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SPIN REORIENTATION IN RARE EARTH
ORTHOFERRITES
G. Durbin, C. Johnson, L. Prelorendjo, M. Thomas
To cite this version:
G. Durbin, C. Johnson, L. Prelorendjo, M. Thomas. SPIN REORIENTATION IN RARE
JOURNAL DE PHYSIQUE Colloque C6, suppl&ment au no 12, Tome 37, Dkcembre 1976, page C6-621
SPIN
REORIENTATION IN RARE EARTH ORTHOFERRITES
G. W. DURBIN, C. E. JOHNSON, L. A. PRELORENDJO and M. F. THOMAS Oliver Lodge Laboratory, University of Liverpool, Liverpool, L69 3BX, England
Rhumb.
-
L'effet Mossbauer etait employ6 pour dkcouvrir la reorientation de spin des ions Fe3+ dans des Bchantillons de cristal simple des orthoferrites de terres fares YFe03, GdFeO3 et EuFeO3. La rkorientation de spin etait produite par moyen des champs magnktiques jusqu78 concurrence de 6 T le long de l'axe a du cristal (l'axe de I'antiferromagnetisme des ions Fe3+). Les champs employks afin de produire la rkorientation complhte dans YFeO, B 293 K et 4,2 K ktaient6,5 f 0,2 T et 7,7 & 0,2 T respectivement. Dans GdFeO, A 293 K, 77 K, 4,2 K et 2 K les champs necessaires Btaient 7,6 -1: 0,2 T, 8,l & 0,2 T, 1,2
+
0,2 T et 0,7 f 0,l T et dans EuFe03 8 4,2 K, 6,7 & 0,2 T. On interprkte les rbsultats en fonction de l'effet de l'action rtkiproque entre la terre rare et le fer sur la reorientation de spin.Abstract. - The Mossbauer effect was used to detect spin reorientation of Fe3+ ions in single crystal samples of the rare earth orthoferrites YFe03, GdFeO3 and EuEeO3. Spin reorientation was induced by applying magnetic fields of up to 6 T along the crystal a axis (the axis of anti- ferromagnetism of the Fe3f ions). The applied fields necessary to cause complete reorientation in YFeO3 at 293 K and 4.2 K were 6.5 f 0.2 T and 7.7 5 0.2 T respectively. In GdFeO3 at 293 K, 77K,4.2Kand2Kthefieldsrequiredwere7.6 &0.2T,8.1 f 0.2T,1.2 & 0.2Tand0.7 & 0.1 T
and in EuFe03 at 4.2 K, 6.7
+
0.2 T. The results are interpreted in terms of the effect of rare earth-iron interaction on spin reorientation.1. Introduction.
-
The orthorhombic unit cell of the rare earth orthoferrites (RFeO,) is illustrated in figure 1 which also shows the arrangement into perovs- kite pseudo cells. Magnetic interaction between Fe3+FIG. I.
-
The structureRFe03 (oxygen atoms omitted) showing the orthorhombic cell and the pseudocubic subcells of the Rand Fe ions. The direction of the applied field and y rays are related to the axial directions as indicated.
ions results in antiferromagnetic ordering below a Niel temperature of
-
650 K but slight canting of the ions gives rise to weak ferromagnetism in a direction perpendicular to the antiferromagnetic axis. At room temperature in all rare earth orthoferrites except SmFeO, the antiferromagnetic axis lies along the crystal a axis with the weak ferromagnetism along the caxis. However, in the orthoferrites TbFeO,, ErFeO, and TmFeO, the axes of antiferromagnetism and weak ferromagnetism begin to rotate as the tempera- ture is decreased below a critical temperature TI and rotation continues with falling temperature until at temperature T, the antiferromagnetic axis is along the c
axis with weak ferromagnetism along the a axis [I]. The change in anisotropy energy causing this rota- tion has been ascribed to the interaction of the Fe3+ system with the system of rare earth R3+ ions 12, 3,4].
The detail of this behaviour depends on the individual R3+ ion. The application of a magnetic field along the antiferromagnetic axis of the Fe3+ system acts to destabilise it owing to the higher magnetic energy of this configuration compared to that when the spins are perpendicular to the applied field. The anisotropy of the Fe3+ system can thus be measured against the field necessary for complete reorientation. Since the anisotropy energy depends on the R3+-Fe3+ interac- tion this method can balance the interaction against the change in reorienting field.
The anisotropy of the Fe3+ system with no rare earth-iron interactions present is investigated in YFeO, with the diamagnetic Y + ion on the rare earth
site [5]. With these results as a <( background >> - spin reorientation without R3+-Fe3 + interaction -
the results of measurements on GdFeO, and EuFeO, are discussed in terms of the rare earth-iron interaction.
