Structural Stabilizing Effect of Zn Substitution on MnAl and Its Magnetic Properties

doi:10.4236/ojm.2011.12003

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Open Journal of Microphysics, 2011, 1, 19-22

doi:10.4236/ojm.2011.12003 Published Online August 2011 (http://www.SciRP.org/journal/ojm)

Structural Stabilizing Effect of Zn Substitution on

MnAl and Its Magnetic Properties

H. X. Wang1, P. Z. Si1,*, W. Jiang1, J. G. Lee2, C. J. Choi2, J. J. Liu3,

Q. Wu1, M. Zhong1, H. L. Ge1

1School of Materials Science and Engineering, China Jiliang University,

Hangzhou, China

2Korea Institute of Materials Science, Changwon, Gyeongnam, R. Korea

3Faculty of Materials Science and Chemical Engineering,

Ningbo University, Ningbo, China

E-mail: [email protected]

Received

May 17, 2011; revised June 28, 2011; accepted July 12, 2011

Abstract

The effect of Zn substitution on the structure and magnetic properties of τ-MnAl has been investigated sys-

tematically. It is found that Zn substitution can stabilize the structure of τ-phase and thus a significant

amount of τ-phase can be produced. Zn increases the coercivity and saturation magnetization of the τ-MnAl

but reduces the Currie temperature. However, excess Zn is detrimental to the magnetic parameters. The op-

timum magnetic performance was found in samples with Zn substitution to 2.9% Mn atoms and 3.5% Al

atoms, respectively.

Keywords: MnAl, Magnetic Property, Zn Substitution

1. Introduction

The development of rare earth free permanent magnets is

becoming more and more important with increasing cost

and decreasing reserve of rare earth resources. Ferro-

magnetic τ-MnAl, as a low cost rare-earth-free magnet,

has received continuous attention since its discovery in

1958 for its superior magnetic performance in compari-

son with magnetically hard ferrites and Alnicos [1-3].

The τ-MnAl has a Currie temperature of 661 K, a man-

ganese moment of 1.94 µB, and an enhanced magnetic

anisotropy. The structure of ferromagnetic τ-MnAl is

tetragonal, which can be interpreted to arise from a non-

magnetic cubic structure by two subsequent steps,

namely an electronic distortion due to spin polarization

followed by a structural distortion into the tetragonal

system [4]. Since the τ-MnAl is metastable, it is difficult

to obtain pure phase of τ-MnAl, which usually is pro-

duced by a rapid quench of the high temperature ε-phase

followed by isothermal annealing at temperatures be-

tween 400˚C and 700˚C for a short while, or by cooling

the τ-phase at a rate of 10˚C/min [2]. Long time anneal-

ing would result in decomposition of the τ-phase to the

equilibrium γ-phase and β-phase. In order to stabilize the

τ-MnAl, the doping effect of several elements, including

Ti, Cu, Ni, C, B, etc., has also been studied [2,5]. It was

found that the addition of carbon to MnAl can stabilize

the τ-phase [6,7]. Reports on the partial substitution of

Mn in MnAl by Ni or Co could also be found [8,9]. The

effect of introducing other elements into MnAl system

has not been studied. The purpose of this work is to in-

vestigate the effect of doping Zn on the structure and

magnetic properties of the Mn-Al system.

2. Experiments

High purity (>99.9%) Mn, Al and Zn in nominal compo-

sition of Mn54Al46-δZnδ (δ = 0, 1.6, 3.3, 4.7, 5), Mn53.5

Zn0.5Al46, Mn53.4Zn1.6Al46, Mn51.8Zn3.2Al46, and Mn49Zn5

Al46 were melted by using induction meting in an argon

atmosphere. Then the melt was quenched in water. After

that the quenched ingots were annealed at 420˚C for 1 h

in vacuum, respectively. The structure of the samples

was determined by using a powder x-ray diffractometer

(XRD) with Cu Kα radiation while the magnetic proper-

ties were measured by using a Vibrating Sample Magne-

tometer (VSM) in fields up to 1.5 T. Thermal analysis on

the samples was carried out in argon atmosphere with a

20 H. X. WANG ET AL.

temperature sweep rate of 20˚C/min.

