Shin Funada

The spin flop of synthetic antiferromagnetic films

The spin flop of synthetic antiferromagnetic pinned layers (SAF), under a magnetic field has been theoretically predicted and recently reported [J. G. Zhu and Y. Zheng, IEEE Trans. Magn. 34, 1063 (1998); J. G. Zhu, IEEE Trans. Magn. 35, 655 (1999)]. However, no experimental data have yet being reported to confirm the theoretical prediction. This article will provide direct experimental evidence to confirm the spin flop phenomenon in SAF layers. A spin valve, [CoFe/NiFe]/Cu/[CoFe(II)/Ru/CoFe(I)]/IrMn, was used to verify the spin flop in SAF layers. The exchange bias direction of CoFe(I)/IrMn was introduced by a magnetic annealing process at 225 °C with a field strength of Han(10 kOe) and the exchange bias direction was found parallel to the magnetic field. These samples serve as the reference for the remaining experiments. By magnetic annealing the reference samples at 225 °C with lower magnetic fields, we found that the magnetic field threshold for SAF spin flop is about 1 kOe. When the field is further i...

Magnetic properties of ultrathin NiFe and CoFe films

In this article, we report the magnetic properties of ultrathin (15–200 A) NiFe and CoFe films deposited using ion beam deposition techniques. They are symmetrically sandwiched between Ta, Cu, or Ta/Cu under and capping layers. NiFe and CoFe films grown between Ta/Cu and Cu/Ta bilayers exhibit the smallest magnetic thickness loss of about 1 A. This interfacial magnetic dead layer thickness, t0, is about 5 A for Cu-sandwiched films and about 15 A for Ta-sandwiched films. As the film thickness becomes thinner than 100 A, the magnetic properties are found to be more sensitive to the choice of material and growth environment. CoFe films show an interfacial contribution, λi, about ten times larger than that for NiFe films. Among others, NiFe and CoFe films sandwiched by Ta/Cu and Cu/Ta bilayers exhibit the smallest values of λi. The magnetic anisotropy in Ta-sandwiched CoFe films appears to be predominantly magnetoelastic in nature.

Voltage-induced barrier-layer damage in spin-dependent tunneling junctions

The effect of a dc stress voltage on the junction resistance and magnetoresistance (MR) of spin-dependent tunneling (SDT) junctions with naturally oxidized barriers was investigated. There is a threshold voltage at which irreversible resistance change begins. Beyond this threshold, device resistance decreases gradually over a transition period prior to breakdown of the tunneling barrier. The onset voltage of irreversible resistance change is much higher than the optimum operating voltage of SDT heads having the precursor aluminum thicknesses here investigated (5–11 A). The MR ratio decreased with increasing stress voltage in a pattern similar to that of the junction resistance.

Low resistance spin dependent tunneling junctions with naturally oxidized tunneling barrier

We investigated the effect of aluminum thickness on RA (resistance area product) and MR ratio in spin-dependent tunnel junctions. Very low RA values below 40 /spl Omega/ /spl mu/m/sub 2/ and a maximum MR ratio of 6% were obtained in Ni/sub 81/Fe/sub 19//Al/sub 2/O/sub 3//Ni/sub 81/Fe/sub 19/ tunneling junctions fabricated using natural oxidation of aluminum. The MR ratio significantly increased with use of Co/sub 90/Fe/sub 10/ electrodes. Using natural oxidation of 6 /spl Aring/ aluminum, RA and MR ratio of these junctions are /spl sim/50 /spl Omega/ /spl mu/m/sup 2/ and /spl sim/28%, respectively. The MR ratio and the junction resistance strongly depend on the aluminum thickness. The optimum thickness of aluminum is 6-7 /spl Aring/ fur natural oxidation.

