Magnetic Structure of Ground and Field Induced Ordered States of

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Magnetic Structure of Ground and Field Induced Ordered States of Low-Dimensional γ‑CoV2O6 M. Lenertz, A. Dinia, and S. Colis* Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France

O. 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

G. André and F. Porcher CEA, Centre de Saclay, DSM/IRAMIS, Laboratoire Léon Brillouin (LLB), F-91191 Gif-sur-Yvette, France

E. Suard Institut Max von Laue-Paul Langevin (ILL), 6 rue Jules Horowitz, BP 156, F-38042 Grenoble Cedex 9, France ABSTRACT: The structural and magnetic properties of γ-CoV2O6 low-dimensional magnetic oxide have been investigated with an emphasis on its magnetic structure. Our main results have been obtained by powder neutron diffraction measurements carried out under magnetic fields of 0, 0.52, and 1 T, corresponding to the antiferromagnetic, ferrimagnetic, and ferromagnetic configurations of the magnetic moments. The magnetic moments are not collinear, but lie mostly along the b direction (i.e., inside structural edge-sharing CoO6 chains), which corresponds to the experimental easy magnetization axis. In the ground state, the moments are ferromagnetically ordered along the b and c directions, and antiferromagnetically ordered along a. The ferromagnetic sheets (bc plane) containing the chains are separated by nonmagnetic sheets containing VO6 octahedra and VO4 tetrahedra. The ground state magnetic structure can be described in a (4a, 2b, c) supercell using two antiferromagnetic propagation vectors kAF1 = (1/2, 0, 0) and kAF2 = (1/4, 1/2, 0). The ferri- and ferromagnetic structures are characterized by kFi = (1/3, 0, −1/3) and k = (0, 0, 0) propagation vectors, respectively. These results are in agreement with macroscopic magnetic measurements performed on single crystals.

1. INTRODUCTION Low-dimensional magnetic oxides integrating one-dimensional magnetic chains present an increasing interest due to their unusual properties such as a stepped magnetization reversal,1,2 a particularly strong magnetic anisotropy,3 interesting magnetodielectric coupling,4,5 giant magnetostriction,6 quantum phase transition,7 or soliton propagation.8 In some materials showing low resistivity, magnetoresistive properties could also be correlated with the magnetic behavior.9,10 Recently, it was also reported that such Ising chain magnets can also have similar magnetization dynamics to what reported previously in molecular magnets.11,12 Many of these properties originates from geometrical frustrations leading to competing antiferromagnetic (AF) and ferromagnetic (F) interactions, from structural modulations, and sometimes from mixed valences magnetic ions. © 2014 American Chemical Society

CoV2O6 is such a one-dimensional magnetic oxide that exhibits an AF ground state, a strong anisotropy and a magnetization plateau at one-third of the saturation magnetization (MS). It presents two polymorphs: one monoclinic called α-CoV2O6 with C2/m space group13 and one triclinic called γ-CoV2O6 (Figure 1) with P-1 space group14 Both are constituted of edge-sharing CoO6 octahedra forming onedimensional chains running along the b axis.15 These chains form planes that are separated by V5+O4/6 nonmagnetic units. While in the α phase, all Co atoms are equivalent and perfectly aligned along chains (CoCoCo angles of 180°), in the γ phase octahedral Co occupies two different atomic positions in Received: April 7, 2014 Revised: June 3, 2014 Published: June 5, 2014 13981

dx.doi.org/10.1021/jp503389c | J. Phys. Chem. C 2014, 118, 13981−13987

The Journal of Physical Chemistry C

Article

2. EXPERIMENTAL SECTION The polycrystalline γ-CoV2O6 powder was prepared by solidstate reaction from vanadium oxide and hydrated cobalt oxalate. Both compounds were ground together in agate mortar and then heated several times at 640 °C in a platinum crucible. The resulting powder was ground in order to obtain a fine powder. Stoichiometry was checked by EDS analysis and showed a good agreement between the measured and the nominal values. The single crystal samples were obtained by flux method as described by Müller-Buschbaum and Kobel.14 A mixture of αCoV2O6 and V2O5 with 3:2 molar ratio was sealed in a quartz tube. The use of V2O5 in excess allows avoiding additional impurities in the final phase. The mixture was melted at 780 °C and kept at this temperature for 20 h in order to ensure an homogeneous solution. The melted solution was cooled down to 660 °C at a rate of 1 °C/h and further to 640 °C at a rate of 1 °C/8 h. The first cooling step to 660 °C results in obtaining α-CoV2O6, while the second operates a transition from α to γCoV2O6. This second step has to be slow enough in order to avoid crystal breaking due to internal stress. Below 640 °C, the samples were cooled down to room temperature in about 5 h, and the resulting crystals (inset of Figure 2a) were separated mechanically from the tube. The magnetic properties of the samples were analyzed using a MPMS SQUID-VSM (Quantum Design) magnetometer. In the case of powders, the measurements were carried out on nonaligned samples (random crystallites orientation) in order to allow a comparison with the PND measurements carried out

