Spin-dependent Tunnel Junctions Research Articles

Ion beam deposition and oxidation of spin-dependent tunnel junctions

Spin dependent tunnel junctions showing tunnel magnetoresistance (TMR) values of 39/spl sim/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 3/spl times/2 /spl mu/m/sup 2/. As-deposited junctions with 15 /spl Aring/ of Al showed TMR of 27/spl sim/29%, independent of the junction area, with resistance-area products of 0.8/spl sim/1.6 M/spl Omega//spl times//spl mu/m/sup 2/. This TMR value reached 40% upon annealing at 290/spl deg/C, with resistance decreasing to 0.5/spl sim/0.8 M/spl Omega//spl times//spl mu/m/sup 2/.

In-situ characterization of oxide growth for fabricating spin-dependent tunnel junctions

Ni/sub 45/Fe/sub 55/-Al/sub 2/O/sub 3/-Ni/sub 45/Fe/sub 55/ spin dependent tunneling junctions with an MR ratio of /spl sim/16.5% have been fabricated. The Al/sub 2/O/sub 3/ barriers are formed by plasma oxidation, and the oxide growth is characterized by in-situ resistive measurements. The oxide barrier growth mode is greatly affected by process parameters, including shutter configuration, gas composition, and plasma power. The changes in oxide thickness can be modeled by a combination of the linear growth law and the generalized Mott-Cabrera law.

Vertical integration of a spin dependent tunnel junction with an amorphous Si diode for MRAM application

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 0.86 V to the tunnel junction-diode series. In the integrated device, the TMR signal of the junction measured alone was 25.3% at 7 mV bias, demonstrating junction robustness and process compatibility. The /spl 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 25 ns is calculated, higher than the 2.5 ns estimated for the tunneling junction.

Magnetoresistance in La/sub 0.8/Sr/sub 0.2/MnO/sub 3-δ/ biepitaxial grain boundary junction

La/sub 0.8/Sr/sub 0.2/MnO/sub 3-/spl delta// (LSMO) films with 45/spl deg/ in-plane grain boundaries were fabricated using two types of biepitaxial multilayers. The low field MR (magnetoresistance) ratio of 32% at 77 K is a significant value due to the artificial grain boundary effect and it maybe useful for developing spin-dependent tunneling junctions using a LSMO biepitaxial grain boundary. The magnetoresistive properties at two different magnetic fields, parallel or perpendicular to the grain boundary junction, can be interpreted by the different ferromagnetic spin-polarization ordering in ab-plane.

Voltage dependence of magnetoresistance in spin dependent tunneling junctions

The voltage dependence of magnetoresistance in spin dependent tunneling (SDT) junctions was studied experimentally and theoretically. Different magnetoresistance (MR)-V dependence in various patterned junctions was observed and correlated with other technologically important parameters, including the magnitude of the MR, linearity of the current–voltage characteristic, temperature dependence of the junction resistance, and the MR. A phenomenological model based on a spin-independent two-step tunneling via defect states in the barrier, in addition to the spin-dependent direct tunneling, is proposed to account for the MR-V dependence. The MR ratio is determined by the ratio of the two currents. The MR-V dependence results from a stronger voltage dependence of the two-step tunneling current compared to that of the direct tunneling current. The same model also satisfactorily predicts other properties of SDT junctions. A high quality barrier is required to minimize the MR-V dependence and improve other junction properties. The approach to achieving desirable junction impedance for data storage applications is discussed.

Tunneling magnetoresistance and current distribution effect in spin-dependent tunnel junctions

In this paper, spin-dependent tunnel junctions fabricated by shadow mask (junction area 0.25 mm2) and by lithography (junction area down to 9×2 μm2) were studied. The junctions have NiFe and CoFe electrodes and the insulating barrier Al2O3 is formed by depositing a 10–30 Å thick Al layer, followed by a 1–3 min plasma oxidation in an O2 atmosphere. The mm2-size junctions show tunneling magnetoresistances (TMR) of 10%–13.5% at room temperature (RT), with 50% decrease in TMR for a bias voltage of 220 mV. The junction resistances range from hundreds of Ω to tens of kΩ. The analysis of current distribution indicates that no geometrically enhanced magnetoresistance occurs in the cross-shaped mm2-size junctions when the measured junction resistance is five times larger than the electrode resistance over junction area. The μm2-size junctions show TMR of 17%–24% at RT, independent of the junction area, and have a resistance between 90 kΩ and 1 MΩ for the 9×2 μm2 size (resistance-area products of ∼3 MΩ×μm2). The μm2-size junctions show 50% decrease in TMR for a bias voltage of 430 mV, and high sensitivity (>20%/Oe).

