Short range magnetic correlations in spinel Li(Mn Co ) 2 O 4

Short range magnetic correlations in spinel Li(Mn Co ) 2 O 4 Nanophysics Laboratory, Department of Physics, National Central University

C. C. Yang, a F. C. Tsao, a S. Y. Wu, a W.-H. Li, a * and K. C. Lee, a J. W. Lynn, b R. S. Liu, c a Department of Physics, National Central Universtiy, Chung-Li, Taiwan 32054, Republic of China b NIST Center for Neutron Research, NIST, Gaithersburg, Maryland c Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, Republic of China

Nanophysics Laboratory, Department of Physics, National Central University Abstract The energy material Li(Mn Co ) 2 O 4 was prepared by standard solid-state reaction techniques. The structures are confirmed by varied temperature neutron scattering experiments. From 300 K to 1.8 K, these samples hold cubic Fd3m spinal phase without any structure change. In ac magnetization experiments, M(T) may be described using the Curie-Weiss law for antiferro- magnetic coupling at high temperatures, which T  = 86 K. At low temperature, two anomaly peaks are observed at 25 K and 13 K, which are mainly contributed by Mn spins. The neutron magnetic scattering discovered Li(Mn Co ) 2 O with varied temp- erature which shows the short-range correlation started from 80 K and saturated around 40 K. _

Nanophysics Laboratory, Department of Physics, National Central University Structural Analysis As a cathode material for rechargeable lithium-ion batteries, the spinel LiMn 2 O 4 is known [1,2] to be economically a more suitable material than currently popular LiCoO 2. Improvement in the rechargeable cycle-performance at room temperature has been reported [3] in Li-rich systems, and a small amount of Co-doping has been found to stabilize the structure. A polycrystalline sample of Li(Mn Co ) 2 O 4 was prepared by employing the standard solid-state reaction techniques. High purity Li 2 CO 3, MnO 2, and CoO powders were evenly mixed at a stoichiometric molar ratio, and then sintered in air at 800°C for 24 h, followed by slowly cooling to room temperature. High-resolution neutron powder diffraction and Rietveld analysis [4] were employed to determine the detailed structural parameters. The diffraction pattern was collected on the BT-1 powder diffractometer at the NIST Center for Neutron Research, employing a Cu(311) monochromator crystal to extract = Å neutrons. The diffraction pattern taken at 300 K displayed a cubic Fd3m symmetry as Fig. 1, occupy their which is the same structure as the reported one the for undoped compound [5,6]. Both the Li and Mn/Co atoms occupy their normal sites, and the Co atoms enter the Mn sites. Analysis of the occupancy factors gave a chemical formula of Li(Mn Co ) 2 O for the present compound as list in the table 1. No traces of any impurity phases were found, as the temperature was reduced to 7 K, showing that 2.4% Co-doping stabilizing the crystalline structure against temperature change. _

The effects of Co-doping on the magnetic properties of the system were studied by means of ac magnetic susceptibility and neutron magnetic diffraction measurements. Neutron magnetic diffraction measurements were also conducted at the NIST Center for Neutron Research, using the BT-9 triple-axis spectrometers, with a pyrolytic graphite PG(002) monochromator crystal and PG filters to extract =2.359 Å neutrons.Figure 2 shows the in-phase component of the ac magnetic susceptibility, χ(T), measured at various applied dc magnetic fields. The main features perceivable in χ(T) are the peaks at ~15 K. Finite values for χ were obtained at low temperatures, cusps in the χ(T) curves are clearly seen, and an applied field suppresses the responses in χ at low temperatures, suggesting the existence of both the ferromagnetic and antiferromagnetic components for the Mn moments. Although the peaks occur at ~15 K, the correlations between the Mn spins develop at a much higher temperature, as indicated by the observations that χ(T) departs from the Curie-Weiss behavior at ~150 K, as can be seen in the 1/χ curve shown in the inset to Fig. 2. Nanophysics Laboratory, Department of Physics, National Central University Magnetic Susceptibility

Nanophysics Laboratory, Department of Physics, National Central University Figure 3 shows the magnetic diffraction pattern obtained at 1.4 K. Two broad peaks at around 2θ=31  and 45 , with very different widths, are clearly revealed, signaling the development of short-range magnetic correlations among the Mn spins, as the temperature was reduced from 160 to 1.4 K. Detail investigations show that the magnetic intensities include three peaks, as marked by the dashed curves shown in Fig. 3. The magnetic diffraction pattern observed for the 2.4% Co-doped compound is similar to that was observed [6] in the undoped compound, but with the widths of the peaks are much broader. As has been observed [6,7] in the undoped compound, there are both the ferromagnetic, characterized by the {111} peak, and antiferromagnetic, characterized by the {01½} and {011} peaks, components for the Mn moments in the 2.4% Co-doped compound. The magnetic correlation lengths that we obtained for the 2.4% Co-doped compound at 1.4 K are 100 Å and 30 Å for the antiferromagnetic and ferromagnetic components, respectively, which are somewhat smaller than the 120 Å and 40 Å observed for the undoped compound [6]. The temperature dependence of the intensity at 2θ=31  is shown in Fig. 3, showing that the magnetic correlations began to develop below Tm=150 K. The Tm observed for the 2.4% Co-doped compound is almost a factor of 2 higher than that of the undoped compound, indicating that the Co- doping enhancing the couplings between the Mn spins. Magnetic Neutron Diffraction

