High Temperature Coercivity Research Articles

Hard magnetic properties of FeCoNiAlCuXTiX based high entropy alloys

High entropy alloys (HEA) contain multiple principal alloying elements, and possess unique properties due to the high configurational entropy and lattice strain in the system. Ferromagnetic FeCoNi-based HEAs exhibit dramatic changes in crystal structure and the type of magnetism expressed when adding non-magnetic elements such as Al, Cr, Ga, Ti, etc. Interestingly, Alnico permanent magnets also contain multiple principal alloying elements, such as Fe, Co, Ni, and Al, along with other minor additions. This well-studied system is similar in concept to high entropy alloys (HEAs). In this paper, we investigate the hard magnetic properties of FeCoNiAl-based HEAs with additions of Cu/Ti. The addition of Cu/Ti to an equimolar FeCoNiAl alloy is effective at enhancing coercivity, due to spinodal decomposition, but at the expense of saturation magnetization. By varying the ratio of Fe and Co, however, with respect to the other alloying elements, the saturation magnetization is increased, while generally retaining or improving the coercivity. In particular, the Fe2CoNiAlCu0.4Ti0.4 HEA shows promising hard magnetic properties as an isotropic cast magnet, with an HC of 1,078 Oe and (BH)max of 2.06 MGOe, slightly better than the performance of isotropic cast Alnico 2 magnets. The thermal stability is also sufficient for use at elevated temperatures over 200 °C. There was also an interesting increase in high temperature coercivity observed at temperatures from ∼650-800 °C, where these alloys often exhibited higher coercivity than that measured at RT.

Overview of composition and technique process study on 2:17-type Sm–Co high-temperature permanent magnet

Sm–Co permanent magnets are widely used in aerospace equipments, tubes, sensors and hybrid electric vehicles due to their excellent magnetic properties at both room and high temperatures. Compared to that of Nd–Fe–B magnets, the capability of reliably operating at temperatures up to 500 °C is the biggest preponderance of Sm–Co permanent magnets. However, the reliable high-temperature magnetic properties are susceptible to composition and technique processes. So, appropriate composition and matched technique processes are prerequisites for good high-temperature magnetic properties. This review mainly emphasizes the influences of Sm, Fe, Cu and Zr contents and technique processes on development of high-temperature performance and discusses the controversial high-temperature coercivity mechanism of 2:17-type high-temperature Sm–Co magnets.

Semi-hard magnetic nanocomposites based on out-of-equilibrium Fe2+δNb and Fe2+δTa Laves phases

Melt-spun Fe100-xNbx and Fe100-xTax alloys consisting of a hexagonal (C14) Laves phase and an iron-based bcc phase were found to exhibit room-temperature coercivities up to 0.45 and 0.48 kOe, respectively. The non-equilibrium C14 structures in the melt-spun alloys are characterized by a smaller unit cell volume, a higher Curie temperature and, presumably, a greater concentration of Fe compared to the C14 structures in the alloys annealed at 1533 K. The room-temperature coercivity, which correlates with the C14 lattice contraction, is believed to be caused by the magnetocrystalline anisotropy of the non-equilibrium Laves phase with the latter being magnetically coupled with the Fe phase via an intergranular exchange interaction. On the other hand, a coercivity of around 0.3 kOe persists in the melt-spun alloys above the Curie temperature of the C14 phase. This high-temperature coercivity may originate from the shape or strain anisotropy of the Fe phase particles. The energy density of the reported two-phase alloys is not sufficiently high to consider them potential hard magnetic materials. However, the pure non-equilibrium Fe2+δNb and Fe2+δTa compounds may be of interest if they can be isolated and textured.

An air-stable Dy(iii) single-ion magnet with high anisotropy barrier and blocking temperature.

