Domain Wall Magnetoresistance Research Articles

Large Magnetoresistance of Isolated Domain Walls in La2/3 Sr1/3 MnO3 Nanowires.

Generation, manipulation, and sensing of magnetic domain walls are cornerstones in the design of efficient spintronic devices. Half-metals are amenable for this purpose as large low field magnetoresistance signals can be expected from spin accumulation at spin textures. Among half metals, La1- x Srx MnO3 (LSMO) manganites are considered as promising candidates for their robust half-metallic ground state, Curie temperature above room temperature (Tc = 360 K, for x = 1/3), and chemical stability. Yet domain wall magnetoresistance is poorly understood, with large discrepancies in the reported values and conflicting interpretation of experimental data due to the entanglement of various source of magnetoresistance, namely, spin accumulation, anisotropic magnetoresistance, and colossal magnetoresistance. In this work, the domain wall magnetoresistance is measured in LSMO cross-shape nanowires with single-domain walls nucleated across the current path. Magnetoresistance values above 10% are found to be originating at the spin accumulation caused by the mistracking effect of the spin texture of the domain wall by the conduction electrons. Fundamentally, this result shows the importance on non-adiabatic processes at spin textures despite the strong Hund coupling to the localized t2g electrons of the manganite. These large magnetoresistance values are high enough for encoding and reading magnetic bits in future oxide spintronic sensors.

Unconventional domain wall magnetoresistance of patterned Ni/Nb bilayer structures below superconducting transition temperature of Nb

Scattering of spin-up and spin-down electrons while passing through a ferromagnetic domain wall (DW) leads to an additional resistance for transport current, usually observed prominently in constricted magnetic structures. In this report we use the resistance of the DW as a probe to find an indirect signatures of the theoretically predicted spin-singlet supercurrent to spin-triplet supercurrent conversion effect of ferromagnetic DWs. Here we examine the DW induced resistance in Ni stripe in a bilayer Ni/Nb geometry in the normal state and in the superconducting state of Nb. By making a 3 μm wide gap in the top Nb layer we routed the transport current through the Ni layer in the normal state and in the superconducting state of Nb. In the normal state of Nb, in-field transport measurements showed a clear domain wall magneto-resistance (DWMR) peak of amplitude ∼5.9 mΩ near the coercive field, where the DW density is expected to be maximum. Interestingly, however, below the superconducting transition temperature of Nb, the DWMR peak of the Ni layer showed a sharp drop in the field range where the number of DWs become maximum. This observation may be a possible signature of magnetic DW induced spin-triplet correlations in the Ni layer due to the direct injection of spin-singlet Cooper pairs from Nb into the magnetic DWs.

Theory of domain-wall magnetoresistance in metallic antiferromagnets

We develop a theory to compute the domain-wall magnetoresistance (DWMR) in antiferromagnetic (AFM) metals with different spin structures. In the diffusive transport regime, the DWMR can be either {\it negative} or positive depending on the domain-wall orientation and spin structure. In contrast, when the transport is in the ballistic regime, the DWMR is always positive, and the magnitude depends on the width and orientation of the domain wall. Our results pave the way of using electrical measurements for probing the internal spin structure in antiferromagnetic metals.

Robust magnetotransport in disordered ferromagnetic kagome layers with quantum anomalous Hall effect

The magnetotransport properties of disordered ferromagnetic kagome layers are investigated numerically. We show that a large domain-wall magnetoresistance or negative magnetoresistance can be realized in kagome layered materials (e.g. Fe$_3$Sn$_2$, Co$_3$Sn$_2$S$_2$, and Mn$_3$Sn), which show the quantum anomalous Hall effect. The kagome layers show a strong magnetic anisotropy and a large magnetoresistance depending on their magnetic texture. These domain-wall magnetoresistances are expected to be robust against disorder and observed irrespective of the domain-wall thickness, in contrast to conventional domain-wall magnetoresistance in ferromagnetic metals.

