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The coercivity of RE2Fe14B-type permanent magnet is strongly influenced by the microstructural features such as grain boundary phases as well as grain size. Micromagnetic simulations are powerful tools for revealing the correlation between microstructure and the evolution of hard magnetic properties, including the effects of grain size and grain boundary phases. We have combined micromagnetic simulations and experiments to elucidate the role of excess RE (Nd/Pr) in determining the resulting hard magnetic properties of Nd-Pr-Fe-B melt-spun ribbons. Also, the effect of non-magnetic grain refining refractory carbide (TiC) on both the microstructure and magnetic hardening was studied. For the simulated polycrystalline microstructure of Nd-Pr-Fe-B magnets (Fig. 1(a)) with a mean grain size of 40 nm, the coercivity increases with decreasing inter-grain magnetic exchange interaction Fig. 1(b). The increasing trend in magnetic coercivity of measured melt-spun (Nd, Pr)–Fe–B alloys is ascribed to weak inter-grain exchange coupling due to non-magnetic RE-rich grain boundary phase (Fig. 1(c)) and/or TiC at grain boundary (Fig. 1(d)). This work provides useful information on the role of non-magnetic grain boundary phase to improve the coercivity in Nd-Pr-Fe-B magnets. Combined with experimental and modeling results, we will discuss the mechanism responsible for the enhancements in coercivity and the suitability of the alloys for high performance permanent magnet development. This research was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office.

MMM 2020

Understanding the Enhancement in Hard Magnetic Properties of Rapidly Solidified (Nd, Pr)-Fe-B Melt-spun Ribbons

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High-frequency ferrite ceramics exhibiting high permeability and low magnetic loss have been recently attracting more and more attention owing to the rapid development of modern communication technologies.1-3 It is known that the magnetic performance of ferrites is strongly associated with the morphology of their polycrystalline structure.4 During the final-stage sintering, densification is typically accompanied by rapid grain growth.5 In this work, we focus on the control of grain growth and densification process for planar Co-Ti doped M-type barium hexaferrites, BaCo1.2Ti1.2Fe9.6O19, by one-step sintering (OSS) and two-step sintering (TSS) techniques. Experimental results in Fig. 1. indicate that the microstructure of the samples prepared by TSS shows visibly finer and more uniformity compared to that of the samples prepared by conventional OSS, evidence in that the grain growth during the second-step sintering of TSS is successfully suppressed while the density continues to increase. More importantly, the TSS technique shifts the resonant frequency to a higher value as shown in Fig. 2. In addition, the uniformity of the microstructure contributes to a narrower resonant peak. The reshaping of the magnetic spectra leads to a significant reduction of magnetic loss below 300 MHz. The ferrites prepared by TSS demonstrate high permeability (μ' = 18-20), low magnetic loss (tan δm = 0.1-0.3), and high Snoek's product (>25 GHz) in the frequency range of 100-400 MHz. By fitting experimental data, we have determined that low magnetic loss is derived from the small damping coefficient of spin rotation in terms of Kittel's theory. Therefore, the TSS technique provides an effective and efficient approach to prepare planar BaM hexaferrite materials for ultra-high frequency (UHF) communication devices requiring low loss and high Snoek's product.References: 1V. G. Harris, Z. Chen, Y. Chen et al., J. Appl. Phys., Vol. 99, p. 08M911 (2006). 2V. G. Harris, A. Geiler, Y. Chen et al., J. Magn. Magn. Mater., Vol. 321, p. 2035 (2009). 3V. G. Harris, IEEE Trans. Magn., Vol. 48, p. 1075 (2012). 4Q. Li, Y. Chen, and V. G. Harris, J. Magn. Magn. Mater., Vol. 453, p. 44 (2018). 5I. W. Chen and X. H. Wang, Nature, Vol. 404, p. 168 (2000).

