United States

Magnetic wires can present outstanding magnetic properties such as giant magnetoimpedance (GMI) or magnetic bistability and related single domain wall (DW) propagation. Magnetically soft glass-coated magnetic microwires prepared by the Taylor-Ulitovsky technique with metallic nucleus covered by flexible, insulating and biocompatible glass are quite demanded for a great number of applications [1]. Recenly, we showed that stress-annealing allows magnetic softening, GMI effect improvement and even grading the magnetic anisotropy in Fe-rich microwires [2].&lt;br/&gt;Although generally thinner wires present certain advantages related to the industrial application, for some applications (like non-contact stress monitoring or magnetic tags) thicker wires are highly requested. Recently, preparation of thicker (above 50 μm in diameter, d) glass-coated Fe-rich microwires is reported [3].&lt;br/&gt;In this paper, we present our recent experimental results on preparation and processing of “thick” Co-rich magnetic microwires (d = 94 μm) with graded magnetic anisotropy. From previous publications related to the effect of stress-annealing on magnetic anisotropy of Fe-rich microwires, is known that the stress-annealing induced anisotropy depends on the annealing temperature, &lt;i&gt;T&lt;sub&gt;ann &lt;/sub&gt;&lt;/i&gt;[2]. Such graded magnetic anisotropy has been obtained by stress-annealing of Co-rich microwire at variable annealing temperature. The hysteresis loops for different sections of the microwire have been evaluated using a short (2 mm long) movable pick-up coil.&lt;br/&gt;A gradual change in the hysteresis loop (measured by the short pick-up coil) of the sample stress-annealed at variable &lt;i&gt;T&lt;sub&gt;ann&lt;/sub&gt;&lt;/i&gt; is observed (see Fig.1). Such microwire, presents also variable squareness ratio, &lt;i&gt;M&lt;sub&gt;r&lt;/sub&gt;/M&lt;sub&gt;0&lt;/sub&gt;&lt;/i&gt;, along the microwire length, &lt;i&gt;L&lt;/i&gt; (see Fig.1). Obtained graded anisotropy has been attributed to a gradual modification of the domain structure along the microwire.&lt;br/&gt;Previously, for preparation of thin films with a graded magnetic anisotropy, rather sophisticated methods were used, including a change in the chemical composition during the deposition of thin films [4]. Accordingly, we propose a rather simple route to design graded magnetic anisotropy in a magnetic microwire.

![](https://assets.underline.io/uploads/markdown_image/1/image/57c8ead2c65eb757399987363b53f20c.jpg)



2022 Joint MMM-INTERMAG

Graded magnetic anisotropy in Co-rich microwires

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

For tickets to the event, please register at the following link: https://magnetism.org/register

Please register for the Joint MMM-Intermag conference to join the event.

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

Magnetic wires can present outstanding magnetic properties such as giant magnetoimpedance (GMI) or magnetic bistability and related single domain wall (DW) propagation. Magnetically soft glass-coated magnetic microwires prepared by the Taylor-Ulitovsky technique with metallic nucleus covered by flexible, insulating and biocompatible glass are quite demanded for a great number of applications [1]. Recenly, we showed that stress-annealing allows magnetic softening, GMI effect improvement and even grading the magnetic anisotropy in Fe-rich microwires [2]. Although generally thinner wires present certain advantages related to the industrial application, for some applications (like non-contact stress monitoring or magnetic tags) thicker wires are highly requested. Recently, preparation of thicker (above 50 μm in diameter, d) glass-coated Fe-rich microwires is reported [3]. In this paper, we present our recent experimental results on preparation and processing of “thick” Co-rich magnetic microwires (d = 94 μm) with graded magnetic anisotropy. From previous publications related to the effect of stress-annealing on magnetic anisotropy of Fe-rich microwires, is known that the stress-annealing induced anisotropy depends on the annealing temperature, Tann [2]. Such graded magnetic anisotropy has been obtained by stress-annealing of Co-rich microwire at variable annealing temperature. The hysteresis loops for different sections of the microwire have been evaluated using a short (2 mm long) movable pick-up coil. A gradual change in the hysteresis loop (measured by the short pick-up coil) of the sample stress-annealed at variable Tann is observed (see Fig.1). Such microwire, presents also variable squareness ratio, Mr/M0, along the microwire length, L (see Fig.1). Obtained graded anisotropy has been attributed to a gradual modification of the domain structure along the microwire. Previously, for preparation of thin films with a graded magnetic anisotropy, rather sophisticated methods were used, including a change in the chemical composition during the deposition of thin films [4]. Accordingly, we propose a rather simple route to design graded magnetic anisotropy in a magnetic microwire.

