Exchange spring hard-soft magnetic nanocomposites (NCs) are proposed as potential high-performance magnets [1,2]. Their relatively poor performance [3-6] compared to predicted values [7] is attributed to insufficient control over the size and distribution of the soft phase. In this work e-beam lithography was used to control the size, shape and position of soft magnetic (FeCo) nano-rods embedded in a micro-patterned hard magnetic matrix (25 nm thick FePt-L10). We report a study of samples with different nanorod widths (24 and 120 nm) and comparable volume content. Their global M(H) loops have similar Hc values, but that of the sample with wide nanorods is concave (Fig. 2a), and First Order Reversal Curve diagrams reveal that this sample has some uncoupled soft magnetic phase. Uncoupled soft phase reversal is confirmed by MFM imaging in different remanent states, where hard and soft phase switching can be distinguished owing to the different length scales of the patterns (Fig. 1). Micromagnetic modelling (MuMax3) was performed to simulate magnetization reversal and MFM contrast. Good agreement is obtained between the measured (Fig. 2a) and simulated (Fig. 2b) data. Magnetization distribution cross-sections during reversal indicate that the wide soft rods reverse independently of the hard matrix, while the narrower rods are coupled during the entire reversal process (Fig. 2c-e). Convoluting results of stray field distributions at the experimental scan height with an MFM probe modelled as a monopole, we obtained comparable images with periodic patterns with the same length scale as the nanorods (Fig. 2f-g). The use of MFM remanent imaging combined with simulations to study magnetization reversal will be discussed in the framework of a broader study of model soft in hard nanocomposites.

![](https://assets.underline.io/uploads/markdown_image/1/image/97f767231afaf37c18391fe56fc92af6.jpg)

MFM images of NCs in remanent state, after applied field of 250 mT (left) and 500 mT (right), for rod widths of 24 nm (a,c) and 120 nm (b,d). Inserts - SEM plan view images of the nanorods (1 µm scale bar).

![](https://assets.underline.io/uploads/markdown_image/1/image/0304cf4965480752d8cca3cfd62a8896.jpg)

a) Experimental M(H) loops; b) Simulated demagnetization curves. c-e). Magnetization maps at the height of the rod (numbers indicate position along the demag curve). Simulated (f) and experimental (g) MFM contrast.

Magnetization reversal in nanofabricated soft-in-hard exchange-spring magnets

La(Fe, Si)13 is a promising candidate for magnetocaloric based technology such as magnetic refrigeration or energy harvesting [1-3]. This alloy family consists of a low content of rare earth (7.1 at. %) and high content of iron, resulting in a cheap and non-toxic compound. La(Fe, Si)13 based alloys exhibit a large magnetocaloric effect (MCE) at the transition between ferromagnetic and paramagnetic states, which occurs at Curie temperatures (Tc) ranging between 200-259K [4]. The magnetic properties (Tc, magnetization, MCE), the crystal structure (cubic or tetragonal) and the type of transition can be tuned by adjusting the Si content. Furthermore, the Curie temperature can be increased to above room temperature by doping and hydrogenation [5]. Recently our group reported for the first time the possibility to fabricate thick films of La(Fe, Si)13 [6]. Here we present a study on compositionally graded films of La-Fe-Si. The films were fabricated by triode co-sputtering of three targets (La-Fe, Fe, Si-Fe) onto thermally oxidised stationary Si substrates (100 mm in diameter). The composition of the as-deposited films was mapped by EDX, as shown in Fig. 1. The films were annealed ex-situ in a rapid thermal annealing furnace at 900°C. For each film, we selected samples from specific regions for characterization using optical and scanning electron microscopy, VSM-SQUID magnetometry and X-ray spectroscopy. Fig. 2 displays M(T) measurements on four La-Fe-Si samples of 5 μm thickness, taken from the regions specified in Fig. 1. The presentation will report results from films of nominal thickness ranging between 150nm – 5μm. The presentation will give details about how the structural and magnetic properties vary as a function of film composition, thickness and annealing conditions. Prospects for further developments and possible applications will also be discussed. Acknowledgement: This research has received funding from the French National Research Agency under the HiPerTherMag project (ANR-18-CE05-0019).

