Journal of Magnetism and Magnetic Materials 104-107 (1992) 2103-2104 North-Holland / H I

Molecular-based magnets studied with an ultrasensitive SQUID magnetometer

Epiphane Codjovi, Pierre Bergerat, Keitaro Nakatani, Yu Pei and Olivier Kahn Laboratoire de Chimie lnol~ganique, URA n ° 420, Universit~ de Paris-Sud, 91405 Orsay, France

We describe our latest results concerning the molecular-based compo'mds exhibiting a spontaneous magnetization. The ordering temperature of one of our compounds is T¢ = 30 K, which presently seems to be a record in this field.

For quite some time, we have looked for synthesiz- ing molecular-based compounds exhibiting a sponta- neous magnetization below a critical temperature T c [1]. Our s t r a t c~ along this line consists in assembling high-spin molecules or i'errimagnetic chains within the crystal lattice in a ferromagnetic fashion. We have already reported cn two compounds of this kind, both containing Mn(II) and Cu(II) ions. The former is M n C u ( p b a O H ) ( H 2 0 ) 3 [pbaOH = 2-hydroxy-l,3- propylenebis(oxamato)] and T c is equal to 4.6 K [2]. The latter is MnCu(obbz) -H20 [obbz=oxamido- bis(N-benzoato)] with T c = 14 K [3]. In this paper, we would like to summarize our latest results.

MnCu(obze)H20) 4 . 2 H 2 0 and MnCu(obze)(H20) 2. The reaction of the copper(II) precursor of formula [Cu(obze)] 2- [obze = oxamido(N-benzoato-N'-ethano- ato)] on Mn(II) affords a compound with a structure that consists of discrete MnCu(obze)(H20) 4 units in which Mn(Il) and Cu(II) ions are bridged by an oxam- ido group, and non-coordinated water molecules (see fig. 1). The magnetic behavior of this compound is that of antiferromagnetically coupled Mn(II)Cu(II) pairs with XM T (,'~'M is the molar magnetic susceptibility per

f2

Fig. 1. Structure of MnCu(obze)(H 20)4 . 2H 20.

MnCu unit) decreasing continuously upon cooling. This behavior, quite surprisingly, is completely modified when treating this compound under vacuum at room temperature. Four water molecules are very easily lost. The XM T versus T curve for the resulting material of formula MnCu(obzeXH20)2 is characteristic of a ferri- magnetic behavior (see fig. 2). The magnetization ver- sus T curves reveal a ferromagnetic transition at 4.6 K (see fig. 3). The structure of MnCu(obzeXH20)2 is not known but the spectroscopic data indicate that Mn(II) remains in octahedral surroundings, which requires the formation of Mn-O(carboxylato) bonds. Instead of iso- lated oxamido-bridged Mn(II)Cu(II) pairs, we have now a polymeric structure. Interestingly, the approach lead- ing to MnCu(obze)(H20) 2 is reminiscent of a mecha- nism suggested by McConnelI [4] as early as 1963. This mechanism is the following: a molecular magnetic en- tity may exhibit regions of non-compensating positive and negative spin densities. If so, the interaction be- tween the positive spin density of a unit and the negative spin density of the adjacent unit may lead to overall ferromagnetic interactions as schematized [5]:

Assembly of non-interacting heterobinuclear molecules

Dehydration and

polymerization

1-II 1-II 1 L' +J t,' "J t.' "J

Molecular-based fcrromag.net

MnCu(pbaOH)(H20) 2. Quite recently, we x~cre wondering whether it wouid be ~c~ssible to manipulate the chain compound MnCu(pbaOH)(H zO)3 in order to shift T c toward higher temperatures. Tc is governed by both intrachain, Jintra, and interchain, Jintcr, interaction

0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

2104 E. Codjoci et al. / Molecular-based magnets studied with a SQUID magnetometer

5.5

5 ~._

4 . 5

'~E 4 Cdl

t-~ 3.5

3

2.5

A + I i l $

t x

t~

l , I I I

0 50 100 150 200 250 300 T I K

Fig. 2. xMT versus T curves for (o) MnCu(obzeXHaO) 4. 2H 2 O, ( ,x ) MnCu(obze)(H ,O),.

