Alloys

Example 1

95 g of manganese (purity: 99.95%; purchased from Taewon Scientific Co., Ltd.) and 5 g of high-purity graphite (purity: 99.5%; purchased from Taewon Scientific Co., Ltd.) were placed in a water-cooled copper crucible of an argon plasma arc melting apparatus (manufactured by Labold AG, Germany, Model: vacuum arc melting furnace Model LK6/45), and melted at 2,000 K under an argon atmosphere. The melt was cooled to room temperature at a cooling rate of 10⁴ K/min to obtain an alloy ingot. The alloy ingot was crushed to a particle size of 1 mm or less by hand grinding. Thereafter, the obtained powders were magnetically separated using a Nd-based magnet to remove impurities repeatedly, and the Mn₄C magnetic powders were collected. The collected Mn₄C magnetic powders were subjected to X-ray diffraction (XRD) analysis (measurement system: D/MAX-2500 V/PO, Rigaku; measurement condition: Cu—K_α ray) and energy-dispersive X-ray spectroscopy (EDS) using FE-SEM (Field Emission Scanning Electron Microscope, MIRA3 LM).

FIGS. 2(a) and 2 (b) show an X-ray diffraction pattern and an energy-dispersive X-ray spectroscopy graph of the Mn₄C magnetic material produced according to Example 1 of the present disclosure, respectively.

As can be seen in FIG. 2(a), the Mn₄C magnetic material showed diffraction peaks of (111), (200), (220), (311) and (222) crystal planes at 2θ values of 40°, 48°, 69°, 82° and 88°, respectively, in the XRD analysis. Thus, it can be seen that the XRD patterns of the Mn₄C magnetic material produced according to Example 1 are well consistent with the patterns of the cubic perovskite Mn₄C. In addition, the Mn₄C magnetic material shows several very weak diffraction peaks that can correspond to Mn₂₃C₆ and Mn. That is, the diffraction peak intensity at 2θ values of 43° and 44°, which correspond to Mn and Mn₂₃C₆ impurities, is as very low as about 2.5% of the diffraction intensity of the peak corresponding to the (111) plane. Through this, it can be seen that the powders obtained in Example 1 have high-purity Mn₄C phase. The lattice parameter of the Mn₄C is estimated to be about 3.8682 Å.

FIG. 2(b) shows the results of analyzing the atomic ratio of Mn:C in the powder by EDS. The atomic ratio of Mn:C is 80.62:19.38, which is very close to 4:1 within the experimental uncertainties. Thus, it can be seen that the powder is also confirmed to be Mn₄C.

The M-T curve of the field aligned Mn₄C powder obtained in Example 1 was measured under an applied field of 4 T and at a temperature ranging from 50 K to 400 K. Meanwhile, the M-T curve of the randomly oriented Mn₄C powder was measured under an applied field of 1 T. The Curie temperature of Mn₄C was measured under 10 mT while decreasing temperature from 930 K at a rate of 20 K/min.

FIGS. 3(a) to 3(c) show the M-T curves of the Mn₄C magnetic material, produced according to Example 1 of the present disclosure, under magnetic fields of 4 T, 1 T, and 10 mT, respectively.

FIG. 3 shows magnetization-temperature (M-T) curves indicating the results of measuring the temperature-dependent magnetization intensity of the Mn₄C magnetic material, produced in Example 1, using the vibrating sample magnetometer (VSM) mode of Physical Property Measurement System (PPMS®) (Quantum Design Inc.).

According to the Néel theory, the ferrimagnets that contain nonequivalent substructures of magnetic ions may have a number of unusual forms of M-T curves below the Curie temperature, depending on the distribution of magnetic ions between the substructures and on the relative value of the molecular field coefficients. The anomalous M-T curves of Mn₄C, as shown in FIG. 3(a), can be explained to some extent by the Néel's P-type ferrimagnetism, which appears when the sublattice with smaller moment is thermally disturbed more easily. For Mn₄C with two sublattices of Mn_I and Mn_II, as shown in FIG. 1, the Mn_I sublattice might have smaller moment.