Figure 1. Crystalline structure of γ-CoV2O6 showing the magnetic chains running along the b direction. The VO6 octahedra (orange) and VO4 tetrahedra (yellow) form nonmagnetic planes separating the chains along the a direction.

the sequence 212212, which gives rise to a canting angle between two adjacent Co bonds (CoCoCo angles of 170° or 180°).8 Due to its higher symmetry crystalline structure and collinear arrangement between the spins, the magnetic structures of the ground and field induced states of αCoV2O6 have been successfully characterized by powder neutron diffraction (PND)16−18 and theoretical studies.18−21 In contrast, the situation is far more complicated for the γ phase. Although polycrystalline and single crystal samples,14,15,22 as well as epitaxial thin films23 have been obtained, and despite the correct characterization of the crystalline structure, results mainly concern the description of the large magnetic anisotropy, the magnetization steps at MS/3, and the Néel ordering temperature of 7 K. The only attempt to resolve the AF ground state magnetic structure was carried out by PND.8 Although a propagation vector k = (1/2, 0, 0) was found, characterizing standard AF and F moment configurations along the a and c axes, respectively, there is no strong proof that the order inside the chains (i.e., along the b axis) is ferromagnetic as this was the case for the α phase.16,18 Moreover, many magnetic satellite peaks could not be indexed.8 The authors have therefore suggested the existence of incommensurate modulated magnetic structures related to the existence of the two different Co sites, as this was evidenced in other frustrated systems such as Ca3Co2O6.24,25 Unfortunately, no theoretical calculation is yet available on the γ phase to confirm or not this hypothesis. Finally, note also that some discrepancies exist on the easy magnetization axis direction in γCoV2O6. While in γ-CoV2O6 epitaxial thin films, the easy magnetization axis was reported to lie along the b direction (i.e., moments along the chains),23 a recent paper on bulk single crystals showed that the easy axis lies in the c direction22 (i.e., perpendicular to chains), as this is indeed the case in the α phase.3,18 The aim of this work is to shed a new light on the magnetic properties of triclinic γ-CoV2O6, and in particular (i) to accurately resolve for the first time the magnetic structure in the ground state and under magnetic field [for the field induced ferrimagnetic (Fi) and ferromagnetic (F) states], (ii) to check its incommensurate or not character, and (iii) to clarify the direction of the easy magnetization axis. This was carried out by neutron diffraction measurements on polycrystalline powder and by SQUID measurements performed on single crystal samples.

Figure 2. (a) Magnetization curves of γ-CoV2O6 single crystal (with the field applied along the b and c axis, and perpendicular to the bc plane) and powder (random oriented crystallites). The inset shows some of the single crystals obtained for this study. (b) Temperature dependence of the magnetization at 0.1 T recorded along the same axes following a field cooling and zero-field cooling procedure. 13982



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Figure 3. Zero field neutron diffraction patterns for γ-CoV2O6 at (a) room temperature (3T2: λ = 0.1225 nm) and (b) 1.7 K (G4.1: λ = 0.2423 nm). Neutron diffraction patterns recorded at (c) 1.9 K and 0.52 T corresponding to the ferrimagnetic state, and (d) 1.9 K and 1 T corresponding to the ferromagnetic state (D2B: λ = 0.2399 nm). In red experimental data, in black the calculated pattern, in blue the difference and in green the Bragg positions. The arrows in parts b, c, and d show the main antiferromagnetic, ferrimagnetic, and ferromagnetic peaks, respectively. The nuclear and magnetic structures are calculated by Rietveld and profile matching method, respectively.

