Magnetic phase transitions and magnetic structures of In2Cu2O5 and Sc2Cu2O5

Solid State Communications, Vol. 102, No. 1, pp. 71-75, 1997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/97 $17.00+.00

Pergamon

PII:SOO38-1098(96)00700-4

MAGNETIC

PHASE TRANSITIONS

AND MAGNETIC

STRUCTURES

OF In2Cu205 AND SczCu205

M.N. Popova,’ S.A. KIimin,” R. Trocb and Z. Bukowski” %stitute of Spectroscopy, Russian Academy of Sciences, 142092 Troitsk, Moscow region, Russia bInstitute for Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroclaw, Poland (Received 19 July 1996; accepted 22 November 1996 by R. Phillips)

Magnetic properties of In2Cuz05 and Sc2Cu205 that crystallize in the Pna21 space group were studied by means of high resolution Fourier transform spectroscopy of the rare earth probe. Magnetic ordering of In2Cu20s at 29 _’ 1 K and of Sc2Cu20s at 16 f 1 K has been confirmed by this method. The most probable magnetic structure in an ordered state of In2Cu205 is suggested to be the same as in R2Cu205 with R = Y, Lu, Er, Tb, namely, ferromagnetically ordered CuO ab layers coupled antiferromagnetically; copper magnetic moments aligned along the crystal axis b. 0 1997 Elsevier Science Ltd. All rights reserved Keywords:

A. magnetically

ordered materials,

D. phase transitions.

link the planes into a 3D structure. Two nonequivalent fourfold low symmetry (Cl) positions for the R3+ ions are inside R(l)Ob and R(2)Oh distorted octahedra and both are located between the two CuO planes in the crystal cell (Fig. 1). All the R2Cu20s cuprates (with the possible exception of Tm2Cu20s) undergo antiferromagnetic ordering [3, 41 and, what is important, all of them (with the possible exception of Tb2Cu20s) show the metamagnetic behaviour [4]. The latter is quite unusual for divalent copper compounds. The Cu2+ ion (S = l/2) is of Heisenberg type and has a comparatively small g-factor anisotropy, while metamagnetism demands the existence of an anisotropic interaction comparable in magnitude with the antiferromagnetic exchange. To explain such an unusual behaviour of the R2Cu205 compounds, the authors of [4] suggested that magnetic dimers with S = 1 are formed due to exchange interaction inside the Cu(l)Cu(2) dimeric units through two Cu(l)-0-Cu(2) bridges with angles close to 90” favourable for a strong ferromagnetic exchange and so the magnetic anisotropy comes from the two-ion mechanism. This idea has been developed in the recent paper [5] to interpret the results of magnetization measurements for R2Cu20s with nonmagnetic R3+ ions (R = Y, Lu, In, SC) in the pulsed magnetic field up to 30 T at 1.7 K. Two successive metamagnetic transitions

1. INTRODUCTION The compounds Sc2Cu20s and In2Cu205 crystallize in the orthorhombic symmetry (space group Pna21) and have a structure of the Ho2Cu202 type [l, 21, they belong to the family of the R2Cu20s cuprates that first received considerable attention as the phases related to high T, superconductors. It turned out later that these cuprates have interesting magnetic properties and also deserve studying in connection with the problem of lowdimensional magnetism [3-51. The detailed analysis shows [4] that the structure consists of low-dimensional units successively forming a three-dimensional (3D) system. Copper ions are inside the distorted elongated oxygen octahedra with four short Cu-9 bonds (1.92-2.00 A) and two long bonds (2.642.78 A). These octahedra joined by their common short edges form dimeric units. Dimers are linked by their long edges into zig-zag chains parallel to the a axis. The chains, in turn, are linked by oxygen atoms and form CuO layers parallel to the ab plane. Within these layers the Cu-Cu distances along the b axis (2 3.5 A) are close to the largest intrachain ones (= 3.2 A). The distances between Cu atoms located in neighbouring layers are twice as long (= 6.5 A). There are no Cu-0-Cu bonds through one oxygen atom between the CuO planes. Cu-0-0-Cu and Cu-0-R-0-Cu interplane bridges 71

