SUCCESSIVE REORIENTATIONS OF IRON MOMENTS IN YbFeO3, TbFeO3 AND ErFeO3

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SUCCESSIVE REORIENTATIONS OF IRON MOMENTS IN YbFeO3, TbFeO3 AND ErFeO3

M. Belakhovsky, J. Chappert, T. Rousokov, J. Sivardière

To cite this version:

M. Belakhovsky, J. Chappert, T. Rousokov, J. Sivardière. SUCCESSIVE REORIENTATIONS OF IRON MOMENTS IN YbFeO3, TbFeO3 AND ErFeO3. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-492-C1-493. �10.1051/jphyscol:19711162�. �jpa-00213983�

JOURNAL DE PHYSIQUE Colloque C I , supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 492

SUCCESSIVE REORIENTATIONS OF IRON MOMENTS IN YbFe03, TbFeO, AM) ErFeO,

M. BELAKHOVSKY, J. CHAPPERT, T. ROUSKOV (*), J. SIVARDIERE Centre d3Etudes NuclBaires, Cedex 85, Grenoble, France

R6sum15. - Nous avons observe par effet Mossbauer diverses rkorientations des moments du fer dans les ortho- ferrites YbFe03, TbFeO3 et ErFe03 : la reorientation bien connue Gx -+ G, a 8 OK, 7 OK et 90 OK respectivement, et une deuxikme reorientation G z ³ Gx a 3 OK dans TbFeO3 et G, 3 Gxy B 4 OK dans ErFeO3.

Abstract. - We report the observation with the Mossbauer technique of several reorientations of the Fe3+ spins in YbFeO3, TbFe03 and ErFe03 : the well known Gx + G B reorientation at 8 OK, 7 OK and 90 OK respectively, and a second reorientation G, -+ Gx at 3 OK in TbFeO3 and G Z ^-+ Gxy at 4 OK in ErFeO3.

The reorientations of the Fe3+ spins in the ortho- ferrites YbFeO,, TbFeO, and ErFeO, have been investigated with the Mossbauer technique. The sam- ples are either single crystals doped with 57Co and used as sources, or natural absorbers partially oriented along the c axis by allowing a mixture of powder and epoxy to harden in a magnetic field at room tempe- rature. Information concerning the angle between the

~ e spins and the direction of the y-rays is derived ~ + from the Am = 0 line intensities.

At high temperature, the Fe3+ spins are ordered according to a G, mode. A first reorientation G, ^-t G, is observed at T , = 8 OK, 7 OK and

-

90 OK in YbFeO,, TbFeO, and ErFeO, respectively ^(l) [l].

When the temperature is further decreased, the change in the Am = 0 line intensities can be interpreted as due t o a second reorientation G, + G, at 3 OK in TbFeO, and G, G,, at 4 OK in ErFeO,. This is the first unambiguous experimental evidence of transitions which where inferred from symmetry properties [2,6].

TbFeO,. - The first reorientation G, + G, is a consequence of the Fe-Tb coupling. When the T b sub- lattice orders cooperatively according to the A, G, structure at 3 OK [2,7], the Fe-Tb coupling vanishes since the modes A, G, and G, or G, do not transform according to the same representation of the group Pbnm ; consequently it can be predicted that the

~ e , + spins should return to X, which is compatible with the observed variation of the Am = 0 line inten- sities as shown in figure 1 (the same process occurs at 4.2 OK with a field applied along the y axis which induces the C, F, structure of Tb [6]).

YbFeO,.

-

The Yb-Yb interactions are too weak to be responsible for a cooperative ordering of the Yb sub-lattice [3], and the G, mode of the Fe3+ spins is stable at very low temperature.

(*) On leave from : Institut de Physique, Acadkmie des Sciences Sofia, Bulgarie.

(1) Evidence for the Gx -+ Gz reorientation in TbFeOs pow- ders has already been given [2,3] ; however no evidence for any spin reorientation (G= -+ G, or Gx -+ Gy 141) is found in our TbFe03 single crystals (we have checked this point by neu- tron diffraction) it is likely that l % or 2 % lead impurities from the flux prevent the reorientation to take place ; such an impurity ratio is not found in YbFeO3 single crystals [51,

where the reorientation is observed.

I I I I I 1

-10 -5 0 5 10 VELOCITY

(mm/s)

FIG. 1. - Mossbauer spectrum of TbFeO3 at 10 OK, 4.2 ^{O K} and 1.5 ^{O K .} The reorientation is indicated by the change in the Am = 0 lines (lines 2 and 5). Since the magnetic orientation of the powder is not complete, the change is smaller than pre-

dicted.

ErFeO,. -The Er sub-lattice orders at 40K according to the G, mode : this ordering is not coope- rative, but is induced by the iron sub-lattice. This implies the existence of a G, component of the Fe3+

spins and consequently a second reorientation G, -+ G,, [6] must take place. This is in agreement with our observations.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711162

SUCCESSIVE REORIENTATIONS OF IRON MOMENTS IN YbFeOs, TbFeOs AND ErFe03 C 1

-

⁴⁹³

Let K , py and pz be the moments of the Er3+ ions the G, phase is stable at high temperature in the X, y and z directions (which are the principal

directions of the g-tensor of the Er3+ ions [S]) ; H,, (T, < T < TN)

,

H; and H, the field acting on the Er3+ ions in the the G, is stable between T, and T2, and the G, ~ h a s e G,, G, and G, phases (in the G, phase, the field is is stable at low temperature (T < T2)> with :

H: on the ions 1 and 2,

-

H: on the ions 3 and 4).

kTl =

p1 ^H:

- 112 If the G mode is oriented in the 0, Q, direction ^(X being

the polar axis), and if Kt, and K,, are the anisotropy and 2 Kl, constants of the iron sub-lattice, the free energy of the

H L ~

^{- P: H:}

systems can be written in a high temperature appro- kT -

ximation 191 : - - - 2(Kly - Ktz) ^'

F(8, Q,) = (K1, cos2 Q,

+

K,, sin2 Q,) sin2 8

-

+

HL2 sin2 8 cos2 Q,) - kTLog 2

.

The equilibrium condition

gives : 8 = O0 (G,) ; 0 = 900 and Q, = O0 (G,) ; and 8 = @ = 900 (G,). If K,, > K,, and

The transitions at T , and T2 are of first order. If the fourth order anisotropy terms

K2y G;

+

^{KZ, G:}

+

^{KZ,, G:G}

2

are introduced in F(6, Q,), each transition is composed of two second order transition [g], implying a rotation and not a jump of the Fe3+ spins.

Let us remark that the G, phase does not appear in ErCrO, [l01 ; the above model does not explain why the low temperature phase in ErFe03 and ErCrO, is G,, and not G, : a Mossbauer experiment in a ErFeO, single crystal is under way to determine the exact Fe3+ spin direction and the order of the transition at 4 OK.

We thank J. C . Picoche for his help for the sample preparation.

References

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