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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.28 No.10 pp.539-543
DOI : https://doi.org/10.3740/MRSK.2018.28.10.539

Effect of Milling Speed on the Structural and Magnetic Properties of Ni70Mn30 Alloy Prepared by Planetary Ball Mill Method

Imad Hussain1, Ji Eun Lee1, So Eun Jeon1, Hyun Ji Cho1, Seok-Hwan Huh2, Bon Heun Koo1, Chan Gyu Lee1†
1School of Materials Science and Engineering, Changwon National University, Changwon 51140, Republic of Korea
2School of Mechatronics Conversion Engineering, Changwon National University, Changwon 51140, Republic of Korea
Corresponding author
E-Mail : chglee1225@gmail.com(C. G. Lee, Changwon Nat'l Univ.)
July 16, 2018 September 10, 2018 September 12, 2018

Abstract

We report the structural, morphological and magnetic properties of the Ni70Mn30 alloy prepared by Planetary Ball Mill method. Keeping the milling time constant for 30 h, the effect of different ball milling speeds on the synthesis and magnetic properties of the samples was thoroughly investigated. A remarkable variation in the morphology and average particle size was observed with the increase in milling speed. For the samples ball milled at 200 and 300 rpm, the average particle size and hence magnetization were decreased due to the increased lattice strain, distortion and surface effects which became prominent due to the increase in the thickness of the outer magnetically dead layer. For the samples ball milled at 400, 500 and 600 rpm however, the average particle size and hence magnetization were increased. This increased magnetization was attributed to the reduced surface area to volume ratio that ultimately led to the enhanced ferromagnetic interactions. The maximum saturation magnetization (75 emu/g at 1 T applied field) observed for the sample ball milled at 600 rpm and the low value of coercivity makes this material useful as soft magnetic material.

초록

Changwon National University

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The NiMn based Heusler alloys have attracted a great deal of attention due to their exceptional magnetic properties associated with the magneto-structural phase transition. The exchange interactions and strong magnetostructural coupling greatly influence the physical properties including magnetocaloric effect, shape memory effect and the magnetoresistance in these materials.1-8) In case of the NiMn system, the Mn and Ni display the contrary types of magnetic alignment in their basic states and the combination of paramagnetic Mn and ferromagnetic Ni exhibit attractive magnetic properties.9) The Mn-Mn interatomic distance is considered as a crucial parameter for determining the nature of the exchange interaction occurring in these materials. Great efforts were recently made to improve the magnetic properties of the NiMn based alloys using different experimental techniques.10-12) One of the strategies used to improve the magnetic properties of the materials is to reduce the particle size or grain size that leads to an increased surface area to volume ratio of the particle. The surface effects introduced in this manner play an important role in altering the magnetic properties of the materials. One of the unique properties of the magnetic nanoparticles is the vanishing of the coercive force below a certain critical size where the random flipping of magnetization direction becomes dominant.13,14) Given that the hysteresis loss depends on the coercive force, it is effectively decreased with the decrease in the particle size.15)

Various methods were used so far to prepare the NiMn based alloys. These methods included the hot extrusion, mechanical milling, gas atomization and melt-spinning etc.16-21) One recent approach is the planetary ball mill method which is a cost effective and efficient method used to prepare materials with submicron or nano-crystalline microstructure. Ball milling is a simple way of producing fine particles by repeated fracturing, cold-welding and micro-forging processes that continuously occur in the elementary metal particles.22) During the ball-mill process, a significant amount of energy is transferred from the balls to the powder. This method is highly attractive for the production of ultrafine crystals and allows a uniform dispersion of rigid materials such as metals and non-crystalline materials. Two important parameters that significantly affect the properties of material during ball milling are the milling speed and time. The effect of the ball milling time on the properties of the alloys has been extensively investigated. B.Tian et al.23) reported that in the case of Ni-Mn-Ga nanoparticles, the structural transition gradually disappeared with the increasing ball milling time. A. L. Alves et al.24) also inquired into the effect of milling time on the ferromagnetic interactions reporting that for the Ni-Mn-Sn alloys the short time milling suppressed the ferromagnetic state considerably. In fact the grain size goes down and the ionic disorder as well as mechanical strain goes up during ball milling that ultimately affects the structural transition and magnetic properties. Up to the best of our knowledge the effect of ball milling speed on the properties of the alloys however has been rarely reported.