3. RESULTS AND DISCUSSION The magnetic measurements performed on single crystals (Figure 2) along the different axes show clearly that b is the easy magnetization axis. Indeed, along this direction, the magnetization loop shows a stepped variation with three plateaus corresponding to the AF, Fi, and F states. A first field induced AF−Fi transition is obtained at about 0.4 T followed by an Fi−F transition at 0.6 T. The Fi plateau (∼1.1 μB) is located at about 1/3 of the magnetization at 1.5 T (∼3 μB). Along the c direction and along the normal to the bc plane (close to the a direction), the magnetization varies slowly and no field induced transition is observed. At 1.5 T, the magnetization along the hard axes is about 10 times smaller than that recorded along the b easy axis. The existence of the AF order is also confirmed by the M-T variations recorded at constant field (0.1 T) after cooling the samples in zero field (ZFC) and 0.1 T (FC). As already reported for polycrystalline γ-CoV2O6,8,15 the ZFC/FC curves are superimposed and show a maximum at 7 K corresponding to the Néel temperature (TN) of the AF ground state. This is typical to what is usually observed in triangular frustrated low dimensional AF systems. With respect to α-CoV2O6,18 some important differences have to be underlined. One important difference is related to the easy magnetization axis which lies along the chains (i.e., along the b direction) in γ-CoV2O6 while in the α phase it lies perpendicular to the chains (i.e., along the c axis). This difference can be due to (i) the shorter CoCo distance in the γ phase [2.98(1) and 3.50(1) Å in γ- and α-CoV2O6, respectively] resulting in a stronger intrachain interaction and (ii) to the deformation of the octahedral oxygen environment

in the same conditions (random orientation) under magnetic field. In the case of single crystals, previous to the magnetic measurements, the crystalline directions were located using a Rigaku diffractometer. Zero-field powder neutron diffraction measurements were performed at the LLB facility at Saclay (France) using the 3T2 high resolution powder diffractometer at room temperature with a 0.1225 nm wavelength and the G4.1 two-axis diffractometer equipped with a cryostat and using a 0.2423 nm wavelength. Using the 3T2 instrument (high resolution) the accurate nuclear structure was refined, while the G4.1 instrument was used to determine the magnetic structure in the ground state at 1.7 K and the evolution of this structure as a function of temperature. Neutron diffraction under magnetic field was carried out at the ILL facility at Grenoble (France) using the D2B high-resolution two-axis diffractometer (0.2399 nm wavelength) equipped with a cryomagnet delivering a magnetic field up to 5 T, which enabled the determination of both the ferrimagnetic (at 0.52 T) and ferromagnetic (at 1 T) states. For all measurements, the sample was placed in an 8 mm diameter vanadium cylinder. For the measurements under magnetic field, the sample was pressed in order to limit the powder reorientation under high magnetic fields. Because of the weak V atoms scattering length for neutrons, the atomic position of V has been refined using X-ray diffraction (XRD) data. The XRD measurements were carried out on a D8 Brücker-AXS diffractometer (Cu Kα1 wavelength λ = 0.154 056 nm) and equipped with a front monochromator. The diffracted beam was energy filtered in order to eliminate the fluorescence background. 13983



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Figure 4. Magnetic order between the chains in the (a) antiferromagnetic, (b) ferrimagnetic, and (c) ferromagnetic configurations of γ-CoV2O6. CoO6, VO6, and VO4 polyhedra are presented in violet, orange, and yellow, respectively.

Figure 5. Zero field neutron diffraction patterns for γ-CoV2O6 at 1.7 K (G4.1: λ = 0.2423 nm) fitted with the Rietveld method (a) using a zigzag trivial solution of the magnetic moments (see also inset) and (b) using magnetic components along a, b, and c directions for both propagation vectors. The numbers on the atoms inside the chain of part a correspond to the Co (in blue) and O (in red) atoms positions inside the γ-CoV2O6 structure. Part c describes the ground state magnetic structure of γ-CoV2O6 in a (4a, 2b, c) supercell. The V atoms and the O neighbors are not shown for visibility reasons. The red arrows indicate the direction of the magnetic moments.

