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ABSTRACT: The application of radio-frequency (rf) currents in spin-torque devices allows control of magnetization reversal through frequency of the rf current. Using this effect, the impact of the rf currents on switching fields of nanostructured metallic spin valves has been investigated. The rf current injection was observed to change the critical field for free-layer magnetization reversal when the intrinsic spin-transfer-induced dynamics are frequency locked with the injected rf. Experimental results are presented by mapping the switching contour plot, and are compared with simulations in the context of macrospin models of spin transfer in metallic-spin-valve structures. Differences between this effect and the effect of rf currents on direct-current-induced magnetization reversal are highlighted by considering the role of a magnetization dispersion in the two sweep methods, which allows insights for device applications assisted by rf currents.
Phys. Rev. B. 08/2011; 84(5).
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ABSTRACT: Magnetic and magnetotransport properties of current-perpendicular-to-the-plane (CPP) giant-magnetoresistance (GMR) spin-valve sensors containing the Heusler alloy Co2MnGe are presented. The geometrical and head integration constraints which exist for recording head applications are discussed and dictate various design compromises which determine the final device properties. Here we show that even for small total sensor thicknesses 400 Å and anneal temperatures < 250 °C we can obtain CPP-GMR signal levels up to ΔRA = 4 mΩ-μm2 at room temperature when inserting Co2MnGe in both the free layer and reference layers of the spin valve. Output levels increase to 10 mΩ-μm2 when reducing the temperature below 100 K, demonstrating the strong temperature dependence of the spin-dependent scattering in the Co2MnGe-based magnetic layers.
Journal of Applied Physics 05/2011; 109(9):093912-093912-11. · 2.17 Impact Factor
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ABSTRACT: A spin-wave Doppler technique is used to measure the drift velocity of the magnetization in current-carrying (CoFe)1−xGex alloys. For a current density of 1011 A/m2, we obtain a large enhancement of drift velocity with increased Ge concentration from 3.1±0.2 m/s for CoFe to 8.2±0.6 m/s for (CoFe)0.7Ge0.3. Interpretation of these values yields current polarization increasing from 0.84±0.04 to 0.95±0.05 as the Ge doping increases. While both spin-dependent conductivities decrease with increasing Ge concentration, the maximum in polarization is associated with a stronger relative change in the minority spin conductivity.
Applied Physics Letters 02/2011; 98(7):072510-072510-3. · 3.84 Impact Factor
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ABSTRACT: We systematically study the changes in the local atomic environments of Co in CoFe–Al alloys as a function of Al content by means of nuclear magnetic resonance. We find that a Co2FeAl Heusler type structure is formed on a local scale. The observed formation of a highly spin-polarized Heusler compound may explain the improved magnetotransport properties in CoFe–Al based current-perpendicular-to-the-plane spin-valves.
Applied Physics Letters 01/2011; 98(1):012506-012506-3. · 3.84 Impact Factor
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ABSTRACT: The magnetotransport properties of current perpendicular to the plane giant magnetoresistive spin-valves utilizing ( CoFe )<sub>100-x</sub> Ge <sub>x</sub> alloys are investigated. The composition range of 21≤x≤32 at . % is determined to be the alloy composition that maximizes magnetoresistance. Δ RA values of 2.6 m Ω–μ m <sup>2</sup> are measured for spin-valves with CoFeGe in the free and reference layer resulting in magnetoresistance values of greater than 6%, which is almost twice the value of similar spin-valves with CoFeAl alloys. An analysis of the physical properties of the CoFeGe thin films in the range of interest is also presented.
Applied Physics Letters 11/2008; · 3.84 Impact Factor
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ABSTRACT: We demonstrate that the addition of Dy capping layers in current perpendicular to the plane giant magnetoresistive spin-valves can increase the critical current density beyond which spin-torque induced instabilities are observed by about a factor of three. While Dy capped samples exhibit nonmagnetic 1/f noise, it is sufficiently small to be unimportant for read head operation at practical data rates.
Applied Physics Letters 10/2008; · 3.84 Impact Factor
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ABSTRACT: It is shown that the maximum stable output of a CPP-GMR sensor is increased significantly by using a synthetic ferrimagnet free layer, provided the electron current flows from free layer to reference layer. This free layer allows a larger magnetoresistance ratio for a given free layer magnetic moment, and in addition results in a greater than three-fold increase in the critical current above which spin-torque instability of the free layer occurs. In read heads with net free layer moments equivalent to only 4.5nm of Ni80Fe20, this effect is shown to result in sustainable sense current densities above 2e8 A/cm2.
09/2008;
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ABSTRACT: It is shown that the maximum stable output of a current-perpendicular to the plane giant magnetoresistance sensor is increased significantly by using a synthetic-ferrimagnet free layer, provided the electron current flows from free layer to reference layer. This free layer allows a larger magnetoresistance ratio for a given free layer magnetic moment, and in addition results in a greater than threefold increase in the critical current, above which spin-torque instability of the free layer occurs. In read heads with net free layer moments equivalent to only 4.5 nm of Ni80Fe20, this effect is shown to result in sustainable sense current densities above 2×108 A/cm2.
