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G. T. DUBOVKA al. : Some Peculiarities of the T-P-C Diagram et
phys. stat. sol. (a) 38,301 (1975) Subject classification: 18.2 and 18.4; 18.1; 21.1; 21.1.1

301

Institute of Solid State Physics, Academy of Sciences of the USSR, Chernogolovka

Some Peculiarities of the T-P-C Diagram of the System Fe65(Nil-zMnz)86
BY

G. T. DUBOVKA, G. PONYATOVSKII, GEORGIEVA, V. E. ANTONOV E. I. PA. and
The form of the T-P-C diagram of the Fe,,(Nil -&nz)35 system is specified. The alloy with x = 0.171 is found to deviate from the linear Curie point dependence on pressure. The interrelation between the magnetic T-C diagram peculiarities and the anomalies of the thermal expansion coefficient and the isotherm of the magnetic susceptibility in the paramagnetic range is investigated.
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1. Introduction
One of the reasons which recently caused an increasing interest in investigations of ternary alloys on Fe-Xi invar base is the possibility to study anomalies of physical properties by varying the ratio of alloying elements. These anomalies always appear simultaneously in binary invars: low magnitude of the Curie points T, and spontaneous magnetization at 0 OK, anomalies of P-V-T relations (including thermal expansion), decreasing stability of the y-phase to y + a transformation, etc. The Fe-Ni-Mn system gives the opportunity to study the transition from alloys being ferromagnetically ordered at low temperature (Xi-rich alloys) to those being ordered antiferromagnetically (Mn-rich alloys) [ 11. In the present work the 1'-P-C phase diagram of the Fe-Xi-Mn system was --2Mnz)35, relation of the peculiarities of the diagram the specified for Fee5(Ki1 with anomalies of linear expansion and magnetic susceptibility in the paramagnetic region was investigated.

2. Experimental
The alloys were prepared from electrolytic Xi, Mn, and Fe carbonyl by melting in argon atmosphere in an induction furnace. The melts were subjected to homogeneous annealing at 1100 "C for 6 h and subsequent quenching in water. was measured by the Faraday-Sucksmith method with The susceptibility e.m.u./g. Alloys So. 1, 2, 3, 8, 9 (Table 1) were inan accuracy of 0.1 x vestigated at hydrostatic pressures up to 20 kbar. The pressure was measured by a inanganin wire gauge with an accuracy of fl00 bar, the temperature by a chromel-alumel therniocouple with an accuracy of f1 deg. Alloys Xo. 4 and 5 were investigated using AgCl as the pressure transmitting medium. In these

