Field-current phase diagrams of in-plane spin transfer torque memory cells with low effective magnetization storage layers

Abstract

Perpendicular spin transfer torque magnetic random access memories with high spin torque efficiency and thermal stability for embedded applications (invited)

Effects of the Heating Current Polarity on the Writing of Thermally Assisted Switching-MRAM

“…The storage layer used here corresponds to a ferromagnetic material with 3 nm thickness and 1 × 10 6 A/m saturation magnetization by accounting for the relative contribution of CoFeB and NiFe in the storage layer. Correcting for the thickness and saturation magnetization in (5), the reported effect [10] converts to 7.5%/(MA/cm 2 ) in our case. In our samples, the current density ranges from 1.3-2.2 MA/cm 2 , so the actual field reduction expected is 9.7%-16.5%.…”

“…With this relation, we can explain the writing field reduction using the switching field dependence on the STT current reported in previous works [10]. They report a writing field dependence with pulse voltage for a 1.1-nm-thick CoFeB storage layer, corresponding to an STT effective field reduction of 12.5%/(MA/cm 2 ), relatively to a typical writing field.…”

“…Equation (4) explicitly shows the dependence of the magnetic switching field on the applied bias current, where H is proportional to the current I as was previously demonstrated [9], [10]. Because of the assumptions taken, (4) is valid only at currents well below the critical current and an effective field small compared with the critical field.…”

“…The write selectivity based on the combination of field and heating offers a number of advantages: 1) perfect write selectivity based on transistor selection; 2) lower write fields compared with toggle MRAM; and 3) reduced power consumption thanks to the possibility of field sharing between all bits of a given word. Typical current density required for heating are in the range of 10 6 A/cm 2 , which are in the range where spin-transfer torque (STT) effects can be observed in MgO-based MTJs [9], [10]. In contrast to heating, STT influence depends on the current polarity favoring either parallel or antiparallel magnetization Manuscript configurations.…”

“…To study the dependence of the switching field with the polarity of the injected current, field hysteresis loops have been measured using constant voltage bias on the junction [8], [9], or by applying voltage pulses at each point along the loop [10]. With this procedure, it is possible to determine either the critical current for STT switching or the relationship between the injected current density and the magnetic switching field.…”