Off-axis electron holography for the quantitative study of magnetic properties of nanostructures: from the single nanomagnet to the complex device.

Electromagnetic properties are one of the keys for understanding and mastering nano systems used in many applications, as in medical treatment, optics, microelectronic or data storage. Various methods exist to map magnetic fields. Some are based on near field microscopy, like magnetic force microscopy, other on X-ray set-ups, like photoemission electron microscopy. Electron holography (EH), a powerful transmission electron microscopy method, is another appropriate tool which combines high sensitivity with a high spatial resolution. EH allow the quantitative measurement of both internal and external fields in individual nano-objects instead of assemblies of nanoobjects. This interferometric method can also be used for performing in situ/in operando experiments. We thus developed and applied EH on very different systems, from the single nanoparticles to the thin layer and the complex magnetic device, for studying their magnetic properties. In this presentation, we will present our investigations on a single Fe nanocube and an FeRh thin layer. - Nanomagnets recently attracted considerable interest due to their possible application as building blocks for hard drive disks and permanent magnets or as nanobiological vectors for drug delivery and hyperthermia. Despite theoretical studies, the size-dependence of spin arrangements in single nanomagnets has not yet been evidenced experimentally due to sensitivity limitations of the investigation tools. The single domain limit, corresponding to the critical nanomagnet size separating vortex/single domain configurations, has never been observed although it will dictate the optimized size for applications. In such small nano-objects, micromagnetic simulations show that the magnetic internal structure changes from single domain (SD) to vortex states as the cube size increases (Fig. 1). Some years ago, we reported symmetrical vortices, i.e. vortex of axis, in isolated 30 nm single crystal Fe cubes with a 14 nm vortex core size [2]. Next, we showed that vortices can also be stabilized in the presence of dipolar interactions thanks to holes in the cubes inducing a pinning of the vortex core [3]. Here we will present the spin configuration phase diagram in size-controlled single Fe nanocubes combining EH experiments and micromagnetic simulations [4]. High sensitivity imaging explicitly reveals how three different spin arrangements can be stabilized within a 3 nm window, evidencing the key importance of nanometric size control of magnetic nanoparticles. Moreover, it gives a deeper understanding of the single domain limit, which is more complex than expected with the appearance of a previously unreported vortex state. Such a measurement opens the door to fine magnetic control of nano-objects. - In situ heating/cooling EH has been used to quantitatively map the magnetization of a cross-sectional FeRh thin film through its magnetic transition [5]. This alloy presents a remarkable and unusual magnetic transition from a low temperature antiferromagnetic state (AFM) to a high temperature ferromagnetic state (FM) close to 370K accompanied by a 1% volume expansion. [6- 8]The transition is obtained for a narrow composition range 0.48regular spacing of the ferromagnetic domains nucleated upon monitoring of the transition is also evidenced. Beyond these findings on the fundamental transition mechanisms, EH also brings insights for in operando analysis of magnetic devices and a very promising approach to investigate the mechanisms of phase transitions in various magnetic systems (as MnAs [13]) at all pertinent scales.