Spin-transfer torque random access memory (STT-RAM) is a potentially revolutionary universal memory technology that combines the capacity and cost benefits of DRAM, the fast read and write performance of SRAM, the non-volatility of Flash, and essentially unlimited endurance. In order to realize a small cell size, high speed and achieve a fully functional STT-RAM chip, the MgO-barrier magnetic tunnel junctions (MTJ) used as the core storage and readout element must meet a set of performance requirements on switching current density, voltage, magneto-resistance ratio (MR), resistance-area product (RA), thermal stability factor (¿) , switching current distribution, read resistance distribution and reliability. In this paper, we report the progress of our work on device design, material improvement, wafer processing, integration with CMOS, and testing for a demonstration STT-RAM test chip, and projections based on modeling of the future characteristics of STT-RAM.

A model anisotropic system, suggestive of the graphite structure, is investigated: It consists of a series of equally spaced parallel planes. A finite two-dimensional density of electrons in each plane is allowed to move freely in the plane, but tunneling between planes does not take place. The dielectric screening of a point charge is evaluated exactly in the random-phase approximation. For realistic electron densities and interlayer separations the screened potential drops off very rapidly both in the plane of the charge and perpendicular to it. The induced charge density is determined, and validity of the Thomas-Fermi approximation is discussed.

We model “soft” error rates for writing (WSER) and for reading (RSER) for spin-torque memory devices that have a free layer with easy axis perpendicular to the film plane by solving the Fokker-Planck equation for the probability distribution of the angle that the free layer magnetization makes with the normal to the plane of the film. We obtain: 1) an exact, closed form, analytical expression for the zero-temperature switching time as a function of initial angle; 2) an approximate analytical expression for the distribution function of the direction of the magnetization and the exponential decay of the WSER as a function of the time the current is applied; 3) comparison of the approximate analytical expressions for the distribution function and WSER to numerical solutions of the Fokker-Planck equation; 4) an approximate analytical expression for the distribution function and WSER for the case in which the pinned layer is not collinear with the perpendicular free layer; 5) an approximate analytical expression for the linear increase in RSER with current applied for reading; 6) comparison of the approximate analytical formula for the RSER to the numerical solution of the Fokker-Planck equation; and 7) confirmation of the accuracy of the Fokker-Planck solutions by comparison with results of direct simulation using the single-macrospin Landau-Lifshitz-Gilbert equations with a random fluctuating field in the short-time regime for which the latter is practical. We find that the WSER decays at long times as exp[-2( i -1)τ] where the reduced time τ is related to the switching time, Gilbert damping and precession frequency through τ = αω ₀ t , and the reduced current i is the ratio of the applied current to the critical current density for switching i = I̅ / I ₀ . This exponentially decaying tail in WSER is not easily reduced by tilting the pinned layer magnetization.

We describe a general Fokker-Planck approach to understanding and calculating magnetization switching rates and noise in the recently observed phenomenon of spin-torque switching. In this phenomenon, which has possible applications to information storage, a large current passing from a pinned ferromagnetic (FM) layer to a free FM layer switches the free layer. Beginning with Brown [Phys. Rev. 130, 1677 (1963)], switching rates in magnetic systems have been calculated using the Fokker-Planck equation. In the small-oscillation limit, the equations have been solved analytically, giving a first-principles justification for phenomenological effective temperature theories: the spin-torque effect increases the Arrhenius factor $\exp (-E∕kT)$ in the switching rate by raising the effective spin temperature $T$. In the present Rapid Communication we generalize the nonlinear Fokker-Planck equation to the case of a Slonczewski spin torque. As an example, we use a linear approximation to calculate telegraph noise rates, leading to good qualitative agreement with recent experiments. However, our nonlinear formulation is also valid for large precessional oscillations. The method also allows the calculation of current-induced magnetic noise in current perpendicular to plane spin valve read heads.

The simple cubic Hubbard model is examined at zero temperature, for the case of strong interactions and average densities near but not at the half-filled band. It is shown that the ferromagnetic state which is usually assumed to be the ground state is unstable against phase separation. A new phase diagram is proposed, featuring a two-phase region in which the equilibrium state is separated into macroscopic antiferromagnetic and ferromagnetic phases (of differing density).