Virgil Simon Speriosu

Magnetotransport properties of magnetically soft spin‐valve structures (invited)

The magnetic and magnetotransport properties of several series of sandwiches consisting of two ferromagnetic layers (Ni, Co, Ni80Fe20) separated by a noble metal (Cu, Ag, Au) are described. In order to vary the relative orientation of the magnetizations of the two ferromagnets, one of them was constrained by exchange anisotropy (e.g., NiFe/Fe50Mn50). The ferromagnetic layers are magnetically soft and not coupled antiparallel, giving very large changes of resistance at low fields. At room temperature relative changes ΔR/R of 4.1% in 10 Oe for Si/Ta 50 A/NiFe 62 A/Cu 22 A/NiFe 40 A/FeMn 70 A/Ta 50 A and 8.7% in 20 Oe has been obtained for a structure based on Co/Cu/Co layers. The magnetoresistance versus the thickness of the ferromagnetic layer shows a broad peak near 80 A for Ni, Co and NiFe, demonstrating the importance of bulk rather than interfacial spin‐dependent scattering, in contrast to Fe/Cr multilayers. The magnetoresistance decreases exponentially with increasing interlayer (Cu and Au) thickness,...

Design, fabrication and testing of spin-valve read heads for high density recording

Spin-valve sensors of the type NiFe/Cu/Co have been designed for optimal biasing behavior and successfully incorporated into a gigabit-type shielded read head configuration with a read trackwidth of 2 /spl mu/m, a read gap of 0.25 /spl mu/m, and a MR sensor height of 1 /spl mu/m. The spin-valve sensor had a structure of 100 /spl Aring/ NiFe/25 /spl Aring/ Cu/22 /spl Aring/ Co/110 /spl Aring/ FeMn, and yielded a net spin-valve coefficient of /spl sim/3.5% at the completion of head processing. Uniform field testing of the read heads after wafer fabrication and lapping showed quiet and stable spin-valve response with near optimal bias performance. Recording tests of the read heads at a head-disk clearance of 1.5 /spl mu/m showed linear, non-saturated signal response on a media with an areal moment as high as 1.25 memu/cm/sup 2/, yielding reasonably symmetrical signals with peak-to-peak amplitudes ranging from /spl sim/750 /spl mu/V//spl mu/m to as high as /spl sim/1000 /spl mu/V//spl mu/m of read trackwidth. Linear density rolloffs and microtrack profiles have also been studied, and results showed behaviors closely agreeing with design targets for high density recording operations. >

Design and operation of spin valve sensors

Two types of patterned, unshielded Giant MagnetoResistance (GMR) spin valve sensors have been fabricated: nano-layered NiFe/Co/Cu/Co/NiFe and simpler NiFe/Cu/Co spin valves. GMR values of 7.6% for /spl Delta/H=10 Oe were measured for the nano-layered structures on coupons. Transfer curves in uniform fields were obtained and were in agreement with theoretical expectations. The sensors were highly linear and well biased. Optimum biasing of the free layer in the spin valve sensor has new features over that in AMR sensors. These were explored in shielded as well as unshielded spin valves using micromagnetic simulation. >

Spin-valve RAM cell

This paper presents the design and the characteristics of a nonvolatile memory cell using giant magneto-resistance effects. Unlike other magnetic memory cells, the present cell design exploits the full /spl Delta/R of the spin valve material. A dc voltage difference between the two cell states of 30 mV range has been realized on a cell stripe only 6-microns long, making it compatible with the high-speed sensing schemes presently employed in silicon RAMs. The cell switches states in sub-nanoseconds. Its performance/density is close to that of the static RAM cell.

Spin-valve effect in soft ferromagnetic sandwiches

Abstract We demonstrated in a variety of systems that the in-plane resistivity of sandwiches of soft ferromagnetic layers separated by nonmagnetic metallic layers depends on the relative angle between their magnetizations. We observe this phenomenon, which we term the spin-valve effect, in sandwiches where we are able to control the relative angle between the magnetizations of two ferromagnetic layers either by constraining one layer through exchange anisotropy or by fabricating layers with different coercivities. In the first case, for example Si/50A Ta/60A NiFe/25A Cu/40A NiFe/50A FeMn/50A Ta we have seen relative changes in resistance of more than 4% at room temperature in a range of in-plane field of 0 to 15 Oe. In a system where the layers have different coercivities, Si/8 × (30A Fe/60A Ag/30A Co/60A Ag), we observed a relative change of 1.6% at room temperature for fields between 0 and 50 Oe. Since the ferromagnetic layers are essentially decoupled and have high squareness, one can rule out any mechanism requiring scattering by domain walls. The usual anisotropic magnetoresistance in these structures is much smaller than the spin-valve effect. In contrast to noble metals, when using Ta, Al, Cr or Pd spacers of similar thickness (20 to 150A) between layers of permalloy, only the anisotropic magnetoresistance is observed. We believe the spin-valve effect to be related to spin-dependent scattering at the interface and within the ferromagnetic layers, in balance with spin-dependent relaxation within the layers. We also report the observation of a weak exchange-like coupling between the ferromagnetic layers.

