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Submitted on 1 Jan 1964
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The magnetic structure of CoPt
B. van Laar
To cite this version:
B. van Laar. The magnetic structure of CoPt. Journal de Physique, 1964, 25 (5), pp.600-603.
�10.1051/jphys:01964002505060001�. �jpa-00205836�
Dr Lovi;. - Je dois rappeler que le moment sur les atomes Fe proches voisins des impuretes appa- rait en fait diminu6, tandis que I’augmentation
semble s’appliquer seulement aux voisins plus
éloignés. Ainsi, toute explication théorique de ces
résultats doit tenir compte des deux perturbations
aussi bien positives que negatives de la distri-
bution des moments magnétiques.
REFERENCES
[1] Low (G. G. E.) and COLLINS (M. F.), J. Appl. Physics, 1963, 34, 1195.
[2] ARROTT (A.) and NOAKES (J. E.), Iron and its dilute solid solutions, Spencer C. W. and Werner F. E.
editors, Interscience Publishers, 1963.
[3] SHULL (C. G.) and WILKINSON (M. K.), Phys. Rev., 1955, 97, 304.
[4] CRANSHAW (T. E.) and RIDOUT (M. S.), Private com-
munication.
THE MAGNETIC STRUCTURE OF CoPt
By B. VAN LAAR
Reactor Centrum Nederland, Petten (N. H.), the Netherlands.
Résumé. 2014 Les résultats en diffraction de rayons X montrent que CoPt est quadratique,
a = 2,677 Å, c = 3,685 Å avec Co0,92Pt0,08 en (000) et Co0,08Pt0,92 en
(1/2, 1/2, 1/2)
pour l’échan- tillon étudié.Les moments magnétiques sont dirigés suivant l’axe c. Co en (0 0 0) et Pt en
(1/2, 1/2, 1/2)
sontcouplés ferromagnétiquement. On montre que le moment magnétique total se partage entre les
deux éléments constituants et on estime la grandeur des moments localisés sur Co et Pt.
Abstract. 2014 X-ray diffraction data show that CoPt is tetragonal, a = 2.677 Å, c = 3.685 Å, with, for the sample under investigation, Co0.92Pt0.08 at (0 0 0) and Co0.08Pt0.92
at (1/2 1/2 1/2).
The magnetic moments point along the c-axis. The Co at (0 0 0) and the Pt
at (1/2 1/2 1/2)
are coupled ferromagnetically. It is shown that the total magnetic moment is divided between the constituent elements and an estimate is made of the magnitude of the moments localized on Co and Pt.
LF; JOURNAL DE PHYSIQUE TOME 25, MAI I 1964,
Introduction. - CoPt has a tetragonal unit cell.
The space group is P4 /mmm. From X-ray data,
taken on a Philips diffractometer, the cell dimen- sions were determined : a = 2 . 677 ui, c = 3 . 685 ui.
In the ideal case there is one Co-atom at (0 0 0)
and one Pt-atom at
I I I
In the investigated222 g
sample, provided by Philips Research Labora- tories, Eindhoven, some disorder was observed.
The disorder parameter, the probability of a Co-
site being occupied by a Pt-atom is :
r = ~0.076 ,T 0.008).
The distribution of the atoms over the two sites
TABLE 1
OCCUPANCY OF POSITIONS IN UNIT CELL
is given in table 1. At room temperature CoPt
has ferromagnetic properties. From measurement of the saturation magnetization, carried out by
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002505060001
Philips Research Laboratories, the total moment
per unit cell was determined as (1.90 z 0.0:1)
The neutron diffraction investigation was under-
taken to reveal the distribution of the magnetic
moment over the constituent elements.
Experimental. - For the unpolarized neutron
work the powder sample was mounted on the
diffractometer at the Petten High Flux Reactor.
The sample was contained in a cylindrical alu-
minium sampleholder of 0.05 mm wall thickness and 20 mm diameter.
