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We have invented a three superconducting quantum interference device (SQUID) gradiometer (TSG) that uses three SQUID magnetometers and a novel feedback method to measure magnetic field gradients. One SQUID, designated the reference SQUID, operates normally except that its feedback loop output is directed to all three SQUIDs through identical nonsuperconducting coils around each SQUID. The feedback loops for the remaining two SQUIDs, the sensor SQUIDs, measure the differences between the magnetic field at the reference SQUID location and those at the sensor SQUID locations. The voltage difference between the two sensor SQUID outputs divided by the gradiometer base line, the distance between the sensor SQUIDs, represents the average magnetic field gradient. We have measured gradient sensitivities of 10−12 and 10−10 T/m√Hz for TSGs made from bare low‐Tc and high‐Tc SQUIDs. An advantage of a TSG is that a sensitive gradiometer, free of hysteresis error, can be made using relatively small substrates.

Doped manganite trilayer junction provides a model system for the study of spin-dependent transport across an artificial interface between manganites. Large interface magnetoresistance is observed in manganite trilayer junctions. Further studies reveal a strongly temperature and junction-bias dependent magnetoresistance, as well as a large interaction between the spin-polarized current and the ferromagnetic moment. The physical and materials origin of these and other junction properties are discussed in this brief review.

Spin-dependent conductance across a manganate-barrier-manganate junction has recently been demonstrated. The junction is a La$_{0.67}$Sr$_{0.33}$MnO$_3$% -SrTiO$_3$-La$_{0.67}$ Sr$_{0.33}$MnO$_3$ trilayer device supporting current-perpendicular transport. Large magnetoresistance of up to a factor of five change was observed in these junctions at 4.2K in a relatively low field of the order of 100 Oe. Temperature and bias dependent studies revealed a complex junction interface structure whose materials physics has yet to be understood., Comment: 20 pages, 14 figures. To appear in Phil. Trans. R. Soc. Lond. A vol.356 (1998)

The circuit element behavior during an active read-out approach is analyzed for the detection of the bit-state of a spin-transfer-torque (STT) switchable magnetic tunnel junction (MTJ) as a storage bit in a random access memory. Fundamentally, such read-out schemes detect the presence or absence of a hysteresis in the current-voltage characteristic of the MTJ depending on its magnetic configuration and the bias direction. A quantitative assessment is given in terms of the range of threshold distribution to be expected, and the read-out time one can achieve as it depends on various device parameters. The quantitative results given are based on an approximate macrospin model for the STT-switched MTJ.

The role of grain boundaries in the magnetoresistance (MR) properties of the manganites has been investigated by comparing the properties of epitaxial and polycrystalline films of ${\mathrm{La}}_{0.67}{D}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}} (D=\mathrm{C}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{o}\mathrm{r}\phantom{\rule{0ex}{0ex}}\mathrm{v}\mathrm{a}\mathrm{c}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{s})$. While the MR in the epitaxial films is strongly peaked near the ferromagnetic transition temperature and is very small at low temperatures, the polycrystalline films show large MR over a wide temperature range down to 5 K. The results are explained in terms of switching of magnetic domains in the grains and disorder-induced canting of Mn spins in the grain-boundary region.

Direct observations of the half-integer flux quantum effect in rings, disks, and blanket films of the high-Tc cuprate superconductorsY Ba 2 Cu 3O 7−δ (YBCO),GdBa 2 Cu 3O 7−δ (GdBCO),Tl 2 Ba 2 CuO 6+δ (Tl2201), andBi 2 Sr 2 CaCu 2O 8+δ (BSCCO) with three tricrystal substrate geometries show that their gaps have sign changes consistent with d-wave symmetry. The blanket geometry gives the Josephson penetration depth directly, and provides a means for symmetry tests without patterning. Tl2201 is a tetragonal, single-layer, cuprate without chains. When combined with photoemission results, our BSCCO results imply that the gap followsd x 2−y2 symmetry closely. Asymmetric 45 degree bicrystal grain boundary samples show spontaneous magnetization, as expected for d-wave superconductors with interface roughness.

Magnetic-memory devices of the future could be based on 'spintronics', through switching the directions of electron spins. New work confirms the physics behind a spin-switching mechanism.

