Progress in Materials Science 44 (1999) 291±433 Amorphous and nanocrystalline materials for applications as soft magnets Michael E. McHenry*, Matthew A. Willard, David E. Laughlin Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA Received 30 November 1998 Abstract This review seeks to summarize the recent developments in the synthesis, structural characterization, properties, and applications in the ®elds 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 modi®cations, 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. # 1999 Elsevier Science Ltd. All rights reserved. Contents 1. Introduction and historical perspective ...............................300 1.1. Technical considerations ....................................302 2. Alloy design issues..............................................306 2.1. Magnetic dipole moments and magnetization......................307 * Corresponding author. Tel.: +1-412-268-2703; fax: +1-412-268-3113. E-mail address: [email protected] (M.E. McHenry). 0079-6425/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0079-6425(99)00002-X 292 M.E. McHenry et al. / Progress in Materials Science 44 (1999) 291±433 2.2. Ferromagnetic ordering (Curie) temperatures......................310 2.3. Magnetocrystalline anisotropy and magnetostriction ................313 2.4. Magnetic domains and domain wall mobility......................315 2.5. AC magnetic response ......................................317 2.6. Considerations of glass forming ability and crystallization ............318 3. Synthesis of nanoparticles, amorphous, bulk amorphous and nanocrystalline alloys .......................................................321 3.1. Powder synthesis of nanoparticles and nanoencapsulates .............322 3.1.1. Carbon arc synthesis .................................322 3.1.2. Plasma torch synthesis................................312 3.1.3. Mechanical milling techniques ..........................327 3.1.4. Powder consolidation ................................330 3.2. Rapid solidi®cation processing ................................331 3.3. Solidi®cation processing of bulk amorphous alloys..................333 3.4. Primary nanocrystallization of amorphous precursors................333 4. Thermodynamic and transformation kinetic considerations for amorphous and nanocrystalline materials .........................................335 4.1. Calorimetry and stability of bulk amorphous materials...............337 4.2. Primary and secondary crystallization events in nanocrystalline alloys ....340 4.2.1. FINEMET alloys ...................................340 4.2.2. NANOPERM alloys .................................342 4.2.3. HITPERM alloys ...................................343 4.2.4. Other alloys .......................................343 4.3. Surface crystallization kinetics ................................343 4.4. Crystallization kinetics......................................344 4.4.1. FINEMET alloys ...................................344 4.4.2. NANOPERM alloys .................................346 4.4.3. HITPERM alloys ...................................346 4.4.4. Other alloys .......................................346 4.5. Ordering and structural phase transitions in nanocrystalline alloys ......347 4.5.1. Nanoparticles and nanoencapsulates. .....................347 4.5.2. FINEMET alloys ...................................349 4.5.3. NANOPERM alloys .................................350 4.5.4. HITPERM alloys ...................................350 5. Structure and microstructure of amorphous and nanocrystalline alloys ........352 5.1. Background information ....................................352 5.1.1. Amorphous alloys ...................................352 5.1.2. Nanocrystalline phases ...............................354 5.1.3. Microstructural characterization and spectroscopic analysis (amorphous and nanocrystalline alloys)....................355 5.2. Experimental observations: X-ray scattering and EXAFS .............358 5.2.1. FINEMET alloys ...................................362 5.2.2. NANOPERM alloys .................................363 5.2.3. HITPERM alloys ...................................363 5.3. Experimental observations: neutron scattering .....................364 5.3.1. Nanoparticle powders ................................364 M.E. McHenry et al. / Progress in Materials Science 44 (1999) 291±433 293 5.3.2. Nanocrystalline materials..............................365 5.4. Experimental observations: