J. A. Katine

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

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Publications (213)642.61 Total impact

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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 01/2014; s 358–359:233–258. · 1.83 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 01/2014; 115(11):113910-113910-5. · 2.21 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 01/2014; 104(11):112410-112410-5. · 3.79 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.
    12/2013; 89(13).
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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; 103:172406. · 3.79 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). · 2.21 Impact Factor
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    ABSTRACT: Stochastic dynamics of spin torque oscillators 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 spin torque oscillator. We show that the effective energy approach provides a simple recipe for predicting spectral linewidths and line shapes near the generation threshold. Our time domain technique also accurately measures the fieldlike component of spin torque in a wide range of the voltage bias values.
    Physical Review Letters 08/2013; 111(8):087206. · 7.73 Impact Factor
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    ABSTRACT: Spin transfer torque can excite ferromagnetic resonance of magnetization in a nanoscale magnetic tunnel junction. Here we describe a strongly nonlinear regime of spin-torque-driven ferromagnetic resonance in which large-amplitude magnetization oscillations are excited by microwave current applied to the junction. In this nonlinear regime, the junction generates a large direct voltage in response to the applied microwave signal and thereby can serve as a sensitive microwave signal detector. We demonstrate a low-temperature detector sensitivity of 2.5 × 104 V/W, which exceeds the sensitivity of metal-semiconductor Schottky diodes.
    Applied Physics Letters 08/2013; 103(8). · 3.79 Impact Factor
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    ABSTRACT: The presented embodiments generally relate to designing an antenna of an optical transducer (e.g., a near-field transducer or near-field optical source) that focuses the optical energy of a radiation source (e.g., a laser) onto a magnetic media, thereby heating the media. Specifically, the antenna is designed to wrap-around an aperture of the optical transducer such that at least a portion of the antenna is between a main pole of a write head and a surface of the aperture that faces the main pole. Moreover, the antenna may wrap-around the aperture such that it directly contacts the main pole.
    Ref. No: US 8498182 B1, Year: 07/2013
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    ABSTRACT: We present temperature dependent switching measurements of the Co/Ni multilayered free element of 75 nm diameter spin-valve nanopillars. Angular dependent hysteresis measurements as well as switching field measurements taken at low temperature are in agreement with a model of thermal activation over a perpendicular anisotropy barrier. However, the statistics of switching (mean switching field and switching variance) from 20 K up to 400 K are in disagreement with a N\'{e}el-Brown model that assumes a temperature independent barrier height and anisotropy field. We introduce a modified N\'{e}el-Brown model thats fit the experimental data in which we take a $T^{3/2}$ dependence to the barrier height and the anisotropy field due to the temperature dependent magnetization and anisotropy energy.
    Physical Review B 07/2013; 88(10). · 3.66 Impact Factor
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    ABSTRACT: We report on room temperature magnetoresistance and low frequency noise in sub-100nm elliptic CoFeB/MgO/CoFeB magnetic tunnel junctions with ultrathin (0.9nm) barriers. For magnetic fields applied along the hard axis, we observe current induced magnetization switching between the antiparallel and parallel alignments at DC current densities as low as 4*106A/cm2. We attribute the low value of the critical current to the influence of localized reductions in the tunnel barrier, which affects the current distribution. The analysis of random telegraph noise, which appears in the field interval near a magnetization switch, provides an estimate to the dimension of the pseudo pinholes that trigger the magnetization switching via local spin torque. Micromagnetic simulations qualitatively and quantitatively reproduce the main experimental observations.
    Applied Physics Letters 05/2013; 102(9). · 3.79 Impact Factor
