K. Takano

Oxygen as a surfactant in the growth of giant magnetoresistance spin valves

We have found a novel method for increasing the giant magnetoresistance (GMR) of Co/Cu spin valves with the use of oxygen. Surprisingly, spin valves with the largest GMR are not produced in the best vacuum. Introducing 5×10−9 Torr (7×10−7 Pa) into our ultrahigh vacuum deposition chamber during spin-valve growth increases the GMR, decreases the ferromagnetic coupling between magnetic layers, and decreases the sheet resistance of the spin valves. It appears that the oxygen may act as a surfactant during film growth to suppress defects and to create a surface which scatters electrons more specularly. Using this technique, bottom spin valves and symmetric spin valves with GMR values of 19.0% and 24.8%, respectively, have been produced. These are the largest values ever reported for such structures.

Magnetoresistance values exceeding 21% in symmetric spin valves

We report values of the giant magnetoresistance (GMR) effect exceeding 21% in symmetric spin valves, the highest values ever reported for such structures. The key elements in this achievement are the use of a Co/Cu/Co/Cu/Co multilayer in which the center Co layer is substantially thicker than the outer Co layers and the use of the antiferromagnetic insulator NiO at the top and bottom to pin the adjacent Co layers magnetically. The relative Co layer thicknesses suggest that some specular scattering of conduction electrons may occur at the metal/insulator interfaces and may enhance the GMR.

Role of interfacial uncompensated antiferromagnetic spins in unidirectional anisotropy in Ni81Fe19/CoO bilayers (invited)

The uncompensated spins on the surfaces of antiferromagnetic CoO films exhibit a thermoremanent magnetization after field cooling from T>TN that has the same temperature dependence as the exchange field of Ni81Fe19/CoO bilayers after field cooling. This suggests that these interfacial uncompensated spins are responsible for unidirectional anisotropy. A model based on a calculation of the density of these interfacial uncompensated spins predicts the correct magnitude of the exchange field, as well as the observed inverse dependence on average interfacial grain size.

Optimizing the giant magnetoresistance of symmetric and bottom spin valves (invited)

We have attempted to optimize the values of the giant magnetoresistance in symmetric spin valves of the type NiO/Co/Cu/Co/Cu/Co/NiO (achieving 23.4%) and in bottom spin valves of the type Co/Cu/Co/NiO (achieving 17.0%), the largest values ever reported for such structures. The key elements in this achievement are improved vacuum conditions and careful attention to the film thicknesses.

Specular electron scattering in giant magnetoresistance spin valves

Specular scattering of electrons at the top surface of bottom spin valves has been found to contribute to the giant magnetoresistance effect. The specular scattering can be increased by the deposition of /spl sim/2 monolayers (ML) of Au, Ag, or Cu, with the result that the GMR increases and the R/sub /spl square// decreases. The deposition of 2 ML of species that disorder the surface, such as Ta, Si, C, or Ni/sub 80/Fe/sub 20/, suppresses the specular scattering. This suppression appears to be useful as a new method for evaluating the importance of specular scattering.

Growth of giant magnetoresistance spin valves using Pb and Au as surfactants

We have investigated the use of Pb and Au as surfactants in an attempt to achieve smoother and sharper interfaces in three types of giant magnetoresistance (GMR) spin valve multilayers: symmetric spin valves, bottom spin valves, and top spin valves. The coupling fields are reduced by a factor of 10 for symmetric and bottom spin valves and by a factor of 3 for top spin valves, presumably by suppressing roughness and interdiffusion at the Co/Cu/Co interfaces, when ∼1 monolayer of Pb is deposited in the early stages of spin valve growth. The Pb has a strong tendency to float out or segregate to the surface during deposition of the spin valve leaving the GMR largely unaltered. Au is almost as effective as Pb, however the Au tends to be left behind in the spin valve, and the GMR is reduced slightly. Attempts to use Hg as a surfactant were unsuccessful. The coupling field increased, and the GMR decreased sharply.

The trade‐off between large magnetoresistance and small coercivity in symmetric spin valves

We have investigated the use of various alloys as substitutes for pure Co in the center film of symmetric spin valves of the type NiO/Co/Cu/Co/Cu/Co/NiO. The aim of this work is to identify magnetic materials that exhibit smaller coercivities than pure Co for the center or ‘‘valve’’ film but which retain much of the giant magnetoresistance associated with a pure Co film. The materials investigated include Co95Fe5, Co90Fe10, Ni80Fe20, Co86Fe10.5Ni3.5, and Co85B15. It appears that each of these alloys scatters electrons more strongly than does pure Co as they cross the center film. This scattering degrades the dual spin‐valve effect, which is the primary advantage of the symmetric spin valve. As a result, a tradeoff exists between large GMR and small coercivity when using these materials.

Spin-flop tendencies in exchange-biased Co/CoO thin films

In order to study the antiferromagnetic (AFM) spin structure near the interface of exchange-biased bilayers, polarized neutron diffraction measurements were performed on a series of (111)u2009Cou2009(7.5 nm)/CoO (X nm) and CoO (X nm) thin films where X=20, 40, and 100 nm. In these samples, field cooling through the Neel temperature of the AFM increases the component of the CoO moment perpendicular to the applied field, relative to the parallel component. The subsequent application of a 500 Oe field perpendicular to the cooling direction rotates both the Co and CoO moments. Experiments on CoO films without Co showed a smaller difference between the parallel and perpendicular CoO moments in response to cooling and applied fields. Exchange coupling between the Co and CoO layers is apparently responsible for the increased projection of the AFM moments perpendicular to the cooling field.

Specular Electron Scattering In GMR Spin Valves

Specular electron scattering is an issue of great importance for the field of giant magnetoresistance (GMR) spin valves.[ I] It is generally agreed that simple spin valves (one Cu layer) could, in principle, exhibit GMR values as large those found in GMR superlattices if both the top and bottom surfaces of the simple spin valve were perfectly specular. The largest roomtemperature GMR valuesever reported are 80% for a Co/Cu superlattice [2] and 17% for a simple Co/CulCo spin valve[3]. Clearly there is room for major improvement in simple spin valves.

Mössbauer study of the interfacial environment of ferromagnetic atoms exchange coupled to CoO (abstract)

Although unidirectional anisotropy was first observed over 40 years ago in surface oxidized Co particles, the basic physics of the mechanism remains elusive. Unidirectional and exchange anisotropies are observed when ferromagnetic (FM) and antiferromagnetic (AFM) materials are coupled to each other. Because of the interfacial nature of the coupling, the exchange strength is sensitive to the interfacial environment. The simple model of an epitaxial atomically smooth interface predicts exchange field strengths two orders of magnitude larger than the observed values. Cross-sectional pictures of NixCo(1−x)O/permalloy exchange couples by transmission electron microscopy (TEM) reveal interfacial complexities such as roughness and strain gradients that could account for the reduction of the interfacial exchange strength. In this work, we studied the interfacial environment of the first few atomic layers on the FM side. We deposited the Mossbauer isotope 57Fe as an interlayer of variable thickness (4⩽t⩽16 A) betw...