Metamagnetic Heusler compounds display a first order structural transformation associated with large changes of magnetization and/or magnetic order. Their properties can
be tuned by suitable changes in composition making them a versatile class of multifunctional materials, thanks to the strong interplay between thermal, mechanical and magnetic degrees of
freedom [1]. In addition, their hierarchically interrelated twin-with-twin microstructure, in the low temperature martensitic phase, and the strong spin-lattice coupling allow to control their
magnetic and multifunctional properties from the atomic to the micro-scale by tuning growth conditions and applying external fields [2, 3].
In my talk I will report on our recent results on nano/microscale materials obtained by different fabrication methods (e.g. epitaxial thin films, patterned and free-standing structures) [3-6].
Thin films and micro/nanostructures are of particular interest not only for the realization of new-concept devices, but also for providing insights into the magnetostructural coupling at the
different length scales. The talk will focus on microstructure engineering and microstructure related effects on the martensitic transformation, also in view of the possible exploitation of
this class of materials in energy related and smart applications.
In particular, by an accurate magnetic and structural investigation at different length-scales, I will demonstrate how growth temperature and simple post-growth treatments, i.e. postannealing
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 [3].
Taking advantage from the possibility to manipulate microstructure we have studied the role of specific martensitic configurations on the martensitic transition path. By advanced magnetic
force microscopy imaging in a wide temperature (260–350 K) and magnetic field range (up to 14 T) we have directly observed the nucleation and the self-accommodation of the martensitic
twinning configurations under zero-field, isofield and isothermal conditions. We have found that between the two possible twinning configurations, the Y-type, which nucleates first, shows
a significantly smaller thermal hysteresis as well as a sharper phase transition with respect to X-type microstructure, for all the three investigated conditions [4].
The effects of lateral size, shape and geometry on the properties of patterned epitaxially grown Ni-Mn-Ga films will also be discussed. In particular, I will present the characterization of
arrays of microstructures, obtained by means of different lithographic techniques, with lateral sizes down to 200 nm range and having different shapes and orientations with respect to the
substrate edges [5].
The last part of my talk will be devoted to free-standing nanostructures in view of their possible exploitation as micro/nano actuators exploiting microstructure-controlled actuation
mechanisms by the combined application of temperature and magnetic field [6]. 

**References:** 

[1] M. Acet, L. Mañosa, A. Planes, Handbook of Magnetic Materials (Ed: K. H. J. Buschow), Elsevier, Amsterdam 2011, pp. 231–289 
[2] P. Ranzieri et al., Adv. Mater. 2015, vol. 27, p. 4760 
[3] M. Takhsha Ghahfarokhi et al. Acta Mater. vol. 187, p. 135–145 (2020) 
[4] M. Takhsha Ghahfarokhi et al. Acta Mater. vol. 23, 117356 (2021) 
[5] M. Takhsha Ghahfarokhi, et al. Appl. Mater. Today vol. 23, p. 101058 (2021). 
[6] M. Campanini et al., Small, vol. 14, p. 1803027 (2018) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/8dcd0007b95b090d53734d3897ab328e.png)

NiMn based Heusler thin films: an amazing class of ferromagnetic shape memory alloys

Co-based Heusler alloys are currently exploited in several interesting spintronics applications, such as magnetic electrodes in spin-valve and magnetic tunnel junction devices, due to their half-metallic ferromagnetic properties [1]. Their magnetic and electronic transport behavior at room temperature are intensely studied and tuned to meet several applications demands. In particular, the electronic transport properties display an unusual trend of Seebeck coefficient and electrical conductivity proving the half-metallic ferromagnetic behavior of these compounds [2]. In this work, Co2HfSn and Co2ZrSn bulk alloys are synthetized by arc-furnace in which the melt contains pure elements in the appropriate stoichiometric quantities; subsequent, annealing at 800 °C for 6 days is performed to release stress and reach the equilibrium phase. Magnetic properties were investigated in the wide temperature range from 2 to 500 K by SQUID and VSM magnetometers; isothermal hysteresis loops M(H) and magnetization as a function of temperature curves M(T) have been measured. Magnetic results are in excellent agreement with the mean-field model of itinerant ferromagnetism (Stoner model) in the limit of very weak ferromagnetism. The linear relationship between M2 and H/M (i.e. Arrott plot) is maintained at all studied temperatures, both well below and above the Curie temperature (see Fig. 1a). The weak itinerant ferromagnetism model, by means of its temperature-dependent coefficients, is successfully used to determine the Curie temperature resulting at 432 K and 458 K for the Co2HfSn and Co2ZrSn alloy, respectively. In addition, the extrapolation of the experimental M(T) curve at T = 0 K reveals a magnetic moment of 2.0 μB/f.u. 1.81 μB/f.u. for the Co2HfSn and Co2ZrSn samples, respectively. Electric and magnetic experimental results are compared with the ab-initio calculation, which provides the spin-polarized density of states (DOS) confirming that Co2HfSn and Co2ZrSn alloys are half-metallic compounds with a majority-spin channel across the Fermi surface, whereas the minority-spin channel shows a small band gap (see Fig. 1b).

