Paulo P. Freitas

Planar Hall effect sensor for magnetic micro- and nanobead detection

Magnetic bead sensors based on the planar Hall effect in thin films of exchange-biased permalloy have been fabricated and characterized. Typical sensitivities are 3 μV/Oe mA. The sensor response to an applied magnetic field has been measured without and with coatings of commercially available 2 μm and 250 nm magnetic beads used for bioapplications (Micromer-M and Nanomag-D, Micromod, Germany). Detection of both types of beads and single bead detection of 2 μm beads is demonstrated, i.e., the technique is feasible for magnetic biosensors. Single 2 μm beads yield 300 nV signals at 10 mA and 15 Oe applied field.

Ion beam deposition of Mn-Ir spin valves

Half-biased spin valves have been prepared by ion beam deposition. The magnetoresistance (MR) signal reaches 7.7% and the exchange field is 350 Oe with a coupling field of 15 Oe and a coercivity of the free layer equal to 4 Oe. The [111] texture induced by a very thin Ta buffer layer (thickness <10 /spl Aring/) has a strong effect in increasing the MR signal and coupling field, while decreasing the exchange field and coercivity. The blocking temperature of the MnIr-biased spin valves is 250/spl deg/C and a thermal stability study shows that the exchange field is constant up to 300/spl deg/C, under consecutive 5-h anneals at each temperature. After these anneals, the MR signal is still equal to 5%. These films show better thermal stability than equivalent samples prepared by sputtering.

Phase coherent precessional magnetization reversal in microscopic spin valve elements

We evidence multiple coherent precessional magnetization reversal in microscopic spin valves. Stable, reversible, and highly efficient magnetization switching is triggered by transverse field pulses as short as 140 ps with energies down to 15 pJ. At high fields a phase coherent reversal is found revealing periodic transitions from switching to nonswitching under variation of pulse parameters. At the low field limit the existence of a relaxation dominated regime is established allowing switching by pulse amplitudes below the quasistatic switching threshold.

Size dependence of the exchange bias field in NiO/Ni nanostructures

NiO/Ni wires have been investigated as a function of their width in order to investigate the size dependence of exchange bias. The samples have been prepared by e-beam lithography and ion milling of ion beam sputtered thin films. For NiO/Ni wires narrower than 3 μm, the exchange bias field significantly depends on the wire width. A NiO/Ni film shows an exchange bias field of −78 Oe whereas the exchange bias field of wires narrower than 200 nm is reduced to approximately −40 Oe. The coercive field of the NiO/Ni film is 28 Oe and increases to 210 Oe for the narrowest wires. The decrease of the exchange bias field for the narrowest wires is consistent with a recent microscopic model of exchange bias where the appearance of a unidirectional anisotropy in ferromagnet/antiferromagnet bilayers has been attributed to the presence of antiferromagnetic domains in the bulk of the antiferromagnet. A possible onset of a transition from a multidomain to a single-domain state of the antiferromagnet as a function of the ...

Femtomolar limit of detection with a magnetoresistive biochip.

In this paper the biological limit of detection of a spin-valve-based magnetoresistive biochip applied to the detection of 20 mer ssDNA hybridization events is presented. Two reactional variables and their impact on the biomolecular recognition efficiency are discussed. Both the influence of a 250 nm diameter magnetic particle attached to the target molecule during the hybridization event and the effect of a magnetic focusing system in the hybridization of pre-labeled target DNA (assisted hybridization) are addressed. The particles carrying the target molecules are attracted to the probe active sensor sites by applying a 40 mA DC current on U-shaped aluminium current lines. Experiments comparing pre-hybridization versus post-hybridization magnetic labeling and passive versus magnetically assisted hybridization were conducted. The efficiency of a passive hybridization is reduced by about 50% when constrained to the operational conditions (sample volume, reaction time, temperature and magnetic label) of an on-chip real-time hybridization assay. This reduction has shown to be constant and independent from the initial target concentration. Conversely, the presence of the magnetic label improved the limit of detection when a magnetically assisted hybridization was performed. The use of a labeled target focusing system has permitted a gain of three orders of magnitude (from 1 pM down to 1 fM) in the sensitivity of the device, as compared with passive, diffusion-controlled hybridization.

