Spin Valve (SV) heads are a subject of intensive experimental and theoretical investigation withi... more Spin Valve (SV) heads are a subject of intensive experimental and theoretical investigation within the last decade due to their ability to support very high areal density. Generally. SV sensors consist of four layers: a free sofl magnetic layer, Cu-spacer, pinned ferromagnetic layer exchange coupled with an antiferromagnetic (AFM) pinning layer. A large variety of AFM materials are used as
We demonstrate the feasibility of a spin-valve read head using an adjacent antiferromagnetic (AFM... more We demonstrate the feasibility of a spin-valve read head using an adjacent antiferromagnetic (AFM) layer to provide a stabilization field, eliminating the hard bias permanent magnet layer. The head stack consists of seed/ AFM/CoFe/Ru/CoFe/Cu/CoFe(and/or NiFe)/Cu/IrMn/cap where the AFM is PtMn or IrMn. The abutted junction is about 0.1 m wide at the free layer and no permanent magnet materials are used. Micromagnetic modeling indicates that a proper bias field is needed to have a stable head response. The bias field from the AFM layer can be modified by a nonmagnetic spacer layer between AFM and FM. The coupling field decays exponentially with the thickness of the spacer layer, consistent with a pinhole-dominated mechanism. Unshielded sensors show much higher sensitivity as compared with a standard PM abutted junction head. Microtrack profile based on the spin-stand test shows a regular shape without apparent lumps. The electrical reader width is about 6 (0.15 m), corresponding to a track density of 112 ktpi. Combined with a linear density of 591 kbpi the heads in this work are capable of areal density around 66 Gb/in 2 .
Shot noise measurements made on low-resistance magnetic tunnel junctions show results inconsisten... more Shot noise measurements made on low-resistance magnetic tunnel junctions show results inconsistent with the resistance of the samples examined. The results yield lower than expected shot noise which is consistent with parallel path conduction (pinholes). A simple electrical model shows this should be true for noise measurements as well as for the tunneling magnetoresistance (TMR) as the resistance-area product (R×A) is reduced. The model suggests that a correlation between TMR and shot noise should exist assuming the presence of pinholes and that shot noise offers a useful experimental monitoring technique. The difficulties of making these measurements related to high frequency 1/f noise are discussed. Noise results are presented for high R×A and low R×A low-resistance samples which, in this particular case, both show the influence of pinhole shunting.
The shape of the dynamic conductance versus voltage of NiFe/AlOx/NiFe tunnel junctions is correla... more 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 Å 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.
Giant magnetoresistance (GMR) responses were studied as a function of applied measurement field a... more Giant magnetoresistance (GMR) responses were studied as a function of applied measurement field angle in FeMn and NiMn pinned spin valves. As the measurement field varies away from the pinning field direction, the peak GMR ratio and the free layer coercivity decrease. The GMR curve changes from unsymmetrical to symmetrical with respect to the applied field polarity. The GMR responses from the component parallel to the applied field are proportional to the cosine of the angle between the pinning and the applied field. The results can be described well by a simple vector model, and an empirical method to measure the pinning field rotation is established.
The giant magnetoresistance response of NiMn pinned spin valves was studied at elevated temperatu... more The giant magnetoresistance response of NiMn pinned spin valves was studied at elevated temperature. Top spin valve films were made by ion beam sputtering and thermally treated to induce the strong unidirectional pinning field in the pinned layer. Both δR and δR/R decrease linearly with temperature. The sheet resistance of the spin valves also increases linearly with temperature. The exchange coupling between pinned layer and free layer decreases slightly and the coercivity of the free layer increases slightly. The temperature dependence of the exchange pinning field is unique in NiMn spin valves. The pinning field has a weakly increasing temperature dependence up to 200 °C, then decreases to zero at the blocking temperature of 380 °C. Samples with different thickness NiMn layers show different temperature dependencies. However, the blocking temperature is unchanged. The pinning fields of NiMn, FeMn, IrMn, and NiO spin valves were also measured up to 200 °C NiMn pinned spin valves show the least dependence of pinning field at elevated temperatures.
Giant magnetoresistance (GMR) responses were studied in micron-sized IrMn pinned spin valve strip... more Giant magnetoresistance (GMR) responses were studied in micron-sized IrMn pinned spin valve stripes. As the stripe height is decreased both the pinning field and free layer bias field increase while the GMR ratio stays almost constant. The free layer coercivity becomes zero for the shortest stripe while the pinned layer coercivity drops slightly at high aspect ratio. The pinned layer switching field is enhanced in the patterned films, indicating a strong magnetostatic coupling between the free and pinned layer that was further simulated micromagnetically.