![](https://assets.underline.io/uploads/markdown_image/1/image/57c8ead2c65eb757399987363b53f20c.jpg)

technical paper

Magnetic two-dimensional (2D) van der Waals (vdW) materials show promising magnetic properties, which can be utilized to build spintronic devices with improved performance and new functionalities. However, most vdW magnets have Curie temperatures below 300 K. With molecular beam epitaxy (MBE) [1], we recently synthesized 1T-phase single-crystal CrTe2 thin films [2], which show ferromagnetism and strong perpendicular magnetic anisotropy at room temperature. We have also studied the magnetic domain evolutions and the anomalous Hall effect (AHE) in these samples at different temperatures and magnetic fields. As shown in Fig. 1, MFM has revealed clear magnetic stripe domains with out-of-plane polarization in the 16 ML CrTe2 film at room temperature. The magnetic stripe domains still exist upon applying an out-of-plane magnetic field, but the width becomes narrower at +1.2 kOe and wider at -1.2 kOe, respectively. We have patterned the samples into Hall bars and measured electron transport properties at different temperatures. Both the 8 ML and 16 ML CrTe2 films show AHE up to room temperature. However, the AHE changes sign between 100 K and 150 K. This indicates intriguing magnetic domain evolution at lower temperatures. Further work has been carried out toward a deeper understanding of the magnetization in CrTe2. Our study has highlighted CrTe2 as a unique platform for studying vdW magnetism and opened a new avenue for developing large-scale 2D magnet-based spintronics devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/56f0f120d0aa30f2da3b95b65bbc1a07.jpg)

Fig.1: MFM images showing stripe domains for the 16 ML CrTe2 at H = 0 (a), +1.2 kOe (b) and -1.2 kOe (c), respectively.

![](https://assets.underline.io/uploads/markdown_image/1/image/8ef809c914f5e6a06754ccaa6457d940.jpg)

Fig.2: a. Anomalous Hall resistance vs. H at different temperatures for 8 ML CrTe2. b, c. Dependence of anomalous hall resistance (RAHE) (b) and coercivity (c) vs. temperature for 8 ML and 16 ML CrTe2.

Magnetization evolution in room-temperature van der Waals magnet CrTe2

Anomalous Nernst effect (ANE) has drawn special attention during the last few years because of its possibility in thermal energy harvesting and fundamental physics involved [1]. Reports on ANE in perovskite oxides are relatively few compared to ferromagnetic metallic alloys. The ANE was previously reported in Sr substituted LaCoO3 [2]. Because of the smaller size of Ca2+ cation compared to Sr2+, magnetic interactions are expected to be modified. Hence, we studied the ANE in La0.5Ca0.5CoO3 for the first time, which shows ferromagnetic ordering at 147 K (Fig. 1(a)). The non-saturating nature of the field dependence of magnetization and its smaller value shown in the inset of Fig. 1(a) indicates the existence of competitive FM-AFM interactions that largely depends on the spin states of the Co3+ and Co4+ ions[2-4]. Such a system can be very interesting to study ANE as the aforementioned competitive effect may dramatically affect the spin-dependent deflection of thermally driven charge carriers. The presence of the ANE in La0.5Ca0.5CoO3 is confirmed from the field dependences of the Nernst thermopower (Sxy) as shown in Fig. 1(b), which shows hysteresis like the magnetization. In this preliminary work, we have carried out ANE measurements only at four selected temperatures. Sxy nearly saturates above 20 kOe at 40 K but not at 100 K, where it has a maximum value at the highest field. The Sxy shows behavior similar to superparamagnetic at 160 K and linear behavior at 200 K with much-reduced values. Magnetic, electrical transport, thermoelectric measurements, and ANE at multiple temperatures will be carried out to realize the mutual effects of magnetism, charge, and heat currents to establish a better relationship between them. This work was supported by the Ministry of Education, Singapore (Grant no. R144-000-422-114 and R144-000-442-114).

