Finite Coercivity Research Articles

Hard ferromagnetic behavior in atomically thin CrSiTe3 flakes.

The research on two-dimensional (2D) van der Waals (vdW) magnets has promoted the development of ultrahigh-density data storage and nanoscale spintronic devices. However, the soft ferromagnetic behavior in most 2D magnets, which means the absence of remanent magnetization, severely limits their applications in realistic devices. Here, we report a layer-controlled ferromagnetic behavior in atomically thin CrSiTe3 flakes, where a transition from the soft to the hard ferromagnetic state occurs as the thickness of samples decreases down to several nanometers. Phenomenally, in contrast to the negligible hysteresis loop in the bulk counterparts, atomically thin CrSiTe3 shows a rectangular loop with finite magnetization and coercivity as the thickness decreases down to ∼8 nm, indicative of a single-domain and out-of-plane ferromagnetic order. We find that the stray field is weakened with decreasing thickness, which suppresses the formation of the domain wall. In addition, thickness-dependent ferromagnetic properties also reveal a crossover from 3 dimensional to 2 dimensional Ising ferromagnets, accompanied by a drop of the Curie temperature from 33 K for bulk to ∼17 K for the 4 nm sample. Our study paves the way towards exploring and learning much more about atomically thin and layered intrinsic ferromagnets.

Tuning the magnetic properties of NiPS3 through organic-ion intercalation.

Atomically thin van der Waals magnetic crystals are characterized by tunable magnetic properties related to their low dimensionality. While electrostatic gating has been used to tailor their magnetic response, chemical approaches like intercalation remain largely unexplored. Here, we demonstrate the manipulation of the magnetism in the van der Waals antiferromagnet NiPS3 through the intercalation of different organic cations, inserted using an engineered two-step process. First, the electrochemical intercalation of tetrabutylammonium cations (TBA+) results in a ferrimagnetic hybrid compound displaying a transition temperature of 78 K, and characterized by a hysteretic behavior with finite remanence and coercivity. Then, TBA+ cations are replaced by cobaltocenium via an ion-exchange process, yielding a ferrimagnetic phase with higher transition temperature (98 K) and higher remanent magnetization. Importantly, we demonstrate that the intercalation and cation exchange processes can be carried out in bulk crystals and few-layer flakes, opening the way to the integration of intercalated magnetic materials in devices.

Observation of exchange-bias in the inverse Heusler alloy Mn1.5Ru1.5Si

Recently, Mn-Ru-Ga based inverse Heusler alloys have been found to be potential candidates for half-metallic fully compensated ferrimagnetic materials. In this work, we have studied another Heusler based alloy, Mn1:5Ru1:5Si, intending to design a compensated ferrimagnet. The alloy Mn1:5Ru1:5Si crystallizes in an inverse Heusler (XA) structure with a small amount of anti-site disorder. Our magnetic study indicates that the alloy orders magnetically below about TN = 50 K, which is characterized by a sharp drop in magnetization. Nevertheless, the isothermal magnetization curve shows a tendency of saturation and finite coercivity at 5 K, which rules out a pure antiferromagnetic state. Interestingly, Mn1:5Ru1:5Si shows vanishingly small magnetic moment, which also rules out possibility of a pure antiferromagnetic state. The alloy shows a significant amount of exchange-bias at low temperature. The alloy fails to obey a Curie-Weiss law between 300 and 50 K, indicating short-range correlations prior to the long-range order at TN. The exchange bias possible arises from the interfacial coupling of the short range ferromagnetic clusters with the long range antiferromagnetic state. The observation of exchange bias for such a system along with a low net magnetization value, has immense potential for magneto-electric applications and switching devices. The resistivity versus temperature data of the sample indicates a metallic character, albeit with a rather large value resistivity. The resistivity data show an upturn below 25 K that follows a logarithmic behaviour. Such upturn may be due to weak localization effect due to the presence of antisite disorder and short range magnetic clusters.

Pressure-Enhanced Ferromagnetism in Layered CrSiTe3 Flakes.

