Spectral studies of magnetic cuprates R2Cu2O5 (R=Yb, Ho, Dy)

PhysicsLettersA 159 (1991) 187—192 North-Holland

PHYSICS LETTERS A

Spectral studies of magnetic cuprates R2Cu2O5 (R=Yb, Ho, Dy) M.N. Popova and LV. Paukov Institute ofSpectroscopy, USSR Academyof Sciences, 142092 Troitsk, Moscow region, USSR Received I July 1991; accepted for publication 2 August 1991 Communicatedby V.M. Agranovich 3~ion or Er3~probe have been measured in cuprates R High resolution optical absorption spectra of an intrinsic R 2Cu2O, (R = Yb, Ho, Dy) related to hjgh-T~superconductors of the 123 type. The magnetic transition temperatures, energies ofcrystal field levels, splittings of the ground state doublet have been determined. Possible directions of3~ the probe copper in them. magnetic moments in an ordered state of R2Cu2O, (R=Y, Tb—Lu) are discussed on the basis of the spectra ofan Er

1. Introduction In a recent publication [1], we have communicated the results of spectral studies of the magnetic cuprates R2Cu2O5 (R = Y, Th, Er, Tm, Lu) related to the high-Ta superconductors of the 123 type. In this Letter, we present the results on R2Cu2O5 with R=Yb, Ho, Dy and discuss the magnetic structure of the copper subsystem in an ordered state in the cuprates R2Cu2O5 on the basis of the spectra of an 3~probe in them. ErThe so-called “blue phases” R 2Cu2O, with R = Y, Tb—Lu, Se, In crystallize in the orthorhombic space group Pna21. Their structure has been analysed in detail in ref. [2]; references to the original works can also be found there. The copper—oxygen pyramides Cu05 are joined by the common edges into Cu (1 )Cu (2)08 dimers which, in their turn, form zigzag chains along thea-axis. Thesechains are linked by oxygen atoms forming goffered ab planes, where the Cu—Cu distance along the b-axis (~3.5 A) is close to the largest one in a chainthe along the a-axis (~3.2 A). The distances between planes are considerably larger (~6.5 A). There are two nonequivalent fourfold low symmetry (C 3~ 1) positions ions in a unit cell. R( 1)06 and R(2)0 for the R 6 distorted octahedra are united in a three-dimensional network. Yb 2Cu2O5, Ho2Cu2O, and Dy2Cu2O5 romagnets with metamagnetic behavior.areAnantiferadditional phase transition was observed in Yb 2Cu2O,

[2—4].Neutron diffraction data at 4.2 K show a doubling of a magnetic unit cell in Ho 2Cu2O5 along the b-axis and in Yb2Cu2O, along the b and c directions [5—7].As for the orientation of the magnetic moments contradictory data are available: according to ref. [6], in Ho2Cu2O5 both holmium and copper magnetic moments are aligned along the c-axis PH0 ~C, 4uc~IIc,in Yb2Cu2O5 p~~Ila, PYb has x and y cornponents, while according to ref. [7], in Ho2Cu2O5 p~~IIa, #‘Ho hasin x,the y, ab z components, in Yb2Cu2O5 p~~IIb, p~lies plane.

2. Experiment X-ray single phase polycrystalline Yb2Cu2O5, Ho2Cu2O5 and Dy2Cu2O5 samples were synthesized by solid state reactions from oxides in air at 1030— 1050°C.Samples with 1 at% of erbium introduced as a probe were prepared in the same way. High resolution diffuse transmittance 3~spectra intrinsiccorresponding ion or Er3~ to f—f were transitions the RKusing the Fourier transprobe taken atin2—130 form spectrometer BOMEM DA3.002. The low symmetry crystal field completely lifts all 3~ion. Krabut Kramerofdegeneracy of a an freeodd R number of mersthedoublets the ions with 3~,Dy3~,Er3~)are further split by electrons interactions (Yb magnetic the spectral lines are split into —