72

MAGNETIC STRUCTURES OF In&u205 AND Sc#Zu205

Vol. 102, No. 1

resolution Fourier transform spectroscopy (FT’S) of a rare earth probe is a powerful tool for investigating their magnetic properties. Review of this work is given in [12]. In the present work we extend the above mentioned studies to the two remaining members of the RzCuZOSfamily, namely, In2Cu205 and SczCuz05. 2. EXPERIMENTAL

lCu(l)

G&?) *R(l)OR(Z) 00

sEr (21 probe

Fig. 1. ac projection of the crystal structure of R2Cuz05. were observed for all these compounds. The exchange and anisotropy fields were calculated by the proposed model. This model assumes that the magnetic structure in an ordered state is the same for all the cuprates studied, namely, ferromagnetic CuO layers parallel to the ab plane coupled antiferromagnetically one with another, copper magnetic moments being aligned along the b direction. Such a structure of copper magnetic moments has been found by neutron diffraction for Y2Cu205 [6,7], Lu2Cu205 IS], ErzCuz05 191 and Tb2Cu205 [lo]. The doubling of magnetic unit cell along the b direction and more complex magnetic structure have been discovered for Sc&u205 1111.As far as we know, there exist no data on the magnetic st~cture of InzCuz05. The present work was unde~aken, mainly, with the aim to investigate the magnetic structure of In2Cu205. Our earlier spectral measurements of the R2Cu205 (R = Y, Tb-Lu) compounds have shown that high

x0.5

II AA-

Polycrystalline X-ray single phase samples of R2Cu205 (R = In or SC) with 1% of Er intr~u~d as a probe have been prepared by solid state reaction as described in 131. Powder samples were mixed with ethanol and put on the sapphire platelet just before the InSb detector inside the helium vapor cryostat. High resolution (down to 0.1 cm-‘) spectra in the region of the 411s,2- 4I13j2optical transition in the Er3+ probe ion were recorded at 3-110 K by the BOMEM DA3.002 Fourier transform spectrometer. The Er3+ probe ion substitutes for the R3+ ion in the R2Cu205 compounds. For both nonequivalent positions of the R3+ ion, the crystal field of low symmetry C, completely removes the level degeneracy of the free ion Er3+ and splits this level into J + l/2 Kramers doublets. The Kramers degeneracy may be removed only by the external magnetic field or by the magnetic field that originates during the magnetic ordering of a system. The spectral line of an optical transition in this case generally splits into four com~nents (see Fig. 2). The detection of splitting and n~rowing of spectral lines during the magnetic ordering in the system institutes the basis of the spectral method for studying the magnetic phase transitions. Earlier, we have shown experimentally (see, e.g.

x0.5 4.2 K

-_-

I

n

PLL_

IA

6400

6600

cm“

ln,Cu,O,: Er 1%

6400

6600

cm-’

Sc2Cu205: Er 1%

Fig. 2. Levels of the Er3+ Kramers ion in a magnetically ordered RzCuz05.

MAGNETIC STRUCTURES

Vol. 102, No. 1

[X2]) in the course of spectral studies of the R2Cu205 cuprates whose magnetic structure was well known from neutron dif~action measurements, that the splitting of Kramers doublets of the Er3+ probe ion in RZCuZ05 depends, mainly, on the exchange interaction Er-Cu of this ion with its nearest copper neighbours the Er-R interactions being negligible and that the Er-Cu exchange is highly anisotropic. So, the spectrum of the Er3+ probe in a magnetically ordered RaCuzOs compound delivers information on the magnetic structure of copper subsystem. One can find a description of the method in the recent review article [13]. 3. RESULTS AND DISCUSSION The spectra of the Er3+ probe in the paramagnetic phase of all the compounds R2CuZOS are similar and demonstrate, for the 4115,2- 41,s,zoptical transition, two groups of lines corresponding to two groups of crystal field levels in the 41,sj2multiplet separated by the energy intervals of about 230 cm - * . The low frequency lines of the transition are relatively narrow while the high frequency ones are broadened by the phonon relaxation. The bottom of Fig. 3 shows the low frequency part of the 4f15,2+41,3,2 optical transition of Er3+ in the paramagnetic In2Cu205 and Sc&!u205. The spectral lines associated with the transitions in the two nonequivalent crystallographic positions overlap which points to a similarity of the crystal field for these two positions. Energies of the lowest crystal field levels of 411s,zand 41,3,2Er3+ multiplets are listed in Table 1 for Sc@t205 : Er, In2Cu205 : Er and for comparison, Er2Cu205. While the spectra of Er3+ probe in the paramagnetic In2Cu205 and Sc&u205 are quite similar, those for