Applied Physics Letters 09/2008; 93(10):102509-102509-3. · 3.84 Impact Factor
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ABSTRACT: We demonstrate that the addition of Dy capping layers in current perpendicular to the plane giant magneto-resistive spin-valves can increase the critical current density beyond which spin-torque induced instabilities are observed by about a factor of three. Current densities as high as 5e7 A/cm2 are measured provided that the electron current flows from the free to the reference layer. While Dy capped samples exhibit nonmagnetic 1/f noise, it is sufficiently small to be unimportant for read head operation at practical data rates. Comment: 13 pages (manuscript form), with 5 figures. Submitted for publication
08/2008;
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J.R. Childress, M.J. Carey,
S. Maat,
N. Smith,
R.E. Fontana,
D. Druist,
K. Carey,
J.A. Katine,
N. Robertson,
T.D. Boone,
M. Alex,
J. Moore,
C.H. Tsang
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ABSTRACT: Read heads using current-perpendicular-to-plane (CPP) giant magnetoresistance sensors have been fabricated and tested under high-density recording conditions. A magnetoresistance of 5.5% and shield-to-shield spacing of 45 nm have been achieved by using an all-metal single-spin-valve with Heusler-alloy-based free and reference magnetic layers. Read heads with magnetic read widths ~45 nm were tested on perpendicular media, resulting in signals above 1 mV and signal-to-noise ratio ~30 dB. Linear densities in excess of 1050 kbpi were achieved with thermal fly-height control, compatible with recording areal densities of ~400 Gb/in<sup>2</sup>. Current-induced spin-torque effects in the recording head were observed to result in rapid performance degradation above a threshold bias voltage of about 75 mV, corresponding to current densities >10<sup>8</sup> A/cm<sup>2</sup>.
IEEE Transactions on Magnetics 02/2008; · 1.36 Impact Factor
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ABSTRACT: The magnetotransport properties of current perpendicular to the plane giant magnetoresistive spin valves utilizing (CoxFe100−x)100−yAly alloys are investigated over a wide alloy composition range. (Co50Fe50)75Al25 is determined to be the approximate alloy composition that maximizes magnetoresistance. An increase in magnetoresistance from 1.7% for spin valves with standard Co50Fe50 to 3.3% for spin valves with (Co50Fe50)75Al25 is observed when substituting the same “magnetic” thickness in both the reference and the free layers. The spin-diffusion length for (Co50Fe50)75Al25 is determined to be less than approximately 35 Å. Spin-torque measurements show that the spin-torque current density threshold is lower in CoFeAl spin valves compared to standard CoFe spin valves.
Journal of Applied Physics 05/2007; 101(9):093905-093905-6. · 2.17 Impact Factor
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J.R. Childress,
M.J. Carey,
M.-C. Cyrille, K. Carey,
N. Smith,
J.A. Katine,
T.D. Boone,
A.A.G. Driskill-Smith,
S. Maat,
K. Mackay,
C.H. Tsang
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ABSTRACT: Magnetic recording read heads have been fabricated and tested using a dual-spin-valve (dual-SV) current-perpendicular-to-plane (CPP) giant magnetoresistive (GMR) sensor stack. Thin IrMn antiferromagnetic layers are used for top- and bottom-pinned layers, resulting in a total sensor stack height of 50 nm. Operational read heads with physical sensor trackwidths ranging from 60 to 30 nm have been achieved. Successful readback testing is obtained using perpendicular recording media, with observed magnetic read widths as low as 46 nm
IEEE Transactions on Magnetics 11/2006; · 1.36 Impact Factor
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ABSTRACT: Dual IrMn-pinned current-perpendicular-to-plane giant magnetoresistive sensor structures have been investigated, and their signal and noise characteristics are compared to similar standard single sensors, using test structures with sizes down to 50 nm. The intrinsic signal ΔRA product is found to be increased by a factor of nearly 2 between single and dual spin valves, although the parasitic resistance due to the second 80-nm-thick IrMn-pinned layer stack limits the net increase in ΔR/R to a factor of 1.5. For the single spin valve with antiparallel free and reference layers, the noise power spectral density shows a large asymmetry between positive and negative current flows due to strong spin-torque effects for positive (electrons flow from reference layer to free layer) current densities above 1×107A/cm2. For the dual sensor, however, the critical current density is symmetric in polarity and is increased to above 5×107A/cm2, resulting in a 5-fold increase in practical sensor output voltage.
Journal of Applied Physics 04/2006; 99(8):08S305-08S305-3. · 2.17 Impact Factor
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ABSTRACT: This paper considers magnetization fluctuations (mag-noise) in current-perpendicular-to-plane (CPP) read sensors, due to either thermal fluctuations or spin-torque induced instability. The former are relevant to both tunneling (TMR) and giant-magnetoresistance (GMR) devices, the latter a potentially more serious limitation most relevant to all-metal GMR sensors. Discussion of thermal mag-noise is here treated by micromagnetic simulation. Discussion of spin-torque instability includes analytical modeling combined with experimental measurements.
