Bulk Magnetization Research Articles

Aging Treatment to Enhance Coercivity Through Grain Boundary Modification in SmFe10V2 Bulk Magnets

We explored the potential for an aging treatment to achieve high coercivity, of 0.859 MA/m, in a SmFe10V2 alloy with a ThMn12-type structure. Bulk magnets were fabricated by sintering ball-milled powders, followed by aging treatment. XRD and SEM analyses revealed that aging treatment promotes the formation of a Sm-rich grain boundary phase with nano-scale thickness. The high Sm content (~60–80 at.%) and low Fe content (~20–30 at.%) in the grain boundary phase led to non-ferromagnetism, enhancing the coercivity by isolating the 1–12 grains and weakening the dipolar interaction between the grains. The aging temperature and duration were optimized to maximize the Sm-rich phase and minimize the soft magnetic SmFe2 phase. This study provides a new fabrication method for ThMn12-type magnets and investigates the relationship between microstructure and coercivity, offering valuable insights for the future design and development of high-performance SmFe12-based magnets.

Emergent topological quasiparticle kinetics in constricted nanomagnets

The ubiquitous domain wall kinetics under magnetic field or current application describes the dynamic properties in nanostructured magnets. However, when the geometrical size of a nanomagnetic system is constricted to the limiting domain wall length scale, the competing energetics between anisotropy, exchange, and dipolar interactions can cause emergent kinetics due to quasiparticle relaxation, similar to bulk magnets of atomic origin. This paper presents a joint experimental and theoretical study to support this argument: constricted nanomagnets, made of antiferromagnetic and paramagnetic neodymium thin film with honeycomb motif, reveal fast kinetic events at picosecond timescales due to the relaxation of topological quasiparticles that persist to low temperature in the absence of any external stimuli. This discovery is especially important considering the fact that paramagnets or antiferromagnets have no net magnetization. Yet, the kinetics in neodymium nanostructures is quantitatively similar to that found in ferromagnetic counterparts and only varies with the thickness of the specimen. This suggests that a universal, topological quasiparticle-mediated dynamical behavior can be prevalent in nanoscopic magnets, irrespective of the nature of the underlying magnetic material. Published by the American Physical Society 2024

High-performance anisotropic Sm2Co17/Fe(Co) bulk nanocomposite magnets fabricated by two-step high-pressure thermal compression deformation

In the realm of Sm2Co17 nanocomposite magnetic materials, it remains a challenge to fabricate anisotropic magnets by forming nanocrystalline Sm2Co17 phases with strong texture, alongside soft phases of small size and high soft phase content. In this paper, we present a novel approach for fabricating anisotropic Sm2Co17/Fe(Co) nanocomposite bulk magnets with a prominent (00l) texture of the Sm2Co17 phase, the 25 wt% content of the Fe(Co) phase, and a refined grain size of 25 nm. This fabrication is achieved using a two-step high-pressure thermal compression (HPTC) deformation process. The fabricated magnets exhibit a maximum energy product [(BH)max] of 20.0 MGOe with a pronounced magnetic anisotropy (Br///Br⊥ = 1.23). This result is 53 % higher than the previously reported largest value [(BH)max = 13.1 MGOe] for Sm2Co17-based nanocomposites. The magnets also exhibit a low remanence temperature coefficient (α = −0.014 %/°C) and a low coercivity temperature coefficient (β = −0.23 %/°C), demonstrating exceptional thermal stability. Our findings may improve the fabrication of anisotropic bulk Sm2Co17 nanostructure magnets for practical applications.

Spin Spiral State at a Ferromagnetic Gd Vacuum Interface.

Centrosymmetric bulk magnets made of layered Gd intermetallics had been discovered recently to exhibit helical spin spirals with a wavelength of ≈2 nm that transform into skyrmion lattices at certain magnetic fields. Here we report on the observation of a spin spiral state at the Gd(0001) surface. Spin-polarized scanning tunneling microscopy images show striped regions with a periodicity of about 2nm. These stripes rearrange upon application of an external magnetic field, thereby unambiguously confirming their magnetic origin. Density functional theory calculations explain that competing exchange interactions in the surface layer of Gd(0001) together with a magnetovolume fine-tuning of the exchange interaction to the next Gd layer favor a chiral 2nm conical spin spiral at the surface, arising as a general behavior of the Gd monolayer.

