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In this paper, we describe the development of multilayer (ML) thin films suitable for flux concentrators (FC). We prepared ML films having magnetic thicknesses n x [t.sub.F] in the range of 400 [Angstrom]. to 3000 [Angstrom] by ion beam deposition (IBD). The films exhibit Ruderman-Kittel-Kasuya-Yosida (RKKY) long range antiferromagnetic (AF) coupling through very thin Ru interlayers. We prepared two types of samples: (CoFeB [t.sub.F]/Ru [t.sub.Ru])xn and (NiFe [t.sub.F]/Ru [t.sub.Ru])xn, where CoFeB is [([Co.sub.70][Fe.sub.30]).sub.80at%][B.sub.20at%] and NiFe is [Ni.sub.81][Fe.sub.19]. The former has good thermal stability and high magnetic moment while the latter, previously reported, has poor thermal stability and low spontaneous magnetic moment. We discuss the magnetic properties of the films in terms of the remanent magnetic moment Mr and saturation field [H.sub.s]. Despite having higher spontaneous magnetic moment of 1200 emu/[cm.sup.3], (CoFeB/Ru)xn films with adequate magnetic properties for sensor-in-gap devices with FC could not be prepared. By contrast, samples consisting of NiFe350 [Angstrom]/(Ru7 [Angstrom]/NiFe350 [Angstrom])x7 exhibited suitable magnetic properties. We lithographically patterned the films into FC and test structures to investigate their magnetization processes by anisotropic magnetoresistance (MR) measurements. The MR curves indicate magnetization processes by uniform rotation and show absence of hysteresis. Index Terms--Flux concentrator, multilayer, RKKY.

An analog isolator device, based on linear magnetic tunnel junctions is presented. Elemental single structures and full Wheatstone bridge devices are designed, fabricated and tested. In addition, the ranges of functionality of the devices are analyzed, mainly in terms of linearity, signal reconstruction and frequency response. Index Terms--Analog isolator, linear sensor, magnetic tunnel junction, magnetoresistance.

A dc to ac magnetic field transformer was developed using a magnetic tunnel junction (MTJ)/microelectromechanical system (MEMS) mixed device. A MEMS torsionator was fabricated with an incorporated magnetic flux guide, that when actuated by a gate electrode, modulates an external dc field into the same frequency of the micro-torsionator oscillation. Attached to it a MgO based MTJ was fabricated, performing the detection of the generated ac magnetic field. This dc to ac field transformation enables the detection of dc magnetic fields in the high frequency thermal noise regime, where the 1/f noise is typically two orders of magnitude lower. Measurements show the detection of a modulated field at 460 kHz, coming from the MEMS torsionator designed to exhibit high modulation efficiency. This efficiency was of 11%, i.e., 11% of the external dc field is modulated. As a result, from a MTJ exhibiting a noise of [S.sup.MTJ.sub.v] = 4.4 nT/[Hz.sup.1/2] at 460 kHz, this hybrid device enables a dc magnetic field detection of 40nT/[Hz.sup.1/2]. This work acts as a major improvement to a previous one [7], mainly through the optimization of the MEMS magnetic field modulation efficiency. Index Terms--Field modulation, magnetic flux guides, MEMS, MgO tunnel junctions, noise, 1/f noise.

Shielded spin-valve tape heads with permanent magnet stabilization are described. Final MR of 3.5% is shown on heads with trackwidths of 5 and 9 [[micro]meter], and sensor heights of 0.5, 1.0 and 1.5 [[micro]meter]. Heads are characterized on 1/2[inch] DLT-IV tape up to 180 kfci. For isolated transitions the heads show a [PW.sub.50] of 0.8 [[micro]meter] (pulse broadening due to partial head saturation) and a baseline shift [approximately]10% that can be positive or negative, and is related with shield remanence and transfer curve hysteresis. At 120 kfci, head output for a sense current of 3 to 4 mA is [approximately] 1 [mV.sub.pp] (up to 30% of the output at 30 kfci). At high linear densities, signal loss is 0.24 dB/kfci for a 0.35 [[micro]meter] shield-to-shield spacing ([D.sub.50] of 80[approximately]100 kfci). At low linear densities the head is partially saturated, and above 60 kfci harmonic distortion is lower than -20 dB. Index Terms - magnetic recording/reading heads, magnetoresistive materials and devices, spin valve heads, tape recording.

