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First-principles calculations were performed to calculate the electronic structures of low temperature phase (LTP) MnBi (Mn50Bi50) and substitutionally and interstitially Sn-doped MnBi [Mn50Bi25Sn25, (Mn0.5Bi0.5)66.7Sn33.3]. Brillouin function predicts the temperature dependence of saturation magnetization M(T). Sn substitution for Bi in MnBi (Mn50Bi25Sn25) changes the magnetocrystalline anisotropy constant (Ku) from −0.202 MJ/m3 (the in-plane magnetization) for LTP MnBi to 1.711 MJ/m3 (the out-of-plane magnetization). In comparison, the Ku remains negative but slightly decreases to −0.043 MJ/m3 when Sn is interstitially doped in MnBi [(Mn0.5Bi0.5)66.7Sn33.3]. The Curie temperature (TC) decreases from 716 K for LTP Mn50Bi50 to 445 K for Mn50Bi25Sn25 and 285 K for (Mn0.5Bi0.5)66.7Sn33.3. Mn50Bi25Sn25 has a lower magnetic moment of 5.034 μB/f.u. but a higher saturation magnetization of 64.2 emu/g than (Mn0.5Bi0.5)66.7Sn33.3 with a magnetic moment of 6.609 μB/f.u. and a saturation magnetization of 48.2 emu/g because the weight and volume of the substitutionally Sn-doped MnBi are smaller than the interstitially Sn-doped MnBi. The low Curie temperature and magnetization for Sn-doped MnBi are attributed to the high concentration of Sn. Thus, future study needs to focus on low Sn-concentrated MnBi. [ABSTRACT FROM AUTHOR]

A conventional energy product calculated by the product of the B-field and the H-field is not sufficient for representing the performance of a magnet because it considers the homogeneous and only the uniaxial magnetic properties of the magnet. The conventional energy product has been compared with another energy product obtained by integrating the scalar product of the B-field and the H-field of each cell composed of the three-dimensional components. We investigated a model system by micromagnetic simulation using finite differential method (FDM) and calculated the full hysteresis of the magnet. The model system of a Nd2Fe14B magnet composed of grains with a diameter of about 100 nm was assumed. In the case of the isotropic multi-grain magnet, the energy product calculated by the integration method was 28% larger than the energy product obtained by the conventional way, although a discrepancy between the distribution of the magnetizations and the demagnetizing fields at the reversal process resulted in the decrease of the energy product. [ABSTRACT FROM AUTHOR]

The magnetic properties of ThMn12-type Fe-rich compounds were investigated by producing Sm(Fe0.8Co0.2)10Si2 ribbons. The produced ribbons, with different conditions by varying the melt-spinning conditions, were characterized to investigate physical and magnetic properties. Weight fraction of ThMn12-type phase decreased from 94.5 to 57.1 wt. % as the melt-spinning wheel speed increased from 6.5 to 39 m/s, and corresponding magnetizations and coercivities were substantially varied; the coercivity increased up to 0.175 T from 0.058 T by increasing the wheel speed from 6.5 to 26 m/s, and their magnetization also increased from 89.81 Am2/kg to 105.58 Am2/kg, even though the content of ThMn12-type phase decreased. Morphologies of the ribbons were also observed to verify the melt-spinning effects on the surface conditions and grain sizes. It was found that the particle and grain sizes in the ribbons became smaller and striped patterns appeared as the wheel speed increased. The grain size decreased from about 1 μm to 250 nm by increasing the wheel speed from 6.5 to 39 m/s. [ABSTRACT FROM AUTHOR]

The low coercivity in MnAl alloys has become a barrier to develop high performance MnAl‐based magnets for more than five decades. Herein, a high room‐temperature coercivity of 0.59 T in MnAl disc is achieved by severe plastic deformation. The grain size of the MnAl decreases significantly when ε‐phase is transformed into τ‐phase and when the τ‐phase is severely deformed. The aging process enhances the magnetization of the deformed τ‐phase significantly and reduces the coercivity of the MnAl disc to 0.396 T. The magnetic domain structure of the severely deformed τ‐MnAl exhibits many pinning sites in the intergranular regions and within the domains, resulting in the high coercivity of the bulk samples. The deformation process produces locally oriented texture of τ‐phase. This study opens a window for development of MnAl‐based bulk magnet with high coercivity and enhanced remanence. [ABSTRACT FROM AUTHOR]

The ferromagnetic MnAl-C powders were prepared by using a one-step solid-state reaction method starting from Mn micro-/nano-particles and Al/C micropowders. The bulk MnAl-C with enhanced coercivity was prepared by high-pressure compaction of these MnAl-C powders. The grain size of the τ-phase was significantly reduced during high-pressure compaction, which may also result in a decomposition of the τ-MnAl. Carbon element stabilizes the τ-phase under both ambient and high pressures. The annealing temperature and time intervals are crucial for the preparation of high purity τ-phase samples. The MnAl-C with smaller particle size were produced from Mn nanoparticles. In comparison with the samples prepared from Mn micropowders, the product prepared from Mn nanoparticles shows lower purity, owing to the surface oxidation of the precursor nanoparticles. After high pressure compaction, the coercivities of the bulk MnAl and MnAl-C were increased from 0.05 T and 0.08 T to 0.39 T and 0.22 T, respectively. The room temperature magnetization of the MnAl-C sample at 4 T reached up to 95 Am2/kg. [ABSTRACT FROM AUTHOR]

