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Journal of Magnetism and Magnetic Materials NO-144 (1995) 4950

Ferromagnetic ar-Mn type Mn-Al alloys produced by mechanochemical methods

K.J. Kim aTb, K. Sumiyama ay*, K. Suzuki a a Institute for Materials Research, Tohoku Unicersi& Sendai 980-77, Jnpan

b Magnetic Alloys Laboratory, Korea Institute of Science nnd Technology, Cheongryanb Seoul, South Korea

Abstract Mechanically milled Mn-Al alloy powders have a-Mn type structure for a wide AI composition, being ferromagnetic at

low temperatures. Upon leaching the milled Mn-85 at % A! powders, the spontaneous magnetization of the residual alloy is 36 emu/g at 290 K, one order larger than that of the as-milled ones.

Mechanical milling is a simple technique using solid state reaction, to produce nonequilibrium materials that cannot be ob!ained by conventional melting. We have succeeded in obtaining a nonequilibrium a-Mn type solid solution in milled Mn-Al alloy powders with a wide composition range. These alloys are ferromagnetic at low temperatures [l I, even though no significant peak shift from those of the pure cw-Mn is detected in their X-ray diffraction patterns: the spontaneous magnetization oS is about 4 emu/g and the coercive force H, about 120 Oe at 290 K for the as-milled Mn-85 at% Al alloy powders. The ferromagnetic ru-Mn type phase is stable up to about 600 K and decomposes into the a-Mn, Al,Mn and Al phh-es above 700 K.

A major portion of Al can be extracted from the as-milled powders without any marked structural changes simply by soaking these powders in a basic solution [Z]. This is probably because the chemical reaction is enhanced by the small particle size and a large amount of strain stored. Since this process is very powerful for producing nonequilibrium phases of elemental powders, such as bee and amorphous Ni [2,3], we applied it to obtain nonequi- librium Mn phases [4j. This report deals with the structure and magnetic properties of Mn-Al alloy powders prepared by leaching Al from the as-milled alloy powders.

Pure Mn and Al powders were blended in a glove box filled with Ar gas. Mechanical milling was carried out in a rod mill evacuated down to 2 X 10s4 Torr. The rod-to- powder weight ratio was 23: 1 and the rotating speed was 100 rpm. The milled alloy powders were soaked in 20 wt%KOH-methanol solution at 353 K.

* Corresponding author. Fax: t Sl-22-215 2076.

Fig. 1 shows transmission electron microscope (TEM) images and selected area diffraction patterns for the pure ol-Mn powders and the mechanically milled Mn-85 at%Al

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alloy powders before and after ieaching Al. The diffraction rings of the c&In phase and the unprocessed Al phase are observed for the as-milled alloy after 300 h milling. The Al-leached sample has a skeleton type structure with many pores, which are probably due to the removal of Al atoms. The corresponding diffraction pattern indicates that the a-Mn phase is predominant. The energy dispersive X-ray chemical analysis of the leached specimen indicates the presence of a small amount of Al in the leached specimen: about 95% of the initial Al content have been removed by leaching. Also it should be noted that no intensity for Fe was found, indicating that the specimen is free from the contamination of ferromagnetic Fe.

Fig. 2 shows X-ray diffraction patterns of the as-milled and leached specimens. Strong Al peaks ac4 observed for the as-milled specimen, but they disappear after leaching. In the leached specimen, moreover, strong a-Mn peaks are seen together with weak p-Mn peaks and very weak MnO peaks, indicating that the c&In phase is predominant. Here, the /3-Mn phase may be attributed to the reaction of Mn and AI, while the MnO phase to the reaction of Mn and KOH solution during leacLing at 353 K.

The magnetization was measured by a SQUID magne- tometer. The magnetization curties shown in Fig. 3 indi- cate that the leached specime,i is ferromagnetic at low temperatures. The spontaneous magnetization, a,, is 38 emu/g at 5 K and 36 emu/g at 290 K, about one order larger than that of the as-milled specimen, while the H, value of this alloy is approximately 95’ Oe at 290 K, smaller than that of the as-milled one.

The marked increase in 4 of ?h? leached specimen is .itributed partly to the removal of Al from the as-milled

Mn-Al alloy: an increase of approximately 12 emu/g in a, is expected by removing all Al content from the ferromagnetic u-Mn type Mn-Al alloy powders [4]. In the Mn-Al alloy system, the metastable T phase is obtained by annealing speciaens rapidly quenched from the high temperature E phase, The 7 phase is a well-known nonfer- rous ferromagnet [5]. However, since no significant Bragg

2 8 (Wcg.)

Fig, 2. X-ray diffraction patterns of 300 h milled Mn-85 at % Al alloy powdereo and leached ones.

Fig. 3. The and 300 K.

HO-)

magnetization curves of leached Mn-Al powders at 5

peak corresponding to this phase was detected in the X-ray diffraction pattern (see Fig. Z), the contribution from the 7 phase to the ferromagnetism i the present milled and leached specimens can he discarded. The MnO phase detected in the present specimen is known to be antiferro- magnetic and gives no increase in us [6]. However, oxy- gen deficits and several defects contained in the leached alloy powders may yield uncompensated magnetic mo- ments at the Mn sites, giving rise to a slight increase in the a, value, as expected for the as-milled Mn-Al powders. Moreover, a considerable amount of hydrogen is usually involved in leached specimens due to the decomposition cf hydroxides [2]. If a manganese hydride is formed in the present specimen, it should become a ferromagnet. How- ever, the anticipated contribution to a, is small: the maxi-

. r. :- kXn’J mum value 01 us 111 rra,,l.U,y4 t’. II mk?qe is &out 2 emu/g [7]. In order to better understand the origin of the ferromag-

netism in the mechanically milled a-Mn type Mn-Al alloys and the leached ones, we are currentiy planning NMR ahld neutron diffraction experiments.

Acknow$edgmenrs: The authors wish to thank Mr. 5. Ono for his experimental support. This work was partially supported by a Grant-in-Aid for General Scientific Re- search (Grant No. 06452323) given by fhp. Ministry of Education, Science and Culture, Japan.

References

[l] K.J. Kim, K. SumiyaEa and K. Suzuki, J. Alloys Compounds, in press.

[2] E. Ivanov, S,A. Makhlouf, K. Sumiyama, H, Yamauchi, K. Suzuki and G. Golubkuva, J. Alloys and Compounds 185 (1992) 25.

[3] S.A. Makhlouf, K. Sumiyama and K. Suzuki, J. Alloys and Compounds 199 (1993) 119.

[4] K.J. Kim, Ph.D. Thesis, Tohoku University, Scndai (1994). [5] H. Kono, J. Phys. Sot. Jpn. 13 (1958) 1444. [6] C.G. Shull and J.S. Smart, Phys. Rev. 76 (1949) 1256. [7] LT. Belash, B.K. Ponomarev, V.G. Tissen, N.S. Afonikova,

V.Sh. Shekhtman and E.G. Pony&&ii, Sov. Phys. Solid State 20 (1978) 244.