A new exchange material, [Mn.sub.78][Rh.sub.22], is described, requiring no post-deposition anneal to obtain the antiferromagnetic phase and leading to spin valve sensors with good corrosion resistance, thermal stability up to 225 [degrees] C, and good exchange coupling characteristics. Potentiodynamic polarization scans performed in a sodium sulfate electrolyte show that [Mn.sub.78][Rh.sub.22] films exhibit a good corrosion resistance, comparable to that of [Ni.sub.81][Fe.sub.19], [Mn.sub.80][Ir.sub.20], and [Mn.sub.50][Ni.sub.50] films but with higher corrosion potential. Spin valve structures prepared with this exchange material show an exchange coupling strength ([J.sub.ex]) of 0.19 erg/[cm.sup.2]. The blocking temperature ([T.sub.B]) of the as-deposited spin valve coupon samples is 235 [degrees] C. Unshielded sensors with trackwidths W = 4-6 [[micro]meter] and height h = 1-2 [[micro]meter] were fabricated. The sensors show well-linearized magnetoresistance (MR) transfer curves, without hysteresis or Barkhausen noise and are thermally stable under consecutive 5 h anneals in vacuum up to 225 [degrees] C. A shielded tape head device was fabricated showing a maximum output of 1.8 m[V.sub.pp]/[[micro]meter], with potential for operating at linear densities near 100 kfci, at which a 580 [Mu][V.sub.pp]/[[micro]meter] output is measured. Index Terms - Corrosion resistance, exchange layers, magnetic recording/reading heads, magnetoresistive materials and devices, spin valve heads.

Unshielded spin-valve sensors were fabricated and annealed to study the effect of high temperature processing on the transfer curves. A micromagnetic simulation program was developed to analyze the changes observed on the transfer curves upon annealing. Analysis of experimental data versus the micromagnetic simulations indicates two causes responsible for the progressive loss of the MR signal upon annealing above the blocking temperature: a rotation of the pinned layer magnetization from the transverse to the longitudinal direction (along the sensor trackwidth) and an increase of the free layer anisotropy. Rotation of the free layer easy axis can also be observed. Cool down in a magnetic field can prevent this MR loss by pinning the magnetization direction during the high temperature stage. Index Terms - Magnetic recording/reading heads, magnetoresistive materials and devices, micromagnetic simulations, spinvalve sensors, thermal stability.

Magnetic tunnel junctions (MTJs) incorporating GeSbTe thermal barriers and weakly pinned free layers ([H.sub.ex] ~ 60 Oe) have been produced with areas between 1 x 1 [micro][m.sup.2] and 3 x 8 [micro][m.sup.2]. Current pulses lasting from 100 [micro]s down to 20 ns were used to heat the MTJs and reverse the free layer pinning field. It is demonstrated that pulses with 7 mW/[micro][m.sup.2] can be used to switch the free layer of MTJs incorporating GeSbTe (20 mW/[micro] [m.sup.2] without GeSbTe). The dependence of the power required to switch the MTJs is studied with respect to sample area, pulse duration, and GeSbTe thickness. Index Terms--Blocking temperature, GeSbTe thermal barriers, magnetic tunnel junctions (MTJs), thermally assisted switching.