The thermal stability of spin valve films with synthetic antiferromagnet (SAF) pinned by antiferr... more The thermal stability of spin valve films with synthetic antiferromagnet (SAF) pinned by antiferromagnetic IrMn, NiO, and NiMn layers were studied. The SAF layer enhances the thermal stability in general; however, the blocking temperature (and the blocking temperature distribution) of the antiferromagnet is still important for the magnetic rigidity of the pinned layer. Once the temperature reaches the blocking temperature the SAF layer can go into either the spin flip or flop state, depending upon the magnetic moment ratio of the reference layer and pinned layers. The GMR linear head response can be distorted for nonlinearity. The NiMn pinned SAF structure shows magnetic and thermal stability which makes it practical for the real products. A high GMR of 11% can be obtained in both bottom and top NiMn SAF spin valves by advanced processes. Recording heads were built using such stacks which demonstrated recording areal density of 20 Gbit/in.2 and beyond.
Spin valve film stacks should be thermally stable both for processing and operation in the head a... more Spin valve film stacks should be thermally stable both for processing and operation in the head application. The fabrication processes involve high temperature annealing and head operation in the drive also has a considerable temperature increase. Engineering of favorable AFM materials such as NiMn and PtMn needs more than 250C thermal anneal to induce the pinning field. A systematic study of the spin valve thermal stability was conducted in this work. It was found that the GMR thermal stability correlates with the microstructure and the degradation mechanism of the GMR may be different in the processing and operating conditions. In the as-made films of FeMn spin valves, the GMR ratios are similar between films with different rocking curve widths; however, after the thermal anneal the GMR changes dramatically between samples with different crystallinity. Both the XRD and cross-sectional TEM pictures revealed that the grain size between those samples change little except for a fully polycrystalline film. It is apparent that the narrower the rocking curve the better the GMR thermal stability. The rocking curve width is an indicator of better crystallinity. The narrower rocking curves have better grain alignment, thus less grain boundaries near the Cu spacer interfaces. Diffusion will be suppressed, resulting in improved thermal stability. NiMn spin valves showed the same trend. A long-term thermal annealing at 200C was carried on NiMn spin valves for one thousand hours. GMR ratio sustained almost constant with only apparent increase of interlayer coupling field. The results will be discussed in terms of the interface evolution under thermal stress.
Spin Valve (SV) heads are a subject of intensive experimental and theoretical investigation withi... more Spin Valve (SV) heads are a subject of intensive experimental and theoretical investigation within the last decade due to their ability to support very high areal density. Generally. SV sensors consist of four layers: a free sofl magnetic layer, Cu-spacer, pinned ferromagnetic layer exchange coupled with an antiferromagnetic (AFM) pinning layer. A large variety of AFM materials are used as
We demonstrate the feasibility of a spin-valve read head using an adjacent antiferromagnetic (AFM... more We demonstrate the feasibility of a spin-valve read head using an adjacent antiferromagnetic (AFM) layer to provide a stabilization field, eliminating the hard bias permanent magnet layer. The head stack consists of seed/ AFM/CoFe/Ru/CoFe/Cu/CoFe(and/or NiFe)/Cu/IrMn/cap where the AFM is PtMn or IrMn. The abutted junction is about 0.1 m wide at the free layer and no permanent magnet materials are used. Micromagnetic modeling indicates that a proper bias field is needed to have a stable head response. The bias field from the AFM layer can be modified by a nonmagnetic spacer layer between AFM and FM. The coupling field decays exponentially with the thickness of the spacer layer, consistent with a pinhole-dominated mechanism. Unshielded sensors show much higher sensitivity as compared with a standard PM abutted junction head. Microtrack profile based on the spin-stand test shows a regular shape without apparent lumps. The electrical reader width is about 6 (0.15 m), corresponding to a track density of 112 ktpi. Combined with a linear density of 591 kbpi the heads in this work are capable of areal density around 66 Gb/in 2 .
Shot noise measurements made on low-resistance magnetic tunnel junctions show results inconsisten... more Shot noise measurements made on low-resistance magnetic tunnel junctions show results inconsistent with the resistance of the samples examined. The results yield lower than expected shot noise which is consistent with parallel path conduction (pinholes). A simple electrical model shows this should be true for noise measurements as well as for the tunneling magnetoresistance (TMR) as the resistance-area product (R×A) is reduced. The model suggests that a correlation between TMR and shot noise should exist assuming the presence of pinholes and that shot noise offers a useful experimental monitoring technique. The difficulties of making these measurements related to high frequency 1/f noise are discussed. Noise results are presented for high R×A and low R×A low-resistance samples which, in this particular case, both show the influence of pinhole shunting.