Anomalous Behavior in Spin Dependent Tunnel Junctions

ABSTRACTSpin dependent tunneling has been investigated in tunnel junctions composed of a variety of materials. The best results thus far have been with either HfO2 or MgO as the barrier layer using CoFe, Fe, or Co as the magnetic electrodes. The maximum magnetoresistive (MR) response of these junctions has been at low temperatures on the order of 30% in HfO2 and 20% in MgO. We have also observed a variety of anomalous behavior in some of our tunnel junctions at low temperature. These include MR effects dependent on the angle of orientation of the tunnel junction in the applied magnetic field, transition fields greater than lOkOe, and negative MR effects on the order of 2%.

Influence of the Sputtering Parameters on the Properties of A12O3 and Ain Insulators in Spin Tunneling Junctions

ABSTRACTThe role of the sputtering parameters on the properties of thin A12O3 and A1N insulator layers for spin dependent tunneling junctions was studied. A12O3 insulators were prepared by rf sputtering from an A12O3 target in an Ar or Ar-O2 atmosphere and also by plasma oxidation in an O2 atmosphere of a thin Al layer. The thin Al layer was deposited by rf sputtering at room temperature. A1N was prepared by reactive sputtering from an Al target in a Ar-N2 atmosphere. The index of refraction n measured by ellipsometry, was used to optimize the sputtering parameters, and chemical etch rates were used for information on pin-hole density. The aimed values were n=1.75 for A12O3 (corundum structure) and n=2.2 for A1N. For reactively sputtered A12O3, under optimized deposition conditions, n reaches 1.65 for films 300 Å thick. A12O3 thin layers fabricated by plasma oxidation of a thin Al film were also prepared but the oxide layer was too thin for ellipsometry characterization. A1N insulating films show a constant n value of 1.92 from 400 Å down to 60 Å thick films. Rutherford backscattering analysis show that the ratio of Ar to N2 must be less than 1 to avoid O2 incorporation in the film. Junctions of the type glass/buffer/FM/insulator/FM, where FM is either Co or NiFe, were made by contact metal masks with a junction area of 0.25 mm.2 The A12O3 deposited by sputtering from an A12O3 target under conditions expected to give n>1.5 had a magnetoresistance at room temperature MR(RT) of 2.5% with a junction resistance Rj of 2.2 m Ω and by fitting the I-V curve to existing tunneling theory gave an effective barrier thicknesst of 20 Å and a barrier heightΦ of 2.2 eV. These occurred in only one sample (9 junctions) and were not reproducible. The A12O3 insulators prepared by plasma oxidation of a thin Al layer gave a MR(RT)= 4%, tj = 22 Å, Rj=250 k,Ω and Φ =1.4 eV. The A1N insulator with n =1.9 gave a MR(RT)= 1.6 %, t = 20 A, Rj = 4.8 kΦ and barrier height 1.2 eV. In the last two cases junctions with MR at room temperature can be produced with reasonable reproducibility, Al2O3 samples showed a 15–20% increase in MR as the temperature was lowered to 17 K, while AIN junctions show an increase of a factor of 2 in MR at 77 K.

Spin-dependent tunneling in HfO2 tunnel junctions

We identified reactively sputtered HfO2 as a particularly good material for making thin insulating barriers for spin-dependent tunnel junctions. This material allows one to form pinhole-free tunnel barriers with good transmission of the spin polarization of the tunneling electrons. Magnetic tunnel junctions consisting of a thin layer of HfO2 sandwiched between transition metal electrodes (Co and Fe, for instance) exhibit changes of tunnel resistance up to 30% at low temperature as a function of applied field. This effect can be used in a variety of magnetic field sensing applications or in magnetic random access memory.