磁性隧道结Ni80Fe20/Al2O3/Co的制备和物性

用等离子体氧化形成绝缘层的方法,重复性地制备出了Ｎｉ８０Ｆｅ２０／Ａｌ２Ｏ３／Ｃｏ磁性隧道结。样品的隧道磁电阻（ＴＭＲ）比值在室温下最高可达６．０％,翻转场（ｓｗｉｔｃｈ ｆｉｅｌｄ）可低于８００Ａ／ｍ,平台宽度约２４００Ａ／ｍ。结电阻的变化范围从几百欧姆到几百千欧。     

Ballistic anisotropic magnetoresistance

Electronic transport in ferromagnetic ballistic conductors is predicted to exhibit ballistic anisotropic magnetoresistance-a change in the ballistic conductance with the direction of magnetization. This phenomenon originates from the effect of the spin-orbit interaction on the electronic band structure which leads to a change in the number of bands crossing the Fermi energy when the magnetization direction changes. We illustrate the significance of this phenomenon by performing ab initio calculations of the ballistic conductance in ferromagnetic Ni and Fe nanowires which display a sizable ballistic anisotropic magnetoresistance when magnetization changes direction from parallel to perpendicular to the wire axis.     

Ballistic magnetoresistance in different nanocontact configurations: a basis for future magnetoresistance sensors

We report ballistic magnetoresistance (BMR) values in magnetic nanocontacts for Ni, Co, and Fe. The samples range from atomic nanocontacts (smaller than 1 nm cross-section) to stable electrodeposited nanocontacts (up to 30 nm cross-section). The experiments are done at room temperature and up to 4 kOe applied field. We obtain values of stable BMR up to 700%. By manipulating the resistance and the contact shape electrochemically in situ we can have any desired value of BMR. We also discuss BMR in Ni microclusters contacted through pinholes on thin oxides with nanometer thick Ni and Co films with BMR up to 15%. All the experiments show that the BMR is a very local effect of the size and shape of the nanocontact. In this respect 2D and 3D domain wall calculations are presented. The experiments reported here show that magnetic nanocontacts have potential for development of highly compacted sensor.     

A study on electrodeposited Ni x Fe1−x alloy films

Ni _x Fe_1−x (0.22 ≤x ≤ 0.62) alloy films were grown by electrodeposition technique. A shift in diffraction peaks of NiFe and Ni₃Fe was detected with increasing Ni content. The highest positive magnetoresistance ratio was detected as 5% in Ni0.₅₁Fe0.₄₉. Positive and negative anisotropic magnetoresistance were observed in longitudinal and transverse geometries respectively. The highest anisotropic magnetoresistance ratio of 9.8% was also detected in Ni0.₅₁Fe0.₄₉. The angular variation of magnetoresistance was studied. Magnetisation loop curves show that NiFe alloy films have a linear decreasing anisotropy constant with increasing Ni deposit content and show a decreasing behavior of coercivity which indicates soft magnetic property with increasing Ni deposit content     

Constricted hysteresis loops in Fe and Ni single crystals

Magnetic hysteresis loops reflect the variety of magnetic domain structures and have been considered to have normal rectangular or leaf-like shapes in standard ferromagnets such as Fe and Ni metals. We report on observations of constricted hysteresis loops in Fe and Ni single crystals with very low defect densities. The constricted loops were observed below T=150 K and in a medium temperature range from 150 to 430 K in Fe and Ni single crystals, respectively. These constricted loops disappear by weak plastic deformation for both single crystals. The origin of constricted hysteresis loops was explained by eddy current effects under less domain wall pinning due to dislocations.     