![](https://assets.underline.io/uploads/markdown_image/1/image/df3139ef06e4c7e8a2379ed3a8f014a3.jpg)

a) Arrott plot for the Co2HfSn sample; b) DOS by ab-initio calculation for the Co2ZrSn sample

Weak ferromagnetism and electronic properties of half-metallic Co2HfSn and Co2ZrSn Heusler alloys

Magnetostrictive materials possess the potential to be used in wide variety applications such as sensing, straintronic, spintronic devices etc due to the coupling between the strain and the magnetization. In this context, FeGa which has a very high magnetostrictive constant, high tensile strength and low coercivity proves to be an efficient material for current research. Commercial producibility of FeGa is cheaper and easier compared to the rare earth alloy families since it does not include any rare earth elements. Fe100-xGax with x = 19% has the highest magnetostriction constant with a value of around 3/2 × λ100= 400 ppm [1]. A thin film with this composition is attributed to the formation of next-nearest-neighbours (NNN) Ga-Ga pairs along [100] directions which leads to the formation of ordered D03 lattice structure [2]. In our report, we characterized the magnetic properties of Fe70Ga30 deposited on Si/SiO2 substrate (rigid material) and Kapton substrate (flexible material) by measuring room-temperature hysteresis loops by AGFM and MFM characterisation as a function of magnetic field. The magnetic hysteresis behaviour is quite different for Fe70Ga30 sputtered Si/SiO2 and Kapton as shown in Fig. 1(a, b). The coercive field obtained for Fe70Ga30/Kapton is approximately 20 times higher than that of Fe70Ga30/SiO2/Si. This can be referred to the interface induced strain which results in higher anisotropy for Fe70Ga30/Kapton. MFM results reveal linear stripe domains for the both Fe70Ga30/SiO2/Si and Fe70Ga30/Kapton shown in Fig. 1(c, d). The periodicity of domains is higher for the samples on Kapton due to high anisotropy. The substrate induced effect largely changes the properties of FeGa. We further try to measure the properties of Fe81Ga19 deposited on both Si/SiO2 and Kapton for comparison. The dependence of strain percentage with magnetic anisotropy will be studied for Fe81Ga19/Kapton.

**References:**

[1] A. E. Clark et al. Journal of Applied Physics 93, 8621 (2003). [2] H. Yangkun et al. Adv. Funct. Mater. 28 (20) 1800858 (2018).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/fe11bced4390bcf46677e66807dd5c28.jpg)
Fig. 1 Hysteresis loops and MFM domain patterns of Fe70Ga30/SiO2/Si (a, c) and Fe70Ga30/Kapton (b, d), respectively.

 Magnetic Properties of FeGa on Rigid and Flexible Substrates

The power released by magnetic nanoparticles (MNPs) submitted to an alternating driving field in the typical frequency range used in therapeutic treatments based on magnetic hyperthermia is studied using a rate-equation approach in which MNPs are modelled as double-well systems with randomly distributed easy axes. Taking into account the temperature dependency of MNP magnetization and anisotropy, a detailed picture of their hysteretic properties and of the resulting heating efficiency. The temperature variation in a host medium is then studied by solving a simple, radial Fourier equation with two typical boundary conditions. The time evolution of the temperature increment of the medium turns out to be strongly affected by the non-monotonic temperature dependence of the power released by NPs (see Figure 1a); the direct consequences on the heating efficiency make the determination of the specific loss power (SLP) and the resulting maximum temperature from the initial slope of the heating curve misleading. An alternative, sounder method to measure the average power released by magnetic nanoparticles and the SLP is proposed [1]. Another way of optimizing the performance of MNPs as heat generators consists in applying non-harmonic driving-field waveforms, with notable advantages with respect to the effects of harmonic magnetizing fields of same frequency and amplitude [2]. This improvement is related to the presence of fast, quasi-adiabatic transformations where the occupancy numbers in the two potential wells are virtually frozen, resulting in largely off-equilibrium magnetization states and in hysteresis loops with a greater area. In particular, changes of the inclination of the trapezoidal waveform’s lateral sides are also shown to induce controlled changes in the specific loss power generated by the activated nanoparticles (Figure 1b). Specific issues typical of the therapeutic practice of hyperthermia, such as the need for fine tuning of the optimal treatment temperature in real time, the possibility of combining sequential treatments at different temperatures, the ability to substantially reduce the heating transient in a hyperthermia treatment are properly addressed and overcome by making use of versatile non-harmonic driving fields of trapezoidal shape [3]. In the light of our results, features, methods, advantages and dangers of MNP-assisted precision nanomedicine are assessed in a more accurate way, and the efficiency of magnetic hyperthermia treatments is improved.

**References:**

[1] G. Barrera, P. Allia, P. Tiberto, Nanoscale, 2020,12, 6360. [2] P. Allia, G. Barrera, P. Tiberto, Phys. Rev. Applied, 2019, 12, 034041. [3] G. Barrera, P. Allia, P. Tiberto, Nanoscale Advances, 2020, 2, 4652.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/aceca2e127ef31688d27f6c19470c419.jpg)
(a) Left: Time evolution of the temperature increment. Right: released power by NPs as a fuction of temperature. (b) Temperature increment as a function of time for selected inclination of the trapezoidal waveform lateral sides.

 Heating efficiency of magnetic nanoparticles for magnetic hyperthermia: effects of temperature and driving-field waveform

NOA: Magneto-Elastic and -Optical Phenomena

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BOA - Magnetoelectronic Materials and Phenomena: Heuslers and Related Materials

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JU: Magneto-elastic Materials and Devices

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Presentations

NiMn based Heusler thin films: an amazing class of ferromagnetic shape memory alloys

Weak ferromagnetism and electronic properties of half-metallic Co2HfSn and Co2ZrSn Heusler alloys

Magnetic Properties of FeGa on Rigid and Flexible Substrates

Heating efficiency of magnetic nanoparticles for magnetic hyperthermia: effects of temperature and driving-field waveform

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