Permeability Spectra of Planar M-type Barium Hexaferrites with High Snoek’s Product by Two-Step Sintering

Hexagonal ferrite materials such as BaFe12O19 and SrFe12O19 have strong anisotropic magnetic field and strong remnant magnetism. These properties enable the working frequency of devices such as phase shifter, isolator and circulator, move up to tens of GHz without a strong external bias field and even achieve self-biasing. Other features such as large saturation magnetization, large anisotropy, and low dielectric loss and most importantly, its potential of achieving self-bias, paves the way to fabrication of planar ferrite device that can play a crucial role in a next generation wireless system. The advantage of ferrite devices are their low insertion loss, high isolation, broad bandwidth and high power handling ability compared to their active counterparts. If the fabrication process of aforementioned devices can be integrated into the mature CMOS process flow as an option, it will gain great competiveness as regards the cost. In this work, thin film ferrite material was deposited by employing spin-casting method [Liu 2013] and patterned with the help of Chemical-Mechanical Polishing (CMP). The spin-casting is a low-temperature process: the nano-sized barium ferrite materials can be processed into composite and spin-casted into thin film layer. This method requires only 120°C baking under external biasing filed. In this research, we present the design, simulation, fabrication and measured results of a self-bias micro-strip line Barium hexagonal Nano-ferrite (BaM) circulator on silicon wafer as Fig. 1. This planar Y-junction circulator is 2mm by 2mm by 0.5mm in size, which is capable of future integration with the top three layers of 180nm CMOS technology. Ferrite thin film is deposited and patterned employing composite spin-casting method. Typical characterization techniques are employed together with free-space quasi-optical spectrometry to study complex permittivity and permeability of deposited film up to 120GHz. S parameters of fabricated circulators are characterized by a set of on wafer probes up to 67GHz as Fig. 2. We observed over 15dB non-reciprocal phenomenon at 55GHz.References: Liu C, Sholiyi O, Afsar M N and Williams J D (2013), “Characterization of micro-structured ferrite materials: course and fine barium, and photoresist composite,” Magnetic, IEEE Transactions on, vol. 49, pp.4319-4322, July 2013.

Hexagonal Nano-ferrites used on a V-band Self-bias On-chip Circulator for CMOS

The present work reports the synthesis of Fe3O4 nanotubes (NTs), nanorods (NRs), and nanodiscs (NDs) prepared by microwave-hydrothermal route with varying ratio of sodium phosphate/sodium sulfate ions. The Rietveld refinement analysis of the X-ray diffraction patterns divulges the presence of a metallic iron phase decreasing gradually from NTs ~16 %, NRs ~11 % to NDs ~ 0% with increasing concentration of phosphate ions as confirmed later by Mossbauer data. A sharp drop in the coercive field from NTs (~252 Oe) to NDs (~182 Oe) indicates clearly a magnetic vortex state in NDs as also reported by Lorenz microscopy and micromagnetic simulation [1]. Further, a reduction in Verwey transition temperature (NTs to NDs) is observed in zero fields cooled magnetization measurement (Figure 1(a)) related to a gradual loss of stoichiometry for Fe3O4 nanostructures. The obtained results are understood based upon a core-shell structure consist of stoichiometric Fe3O4 surrounded by a non-stoichiometric one. The magnetic vortex NDs are further analyzed for their utilization in magnetic hyperthermia applications [2,3]. The specific absorption rate (SAR) for NDs is calculated by orientation-dependent hysteresis loss through micromagnetic simulation and found to be 16 % higher at θ = 15o as compared to randomly oriented NDs, where q is the angle between magnetization (M) and applied field (H). Interestingly, the switching field (HS) of magnetic vortex NDs is responsible for a larger hysteresis loss at θ = 15o which is understood by developing a relationship with anisotropy constant (K). The theoretical SAR (~ 182 W/gNPs) of NDs is almost constant for aspect ratio β( = t/d) ≤ 0.13 whereas it is dramatically enhanced by ~5 times (~ 862 W/gNPs) at β = ~ 0.26, which is assigned due to the contribution of shape anisotropy. Our results indicate the implication of switching field (HS), orientation, anisotropy (K), and geometrical parameters (d & t) for improving the heating performance of Fe3O4 nanodiscs.References: 1. H. M. Fan, J. B. Yi, Y. Yang. ACS Nano 3:2798–2808 (2009). DOI: 10.1021/nn9006797 2 Y. Yang, X. Liu, and Y. Lv, Adv Funct Mater 25:812–820(2015) . DOI: 10.1002/adfm.201402764 3. D. H. Kim, E.A. Rozhkova, and I. V. Ulasov. Nat Mater 9:165–171(2010). DOI: 10.1038/nmat2591

