Single Spin Valves Research Articles

Specularity in GMR spin valves and in situ electrical and magnetotransport measurements

An experimental four-point probe for in situ electrical and magnetotransport measurements during film growth is described. This method is unique in that it can capture thickness-dependent evolution of not only conductance but also magnetoresistance in a UHV deposition chamber. It has been used to monitor the growth of single layer, spin valve and multilayer structures and to measure specularity and interface scattering, both spin-independent and spin-dependent. Addition of /spl sim/1 monolayer of Co to Cu surface causes the net film conductance to decrease sharply, but the reverse case of Cu on Co shows a strong positive curvature of the thickness-dependent conductance. Most of the observed interface scattering may be intrinsic, arising from a high density of empty d-states at Cu boundaries that result in increased electron scattering. It is confirmed that exposing a spin valve or metal thin film to oxygen for an appropriate time may lead to an appreciable enhancement of specularity of its top surface. This leads to an increase in the sheet conductance, much more so when the free layer and pinned layer magnetizations are parallel than antiparallel. In contrast, previously reported specularity enhancement due to noble metal overlayers has not been observed by the in situ experiments.

PtMn single and dual spin valves with synthetic ferrimagnet pinned layers

Spin valve films with the PtMn/Co/Ru/Co synthetic ferrimagnet pinned layer, in which the magnetization of the two Co layers is strongly coupled in an antiparallel orientation and the magnetization direction of one of the Co layers is pinned by unidirectional exchange coupling with PtMn, were investigated. PtMn synthetic ferrimagnet pinned layers were demonstrated to exhibit strong unidirectional exchange field Hex exceeding 2500 Oe at an optimum PtMn/Co/Ru/Co thickness combination for either PtMn on top or at the bottom. The Hex of PtMn/Co/Ru/Co rapidly increased at the PtMn layer thickness around 10–12 nm and was nearly constant from 12 to 30 nm. It was also found that the saturation field of the Co(3 nm)/Ru(0.8 nm)/Co(2 nm) sandwich, which represents the antiparallel interlayer coupling strength through Ru, remained 2200 Oe even at 300 °C. Thus the combination of the high blocking temperature of 380 °C for PtMn and the large antiparallel exchange interaction of Co/Ru/Co up to 300 °C makes the pinned layer highly stable against various magnetic fields during Joule heating of spin valve sensor stripes. Further, the PtMn synthetic-ferri-pinned-type dual spin valve films showed a large unidirectional exchange field of 2000 Oe and a giant magnetoresistance ratio of 11.0%.

Spin valve and dual spin valve heads with synthetic antiferromagnets

Analytical and micromagnetic analysis have been performed for synthetic antiferromagnet (SAF) biased single and dual spin valve heads. It is found that for a un-pinned SAF under an external field, the antiparallel axis will flop to the direction orthogonal to the field direction. Introducing a thickness differential in the SAF can help to prevent the magnetization flop in addition to the exchange pinning. Design principles for SAF biased single and dual spin valve heads are presented. An SAF biased dual spin valve head with appropriate design can have significant advantages over the single spin valve besides higher GMR values.

Top, bottom, and dual spin valve recording heads with PdPtMn antiferromagnets

We fabricated top-type (Substrate/free layer/Cu/pinned layer/antiferromagnetic layer) and bottom-type (Sub./antiferro/pinned/Cu/free) spin valve heads using PdPtMn antiferromagnets. Bottom type spin valve head with a buffer layer of Ta/NiFe showed similar exchange bias field and MR ratio as top-type spin valve head. Dual spin valve (DSV) films with two pinning layers of PdPtMn had an enhanced MR ratio by 40% compared to the top-type spin valve. Patterned DSV elements with 3 /spl mu/m MR height showed relatively large deviation from the optimum bias state. In order to adjust the bias point, asymmetric DSV elements with Cu layers of different thicknesses were investigated. Asymmetric DSV with the structure of Ta/NiFe/PdPtMn/CoFe/Cu(t/sub 1/)/CoFe/NiPe/CoFe/Cu(t/sub 2/)/CoFe/PdPtMn (t/sub 1//spl ne/t/sub 2/) showed an improvement for just bias using sense current magnetic field. The possibility of reducing the PdPtMn critical thickness for exchange coupling was investigated using ultra high vacuum (UHV) deposition environment. UHV technique is significantly useful to obtain exchange bias field (H/sub ua/) of PdPtMn(t)/ferro at t<15 nm. The thickness of pinning PdPtMn was reduced from 25 nm to 13 nm and we fabricated single spin valve with total thickness of 26.5 nm which exhibited large MR ratio over 8.0%, sufficient H/sub ua/ (>500 Oe), large sheet resistance change (/spl Delta//spl rho///sub total/=1.6 /spl Omega/), and good thermal stability up to 300/spl deg/C. The total thickness of DSV was also reduced from 80 to 53 nm. Single and dual spin valves with thin PdPtMn pinning layer are very promising sensors for ultra high density recording applications.

Design and modelling of dual spin-valve heads

Two types of dual spin-valve head structure, the anti-parallel and parallel dual spin-valve heads, were proposed. Both dual spin-valve heads are thermal noise immune due to differential voltage sensing. The transfer functions of the two dual structures were analysed and transfer curves of shielded heads were simulated using a 3-D finite element method. Results have shown that the anti-parallel dual spin-valve head exhibits greater linear dynamic range than a single spin-valve head and almost doubled output of a single spin-valve head. It also has an ideal transfer curve passing through the origin independent of the bias state of single spin-valves.

High sensitive magnetoresistance in evaporated spin valves with growth-induced uniaxial anisotropy

A highly sensitive magnetoresistance behavior related to the well defined induced uniaxial anisotropy has been observed in a single spin valve deposited with conventional electron beam evaporation. An external magnetic field of 20 Oe (deposition field) was applied during the deposition to induce the anisotropy, which resulted in a marked increase of the MR ratio. Magnetoresistence changes of 4.1% (improved from 3.1% due to the application of the deposition field) for NiFe/Cu/Co and 6.3% for NiFe/Co/Cu/Co were obtained in a micro strip pattern prepared by an Ar ion milling through photoresist mask. A hysteresis free linear MR behavior has been realized in the patterned samples.

Novel magnetoresistance behavior in single trilayer spin valves

Single trilayer spin valves consisting of uncapped FeNi(60 Å)/Cu(60 Å)/Co(40 Å) have been grown using ultrahigh vacuum (UHV) electron beam evaporation methods. Room temperature magnetoresistance (MR) measurements exhibit well defined plateaux corresponding to the full antiparallel alignment of the FeNi and Co magnetizations. This sharp switching behavior and precisely defined antiferrimagnetic state is in contrast with the corresponding behavior reported for FeNi/Cu/Co multilayers.