OF In2Cu205 AND Sc&u205

73

magnetically ordered phase differ considerably (see the top of Fig. 3). This fact points to different exchange splittings of Ersf probe in InzCuzOz and Sc2Cu205. As has already been mentioned above, the splittings of the Er3+ probe Kramers doublets in a magnetically ordered state of the RzCu,05 cuprates are governed by the anisotropic exchange interactions with the nearest copper ions and, thus, depend on the directions of their magnetic moments. Each Er?’ probe ion is bound through one oxygen ion with seven copper ions. Three of them, labeled 1, 2, 3 in Fig. 1, lie in the Cu-0-Cu chain nearest to the given Er3+ ion, then there are three ions l’, 2’, 3’ in the chain situated under the nearest one. These six copper ions belong to the CuO plane nearest to the given Er3+ ion. The seventh ion 4 lies in the neighbouring chain (and plane). We have shown earlier [12] that for R&u205 with R = Y, Lu, Er, Tb where the magnetic structure of an ordered copper subsystem consists of the ferromagnetic CuO ab layers coupled antife~omagnetically and copper magnetic moments are parallel to the b axis, the ground state splitting of Er3+ probe is A = 18-20 cm- ’ while for Tm&uz02 where the same CuO fe~oma~etic layers coupled antife~omagnetically exist but copper ~gnetic moments are parallel to the a axis A = 2 cm - ‘. Let us examine the low temperature spectra of In2Cu205 and Sc2Cu205 in more detail. 3.1. In2Cu205 Figure 4 shows the most intense lowest frequency line at 6531 cm- ’ in the 4I15,2--‘4113j2transition at different temperatures. Spectral lines due to transitions in two nonequivalent structural positions are super-

.__ G _.-

411312

-_L_ _- AA f

411512

-i-

A t Free ES ion

’

+

crystal fie,d

+

exchange

Fig. 3. The low frequency part of the 411~,2d4 I,, optical transition of Er”+ probe ion in InzCuZOS and Sc2Cu205 at T = 100 K > TN (bottom spectra) and at T = 42 K < TN (top spectra).

I

6516

I

6526

I

6536

cm-l

1

Fig. 4. The lowest frequency line of the 4fr,,2 -+4 1,312 absorption of the Er3+ probe in In2Cu205 at different temperatures.

74

MAGNETIC

STRUCTURES

OF In,Cu205

AND SczCu205

Vol. 102, No. 1

Table 1. Energy levels E, (in cm-‘) of the 41t5~ and 41t312crystal field manifolds of Er3+ in ScrCu~Os, InzCu~Os and Er2Cu205 n

4z15/z

E,

S&u205 IR@r~O~ Er2Cu20s

n sc*cu?o5 4I 1312

En

In2Cu205

Er2Cu205

I

II

III

IV

0 0 0

50 53 45

85 76 65

110 111 109

A

B

C

D

E

F

G

6537 6531 6523

6558 6556 6551

6571 6575 6568

6609 6605 6597

6841 6837 6822

6858 6850 6840

6886 6882(?) 6895

imposed. It is possible to separate them in the case of sharp lines at low temperatures. The ground doublet splittings A at 4.2 K are listed below for erbium ions in two nonequivalent positions Erl, Er2 respectively: A(1) = 19.2cm-‘,

A(2) = 19.5cm - I.

As these splittings are very close to the splittings of the Er3+ probe ground levels in the R7Cu205 cuprates with R = Y, Tb, Er, Lu (121, we suppose that the structure of the ordered Cu magnetic moments in InzCuz05 is the same as in these cuprates, nameIy, quasi~~o-dimensiona ferromagnetic layers coupled antiferromagnetically, with Cu moments aligned along the b axis. The splitting versus temperature is plotted in Fig. 5. The strong broadening of the closely spaced lines near the temperature of a magnetic ordering and the low intensity of the high-frequency components of the line make it difficult to trace the splitting of the ground state A (see Fig. 4); therefore, Fig. 5 shows the temperature dependence of some splitting averaged over two crystallographic positions, i.e. Av = (A - AA>. Our spectral measurements gave the value 29 K for the magnetic ordering temperature (T,,, = 30 K from the results of magnetic susceptibility measurements [3]). This temperature was determined as the abscissa of the

point of inflection in the Av(T) curve. The splittings at elevated temperatures, when a single line remains, were estimated as the difference in the half-width of the line at a given temperature and the most narrow line at T > TN. The “tail” of the splittings at T > TN is due to the short range order that remains at the temperatures higher than TN+