The role of spin‐dependent impurity scattering in Fe/Cr giant magnetoresistance multilayers

To probe the mechanism of giant magnetoresistance (GMR) observed in Fe/Cr multilayers, we have sputter deposited at the interfaces of Fe(15 A)/Cr(12 A) multilayers an additional, ultrathin (0–4 A) layer of a variety of third elements (V, Mn, Ge, Ir, and Al). When alloyed with Fe in dilute concentrations, the elements chosen have known resistivities for spin‐up (ρ↑) and spin‐down (ρ↓) currents arising from spin‐dependent impurity scattering. The results show a clear correlation between α=ρ↓/ρ↑ of the respective element and the way in which GMR varies with the ultrathin layer thickness. In addition, little difference in GMR is observed between multilayers where the ultrathin layer thickness t/2 is deposited on every Fe/Cr interface and those with a thickness t deposited on alternate interfaces. This investigation demonstrates the importance of the type and total number of scattering centers per multilayer period to the GMR effect.

Effect of annealing on the interfaces of giant‐magnetoresistance spin‐valve structures

Si/Ta/NiFe/Cu/NiFe/FeMn/Ta spin‐valve layered structures were analyzed by x‐ray reflectivity, before and after annealing at 240, 320, and 360 °C. Specularly reflected x‐ray data were collected using a high‐resolution reflectometer and were analyzed by least‐squares refinement. The thicknesses of the individual layers in the NiFe/Cu/NiFe/FeMn magnetic sandwich remained essentially unchanged. With the exception of the FeMn/Ta interface, the widths of the buried interfaces increased rapidly with annealing temperature. The increase in widths at each of the NiFe/Cu and Cu/NiFe interfaces from 6.8 to 22.6 A caused a fivefold increase in the magnetically inactive layer in NiFe and a tenfold decrease in magnetoresistance. An increase in the total film thickness with annealing temperature was found to be caused by the surface oxidation of the Ta capping layer and the growth of a Ta silicide layer between the Ta buffer layer and the Si substrate.

High sensitivity spin-valve strain sensor

A technique for detecting strain has been demonstrated based on a spin-valve sensor. The 400 A thick sensor has been integrated onto an atomic force microscope cantilever. An applied strain caused by bending of the cantilever changes the orientation of the free-layer magnetization due to magnetostriction. This in turn results in a change in the electrical resistance because of the giant magnetoresistance effect. With the proper magnetic bias, a base-line strain sensitivity of 10−10/Hz1/2 has been achieved. The corresponding gauge factor of 150 is roughly 1.6× that of similar silicon piezoresistive cantilevers. In the future, one might be able to enhance the sensitivity by another factor of 3–5.

MAGNETIC AND MAGNETO-OPTIC PROPERTIES OF THICK FACE-CENTERED-CUBIC CO SINGLE-CRYSTAL FILMS

The present letter describes for the first time the magnetic and magneto‐optical properties of thick fcc cobalt single‐crystal films (≂1000‐A thickness). The magneto‐crystalline anisotropy constants K1 and K2 are about −7.2×10 erg/cc and 2×10 erg/cc at 77 K, respectively. Both decrease in magnitude with increasing temperature in a range from 77 K to about 450 K. The easy axis is found to be 〈111〉. The polar Kerr angle for the fcc phase is much larger than that of the hcp phase by 0.1°–0.2° in a photon energy range from about 1 to 5 eV.

New approaches to epitaxy of transition metals and rare earths: Heteroepitaxy on lattice‐matched buffer films on semiconductors (invited)

Examples of new approaches to epitaxy of magnetic transition metals and rare earths are described. Epitaxy of bcc Fe on lattice‐matched InxGa1−xAs films is accompanied by an interfacial reaction. This is indicative of a more general problem of instability of interfaces between Ni, Fe, Co, and compound semiconductors. Such reactions can be avoided if an intermediate epitaxial film of Ag is interposed between the semiconductor and the Fe. However, this results in long‐range coherency strain and tilted epitaxy of Fe. Epitaxial growth of the rare‐earth metal Dy onto epitaxial films of LaF3 on GaAs(111) is described and the magnetic properties of epitaxial sandwich structures of LaF3/Dy/LaF3 summarized.Examples of new approaches to epitaxy of magnetic transition metals and rare earths are described. Epitaxy of bcc Fe on lattice‐matched InxGa1−xAs films is accompanied by an interfacial reaction. This is indicative of a more general problem of instability of interfaces between Ni, Fe, Co, and compound semiconductors. Such reactions can be avoided if an intermediate epitaxial film of Ag is interposed between the semiconductor and the Fe. However, this results in long‐range coherency strain and tilted epitaxy of Fe. Epitaxial growth of the rare‐earth metal Dy onto epitaxial films of LaF3 on GaAs(111) is described and the magnetic properties of epitaxial sandwich structures of LaF3/Dy/LaF3 summarized.