(198i) Phase Transformation Induced By Tertragnility Variation of Metal-Redox Synthesised NiMn Nanoalloys

Phase Transformation
Induced by Tertragnility Variation of Metal-redox Synthesised NiMn Nanoalloys

Jian Shen^1,2*, Jin Xin^2*

1 Water
Environment Technology Research Institute, MCC, Nanjing 210019, China

2 State
Key Laboratory for Heavy Oil Processing, China
University of Petroleum, Qingdao 266580, China

Introduction

Advanced
permanent magnets have emerged as a critical component for modern society from
power electronics and data storage to green energy-related generators and
electric motors. Manganese (Mn), as a rare-earth-free
metal, attracts wide interest because of the largest magnetic moment among all
transition metals.Such large unpaired electrons and
high magnetic moment in Mn-based alloys are believed
to be responsible for their complex structural and magnetic phases. One of the
long range chemically ordered MnNi alloys is found in
the antiferromagnetic equiatomic Mn₅₀Ni₅₀
(MnNi ¦È phase) compound, which adopts a face centered
tetragonal (fct) L1 0 AuCu
structure. Generally, the MnNi ordered structure
tends to phase transform from a body centered cubic (bcc) structure after
annealing at 973 K. At present, most studies in tetragonal MnNi
growth are reliant on the energy-consuming processing (ball milling or arcmelting). The challenge of MnNi
nanosynthesis arises from the simultaneous reducing
and alloying between manganese and nickel due to their different reduction
potentials and rates.

Iron
cobalt (FeCo), as a ¡°soft¡± magnetic materials, has a
body-centered cubic (bcc) nanostructure with small magnetocrystal-line
anisotropy energy (MAE), limiting their application in energy-critical
technologies requiring high energy product. Though computational efforts have
predicted a giant uniaxial MAE in tetragonal FeCo
nanostructures, it is still a challenging task to grow metastable tetragonal FeCo nanostructures. Here we adopt a unique synthesis
utilizing the metal-red[1]ox method for MnNi synthesis^[1]. Moreover, we report a
facile epitaxial metal-redox growth method to deposit the FeCo
shell onto NiMn core (Figure 1) to enable the
core/shell nanostructures, where we select the lattice-matched NiMn and FeCo phases as a
prototypical example to illustrate the effect of tetragonal-effects-induced
phase transformation on the magnetic performance of FeCo
shell.

Materials and
Methods

Manganese
carbonyl, nickel(II) acetylacetonate,
iron pentacarbonyl, cobalt carbonyl, 1-octadecene,
oleic acid, oleylamine, hexane, chloroform, all the
chemicals and solvent were purchased from Sigma-Aldrich, and used without
further purification. All synthesis procedures were operated in the Schlenk-line and the final samples were stored in the
glove-box. TEM and VSM was adopted to characterize
structural and magnetic properties, respectively.

Figure 1. The
schematic structural evolution of an FeCo shell
induced by the tetragonality change of the MnNi phase under phase transformation.

Results and
Discussion

To improve its MAE and utilize tetragonality change of MnNi
phase, the FeCo shell is epitaxially
grown on the MnNi nanocrystals.
Figures 2(a)-(f) show the structural and elemental mapping images of MnNi-FeCo core-shell nanostructures. As shown in figure
2(g), the optimum coercivity of FeCo
reaches 714Oe with an average shell thickness of 2.0 nm after thermal annealing
at 673 K.

Figure 2.
(a)–(f) TEM image, HRTEM image, STEM image and elemental mapping images of MnNi–FeCo nanoalloys.
(g) The shell thickness (the stoichiometry of FeCo
shell at 45:55, blue) and stoichiometry of FeCo (the
average FeCo shell thickness of 2.0 nm, red)
dependent coercivity of MnNi@FeCo
core@shell nanoalloys.

The annealing temperature
dictates the tetragonality of MnNi
¦È phase, which directly controls the tetragonal distortion in the FeCo shell. As shown in Figure 3(a)-(b), the results
matches the concepts on temperature-dependent tetragonality
of the MnNi ¦È phase. In order to verify phase
transformation induced by tertragnility variation, we
calculated the Bain strain mismatch of the (111)-(110) structure relationship
between MnNi core and FeCo
shell^[2]. The calculated mismatches between L10-MnNi/bct-FeCo and L10-MnNi/bcc-FeCo
were 3.7% and 6.9%, respectively, which confirms the stable interfacial
structure in (111) L10-MnNi core and (110) bct-FeCo
shell.. Therefore, the tetragonality change of the
L10 MnNi phase triggers tetragonal distortion in the FeCo shell phase for the controlled magnetic coercivity. Ultimately, MnNi-FeCo
nanocrystals with high coercivity
and saturation magnetization were obtained (940 Oe
and 154 emug^-1).

Figure 3. (a)
The magnetic hysteresis (M–H) loops of MnNi-FeCo nanocrystals with an average shell thickness of 2.0 nm and thestoichiometry of FeCo 45:55,
annealed at temperatures from 573 K to 873 K. (b) The annealing
temperature-dependent coercivity of MnNi-FeCo nanocrystals with an
average shell thickness of 2.0 nm and the stoichiometry of FeCo
45:55. (c)–(g) HRTEM image, STEM image and elemental mapping images of MnNi–FeCo nanoalloys
with an average shell thickness of 2.0 nm and the stoichiometry of FeCo 45:55 after annealing at 673 K.

Significance

Phase transformation induced by tetragonality change for tunable MAE, which open up a new
strategy to grow metastable tetragonal nanostructures with high MAE.

Reference

1£®Alec,
K., Shen, J., Gong, M., Cui, J., Ren,
S. Chemistry of Materials 27 (2015) 4677-4681.

2£®Shen, J., Dai, Q., Ren, S. Nanotechnology 27 (2016) 10LT01.