Exchange biasing and thermal stability of CoFe/PtPdMn films

Two sets of PtPdMn exchange biased films Ta 50 A/CoFe 100 A/PtPdMn (tAF) A/Ta 50 A, with PtPdMn thickness, tAF=350, 600 A, were deposited on Si substrates by dc magnetron sputtering techniques. After magnetic annealing, these two sets of films exhibited values of exchange bias field, Hex=229 and 254 Oe, respectively. The PtPdMn layer was then thinned to various thicknesses from 600 down to 50 A by ion beam etching. Hex does not retain its original value. It decreases with decreasing tAF and becomes zero at tAF∼75 A. In addition, we have observed that the training effect or the anomalous hysteresis loss becomes more pronounced with decreasing tAF. This confirms that not only face-centered-tetragonal phase but, more critically, tAF plays role in determining exchange biasing and its thermal stability. The blocking temperature, TB, appears unaffected by the thinning of the PtPdMn layer, and no apparent change occurs in the local blocking temperature distribution, as suggested by the finite size effect.

ALCVD AlO/sub x/ barrier layers for magnetic tunnel junction applications

Ultrathin AlO/sub x/ layers 5-25 /spl Aring/ thick were deposited using the atomic-layer chemical vapor deposition technique. A magnetic thickness loss of /spl sim/ 1 /spl Aring/ has been estimated at CoFe-AlO/sub x/ or NiFe-AlONiFe-AlO/sub x/ interfaces. The two-dimensional integrity of the thin AlO/sub x/ films at thickness >7 /spl Aring/ has been validated by the differential switching of two magnetic layers sandwiching a ultrathin AlO/sub x/ layer and the distinct cross-sectional transmission-electron-microscopy images. No appreciable tunnel magnetoresistance effect has been measured from the fabricated magnetic tunnel junction devices. A proper in situ treatment prior to and/or after AlO/sub x/ deposition and a proper protection of the underlying magnetic layer are expected for further improvements.

Spin-dependent tunneling junctions with parallel hard bias for read heads

We investigated the feasibility of a spin-dependent tunneling (SDT) read head with a parallel hard bias. In this scheme, the longitudinal biasing to the free layer is provided by fringe fields from a hard magnet which is fabricated over or under the free layer. A linear response to the applied field is achieved for a SDT junction biased with 400 A CoCrPt underneath. Thinner CoCrPt layers yield Barkhausen jumps in the free layer. Micromagnetic simulation indicates the bias field at the edge of the free layer is smaller than that which would result from an abutted magnet. The simulation results are similar to experimental data, and indicate that shielded devices with 400 A permanent magnet will provide stable transfer curves.

Lifetime of magnetic tunnel junctions under voltage stress

Summary form only given. Magnetic tunnel junctions (MTJ) are being investigated for possible application in magnetic sensors and memories. Due to the thin tunnel barrier, the junctions are very sensitive to electrical breakdown and degradation. This paper describes the lifetime of junctions under voltage stress with extrapolation to use conditions assuming an exponential acceleration factor. The tunneling films were deposited with a DC magnetron sputtering system. The aluminum layer is plasma oxidized in an oxidation module to form an Al/sub 2/O/sub 3/ tunneling barrier. Aluminum thickness, oxidation times, and plasma powers were used to create barriers of different thickness. The film structure is NiFe/PtMn/CoFe/Ru/CoFe/AlO/CoFe/NiFe/Ta. The TMR response of the junction decreases with voltage resulting in a peak for the TMR signal versus voltage. The acceleration factor calculated in stress testing can be used to estimate the junction lifetime at the optimum operating voltage.

Effects of CoFe surface oxidation on tunneling magnetoresistance

Summary form only given. TMR is being investigated for its possible applications in magnetic sensors and memories. One of the key processes in TMR junctions is the fabrication of the tunneling barrier. The oxidation status of Al and the ferromagnet at the ferromagnet/Al interface is very important in determining the MR ratio, junction resistance, and magnetic properties. Also, the surface state of the ferromagnet affects the growth mode of Al, which affects the resistance uniformity. The authors report the effects of surface oxidation of CoFe prior to Al deposition on the MR ratio, RA (resistance/spl times/area product), and wafer uniformity.

Micromagnetic simulation of tunneling magnetoresistance junctions with parallel hard bias

Tunneling magnetoresistance (TMR) films and devices were simulated to understand the response of the free layer with a parallel hard bias. In order to determine the effect of the granular hard bias material micromagnetic simulation was used to model both the hard bias and TMR material. Minimizing hysteresis and Barkhausen jumps in the response of the device involves an optimization of the spacing between the free layer and the hard bias coupled with the shape of the device edges.