FIG. 3(a) shows the temperature dependence of magnetization of the Mn₄C magnetic material produced in Example 1. The magnetization of Mn₄C measured at 4.2K is 6.22 Am²/kg (4 T), corresponding to 0.258μ_B per unit cell. The magnetization of the Mn₄C magnetic material varies little at temperatures below 50 K, and is quite different from that of most magnetic materials, which undergo a magnetization deterioration with increasing temperature due to thermal agitation. Furthermore, the magnetization of the Mn₄C magnetic material increases linearly with increasing temperature at temperatures above 50 K. The linear fitting of the magnetization of Mn₄C at 4 T within the temperature range of 100 K to 400 K can be written as M=0.0072T+5.6788, where M and T are expressed in Am²/kg and K, respectively. Thus, the temperature coefficient of magnetization of Mn₄C is estimated to be about ˜2.99*10⁻⁴μ_B/K per unit cell. The mechanisms of the anomalous thermomagnetic behaviors of Mn₄C may be related to the magnetization competition of the two ferromagnetic sublattices (Mn_I and Mn_II) as shown in FIG. 1.

FIG. 3(b) shows the M-T curves of the Mn₄C powders at temperatures within the range of 300 K to 930 K under 1 T. The linear magnetization increment stops at 590 K, above which the magnetization of Mn₄C starts to decrease slowly first and then sharply at a temperature of about 860 K. The slow magnetization decrement at temperatures above 590 K is ascribed to the decomposition of Mn₄C, which is proved by further heat-treatment of Mn₄C as described below.

According to one embodiment of the present disclosure, the saturation magnetization of Mn₄C increases linearly with increasing temperature within the range of 50 K to 590 K and remains stable at temperatures below 50 K. The increases in anomalous magnetization 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 are partially oxidized into the manganosite when exposed to air. The remanent magnetization of Mn₄C varies little with temperature. The Curie temperature of Mn₄C is about 870 K. The positive temperature coefficient (about 0.0072 Am²/kgK) of magnetization in Mn₄C is potentially important in controlling the thermodynamics of magnetization in magnetic materials.

The Curie temperature T_e of Mn₄C is measured to be about 870 K, as shown in FIG. 3(c). Therefore, the sharp magnetization decrement of Mn₄C at temperatures above 860 K is ascribed to both the decomposition of Mn₄C and the temperature near the Tc of Mn₄C.

FIG. 4 is a graph showing the magnetic hysteresis loops of the Mn₄C magnetic material, produced according to Example 1 of the present disclosure, at 4.2 K, 200 K and 400 K. The magnetic hysteresis loops were measured by using the PPMS system (Quantum Design) under a magnetic field of 7 T while the temperature was changed from 4 K to 400 K.

As shown in FIG. 4, the positive temperature coefficient of magnetization was further proved by the magnetic hysteresis loops of Mn₄C as shown in FIG. 4. The Mn₄C shows a much higher magnetization at 400 K than that at 4.2 K. Moreover, the remanent magnetization of Mn₄C varies little with temperature and is Δ3.5 Am²/kg within the temperature range of 4.2 K to 400 K. The constant remanent magnetization of Mn₄C within a wide temperature range indicates the high stability of magnetization against thermal agitation. The coercivities of Mn₄C at 4.2 K, 200 K, and 400 K were 75 mT, 43 mT, and 33 mT, respectively.

The magnetic properties of Mn₄C measured are different from the previous theoretical results. A corner Mn_I moment of 3.85μ_B antiparallel to three face-centered Mn_II moments of 1.23μ_B in Mn₄C was expected at 77 K. The net moment per unit cell was estimated to be 0.16μ_B. In the above experiment, the net moment in pure Mn₄C at 77 K is 0.26μ_B/unit cell, which is much larger than that expected by Takei et al. It was reported that the total magnetic moment of Mn₄C was calculated to be about 1μ_B, which is almost four times larger than the 0.258μ_B per unit cell measured at 4.2 K, as shown in FIG. 4.

FIG. 5 is an enlarged view of the temperature-dependent XRD patterns of the Mn₄C magnetic material produced according to Example 1 of the present disclosure.