Engineering hard ferrite composites by combining nanostructuring and Al3+ Substitution: From nano to dense bulk magnets

We have investigated the bottom-up sol-gel synthesis of nanocomposite powders comprising two magnetic phases (hexagonal Sr ferrite and spinel Co ferrite) in order to outline a strategy to obtain permanent magnets with large coercivities via low-cost and scalable syntheses. The correlation between morphological, structural and macroscopic magnetic properties of Al-substituted SrFe12O19 and SrFe12O19/CoFe2O4 nanocomposites was analyzed in detail. The hysteretic behavior can be tuned by cation substitution and/or modulation of the super-exchange coupling at the interface of the constituting phases. The magnetic data, supported by Monte Carlo simulations, indicates enhanced magnetic coupling within the composite: this observation underscores the significance of soft crystallite size and epitaxial growth quality at the interface as key factors influencing super-exchange coupling strength, ranging from fully coupled to essentially decoupled composites. Bulk magnets with high density were manufactured by compacting these nanostructured phases using spark plasma sintering, without an applied magnetic field. Consolidation of powders significantly impacted magnetic properties, by increasing remanent magnetization and decreasing coercivity due to enhanced super-exchange coupling. The presence of two phases hindered reciprocal growth, influencing coercivity differently in various compositions. Overall, the compaction enhanced magnet performance through improved particle alignment and super-exchange coupling, offering the potential for optimized magnet design.

Excellent field–trapping properties of large ring–shaped REBCO melt–textured bulks fabricated by the single–direction melt growth (SDMG) method

Abstract Ring-shaped homogeneous YBCO and DyBCO bulks were successfully fabricated using the Single-Direction Melt Growth (SDMG) method. The bulks were directly grown from ring-shaped compacted powder using ring-shaped molds with an outer diameter of 50 mm and inner diameters of 15, 20, and 25 mm. The ring-shaped bulks exhibited high trapped fields inside the rings up to 1.2 T at 77 K. Analyses of trapped field distributions revealed uniform current density distributions along the orbital direction. Stacked ring bulks demonstrated even higher trapped fields, reaching 2.0 T at 77 K. It was confirmed for the stacked bulks that time-independent uniform trapped fields can be achieved by magnetizing at lower fields than fully magnetizing conditions. Observed paramagnetic magnetization of the SDMG-processed YBCO bulk was negligibly small below the detection limit, which is considered to be more suitable for bulk NMR/MRI applications than DyBCO. Additionally, we proposed a method to quantitatively evaluate trapped fields of superconducting bulks with various diameters and thicknesses, where the estimated average current densities from the maximum trapped fields for all the obtained ring-shaped bulks were above 104 A/cm2 at 77 K. These results indicate that SDMG is an effective method for fabricating high-quality, large-scale ring-shaped bulks with superior field-trapping properties.

Tunning Pr to achieve ultra-high magnetic properties of anisotropic nanocrystalline (Sm,Pr)Co5 magnets

In this study, we reported on the bulk anisotropic nanocrystalline Sm1-xPrxCo5 (x = 0.2–0.6) magnets aimed at achieving excellent energy products. Through a combination of advanced processing techniques, including melt spinning, hot pressing, and hot deformation process, bulk magnets with a fine nanocrystalline structure were successfully synthesized. The results revealed that the substitution of Pr strengthened the c-axis texture and the saturation magnetization, facilitating the enhancement of remanence (Mr) and maximum magnetic energy product [(BH)max]. However, Pr substitution decreased the coercivity (Hci) due to the reduced magnetocrystalline anisotropy field. Excellent magnetic properties are obtained for Sm0.4Pr0.6Co5 magnets with the (BH)max of 23.2 MGOe and Hci of 11.8 kOe. The prepared (Sm,Pr)Co5 magnets with different Pr contents have essentially the same microstructure with RECo5 main phase grains of about 400 nm in diameter and fine dispersed rare earth-rich nanoprecipitates. Our findings can provide reliable reference for manufacturing nanocrystalline permanent magnet materials and promote the development of rare earth permanent magnet new materials.

