Ferromagnetic-shape-memory (FSM) Heuslers are a class of smart materials, promising for integration into miniaturized thermo/magnetomechanical devices, applicable in automotive, aerospace, biology, and robotics fields. In addition to compactness and mechanical simplicity, it is crucial for the material to maintain its properties at micro and nanometer scales. This study evidences the effects of lateral dimension and geometry on the properties of FSM Heuslers in patterned epitaxially grown Ni-Mn-Ga films. In particular, arrays of microstructures with lateral sizes down to the micrometer range, having different shapes and orientations with respect to the substrate edges, are investigated. The key properties of the material are stable after the microfabrication process: the martensitic transition temperatures increase by less than 3 K and thermal hysteresis changes by only 2 K. Notably, the size and geometry (i.e. shapeand orientation) of the patterned microstructures are reported to be a suitable tool for controlling the martensitic configuration. The study demonstrates selective response of a specific type of martensitic twin boundaries, i.e., X-type twin boundaries along [110] MgO and [1-10] MgO, to the shape and orientation of the microstructures showing a twin boundary selection of up to ~96%. The effects of lateral size, shape, and orientation on the martensitic and magnetic properties of the lithographically patterned structures are discussed

This work presents a top-down method, based on ball-milling techniques, for the preparation of and micro-meter and sub-micro-meter sized particles. The structural, morphological, and magnetic features of the as-milled and annealed particles are investigated. First, we observe a detrimental effect on the magnetic properties and magneto-structural phase transitions of the induced lattice defects and atomic disorder, and second, we demonstrate the possibility to recover the original configuration by subsequent annealing. The optimization of the latter is strongly dependent on the initial milling energy, but, generally, a fast recovery of the Heusler and martensitic phases is observed. Further, we can tune the transformation temperatures or even improve the magnetic properties, in terms of increase of phase homogeneity, decrease of thermal hysteresis and increase of saturation magnetization, by decreasing the particles' size of the examined compounds.

We show the application of the singular point detection (SPD) technique to determine the magnetic anisotropy of different hard nanomaterials

Magnetic shape memory Heuslers have a great potential for being exploited into the next generation of cooling devices and actuating systems due to their "giant" caloric and thermo/magnetomechanical effects, arising from the combination of magnetic order and a martensitic transition. Thermal hysteresis, broad transition range, and twinning stress are among the major obstacles preventing the full exploitation of these materials. In order to find possible solutions to overcome these unfavorable obstacles, it is necessary to gain a comprehensive view of the configuration of the twin variants at the different length scales and its evolution upon martensitic phase transition. In the literature, there are a few works focused on the crystallographic structures and the martensitic configurations of epitaxial Ni-Mn-Ga thin films through experiments and models [1-4]. However, the knowledge about the multiscale hierarchical self-accommodation of the twin variants in the martensitic phase and its possible links to the transition route is still limited, mainly due to lack of direct multiscale observations. In the present study, we directly visualize the crystallography of seven-fold modulated (7M) Ni-Mn-Ga epitaxial films, the symmetry relations between the twin boundaries [5], and the interfaces [6] between colonies of different twin boundaries in the martensitic phase by means of different transmission electron microscopy (TEM) techniques. We combine our direct observations through TEM techniques to atomic force microscopy (AFM) topography imaging vs. temperature [7]. We propose a route for the martensitic forward and reverse transitions, highlighting the major role played by the different martensitic interfaces [7]. The present results represent a step forward in the understanding of the transition processes, and pave the way to the possibility of tuning the characteristics of the transition, e.g. hysteresis and transition width, by microstructure engineering aimed at the full exploitation of martensitic Heuslers for applications requiring cyclic phase transition.