CoFeB-based magnetic tunnel junctions with perpendicular magnetic anisotropy are used as a model system for studies of size dependence in spin-torque–induced magnetic switching. For integrated solid-state memory applications, it is important to understand the magnetic and electrical characteristics of these magnetic tunnel junctions as they scale with tunnel junction size. Size-dependent magnetic anisotropy energy, switching voltage, apparent damping, and anisotropy field are systematically compared for devices with different materials and fabrication treatments. Results reveal the presence of sub-volume thermal fluctuation and reversal, with a characteristic length-scale of the order of approximately 40 nm, depending on the strength of the perpendicular magnetic anisotropy and exchange stiffness. To have the best spin-torque switching efficiency and best stability against thermal activation, it is desirable to optimize the perpendicular anisotropy strength with the junction size for intended use. It also is important to ensure strong exchange-stiffness across the magnetic thin film. These combine to give an exchange length that is comparable or larger than the lateral device size for efficient spin-torque switching.

Spin-transfer torque magnetic random access memory (STT-MRAM) is one of the most promising emerging non-volatile memory technologies. MRAM has so far been demonstrated with a unique combination of density, speed, and non-volatility in a single chip, however, without the capability to replace any single mainstream memory. In this paper, we demonstrate the basic physics of spin torque switching in 20 nm diameter magnetic tunnel junctions with perpendicular magnetic anisotropy materials. This deep scaling capability clearly indicates the STT MRAM device itself may be suitable for integration at much higher densities than previously proven.

The superconducting transport characteristics of HgBa(2) CaCu(2)O(6+delta) (Hg-1212) films and grain-boundary junctions grown on (100)-oriented SrTiO(3) bicrystal substrates have been investigated. The films exhibit a zero-resistance temperature of approximately 120 kelvin and sustain large critical current densities, with values as high as 10(6) amperes per square centimeter at around 100 kelvin. On the other hand, the grain boundaries behave as weak links, with substantially lower critical currents, as is observed for other cuprate superconductors. A reduction of three orders of magnitude in critical current was observed for transport across a 36.8 degrees grain boundary. The current-voltage characteristics of bridges across such a grain boundary show weak-link behavior qualitatively resembling that of a resistively shunted junction. Single-level direct-current superconducting quantum interference devices (SQUIDs) have been fabricated with such bicrystal junctions. These SQUIDs show clear periodic voltage modulations when subjected to applied magnetic fields. The SQUIDs operate at temperatures as high as 111.8 kelvin, which makes them attractive for operation in portable sensors and devices that utilize nonconventional cooling methods.

We report on a numerical study of the micromagnetic switching process of a nanostructured spin torque device. We show that incoherent spin waves can be excited over a wide range of current and field even at zero temperature. These large amplitude, incoherent, and nonzero k spin wave modes are shown to alter the switching phase boundary from that calculated within a macrospin model. The presence of telegraphic transitions between different spin wave modes may also contribute to the so-called back-hopping phenomenon where the switching probability varies nonmonotonically with increasing bias current.

Spin torque switching is investigated in perpendicular magnetic tunnel junctions using Ta∣CoFeB∣MgO free layers and a synthetic antiferromagnet reference layer. We show that the Ta∣CoFeB interface makes a key contribution to the perpendicular anisotropy. The quasistatic phase diagram for switching under applied field and voltage is reported. Low switching voltages, Vc 50 ns=290 mV are obtained, in the range required for spin torque magnetic random access memory. Switching down to 1 ns is reported, with a rise in switching speed from increased overdrive that is eight times greater than for comparable in-plane devices, consistent with expectations from a single-domain model.

Bit error rates below 10-11 are reported for a 4-kb magnetic random access memory chip, which utilizes spin transfer torque writing on magnetic tunnel junctions with perpendicular magnetic anisotropy. Tests were performed at wafer level, and error-free operation was achieved with 10 ns write pulses for all nondefective bits during a 66-h test. Yield in the 4-kb array was limited to 99% by the presence of defective cells. Test results for both a 4-kb array and individual devices are consistent and predict practically error-free operation at room temperature.

CeO2 thin films have been deposited on patterned (100) LaAlO3 substrates using ion beam assisted deposition (IBAD) with ion beam energies between 350 and 500eV. Deposition temperatures were varied between 400°C and 600°C and deposition rates from 0.2Å/s to 1.2Å/s. Both normal and off-normal incidence ion bombardment have been studied. A trend towards planarization is observed when the ion to atom ratio is adjusted to obtain the proper degree of etching. The planarization mechanism for normal incidence bombardment appears to be similar to that previously observed for bias sputtering. X-ray diffraction shows that an initial layer of evaporated epitaxial CeO2 is required for continued epitaxial development during IBAD processing. The extent of planarization via off-normal ion incidence can be related to the direction of the ion beam with respect to the patterned features. X-ray pole figure measurements show that these films possess an in-plane orientation that is directly related to the ion beam parameters.