Observation of Néel-type magnetic skyrmion in a layered non-centrosymmetric itinerant ferromagnet CrTe1.38

Magnetic skyrmions are nanoscale vortex-like magnetization textures that hold great promise for next-generation memory and spintronic devices. While extensive research has focused on discovering such localized spin textures in bulk magnets and multilayers with heavy metals, there is a growing interest in finding them in two-dimensional (2D) magnetic materials. In this research work, we report two distinct phases of the 2D CrxTey family: non-centrosymmetric CrTe1.38 and centrosymmetric CrTe0.96, with a Curie temperature of around 200 and 300 K, respectively. Detailed magnetic study indicates a prominent out-of-plane magnetic anisotropy in CrTe1.38. In contrast, CrTe0.96 exhibits a weak uniaxial magnetic anisotropy. Lorentz transmission electron microscopy shows that the spontaneous ferromagnetic ground state of CrTe1.38 consists of Néel skyrmions, whereas CrTe0.96 exhibits Bloch domain walls, consistent with their crystalline symmetries. This research expands the quasi-2D CrxTey family and opens up new avenues for exploring non-trivial spin structures and their potential applications in spintronic devices.

Anisotropic SmFe10V2 Bulk Magnets with Enhanced Coercivity via Ball Milling Process.

Anisotropic bulk magnets of ThMn12-type SmFe10V2 with a high coercivity (Hc) were successfully fabricated. Powders with varying particle sizes were prepared using the ball milling process, where the particle size was controlled with milling time. A decrease in Hc occurred in the heat-treated bulk pressed from large-sized powders, while heavy oxidation excessively occurred in small powders, leading to the decomposition of the SmFe10V2 (1-12) phase. The highest Hc of 8.9 kOe was achieved with powders ball-milled for 5 h due to the formation of the grain boundary phase. To improve the maximum energy product ((BH)max), which is only 2.15 MGOe in the isotropic bulk, anisotropic bulks were prepared using the same powders. The easy alignment direction, confirmed by XRD and EBSD measurements, was <002>. Significant enhancements were observed, with saturation magnetization (Ms) increasing from 59 to 79 emu/g and a remanence ratio (Mr/Ms) of 83.7%. (BH)max reaching 7.85 MGOe. For further improvement of magnetic properties, controlling oxidation is essential to form a uniform grain boundary phase and achieve perfect alignment with small grain size.

Using magnons as a quantum technology platform: a perspective

Traditional electronics rely on charge currents for controlling and transmitting information, resulting in energy dissipation due to electron scattering. Over the last decade, magnons, quanta of spin waves, have emerged as a promising alternative. This perspective article provides a brief review of experimental and theoretical studies on quantum and hybrid magnonics resulting from the interaction of magnons with other quasiparticles in the GHz frequency range, offering insights into the development of functional magnonic devices. In this process, we discuss recent advancements in the quantum theory of magnons and their coupling with various types of qubits in nanoscale ferromagnets, antiferromagnets, synthetic antiferromagnets, and magnetic bulk systems. Additionally, we explore potential technological platforms that enable new functionalities in magnonics, concluding with future directions and emerging phenomena in this burgeoning field.

Correlations of Plasma Properties Between the Upstream Magnetosheath and the Downstream Outflow Region of Magnetopause Reconnection

AbstractThe impact of upstream conditions on magnetopause reconnection has been an intriguing question in solar wind‐magnetosphere coupling. In this study, we conduct a statistical analysis of plasma properties in the reconnection outflow region and the associated upstream solar wind/magnetosheath. We observe that the normalized ion density (N/Nsw) decreases and the flow speed (V/Vsw) increases in the upstream magnetosheath with distance from the subsolar point, consistent with previous models and observations. The magnetic field strength (|B|), ion density (N), and ion bulk speed (|V|) in the upstream magnetosheath exhibit close correlations with those in the reconnection outflow region. This upstream‐downstream correlation likely arises from the process of forming reconnection outflows, where most upstream ions cross the separatrix and mix with ion outflow already accelerated near the X‐line. High‐speed part of reconnection outflow is mostly located on the magnetosphere side of the magnetopause current layer, with outflow velocities peaking close to the upstream magnetosheath Alfvén speed. The spatial extent of high‐speed outflow is greater in conditions of lower solar wind Alfvén Mach number (MA,sw). Additionally, the southward magnetic field in the magnetosheath and |B| of magnetopause current layer are larger in the cases of lower MA,sw. These findings indicate a close connection of plasma properties between the outflow region of magnetopause reconnection and the upstream magnetosheath.