a maximum of four components in a magnetically

0375-960l/91/$ 03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

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PHYSICS LETTERS A

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3. Experimental results 3.1. YB~Cu 2O5 The optical spectrum of Yb 2Cu2O5 single f—ftransition in the Yb3~ion 2F is due 2F to the 712—+ 512 and in a paramagnetic state consists of three pairs oflines near 10250, 10485 and 10670 cm—’. The lines of a pair correspond, probably,For toeach the transitions two nonequivalent positions. position theinlevel 2F 512 is split by a crystal field into three Kramers doublets. We failed to observe the transitions from the excited levels plet up to 130 K. of the ground crystal field multiFig. 2a shows the transformation of the low frequency absorption line of Yb3~when changing the temperature. At the elevated temperatures the line is broad (12 cm—’), it begins to narrow at about 24 K, and a poorly resolved structure appears at 9—8 K, probably due to the splitting of the Yb3~Kramers doublets. At the temperature of 6.8 K of the sharp symmetric peak in the heat capacity suggesting the first order phase transition [4] the spectrum changes suddenly in a temperature interval ofless than 0.7 K, where the spectrum is a superposition of the spectra before and after the transition. It is natural to ascribe such a behavior to the coexistence of the domains of two phases in the vicinity of the first order spin-reorientational phase transition [8]. Different orientations ofthe magnetic moments in two phases lead, 188

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3~absorption line Fig. 2. dependences of (a) the Yb 2FTemperature 2F,, 3~probe absorption IA 2) in Yb2Cu2O,, (b) the Ernotation refers to the lines( IB,112-. j~(~II5,2—.~I,3,2). The underlined

second position in the structure, (c) the halfwidth Si’ (x) and the maximum position v 3~probe spectral line of (b) (~)and 0 (o) for the Yb spectral line of (a), the halfwidth splitting i~p Er frequency component (0). the 6vof of the its low

provided the exchange is anisotropic, different 3 + Kramers doubletsto and, hence, splittings of spectral the Yb patterns. to different Becai~isethe lines in the spectrum ofYb3~are broad but their splittings small, it is difficult to extract the splittings from the spectrum. In fig. 2c the temperature dependences of the halfwidth of the line of fig. 2a and ofthe position of its maximum are presented. The low temperature steps are due to the already discussed sudden change of the spectral shape in the region of the first order phase transition at 6.8 K. The smooth change of the maximum position in the interval 18—11 K is, most likely, due to the smooth evolution of the exchange splittings and, as a result, of the line shape when a magnetic ordering establishes. The X-type heat capacity anomaly typical for the second order phase transition was observed in Yb 2Cu2O5 at 15 K [4]. The onset of magnetic ordering at TN =15 ±1 K is

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PHYSICS LETFERS A

confirmed by the temperature dependence of the line splitting Av clearly seen in the spectrum of the Er3~ probe in Yb 2Cu2O5 (fig. 2b) ~. This temperature was determined as the~abscissaof the point of inflection in the ~ sidual splitting ii ( T) curve due to(see short-range fig. 2c). The order“tail” remains of reat higher temperatures. Two jumps are seen in the dependence 8 v ( T) of the3~ halfwidth ofsharp a component in probe. The narrowing the6.8 split of the Ercomponents of the line is also at K line of separate well seen in the spectrum of Yb3~(fig. 2a). These facts together with the absence ofYb3~line splitting in the vicinity of TN force us to assume that the copper magnetic system orders first (at 15 K) while the ytterbium system remains unordered till 6.8 K when its ordering occurs as a first order phase transition, in agreement with neutron diffraction data [7]. Between 15 and 6.8 K the Yb magnetic system gets weakly polarized by the Cu system, at 6.8 K Yb— Yb anisotropic interactions causes its ordering in some different directions together with a spin-reorientational transition in the Cu system.