IEEE Transactions on Magnetics 03/2006; · 1.36 Impact Factor
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ABSTRACT: The magnetics and magnetotransport of current perpendicular to the plane (CPP) giant magnetoresistive (GMR) spin valves utilizing thin CoPt hard magnet pinning layers on Cr seed layers are investigated. 50-Å-thick CoPt18 films grown on >20 Å Cr seed layers exhibit a coercivity of ∼ 1.5 kOe and a resistivity of 31 μΩ cm. The low critical thickness of the CoPt hard magnet compared to IrMn or PtMn antiferromagnets makes it an attractive pinning material for small gap CPP sensors required for high magnetic recording densities. Furthermore, CoPt/Ru/CoFe trilayers exhibit strong antiparallel coupling and the low CoPt resistivity minimizes parasitic resistance yielding a higher magnetoresistance. Moreover, free layers with coercivities and coupling fields in a range of 5–10 Oe have been measured, demonstrating that free layer softness does not suffer from the proximity to the CoPt hard magnet due to the relatively thick spacer layers compatible with CPP GMR sensors. In particular we deposited CoPt18 pinned antiferromagnetically coupled spin with a variety of reference layers. The films were patterned into pillars with diameters ranging from 50 to 200 nm by using a combination of electron beam lithography and ion milling. The devices exhibit a magnetoresistance up to 3.6% and a resistance area product of only 20 mΩ μm2.
Journal of Applied Physics 12/2005; 98(11):113907-113907-4. · 2.17 Impact Factor
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ABSTRACT: This paper employs (analytical) micromagnetic modeling to derive expressions for the critical current at the onset of spin-transfer-torque (STT) instability in CPP-GMR read heads, as a function of the relative angle between the free and reference layer magnetizations, including a general angular dependent STT coefficient. Experimental measurement of the angular dependence of the critical currents are made on 50-nm-sized CPP-GMR devices with synthetic antiferromagnet pinned layers, and fabricated using e-beam lithography. The results are consistent with prior theoretical models, but indicate perhaps unanticipated implications for read head operation.
IEEE Transactions on Magnetics 11/2005; · 1.36 Impact Factor
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ABSTRACT: Nanoscale heat conduction across tunnel junctions prepared through natural oxidation of metal electrodes is experimentally studied. The thermal resistance of AlOx tunnel barriers increases linearly with barrier thickness, which is consistent with the prevailing theory of heat conduction in highly disordered materials. Heat conduction across tunnel junctions is strongly impeded by finite thermal resistance at interfaces between barrier and electrode layers, which can be exploited to create superior thermal barrier coatings. The thermal conductivity of nanolaminates consisting of a series of Ta/TaOx tunnel junctions is determined to be well below the minimum thermal conductivity limit.
Applied Physics Letters 05/2005; 86(20):203113-203113-3. · 3.84 Impact Factor
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ABSTRACT: The effects of thermal fluctuations on the magnetic-field dependence of spin-transfer-induced magnetization reversal was studied using a pinned synthetic antiferromagnet as a reference layer. These effects were analyzed by numerical solution of the thermal-field plus spin-transfer-augmented LLG equations for a single domain free layer.
Magnetics Conference, 2005. INTERMAG Asia 2005. Digests of the IEEE International; 05/2005
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ABSTRACT: The effect of imperfections on the bandstructure of Co <sub>2</sub> Mn Ge is explored. In particular, the effect of uniform and tetragonal distortions, and Cu, Al, O, Rh, and Sn impurities on the half-metallic behavior are examined. We find that a high spin polarization can be maintained even under significant strains and distortions. However, we find that impurities with as little as 3% concentration will generally have a significant detrimental effect on the half-metallic behavior of the Heusler alloy.
Applied Physics Letters 12/2004; · 3.84 Impact Factor
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ABSTRACT: We have developed a self-aligned, high-yield process to fabricate current-perpendicular-to-plane giant magnetoresistance (GMR) spin-valve sensors of sub-100-nm dimensions. A pinned synthetic antiferromagnet is used as the reference layer which minimizes dipole coupling to the free layer and field-induced rotation of the reference layer. We find that the critical currents for spin-transfer-induced magnetization reversal of the free layer vary dramatically with relatively small changes in the in-plane magnetic field, in contrast to theoretical predictions based on stability analysis of the Gilbert equations of magnetization dynamics, including Slonczewski-type spin-torque terms. The discrepancy is believed due to thermal fluctuations over the time scale of the measurements. Once thermal fluctuations are taken into account, we find good quantitative agreement between our experimental results and numerical simulations.
Applied Physics Letters 11/2004; 85(20):4681-4683. · 3.84 Impact Factor
