J. A. Katine

University of California, Irvine, Irvine, California, United States

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Publications (228)659.64 Total impact

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    ABSTRACT: The high contrast in the electrical resistivity between amorphous and crystalline states of a phase change material can potentially enable multiple memory levels for efficient use of a data storage medium. We report on our investigation of the role of the current injection site geometry (circular and square) in stabilizing such intermediate states within a nanoscale single-phase change material system (Ge 2Sb2Te5). We have developed a three dimensional multiphysics model, which includes phase change kinetics, electrical, thermal, thermoelectric, and percolation effects, all as a function of temperature, using an iterative approach with coupled differential equations. Our model suggests that the physical origin of the formation of stable intermediate states in square top contact devices is mainly due to anisotropic heating during the application of a programming current pulse. Furthermore, the threshold current requirement and the width of the programming window are determined by crystallite nucleation and growth rates such that a higher crystallization rate leads to a narrower range of current pulses for switching to intermediate resistance level(s). The experimentally determined resistance maps, those that are indicative of the crystallinity, show good agreement with the simulated phase change behavior confirming the existence of stable intermediate states. Our model successfully predicts the required programming conditions for such mixed-phase levels, which can be used to optimize memory cells for future ultra-high density data storage applications.
    Journal of Applied Physics 06/2015; 117(21):214302. DOI:10.1063/1.4921827 · 2.19 Impact Factor
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    ABSTRACT: We present a comparative study of high frequency dynamics and low frequency noise in elliptical magnetic tunnel junctions with lateral dimensions under 100 nm presenting current-switching phenomena. The analysis of the high frequency oscillation modes with respect to the current reveals the onset of a steady-state precession regime for negative bias currents above $J=10^7 A/cm^2$, when the magnetic field is applied along the easy axis of magnetization. By the study of low frequency noise for the same samples, we demonstrate the direct link between changes in the oscillation modes with the applied current and the normalised low frequency (1/f) noise as a function of the bias current. These findings prove that low frequency noise studies could be a simple and powerful technique to investigate spin-torque based magnetization dynamics.
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    ABSTRACT: Vortex dynamics within arrays of square ferromagnetic nanoelements have been studied by time-resolved scanning Kerr microscopy (TRSKM), while x-ray photoemission electron microscopy has been used to investigate the equilibrium magnetic state of the arrays. An alternating field demagnetization process was found to initialize a distribution of equilibrium states within the individual elements of the array, including quasiuniform states and vortex states of different chirality and core polarization. Repeated initialization revealed some evidence of stochastic behavior during the formation of the equilibrium state. TRSKM with a spatial resolution of ∼300 nm was used to detect vortex gyration within arrays of square nanoelements of 250-nm lateral size. Two arrays were studied consisting of a 9×9 and 5×5 arrangement of nanoelements with 50- and 500-nm element edge-to-edge separation to encourage strong and negligible dipolar interactions, respectively. In the 5×5 element array, TRSKM images, acquired at a fixed phase of the driving microwave magnetic field, revealed differences in the gyrotropic phase within individual elements. While some phase variation is attributed to the dispersion in the size and shape of elements, the vortex chirality and core polarization are also shown to influence the phase. In the 9×9 array, strong magneto-optical response due to vortex gyration was observed across regions with length equal to either one or two elements. Micromagnetic simulations performed for 2×2 arrays of elements suggest that particular combinations of vortex chirality and polarization in neighboring elements are required to generate the observed magneto-optical contrast.
    Physical Review B 05/2015; 91(17):174425. DOI:10.1103/PhysRevB.91.174425 · 3.66 Impact Factor
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    ABSTRACT: We study thin films and magnetic tunnel junction nanopillars based on Ta/Co20Fe60B20/MgO multilayers by electrical transport and magnetometry measurements. These measurements suggest that an ultrathin magnetic oxide layer forms at the Co20Fe60B20/MgO interface. At approximately 160 K, the oxide undergoes a phase transition from an insulating antiferromagnet at low temperatures to a conductive weak ferromagnet at high temperatures. This interfacial magnetic oxide is expected to have significant impact on the magnetic properties of CoFeB-based multilayers used in spin torque memories.
