X.B. Liu

High Energy Product Developed from Cobalt Nanowires

Cobalt nanowires with high aspect ratio have been synthesized via a solvothermal chemical process. Based on the shape anisotropy and orientation of the nanowire assemblies, a record high room-temperature coercivity of 10.6 kOe has been measured in Co nanowires with a diameter of about 15 nm and a mean length of 200 nm. As a result, energy product of the wires reaches 44 MGOe. It is discovered that the morphology uniformity of the nanowires is the key to achieving the high coercivity and high energy density. Nanowires of this type are ideal building blocks for future bonded, consolidated and thin film magnets with high energy density and high thermal stability.

High temperature magnetic properties of SmCo5/α-Fe(Co) bulk nanocomposite magnets

To find alternative high temperature magnets containing no heavy rare earths for power applications, SmCo5/Fe bulk nanocomposite magnets with enhanced energy density and high thermal stability have been produced by using a ball-milling plus warm-compaction route. Up to 30% of the Fe soft magnetic phase has been added to the composites with grain size <20 nm distributed homogenously in the matrix of the SmCo5 hard magnetic phase. It was observed that the microstructure does not change with temperature up to 500 °C. It is also observed that the thermal stability of bulk nanocomposite samples is closely related to bulk density. Energy products above 11 MGOe have been obtained at 300 °C in fully dense bulk SmCo5/Fe nanocomposite magnets, which is 65% higher than that of a single-phase counterpart at the same temperature.

Processing of MnBi bulk magnets with enhanced energy product

We report magnetic properties and microstructure of high energy-product MnBi bulk magnets fabricated by low-temperature ball-milling and warm compaction technique. A maximum energy product (BH)max of 8.4 MGOe and a coercivity of 6.2 kOe were obtained in the bulk MnBi magnet at room temperature. Magnetic characterization at elevated temperatures showed an increase in coercivity to 16.2 kOe while (BH)max value decreased to 6.8 MGOe at 400 K. Microstructure characterization revealed that the bulk magnets consist of oriented uniform nanoscale grains with average size about 50 nm.

Novel processing of high-performance MnBi magnets

Rare-earth-free MnBi magnets have attracted much attention recently due to their positive temperature coefficient of coercivity. In this work, the preparation, microstructure and magnetic properties of bulk MnBi magnets have been investigated. A low-temperature (−120 °C), low-energy ball-milling (LTLEBM) process has been adopted in the initial MnBi powder preparation that reduces the particle size to 1–5 μm from the 35–75 μm size of raw material powders of the MnBi low-temperature phase (LTP) (~97 wt%) made by melting and annealing. The LTLEBM process has significantly suspended the decomposition of the LTP MnBi that occurs excessively during ordinary room-temperature ball milling. After the LTLEBM, the coercivity iHc of the MnBi powder was increased from 1 kOe to 12 kOe while the LTP content in the powder was retained as high as 95 wt%. The as-milled powders were then aligned in an 18 kOe field and warm-compacted into a dense bulk magnet at 300 °C for 10 min to reach a mass density of ~8.4 g cm−3. The bulk magnets have a maximum energy product of 7.8 MGOe and coercivity of 6.5 kOe at room temperature. When the temperature is increased to 475 K, the coercivity is increased to 23 kOe.

First-principles survey on the doping of Ga in Nd2Fe14B

The preferential site substitution of Fe by Ga and its effect on magnetic moments have been studied in Nd2Fe14B by a first-principles density functional calculation. The calculated substitution energies of Ga are positive values and unfavorable for the Ga substitution at the sites of 8j2, 16k1, and 4e, while those are negative values and favorable for the Ga substitution at the sites of 4c, 8j1, and 16k2, in good agreement with the experimental results. The Ga atom shows a small induced negative moment with a size of about 0.2 μB. The magnetic moments of Fe with the nearest neighbor of Ga reduce by about 0.1–0.2 μB while the moments of Nd with the Ga nearest neighbors are enhanced slightly. These magnetic moment changes originate from the hybrid of the Ga 4p- with the Fe 3d- or the Nd 5d-electron states.

Preparation and magnetic properties of MnBi-based hard/soft composite magnets

Bulk anisotropic composite magnets based on MnBi/Co(Fe) exhibiting the different morphology of the soft magnetic phase were prepared by powder metallurgy processing. First, single-phase MnBi bulk magnets were produced with a maximum energy product [(BH)m] of 6.3 MGOe at room temperature. The nanoscale soft phase with the different morphology was then added to form a composite magnet. It was observed that addition of magnetic soft-phase nanoflakes causes a dramatic coercivity reduction. However, the addition of soft magnetic phase nanowires enhanced the composite magnetization without sacrificing the coercivity. Nevertheless, a kink was still observed on the demagnetization curves and the coercivity decreased when the soft-phase content was larger than 10 wt. %, which was caused by the agglomeration of the soft phase nanowires that also led to a decreased degree of texture.

High-Performance MnBi Alloy Prepared Using Profiled Heat Treatment

The profiled heat treatment (PHT) method has been used to synthesize MnBi alloys with high-purity low-temperature phase (LTP). In the PHT method, the arc-melted MnBi alloy was remelted then slowly cooled by a pseudo-equilibrium solidification process to promote the formation of LTP phase. The PHT-treated MnBi alloys had an LTP phase up to 94 wt.% and a magnetization of 73 emu/g under a field of 9 T. Scanning electron microscopy showed that there exist some micrometer-sized Mn-rich inclusions in the LTP matrix of the PHT MnBi alloy. The PHT MnBi alloys were crushed into powders with an average size of ~3 μm by low-energy ball milling. These MnBi powders were aligned in an 18 kOe field and warm compacted into a bulk magnet at 300 °C for 30 min. The magnet had a density of 8.2 g/cm3 and magnetic properties of Ms = 6.7 kG, Mr = 5.3 kGs, i Hc = 5 kOe, and (BH)max = 6.1 MGOe.

Exchange interaction in hexagonal MnRhP from first-principles studies

Electronic structure and magnetic properties for MnRhP have been studied from a first-principles density functional calculation. The calculated lattice constants, a = 6.228 A and c = 3.571 A, are in good agreement with the experimental values of a = 6.223 A and c = 3.585 A. The calculated moment of Mn is 3.1 μB/atom, resulting in a total moment of 3.0 μB/atom due to small moments induced at Rh and P sites. The magnetic moment of Mn decreases with unit cell size. The exchange interactions are dominated by positive Mn-Mn exchange coupling (JMn−Mn), implying a stable ferromagnetic ordering in Mn sublattice. In particular, JMn−Mn shows a maximum value (1.5 mRy) at the the optimized unit cell size. The structural distortion or unit cell size change will affect JMn−Mn, which is intimately related to the magneto-elastic and magneto-caloric effect.