Ioan Lucian Prejbeanu

The 2014 Magnetism Roadmap

Magnetism is a very fascinating and dynamic field. Especially in the last 30 years it has experienced many major advances in the full range from novel fundamental phenomena to new products. Applications such as hard disk drives and magnetic sensors are part of our daily life, and new applications, such as in non-volatile computer random access memory, are expected to surface shortly. Thus it is timely for describing the current status, and current and future challenges in the form of a Roadmap article. This 2014 Magnetism Roadmap provides a view on several selected, currently very active innovative developments. It consists of 12 sections, each written by an expert in the field and addressing a specific subject, with strong emphasize on future potential. This Roadmap cannot cover the entire field. We have selected several highly relevant areas without attempting to provide a full review - a future update will have room for more topics. The scope covers mostly nano-magnetic phenomena and applications, where surfaces and interfaces provide additional functionality. New developments in fundamental topics such as interacting nano-elements, novel magnon-based spintronics concepts, spin-orbit torques and spin-caloric phenomena are addressed. New materials, such as organic magnetic materials and permanent magnets are covered. New applications are presented such as nano-magnetic logic, non-local and domain-wall based devices, heat-assisted magnetic recording, magnetic random access memory, and applications in biotechnology. May the Roadmap serve as a guideline for future emerging research directions in modern magnetism.

In-plane reversal mechanisms in circular Co dots

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.

Magnetization processes in hard Co-rich Co–Pt films with perpendicular anisotropy

We present a study of the magnetic properties and magnetization processes in hard Co-Pt (Pt∼20at.%) films. Co-rich Co-Pt films, with thickness t ranging from 5nm up to 2μm, 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 dom...

Configurational anisotropy in square lattices of interacting cobalt dots

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.

Domain structures in epitaxial (101~0) Co wires

A systematic investigation of the magnetic domain structure is presented for epitaxial sub-micron (101~0) 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.

Perpendicular magnetic tunnel junctions with a synthetic storage or reference layer: A new route towards Pt- and Pd-free junctions.

We report here the development of Pt and Pd-free perpendicular magnetic tunnel junctions (p-MTJ) for STT-MRAM applications. We start by studying a p-MTJ consisting of a bottom synthetic Co/Pt reference layer and a synthetic FeCoB/Ru/FeCoB storage layer covered with an MgO layer. We first investigate the evolution of RKKY coupling with Ru spacer thickness in such a storage layer. The coupling becomes antiferromagnetic above 0.5 nm and its strength decreases monotonously with increasing Ru thickness. This contrasts with the behavior of Co-based systems for which a maximum in interlayer coupling is generally observed around 0.8 nm. A thin Ta insertion below the Ru spacer considerably decreases the coupling energy, without basically changing its variation with Ru thickness. After optimization of the non-magnetic and magnetic layer thicknesses, it appears that such a FeCoB/Ru/FeCoB synthetic storage layer sandwiched between MgO barriers can be made stable enough to actually be used as hard reference layer in single or double magnetic tunnel junctions, the storage layer being now a single soft FeCoB layer. Finally, we realize Pt- or Pd-free robust perpendicular magnetic tunnel junctions, still keeping the advantage of a synthetic reference layer in terms of reduction of stray fields at small pillar sizes.

Vortex states stability in circular Co(0001) dots

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.

Perpendicular magnetic tunnel junctions with double barrier and single or synthetic antiferromagnetic storage layer

The magnetic properties of double tunnel junctions with perpendicular anisotropy were investigated. Two synthetic antiferromagnetic references are used, while the middle storage magnetic layer can be either a single ferromagnetic or a synthetic antiferromagnetic FeCoB-based layer, with a critical thickness as large as 3.0 nm. Among the different achievable magnetic configurations in zero field, those with either antiparallel references, and single ferromagnetic storage layer, or parallel references, and synthetic antiferromagnetic storage layer, are of particular interest since they allow increasing the efficiency of spin transfer torque writing and the thermal stability of the stored information as compared to single tunnel junctions. The latter configuration can be preferred when stray fields would favour a parallel orientation of the reference layers. In this case, the synthetic antiferromagnetic storage layer is also less sensitive to residual stray fields.

Enhanced blocking temperature in (Pt/Co) 3 /IrMn/Co and (Pd/Co) 3 /IrMn/Co trilayers with ultrathin IrMn layer

IrMn blocking temperature (TB) is compared between IrMn/Co bilayers and (Pt/Co)3/IrMn/Co and (Pd/Co)3/IrMn/Co trilayers for different IrMn thicknesses. Exchange bias field (Hex) is measured from 5 to 400 K. Trilayers show a more concave thermal decrease of Hex(T) compared to bilayers and, for thin IrMn layers, larger TB and sharper peak of coercive field Hc around TB. This Hex(T) behaviour presents improved characteristics for thermally assisted-MRAM (TA-MRAM) applications, with large Hex/Hc ratio. Two physical explanations are proposed: an indirect IrMn intergrain coupling through the (Pt(Pd)/Co)3 layer and a reduction of IrMn/Co interfacial coupling due to out-of-plane canting of the IrMn spins.

Magnetotransport measurements as a tool to probe the micromagnetic configurations in epitaxial Co wires

The micromagnetic structure in epitaxial Co wires is investigated by using the magnetoresistance effect associated with the domain wall formation. This provides an efficient tool to monitor the magnetization reversal in systems with reduced lateral size. The influence of the lateral size and of the magnetic history on the micromagnetic configuration of the wires is considered.