M. A. Willard

Amorphous and nanocrystalline materials for applications as soft magnets

Abstract This review seeks to summarize the recent developments in the synthesis, structural characterization, properties, and applications in the fields of amorphous, bulk amorphous, and nanocrystalline soft magnetic materials. Conventional physical metallurgical approaches to improving soft ferromagnetic properties have relied on the optimization of chemical and microstructural features. Within the last decade, the development and rapid increase in research of nanocrystalline materials has shown that through proper modifications, revolutionary contributions can be made to better materials’ properties. A wide range of materials’ properties are examined in this review, including: kinetics and thermodynamics, structure, microstructure, and intrinsic and extrinsic magnetic properties.

Magnetic Materials and Devices for the 21st Century: Stronger, Lighter, and More Energy Efficient

A new energy paradigm, consisting of greater reliance on renewable energy sources and increased concern for energy efficiency in the total energy lifecycle, has accelerated research into energy-related technologies. Due to their ubiquity, magnetic materials play an important role in improving the efficiency and performance of devices in electric power generation, conditioning, conversion, transportation, and other energy-use sectors of the economy. This review focuses on the state-of-the-art hard and soft magnets and magnetocaloric materials, with an emphasis on their optimization for energy applications. Specifically, the impact of hard magnets on electric motor and transportation technologies, of soft magnetic materials on electricity generation and conversion technologies, and of magnetocaloric materials for refrigeration technologies, are discussed. The synthesis, characterization, and property evaluation of the materials, with an emphasis on structure-property relationships, are discussed in the context of their respective markets, as well as their potential impact on energy efficiency. Finally, considering future bottlenecks in raw materials, options for the recycling of rare-earth intermetallics for hard magnets will be discussed.

Chemically prepared magnetic nanoparticles

Abstract Nanotechnology has spurred efforts to design and produce nanoscale components for incorporation into devices. Magnetic nanoparticles are an important class of functional materials, possessing unique magnetic properties due to their reduced size (below 100 nm) with potential for use in devices with reduced dimensions. Recent advances in processing by chemical synthesis and the characterisation of magnetic nanoparticles are the focus of this review. Emphasis has been placed on the various solution chemistry techniques used to synthesise particles, including: precipitation, borohydride reduction, hydrothermal, reverse micelles, polyol, sol–gel, thermolysis, photolysis, sonolysis, multisynthesis processing and electrochemical techniques. The challenges and methods for examining the structural, morphological, and magnetic properties of these materials are described.

Structure and magnetic properties of (Fe0.5Co0.5)88Zr7B4Cu1 nanocrystalline alloys

The development of Fe73.5Si13.5B9Nb3Cu1 (FINEMET) by Yoshizawa et al. and Fe88Zr7B4Cu1 (NANOPERM) by Inoue et al. have shown that nanocrystalline microstructures can play an important role in the production of materials with outstanding soft magnetic properties. The FINEMET and NANOPERM materials rely on nanocrystalline α-Fe3Si and α-Fe, respectively, for their soft magnetic properties. The magnetic properties of a new class of nanocrystalline magnets are described herein. These alloys with a composition of (Fe,Co)–M–B–Cu (where M=Zr and Hf) are based on the α- and α′-FeCo phases, have been named HITPERM magnets, and offer large magnetic inductions to elevated temperatures. This report focuses on thermomagnetic properties, alternating current (ac) magnetic response, and unambiguous evidence of α′-FeCo as the nanocrystalline ferromagnetic phase, as supported by synchrotron x-ray diffraction. Synchrotron data have distinguished between the HITPERM alloy, with nanocrystallites having a B2 structure from the ...

MAGNETIC PROPERTIES OF HITPERM (FE, CO)88ZR7B4CU1 MAGNETS

A new class of nanocrystalline alloys with composition Fe44Co44Zr7B4Cu1 has been developed. This and similar alloys of general composition (Fe, Co)–M–B–Cu (where M=Zr, Hf, Nb, etc.) have been named HITPERM. They offer large magnetic inductions and excellent soft magnetic properties at elevated temperatures. Thermomagnetic properties, permeability, and frequency dependent losses are described in this report. These alloys exhibit high magnetization that persists to the α→γ phase transformation at 980 °C. Alternating current permeability experiments reveal a high permeability at 2 kHz with a loss value of 1 W/g at Bs=10 kG and f=10 kHz.