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    ABSTRACT: Element-specific phase-resolved x-ray ferromagnetic resonance (FMR) was used to study spin pumping within Co50Fe50(3)/Cu(6)/Ni80Fe20(5) (thicknesses in nanometers) spin valve structures with large areas, so that edge effects typical of nanopillars used in standard magnetotransport experiments could be neglected. The phase of precession of the Co50Fe50 fixed layer was recorded as FMR was induced in the Ni80Fe20 free layer. The field dependence of the fixed layer phase contains a clear signature of spin transfer torque (STT) coupling due to spin pumping. Fitting the phase delay yields the spin-mixing conductance, the quantity that controls all spin transfer phenomena. The STT coupling is destroyed by insertion of Ta into the middle of the Cu layer.
    Physical review. B, Condensed matter 05/2013; 87(18). · 3.77 Impact Factor
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    ABSTRACT: The spin-transfer nano-oscillator (STNO) offers the possibility of using the transfer of spin angular momentum via spin-polarized currents to generate microwave signals. However, at present STNO microwave emission mainly relies on both large drive currents and external magnetic fields. These issues hinder the implementation of STNOs for practical applications in terms of power dissipation and size. Here, we report microwave measurements on STNOs built with MgO-based magnetic tunnel junctions having a planar polarizer and a perpendicular free layer, where microwave emission with large output power, excited at ultralow current densities, and in the absence of any bias magnetic fields is observed. The measured critical current density is over one order of magnitude smaller than previously reported. These results suggest the possibility of improved integration of STNOs with complementary metal-oxide-semiconductor technology, and could represent a new route for the development of the next-generation of on-chip oscillators.
    Scientific Reports 03/2013; 3:1426. · 5.08 Impact Factor
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    ABSTRACT: Recent experiments have established that thermally activated switching in perpendicularly magnetized spin-valve (SV) nanopillars larger than about 40 nm in diameter is dominated by sub-volume nucleation and domain wall propagation. Despite this complex behavior, room temperature measurements of the switching field distributions indicate thermal activation over a single energy barrier [1]. To better understand the magnetization reversal process, we conducted temperature dependent studies of the switching statistics in nanopillars in which we stabilize non-uniform magnetization states formed by a sub-volume nucleation event. We present results on Co|Ni free layers in SV nanopillars, which include a perpendicularly magnetized fixed layer. Here we measure the distribution of switching events as a function of temperature from 20 K to 300 K. The temperature dependence of both nucleation and propagation distributions is consistent with a thermal activation model, with distinct field-dependent barrier heights for each stage in the reversal process. This is evidence of an energy landscape for switching, which should be relevant for understanding the switching of SV devices even at temperatures that no longer show metastable non-uniform states. [1] Appl. Phys. Lett. 100, 062404 (2012)
    03/2013;
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    ABSTRACT: Precessional dynamics of a Co50Fe50(0.7)/Ni90Fe10(5)/Dy(1)/Ru(3) (thicknesses in nm) thin film have been explored by low temperature time-resolved magneto-optical Kerr effect and phase-resolved x-ray ferromagnetic resonance measurements. As the temperature was decreased from 300 to 140 K, the magnetic damping was found to increase rapidly while the resonance field was strongly reduced. Static x-ray magnetic circular dichroism measurements revealed increasing ferromagnetic order of the Dy moment antiparallel to that of Co50Fe50/Ni90Fe10. Increased coupling of the Dy orbital moment to the precessing spin magnetization leads to significantly increased damping and gyromagnetic ratio of the film while leaving its magnetic anisotropy effectively unchanged.
    Applied Physics Letters 02/2013; 102(6). · 3.79 Impact Factor
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    ABSTRACT: This letter presents a diode-magnetic tunnel junction (MTJ) magnetic random access memory cell in a 65-nm complimentary metal-oxide-semiconductor compatible process. A voltage-controlled magnetic anisotropy switching mechanism, in addition to STT, allows for a unipolar set/reset write scheme, where voltage pulses of the same polarity, but different amplitudes, are used to switch the MTJs. A small crossbar array is constructed from 65-nm MTJs fabricated on a silicon wafer, with switching voltages ${sim}{rm 1}~{rm V}$ and thermal stability greater than 10 years, with discrete germanium diodes as access devices to allow for read/write operations. The crossbar architecture can be extended to multiple layers to create a 3-D stackable, nonvolatile memory with a sub-$1{rm F}^{2}$ effective cell size.
    IEEE Electron Device Letters 01/2013; 34(6):753-755. · 2.79 Impact Factor
