| AVS 59th Annual International Symposium and Exhibition | |
| Thin Film | Wednesday Sessions |
| Session TF+MI-WeA |
| Session: | Thin Films for Memory and Data Storage |
| Presenter: | R.H. Victora, University of Minnesota |
| Authors: | R.H. Victora, University of Minnesota S.H. Hernandez, University of Minnesota T. Qu, University of Minnesota |
| Correspondent: | Click to Email |
Since the discovery of giant magnetoresistance (GMR) in 1988, spin transport has rapidly evolved as a research area examining effects such as Current Perpendicular to Plane (CPP) GMR and spin torque transfer (STT). Giant Magnetoresistance is caused by spin-dependent scattering. High electrical resistance (RAP) is measured for antiparallel magnetizations of adjacent layers, while low resistance (RP) is measured for parallel magnetizations. CPP GMR shows an advantage in MR ratio ((RAP-RP)/RP), because all electrons must pass through all layers. This geometry is widely used as the reader in high areal magnetic recording, where it is likely that the current non-magnetic insulator will ultimately be replaced by a metallic layer in order to limit resistance. The reciprocal effect, STT, occurs when an electric current passes through a pinned ferromagnetic layer and the angular momentum (magnetic moment) is transferred to a neighboring free magnetic layer. The magnetization in the free layer may stably oscillate or may achieve a collinear state to the pinned layer. Magnetization switching with the help of a current has been proposed as potential magnetoresistive random access memory (MRAM). However, the mechanism of spin transport is not fully understood for these effects.
We consider multiple reflections between the interfaces of the adjacent magnetic layers. If the ferromagnetic material is not 100% polarized, electrons with different polarizations are not perfectly transmitted or reflected. We show that reflections, although typically neglected, strongly affect the spin transport properties. They explain1 the experimentally observed nonlinearity of GMR dependence on β =cos2(θ/2 ) (θ is the angle between the magnetizations of the fixed and free layers ). Also the spin torque is decreased2 by the reflection. The more orders of reflection we include in the spin torque, the more critical current is needed to switch the magnetization state. The spin torque oscillator (STO) is an attractive replacement for current microwave devices owing to its very small (nanoscale) size. However, a single STO does not provide sufficient power for many applications. An array of oscillators in series or parallel has been proposed to generate more power. The problem is to phase lock the non-uniform oscillators. We calculate the power spectrum of serial oscillators. We show that the oscillators’ could be closely synchronized by a feedback ac current, even at room temperature.
1. T. Qu and R.H. Victora, J. Appl. Phys. 111, 07C516 (2012)
2. S. Hernandez and R.H. Victora, Appl. Phys. Lett. 97, 062506 (2010)