Magnetic Nanopillars Research Articles

Current-induced magnetization dynamics in single and double layer magnetic nanopillars grown by molecular beam epitaxy

Molecular beam epitaxy is used to fabricate magnetic single and double layer junctions, which are deposited in prefabricated nanostencil masks. For all Co | Cu | Co double layer junctions we observe a stable intermediate resistance state, which can be reached by current starting from the parallel configuration of the respective ferromagnetic layers. The generation of spin waves is investigated at room temperature in the frequency domain by spectrum analysis, demonstrating both in-plane and out-of-plane precessions of the magnetization of the free magnetic layer. Current-induced magnetization dynamics in magnetic single layer junctions of Cu | Cu | Cu has been investigated in magnetic fields, which are applied perpendicular to the magnetic layer. We find a hysteretic switching in the current sweeps with resistance changes significantly larger than the anisotropic magnetoresistance effect.

Spin transfer induced coherent microwave emission with large power from nanoscale MgO tunnel junctions

In low resistance-area product MgO magnetic tunnel junction nanopillars, we observe high integrated power (up to 43nW), narrow linewidth (down to 10MHz), spin transfer induced microwave emission at frequencies up to 14GHz due to precession of the free layer magnetization at room temperature. Although all devices were fabricated on the same wafer, they present bimodal transport and precessional characteristics. The devices in which the narrowest linewidths were observed exhibited low resistance and tunneling magnetoresistance (30%), while maintaining large integrated power.

Thermal dynamics in symmetric magnetic nanopillars driven by spin transfer

We study the effects of spin transfer on thermally activated dynamics of magnetic nanopillars with identical thicknesses of the magnetic layers. The symmetric nanopillars exhibit anomalous dependencies of switching statistics on magnetic field and current. We interpret our data in terms of simultaneous current-induced excitation of both layers. We also find evidence for coupling between the fluctuations of the layers due to the spin transfer.

Effect of vortex handedness on spin momentum torque dynamics in dual-vortex ferromagnetic nanopillar structures

An investigation of the dynamics of vortices driven by spin-momentum transfer in magnetic tunnel junction nanopillars containing a vortex in the hard ferromagnetic pinned layer (PL) and a vortex in the soft ferromagnetic free layer (FL) is presented. This dual vortex configuration is interesting because the handedness of the vortex in the PL can be set so that the SMT is either assisted or opposed by the torque due to the Amperean magnetic field produced by the current passing through the nanopillar. It is shown that the handedness of the vortex in the PL controls the dynamics of the nanopillar device. Micromagnetic simulations of the three-dimensional nanopillar structures were performed as a function of PL vortex handedness, spin polarization (η), and nanopillar dimensions. Generally, for positive η, it is found that when the PL vortex is set counter-clockwise (CCW), the FL vortex shows a well-defined switching behavior, where the handedness of the final vortex state in the FL is dependent on the current direction through the nanopillar, and the switching current is decreased as η is increased. In devices where the PL is magnetized clockwise (CW), the FL magnetization dynamics show a more complicated dependence on η. The CW magnetized PL nanopillars show high-current Amperean field-induced vortex switching at low η, chaotic oscillation at intermediate η, and well-defined low-current switching at high η. For negative η, the CCW and CW PL results invert.

Domain nucleation mediated spin-transfer switching in magnetic nanopillars with perpendicular anisotropy

Spin-transfer-driven switching is investigated by micromagnetic simulation in perpendicular spin valve nanopillars with a free layer structure which contains two in-film-plane regions: a main perpendicularly magnetized hard region and a soft nanocore with intrinsic in-plane anisotropy. The temporal magnetization snapshots demonstrate that the current-induced magnetization rotation starts from the nanocore, followed by an incoherent switching process mediated by domain nucleation and expansion. The initial magnetization rotation of nanocore to in-plane direction generates driving force acting on the hard region via exchange coupling, together with locally enhanced spin torque, leading to considerable reduction in both critical current and switching time.

Evolution of magnetic pillars using 200 MeV Ag15+ ion irradiation

We have studied the evolution of micro and nano magnetic pillars, created by 200MeV Ag15+ ion irradiation on the surface of Mg0.95Mn0.05Fe2O4 thin films, using X-ray diffraction, atomic force and dc magnetization. Thin films of Mg0.95Mn0.05Fe2O4 were deposited using pulsed laser deposition technique on platinum coated silicon (Pt–Si) substrate. The X-ray diffraction study clearly indicates that the evaluated magnetic pillars have single phase spinel cubic structure. From dc magnetization hysteresis loop, it is observed that the film deposited on Pt–Si exhibits a ferrimagnetic ordering at room temperature and the saturation magnetization increases with fluence value up to 5×1011ions/cm2 which may be due to grain growth. However, with further increase in fluence value, the magnetization decreases, but this value is still larger than that of as-deposited film. The average size of magnetic pillars calculated from atomic force microscopy is found to be ∼63 and ∼121nm at a fluence of 1×1011ions/cm2 and then increases up to ∼136nm at a fluence of 5×1011ions/cm2 and finally, attains a value of ∼107nm at a fluence of 1×1012ions/cm2. The formations of these magnetic nano-pillars are explained on the basis of metallic catalyst assisted growth after SHI irradiation.