Switching Field, Orientation and Shape Anisotropy Contribution on the Magnetic Heating of Fe3O4 Nanodisc

With the development of AC/DC UHV grid, large capacity transformer is widely used in power grid. After DC resistance measurement, no-load test and other operations of power transformer, mainly due to hysteresis phenomenon of ferromagnetic materials, a certain magnetic flux is left in the core, which is called residual flux. When transformer switching on without load, residual flux will lead to inrush current which is several times of the rated current. Inrush current endangers the mechanical stability and insulation strength of power transformer windings, and affects the normal operation of power system, resulting in many adverse effects. In this paper, an improved method is proposed to establish the relationship between residual flux and transient current characteristics by analyzing the transient current waveform of winding under different residual flux with different polarity DC voltage. DC voltage amplitude is determined by a criterion that the residual flux in the core should not change over 10% after the DC voltage unloaded. Based on the previous studies on positive and negative transient current, the relationship between transient characteristics and residual flux is further studied and analyzed. Combined with theory and experiment, the form of empirical formula and parameter calculation flow are determined. Compared with directly using transient current amplitude at a certain moment to calculate, using waveform characteristics to calculate residual flux is more accurate and less susceptible to measurement error and noise.References: C. Huo, Y. Wang, S. Wu, Y. Yang, and Z. Zhao, "Residual Flux Density Measurement Method for Transformer Core Considering Relative Differential Permeability," IEEE Trans. Magn., pp. 1-1, 2020. G. Wenqi, W. Youhua, Z. Zhenghan, Y. Xiaoguang, and L. Yongjian, "Residual Flux in the Closed Magnetic Core of a Power Transformer," IEEE Trans. Appl. Supercond., vol. 24, no. 3, pp. 1-4, 2014. W. Ge, J. Zhao, and Y. Wang, "Analysis of the residual flux influence on inrush current and electromagnetic force in large power transformer," The Journal of Engineering, vol. 2019, no. 16, pp. 2426-2429, 2019.

Improved Residual Flux Measurement Method based on Transient Current Characteristics used in Single-Phase Power Transformer

Spintronic with garnets materials is an emerging field that promises the use of pure spin currents to realize low power electronics thanks to the insulating character and low Gilbert dampings of this family of materials. The recent demonstration of the possibility to control the magnetization dynamics with a DC current through spin-orbit-torques (SOT) provides some great perspectives in tunable spin wave devices for magnonic applications. An example illustrating these opportunities is the auto-oscillations that can be induced in YIG/Pt bilayers using SOT [1] or compensate the damping of propagating spin-waves [2]. In Bi doped YIG/Pt that shows a perpendicular anisotropy, coherent spin-waves emission can be generated by SOT [3]. However, SOT induced irreversible switching of magnetization has only been observed so far in Tm3Fe5O12 (TmIG) thin films covered with Pt, with the disadvantage that TmIG has a much larger Gilbert damping than YIG and BiYIG [4], which limits the scope of applications. In this study, we grew some series of epitaxial TmIG with different thicknesses (from 6 to 50nm) by pulsed laser deposition (PLD) on sGGG(111)-oriented substrate. Their structural characterization demonstrated the high epitaxial strain of the films up to a thickness of 25nm. Epitaxial strain could be maintained up to films thickness of 50 nm as shown in the XRD pattern in Figure 1. Kerr microscopy was used to image the magnetics domains of the samples and their hysteresis loops. The results attest the strong perpendicular magnetic anisotropy of these TmIG samples as depicted in Figure 2. Magnetic dynamical properties were probed by broadband ferromagnetic resonance with a coplanar waveguide. We hence found that the Gilbert damping of TmIG can still be improved for very thin films by tailoring the growth parameter to achieve a ferromagnetic linewidth of 72 Oe at 12 GHz, narrower than recent results in literature [5,6]. This work has been funded by the European Union’s Horizon 2020 research and innovation program within the FET-OPEN project CHIRON under grant agreement No. 801055.References: [1] V. E. Demidov, S. Urazhdin, G. De Loubens, et al., Physics Reports, 673, 1-31 (2017) [2] V. E. Demidov, S. Urazhdin, A. Anane, et al., Journal of Applied Physics, 127, 170901 (2020) [3] M. Evelt, L. Soumah, A. B. Rinkevich, et al., Physical Review Applied, 10 (4), 041002 (2018) [4] L. Soumah, N. Beaulieu, L. Qassym, et al., Nature Communications, 9, 3355 (2018) [5] C. Tang, P. Sellappan, Y. Liu et al., Physical Review B, 94, 140403(R) (2016) [6] O. Ciubotariu, A. Semisalova, K. Lenz et al., Scientific Reports, 9, 17474 (2019)

