PHYSICAL REVIEW B 86, 184403 (2012) Dramatic effect of A-site substitution upon the structure and magnetism of the “114” CaBaCo 4 O 7 cobaltite Md. Motin Seikh, * Tapati Sarkar, V. Pralong, V. Caignaert, and B. Raveau Laboratoire CRISMAT, UMR 6508 CNRS ENSICAEN, 6 bd Mar´ echal Juin, 14050 CAEN, France (Received 7 September 2012; published 5 November 2012) Electron and hole doping at the cobalt sites of CaBaCo 4 O 7 has been successfully realized by heterovalent substitution on A sites (Ln at Ca sites and K at Ba sites) with a narrow range. Both electron and hole doping have a dramatic impact upon the structure and magnetism, decreasing the orthorhombic distortion and weakening considerably the ferrimagnetism at the benefit of magnetic frustration. The hole doping very drastically kills the ferrimagnetism of the parent phase leading to a cluster-glass phase at lower temperatures, whereas the electron doping with a smaller cation like yttrium leads to an admixture of cluster and spin glass along with the preservation of weak ferrimagnetism. The frequency dependence of the peak position in χ ′ (T ) curves was quantitatively analyzed using the power law, τ = τ 0 (T f /T g − 1) −zν . Moreover, the electron doping with larger lanthanides (Pr and Nd) leads to spin-glass states, which are manifested at two temperatures vis-` a-vis ∼60 K and ∼110–115 K. The appearance of magnetic frustration at the expense of ferrimagnetism in both cases is interpreted as the result of deviation of the Co 2+ /Co 3+ ratio from unity and cationic disordering on cobalt sites, even if the crystal remains orthorhombic. DOI: 10.1103/PhysRevB.86.184403 PACS number(s): 75.47.Lx I. INTRODUCTION The cobaltites LnBaCo 4 O 7 with Ln = Dy, Ho, Er, Tm, Yb, Ln, and Y 1,2 and CaBaCo 4 O 7 3 form a large family called “114,” which has been extensively studied for its complex magnetic properties, especially magnetic transitions possibly connected with a structural transition. 4–11 One important fea- ture of these compounds deals with the fact that the transition element, in its mixed valent state Co 2+ /Co 3+ , accommodates a pure tetrahedral coordination. This is quite unique with respect to other strongly correlated oxides, where the framework is octahedral, and it should have a great impact upon the magnetic properties of these cobaltites, leading to cobalt and oxygen to triangular (T) sublattices favorable to a geometric frustration. In this respect, the orthorhombic structure of CaBaCo 4 O 7 12 [Fig. 1(a)], although it is closely related to that of the hexagonal LnBaCo 4 O 7 oxides 5,6,13 [Fig. 1(b)] having identical T layers, differs from the latter by a strong puckering of its kagom´ e (K) layers in the structure. Importantly, it causes CaBaCo 4 O 7 to be the only member of the “114” family that exhibits, instead of an average Co 2.5+ , an unequal valence distribution at different cobalt sites, with a realization of Co 2+ ions sitting in the K layers (Co2 and Co3 sites), whereas the Co1 and Co4 tetrahedra of the T and K layers are occupied by trivalent Co 3+ ions to satisfy the oxygen stoichiometry “O 7 .” The stabilization of such an exceptional tetrahedral coordination of Co 3+ was anticipated to be related to the probable ligand to metal charge transfer. 12 Such a valence distribution has a particular role in the exceptional ferrimagnetic structure of CaBaCo 4 O 7 [Fig. 1(c)], which consists of ferromagnetic (FM) “Co 2+ zigzag chains antiferromagnetically coupled with the Co 3+ cations. In the observed ferrimagnetic magnetic structure, the Co 2+ /Co 2+ and Co 3+ /Co 3+ interactions are FM, whereas the Co 2+ /Co 3+ interactions are antiferromagnetic (AFM) in nature. 12 Bearing in mind the specific cobalt valence distribution that appears in CaBaCo 4 O 7 with respect to the other members LnBaCo 4 O 7 , we believe that the ratio Co 3+ /Co 2+ = 1 in this oxide, being much larger than that observed in the other “114” cobaltites (Co 3+ /Co 2+ = 1/3), plays a crucial role in the ferrimagnetic properties of this oxide. Thus, we have studied the doping of the cobalt sites with holes and with electrons by introducing potassium and yttrium/lanthanides on the A sites, i.e., on the barium and calcium sites, respectively. We show herein that a fluctuation of the cobalt valence by less than 2.5% weakens considerably the ferrimagnetism of this oxide and induces a strong magnetic frustration. This high sensitivity of the magnetism of doped CaBaCo 4 O 7 is correlated to a significant decrease in the orthorhombic distortion of the structure. II. EXPERIMENTAL The samples Ca 1−x Ln x BaCo 4 O 7 (Ln = Pr, Nd, and Y) for x = 0.1 and CaBa 1−x K x Co 4 O 7 for x = 0.1 and 0.2 were synthesized from the respective mixtures of oxides Pr 7 O 12 (99.5%), Nd 2 O 3 (99.9%), Y 2 O 3 (99.9%), CaCO 3 (99.8%), BaCO 3 (99.8%), K 2 CO 3 (99.0%), and Co 3 O 4 (99.7%) all from Alfa Aesar. The oxygen stoichiometry of the cobalt oxide was found to be CoO 1.28 instead of CoO 1.33 . Pr 7 O 12 , Nd 2 O 3 , and Y 2 O 3 were dried at 1173 K prior to mixing. The appropriate proportions of the starting materials were weighed and thoroughly mixed with mortar pestle, adding ethanol for homogeneous mixing. The intimate mixtures were first heated overnight in air at 1173 K. They were then ground and pressed in the form of parallellepipedic bars and heated in air for 24 h at 1373 and 1323 K, respectively, for Ca 1−x Ln x BaCo 4 O 7 and CaBa 1−x K x Co 4 O 7 compositions. All the samples finally were quenched to room temperature in order to stabilize the “114” phase. Bearing in mind that in the oxides with nominal compositions CaBa 0.9 K 0.1 Co 4 O 7 and CaBa 0.8 K 0.2 Co 4 O 7 , a potassium deficiency can appear due to the volatility of this element, energy dispersive x-ray (EDX) analyses were carried out with an analyzer (LINK) mounted on a scanning electron microscope (Oxford Instru- ments). The cationic compositions, “Ca 1.01 Ba 0.92 K 0.07 Co 4 ” 184403-1 1098-0121/2012/86(18)/184403(9) ©2012 American Physical Society