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The magnetic cell doubling in Yb 2Cu2O, [5—7] manifests itself by the appearance of new spectroscopically nonequivalent positions: note the doubled number3~of components probe in fig. 5. in the split spectral line of the Er 3.2. Ho 2Cu2O5 Fig. 3a presents the low frequency part of the ~I8—+’I7 transition in Ho 2Cu2O5 at different temperatures (the high frequency lines are dramatically broadened even at low temperatures due to the phonon relaxation). The spectral lines are split at low temperatures. This splittinggoes to zero at about 12.7 K, close to the temperature TN of the antiferromagnetic ordering in Ho2Cu2O5 [2—4]. The lines strongly broaden and shift in the vicinity of TN (see fig. 3b). As the relative intensities ofthe components in the split lines IA, IA do not depend upon the temperait would be in natural to assume the of existence of ature quasidoublet the excited state the non‘~‘

Spectral lines due to transitions in two nonequivalent posi.. tions Erl and Er2 are superimposed. ~v is some splitting averaged over the two positions (see also ref. [1]).

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Fig. 3. Temperature dependences of (a) the low frequency part of the spectral transition 2I8_*517 in components Ho2Cu2O5, the splitting and ofthe halfwidth Si’ ofone of (X). iti’ (~0~ ) and central frequency v, its (o) forthe line (b) near 5180cm—’

3 + which is split by an effectivefield Kramers ion Ho ordered state. But the different in a magnetically splittings of IA and hA lines (1.5 and 2.0 cm’ at 10K) are in contradiction with such an assumption. Another possibility is that there appear new nonequivalent positions due to the doubling of a magnetic unit cell in Ho2Cu2O5 [5—7].In one of them the distance between the ground and the lowest excited crystal field level of Ho! grows from 6.5 to 11 cm—’ in the course of a magnetic ordering while in the other it remains practically unchanged. For the second structural position Ho2 the situation is similar. In favor of the appearance of new nonequivalent3~probe positions in Ho2Cu2O5 is the structure ofEach the spectral lines at low temperatures. Er consists of a greater number ofcomponents than line is to be expected for two nonequivalent positions in the structure (see fig. 5).

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7 October 1991

Table 1 Ordering temperatures for R

3~ion in two positions. 2Cu2O, and energies ofthe lowest crystal field levels El, E2 for the R

R

Yb Ho Dy

T~(K)

El/E2 (cm’)

ref. [4]~)

ref. [2]

15, 6.8 12.3 9.5

13/8 12 10/6

b)

ref. [3]

our data

b)

15, 6.8 12.7 10.6

15/7.5 13 11

>500 6.5 30/22 54/52 103/104 155/175 200/230 310/340

~ Heat capacity data. b) From the maximum of the magnetic susceptibility x. The value after the backslash is the temperature of the additional anomaly in X(T).

3.3. Dy~Cu~O 5 For all the recorded transitions (6H 6H, i/2, 1512—+ 6H 6F,,, 6H 912+ 2, 712+’F912) the lines group in pairs and their number in a paramagnetic state corresponds to the presence of two 3~ion. low symmetry nonThe energies equivalent positions for the Dy of the crystal field levels of the ground multiplet found from the absorption spectra at elevated temperatures are listed in table 1. The lines in the spectra of Dy 2Cu2O5 are quite broad (4 cm at 2 K) and their splittings in a magnetically ordered state are not seen directly but manifest themselves in a temperature dependent asymmetry (see fig. 4a). We extracted the line splitting Av by dividing the lineshape into two components. The A v (T) dependence shown in fig. 4b reveals the temperature of magnetic ordering of 10.6 K, in agreement with the magnetic and specific heat data [2—41.Analysis of the spectra gives the following 3~ values the ground doublet positions: splittings ofAlthe Dy±1 ions infortwo nonequivalent = 10 cm’, A2 = 13 ±1 cm—’. No peculiarity is seen on the Av (T) curve in the region of an additional anomaly for susceptibility [2]. But one has to keep in mind the poor precision of Av obtained in the way mentioned above. On the other hand, the position ofthe maximum of the line v0 can be determined with a very good precision. The v0 (T) dependence shows a small step at 7.3 K (see fig. 4b). Probably it is connected with the second phase transition in Dy2Cu2O5. 3~probe in Dy The spectrum of the Er 2Cu2O, (fig. 5) has a the verypresence rich structure at low temperatures indicating of additional nonequivalent 190

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6013 6051 crf1 Fig. 4. Temperature dependences of (a) the low frequency part of the spectral transition 6H,,, 6H,,, 2—+ 2 in Dy2Cu2O,, (b) the splitting Lip of the line 6078 cm’ (Lx), the position v0 of its maximum (o), the halfwidth Si’ of its high frequency component (x).

positions, most likely due to the doubling of a magnetic unit cell. Unfortunately, owing to the strong broadening of the lines and their small separation it is difficult to follow the spectral evolution when the temperature rises anddata to obtain from the the transitions spectra of 3 + probe some concerning theDy Er in 2Cu2O5.