Narrow Ferromagnetic Resonance Linewidth in Perpendicularly Magnetized Tm3Fe5O12 Thin Films Grown by Pulsed Laser Deposition

Magnetite, Fe3O4, is a frequently studied magnetic oxide growing in the inverse spinel structure Fe3O4 exhibits a ferrimagnetic ground state which stems from the interplay of its three cation species due to super exchange and double exchange depending on site and oxidation state: Octahedrally coordinated Fe2+ and Fe3+ ions, and tetrahedrally coordinated Fe3+ ions. The coupling among octahedral sites is ferromagnetic, while tetrahedral and octahedral sites couple antiferromagnetically. One of the most successful ways to discriminate between the magnetic contributions of these cations are X-Ray Magnetic Circular Dichroism (XMCD) experiments across the Fe L2,3 edges, making use of a slight difference of their L edge energies and the easily resolvable antiferromagnetic coupling between the tetrahedral and the octahedral sites (c.f. Fig. 1).On the other side, X-Ray Resonant Magnetic Reflectivity(XRMR) is a technique combining the depth resolution of X-Ray Reflectivity and the site- and magnetism-sensitivity of XMCD. This enables us to determine the magnetooptical depth profiles at the energies characteristic for Fe2+oct, Fe3+tet, and Fe3+oct in the XMCD of Fe3O4/MgO(100) ultrathin films (c.f. Fig. 2). We find a ~3.9Å layer of enhanced magnetooptical absorption at the surface for both Fe3+ species but not for Fe2+ [1]. The magnetically enhanced Fe3+tet layer is additionally shifted about 3±1.5 Å into the sample compared to the Fe3+oct layer. We attribute this to the first unit cell from the surface containing an excess to Fe3+ cations.References: [1] T. Pohlmann et al., arXiv:2005.01657 [2] S. Macke and E. Goering, J. Phys.: Cond. Matter 26, 363201 (2014).

Cation-Specific Magnetic Depth Profiles of Ultrathin Fe3O4 Films Obtained by X-Ray Resonant Magnetic Reflectivity (XRMR)

Nanostructured Ni-Zn ferrite (NZF) thin film, deposited by a CMOS-compatible microwave-assisted solvothermal (MAS) process, exhibits ‘far-from-equilibrium’ arrangement of cations in the spinel structure with significant anisotropy, thus promising for miniaturized passive devices in GHz regime [1, 2]. Achieving extreme cationic distribution at sub-200oC is the hallmark of this process [3] where, selected metal precursors dissolved in a solvent transform into metal oxide and nucleate on an immersed substrate under microwave heating at 2.45 GHz interacting with solvents of high loss-tangent (tanδ). The objective here is to study the impact of the solvent tanδ on the film’s magnetic property, especially the surface spin-canting layer and interparticular dipolar interaction. NZF films were deposited on Si(100) substrates using various ratios of ethanol (tanδ≈0.941), 1-decanol (tanδ≈0.1), and benzyl alcohol (tanδ≈0.667). The best characteristics were for ethanol:benzyl alcohol ratio (vol.) of 3:5. A smooth, densely packed NZF film (~ 200 nm) composed of highly crystalline nanoparticles (8.6 nm) resulted. Raman analysis of A1g mode shows a degree of inversion for Fe3+ ions x = 0.21 corresponding to 89% population of Fe3+ in octahedral site which led to a net magnetic moment enhancement (Ms of 79 emu/cc@300K), much higher than with 1-decanol [1]. The magnetic size deduced from the Langevin function (DL:9.4 nm) is consistent with that of Scherrer’s crystallite size with surface spin canting layer of 0.5 nm at 300 K. Temperature-dependent magnetization at different dc fields shows strong inter-particle dipolar interaction, estimated as EDipole≈ 1.9×10-17 J (300 K); 1.3×10-16 J (5 K). The blocking process shows individual 2-level energy system replaced by a multi-valley energy system. Thus, TB distribution and anisotropy energy barrier were ascertained from the derivative of thermoremnant magnetization with temperature. A close association between the anisotropy constants and strength of dipolar interaction is realized, driven by the influence of high microwave absorbing solvents.References: 1 R.R. Kahmei, R. Sai, S. Arackal, S.A. Shivashankar, and N. Bhat, IEEE Magn. Lett. 10, 1 (2019). 2 R. Sai, S.D. Kulkarni, M. Yamaguchi, N. Bhat, and S.A. Shivashankar, IEEE Magn. Lett. 8, 3 (2017). 3 I. Bilecka and M. Niederberger, Nanoscale 2, 1358 (2010).

