M. Mao

Giant magnetoresistance in ion beam deposited spin-valve films with specular enhancement

Three different techniques, natural oxidation, remote plasma oxidation and low energy ion beam oxidation, have been proved to be equally effective in forming nano-oxide layers (NOLs) in spin-valve films for specular enhancement of giant magnetoresistance (GMR) effect. GMR values over 12% have been routinely obtained in spin-valve films with NOL, corresponding to a 30% specular enhancement over those without NOL. The consistency and robustness of the oxidation processes has been demonstrated by a very large GMR value ∼19% in a dual spin-valve film with the NOLs formed in both pinned layers, the oscillatory dependence of the interlayer coupling field on Cu layer thickness in specular enhanced spin-valve films and the uniform and repeatable film performance over 5 in. substrates.

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.

Influence of base pressure on FeMn exchange biased spin-valve films

Spin-valve films of structure NiFeCo/Co/Cu/NiFeCo(Co)/FeMn/Cu were deposited on Si substrates by DC planetary magnetron sputtering techniques. The influence of base pressure, P{sub b}, on spin-valve properties was studied by varying P{sub b} over two decades from 3 x 10{sup -8} to 7 x 10{sup -6} Torr. The GMR ratio show a slight increase with increasing P{sub b} until a large decrease occurs at P{sub b} > 3.3 x 10{sup -6} Torr. Exchange bias field and blocking temperature remain constant in the base pressure range between 3 x 10{sup -8} and 5 x 10{sup -7} Torr before a large reduction begins. An upper bound base pressure, {sup u}P{sub b} {approx} 5 x 10{sup -7} Torr, is noted from the data, above which significant performance modification begins. The degradation in exchange bias field and blocking temperature, in particular, in spin-valve films using a NiFeCo pinned layer, is the result of deterioration in the crystallographic texture and can be understood due to the contamination both at the ferromagnetic/antiferromagnetic interface and in the bulk of FeMn layer.

Enhanced spin-valve giant magneto-resistance in non-exchange biased sandwich films

A large giant magnetoresistance (GMR) value of 7.5% has been measured in simple NiFeCo(1)/Cu/NiFeCo(2) sandwich films grown on a 30 /spl Aring/ Cr seed layer. This spin-valve GMR effect is consistent with the differential switching of the two NiFeCo layers due to an enhanced coercivity of the NiFeCo(1) layer grown on the Cr seed layer. A change in growth texture of the NiFeCo(l) layer from fcc [111] to bcc [110] crystallographic orientation leads to an increase in magnetic anisotropy and an enhancement in coercivity. The GMR value increases to 8.7% when a thin CoFe interfacial enhancing layer is incorporated. Further enhancement in GMR values up to 14% is seen in the sandwich films by nano-oxide layer formation. The specular reflection at the oxide/magnetic layer interface further extends the mean free path of spin-polarized electrons.

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.

Comparison of semiclassical calculations to experiment for spin valve films grown on oxide surfaces

The physical properties of spin valve films grown on the surface of nano-oxide layers were studied as a function of Cu spacer layer thickness. In comparison to identical structures without the oxide surface, the films exhibited an increase in ΔR/R of 30% accompanied by a reduction of only 5% in resistance. Semiclassical calculations were preformed on these films with a close match to experiment. Assuming the oxide layer did not cause drastic changes in the properties of the other film layers, the specular reflection was changed to obtain a match with experiment. The increase in giant magnetoresistance response was achieved by increasing the specular reflection at the metal/oxide interfaces from 15% to 85%, indicating high efficiency for specular reflection at the nano-oxide interface.

Temperature dependent specular scattering enhancement in Cu-wedge spin valves

Magneto-transport properties have been systematically studied in Cu-wedge regular and specular spin valves prepared by magnetron sputtering. In the specular spin valve, a thin oxide layer is inserted between the Ta underlayer and the free layer to provide a specular interface. Up to 50% enhancement in giant magnetoresistance (GMR) is realized by the specular interface at room temperature. The observed Cu layer thickness dependence of both the resistivity and GMR in both types of the spin valves has been satisfactorily modeled using a semiclassical Boltzmann approach. Temperature dependence of the GMR has been measured from 325 to 15 K for several Cu-layer thicknesses. The specular scattering enhancement initially increases as the temperature is lowered from 325 K, but rapidly decreases below 100 K. This decrease is correlated with the reduced degree of the full antiparallel alignment between the magnetic moments of the pinned and free layers in the specular spin valve, which is caused by the antiferromagn...

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.

Thermal stability of magnetron and ion beam sputtered top and bottom spin-valve films

The thermal stability of top and bottom IrMn exchange-biased spin-valve films prepared by ion beam deposition (IBD) and magnetron sputtering physical vapor deposition (PVD) is compared. These films exhibit identical temperature dependence for the exchange bias field Hex, with a blocking temperature of TB=250 °C, that is independent of preparation technique. Isothermal annealing at temperatures below TB led to a ln(t) dependent degradation in Hex, suggesting a thermal activation process. The high crystallographic quality of the IBD films leads to a superior stability compared to PVD films. Top spin-valve films are also found to be more stable than bottom spin-valve films.

High Yield Second Peak Cu-Co CPP GMR Multilayer Sensors

We have fabricated and tested GMR magnetic flux sensors that operate in the CPP mode. This work is part of the ongoing effort to develop an ultra-high density magnetic sensor introduced at INTERMAG 96 [l]. We have addressed three problems encountered during testing of the devices described in [l]: the MR response was anisotropic, the response was non-uniform, and the operational yield of devices was low. We have also changed from the Cu-CO third peak layer spacing to the second peak layer spacing to take advantage of the larger M R response.