2. Experimental procedure. - Orthoferrite samples were prepared from single crystals grown with lead oxide based flux and ultra pure materials. Slices were
C6-622 G. W. DURBIN, C. E. JOHNSON, L. A. PRELORENDJO A N D M. F. THOMAS
cut with the c axis normal to the plane of the sample, pattern displayed by the intensity of the Am = 0 lines these were thinned to an equivalent thickness of 10- in the spectra. The relative intensities of a thin absor- 20 mg cm-2 of natural iron and set in epoxy. The
ber, six-line Mossbauer spectrum are 3 :
spin direction in the plane of the sample was assigned
from morphological- examination and verified by 1 where a is the angle between the spin'direction a i d Mossbauer absorption using a source of polarised the direction of propagation of the gamma rays and 14.4 keV gamma rays [6]. the intensity of the Am = 0 lines are given by the mid-
A conventional Mossbauer spectrometer was used dle expression.
with a 100 mCi s7Co in rhodium source and an An alternative method of detecting this orientation argon/methane filled proportional counter to detect the
radiation. After amplification and selection of the signal of the 14.4 keV radiation, the counts were gated into a 256 channel subsection of a 4 096 channel analyser used in the multiscaler mode to provide a velocity spectrum.
Fields of up to 6 T were applied to the samples
using a superconducting magnet. Measurements were made with the various magnetic fields applied at sample temperatures of 2 K, 4.2 K, 77 K and 293 K. The crystal slices were mounted inside the magnet with the field direction coaxial with the antiferroma- gnetic vector (a axis) and with the weak ferromagnetic moment (along the c axis) perpendicular to the field direction. The source-detector system was arranged such that the gamma ray direction was perpendicular to the field direction, i. e. the propogation of gamma rays was along the c axis. With this arrangement, deviation of the antiferromagnetic vector from the a axis was observable from the change in the radiation
is by measurement of the different effective fields
~ $ 1 , )
and Hi&), at the nuclei of the two antiferromagnetic
sublattices. This is caused by the applied field adding to the hyperfine field of one sublattice and subtracting from that of the other. An average value for the angle 0 between the spin direction and field direction
is given, for fields less than the critical field required for reorientation, by
where
~ $ 2
and H:f"f' are the measured effective fields and Ha is the applied field.3. Results.
-
A series of spectra taken at fields up to 6 T on a 20 mg cm-2 sample of YFeO, at 4.2 K is shown in figure 2. The spectra exhibit aprogressive reduction in the intensity of the Am = 0 lines and a splitting of the 6 line pattern into two
Velocity
t m m
s-'1 V e l o c ~ t y (mm s-'1FIG. 2.
-
Mossbauer spectra of 57Fe in YFeOj at 4.2 K with increasing magnetic field. The full line is a least-squares fit of two series of 3 pairs of Lorentzian lines to the experimental data. The approximate orientation of the spins to the appliedSPIN REORIENTATION IN RARE EARTH ORTHOFERRITES C6-623
series of 6 lines each associated with one sublattice.
The spectra were analysed to provide values of the angle of orientation 0, the effective fields at nuclei of each sublattice, quadrupole splitting and isomer shift.
In the series of spectra illustrated the method of sublattice splitting was used to evaluate 8 as the splittings are sufficiently large to be fitted with accu- racy and because the thickness of this sample makes the intensity ratio of outer to inner lines deviate appreciably from 3 : 1. With thinner (- 10 mg ~ m - ~ ) samples of GdFeO, and EuFeO, this intensity ratio approached 3 : 1 and for GdFeO, at 2 K and 4.2 K the method of intensity ratios was used to obtain 0 as the small fields involved caused little splitting of the sublattices. When both methods were appropriate as in EuFeO, at 4.2 K values of 8 obtained by the two methods agreed to within
+
3O. The rotation of the antiferromagnetic axis (as sin 0) is plotted against0
I/'
1 , I I I 10 2 4 6 8
APPLIED FIELD. IT).
FIG. 3. - Field dependence of the orientation of Fe3f spins in YFe03 with respect to the field direction at 4.2 K and 293 K . ~
FIG. 4. -Field dependence of the orientation of Fe3f spins in GdFe03 with respect to the field direction at 4.2 K and 293 K.
Applied field ( T )
Fig. 5. - Field dependence of the orientation of Fe3+ spins in EuFe03 with respect to the field direction at 4.2 K.
applied field for the YFeO, sample at 293 K and
4.2 K in figure 3. Similar graphs for GdGeO, at 293 K and 4.2 K are shown in figure 4 and for EuFeO, at 4.2 K in figure 5. Values of the critical applied field
HZ
necessary to cause complete reorientation (sin8 = 1) at temperature T were taken from these and similar plots and are collected in table I.