Magnetic-shape-memory Heuslers are an important class of ferroic materials, constantly opening new fields of research and application (e.g. magnetic actuation, energy harvesting, ferroic cooling) arising from the strong interplay between magnetism and structure. The exploitation of thin films opens up barely new perspectives in the realization of micro/nanomachines, memories and smart devices, taking advantage also of their possible integration in microsystems. The control of microstructure is a crucial goal for the full exploitation of their functional properties (e.g. shape memory, magneto-mechanical) since the twin variants configuration plays a major role. Few recent works have been addressed to microstructure engineering, mainly exploiting the epitaxial growth on different substrates and varying film thickness and growth parameters. In previous papers we have shown the correlation between structure, microstructure and magnetic properties at the different legth-scales of substrate-constrained Ni-Mn-Ga films grown on Cr/MgO(100) [1]. More recently, we have studied the effects of size confinement and reported the possible thermo-magnetic actuation of free-standing Ni-Mn-Ga nano-disks, enabled by the peculiar martensitic microstructure [2] In the present talk we present an important step forward [3]. By means of a thorough multiscale magnetic and structural study, we demonstrate that a variety of simple post-growth treatments, i.e. post-annealing at low T (350°C), magnetic field cooling and mechanical stress, open up the possibility to tailor the twin variant configuration of epitaxial thin films. In particular, depending on the selected treatment, it is possible to tune the type of variant, i.e. X-type with out-of-plane magnetic easy axis or Y type with in-plane magnetic easy-axis, and their geometrical distribution. The mechanisms underlying the overall and local manipulation of microstructure are discussed by taking into account the role of microstructural defects and disorder together with the influence of external fields on the martensitic transformation path. Our findings definitely provide a platform to easily control and manipulate microstructural and magnetic patterns in magnetic shape memory thin films, paving the way to their full exploitation in smart applications. In a broader context, this remarkable "magnetic flexibility", enabled by the peculiarity of martensitic microstructure, makes magnetic shape memory Heusler thin films, a unique class, among magnetic materials, for the easy manipulation of their magnetic configuration by simple after-growth treatments.

This study investigates the magnetisation process of a martensitic Ni-Mn-Ga thin film with microstructure optimized to obtain a unidirectional and reversible magnetisation jump. The study has been realised by a thorough vector magnetometry characterisation and supported by micromagnetic modelling, considering different orientations of the applied field with respect to the symmetry directions of the sample. The model has been built on the film microstructure and experimental characteristics. The main features of the magnetisation curves measured along the film symmetry directions can be well reproduced by the micromagnetic model, that is, neglecting structural and magnetostructural contributions to the free energy. The model also well reproduces the field-dependent behaviour of the transverse magnetisation components. The agreement demonstrates that the spatial organisation of magnetocrystalline anisotropy axes due to martensitic twinning has a dominant effect on the magnetisation process, giving rise to magnetisation jumps when the magnetic field is applied along the alignment direction of the twin boundaries. When a reverse field is applied along this direction, simulations show that magnetisation reversal proceeds through the formation and three-dimensional expansion of magnetic domains, passing around the zero field through a closed-flux domain configuration, with the domain walls showing perpendicular orientation of the magnetic moments.

Magnetic shape memory Heuslers have a great potential for their exploitation in next-generation cooling devices and actuating systems, due to their "giant" caloric and thermo/magnetomechanical effects arising from the combination of magnetic order and a martensitic transition. Thermal hysteresis, broad transition range, and twinning stress are among the major obstacles preventing the full exploitation of these materials in applications. Using Ni-Mn-Ga seven-modulated epitaxial thin films as a model system, we investigated the possible links between the phase transition and the details of the twin variants configuration in the martensitic phase. We explored the crystallographic relations between the martensitic variants from the atomic-scale to the micro-scale through high-resolution techniques and combined this information with the direct observation of the evolution of martensitic twin variants vs. temperature. Based on our multiscale investigation, we propose a route for the martensitic phase transition, in which the interfaces between different colonies of twins play the major role of initiators for both the forward and reverse phase transition. Linking the martensitic transition to the martensitic configuration sheds light onto the possible mechanisms influencing the transition and paves the way towards microstructure engineering for the full exploitation of shape memory Heuslers in different applications.

Magnetic-shape-memory Heusler thin films have a great potential for new-concept integrated devices, such as microactuators, energy harvesters and solid-state microrefrigerators, thanks to the intimate coupling between structure and magnetism. The control of microstructure in their martensitic phase is crucial for their full exploitation. Here, by an accurate magnetic and structural investigation at different length-scales, we demonstrate how growth temperature and simple post-growth treatments, i.e. post-annealing at low T, magnetic field cooling and mechanical stress, are suitable to manipulate the twin variant configuration in epitaxial thin films. X-type variants with out-of-plane magnetic easy axis or Y-type variants with in-plane magnetic easy-axis can be selected, as well as their geometrical distribution in films with mixed X/Y-type microstructure. The mechanisms behind the overall and local manipulation of microstructure are discussed by taking into account the role of microstructural defects, disorder and external fields on the martensitic transformation path. Our findings provide a platform for a controlled manipulation of microstructure and magnetism in magnetic-shape-memory thin films, which opens up a window towards engineering smart magnetic materials for multiple purposes. This remarkable "magnetic flexibility" makes magnetic-shape-memory alloys a unique class, among magnetic materials, for the easy tuning of the magnetic configuration at different length scales.

