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THE TEMPERATURE ROTATION OF THE EASY AXIS DIRECTION IN DyFe3
S. Japa, K. Krop, R. Radwanski, J. Wolinski
To cite this version:
S. Japa, K. Krop, R. Radwanski, J. Wolinski. THE TEMPERATURE ROTATION OF THE EASY AXIS DIRECTION IN DyFe3. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-193-C2-195.
�10.1051/jphyscol:1979268�. �jpa-00218666�
THE TEMPERATURE ROTATION OF THE EASY AXIS DIRECTION IN DyFe
3S. Japa, K. Krop, R. Radwanski and J. Wolinski
Department of Solid State Physios IM, Academy of Mining and Metallurgy, 30-059 Craoow, Poland
Abstract.- The orientation of easy magnetization axis and its temperature dependence was determined for DyFe3 by Mossbauer spectroscopy. Below the room temperature the easy axis does not overlap any of the principal crystallographic axes. In the temperature region where the easy axis rotates the zero thermal expansion cone was observed.
1. Introduction.- In the past few years the rotation of the easy magnetization axis in some R-T interme- tallics was observed by Mossbauer spectroscopy. In pseudobinary Laves phases (A B _ )Fe changes of the easy axis occure as a function of x/1/. Rotation of easy axis with temperature was observed in RFe3
/2-4/ and R2 F e1 7 /5/.
Generally the reorientation takes place between the principal crystallographic axes, which is howe- ver not the case for DyFe3 at low temperatures.
2. The orientation of easy axis and the shape of Moss- bauer spectrum.- In rhombohedral DyFe3 (R 3m space
group) the iron atoms occupy three unequivalent crys- tallographic positions : (b), (c) and (h). Due to that one could expect the Mossbauer spectrum to be a superposition of three sextets with the intensity ratio equal to the occupation ratio of these posi- tions e.g. 1:2:6. In reality the situation can be much more complicated because the electric quadrupol
and magnetic dipole interaction can introduce addi- tional inequality within a given group of positions.
Positions (b) and (c) with 3m and 3m point symmetry respectively, where the triple axis coincides with the c axis, do not split into subgroups whatever the direction of easy axis is. For (h) positions with m point symmetry the situation differs. The electric field gradient (EFG) tensor is not axially symmetric and for different easy directions the (h) positions split into maximum three subgroups (hi), (I12) and
(I13) occupied in proportion 1 : 1 : 1 . Following instances are possible :
1/ easy axis coincides with c axis- (hi), (ha) and (b.3) are equivalent.
2/ easy axis coincides with a axis or "be" pla-
ne-subgroups (hi) and (h^ + I13) exist,
3/ easy axis has an arbitrary direction - (hi), (112) and (I13) are unequivalent.
According to the easy axis orientation then, Mossbauer spectrum might consist of three, four or five sextets.
3. Electric quadrupole interaction in DyFe3-- For the lack of axial symmetry the Hamiltonian has off- diagonal elements of the EFG tensor and the eigen- values can only be calculated approximately. This is the case for h-positions in DyFe3 /6/. The energy eigenvalues are determined through the nonvanishing components of the EFG tensor and the angles 0 and <|>, the chosen coordinate system has to be turned about, to overlap the magnetic system in which the z-axis overlaps the easy magnetization direction.
4. Experimental procedure.- DyFe3 samples were pre- pared by arc melting of 99.9% purity components in argon atmosphere. The samples were proved bu X-rays to be homogeneous and uniphase with tiny amount of dysprosium oxide. Because of brittleness powdered absorbers were prepared with effective thickness of 0.1 mg 57Fe/cm2 in organic glass containers. Moss- bauer spectra were measured at temperatures from 78 K to 300 K with 57Co-Cr as a source. Lattice cons- tants were measured with DRON-2 diffractometer.
5. Data analysing and results.- A selection of Moss- bauer spectra measured at different temperatures is presented in figure 1. It is difficult to decide a priori how many sextets they consist of. It seems however, that easy axis does not coincide with c axis. Fitting the spectra as superposition of four overlapping sextets did not give good results. Hence the conclusion was drown that the easy axis should not overlap any principal crystallographic axis.
JOURNAL DE PHYSIQUE
Colloque C2, supplément au n° 3, Tome 40, mars 1979, page C2-193
Résumé.- Dans le composé DyFe3 on a déterminé, en utilisant l'effet Mossbauer, la direction d'un axe d'aimantation facile et les changements de cette direction avec la température. On a constaté qu'au- dessous de la température ambiante cet axe n'est parallèle à aucun axe crystallographique principal.
En outre dans cette région de température où l'axe facile change de direction, on a observé le cône de dilatation thermique nulle.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979268
JOURNAL DE PHYSIQUE
appears with the crystallographic c axis as the cone
u
axis. The apex angle of the cone is 26 where :-5 -.4 -3 -2 -1 0 1 2 3 .4 5
velocify [mmlsl
I I I I I
Fig. 1 : A selection of ~gssbauer spectra of DyFe3 vs. temperature.
I I I I I
Another two fitting procedures have been tried : I / five sextets fitted with standard programe,
2 / five sextets fitted with the programme that
takes into account the off-diagonal elements of the EFG tensor 1 6 1 .
In the first procedure the fitting parameters were : background and IS, Heff, QS for each sextet,
rl = rs, r2 = r 5 ,
r3
=rr,
the same for all sextets.The sextets intensity ratios were taken to be equal to the occupation ratios, ie. 1 : 2 : 2 : 2 : 2 . Isomer shifts for hl, h2, h3 were taken to be equal. In the second procedure the fitting parameters were : back- ground, IS, Heff and the EFG tensor parameters P20 (b), P20 (c), P20 (h), p21S(h), p2zC(h), @,
a.
Line widths, intensity ratios and IS (h) like in the first procedure. @ and are the polar angles easy direction makes with c and a axes.
The standard programme fitting procedure gave high
x2
values, especially at low temperatures and they were decreasing with increasing temperature.The second fitting procedure gave much smaller
x2
values. It was also assumed that the easy axis is in the (a c) plane ie. 9 = 0 1 2 1 . Determined@ values vs. temperature are shown in figure 2 . There has been also an attempt to fit the spectra with the free parameter, but it did not improve the X2 va- lues. When was fixed at 28.5' (close to the b c plane) the obtained @ values, within experimental error, were the same like in the case4
0 .X-ray measurements reveal that below 190 K the thermal expansion coefficient along hexagonal axis (a1l)becomes negative and the zero expansion cone
Fig. 2 : Temperature dependence of the angle @,bet- ween c-axis and the easy axi,s direction.
Fig. 3 : Temperature dependence of lattice constants a and c and the apex angle of the zero thermal expansion cone.
6 . Conclusions.- 1 . Below room temperature the easy axis in DyFe3 does not overlap any of the principal crystallographic axes. This necessitates the off- diagonal elements of the EFG tensor to be taken into account.
2. Rotation of the easy magnetization axis towards the basal plane is observed between 150 K and 200 K.
3. The apex angle of zero expansion cone decrea- ses to zero as the easy axis rotates towards the ba- sal plane. This is just opposite what one observes in gadolinium /7,8/ where the zero expansion direc- tion follows the easy direction.
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