![](https://assets.underline.io/uploads/markdown_image/1/image/70f4701c2741c601a6cc175160e1bb04.jpg)

Fig. 1: (a) Temperature dependence of magnetization (M) in La0.5Ca0.5CoO3 . Inset: Field dependence of (b) M at 5 K. and (b) Nernst thermopower (Sxy) measured at T = 40, 100, 160 and 200 K.

Anomalous Nernst effect in perovskite La0.5Ca0.5CoO3

Controlling ion migration of transition metal oxide(TMO) to recast crystal structure and macroscopic properties is current research emphasis [1, 2], which has potential applications in solid fuel cells, resistance switching etc. Among cobalt-based TMO, B-site atom determines functionality, but oxygen atom is hard to ignored. Electronic structure of B-site atom is profoundly affected by oxygen content due to coupling of lattice, charge, orbit, and spin. Vacuum annealing is a stable method to extract O2--ions [3, 4]. 40nm La0.5Sr0.5CoO3(P-LSCO) film are grown on LSAT(100) substrate. Then films are respectively annealed at 400 centigrade for 2 hours(B-LSCO) and 500 centigrade for 35 hours(S-LSCO) to extract O2--ions. Fig. 1a shows soft X-ray absorption spectrum(XAS) for cobalt L-edge, and an obvious peak shift to low energy (L3-peak of S, B and P-LSCO appear at 782.2eV, 781.5eV and 780.4eV respectively) indicates continuous decrease in average valence states of Co and oxygen content during O2--ions extracting process. For XAS of oxygen K-edge (Fig. 1b), the A and C peaks, representing hybridization of Co-2p/3d and O-2p orbital, gradually decrease in three samples. While B-peak, which represents content of oxygen vacancy, exhibits an increasing trend, consistent with above results. As a result, dramatic magnetism transformation was observed in film upon annealing. Fig. 1c and 1d show magnetic moment-temperature(M-T) curves and magnetic moment- magnetic field (M-H) curves for three samples, demonstrating ferromagnetism(FM) with Tc~220K, antiferromagnetism(AFM) and ferromagnetism with Tc&gt;300K in P-LSCO, B-LSCO and S-LSCO, respectively. In sum, we achieved giant magnetic transformation from FM to AFM to FM accompanying with O2--ion migration in LSCO film by vacuum annealing, which provides idea for regulating physics properties by ion migration engineering.

![](https://assets.underline.io/uploads/markdown_image/1/image/2f9d13eeec602e127c33a77c98f889f5.jpg)

Fig. 1 XAS for a) cobalt L-edge, b) oxygen K-edge, c) M-T curve, d) M-H curve

Oxygen ion migration regulates magnetism in La0.5Sr0.5CoO3 film

Nowadays, chromium-based normal spinel oxides ACr2O4 are one of the most studied materials in the condensed matter community due to the interplay between its magnetic, electric and structural properties as well as to its potential application to different key industry sectors. In these compounds, several physical effects have been observed, which include magnetostriction, colossal magnetoresistance, multiferroic, spin frustration and more [1-3]. In particular, for MnCr2O4, the ground state magnetic structure is still controversial because the magnetic structures reported by different groups and investigated by independent techniques are inconsistent [1-6]. The magnetic structure of this compound was reinvestigated by magnetization, specific heat and neutron diffraction experiments at different temperatures. The results suggested that a new magnetic phase, not previously reported, is developed under 18 K when the sample is synthesized under a reductive atmosphere. The magnetic phases present in this sample were: long-range ferrimagnetic order below TC = 45 K; incommensurate conical spin order with propagation vector kS1 = (0.62(1), 0.62(1), 0) below TS1 = 20 K; and incommensurate conical spin order with propagation vector kS2 = (0.660(3), 0.600(1), 0.200(1)) below TS2 = 18 K. Using the superspace group formalism [7, 8], the symmetry of the nuclear and magnetic structures is described. The presence of transverse conical magnetic structures in the lower-temperature phases implies the existence of multiferroicity. Using simple theoretical calculations, we derive the directions along which the electric polarization lies for each magnetic phase.