![](https://assets.underline.io/uploads/markdown_image/1/image/e47b4016b72ccd38f00fea3375cd00d9.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/a06f84bea07fbcfb4eb5845d9b87a6ea.jpg)

Compositionally graded La-Fe-Si films

Compositionally graded La-Fe-Si films

Internet of Things devices prompt a rising demand for autonomous and compact power supply generators to sustain low-power wireless sensor nodes. Due to the ubiquity of waste heat in buildings and urban environments, low-grade heat sources are very attractive for applications. Thermomagnetic generators (TMG) can be small and autonomous. Ahmim’s TMG [1] produces 4.2 μW (0.24 mW/cm3 over a 35°C temperature difference. While this device exploits direct thermal contact with both reservoirs, a key element of a thermal device [2], the output power is penalized by a sub-Hertz working frequency. Khol’s TMG [3] produces 118 mW/cm3 at 84 Hz but over a larger temperature difference of 138°C. Their use of 5 μm thick Heusler Alloy films, compared to a 250 μm thick plate of LaFeSi by Ahmim, is an important factor to explain the different in the working frequency. We aim to explore the possibility to use thin films in Ahmim’s TMG while keeping its low temperature difference capability. Ahmim’s TMG has specific characteristics. Firstly, it needs to have a freestanding magnetocaloric film of several micrometers. Our recent work on Gd film deposition has shown a 17 μm film with significant magnetocaloric effect [4]. Secondly, it needs to have a magnetic field source that gives a maximum field variation but also confines the magnetic field in order to reduce the film displacement. By combining Gd films and high performance NdFeB micro-magnets [5], [6] with our design experience on macro devices (Fig.1), we will discuss the key steps of the design, e.g., the spring characteristics to achieve self-oscillating micro-devices. Several parameters need to be optimized, such as the micro-magnet pattern, the air gap and Gd film thickness. 3D numerical simulations of the magnetic force as a function of the material position and temperature (Fig. 2), which are being used to optimize the system design, will be discussed in detail. With our proposed design, if we assume a 50 Hz working frequency, there is 5mW of mechanical energy available.

![](https://assets.underline.io/uploads/markdown_image/1/image/6c8def3c8eadc39bbbf231f211465813.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/d44ec31fe73d8d8565d56c704b72f012.jpg)

Key steps in designing a micro thermomagnetic generator

Key steps in designing a micro thermomagnetic generator

Gadolinium is the simplest material with a strong magnetocaloric effect close to room temperature (Tc = 293 K) and no thermal hysteresis [1], making it an ideal test-bench material for magnetic refrigeration and energy harvesting prototypes [2,3]. Gd ribbons of thickness 36-215 μm were prepared by cold-rolling [4] while films of thickness 20-150 μm were deposited by evaporation onto dimpled Hastelloy substrates [5]. More recently, films of thickness in the µm range were made by magnetron sputtering [6,7]. Here we present a study of Gd films of thickness in the range 3.5 - 40 μm, deposited by triode sputtering onto Si substrates at RT or 400°C, with Ta buffer and capping layers. As has been reported in magnetron sputtered films [7], our triode sputtered films can be removed from the substrate to produce free-standing films, and peel-off is easiest for films deposited at elevated temperature. Films were characterized structurally (optical and Scanning Electron Microscopy, XRD), magnetically (VSM-SQUID, extraction magnetometer) and mechanically (interferometer, nanoindenter). An SEM image of the fractured cross section of a 20 µm thick film deposited at 400°C is shown in Fig. 1, together with θ-2θ XRD patterns from the upper and lower surfaces of the film. It can be seen that the film texture changes from (00l) at the lower surface to (103) at the upper surface. A spin-reorientation transition is clearly seen in the M(T) curve of a 3.5 µm thick film, while it becomes smeared in thicker films (Fig. 2), owing to the evolution in crystallographic texture. In this presentation we will report on the influence of film thickness and deposition temperature on the structural, magnetic and mechanical properties of Gd films, which are being developed for integration into micro-scaled thermal energy harvesting devices [3]. Acknowledgement: This research has received funding from the French National Research Agency under the HiPerTherMag project (ANR-18-CE05-0019).

