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The temperature required to set the exchange bias of a ferro/antiferromagnetic (F/AF) storage bilayer as a function of the heating pulse width was studied on magnetic tunnel junctions (MTJs) of thermally assisted magnetic random access memories. Heating is produced by a pulse of electric current flowing through the junction. For sufficiently long heating pulse (>20 ns), a quasiequilibrium temperature profile is reached in the MTJ. In this stationary regime, a relationship between the temperature of the storage layer and the power of the pulse was established by using an Arrhenius–Néel model of thermal relaxation. The introduction of thermal barriers between the junction tunnel barrier and the electrodes allows a significant reduction of the power required to achieve a given temperature rise of the storage layer. When the heating pulse duration is reduced from 1 s to 2 ns, the heating power required for setting the F/AF storage bilayer increases by about 80%. This experimental observation is quantitatively interpreted by combining the Arrhenius–Néel model with thermodynamic simulations of heat diffusion with source term given by the experimentally known heating power dissipated in the tunnel barrier by Joule effect. [ABSTRACT FROM AUTHOR]

We present a study of the magnetic properties and magnetization processes in hard Co-Pt (Pt∼20 at. %) films. Co-rich Co-Pt films, with thickness t ranging from 5 nm up to 2 μm, were prepared by electrodeposition on (0001)-oriented Ru underlayers. All samples displayed strong perpendicular magnetic anisotropy and high coercivity. Virgin magnetic domain structures for varying thickness were investigated by magnetic force microscopy (MFM). The observed increase of domain width with film thickness is well understood by full two-dimensional micromagnetic computations with no adjustable parameters. The easy-axis magnetization process, as observed by measuring virgin curves by magnetometry and imaging the corresponding magnetization configurations by MFM in variable field, consists of two stages separated by a well-defined critical field, marking the onset of domain wall propagation. A thorough analysis of the out-of-plane angular dependence of the switching field points out that unpinning of domain walls is the dominant magnetization process. [ABSTRACT FROM AUTHOR]

The tunnel junction heating process of micrometer and sub micrometer size junctions to be used in a thermally assisted magnetic random access memories write scheme is studied using the blocking temperature of the pinned storage layer as a reference threshold temperature. The results reveal two different heating regimes depending on the heating current pulse width.

Spintronics devices and in particular thermally assisted magnetic random access memories require a wide range of ferromagnetic/antiferromagnetic (F/AF) exchange bias (EB) properties and subsequently of AF materials to fulfil diverse functionality requirements for the reference and storage. For the reference layer, large EB energies and high blocking temperature (TB) are required. In contrast, for the storage layer, mostly moderate TB are needed. One of the present issues is to find a storage layer with properties intermediate between those of IrMn and FeMn and in particular: (i) with a TB larger than FeMn for better stability at rest-T but lower than IrMn to reduce power consumption at write-T and (ii) with improved magnetic interfacial quality, i.e., with reduced interfacial glassy character for lower properties dispersions. To address this issue, the EB properties of F/AF based stacks were studied for various mixed [IrMn/FeMn] AFs. In addition to EB loop shifts, the F/AF magnetic interfacial qualities and the AF grains thermal stability are probed via measurements of the low- and high-temperature contributions to the TB distributions, respectively. A tuning of the above three parameters is observed when evolving from IrMn to FeMn via [IrMn/FeMn] repetitions. [ABSTRACT FROM AUTHOR]

A systematic investigation of the magnetic domain structure is presented for epitaxial sub-micron (1010) Co wires characterized by a strong in-plane uniaxial magneto-crystalline anisotropy whose easy axis is oriented perpendicular to the long wire axis. Wires of varying width (100 to 1000 nm) and thickness (30 to 80 nm) were patterned by electron beam lithography and lift-off process. We establish experimentally the boundaries between the ground state transverse single domain state and the open stripe structure as a function of wire thickness and width. Moreover, the stability of the transverse single domain state is investigated as a function of the magnetization history. Index Terms--MFM, micromagnetics, stripe domains.

The possible magnetization configurations of individual circular Co(0001) dots are investigated by means of 3D micromagnetic simulations as a function of dot dimensions. In zero applied field, the vortex state corresponds to the ground state for diameters larger than 60 nm and up to a thickness of 25 nm where a transition into a weak stripe structure occurs. Index Terms--Co(0001), MFM, micromagentism, vortex state.