Influence of Co and Ni addition on the magnetocaloric effect in Fe88−2xCoxNixZr7B4Cu1 soft magnetic amorphous alloys

We have studied the magnetocaloric effect in a series of Fe88−2xCoxNixZr7B4Cu1 alloys. The partial substitution of Fe by Co and Ni leads to a monotonic increase in the Curie temperature (TC) of the alloys from 287 K for x=0 to 626 K for x=11. The maximum magnetic entropy change (ΔSMpk) at an applied field of 1.5 T, shows a value of 1.98 J K−1 kg−1 for x=8.25. The refrigerant capacity (RC) has maximum values near 166 J kg−1 (for x=0 and 2.75). These values place the present series of alloys among the best magnetic refrigerant materials, with an RC ∼40% larger than Gd5Si2Ge1.9Fe0.1 and ∼15% larger than Fe-based amorphous alloys.

Optimization of the refrigerant capacity in multiphase magnetocaloric materials

The refrigerant capacity (RC) of magnetocaloric materials can be enhanced using multiphase materials or composites, which expand the temperature range over which a significant magnetic entropy change can be obtained. Numerical simulations show that by controlling the parameters of the composite (the fraction of the different phases and their Curie temperatures) improvements of RC of ∼83% are possible. The maximum applied field plays a crucial, nonmonotonic, role in the optimization. As a proof of concept, it is shown that the combination of two Fe88−2xCoxNixZr7B4Cu1 alloys produces an enhancement in RC of ∼37%, making it ∼92% larger than that of Gd5Si2Ge1.9Fe0.1.

Magnetic, structural, and transport properties of thin film and single crystal Co2MnSi

The magnetic, structural, and transport properties of the Heusler alloy Co2MnSi are reported for sputtered thin films and a single crystal. X-ray diffraction reveals a phase pure L21 structure for all films grown between 573 and 773 K. Films grown at 773 K display a four-fold decrease in the resistivity relative to those grown at lower temperatures and a corresponding 30% increase in the residual resistivity ratio (ρ300 K/ρ5 K). We show that the higher growth temperature results in lattice constants, room temperature resistivities, and magnetic properties that are comparable to that of the bulk single crystal.

Microstructural characterization of (Fe0.5Co0.5)88Zr7B4Cu1 nanocrystalline alloys

The distribution of Cu atoms in an Fe44Co44Zr7B4Cu1 nanocrystalline alloy (HITPERM) has been studied by the three-dimensional atom probe technique. Cu atoms do not form clusters prior to the crystallization reaction, and they are partitioned in the remaining amorphous phase after crystallization. A Cu-free Fe44.5Co44.5Zr7B4 alloy has the same microstructure as the alloy with 1% Cu.

Recent advances in the development of (Fe,Co)88M7B4Cu1 magnets (invited)

Annealing of amorphous precursor alloys, with compositions (Fe,Co)88M7B4Cu1 (M=Zr, Nb, Hf), above their primary crystallization temperature results in the nanocrystallization of the ferromagnetic α′-FeCo phase. This work describes results of the characterization of these alloys, including morphological and chemical stability of the α′-FeCo phase, examination of alloy compositions, and development of a pseudo-Slater–Pauling curve for the amorphous precursor alloys. Samples with the composition Fe44Co44Zr7B4Cu1 were annealed at 600 °C for 10, 31, 100, 308, 1000, and 3072 h in Ar and examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Scherrer analysis of x-ray peak breadths was used to infer only a slight increase in the grain size of the sample annealed for 3072 h (∼60 nm) compared to the samples annealed for short times (∼40 nm). TEM studies revealed a distribution of grain sizes in the material with an average grain size of 42 nm for the 3072 h annealed sample. Samples anneale...