around the Co atoms which is much larger in the α phase with respect to the γ one. Both effects should modify the Co2+ spin− orbit coupling tailoring the magnetic anisotropy. The two Coindependent sites forming a zigzag inside the chains could explain the difficulty to reach a complete saturation of the magnetization in γ-CoV2O6 along the easy magnetization axis. Indeed, when increasing further the field from 1.5 to 7 T (not shown here), the magnetization of the γ phase increases slowly from 2.95 to 3.12 μB. This is even more intriguing if we keep in mind that the AF interactions (TN = 7 K) are weaker than those of the α phase (TN = 15 K) and where a full saturation is reached at 5 T. This suggests that the magnetic moments do

not lie completely along the chain direction (i.e., b axis), but may exhibit a noncollinear arrangement giving rise to a modulated magnetic structure in a similar manner with what is observed in other single chain magnetic systems.11 Finally, note that the smaller saturation magnetization in the γ phase (3.12 μB at 7 T) with respect to the α phase (∼4.5 μB3,15) supports this assessment while effective moments are comparable in the paramagnetic domain (5.3 μB for the γ phase,26 close to the values reported in ref 8, and 5.4 μB for the α phase6). Besides the different chains topologies in the two phases, recent XMCD measurements attributed this disparity to the different orbital and spin contributions to the magnetic moment (morb/mspin = 13984



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1.9 μB/2.5 μB for the α phase and morb/mspin = 0.7 μB/1.8 μB for the γ phase). These morb/mspin values are explained on the basis of different distortions of the CoO6 octahedra in the two phases.27 Before analyzing the magnetic properties by neutron diffraction, magnetization measurements were also carried out on a polycrystalline (random) sample. Figure 2a shows that in this case, the plateaus are much less defined, the transitions less sharp and the magnetization smaller than that recorded on single crystals. While each plateau corresponds in single crystals to only one magnetic configuration (AF, Fi, or F), due to the random distribution of the anisotropy axes, the magnetic state will be characterized by a mixture of the different magnetic configurations: AF + Fi for a field between 0.4 and 0.6 T and AF + Fi + F above 0.6 T. This mixture of magnetic configurations is therefore expected during the PND measurements recorded under magnetic field. Note that such behavior was already reported for α-CoV2O618 and other polycrystalline low-dimensional compounds.28 The atomic parameters refined on PND measurement at 300 K (Figure 3a) are in agreement with the ones obtained on single crystal [a = 7.1750(3) Å, b = 8.8862(3) Å, c = 4.8079(2) Å, α = 90.310(2)°, β = 93.824(2)°, γ = 102.185(2)°]. Additional measurements (not shown here) performed at 15 K showed no structural transition. Below the Néel temperature, magnetic peaks appear (Figure 3b). Most of the peaks were indexed with the kAF1 = (1/2, 0, 0) propagation vector according to the results reported in ref 8. However, some small peaks could not be indexed in absence of a second propagation vector kAF2 = (1/4, 1/2, 0) necessary to fully describe the magnetic structure. By using a second propagation vector, all peaks could be indexed (Figure 3b). It is important to note that the magnetic reflections of both propagation vectors display the same thermal behavior and totally disappear above TN = 7 K. This supports a magnetic structure described by two propagation vectors rather than the coexistence of multiple magnetic phases. The first propagation vector kAF1 defines a ferromagnetic order along the c axis and a spin inversion between two chains (i.e., antiferromagnetic order) along the a axis (Figure 4a). The intrachain ordering (b axis) along edge-sharing CoO6 octahedra is expected ferromagnetic (see ref 26) as usually observed for plethora of systems with similar topologies.2,16−18,23,29 The second propagation vector modulates the first one, tuning the orientation of the spin in a 8-fold unit cell. This hypothesis is supported by the fact that the magnetic saturation is not totally reached at the F plateau and could explain the slope of the magnetization curve above 1 T (Figure 2a). Because of the low structural symmetry (P-1 space group), the two independent crystallographic sites of the Co atoms and the two propagation vectors, there are at least 12 independent parameters (amplitude and direction of the magnetic moments of the two Co sites and the two propagation vectors) to refine. This number of parameters is large if compared to (i) the number of magnetic reflections, (ii) the high rate of overlap, and (iii) the very weak intensity of the peaks related to the second propagation vector. In order to determine the magnetic structure, different strategies were employed. First, a trivial solution has been tested, followed by a simulated annealing method with the Fullprof software. The trivial solution corresponds to (i) the magnetic moments related to the first propagation vector lay purely along the easy magnetization axis (i.e., b direction),