2507+++J.+,1 . . . . I + + + + i + r v r F i i i i + ' '

. =u=.=.,,.., = 3O K: - mm l -

200 Tc B I I ~1 I l l •

mll I &&,t &A I -

o 150 - " , , , "% [ - o~,..E , , , , , , % , +

100 A m -

+ 50 " ' " " ' " " ' " " " X + A

0 i I . . . . I I .... l u~ l ; ' ~ h , ~ 0 5 10 15 20 25 30 35

T I K Fig. 4. Magnetization versus T curves of MnCu(pbaOH) (H zO)z. ( • ) FCM; ( , ) ZFCM; ( • ) remnant magnetization.

parameters. Jintra is essentially determined by the na- ture of the bridges, therefore, by the nature of the Cu(ll) precursor [Cu(pbaOH)] 2-. As for Jinter, it might be related to the interchain distances. Along this line, it appeared to us that we could make the chains closer to each other by removing the weakly coordinated water molecule occupying the apical position in t h e copper coordination sphere [2]. Heating MnCu (pbaOH)(H20) 3 at 100 ° under vacuum affords MnCu (pbaOH)(H20) 2. Spectroscopic data indicate that the water molecule which has been removed actually be- longed to the copper(II) chromophore.

In the 300-80 K temperature range, the Xu T ver- sus T curve for MnCu(pbaOH)(H20) 2 is essentially identical to that of MnCu(pbaOH)(H20)3 . In particu- lar, the minimum characteristic of the one-dimensional ferrimagnetism is observed at the same temperature.

0 i L i i i ; ~ +

rD 50 m ~

• -: 40

~E 30

N 20

10 a o o

i i L L ~ l ~ J ~ . . . . . L . . . . J . . . . L . t i ~

0 0 2 4 ...... + ....... 8 ~ I0 T/K

Fig. 3. Magnetization versus T curves for MnCu(obze)(H 20)2. ( [] ) FCM: (©) ZFCM; ( ,_'~ ) remnant magnetization.

Upon cooling down further, Xm T for MnCu(pbaOH) (H20) 2 increases even faster than for MnCu(pbaOH) ( H 2 0 ) 3. The magnetization versus temperature curves reveal a magnetic transition at 30 K (see fig. 4). Through a mild thermal treatment, we succeeded in shifting T c from 4.6 to 30 K [6], apparently a record in the area of the molecular-based magnetic materials [7-10].

References

[1] See Magnetic Molecular Materials, eds. D. Gatteschi, O. Kahn, J.S. Miller, F. Palacio, NATO ASI Series (Kluwer, Dordrecht, 1991).

[2] O. Kahn, Y. Pei, M. Verdaguer, J.P. Renard and J. Sletten, J. Am. Chem. Soc. 110 (1988) 7d2.

[3l K. Nakatani, J.Y. Carriat, Y. Journaux, O. Kahn, F. Lloret, J.P. Renard, Y. Pei, J. Sietten and M. Verdaguer, J. Am. Chem. Soc. 111 (1989) 5739.

[4] H.M. McConnell, J. Chem. Phys. 30 (1963) 1910. [5] Y. Pei, O. Kahn, K. Nakatani, E. Codjovi, C. Mathoni~re

and J. Sletten, J. Am. Chem. Soc. 113 (1991) 6558. [6] K. Nakatani, P. Bergerat, E. Codjovi, C. Mathoni~re, Y.

Pei and O. Kahn, lnorg. Lhem. 30 (1991) 3977. [7] J.S. Miller, J.C. Calabrese, H. Rommelmann, S.R. Chit-

tipeddi, J.H. Zhang, W.M. Reiff and A.J. Epstein, J. Am. Chem. Soc. 109 (1987) 769.

[8] A. Caneschi, D. Gatteschi, R. Sessoli and P. Rey, Acc. Chem. Res. 22 (1989) 392.

[9] W.E. Broderick, J.A. Thomson, E.P. Day and B.M. Hoff- man, Science 24q (1990) 401.

[10] N. Matsumoto, M. Sakamoto, H. Tamaki, H. Okawa and S. Kida, Chem. Lett. (1990) 853.