Herein we report air-stable Dy(iii) and Er(iii) single-ion magnets (SIMs) with pseudo-D5h symmetry, synthesized from a sterically encumbered phosphonamide, t BuPO(NHiPr)2, where the Dy(iii)-SIM exhibits a magnetization blocking (TB) up to 12 K, defined from the maxima of the zero-field cooled magnetization curve, with an anisotropy barrier (Ueff) as high as 735.4 K. The Dy(iii)-SIM exhibits a magnetic hysteresis up to 12 K (30 K) with a large coercivity of ∼0.9 T (∼1.5 T) at a sweep rate of ∼0.0018 T s-1 (0.02 T s-1). These high values combined with persistent stability under ambient conditions, render this system as one of the best-characterized SIMs. Ab initio calculations have been used to establish the connection between the higher-order symmetry of the molecule and the quenching of quantum tunnelling of magnetization (QTM) effects. The relaxation of magnetization is observed via the second excited Kramers doublet owing to pseudo-high-order symmetry, which quenches the QTM. This study highlights fine-tuning of symmetry around the lanthanide ion to obtain new-generation SIMs and offers further scope for pushing the limits of Ueff and TB using this approach.

Paleomagnetism of the Neoproterozoic diamictites of the Qiaoenbrak formation in the Aksu area, NW China: Constraints on the paleogeographic position of the Tarim Block

The middle to late Neoproterozoic records the transition from the dispersal of supercontinent Rodinia to the subsequent assembly of supercontinent Gondwana. Determining paleogeographic positions of the major cratons during this transition is crucial to understand the displacement histories of these blocks. However, high-quality paleomagnetic results from well-dated rocks during this period remain sparse for the cratons. In this paper, we have carried out a paleomagnetic study of the succession of the Cryogenian Qiaoenbrak formation (Fm) in the Aksu area, NW China, which contains diamictites that are of probable glacial origin. The age of the studied section is constrained at around 730Ma based on the stratigraphy and recent zircon U–Pb chronologic data. Stepwise alternating field (AF) and thermal demagnetizations generally reveal two-component magnetizations. The low temperature (coercivity) component was removed by ∼300°C (∼30mT) and the high temperature (coercivity) component decayed toward the origin, defining the characteristic remanent magnetization (ChRM). Rock magnetic data indicate that magnetite is the major remanence carrier. A negative fold test for an isoclinal soft-sediment fold, the presence of both normal and reversed polarities, and positive conglomerate tests in conjunction with the presence of pristine sedimentary features by microscopic inspections of thin sections suggest that the remanence was acquired shortly after the deposition, i.e. post-depositional remanent magnetization (pDRM), which is very close to the magnetic field at the time of deposition. Fourteen out of 17 sites yield stable ChRMs with a mean direction of Ds=246.1°, Is=30.4°, ks=14.1, α95=11.0° in stratigraphic coordinates, corresponding to a paleopole (Q=6) at λ=6.3°S, φ=17.5°E, A95=9.1°. Together with geologic constraints, the new paleomagnetic results place the Tarim Block at 16.3±5.6°N adjacent to either the west or east of Australia (WOA or EOA) at around 730Ma, but the WOA position is preferred based on the overall relatively favorable evidence. In addition, since the studied diamictites of the Qiaoenbrak Fm that are of probable glaciogenic origin yield a low paleolatitude, this study may lend support to the “Snowball Earth” hypothesis during the Neoproterozoic times.

High temperature coercivity of Nb-containing HITPERM alloys: Effect of Cu addition

Hysteresis loops of nanocrystalline samples of Fe39Co39Nb6B16−yCuy (y=0, 1) HITPERM-type alloys were measured from room temperature up to 773K. The alloy with Cu shows a nearly constant value of coercivity (∼135A/m) in all the studied temperature range, whereas for the Cu-free alloy, the coercivity increases from 125A/m at 300K up to 190A/m at 773K. Room temperature Mössbauer spectra can be interpreted on the basis of the presence of three different regions: amorphous, crystalline and interface. The observed nanocrystalline microstructure is similar for the two alloys, although bigger crystallites are found in the alloy without Cu. The very small grain size ensures the averaging out of the magnetocrystalline anisotropy but, as the expected value of magnetostriction constant for the crystalline phase is ∼60ppm, the magnetoelastic anisotropy cannot be neglected.

