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The lattice parameters of Pt3MnxCr1−x alloys
337
Journal of the Less-Common Metals, 44 (1976) 337 - 339 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
Short Communication
The lattice parameters of PtsMn,Cr,_,
alloys
B. G. LEWIS and D. E. G. WILLIAMS Department of Physics, Loughborough (Gt. Britain)
University of Technology,
Lough borough
(Received June 25,1975)
It is well known that the alloys in the Pt-Mn and Pt-Cr binary alloy systems which assume the Lla structure (with Pt concentrations between 0.8 and 0.6) have ferromagnetic properties [ 1, 21. This ferromagnetism disappears when the Lls structure is disordered by cold working and the disordered alloy has a c.c.p. structure with a lattice parameter larger than that in the ordered state [ 31. This note reports the results of measurements made on the pseudobinary alloys PtsMn,Cri_,, for which the room temperature values of the lattice parameters, both in the ordered and disordered states, and the long range order parameters, S, have been obtained. The alloys were prepared from Johnson Matthey “Specpure” platinum and manganese, and Metals Research “5N” chromium, by arc-melting in an argon atmosphere. The arc-melted buttons were subjected to an homogenizing anneal, and then reduced to 300-mesh powders by crushing in a ball mill. The cold working necessary to produce these powders disordered the alloys, and their ferromagnetism disappeared. The crystallographically ordered state was regained by annealing the powders in evacuated silica tubes at 950 “C! for up to 72 h. The X-ray diffraction measurements were made using a Philips PW1050 powder diffractometer and a PW1130 X-ray generator producing Cu radiation. The diffractometer was scanned at a rate of %‘/min to locate the positions of the Bragg peaks for the lattice parameter calculations. For the ordered alloys, the diffractometer was also step scanned about the positions of the superlattice and fundamental Bragg reflections. The integrated intensities obtained from these step scans were compared amongst themselves to provide a measure of the ordering parameter, S. One of the alloy speciments (x = 0.5), was made the subject of a neutron diffraction experiment; this experiment was carried out at UKAEA, Harwell, using a powder diffractometer (we are indebted to Dr. B. D. Rainford for his assistance). There were two objectives in performing the neutron experiment: (i) to discover if the Cr and Mn atoms were ordered on their sites, and (ii) to provide an extinction-independent measure of the order parameter, S. No evidence of Mn/Cr order was found, and the value of S obtained agreed well with the X-ray value. The results of these experiments showed that all the alloys
338 TABLE
1
The lattice parameters of PtsMnx Cr(,_,) Composition (x)
Lattice parameter a (A) Disordered
Ordered 1.0 0.9 0.7 0.5 0.3 0
alloys
3.8931 3.8936 3.8873 3.8848 3.8825 3.8768
+ 0.0006
3.9098 3.9053 3.8991 3.8946 3.8837 3.8771
t 0.0008
3.90
a
(A, 3.88
0
0.5 X
3 1.0
Fig. 1. The lattice parameters, o (A), of the alloys PtaMn,Cr(~,) 0, ordered alloys; 0, disordered alloys.
as functions of x.
assumed a c.c.p. structure, and that the ordered alloys were all fully ordered (S = 1) within experimental error. The lattice parameters, a, are given in Table 1, and displayed in Fig. 1; the values obtained for ordered PtsCr and ordered Pt,Mn agree closely with those of previous workers [2, 41. The difference between the lattice parameters of the ordered and disordered states can be explained in terms of a combination of the effects of spontaneous volume magnetostriction and the small increase in lattice parameter (-0.1%) which generally occurs tihen an ordered alloy is disordered. The difference, Au, between the room temperature (Z’,) values of the lattice parameter for the disordered and ordered states can be written as
339
where T, is the ferromagnetic Curie temperature, (aM/a V)T is the isothermal variation of the magnetic moment, M, with volume, V, and C is the effect of crystallographic disorder. For the alloys in the composition range 0.5 < x < 1, 0.2% < Au/u < 0.596, which is much larger than we would expect from the simple disorder effect; for the composition range 0.3 > x > 0, A& is much smaller than would be expected from the disorder effect. This leads us to conclude that the integral in eqn. 1 is positive for x > 0.5 and negative for x G 0.3. Since dM/dT < 0, this implies that (a V/aM)T is predominantly negative in the temperature range T, > T > T, for the alloys with x > 0.5, and predominantly positive for the alloys with x < 0.3. We may thus surmise that both the spontaneous magnetostriction and the forced magnetostriction change sign at some point in the composition range 0.5 > x > 0.3 in this pseudobinary alloy system. It is a pleasure to acknowledge both the support given by the SRC to this work and its provision of neutron beam facilities. 1 2 3 4
E. Friedrich and A. Kussman, +??.Phys., 36 (1935) 187. M. Auwarter and A. Kussman, Ann. Phys. (Leipzig), 7 (1950) A. J. P. Meyer and M. Besnus, Phys. Status Solidi, b55 (1973) W. Bronger and W. Klemm, Z. Anorg. Chem., 39 (1962) 58.