Spin-transfer switching with short current pulses has been studied in spin-valve nanopillars with perpendicularly magnetized free and reference layers. Magnetization switching with current pulses as short as 300 ps is demonstrated. The pulse amplitude needed to reverse the magnetization is shown to be inversely proportional to the pulse duration, consistent with a macrospin spin-transfer model. However, the pulse amplitude duration switching boundary depends on the applied field much more strongly than predicted by the zero temperature macrospin model. The results also demonstrate that there is an optimal pulse length that minimizes the energy required to reverse the magnetization.

A three-terminal spin-torque-driven magnetic switch is experimentally demonstrated. The device uses nonlocal spin current and spin accumulation as the main mechanism for current-driven magnetic switching. It separates the current-induced write operation from that of a magnetic tunnel junction based read. The write current only passes through metallic structures, improving device reliability. The device structure makes efficient use of lithography capabilities, important for robust process integration.

For CoFeB∕MgO-based magnetic tunnel junctions, the switching probability has an unusual dependence on bias voltage V and bias magnetic field H for bias voltage pulse durations t long enough to allow thermally activated reversal. At high junction bias close to 1V, the probability of magnetic switching in spin-torque-driven switches sometimes appears to decrease. This is shown to be due to a backhopping behavior occurring at high bias, and it is asymmetric in bias voltage, being more pronounced in the bias direction for antiparallel-to-parallel spin-torque switch, i.e., in the direction of electrons tunneling into the free layer. This asymmetry hints at processes involving hot electrons within the free-layer nanomagnet.

In some cases such as junctions with low magnetic thermal activation energy, the magnetization of the free layer in MgO-based magnetic tunnel junctions (MTJs) can back hop to its original direction after successful spin torque induced switching. The back-hopping is observed in both current directions corresponding to parallel-to-antiparallel and antiparallel-to-parallel switchings. For bias voltage pulses with increasing pulse width, the threshold voltage for back-hopping appears to decrease together with spin-torque switching and junction breakdown thresholds, but its rate of decrease is less. Increasing the anisotropy field Hk by increasing the MTJ aspect ratio can raise the threshold voltage of back-hopping significantly.

We present room-temperature measurements of high-frequency magnetization fluctuation (mag noise) in MgO-based nanopillar magnetic tunnel junctions (MTJs) biased with a direct current (dc). In the frequency range of 1–13 GHz, double mag-noise peaks are observed for some MTJs while others only show a single mag-noise peak. The in-plane field dependence of the mag-noise peak frequency is consistent with the Kittel formula. For all MTJs measured, the bias-dependent shift in the mag-noise peak frequency has a pronounced asymmetry. In addition, we find nonmonotonic variations in peak linewidth as a function of the external in-plane magnetic field and of the dc bias current. These suggest the possible involvement of nonmacrospin modes in spin-torque-dependent thermal mag-noise generation.

Current-induced spin-torque switching was demonstrated on sub-100 nm magnetic tunnel junction devices fabricated on 200 mm substrates utilizing 180 nm complimentary metal–oxide–semiconductor back-end-of-the-line (BEOL) technology. Low resistance-area (RA) product and high tunneling magnetoresistance (TMR) were achieved by using substrates containing a CoFeB free layer and a thin MgO barrier. To obtain the desired sub-100 nm features, photoresist trimming was applied on patterns created by a 248 nm lithography tool. Furthermore, the magnetic stack was defined using an ion beam etch that stopped on the thin MgO barrier. Field-sweep measurements on elliptical devices that are 80 nm wide and 160 nm long indicated RA∼4 Ω μm2 and TMR∼90%. Upon injecting current into the devices while applying an external offset field of 28 Oe, current-induced switching occurred from parallel (P) to antiparallel (AP) state at +1.3 mA, and from AP to P state at −1.25 mA. BEOL process integration on 200 mm substrates enabled stati...

