Magnetic properties of FeMnAl alloys

Journal

of Magnetism and Magnetic Materials 140-1U (1995) 63-64

Magneticpropertiesof FeMnAl alloys H. Bremers a7*, CE. Jarms a, J. Hesse a, S. Chadjivasiliou b, KG. Efthimiadis b, I. Tsoukalas b a Institutfiir

Metallfhysik und Nukleare Festtirperphysik, TV, Meadelssohnstr. 3, D-38106 Braunschweig, Gemmy 3rd Laboratory of Enuirontnental Physics, University of Thessaloniki, Greece

Abstract The magnetical properties of Fe,,-,Mn,, Al, alloys have been studied by MiSssbauer spectroscopy and magnetization measurements. The presented examples show both ferromagnetic (x = 14) and re-entrant ferromagnetic (x = 40) behaviour.

1. Introduction The magnetic properties of the ternary FeMruU alloy system are not yet known in all details. In our investigation we have chosen the series Fes+.xMn,,Alx which by varying x crosses two predicted phase boundaries in the triangular phase diagram. These are a structural phase transition from fee to bee [l] at about x = 6 at% Al and a magnetical transition from ferromagnetism to paramagnetism [2] at about 40 at% Al at room temperature. We prepared four alloys in the region x = 5-14 and five alloys with x = 34-46. In this paper we confine ourself to one sample of each, i.e. x = 14 and x = 40. 2. Experimental

details and results

The alloys were melted in an induction furnace under an argon atmosphere using Fe, Mn and Al with more than 99.85% purity. The bulk samples with the lower Al content were cut and afterwards polished to 20-40 pm thick foils. The alloys with higher Al r,mtent have first been homogenized at 1000°C for 3 d and then quenched in water. Finally these samples were powdered. As experimental methods we used 57Fe MBssbauer effect, X-ray diffraction and magnetization measurements. All specimen exhibited pure hcc structure. The magnetization measurements at high temperatures were performed in a Foner-type magnetometer with an external field of 450 mT and a heating resp: tively cooling rate of 3 K/min, whereas the low temperature measurements were done in a Faraday balance in a field of 10 mT. Fig. 1 presents the Miissbauer spectra of Fe7SMn, ,Al ,4 as a function of temperature. The shape of the as casted

alloy spectrum is typical for disordered ferromagnetic alloys. With increasing temperature we see the normal decrease in the splitting of the lines, caused by the competition of thermal motion and ferromagnetic coupling. But above 740 K a new subspectrum with a bigger hypefine field begins to grow up. The hyperfiie field value for this subspectrum corresponds with that of pure iron. All the Miissbauer spectra were evaluated by means of hypertine field distributions. The mean hyperfme field H, together with the results of the magnetization measurements are shown versus temperature in Fig. 2. Both curves lead to the same shape (after normalization) and deliver the same Curie-temperature T, = (835 f 5) K. Above this temperature we observe a secoad phase in the magnetization curve with T, = (1045 f 5) K, which is in agreement with that of pure iron (T, = 1043 K). After cooling down from T,,, = 1170 K, the second magnetic phase disappears, T,

-8 -6 ’ Corresponding author. Fax: i2010§[email protected],tu-bs.de.

t49-531-391

5129; email:

03048853/95/%09.50 8 1995 Elsevier Science B.V. All rights rescrvcd SSDf 0304-8853(94)01133-8

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Fig. 1. Miksbauer effect spectra of FewMn,,All,

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H. Bremers et al. /Journal

of Magnetism and Magnetic Materiuis 140-144 (1995) 63-64

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Fig. 2. Heating up (MU,) and cooling down (MdOWs) curve of the magnetization (Ieft axis) as welt as mean hyperfine field (H, 1 vs. temperature for Fe,5Mn,,Al,,.

Fig. 4. Fe,gMn,,AJ,,

field warming magnetization measure-

is shifted towards higher temperatures and the saturation magnetization value decreases. For the second example, the alloy Fe,,Mn,,Al,, low temperatures are of interest. This is due to the high Al content of x = 40 at%. Fig. 3 presents the Mijssbauer spectra for this sample collected vs. temperature. This alloy is reported as ferromagnetic at room temperature [2]. From all the spectra the mean hypertine field was derived and plotted vs. temperature (Fig. 4). Starting from room temperature normal ferromagnetic behavior is observed, but below (65 & 5) K an increase of the mean hyperfine field can be seen. The magnetization measurements displayed in the same figure also show a deviation from the

‘normal’ spontaneous magnetization curve. This can be seen in a field warming after field cooling (FC-FW) experiment. Starting from 4 K, we observe an increase of the magnetization at (50 f 5) K. After passing a maximum around 150 K the magnetization diminishes again corresponding to conventional ferromagnetism. These observations together with a ZFC-FW experiment indicate a re-entrant transition from a spin glass to a ferromagnetic phase. Because of the competing exchange interactions introduced by the Fe and Mn atoms, some spins are coupled ferromagnetic and antiferromagnetic with their next neighbours. Since the Mijssbauer effect is measuring only the Zeeman energy splitting due to the hyperfme field produced by magnetic moments we see an increase of the mean hyperfine field just at temperatures where the re-entrant transition happens.

ments: field cooling (FC-FW) and zero field cooling (ZFC-FW) (left axis); mean hyperfine field (H,) vs. temperature (right axis).

3. Conclusion The ternary system Fe,,-,Mn,,Al, in the examined concentration range exhibits different magnetical properties, i.e. ferromagnetism, re-entrant transitions from spin glass to ferromagnetism and paramagnetism. The predicted, estimated phase boundary behveen fee and bee phase has to be shifted to aluminum contents lower than 5 at% [l]. References -8

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Fig. 3. Fe,,Mn ,, At,, Mtissbauer effect spectra.

[l] D.J. Chakrabarti, Metallurgical Trans. B 8 (1977) 121.

[2] G. Pirez Alcazar, J.A. Plascak and E. GalvLo da Silva, Phys. rev. B 38 (1988) 2816.