169. 521.
Journal of the Less-Common Metals, 44 (1976) 337 - 339 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
Short Communication
The lattice parameters of PtsMn,Cr,_,
alloys
B. G. LEWIS and D. E. G. WILLIAMS Department of Physics, Loughborough (Gt. Britain)
University of Technology,
Lough borough
(Received June 25,1975)
It is well known that the alloys in the Pt-Mn and Pt-Cr binary alloy systems which assume the Lla structure (with Pt concentrations between 0.8 and 0.6) have ferromagnetic properties [ 1, 21. This ferromagnetism disappears when the Lls structure is disordered by cold working and the disordered alloy has a c.c.p. structure with a lattice parameter larger than that in the ordered state [ 31. This note reports the results of measurements made on the pseudobinary alloys PtsMn,Cri_,, for which the room temperature values of the lattice parameters, both in the ordered and disordered states, and the long range order parameters, S, have been obtained. The alloys were prepared from Johnson Matthey “Specpure” platinum and manganese, and Metals Research “5N” chromium, by arc-melting in an argon atmosphere. The arc-melted buttons were subjected to an homogenizing anneal, and then reduced to 300-mesh powders by crushing in a ball mill. The cold working necessary to produce these powders disordered the alloys, and their ferromagnetism disappeared. The crystallographically ordered state was regained by annealing the powders in evacuated silica tubes at 950 “C! for up to 72 h. The X-ray diffraction measurements were made using a Philips PW1050 powder diffractometer and a PW1130 X-ray generator producing Cu radiation. The diffractometer was scanned at a rate of %‘/min to locate the positions of the Bragg peaks for the lattice parameter calculations. For the ordered alloys, the diffractometer was also step scanned about the positions of the superlattice and fundamental Bragg reflections. The integrated intensities obtained from these step scans were compared amongst themselves to provide a measure of the ordering parameter, S. One of the alloy speciments (x = 0.5), was made the subject of a neutron diffraction experiment; this experiment was carried out at UKAEA, Harwell, using a powder diffractometer (we are indebted to Dr. B. D. Rainford for his assistance). There were two objectives in performing the neutron experiment: (i) to discover if the Cr and Mn atoms were ordered on their sites, and (ii) to provide an extinction-independent measure of the order parameter, S. No evidence of Mn/Cr order was found, and the value of S obtained agreed well with the X-ray value. The results of these experiments showed that all the alloys
338 TABLE
1
The lattice parameters of PtsMnx Cr(,_,) Composition (x)
Lattice parameter a (A) Disordered
Ordered 1.0 0.9 0.7 0.5 0.3 0
alloys
3.8931 3.8936 3.8873 3.8848 3.8825 3.8768
+ 0.0006
3.9098 3.9053 3.8991 3.8946 3.8837 3.8771
t 0.0008
3.90
a
(A, 3.88
0
0.5 X
3 1.0
Fig. 1. The lattice parameters, o (A), of the alloys PtaMn,Cr(~,) 0, ordered alloys; 0, disordered alloys.
as functions of x.
assumed a c.c.p. structure, and that the ordered alloys were all fully ordered (S = 1) within experimental error. The lattice parameters, a, are given in Table 1, and displayed in Fig. 1; the values obtained for ordered PtsCr and ordered Pt,Mn agree closely with those of previous workers [2, 41. The difference between the lattice parameters of the ordered and disordered states can be explained in terms of a combination of the effects of spontaneous volume magnetostriction and the small increase in lattice parameter (-0.1%) which generally occurs tihen an ordered alloy is disordered. The difference, Au, between the room temperature (Z’,) values of the lattice parameter for the disordered and ordered states can be written as
339
where T, is the ferromagnetic Curie temperature, (aM/a V)T is the isothermal variation of the magnetic moment, M, with volume, V, and C is the effect of crystallographic disorder. For the alloys in the composition range 0.5 < x < 1, 0.2% < Au/u < 0.596, which is much larger than we would expect from the simple disorder effect; for the composition range 0.3 > x > 0, A& is much smaller than would be expected from the disorder effect. This leads us to conclude that the integral in eqn. 1 is positive for x > 0.5 and negative for x G 0.3. Since dM/dT < 0, this implies that (a V/aM)T is predominantly negative in the temperature range T, > T > T, for the alloys with x > 0.5, and predominantly positive for the alloys with x < 0.3. We may thus surmise that both the spontaneous magnetostriction and the forced magnetostriction change sign at some point in the composition range 0.5 > x > 0.3 in this pseudobinary alloy system. It is a pleasure to acknowledge both the support given by the SRC to this work and its provision of neutron beam facilities. 1 2 3 4
E. Friedrich and A. Kussman, +??.Phys., 36 (1935) 187. M. Auwarter and A. Kussman, Ann. Phys. (Leipzig), 7 (1950) A. J. P. Meyer and M. Besnus, Phys. Status Solidi, b55 (1973) W. Bronger and W. Klemm, Z. Anorg. Chem., 39 (1962) 58.
169. 521.