    Applied Physics Letters 05/2015; 106(19):192407. DOI:10.1063/1.4921306 · 3.52 Impact Factor
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    ABSTRACT: Picosecond magnetization dynamics in the free and pinned layers of a microscale magnetic tunnel valve have been studied using time-resolved scanning Kerr microscopy. A comparison of the observed dynamics with those of individual free and pinned layers allowed the effect of interlayer coupling to be identified. A weak interlayer coupling in the tunnel valve continuous film reference sample was detected in bulk magnetometry measurements, while focused Kerr magnetometry showed that the coupling was well maintained in the patterned structure. In the tunnel valve, the free layer precession was observed to have reduced amplitude and an enhanced relaxation. During magnetization reversal in the pinned layer, its frequency approached that of the low frequency mode associated with the free layer. At the pinned layer switching field, the linewidth of the free layer became similar to that of the pinned layer. The similarity in their frequencies promotes the formation of precessional modes that exhibit strong collective properties such as frequency shifting and enhanced linewidth, while inhomogeneous magnetization of the pinned layer during reversal may also play a role in these observations. The collective character of precessional dynamics associated with mixing of the free and pinned layer magnetization dynamics must be accounted for even in tunnel valves with a small interlayer coupling.
    Journal of Applied Physics 05/2015; 117(17):17B301. DOI:10.1063/1.4907701 · 2.19 Impact Factor
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    ABSTRACT: We study thin films and magnetic tunnel junction nanopillars based on Ta/Co$_{20}$Fe$_{60}$B$_{20}$/MgO multilayers by electrical transport and magnetometry measurements. These measurements suggest that an ultrathin magnetic oxide layer forms at the Co$_{20}$Fe$_{60}$B$_{20}$/MgO interface. At approximately 160 K, the oxide undergoes a phase transition from an insulating antiferromagnet at low temperatures to a conductive weak ferromagnet at high temperatures. This interfacial magnetic oxide is expected to have significant impact on the magnetic properties of CoFeB-based multilayers used in spin torque memories.
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    ABSTRACT: The prediction and realization of magnetic excitations driven by electrical currents via the spin transfer torque effect, enables novel magnetic nano-devices where spin-waves can be used to process and store information. The functional control of such devices relies on understanding the properties of non-linear spin-wave excitations. It has been demonstrated that spin waves can show both an itinerant character, but also appear as localized solitons. So far, it was assumed that localized solitons have essentially cylindrical, $s-$like symmetry. Using a newly developed high-sensitivity time-resolved magnetic x-ray microscopy, we instead observe the emergence of a novel localized soliton excitation with a nodal line, i.e. with $p-$like symmetry. Micromagnetic simulations identify the physical mechanism that controls the transition from $s-$ to $p-$like solitons. Our results suggest a potential new pathway to design artificial atoms with tunable dynamical states using nanoscale magnetic devices.
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    ABSTRACT: We have used a MHz lock-in x-ray spectro-microscopy technique to detect the accumulation of transient spins flowing across a Co/Cu interface. The elemental and chemical specificity of x-rays allows us to distinguish spin accumulation on Cu atoms located at the interface from those within the 28 nm thick Cu film. Spin accumulation in the film gives rise to an average transient magnetic moment per Cu atom of $3\times 10^{-5}$ $\mu_\mathrm{B}$, which is explained by Mott's two current model. We find a greatly enhanced transient moment of $4\times 10^{-3}$ $\mu_\mathrm{B}$ on the Cu interface atoms which is attributed to enhanced spin dependent scattering via localized interface states.
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    ABSTRACT: A current-perpendicular-to-the plane magnetoresistive sensor has top and bottom electrodes narrower than the sensor trackwidth. The electrodes are formed of one of Cu, Au, Ag and AgSn, which have an ion milling etch rate much higher than the etch rates for the sensor's ferromagnetic materials. Ion milling is performed at a high angle relative to a line orthogonal to the plane of the electrode layers and the layers in the sensor stack. Because of the much higher etch rate of the material of the top and bottom electrode layers, the electrode layers will have side edges that are recessed from the side edges of the free layer. This reduces the surface areas for the top and bottom electrodes, which causes the sense current passing through the sensor's free layer to be confined in a narrower channel, which is equivalent to having a sensor with narrower physical trackwidth.