A spin-valve sensor biochip was used to detect cystic fibrosis related DNA targets for the purpose of developing an affordable diagnostic chip. The strategy used was based on the ac field focusing of magnetically labeled target DNA at sensor sites using U-shaped current lines. U-shaped spin-valve sensors, fabricated within the line structures, detected in real-time the hybridization of DNA targets to complementary DNA probes, previously immobilized onto the chip surface. Hybridization occurred in relatively short times (15-30 min) in comparison with conventional hybridization approaches (3 to 12 h). Statistical data on detection signals for single probe and multiprobe experiments was obtained, showing a significant difference between complementary binding signals and noncomplementary and background ones. Index Terms--Biomedical transducers, DNA, magnetic particles, magnetoresistive devices, medical diagnosis.

On-chip spin valve sensors were used to measure the flow velocity within fabricated poly(dimethylsiloxane) (PDMS) microchannels. Superparamagnetic particles with dimensions of 250 nm in diameter were introduced into a 100 [micro]m wide microchannel, bonded to the chip surface. The flow velocity was determined by measuring the changes in resistance of a pair of spin-valve sensors, 1.65 mm apart from each other, caused by the passage of ensembles of superparamagnetic particles. Flow velocities ranging from 50 to 300 [micro]m/s were estimated, depending on the pressure used to drive the flow. The application of this system provides a means to measure and control flow velocities on microfluidic devices. Index Terms--Fluidics, magnetoresistive devices, velocity measurement.

In this paper, junctions with reduced [H.sub.f] coupling were fabricated by ion beam deposition and oxidation, using CoFeB electrodes. The CoFeB layer has a strong (111) texture that can be the origin of lower [H.sub.f] and coercivity when compared with CoFe. Junctions processed down to 2 x 4/[micro][m.sup.2] with 40-[Angstrom]-thick CoFeB bottom eiectrodes have 42% of tunneling magnetoresistance (TMR), (R x A ~ 400 [OMEGA]x[micro][m.sup.2]), [H.sub.c] of ~10 Oe and [H.sub.f] of ~2 Oe. CoFe-based junctions (R x A ~ 500 [OMEGA]x[micro][m.sup.2]) have lower TMR (~35%) and larger [H.sub.f] (~5-6 Oe) and [H.sub.c] (~12-14 Oe). Local chemical composition analysis of the cross section indicated Fe-O segregation with very little Co grown on top of the barrier for CoFe-based junctions and not for CoFeB ones. Index Terms--CoFeB electrodes, ion beam deposition, low ferromagnetic coupling, tunnel junctions.

Spin-valve sensors were used to measure on chip, the vibration of microelectromechanical systems microbridges excited by dc + ac voltages. A spin-valve sensor (10 x 2 [micro][m.sup.2], MR = 6.5%) was placed 3 [micro]m away and 2.6 [micro]m below the central region of an a-Si:H/Al bridge, with a 1-[micro]m air gap. This sensor detects the fringe field created by a [Co.sub.78][Pt.sub.22] micromagnet deposited on top of the bridge. The bridge movement was controlled by an applied voltage on the gate. An unforeseen capacitance coupling effect was found between the control gate and the spin-valve sensor, affecting all ac measurements. Once this coupling was isolated and taken into account, bridge oscillation at joint dc + ac excitation voltages was monitored. Bridge vibration amplitudes of several tens of angstroms peak to peak were detected for dc and ac gate voltages up to 30 V. Index Terms--MEMS, micromagnets, spin-valve sensors.

In this paper, a remote [O.sub.2] ion source is used for the formation of nano-oxide layers. The oxidation efficiency was measured in CoFe-oxide films, and a decrease of the oxide layer with the pan angle and the oxidation pressure is observed. For the same oxidation pressure, the oxidation efficiency depends on the [O.sub.2] content in the Ar-[O.sub.2] plasma. These results were applied in optimizing the fabrication of [Al.sub.2][O.sub.3] barrier for tunnel junctions. This method was also used to fabricate junctions with Fe-oxide layers inserted at the [Al.sub.2][O.sub.3]-CoFe interface. TEM and magnetization data indicate that after anneal at 385[degrees]C, a homogeneons ferromagnetic Fe-oxide layer ([Fe.sub.3][O.sub.4]?) is formed. Index Terms--Ion oxidation, nano-oxide layers, transmission electron microscopy, tunnel junctions.