The impact of the solvent tanδ on the magnetic characteristics of nanostructured NiZn-ferrite film deposited by microwave-assisted solvothermal technique

Recently, additive manufacturing (AM) of functional materials started integrating an additional physical property available besides the mechanical stability in printed parts. This additional property can be an increased electrical conductivity or as addressed here a specific magnetic stray field. AM allows near-net-shape manufacturing of complex magnet shapes, avoiding significant machining waste during conventional manufacture of the final shape from sintered magnet blocks. However, the development of the AM process in general requires a qualified starting material for each process such as powder bed fusion [1], binder jetting or material extrusion. Since the supply of metal powders for AM is limited in general, and there is no Nd-Fe-B based powder designed for use in AM, the development of a Nd-Fe-B-based starting material for commercial production is the goal of the EIT Raw Materials funded project 3DREMAG [2]. Commercial powders designed for injection molding are already used for different AM techniques in combination with different polymers and binders [3 and references therein]. E.g. self-produced Mn-Al powders were used in electron beam melting to produce polymer (and rare-earth element) free hard magnetic fully dense bodies [4]. The printing process was followed by a heat treatment to achieve 90% of the hard ferromagnetic tau-phase in the samples. It is very challenging to obtain fully dense Nd-Fe-B magnets by AM due to the specific microstructure of uniform micrometer sized Nd2Fe14B grains separated by a rare-earth rich grain boundary phase that has to be established to achieve high remanence and coercivity. To obtain sufficient amounts of powders that can be processed by powder bed fusion, Nd-Fe-B strip cast flakes are milled and further plasma spheroidized. First results of lab scale production of the powders and first AM tests will be presented and discussed.References: [1] Mateusz Skalon, Michael Görtler, Benjamin Meier, Siegfried Arneitz, Nikolaus Urban, Stefan Mitsche, Christian Huber, Joerg Franke and Christof Sommitsch, Influence of Melt-Pool Stability in 3D Printing of NdFeB Magnets on Density and Magnetic Properties, Materials 13 (2020), 139. [2] http://3dremag.eu [3] Christian Huber, Gerald Mitteralmskogler, Michael Goertler, Iulian Teliban, Martin Groenefeld, Dieter Suess, Additive manufactured isotropic NdFeB magnets by stereolithography, fused filament fabrication, and selective laser sintering, arXiv:1911.02881 [4] I. Radulov, V. Popov Jr, A. Koptyug, F. Maccari, A. Kovalevsky, S. Essel, J. Gassmann, K. Skokov, M. Bamberger, Production of net-shape Mn-Al permanent magnets by electron beam melting, Additive Manufacturing 30 (2019) 100787.

Nd-Fe-B based permanent magnetic materials design for additive manufacturing: 3DREMAG

