Article pubs.acs.org/JPCC
Magnetization Steps Promoted by Structural Modulation in BaCoX2O7 (X = As, P) Rénald David,† Houria Kabbour,† Silviu Colis,‡ Alain Pautrat,§ Emmanuelle Suard,∥ and Olivier Mentré*,† †
Université Lille Nord de France, UMR 8181 CNRS, Unité de Catalyse et de Chimie du Solide (UCCS USTL), F-59655 Villeneuve d’Ascq, France ‡ Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS and Université de Strasbourg (UDS-ECPM), 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France § Laboratoire CRISMAT, UMR 6508 du CNRS, ENSICAEN et Université de Caen, 6 Bd Maréchal Juin, F-14050 Caen 4, France ∥ Institut Laue-Langevin, BP 156, 6 rue Jules Horowitz, 38042 Grenoble Cedex 9, France S Supporting Information *
ABSTRACT: BaCo2X2O7 (X = P, As) compounds are formed of chains of Co2+O8 dimers. In a first approximation, the structural chains appear strongly isolated, which gives a pronounced one-dimensiona (1D) magnetic character to the systems. GGA+U calculations show ferromagnetic (FM) exchanges inside the dimers and predominant antiferromagnetic coupling between them along the chains, as confirmed by the refined magnetic structure. This 1D antiferromagnetic topology between S = 3 FM dimers does not support the existence of two broad overlapped magnetization plateaus observed around Hc1 = 5 T (ferrimagnetic state → ∼Ms/3) and Hc2 = 7 T (ferromagnetic state → Ms). In the crystal, an incommensurate structural modulation promotes important atomic displacement waves that transform the 1D chains into two-dimensional layers. In the layers, strongly frustrated topologies between Co2+ triangles of S = 3 dimers are distributed accordingly to the modulation wave. The exchanges calculated for the maximal frustrated geometry offered by the modulation show that they are mainly responsible for the stepped magnetization.
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the field of materials for spintronics, SrCo6O11 and Cr-doped Ca3Ru2O7 both formed of separated magnetic units show magnetization steps associated with different resistive states,12,13 which implies that the electron transport across the layers is controlled by the relative spin alignment of two magnetic layers as usually observed in magnetic tunnel junction systems. 14 In essence, the discovery of novel low-D ferromagnetic or metamagnetic compounds deserves particular attention at both the theoretical and functional viewpoints. In this context, we paid attention to the family of compounds AMX2O7 (A = Ba, Pb, Sr, or Cd; M = a transition metal; X = As, P, or V). They crystallize into two polymorphs, the β and the α forms, with the P21/n and P-1 space groups, respectively. Table S1 (Supporting Information) summarizes in a nonexhaustive way the members found in the literature. Among them, α-BaCoP2 O7 was reported to show two broad metamagnetic transitions (two magnetic plateaus) centered at H ∼ 5.8 T (M ∼ 1.1 μB/Co) and H ∼ 7.8 T (M ∼ 2.8 μB/Co) below 5 K.15 Its crystal structure consists of isolated infinite chains oriented along the a axis and built from Co2O8 edge-
INTRODUCTION Low-dimensional oxides with disconnected magnetic units are increasingly attractive due to their amazing properties offered by the versatile interactions between individual magnetic moments when placed into an external magnetic field. Indeed, one-dimensional (1D) or two-dimensional (2D) topologies between magnetic ions generally favor strong magnetic anisotropy and form a rich playground to investigate exotic fundamental states and other phenomena such as field-induced transitions or magnetization steps with potential applications for high-density storage. For instance, BaFe2(PO4)2 was recently proposed as a unique example of a 2D ferromagnetic (FM) Ising-like oxide built of well-separated Fe2+-based FM honeycomb layers. 1 Also, several compounds such as Ca3Co2O62 and α-CoV2O63,4 with both 1D cobalt chains arranged in a (pseudo-) triangular lattice have been investigated but remain controversial about the real nature of the magnetic exchanges inside and between chains.5−9 An additional interest emerges from the properties coupled to the magnetic specificities of such systems. For instance, the Ban+1ConO3n−1Br (n = 5 and 6) perovskite derivatives have important magnetocaloric effects associated to the spin flop transitions between individual FM blocks.10 α-CoV2O6 displays magnetodielectric couplings following the magnetic plateau.11 Also, in © 2013 American Chemical Society
Received: May 31, 2013 Revised: August 1, 2013 Published: August 1, 2013 18190
dx.doi.org/10.1021/jp405382t | J. Phys. Chem. C 2013, 117, 18190−18198
The Journal of Physical Chemistry C
Article
Figure 1. Magnetic measurements (χ, χ−1, and d(χT)/d(T) in the inset) as a function of temperature on aligned powder (H = 0.1 T) of (a) BaCoP2O7 and (b) BaCoAs2O7. (c) Magnetization as a function of the applied field at 2 K for BaCoAs2O7 (red) and BaCoP2O7 (blue) aligned powder. (d) Magnetization data collected along the three crystallographic axes for aligned crystals of BaCoAs2O7 at 2 K. dM/dH along the a axis is shown in the inset with a minimum of inflection for the ferromagnetic plateau. (e, f) Deconvolution of M(H) at 2 K for BaCoP2O7 and BaCoAs2O7 into FERRI and FM components as described in the text.