Effects of imidazolium-based ionic liquids on the elasticity of DNA revealed by magnetic tweezers

The functions of DNA are related to its mechanical properties, including elasticity and conformational transitions. Ionic liquids, known as green solvents, are used for DNA separation and as a solvent medium for long-term DNA storage. This paper elucidated DNA interaction mechanisms with two imidazolium-based ionic liquids: 1-butyl-3-methylimi-dazolium chloride ([bmim]Cl) and 1-octyl-3-methylimidazolium chloride ([omim]Cl), via magnetic tweezers, bulk techniques and molecular dynamic simulation (MD). By stretching and twisting individual DNA molecules, we measured the elasticity of DNA in the presence of ILs ([bmim]Cl and [omim]Cl). As results, the persistence length, stretch modulus, torsional stiffness and twist-stretch coupling exhibited a significant decrease compared to the DNA without ILs binding. Isothermal titration calorimetry analysis indicated that the ILs in the presence of DNA were thermodynamically favored driven by enthalpy and entropy. DNA underwent size transition and conformational change induced by ILs, and the charges of DNA were neutralized by the added ILs. The binding of ILs induced unwinding and softening of DNA. Circular dichroism spectra and MD results indicated that DNA maintained B-conformation after interacting with ILs. Compared to [bmim]+, [omim]+ showed stronger binding affinity to DNA and tended to aggregate on the DNA surface. ILs binding reduced Na+ and water binding on DNA, which likely prevented the hydrolytic reactions that denatured DNA and helped stabilize DNA for the long term. This research provides a critical theoretical foundation for the utilization of ionic liquids in life sciences.

Role of (Pr20Nd80)60Tb15Cu15Ga10 intergranular addition on the magnetic properties of Nd-Fe-B sintered magnet

Intergranular addition using heavy rare earth (HRE) Dy/Tb containing alloys has been an effective way to modulate the microstructure and chemical distribution towards fabricating high-coercivity Nd-Fe-B sintered magnets. In this work, (Pr20Nd80)60Tb15Cu15Ga10 (at%) intergranular alloy powders were designed and introduced with dual purposes of forming Tb-rich matrix shell and antiferromagnetic RE6Fe13Ga grain boundary phase (GBP). With 3 wt% (Pr20Nd80)60Tb15Cu15Ga10 addition, the coercivity Hcj is significantly increased from 11.1 kOe to 17.9 kOe, accompanied with a high utilization efficiency of Tb (11.8 kOe/%) and a superior temperature coefficient of coercivity β(−0.531%/K). Microstructural analysis suggests that the improved wettability of grain boundary by (Pr20Nd80)60Tb15Cu15Ga10 addition is beneficial to form the Tb-rich matrix shell and antiferromagnetic RE6Fe13Ga GBP throughout the bulk magnets. Micromagnetic simulation is implemented to study the effect of formed ferromagnetic/antiferromagnetic GBP and Tb-rich matrix shell on the magnetization reversal process. Results show that the synergy of Tb-rich shell surrounding Nd/Pr-rich core and antiferromagnetic Nd/Pr-rich RE6Fe13Ga GBP is an effective way to promote the coercivity with high utilization efficiency of HRE Tb.

Chemical pressure induced spin reorientation in the magnetic structure of Ca3Mn2−xSnxO7 ( x=0,0.03,0.05 )

The effect of partially substituting Tin (Sn) at the Manganese (Mn) site of Ca3Mn2O7 , viz. Ca3Mn2−xSnxO7 with x=0.03,0.05 , on its structural and magnetic properties have been investigated using synchrotron diffraction, neutron diffraction, and bulk magnetization measurements. It is observed that with a substitution of x=0.03 , the minor (≈8%) tetragonal ( I4/mmm ) structural phase that is present in the predominantly orthorhombic ( Cmc21 ) undoped Ca3Mn2O7 , completely disappears. The compounds order antiferromagnetically, the ordering temperature decreases with increasing Sn-content, indicating a weakening of the antiferromagnetic exchange interactions. Interestingly, in the ordered state, the spin magnetic moments which were aligned along the a-axis of the unit cell in the undoped compound, are observed to have reoriented with their major components lying in the b − c plane in the Sn-doped compounds. The above influence of Sn-doping is seen to be stemming from a significant modification of the octahedral rotation and tilt mode geometry in the unit cell, that is known to be responsible for driving ferroelectricity in these compounds.

