THE HYPERFINE STRUCTURE OF Sm149 IN SmFeO3 and SmCrO3

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THE HYPERFINE STRUCTURE OF Sm149 IN SmFeO3 and SmCrO3

M. Eibschütz, R. Cohen, L. van Uitert

To cite this version:

M. Eibschütz, R. Cohen, L. van Uitert. THE HYPERFINE STRUCTURE OF Sm149 IN SmFeO3 and SmCrO3. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-922-C1-923.

�10.1051/jphyscol:19711328�. �jpa-00214361�

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JOURNAL DE PHYSIQUE Colloque C I , suppliment au no 2-3, Tome 32, Fhrier-Mars 1971, page C 1

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THE HYPERFINE STRUCTURE OF Sm149 IN SmFeO, and SmCrO,

M. EIBSCHUTZ, R. L. COHEN and L. G . Van UITERT Bell Telephone Laboratories, Incorporated Murray Hill, New Jersey 07974

Rksumk. - La technique Mossbauer a ete employee pour etudier la structure hyperfine du niveau h 22,5 keV (transition 512-712) du Smi49 dans les perovskites deformees et antiferromagnktiques obliques, SmFeO, et SmC03, de 1,3 ^O^K a 300 OK. A 1,3 OK nous observons une structure hyperline bien resolue. revelant un champ magnttique et un gradient de champ electrique uniques pour tous les noyaux de samarlum avec Herr = 2,09 5 0,04 MOe dans SmFeO! et 2,28 f 0.04 MOe dans SniCrO3. En-dessous de 20 OK la varlatlon avec la temperature de Herr obht a la relatlon :

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Herr = H e f f tanh (412 kT) avec A = 4,8 et 8 ^O^Kpour SmFeO, et SmCr03, respectivement.

Abstract. - The Mossbauer technique has been used to investigate the hyperfine structure of the 22.5 keV level (512-712 transition) of Sm149 in the canted antiferromagnetic distorted perovskites, SmFeO:, and SmCrO,, from 1.3 O K

to 3C0 OK. At 1.3 OK we observed a well-resolved hyperfine pattern, revealing a unique magnetic field and EFG at all samarium nuclei with Herr = 2.09 0.04 MOe for SmFeO3 and 2.28 0.04 MOe for SmCrO3. Below 20 OK, the tem-

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perature dependence of the Heft follows the equation Herr = Herr tanh (412 kT) with A = 4.8 and 8 ^{O K}for SmFeO3 and SmCrO3, respectively.

Introduction. - The rare earth orthoferrites RFeO, x 104

and orthochromites RCr0, (R = rare earth or

~5

are two families of distorted perovskites which show an unusual variety of magnetic properties and have attracted considerable attention recently [I, 21.

The magnetic properties of SmFeO, and SmCrO, are very similar. The compounds are both canted antiferromagnets below the ordering point of the iron (T,(Fe) = 674 OK) and chromium (T,(Cr) = 192 OK) sublattices. Both materials have the c( G, ⁾⁾ spin configuration, for the Fe and Cr ions at low tcmpc- ratures [I, 21. A spin reorientation was observed from the spin configuration G, to G, with increasing temperature, T,(Fe) ²¹450OK [3] and T,(Cr) ⁼38OK [4].

No ordering of the Sm sublattice was observed by magnetic [ I , 41 and specific heat measurements [S, 61 down to 1.3 O K .

The temperature dependence of the iron hyperfine field (proportional to Fe sublattice magnetization) in SmFeO, has already been measured and compared with various statistical mechanical theories [7]. In this work we have used the Mossbauer effect of 22.5 keV transition of Sm149 in order to study the magnetic hyperfine interactions of Sm nuclei in SmFeO, and SmCrO, from 1.2 OK to 300OK.

Experimental. - The sources used were made by a (p, 2 n) reaction in a cyclotron irradiation.

The absorbers were polycrystalline materials prepared by the ceramic method from enriched Sm14', or powder obtained by crushing single crystal material made from unenriched Sm. Ceramic and single crystal materials showed identical spectra. The y rays were detected by a silicon detector and the Mossbauer spectra were taken in a standard transmission geo- metry using a conventional constant acceleration spectrometer. Typical spectra are given in figure 1.

These are the first reported measurements of resolved hfs in these materials [8].

I I I B I

-30 - 2 0 -10 0 10 2 0 3 0

S O U R C E VELOCI-TY (rnn~/s)

FIG. ^1.^-- Mossbauer absorption spectra of 22.5 keV Sml22

y rays in SmCrO3 and SmFe03. The solid lines are leats-squares

fits to the data with the peak positions and intensities as shown.

