Giant Coercivity Research Articles

Colossal Zero-Field-Cooled Exchange Bias via Tuning Compensated Ferrimagnetic in Kagome Metals.

Exchange bias (EB) is a crucial property with widespread applications but particularly occurs by complex interfacial magnetic interactions after field cooling. To date, intrinsic zero-field-cooled EB (ZEB) has only emerged in a few bulk frustrated systems and their magnitudes remain small yet. Here, enabled by high temperature synthesis, we uncover a colossal ZEB field of 4.95 kOe via tuning compensated ferrimagnetism in a family of kagome metals, which is almost twice the magnitude of known materials. Atomic-scale structure, spin dynamics, and magnetic theory revealed that these compensated ferrimagnets originate from significant antiferromagnetic exchange interactions embedded in the holmium-iron ferrimagnetic matrix due to supersaturated preferential manganese doping. A random antiferromagnetic order of manganese sublattice sandwiched between ferromagnetic iron kagome bilayers accounts for such unconventional pinning. The outcome of the present study outlines disorder-induced giant bulk ZEB and coercivity in layered frustrated systems.

Tunable sub-terahertz resonance absorption in high-coercivity magnetodielectric ceramics.

Recently, giant coercivities (20-42 kOe) and sub-terahertz natural ferromagnetic resonance (NFMR) at 100-300 GHz were observed for single-domain M-type hexaferrite particles with high aluminum substitution. Herein, we fabricated dense ceramics of Sr0.67Ca0.33Fe8Al4O19 and, for the first time, investigated their magnetostatic and magnetodynamic properties in the temperature range of 5-300 K. It was shown that dense ceramics maintain their high magnetic hardness (a coercivity of 10-20 kOe) and NFMR frequencies of 140-200 GHz durably in the entire temperature range. Magnetizing the initially non-magnetized ceramics leads to a considerable decrease in the resonance absorption and to almost complete vanishing of the resonance line at 5 kOe. At the same time, an efficient linear frequency tuning by the external magnetic field was observed for the remanent sample. These findings open new horizons for developing industrial terahertz electronics based on dielectric ferrimagnets.

Giant coercivity induced by perpendicular anisotropy in Mn2.42Fe0.58Sn single crystals

We report the discovery of a giant out-of-plane coercivity in the Fe-doped Mn3Sn single crystals. The compound of Mn2.42Fe0.58Sn exhibits a series of magnetic transitions accompanying with large magnetic anisotropy and electric transport properties. Compared with the ab-plane easy axis in Mn3Sn, it switches to the c-axis in Mn2.42Fe0.58Sn, producing a sufficiently large uniaxial anisotropy. At 2 K, a giant out-of-plane coercivity (Hc) up to 3 T was observed, which originates from the large uniaxial magnetocrystalline anisotropy. The modified Sucksmith-Thompson method was used to determine the values of the second-order and the fourth-order magnetocrystalline anisotropy constants K1 and K2, resulting in values of 6.0 × 104 J/m3 and 4.1 × 105 J/m3 at 2 K, respectively. Even though the Curie temperature (TC) of 200 K for Mn2.42Fe0.58Sn is not high enough for direct application, our research presents a valuable case study of a typical uniaxial anisotropy material.

Low-temperature giant coercivity in Co6.2Ga3.8−x Ge x (x = 2.4 to 3.2)

The observation of giant coercivity exceeding 20 kOe at low temperatures in several transition-metal-based compounds has attracted significant attention from a fundamental perspective. This research is also relevant to developing rare-earth-free permanent magnets, wherein cobalt is one of the primary elements used. To facilitate easy fabrication, rare-earth-free and Co-based inorganic bulk magnets that exhibit giant coercivity are highly demanded but rarely reported. Herein, we report the observation of low-temperature giant coercivity in polycrystalline metallic Co6.2Ga3.8−x Ge x (x = 2.4 to 3.2) with the hexagonal Fe13Ge8-type structure composed of Kagome and triangular lattices. As the Ge content x decreases from 3.2, the magnetic ground state changes from ferrimagnetism to ferromagnetism at x = 2.6. In the ferrimagnetic state, we observed a signature of spin frustration arising from the Kagome and/or triangular lattices of Co atoms. The ferromagnetic ordering temperatures for the x = 2.6 and 2.4 samples are 46 K and 60 K, respectively. The coercive fields rapidly increase upon cooling and reach values of 26 kOe and 44 kOe in the x = 2.6 and 2.4 samples, respectively, at 2 K.