![](https://assets.underline.io/uploads/markdown_image/1/image/ab2df24cc036f478f8e3383173532d06.jpg)

Incommensurate Magnetic Phases of the Multiferroic Compound MnCr2O4 Described with the Superspace Formalism

Incommensurate Magnetic Phases of the Multiferroic Compound MnCr2O4 Described with the Superspace Formalism

Single-phase multiferroic materials that allow the coexistence of ferroelectric and magnetic ordering above room temperature are highly desirable, motivating an ongoing search for mechanisms for unconventional ferroelectricity in magnetic oxides. Here, we report an antisite defect mechanism for room temperature ferroelectricity in epitaxial thin films of yttrium orthoferrite, YFeO3, a perovskite-structured canted antiferromagnet [1]. A combination of piezoresponse force microscopy, atomically resolved elemental mapping with aberration corrected scanning transmission electron microscopy and density functional theory calculations reveals that the presence of YFe antisite defects facilitates a non-centrosymmetric distortion promoting ferroelectricity. This mechanism is predicted to work analogously for other rare earth orthoferrites, with a dependence of the polarization on the radius of the rare earth cation. Furthermore, a vertically aligned nanocomposite consisting of pillars of a magnetoelastic oxide CoFe2O4 embedded epitaxially in the YFeO3matrix exhibits both robust ferroelectricity and ferrimagnetism at room temperature, as well as a noticeable strain-mediated magnetoelectric coupling effect [2]. Our work uncovers the distinctive role of antisite defects in providing a novel mechanism for ferroelectricity in a range of magnetic orthoferrites and further augments the functionality of this family of complex oxides for multiferroic applications.

Defect mechanism for ferroelectricity in orthoferrites

One of the most promising ways of improvement of thermoelectric energy conversion leads via band gap engineering towards the enlargement of the density of states at the Fermi level, e.g. via a co-doping to induce a resonant level in the electric transport relevant band [1]. Cr in PbTe is a particularly interesting example since Cr atoms form mixed valence Cr2+/3+ donor centers resonant with the PbTe conduction band [2]. We report on detailed temperature and magnetic field dependence of magnetization of PbTe doped with transition metal Cr2+/3+. The material is studied by a SQUID magnetometer in order to quantitatively determine the contribution of single substitutional Cr2+/3+ as well as of various Cr-Te magnetic nanocrystals, including their identification. The applied experimental procedure reveals the presence of about 1019 cm-3 paramagnetic Cr ions, of which 2/3 are the Cr3+ ions formed via self-ionization of Cr2+ resonant donors. The magnetic finding agrees with previous Hall effect studies thus providing a new experimental support for the proposed electronic structure model of PbTe:Cr system with resonant Cr2+/3+ state located (at low temperatures) about 100 meV above the bottom of the conduction band. Below room temperature a ferromagnetic-like signal points to the presence of Cr-rich nanocrystalline precipitates. Two most likely candidates, namely: Cr2Te3 and Cr5Te8 are identified upon dedicated temperature cycling of the sample at the remnant state. As an ensemble, the nanocrystals exhibit (blocked) superparamagnetic properties. Also HR-TEM confirmed the presence of numerous single nanometer size precipitates of Cr-Te compounds. The magnetic susceptibility of both n- and p-type PbTe in the temperature range 100 &lt; T &lt; 400 K has been established. These magnitudes are essential to properly account for the high temperature magnetic susceptibility of PbTe:Cr [3]. This study has been supported by the National Science Centre (Poland) through project OPUS (UMO - 2017/27/B/ST3/02470) and by the National Science Centre for Development (Poland) through grant TERMOD No TECHMATSTRATEG2/408569/5/NCBR/2019 and by the Foundation of Polish Science through the IRA Programme co-financed by EU within SG OP.