3. Results and Discussion

The x-ray powder diffraction patterns of the Mn54Al46-δ

Znδ (δ = 0, 1.6, 3.3, 5) water quenched samples and their

counterparts after 420˚C and one hour vacuum heat-

treatment are shown in Figure 1, respectively. The XRD

patterns for the as-quenched Mn54Al46, as shown in Fig-

ure 1(a), could be mainly indexed with orthorhombic

ε-MnAl phase, which is stable at temperatures above

870˚C and is maintained to room temperature during

quench. The weak broadened peak in the vicinity of 43˚

indicates the presence of γ and/or β-phase as minor phase

in the sample. Figure 1(b) shows that the heat-treatment

to Mn54Al46 induces the transformation of ε-phase to a

more stable Al-rich γ-phase and Mn-rich β-phase. The

diffraction peaks for meta-stable τ-phase could also be

found but are very weak. Strong diffraction peaks appear

in both water-quenched and heat-treated Mn54Al44.4Zn1.6,

as shown in Figures 1(c) and (d), indicating stabilizing

effect of Zn on the structure of τ-phase. Trace amount of

ε-phase, β-phase and γ-phase are detected in the as-

quenched Mn54Al44.4Zn1.6. After heat-treatment, the ε-

phase disappears while the diffraction intensity of β- and

γ-phase made almost no change, indicating a phase

transformation of ε-phase to τ-phase during heat-treat-

ment. For Mn54Al42.7Zn3.3, the τ-phase presents in the

as-quenched samples but disappears after heat-treatment,

as seen in Figures 1 (e) and (f). These phenomena indi-

cate that the τ-phase formed in Mn54Al44.4Zn1.6 is more

stable than that formed in Mn54Al42.7Zn3.3. With increas-

ing Zn content, the τ-phase, β-phase and γ-phase coexists

in both water-quenched and heat-treated Mn54Al41Zn5

samples as seen in Figures 1(g) and (h).

It should be noted that for sample with δ = 1.6, τ-phase

is the major phase while for other samples β-phase is the

major one. This result indicates that substitution of a

certain amount of Zn to Al is beneficial for the formation

of τ-phase. We speculate that atomic size may play an

important rule in this process. It is known that the atomic

size of Zn is slightly smaller than that of Al. Since the

minimum internal energy in MnAl occurs very close to

c/a = 1 (c and a are the lattice parameters), thus the

tetragonal τ-phase is reported to be meta-stable [4].

When a small number of Al atoms were substituted by

smaller Zn, a local lattice distortion that make tetragonal

τ-phase more stable might occur to maintain internal

energy minimum. However, for samples with increasing

substitution δ ≥ 3.3, β-phase rather than ε or τ-phase

formed as major phase in water quenched samples. Since

β-phase is very stable and thus the heat-treatment has

little effect on the structure of these samples.

Figure 2 shows the effect of Zn substitution to Al on

the magnetic properties of Mn54Al46-δZnδ (δ = 0, 1.6, 3.3,

5). For heat-treated samples, the coercivity and the satu-

ration magnetization increase first and then decrease with

increasing Zn substitution to Al. A maximum coercivity

of 0.157 T was observed in heat-treated Mn54Al44.4Zn1.6.

The 0.157 T coercivity in our Mn-Al-Zn system is lower

than the 0.34 T coercivity as reported in the Mn-Al-C

samples [7]. Since the parameter of coercivity is very

sensitive to microstructures, we tend to believe that cer-

tain partial substitution of Zn to Al might result in the

formation of localized lattice defects that hinder mag-

netization reversal. The increasing saturation magnetiza-

tion with Zn substitution was ascribed to the structural

stabilizing effect of Zn on ferromagnetic τ-MnAl and its

presence as major phase in the samples. Excess Zn

would result in the formation of more β-phase and

γ-phase that are detrimental to the magnetic performance.

In comparison with heat-treated samples, most wa-

ter-quenched samples exhibit a lower coercivity except

Mn54Al42.7Zn3.3, as seen in Figure 2. However, for sam-

ples with δ ≥ 3.3, the effect of heat-treatment on the coer-

Figure 1. XRD patterns for (a) Mn54Al46, (b) Heat-treated

Mn54Al46, (c) Mn54Al44.4Zn1.6, (d) Heat-treated Mn54Al44.4

Zn1.6, (e) Mn54Al42.7Zn3.3, (f) Heat-treated Mn54Al42.7Zn3.3, (g)

Mn54Al41Zn5, and (h) Heat-treated Mn54Al41Zn5. The heat-

treatment for sample (b), (d), (f), and (h) was carried out in

vacuum under 420˚C for one hour.

