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Journal of Magnetism and Magnetic Materials 140-144 (1995) 1539-1540

Magnetic field induced transitions in MnsSi 3

H.J. A1-Kanani, J.G. Booth *

Joule Physics Laboratory, University of Salford, Salford M5 4WT, UK

Journal of magnetism and magnetic materials

Abstract The magnetic properties of the intermetallic compound MnsSi 3 with D88-type crystallographic ordering have been

investigated at high fields and low temperatures. Magnetisation measurements in fields up to 12 T reveal that it is possible to induce transitions from the low temperature non-coUinear antiferromagnetic phase by the application of a critical field which reduces in an approximately quadratic fashion with increasing temperature. The transition takes place over a field range of about 4 T wide and is accompanied by a large increase in susceptibility and hysteresis. At higher fields the behaviour seems to be unsaturated and similar to that observed for the collinear antiferromagnetic phase which is stable in the range 70-95 K. In addition, for both phases another transition which appears to be of second order occurs at a much smaller field involving a decrease in susceptibility for the non-collinear phase and an increase in susceptibility for the collinear phase.

1. Introduction 2. Experimental details

The intermetallic compound MnsSi 3 with the D88 structure has been shown by previous workers to have a complex non-collinear modulated magnetic structure at very low temperatures. Studies of its physical properties in zero field as the temperature is increased reveal the occur- rence of two successive transitions near the temperatures ~68 K and ~95 K [1-3]. It is thought that these represent the sequence: non-collinear antiferromagnetism- collinear antiferromagnetism-paramagnetism [1]. At 4.2 K the commensurate magnetic unit cell is orthorhombic and recent polarised neutron results have determined the mag- netic structure and the moments on Mn atoms for the non-collinear phase [4]. The present work shows that it is possible to induce transitions from this low temperature non-collinear phase by the application of strong fields. High field magnetisation measurements in fields up to 12 T over the temperature range 2-120 K reveal a first order transition with associated hysteresis which occurs at a critical field which is dependent on the temperature. The critical field reduces in an approximately quadratic fash- ion, with the maximum critical field being ~ 11.5 T at about 2 K. Above the critical field the material appears to be unsaturated and its possible magnetic structure is dis- cussed. Another transition, which appears to be of second order, is also found at much smaller fields.

The material used for this investigation was fabricated in the form of a 25 g ingot by melting together the appropriate quantities of spectrographically pure elements in an argon arc furnace. Specimens for magnetic measure- ments were cut from different parts of the ingot and the remainder was crushed to provide a powder sample for X-ray analysis. The powder and the pieces were sealed in an evacuated quartz ampoule and heat treated at 900°C for 24 h before quenching into water. X-ray examination of the alloy indicated a single phase hexagonal structure. Magnetisation measurements were obtained using the Manchester/Salford vibrating sample magnetometer facil- ity in fields up to 12 T over a temperature range 2-120 K.

7

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* Corresponding author. Fax: +44-61-745-5903; email: j.g.booth@physics.salford.ac.uk.

H [ .~/e ]

Fig. 1. Typical magnetisation curves of MnsSi 3 in the temperature range 2-100 K.

0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-885 3(94)0115 7-5

1540 HI. Al-Kanani, J.G. Booth/Journal of Magnetism and Magnetic Materials 140-144 (1995) 1539-1540

140 , , , , i , i • i . r ,

120 -

100

8 0

~ 6 0 r o

4 O

2 O

0 ' 10 2 0 30 40 5 0 60 7 0 8 0

T [ K ]

Fig. 2. Variation of the transition fields as a function of tempera- ture for MnsSi3:

3. Results and discussion

In Fig. 1 the magnetisation is given as a function of field for temperatures in the range 2-100 K. The low field susceptibility in this range has a peak at ~ 68 K which is taken to be the temperature of the transition from non-col- linear to collinear antiferromagnetism in accordance with previous work [1]. The isothermal susceptibility increases abruptly at a critical field which decreases from about 12 T at 2 K to 1.5 T at 65 K. It is seen in Fig. 2 that the fall-off with temperature is approximately quadratic reaching zero at ~ 70 K. The transition takes place over a wide field range and is accompanied by hysteresis suggesting a first order transition possibly associated with magnetostriction. At high fields the susceptibility indicates a lack of satura- tion and it is clear, particularly from the isotherms for 65 and 70 K in Fig. 1, that the properties of the high field phase are similar to those found for the collinear antiferro- magnetic phase which exists in the temperature range 70-95 K rather than those of the ferromagnets found in the Mns(SixGel-x)3 system [5]. At smaller fields the suscepti- bility displays another (second order) transition. This is

4 . 5

4 . 0

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2 0 7 0

1 0 ~' 8 0 _

o 5000 1 oooo 15000 20000 25000 3oooo

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Fig. 3. Magnetisation curves of MnsSi 3 in the non-collinear and coUinear phase, o" a and o" b represent magnetisation for each phase respectively.

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I ooo

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50 40K

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65

70

80 3 0

9 0 5

100 110

120

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1000 2000 3000 4000 5000 6000 7000 8000 9000

H / a [ Oe g e m u ~ ]

Fig. 4. Arrott plots of MnsSi 3.

shown in Fig. 3 for the non-collinear and collinear phases. The transition field is ~ 0.5 T for the non-collinear phase and involves a decrease in susceptibility. In contrast, above 70 K for the collinear phase the transition field is slightly larger ( ~ 1 T) and involves an increase in susceptibility. In view of these relatively small fields it is hoped to use neutron diffraction to provide information on the structures of these two field-induced phases. Fig. 4 gives Arrott plots for the system and it is found that although the isotherms in the paramagnetic regime (T > 95 K) give intercepts on the H/cr axis which approach the origin as the tempera- ture is lowered towards the ordering temperature, in the antiferromagnetic regimes the intercept retracts away from the origin as the temperature is further lowered as expected theoretically [6]. The change in magnetisation which oc- curs at the transition is seen in Fig. 1 to be ~ 0.3~ B per Mn atom. It is clear from a consideration of the magnetic moments on the Mn atoms given in Ref. [4] that this cannot correspond to a complete alignment of the Mn moments and one may therefore expect a further transition at higher fields than we have available, possibly to a state similar to the Mn5(SixGel_x) 3 ferromagnetism. Further comments on the structures of these field-induced phases must await neutron diffraction results.

References

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[2] R.P. Krentsis, I.Z. Radovskii, P.V. Gel'd and L.P. Andreeva, Zh. Neorg. Khim. 10 (1965) 2192.

[3] K.I. Meizer, R.P. Krentsis, A.A. Frolov and P.V. Gel'd, Fiz. Tverd. Tela 15 (1973) 3123.

[4] P.J. Brown, J.B. Forsyth, V. Nunez and F. Tasset, J. Phys. Condens. Matt. 4 (1992) 10025.

[5] M.E. Sheinker, R.P. Krentsis and P.V. Gel'd, Sov. Phys. Solid State 19 (1977) 1109.

[6] K.-U. Neumann, J. Crangle, N.K. Zayer and K.R.A. Ziebeck, Proceedings of summer school (Bialowieza, Poland, 1993) (Warsaw Univ. Press, 1994), to be published.