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RESULTS AND DISCUSSION Magnetic Properties. The magnetic susceptibility as a function of temperature χ(T) measured on aligned powder is shown in Figure 1a,b for α-BaCoAs2O7 and α-BaCoP2O7. In the paramagnetic regime, a Curie−Weiss law was fitted with parameters μeff = 5.1 μB/Co and θCW = −21.7 K for X = P (in good agreement with data from ref 15) and μeff = 4.35 μB/Co and θCW = −3.75 K for X = As. Both μeff values lie in the expected range for the Co2+ cation taking into account an important orbital contribution. The weak but negative Weiss constants indicate weakly predominant anti-ferromagnetic (AFM) exchanges. For both compounds, the χ(T) curves show broad maxima at TM = 20 K/15 K for BaCoP2O7/ BaCoAs2O7, while d(χT)/dT exhibits sharp peaks at the Néel temperature, 11 K/10 K, respectively. For BaCoP2O7, this estimation of the TN value was corroborated by specific heat measurements.15 The large difference between TN and TM values suggests the presence of a large contribution from the short-range correlations. Our magnetization measurements up to 14 T, carried out on aligned powders of BaCoAs2O7 and BaCoP2O7, are represented in Figure 1c. Data for BaCoP2O7 reproduces well the M(H) plot previously reported.15 For both compounds, it shows two overlapped broad magnetization steps maximal at ∼Mmax/3 and Mmax. At the critical fields (determined from dM/dH maxima), Hc1 = 5.2/5.3 T, and Hc2 = 7.1/7.4 T for BaCoP2O7/ BaCoAs2O7, respectively. These states likely correspond to an intermediate ferrimagnetic (FERRI) state, followed by a ferromagnetic (FM) alignment of spins. Note that FM and
sharing dimers of square pyramids interconnected by PO4 groups. At first glance, the arrangement between the disconnected chains could not explain the sequence of magnetic events under a magnetic field. It is noteworthy that evidence for incommensurate structural modulations have been reported for both α-BaCoP2O7 and α-BaNiP2O7.16,17 Their possible influences on the magnetic properties was occulted so far. Recently, we have prepared the new α-BaCoAs2O7 (already reported in its β form18) that behaves similarly to the phosphate α homologue. Here, we describe the unconventional effect of the aperiodic modulation on its magnetic properties. Especially, we point out the most probable relationships between the field-dependent magnetization and the atomic shift driven by the structural modulation. In this sense, we combined density functional theory (DFT) calculations on various local structures created in the real space by the modulation wave, leading to quantitative and qualitative insights on aperiodic magnetic paths and coupling exchanges. The validity of our calculations is confirmed by the magnetic structure refined from powder neutron diffraction data. In this study, first the magnetic properties will be characterized to confirm similar steplike magnetization for both As and P compounds. After resolution of the incommensurate modulated structure for the As phase, it will be shown that strong atomic shifts are responsible for the magnetization steps using the adapted J determinations. 18191
dx.doi.org/10.1021/jp405382t | J. Phys. Chem. C 2013, 117, 18190−18198
The Journal of Physical Chemistry C
Article
(accordingly with the presence of third-order satellites) for Ba, Co, and As atoms, concomitantly with a sensitive improvement of the reliability factors (Table S4, Supporting Information). A first-order modulation wave was used for all oxygen atoms. The refined modulation waves are given in Table S5 (Supporting Information). It leads to the final reliability factors R1 = 3.57%, Rfirst‑order = 2.11%, Rsecond‑order = 4.67%, Rthird‑order = 9.18%, Rall = 13.23% for all I > 3σ(I) using 215 refined parameters and 10 454 independent reflections (4640 I>3σ(I) + 5814 I