Bulk MnAl-C magnets were prepared via high-pressure compaction of melt-spun ribbons. The ε → τ phase transformation (PT) in the melt-spun MnAl and MnAlC ribbons occurs mainly via the massive mode, whereas the displacive mode is not prominent. The massive PT temperatures of the melt-spun MnAl and MnAlC ribbons were measured to be in the range 660-739 K and 700-758 K, respectively. The PT temperatures of the melt-spun MnAlC ribbons are lower than those of water-quenched MnAlC. The annealing process for producing τ-phase from ε-phase was optimized based on the measured PT temperature range. As a result, no decomposition of the metastable ε/τ-phase MnAl and MnAlC were observed during the optimized high temperature annealing. High-purity τ-phase MnAlC ribbons with high magnetization (122 Am2/kg at 8 T) and low coercivity (0.13 T) were obtained. The coercivity of the bulk MnAlC obtained by high-pressure compaction was significantly enhanced and reached 0.38 T partially due to the grain refinement of the sample during high-pressure deformation. [ABSTRACT FROM AUTHOR]

Little is known about the physical properties of Mn4C for which is unstable and difficult to prepare. We herein report on the unusual thermomagnetic properties of high purity Mn4C powders obtained by plasma melting and magnetic separation processes. The saturation magnetization of Mn4C increases linearly with increasing temperature in the range of 50 K–590 K and remains stable at temperatures below 50 K. The anomalous magnetization increases of Mn4C with increasing temperature can be considered in terms of the Néel's P-type ferrimagnetism. At temperatures above 590 K, the Mn4C decomposes into Mn23C6 and Mn, which would be partially oxidized into manganosite when exposed to air. The remanent magnetization of Mn4C varies little with temperature. The Curie temperature of Mn4C is around ∼870 K. The positive temperature coefficient (∼0.0072 Am2 kg−1 K−1) of magnetization in Mn4C makes it potentially important in controlling the thermodynamics of magnetization in magnetic materials. [ABSTRACT FROM AUTHOR]

In this paper, a general energy expression is developed for a cluster of multilayer exchange spring media grains. We expand the rotation of the magnetization vectors to second order in elements of an exponential unitary transformation. The first and second derivatives of the energy are determined and a damped optimization procedure is used to efficiently locate minima and transition states on the potential energy surface. In this method, the interaction of neighboring grains is taken into account and thus the effect of inter-granular exchange on the energy barrier is explicitly taken into consideration. Thus, cluster formation is seen as coherent switching of a group of grains and can be studied as a function of anisotropy and exchange distributions in the media. The lowest energy barrier for a grain or cluster is then used to construct a rate constant. We then determine rate constants for an ensemble of interacting grains from which M-H loops and magnetization decay are simulated as a function of an applied field. Both kinetic Monte Carlo and direct integration of the rate equations are used to study the kinetics of these systems. We find that thermal decay rates have better correlation with energy barriers than with nucleation fields. [ABSTRACT FROM PUBLISHER]

The scaling effect of permalloy s-shaped element, a rectangular element with appendages, on operating fields, Hx and Hy, was investigated by micromagnetic computer simulations for and or or logic. The optimized combination of operating fields (Hx, Hy) was found to be (27.7 \pm 9.9, -16.7 \pm 8.8), (37.9 \pm 12.4, -25.9 \pm 6.0), and (42.2 \pm 8.8, -23.9 \pm 4.0) in kA/m for the 100, 50, and 30 nm long s-shaped elements, respectively. As the s-shaped element is scaled down, the allowable deviation from the optimized operating fields becomes smaller and optimized operating fields shift to higher field. [ABSTRACT FROM AUTHOR]

We have experimentally and analytically studied the writeability and recording characteristics of perpendicular magnetic recording media in which the anisotropy (Ku) of the granular oxides and the magnetization (Ms) and lateral exchange coupling (Aex) of the cap layer were varied. The analytical results are obtained with a 9-spin model, which consists of 3 grains with 3 layers each, representing dual oxide media with a capping layer. The 9-spin model estimates bit error rate (BER) using an error-pattern-correcting code (EPCC) based iterative channel model. The experimental results qualitatively agree well with the analytical results of the 9-spin model. High Ku gradient oxides and high Ms cap layer improve writeability and BER, whereas high Aex in the cap layer degrades switching field distribution and off-track capability. Media switching field and switching field distribution strongly depend on spatial distribution of the external field. Both experimental and analytical results indicate that an appropriate design of Ku graded media with an optimum cap layer can effectively reduce jitter and switching field distribution for given head fields, resulting in higher recording density. [ABSTRACT FROM AUTHOR]