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PHYSICS LETTERS A

4. Discussion We would like to discuss here the possible direc-

7 October 1991

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tions of the copper magnetic moments in a magnetically ordered state in the cuprates R2Cu2O5 on the 3~probe in them. basis As of the have spectra shown ofinthe ref. Er[1], the level of thewe Er3~ion, intrinsic or introduced as splittings a probe, Those are very aresimilar cuprates in R with strongly different R ions (Y 2Cu2O5 with R=Y, Tb, Er, Lu. and Lu have no magnetic moments, the ordering of the Th and Er magnetic moments is different [5,9]) but with an identical structure of the copper magnetic moments #c~ (ferromagnetically ordered ab planes coupled antiferromagnetically one with another, pc~aligned thedoublet b-axis in [5,9—11]). The 3 +along ground them exceeds splitting of thethat Er of the terminal level of an optical considerably transition and, as a result, all the spectral components that grow in intensity when the temperature rises are situated to the left from the main components, and so does the unsplit line at T> TN. The spectral patterns for R 2Cu2O5 with R = Y, Tb, Er, Lu are all of the type shown in fig. 5 for R = Er (see also ref. [1]). In Tm2Cu2O5, where the same copper magnetic planes 3~ground exist but PCu is aligned along theorder a-axis state splitting is an of[5,7], magthe Er less than in R nitude 2Cu2O5 with R = Y, Th, Er, Lu and less than the excited level splitting. As a result, the main and growing components are mixed, the unsplit line at(see T>fig. TN 5). is situated between the main components Our results of ref. [1] are consistent with the following conclusions: (i) The level splittings of the Er3~probe and, consequently, the shape ofthe spectral lines in a magnetically ordered state of the cuprates R2Cu2O5 are governed by the Er—Cu interactions, the Er—R interactions being negligible, (ii) The Er—Cu interactions are highly anisotropic. 3~probe turn now in Let the us cuprates R to the spectra of the Er Dy studied in this work. 2Cu2O5 We notewith firstR=Yb, that theHo, spectrum of 3~probe in Yb the Er 2Cu2O5 at low temperatures resembles that in Tm2Cu2O5. Thesplit same of components is clearly seen in the linesequence IA, IA (see fig. 5) with the difference that in Yb 2Cu2O5 each component is split, probably due to the magnetic cell

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6510

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Fig. 5. Absorption of the Er3~probe in R 4IIi/2_~+4I~3/2 at (1) T=5 K for 2Cu2O5 in the low frequency part of the transition R=Er, Tm, Yb, T=2 K for R = Ho, Dy and (2) T~3TN. The lines that grow in intensity whentemperature rises are shown by arrows. * labels the lines to some other Schematic ab3~iondue surrounded by itsphase. closestCu neighbors projections ofthe R shown for R =Tm, Yb. with their~ucu Ia are

doubling. The level splittings of the Er3 + probe in both Yb 2Cu2O5 and Tm2Cu2O5 are listed in table 2. As one can see, their values are close. In our opinion, this favors the alignment ofthe copper magnetic moments in Yb2Cu2O5 along the a-axis, in accordance with [6], rather than along the b-axis of [7]. Theref. alignment ofthe magnetic moments copper 3~probe is sketchedscheions closest given matically in to fig.the 5 for Tm Er 2Cu2O5 [5,7] and Yb2Cu2O5 [6]. The ions designated circles Cu—O—R are con3~via the by twofullshortest nected with Er bridges each and must give the main contribution, the same in both Tm2Cu2O5 and Yb2Cu2O5, to the Er—Cu exchange interaction. It is worth mentioning 191