Early Paleoproterozoic–Archean dykes and gneisses in Russian Karelia of the Fennoscandian Shield—New paleomagnetic, isotope age and geochemical investigations

We present here new palaeomagnetic, isotopic age and geochemical data from Archean and Early Palaeoproterozoic rocks in the eastern Fennoscandian Shield. We have studied NE–SW trending gabbronorite dyke sets and their host Archean basement rocks in the Vodlozero block near the 2449 Ma Burakovka layered intrusion in southern Russian Karelia. Both dyke sets are genetically related to the Burakovka intrusion. The other, ca. 25 km long Avdeev dyke, locating a few kilometers south from the Burakovka intrusion, yields a stable single component remanence direction that is in agreement with the direction previously obtained from the Burakovka intrusion. Another NE–SW trending dyke, 0.8 m wide Shalskiy diabase dyke, about 30 km south of the Burakovka intrusion yields a similar remanence direction as the Avdeev dyke. The overall mean remanence direction has a palaeopole at Plat = −12.3°N, Plong = 243.5°E ( A95 = 15.4°, 4 sites, 28 samples). The thin Shalskiy diabase dyke transects a similarly NE–SW trending 500 m wide coarse grained gabbronorite dyke which has now been dated by Sm–Nd method as 2608 ± 56 Ma. Geochemically all the dykes are quite similar showing slight calc-alkaline affinity and low TiO 2 and high SiO 2 with moderate MgO and low Cr and Ni. Furthermore, the dykes are geochemically identical to the 2.45 Ga dyke swarm in the northern Karelian Province. The remanence direction of the thin Shalskiy diabase dyke differs significantly from the high temperature and high coercivity remanence component of the unbaked Archean gabbronorite dyke which yields a palaeopole at Plat = 22.7°N, Plong = 222.1°E (dp = 8.2°, dm = 16.2°, five samples). On the basis of different remanence directions of the diabase dyke and the unbaked Archean gabbronorite dyke, the baked contact test for the diabase dyke is positive. In addition to the high temperature and high coercivity component of the baked and unbaked Archean gabbronorite dyke, in low temperatures and coercivities we isolated a similar component as in the diabase dyke. A comparable remanence component was also obtained from the Archean basement at ca. 8 km from the dykes. We propose that in the studied area, the Archean basement and the Archaean dyke were partly remagnetized due to emplacement and subsequent uplift and cooling of the large Burakovka layered intrusion and related dykes at about 2.40 Ga ago. This interpretation lends support from a new 40Ar/ 39Ar dating of hornblende from another area, Lake Paajarvi area, in northern Karelia. There, a negative baked contact test was previously obtained for the remanence of the dated ca. 2.45 Ga dyke rocks related to the ca. 2.45 Ga Oulanka layered intrusion. The 40Ar/ 39Ar dating of the unbaked Archean basement which yields the same remanence component as the dykes, shows a plateau age of ca. 2.6 Ga, but in addition, it also shows resetting of the basement at ca. 2.4 Ga ago. The dating thus supports reactivation and partial remagnetization of the Archean basement at ca. 2.4 Ga ago. Our new palaeomagnetic results from the Burakovka dykes and the new 40Ar/ 39Ar dating from the Lake Paajarvi area give support to our previous interpretation that at Lake Paajarvi area the remanence component suggested to be 2.4 Ga, despite to negative baked contact test, is indeed of this age. Therefore, it is implied that the results can be used for continental reconstructions.