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    ABSTRACT: Using transport measurements and micromagnetic simulations we have investigated the domain wall motion driven by spin-transfer torques in all-perpendicular hexagonal nanopillar spin-valves. In particular, we probe domain walls nucleated in the free layer of the spin-valves, which are then pinned in the devices. We have determined both the field-current state diagrams for the domain-wall state and the thermally activated dynamics of the nucleation and depinning processes. We show that the nucleation process is well-described by a modified Néel-Brown model taking into account the spin-transfer torque, whereas the depinning process is independent of the current. This is confirmed by an analytical calculation which shows that spin-torques have no effect on the Arrhenius escape rate associated with thermally activated domain wall depinning in this geometry. Furthermore, micromagnetic simulations indicate that spin-transfer only weakly affects the domain wall motion, but instead modifies the inner domain wall structure.
    Physical review. B, Condensed matter 12/2012; 86(21). · 3.77 Impact Factor
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    ABSTRACT: A spin-torque oscillator (STO) has a single free ferromagnetic layer that forms part of both a giant magnetoresistance (GMR) structure with a nonmagnetic conductive spacer layer and a tunneling magnetoresistance (TMR) structure with a tunnel barrier layer. The STO has three electrical terminals that connect to electrical circuitry that provides a spin-torque excitation current through the conductive spacer layer and a lesser sense current through the tunnel barrier layer. When the STO is used as a magnetic field sensor, the excitation current causes the magnetization of the free layer to oscillate at a fixed base frequency in the absence of an external magnetic field. A detector coupled to the sense current detects shifts in the free layer magnetization oscillation frequency from the base frequency in response to external magnetic fields.
    Ref. No: 8,320,080, Year: 11/2012
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    ABSTRACT: We studied the spin transfer torque (STT) driven switching voltage distribution systematically by characterizing the switching probability density function (PDF) with large statistics (105 trials) across a wide time scale from 5 ns to 1 μs. The skew normal distribution function is found to be a good one to fit the measured switching PDF down to low values, which would be used as a guideline to extrapolate read disturb rate (RDR) and write error rate (WER) in STT-RAM design. Moreover, the asymmetry of switching probability density function is observed to flip when the pulse width decreases. It is related to the fluctuation mechanism transition from the thermal agitation to the initial magnetization trajectory dispersion.
    IEEE Transactions on Magnetics 11/2012; 48(11):3818-3820. · 1.42 Impact Factor
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    ABSTRACT: In an isochronous auto-oscillator, where the auto-oscillation frequency f does not depend on the oscillation amplitude, the linewidth of the nth harmonic is n2 times larger than the linewidth of the main auto-oscillation mode. Here we present a theoretical description that predicts that the increase in the linewidth of higher harmonics with harmonic number n is substantially slower in the case of a nonisoschronous oscillator, where f depends strongly on amplitude. Using spin-torque nano-oscillators as an example of a nonisochronous oscillator, this description is confirmed numerically and experimentally. The presented model allows one to extract important nonlinear parameters of an auto-oscillator of any physical nature from the measurement of higher harmonic linewidths.
    Physical review. B, Condensed matter 09/2012; 86(10). · 3.77 Impact Factor

Publication Stats

3k Citations
642.61 Total Impact Points

Institutions

  • 2009–2013
    • University of California, Irvine
      • Department of Physics and Astronomy
      Irvine, CA, United States
  • 2010
    • University of California, San Diego
      • Department of Electrical and Computer Engineering
      San Diego, California, United States
  • 2009–2010
    • University of Exeter
      • Department of Physics and Astronomy
      Exeter, England, United Kingdom
  • 2006–2010
    • Miami University
      • Department of Physics
      Oxford, OH, United States
    • San Jose State University
      San Jose, California, United States
    • Université Paris-Sud 11
      • Institut d'Electronique Fondamentale
      Orsay, Île-de-France, France
  • 2000–2010
    • Cornell University
      • School of Applied and Engineering Physics
      New York City, NY, United States
  • 2008
    • CSU Mentor
      Long Beach, California, United States
    • Stanford University
      • Department of Applied Physics
      Stanford, CA, United States
  • 2005–2006
    • National Institute of Standards and Technology
      • Electromagnetics Division
      Gaithersburg, MD, United States