With the unprecedented surge in wireless communication systems, microwave (MW) absorbers are becoming increasingly important for civil as well as military applications. When a MW signal is incident on a surface, the signal energy is divided into three parts: (i) reflected, (ii) transmitted and (iii) absorbed energies. For the usual metal-backed MW absorbing coating, transmitted energy=0; thus, minimizing reflection is the key for good absorbers. The reflection of EM energy can be curtailed by two methods: (i) an increase in absorption, and (ii) the quarter wavelength cancellation [1,2]. The loss of EM energy may originate from a number of processes within the absorber such as conduction, resonances, and relaxations etc. MW signal, on passing through an absorber dissipates its energy in the form of heat through interactions with the dielectric and magnetic structures of a material. Hence, in terms of two characteristic material parameters: relative permeability (μ*) and relative permittivity (ε*) in the absorbing medium are to be tailored for attaining a strong and broadband MW absorption. For MW absorbing applications, U-type hexaferrite phase with chemical formula Ba4Me2Fe36O60 (Me2U, Me: divalent transition-metal cation) has most complicated crystal structure and is least studied [3-11]. In present work, we investigate the preparation and MW absorbing properties of the nanoscale U-type hexaferrites (NANO-U) for development of a broadband MW absorber. The nano-powders are prepared using two different methods: (1) mechanical jet-milling of bulk U-type hexaferrite prepared by the standard solid-state reaction route at ∼1250 °C, designated here as (NANO-U)JM and (2) a novel one-pot chemical route, designated here as (NANO-U)C. In the first method, average particle size ∼20–50 nm powders were achieved; whereas, the (NANO-U)C powders of controlled particle sizes (∼65 to ∼170 nm) were prepared by changing the holding time (4, 6, 8 and 10 h) at a constant optimized working temperature of ∼900 °C. Four samples of (NANO-U)C were prepared that are designated as per their holding time as- (NANO-U)C4, (NANO-U)C6, (NANO-U)C8, (NANO-U)C10. A blend of all powders in equi-proportion, is designated as (NANO-U)CMIX, The advantage of one-pot chemical method is attainability of comparable particles sizes of a few tens of nm, at much lower working temperatures. We compare the EM parameters and MW absorbing properties of (NANO-U)JM , (NANO-U)C and bulk U-type hexaferrite powders. It is interesting to note that the (NANO-U)JM demonstrates excellent reflection loss (RL) of ∼ -43.8 dB (99.99% MW absorption) at ∼11.3 GHz frequency with a broad bandwidth of 8.6 GHz (8.9 ≤ f ≤ 17.5 GHz) for 90% or more MW absorption. The values for (NANO-U)C too are comparable, with RL of -44.2 dB (99.99% MW absorption) at 11.6 GHz frequency for (NANO-U)C8 and a broadband MW absorption with an excellent bandwidth of 8.4 GHz (9.3 ≤ f ≤ 17.7 GHz) for (NANO-U)CMIX. Significantly improved MW absorption performance are demonstrated in NANO-U hexaferrites, in comparison to the previously reported nanoscale spinel ferrites (-38.9 dB or 99.8% MW absorption along with a bandwidth of ∼3.7 GHz only). We believe that nanoscale particle sizes in U-type hexaferrites result in synergistic enhancements of the dielectric and magnetic losses. A self-explanatory schematic representation of the various possible loss mechanisms in NANO-U absorbers is shown in Fig. 1. Easy tunability, large magnetic anisotropy, presence of disordered surface spin states, defect polarization centres, greater magnetic and dielectric losses and large hopping conduction are responsible for large and broad-band MW absorption in NANO-U hexaferrites. These economical and easily processed U-type hexaferrite ultrafine particles can be used in the paints and sheets for wide-band and strong MW absorbing coating applications.References: (1) Y. Huang, H. Zhang, G. Zeng, Z. Li, D. Zhang, H. Zhu, R. Xie, L. Zheng, J. Zhu, J. Alloys Compd., 682 (2016) 138-143. (2) L. Wan, J. Zhang, Y. Chen, H. Wang, W. Hu, L. Liu, Y. Deng, J. Phys. D: Appl. Phys., 48 (2015) 355302-355310. (3) D. Lisjak, M. Drofenik, J. Appl. Phys., 93 (2003) 8011-8013. (4) M.N. Afsar, D. Lisjak, A. Bahadoor, Y. Wang, IEEE Trans. Magn., 41 (2005) 3472–3474. (5) D. Lisjak, V.B. Bregar, M. Drofenik, J. Magn. Magn. Mater., 310 (2007) 2558-2560. (6) R.S. Meena, S. Bhattacharya, R. Chatterjee, J. Magn. Magn. Mater., 322 (2010) 2908-2914. (7) R.S. Meena, S. Bhattacharya, R. Chatterjee, Mater. Sci. Eng., B, 171 (2010) 133-138. (8) R.S. Meena, S. Bhattacharya, R. Chatterjee, J. Magn. Magn. Mater., 322 (2010) 1923-1928. (9) R.S. Meena, S. Bhattacharya, R. Chatterjee, Mater. Des., 31 (2010) 3220–3226. (10) S. Padhy, S. Sanyal, R.S. Meena, R. Chatterjee, A. Bose, Journal of Electromagnetic Waves and Applications, 27 (2013) 191-204. (11) S. Padhy, S. Sanyal, R.S. Meena, R. Chatterjee, A. Bose, IET Microw. Antennas Propag., 8 (2014) 165–170.



