Field-free synthetic-ferromagnet spin torque oscillator

Abstract

We study the magnetization dynamics of spin valve structures with a free composite synthetic ferromagnet (SyF) that consists of two ferromagnetic layers coupled through a normal metal spacer. A ferromagnetically coupled SyF can be excited into dynamical precessional states by an applied current without external magnetic fields. We analytically determine the stability of these states in the space spanned by the current density and SyF interlayer exchange coupling. Numerical simulations confirm our analytical results. The transfer of angular momentum between the magnetic layers of current-driven spin valves (spin-transfer torque) has not been so long ago predicted 1,2 and experimentally confirmed.3,4 The implied efficient electrical control of magnetizations motivated the pursuit of new research directions. When the current density exceeds a critical value, the spintransfer torque can switch the magnetization to a different static configuration without the necessity of applied magnetic fields, which makes it attractive for next generation magnetoresistive random access memory (MRAM) application. 3,[5][6][7] Under an external magnetic field, the spin-transfer torque can also drive the magnetization into sustainable coherent oscillations spanning a wide frequency range from a few MHz to several hundred GHz.3,5-11 High frequency magnetic oscillations generate a coherent microwave voltage signal through the giant magnetoresistance (GMR) in metallic spin valves or through the tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs). This effect can be used in so-called spin-torque oscillators (STO), which has many advantages including wide tunability, 12 very high modulation rates, 13,14 compact device size, and high compatibility with standard CMOS processes. 15,16 Thus STO is appealing for high frequency microwave applications including microwave emitters, modulators, and detectors.17 However, the necessity of an applied magnetic field up to ∼1 T has severely limited the potential of these STOs for microwave generation and wireless communication applications. Recently, various solutions have been proposed to enable zero-field operation, viz., STO with a perpendicularly magnetized fixed 18 or spin valves with out-of-plane magnetized free layer, 19,20 magnetic vortex oscillators, 21-27 wavy-torque STO by judicially choosing free and fixed layer materials with different spin diffusion lengths, 28 and a tilted magnetization of the fixed layer with respect to the film plane. [29][30][31][32][33] Recently, synthetic ferromagnets (SyFs) composed of two ferromagnetic layers separated by a very thin nonmagnetic spacer have been used to replace the free layer of a spin valve or MTJ. [34][35][36][37][38][39][40] SyF based spintronic devices have the advantage of higher thermal stability, smaller stray magnetic fields, faster switching speed, and reduced threshold switching current as compared to single ferromagnetic free layers. [34][35][36][37][38][39][40] Klein et al. 40 predicted that an antiferromagnetically co...

Ultrafast Spin-Transfer-Torque Switching of Synthetic Ferrimagnets

“…Synthetic antiferromagnets have been of special interest due to the inherent advantages of antiferromagnets such as stray-field free magnetic stacks that can potentially operate at THz frequencies [6][7][8]. Spin-torque nano oscillators based on synthetic antiferromagnets have been proposed theoretically [9,10]. In addition, spin-transfer-torque driven magnetic structures comprising high and low magnetic anisotropy materials have been theoretically analyzed [11,12] and experimentally investigated [13][14][15].…”

Current-Induced Pinwheel Oscillations in Perpendicular Magnetic Anisotropy Spin Valve Nanopillars

“…where A is the dynamic parameter which governs the switching rate, I is current and I c0 is the zero temperature critical current. Equation (1) has been shown experimentally to describe the reversal of spin-torque devices with PMA layers [14]. The value of I c0 can be estimated for both the free and reference layers…”

Current-induced spin-wave excitation in Pt/YIG bilayer

“…The spatially dependent dynamics of the magnetization unit vector m(r,t) is described by the Landau-Lifshitz-GilbertSlonczewski (LLGS) equation: [16][17][18] …”