S. S. P. Parkin

Exchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory (invited)

Exchange biased magnetic tunnel junction (MTJ) structures are shown to have useful properties for forming magnetic memory storage elements in a novel cross-point architecture. MTJ elements have been developed which exhibit very large magnetoresistive (MR) values exceeding 40% at room temperature, with specific resistance values ranging down to as little as ∼60 Ω(μm)2, and with MR values enhanced by moderate thermal treatments. Large MR values are observed in magnetic elements with areas as small as 0.17 (μm)2. The magnetic field dependent current–voltage characteristics of an MTJ element integrated with a silicon diode are analyzed to extract the MR properties of the MTJ element itself.

Magnetotransport properties of magnetically soft spin‐valve structures (invited)

The magnetic and magnetotransport properties of several series of sandwiches consisting of two ferromagnetic layers (Ni, Co, Ni80Fe20) separated by a noble metal (Cu, Ag, Au) are described. In order to vary the relative orientation of the magnetizations of the two ferromagnets, one of them was constrained by exchange anisotropy (e.g., NiFe/Fe50Mn50). The ferromagnetic layers are magnetically soft and not coupled antiparallel, giving very large changes of resistance at low fields. At room temperature relative changes ΔR/R of 4.1% in 10 Oe for Si/Ta 50 A/NiFe 62 A/Cu 22 A/NiFe 40 A/FeMn 70 A/Ta 50 A and 8.7% in 20 Oe has been obtained for a structure based on Co/Cu/Co layers. The magnetoresistance versus the thickness of the ferromagnetic layer shows a broad peak near 80 A for Ni, Co and NiFe, demonstrating the importance of bulk rather than interfacial spin‐dependent scattering, in contrast to Fe/Cr multilayers. The magnetoresistance decreases exponentially with increasing interlayer (Cu and Au) thickness,...

Microstructured magnetic tunnel junctions (invited)

We have used a simple self-aligned process to fabricate magnetic tunnel junctions down to submicron sizes. Optical and electron-beam lithographies were used to cover a range of areas spanning five orders of magnitude. The bottom magnetic electrodes (Co or permalloy) in our junctions were exchange biased by an antiferromagnetic layer (MnFe). The top electrodes were made of soft magnetic materials (Co or permalloy). We have consistently obtained large magnetoresistance ratios (15%–22%) at room temperature and in fields of a few tens of Oe. The shape of the field response of the magnetoresistance was varied from smooth to highly hysteretic by adjusting the shape anisotropy of one junction electrode.

Oscillations in giant magnetoresistance and antiferromagnetic coupling in [Ni81Fe19/Cu]N multilayers

We report giant magnetoresistance in [Ni81Fe19/Cu]N multilayers. Saturation magnetoresistance values exceeding 16% for saturation fields of only 600 Oe are found at 300 K. In addition, we show evidence for well‐defined oscillations in saturation magnetoresistance as a function of Cu spacer layer thickness at 4.2 K, with an oscillation period and phase similar to that in Co/Cu multilayers. However, the temperature dependence of the magnetoresistance, while weak for thin Cu layers, is much stronger for thicker Cu layers. Consequently at 300 K only a single oscillation in magnetoresistance for thin Cu layers is found. We show that the properties of the Ni81Fe19/Cu multilayers are very sensitive to annealing at moderate temperatures, which may limit the possible technological applications of such structures.

Magnetic domain wall motion triggered by an electric current

The current-induced propagation of magnetic domain walls is studied in CoFe nanoconstrictions patterned by electron beam lithography. Propagation of the walls was confirmed by magnetic force microscopy imaging. The device geometry allows us to distinguish between various mechanisms of interaction between electric current and domain walls: a mechanism in which spin transfer associated with current traversing a domain wall dominates. As expected for such a mechanism the domain wall propagation occurs in the direction of electron current flow and has a current threshold of the order ∼1011 A/m2.

Spin polarization of tunneling current from ferromagnet/Al2O3 interfaces using copper-doped aluminum superconducting films

Superconducting Cu-doped Al films sputtered at room temperature (RT) show improved characteristics for superconductor–insulator–ferromagnet spin-polarization measurements as compared to dirty Al films evaporated at 77 K. Detailed analysis including temperature, depairing and spin-orbit effects reveal spin-polarization values for a large variety of ferromagnetic transition-metal alloys. The high-spin polarization for these RT-sputtered junctions is consistent with large tunnel magnetoresistance values observed in similarly sputtered magnetic tunnel junctions.

Dramatic enhancement of interlayer exchange coupling and giant magnetoresistance in Ni81Fe19/Cu multilayers by addition of thin Co interface layers

The giant magnetoresistance (MR) of Ni81Fe19/Cu multilayers is dramatically enhanced by inserting thin Co layers at each Ni81Fe19/Cu interface, unambiguously demonstrating the predominant interfacial origin of the giant MR effect. Similarly the long‐range oscillatory MR and interlayer exchange coupling only present in magnetron sputtered Ni81Fe19/Cu multilayers at low temperatures is restored at higher temperatures by the addition of thin Co interface layers. Multilayered structures comprised of Co/Ni81Fe19/Co/Cu exhibit MR of more than 17% for field changes of ±100 Oe at room temperature.

Superconducting Tl-Ca-Ba-Cu-O thin films with zero resistance at temperatures of up to 120 K

We have prepared superconducting Tl‐Ca‐Ba‐Cu‐O thin films on a variety of substrates with transition temperatures as high as ≂120 K, confirmed by sharp onsets of substantial Meissner and shielding signals at the same temperatures. The properties of the films are found to depend sensitively on the post‐annealing conditions. Highly textured c‐axis‐oriented films comprised mostly of Tl2Ca1Ba2Cu2Ox, Tl1Ca2Ba2Cu3Ox, and Tl2Ca2Ba2Cu3Ox were synthesized by varying the annealing procedure with corresponding maximum superconducting transition temperatures of ≂100, 110, and 120 K, respectively.

Shape-anisotropy-controlled magnetoresistive response in magnetic tunnel junctions

We show that shape anisotropy can be used to control the response characteristics of magnetic tunnel junctions. By varying the junction shape, the resistance versus field curve was made to vary from a nonhysteretic linear curve with a high-field sensitivity (0.3%/Oe) to a hysteretic response curve with high squareness.

Néel “orange-peel” coupling in magnetic tunneling junction devices

We present measurements of the magnitude of Neel “orange-peel” coupling due to interface roughness in a series of magnetic tunneling junction devices. Results from magnetometry and transport measurements are shown to be in good agreement with the theoretical model of Neel. In addition, we have used transmission electron microscopy to directly probe the sample interface roughness and obtain results consistent with the values obtained by magnetometry and transport methods.