Hua-Ching Tong

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.

CHARACTERIZATION OF ION BEAM AND MAGNETRON SPUTTERED THIN TA/NIFE FILMS

Thin Ta/NiFe films were deposited using ion beam deposition (IBD), pulsed, and static magnetron sputtering techniques. These NiFe films show anisotropy field values ∼4 Oe, easy axis coercivities ⩽1 Oe, and hard axis coercivities ⩽0.3 Oe. IBD films exhibit higher magnetoresistance ratios (ΔR/R), while little difference is noted between different deposition techniques in the sheet resistance of NiFe films. A ΔR/R value of 1.8% has been measured for a 90 A IBD NiFe films. X-ray diffraction measurements indicate that NiFe films of the same thickness have about the same grain size regardless of deposition technique, however, IBD films exhibit superior (111) texture and crystallinity. Our results clearly indicate that the superior magnetic properties of thin IBD Ta/NiFe films are a result of high crystallographic quality of these films.

Ion beam sputtered spin-valve films with improved giant magnetoresistance response

A large value of giant magnetoresistance ΔR/R=9% with an exchange field Hex=350 Oe has been measured from simple NiFe/CoFe/Cu/CoFe/IrMn top spin-valve films prepared by ion beam deposition (IBD) techniques. The exchange biasing was greatly enhanced when a synthetic pinned layer, CoFe/Ru/CoFe, is used in the spin-valve structures. Apparent exchange field values in excess of 2000 Oe and ΔR/R values above 8% have been obtained in synthetic spin-valve films. These IBD spin-valve films show excellent thermal stability and they are suitable for the applications in high density magnetic recording heads.

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.

Magnetic properties of ion beam deposited CoCrPt hard bias films for magnetoresistive heads

Ion beam deposition techniques have been employed to prepare Cr/CoCrPt bilayers for hard bias applications in giant/anisotropic magnetoresistive recording heads. The bilayers were deposited at different deposition angles, which is defined as the angle between deposition beam direction and substrate plane. The magnetic properties of CoCrPt films, such as coercivity (Hc), remnant magnetization–thickness product (Mrt), and coercive squareness (S*) as a function of CoCrPt layer thickness and deposition angle have been studied and correlated to film microstructures. Typical values of Hc, Mrt, and S* for a Cr100 A/CoCrPt490 A bilayer deposited at 42° are 1580 Oe, 2.90 memu/cm2, and 0.90, respectively. The increase in Mrt with increasing CoCrPt layer thickness at a fixed deposition angle is accompanied by a linear decrease in Hc. We have observed, however, that for the same bilayer geometry both Hc and Mrt of CoCrPt layer increase with decreasing deposition angle, whereas S* remains almost unchanged. High angle ...

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.

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.

Laminated FeRhN films for high speed writers

High moment FeRhN nanocrystalline films were laminated by Co/sub 90/Zr/sub 9/Cr (CZC) amorphous films, in order to improve the anisotropy definition and high frequency permeability. The experimental results also showed improved soft magnetic properties, higher resistivity, and better magnetic properties on sloped surfaces.