In this paper we report our recent work on the preparation and investigation of the magnetic properties of the Ni70Mn30 alloys. The effect of different ball milling speeds on the structural, morphological and magnetic properties is investigated.

2. Experimental Procedure

The Ni70Mn30 powder samples were prepared using a planetary ball mill machine(HPM-700). High purity elemental powders of Ni (Alfa Aesar-99.5 %) and Mn (Alfa Aesar-99.8 %) were used as starting materials. Steel balls (with 10 mm diameter) and vials were used for milling. The ball to powder mass ratio was kept constant as 10:1. The process was performed for a fixed time interval of 30 h at various rotational speeds of 0, 100, 200, 300, 400, 500 and 600 rpm. The phase and crystal structure were investigated by X-ray diffraction using a Bruker D8 Advance diffractometer. The morphology was investigated by the scanning electron microscope( SEM, JSM5610). The average particle size was estimated by image-J software. The magnetic measurements were performed using a vibrating sample magnetometer( VSM-7360).

3. Results and Discussion

The room temperature X-ray diffraction(XRD) profiles of the Ni70Mn30 powder samples milled for 30 h at different milling speed are shown in Fig. 1. The XRD pattern of the 0 h milled sample showed the characteristic peaks of Ni (fcc) [ICDD Card-04-0850] and Mn (bcc) [ICDD Card-32-0637]. The XRD pattern obtained for the sample milled at 200 rpm did not show a significant change in the structure but only a slight increase in the peak width was observed. All the characteristic peaks corresponding to the constituent elements were still very clear at this early stage of milling. For the powder sample milled at 300 rpm, the diffraction peaks became broader indicating the refinement of particles with the increased milling speed. Furthermore, a reduction in the intensity of each elemental diffraction peak was observed which indicated the dissolution of the Mn and Ni atoms. At an increased milling speed of 400 rpm, all the characteristic diffraction peaks corresponding to the Mn and Ni atoms completely disappeared and new peaks were observed. The newly obtained diffraction profile clearly established the Ni70Mn30 alloy with bcc crystal structure. No significant changes were observed in the XRD patterns of the samples milled at higher speed of 500 and 600 rpm.

Fig. 2 shows the possible change in the peak position and the peak width of the XRD lines with the increased milling speed in the range of 2θ = 40.3 to 47.6. The diffraction peak for Mn and Ni labelled as (330) and (111) can be clearly observed in the XRD profile for the unmilled sample. Up to 300 rpm, the XRD peaks appeared broadened, the peak position however remained nearly unaffected. The broadening in the XRD peaks can be attributed to the reduction in particle size, increase microstrain and lattice distortions produced by milling.22) The no change in the peak position up to 300 rpm showed that the lattice parameter did not change significantly with partial dissolution of Ni at Mn lattice sites. In fact the ionic radii of Ni(1.24 Å) and Mn(1.26 Å) are very close. Therefore the peak position and hence the lattice parameter did not change significantly due to the small difference in their radii. At 400 rpm, the Ni peaks disappeared and the individual peaks of Mn and Ni centered at around 2θ ≈ 43o and 44.5o respectively merged in to a single broad peak indicating the formation of Ni70Mn30 alloy. However, milling the sample at higher speed of 500 and 600 rpm did not bring about any change in the peak shift, peak width or lattice parameter. The lattice parameter ‘a’ for the cubic structure was calculated using the Bragg’s law and the following formula; Where ‘a’ is the unit cell parameter and ‘d’ is the interplaner distance of (h k l) planes. The variation in lattice parameter with the increasing milling speed is shown in Fig. 3.

