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MAGNETIC PROPERTIES OF IRON COMPOUNDS WITH YTTRIUM, LUTETIUM AND GADOLINIUM
D. Givord, F. Givord, R. Lemaire
To cite this version:
D. Givord, F. Givord, R. Lemaire. MAGNETIC PROPERTIES OF IRON COMPOUNDS WITH YTTRIUM, LUTETIUM AND GADOLINIUM. Journal de Physique Colloques, 1971, 32 (C1), pp.C1- 668-C1-669. �10.1051/jphyscol:19711232�. �jpa-00214060�
JOURNAL DE PHYSIQUE CoIloque C 1, supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 668
MAGNETIC PROPERTIES OF IRON COMPOUNDS WITH YTTRIUM, LUTETIUM AND GADOLINIUM
D. GIVORD, F. GIVORD, R. LEMAIRE
Laboratoire d'Electrostatique et de Physique du Mttal, C . N. R. S., Cedex 166, 38, Grenoble, France R&urnh. - Des mesures d'aimantation entre 2 et 850OK sur les corn osCs TFe2, TFe3, TsFe23, et T2Fe17, oh T = Y , Lu ou Gd, out confirm6 leur caractkre ferromagnktique (T = Y ou ~ u r o u fenirnagn6tique (T = Gd). Le compose Lu2Fe17 prksente une transition ferro-antiferromagnktique et I'ktude par diffraction neutronique a r6ve16 un arrangement hklimagnetique des moments. Une variation des interactions entre atomes de fer avec la distance semblable A celle mise en evidence par NCel et Slater permet d'expliquer cette transition ainsi que la variation des temperatures d'ordre.
Abstract. - Magnetization measurements, between 2 and 850 OK, on RFe2, RFe3, RsFe23 and R2Fel7 compounds, for R = Y, Lu or Gd, have confirmed their ferromagnetic (R = Y or Lu) or ferrimagnetrc (R = Gd) character. The LuzFel7 compound shows a ferro-antiferromagnetic transition and a neutron diffraction study revealed an helical spin configuration. A distance dependence of the magnetic interactions between iron atoms as has been shown by Nkel and Slater can explain this transition as well as the order temperatures variation.
The intermetallic compounds that exist in the rare earth-iron system are RFe,, RFe,, R6Fe,, and R,Fe,, [I to 31. Much work has already been carried out on the magnetic properties of these compounds, but the results are somewhat inconsistent, which renders difficult an interpretation of the observed properties.
The RFe, compounds have been more frequently investigated [4 to 71. O'Keefe et al. [8] determined the magnetic structure of HoFe, and Ho6Fe2, : they proposed an antiparallel coupling between the hol- mium and iron moments. Hoffer and Salmans [9]
measured the temperature dependence of the magne- tization of the RFe, compounds. Strnat et al. -[lo]
observed on the R2Fe,, compounds, where R = Ce, Gd, Tm, Lu, a peak of susceptibility above the tempe- rature at which the spontaneous magnetization vani- shes. We have more particularly investigated magnetic properties of these compounds for R = yttrium, gado- linium or lutetium. Yttrium and lutetium have no iritririsic magnetic moment but their atomic radii are different, yttrium being the larger. The state configu- ration of the gadolinium atom is 'S,,, and is fairly insensitive to the crystal field. Its magnetic moment is 7 P B .
I. Experimental procedure and results. - The sam- ples were prepared by melting the elements at about 1700 OC in a high-frequency levitation furnace. Rapid quenching prevents the various peritectic transforma- tions from occuring.
Magnetizations were measured in fields up to 24 k o e and at temperatures ranging from 2 to 850 OK.
Results are presented in table I. The LuFe, and Gd,Fe,, compounds do not exist. Within the limit of experimental accuracy, each type of intermetallic compounds with yttrium and with lutetium exhibits the same absolute saturation magnetization, and therefore the same mean moment per iron atom.
Nevertheless, their magnetic order temperatures are different. Absolute saturation magnetization of com- pounds with gadolinium is small compared to that of the same compounds with yttrium. Furthermore, GdFe, exhibits a compensation point at 613 OK. The magnetic moment of the gadolinium atoms therefore lies antiparallel to that of the iron atoms. The iron mean moment is then deduced from absolute satura- tion magnetization.
TABLE I
Mean iron moments a,, and order temperatures 8, of the compo~rnds of iron with yttrium, lufetium and gadolinium.