The increase of the spin-transfer torque threshold current density in coupled vortex domain walls

We have studied the dependence on the domain wall structure of the spin-transfer torque current density threshold for the onset of wall motion in curved, Gd-doped Ni(80)Fe(20) nanowires with no artificial pinning potentials. For single vortex domain walls, for both 10% and 1% Gd-doping concentrations, the threshold current density is inversely proportional to the wire width and significantly lower compared to the threshold current density measured for transverse domain walls. On the other hand for high Gd concentrations and large wire widths, double vortex domain walls are formed which require an increase in the threshold current density compared to single vortex domain walls at the same wire width. We suggest that this is due to the coupling of the vortex cores, which are of opposite chirality, and hence will be acted on by opposing forces arising through the spin-transfer torque effect.     

Ballistic transport through a sharp domain wall in a semiconducting ferromagnetic nanoconstriction and in the presence of the Dresselhaus spin-orbit coupling

We study the effect of the Dresselhaus spin-orbit interaction on the magnetoresistance (MR) of a quasi-one-dimensional ferromagnetic semiconductor containing a sharp domain wall. The MR is calculated in the ballistic regime, within the Landauer-Büttiker formalism. The results show that the Dresselhaus spin-orbit coupling which induces an effective magnetic field along the wire, reduces the domain wall MR.     

Temperature dependence of the spin torque effect in current-induced domain wall motion

We present an experimental study of domain wall motion induced by current pulses as well as by conventional magnetic fields at temperatures between 2 and 300 K in a 110 nm wide and 34 nm thick Ni80Fe20 ring. We observe that, in contrast with field-induced domain wall motion, which is a thermally activated process, the critical current density for current-induced domain wall motion increases with increasing temperature, which implies a reduction of the spin torque efficiency. The effect of Joule heating due to the current pulses is measured and taken into account to obtain critical fields and current densities at constant sample temperatures. This allows for a comparison of our results with theory.     

Spin-flip and domain wall magnetoresistance in quantum magnetic nanocontacts

The theory of nanosize point contacts made of ferromagnetic metals is developed. A general quantum scattering theory is applied to calculate magnetoresistance of a nanocontact with a domain wall located in the constriction. The exact solution of the electron motion in a potential of the linear domain wall is used as a zero-order approximation. Spin-flip and spin-conserving quantized conductances of the nanocontact are calculated by the perturbation theory by the difference between the model and the Cabrera-Falicov potentials of the domain wall. It is explicitly shown that spin-flip conductance imposes natural limitation on magnetoresistance of the point contact, which otherwise diverges in the regime of complete spin-rectified conductance through the contact.     

From ballistic to non-ballistic magnetoresistance in nanocontacts: theory and experiments

This paper reports on a large set of experiments using different materials and samples to explore the ballistic and non-ballistic magnetoresistence in magnetic nanocontacts. Ballistic contacts are obtained with 3-d magnetic crystalline metals and non-ballistic by using permalloy, amorphous metal, ceramic perovskites and Heusler alloys that have large resistivity values. It is shown that while the ballistic nanocontacts can present 200% magnetoresistance, the non-ballistic exhibit a non-noticeable value (<2%) at room temperature. Two criteria are given according to theory and in good agreement with experiments to obtain large magnetoresistance values.     

Magnetoresistance due to domain wall bulging

The effect of bulging of domain wall (DW) on the magnetoresistance (MR) is investigated. With taking into account the auto-correlation between the points of the interface, one can formulate the mobility within the relaxation time approximation scheme. The results show that the bulging of DW, evaluated with the commonly accepted magnetic parameters for typical ferromagnetic materials of Co, Fe and Ni, has a countable role into the MR.     

Negative and positive magnetoresistance manipulation in an electrodeposited nanometer Ni contact

We show that, in a nanometric size stable electrodeposited Ni contact, it is possible to modify the magnetoresistance by applying current pulses and external magnetic fields whereby the same current path is used for detection and modification. We can pass from positive to negative magnetoresistance with values as large as 25% at room temperature, all in the same contact. We propose that the effect may be due to switching and moving domain walls in the contact region under the combination of current effects and external fields.     

Evidence for local orbital moments on Ni and Co impurities in Pd

The spontaneous magnetoresistance anisotropy has been measured for the ferromagnetic alloys PdNi, PdCo and PdFe. The results confirm the conclusion drawn from other data that in Pd, Ni and Co (but not Fe) possess local orbital moments.     