Despite recent advances in layered ferromagnets, ferromagnetic interactions in these materials are rather weak. Here, we report pressure-enhanced ferromagnetism in layered CrSiTe3 flakes revealed by high-pressure magnetic circular dichroism measurements. Below ∼3 GPa, CrSiTe3 undergoes a paramagnetic-to-ferromagnetic phase transition at ∼32 K, and the field-induced spin-flip in the ferromagnetic phase produces nearly zero hysteresis loops, demonstrating soft ferromagnetism. Above ∼4 GPa, a soft-to-hard ferromagnetic transition occurs, signaled by rectangular-shaped hysteresis loops with finite coercivity and remanent magnetization. Interestingly, as pressure increases, the Curie temperature and coercivity dramatically increase up to ∼138 K and 0.17 T at 7.8 GPa, respectively, in contrast to ∼36 K and 0.02 T at 4.6 GPa. It indicates a remarkable influence of pressure on exchange interactions, which is consistent with DFT calculations. The effective interaction between magnetic couplings and external pressure offers new opportunities in pursuit of high-temperature layered ferromagnets.

Structural and Magnetic properties of Room Temperature Multiferroic Lu0.9Ho0.1FeO3

In recent times, LuFeO3 is found to be an interesting material exhibiting room temperature multiferroic properties both in its stable orthorhombic and meta-stable hexagonal phase [1, 2]. To stabilize the single phase and also to enhance the magnetic and ferroelectric properties rare earth doping is a possibility [3, 4]. In this work, magnetic rare earth atom, Holmium is doped in place of Lu, the structural, magnetic and ferroelectric properties are investigated. Bulk nanocrystallineLu0.9Ho0.1FeO3 compound is synthesized by hydrothermal method. Doping with ionic radius larger than Lu, helps to stabilize the structure in a stable orthorhombic- Pnma phase. Rietveld refinement of structural data is carried out on the synthesized sample and the lattice parameters are found to be increasing as expected. Also, Ho being an element with a large magnetic moment, the effect of doping on magnetic properties is investigated. M-H isotherms didn’t saturate even at magnetic field, H= 9T reflecting a strong antiferromagnetic interaction that arises from the super exchange Fe3+-O-Fe3+ interactions. Finite remnant and coercivity values obtained from the M-H loop measured at 300 K confirms that the TN lies above room temperature (RT). Observation of P-E loop at 300 K confirms ferroelectric ordering at RT.

Effect of hole doping on the structure and magnetic properties of hexagonal Zr0.2Lu0.8FeO3 ceramics

Zr4+ doped multiferroic LuFeO3 (LFO) samples were prepared by a sol-gel reaction method and the effect of doping on structural, electric and magnetic properties are evaluated and compared with pure LFO. X-ray diffraction and Rietveld refinement confirm that the pure and Zr doped LFO exists in the hexagonal crystal structure with space group P63cm. The unit cell parameters and the volume of Zr0.2Lu0.8FeO3 (ZLFO) are greater than that of the undoped LFO. Magnetic measurements for ZLFO sample show a weak ferromagnetic-like behaviour at room temperature with Mr ∼ 0.013 emu/g and finite coercivity. Hence, the paramagnetic LFO transformed to the weak ferromagnetic at room temperature upon Zr doping.

Enhanced permeability dielectric FeCo/Al2O3 multilayer thin films with tailored properties deposited by magnetron sputtering on silicon