Nanoscale U-Type Hexaferrites as Microwave Absorbers INVITED

Nd–Fe–B permanent magnets are indispensable materials for high-technology commercial products such as motors, electronic devices, etc., because of their high coercive force. However, the mechanism of the coercive force has not been well understood [1]. For the understanding of the microscopic mechanisms, atomistic modeling, which is a new trend of permanent-magnet modeling, is important. We have studied dynamical and static properties of the Nd magnet, Nd2Fe14B, by applying the stochastic LLG equation [2] and Monte Carlo methods to an atomistic model, in which microscopic parameters were introduced by first-principles calculations [2-7]. We have shown the temperature dependencies of the magnetization, domain-wall profiles, dipolar-interaction effect, ferromagnetic resonance, inhomogeneity effect, surface Nd anisotropy effect, etc. [2-7] In this presentation, we mainly show dynamical aspects of the magnet, focusing on the effect of the interface between grains and grain boundaries. There exist difficulties to estimate long relaxation times of barrier-crossing magnetization reversal quantitatively due to the limitation of simulation time and the dependence on the damping factor. Here we propose a statistical method to estimate precisely long relaxation times in the stochastic region, by which one can identify an initial transient process and a long-time regularly relaxation process. We show an estimation of the coercive force of a single grain using this method [6]. Local environments around rare-earth ions at the interfaces of rare-earth magnets may be different from those in the bulk. First-principles studies have suggested that the magnetocrystalline anisotropies of the Nd atoms at the (001) surface of grains in the Nd magnet have in-plane (c-plane) anisotropy, while those in the bulk have out-of-plane anisotropy. Thus, we investigated the effect of the surface magnetic anisotropy of the Nd atoms on the coercivity in the Nd magnet (Fig. 1). We analyzed the coercive force in three cases, in which the Nd atoms in surface layers have (I) no anisotropy, (II) in-plane anisotropy, and (III) doubly reinforced anisotropy for not only the (001) surface but also (100) surface, with the layer-depth dependence [6]. We find that at zero temperature the modification of the anisotropy of the Nd atoms at the first-surface layer reduces the coercive force in cases I and II and enhances the force in case III, but at room temperature, this modification does not affect the coercive force, and the modification at several surface layers is necessary to reduce or enhance the coercivity at room temperature. We also present the nucleation and pinning fields of the Nd magnet as a function of the strength of the anisotropy energy of the soft magnet and also as a function of the thickness of the soft magnet (Fig. 2) [7]. In the Nd magnet, reflecting the lattice structure, the properties of domain walls (DWs) depend on their moving directions. Bloch and Néel DWs move along the a (or b) and c axes, respectively. We show the difference in the nucleation and pinning fields between the Bloch and Néel DWs. We find that the thermal fluctuation effect affects the threshold fields in the Nd magnet significantly. We also find that the strength of the anisotropy energies of the soft magnet phase is not so important for the pinning field, while it is essential for the nucleation field, etc.References: [1] S. Hirosawa et al., Adv. Nat. Sci.: Nanosci. Nanotechnol. 8, 013002 (2017). [2] M. Nishino and S. Miyashita, Phys. Rev. B 91, 134411 (2015); 100, 020403(R) (2019). [3] M. Nishino et al., Phys. Rev. B 95, 094429 (2017). [4] S. Miyashita et al., Scr. Mater. 154, 259 (2018). [5] T. Hinokihara et al., Phys. Rev. B. 97, 104427 (2018). [6] M. Nishino et al., submitted. [7] I. E. Uysal et al., Phys. Rev. B 101, 094421 (2020).

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Permeability Spectra of Planar M-type Barium Hexaferrites with High Snoek’s Product by Two-Step Sintering

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