Figure 2. The effect of Zn substitution to Al on the coerciv-

ity and saturation magnetization of Mn54Al46-δZnδ (δ = 0,

1.6, 3.3, 5).

H. X. WANG ET AL.

civity is decreasing due to the presence of stable β-phase

and γ-phase in the samples. Heat-treatment has a strong

effect on the magnetic properties of Mn54Al44.4Zn1.6.

Figure 3 shows the XRD patterns of the water-

quenched and the heat-treated ingots of Mn53.5Zn0.5Al46,

Mn53.4Zn1.6Al46, Mn51.8Zn3.2Al46, and Mn49Zn5Al46. It is

interesting that τ-phase formed in all water-quenched

samples, indicating a strong structural stabilizing effect

of Zn substitution to Mn on τ-MnAl. However, the

β-phase and γ-phase were also formed in all samples

during water-quench. For Mn53.5Zn0.5Al46, as seen in

Figures 3(a) and (b), by comparing the relative diffrac-

tion intensity of τ-phase and β-/γ-phase, we noticed that

the amount of τ-phase increased after heat-treatment.

Figures 3(c) and (d) shows that the heat-treatment has

little effect on the structure of Mn53.4Zn1.6Al46. For

Mn51.8Zn3.2Al46, ε-phase was detected in water-quenched

sample while it disappears after heat-treatment, as shown

in Figures 3(e) and (f). The intensity of τ-phase peaks

were enhanced after heat-treatment, indicating a ε → τ

transformation in this process. Figures 3(g) and (h) shows

that both water-quenched and heat-treated Mn49Zn5Al46

were composed of τ-phase, β-phase and γ-phase, while

heat-treatment had almost no effect on the structure.

Figure 3 shows that no pure τ-phase could be obtained in

Zn substituting Mn process, for comparison Figures 1(c)

and (d) shows that pure τ-phase could be obtained in Zn

substituting Al process.

The effect of Zn substitution to Mn on the magnetic

properties of Mn54Al46, Mn53.5Zn0.5Al46, Mn53.4Zn1.6Al46,

Mn51.8Zn3.2Al46 and Mn49Zn5Al46 are shown in Figure 4.

Both the saturation magnetization and the coercivity of

the heat-treated samples increase first and then decrease

with increasing Zn content. The maximum coercivity

was observed in Mn53.5Zn0.5Al46 while the maximum

saturation magnetization was observed in Mn53.4Zn1.6Al46.

The enhancement of the magnetic performance for sam-

ples with lower Zn substitution indicates that Zn is bene-

ficial for the formation of τ-phase. However, Figure 4

also shows that higher Zn substitution is detrimental to

the magnetic properties, suggesting a limited effect of Zn

addition. It is interesting to note that heat-treatment in-

creases the coercivity when Zn substitution is low and

decreases the coercivity when Zn content is high.

Figure 5 plots the M-T and ΔT-T curves for the sam-

ples. The Currie temperature of Mn54Al42.7Zn3.3, Mn54

Al44.4Zn1.6, Mn53.5Zn0.5Al46 determined by using dM/dT

method is in the vicinity of 371K-373 K, which is lower

than the 388 K reported for τ-MnAl[3]. The result indi-

cates that Zn substitution reduces the Currie temperature

of τ-MnAl. The endothermal peak observed in Figure

5(d) was ascribed to the ferromagnetic-paramagnetic

transition of τ-MnAl in Mn54Al42.7Zn3.3.

Figure 3. XRD patterns for (a) Mn53.5Zn0.5Al46, (b) Heat-

treated Mn53.5Zn0.5Al46, (c) Mn53.4Zn1.6Al46, (d) Heat-treated

Mn53.4Zn1.6Al46, (e) Mn51.8Zn3.2Al46, (f) Heat-treated Mn51.8

Zn3.2Al46, (g) Mn49Zn5Al46, and (h) Heat-treated Mn49Zn5Al46.