We have studied the structural and magnetic properties of enhanced-permeability-dielectric FeCo/Al2O3-multilayer thin films deposited on 8"-Si wafers in an industrial magnetron sputtering system. The EPD-multilayers consist of 25 periods of alternating nanometer-thick FeCo-layers deposited by DC sputtering from a Fe60Co40 target and Al2O3-interlayers deposited by RF sputtering from an Al2O3 target. We tuned the magnetic properties of these thin films by varying the thickness of FeCo-layers from 1.1nm to 2.1nm, while the thickness of Al2O3-interlayers remained unchanged (3.5nm). The formation of layers of disconnected FeCo-nanoparticles separated by an Al2O3-matrix was revealed by grazing incidence small angle X-ray-scattering. Further insight into the microstructure of these layers was obtained from X-ray-reflectivity, highly asymmetric-X-ray-diffraction and non-coplanar grazing-incidence-diffraction. The Fe/Co ratio in the FeCo-layers obtained from X-ray-fluorescence measurements was (59±1)/(41±1), which is in very good agreement with the nominal value in the Fe60Co40 target. Using the standing wave technique we found that most of the Fe and Co atoms were located inside the polycrystalline grains, except for a small fraction that diffused into the Al2O3-matrix, and that the thinner the FeCo-layers thickness the higher the fraction of diffused atoms with respect to those in the grains. Zero-field-cooled, field-cooled, and hysteresis (B-H) and (M-H) measurements showed that the FeCo/Al2O3-multilayers with FeCo-layers thinner than 1.7–1.8 nm exhibit superparamagnetic behavior (no coercivity and remanence) at room temperature with peak relative low-field permeability up to 887. By exceeding this critical thickness, the neighboring FeCo-aggregates started to coalesce, and this led to the ferromagnetic behavior revealed by a finite coercivity and remanence in the hysteresis loops.

Microstructural, Optical, and Magnetic Properties of Vanadium-Substituted Nickel Spinel Nanoferrites

The current study investigates the impact of vanadium substitution on the structural, magnetic, and optical properties of NiFe2−xVxO4 (x ≤ 0.3) nanoparticles (NPs) produced by the cost-effective sol-gel route. The as-prepared spinel ceramic powders were examined by X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (DRS), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM). The functional groups, spinel phase, and crystal structure were confirmed by XRD and FT-IR, respectively. The crystallites size decreased from 45.24 to 36.56 nm as the doping process increases. The plots of Tauc were drawn to determine optical band gap magnitudes of 1.291, 1.302, and 1.312 eV for x = 0.0, 0.2, and 0.3, respectively. The estimated saturation magnetization is maximum for pristine NiFe2O4 NPs and decreases to minimum for NiFe1.7V0.3O4 NPs. The σ-H hysteresis loops have finite coercivity (between 125 and 169 Oe) and retentivity (between 9.36 and 14.04 emu/g) values. The calculated σr/σs ratios are lower than 0.500, assigning the uniaxial anisotropy for NiFe2−xVxO4. The effective anisotropy constants (Keff) are in the range of 0.824 × 105 and 1.303 × 105 Erg/g. The magnetocrystalline anisotropy field (Ha) values are around 5.0 kOe. The characteristics of hysteresis (σ-H) curves and the order of magnetic data reveal the soft ferrimagnetic feature of as-prepared nanoparticle samples. From Mossbauer analysis, the variations in hyperfine magnetic field, quadrupole splitting, line width, and isomer shift have been evaluated. The distribution of cations showed that the octahedral B sites are occupied by all the ions of V3+. Mossbauer spectra are composed of four Zeeman sextets and one doublet.

Magneto Optical Properties and Hyperfine Interactions of Cr3+ Ion Substituted Copper Ferrite Nanoparticles

Spinel CuCrxFe2−xO4 (0.0 ≤ y ≤ 1.0) nanoparticles were fabricated by co-precipitation. X-ray diffraction proved the pureness and cubic crystal assembly of products which have the crystallite sizes varying between 16 and 33 nm. The cubic morphology and expected chemical composition, spectral analyses of all sample were accomplished via Scanning electron microscopy along with Energy Dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy respectively. Magnetic and optical characterization of samples were done via Vibrating sample magnetometer, Mossbauer spectroscopy and Diffuse reflectance spectroscopy. Kubelka–Munk model was applied to UV–Vis data for calculating the optical Eg (band gap) values between minimum 1.20 and maximum 1.80 eV. Mossbauer analysis determined the consequence of Cr3+ substitution on isomer shift and quadrupole splitting of all products etc. The specific magnetization (σ-H) hysteresis curves have finite coercivity (in a range of 35–410 Oe) and retentivity (in a range of 0.33–3.84 emu/g) values and reveal the soft ferrimagnetic nature of spinel CuCryFe2−yO4 nanoparticles (NPs). The estimated saturation magnetization Ms value of 35.17 emu/g is maximum for pristine CuFe2O4 NPs and decreases to minimum value of 1.57 emu/g for CuCr0.4Fe1.6O4 NPs due to Cr3+ ion substitution. The calculated squareness ratios less than 0.5 assign the uniaxial anisotropy for all CuCryFe2−yO4 NPs. The magneto-crystalline anisotropy field (Ha) values less than 10.0 kOe except for the composition of CuCr0.2Fe1.8O4 NPs are other magnetic data to reveal the soft magnetic character of samples.

