Low-frequency noise was measured in the frequency range from 0.1Hzto10kHz on a variety of commercially available magnetic sensors. The types of sensors investigated include anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR) effect devices. The 1∕f noise components of electronic and magnetic origin are identified by measuring sensor noise and sensitivity at various applied magnetic fields. Commercial magnetometers typically consist of four elements in a Wheatstone bridge configuration and are biased with either a constant voltage or current. Voltage fluctuations at the sensor output are amplified by a pair of battery powered low-noise preamplifiers and input to a spectrum analyzer. A two-channel cross-correlation technique is used when the performance of a single preamplifier is not sufficient. For the AMR and GMR sensors investigated, both electronic and magnetic components contribute to the overall sensor noise. Maximum noise occurs at the bias field which gives maximum sensitivity. The noise of TMR based sensors is primarily due to resistance fluctuations in the tunnel barrier, having little to no field dependence. The best low-field detectivity of the sensors that have been measured is on the order of 100pT∕Hz0.5 at 1Hz.

The high-frequency noise of micrometer-dimension spin valve devices has been measured as a function of applied field and temperature. The data are well fit with single-domain noise models that predict that the noise power is proportional to the imaginary part of the transverse magnetic susceptibility. The fits to the susceptibility yield the ferromagnetic resonance ͑FMR͒ frequency and the magnetic damping parameter. The resonant frequency increases, from 2.1 to 3.2 GHz, as the longitudinal field varies from Ϫ2 to 4 mT and increases from 2.2 to 3.3 GHz as the temperature decreases from 400 to 100 K. The shift in the FMR frequency with temperature is larger than that expected from the temperature dependence of the saturation magnetization, indicating that other temperature-dependent anisotropy energies are present, in addition to the dominant magnetostatic energies. The measured magnetic damping parameter ␣ decreases from 0.016 to 0.006 as the temperature decreases from 400 to 100 K. The value of the damping parameter shows a peak as a function of longitudinal bias field, indicating that there is no strict correlation between the damping parameter and the resonant frequency.

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The design and performance of a novel 4-arm equiangular slot spiral antenna operating in the 2nd mode are discussed in this letter. A straightforward feeding method consisting of a single vertical coaxial probe and no underlying mode forming network is proposed. Along with inherent simplicity, this feed enables design flexibility with respect to the slot-to-metal ratio, growth rate, and other spiral parameters. Numerical tools including an in-house finite element-boundary integral (FE-BI) code and Ansoft HFSS are used in designing the antenna. Good performance over an octave bandwidth is achieved, with axial ratio and WoW less than 3 dB, nominal gain of 3 dBic, and VSWR less than 1.6. Performance is verified with measurements. A method for increasing the bandwidth up to 3 : 1, which is the theoretical value determined by the excitation of the first higher order mode, is demonstrated computationally.Index Terms-Log spiral antennas, slot antennas.

The effects of circuit-level stress on both inverter operation and MOSFET characteristics have been investigated. Individual MOSFETs, with gate oxide thicknesses of 3.2 nm and active dimensions of 25 p x 25 p, are connected in an inverter configuration off-wafer via a low-leakage switch matrix. Inverters are stressed with a ramped voltage stress (RVS) of various magnitudes to induce different degrees of gate oxide degradation. In addition, voltage transfer curves (VTCs) of degraded inverters are simulated using a new circuit model. At the transistor level, both the PMOSFET and NMOSFET show increased gate leakage current up to eight orders of magnitude, severely reduced on-currents and transconductances (gm), and large threshold voltage (V,) shies of 100 mV or more. Different trends in inverter performance are observed following positive and negative stress. However, regardless of the stress polarity, circuit-level stress results in inverter performance degradation, such as reduced output swing, switching point shifts, and increased risdfall times. After the largest positive RVS, the output voltage swing has decreased from 1.8 V fresh, to 1.54 V poststress. Much larger changes in the inverter voltage (V-t) time domain performance are observed. The minimum output low voltage is similar to that of the VTC, but the rise time increased significantly enough that the output voltage is only pulled up to 660 mV (VDD =

Temperature and field dependence of high-frequency magnetic noise in spin valve devices Appl. Phys. Lett. 82, 91 (2003); 10.1063/1.1534386 Thermal magnetization fluctuations in CoFe spin-valve devices (invited)High-frequency measurements of spin-valve films and devices, made using several different measurement techniques, are presented and compared. Pulsed inductive measurements were made on sheet films and provide insight into the intrinsic dynamical properties of the component films and multilayer stacks. The damping parameter, in the completed spin-valve stack, is larger than in the constituent films. Direct time and frequency domain measurements of the dynamical response of micrometer-size spin-valve devices, made using high-bandwidth magnetoresistance techniques, showed damping parameters comparable to these measured on spin-valve sheet films. The small-angle magnetization response was also determined by high-frequency magnetic noise measurements. The damping parameters were smaller than those obtained by direct susceptibility measurements. The device-level measurements show a different dependence of the damping parameter on the easy-axis field as compared to sheet-level measurements. In addition to the uniform rotation mode, other peaks can be observed in the noise spectra that correspond to fluctuation modes arising from the micromagnetic structure. Electrical device measurements have much greater sensitivity than other high-frequency magnetic measurement techniques, which allow the direct observation of magnetization motion in submicrometer elements without averaging. This technique is used to directly examine thermally activated events and nonrepetitive dynamical motions.

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