High-temperature magnetic properties of mechanically alloyed SmCo5 and YCo5 magnets

The high-temperature coercivity of mechanically alloyed and subsequently annealed RCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> (R=Sm and Y) is studied. The annealed materials have the hexagonal CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> structure with 2:17 (or 1:7) regions as a minor phase. High-temperature magnetic measurements show that the coercivities of materials decrease with increasing temperature from room-temperature to 873 K, but that the temperature coefficient of the coercivity of YCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> is much smaller than that of SmCo <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> . This behavior is explained in terms of the intrinsic temperature variation of the magnetocrystalline anisotropy.

Effect of Iron Substitution on the High-temperature Properties of Sm(Co,Cu,Ti)z Permanent Magnets

ABSTRACTRecently, Ti-substituted Sm-Co permanent magnets have attracted renewed attention due to their interesting high-temperature coercivity. Our presentation deals with the effect of iron substitutions on the magnetic properties of the materials. X-ray diffraction shows that the investigated Sm(Co,Fe,Cu,Ti)z materials (z = 7.0 - 7.6) are two-phase magnets, consisting of 1:5 and 2:17 regions. The iron content affects both the coercivity and the magnetization. Depending on composition and heat treatment, some samples show a positive temperature coefficient of the coercivity in the temperature range from 22 °C to 550 °C. Moderate amounts of iron enhance the room-temperature coercivity. For example, the room-temperature coercivity of Sm(Co6.0Fe0.4Cu0.6Ti0.3) is 9.6 kOe, as compared to 7.6 kOe for Sm(Co6.4Cu0.6Ti0.3). At high temperatures, the addition of Fe has a deteriorating effect on the coercivity, which is as high as 10.0 kOe at 500 °C for Sm(Co6.4Cu0.6Ti0.3). The room-temperature magnetization increases on iron substitution, from 73 emu/g for Sm(Co6.4Cu0.6Ti0.3) to 78 emu/g for Sm(Co6.0Fe0.4Cu0.6Ti0.3). The observed temperature dependence is ascribed to the preferential dumbbell-site occupancy of the Fe atoms.

Micromagnetism and high temperature coercivity of MnBi/Al multilayers

The micromagnetic properties of multilayered MnBi/Al films have been investigated and compared to pure MnBi thin films. Pure MnBi films reveal an anomalous increase of the high temperature coercivity, which has been explained on the basis of a hybrid domain wall pinning model. The multilayer-type preparation of MnBi/Al thin films results in significantly reduced MnBi particle size of approximately 40 nm. The smaller particle size leads to a change of the dominant magnetization reversal process from one driven by domain wall movement toward coherent rotation. This was investigated via magnetic force microscopy imaging and micromagnetic calculations. The absence of domain walls during magnetization reversal results in a clear suppression of the increase of the high temperature coercivity observed in pure MnBi films.

Sm–Co–Cu–Ti high-temperature permanent magnets

A class of promising permanent-magnet materials with an appreciable high-temperature coercivity of 8.6 kOe at 500 °C is reported. The Sm–Co–Cu–Ti magnets are prepared by arc melting and require a suitable heat treatment. Magnetization measurements as a function of temperature and x-ray diffraction patterns indicate that the samples are two-phase mixtures of 2:17 and 1:5 structures. Depending on heat treatment and composition, some of the magnets exhibit a positive temperature coefficient of coercivity. The promising high-temperature behavior of the coercivity is ascribed to the temperature dependence of the domain-wall energy, which affects the curvature of the walls and the pinning behavior.

High-temperature coercivity and Kerr spectroscopy on MnBi/Al multilayers

The magneto-optic properties of multilayered MnBi/Al films have been investigated and compared to pure MnBi films. The Al interlayers act as diffusion barrier during the MnBi formation process leading to a reduced MnBi particle size of approximately 50 nm. The smaller particle size changes the dominant magnetization reversal process from domain wall movement towards coherent rotation. This leads to a suppression of the anomalous increase of the high-temperature coercivity observed in pure MnBi. The comparison of the experimental polar Kerr spectra with theory elucidates the role of oxygen in MnBi films.

