NEU
Nanomagnetism Group
research

Lewis Lab    |    Heiman Lab

Nanomagnetism Research Group

The Nanomagnetism Group, led by Dr. L.H. Lewis, focuses on the synthesis and characterization of magnetic nanomaterials with the aim to contribute fundamental knowledge to enable advanced applications towards energy management, memory storage, and multifunctional devices. Currently, our lab focuses on projects examining magnetostructural transitions, advanced permanent magnet nanomaterials, ferromagnetic semiconducting nanomaterials and magnetic nanoparticles.

Our lab, in collaboration with the Heiman Lab, carries out materials synthesis as well as characterization and analyses. Synthesis methods available to the group include arc-melting, rapid solidification (melt-spinning), and thin film synthesis via Molecular Beam Epitaxy (MBE), thermal evaporation, and anodization. Characterization of the magnetic and phase state is carried out via X-ray diffraction (XRD), electron microscopy, atomic/magnetic force microscopy, magnetometry (Superconducting Quantum Interference Device (SQUID), and thermal analysis. Synchrotron-based spectroscopy and scattering probes are also employed for advanced materials characterization.

Ongoing Research

Rare-Earth-Free Exchange Coupled Permanent Magnets 

The demand for rare-earth elements is currently outstripping supply and leading to what is known popularly in the press as the "rare earth crisis" (Lewis, L. H. & Jiménez-Villacorta, F. Metall. Mater. Trans. A 44, 2–20 (2012). This demand arises from the many applications for rare-earth elements including the rare-earth-based "supermagnets", materials for military applications, and alternative energy technologies. To that end, we seek to produce nanostructured permanent magnets that produce anisotropies and energy products that are comparable to those of the current supermagnets using only cheap, abundant and more sustainably-produced metals.

One example we are working on is to combine τ-MnAl with α-Fe at the nanoscale to produce an exchange-spring magnet with better performance than its constituent materials. The τ-phase of MnAl is metastable, but is ferromagnetic with high coercivity. We are able to produce this phase through the rapid solidification of molten MnAl through melt-spinning (Jiménez-Villacorta, F. et al. ; Jiménez-Villacorta, F. et al. Metals (Basel). 4, 8–19 (2014). The soft but highly magnetic α-Fe phase is then mixed with τ-MnAl at the nanoscale through high energy ball milling, to produce exchange-spring coupling.

This work is supported by the Office of Naval Research Grant #XXXXX.

figure 1

  figure 2

Phase and magnetic diagram at the near-equiatomic region of Al-Mn alloys. From Jiménez-Villacorta, F. (2012).

 

Room temperature M(H) curve taken for Mn53.8Al46.2. The paramagnetic signal was linearly fit and subtracted to obtain a corrected FM signal. By comparing the MS of this sample to the theoretical MSof τ-MnAl, it is determined that the sample contains ~21 wt% τ-phase.


Chemically ordered L10 FeNi: a candidate material for advanced permanent magnets

Ferrous compounds with the L10 crystal structure have recently attracted considerable attention for the development of advanced rare-earth free permanent magnets. Among these, L10 FeNi (a.k.a. tetrataenite) is particularly promising for next-generation rare-earth-free permanent magnets as Fe and Ni are both inexpensive and readily available, amenable to standard metallurgical processing techniques, and resistant to corrosion. L10 FeNi has a theoretical maximum energy product comparable to the best rare-earth permanent magnets due to its inherently large saturation magnetization and large magnetocrystalline anisotropy (MS = 1300 emu/cc, Ku = 1.3 x 107 erg/cc). However, the phase is only found naturally in meteorites that form over 4.5 billion years; it has not been produced in quantities by laboratory methods due to extremely low atomic Fe and Ni mobilities (1 atomic jump per 2,600 years) below the critical temperature of 320 °C, where the fcc chemically-disordered phase transforms into the L10 chemically-ordered phase. (L.H. Lewis, et al. J Phys: Conds Matter. 26. 2014.)

Ongoing research utilizes tetrataenite extracted from meteorites to examine structure-magnetism correlations in L10-structured FeNi and characterize the chemical-order/disorder transformation. Results have shown that L10 FeNi, once formed, is kinetically stable and magnetization remains high until a simultaneous magnetic and structural transformation occurs (L10 to fcc, ferro to paramagnetic) near 800 K. Efforts are being directed towards the understanding of the various factors that influence L10 phase formation and stability, in pursuit of fundamental information to guide laboratory-time-scale synthesis of tetrataenite.

schematic
graph

Schematic of the FCC and L10 crystal structures. The tetragonal L10 structure is composed of crystallographic planes oriented perpendicular to the c-axis which alternate in atomic species.

 

Magnetization as a function of temperature for FeNi from the NWA6259 meteorite. The red branch begins at 300 K in the ferromagnetic L10 state, then is heated through a 1st order magnetostructural transition at 805 K. The blue branch begins at 900 K in the chemically-disordered paramagnetic state and is cooled through a 2nd order Curie transition at 750 K.