The monochromatic radiation with a wavelength
of 1.093 A was obtained from a copper (200) plane.
Soller-slit systems with an angular divergence
of 30’ were mounted between the monochromator
crystal and the sample and in front of the BF3
detector.
The intensities were obtained in about one week with the reactor operating at 20 MW.
The observed intensities were brought on an
absolute scale by comparing with the intensities from a nickel powder diagram obtained under identical conditions.
Neutron diffraction results - Because of the
rapid dropping off of the magnetic form factors most of the information concerning the magnetic
structure will be obtained from the first reflections which have small intensities. The peak-to-back- ground ratio of the first two reflections is about 1.
FIG. 1. - Neutron difraction pattern of CoPt.
Due to the relatively high intensity, available at
the Petten reactor it was possible to obtain an acceptable accuracy (probable errors due to coun- ting statistics less than 1 %). The observed inten-
sities are shown in the last column of table 2. By comparison of these intensities with the calculated nuclear part of the intensities it is seen that the
(0 0 I)-reflection can be fully explained by the
nuclear scattering, which means that the magnetic spins are aligned along the c-axis. In the calcu-
lation of the nuclear structure factors it is neces-
sary to realize that there exists a probable error
TABLE 2 .
COMPARISON OF OBSERVED AND CALCULATED INTENSITIES
TABLE 3
CALCULATION OF SPIN QUANTUM NUMBERS
of J~ 0.01 X 10-12 cm in the tabulated scattering lengths:
and
Therefore the nuclear structure factor, derived
from the observed (0 0 1) intensities was used in the calculation of the magnetic contribution in the remaining odd reflection.
The magnetic structure f actors are shown in the second column of table 3.
It was possible to choose the proper sign because
other choices would lead to sets of spin values
which are highly improbable. To find out how
the total magnetic moment is distributed over the two sites in the lattice, the following quantities are
introduced :
So = total effective, spin quantum number at ( 0 )
= total effective spin quantum number at
(111)
222
A form factor f is assigned to these spin num-
bers.
Then :
Using these expressions, and taking f or f the magnetic form factor given by Nathans and Pao- letti [1] for cubic cobalt, one arrives at the values
f o Sn + and So - given in the third and fourth column of table 3.
The measured saturation magnetization of
41.8 erg Oe-I gr-l results in
assuming a g-f actor of 2.
Combination of this results gives :
Total effective spin quantum number at (0 0 0) :
Total effective spin quantum number at
(111) :
222First it was assumed that an eff ective spin is
connected to the cobalt atoms only, with different
spin values for the cobalt atoms at the (0 0 0) and
(111)
222
sites. With this model 1L follows that the effective spin value of the cobalt atoms at the(111) -sites is about 2.3 which is very unlikely.
Obviously
222
the platinum atoms are carrying a magnetic moment too.The same moment can then be assigned to the
same kind of atoms at different sites.
Since both elements are present at both sites,
two possibilities should be considered. In the one
the spins of one element in the two positions are f erromagnetically coupled, in which case :
In the other case they are antif erromagnetically coupled,
Here 8co and 8Pt are the spin components of
Co at (0 0 0) and of Pt at
(1 1 1) 222
along the posi-tive c-axis.
Then one obtains in the f erromagnetic case :
and in the antif erromagnetic case :
The most important conclusion is that both Sco and SPt are positive, so that the two possibilities
are as shown in table 4. In both arrangements
TABLE 4
DIRECTION OF SPINS FOR TWO POSSIBLE ARRANGEMENTS
the Co at (0 0 0) is coupled ferromàgnetically with
the Pt
at (111), 222 which means that apart from
minor details both arrangements are the same. Jut should be emphasized that the quoted probable
errors do only include the uncertainties due to
counting statistics and the tabulated nuclear scat-
tering lengths. There are large probable errors
introduced in the calculations by using the cobalt magnetic f orm factor for the platinum and this
will certainly influence the result for the spin quantum numbers. Though there are no data
available for the form factor of platinum it is
certain that this form factor drops off more rapidly
with sin 0 /x than the one of cobalt. This does not change the overall conclusion, because a de-
crease in the f orm-f actor values of platinum causes
an increase of the platinum spin quantum numbers and a decrease of the cobalt spin quantum num-
bers. The conclusion that a magnetic moment is
localized on the platinum would still hold.