A comparative analysis of tape heads with two spin-valve designs is presented: spin valves with synthetic free and pinned layers (SF-SAF/AF) versus spin valves with a conventional free layer and a synthetic pinned layer (SAF/AF). The SF layer, corresponding to a smaller free layer's effective magnetic thickness ([t.sub.eff,SF] ~ 28 [Angstrom]) and lower magnetic moment ([M.sub.eff,SF] ~ 398 emu/cc), is more sensitive to the field flux from the medium and has a higher sensitivity (~ 3.4 mV/[micro]m) than the conventional NiFe/CoFe free layer, with D50 ~ 130-138 kfci, in comparison with D50 ~ 99 kfci for SAF/AF spin-valve heads. Thick media limitations become visible on the heads' performance above ~ 112 kfci and the heads are shown to be mostly time-instability free. Index Terms--Head characterization, magnetic read heads, spin valves, synthetic ferrimagnets/antiferromagnets.

A micromagnetic model was developed to study a 4-terminal, insulating spacer Dual-Stripe GMR head (DSGMR) operating in differential mode, and a 2-terminal, high resistivity conducting spacer, Dual-GMR head. Pulse shape and magnetization distributions were correlated with current biasing and non-linear effects. For two (Ni[Fe.sub.20[Angstrom]]/[Cu.sub.20[Angstrom]]).sub.10] GMR elements and a gap of 500[Angstrom], the DSGMR shows higher output than the Dual-GMR at frequencies < 400kfci but has a gap null at 570kfci. The Dual-GMR has a higher output above 400kfci, but shows no common-mode effect cancellation, characteristic from dual-stripe heads in differential mode. A single element GMR tape head was fabricated and results compared with simulation. Index Terms - Magnetoresistive heads, Magnetic Recording, Dual-Stripe Sensors, GMR heads, Micromagnetic Simulation.

Giant magnetoresistance (GMR) materials are used for random access, non-volatile memories. Different memory architectures have been proposed, both using GMR multilayers and spin-valve (SV) sandwich structures. In most of these approaches extra word lines are needed for writing purposes in addition to read contacts. This makes three interconnect levels. We show results on a simpler SV memory architecture, where writing is achieved using the read current contacts. This is relevant in terms of fabrication, as only two interconnecting layers are necessary to address a matrix of bits. A 400 bit memory matrix was fabricated and tested. A 2 mV signal between '0' and '1' states was measured for a bit inside this matrix.

In this paper, a new head design, the dual stripe GMR (DS-GMR) is proposed. It consists of two self biased GMR sensors of thickness t, carrying opposite sense currents I, separated by an oxide layer with thickness g. Modeling of the head shows that outputs of 1 mV/[[micro]meter] trackwidth, with a [D.sub.50] of 300 kfci and [PW.sub.50] [approximately equals] 0.1 [[micro]meter] can be achieved, which make it suitable for very high density. operation. In order to test this DS-GMR sensor, [(NiFe/Cu).sub.xn] GMR multilayers were developed with MR = 12% and sensitivity of 0.25%/Oe. DS-GMR sensors were fabricated and tested at wafer level with bottom and top GMR of 4 to 5% for 5 [[micro]meter] trackwidths and a gap g=0.2 [[micro]meter].

The influence of the ion milling parameters on the magnetization reversal of patterned NiFe/CoFe layers is investigated. In 4 [micro]m x 2 [micro]m ellipse elements switching of the magnetization occurs through a two step process. The first step is associated with domain wall propagation and the second with vortex annihilation. Reversal fields are reduced by ion milling at low angles (25 [degrees]-35 [degrees]). Magnetization reversal fields also show a dependence on ion energy. Milling at low ion energies reduces the switching field and dispersion values. Lower magnetization reversal fields are obtained for thinner layers. Index Terms--Ion beam milling, magnetization reversal, MRAM.

