Finite size effects on spin-torque driven ferromagnetic resonance in spin valves with a Co∕Ni synthetic free layer

Abstract

Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study magnetic excitations in Co/Ni synthetic layers confined in nanojunctions. Field swept ST-FMR measurements were conducted with a magnetic field applied perpendicular to the layer surface. The resonance lines were measured under low amplitude excitation in a linear response regime. The resulting resonance fields were compared with those obtained using conventional rf field driven FMR on extended films with the same Co/Ni layer structure. A lower resonance field is found in confined structures. The effect of both dipolar fields acting on the Co/Ni layer emanating from other magnetic layers in the device and finite size effects on the spin wave spectrum are discussed.One approach to study ferromagnetic resonance (FMR) of a magnetic layer in a confined structure is to use the spin transfer interaction [1, 2] in a currentperpendicular (CPP) nanojunction. An rf current is applied to a magnetic tunnel junction [3] or spin valve [4], to drive FMR, in a method known as the spin-torque-driven ferromagnetic resonance (ST-FMR). This new technique enables quantitative studies of magnetic properties of materials in nanopillars, such as their magnetic excitations, anisotropy and damping.Spin-transfer devices that incorporate materials with perpendicular magnetic anisotropy are of great interest. This is because of their potential to lead to faster STdevices, with lower power dissipation [5] and critical current [6]. Recently, Mangin et al. studied perpendicular spin valves with a Co/Ni multilayer free layer, where a large magnetoresistance value and a high spin torque efficiency were observed [7].In this work, we present ST-FMR studies on bilayer spin valves, where the thin (free) layer is composed of a Co/Ni synthetic layer and the thick (fixed) layer is pure Co. By comparing the ST-FMR resonance fields with those of conventional rf field driven FMR of extended Co/Ni films with the same layer structure, we illustrate interactions important in ST-FMR of nanojunctions. Specifically, we discuss both dipolar interactions between the Co/Ni layer and other magnetic layers in the device, and finite size effects on the magnetic excitation spectrum.Pillar junctions with submicron lateral dimensions and rectangular shape, shown in Fig. 1a, were patterned on a silicon wafer using a nanostencil process [8]. Junctions were deposited by evaporation, and have the layer structure 1.5 nm Cr| 100 nm Cu| 20 nm Pt| 10 nm Cu| [0.4 nm Co| 0.8 nm Ni]×3 | 10 nm Cu| 12 nm Co| 200 nm Cu . The ST-FMR measurement setup is shown in Fig. 1(a). An rf current generated by a high frequency source is coupled with a dc current through a bias-T (the dashed-line box in Fig. 1(a)) into the spin valve. At resonance, the rf current and spin valve resistance oscillate at the same frequency. This results in a dc voltage (V =< I(t)R(t) >) [3,4]. Assuming a small angle circular precession of the free layer on resonance,where β is the angle between the free and fixed layers (before applying the rf curre...

Spin wave excitations of a magnetic pillar with dipolar coupling between the layers

“…Since the first observations of microwave generation by STO [4], extensive experimental work have been reported concerning the frequency measurements of this generation in low-amplitude regime for the large variety of magnetic multi-layer nano-structures. Typically, the experimentally observed dependence of the measured frequency on the bias magnetic field can be well described by the traditional Kittel' expression for both in-plane and perpendicularly magnetized cases, provided that the saturation magnetization is decreased by 30 to 75 percent depending on a system [4,5,6,7,8]. This apparent reduction of the saturation magnetization in nano-patterned systems was sometimes attributed to the effect of patterning and sometimes to the influence of the dipole-dipole interaction between the magnetic layers.…”

Identification and selection rules of the spin-wave eigenmodes in a normally magnetized nanopillar

“…22,23 It is also possible to use spintorque-driven ferromagnetic resonance (ST-FMR). [24][25][26][27][28][29][30] In this approach, an rf current flowing through the magnetoresistive device is used to excite the precession of magnetization and to detect it through a rectification effect. Direct excitation of SW modes by the rf field generated by microantennas and their detection through dc rectification 31 or high-frequency GMR measurements 32 has also been reported in spin-valve sensors.…”

Spin-torque driven ferromagnetic resonance in a nonlinear regime

“…5,6 The Co͉ Ni layer magnetic anisotropy was measured with rf currents in the submilliampere range, well below the devices' spin-transfer switching threshold of Ӎ8 mA. 5,6 The Co͉ Ni layer magnetic anisotropy was measured with rf currents in the submilliampere range, well below the devices' spin-transfer switching threshold of Ӎ8 mA.…”