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The spontaneous moment of nickel-chromium alloys
Volume 34A, number 5
THE SPONTANEOUS
PHYSICS LETTERS
22 March 1971
MOMENT OF NICKEL-CHROMIUM
ALLOYS
H. CHIFFEY and T. J. HICKS Physics Department, Monash University, Clayton, Victoria 3168, Australia
Received 2 March 1971 11B per atom. The decrease of spontaneous moment to nickel is Near theinitial critical concentration the decrease of adding momentchromium with concentration is 5.2 much 0.3 larger than that inferred from neutron scattering.
The change of spontaneous magnetization with concentration for nickel rich nickel-chromium alloys is of interest with regard to the diffuse neutron scattering measurements of Collins and Low [1] on this system. Previous magnetization measurements by Sadron [2] and by van Elst et al. [3] which are extrapolated to infinite field do not agree, and are not useful for comparing with the neutron data which are taken at H = 0. Specimens varying in concentration from pure nickel to 14 at.% Cr were made at 2 at.% intervals by adding nickel to a master alloy. Alter thorough melting in an argon arc furnace the alloys were homogenized at 950°Cfor three days, quenched, machined, and then reannealed at 950°Cfor a further day before final quenching. All alloys were a single face centred cubic phase with lattice parameter decreasing with chromium content [4]. The magnetization measurements used a Sucksmith ring balance [5] in which the temperature could be varied above 80°Kand the field up to 18 kOe. The specimens were cylinders 3 mm long and with diameters varying from 0.5 mm for pure nickel to 2 mm for those alloys with a low moment. These cylinders were arranged parallel to the field to reduce the demagnetization correction applied. Because the Curie temperatures of alloys contaming 6 at.% Cr. and less were high compared with 80°K, the magnetizations at 0°Kcould be extrapolated quite reliably. The magnetization of the 8 at.% Cr alloy closely followed a law at each field and the extrapolation to 0°Kwas made on this basis for each field used. The magnitization at 0°Kfor each field was then extrapolated to H = 0. The 10 at.% Cr alloy was strongly paramagnetic at 80°K. Fig. 1 shows the spontaneous moment per atom at 0°Kand H = 0 obtained from these measurements.
OS
-
-
03
-
02
01
-
3 Co,~osl~ At/.Cr
Fig. 1. The variation of spontaneous moment (T = 0°K. H = 0) with composition for nickel-chromium alloys. The initial decrease of moment with added chromium is 5.2 ± 0.3 ILB per atom, which might be expected if the virtual 3d levels at the chromium impurity sites were above the Fermi level so that five electrons per impurity were added to the nickel 3d band. The experimental value is slightly higher than SILB per atom and this may be due to the associated weakening of the average exchange field splitting the nickel 3d band. These results can also be compared with the diffuse neutron scattering measurements of Collins and Low [1]. Marshall [61has derived 267
Volume 34A, number 5
PHYSICS
~
3
-
2
-
—
-
I
2
1.
6 8 Composition At ‘I. Cr
10
2 from the present magnetic with those inferred from the extra— Fig. 2.measurements Comparison of (dp/dc) polation to K = 0 of Collins and Low’s [1] neutron scat— tering results. The 1extrapolatAon toK = 0 was made by plotting (da/dll) versus K~as a straight line [8].
that in the forward direction (scattering vector, K — 0) the magnetic neutron scattering is 2barn/atom sterad. da/dfl — 0.048 c(1 - c)(~i/~c) from an unmagnetized ferromagnetic binary alloy, where c is the alloy composition, and a~.L/~cis the rate of change of atomic moment with composition in Bohr ma~netons. Fig. 2 shows the cornparison of (~i/~c)~ inferred from the neutron and
268
LETTERS
22 March 1971
obtained from the present magnetic measuremerits. At low chromium content the two agree tolerably well. The small difference may be due to the neutrons sampling the 3d moment in the alloy rather than the bulk moment, or there may be a small amount of atomic clustering in the alloys. Both effects would raise the neutron cross-section with respect to the bulk magnetic measurements. At higher concentrations however the discrepancy between the two sets of results is quite marked; the neutron result being much too low. The same discrepancy is evident in the results for Ni-Cu near the critical concentration for ferromagnetism [~I~In both alloy systems for there to be a ferromagnetic critical concentratiori. the distrubauces in the moment due t 0 the impurities must not superpose, and a model which takes this into account [8] must be used. References [1] M. F. Collins and G. G. Low, Proc. Phys. Soc. 86 (1965) 535. [2] C.Sadron, Ann. Phy~.Paris 17 (1932) 371. [3] H.C.Van Elst, B. Lubach and G.J.Van den Berg, Physica 28 (1962) 1297. [4] A. Taylor and R. W. Floyd, J. Inst. Metals 80
(1951-52) 577.
(1965)Pepper 328. snd .T.HSmith, J Sri. Instrum [F.] A.R
42
[6] W. Marshall, Proc. Phys. Soc. C, Ser. 2, 1 (1968) 88. [7] T.J.Hicks, B. Rainford, J.S.Kouvel, G.G. Low and J. B. Conily. Phys. Rev. Letters 22 (1969) 531. [81 T. J. Hicks, Phvs. Letters 32A (1970) 410.