while (ii) the second propagation vector defines the modulation in the ac plane. In such a configuration, the magnetic moment follows a zigzag structure between the oxygen atoms in the order 4224334 (inset of Figure 5a) as this is the case inside the CoO6 chains of other compounds such as BaCo2(As3O6)2·H2O.11 Unfortunately, this elegant solution does not fit properly the experimental pattern (Figure 5a). It appears that the solution is more complicated and requires a more complex configuration with magnetic components along a, b, and c directions for both propagation vectors. In order to simplify the problem, a magnetic supercell composed of (4a, 2b, c) has been considered with a unique k = (0, 0, 0) propagation vector. The magnetic moment directions of the resulting 4 × 2 × 1 × 3 = 24 magnetic atoms have been constrained in such a way that it reproduces the order imposed by the combination of the kAF1 and kAF2 propagation vectors. With such hypothesis, the number of parameters to refine is still 12. The reason to consider the supercell comes from a first additional simplification, constraining all magnetic moments to have equal amplitude, which decreased the number of free variables to 9. However, examination of the refined parameters show that the in-plane rotation angles θ (Rtheta in Fullprof) for the different magnetic atoms are highly correlated and exhibit a large standard deviation around a mean value of 87(9)°. Consequently, we choose to restrain these angles to a common value, meaning that all the magnetic moments within a CoO chain lie in a plane containing the b axis tilted from the a axis. The remaining 6 independent parameters describe satisfyingly the magnetic structure with agreement factors similar to those obtained without restrains. The best solution, presented in Figure 5b, corresponds to a magnetic R-factor of 8.0% and fits correctly the small peaks. The refined magnetic moment is 3.1(6) μB/Co2+ and lies mainly along the b direction in agreement with the easy magnetization axis evidenced on single crystal. As discussed before, there is a large uncertainty on the small tilt angle with the a axis that suggests that the magnetic moments do not lie exactly in this plane, but the accuracy of our powder diffraction data precludes precise refinement of this angle. We also note that an independent refinement of Co1 and Co2 moments gives similar results for both values. Although the quality of the Rietveld refinement sustains our model, this latter should be confirmed and improved by complementary measurements such as single crystal neutron diffraction. Nevertheless, it is a step forward in the comprehension of the magnetic properties of γ-CoV2O6. At least at this stage, it is clear that a unified anisotropy in each CoO6 is not respected anymore due to the influence of kAF2, while for ions with strong spin−orbit coupling such as Co2+ or Fe2+, the spin orientation is generally governed by local crystal field. It most probably denotes significant antiferromagnetic interactions between the chains for a modulated spin tilting along a 4a period. In order to probe the Fi state, PND under a magnetic field of 0.52 T was carried out. New peaks are observed while the peaks corresponding to the AF ground state which are still present decrease in intensity (Figure 3c). As mentioned above, this is related to the strong uniaxial anisotropy as this is the case in αCoV2O6.18 The new peaks can be indexed using the kFi = (1/3, 0, −1/3) propagation vector. This corresponds to an ”up−up− down” configuration of the Co magnetic moments along the a and c directions (Figure 4b) like in α-CoV2O6. The modulation of the magnetization along the b direction is certainly still present at this field and could be described by the same kAF2 = (1/4, 1/2, 0) propagation vector as for the AF ground state. 13985