Spin dependent tunnel junctions with TMR exceeding 30-40% can now be prepared with Al[O.sub.x] and AlN barriers, with junction resistance tuned from 30-40 [Omega] x [[micro]m.sup.2] to [10.sup.8] [Omega] x [[micro]m.sup.2]. Thermal stability is better than 300 [degrees] C for thicker barriers ([is greater than] 11 [Angstrom]) but is degraded to 220 [degrees] C for 6 [Angstrom] barriers. A 9 bit MRAM cell is demonstrated using tunnel junctions and vertically integrated a:Si diodes. Junction switching is achieved with on chip 20 ns field pulses. Requirements for tunnel junctions for 100 Gbit/[in.sup.2] read head applications are discussed. First prototypes of tunnel junction read heads with 600 [Angstrom] read gaps were fabricated, where the TJ is at the air bearing surface. TMR loss was observed during the final head lapping steps. Index Terms--Spin-dependent tunnel junctions, MRAM, a:Si diodes, tunnel junction read heads.

A dual integrated GMR bridge sensor, which is able to simultaneously detect x and y components of an external magnetic field, was demonstrated, and applied to the precision robotic placement of objects. Two orthogonal GMR bridges are used, biased by two sets of integrated [Co.sub.78] [Pt.sub.22] film permanent magnets with different coercivities (1230 Oe and 522 Oe, respectively). The field sensitivity is 0.35 mV/(V x Oe) in each field direction. For the positioning application, an absolute accuracy of 10 [micro]m in x-direction has been achieved over more than 1900 cycles which is insensitive to sensor offsets and thermal drifts. Index Terms--Magnetic field sensor, dual integrated GMR bridge sensor, robot position control system.

Spin dependent tunnel junctions aimed for storage element in MRAM cells were switched between low and high resistance states using 20 ns current pulses on two perpendicular conductors. Switching was done in quasi static mode using an external field and dynamically with field pulses created on chip. For 20 ns pulsewidths the introduction of a delay between the two pulses promotes the switching at lower fields and reduces the dispersion in remanent resistance values after switching. Index Terms--MRAM, magnetic tunnel junctions, magnetization reversal.

MnIr-biased spin valves have been prepared by ion beam deposition. The magnetoresistance (MR) signal reaches 7.7% and the exchange field is 450 Oe with a coupling field of 15 Oe and a coercivity of the free layer equal to 40e. The (111) texture induced by a very thin Ta buffer layer (thickness [is less than] 10 [Angstrom]) 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 [degrees] C and a thermal stability study shows that the exchange field is constant up to 300 [degrees] 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. Index Terms--Exchange coupling, low beam deposition, magnetoresistive materials and devices, MnIr, spin valves, thermal stability.

Dual-stripe giant magnetoresistive (DSGMR) devices with an insulating spacer were fabricated and tested on tape media. The head design consists of two mutually biased, unshielded GMR elements based on [Cu 19 [Angstrom]/NiFe 13 [Angstrom]/CoFe 4 [Angstrom]]x10 antiferromagnetically coupled multilayers, separated by a 600-[Angstrom]-thick Si[O.sub.2] spacer. The sensor is patterned into a U shape, with leads connected away from the active region to avoid topography. Simultaneous etching of both GMR elements avoids misalignment problems along or across the track width. A DSGMR tape head operating in differential mode (track width of 2 [micro]m and height of 0.8 [micro]m) was tested versus high density dibits (0.05 [is less than] bit [is less than] 4 [micro]m) recorded on tape media, using a current bias of J = 2.8 x [10.sup.7] A/[cm.sup.2] on each GMR. The DSGMR responds to the dibit with a bipolar pulse independently of bit separation even when the individual GMR elements are partially saturated. Peak-to-peak output is 1.7 mV/[micro]m, for bit sizes [is greater than or equal to] 0.4 [micro]m. Experimental pulse amplitude and shape are in agreement with a two-dimensional micromagnetic simulation. The head response to a dibit signal loses half its maximum amplitude for a recorded bit size of 0.2 [micro]m. Pulse broadening (read bit cell is 0.8 [micro]m larger than recorded bit, for bit sizes [is less than] 0.4 [micro]m) is correlated with sensor geometry. Index Terms--Dual stripe giant magnetoresistive (GMR) heads, GMR multilayers, magnetic recording heads, mutual current biasing, pulse shape analysis.