Microstructure and coupling mechanisms in MnBi–FeSiB nanocomposites obtained by spark plasma sintering

Fabrication and extensive characterization of hard-soft nanocomposites composed of hard magnetic low-temperature phase LTP-MnBi and amorphous Fe70Si10B20 soft magnetic phase for bulk magnets are reported. Samples with compositions Mn55Bi45 + x⋅(Fe70Si10B20) (x = 0, 3, 5, 10, 20 wt.%) were prepared by spark plasma sintering of powder mixtures. Characterization has been performed by X-ray diffraction, scanning and transmission electron microscopy, magnetometry and 57Fe Mӧssbauer spectroscopy. It was shown that samples contain crystallized and nanometric LTP-MnBi phases with various elemental compositions depending on the degree of Bi clustering. Complex correlations between starting compositions, processes during fabrication, and functional magnetic characteristics were observed. Unexpected special situations of the relation between microstructure and magnetic coupling mechanisms are discovered. Exchange spring effects of different strengths occur, being very sensitive to morpho-structural and compositional features, which in turn are controlled by processing conditions. An in-depth analysis of related microscopic characteristics is provided. Results of this work suggest that fabrication by powder metallurgy routes, such as spark plasma sintering of hard and soft magnetic powder mixtures, of MnBi-based composites with exchange spring phenomena have a high potential in designing and optimization of suitable materials with tunable magnetic properties towards rare-earth–free permanent magnet applications.

SrCu(OH)3Cl , an isolated equilateral triangle spin S=1/2 model system

We have investigated the magnetic ground state properties of the quantum spin trimer compound strontium hydroxy copper chloride SrCu(OH)3Cl using bulk magnetization, specific heat measurements, nuclear magnetic resonance (NMR), and electron spin resonance (ESR) spectroscopy. SrCu(OH)3Cl consists of layers with isolated Cu2+ triangles and hence provides an opportunity to understand the magnetic ground state of an isolated system of S = 1/2 arranged on an equilateral triangle. Although magnetization measurements do not exhibit a phase transition to a long-range ordered state down to T = 2 K, they reveal the characteristic behavior of isolated trimers with an exchange of J=154 K. The Curie-Weiss behavior changes around 50–80 K, as is also seen in the NMR spin-lattice relaxation rate. In zero magnetic field, our specific heat data establish a second-order phase transition to an antiferromagnetic ground state below T= 1.2 K. We have drawn a magnetic field-temperature (H−T) phase diagram based on the specific heat measurements. The ESR data show divergence of the linewidth at lower temperatures, which precedes the phase transition to an antiferromagnetic long-range ordered state with unconventional critical exponents. The temperature variation of the g-factor further confirms the antiferromagnetic phase transition and reflects the underlying magnetocrystalline anisotropy of the compound. Published by the American Physical Society 2024

Structures and magnetic properties of Sm2Fe17N3 bulk magnets prepared by spark plasma sintering with dynamic compression

The spark plasma sintering (SPS) method can consolidate powders at low temperatures over a short period. The addition of dynamic compression (DC) can produce bulk materials at lower temperatures. To demonstrate the effect, the study used SPS-DC to consolidate Sm2Fe17N3 powder, which decomposes into α−Fe and SmN phases above 873 K. The SPS-DC method was able to consolidate Sm2Fe17N3 powder into bulk material at 473 K. The Sm2Fe17N3 magnet produced at 473 K by the SPS-DC method showed a relative density of 88 %, comparable to that of the Sm2Fe17N3 magnet produced at 673 K by the conventional SPS method. The Sm2Fe17N3 bulk magnet produced at 473 K by the SPS-DC method exhibited a high coercivity of 9.45 kOe.