Discussion. - The well resolved hf spectra observed at low temperatures have been analyzed using a nuclear spin Hamiltonian corresponding to a magnetic field (He,,) and collinear axially symmetric E F G tensor. This Hamiltonian is exact if the 4 f electrons are the sole source of the hf interaction. Using the nuclear parameters [9, 101 I, = 712, I,, ⁼512, g,/gg = 1.300, Q,/Q, ⁼8, gg = - 0.1914, and an M 1 transition, an 18 line hf spectrum is obtained which provides an excellent fit (Fig. 1) t o the observed spectra. The hf fields obtained from the least- squares fits are shown in figure 2. Although the G, spin configuration produces two magnetically ine-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711328

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THE HYPERFINE STRUCTURE OF Sm149 IN SmFeO, AND SmCrO3 C 1 - 923

FIG. 2. - Tcmperature dependcnce ^of Herr of Sm3+ ions in

SmCr03 and SmFe03. The experimental error is about the size of ^thepoints. The solid lines are theoretical curves for H e r r

calculated on the basis of the lowest crystal field doublet of

6H5I2 ground state, split at 1.3 OK by 8 and 4.8 OK respectively.

quivalent Sm sites, they appear to have the same hf parameters. The width of the individual lines is

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2.5 mm.s-', comparable to that obtained in other Sm compounds. Thc isomer shift is appropriate for Sm3+ in insulators.

The Sm3+ ion has a 6H,,, ground state, with the first excited state (6H,l,) at 1.100 cm-'. The mono- clinic crystal field splits the 6 ~ 5 : 2 state into 3 Kramers

doublets, spread by -- 300 cm-l. The doublets are split by A x 10 cm-' by the exchange interaction.

Below -- 20 ^OK, our results are consistent with the expectation that only the lowest Kramers doublet is populated, and that HCff comes from thermal ave- raging within this doublet. Under these conditions, Heff(T) = H!:! tanh (412 kT). Figure 2 shows that this function fits the temperature dependence of the observed data very well, with H$: = 2.17 MOe and A = 4.8

+

0.2 ^O^Kfor SmFeO,, and H::! = 2.29 MOe and A ⁼8

+

0.2OK for SmCt-0,.

Since a constant value of A fits the observed tem- perature dependence of Hcff, we may conclude that the exchange comes predominantly from the Fe (or Cr) sublattice. This conclusion is consistent with the observation [6] that TN = 1.30K in SmAIO,, so that the Sm-Sm interactions would be expected to be much weaker than the Sm-Cr interactions.

The agreement between the experimental points and the theoretical curve is good up to about 10OK for SmFeO, and 200K for SmCrO,. Above these temperatures the mixing of the upper electronic levels with the ground state splitting has to be taken into considcration. The splittings are in very good agreement with the splitting of the lowest doublet calculated from the Schottky anomalies observed in specific heat measurements A ⁼4.7

+

0.3 OK [5]

lor SmFe03 and A = 7.5 -- 8 OK in SmCrO, [6].

The Sm6H,,, ^<(free ion ^)>HCff (i. e., with A g crys- tal field splitting) is 3.3 MOe, and this value actually occurs in Sm metal and SmIG at saturation. If we neglect the coupling of the 6 ~ , 1 2 and higher states by the crystal field, He,, is proportional to the Sm ion moment, so we can write ps, = g,. J. ~:::/3.3 MOe.

This leads to ^p⁼0.45 and 0.49 ^ppfor Sm ions in the orthofcrrite and orthochromite. Using these values for p and A and assuming that the exchange field H,, from Cr (or Fe) acts only on the spin of the Sm3+ ion [Ill, we found H,, = 16 and 24 kOe for SmFeO, and SmCrO,, respectively. These results are similar in magnitude exchange field values obtained [12] in ErFeO, and ErCrO,.

References

[I] See for example WHITE (R. L.), J. AppI Phys., 1969, [8] OFER (S.), et al., Phys. Rev., 1965, 137, A 627.

40, 1061 and refercnces therein. ^{[9] OFEK}^(S.)and NOWIK (I.), Nucl. Phys., 1967, A93,689.

[2] BERTAUT (E. F.), et d., IEEE Trans. Mugn., 1966, [lo] EIBSCHUTZ (M.), COHEN (R. L.) and WERNICK (J. H.),

2, 453. Int. Conf. on Hyperfine Interactions Detected

[3] SHREQOOD (R. C.), REMEIKA (J. P.) and WILLIAMS by Nuclear Radiation, Israel, 1970 (in press).

(H. J.), J. Appl. Phys., 1959, 30, 217. [ I l l WOLF (W, P.) and VAN VLECK (J. K.), Phys. Rev., [4] TSUSHIMA (K.), AOYAGI (K.) and SUGAKO (S.), J. 1960, 118, 1490 ; WHITE (J. A,) and VAN VLECK

Appl. Phys., 1970, 41, 1238. (J. H.), Phys. Rev. Letters, 1961, 6, 412.

[ S ] MAITA (J. P.), private communication. ^[I21WOOD (D. L.), HOLMES (L. M.) and R E M ~ I K A (J. P.), [6] DE COMRARIEU (A.), et al., Acad. Sci. Paris, 1968, Phys. Rev., 1969, 185, 689 ; EIBSCHUTZ (M.),

267, 1169. COHEN (R. L.) and WEST (K. W.), Phys. Rev.,

[7] EIBSCHUTZ (M.), SHTRIKMAN (S.) and T R E V ~ S (D.), 1969, 118, 1932.

Phys. Rev., 1967, 156, 562.