Suppression of the increase of high-temperature coercivity in MnBi thin films by Al interlayers

By tailoring the microcrystalline structure of MnBi films, using Al interlayers, a reduction of the high-temperature coercivity by a factor of 3 is achieved. The separation of Bi/Mn bilayers by Al interlayers acts as a diffusion barrier perpendicular to the surface. After annealing, the MnBi layers contain single-domain particles surrounded by an Al matrix exhibiting no significant increase of the coercive field with increasing temperature.

Magneto-optic study of the high-temperature coercivity of MnBiAl and MnBiPt films

We present evidence that domain-wall pinning in MnBi films is responsible for the unusual increase of the high-temperature coercivity. The MnBi crystallite size has been reduced by depositing Bi/Mn/Al multilayers. The average diameter of the MnBi particles is smaller than the critical diameter of single-domain particles. Therefore, domain-wall pinning does not enhance the high-temperature coercivity as observed for pure MnBi films.

Paleomagnetism of upper Miocene ignimbrites at the Puna: An analysis of vertical‐axis rotations in the Central Andes

The origin of the rotations detected paleomagnetically in the Central Andes is controversial. Tectonic models proposed to explain it involve Late Cenozoic oroclinal bending (the Bolivian Orocline), and/or small‐block rotations driven by oblique subduction. In this paper, we report paleomagnetic data from upper Miocene ignimbrites from the Puna, in the presumed southern limb of the orocline. These rocks showed high unblocking temperature and high coercivity magnetization. The resulting pole position (latitude 85.7°S, longitude 80.5°E, A95 = 7.9°, K = 22, N = 17) indicates that the study area has not been significantly affected by vertical‐axis rotation since late Miocene onward. This implies that the whole Puna has not undergone significant regional rigid‐body rotation during the same time span. A farther analysis based on Neogene paleomagnetic data from 31 Andean localities distributed from 11°S to 31°S shows the presence of rotated and unrotated areas. Systematic counterclockwise rotations in Perü and northern Bolivia are observed, whereas clockwise rotations are present in southern Bolivia, northern Chile, and perhaps northwestern Argentina. No systematic rotations are found farther south. The overall time‐spatial distributions of these Neogene paleomagnetic data suggest that if orogenic bending occurred, it must have taken place before the middle Miocene. Thus the oroclinal hypothesis cannot explain the rotations observed in middle Miocene rocks or younger ones, which also show orocline‐like declination anomalies. We suggest that small‐block rotations driven by distributed shear may be a single process that can account for the whole rotations in the Central Andes. It is hypothesized that horizontal shear in the Andean crust could be controlled by ancient structures.

A Nd-Fe-O intergranular phase in Nd-Fe-B sintered magnets and its effect on coercivity

The intergranular microstructure and coercivity of Nd richer Nd/sub x/Fe/sub 93-x/B/sub 7/ (x=16-28) sintered magnets have been systematically studied by means of scanning electron microscopy, analytical transmission electron microscopy, scanning Auger multiprobe, thermomagnetic analysis, magnetic measurement and annealing experiments. A new stable Nd-Fe-O ferromagnetic phase with a composition NdFe/sub 2/O/sub x/ (x/spl ap/0.3) and a Curie temperature of 145/spl deg/C is found in the intergranular regions of the magnets, due to the introduction of a considerable amount of oxygen during magnet processing and its segregation in the intergranular regions. The phase forms at /spl sim/650/spl deg/C and volume fraction reaches a maximum of /spl sim/4% in the Nd/sub 22/Fe/sub 71/B/sub 7/ magnets. This new phase has a strong hindrance effect on the propagation of reversed domain walls between Nd/sub 2/Fe/sub 14/B grains, and thus leads to a considerable enhancement of coercivity. Raising the Curie temperature of this phase would be a new important approach to improve the high-temperature coercivity of Nd-Fe-B magnets.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>