Topology optimization of reduced rare-earth permanent magnet arrays with finite coercivity

The supply chain risk of rare-earth permanent magnets has yielded research efforts to improve both materials and magnetic circuits. While a number of magnet optimization techniques exist, literature has not incorporated the permanent magnet failure process stemming from finite coercivity. To address this, a mixed-integer topology optimization is formulated to maximize the flux density of a segmented Halbach cylinder while avoiding permanent demagnetization. The numerical framework is used to assess the efficacy of low-cost (rare-earth-free ferrite C9), medium-cost (rare-earth-free MnBi), and higher-cost (Dy-free NdFeB) permanent magnet materials. Novel magnet designs are generated that produce flux densities 70% greater than the segmented Halbach array, albeit with increased magnet mass. Three optimization formulations are then explored using ferrite C9 that demonstrates the trade-off between manufacturability and design sophistication, generating flux densities in the range of 0.366–0.483 T.

Room-Temperature Ferromagnetism and Extraordinary Hall Effect in Nanostructured Q-Carbon: Implications for Potential Spintronic Devices

We report extraordinary Hall effect and room-temperature ferromagnetism in undoped Q-carbon, which is formed by nanosecond pulsed laser melting and subsequent quenching process. Through detailed structure–property correlations in Q-carbon thin films, we show the excess amount of unpaired electrons near the Fermi energy level give rise to interesting magnetic and electrical properties. The analysis of the extraordinary Hall effect in Q-carbon follows nonclassical “side-jump” electronic scattering mechanism. The isothermal field-dependent magnetization plots confirm room-temperature ferromagnetism in Q-carbon with a finite coercivity at 300 K and a Curie temperature of 570 K, obtained by the extrapolation of the fits to experimental data using modified Bloch’s law. High-resolution scanning electron microscopy and transmission electron microscopy clearly illustrate the formation of Q-carbon and its subsequent conversion to single-crystalline diamond. Further, we found n-type conductivity in Q-carbon in the e...

From Mn3O4/MnO core–shell nanoparticles to hollow MnO: evolution of magnetic properties

Manganese oxide nanoparticles (MNOPs), when dispersed in a water solution, show a magnetic behavior that drastically changes after an aging process. In this paper, the variation in the magnetic properties has been correlated with the structural evolution of the nanoparticles: in particular, the as prepared Mn3O4/MnO core/shell system manifests a low temperature magnetization reversal that is strongly affected by the presence of the MnO shell and, in particular, by the existence of a frustrated interfacial region playing a key role in determining the low temperature irreversibility, the finite coercivity slightly above the Curie temperature of the Mn3O4 phase and the horizontal displacement of the FC-hysteresis loop. On the other hand, the magnetic behavior of the aged system results dominated by the presence of Mn3O4 whose highly anisotropic character (i.e. high coercivity and high magnetization remanence) is attributed to the presence of a large fraction of surface spins. Such a result is consistent with the structural evolution, from core/shell to hollow nanoparticles, as shown by TEM observation.

