Soft Free Layer Research Articles

Two oscillation states in free/hard bilayered nano-pillars

The magnetization oscillation driven by spin transfer torque (STT) in a nano-pillar composed of a large in-plane anisotropy fixed layer and a soft free layer is studied. It is found that instead of frequency continuously changing with current as most nano-oscillators do, this kind of nano-oscillator can only oscillate in two stable states with specific frequencies. In each state, the frequency is almost invariant with current density. The oscillation state could be easily manipulated by the magnetization state of the free layer or an applied pulse magnetic field as the working current density is lower than a critical value (Jc). The critical current density and the frequency difference of the two states can be tuned by the saturation magnetization (Ms) of the two layers and the anisotropy constant K of fixed layer. Phase-locked oscillation is obtained in a two-nanopillar system, suggesting that it may be possible to amplify the oscillation signal by building an array of this kind of nanopillar system. This kind of STT-based nano-oscillator may have various applications in the field of spintronics.

Current-driven transverse domain wall oscillations in perpendicular spin-valve structures

Spin-transfer-driven oscillations of a transverse domain wall confined to a perpendicular spin-valve structure are investigated using a one-dimensional model. The stack consists of a polarizer, nonmagnetic spacer, soft free layer, and pinned magnetic layer. It is found that the domain-wall oscillation frequency is a nonmonotonic, highly asymmetrical function of applied electric current, showing a strong dependence on the current direction and the relative strengths of the interfacial and bulk spin-transfer torques. Micromagnetic analysis reveals that the surprising and atypical oscillator response is due to an interplay between the interfacial spin-transfer torque, the bulk spin-transfer torque, the exchange torque, and the damping torque. The underlying physical and material responses are examined, including the important role of the domain-wall twist. The competitions between the involved torques under different operating conditions suggest that the oscillator could serve as a model system to investigate magnetic and spintronic phenomena at the nanoscale. The observed current-dependent twisting of the free-layer magnetization about the axis of precession may further be found interesting for investigations of the interaction between spin-polarized current and chiral spin structures.

Phase controllable Josephson junctions for cryogenic memory

Josephson junctions containing ferromagnetic layers have generated interest for application in cryogenic memory. In a junction containing both a magnetically hard fixed layer and soft free layer with carefully chosen thicknesses, the ground-state phase difference of the junction can be controllably switched between 0 and π by changing the relative orientation of the two ferromagnetic layers from antiparallel to parallel. This phase switching has been observed in junctions using Ni fixed layers and NiFe free layers. We present phase-sensitive measurements of such junctions in low-inductance symmetric SQUID loops which simplify analysis relative to our previous work. We confirm controllable 0−π switching in junctions with 2.0 nm Ni fixed layers and 1.25 nm NiFe free layers across multiple devices and using two SQUID designs, expanding the phase diagram of known thicknesses that permit phase control.

Magnetic noise spectra and spin transfer torque of a magnetic tunnel junction with an exchange biased synthetic ferrimagnetic reference layer

We investigate magnetic thermal noise spectra in magnetic tunneling junction with spin transfer torque. The basic structure of the examined magnetic tunneling junction is PtMn/F 3/Ru/F 2/MgO/F 1, where F 1,2,3 are ferromagnetic layers. F 1 acts as a soft free layer, while PtMn/F 3/Ru/F 2, an exchange biased synthetic ferrimagnet structure, serves as a fixed reference layer. With the assumption of rigid fixed reference layer, the magnetic thermal noise is only related with the thermal motion of the free layer spins. However, we find additional resonance peaks in the magnetic noise spectra, which correspond to the thermal motion of the reference layer spins. Furthermore, the magnetic noise spectra show intricate field dependences, which are related with the spin dynamics of the reference layer induced by spin transfer torque.

Current Induced Switching of the Hard Layer in Perpendicular Magnetic Nanopillars

We report the spin-torque switching of the magnetization of the reference layer in a perpendicular magnetic anisotropy nanopillar with a spin-valve structure made of a combination of [Co/Pd] and [Co/Ni] multilayers. Around critical current, the switching is random, but becomes deterministic for larger currents. The threshold current is different for the two current directions, which shows that heating is not the main driving force but the spin torque interactions. A possible memory application is to store the magnetic bit in the magnetization of the hard magnetic reference layer and use the soft free layer to read and write it.

Magnetic tunnel junctions with large tunneling magnetoresistance and small saturation fields

There is a continuing need for greater sensitivity in magnetic tunnel junction (MTJ) sensors. We have found a new approach to achieving large tunneling magnetoresistance (TMR) with a very soft free layer. The high TMR is achieved by conventional means of annealing a bottom pinned MTJ that has Ta and Ru capping layers. The soft free layer is achieved by etching almost to the MgO tunnel barrier and depositing a thick soft magnetic film. The results are far superior to annealing the MTJ with the thick soft layer already deposited.