Critical fields H,, required to cause complete reorientation of ~ e , + from the crystal
a axis to the c axis
Sample
-
YFeO, YFeO, GdFeO, GdFe03 GdFeO, GdFeO, EuFeO, Temperature (K)-
Hc* (T)-
293 6.51-
0.2 4.2 7.71-
0.2 293 7.6+
0.2 77 8.11-
0.2 4.2 1.2+
0.2 2 0.7+
0.1 4.2 6.7 _+ 0.2The values of isomer shift and hyperfine field and quadrupole splitting at zero applied field are in good general agreement with those in the literature [7].
4. Discussion.
-
In a pure (non canted) antifer- romagnet an applied field along the axis of antiferro- magnetism causes a sharp spin reorientation through 90° at a critical field H: given byHZ
= (2 HE HA)'I2where HE is the exchange field of the antiferromagnetic
system and HA is the anisotropy field. When a field is
applied along the antiferromagnetic axis of a canted antiferromagnet the reorientation occurs continuously
18, 51 reaching completion at a field H: given by
where HD is the Dzyaloshinsky field that causes the
C6-624 G. W. DURBIN, C. E. JOHNSON, L. A. PRELORENDJO AND M. F. THOMAS
In YFeO, the diamagnetic Y3+ ion occupies the
rare earth site and no interaction with the Fe3+ system occurs. The resuIts show that the bare Fe3+ system
has an anisotropy which increases by
-
12%
as the temperature decreases from 293 K to 4.2 K.In GdFeO, the Gd3+ ions ('s,~,) possess magnetic moments and interact with the Fe3+ system. The Gd3+ ions order themselves at a temperature of 1.5 K 191 thus in the temFerature range used in these experi- ments the Gd3+ ions behave paramagnetically. The applied field aligns these ions to give a net moment
<
p >Gd along the crystal a axis that can be estimatedfrom the Brillouin function B7!,(H/T) [lo].
The total effective field required to cause complete reorientation of the Fe3+ spin system at temperature T,
H:, contains contributions from the applied critical
field at this temperature,
HZ
and from the exchangefield of the Gd3+ ions. The value of H: (the field that
would be required for complete reorientation in GdFeO, in the absence of Gd3+-Fe3+ interactions) was obtained by scaling the YFeO, results to the value of in GdFeO, where the value of
<
p>,,
is small. The effect of the iron-rare earth interactions are then represented in experimental terms byIn figure 6 the graph of AH against the mean moment
<
p > G d indicates that the interaction between theparamagnetic Gd3' system and the antiferromagnetic Fe3+ system is proportional to
<
p>,,
and can be represented bywhere the molecular field constant of the Gd3+ ions, 11 = 2.3 T per Bohr magneton.
Preliminary measurements on EuFeO, show that at
4.2 K the observed value of
~2~
is less than that expected for the bare Fe3+ system as measured in YFeO,. This difference between YFeO, and EuFeO, at 4.2 K may be ascribed to a weak rare earth-iron interaction in EuFeO,. The ground state of the Eu3' ion(7F,)
is diamagnetic but a J = 1 state at an excitation of 255 cm-' is mixed into the ground state by the exchange field of the Fe3+ system and the applied field. This results in a small net moment for the system of Eu3+ and a weak rare earth-iron interac- tion. At higher temperatures the excited state of Eu3+FIG. 6. - Graph of AH, representing the effect of GdjC-Fe3+ will become more populated changing the mean interactions on the reorienting field, versus the mean Gd3+
<
P >E", the rare earth-ion interaction and cons+moment
<
N >ad. quently the reorienting field.References
[I] GORODETSKY, G., LEVINSON, L. M., SHTRICKMAN, S., TREVES, D. and WANKLYN, B. M., Phys. Rev. 187 (1969) 637.
[2] YAMAGUCHI, T., J. Phys. Chem. Solids. 35 (1974) 479.
[3] BIDAUX, R., BOUREE, J. E. and HAMMANN, J., J. Phys. Chem. Solids 35 (1974) 1645.
[4] BIDAUX, R., BOUREE, J. E. and HAMMANN, J., J. Phys. Chem. Solids 36 (1975) 655.
151 DURBIN, G. W., JOHNSON, C. E. and THOMAS, M. F.,
J. Phys. C : Solid State Phys. 8 3051.
[61 GRANT, R. W., ArP Con$ A o c . Magnetism and Magnetic Materials no. 5 part 2, eds C. D. Graham Jr. and
J. J. Rhyne (New York : AIP) 1395, 1971.
[7J EIBSHUTZ, M., SHTRICKMAN, S. and TREVES, D., Phys.
Rev. 156 (1966) 562.
[8] JACOBS, I. S., BURNE, H. F. and LEVINSON, L. M., J. Appl. Phys. 42 (1971) 1631.
[9] CASHION, J. D., COOKE, A. H., MARTIN, D. M. and WELLS, M. R., J. Phys. C : Solid State Phys. 3 (1970) 1612.
