Janusz Nowak

Spin vortex states and hysteretic properties of submicron size NiFe elements

Micromagnetic structures in submicron size circular permalloy elements of various thickness (5–50 nm) have been experimentally studied using a magnetic force microscope (MFM). One and two vortex micromagnetic states were observed in the elements. It was found that one vortex state is more favorable under external field of low values, while two vortex state forms under high fields. Switching between one and two vortex states results in a hysteresis in the micromagnetic response of the elements. The transition field from one to two vortex state increases with the increase of element thickness and, therefore, with the increase of the demagnetizing effect of the element edges. The two vortex state was not observed in the elements with a thickness of 50 nm or greater. The micromagnetic behavior correlates with hysteresis loops of a free layer in a magnetic tunnel junction of size, shape, and free layer thickness similar to elements examined with the MFM.

Dielectric breakdown in magnetic tunnel junctions having an ultrathin barrier

Magnetic tunnel junctions have been fabricated by magnetron sputtering and patterned by deep ultraviolet photolithography. The tunnel magnetoresistance was 15%–22% and resistance times area product (R×A) 7–22 Ω μm2 for junctions having 4.75–5.5-A-thick Al layer oxidized naturally. Two types of breakdown were observed: abrupt dielectric breakdown at an effective field of 10 MV/cm determined by the thickness of the tunnel barrier, and a gradual breakdown related to defects in the tunnel barrier. After the breakdown a metallic pinhole is created, the size of which depends on the maximum current applied to the junction. The current flowing through the pinhole creates a strong circular magnetic field that curls the local magnetization in the free layer around the pinhole. The subsequent free-layer reversal is very sensitive to the pinhole location. The electric properties after breakdown can be well described by an Ohmic resistor and a tunnel magnetoresistor connected in parallel.

Two breakdown mechanisms in ultrathin alumina barrier magnetic tunnel junctions

Two breakdown mechanisms are observed in magnetic tunnel junctions having an ultrathin alumina barrier. The two breakdown mechanisms manifest themselves differently when considering large ensembles of nominally identical devices under different stress conditions. The results suggest that one type of breakdown occurs because of the intrinsic breakdown of a well-formed oxide barrier that can be described by the E model of dielectric breakdown. The other is an extrinsic breakdown related to defects in the barrier rather than the failure of the oxide integrity. The characteristic of extrinsic breakdown suggests that a pre-existing pinhole in the barriers grows in area by means of dissipative (Joule) heating and/or an electric field across the pinhole circumference.

Demonstrating a tunneling magneto-resistive read head

We demonstrate the feasibility of a read head using a high-resistance spin-dependent tunneling (SDT) junction. The head consists of a 1.5 /spl mu/m or 1.7 /spl mu/m wide flux guide on top of the junction. The junction is about 0.8 /spl mu/m wide and varies in height, There is a pinned layer at the bottom of the stack. The tunnel barrier is made by plasma oxidation of about 10 /spl Aring/ of aluminum. The barrier has resistance of about 3.7 k/spl Omega/ /spl mu/m/sup 2/, leading to a typical head resistance of a few k/spl Omega/. The heads are lapped to the edge of the junction, and show 5% to 8% TMR during transfer curve measurement. Isolated pulses during the spin stand test shows large signal up to 5.7 mV. We find a voltage sensing preamp is a better choice over a current sensing one.

Proper oxidation for spin-dependent tunnel junctions

From wafers where the AlOx barrier varies from junction to junction, we determine the proper oxidation conditions for spin-dependent tunnel junctions. We obtain large variation in the resistance×area product (R×A) of the junctions within a wafer by either using nonuniform plasma oxidation on uniform Al films, or using uniform oxidation on Al films with a wedge thickness profile. When plotted against R×A, the tunneling magneto-resistance (TMR) for all junctions on the wafer falls on one curve that exhibits a broad maximum in the TMR. We propose that this maximum is where most metal Al has been oxidized while the oxidation of the bottom electrode is minimal. With annealing, we achieved our highest TMR, 38%, in highly resistive Co–AlOx–Co junctions. The most conductive junctions we made have about 18% TMR and R×A of 140 Ω μm2. They are made by natural oxidation on about 5 A of Al. For barriers thinner than 13 A Al, we start to lose TMR for junctions larger than 2 μm2. This is possibly caused by pinholes in t...

