In MaChiNaCo, new nanocomposites based on magnetic objects organized on silica nanohelices will be designed, synthesized and characterized for the study and optimization of induced magnetochiral dichroism (MChD). MChD is a cross-effect between natural circular dichroism (NCD) and magnetic circular dichroism (MCD), and manifests as a differential absorption of non-polarized light by a chiral medium according to the direction of an external magnetic field. MChD was experimentally validated in 1997, and has been used to preferentially enrich one of the two enantiomers of a racemic mixture, without the use of circularly-polarized light, providing a possible mechanism for the homochirality of life. While the existence of MChD is indisputable, only a handful of examples of this phenomenon are known in the literature. These examples encompass widely disparate systems, from small molecules in solution, complexes in the solid state, to metal-organic frameworks (MOFs), and it is still difficult to successfully predict which chiral compounds will show a strong magnetochiral response. Moreover, it is believed that the chirality and the magnetism must be present simultaneously on the same molecule or particle. Here we wish to tackle simultaneously these two statements by proposing a thorough study of MChD using a composite approach where achiral objects with a strong magneto-optical response are grafted onto chiral silica nanohelices with controllable handedness and pitch. This bottom-up strategy takes advantage of induced circular dichroism (ICD), a phenomenon observed for achiral chromophores strongly interacting with a chiral component, which must transform into induced MChD under the influence of a magnetic field. The indisputable novelty of MaChiNaCo is to avoid the often complex synthesis of chiral magnetic objects by exploiting ICD based on chiral inorganic templates at the nanoscale. This strategy considerably increases the number of magnetic objects that can be studied, as they no longer require intrinsic chirality. The original exploitation of the induced MChD effect will allow us to envisage a wide range of nanocomposites based on different magnetic objects and helix shapes, thus significantly broadening the field of MChD research. A better understanding of this effect and the factors that govern it will clearly benefit from the proposed study. MaChiNaCo is a 48 month project proposed by a consortium uniting French expertise in magnetism (CRPP), nanotechnology (CBMN, IPCMS) and magneto-optical phenomena (LNCMI). Our work program will tackle the following challenges: WP1) Optimize the grafting of model magnetic complexes onto silica nanohelices. WP2) Evaluate the influence of the template (silica helices) morphology on the ICD (and eventually, the MChD) signals. WP3) Study the influence of the nature of metal center of the magnetic complexes on the MCD and MChD signals. WP4) Amplify the magneto-optical component with superparamagnetic clusters and nanoparticles. WP5) Amplify the magneto-optical component by the organization of the magnetic objects to induce magnetic or magnetoplasmonic coupling.

The emergence of atomically-thin van der Waals materials offers great opportunities to develop next-generation devices for nano- electronics and photonics. The recent demonstration of an intrinsic magnetic order in atomically-thin van der Waals materials (MVM) opens a vast playground to understand and control magnetism at the nanoscale. This nascent class of magnetic materials will embed magnetic degrees of freedom in van der Waals heterostructures that could be controlled by proximity effects, doping or strain. The objective of this proposal is to investigate the properties of 2D magnets integrated in hybrid heterostructures as building blocks of future ultrathin devices for nano-optics and spintronics, through the perspective of nano-optomechanics and optical spectroscopy. We will exploit magnetic proximity effects on monolayers of semiconducting transition-metal dichalcogenides (TMD). Offering a direct bandgap, large excitonic binding energy in the visible/near-infrared region, and locked valley and spin degrees of freedom that can be selectively addressed by the helicity of the incoming light, this class of materials draws a lot of interests for ultrathin optoelectronics and quantum optics devices. Their optical properties can be tuned by the interfacial exchange field inherited by an adjacent magnetic material. We will engineer the optical properties of the heterostructures by the diverse magnetic states offered by the MVM, finely tuned through nanomechanics. Toward this elaborated goal, the following objectives will be pursued: O1) building ultraclean suspended van der Waals magnetic heterostructures O2) creating an optomechanical platform to observe and control their magnetic order O3) investigating strain-mediated magnetic proximity effects in suspended heterostructures The unique conjunction of techniques and expertise inspired from nanomechanics, nano-optics, optical spectroscopy, magnonics and nanomagnetism will allow us to monitor and control exquisitely the strain in the system, the magnetic order and induced proximity effects. Since the seminal works on MVM in 2016-2017, this area has been very prolific and competitive, but is still in its infancy, considering the tremendous variety of structures and phenomena to be explored. By an optomechanical detection, we aim at observing the Brownian motion of van der Waals magnetic materials monolayers at 4K. We will observe change in the mechanical properties through the magnetic transition by magnetostriction correlated with Raman fingerprints, a minimally invasive approach that will provide a specific signature of the elementary excitations (phonons, magnons...). This combined method will be powerful to disentangle the physical origin of the observed effects. Strain control of the magnetic order will be investigated. We will study the proximity coupling between magnons at microwave frequencies and excitons, first in a heterostructure made of TMD and a microstructured magnetic substrate before replacing it by few-layer MVM. The static proximity effects induced by MVM will be investigated by optical spectroscopy. Finally, we will consider the vibrations of suspended heterostructures made of MVM and transition-metal dichalcogenides to investigate strain-mediated proximity effects. The understanding and control over these novel objects, combining 2D material physics, nanomagnetism and nano-optomechanics with a unique approach, will open up new frontiers for ultrathin nanodevices and fundamental hybrid optomechanics.