Figure 6 shows the most intense lowest frequency line at 6537 cm-’ in the “it, -+4Zr3a transition of the Er”’ probe ion in Sc2Cuz05 at different temperatures. The line becomes narrower when lowering the temperature and shoulders appear at both the low and high frequency sides. Near 3 K the low frequency shoulder disappears. Such a behaviour points to a splitting of the line into several broad components. The component originating from the upper level of the Kramers ground doublet split by exchange interaction disappears near 3 K. The reason for a large inhomogeneous broadening lies, probably, in the big difference between the ionic radii of Sc3+ (0.75 & and Er3+ (0.89 A). Notwithstanding the fact that the spectral lines of the Er”+ probe in Sc2Cu20s are broad, it is evident that the picture of the Er3+ probe level splittings is completely

I

6525

Fig. 5. In2Cu20d. The splitting Au of the 6531 cm-’ line of the 411,,, - 1t3,* absorption of the Er”’ probe vs temperature.

I

6535

’

I

cm-1

Fig. 6. The lowest frequency line of the 4115/ze4 113,2 absorption of the Er’+ probe in ScrCu205 at different temperatures.

Vol. 102, No. 1

MAGNETIC STRUCTURES OF InzCu205 AND Sc2Cu205

64 _

40

CllT'

0

0

cm-l -6537,3

cl---

75

are grateful to Yu.A. Hadjiiskii who participated in spectral measurements. This work was supported in part by the Russian Fund for Basic Research under Grant number 9502-03796a.

Acknowledgements-We

8-

REFERENCES Freund, R. and Miiller-Buschbaum, Hk., 2. Natur- 6536,7 0'

I 10

1 20

1 30

I T,K

Fig. 7. ScQ1205. The halfwidth 6v (circles) and central yl v. (squares) of the 2237 cm line of the 113,2 absorption of the Er probe vs temperature. 15/2different from that observed for In2Cu20s. So, the magnetic structure of Sc2Cu205 is quite different from that of R2Cu205 (R = Y, Tb, Er, Lu, In). This conclusion does not contradict the results of neutron scattering studies of [ll]. Unfortunately, the absence of a resolved fine structure of the Er3+ probe spectra in a magnetically ordered Sc2Cu205 (because of large inhomogeneous broadening) does not permit to draw more definite conclusion concerning magnetic structure of this compound. The temperature dependences of the halfividth 6v and central frequency v. of this line presented in Fig. 7 reveal the temperature 16 K for a magnetic phase transition in Sc2Cu205, in accordance with the results of the magnetic susceptibility measurements [3]. 4. CONCLUSIONS In conclusion, the use of high resolution ITS enabled us to detect magnetic phase transitions in In2Cu205 and Sc2Cu205 and to specify the transition temperatures. The most probable magnetic structure in an ordered state of In2CuZOS was suggested, namely, ferromagnetic CuO layers parallel to the crystal ab planes coupled antiferromagnetically; the easy axis for the magnetic moments is along the b axis.

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J.L., Rodrigues-Carvajal, J., 7. Garcia-Muiioz, Obradors, X., Vallet-Regi, M., Gonzalez-Calbet, J. and Parras, M., Phys. Rev., B44,1991,4716. 8. Plakhtii, V., Golosovskii, I., Zoubkova, Ja., Kuznetsov, S., Mill’, B. and Kharchenkov, V., Pis’ma Zh. Eksp. Teor. Fiz., 51,1990, 45. 9. Plakhtii, V., Bonne, M., Golosovskii, I., Mill’, B., Rudo, E. and Fedorova, E., Pis’ma Zh. Eksp. Teor. Fiz., 51, 1990, 637. 10. Golosovskii, I., Mill’, B., Plakhtii, V. and Kharchenkov, V., Fiz. Tverd. Tela, 33, 1991,3412. 11. Murasik, A., Fisher, P., TroC, R. and Bukowski, Z., J. Magn. Magn. Mater., 127, 1993, 365.

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A. Ryskin and V. Masterov), p.182. Proc. SPIE 2706, 1996.