Scaling of intrinsic domain wall magnetoresistance with confinement in electromigrated nanocontacts

In this work we study the evolution of intrinsic domain wall magnetoresistance (DWMR) with domain wall confinement. Clean permalloy notched half-ring nanocontacts are fabricated using a special ultra-high vacuum electromigration procedure to tailor the size of the wire in-situ and through the resulting domain wall confinement we tailor the domain wall width from a few tens of nm down to a few nm. Through measurements of the dependence of the resistance with respect to the applied field direction we extract the contribution of a single domain wall to the MR of the device, as a function of the domain wall width in the confining potential at the notch. In this size range, an intrinsic positive MR is found, which dominates over anisotropic MR, as confirmed by comparison to micromagnetic simulations. Moreover, the MR is found to scale monotonically with the size of the domain wall, $\delta_{DW}$, as 1/$\delta_{DW}^b$, with $b=2.31\pm 0.39 $. The experimental result is supported by quantum-mechanical transport simulations based on ab-initio density functional theory calculations.

Domain wall magnetoresistance in BiFeO3 thin films measured by scanning probe microscopy

We measure the magnetotransport properties of individual 71° domain walls in multiferroic BiFeO3 by means of conductive—atomic force microscopy (C-AFM) in the presence of magnetic fields up to one Tesla. The results suggest anisotropic magnetoresistance at room temperature, with the sign of the magnetoresistance depending on the relative orientation between the magnetic field and the domain wall plane. A consequence of this finding is that macroscopically averaged magnetoresistance measurements for domain wall bunches are likely to underestimate the magnetoresistance of each individual domain wall.

Anisotropic magnetoresistance state space of permalloy nanowires with domain wall pinning geometry.

The domain wall-related change in the anisotropic magnetoresistance in L-shaped permalloy nanowires is measured as a function of the magnitude and orientation of the applied magnetic field. The magnetoresistance curves, compiled into so-called domain wall magnetoresistance state space maps, are used to identify highly reproducible transitions between domain states. Magnetic force microscopy and micromagnetic modelling are correlated with the transport measurements of the devices in order to identify different magnetization states. Analysis allows to determine the optimal working parameters for specific devices, such as the minimal field required to switch magnetization or the most appropriate angle for maximal separation of the pinning/depinning fields. Moreover, the complete state space maps can be used to predict evolution of nanodevices in magnetic field without a need of additional electrical measurements and for repayable initialization of magnetic sensors into a well-specified state.

Fabrication of Sub-20 nm Width Ferromagnetic Nanocontact Structures by Shadow Evaporation

The movement of the magnetic domain wall could result in the changing of the contact resistance. Such a resistance change is named as the domain wall Magnetoresistance (DWMR), which can be used as a basic signal of nanodevices. For application, a large DWMR is necessary to improve the device performance. An approach to improve the DWMR value is to fabricate magnetic structures with narrow contact width. However, due to the proximity effect during the process of electron beam lithography (EBL), it is not easy to fabricate sub-20 nm width structures by EBL technique directly. In this paper, we investigated the fabrication of sub-20 nm width nanocontact structures by combined techniques of EBL and shadow evaporation. Upon optimizing the resist thickness, opening width, and the evaporation angle, the contact width was tuned and the corresponding variation trends with these parameters were explored. Using the optimized fabrication conditions, 14 nm wide ferromagnetic contact structures were successfully fabricated.

Helium ion beam milling to create a nano-structured domain wall magnetoresistance spin valve

We have fabricated and measured single domain wall magnetoresistance devices with sub-20 nm gap widths using a novel combination of electron beam lithography and helium ion beam milling. The measurement wires and external profile of the spin valve are fabricated by electron beam lithography and lift-off. The critical bridge structure is created using helium ion beam milling, enabling the formation of a thinner gap (and so a narrower domain wall) than that which is possible with electron beam techniques alone. Four-point probe resistance measurements and scanning electron microscopy are used to characterize the milled structures and optimize the He ion dose. Successful operation of the device as a spin valve is demonstrated, with a 0.2% resistance change as the external magnetic field is cycled. The helium ion beam milling efficiency as extracted from electrical resistance measurements is 0.044 atoms/ion, about half the theoretical value. The gap in the device is limited to a maximum of 20 nm with this technique due to sub-surface swelling caused by injected ions which can induce catastrophic failure in the device. The fine patterning capabilities of the helium ion microscope milling technique indicate that sub-5 nm constriction widths could be possible.

