Search

A systematic investigation of the switching behavior and writeability characteristics of perpendicular magnetic oxide media was performed by varying the number of oxide layers as well as the anisotropy Ku gradient in multilayered oxides. The media switching behavior highly depends on the magnetic volume of laterally exchange-coupled cap layer as well as the vertical exchange coupling between oxide and cap layers. The media switching field is significantly reduced with incoherent switching mode. The media writeability is further enhanced by employing the multilayered oxides whose anisotropy values vary. Higher Ku graded oxide media exhibits improved resolution, sharper transition, and higher signal-to-noise ratio. An analytical 9-spin model is used to study the impact of anisotropy grading, intergranular and interlayer exchange coupling on writeability and noise performance. The 9-spin model is found to qualitatively agree with the experimental results. Both model and experiment indicate that optimal tuning of the media anisotropy and exchange to the head design is critical in attaining good recording performance.

A nano-sized oscillator for RF applications is presented which is based on two spintronic effects, the tunneling magnetoresistance (TMR) and the spin momentum transfer torque. The oscillation frequency is several GHz and can be tuned by both a DC bias current and an external DC magnetic field. High compactness, high tunability and full compatibility with standard CMOS process make this spin torque nano-oscillator (STNO) a promising candidate for future RF transceivers. The main issues to be addressed are spectral purity and output power. First measurements on a hybrid built connecting the STNO to a dedicated wideband amplifier show that today's performance in terms of power is close to but not yet compatible with telecommunication standard requirements. Using time domain analysis we show that frequency fluctuations are an issue for spectral purity. Frequency synthesis concepts based on STNOs are also discussed.

By analyzing the magnetic cluster size of various media, we have investigated the variation of inter-granular exchange coupling with grain size. With the reduction of the grain size below 8.0 nm, higher inter-granular exchange coupled media showed a large increase in magnetic cluster size, while lower exchange media did not show as dramatic a cluster size change. This cluster size “knee” position was consistent with reduction in signal-to-noise ratio. To utilize small grain media for high density recording, it is desirable to push this “knee” position toward smaller grain size.

This study focuses on domain wall resistance in Ni_(80) Fe_(20) nanowires containing narrow constrictions down to 15 nm in width. Distinct differences in the magnetoresistance curves were found to depend on the constriction size. Wider constrictions are dominated by the overall anisotropic magnetoresistance of the structure, while constrictions narrower than ∼40 nm exhibit an additional distinct contribution from a domain wall. The effect is negative and typically varies from 1% to 5%.

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.

In this work we investigate the highly unexplored PS dynamical regime as well as the effects of additional rf currents on the PS and switching modes. The PS modes are found to interact with the applied rf currents having frequencies in the range of the dc-only driven modes, and under certain conditions are found to frequency lock. In contrast to previous frequency locking results [7, 8, 9], by looking at these effects in the PS regime, we find a measurable dependence of the dc switching threshold on the external rf frequency, finj. In the locking range the rf appears to have a stabilizing effect which hinders switching, however, at frequencies just below this range, we find a measurable reduction in the direct critical current for switching, Ic. While we have already reported this effect [10], an explanation for the origin of this effect has not been given. In this work, by studying the behavior of this system in frequency domain and through macrospin simulations (MS) (using the Landau-Lifshitz-Gilbert equation with Slonczewski STT [2]) we provide a description of the magnetization dynamics originating from the interaction between the dc driven switching modes and the applied rf. The results show good agreement with our experimental data. EXPERIMENT

We systematically vary the strength of the vertical interlayer exchange coupling (VIEC) between the magnetic layers (MLs) of an advanced exchange coupled composite media and experimentally investigate, through magnetic viscosity and dynamic coercivity measurements, the impact on the thermal stability. The strength of the VIEC is controlled through exchange coupling layers of varied thicknesses, whereas MLs are unchanged. We estimate the thermal energy barrier ΔEo as a function of the VIEC within the standard Arrhenius-Neel relaxation and Sharrock formalisms and characterize the effect of the VIEC on the degree of incoherency in the grain reversal mechanism. The analysis shows that while ΔEo drops for weaker VIEC, the ratio of ΔEo to MsHo energy per activation volume is maintained near 2 and weakly optimizes for intermediate VIEC.

We analyze the magnetic design for different generations of perpendicular magnetic recording (PMR) media using resonant soft x-ray small angle x-ray scattering. This technique allows us to simultaneously extract in a single experiment the key structural and magnetic parameters, i.e., lateral structural grain and magnetic cluster sizes as well as their distributions. We find that earlier PMR media generations relied on an initial reduction in the magnetic cluster size down to the grain level of the high anisotropy granular base layer, while very recent media designs introduce more exchange decoupling also within the softer laterally continuous cap layer. We highlight that this recent development allows optimizing magnetic cluster size and magnetic cluster size distribution within the composite media system for maximum achievable area density, while keeping the structural grain size roughly constant.