Giant coercivity, resistivity upturn, and anomalous Hall effect in ferrimagnetic FeTb

Giant coercivity, resistivity upturn, and anomalous Hall effect in ferrimagnetic FeTb

Hard ferrite magnetic insulators revealing giant coercivity and sub-terahertz natural ferromagnetic resonance at 5-300 K.

The temperature behavior of the magnetic properties is crucial for the application of magnetic materials. Recently, giant room temperature coercivities (20-36 kOe) and sub-terahertz natural ferromagnetic resonance (NFMR) frequencies (160-250 GHz) were observed for single-domain M-type hexaferrites with high aluminum substitution. Herein, the temperature dependences of the magnetic properties and natural ferromagnetic resonance are studied at 5-300 K for single-domain Sr1-x/12Cax/12Fe12-xAlxO19 (x = 1.5-5.5) particles. It is shown that the samples maintain their magnetic hardness over the whole temperature range. The coercivity and NFMR frequencies have a maximum shifting to the low-temperature region with a rise in aluminum concentration. The highest coercivity of 42 kOe and the maximum NFMR frequency of 297 GHz are observed for x = 5.5 at 180 K.

Nanoceramics of metastable ε-Fe2O3: effect of sintering on the magnetic properties and sub-terahertz electron resonance.

In this study, we demonstrate the sintering of metastable ε-Fe2O3 nanoparticles into nanoceramics containing 98 wt% of the epsilon iron oxide phase and with a specific density of 60%. At room temperature, the ceramics retain a giant coercivity of 20 kOe and a sub-terahertz absorption at 190 GHz inherent in the initial nanoparticles. The sintering leads to an increase in the frequencies of the natural ferromagnetic resonance at 200-300 K and larger coercivities at temperatures below 150 K. We propose a simple but working explanation of the low-temperature dynamics of the macroscopic magnetic parameters of the ε-Fe2O3 materials via the transition of the smallest nanoparticles into a superparamagnetic state. The results are confirmed by the temperature dependence of the magnetocrystalline anisotropy constant and micromagnetic modeling. In addition, based on the Landau-Lifshitz formalism, we discuss features of the spin dynamics in ε-Fe2O3 and the possibility of using nanoceramics as sub-terahertz spin-pumping media. Our observations will expand the applicability of ε-Fe2O3 materials and promote their integration into telecommunication devices of the next generation.

Controllable Itinerant Ferromagnetism in Weakly Correlated 5d SrIrO3.

The weakly correlated nature of 5d oxide SrIrO3 determines its rare ferromagnetism, and the control of its magnetic order is even less studied. Tailoring structure distortion is currently a main route to tune the magnetic order of 5d iridates, but only for the spatially confined insulating counterparts. Here, we have realized ferromagnetic order in metallic SrIrO3 by construction of SrIrO3/ferromagnetic-insulator (LaCoO3) superlattices, which reveal a giant coercivity of ∼10 T and saturation field of ∼25 T with strong perpendicular magnetic anisotropy. The Curie temperature of SrIrO3 can be controlled by engineering interface charge transfer, which is confirmed by Hall effect measurements collaborating with EELS and XAS. Besides, the noncoplanar spin texture is captured, which is caused by interfacial Dzyaloshinskii-Moriya interactions as well. These results indicate controllable itinerant ferromagnetism and an emergent topological magnetic state in strong spin-orbit coupled semimetal SrIrO3, showing great potential to develop efficient spintronic devices.