![](https://assets.underline.io/uploads/markdown_image/1/image/dd535808ff17c217e3a7ead00118fb60.jpg)


Fig. 1 Left: SEM image of a 20 µm thick Gd film deposited at 400 °C. Right: θ-2θ XRD spectra from the upper and lower surfaces of the film, following peel-off from the substrate.


![](https://assets.underline.io/uploads/markdown_image/1/image/f29d014564fd1b808054343a926d5a44.jpg)

Fig. 2 Magnetization vs temperature, measured under 50 mT, of Gd films deposited at 400°C.

Structural, magnetic and mechanical properties of Gd films

Excellent hard magnetic properties have been achieved in rare earth transition metal films (Nd-Fe-B, Sm-Co) produced by sputtering, pulsed laser deposition and molecular beam epitaxy. Such films can serve as model systems to probe intrinsic magnetic properties or to study the link between microstructure and extrinsic magnetic properties. They can also be used to produce miniaturized magnetic field sources for applications. In the latter case, the films must be patterned at a length scale comparable to the film thickness, because of the demagnetising field effect. The choice of film thickness will depend on the targeted application, nm-sized field sources require nm-thick films while µm-sized sources require µm-thick films. The film deposition rate becomes very relevant as the film thickness is increased, especially if the ultimate aim is for commercial production. What is more, deposition on large surface areas is needed to allow batch fabrication and integration of micro-magnets. We have previously reported on triode sputtering of 5 µm thick Nd-Fe-B films at deposition rates of up to 18 µm/h [1]. We then demonstrated how such films could be physically patterned at the micro-scale using a topographic approach based on pre-patterning of the substrate or post-deposition etching of the magnetic layer [2]. We also showed that localised heating by laser irradiation through a mask can be exploited to produce magnetisation reversal at the µm-scale in the so-called thermo-magnetic patterning approach [3]. Small arrays of micro-magnets produced using these approaches have already been used in a range of bio-medical experiments [4]. In this work we will demonstrate the potential that triode sputtering holds for batch fabrication of even thicker micro-magnets, for future integration into micro-scaled devices. Deep Reactive Ion Etching (DRIE) was used to anisotropically etch 55 µm deep trenches into Si substrates of diameter 100 mm and thickness 525 µm. Patterns consisting of squares with lateral size varying from 20 to 400 μm, and 4 mm long stripes with width varying from 20 to 400 μm, were used as test structures (Fig 1). Ta (100 nm) / Nd-Fe-B (50 µm) / Ta (100 nm) trilayers were deposited onto these topographically patterned substrates heated to a temperature of 500°C or 700°C. The NdFeB layer deposited at 500°C was amorphous, while that deposited at 700°C was crystallised. Post-deposition crystallisation of the former was achieved by annealing the full wafer in a Rapid Thermal Annealing (RTA) furnace, under secondary vacuum (10-6 mbar) at 600 °C, with a heating rate of 20°C/s and a dwell time of 10 minutes. Hereafter the samples will be referred to as 1-step processed (Tdep = 700°C) or 2-step processed (Tdep = 500°C + Tanneal = 600°C). Despite the relatively large thickness of the NdFeB layers, film peel-off does not occur. It should be noted that NdFeB films deposited onto non-patterned Si substrates start to peel off the substrate at a film thickness greater than 10 µm. The stress relief afforded by topographic patterning of the substrate depends on the dimensions of the pattern features and the film thickness, and some peel off was observed in 50 µm thick NdFeB layers deposited on square structures of width 500 µm prepared in another study. For the samples detailed here, some cracking of the deposit was observed in expansive regions far from etched features. SEM images of fractured cross-sections of stripes of two different width (200 µm and 30 µm) of each sample type are compared in Fig 2. Deposition on the sidewalls of the etched trenches leads to the deposition on the “hills” being wider than the underlying Si pillars. A shadowing effect results in the profiles of the deposit in the “valleys” being different to that on the neighbouring “hills”, and the effect becomes stronger as the feature size is reduced. The shadowing effect can be enhanced by increasing the depth of the trenches, as will be shown elsewhere. SEM imaging at higher resolution reveals that the NdFeB grains of the 1-step processed sample are columnar and have widths as large as 1 µm while those of the 2-step processed film are almost equiaxed with diameters in the range of 200 – 500 nm (not shown). Hysteresis loops measured on pieces taken from each sample are shown as insets to Fig 2. The 1-step sample has a coercivity of 1.2 T and an M0T/M7.5T of 0.79 while the corresponding values for the 2-step sample are 2.2 T and 0.85. The higher coercivity of the latter is due to its much finer microstructure and the 2-stage initial magnetization curve may be explained by easy domain wall motion in grains coarser than 300 nm, the single domain particle size of Nd2Fe14B, and domain wall pinning in finer grains. An in-house developed Scanning Hall Probe Microscope (SHPM) equipped with a Hall cross of active area size 4 x 4 µm2 [5], was used to measure the stray magnetic field produced at different heights above out-of-plane magnetized pieces of each sample. In this presentation we will provide full details of structural and magnetic characterization and will compare measured field profiles with simulated profiles. We will finish by discussing future prospects for the fabrication, integration and use of high-performance NdFeB micro-magnets.