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(4) Singh, K.; Maignan, A.; Pelloquin, D.; Perez, O.; Simon, Ch. Magnetodielectric coupling and magnetization plateaus in α-CoV2O6 crystals. J. Mater. Chem. 2012, 22, 6436−6440. (5) Bellido, N.; Simon, Ch.; Maignan, A. Magnetodielectric Coupling in a Triangular Ising Lattice: Experiment and Modeling. Phys. Rev. B 2008, 77, 054430. (6) Nandi, M.; Khan, N.; Bhoi, D.; Midya, A.; Mandal, P. FieldInduced Spin-Structural Transition and Giant Magnetostriction in Ising Chain α-CoV2O6. J. Phys. Chem. C 2014, 118, 1668−1673. (7) Coldea, R.; Tennant, D. A.; Wheeler, E. M.; Wawrzynska, E.; Prabhakaran, D.; Telling, M.; Habicht, K.; Smeibidl, P.; Kiefer, K. Quantum Criticality in an Ising Chain: Experimental Evidence for Emergent E8 Symmetry. Science 2010, 327, 177−180. (8) Kimber, S. A. J.; Mutka, H.; Chatterji, T.; Hofmann, T.; Henry, P. F.; Bordallo, H. N.; Argyriou, D. N.; Attfield, J. P. Metamagnetism and Soliton Excitations in the Modulated Ferromagnetic Ising Chain CoV2O6. Phys. Rev. B 2011, 84, 104425. (9) Ishiwata, S.; Terasaki, I.; Ishii, F.; Nagaosa, N.; Mukuda, H.; Kitaoka, Y.; Saito, T.; Takano, M. Two-Staged Magnetoresistance Driven by the Ising-Like Spin Sublattice in SrCo6O11. Phys. Rev. Lett. 2007, 98, 217201. (10) Cao, G.; Korneta, O.; Chikara, S.; DeLong, L. E.; Schlottmann, P. Ca3(Ru1−xCrx)2O7: A New Paradigm for Spin Valves. J. Appl. Phys. 2010, 107, 09D718. (11) David, R.; Kabbour, H.; Colis, S.; Mentré, O. Slow Spin Dynamics between Ferromagnetic Chains in a Pure-Inorganic Framework. Inorg. Chem. 2013, 52, 13742−13750. (12) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Magnetic Bistability in a Metal-Ion Cluster. Nature 1993, 365, 141−143. (13) Jasper-Tönnies, B.; Müller-Buschbaum, H. Synthesis and Structure Determination of CoV2O6. Z. Anorg. Allg. Chem. 1984, 508, 7−11. (14) Müller-Buschbaum, H.; Kobel, M. Zur Kristallchemie von Oxovanadaten: γ-CoV2O6 und MnV2O6. J. Alloys Compd. 1991, 176, 39−46. (15) Lenertz, M.; Alaria, J.; Stoeffler, D.; Colis, S.; Dinia, A. Magnetic Properties of Low-Dimensional α and γ CoV2O6. J. Phys. Chem. C 2011, 115, 17190−17196. (16) Markkula, M.; Arévalo-López, A. M.; Attfield, J. P. Field-Induced Spin Orders in Monoclinic CoV2O6. Phys. Rev. B 2012, 86, 134401. (17) Markkula, M.; Arévalo-López, A. M.; Attfield, J. P. Neutron Diffraction Study of Monoclinic Brannerite-Type CoV2O6. J. Solid State Chem. 2012, 192, 390−393. (18) Lenertz, M.; Alaria, J.; Stoeffler, D.; Colis, S.; Dinia, A.; Mentré, O.; André, G.; Porcher, F.; Suard, E. Magnetic Structure of Ground and Field-Induced Ordered States of Low-Dimensional α-CoV2O6: Experiment and Theory. Phys. Rev. B 2012, 86, 214428. (19) Saúl, A.; Vodenicarevic, D.; Radtke, G. Theoretical Study of the Magnetic Order in α-CoV2O6. Phys. Rev. B 2013, 87, 024403. (20) Kim, B.; Kim, B. H.; Kim, K.; Choi, H. C.; Park, S. Y.; Jeong, Y. H.; Min, B. I. Unusual Magnetic Properties Induced by Local Structure in a Quasi-One-Dimensional Ising Chain System: α-CoV2O6. Phys. Rev. B 2012, 85, 220407(R). (21) Yao, X. 1/3 Magnetization Plateau Induced by Magnetic Field in Monoclinic CoV2O6. J. Phys. Chem. A 2012, 116, 2278−2282. (22) He, Z.; Itoh, M. Single Crystal Flux Growth of the Ising SpinChain System α-CoV2O6. J. Cryst. Growth 2014, 388, 103−106. (23) Lenertz, M.; Colis, S.; Ulhaq-Bouillet, C.; Dinia, A. Epitaxial Growth of γ-CoV2O6 Thin Films: Structure, Morphology, and Magnetic Properties. Appl. Phys. Lett. 2013, 102, 212407. (24) Agrestini, S.; Chapon, L. C.; Daoud-Aladine, A.; Schefer, J.; Gukasov, A.; Mazzoli, C.; Lees, M. R.; Petrenko, O. A. Nature of the Magnetic Order in Ca3Co2O6. Phys. Rev. Lett. 2008, 101, 097207. (25) Moubah, R.; Colis, S.; Ulhaq-Bouillet, C.; Drillon, M.; Dinia, A. Effect of the Nanometric Scale Thickness on the Magnetization Steps in Ca3Co2O6 Thin Films. J. Phys.: Condens. Matter 2011, 23, 276002. (26) μeff = 5.3(2) μB was calculated from (1/χ) vs T variations on the 150−300 K range. These variations were deduced from the M-T variations measured on the single crystal sample along the different