The heat-treatment for sample (b), (d), (f), and (h) was car-

ried out in vacuum under 420˚C for one hour.

Figure 4. The effect of Zn substitution to Mn on the mag-

netic properties of Mn54Al46, Mn53.5Zn0.5Al46, Mn53.4Zn1.6

Al46, Mn51.8Zn3.2Al46 and Mn49Zn 5Al46.

Figure 5. Temperature dependence of magnetization of

heat-treated samples (a) Mn54Al42.7Zn3.3, (b) Mn54Al44.4Zn1.6,

rate of 5˚C/min. Line (d) plots the T-ΔT data from thermal

analysis on Mn54Al42.7Zn3.3.

4. Conclusions

Zn substitution can stabilize the structure of τ-phase. Zn

increases the coercivity and saturation magnetization of

the τ-MnAl but reduces the Currie temperature. It should

be noted that excess Zn is detrimental to the magnetic

H. X. WANG ET AL.

parameters. The optimum magnetic performance was

found in samples with Zn substitution to 2.9% Mn atoms

and 3.5% Al atoms, respectively.

5. Acknowledgements

This work was supported by the Natural Science Foun-

dation of China (Nos. 10874159, 11074227, 50801039),

Zhejiang Provincial Natural Science Foundation of

China (No. R6110362), and Fundamental R&D Program

for Core Technology of Materials funded by the Ministry

of Knowledge Economy, R. Korea.

6. References

[1] H. Kono, “On the Ferromagnetic Phase in Manga-

nese-Aluminum System,” Journal of the Physical Society

of Japan, Vol. 13, No. 12, 1958, pp. 1444-1451.

[2] Q. Zeng, I. Baker, J. B. Cui and Z. C. Yan, “Structural

and Magnetic Properties of Nanostructured Mn-Al-C

Magnetic Materials,” Journal of Magnetism and Mag-

netic Materials, Vol. 308, No. 2, 2007, pp. 214-226.

[3] J. H. Park, Y. K. Hong, S. Bae, J. J. Lee, J. Jalli, G. S.

Abo, N. Neveu, S. G. Kim, C. J. Choi and J. G. Lee,

“Saturation Magnetization and Crystalline Anisotropy

Calculations for MnAl Permanent Magnet,” Journal of

Applied Physics, Vol. 107, No. 9, 2010, pp. 09A731.

[4] Y. Kurtulus and R. Dronskowski, “Electronic Structure,

Chemical Bonding, and Spin Polarization in Ferromag-

netic MnAl,” Journal of Solid State Chemistry, Vol. 176,

No. 2, 2003, pp. 390-399.

doi:10.1016/S0022-4596(03)00242-1

[5] Y. Sakka, M. Nakamura and K. Koshimoto, “Rapid

Quenching and Properties of Hard Magnetic Materials in

MnAl-X (X=Ti, Cu, Ni, C, B) Systems,” Journal of Ma-

terials Science, Vol. 24, No. 12, 1989, PP. 4331-4338

[6] W. H. Dreizler and A. Menth, “Transformation Kinetics

of the Ferromagnetic Alloy Mn-Al-C,” IEEE Transac-

tions on Magnetics, Vol. 16, No. 3, 1980, PP. 534-536.

[7] D. C. Crew, P. G. McCormick and R. Street, “MnAl and

MnAlC Permanent Magnets Produced by Mechanical

Alloying,” Scripta Metallurgica et Materialia, Vol. 32,

No. 3, 1995, PP. 315-318.

[8] M. Mat, A. Mor and N. Koh, “Crystal Structure and

Magnetic Properties of Mn-Al-Ni Ferromagnetic Films,”

IEEE Translation Journal on Magnetics in Japan, Vol. 6,

No. 2, 1991, pp. 134-140

doi:10.1109/TJMJ.1991.4565123

[9] R. Kainuma, M. Ise, K. Ishikawa, I. Ohnuma and K.

Ishida, “Phase Equilibria and Stability of the B2 Phase in

the Ni–Mn–Al and Co–Mn–Al systems,” Journal of Al-

loys and Compounds, Vol. 269, 1998, pp. 173-180.

doi:10.1016/S0925-8388(98)00127-3