Current-induced excitations in bilayer magnetic nanopillars have been studied with large magnetic fields applied perpendicular to the layers at low temperature. Junctions investigated all have Cu/Co/Cu/Co/Cu as core layer stacks. Two types of such junctions are compared, one with the core stack sandwiched between Pt layers (type A), the other with Pt only on one side of the stack (type B). Transport measurements show that these two types of junctions have similar magnetoresistance and slope of critical current with respect to field, while A samples have higher resistance. The high-field bipolar excitation, as was previously reported [Oezyilmaz et al., Phys. Rev. B 71, 140403(R) (2005)], is present in B samples only. This illustrates the importance of contact layers to spin-current-induced phenomena. This also confirms a recent prediction on such spin-wave excitations in bilayers., Comment: 3 pages, 3 figures

Spin-transfer-induced magnetic excitation in large magnetic field applied perpendicular to the thin film junction surface reveals both a current threshold Ic and a voltage threshold. The current threshold follows the Slonczewski-type of magnetic field dependence [J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996)]. The voltage step at Ic is ΔV which appears to scale with the applied field with a prefactor of the order of 2μB∕e, suggesting a threshold to magnetic excitation. Furthermore, experimentally it is observed that ΔV≈IcδR, where δR is the magnetoreistance between the parallel and the antiparallel states. This apparent coincidence can be unified when one includes the effect of spin-pumping-related nonlocal damping. The spin-pump damping relates magnetic instability threshold Ic to δR, producing (dIc∕dH)δR that is about 2μB∕e, explaining the origin of the coincidence.

Recent progress in oxide perovskite thin-film technology has led to the discovery of a large negative magnetoresistance at room temperature in the doped manganate perovskite thin films. For applications such as magnetic-field sensing, the saturation magnetic field for large magnetoresistance has to be significantly lowered. The magnetic and transport properties of the doped manganates involve a curious magnetic-field scale, on the order of 1–10 T. Upon the application of a field on this scale, the magnetoresistance saturates, and a significant broadening of the temperature-dependent magnetization is seen. An understanding of the materials physics that underlie such behavior can point to new ways of lowering the saturation field in this class of materials. We argue that this characteristic field is suggestive of an inhomogeneous magnetic state in the system. We will discuss the basic phenomena and physics of magnetotransport in this class of materials. We will also report the successful fabrication of a tr...

We have explored the colossal magnetoresistance (CMR) effect in the antiferromagnetic La0.5Ca0.5MnO3 compound. In the absence of a magnetic field (H), the solid is a canted antiferromagnetic (AFM) insulator. An applied H in the Tesla scale induces a first order AFM to FM phase transition, and correspondingly, an insulator to metal transition. The observed CMR is attributed to the H‐induced charge localization‐delocalization behavior associated with the AFM–FM transition. At low temperatures (T

We have fabricated epitaxial La0.67Ca0.33MnO3−δ thin films using pulsed laser deposition and performed subsequent thermal treatments. Magnetoresistance values as large as 14 000% (Δρ/ρH) were obtained. Above magnetic ordering temperatures, we observed a thermally activated conductivity that also has a quadratic magnetic field dependence. At low temperatures, we found a large hysteresis in the magnetoresistance that correlates with an unusual magnetic hysteresis curve. We interpret the hysteretic behavior as the result of a ‘‘frozen‐in’’ canted spin structure. Our results suggest that the electron conduction in this material is most sensitive to the canting angle of the neighboring Mn spins.

Epitaxial thin films of La1−x⧠xMnO3−δ(0≤x≤0.33) where ⧠ is a vacancy have been grown on (100) SrTiO3 substrates by pulsed laser deposition. The lanthanum deficiency results in self doping of the system without the need for divalent ion substitution. Both the films and the bulk targets used for deposition were ferromagnetic and showed giant magnetoresistance (GMR). The films increased in saturation magnetization Ms and Curie temperature Tc with increasing x. The Tc, however, was about the same as found in Cax substitutions. Bulk samples showed little change in Ms and Tc with vacancy concentration. Ms in bulk samples is about half the value found in the films which have theoretical saturations for x≥0.2. GMR in the films is a few hundred percent but the bulk values are 25% at H=1.8 T.

We report on the thermal-activation nature of magnetic switching in magnetic nanostructures, using the junction magnetoresistance of a current-perpendicular magnetic spin-valve device as a probe. A spin-valve junction structure was fabricated using electron-beam lithography. A sweep-rate-dependent magnetic switching field was obtained in the quasi-static limit. Results confirm the predictions of a single-domain thermal activation model. The scaling relation between the magnetic field sweep rate, the magnetic switching field, and the sample size is verified for sample dimensions of 0.1×0.2 μm 2 .