We present a new approach for a large-scale production of the rare-earth free NiCoMnIn Heusler alloy for room temperature magnetic refrigeration applications. This class of compounds has recently attracted attention, thanks to the large reversible isothermal entropy change (Delta S-iso) and adiabatic temperature change (Delta T-ad) associated to a first-order magnetostructural phase transition. A large-scale production method, however, has not yet been proposed. For giant magnetocaloric materials and especially for Heusler compounds, the synthesis has a predominant role in tailoring the physical-chemical properties, due to the high sensitivity of the first-order transition characteristics on chemical composition and microstructure. Up to 250 g of the nominal composition Ni45.7Co4.2Mn36.6In13.3 alloy was prepared in a unique sample starting from industrial-grade powdered elements. The phase transition temperatures and magnetocaloric properties were investigated by magnetic and direct adiabatic temperature measurements and were found to be homogeneous in the whole sample. The mechanical stability of the produced alloy and its workability were investigated. A low-temperature thermal treatment was identified and showed promising results by reducing hysteresis and transition width.

Ferromagnetic shape memory Heuslers, such as NiMnGa, show multifunctional properties arising from a strong coupling between magnetic, thermal and mechanical degrees of freedom. Thin films have recently attracted much interest for their possible integration in microsystems for the realization of new-concept devices (e.g. microactuators, energy harvesters, solid-state microrefrigerators). Ni2MnGa shows a martensitic phase transformation from a cubic L21 phase (austenite) to a lower symmetry phase (martensite), by decreasing T. In the presently investigated composition (i.e. Ni54Mn22Ga24) the martensitic phase is ferromagnetic with a monoclinic 7M incommensurated structure. The martensitic microstructure is made of complex arrays of differently oriented hierarchical twin microstructures, i.e. X-type and Y-type, where the easy-magnetization directions are respectively out-of-plane and in-plane. In a previous paper we showed that growth conditions (including stress applied during growth) strongly influence the martensitic microstructure of films grown on a Cr underlayer [1]. In the present study we focus on microstructure engineering by post-growth treatments of films directly grown on MgO. Ni54Mn22Ga24 thin films of thickness 200 nm were epitaxially grown on MgO (001) in the range T=200-400 °C by rf sputtering. A multiscale thorough magnetic and structural study was performed be means of several techniques. AFM, SEM, TEM, XRD were used for characterizing microstructure, while the magnetic properties were studied by MFM, Lorentz microscopy, AGFM and SQUID magnetometry. By applying T, magnetic fields and stress to the substrate after growth, a variety of martensitic patterns was obtained, demonstrating that it is possible to engineer the magnetization patterns and strongly influence the magnetization processes. The intimate link between magnetic and structural degrees of freedom and the flexible twin-within-twin martensitic structure makes epitaxial NiMnGa films a unique platform for the precise control of the magnetic configuration from the atomic to the macro-scale also by means of easy and suitable post-growth treatments.

Ferromagnetic shape memory alloys (FSMA) such as NiMnGa thin films show a strong coupling between magnetic and structural degrees of freedom, which makes it interesting for promising application in smart micro and nano-devices [1]. The ability to control the microstructure at different length scales is of particular interest for the magnetic field induced strain applications. In low-temperature ferromagnetic phase, NiMnGa film consists of differently oriented twin microstructures [2,3]. Magnetic properties can be tuned by engineering these microstructures [4]. In the present study, NiMnGa films (75-200nm) were epitaxially grown on MgO (100) at 200-380°C using RF sputtering technique. The deposition rate was 38.3 to 60.3Å/min. Morphology, composition, and microstructural characterizations were performed using AFM, SEM, EDS, XRD and TEM. Magnetic configuration and behavior were studied by MFM, AGFM, and SQUID. Samples were post-treated by annealing, mechanical stress, and magnetic field cooling. We found that hierarchical twin structure and magnetic properties of the substrate-constrained films can be manipulated by growth temperature, post-heating, mechanical stress, and field cooling.