The magnetostatic exchange length is an important parameter in magnetics as it measures the relative strength of exchange and self-magnetostatic energies. Its use can be found in areas of magnetics including micromagnetics, soft and hard magnetic materials, and information storage. The exchange length is of primary importance because it governs the width of the transition between magnetic domains. Unfortunately, there is some confusion in the literature between the magnetostatic exchange length and a similar distance concerning magnetization reversal mechanisms in particles known as the characteristic length. This confusion is aggravated by the common usage of two different systems of units, SI and cgs. This paper attempts to clarify the situation and recommends equations in both systems of units. [ABSTRACT FROM PUBLISHER]

Magnetic properties and ferromagnetic resonance (FMR) frequency of Ni0.7Mn0.3-xCoxFe2O4 ferrites (NiMnCo: x = 0.00, 0.04, 0.06, and 0.10) were controllable with the concentration of cobalt (Co). The measured saturation magnetization remains almost constant at 57 emu/g in the studied range of the Co content. However, the coercivity increased to 72 Oe (x = 0.10) from 12 Oe (x = 0.00). Ferromagnetic resonance frequency increased from 129 MHz to the GHz frequency with increasing the Co content from x = 0.00 to x = 0.10, but the real part of permeability decreased to 1.6 from 16.7. This is because FMR frequency increases with the magnetocrystalline anisotropy (Hk) of the Co according to fr = (γ/2π)Hk. The permeability decreased with the Co content by µ = 1 + (4πMs/Hk). Temperature coefficient, in the range of 20-100 °C, was 0.002 for Ni0.7Mn0.3Fe2O4 (x = 0.00) and 0.008 for Ni0.7Mn0.2Co0.1Fe2O4 (x = 0.10). The results suggest that Ni0.7Mn0.3-xCoxFe2O4 ferrites have excellent temperature stability, and also are applicable to very high frequency: 30-300 MHz) and ultra high frequency: 300 MHz-3 GHz) devices. [ABSTRACT FROM AUTHOR]

The magnetization of most materials decreases with increasing temperature due to thermal deterioration of magnetic ordering. Here, we show that Mn4C phase can compensate the magnetization loss due to thermal agitation. The Mn–C nanoparticles containing ferrimagnetic Mn4C and other Mn–C/Mn-O phases were prepared by using the traditional arc-discharge method. A positive temperature coefficient of magnetization (~0.0026 Am2 kg−1 K−1) and an exchange bias up to 0.05 T were observed in the samples. We ascribe the exchange bias to the co-existence of ferrimagnetic Mn4C/Mn3O4 and antiferromagnetic α-Mn(C)/MnO phases. The positive temperature coefficient of magnetization of the samples was ascribed to the presence of Mn4C phase, which is considered as a Néel's P-type ferrimagnet. [ABSTRACT FROM AUTHOR]

A correction is presented to the article "Enhancing the magnetization of Mn4C by heating" which appeared in the May 10, 2018 issue.

Exchange coupled hard/soft MnBi/Fe–Co core/shell structured composites were synthesized using a magnetic self-assembly process. MnBi particles were prepared by arc-melting, and Fe–Co nanoparticles were synthesized by an oleic acid assisted chemical reduction method. Grinding a mixture of micron-sized MnBi and Fe–Co nanoparticles in hexane resulted in MnBi/Fe–Co core/shell structured composites. The MnBi/Fe–Co (95/5 wt%) composites showed smooth magnetic hysteresis loops, enhanced remanent magnetization, and positive values in the Δ M curve, indicating exchange coupling between MnBi and Fe–Co particles. [ABSTRACT FROM AUTHOR]

Abstract The low temperature phase Mn 55 Bi 45- x Ga x (x = 0, 1, 3, 5, and 10) alloys were prepared by induction melting process with subsequent low temperature annealing. The effects of Ga-doping on the crystal structure and magnetic properties of the alloys were systematically studied. The room temperature coercivities of Mn 55 Bi 45- x Ga x after ball milling increased from 1.43 T for x = 0 to 1.66 T for x = 5, while the saturation magnetization decreased from 60.7 Am2/kg (x = 0) to 45.1 Am2/kg (x = 5). The maximum energy product (BH) max of Mn 55 Bi 44 Ga powders reached 7.87 MGOe. The Curie temperature of the Mn 55 Bi 45- x Ga x alloys increased from 633 K to 658 K with increasing Ga concentration in the range of 0 ≤ x ≤ 5. Highlights • Effects of doping Ga on the microstructural and magnetic properties of MnBi alloy. • The MnBi-Ga powders are achieved by surfactant assisted high energy ball milling. • The maximum energy produce (BH) max shows 7.87 MGOe for Mn 55 Bi 44 Ga sample. • The coercivity of Mn 55 Bi 40 Ga 5 after ball milling reached 1.66 T at room temperature. • The elevated curie temperature (T c) by doping Ga makes it a possible candidate for high temperature applications. [ABSTRACT FROM AUTHOR]