Unveiling a New High-Temperature Ordered Magnetic Phase in ε-Fe2O3

Iron oxides are among the most abundant materials on Earth, and yet there are some of their basic properties which are still not well-established. Here, we present temperature-dependent magnetic, X-ray, and neutron diffraction measurements refuting the current belief that the magnetic ordering temperature of e-Fe2O3 is ∼500 K, i.e., well below that of other iron oxides such as hematite, magnetite, or maghemite. Upon heating from room temperature, the e-Fe2O3 nanoparticles’ saturation magnetization undergoes a monotonic decrease while the coercivity and remanence sharply drop, virtually vanishing around ∼500 K. However, above that temperature the hysteresis loops present a nonlinear response with finite coercivity, making evident signs of ferrimagnetic order up to temperatures as high as 850 K (TN1). The neutron diffraction study confirms the presence of ferrimagnetic order well above 500 K with Pna'21' magnetic symmetry, but only involving two of the four Fe3+ sublattices which are ordered below TN2 ≈ 480...

Enhanced magnetization in morphologically and magnetically distinct BiFeO3 and La0.7Sr0.3MnO3 composites

Nanomaterials exhibit properties different from those of their bulk counterparts. The modified magnetic characteristics of manganite nanoparticles were exploited to improve magnetization in multiferroic BiFeO3 compound. We studied the composite of two morphologically and magnetically distinct compounds BiFeO3 (BFO) and La0.7Sr0.3MnO3 (LSMO). The microcrystalline BiFeO3 sample was prepared by solid state reaction method and the nanocrystalline La0.7Sr0.3MnO3 by sol-gel method. Composites with nominal compositions (1−x)BiFeO3–(x)La0.7Sr0.3MnO3 were prepared by modified solid state reaction method. The phase purity and crystal structures were checked by using X-ray diffraction. The formation of composites with phase separated BFO and LSMO was confirmed using Raman and Fourier Transform Infrared spectroscopy studies. The composite samples showed relatively high value of magnetization with finite coercivity. This improvement in magnetic behavior is ascribed to the coexistence of multiple magnetic orderings in composite samples. We scrutinized the possibility of oxygen vacancy or Fe mixed valency formation in the samples using X-ray photoelectron spectroscopy technique.

Analytic solution of field distribution and demagnetization function of ideal hollow cylindrical field source

The Halbach type hollow cylindrical permanent magnet array (HCPMA) is a volume compact and energy conserved field source, which have attracted intense interests in many practical applications. Here, using the complex variable integration method based on the Biot-Savart Law (including current distributions inside the body and on the surfaces of magnet), we derive analytical field solutions to an ideal multipole HCPMA in entire space including the interior of magnet. The analytic field expression inside the array material is used to construct an analytic demagnetization function, with which we can explain the origin of demagnetization phenomena in HCPMA by taking into account an ideal magnetic hysteresis loop with finite coercivity. These analytical field expressions and demagnetization functions provide deeper insight into the nature of such permanent magnet array systems and offer guidance in designing optimized array system.

Synthesis and Magnetic Properties of Nanocrystalline FeTiO3 Materials

Ilmenite (FeTiO3) nanoparticles with different size distributions were prepared through a sol-gel auto-ignition method. As-prepared powder is annealed at moderate temperatures to get nanocrystalline FeTiO3, and structural, micro-structural, and magnetization characterizations are carried out. Magnetization vs temperature shows antiferromagnetic ordering around 52 K and below 15 K a weak ferromagnetic order develops. The Curie-Weiss law is well obeyed above the Neel temperature (T N) for all samples. Hysteresis curves show finite coercivity below the peak temperature and none at room temperature. M-H curves for all the samples cooled with a bias magnetic field show exchange bias below T N which is interpreted due to the finite size and surface effects. The exchange bias is attributed to weak ferromagnetism due to surface spin disorder (Dzyaloshinskii-Moriya interaction) and ferromagnetic interaction (double exchange) due to mixed valence states of Fe ions.

