We present measurements of the magnitude of Néel “orange-peel” coupling due to interface roughness in a series of magnetic tunneling junction devices. Results from magnetometry and transport measurements are shown to be in good agreement with the theoretical model of Néel. In addition, we have used transmission electron microscopy to directly probe the sample interface roughness and obtain results consistent with the values obtained by magnetometry and transport methods.

We have systematically investigated the dependence of tunnel magnetoresistance in MgO-based magnetic tunnel junctions as a function of Ar pressure during sputtering. The MgO surface roughness, and therefore device magnetoresistance, depends strongly on Ar gas pressure. Magnetoresistance of up to 236% was achieved at room temperature after thermal annealing at 425°C and with optimal sputtering conditions. The long mean free path of target atoms at low background pressures increases their kinetic energy at the substrate surface, resulting in smooth surface morphology and correspondingly improved device performance.

We have demonstrated the detection of 2.5 M target DNA labeled with 16 nm Fe 3 O 4 nanoparticles ͑NPs͒ and 50 nm commercial MACS™ NPs using arrays of magnetic tunnel junction sensors with ͑001͒-oriented MgO barrier layers. Signal-to-noise ratios of 25 and 12 were obtained with Fe 3 O 4 and MACS™ NPs, respectively. These data show conclusively that MgO-based MTJ sensor arrays are very promising candidates for future applications involving the accurate detection and identification of biomolecules tagged with magnetic nanoparticles.

We studied low-frequency noise in NiFe-Al 2 O 3 -NiFe based magnetic tunnel junctions ͑MTJ's͒ with and without a hard-axis bias field. The 1/f noise is observed to be magnetic-field dependent and reduced with the application of hard-axis bias fields, attributed to thermally activated magnetization fluctuations in the magnetic electrodes. A linear dependence of noise on derivative of magnetoresistance has been observed, and the magnetic-field noise for MTJ sensing elements is defined and evaluated to be as low as 60 nT m/Hz 1/2 .The study of noise of magnetoresistive materials is important for understanding the performance of spin-dependent electronic devices as well as the physical processes that govern their magnetic properties. There are many processes that can contribute to the noise of a magnetoresistive based device. Based on its physical origin, the noise is categorized into three types. Thermal resistance noise, also known as Johnson or Nyquist noise, results from a thermal smearing of the density of states near the Fermi level. Shot noise is due to the stochastic nature of electron transport with an applied current. The third kind of noise, 1/f noise, also called flicker noise or excess noise, arises from the coupling of electronic processes with magnetization fluctuations. 1,2 The first two noise sources are frequency independent, while 1/f noise scales inversely with frequency, as suggested by its name.Usually 1/f noise is an indirect noise and manifests itself as resistance fluctuations in different physical systems in lowfrequency range. As examples, resistance fluctuations in twodimensional electron system in semiconductor heterostructures 3 and high-T C superconductor 4 have different physical origin. Due to its nature, there is not a universal explanation to physical origin of 1/f noise.In this paper we are interested in magnetoresistance ͑MR͒ fluctuations at low fields in Py-Al 2 O 3 -Py ͑PyϭPermalloy, Ni 79 Fe 21 ) based magnetic tunnel junctions ͑MTJ's͒. This spin-dependent system shows great potential for high sensitivity, low-noise magnetic-field sensing applications, due to its low saturation field and high MR ratio ͑defined as ⌬R/R, where ⌬R and R are the maximum resistance change and the minimum resistance, respectively͒. In addition, the exponential dependence of junction resistivity on the barrier thickness allows for a large degree of control over device resistance. A typical tunnel junction consists of two ferromagnetic ͑FM͒ metal layers separated by an ultrathin insulating barrier. Changes in the relative magnetization orientations between these two FM layers in applied magnetic field͑s͒ determine the final resistance, i.e., MR. Although many fundamental aspects of this system, including magnetization reversal processes, 5 scattering mechanisms, 6 and electron transport behavior have been studied in detail, the lowfrequency noise characteristics have not been fully explored.In a recent work, Ingvarsson et al. have studied magnetic noise produced by magnetization fluctuations in MTJ's ͑...