High‐Quality Magnetically Hard ε‐Fe2O3 Thin Films through Atomic Layer Deposition for Room‐Temperature Applications

The critical‐element‐free ε‐Fe2O3 ferrimagnet exhibits giant magnetic coercivity even at room temperature. It is thus highly attractive material for advanced applications in fields such as spintronics, high‐density data storage, and wireless communication. However, a serious obstacle to overcome is the notoriously challenging synthesis of ε‐Fe2O3 due to its metastable nature. Atomic layer deposition (ALD) is the state‐of‐the‐art thin‐film technology in microelectronics. Herein, it is demonstrated that it has also true potential for the fabrication of amazingly stable in situ crystalline and high‐performance ε‐Fe2O3 thin films from simple (FeCl3 and H2O) chemical precursors at a moderately low deposition temperature (280 °C). Standard X‐ray diffraction and Fourier transfer infrared spectroscopy characterization indicates that the films are of high level of phase purity. Most importantly, precise temperature‐dependent 57Fe Mössbauer spectroscopy measurements verify that the hematite (α‐Fe2O3) trace in the films is below 2.5%, and reveal the characteristic low‐ and high‐temperature transitions at 208–228 K and ≈480 K, respectively, while magnetization measurements confirm the symmetric hysteresis loops expected for essentially phase‐pure ε‐Fe2O3 films. Excitingly, the highly c‐axis oriented film growth, the overall film quality, and the unique magnetic properties remain the same, independently of the substrate material used.

Stability of FePt, FePt3 Nanoclusters of Different Habits

Calculations of the total energy of Fe-Pt nanoclusters, corresponding in the phase diagram to the compositions of FePt, FePt3 intermetallics and possessing either characteristic structures L10 and L12, respectively, or non-characteristic disordered structure A1, as well as various particle habits (cuboctahedra, icosahedra) are carried out by molecular dynamics for the first time. The dependences of cluster stability on their size and temperature are plotted, along with the schemes of temperature transformations of cluster morphology and the dependence of the melting points of the clusters with these structures and habits on their size. The size range (2–8 nm) corresponds to the sizes of particles observed by high-resolution electron microscopy. It is shown that the species play an essential part in the phase transformations proceeding under heating in the nanostructured system Fe-Pt and leading to the formation of nanocrystals with highly ordered L10 structure possessing giant coercivity are cubic nanoclusters with the ordered structures L10 and L12. With an increase in cluster size, their stability and melting points increase, tending to saturation of the dependencies within the size range above 10 nm. The least stable clusters are those of intermetallics with non-characteristic disordered structure A1 and icosahedral habit.

Hard Ferromagnetism Down to the Thinnest Limit of Iron-Intercalated Tantalum Disulfide.

Two-dimensional (2D) magnetic crystals hold promise for miniaturized and ultralow power electronic devices that exploit spin manipulation. In these materials, large, controllable magnetocrystalline anisotropy (MCA) is a prerequisite for the stabilization and manipulation of long-range magnetic order. In known 2D magnetic crystals, relatively weak MCA typically results in soft ferromagnetism. Here, we demonstrate that ferromagnetic order persists down to the thinnest limit of FexTaS2 (Fe-intercalated bilayer 2H-TaS2) with giant coercivities up to 3 T. We prepare Fe-intercalated TaS2 by chemical intercalation of van der Waals-layered 2H-TaS2 crystals and perform variable-temperature transport, transmission electron microscopy, and confocal Raman spectroscopy measurements to shed new light on the coupled effects of dimensionality, degree of intercalation, and intercalant order/disorder on the hard ferromagnetic behavior of FexTaS2. More generally, we show that chemical intercalation gives access to a rich synthetic parameter space for low-dimensional magnets, in which magnetic properties can be tailored by the choice of the host material and intercalant identity/amount, in addition to the manifold distinctive degrees of freedom available in atomically thin, van der Waals crystals.

Ultra-high coercivity Sm-Co bulk magnets with remarkable thermal stability

The Sm-Co bulk magnet prepared in this work using unique Sm-Co alloy nanopowder as a precursor prepared by the lox oxygen-induction thermal plasma (LO-ITP) process displays a giant coercivity of 4.1 MA/m (52 kOe) at 300 K, which is the highest coercivity among the reported rare-earth permanent bulk magnets with high density. The Sm-Co magnet has a high coercivity of 2.4 MA/m (30 kOe) at 473 K and 1.6 MA/m (20 kOe) at 573 K, which shows the good temperature coefficient of coercivity of −0.22%/K. The well-organized microstructure with fine Sm-Co alloy single-crystal grains with an average grain size of 0.63 μm, sharp grain boundary, and better crystallinity, leads to a huge coercivity and remarkable thermal stability.