We have determined the write power density dependence on heating pulse width and antiferromagnet (AF) thickness using magnetic tunnel junctions whose storage layer is exchange biased with IrMn or FeMn. An increase in write power density needed to write the storage layer has been observed for both AF as pulse width is decreased from 0.1 ms to 10 ns. Quasistatic blocking temperatures (Tb) were measured on both full sheet and patterned samples, showing a reduction in Tb for patterned samples which we link to process induced damages. Power-temperature relationship was established based on a correspondence between writing power for long pulse duration and quasistatic Tb measurement. Both write power density and associated temperature results show a dependence of the Tb on AF thickness at pulse width as short as 10 ns. Particularly for IrMn, the relationship between write power and pulse width becomes weakly dependent on the AF thickness above a certain AF thickness. The write power density is significantly lower for thinner AF layer. This suggests a critical thickness below which the AF writing process deviates from bulk behavior. [ABSTRACT FROM AUTHOR]

In order to improve the magnetic and electrical properties of low resistance alumina based magnetic tunnel junctions, an off-axis method of sputtering has been investigated. It is shown that the tunnel magnetoresistance ratio can be greatly increased when there is an offset between the target and the wafer axes during the deposition of the ultrathin aluminum layer (off-axis sputtering) prior to its natural oxidation. The ferromagnetic coupling between the pinned and the free layer through the alumina barrier is also reduced compared to a classical on-axis deposition. This observation is interpreted as an improvement of the barrier quality, reducing both the roughness and the pinholes density. We assume that when the Al layer is sputtered off-axis, the magnetic and aluminum layers are protected from energetic neutralized Ar atoms bombardment. [ABSTRACT FROM AUTHOR]

The magnetic switching of the exchange biased storage layer in thermally assisted magnetic random access memory cells has been studied in the nanosecond time domain. Under reversed static external field, the magnetic tunnel junctions (MTJs) were subjected to current heating pulses long enough to heat the structure above the blocking temperature of the antiferromagnetic layer. The magnetic response of the storage layer was characterized by single-shot real-time measurement of MTJ resistance. The switching of the storage layer exhibits stochastic fluctuations. Nevertheless, using a heating current density of 4.7×106 A/cm2 corresponding to a bias voltage of 1.8 V, the switching takes place in less than 4 ns under 5 mT. Interestingly, the probability of switching versus pulse duration exhibits characteristic periodic steps which are ascribed to a combined effect of the applied field and spin transfer produced by the heating current pulses. [ABSTRACT FROM AUTHOR]

The dynamic heating of submicron size junctions developed for the thermomagnetic write scheme was investigated. The dependency of the heating power density with the voltage pulse width (Δt) was measured and shows a [1-exp(-Δt / t0dg phenomenological variation in the investigated range of pulse widths (5 ns-1 s). For submicron junction areas and constant pulse width, the heating power required for writing decreases as the junction area is reduced. The thermomagnetic write scheme was demonstrated through cycling of the cell between the two low and high resistance levels. This scheme was also shown to be insensitive to stray magnetic fields even in absence of any magnetic shielding of the memory. [ABSTRACT FROM AUTHOR]

In this work the thermal heating of tunnel junctions is investigated doing one-dimensional numerical simulations by solving the heat equation. The temperature increase is proportional to the power density jV, which is confirmed by transport measurements on patterned μm size junctions. The observed temperature increase was 2.3 K/(mW μm-2) compared to a simulation result of 0.9 K/(mW μm-2). Tunneling AFM measurements on 4, 5.4, and 6.7 Å barrier half-junctions, show hot spots with current densities up to 13 times the average density value. The current concentration could explain the higher heating observed experimentally compared to the simulated value. The hot spot density is 600–900/μm2 occupying roughly 5% of the barrier surface.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reversal process of polycristalline circular Co dots is investigated for dot sizes for which the vortex state corresponds to the ground state configuration. Using magneto-optic Kerr magnetometry and magnetic force microscopy imaging, it is found that during the reversal of the magnetization, the system does not necessarily pass through its ground state configuration. For large dot diameters and thickness, the reversal occurs via nucleation, propagation, and expulsion of a single vortex, whereas for a dot thickness below 20 nm the reversal process may occur by a coherent rotation of the quasi-single-domain configuration. Furthermore, a double-vortex state formation has been evidenced for 1000-nm-large dots. [ABSTRACT FROM AUTHOR]

Magnetostatic coupling effects are investigated in square lattices of polycrystalline circular Co dots as a function of dot diameter, spacing, and thickness. We observe a quadratic anisotropy for closely spaced dots when the dot thickness exceeds 15 nm, correlated to a change in the magnetization reversal process from coherent rotation to vortex nucleation/annihilation. To understand the observed anisotropy micromagnetic simulations have been performed which show that vortex nucleation is strongly dependent on the micromagnetic configurations. [ABSTRACT FROM AUTHOR]