics of this double perovskite material, was enhanced with increasing oxygen pressure. It is believed that phase segregation of highly disordered thin films is responsible for the increased magneto-resistance of thin films grown at high oxygen pressure. The anomalous Hall effect, which behaves hole-like, was also observed due to spin-polarized itinerant electrons under low magnetic field below 1 T and the ordinary electron-like Hall effect was dominant at higher magnetic fields.

Magnetoresistance of Manganese-Doped Colloidal Quantum Dot Films

The magnetoresistance of films of manganese-doped colloidal quantum dots of CdSe, ZnO, HgS, and ZnTe is investigated. At low concentration of manganese ions (1% or less), the hyperfine splitting of the Mn2+ electron spin resonance is resolved and similar to that of the bulk doped materials, indicating successful doping into the nanocrystals. At high Mn concentration (∼10%), the hyperfine splitting disappears because of interaction between the Mn2+ ions. Thin films of Mn:CdSe, Mn:ZnO, and Mn:HgS quantum dots are charged negative by applying an electrochemical potential, and the magnetoresistance is measured down to 2 K and up to 9 T. At low charging level, the magnetoresistance of thin films is positive, exhibits little effect of the manganese dopant, and is instead consistent with predictions from the variable range hopping model and the squeezing of the wave function of the quantum dots. At high charging level, the magnetoresistance becomes linear both for Mn:CdSe and Mn:ZnO, and this is not explained. A...

Magnetoresistance of thin films due to weak localization under the variation of the dimensionality induced by the magnetic field and temperature

The magnetoresistance due to the weak localization of electrons for films with thickness comparable with the electron diffusion length during the phase relaxation time of the electron wavefunction or with the magnetic length has been analyzed theoretically and studied experimentally. The expression for the magnetoresistance of thin films at the arbitrary relation between the film thickness and the electron diffusion length during the phase relaxation time of the electron wavefunction has been derived. It has been shown that the derived expression describes well the magnetoresistance of gallium-doped zinc oxide films under conditions of the variation of their effective dimension with respect to weak localization induced by the magnetic field and temperature.

Hall effect in carbon nanotube thin films

We have investigated Hall coefficient and magnetoresistance in thin films of single-walled carbon nanotubes, prepared in four different ways. Hall voltages are linear for all samples in magnetic fields up to 6 T, and the measured carrier density lies in ∼1021–1022cm−3. Whereas earlier Hall-effect experiments reported ∼1018–1019cm−3 for the carrier density, our results are consistent with the theoretically predicted value of ∼1022cm−3, calculated for the aligned metallic CNTs. The signs of the Hall coefficients are positive in general, indicating that majority carriers are holes in these films. In a nanotube film with the lowest conductivity, however, we find the Hall coefficient reverses the sign at low temperature around T=15K. The origin of the sign change is not clear. In strongly localized regime, the Hall effect can be anomalous.

Nonlinear transport in nanoscale phase separated colossal magnetoresistive oxide thin films

We report a study of the I-V characteristics of 2.5–5.4 nm epitaxial La1−xSrxMnO3 (x = 0.33 and 0.5) and La0.7Ca0.3MnO3 thin films. While La0.67Sr0.33MnO3 films exhibit linear conduction over the entire temperature and magnetic field ranges investigated, we observe a strong correlation between the linearity of the I-V relation and the metal-insulator transition in highly phase separated La0.5Sr0.5MnO3 and La0.7Ca0.3MnO3 films. Linear I-V behavior has been observed in the high temperature paramagnetic insulating phase, and an additional current term proportional to Vα (α = 1.5–2.8) starts to develop below the metal-insulator transition temperature TMI, with the onset temperature of the nonlinearity increasing in magnetic field as TMI increases. The exponent α increases with decreasing temperature and increasing magnetic field and is significantly enhanced in ultrathin films with thicknesses close to that of the electrically dead layer. We attribute the origin of the nonlinearity to transport through the nanoscale coexisting metallic and insulating regions. Our results suggest that phase separation is not fully quenched even at low temperatures and high magnetic fields.

Noise reduction by magnetostatic coupling in geomagnetic-field sensors

A new magnetoresistive (MR) thin film with a structure of “antiferromagnetic layer/pinned soft magnetic layer/non-magnetic MgO spacer layer/sensitive NiFe layer” was designed. The barber-pole MR elements with a Wheatstone bridge circuit were fabricated using photolithographic methods. The testing results show that, in comparison to the element with a typical structure of Ta/NiFe/Ta, the fabricated MR element shows significant reduction in the Barkhausen noise and the 1/f noise and good magnetic stability while maintaining high magnetic field sensitivity. This element with improved signals can be attributed to the magnetostatic coupling between the pinned soft magnetic layer and the sensitive NiFe layer, which can act as a small stabilizing field, leading to the coherent rotation of magnetic moment in the sensitive NiFe layer.

Thin film underlayer effects on mass resolving power in laser-assisted atom probe tomography

The effects of varying Co thickness on the mass resolving power for Cu in a multilayer stack are characterized by laser-assisted atom probe. As the Co layer thickness increased, Cu exhibited poorer mass resolving power. By reducing the pulse repetition rate from 250kHz to 10kHz, the mass resolving power improved to match that of Cu with no Co underlayer. These results support that thermal mechanisms for field evaporation dominate in this Co–Cu multilayer, which is a common architecture in giant magneto-resistance thin film devices.