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PHYSICS LETTERS A

Table 2 The splittings of the ground doublet Li and of low lying levels in 3~probe in the crystal field manifold ~I,3,2 LiA, Li R 8 for the Er 2Cu2O5 in a magnetically ordered state. 1 and 2 refer to two nonequivalent positions in the crystal structure. 2 LiBl /Li R Li 1 /Li2 LiA 1 /LiA 82 Er Tm Yb’~

20.6/19.7 2.5/2.0 2.6/1.8 2.3/1.3

3.3/5.7 4.8/5.0 5.0/4.8 4.3/4.5

9.0/7.2 5.6/5.0 3.5/3.4 3.0/4.2

7 October 1991

Acknowledgement The authors thank B.V. Mill of the Moscow State University for kindly supplying the samples used in this investigation and for stimulating conversations. We are grateful to R.Z. Levitin for helpful discussions. G.G. Chepurko’s aid in taking the data is acknowledged. References

‘~Thetwo rows for Yb2Cu2O5 where the doubling of a magnetic unit cell takes place [5—7]are due to the doubling of spectroscopic positions.

that in Yb2Cu2O5 the pattern ofthe copper magnetic moments, as follows from ref. [6], is the same for all Yb (or Er) positions in a magnetic unit cell (as in fig. 5 or turned at 180°).New spectroscopically nonequivalent positions observed in our experiment are probably connected with a local striction. 3~probe forofthem Ho2Cu2O5 and aDy2Cu2O5, thespectrum Er in As both exhibits third type of different from the spectra of both Er 2Cu2O5 (the first type) and Tm2Cu2O3 (the second type). We failed to make an unambiguous identification there, var3~ground ious versions yield 3.3—4.5 cm~for the Er doublet splittings. Most likely, the copper magnetic moments in Ho 2Cu2O5 are aligned along the c-axis, in accordance with ref. [6], rather than along the aaxis [7]. The same alignment pc~ic is expected, according to our spectral data, in Dy2Cu2O5 where no neutron diffraction data exist.

[1] G.G. Chepurko, I.V. Paukov, MN. Popova and Ja. Zoubkova, Solid State Commun., in press. [2] Z.A. Kazei, N.P. Kolmakova, R.Z. Levitin, B.V. Mill, V.V. Moshchalkov, V.N. Orlov, V.V. Snegirev and Ja. Zoubkova, J. Ma~n.Magn. Mater. 86 (1990) 124. [3] R. Trod, J. Klamut, Z. Bukowski, R. Hor~’nand J. StepienDamm, Physica B 154 (1989) 189. [4] V.V. Moshchalkov, N.A. Samarin, Y. Zoubkova and By. Mill, Physica B 163 (1990) 237. [5] V.P. Plakhtii et al., Intern. Conf. on Neutron Bombay, January 21—25, 1991, abstracts, p. 54. scattering, [6] A. Murasik, P. Fischer, R. Trod and Z. Bukowski, Solid State Commun. 75 (1990) 785. [7] J.L. Garcia-Munoz, J. Rodriguez-Carvajal, X. Obrados, M. Vallet-Regi, J. Ganzález-Calbet and M. Parras, Complex magnetic structures rare earth cuprates R2Cu2O5 (R=Y, Ho, Yb, Tm),oftothe be published. [8] K.P. Belov, A.K. Zvezdin, A.M. Kadomtseva and R.Z. Levitin, Spin-reorientational transitions in rare earth magnetic materials (Nauka, Moscow, 1979) [in Russian]. Plakhtii, M. Bonnet, I.V. Golosovskii, B.V. Mill, E. Roudaut and E.I. Fedorova, Pis’ma Zh. Eksp. Teor. Fiz. 51 (1990) 637. [l0]V.P. Plakhtii, IV. Golosovskii, Ja. Zoubkova, S.A. Kuznetsov and B.V. Mill, Pis’ma Zh. Eksp. Teor. Fiz. 51

[91V.P.

(1990)45. [11] J. Aride, S. Flandrois, M. Taibi, A. Boukhari, M. Drillon and J.L. Soubeyroux, Solid State Commun. 72 (1989) 459.

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