Manipulating Nanomagnets with a Spin-Polarized Current

By sending current densities as high as 109A/cm2 perpendicularly through ferromagnetic/nonmagnetic/ferromagnetic (F/N/F) nanostructures, we have found that the exchange coupling between the conduction electron spins and the magnetic moments can measurably alter the direction of the magnetization. We can in fact controllably switch the moments of the two magnetic layers between stable parallel and antiparallel configurations by applying a current of the appropriate sign. This phenomenon has application potential for new kinds of magnetic memory storage devices.

See Ed's APS Meeting 2002 Talk and a preprint

People involved

Edward Myers, Jack Sankey, and Dan Ralph. We collaborate closely with Buhrman Group in Applied and Engineering Physics, in particular Frank Albert, Nathan Emley, and Robert Buhrman.

Spin transfer

Magnetic multilayers, devices which consist of alternating nanometer-scale-thick films of magnetic and nonmagnetic materials, have been the focus of scientific and technological interest since the discovery of their so-called "giant" magnetoresistance over 10 years ago [1]. While the effect of the magnetic layers on the current has been widely studied, the converse phenomenon, the effect of the current on the layers themselves, has received little attention. Recent theories [2,3] predict that a large spin-polarized current (~107-109A/cm2) passing perpendicularly through the layers can exert torques on the magnetic moments in the magnetic layers, due to the exchange coupling between the conduction electron spins and the moments (spin-transfer). These torques may excite spin-waves, or even reverse magnetic domains, in the layers.

Fabrication

Using lithography techniques developed at the Cornell Nanofabrication Facility (CNF), we have observed the above spin-transfer phenomena in two distinct geometries, and are using the effect to probe the microscopic interactions between spin-polarized conduction electrons and magnetic moments. In the nanoconstriction geometry [4], electron-beam lithography and reactive ion etching are used to create a bowl shaped hole in a suspended silicon nitride membrane. The etch can be timed such that the bottom of the hole is as small as 5-10 nm. By depositing metal layers on either side of the hole, we can easily generate current densities as high as 109 A/cm2 through the hole. In a second geometry developed by Jordan Katine, Frank Albert, and Robert Burhman here at Cornell [5], a pillar-shaped nanomagnet is created by ion milling through metal layers on an insulating substrate, using a lithographically-defined mill mask. The nanopillars are approximately 100 nm in lateral extent, resulting in lower current densities than in the nanoconstriction geometry (roughly 107 A/cm2). However, the size and shape of the nanomagnets can be more precisely controlled with the nanopillar technique, facilitating a more quantitative exploration of the spin-transfer effect.


In the nanoconstriction geometry, e-beam lithography and reactive ion etching define a hole 5-10 nm in diameter, through which metallic contact can be made to a magnetic trilayer. In the nanopillar geometry a freestanding trilayer less than 100 nm in diameter is formed with leads connected to top and bottom to make a vertical transport structure. Higher current densities can be achieved in the nanoconstrictions, but magnetic structure and shape can be more closely controlled with nanopillars.

Results

We have created devices consisting of two magnetic layers separated by a nonmagnetic spacer layer; although several materials combinations have been fabricated, we have predominantly utilized cobalt and copper. If one of the magnetic layers is made much thinner than the other, the thinner layer can be controllably flipped so that the magnetic moments are either parallel or antiparallel to the thicker layer moments, simoply by choosing a current of the appropriate sign and magnitude. These two states are easily distinguished, as the antiparallel configuration has a higher resistance than the parallel configuration. The nanopillar trilayers can be switched in a stable manner at room temperature, suggesting that the effect could be used commercially as a magnetic memory element. We are currently varying the shape of the nanopillars to investigate the effect of anisotropy on the magnetic response to spin-transfer, and are trying various materials combinations to give insight into the microscopic properties that are relevant for the effect.


The magnetic layers of a Co/Cu/Co nanopillar can be controllably switched between parallel (low resistance state) and antiparallel (high resistance state) configurations by applying a current of appropriate magnitude and sign.

References


Last updated: 2001-12-02

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