Adjustable spin torque in magnetic tunnel junctions with two fixed layers

Abstract

We have fabricated nanoscale magnetic tunnel junctions (MTJs) with an additional fixed magnetic layer added above the magnetic free layer of a standard MTJ structure. This acts as a second source of spin-polarized electrons that, depending on the relative alignment of the two fixed layers, either augments or diminishes the net spin-torque exerted on the free layer. The compound structure allows a quantitative comparison of spintorque from tunneling electrons and from electrons passing through metallic spacer layers, as well as analysis of Joule self-heating effects. This has significance for current-switched magnetic random access memory (MRAM), where spin torque is exploited, and for magnetic sensing, where spin torque is detrimental. Recent demonstrations1,2 of magnetic random access memory (MRAM) have shown that magnetic memory may compete with silicon based memory in some applications. In order to scale cell size and decrease write currents, future generations of MRAM may use spin transfer as the write mechanism 3,4 . A key to spin-transfer switched MRAM is the use of magnetic tunnel junctions 5,6 (MTJs) to obtain an impedance match to CMOS technology. Initial demonstrations of spin transfer in MTJs indicate critical current densities similar to those observed in current perpendicular to the plane (CPP) spin valves 3 , but with higher bias voltages (~0.5 V). This poses questions regarding self-heating effects and the possible degradation and breakdown of the ultra-thin insulating barrier layer. Therefore, it is beneficial to boost the magnitude of spin torque to enable switching at lower current and voltage levels. In contrast, for magnetic sensing, spin-transfer torques introduce noise and instability 7,8 . In this letter, we adapt a proposal due to Berger 9 to demonstrate that adding a copper spacer and a third magnetic fixed layer above the MTJ structure can, depending on the relative orientation of the two fixed layers, either increase by ~70% or nearly cancel the net spin-torque acting on the free magnetic layer. We analyze results by considering the effects of the spintorque and of the substantial Joule heating that occurs in tunnel junction devices at switching current levels. This investigation provides fundamental insight into the nature of spin torques in MTJs and spin valves by allowing a direct comparison, in the same device, of the magnitude of the spin-torque exerted by tunnel currents and by electrons that transport spin through the copper spacer layer.The multilayers for our samples were prepared on a thermally oxidized Si wafer in a DC magnetron sputtering chamber with a base pressure of 3×10 -8 torr. The 4 nm of Py above the AlO x serves as the magnetic free layer and the two CoFe layers are both magnetic fixed layers in our studies. Since the tunnel barrier limits the conductance, the resistance state reflects whether the bottom fixed layer (8 nm CoFe) and the free layer are aligned parallel (P, low resistance) or antiparallel (AP, high resistance). The role of the top fixed layer (5 nm Co...

Current-driven magnetization switching in CoFeB/MgO/CoFeB magnetic tunnel junctions

“…[19][20][21][22][23] In this letter, we report current-driven magnetization switching with J c on the order of 10 5 A/cm 2 in low-resistance…”

Steady State and Dynamics of Joule Heating in Magnetic Tunnel Junctions Observed via the Temperature Dependence of RKKY Coupling

“…We observe in Fig. 5 (a) that the interpolated voltage dependence of temperature clearly follows a quadratic power law (without low temperature effect such as in [11]):…”

“…One should notice that the spin-flop fields of P→AP and AP→P switching are different. In fact, the AP state is favored which can be attributed to field-like STT [11,12]. The assumption that STT effects are cancelled out by averaging opposite polarities is justified, since the spin flop field characterizes a rotation process.…”

“…Solutions to reduce the current flow and the associated power consumption are currently under investigation, in particular by using the voltage control of magnetic anisotropy [10]. The switching behavior of spin torque driven devices [11][12][13][14] is greatly influenced by a temperature increase, possibly larger than 100°C even during pulses in the nanosecond range. The temperature variations affect the magnetic parameters, such as magnetization, magnetocrystalline anisotropy, and change the thermal activation energy.…”

Magnetic Multilayer Structures for Spin Torque Nano Oscillator

“…(2-27). For example, reducing M S by using CoFeB as the free layer [41]; increasing P by using a double spin-filter structure [42], a synthetic antiferromagnetic pinning structure [43], inserting a Ru spin scattering layer [44]; or by reducing the damping constant by rare-earth doping [45]. Another approach is to introduce inhomogeneity into the nano-magnets and to achieve the reversal by domain wall propagation [46,47].…”