Magnetic Nanopillars Research Articles

Influence of capping layer on the current-induced magnetization switching in magnetic nanopillars

Taking into account the thermal effect, we verify that the strong spin relaxation in the capping layer of a magnetic nanopillar significantly affects the current-induced magnetization switching behavior by reducing the critical switching current density. Theoretical calculations reveal that increasing the spin relaxation in the capping layer enhances the spin-polarized current while suppresses the contribution of the spin accumulation to the magnetization switching. The results suggest that the spin-polarized current, rather than the spin accumulation, dominates the current-induced magnetization switching.

Large-Scale Fabrication of Periodic Nanostructured Materials by Using Hexagonal Non-Close-Packed Colloidal Crystals as Templates

This letter reports a versatile nonlithographic technique for mass fabricating three types of technologically important materials-polymer microwell arrays, 2D-ordered magnetic nanodots, and semiconductor nanopillar arrays, each with high crystalline qualities and wafer-scale sizes. Spin-coated hexagonal non-close-packed silica colloidal crystals embedded in a polymer matrix are used as starting templates to create 2D polymeric microwell arrays. These through-hole arrays can then be used as second-generation templates to make periodic magnetic nanodots and semiconductor nanopillars. This self-assembly approach is compatible with standard semiconductor microfabrication, and complex micropatterns can be created for potential device applications. The wafer-scale technique may find important applications in biomicroanalysis, high-density magnetic recording media, and microphotonics.

Influence of top electrode on the current-induced magnetic switching in magnetic nanopillars

Magnetic nanopillars with variable top electrodes were fabricated to clarify the roles of the spin current and the spin accumulation in the current-induced magnetic switching. The critical switching current is significantly increased when the size of the top electrode is comparable to that of the nanopillar. This result implies that the dominant contribution in the current-induced magnetic switching is not the spin accumulation, but the spin current.

Roles of spin-polarized current and spin accumulation in the current-induced magnetization switching

To clarify the contributions of spin-polarized current and spin accumulation to the current-induced magnetization switching, the effects of the top electrode size of the magnetic nanopillar are investigated both theoretically and experimentally. Theoretical calculation demonstrates that the spin-polarized current and the spin accumulation can be adjusted in opposite directions by modifying the size of the top electrode. Increase in the size of the top electrode suppresses the spin accumulation but enhances the spin-polarized current inside the nanopillar. On the other hand, it is shown experimentally that the nanopillar with a wide top electrode exhibits small critical switching current compared to the nanopillar with a narrow top electrode. The results suggest that the spin-polarized current contributes to the current-induced magnetization switching dominantly over the spin accumulation.

Reversal modes of simulated iron nanopillars in an obliquely oriented field

Stochastic micromagnetic simulations are employed to study switching in three-dimensional magnetic nanopillars exposed to highly misaligned fields. The switching appears to proceed through two different decay modes, characterized by very different average lifetimes and different average values of the transverse magnetization components.

Static, periodic, and chaotic magnetization behavior induced by spin-transfer torque in magnetic nanopillars

The magnetization dynamics of a square magnet in a pillar structure under the influence of a dc spin current is studied by micromagnetic simulation based on the modified Landau-Lifshitz-Gilbert equation. Even in the situation with neither external field perturbation nor thermal activation, the magnetization exhibits intricate behaviors ranging from static configurations to periodic domain wall rotations to chaotic processes. It arises from the nonlinearity of the spin-transfer torque and is explained by the excitation of nonlinear spin waves in the magnet.

Non-lithographic Nanofabrication Using Porous Alumina Membranes

ABSTRACTWe describe a fabrication method to prepare highly ordered Si nanopore arrays. A nanoporous alumina template of thickness ∼1μm is prepared by means of anodization of an aluminum film. The template has a highly ordered hexagonal array of pores of diameter ∼50nm. The template is detached from the aluminum layer and placed on a Si substrate. The nanoporous pattern is transferred onto silicon substrate by means of a dry plasma etch process. This produces an array of nanopores in silicon with a diameter of ∼50nm and depth of ∼300nm. We have used such an array to prepare Fe nanopillars inside the pores by means of thermal evaporation. Magnetization versus applied magnetic field measurements for the Fe nanoarrays, demonstrate large perpendicular anisotropy typical of high aspect ratio magnetic nanopillars. The value of coercivity is about 500Oe in the perpendicular direction and 40Oe in the parallel direction.

Subnanosecond magnetization reversal in magnetic nanopillars by spin angular momentum transfer

Sub-ns magnetization switching has been triggered by spin momentum transfer in pulsed current in pillar shaped CoFe∕Cu∕CoFe trilayers. By analyzing the change in magneto-resistance induced after the application of individual short current pulses (100ps–10ns), we measured the probability of magnetization reversal as a function of the current pulse magnitude, polarity and duration, at various temperatures between 150 and 300K. At all studied temperatures, the reversal process can take place within a few 100ps. The energy cost of the reversal scales favorably with the switching speed and decreases in the 1pJ range when using 100ps current pulses at 300K. Significantly higher switching speeds are obtained at lower temperatures, which is opposite to a thermal activation of the reversal.