Magnetoresistance enhancement in specular, bottom-pinned, Mn83Ir17 spin valves with nano-oxide layers

Bottom-pinned Mn83Ir17 spin valves with enhanced specular scattering were fabricated, showing magnetoresistance (MR) values up to 13.6%, lower sheet resistance R□ and higher ΔR□. Two nano-oxide layers (NOL) are grown on both sides of the CoFe/Cu/CoFe spin valve structure by natural oxidation or remote plasma oxidation of the starting CoFe layer. Maximum MR enhancement is obtained after just 1 min plasma oxidation. Rutherford backscattering analysis shows that a 15±2 A oxide layer grows at the expense of the initial (prior to oxidation) CoFe layer, with ∼12% reduction of the initial 40 A CoFe thickness. X-ray reflectometry indicates that Kiessig fringes become better defined after NOL growth, indicating smoother inner interfaces, in agreement with the observed decrease of the spin valve ferromagnetic Neel coupling.

Spin valve sensors

Abstract This paper demonstrates spin valve sensor applications as read elements in storage systems, or, when in a Wheatstone bridge configuration, as rotational speed control devices (for ABS systems), high current monitoring devices for power lines, and positioning control devices in robotic systems. For recording heads, shielded spin valve sensors are adequate for high linear density recording. A tape head was fabricated with output 400 μV per micron of trackwidth, with a D50 value of 100 kfci, and signal loss of −0.34 dB/kfci. For rotational speed measurement, spin valve bridge sensors with flux guides were used, yielding a 400-mVpp amplitude, and square wave output with rise/fall times below 70 μs, when excited by a magnetized wheel. Amplitude is independent of speed (0–3000 rpm), and of sensor to wheel separation (0.5–2.0 mm). For power line applications, currents up to 2100 A (at 50 Hz) could be measured with a sensitivity of 35 μVrms/A and deviations from linearity of ±1.5%. In robot position control, a maximum error of ±9 μm over a ±0.5-mm span was obtained. In the two last cases, bridges without flux guides are used to maximize linear response.

Current-induced magnetization switching in magnetic tunnel junctions

Current-induced magnetization switching (CIMS) in low-resistance magnetic tunnel junctions was shown at average critical current densities (Jc=1.33×106 A/cm2). When large vertical currents pass through the junctions, spin-transfer torque, and vortex fields can rotate the magnetization of the free layer from the initial parallel state to a vortex state, resulting in 10.8% CIMS resistance change at zero-bias current, which is about half of the resistance change (22%) induced when switching is created by an external field. A micromagnetic simulation including the spin-transfer torque and the vortex field correctly predicts the critical negative-current-inducing switching from the parallel state into the vortex state, but fails to explain the reverse switching from the vortex state into the parallel state at an approximately symmetric positive critical current. Lead fields were analyzed and found to be not the cause of the observed switching. The very small dependence of the switching currents on an external ...

Radiation of spin waves by a single micrometer-sized magnetic element

Dynamic magnetic properties of a single micrometer-sized magnetic element consisting of a permalloy and a partially patterned CoFe layer separated by an intervening Cu spacer layer have been studied by means of a micro-focus Brillouin light scattering setup, which allows for local measurements of the magnetization dynamics on the submicrometer scale. It is shown that quantized spin-wave modes excited in the magnetic element act as radiation sources for spin waves in the surrounding magnetic film. It is found that the intensities of spin waves excited by different quantized modes follow different distance laws when traveling away from the region of excitation.

Tunnel magnetoresistance and magnetic ordering in ion-beam sputtered Co80Fe20/Al2O3 discontinuous multilayers

Discontinuous multilayered Co80Fe20(t)/Al2O3(30 A) thin films have been prepared by ion-beam sputtering. We report on structural, magnetic, and transport (for current in plane geometry) results obtained in this system. With growing nominal thickness t of the metal layers, which effectively characterizes the granular structure, a transition from tunnel to metallic conductance is observed, indicating the onset of infinite conducting paths at t>18 A. At t 13 A was detected from the magnetization data which display here a transition from superparamagnetic to ferromagnetic behavior. The measurements of tunnel magnetoresistance (MR) show that a sharp maximum of MR sensitivity to field takes place at this thickness, reaching ∼24%/kOe at room temperature. At least, MR itself as a function of t has a break at the same value. All these features suggest that some specific kind of percolation with respect to magnetic order occurs in o...