Ferromagnetic shape memory Heuslers show multifunctional properties arising from a strong coupling between magnetic, thermal and mechanical degrees of freedom. Within this class of compounds, Ni2MnGa is a model system, which shows a martensitic phase transformation from a cubic L21phase (austenite) to a lower symmetry phase (martensite) by decreasing temperature. We have investigated the possibility of modifying the martensitic microstructure in Ni-Mn-Ga films with thickness between 75 and 200 nm, grown by r.f. sputtering on MgO(100) or Cr/MgO(100). Films were grown in the temperature range 200 - 400 °C, with different growth parameter (i.e., sputtering rate, sputtering power) and composition Ni54Mn22Ga24. A multiscale magnetic and structural study was performed by means of several techniques: AFM, SEM, TEM, XRD were used for characterizing microstructure, while the magnetic properties were studied by MFM, Lorentz microscopy, AGFM and SQUID magnetometry. The L21austenitic phase grows epitaxial at high temperature both on MgO(100) and Cr/MgO(100). The martensitic phase, which is stable at room temperature, has a monoclinic 7M incommensurated structure. Both the phases are ferromagnetic, but the martensitic phase shows higher magnetocrystalline anisotropy. The martensitic microstructure is made of complex arrays of differently oriented hierarchical twin microstructures, i.e., X-type and Y-type, where the easy-magnetization directions are respectively out-of-plane and in-plane [1]. Controlling the orientation and organization of X- and Y-type twins in epitaxial films and nanostructures could pave the way towards multifunctional applications such as thermomagnetic actuation [2], magnetic storage [3] and magnetic anisotropy dependent properties in fluids [4]. We have focused on microstructure/magnetic pattern engineering by post-growth treatments such as post-annealing, stress, annealing in magnetic field. Through these post-growth treatments, a variety of martensitic patterns (i.e. orientation and spatial organization of the martensitic twin variants) were obtained, demonstrating that it is possible to engineer the magnetic patterns and strongly influence the magnetization processes (Figure 1). The intimate link between magnetic and structural degrees of freedom and the flexible twin-within-twin martensitic structure makes epitaxial Ni-Mn-Ga films a unique platform for the precise control of the magnetic configuration from the atomic to the macro-scale also by means of easy and suitable post-growth treatments.

Magnetic shape memory materials show outstanding and multifunctional properties (e.g. "giant" magnetomechanical, magnetocaloric, barocaloric), originating from the occurrence of both a martensitic transformation and magnetic order. Thin films and nanostructures of these materials have a great potential for different applications, such as micro- or nano-actuators, energy harvesters, valves and solid-state microrefrigerators [1]. We have demonstrated that the microstructure and magnetic properties of Ni-Mn-Ga thin films can be engineered by properly choosing substrate, growth conditions [2] and post- growth treatments. The films have been epitaxially grown on MgO(100) or on Cr/MgO(100) by RF sputtering, with thicknesses up to 200 nm. We have examined the relation between microstructure and magnetization process, simulating magnetization processes in films with different orientation and spatial organization of the martensitic twin variants. The micromagnetic simulations show a good agreement with the experimental results (Figure 1). Starting from the films grown on Cr/MgO, we have also realized Ni-Mn-Ga nanodots (d=160, 650 nm) by polystyrene-nanosphere lithography, and freestanding nanodisks, by subsequently removing the Cr underlayer via chemical etching. The microstructure and magnetic configuration of the nanostructures are influenced by the lateral confinement and release from the substrate. Furthermore, by varying temperature and applying a magnetic field to the free-standing nanodisks, we have obtained important microstructural changes, enabling different actuation modes [3]. [1] A. Backen et al., Adv. Eng. Mater. 14, 696-709 (2012) [2] P. Ranzieri et al., Adv. Mater. 27, 4760 (2015) [3] M. Campanini et al., Small 2018, 1803027

We present a study of the magnetic properties and of the reversal process of magnetic-hard Strontium Hexaferrite (SFO) particles with sizes below and above the single-domain size.

We present a study on the origin of the magnetic properties of Al and Cr doped Sr hexaferrites. M- type hexaferrites are the most employed rare-earth free ceramics in the permanent magnet industry

In this study we have determined for first time the magnetic anisotropy field (HA) of Strontium Ferrite (SFO, SrFe12O19) nanoparticles of different morphologies using the Singular Point Detection (SPD) technique.