$d = a h 2 + k 2 + l 2$
(1)

The morphology and average grain size of the samples were investigated by scanning electron microscope(SEM) and the obtained images are shown in Fig. 4. The morphology of the samples changed remarkably with the increase in milling speed. Relatively larger particles were observed in the un-milled sample. A gradual reduction in the particle size was observed in the samples milled at 200 and 300 rpm respectively. This reduction in the particle size was attributed to the mutual collision between the balls and alloy particles. However, for the sample milled at 400 rpm a noticeable increase in the particle size was observed. This increase in the particle size could be mainly attributed to the cold welding and microforging processes that simultaneously occur at this stage. For samples milled at further higher speeds of 500 and 600 rpm, the grain size reduced only slightly. Fracturing of the particles must have occurred that resulted in reduced particle size at higher speeds. The variation in the average grain size with the increasing milling speed can be clearly observed in Fig. 5.

The magnetic properties of the Ni70Mn30 alloy powders ball milled at different speeds were measured in an applied magnetic field of up to 1 T using vibrating sample magnetometer. All the measurements were performed at room temperature. The hysteresis loops obtained are collectively shown in Fig. 6. All the samples showed a typical ferromagnetic behavior. The saturation magnetization and coercivity for each sample were obtained from the corresponding hysteresis loop and are plotted in Fig. 7. A gradual decrease in magnetization was observed for the samples milled at 200 and 300 rpm respectively. It is well known that in case of the milled NiMn based alloys the magnetic properties are determined by the spin rotation and are mainly governed by the Mn-Mn interaction. 25-28) However, the spin rotation is significantly affected by the potential well produced by various factors including internal stress due to lattice distortion and defects such as dislocations and vacancies. The reduced magnetization obtained for these samples (200 and 300 rpm) can thus be attributed to the increased lattice strain and disorder that ultimately suppressed the Mn-Mn exchange interaction. Furthermore, the decrease in the magnetization observed for the samples ball milled at 200 and 300 rpm respectively could also be attributed to the increased thickness of the magnetically dead layer formed at the surface of the particles. With the decrease in particle size, the surface to volume ratio was increased and hence the surface effects became more prominent in small sized particles. Since the net magnetic moment of the outer layer is zero, the total magnetization value is decreased with the decrease in particle size. However, in the samples obtained at 400, 500 and 600 rpm, the magnetization was increased and the coercivity decreased sufficiently. The increased magnetization may be attributed to the relative decrease in the thickness of the outer magnetically dead layer with the increasing particle size thereby leading to the enhancement of the ferromagnetic interaction between Mn atoms.

4. Conclusions

We systematically investigated the effect of different ball milling speeds on the structural and magnetic properties of the Ni70Mn30 alloy powder prepared by planetary ball mill method. Our results showed that the particle size and hence magnetization was first decreased with the increase in ball milling speed. This decrease in magnetization was attributed to the increased lattice strain, disorder and surface effects which became prominent due to the increase in the thickness of the outer magnetically dead layer. However, the magnetization increased whereas the coercivity decreased with the increase in particle size. This study indicated that the particle size and magnetization can be customized by adjusting the ball milling speed during synthesis process.

Acknowledgement

This research was supported by Changwon National University in 2017~2018.

Figure

Room temperature XRD patterns of the Ni70Mn30 powder milled at different speeds.

The shifting and broadening of the XRD peaks with the increasing milling speed.

Variation in Lattice parameter with increasing milling speed.

SEM images of the Ni70Mn30 samples milled at different speeds.

Average particle size of the Ni70Mn30 powder ball milled at different speeds.

M-H loops for Ni70Mn30 powder samples milled at different speeds.

Saturation magnetization and coercivity of the Ni70Mn30 alloy powders milled at different speeds.

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