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Submitted on 1 Jan 1971
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NEUTRON DIFFRACTION STUDY OF SOME RARE EARTH IRON GARNETS RIG (R = Er, Dy, Yb, Tm)
F. Tchéou, E. Bertaut, H. Fuess
To cite this version:
F. Tchéou, E. Bertaut, H. Fuess. NEUTRON DIFFRACTION STUDY OF SOME RARE EARTH
IRON GARNETS RIG (R = Er, Dy, Yb, Tm). Journal de Physique Colloques, 1971, 32 (C1),
pp.C1-202-C1-204. �10.1051/jphyscol:1971162�. �jpa-00214489�
JOURNAL DE PHYSIQUE Colloque C I, supple'ment au no 2-3, Tome 32, Fe'vrier-Mars 1971, page C 1 - 202
NEUTRON DIFFRACTION STUDY
OF SOME RARE EARTH IRON GARNETS RIG (R = Er, Dy, Yb, Tm)
F. TCHEOU, E. F. BERTAUT and H. FUESS C. E. N.-G., rue des Martyrs, Grenoble, France
Rbume. - Dans les ferrites grenats de Terres Rares les moments magnetiques des ions Fe3+ et Terres Rares sont gkneralement orientes selon [lll]. Cependant I'etude de ErIG, DyIG et YbIG B basse temperature met en evidence une deviation des moments magnetiques de Terres Rares par rapport A la direction [Ill]. Nous avons determink les structures magnetiques ainsi que les angles de deviation de ces grenats a I'aide des valeurs du tenseur d'anisotropie g.
Comme le diagramme de TmIG 1,5 OK ne contient aucune raie de surstructure magnetique, nous dtduisons que l'angle de dtviation est infkrieur a 15O.
Abstract. - In rare-earth-iron garnets, the Iron and rare-earth moments are generally oriented along [Ill]. Devia- tion from [I 111 at low temperature have been observed for ErIG, DylG and YbIG.
Following the values of the magnetic g-tensors we have elaborated models for the magnetic structures of these garnets.
The study of TmIG showed no evidence of magnetic superstructure lines at 1.5 OK.
This result leads to the conclusion that a canting angle of rare earth moments in TmIG, if any, could be smaller than 15O [Ill].
Introduction. - Since their discovery [l] the rare- earth iron garnets have been studied extensively in the world.
Magnetic measurements [2] and neutron studies (3a, 3b) confirmed ferrimagnetic ordering along the [111] direction at room temperature. At liquid helium temperature, however, a non-collinear component of the rare earth moment was found in HoIG (3b) and in TbIG (4) giving rise to superstructure lines in the neutron diffraction diagram.
Wolf et al. (5) pointed out that at low temperature and without applied field the rare-earth moments cant away from the [ I l l ] direction but remain in principal planes of the g-tensors of the individual rare-earth ions.
We describe here the results of our neutron diffrac- tion studies on ErIG, DyIG, YbIG and TmIG at low temperature.
Experimental results. - Neutron diffraction mesu- rements were taken on polycristalline ErIG, DyIG,
,
Il o 15 2 o e
+
FIG. la. - Neutron diffraction diagram of ErIG at room temperature.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971162
NEUTRON DIFFRACTION STUDY O F SOME RARE EARTH IRON GARNETS RIG C 1 - 203
FIG. lb. - Neutron diffraction diagram of ErIG at 1.5
O K .YbIG and TmIG at room temperature and 1.5 OK and also at 0.4 OK for YbIG.
The neutron diffraction pattern of DyIG at 1.5 OK shows the same superstructure lines as that of ErIG.
Typical results are shown for ErIG in figure 1. Only one superstructure line indexed as (200) was found for YbIG. N o superstructure lines were observed for TmIG.
Superstructure lines, forbidden by the space group, but still conserving a bcc-lattice, indicate the decompo- sition into six magnetically inequivalent sublattices of the rare-earth ions. Due t o the fact that the local exchange field is parallel to the ternary axis [ I l l ] , these sublattices reduce to two species C j and C>
( j = 1, 2, 3).
Analysis. - Following the analysis of Wolf et al. (5) we can specify three possible rare-earth sublat- tice arrangements according to the relative magni- tudes of the components of g-tensors. In the first case when g, > g, > gy the magnetic moments of the two inequivalent sublattices C , in $ 0 and C ; in $ 0 $ will be turned towards [loo], in the second case g, > gy > g, they would incline towards [Oll]
and in the last case g, > g, > gy the moments of C , are turned to [Oll] and that of C; t o [loo]. A typical rare-earth sublattice spin arrangement is shown in figure 2 for the case of YbIG.
Discussion and conclusion. - ERBIUM IRON GAR- NET (ErIG). - Orlich and Hiifner (9) have deter- mined the order of magnitude of the g-tensors g, > g, > gy in ErGG and have shown the strong
J x
FIG. 2. - The rare-earth sublattice spin arrangement of YbIG at 1.5
O K .anisotropy of the exchange interaction and the simi-
larity of the g-factors in ErIG and ErGG for the
'Igl2 state. With the same assumption for the ground
state in ErIG and ErGG we worked out a model
for the magnetic structure which was fitted to the
experimental data. Two canting angles of 140 ( C , )
and 420 (C;) between [l 1 11 and the moments of the
respective sublattices were obtained. The magnetic
moments are found to be 6.7 pB and 4.4 pB (average
C I - 2 0 4 F. T C H ~ O U , E. F. BERTAUT AND H . FUESS
5.44 p,) in good agreement with the moments result- ing from magnetization measurements of Harrison et al. (6) (4.95 p,), Geller et al. (7) (5.1 p,) also with the results of Hufner et al. (8), Orlich and Hufner (9) who reported 5.45 p, and 5.0 p, respectively.
DYSPROSIUM IRON GARNET (DyIG). - The neutron diffraction pattern of DyIG at 1.5 OK shows the same sublattice reflections as that of ErIG leading to essen- tially the same magnetic structure with angles of 130 (C,) and 550 (C;). So we would have g, > g, > g, or a t least g, G, > g, G, > g, G,.
YTTERBIUM IRON GARNET (YbIG). - In the neU- tron diffraction pattern a t 1.5 OK and 0.40K only one superstructure line, indexed as (200), is found.
Calculation of magnetic intensities shows that for angles of about 200 only this line will be present, all the other superstructure lines being unobservable.
A fit gives best agreement for angles of 220 and mo- ments of 1.9 p, for both sites. Our findings are riot precise enough to distinguish different magnitudes of the moments of C, and C;. The canting angles are in good agreement with the calculation of Wolf et aI. [5] and the moment of 1.9 pB leads to a compo- nent parallel to [I 111 of 1.7 pg in agreement with magnetic measurements of Geller et al. [7] who repor-
ted 1.66 p, and with calculations of Henderson and White (1.82
11,)[lo].
THULIUM IRON GARNET (TmIG). - In the 1.5
OKpattern we have not observed any superstructure line.
Assuming a magnetic moment of 5.0 p, for the iron ions we refined the moment of thulium and found 1.28 pB with experimental [l l ] and 1.30 p, using theo- retical [I21 form factor tables for iron. This moment is in quite good agreement with Cohen's [13] average moment of 1.22 pt, and with the result of magnetization measurement of Geller et al. 171 who found 1.27 p ~ . Canting angles smaller than 150 don't give rise t o observable intensities of superstructure lines. Thus the canting angles, if any, should be smaller than 150.
The findings of Pickart et al. 1141 in their paper on
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