Magnetic Constitution of PbTe:Cr - Experimental Aspects

We find the magnetic susceptibility of NH4Al(SO4)2-12H2O below its ferroelectric Curie temperature of TC = 58 K can be tuned to become positive or negative simply by controlling the cooling speed. We explain this by taking into consideration that reorienting NH4+ ions possess weak magnetic moments that are linearly coupled to dipole moments which in turn are aligned by the crystal field with cooling but at the same time experiencing competition from geometric frustration. With fast cooling such as a rate of 1.0 K/min, the magnetic moments are not able to become ordered in time so the system remains mostly diamagnetic. In contrast, with a slow cooling rate of 0.1 K/min, the magnetic moments are allowed to access a more ordered state so the magnetic susceptibility becomes positive. From such, four -ferroic states can be accessed below TC by applying either a positive or negative bias field and cooling the sample slow or fast to obtain polarization and magnetization of different polarities. Specific heat and dielectric constant measurements near TC indicate the origin of multiferroic behavior as originating from proton orbital degeneracies. Our findings identify an entire new class of multiferroic materials in the ammonium alums and possibly other hydrogen-bonding compounds.

Multiferroic Behavior in NH4Al(SO4)2-12H2O: Magnetism and Ferroelectricity Based on Proton Orbital Degeneracies

Magnetic materials in which it is possible to control the topology of their magnetic order in real space or the topology of their magnetic excitations in reciprocal space are highly sought-after as platforms for alternative data storage and computing architectures. Here we show that multiferroic insulators, owing to their magneto-electric coupling, offer a natural and advantageous way to address these two different topologies using laser fields. We demonstrate that via a delicate balance between the energy injection from a high-frequency laser and dissipation, single skyrmions—archetypical topological magnetic textures—can be set into motion with a velocity and propagation direction that can be tuned by the laser field amplitude and polarization, respectively. Moreover, we uncover an ultrafast Floquet magnonic topological phase transition in a laser-driven skyrmion crystal and we propose a new diagnostic tool to reveal it using the magnonic thermal Hall conductivity.

![](https://assets.underline.io/uploads/markdown_image/1/image/7db0278173bbc5cba45a88101bb6611a.jpg)

Real- and reciprocal-space topology can be controlled by lasers. (a) The topological spin structure of a skyrmion carrying in-plane electric polarization Ρ undergoes translational motion under a circularly polarized laser. (b) An ultrafast topological phase transition occurs in the Floquet magnon band structure of skyrmion crystals due to the effective magnetic field induced by circularly polarized laser irradiation.



Laser-Controlled Real and Reciprocal Space Topology in Multiferroic Insulators

Ferrites and magnetic nanoparticles (NPs) are widely used in sectors such as energy, electronics and biomedicine [1,2]. This work shows an optimum combination of co-precipitation, milling and reduction process of ferrite precursors (CoFe2O4 and NiFe2O4) to obtain CoFe and NiFe NPs, respectively, with tailored magnetic properties and improved heat generation for magneto-thermal applications. Ferrite precursor powders have been synthesized by co-precipitation [3] followed by nanostructuring through self-developed ultrafast milling (24 min). Reduction at different temperatures under Ar+H2 has led to NPs with a tuneable composition going from pure ferrite to CoFe and NiFe NPs, with core-shell nanostructures (CoFe2O4-CoFe and NiFe2O4-NiFe) at intermediate reduction stages. An optimized temperature has resulted in CoFe and NiFe NPs with a high saturation magnetization of 235 and 149 emu/g, respectively. Milling has allowed for diminishing the reduction temperature in 100 °C compared to that required for non-milled particles, which is of relevance in view of upscaling (Fig. 1) [4]. The magneto-thermal response of the NPs has been studied by exposure to alternating magnetic fields at 103 kHz for 5 min. Remarkably, an increase of 28% in the heat generated (under an applied field of 20 mT) has been obtained for the reduced milled CoFe NPs in comparison to the non-milled NPs (Fig. 2), due to particle refinement and high saturation magnetization. The milled NiFe NPs has shown a lower heat generation due to agglomeration effects. This cost-efficient and scalable approach eases the synthesis of NiFe and CoFe NPs for magneto-thermal applications. Acknowledgments. Authors acknowledge financial support from EU M-ERA.NET through COSMAG project (PCI2020-112143); from MICINN through NEXUS project (PID2020-115215RB-C21); and from CM through NanoMagCOST project (P2018/NMT-4321). A.J.C.-H. acknowledges support from "La Caixa" by the Doctoral INPhINIT Incoming program (LCF/BQ/DI20/1178002).