Magnetic behavior of Ni substituted LiCoO2 – Magnetization and electron paramagnetic resonance studies

Single phase Ni substituted LiCo1-xNixO2 solid solutions with x ≤ 0.15 have been synthesized to study the effect of substitution on the magnetic behavior. Two different techniques, magnetic susceptibility and electron paramagnetic resonance (EPR) which provide information in two different time windows have been used. The solid solutions have been found to be single phase with large grains conforming to R3¯ m rhombohedral structure. The lattice parameters ‘c’ and ‘a’ increase with increasing Ni substitution but with a nearly constant c/a ratio of ≈4.99 in all the cases indicating that the CoO6 and LiO6 octahedra do not undergo any Jahn-Teller distortions. The room temperature EPR absorption spectra clearly shows a peak in all the compounds at a field of 314 mT corresponding to a g-factor of 2.14. The peak width however is found to be a strong function of Ni substitution; increasing with increasing Ni from 3.6 mT to 6.5 mT for x = 0.08. The magnetization increases with decreasing temperature in all the compounds, a paramagnetic behavior, unlike Li-deficient compounds which show a Pauli paramagnetic susceptibility. Also, the magnetization exhibits thermal irreversibility which vanishes at large magnetic fields in all the compounds. The unsubstituted compound has discontinuities at 200 K and 50 K corresponding to magnetic transitions which disappear on substitution with 2% Ni for Co. The effective magnetic moment μeff is found to increase from 0.32 μB to 0.66 μB on increasing the substitution to 0.15. The unique feature however is that all the compounds exhibit a clear magnetic hysteresis at room temperature with a finite coercivity. The coercivity increases from 31 Oe to 168 Oe for x = 0.04 and then decreases on further increasing of x. A deconvolution of the hysteresis loops clearly shows an increasing paramagnetic component with substitution.

Comment on “Performance of Halbach magnet with finite coercivity” [J. Magn. Magn. Mater. 407, 369–376 (2016)

In the recent study given by Insinga et al. [J. Magn. Magn. Mater. 407, 369–376 (2016)], two kind of magnetic hysteresis loops, magnetization versus magnetic field strength (M-H), and magnetic induction versus magnetic field strength (B-H) of the permanent magnetic material with finite coercivity are involved. However, M-H and B-H functions are sensitive to the shape of magnetic element, moreover, M and B are possibly the multivalue functions at H=Hc, perhaps the third kind magnetic hysteresis loop of magnetization versus magnetic induction (M-B) is more appropriate for predicting the phenomena of demagnetization and reversal of original remanence in magnet array.

Performance of Halbach magnet arrays with finite coercivity

A numerical method to study the effect of finite coercivity on the Halbach cylinder geometry is presented. Despite the fact that the analytical solution available for this geometry does not set any limit to the maximum air gap flux density achievable, in real life the non-linear response of the magnetic material and the fact that the coercivity is not infinite will limit the attainable field. The presented method is able to predict when and where demagnetization will occur, and these predictions are compared with the analytical solution for the case of infinite coercivity. However, the approach presented here also allows quantification of the decrease in flux density and homogeneity for a partially demagnetized magnet. Moreover, the problem of how to realize a Halbach cylinder geometry using a mix of materials with different coercivities without altering the overall performance is addressed. Being based on a numerical approach, the presented method can be employed to analyze the demagnetization effects due to coercivity for any geometry, even when the analytical solution is not available.

Retraction Note to: Sol–gel synthesized high anisotropy magnetic nanoparticles of NiCrxFe2−xO4

The nanoparticles of NiCr x Fe2−x O4 were synthesized through sol–gel reactions involving nitrates of Ni, Cr and Fe in an aqueous medium containing citric acid. The cubic spinel structure in single phase with nanometric crystallite size of ~5 nm, the spherical morphology and magnetic relaxations were examined through XRD, TEM and Mossbauer techniques. The abnormal occurrence of finite remanance (M r ) and coercivity (H c ) resulted in the room temperature dc magnetization measurements for the small particles authenticate the ferrimagnetic regime, as proposed by the room temperature Mossbauer results of the samples, with a proximate superparamagnetic regime still at lower particle volumes. This could be attributed to the antiferromagnetic spin interactions of chromium ions at octahedral sites and subsequently the over-occupancy of the rest of the cations at tetrahedral sites. In justification to this, the magnetocrystalline anisotropy constant, K, is estimated to have value relatively high of the order of 107 erg/cm3 at room temperature for all studied concentrations.