Tunable ferromagnetic resonance peak in tunneling magnetoresistive sensor structures

Noise properties of submicron scale tunneling magnetoresistive (TMR) sensors were investigated at frequencies up to 3 GHz. Noise spectral density was measured as a function of frequency, applied field, and bias current. Noise spectral density versus frequency dependence exhibits a pronounced peak, tunable over a wide frequency range. This peak appears to originate from current-driven precession of magnetization. The peak center frequency can be as low as 200 MHz and has a strong dependence on applied field and bias current. The damping constant α of the main precession mode in the TMR sensor free layer was found to be in the range of 0.05–0.18. It is shown that the magnetic state of a magnetoresistive sensor depends on the bias current and may be characterized by noise properties. The magnetoresistive element can operate as a source of high-frequency radiation with 1 nW emitting power from a 0.1 μm2 junction and signal to noise ratio of 10 dB.

Magnetic tunnel junction performance versus barrier thickness: NiFe/AlOx/NiFe junctions fabricated from a wedged Al layer

The resistance–area product (R*A) and the magnetoresistance (MR) of NiFe/AlOx/NiFe spin-dependent tunnel junctions exhibit a strong dependence on the thickness of Al before oxidation. We obtain these data from wafers where we uniformly oxidize an Al layer with a wedged thickness profile, enabling us to reliably characterize the effect of Al thickness variations with subangstrom precision. The R*A drops from 104 to 102 Ω μm2 as the Al thickness decreases from 9 to 4 A, respectively. The MR is highest (21%) for an Al thickness of 7 A, where the Al layer is fully oxidized and the oxidation of the bottom NiFe electrode is minimal.

Dynamic conductance of Ni80Fe20/AlOx/Ni80Fe20 tunnel junctions

The shape of the dynamic conductance versus voltage of NiFe/AlOx/NiFe tunnel junctions is correlated with the intensity and duration of oxidation. A shift of the conductance minimum towards positive voltage (up to 100 mV) indicates that the Al layer is only partially oxidized. In contrast, a shift of the conductance minimum towards negative voltage indicates oxidation of the bottom electrode and/or damage to the top surface of the barrier. Annealing of the junctions makes the tunnel barrier effectively thinner and taller, as implied by a fit to the parabolic conductance. These qualitative trends are observed for Al layers ranging from 10 to 35 A in thickness. In conjunction with these effects, we observe the highest magnetoresistance (up to 26.6%) and the best bias dependence of the magnetoresistance when the conductance is symmetric.

Tunneling criteria and breakdown for low resistive magnetic tunnel junctions

The tunneling criteria are evaluated using magnetic tunnel junctions having ultrathin alumina barrier with and without pinholes. It is shown that the tunneling criteria formulated by Rowell [J. Appl. Phys. 41, 1915 (1970)] clearly do not rule out the presence of pinholes in an ultrathin insulating barrier. In particular, the third criterion, a downward temperature dependence of resistance, cannot be used to decisively rule out the presence of pinholes. Examination of the breakdown mechanism will reveal the true nature of the barrier quality, and thus should be applied alongside the tunneling criteria to identify tunneling and the presence of pinholes.

Spin tunneling heads above 20 Gb/in/sup 2/

Spin tunneling recording heads above 20 Gb/in/sup 2/ have been fabricated using a bottom tunneling junction stack. The spin tunneling stack is made of Ta/PtMn/CoFe/Ru/CoFe/AlO/NiFe/Ta and stabilized by a permanent magnet abutted junction. The effective junction width is about 0.4 /spl mu/m wide and lapped to the junction with an optimum stripe height. The barrier has resistance area product of 15-20 /spl Omega//spl mu/m/sup 2/, leading to a typical head resistance of around 50 /spl Omega/. Isolated pulses during the spin-stand test shows large signal up to 10 mV. On track error rate floor is better than 10/sup -9/ and the head signal-to-noise ratio is also better than that of. a conventional spin valve GMR head. The areal density estimated (using BER of 10/sup -5/) is above 20 Gb/in/sup 2/.