The effects of thermal fluctuations are characterized in granular perpendicular media of two different grain sizes. We extract short time switching fields Ho and intrinsic switching field distributions and find that after correcting for thermal effects and the demagnetizing fields, the switching field distributions are almost independent of grain size, in contrast to the long time scale behavior with broader distributions observed for the small grain media. In addition, the shape anisotropy fields of the magnetic clusters are shown to decrease the observed Ho and are more significant as grain size is reduced.

We report on anisotropy measurements of FePt thin films prepared at different temperatures and find that the anisotropy increases with increasing chemical ordering. Within measurement accuracy, we conclude that the second-order anisotropy constant K2 is negligibly small compared to the first-order anisotropy constant K1. We assert that the presence of a small hard-axis hysteresis causes spatial fluctuations of the field direction within the sample, which changes the shape of the hard-axis loop, such that it is misinterpreted as a K2 in the Sucksmith–Thompson formalism.

Model exchange coupled composite media, namely [Co/Pd]5/Pd(t)/[Co/Ni]3/Co multilayers, deposited onto prepatterned substrates have been investigated for potential use in bit patterned recording. Optimizing the thickness of the Pd interlayer allows decreasing the switching field (HC) and switching field distribution (σSFD) while maintaining thermal stability. The dependence of the remanent coercivity on the external field angle indicates that the gain in HC and σSFD originates from the incoherent magnetization reversal introduced by slightly decoupling the hard and soft multilayer stacks. This tendency is confirmed by further reductions in HC and σSFD, when inserting another Pd interlayer within the hard [Co/Pd]5 stack.

We present a study of the magnetic reversal mechanism for exchange coupled composite perpendicular media, i.e., hard granular layer/exchange-coupling layer (ECL)/soft layer (SL) systems, without soft underlayer. Using a vibrating sample magnetometer, the out-of-plane and in-plane components of the magnetization are monitored simultaneously, while sweeping an external out-of-plane magnetic field. The in-plane signal reveals the reversible rotation of the magnetization during the switching process. The amplitude of this rotation is correlated with the variations in coercivity and angular dependence of the remanent coercivity for different ECL and SL thicknesses.

International audience; We demonstrate a measurement technique with zero-applied magnetic field to deduce and spatially map the activation energy barrier distribution of strongly exchange-coupled magnetic-multilayer thin films, which is otherwise inaccessible with conventional methods in the presence of an applied magnetic field. Our technique involves the analysis of magnetic force microscopy images of magnetic microwires, whose magnetizations have been subject to thermal decay due to Joule heating from applied nanosecond scale current pulses. Fitting the results of such measurements on CoNi/Pd magnetic-multilayer microwires to a modified Arrhenius–Neel formalism yields an energy barrier distribution with 8% sigma, in good agreement with complementary fits of the switching-field-distribution measurements on patterned CoNi/Pd magnetic-multilayer islands. In order to achieve reliable operation of nanoscale magnetic devices, it is paramount to understand the role of spatial inhomogeneity of intrinsic magnetic properties, such as mag-netization, exchange, anisotropy, and activation energy for reversal in the overall performance of these devices. This evaluation is also necessary to separately diagnose the complementary contributions to the element-to-element performance variations from extrinsic ͑i.e., patterning induced͒ factors, such as ion-mill edge damage 1 and sidewall oxidation effects. 2

In low resistance-area product MgO magnetic tunnel junction nanopillars, we observe high integrated power (up to 43nW), narrow linewidth (down to 10MHz), spin transfer induced microwave emission at frequencies up to 14GHz due to precession of the free layer magnetization at room temperature. Although all devices were fabricated on the same wafer, they present bimodal transport and precessional characteristics. The devices in which the narrowest linewidths were observed exhibited low resistance and tunneling magnetoresistance (30%), while maintaining large integrated power.

We characterize the spin transfer torque induced reversal for current-perpendicular-to-plane spin valves with in-plane magnetic anisotropy at low temperature when driven by both direct and rf currents. We find that for rf currents with frequencies in the range of the dc driven resonances, the rf current strongly influences the preswitching precessional modes leading to a clear dependence of the critical dc switching current on the external rf frequency excitation.

We have performed experiments on current-induced domain-wall motion (CIDWM) in the case of the domain walls (DW) trapped within the nanoscale constrictions in patterned NiFe structures. Direct observation of current-induced magnetization reversal was achieved and critical current densities jc were measured in the presence of easy-axis magnetic fields. The direction of CIDWM was found to be along the direction of the electron motion in absence of an applied magnetic field and in the direction of the field when in the presence of even relatively weak fields. Data for the field dependence of jc for both uniform and fast rising pulses suggest that the current, regardless of polarity, assists in the depinning of the DW. Only for the dc case does the data strongly reveal the influence of the electron pressure in promoting or hindering DW motion.