microscopy ..........................365 5.4.1. Nanoparticle powders ................................365 5.4.2. FINEMET alloys ...................................367 5.4.3. NANOPERM alloys .................................370 5.4.4. HITPERM alloys ...................................374 5.4.5. Other alloys .......................................376 5.5. Experimental observations: MoÈ ssbauer spectcroscopy................377 5.5.1. FINEMET alloys ...................................377 5.5.2. NANOPERM alloys .................................377 5.5.3. HITPERM alloys ...................................382 6. Intrinsic magnetic properties ......................................382 6.1. Magnetic dipole moments ...................................382 6.1.1. Amorphous alloys ...................................382 6.1.2. Nanocrystalline alloys ................................385 6.2. Magnetic exchange interactions and T-Dependent ferromagnetic response . 387 6.2.1. Amorphous alloys ...................................387 6.2.2. Nanocrystalline alloys ................................391 7. Extrinsic magnetic properties ......................................395 7.1. Magnetic anisotropy and magnetostriction .......................395 7.1.1. Amorphous alloys ...................................395 7.1.2. Weakly exchange coupled nanocrystalline alloys .............396 7.1.3. Strongly exchange coupled nanocrystalline alloys.............396 7.2. Induced anisotropies .......................................402 7.3. Magnetic coupling in two phase microstructures ...................403 7.4. Experimental observations in bulk amorphous alloys ................410 7.5. Experimental observations in nanocrystalline alloys .................412 7.5.1. FINEMET alloys ...................................412 7.5.2. NANOPERM alloys .................................417 7.5.3. HITPERM alloys ...................................419 8. Applications of amorphous and nanocrystalline materials..................420 8.1. Amorphous materials ......................................420 8.2. Nanocrystalline materials ....................................422 8.2.1. FINEMET alloys ...................................422 8.2.2. NANOPERM alloys .................................422 8.2.3. HITPERM alloys ...................................423 9. Conclusions ..................................................425 Acknowledgements..................................................426 References ........................................................426 294 M.E. McHenry et al. / Progress in Materials Science 44 (1999) 291±433 Nomenclature a Lattice parameter A Exchange stiness Aam Exchange stiness amorphous phase Acx Exchange stiness, crystalline phase AW Domain wall area A(r) Local exchange stiness B, B Induction, ¯ux density BY Breadth of an X-ray peak Bhf Hyper®ne ®eld Bmax (AC) induction amplitude Br Remanent induction Bs Saturation induction BJ x Brillouin function BS x Spin-only Brillouin function c Speed of light C Speci®c heat CV,CP Speci®c heat, constant V, P CM,CH Speci®c heat, constant M, H Ch,Ce Power loss coecients d Skin depth dp Atomic plane spacing dl Displacement D Diusivity DA A-atom diusivity DB B-atom diusivity D, Dg Grain size E Activation energy EF Fermi energy Ep Potential energy Eg Energy of gamma radiation f Frequency Fm Helmholtz free energy g r Pair correlation function g E Density of states g E Spin up density of states g E Spin down density of states h Planck's constant H Magnetic ®eld Ha Applied magnetic ®eld vector Hc Coercive ®eld HINT Internal magnetic ®eld M.E. McHenry et al. / Progress in Materials Science 44 (1999) 291±433 295 HK Anisotropy ®eld Hm Field amplitude (AC) Hmix Heat of mixing HN Domain nucleation ®eld H0 Hyper®ne splitting HW Domain wall pinning ®eld I Current I Intensity J, Jex Exchange energy density kB Boltzmann constant k Equilibrium constant K Magnetic anisotropy Ke Eective magnetic anisotropy Kl Magnetic anisotropy lead term Kl(r) Local anisotropy lead term Ku Uniaxial magnetic anisotropy kà `Structural' anisotropy k0 Macroscopic anisotropy l Length ` Angular momentum quantum number `s Structural ¯uctuation length l(R ) Dipolar magnetic anisotropy L Latent heat Lex Exchange correlation length Lam Lex-amorphous phase à L Modi®ed Lex Ln Lanthanide M, M Magnetization (avg), magnitude M Metalloid Ms Saturation magnetization am Ms Ms-amorphous phase m, m Reduced magnetization, magnitude n Power law exponent ne Number of electrons