Comparison with related investigations. - In
the CoPt-sample under examination a disorder
parameter r - (0.076 ~ 0.08) is observed.
There is a definite indication that the total ma-
gnetic moment is divided between the constituent elements. Though it is difficult to determine the exact value of the magnetic moments localized
on the different kind of atoms, it is possible to
say that in CoPt the moment on the Pt-atoms
can be given as : > 0 . 3 whereas for the mo-
ment of the Go-atoms : 1.6 yB (again assuming
a g-f actor of 2).
The existence of localized magnetic moments on
Pt-atoms in the CU3Au-type alloys Mnpt3 and CrO.3PtO.7 is reported by Pickart and Nathans [2].
Bertaut et al. [3, 4, 5] found that in FeRh,
which is ot the CsCl-type, and thus related to the CoPt-structure, Rh has a small moment in the f erromagnetic region.
Discussion
Dr PICKART. - Comment avez-vous prepare
votre 6chantillon ? Avez-vous not6 des eff ets d’orientation préférentielle ?
Dr VAN LAAR. - L’6ehantillon, fourni par les laboratoires Philips, a été prepare par frittage de
CoPt (CN)6. Nous avons utilise un 6ehantillon
cylindrique de 20 mm de diametre, aussi une
orientation préiéientielle était tr6s improbable et
en fait n’a pas été observée.
Dr CABLE. - J’aimerais faire remarquer qu’a
Oak Ridge nous avons f ait des mesures de diffusion
magnétique sur un alliage de CoPt. Nous trouvons
aussi que le platine a un moment (environ 0,25
et que le moment de Co est à peu près 1,6
VAN LAAR. - Ceci confirme les conclusions de la présente 6tude.
Dr IBERS. - Puisque vous avez des intensit6s nucleaires precises, pouvez-vous en déduire des
valeurs, des longueurs de diffusion nucleaire pour Co et Pt meilleures que celles de la littérature et utilisées par vous ?
VAN LAAR. - Une fois que l’on sait que les spins
sont align6s selon I’axe c il est possible de calculer a partir de la reflection (0 01)
bc, - bPt = (0,71 ± 0,01) 10-12 cm.
Du fait que la reflection (0 0 2) ne peut etre séparée
de (1 1 0) on ne peut évaluer la valeur de bca + bm.
En acceptant la valeur
6pt = (0,95 ~ 0,01) X 10-12 cm,
on arrive a bco = (0,24 ~ 0,02) X 10-12 cm ce qui
est proche de la valeur mentionn6e par Dr Roth.
Dr ROTH. - La longueur de diffusion nuel6aire utilisée dans ce travail pour le cobalt est celle que
j’ai obtenue pour CoO. Dans un travail recent sur
C03O4, nous avons mesure (0,23 ~ 0,02) X ~0-12 cm
en accord aux erreurs expérimentales pr6s avec la
valeur précédente.
REFERENCES .~
[1] NATHANS (R.) and PAOLETTI (A.), Phys. Rev. Letters, 1954, 2, 254.
[2] PICKART (S. J.) and NATHANS (R.), J. Appl. Physics, Suppl., 1962, 33, 1336-1338.
[4] BERTAUT (E. F.) et al., J. Appl. Physics, Suppl., 1962, 33, 1123-1124.
[3] BERTAUT (E. F.) et al., C. R. Acad. Sc. 1963, 256, 1688.
[5] BERGEVIN (F.) et al., J. Chem. Physics, 1961, 35, 1904