    Ref. No: US 8,988,833, Year: 03/2015
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    ABSTRACT: Microwave detectors based on the spin-transfer torque diode effect are among the key emerging spintronic devices. By utilizing the spin of electrons in addition to charge, they have the potential to overcome the theoretical performance limits of their semiconductor (Schottky) counterparts, which cannot operate at low input power. Here, we demonstrate nanoscale microwave detectors exhibiting record-high detection sensitivity of 75400 mV mW$^{-1}$ at room temperature, without any external bias fields, for input microwave power down to 10 nW. This sensitivity is 20x and 6x larger than state-of-the-art Schottky diode detectors (3800 mV mW$^{-1}$) and existing spintronic diodes with >1000 Oe magnetic bias (12000 mV mW$^{-1}$), respectively. Micromagnetic simulations supported by microwave emission measurements reveal the essential role of the injection locking to achieve this sensitivity performance. The results enable dramatic improvements in the design of low input power microwave detectors, with wide-ranging applications in telecommunications, radars, and smart networks.
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    ABSTRACT: For practical applications of spin torque nano-oscillators (STNO), one of the most critical characteristics is the speed at which an STNO responds to variations of external control parameters, such as current or/and field. Theory predicts that this speed is limited by the amplitude relaxation rate Gamma(p) that determines the timescale over which the amplitude fluctuations are damped out. In this study, this limit is verified experimentally by analyzing the amplitude and frequency noise spectra of the output voltage signal when modulating an STNO by a microwave current. In particular, it is shown that due to the non-isochronous nature of the STNO the amplitude relaxation rate Gamma(p) determines not only the bandwidth of an amplitude modulation, but also the bandwidth of a frequency modulation. The presented experimental technique will be important for the optimisation of the STNO characteristics for applications in telecommunications or/and data storage and is applicable even in the case when the STNO output signal is only several times higher than noise. (C) 2014 AIP Publishing LLC.
    Applied Physics Letters 10/2014; 105(15). DOI:10.1063/1.4898093 · 3.52 Impact Factor
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    ABSTRACT: We present a systematic experimental study of the spin-torque-induced magnetic switching statistics at room temperature, using all-perpendicularly magnetized spin-valves as a model system. Three physical regimes are distinguished: a short-time ballistic limit below a few nanoseconds, where spin-torque dominates the reversal dynamics from a thermal distribution of initial conditions; a long time limit, where the magnetization reversal probability is determined by spin-torque-amplified thermal activation; and a cross-over regime, where the spin-torque and thermal agitation both contribute. For a basic quantitative understanding of the physical processes involved, an analytical macrospin model is presented which contains both spin-torque dynamics and finite temperature effects. The latter was treated rigorously using a Fokker–Plank formalism, and solved numerically for specific sets of parameters relevant to the experiments to determine the switching probability behavior in the short-time and cross-over regimes. This analysis shows that thermal fluctuations during magnetization reversal greatly affect the switching probability over all the time scales studied, even in the short-time limit.
    Journal of Magnetism and Magnetic Materials 05/2014; s 358–359:233–258. DOI:10.1016/j.jmmm.2014.01.061 · 2.00 Impact Factor
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    ABSTRACT: We present a study of the temperature dependence of the switching fields in Co/Ni-based perpendicularly magnetized spin-valves. While magnetization reversal of all-perpendicular Co/Ni spin valves at ambient temperatures is typically marked by a single sharp step change in resistance, low temperature measurements can reveal a series of resistance steps, consistent with non-uniform magnetization configurations. We propose a model that consists of domain nucleation, propagation, and annihilation to explain the temperature dependence of the switching fields. Interestingly, low temperature (<30 K) step changes in resistance that we associate with domain nucleation have a bimodal switching field and resistance step distribution, attributable to two competing nucleation pathways.
    Journal of Applied Physics 03/2014; 115(11):113910-113910-5. DOI:10.1063/1.4868159 · 2.19 Impact Factor