Ultra-fast magnetization reversal in magnetic nano-pillars by spin-polarized current

We study the speed limitations of the magnetization switching resulting from spin transfer in pillar-shaped CoFe/Cu/CoFe spin valves. The quasi-static critical currents are Ic-=-2mA for the antiparallel (AP) to parallel (P) configuration and Ic+=+4.6mA for the P to AP transition. Current pulses of duration down to 100ps and amplitude of 4Ic trigger switching at 300K. The switching is probabilistic for lower current pulses. The P to AP transition speed is not much temperature dependant from 50 to 300K. In contrast, the AP to P transition is thermally inhibited and is much faster at 150K than at 300K. This thermal inhibition highlights the importance of the macrospin coherency and of the thermally excited spin waves with finite wave vector parallel to the magnetization. Our results validate spin-transfer switching for fast memory applications.

Controlled Normal and Inverse Current-Induced Magnetization Switching and Magnetoresistance in Magnetic Nanopillars

By combining pairs of ferromagnetic metals with the same or different signs of scattering anisotropies in ferromagnetic-nonmagnetic-ferromagnetic metal nanopillars, we independently invert just the magnetoresistance, just the direction of current-induced magnetization switching, or both together, at room temperature (295 K) and at 4.2 K. In all cases studied, the switching direction is correctly predicted from the net scattering anisotropy of the fixed ferromagnet, including both bulk and interfacial contributions.

Current-driven switching in magnetic multilayer nanopillars (invited)

We summarize our recent findings on how the current-driven magnetization switching in nanofabricated magnetic multilayers is affected by an applied magnetic field, changes of temperature, magnetic coupling between the ferromagnetic layers, variations in the multilayer structure, and the relative rotation of the layers’ magnetizations. We show how these results can be interpreted with a model describing current-driven excitations as an effective current-dependent magnetic temperature.

Substantial reduction of critical current for magnetization switching in an exchange-biased spin valve.

Great interest in current-induced magnetic excitation and switching in a magnetic nanopillar has been caused by the theoretical predictions of these phenomena. The concept of using a spin-polarized current to switch the magnetization orientation of a magnetic layer provides a possible way to realize future 'current-driven' devices: in such devices, direct switching of the magnetic memory bits would be produced by a local current application, instead of by a magnetic field generated by attached wires. Until now, all the reported work on current-induced magnetization switching has been concentrated on a simple ferromagnet/Cu/ferromagnet trilayer. Here we report the observation of current-induced magnetization switching in exchange-biased spin valves (ESPVs) at room temperature. The ESPVs clearly show current-induced magnetization switching behaviour under a sweeping direct current with a very high density. We show that insertion of a ruthenium layer between an ESPV nanopillar and the top electrode effectively decreases the critical current density from about 10(8) to 10(7) A cm(-2). In a well-designed 'antisymmetric' ESPV structure, this critical current density can be further reduced to 2 x 10(6) A cm(-2). We believe that the substantial reduction of critical current could make it possible for current-induced magnetization switching to be directly applied in spintronic devices, such as magnetic random-access memory.

Current-driven excitations in magnetic multilayer nanopillars from 4.2 K to 300 K

We report direct experimental evidence of an energy threshold for spin-transfer-induced magnetic excitations in Co/Cu/Co nanopillars from temperature dependent measurements of current-induced excitations. The current threshold of excitations decreases when the temperature is increased from 4.2 to 300 K. However, the product of the current threshold and the pillar resistance stays almost temperature independent up to a temperature of $\ensuremath{\sim}150\mathrm{K}.$ This behavior supports the energy-threshold mechanism for spin-transfer-induced magnetic excitations.

Quantitative study of magnetization reversal by spin-polarized current in magnetic multilayer nanopillars.

We have studied magnetic switching by spin-polarized currents and also the magnetoresistance in sub-100-nm-diam thin-film Co/Cu/Co nanostructures, with the current flowing perpendicular to the plane of the films. By independently varying the thickness of all three layers and measuring the change of the switching currents, we test the theoretical models for spin-transfer switching. In addition, the changes in the switching current and magnetoresistance as a function of the Cu layer thickness give two independent measurements of the room-temperature spin-diffusion length in Cu.

Fabrication of planar quantum magnetic disk structure using electron beam lithography, reactive ion etching, and chemical mechanical polishing

A planar quantum magnetic disk (QMD) with a magnetic storage density of 65 Gbit/in.2, over two orders of magnitude greater than the state-of-the-art magnetic storage density, has been fabricated. The planar QMD structure consists of single-domain nickel (magnetic) nanopillars uniformly embedded in a SiO2 (nonmagnetic) disk. Electron beam lithography was used to define the QMD bit’s size and location, and reactive ion etching was used to form an SiO2 template. Nickel electroplating was used to selectively deposit nickel into the template openings, and chemical mechanical polishing was used to planarize the surface. The resulting QMD consists of ultrahigh density arrays of single-domain magnetic pillars with a 50 nm diameter and 100 nm period uniformly embedded in 200-nm-thick SiO2 and with a surface roughness of 0.5 nm root mean square. Each single-domain structure has a quantized magnetic moment and acts as a single bit to store one bit of binary information. Furthermore, a method for mass production of QMDs, the nanoimprint technique, is discussed.