The shape of the dynamic conductance versus voltage of NiFe/AlOx/NiFe tunnel junctions is correla... more 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 Å 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.
Giant magnetoresistance (GMR) responses were studied as a function of applied measurement field a... more Giant magnetoresistance (GMR) responses were studied as a function of applied measurement field angle in FeMn and NiMn pinned spin valves. As the measurement field varies away from the pinning field direction, the peak GMR ratio and the free layer coercivity decrease. The GMR curve changes from unsymmetrical to symmetrical with respect to the applied field polarity. The GMR responses from the component parallel to the applied field are proportional to the cosine of the angle between the pinning and the applied field. The results can be described well by a simple vector model, and an empirical method to measure the pinning field rotation is established.
The giant magnetoresistance response of NiMn pinned spin valves was studied at elevated temperatu... more The giant magnetoresistance response of NiMn pinned spin valves was studied at elevated temperature. Top spin valve films were made by ion beam sputtering and thermally treated to induce the strong unidirectional pinning field in the pinned layer. Both δR and δR/R decrease linearly with temperature. The sheet resistance of the spin valves also increases linearly with temperature. The exchange coupling between pinned layer and free layer decreases slightly and the coercivity of the free layer increases slightly. The temperature dependence of the exchange pinning field is unique in NiMn spin valves. The pinning field has a weakly increasing temperature dependence up to 200 °C, then decreases to zero at the blocking temperature of 380 °C. Samples with different thickness NiMn layers show different temperature dependencies. However, the blocking temperature is unchanged. The pinning fields of NiMn, FeMn, IrMn, and NiO spin valves were also measured up to 200 °C NiMn pinned spin valves show the least dependence of pinning field at elevated temperatures.
Giant magnetoresistance (GMR) responses were studied in micron-sized IrMn pinned spin valve strip... more Giant magnetoresistance (GMR) responses were studied in micron-sized IrMn pinned spin valve stripes. As the stripe height is decreased both the pinning field and free layer bias field increase while the GMR ratio stays almost constant. The free layer coercivity becomes zero for the shortest stripe while the pinned layer coercivity drops slightly at high aspect ratio. The pinned layer switching field is enhanced in the patterned films, indicating a strong magnetostatic coupling between the free and pinned layer that was further simulated micromagnetically.
The thermal stability of spin valve films with synthetic antiferromagnet (SAF) pinned by antiferr... more The thermal stability of spin valve films with synthetic antiferromagnet (SAF) pinned by antiferromagnetic IrMn, NiO, and NiMn layers were studied. The SAF layer enhances the thermal stability in general; however, the blocking temperature (and the blocking temperature distribution) of the antiferromagnet is still important for the magnetic rigidity of the pinned layer. Once the temperature reaches the blocking temperature the SAF layer can go into either the spin flip or flop state, depending upon the magnetic moment ratio of the reference layer and pinned layers. The GMR linear head response can be distorted for nonlinearity. The NiMn pinned SAF structure shows magnetic and thermal stability which makes it practical for the real products. A high GMR of 11% can be obtained in both bottom and top NiMn SAF spin valves by advanced processes. Recording heads were built using such stacks which demonstrated recording areal density of 20 Gbit/in.2 and beyond.
Spin valve film stacks should be thermally stable both for processing and operation in the head a... more Spin valve film stacks should be thermally stable both for processing and operation in the head application. The fabrication processes involve high temperature annealing and head operation in the drive also has a considerable temperature increase. Engineering of favorable AFM materials such as NiMn and PtMn needs more than 250C thermal anneal to induce the pinning field. A systematic study of the spin valve thermal stability was conducted in this work. It was found that the GMR thermal stability correlates with the microstructure and the degradation mechanism of the GMR may be different in the processing and operating conditions. In the as-made films of FeMn spin valves, the GMR ratios are similar between films with different rocking curve widths; however, after the thermal anneal the GMR changes dramatically between samples with different crystallinity. Both the XRD and cross-sectional TEM pictures revealed that the grain size between those samples change little except for a fully polycrystalline film. It is apparent that the narrower the rocking curve the better the GMR thermal stability. The rocking curve width is an indicator of better crystallinity. The narrower rocking curves have better grain alignment, thus less grain boundaries near the Cu spacer interfaces. Diffusion will be suppressed, resulting in improved thermal stability. NiMn spin valves showed the same trend. A long-term thermal annealing at 200C was carried on NiMn spin valves for one thousand hours. GMR ratio sustained almost constant with only apparent increase of interlayer coupling field. The results will be discussed in terms of the interface evolution under thermal stress.
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Papers by Ed Murdock