Spin-current pulse induced switching of vortex chirality in permalloy∕Cu∕Co nanopillars

Dynamic response of the vortex magnetization in multilayered magnetic nanopillars to the spin-polarized current pulse has been investigated numerically. The equilibrium magnetization configurations in both magnetic layers are the vortex states with single magnetization cores at the disk center. It was found that the chirality of the vortex state in magnetic free layer can be controllably switched by applying current pulse with appropriate amplitude, polarity, and duration. The critical current density required for the chirality switching is found to be on the order of 108A∕cm2.

Micromagnetic Investigation of Precession Dynamics in Magnetic Nanopillars

This paper interprets and reproduces, by means of full micromagnetic simulations, the pioneering experimental data on magnetization dynamics driven by spin polarized current of the experiment by Kiselev The effect of the spatial dependence of the polarization function together with either nonuniform magnetostatic coupling from the fixed layer and classical Ampere field are shown to play a fundamental role in the magnetization dynamics. A detailed study of the stable magnetization self-oscillations shows that for high field and high current regimes, the dynamics is localized in the sides of the structure, where the energy dissipated by damping and the energy provided by the spin flow compensate exactly.

Dynamic Write/Read Characteristics of Alumina Nanohole Patterned Media With a Soft Underlayer Measured With a Perpendicular Magnetic Head

We present dynamic write/read characteristics of alumina nanohole patterned media having a soft magnetic underlayer (SUL), which offer good flyability of a perpendicular magnetic head with a flight height of 10 nm. Increase in the head-field gradient in the recording layer owing to the SUL was confirmed by measurements of signal-to-noise ratio and isolated magnetic transition width. The writability of the magnetic nanopillars was clearly demonstrated by visualization of the recorded patterns with a magnetic force microscope

Theory of spin current in magnetic nanopillars for zero-field microwave generation

In a magnetic nanopillar, microwave oscillations of the magnetization of one magnetic layercan be driven by spin-polarized current emitted from another magnetic layer. Theconditions for this to occur in zero applied field are formulated in terms of the twocomponents of the spin-transfer torque. One simple route to achieve microwave generationis to ensure that these components have opposite sign. Quantum-mechanical calculationsare presented that show how this may be achieved by a suitable choice of the oscillatingmagnet thickness.

Magnetization dynamics driven by spin-polarized current in nanomagnets

In this report, micromagnetic simulations of magnetization dynamics driven by spin-polarized currents (SPCs) on magnetic nanopillars of permalloy/Cu/permalloy with different rectangular cross-sections are presented. Complete dynamical stability diagrams from initial parallel and antiparallel states have been computed for 100 ns. The effects of a space-dependent polarization function together with the presence of magnetostatic coupling from the fixed layer and classical Ampere field have been taken into account.

Manipulating Spin Current in the Magnetic Nanopillar

Because of the capability to switch the magnetization of a nanoscale magnet, the spin transfer effect is critical for the application of magnetic random access memory. For this purpose, it is important to enhance the spin current carried by the charge current. Calculations based on the diffusive spin-dependent transport equations reveal that the magnitude of spin current can be tuned by modifying the ferromagnetic layer and the spin relaxation process in the device. Increasing the ferromagnetic layer thickness is found to enhance both the spin current and the spin accumulation. On the other hand, a strong spin relaxation in the capping layer also increases the spin current but suppresses the spin accumulation. To demonstrate the theoretical results, nanopillar structures with the size of approximately 100 nm are fabricated and the current-induced magnetization switching behaviors are experimentally studied. When the ferromagnetic layer thickness is increased from 3 nm to 20 nm, the critical switching current for the current-induced magnetization switching is significantly reduced, indicating the enhancement of the spin current. When the Au capping layer with a short spin-diffusion length replaces the Cu capping layer with a long spin-diffusion length, the reduction of the critical switching current is also observed.

Temperature dependence of intrinsic switching current of a Co nanomagnet

The temperature dependence of the switching current of a magnetic nanopillar is investigated from 10to290K. According to the switching probability measurement with the pulsed current, and the differential resistance measurement with sweeping the dc current, the intrinsic switching currents increase with decreasing the temperature. Transport calculations show that this temperature dependence is closely related to the reduced spin accumulation and spin polarization of the electrical current at low temperatures, attributed to the varied transport parameters. The conclusion is in accordance with the temperature dependence of the resistance difference between antiparallel and parallel magnetic configurations.

