Kamaram Munira

Computational investigation of half-Heusler compounds for spintronics applications

The authors investigate the properties of 378 half-Heusler compounds using density functional theory with the goal of identifying promising candidates for spintronic applications, e.g. half-metals. Although DFT has often been applied to the search for half-metals, this study may be the most comprehensive attempt to identify which of the compounds predicted by DFT to be half-metals are likely to be fabricated. The calculated formation energy of each of the 378 potential half Heuslers was compared to that of all competing phases and combination of phases in the Open Quantum Materials Database. Those semiconductors, half-metals, and near half-metals within an empirically determined 0.1 eV/atom hull distance margin for neglected effects were deemed of interest for further experimental investigation.

Switching of Dipole Coupled Multiferroic Nanomagnets in the Presence of Thermal Noise: Reliability of Nanomagnetic Logic

The stress-induced switching behavior of a multiferroic nanomagnet, dipole coupled to a hard nanomagnet, is numerically studied by solving the stochastic Landau-Lifshitz-Gilbert equation for a single-domain macrospin state. Different factors were found to affect the switching probability in the presence of thermal noise at room temperature: 1) dipole coupling strength, 2) stress levels, and 3) stress withdrawal rates (ramp rates). We report that the thermal broadening of the magnetization distribution causes large switching error rates. This could render nanomagnetic logic schemes that rely on dipole coupling to perform Boolean logic operations impractical whether they are clocked by stress or field or other means.

A Quasi-Analytical Model for Energy-Delay-Reliability Tradeoff Studies During Write Operations in a Perpendicular STT-RAM Cell

One of the biggest challenges that the current spin-transfer-torque-based random access memory (STT-RAM) industry faces is maintaining high thermal stability while trying to switch within a given voltage pulse and energy cost. In this paper, we present a physics-based analytical model that uses a modified Simmons tunneling expression to capture the spin-dependent tunneling in a magnetic tunnel junction (MTJ). Coupled with an analytical derivation of the critical switching current based on the Landau–Lifshitz–Gilbert equation and the write error rate derived from a solution to the Fokker–Planck equation, this model provides us a quick estimate of the energy-delay-reliability tradeoffs in perpendicular STT-RAM devices due to thermal fluctuations. In other words, the model provides a simple way to calculate the energy consumed during write operation that ensures a certain error rate and delay time while being numerically far less intensive than a full-fledged stochastic calculation. We calculate the worst case energy consumption during antiparallel (AP)-to-parallel (P) and P-to-AP switchings and quantify how increasing the anisotropy field

Self consistent parameterized physical MTJ compact model for STT-RAM

H_{K}

Comparative material issues for fast reliable switching in STT-RAMs

and lowering the saturation magnetization

Achieving perpendicular anisotropy in half-metallic Heusler alloys for spin device applications

M_{S}

Calculation of energy-barrier lowering by incoherent switching in spin-transfer torque magnetoresistive random-access memory

can significantly reduce the energy consumption. A case study on how manufacturing variations of the MTJ cell can affect the energy consumption and delay is also reported.

Anisotropy in layered half-metallic Heusler alloy superlattices

We present a physical compact model for magnetic tunnel junction (MTJ). Landau-Lifshitz-Gilbert (LLG) differential equation is solved in SPICE to derive the transient characteristics of MTJ. A modified version of the Simmons tunnel current equation captures the steady state properties of MTJ. The model results are validated with published experimental data.

Voltage-controlled magnetization switching in MRAMs in conjunction with spin-transfer torque and applied magnetic field

With its fast write and read, small cell size, non-volatility and excellent endurance, Spin Transfer Torque-RAM STT-RAM) has a high potential of dominating the embedded and standalone memory world in the near future. In this paper, the suitability of different classes of magnetic materials constituting the STT-RAM free layer is reviewed for faster switching and thermal stability. We identify the following material classes for faster switching in the thermally stable free-layer of a STT-RAM: (a) In-plane materials with high HK and low MS. While the high HK deters the magnetization during the easy to hard axis switching, it helps with switching past the equator, making the switching speeds for high and low HK materials comparable. However, high HK materials benefit from higher thermal stability. (b) Perpendicular materials with low damping have the same switching speed as in-plane materials but greater switching probability because of a lower critical current. The demagnetization field helps the free layer to start switching to the hard axis, but hinders it from switching further to the easy axis beyond the equator. (c) Anti-ferromagnetically capped partially-perpendicular materials. Capping with a Va layer decreases the demagnetization field, which promotes faster switching.

Balancing stress & dipolar interactions for fast, low power, reliable switching in multiferroic logic

Various full Heusler alloys are interfaced with MgO and the magnetic properties of the Heusler-MgO junctions are studied. Next to MgO, the cubic Heusler system distorts to a tetragonal one, thereby inducing an anisotropy. The half-metallicity and nature of anisotropy (in-plane or perpendicular) in the Heusler-MgO system is governed mostly by the interface Heusler layers. There is a trend that Mn-O bonding near the MgO-Heusler junction results in perpendicular anisotropy. The ability to remain half-metallic and have perpendicular anisotropy makes some of these alloys potential candidates as free-layers in Spin Transfer Torque Random Access Memory (STT-RAM) devices, particularly, Cr2MnAs-MgO system with MnAs interface layers and Co2MnSi-MgO system with Mn2 interface layers.