Understanding Phase Formation and Coexistence in FeMn-based alloys

Exchange biased systems find use in many mainstream and prospective technologies; these systems are commonly grown in thin film form. The primary prerequisite for the exchange bias phenomenon, or unidirectional anisotropy resulting in a horizontal shift in the magnetic hysteresis curve, is the coexsistence of ferromagnetic (FM) and antiferromagnetic (AF) phases in close contact.

FeMn has long been studied as the antiferromagnetic portion of an exchange biased thin film system. In many examples the ferromagnetic phase is another Fe-based magnetic alloy such as Permalloy; the transition from thin films to bulk systems often proves to be difficult. Our research enables the synthesis of nanostructured bulk exchange biased FeMn-based alloy systems using rapid solidification. We are able to synthesize multiphase α-(FeMn) (FM) / γ-(FeMn) (AF) bulk material from a single starting material. Phase fraction is then controlled through a thermally hysteretic martensitic-austenitic phase transformation.

This work is supported by the Office of Naval Research Grant # XXXXX

Figure 1

Vibrating Sample Magnetometry showing a reduction in magnetic moment indicative of an
incremental transformation from ferromagnetic α-FeMn to antiferromagnetic γ-FeMn.


Thermoelectric Materials

Thermoelectric Materials can be designed for utilization in low temperature applications such as solid state refrigerators. It is desired to increase the dimension-less thermoelectric figure-of-merit ZT in these materials within the temperature range of their functionality. The thermoelectric figure-of-merit is related to the Seebeck coefficient S, total thermal conductivity κ, and electrical resistivity ρ of the material through the equation, equation, where T is the absolute temperature. Development of a material with a high Z value is challenging, due to the interrelations between the three physical quantities that define its thermoelectric figure-of-merit. In this project, we aim to understand the effects of nanostructuring and doping on the thermoelectric properties of specific metallic compounds, towards the development of an efficient low temperature thermoelectric material.


Nanotube Arrays

TiO2 Nanotube Arrays doped with iron are anticipated to exhibit multifunctional behavior that combines effects from electronic charge, magnetic spin and optical response. Properly engineered, these materials hold promise for potential application in devices to efficiently absorb and transfer solar energy and/or to process data with increased speed, precision and accuracy. We investigated the correlations of the morphology, crystallinity, crystal structure, electronic structure and magnetic properties of electrochemically-anodized TiO2 nanotubes, with relevance to their functionality. This project was sponsored by the National Science Foundation under Grant No DMR-0906608.

 

SEM image Titania

SEM micrograph of TiO2 nanotube arrays.


Past Research

Exchange Coupling in Magnetic Nanocomposites

Local magnetic interactions between magnetic components exchange-spring systems strongly depend upon the chemical and topographical nature of the interface. Relationships between the exchange coupling, the microstructure and the magnetic response have been studied in a variety of technologically-relevant materials. In particular, the stability and character of the exchange coupling in the system Nd2Fe14B/α-Fe has been uniquely quantified with recoil curves, an advance beyond the standard assessment using the remanence ratio Brecoil/Br that highlights the origins of the differences in their technical magnetic behavior and suggest structural modification pathways for property improvement. (C. L. Harland, L. H. Lewis, Z. Chen and B.-M. Ma. J MMM., 271. 1. 2004.)

micrograph

  recoil loop

TEM micrograph of nanocrystalline Nd9Fe86B5

  Recoil loopremanence ratio Brecoil/Br as a function of reverse internal field for the Nd9Fe86B5 and Nd2Fe14B samples at a selection of temperatures

Magnetostructural Materials

Magnetostructural materials exhibit simultaneous magnetic and structural phase changes of an abrupt and hysteretic nature that are fundamentally different from the canonical ferromagnetic transition of second-order thermodynamic character, such as that exhibited by iron, for example. The binary alloy iron-rhodium (FeRh) undergoes a ferromagnetic/antiferromagnetic phase transition. That displays unique responses placed under physical, thermal, or magnetic strain or subjected to nanostructuring. This makes FeRh an ideal material for various sensing technologies. Our research aims to better understand the various characteristics of this alloy in nanostructured form.

ReRh magnetic transition   structural transition
FeRh magnetic transition from antiferromagnetic to ferromagnetic in an applied field of 0.1 T.   Corresponding structural transition as a function of temperature

Complex Oxides

Complex oxide materials are capable of exhibiting two or more ferroic features such as ferromagnetism, ferroelectricity, or ferroelasticity. They have strong potential for a broad range of applications, which include multiple state memory storage devices, networked environmental sensors and spintronic devices. Our lab focuses on thin films of complex oxides, such as (La,Sr)MnO3 (LSMO) to better understand their physical and magnetic properties and how changing them affects their multiferroic capability.

lsmo

Schematic of A-site ordered and A-site LSMO in the film form.


Magnetic Nanoparticles in Microfluidic Systems

Magnetic nanoparticle-based microfluidic separation of circulating tumor cell populations enables understanding of the spreading of cancer and fosters development of various drug strategies. In this work we developed a microfluidic magnetic-activated cell sorting (MACS) system using a "bottom-up" approach employing both computational and experimental approaches to lead to optimized elements of device design.

MACS apparatus schematic
A schematic of a MACS apparatus

 

Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF), Office of Naval Research (ONR), Department of Energy (DOE) or any other funding agency.