![](https://assets.underline.io/uploads/markdown_image/1/image/3d3878bca20e35efefe648bf0daa5801.jpg)

Fig.1. (a) Room temperature hysteresis loops and (b) XRD patterns of milled cobalt ferrite NPs reduced at different temperatures.

![](https://assets.underline.io/uploads/markdown_image/1/image/1e335e0631e7215287a7642ecf21df92.jpg)

Fig.2. Magneto-thermal measurements of milled and non-milled CoFe NPs.

Scalable Synthesis of CoFe and NiFe Nanoparticles from Ferrite Precursors with Tailored Magneto-Thermal Properties by a Combined Co-precipitation, Milling and Reduction Process

Metal Amorphous Nanocomposites (MANCs) are an evolving class of soft magnetic materials consisting of an Fe- or Co-based crystalline phase embedded in an amorphous matrix. In thin-film form, they possess highly tunable magnetic properties, low eddy current and hysteresis losses, and are capable of performing under high-frequency magnetic field reversal. Therefore they are good candidates for a range of applications including use in power electronics, transformers, and electric motors[1, 2]. We investigated the structural, static and dynamic properties of magnetron sputtered and vacuum annealed (5 minutes and 20 minutes at 520oC) films of CoFeMnNbSiB using Transmission Electron Microscopy (TEM), Atom Probe Tomography (APT), Vibrating Sample Magnetometry (VSM) and Ferro-Magnetic Resonance (FMR). The sputtered films did not show crystallinity, however, an Mn-rich columnar structure that spanned the thickness of the film parallel to the growth direction was observed. After annealing for 5 minutes and 20 minutes, the columnar structure transformed to a partially crystallized microstructure with Co and Fe enriched crystallized phases with ~7nm grain size enclosed by an amorphous intergranular region as shown in Fig. 1. The magnetization dynamics of the films were measured using a custom-built instrument with a frequency range of 1-65 GHz and fields up to 1.6 T. The FMR spectrum of annealed samples shows two overlapping resonances (Fig. 2) due to the two phases present in the samples whereas the as-sputtered sample shows only one resonance. The effective magnetization remained constant whereas the linewidth increased from 13 to 55 Oe after annealing. The magnetic coercivity was observed to be (~26 Oe) independent of annealing while the saturation magnetization increased from 828 to 953 emu/cm3. The observed changes in magnetic properties with annealing indicate that short-time annealing can be useful in optimizing the performance of these MANCs and tailoring properties for various applications.

![](https://assets.underline.io/uploads/markdown_image/1/image/8e0cc2fd197e33d1445aaa05db782123.jpg)

Fig. 1.a) TEM image of 20 minutes annealed sample b) Magnified section of a crystalline region

![](https://assets.underline.io/uploads/markdown_image/1/image/5a0af14f69f8ff951824fdad5f91dd49.jpg)

Fig. 2. FMR spectrum of 20 minutes annealed sample at 40 GHz reflecting two overlapping peaks due to dual-phase in the sample

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Magnetization evolution in room-temperature van der Waals magnet CrTe2

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Scalable Synthesis of CoFe and NiFe Nanoparticles from Ferrite Precursors with Tailored Magneto-Thermal Properties by a Combined Co-precipitation, Milling and Reduction Process

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Next from 2022 Joint MMM-INTERMAG

Magnetization evolution in room-temperature van der Waals magnet CrTe2

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Scalable Synthesis of CoFe and NiFe Nanoparticles from Ferrite Precursors with Tailored Magneto-Thermal Properties by a Combined Co-precipitation, Milling and Reduction Process

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