Mn1-tCrtAs UNDER PRESSURE, COMPETITION BETWEEN MAGNETIC ORDERINGS

(1)

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Mn1-tCrtAs UNDER PRESSURE, COMPETITION

BETWEEN MAGNETIC ORDERINGS

A. Zieba, R. Zach, H. Fjellvåg, A. Kjekshus

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment au no 12, Tome 49, d6cembre 1988

Mnl-&CrtAs UNDER PRESSURE, COMPETITION BETWEEN MAGNETIC

ORDERINGS

A. Zieba (I), R. Zach (2), H. Fjelldg (3) and A. Kjekshus (3) (I) Academy of Mining and Metall~~gy, Cracow, Poland

(2) Technical University of Cracow, Cracow, Poland (3) University of Oslo, Oslo, Noway

Abstract. - The pressure phase diagrams for Mnl-tCrtAs with t = 0.355 and 0.390 were determined. The observed topology around the two types of heli-ferro-para triple points may indicate multicritical behaviour. The value of dTc/dP =

+11 K/kbar for the induced ferromagnetic phase (t = 0.390) represents the highest positive pressure slope observed for a metallic magnet.

1. Introduction

The Mnl-tCrtAs system is dominated by two heli- magnetic phases, Ha for t

5

0.385 and H, for t

2

0.335. These notations reflect the spiral propagation direc- tion, along the a and c axis, respectively. The spin arrangement of the Ha phase is close t o ferromagnetic (F). (The periodicity of the spirals amounts t o some 15 a lengths [I] and a rather small field is sufficient to align the magnetic moments [2].) The H, phase is profoundly antiferromagnetic.

The specific volumes of the Ha and H, phases are, respectively, smaller and larger than that extrapolated from the high-temperature paramagnetic phase. The application of external pressure should thus favour the Ha phase and suppress the H, phase. The decrease in volume for the Ha phase should be associated with a positive d T ~ l d P slope, which is a quite unusual situ- ation for itinerant magnets.

Earlier studies have shown that the Ha phase for t = 0.385 is transformed into a ferromagnetic (F) state on application of a pressure of some 5 kbar [3]. Recently the pressure-induced transitions of the whole MnAs-CrAs system were investigated by Grib- anov and Melnik [4]. The present work is concentrated on precise determination of the phase diagrams for selected compositions t, reflecting the competition of Ha, F, H, and P phases.

The polycrystalline samples are the same as those used in previous studies [I, 21. The pressure was gen- erated inside a pressure chamber connected to a high pressure helium gas compressor. This gives fully hy- drostatic conditions for the investigated temperature region (helium does not freeze) and allows pressure tuning with a precision of 0.01 kbar. The phase bound- aries were located using an ac susceptibility method. Examples of Xac (T) curves are given in figure 1. The magnetization under pressure was investigated using the pulse field method.

170 180

2.10

1,

214 kbar

Fig. 1.

-

Selected as susceptibility vs. temperature curves for Mno.610Cro.agoAs measured at pressures between 1.97 and 2.14 kbar. Points for heating ( 0 ) and cooling ( 0 ) are used to visualise the hysteresis of continuous X, (T) runs. Curves are shifted 10 K to improve clarity.

2. T h e heli

Ha

t o ferro

F

transition

The pressure-induced Ha to F transition occurs for all compositions with t

<

0.385 (including MnAs), The pressure necessary to induce the F phase decreases with the Cr content to a value as low as 1.4 kbar for t = 0.355. Figure 2 shows the P

-

T phase diagram, and inset gives the pressure dependence of the critical field (HCRIT ; manifested as the inflection point of the sigmoidal magnetization curve) as measured at 77 K. HCRIT seemb to decrease linearly to zero on approach- ing the Ha - F phase boundary.

The pressure phase diagram of Mno.645Cro.355As confirms the similarity between the Ha and F phases by a similar rate of increase for

TN

and Tc, respec-

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C8 - 204 JOURNAL DE PHYSIQUE

P (kbar)

.

HELl

o

t

-I

0 1 2 3 4 5 6

P

(kbar)

Fig. 2.

-

Pressure phase diagram for Mno.645Cro.355.4~. Fig. 3. - Pressure phase diagram for Mno.610Cro.390As. Circles and crosses represent transition temperatures ob- Experimental points ( v

,

o ) and ( I\

,

+) are obtained from tained under heating and cooling conditions, respectively. Xac (T) runs, respectively on heating and cooling, for two Inset shows variation of critical field with external pressure, measurement series. The presumed H,

-

P phase boundary as measured by pulse field technique at 77 K. starts at TN = 234 K (P = 0) [2].

tively, with pressure (- 7 K/kbar). In the vicinity of 4. Conclusions the triple point (2.20 kbar, 198 K) the phase boundary

between Ha, F and P shows a N 3 K anomaly indicat- The pressure experiments confirm the anomalously

ing the possibility of a multicritical point. strong influence of pressure in the MnAs-CrAs system, both in suppressing (H,) and enhancing (Ha or

3. T h e heli HC to ferro

I?

transition F) magnetically ordered phases. The -phase diagrams seem to be determined precisely enough t o

_-

make visible a multicritical behaviour resulting from Also the samples with t

>

0.385 exhibit a pressure-

the competition between different kinds of magnetic induced transition to F phase. This is exemplified by

ordering. the T

-

P phase diagram of Mn0.~1oCro.3goAs (Fig. 3).

Above 2.5 kbar Tc increases linearly with dTc/dP = _~ _~ _k _~ _~ _~ _l _~ _d _~ _~ _~ _~ _~ _t

+11 K/kbar. This is the highest positive pressure slope

observed for a metallic ferromagnet and perhaps the This work was partly supported by the highest value for any transition between a magnetically CPBP 01.12 research programme.

ordered and a paramagnetic phase.

On approaching the presumed triple point

(2.10 kbar, 185 K) the Tc ( P ) phase boundary seems Fjellv%g, Kjekshus, A., Chem. Stand

to bend slightly downwards. The hysteresis of the H, A 39 (1985) 671.

to F transition decreases linearly down to

-

3 K on ap- [2] Ziqba1 -4.1 FJellvk, H.1 Kjekshus, A*, J. Magn.

proaching the triple point and disappears then rapidly Magn. Muter. 68 (1987) 115.

in a pressure interval of N 0.04 kbar (see Fig. I). This 131 Andresen, A. F., Fjellv%g, H., Kjekshus, A.,

may suggest the existence of some kind of multicriti- Lebech, B., J. Magn. Magn. Muter. 62 (1986) cal behaviour, probably of the bicritical point type [5]. 247.

The other possibility is that the

Tc

transition is close [4] Gribanov, I. F., Melnik, V. A., J. Magn. Magn.

to first order (as discussed in Ref. [2]). To clarify this Muter. 71 (1988) 219.