Effect of Non-polar Longitudinal Optical Phonons on Domain Wall Resistance in Diluted Magnetic Semiconductor Nanowires

In the present paper, the effect of non-polar longitudinal optical phonons on domain wall resistance has been studied using the semiclassical approach. The analysis has been based on the Boltzmann transport equation, within the relaxation time approximation. We have determined the magnetoresistance of domain wall in nanowires based on diluted magnetic semiconductors. The modulation of the exchange and the impurity interactions by lattice vibrations, and the hole–phonon interaction are considered. The results indicate that the scattering role of phonons becomes significant by increasing temperature, which leads to the resistivity enhancement. It is also found that the magnetic impurities play an impressive role in enhancing the contribution of the domain wall in the resistance. In providing room temperature spintronics devices, understanding the effects of phonons and magnetic impurities on the domain wall resistance is crucial.

Ballistic transport through a semiconducting magnetic nanocontact in the presence of Rashba and Dresselhaus spin–orbit interactions

AbstractWe study the magnetoresistance (MR) of a one‐dimensional electron gas in a ferromagnetic semiconducting nanocontact containing an atomic‐size domain wall, and in the presence of the Rashba and the Dresselhaus spin–orbit interactions. The MR is calculated in the ballistic regime, within the Landauer–Büttiker formalism. It is shown that, the Dresselhaus spin–orbit interaction which is the result of bulk‐induced inversion asymmetry makes a reduction in the domain wall MR. In contrast, the Rashba coupling may lead to a negative or positive change in the domain wall MR, depending on the Fermi energy and the lateral confinement potentials which can be induced by both parabolic lateral confinement and lateral gate potentials.

High domain wall magneto-resistance in amorphous TbFeCo wires

By introducing artificial pinning sites to locally modify coercivity in perpendicularly magnetized TbFeCo wires, strip domains were constructed without expense of specimen damage. The construction of strip domains with well-defined shape and controllable number enables a determination of magneto-resistance contributing from singe domain wall. Contrast to crystalline materials, a rather high domain wall magneto-resistance ratio near 20% was found at room temperature, indicating the amorphous TbFeCo is a potential candidate for spintronic devices.

Positive antiphase boundary domain wall magnetoresistance in Fe3O4(110) heteroepitaxial films

We observe a strong crystallographic direction dependence on the low-field magnetoresistance (MR) behavior of the epitaxial Fe${}_{3}$${\mathrm{O}}_{4}$ (110) films grown on MgO (110) substrates. The sign of MR is positive when the current and field are parallel to [001], whereas along the $[\stackrel{\underline{}}{1}10]$ direction its sign is negative, similarly to that commonly observed for (100) oriented Fe${}_{3}$${\mathrm{O}}_{4}$ films. We relate this effect to the presence of antiphase boundaries (APB) and subsequent reduction in the width of canted spin structure in its vicinity, due to the hard axis behavior of Fe${}_{3}$${\mathrm{O}}_{4}$ (110) films along this crystallographic direction. At fields greater than the anisotropy field, usual negative MR behavior related to a reduction in spin scattering at the APBs is observed. An analytical model based on the half-infinite spin chains across the APBs is provided to show that the positive MR is due to the domain walls along APBs. The temperature and film thickness dependency of the APB domain wall magnetoresistance is discussed.

The Effect of a Pulsed Magnetic Field on Domain Wall Resistance in Magnetic Nanowires

The effect of a pulsed magnetic field on domain wall magnetoresistance for an ideal one-dimensional magnetic nanowire with a domain wall has been investigated. The analysis has been based on the Boltzmann transport equation, within the relaxation time approximation. The results indicate that the domain wall resistance increase when enhancing the magnetic field. The evaluation of local magnetization has been considered in the presence of a pulsed magnetic field. The time evaluation of the magnetization also has an effect on the domain wall resistance. The resistance depends on the contribution of the Zeeman and exchange interactions.

Influence of longitudinal acoustic phonons on domain wall magnetoresistance in magnetic nanowires

The influence of phonons on domain wall magnetoresistance in magnetic nanowires has gained crucial importance. In the present work, the resistance of a magnetic domain wall in a magnetic nanowire has been investigated in the presence of a longitudinal acoustic phonon using the semiclassical approach. The analysis has been based on the Boltzmann transport equation, within the relaxation time approximation. The resistance resulting from the one-dimensional N\'eel-type domain wall has been studied when the modulation of the exchange interaction by lattice vibrations and the electron-phonon interaction are considered. The results indicate that phonons play a significant role in resistance, and increasing the number of phonons leads to enhancement of the domain wall resistance. Increasing and decreasing the domain wall resistance is observed via enhancing the temperature and mass of the domain wall, respectively. The effect of phonons on the resistance of the domain wall is considerable in providing new spintronics devices based on the nanowires at high temperatures.