On the pre-treatment for recycling spent NdFeB permanent magnets: from disassembling, characterisation to de-coating

An evaluation of neodymium‑iron‑boron permanent magnets (NdFeB PMs) from different end-of-life products, as a secondary resource of rare-earth elements (REEs), is presented. De-coating of PM was investigate as pre-treatment to facilitate efficient direct magnet recycling. Thus, critical aspects from disassembling to the de-coating of the magnets were addressed. A challenge for the de-coating process is that the magnets have different sizes, weights, and their coating compositions are not known beforehand. It was shown that ammonia-based solutions was thermodynamically suitable to dissolve nickel and zinc coatings selectively, while keeping the bulk magnet stable. Nevertheless, the dissolution of Zn was much faster than the Ni one, and more efficient.

Coercive Fields Exceeding 30 T in the Mixed-Valence Single-Molecule Magnet (CpiPr5)2Ho2I3.

Mixed-valence dilanthanide complexes of the type (CpiPr5)2Ln2I3 (CpiPr5 = pentaisopropylcyclopentadienyl; Ln = Gd, Tb, Dy) featuring a direct Ln-Ln σ-bonding interaction have been shown to exhibit well-isolated high-spin ground states and, in the case of the Tb and Dy variants, a strong axial magnetic anisotropy that gives rise to a large magnetic coercivity. Here, we report the synthesis and characterization of two new mixed-valence dilanthanide compounds in this series, (CpiPr5)2Ln2I3 (1-Ln; Ln = Ho, Er). Both compounds feature a Ln-Ln bonding interaction, the first such interaction in any molecular compounds of Ho or Er. Like the Tb and Dy congeners, both complexes exhibit high-spin ground states arising from strong spin-spin coupling between the lanthanide 4f electrons and a single σ-type lanthanide-lanthanide bonding electron. Beyond these similarities, however, the magnetic properties of the two compounds diverge. In particular, 1-Er does not exhibit observable magnetic blocking or slow magnetic relaxation, while 1-Ho exhibits magnetic blocking below 28 K, which is the highest temperature among Ho-based single-molecule magnets, and a spin reversal barrier of 556(4) cm-1. Additionally, variable-field magnetization data collected for 1-Ho reveal a coercive field of greater than 32 T below 8 K, more than 6-fold higher than observed for the bulk magnets SmCo5 and Nd2Fe14B, and the highest coercive field reported to date for any single-molecule magnet or molecule-based magnetic material. Multiconfigurational calculations, supported by far-infrared magnetospectroscopy data, reveal that the stark differences in magnetic properties of 1-Ho and 1-Er arise from differences in the local magnetic anisotropy of the lanthanide centers.

Construction of high-performance multi-main-phase Nd–Dy–Fe–B sintered magnets with strengthened chemical heterogeneity

By designing the [(Pr20Nd80)100-xDyx]30.5FebalM1.3B1.0 components (x = 0, 10, 25, 50, 75 wt.%, denoted as Dy0, Dy10, Dy25, Dy50, Dy75), we constructed a series of multi-main-phase (MMP) and single-main-phase (SMP) Nd–Dy–Fe–B bulk magnets with same average composition of (Pr20Nd80)27.5Dy3.0FebalM1.3B1.0 but with different chemical heterogeneities, as evaluated by Dy concentration discrepancy between Dy-rich shell and Dy-lean core (△C). △C increases from 2.8 wt.% for MMP Dy3-I magnet (∼39.4 % Dy25 and ∼60.6 % Dy0), to 3.1 wt.% for MMP Dy3-II magnet (∼19.7 % Dy50 and ∼80.3 % Dy0), further to 3.4 wt.% for MMP Dy3-III magnet (∼13.1 % Dy75 and ∼86.9 % Dy0), compared to 0 wt.% for SMP Dy3 magnet (100 % Dy10). Corresponding magnetic performance at 20∼180 °C is positively related with △C value, as evidenced by the best magnetic properties of MMP Dy3-III magnet with highest △CIII value than others. Further phase-field simulation underlies the governing mechanism for strengthened chemical heterogeneity in MMP Nd–Dy–Fe–B magnets, while also provides design principles for concentration graded magnetic materials.