**References:**

[1] N.M. Dempsey et al., “High performance hard magnetic NdFeB thick films for integration into Micro-Electro-Mechanical-Systems”, Appl. Phys. Lett. 90 092509 (2007) [2] A. Walther et al., “Micro-patterning of NdFeB and SmCo magnet films for integration into Micro-Electro-Mechanical-Systems”, J. Magn. Mag. Mat. 321 (2009) 590–594 [3] F. Dumas-Bouchiat et al., “Thermo-magnetically patterned micro-magnets”, Appl. Phys. Lett. 96, 102511 (2010). [4] N.M. Dempsey, “Magnetism and Biology” in " Handbook of Magnetic Materials", J.M.D. Coey et al. (eds), Springer, in-press [5] Gorky Shaw et al., “A Scanning Hall Probe Microscope for high resolution, large area, variable height Magnetic Field Imaging”, Rev Sci Inst 87 (2016) 113702

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/71d2432dd00b4772234c43bc8e6f423d.jpg)
Fig 1: Schematic of the pre-patterned substrates.

![](https://assets.underline.io/uploads/markdown_image/1/image/d17c28d143c41c74d8f313da1a848cdf.jpg)
Fig 2: SEM images of fractured cross-sections of stripes of width 200 µm (top) and 30 µm (bottom) prepared using 1-step (left) and 2-step processing. Insets: M(H) loops of pieces taken from 1-step (left) and 2-step processed samples.