THE SPONTANEOUS
PHYSICS LETTERS
22 March 1971
MOMENT OF NICKEL-CHROMIUM
ALLOYS
H. CHIFFEY and T. J. HICKS Physics Department, Monash University, Clayton, Victoria 3168, Australia
Received 2 March 1971 11B per atom. The decrease of spontaneous moment to nickel is Near theinitial critical concentration the decrease of adding momentchromium with concentration is 5.2 much 0.3 larger than that inferred from neutron scattering.
The change of spontaneous magnetization with concentration for nickel rich nickel-chromium alloys is of interest with regard to the diffuse neutron scattering measurements of Collins and Low [1] on this system. Previous magnetization measurements by Sadron [2] and by van Elst et al. [3] which are extrapolated to infinite field do not agree, and are not useful for comparing with the neutron data which are taken at H = 0. Specimens varying in concentration from pure nickel to 14 at.% Cr were made at 2 at.% intervals by adding nickel to a master alloy. Alter thorough melting in an argon arc furnace the alloys were homogenized at 950°Cfor three days, quenched, machined, and then reannealed at 950°Cfor a further day before final quenching. All alloys were a single face centred cubic phase with lattice parameter decreasing with chromium content [4]. The magnetization measurements used a Sucksmith ring balance [5] in which the temperature could be varied above 80°Kand the field up to 18 kOe. The specimens were cylinders 3 mm long and with diameters varying from 0.5 mm for pure nickel to 2 mm for those alloys with a low moment. These cylinders were arranged parallel to the field to reduce the demagnetization correction applied. Because the Curie temperatures of alloys contaming 6 at.% Cr. and less were high compared with 80°K, the magnetizations at 0°Kcould be extrapolated quite reliably. The magnetization of the 8 at.% Cr alloy closely followed a law at each field and the extrapolation to 0°Kwas made on this basis for each field used. The magnitization at 0°Kfor each field was then extrapolated to H = 0. The 10 at.% Cr alloy was strongly paramagnetic at 80°K. Fig. 1 shows the spontaneous moment per atom at 0°Kand H = 0 obtained from these measurements.
OS
-
-
03
-
02
01
-
3 Co,~osl~ At/.Cr
Fig. 1. The variation of spontaneous moment (T = 0°K. H = 0) with composition for nickel-chromium alloys. The initial decrease of moment with added chromium is 5.2 ± 0.3 ILB per atom, which might be expected if the virtual 3d levels at the chromium impurity sites were above the Fermi level so that five electrons per impurity were added to the nickel 3d band. The experimental value is slightly higher than SILB per atom and this may be due to the associated weakening of the average exchange field splitting the nickel 3d band. These results can also be compared with the diffuse neutron scattering measurements of Collins and Low [1]. Marshall [61has derived 267
Volume 34A, number 5
PHYSICS
~
3
-
2
-
—
-
I
2
1.
6 8 Composition At ‘I. Cr
10
2 from the present magnetic with those inferred from the extra— Fig. 2.measurements Comparison of (dp/dc) polation to K = 0 of Collins and Low’s [1] neutron scat— tering results. The 1extrapolatAon toK = 0 was made by plotting (da/dll) versus K~as a straight line [8].
that in the forward direction (scattering vector, K — 0) the magnetic neutron scattering is 2barn/atom sterad. da/dfl — 0.048 c(1 - c)(~i/~c) from an unmagnetized ferromagnetic binary alloy, where c is the alloy composition, and a~.L/~cis the rate of change of atomic moment with composition in Bohr ma~netons. Fig. 2 shows the cornparison of (~i/~c)~ inferred from the neutron and
268
LETTERS
22 March 1971
obtained from the present magnetic measuremerits. At low chromium content the two agree tolerably well. The small difference may be due to the neutrons sampling the 3d moment in the alloy rather than the bulk moment, or there may be a small amount of atomic clustering in the alloys. Both effects would raise the neutron cross-section with respect to the bulk magnetic measurements. At higher concentrations however the discrepancy between the two sets of results is quite marked; the neutron result being much too low. The same discrepancy is evident in the results for Ni-Cu near the critical concentration for ferromagnetism [~I~In both alloy systems for there to be a ferromagnetic critical concentratiori. the distrubauces in the moment due t 0 the impurities must not superpose, and a model which takes this into account [8] must be used. References [1] M. F. Collins and G. G. Low, Proc. Phys. Soc. 86 (1965) 535. [2] C.Sadron, Ann. Phy~.Paris 17 (1932) 371. [3] H.C.Van Elst, B. Lubach and G.J.Van den Berg, Physica 28 (1962) 1297. [4] A. Taylor and R. W. Floyd, J. Inst. Metals 80
(1951-52) 577.
(1965)Pepper 328. snd .T.HSmith, J Sri. Instrum [F.] A.R
42
[6] W. Marshall, Proc. Phys. Soc. C, Ser. 2, 1 (1968) 88. [7] T.J.Hicks, B. Rainford, J.S.Kouvel, G.G. Low and J. B. Conily. Phys. Rev. Letters 22 (1969) 531. [81 T. J. Hicks, Phvs. Letters 32A (1970) 410.