Anisotropy of domain wall resistance

The resistive effect of domain walls in FePd films with perpendicular anisotropy was studied experimentally as a function of field and temperature. The films were grown directly on MgO substrates, which induces an unusual virgin magnetic configuration composed of 60 nm wide parallel stripe domains. This allowed us to carry out the first measurements of the anisotropy of domain wall resistivity in the two configurations of current perpendicular and parallel to the walls. At 18 K, we find 8.2% and 1.3% for the domain wall magnetoresistance normalized to the wall width (8 nm) in these two respective configurations. These values are consistent with the predictions of Levy and Zhang.     

大规模制备Ni80Fe20纳米线阵列及其磁学特性研究

利用电化学沉积方法在高度有序纳米孔氧化铝模板中大规模制备了Ni80₈₀Fe 20₂₀纳米线阵列.该方法得到的Ni80₈₀Fe20₂₀ 纳米线产率高（约1012^{12—101313/cm22）, 而且这些纳米线阵列具有（111）择优生 长取向和很高的纵横比.与体材料相比,这些Ni80₈₀Fe20₂₀纳 米线阵列具 关键词： 纳米线阵列 磁性}     

Domain wall resistance in epitaxial Fe wires

We studied the magnetoresistance behavior of epitaxial Fe wires grown on GaAs(1 1 0) with varying widths at room temperature. Single nanowires show a wire width (w) dependence of the coercive field, which increases with 1/w for decreasing wire widths. This enables the pinning of a single domain wall in the connection area of two wires with different widths. Magnetoresistance measurements of such wire structures clearly reveal resistance contributions arising from a domain wall. The presence of the domain wall is proven by photoemission electron-microscopy with synchrotron radiation. Moreover, micromagnetic simulations are performed to determine the spin orientations, especially within the domain wall. This permits us to calculate the anisotropic magnetoresistance caused by the domain wall. Taking this into account, we determine the intrinsic domain wall resistance, for which we found a positive value of 0.2%, in agreement with theoretical predictions.     

Magnetic and magnetoresistive properties of Fe10Ni90 films

We study the effect of the substrate temperature and the temperature and conditions of thermomagnetic treatment on the magnetic and electrical properties of Fe10Ni90 films deposited on single-crystal silicon substrates by ion-beam sputtering. The laws found to govern these properties are associated with the transformations of the microstructure of the films. The maximum magnetoresistance ratio is shown to occur in films deposited on slightly heated substrates and vacuum-annealed at about 500C. The coercive force, anisotropy field in the plane of the films, the density of bonds at domain walls, and the magnetoresistance ratio are determined as functions of the thickness.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 9–13, January, 1991.We thank E. I. Teitel and N. N. Schchegoleva for measuring the microstructure parameters of the films.     

工艺参量对Ni80Fe20薄膜结构与磁电阻特性的影响

NiFe薄膜在室温下具有较高的各向异性磁电阻率,可广泛应用于磁记录和磁传感器.本文研究了工艺条件对电子束蒸发方法制备的Ni80Fe20薄膜磁电阻特性及微结构的影响,获得了制备各向异性磁电阻率达3%~4%的Ni80Fe20薄膜的工艺条件.     

Very large magnetoresistance in lateral ferromagnetic (Ga,Mn)as wires with nanoconstrictions

We have fabricated (Ga,Mn)As nanostructures in which domain walls can be pinned by sub-10 nm constrictions. Controlled by shape anisotropy, we can switch the regions on either side of the constriction to either parallel or antiparallel magnetization. All samples exhibit a positive magnetoresistance, consistent with domain-wall trapping. For metallic samples, we find a magnetoresistance up to 8%, which can be understood from spin accumulation. In samples where, due to depletion at the constriction, a tunnel barrier is formed, we observe a magnetoresistance of up to 2000%.