 Batch fabrication of 50 µm thick anisotropic NdFeB micro-magnets

Hard magnetic nanocomposites, consisting of a fine mixture between a large magneto crystalline anisotropy phase and a large magnetization phase, are attractive materials both for integration of hardness-tuned magnets in microsystems and for building next-generation high performance permanent magnets for energy conversion technologies. Instead of relying on the hypothetical discovery of new hard magnetic compound, this approach is based on the mastering of the microstructure in a mixture of already known magnetic phases. Theoretical descriptions predicted a potential energy product of 1 MJ/m3, which is twice as large as the one of best Nd2Fe14B magnets produced today [1]. However, exploiting their full potential has appeared to be very challenging since the first reports on exchange spring structures in the 90’s. Indeed, calculations pointed out the absolute necessity to confine the softer phase in nano-sized grains, smaller than twice the domain wall width of the hard phase. In parallel, to maximize the magnetization, the hard phase volume fraction should be restricted to the minimum needed to develop enough coercivity. These requirements cannot be obtained with conventional process for mass scale material fabrication. In this context, investigations of model systems in films can give insights about the interplay between the nanoscale microstructure and the magnetic properties [2]. In particular, the degree of interdiffusion at the soft-hard interface is expected to significantly impact the magnetization reversal but is difficult to address experimentally. Here we focus on the impact of a graded soft-hard interface on magnetic properties. To do so, we recently prepared Co@FePt transition metal-based nanocomposite (NC) films from mass-selected low energy cluster beam deposition technique (MS-LECBD) of Co clusters, in situ embedded in FePt matrix independently produced by alternative electron gun evaporation on the same substrate [3,4,5]. The Co cluster inclusions can be selected in size prior to their deposition, between 1 and 10 nm. The cluster to matrix volume ratio is adjusted controlling the thickness of each Co, Fe, Pt individual layers. The as-prepared Co clusters have been observed by transmission electron microscopy (TEM) and found to crystallise in magnetically soft fcc phase. In this study, we focused on 8 nm Co clusters. The Co to FePt volume fraction was varied from 0 to 50%. Post-deposition annealing of the whole stack is a crucial step to drive the initial Fe and Pt multilayers to the high-magnetic anisotropy L10 phase. However, this inevitably comes with partial intermixing at the interface between the clusters and the matrix. We systematically compared the nanoclusters-based film with same stoichiometry and alloyed films (NF), obtained by sequential evaporation of Co, Fe, Pt thin and continuous layers. Local atomic structures have been thoroughly investigated using polarization-dependent hard x-ray absorption and element-specific spectroscopies (EXAFS, XLD). Transmission electron microscopy (TEM) equipped with energy-dispersive x-ray spectroscopy (EDX) was used to explore the local chemical profile of the composite films (figure 1). The macroscopic magnetic properties were measured in a Superconducting quantum interference device (SQUID) magnetometer together with local and element specific characterisations using circular dichroism (XMCD). We demonstrate the persistence of soft and hard regions and rule out the fully diluted hypothesis. Magnetic measurements (figure 2) showed a very different behaviour upon annealing between NF and NC films. Using a combinatorial approach, we investigated the coercive magnetic field decay when increasing the Co content. The magnetic coercivity is found to be systematically higher in the NC samples, over the whole range of studied composition. EXAFS spectra fitting and numerical modelling of the diffusion mechanism show that magnetically soft Co-rich inclusions remain after annealing, even at the high temperature required to obtain the L10 phase of the matrix. In the case of multilayer nanofilms with Co thin layers, the diffusion leads to homogeneous alloying. Local XMCD revealed the evolution of spin and orbital moments of Fe and Co as a function of the composition, further distinguishing the NC system with respect to fully homogeneous alloy. Studying these materials in model systems synthesised by nanofabrication routes provides interesting insights into the interplay between the microstructure and the magnetic performances in view to develop mass-scale production of such materials. This work is supported by the ANR collaborative project "SHAMAN": Soft in Hard MAgnetic Nanocomposites (2017-2020).

**References:**

1. Skomski R. and Coey J. M. D. Phys. Rev. B 48, 15812 (1993). 2. Balasubramanian B. et al, Scientific reports 4, 6265 (2014) 3. Tournus F. et al, Journal of Magnetism and Magnetic Materials, vol. 323, p. 1868 (2011) 4. Liu Y. et al, Appl. Phys. Lett. 99, 172504 (2011) 5. Paleo C. et al, Interplay between local structure and magnetic properties of graded exchange-coupled Co@FePt nanocomposite films. Phys. Rev. B 7 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/854d17b4235dd33402f5177028c56401.jpg)
Energy-dispersive x-ray images of a FePt/Co cluster/FePt sample on a Si3N4 substrate, along with the scanning transmission electron microscopy-annular dark-field (STEM-ADF) image.

![](https://assets.underline.io/uploads/markdown_image/1/image/8ac33347c0e0ae0e397c52a2e4bb7528.jpg)
Hysteresis loops at 300 K for two samples with the same Co content and a L10-FePt sample without Co. The NC sample has higher saturation magnetization than the bare L10-FePt ref., which is brought by the Co but retains a sizeable coercivity, whereas the NF is magnetically soft. Inset: Dependence of the coercive field μ0Hc on the Co content for the NF and NC.

 Fcc-Co Clusters in L10-FePt Matrix as Graded-Interface Magnetic Nanocomposite

**Session chairs:** Adrian Quesada, Institute of Ceramics and Glass (CSIC) and Alberto Bollero, IMDEA Nanociencia



DOC - Non-Rare Earth Permanent Magnet Materials and Nanocomposites I

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Presentations

Magnetization reversal in nanofabricated soft-in-hard exchange-spring magnets

Compositionally graded La-Fe-Si films

Key steps in designing a micro thermomagnetic generator

Structural, magnetic and mechanical properties of Gd films

Batch fabrication of 50 µm thick anisotropic NdFeB micro-magnets

Fcc-Co Clusters in L10-FePt Matrix as Graded-Interface Magnetic Nanocomposite

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