Co substituted Ni–Zn ferrites with tunable dielectric and magnetic response for high-frequency applications

Microwave ferrites are the most promising materials for high frequency and high-power devices. In this work, Co substituted Ni–Zn ferrites (Ni0.5-xZn0.5CoxFe2O4; x = 0–0.4) are synthesized, and the structural, morphology, dielectric, magnetic response are systematically investigated. The X-ray diffraction pattern (XRD) pattern confirmed the formation of pristine cubic phase alone without any secondary phase in all the samples. Substitution of Co2+ resulted in increased crystallite size and lattice constant. Raman peak intensity is suppressed, and a shift towards a lower wavenumber is observed with Co substitution. The change in position and intensity is analyzed and attributed to the change in cationic distribution in tetrahedral and octahedral positions by deconvoluting the Raman peaks. The Energy-dispersive X-ray spectroscopy (EDAX) analysis confirms the stoichiometry of compounds. The real permittivity and tan δ are decreased with the dilution of Co (x = 0: ε' = 24, tan δ = 0.015; x = 0.4: ε' = 11, tan δ = 0.035 @ 1 MHz). Further, permeability initially increased in the X band frequency range and decreased with increasing Co concentration. An increase in the saturation magnetization was observed initially with increased Co content and a decrease after x = 0.2. A maximum saturation magnetization (Ms = 63.3 emu/g) observed for x = 0.2 composition. Interestingly, a giant coercivity and high effective magnetocrystalline anisotropy are observed with the incorporation of Co at low temperatures (5 K). The low dielectric loss, high saturation magnetization, and low coercivity at room temperature of Co substituted Ni–Zn ferrites suggest that these compounds are potential candidates for high-frequency devices (circulators).

Giant coercivity and spin clusters in high pressure polymorphs of Mn2LiReO6.

New high-pressure ordered-ilmenite and double-perovskite polymorphs of Mn2LiReO6 are unprecedented A-site manganites with B/B′ +/7+ oxidation states. They show cluster spin-cluster-glass behaviour and giant coercivity without spin–orbit-coupling.

High-coercivity hexaferrite ceramics featuring sub-terahertz ferromagnetic resonance.

Herein, we demonstrate for the first time compact ferrite ceramics with giant coercivity. The materials are manufactured via sintering single-domain Sr0.67Ca0.33Fe8Al4O19 particles synthesized by a citrate-nitrate auto-combustion method. The obtained ceramics show coercivities up to 22.5 kOe and natural ferromagnetic resonance frequencies (NFMR) in a sub-THz range of 160-282 GHz. At a maximum density of 95%, the sample displays coercivity of 18.5 kOe, which is the highest value among dense ferrite materials reported so far. In addition, we report an unusual blueshift of the NFMR frequency from 160 to 200 GHz, which occurs during material sintering.

Negative zero‐field‐cooled magnetization and magnetic switching in multiferroic Lu0.5Sc0.5FeO3 ceramics

AbstractIn this study, we have investigated the negative magnetization of Lu0.5Sc0.5FeO3 (LSFO) ceramics in the zero‐field‐cooled (ZFC) mode. The negative magnetization in ZFC measurement appears below the Neél temperature (TN) in LSFO, which is probably caused by the residual negative trapping field in superconducting magnet, and is closely related to the giant coercivity in LSFO. By fitting the high‐temperature paramagnetic data under nominal zero magnetic field, the negative trapping field in superconducting magnet is found to be −3.28 Oe. The giant coercivity is ascribed to the strong uniaxial magnetocrystalline anisotropy in LSFO, which can cause the strong magnetic domain wall pinning effect to limit the nucleation of reversed domains under reverse field. In addition, a switchable magnetization switching is realized in LSFO. The magnetization of LSFO can stably switch between positive and negative values when the applied magnetic field was switched between 0.5 and 3 kOe, which suggests that the LSFO samples are suitable for potential applications in magnetic storage.