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    ABSTRACT: In this work, we experimentally study the temperature dependence of the perpendicular magnetic anisotropy (PMA) and of the voltage-controlled magnetic anisotropy (VCMA) in nanoscale MgO|CoFeB|Ta-based magnetic tunnel junctions. We demonstrate that the temperature dependences of both the PMA and the VCMA coefficient follow power laws of the saturation magnetization, but with different exponents. We also find that the linear dependence of the PMA on electric field is maintained over a wide temperature range, although the VCMA strength decreases faster as a function of temperature as compared to the PMA. Possible mechanisms leading to the different exponents are discussed.
    Applied Physics Letters 03/2014; 104(11):112410-112410-5. DOI:10.1063/1.4869152 · 3.52 Impact Factor
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    ABSTRACT: We present switching field distributions of spin-transfer assisted magnetization reversal in perpendicularly magnetized Co/Ni multilayer spin-valve nanopillars at room temperature. Switching field measurements of the Co/Ni free layer of spin-valve nanopillars with a 50 nm x 300 nm ellipse cross section were conducted as a function of current. The validity of a model that assumes a spin-current dependent effective barrier for thermally activated reversal is tested by measuring switching field distributions under applied direct currents. We show that the switching field distributions deviate significantly from the double exponential shape predicted by the effective barrier model, beginning at applied currents as low as half of the zero field critical current. Barrier heights extracted from switching field distributions for currents below this threshold are a monotonic function of the current. However, the thermally-induced switching model breaks down for currents exceeding the critical threshold.
    Physical Review B 12/2013; 89(13). DOI:10.1103/PhysRevB.89.134427 · 3.66 Impact Factor
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    ABSTRACT: We demonstrate a technique of broadband spin torque ferromagnetic resonance (ST-FMR) with magnetic field modulation for measurements of spin wave properties in magnetic nanostructures. This technique gives great improvement in sensitivity over the conventional ST-FMR measurements, and application of this technique to nanoscale magnetic tunnel junctions (MTJs) reveals a rich spectrum of standing spin wave eigenmodes. Comparison of the ST-FMR measurements with micromagnetic simulations of the spin wave spectrum allows us to explain the character of low-frequency magnetic excitations in nanoscale MTJs.
    Applied Physics Letters 10/2013; 103(17):172406. DOI:10.1063/1.4826927 · 3.52 Impact Factor
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    ABSTRACT: Switching field measurements of the free layer element of 75 nm diameter spin-valve nanopillars reveal a bimodal distribution of switching fields at low temperatures (below 100 K). This result is inconsistent with a model of thermal activation over a single perpendicular anisotropy barrier. The correlation between antiparallel to parallel and parallel to antiparallel switching fields increases to nearly 50% at low temperatures. This reflects random fluctuation of the shift of the free layer hysteresis loop between two different magnitudes, which may originate from changes in the dipole field from the polarizing layer. The magnitude of the loop shift changes by 25% and is correlated to transitions of the spin-valve into an antiparallel configuration.
    Journal of Applied Physics 09/2013; 115(17). DOI:10.1063/1.4855019 · 2.19 Impact Factor
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    ABSTRACT: Stochastic dynamics of spin torque oscillators (STOs) can be described in terms of magnetization drift and diffusion over a current-dependent effective energy surface given by the Fokker-Planck equation. Here we present a method that directly probes this effective energy surface via time-resolved measurements of the microwave voltage generated by a STO. We show that the effective energy approach provides a simple recipe for predicting spectral line widths and line shapes near the generation threshold. Our time domain technique also accurately measures the field-like component of spin torque in a wide range of the voltage bias values.
    Physical Review Letters 08/2013; 111(8):087206. DOI:10.1103/PhysRevLett.111.087206 · 7.73 Impact Factor

Publication Stats

6k Citations
659.64 Total Impact Points

Institutions

  • 2012
    • University of California, Irvine
      • Department of Physics and Astronomy
      Irvine, California, United States
  • 2010
    • University of California, San Diego
      • Department of Electrical and Computer Engineering
      San Diego, California, United States
  • 2006
    • San Jose State University
      San Jose, California, United States
  • 1999–2000
    • Cornell University
      • School of Applied and Engineering Physics
      Ithaca, New York, United States