Compounds R2Fel7 R6Fe23 RFe, RFez
- - - - -
Y or Lu
me, PB ( Gd 2.04 2.16 - 1.91 1.67 1.83 1.45 1.60
Y 310 478 539 537
00, OK ( & 270 49 1 - 583
379 - 733 798
Difference
(OO)L" - (BO)Y, OK - 4 0 f 1 3 - + 46
In order to confirm the influence of the interatomic distances upon magnetic interactions, measurements were made of the dependence of Curie temperatures of YFe,, Y,Fe2, and Y2Fe,, with hydrostatic pressure up to 4.5 kbar. One obtains + 5.0, + 0.1 and
- 9.8 OK/kbar (+ 0.5 OK/kbar) for respectively YFe,, Y6Fe2, and Y,Fe,,. These variations are in agreement with the observed differences (O,),, - (0,), between the magnetic order temperatures of compounds with lutetium and with yttrium (Table I).
The Lu,Fe,, compound has a particular behaviour : it is ferromagnetic below 100 OK and becomes meta- magnetic as temperature increases. The transition of a sample annealed at 1000 OC for 3 days was studied in a 100 Oe field (Fig. 1). The temperatures 0, = 142 OK and 0 , = 107 OK characterize the transition with increasing and decreasing temperature respectively. At 270 OK, the magnetization in a 100 Oe field shows a peak characteristic of a NCel temperature. The transi- tion occurs therefore between a ferromagnetic and an antiferromagnetic state. Neutron diffraction patterns at 77 OK and also above the order temperature show the same diffraction lines, which confirms the ferro- magnetic nature of this compound at low tempera- tures. Magnetic moments are perpendicular to the c-axis. A diffraction line appears between 100 and 270 OK at a low angle, for example, at 160 OK
sin 0/A = 86 x A-'
(Fig. 2). Its position varies linearly from 270OK to 1500K, at lower temperatures the variation is less rapid and vanishes at about 125 OK. In the same range of temperatures, two satellites lines (101+) and (101-) were observed on both sides of line (101). These results
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711232
MAGNETIC PROPERTIES OF IRON COMPOUNDS WITH YTTRIUM, LUTETIUM AND GADOLINIUM C 1 - 669
A distance dependence of the interaction energy as was proposed by Nkel and Slater [ l l ] can explain the Lu2Fe17 transition and the order temperatures varia- tion. There are, in the R2FeI7 compounds, some very short distances (2.4 tf) between iron atoms in the substitution zones with which negative interactions can be associated. Positive interactions, near the peak of the Nkel-Slater curve, are associated to the other distances. Thermal motion involves oscillations of interatomic distances and leads to a decrease of the mean value of the positive interactions, whereas the mean value of the negative interactions is unchanged.
At a certain temperature, positive interactions become sufficiently weak to make the compound helimagnetic.
In Y2Fe17, interatomic distances are larger than in Lu2Fel,. Negative interactions are then weaker, and there is no transition.
TEMPERATURE. K
FIG. 1. - Magnetization of the LuzFe17 comvound : temve- rature depend& in a field of 100 d e and field dependence at
2 OK and 250 OK. The magnetic interactions between iron atoms may be represented by the molecular field constant n, which can be expressed as a function of the Curie temperature 0, and the absolute saturation magne- tization c, by the relation
where J represents the total angular moment and k the Boltzmann constant. Owing to the fact that absc- lute saturation magnetization is the same for com- pounds with yttrium and with lutetium, it is therefore independent of volume. The distance interaction energy dependence is then equal to the Curie tempe- rature depepdence. It was found that, as the volume was increased, interaction energy decreased to the 1.59 th power for the RFe, compounds and to the 0.72 th for the R,Fe2, compounds. These compounds can therefore be placed on the right-hand side of the Nkel-Slater curve, the R,Fe2, compounds being nearer the peak. However, interaction energy increases with volume for the R2Fe17 compounds: so they can be placed on the left-hand side of the curve. This expla- nation compares well with the interpretation given for the Lu2Fe,, transition.
DIFFERENCE PATTERN
BRAGG ANGLE
FIG. 2. - Neutron diffraction patterns of the LuzFet7 compound.
are typical of an helical spin configuration, the screw axis is the c-axis. The magnetic structure is therefore formed by ferromagnetic layers perpendicular to the c-axis. The angle between two consecutive layers varies from 120 at 110 OK to 190 at 270 OK.
11. Discussion. - When the number of rare earth atoms alloyed with one iron atom increases (from R2FeI7 to RFe2) the mean iron moment decreases more slowly than that of cobalt or nickel in the corresponding compounds. Since the two 3 d' and 3 d- half bands are unfilled, the filling of the transi- tion metal 3 d. band by the conduction electrons contributed by the rare earth atoms leads to a less rapid decrease of iron moment.
In conclusion, order temperatures of compounds with iron are in close dependence on the crystal symmetry, that is on the number of nearest neighbour iron atoms, and on their distance, instead of on the value of the magnetic moment. The increase in Curie temperatures when rare earth atoms are magnetic is essentially due to the interactions the latter cause, whereas in cobalt compounds the additional induced moment has a preponderant part.
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