Large Magnetocrystalline Anisotropy and Giant Coercivity in the Ferrimagnetic Double Perovskite Lu2NiIrO6

We discover that large uniaxial magnetocrystalline anisotropy driven by the simultaneous presence of spin-orbit coupling and structural distortions is the origin of the giant coercivity observed experimentally in the double perovskite Lu2NiIrO6. The magnetic easy axis turns out to be the monoclinic b-axis with an anisotropy constant as high as 1.9 × 108 erg/cm3. The predicted coercive field of 50 kOe and Curie temperature of 220 K agree with the experimentally observed values and point to potential of Lu2NiIrO6 in spintronics applications. We find that the spin-orbit coupling induces a rare Ir4+ Jeff = 1/2 Mott insulating state, suggesting that Lu2NiIrO6 provides a playground to study the interplay between spin-orbit coupling and electronic correlations in a 5d transition metal oxide. The spin-orbit coupling also results in a direct band gap with the valence and conduction states localized on different transition metal sublattices, i.e., efficient electron-hole separation upon photoexcitation and low electron-hole recombination.

Charge-transfer-induced 2D ferromagnetism and realization of thermo-remnant memory effect in ultrathin ß-NiOOH-encapsulated graphene

For graphene-based 2D materials, charge transfer at the interface between graphene and ferromagnetic metal leads to many intriguing phenomena. However, because of the unidirectional spin orientation in ferromagnetic transition metals, interface interaction plays a detrimental role in diminishing the magnetic parameters on 2D surfaces. To overcome this issue, we have synthesized ultrathin 2D weak antiferromagnetic β-NiOOH layers on a graphene surface. By exploiting the charge transfer effect and tuning the thickness of the thin β-NiOOH layers, conversion of ferromagnetism along with giant coercivity and the thermo-remnant magnetic memory effect were observed. As antiferromagnets have two spin orientations, transfer of charge at the interface breaks the nullifying effect of zero magnetization in antiferromagnets and the combined system behaves like a 2D ferrimagnet. Whenever, the sandwich structure of β-NiOOH/graphene/β-NiOOH is formed, it also shows interlayer exchange coupling those results in huge exchange bias and anomalous temperature dependence of coercivity. Due to the strong exchange interaction between the layers, the combined system also shows a robust temperature-based memory effect. Spin-polarized density functional theory was also calculated to confirm the interface interaction and its quantitative evaluation by means of Bader charge analysis and charge-density mapping.

Organic chiral ferromagnets with strong spin-chiroptical interactions

The coexistence of ferromagnetism and chirality broadens the fundamental physical properties of chiral materials; however, organic chiral materials exhibiting ferromagnetism are still limited. Here, we fabricate organic chiral ferromagnets, where left-handed and right-handed organic chiral ferromagnets present a giant difference in saturation magnetization and coercivity. Such behavior can be attributed mainly to the helical arrangement of molecules generating orbital angular momentum. The chirality generates orbital angular momentum, acting as a key intermediate, and induces an interaction between the polarized spin and photon to realize spin-chiroptical coupling, where a series of circularly polarized transmissivity-magnetic field (T-H) loops is induced. Organic chiral ferromagnetic enantiomers with giant different coercivity are designed Spin interacts with photons through chirality-generated orbital angular momentum Circularly polarized transmission-magnetic field loops are realized Gao et al. report that, because of the coexistence of organic ferromagnetism and chirality, the difference in spin-lattice interaction between left-handed and right-handed chiral ferromagnets leads to a spin-chiroptical coupling effect, where the transmissivity of the incident circularly polarized light presents a hysteresis loop with a magnetic field.

Flexible Epsilon Iron Oxide Thin Films

Metastable ε-Fe2O3 is a unique phase of iron oxide, which exhibits a giant coercivity field. In this work, we grew epitaxial ε-Fe2O3 films on flexible two-dimensional muscovite substrates via quasi van der Waals epitaxy. It turns out that twinning and interface energies have been playing essential roles in stabilizing metastable ε-Fe2O3 on mica substrates. Moreover, the weak interfacial bonding between ε-Fe2O3 and mica is expected to relieve the substrate clamping effect ubiquitously encountered in films epitaxially grown on rigid substrates, such as SrTiO3. It is anticipated that these flexible ε-Fe2O3 thin films can serve as a platform for exploring possible interesting emergent physical properties and eventually be integrated as flexible functional devices.