Spin dependent tunnel junctions showing tunnel magnetoresistance (TMR) values of 39~41% were fabricated using Ion Beam Deposition (IBD). Both the electrodes and the aluminum layer deposition were done by IBD. The aluminum oxidation was performed using the assist gun with an oxygen beam (+30 V acceleration voltage applied on the grids) using mixed [O.sub.2]/Ar plasma. The oxidation was monitored in real time with a residual gas analyzer (RGA). The junction area is defined by lithography, down to 3x2 [micro][m.sup.2]. As-deposited junctions with 15 [Angstrom] of Al showed TMR of 27~29%, independent of the junction area, with resistance-area products of 0.8-1.6 M [Ohms]x[micro][m.sup.2]. This TMR value reached 40% upon annealing at 290 [degrees] C, with resistance decreasing to 0.5-0.8 M [Ohms]x[micro][m.sup.2]. Index Terms--Ion beam deposition/oxidation, spin polarized tunnel junctions

This work demonstrates the successful vertical integration of magnetic tunneling junctions with hydrogenated amorphous silicon (a-Si:H) diodes. In the finished device over 11.4% current change was measured when the junction free layer is switched in an external magnetic field, applying 0.86V to the tunnel junction-diode series. In the integrated device, the TMR signal of the junction measured alone was 25.3% at 7mV bias, demonstrating junction robustness and process compatibility. The [Delta]I/I signal of the junction-diode series was optimized by changing the junction resistance. With the present a-Si:H technology, an RC time constant below 25ns is calculated, higher than the 2.5ns estimated for the tunneling junction. Index Terms--MRAM, spin polarized tunnel junctions, diode, hydrogenated amorphous Si

We describe the DRAM-like approach towards a non-volatile magnetoresistive memory integrating magnetic and semiconductor devices into one cell. The speed at which the magnetic memory signal can be read depends on many factors. An important factor is the magnetic element itself, the size, magnetic characteristics and absolute resistance. Secondly, the design of the read-out electronics is a key issue. A third determining factor is the technology in which the electronics are fabricated. Some features are indicated that are essential in optimizing MRAM in future. Index Terms--Magnetic Random Access Memories, Spin-Valve, Tunnel-junction.

Spin valve structures with both synthetic ferrimagnet [F.sub.a]/Re/[F.sub.b] free layers and synthetic antiferromagnet [F.sub.1]/Re/[F.sub.2]/AF pinned layers are demonstrated. Physical thicknesses for the free layer of [approximately equals] 50 [Angstrom] can be maintained while decreasing the effective magnetic thickness ([t.sub.eff]) to 8-10 [Angstrom], without significant signal loss. Inverted structures with the synthetic free layer on top present magnetoresistance (MR) values up to 7% for [t.sub.eff] ~ 17 [Angstrom]. The synthetic pinned layer gives high unidirectional exchange [H.sub.ex] (~1250 Oe) and saturation fields [H.sub.s] (~3-4 KOe) with no significant change after several hours anneal at 300 [degrees] C. MR decreases linearly as the temperature increases ((1/MR(RT)) x dMR(T)/dT [approximately equals]-0.0017/[degrees] C). Index Terms--Magnetic recording/read heads, spin valves, synthetic ferrimagnets/antiferromagnets, effective magnetic thickness, magnetic and thermal stability.