Tumor oxygenation is a critical issue for enhancing radiotherapy (RT) effectiveness. Alternating RT with hyperthermia improves tumor radiosensitivity by inducing a massive vasodilation of the neoangiogenic vasculature provided the whole tumor is properly heated. The aim of this work was to develop superparamagnetic oxygen-loaded nanobubbles (MOLNBs) as innovative theranostic hyperthermic agents to potentiate tumor oxygenation by direct intracellular oxygen administration. Magnetic oxygen-loaded nanobubbles were obtained by functionalizing dextran-shelled and perfluoropentane-cored nanobubbles with superparamagnetic iron oxide nanoparticles. Magnetic oxygen-loaded nanobubbles with sizes of about 380 nm were manufactured, and they were able to store oxygen and in vitro release it with prolonged kinetics. In vitro investigation showed that MOLNBs can increase tissue temperature when exposed to radiofrequency magnetic fields. Moreover, they are easily internalized by tumor cells, herein releasing oxygen with a sustained kinetics. In conclusion, MOLNBs can be considered a multimodal theranostic platform since, beyond their nature of contrast agent for magnetic resonance imaging due to magnetic characteristics, they showed echogenic properties and can be visualized using medical ultrasound.

In this work, we highlight the occurrence of different physical mechanisms that independently control the saturation magnetization and the ferro-paramagnetic transition temperature of Ni-Mn-based Heusler compounds, opening new possibilities in mastering the functional properties of this wide class of magnetic materials. We present the magnetic, structural and magnetocaloric features of a complete Ni48Mn36In16-xSnx (x = 0-16)series. The observed different trends of the critical temperature and of the saturation magnetization on varying the Sn to In ratio are discussed with the help of first-principles calculations of the electronic structure and magnetic interactions of the compound.

Ferromagnetic shape memory alloys (FSMA) such as NiMnGa thin films show a strong coupling between magnetic and structural degrees of freedom, which makes them a promising candidate for smart micro and nano-device applications [1]. The ability to control the microstructure at different length scales is of particular interest for the magnetic field induced strain applications. In low-temperature ferromagnetic phase, NiMnGa film consists of differently oriented twins [2,3]. Magnetic properties can be tuned by engineering these twin microstructures [4]. In the present study, NiMnGa films (75-200nm) were epitaxially grown on MgO (100) at 200- 380°C using RF sputtering technique. The deposition rate was 3.83 to 6.03nm/min. Morphology, composition, and microstructural characterizations were performed using AFM, SEM, EDS, XRD, and TEM. Magnetic configuration and behavior were studied by MFM, AGFM, and SQUID. Samples were post-treated by annealing, mechanical stress, and magnetic field cooling. We found that hierarchical configuration of twins and magnetic properties of substrateconstrained films can be manipulated by growth temperature, post-heating, mechanical stress, and field cooling.

Magnetic shape memory materials display multifunctional properties (e.g. magnetomechanical, magnetocaloric, magnetoresistive...) arising from the presence of a martensitic transformation and magnetic states [1]. Low-dimensional materials, mainly thin films, have recently attracted much interest for their great potential in novel applications (e.g. microactuators, energy harvesters, solid- state microrefrigerators) [2]. We have shown that in epitaxial thin films the magnetic and structural properties can be optimized at the different length-scales by an appropriate choice of substrates/underlayers, thickness and growth parameters, including temperature and stress applied during growth [3]. In the present talk we will focus on patterned structures and free-standing nanodisks. Patterned thin films were obtained by polystyrene-nanosphere lithography of epitaxial NiMnGa- based thin films grown by sputtering r.f. on MgO substrates with a Cr underlayer. Free-standing nanodisks (d=160, 650 nm) were subsequently obtained by removing the Cr underlayer by a selective chemical etching. A multiscale structural and magnetic study was performed by means of electron microscopy (HREM, STEM-HAADF, electron diffraction, Lorentz microscopy), X-ray diffraction, AFM/MFM, and SQUID magnetometry. Patterned thin films maintain the same macroscopic martensitic and magnetic properties of continuous thin films (e.g. martensitic transformation temperature, crystalline structures, magnetization loops). On the other hand, their microstructural and magnetic configurations are influenced by lateral confinement and release from substrate. Remarkably, the combined application of temperature and field to free-standing nanodisks gives rise to substantial microstructural changes, enabling different actuation modes. Areal variation of the order of some percent, and tunable in intensity and sign by the application of T and magnetic fields have been obtained These features, arising from the combination of ferromagnetic and martensitic properties, pave the way to the realization of ferromagnetic shape memory nanoactuators. Possible new-concept biomedical applications will also be discussed. [1] M. Acet, et al., Handbook of Magnetic Materials vol. 19, Elsevier, Amsterdam, 2001 [2] A. Backen et al., Adv. Eng. Mat. 14,(2012) 696 [3] P. Ranzieri et al., Acta Mater. 61 (2013) 263,P. Ranzieri et al., Adv. Mater. 32, (2015) 4760