Spin transfer in bilayer magnetic nanopillars at high fields as a function of free-layer thickness

Spin transfer in asymmetric Co-Cu-Co bilayer magnetic nanopillars junctions has been studied at low temperature as a function of free-layer thickness. The phase diagram for current-induced magnetic excitations has been determined for magnetic fields up to $7.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ applied perpendicular to the junction surface and free-layers thicknesses from $2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The junction magnetoresistance is independent of thickness. The critical current for magnetic excitations decreases linearly with decreasing free-layer thickness, but extrapolates to a finite critical current in the limit of zero thickness. The limiting current is in quantitative agreement with that expected due to a spin-pumping contribution to the magnetization damping. It may also be indicative of a decrease in the spin-transfer torque efficiency in ultrathin magnetic layers.

Effect of asymmetric leads on critical switching current in magnetic nanopillars

Manschot et al. [Appl. Phys. Lett. 85, 3250 (2004)] predicted that the critical current to switch the magnetizations in a ferromagnetic/nonmagnetic/ferromagnetic nanopillar from parallel to antiparallel could be reduced by up to a factor of 5 by pairing nonmagnetic leads with different effective resistances (resistivity times spin diffusion length). Comparing switching currents for Co∕Cu∕Co nanopillars with Pt and AgSn(5%) leads on alternate sides of the nanopillar did not give the large reduction predicted by Manschot et al. Possible reasons for this lack are discussed.

Spins on the move

Sankey and co-workers are interested in magnetic nanopillars that consist of two ultrathin ferromagnetic layers separated by a non-magnetic spacer. The resistance of such a structure depends on the relative orientation of the magnetization in the magnetic layers – a phenomenon known as magnetoresistance – and the device resistance will oscillate in time if the magnetization in either layer is made to precess.

Current-induced switching in single ferromagenetic layer nanopillar junctions

Current-induced magnetization dynamics in asymmetric Cu∕Co∕Cu single magnetic layer nanopillars has been studied experimentally at room temperature and in low magnetic fields applied perpendicular to the thin film plane. In sub-100nm junctions produced using a nanostencil process a bistable state with two distinct resistance values is observed. Current sweeps at fixed applied fields reveal hysteretic and abrupt transitions between these two resistance states. The current induced resistance change is 0.5%, five times greater than the anisotropic magnetoresistance effect. We present an experimentally obtained low field phase diagram of current-induced magnetization dynamics in single ferromagnetic layer pillar junctions.

Micromagnetic simulations of nanosecond magnetization reversal processes in magnetic nanopillar

In this paper we study by means of the spin torque model the fast switching behavior of the Co(20nm)∕Cu(5nm)∕Co(2.5nm) magnetic multilayers of two different cross sections: ellipse (130×70nm2) and ellipse (130×40nm2). Simulations have been performed at zero and room (300K) temperatures, these point out that the magnetization inversion occurs by nucleation processes in three main steps, for both structures. In particular, for zero temperature the third step of the switching depends on the value of the spin-polarized current. Furthermore, for all of the simulated currents the switching processes are thermally activated and smoother with respect to zero temperature.

Micromagnetic simulation of spin transfer torque switching by nanosecond current pulses

Magnetization reversal by spin transfer torque due to current flowing in magnetic tunnel junction nanopillar with 200nm×125nm elliptical cross section is studied by micromagnetic simulation assuming the Slonczewski model for spin transfer torque. The magnetization dynamics during the switching is found to be highly inhomogeneous with formation of complex domain structure and spin wave generation even for artificially increased exchange length. The Oersted field influence on onset of precession leading to switching is described in terms of spin wave generation. The switching time as a function of current is found to have nonmonotonic behavior, which is not completely understood but is found to be related to the presence of Oersted field.

Spin-transfer-induced excitations in bilayer magnetic nanopillars at high fields: The effects of contact layers

Current-induced excitations in bilayer magnetic nanopillars have been studied with large magnetic fields applied perpendicular to the layers at low temperature. Junctions investigated all have Cu∕Co∕Cu∕Co∕Cu as core layer stacks. Two types of such junctions are compared, one with the core stack sandwiched between Pt layers (type A), the other with Pt only on one side of the stack (type B). Transport measurements show that these two types of junctions have similar magnetoresistance and slope of critical current with respect to field, while A samples have higher resistance. The high-field bipolar excitation, as was previously reported [Özyilmaz et al., Phys. Rev. B 71, 140403(R) (2005)], is present in B samples only. This illustrates the importance of contact layers to spin-current-induced phenomena. This also confirms a recent prediction on such spin-wave excitations in bilayers.