x


302

G. T. DUBOVKA, G. E.

PONYA4TOVSKII,

I. YA. GEORGIEVA, V. E. ANTONOV and

Table 1 --dTc,,ldP (deg kbar-l)
~~ ~~

35 33.5 32 30.5 29 27 11
6

0

0.000 0.043 0.086 0.129 0.171 0.229 0.686 0.829 1.000

467 402 353 228 190 90 253 341 442

4.4 0.1 5.0 & 0.1 4.8 & 0.15 4.2 & 0.4 3.7 0.3

*

~~~

*
-

0.6 & 0.2 0.9 0.1

*

cases the pressure up to 25 kbar was measured with an accuracy of +1 kbar. The Curie points T, were determined by the differential transformer method [2] using the temperature dependence of the initial magnetic permeability with an accuracy of 1 3 deg. The NBel points T, were determined by controlling the position of the bend of the temperature dependence curve of electrical resistance with an accuracy of -+5 deg. The fact that these bends in alloys No. 8 and 9 correspond to the S6el points was proved by means of neutron diffraction investigation.

3. Results and Discussion

All the investigated alloys had a disordered f.c.c. structure. The obtained magnitudes of T,, T, and initial values of dT,ldP and dT,IdP are given in Table 1 and Fig. 1. The 7'-C diagram at atmospheric pressure is in good agreement with data given in [l,31. The considerable disagreement of the values dT,ldP obtained in [3] with our data might possibly be due to great errors in the measurements [3] resulting from too small maximal pressure (3 to 5 kbar) used in the work. For all the alloys studied under pressure except the alloy No. 5, Tc and T, decrease linearly with increasing pressure. In the alloy KO. 5, however, the marked divergence of the Tc(P) dependence from the linear one is noticed (Fig. 2). It is worth pointing out that from all the alloys studied under pressure

2

>
?.
I

X

--

Fig. 1. The phase diagram of magnetic transitions in the Fe,,(Nil -zMn,),, system at atmospheric pressure and dependences of dTc/dP and dT,/dP on composition. o Tc; dTc/dP; T,; dT,/dP


Some Peculiarities of the T-P-C Diagram of Fe,,(Sil -zMn2)35

303

Fig. 2. The

Tc(P)and T;(P) dependences
alloy No. 5 (z = 0.171)

for

Fig. 3. The dependences of the linear expansion coefficient LY and magnetic susceptibility x at 573 OK on alloy composition

this specimen had the lowest Tc and that for the alloy with similar composition (No. 4) investigated under the same (quasi-hydrostatic) conditions the T,(P) dependence deviates from the linear one only slightly and lies within the experimental error. It is seen in Fig. 2 that the P(Tc)dependence for the alloy KO. is nearly 5 parabolic in the investigated temperature range. The very dependence dT, ~dP
--

A Tc '

where A is the constant given by Wohlfarth's approximation [4] of a very weak itinerant ferromagnetism (in our case A = 700 degZkbar-I). The fact that such an approximation may be used to describe the temperature dependence of magnetization of the system Fe65(Sil-.Mn,),S for 0.15 x 0.3 was already illustrated in [5]. The results of measurements of the linear expansion coefficient LY at different temperatures and the isotherm of magnetic susceptibility at 573 OK (in the paramagnetic range) are given in Fig. 3. It follows from Fig. 3 that on substituting Ni by manganese iy increases sharply near the composition Fe65Ki,5 and decreases more smoothly near Fe,,Mn,, at all temperatures. For one can observe a sharp drop in the 0 x 0.3 range and a weak dependence on concentration at higher Mn contents. Thus it is seen from Fig. 3 that both the functional dependences a(x) and x(x) are different in the regions x & 0.3 and x 2 0.3 up to temperatures which are considerable higher than those of magnetic ordering of the investigated samples (the boundary between these regions is conventionally shown in Fig. 1 by a dashed line). All the investigated alloys had the same f.c.c. lattice at all temperatures, so the difference in the behaviour of the a(.) and x(x) dependences in the mentioned regions may be due to the difference in the electron configuration in the paramagnetic state of the samples which are ferromagnetics (z 0.3) and antiferromagnetics (x 2 0.3) at low temperatures. It is seen from Pig. 1 that a region of alloys paramagnetic down to temperatures close to 0 OK, may appear or extend with increasing pressure. Thus, for example, extrapolation gives T, = 0 OK at P 30 kbar (Fig. 2) for the alloy

<<
x

<<

x

<


304

G. T. DUBOVKA al. : Some Peculiarities of the T-P-C Diagram et

KO. 5. Similar speculations are true for Fe-Xi-Mn aiitiferromagnetic alloys as

well. The number of the s d external electrons is considered as a criterion of a magnetic ordering to exist in alloys on the basis of d-metals in some papers [6, 71. High pressure does not change this number in our Pe-Ni-Mn alloys, but may transform, for example, the state of alloy KO. 5 from ferromagnetic at temperatures below 190 OK to paramagnetic at temperatures close to 0 OK. This fact seems to be unfavourable for the use of the number of external electrons as a criterion of magnetic ordering in alloys of d-metals.
Acknowledgements

+

The authors thank R. Y. Sizov for the Nkel point determination by nieans of neutron diffraction and E. I. Potapov for his help in carrying out the experiments.

References
[l] M. SHIGA, Phys. SOC. J. Japan 2'2, 539 (1967). and [2] G. T. DUBOVKA E. G. PoNYxcovsI(ii, Fiz. Metallov i Metallovedenie 33,640 (1972). [3] J. NAKAMURA, HAYASE, SHIGA, MIYAMOTO, and N. KAWAI, Phys. SOC. M. M. Y. J. Japan 30, 720 (1971). 141 E. P. WOHLFARTH, Phys. Letters A 28, 569 (1969). [5] B. K. PONOMARYOV v. ALEKSANDROVICH, Zh. eksper. teor. Fiz. 67, 1965 (1974). and s. [6] S. CHiKAMUZi, T. N~ZOGUCHI, and N. YAMAGUCHI, appl. Phys. 3'3, 935 (1968). J. [71 V. M. KALININ, Fiz. Metallov i Metallovedenie 39, 439 (1975). (Received September 3, 1975)