The thermomagnetic behaviors of Mn₄C are related to the variation in the lattice parameters of Mn₄C with temperature. It is known that the distance of near-neighbor manganese atoms plays an important role in the antiferro- or ferro-magnetic configurations of Mn atoms. Ferromagnetic coupling of Mn atoms is possible only when the Mn—Mn distance is large enough. FIG. 5 shows the diffraction peaks of the (111) and (200) planes of Mn₄C at temperatures from 16 K to 300 K. With increasing temperature, both (111) and (200) peaks of Mn₄C shifted to a lower degree at temperatures between 50 K and 300 K, indicating an enlarged distance of Mn—Mn atoms in Mn₄C. No peak shift is obviously observed for Mn₄C at temperatures below 50 K. The distance of nearest-neighbor manganese atoms plays an important role in the antiferro- or ferro-magnetic configurations of Mn atoms and thus has a large effect on the magnetic properties of the compounds.

Thus, it can be seen that the abnormal increase in magnetization of Mn₄C with increasing temperature occurs due to the variation in the lattice parameters of Mn₄C with temperature.

The powder produced in Example 1 was annealed in vacuum for 1 hour at each of 700 K and 923 K, and then subjected to X-ray spectroscopy, and the results thereof are shown in FIG. 6.

The magnetization reduction of Mn₄C at temperatures above 590 K is ascribed to the decomposition of Mn₄C, which is proved by the XRD patterns of the powders after annealing Mn₄C at elevated temperatures. FIG. 6 shows the structural evolution of Mn₄C at elevated temperatures. When Mn₄C is annealed at 700 K, a small fraction of Mn₄C decomposes into a small amount of Mn₂₃C₆ and Mn. The presence of manganosite is ascribed to the spontaneous oxidation of the Mn precipitated from Mn₄C when exposed to air after annealing. The fraction of Mn₂₃C₆ was enhanced significantly for Mn₄C annealed at 923 K, as shown in FIG. 6.

These results prove that the metastable Mn₄C decomposes into stable Mn₂₃C₆ at temperatures above 590 K. The presence of Mn₄C in the powder annealed at 923 K indicates a limited decomposition rate of Mn₄C, from which the Tc of Mn₄C can be measured. Both Mn₂₃C₆ and Mn are weak paramagnets at ambient temperature and elevated temperatures. Therefore, the magnetic transition of the Mn₄C magnetic material at 870 K is ascribed to the Curie point of the ferrimagnetic Mn₄C.

The Mn₄C shows a constant magnetization of 0.258μ_B per unit cell below 50 K and a linear increment of magnetization with increasing temperature within the range of 50 K to 590 K, above which Mn₂₃C₆ precipitates from Mn₄C. The anomalous M-T curves of Mn₄C can be considered in terms of the Néel's P-type ferrimagnetism.

Example 3

FIG. 8 is a photograph illustrating the microstructure of an example alloy composition including an iron nitride foil, with an average grain size of 8±1.5 μm. The example alloy composition of FIG. 8 was prepared using melt spinning. The composition had a coercivity of 200 Oe. The grains were relatively large, and ferromagnetically coupled.

Example 4

FIG. 9 is a photograph illustrating the microstructure of an example alloy composition including an iron nitride foil, with an average grain size of 6±1.3 μm. The example alloy composition of FIG. 9 was prepared using melt spinning. The composition had a coercivity of 2037 Oe. The grain boundaries were thicker compared to the grains of the example alloy composition of Example 3, and the ferromagnetic grains were separated by non-magnetic material.

Example 6

A sample alloy composition including a plurality of grains with a predetermined average grain size may be prepared according to the present prophetic example. Pure iron foil, for example, having a thickness of between about 1 μm to about 1 cm, is used as a precursor. The iron foil is heated at a temperature between about 650° C. and about 1600° C. for a period of time between about 0.5 hours and about 10 hours, followed by quenching in a liquid medium. The liquid medium includes cold water, brine, oil, liquid nitrogen, or liquid CO₂. A grain structure associated with an average grain size between about 20 nm and about 100 nm is formed. The sample is nitrided using ammonia, at a temperature between about 120° C. and about 500° C., subject to a pressure between about 1 atmosphere and about 100 atmospheres. The nitride sample is annealed by a strained workpiece technique.