Magnetoresistance in a lithography defined single constrained domain wall spin-valve

We have measured domain wall magnetoresistance in a single lithographically constrained domain wall. An H-shaped Ni nanobridge was fabricated by e-beam lithography with the two sides being single magnetic domains showing independent magnetic switching. The connection between the sides constrains the domain wall when the sides line up antiparallel. The magnetoresistance curve clearly identifies the magnetic configurations that are expected from a spin-valvelike structure. The value of the magnetoresistance at room temperature is around 0.1% or 0.4 Ω. This value is shown to be in agreement with a theoretical formulation based on spin accumulation. Micromagnetic simulations show that it is possible to reduce the size of the domain wall further by shortening the length of the bridge.

The nanofabrication and transport properties of ferromagnetic metal nanocontacts

In this paper, the invar alloy nanocontact structures with different widths were fabricated using the electron beam lithography (EBL) and lift-off technique. Through the I– V measurement, we found that a domain wall (DW) could be pinned at the contact position and pushed away by the spin-polarized current when the width of nanocontact less than 300 nm, which induced the change of the nanocontact resistance, i.e., domain wall magnetoresistance (DWMR). The DWMR decreased with the increase of the contact width, but the critical current density kept constant for the different contact widths, which was about 1.8 × 10 7 A/cm 2. For the string nanostructure with several different widths of nanocontact, the domain wall was pushed away one by one with increasing spin-polarized current. However, the domain walls were pushed away simultaneously for the string nanostructure with the same width of nanocontact and a larger DWMR could be obtained in this kind of nanostructure. Based on the controlling to domain wall motion in nanocontacts, the potential application for ferromagnetic metal nanocontacts on the magnetic logic gate was possible.

Fabrication and simulation of nanostructures for domain wall magnetoresistance studies on nickel

We report the use of electron beam lithography and a bilayer liftoff process to fabricate magnetic Ni nanostructures with constriction widths in the range of 22–41 nm. The structures fabricated correspond to the nanobridge geometry. Reproducibility and control over the final nanostructure geometry were observed when using the fabrication process introduced, these two qualities are important in order to carry out a more systematic analysis of domain wall magnetoresistance (DWMR). On the other hand, micromagnetic simulations of structures with the nanobridge geometry were carried out using not only the dimensions of the fabricated nanostructures but also smaller dimensions thought to be achievable with further optimization of the fabrication process. It was found that domain walls with a reduced length of 42.5 nm can be obtained using the nanobridge geometry. Furthermore, the anisotropic magnetoresistance (AMR) effect was calculated numerically and it was found to be smaller than the DWMR, this makes the nanobridge geometry a good candidate for future measurements of the magnetoresistive effect due to domain wall scattering.

Control of ultranarrow Co magnetic domain wall widths in artificially patterned H-bar structures

Micromagnetic simulations of Co domain walls on nanometer crossbars that join two oppositely magnetized parallel legs of “H” shaped patterns are studied. The crossbar domain walls can twist in the plane of the H-pattern, out of the plane, or swirl, forming Néel, Bloch, or vortex structures, respectively, depending on the initial configurations. An energy phase diagram as a function of the crossbar constriction yields the Néel wall as the energetically most favorable, followed by the Bloch wall, which becomes unstable and changes into a vortex-like wall with increasing crossbar size. Most interestingly, the Néel wall width can either shrink or expand depending on the crossbar dimensions. In the case that both the crossbar length and width are small, desirable, ultranarrow domain walls can be obtained. These findings are useful for spintronic device design based on domain wall pinning via nanonotch and domain-wall magnetoresistance approaches.

Room temperature magnetoresistance switching of Permalloy thin films induced by iron nanoparticles

Arrays of iron nanoparticles grown by a combination of chemical vapor deposition and scanning tunneling microscopy have been placed on top of a narrow stripe of Permalloy (Py). The magnetic coupling between the nanoparticles and the Py film results in distinct negative jumps of the Py magnetoresistance (MR). The switching of the magnetization orientation of individual particles is clearly reflected in the Py resistance as a consequence of anisotropic MR and domain wall MR, with a homogeneous particle magnetization orientation yielding the highest resistances.