Unfortunately, because of the simultaneous presence of the peaks corresponding to the AF and Fi states, it is not possible to give any experimental proof at this stage of the study. Indeed, it is not possible to make any difference between a preferential orientation of the magnetic structure and the alinement of the magnetic moment along the magnetization easy axis (also resulting in a disappearance of the modulation and, consequently, of the associated propagation vector). Neutron diffraction measurements on single crystal are necessary to confirm this hypothesis as single crystal samples present well separated magnetic states. In the same way, by increasing the field up to 1 T, a new series of peaks appear at the same positions as the nuclear ones and can be indexed with the k = (0, 0, 0) propagation vector (Figure 3d). They correspond naturally to the ferromagnetic state describing the magnetic chains ordered ferromagnetically along a and c (Figure 4c). All remarks concerning the modulation of the Fi state are still valid and the modulation can be still present in the F state.

4. CONCLUSIONS We have shown in this work that γ-CoV2O6 is a complex system in which we have succeeded to address some crucial points related to its magnetic properties. Notably, magnetization measurements performed on single crystal samples indicated clearly that the easy magnetization axis lies along the magnetic chains direction (i.e., b axis) while the spin arrangement is not fully collinear. Further investigations carried out by PND showed that the AF ground state is composed of ferromagnetically ordered magnetic planes antiferromagnetically coupled along the a axis. The AF, Fi, and F states are characterized by the kAF1 = (1/2, 0, 0), kFi = (1/3, 0, −1/3), and kF = 0 propagation vectors, respectively. The AF ground state needs also a second vector kAF2 = (1/4, 1/2, 0) for a complete description of the magnetic structure, suggesting that an incommensurable character of the magnetic structure is not necessary to explain the magnetic properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out under the framework of the MADBLAST project supported by the French Agence Nationale de la Recherche (ANR) under the reference ANR-09-BLAN-018703.

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REFERENCES

(1) Moubah, R.; Colis, S.; Ulhaq-Bouillet, C.; Drillon, M.; Dinia, A. Magnetization Plateaus in Ca3Co2O6 Thin Films. J. Mater. Chem. 2008, 8, 5543−5546. (2) David, R.; Kabbour, H.; Colis, S.; Pautrat, A.; Suard, E.; Mentré, O. Magnetization Steps Promoted by Structural Modulation in BaCoX2O7 (X = As, P). J. Phys. Chem. C 2013, 117, 18190−18198. (3) He, Z.; Yamaura, J. I.; Ueda, Y.; Cheng, W. CoV2O6 Single Crystals Grown in a Closed Crucible: Unusual Magnetic Behaviors with Large Anisotropy and 1/3 Magnetization Plateau. J. Am. Chem. Soc. 2009, 131, 7554−7555. 13986



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magnetization axes as reported in Figure 2b. It is interesting to note that the Curie−Weiss temperature along the b direction is positive (about 37 K) indicating a ferromagnetic interaction inside the chains, while it is negative along the c axis (about −90 K) and perpendicular to the bc plane (close to the a direction) (about −15 K), showing antiferromagnetic interactions along these directions. (27) Hollmann, N.; Agrestini, S.; Hu, Z.; He, Z.; Schmidt, M.; Kuo, C.-Y.; Rotter, M.; Nugroho, A. A.; Sessi, V.; Tanaka, A.; Brookes, N. B.; Tjeng, L. H. arXiv:1307.6690v1 [cond-mat.str-el] 25 Jul 2013. (28) Hardy, V.; Martin, C.; Martinet, G.; André, G. Magnetism of the Geometrically Frustrated Spin-Chain Compound Sr3HoCrO6: Magnetic and Heat Capacity Measurements and Neutron Powder Diffraction. Phys. Rev. B 2006, 74, 064413. (29) Mentré, O.; Bouree, F.; Rodriguez-Carvajal, J.; El Jazouli, A.; El Khayati, N.; Ketatni, El M. Magnetic Structure and Analysis of the Exchange Interactions in BiMO(PO4) (M = Co, Ni). J. Phys.: Condens. Matter 2008, 20, 415211.

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