Acknowledgements Nanophysics Laboratory, Department of Physics, National Central University The work at was supported by the NSC of the ROC under Grant No. NSC M Reference 1. M. Thackeray et al., Mater. Res. Bull. 18, 461 (1983). 2. D. Guyomard et al., Solid State Ionics 69, 222 (1994). 3. R. J. Gummow et al., Solid State Ionic 69, 59 (1994). 4. H. M. Rietveld, J. Appl. Cryst. 2, 65 (1969). 5. W. I. F. David et al., J. Solid State Chem. 67, 316 (1987). 6. C. C. Yang et al., Mat. Sci. Eng. B 95, 162 (2002). 7. I Tomeno et al., Phys. Rev. B 64, (2001).

Scattering Angle 2  ( deg. ) Neutron Counts Li(Mn Co ) 2 O T = 300 K, Fd3m λ = Å, 15'-20'–7' a = (8) Å Li (¼, ¼, ¼), Mn(½, ½, ½) O(x, x, x), x= (4) _ Nanophysics Laboratory, Department of Physics, National Central University The neutron-powder-diffraction pattern of sample Li 0.96 (Mn Co ) 2 O at room temperature. Observed (crosses) and Fd3m-fitted (solid lines) patterns with their differences plotted at the bottom. The inset table shows the fitting parameter at other different temperatures. _ Fig. 1.

150 K Nanophysics Laboratory, Department of Physics, National Central University Temperature dependence of  (a) and  (b), measured using a probing field with an rms strength of 10 Oe and a frequency of 10 3 Hz, and the insert shows the dependence of applied field. The main feature is the cusp at ~13 K, which signifies the ordering of the Mn spins with an antiferromagnetic character. Anomalies observed around 25 K which is govern by the ratio of Mn 3+ / Mn 4+ ion. Fig. 2.

Nanophysics Laboratory, Department of Physics, National Central University Differences between the diffraction patterns token at 1.4 and 140 K. The broaden peak between 27  ~ 37  show the short-range magnetic ordering of Mn. Fig. 3.

Nanophysics Laboratory, Department of Physics, National Central University Temperature dependence of the 31  peak intensity where the solid lines are only guides to the eye. The 31  intensity disappears at 130 K. Fig. 4.

Nanophysics Laboratory, Department of Physics, National Central University Neutron diffraction pattern taken at various temperatures. No structure change observed between 9 K~300 K. The inset show the temperature dependence of fitted lattice parameters, Mn-O length, and Mn-O-Mn angle. The lattice parameters and Mn-O length increasing monotonically as thermo expansion. No obvious changes on the Mn-O-Mn angle were seen. Fig. 5. Li(Mn Co ) 2 O 4

Nanophysics Laboratory, Department of Physics, National Central University Li(Mn 0.95 Co 0.05 ) 2 O 4 Neutron scattering pattern taken at different temperature. No structure change observed between 7 K~300 K. The inset show the temperature dependence of lattice parameters, Mn-O length and Mn-O-Mnangle got from structure refinement. The lattice parameters and Mn-O length increasing monotonically as thermo expansion. No observed change of Mn-O-Mn angle. Fig. 6.

Nanophysics Laboratory, Department of Physics, National Central University Temperature dependence of  (a) and  (b), measured using a probing field with an rms strength of 10 Oe and a frequency of 10 3 Hz, and the insert shows the dependence of applied field. The main feature is the cusp at ~13 K, which signifies the ordering of the Mn spins with an antiferromagnetic character. Anomalies observed around 25 K which is govern by the ratio of Mn 3+ / Mn 4+ ion. The red curve was Curie-Weiss fit of zero field experiment. We can get the fitting parameter T θ ~57 K and  eff ~ 3.33  B. Fig. 7.

Li(Mn 0.95 Co 0.05 ) 2 O 4 λ = A 40’-48’-40’ (a)(b) (c)(d) Nanophysics Laboratory, Department of Physics, National Central University The neutron magnetic diffraction patterns of Li(Mn 0.95 Co 0.05 ) 2 O 4, taken at various temperatures, Showing the development of magnetic correlations when the temperature was reduced. Fig. 8.

(e) (g) (f) (h) Nanophysics Laboratory, Department of Physics, National Central University The neutron magnetic scattering pattern of Li(Mn 0.95 Co 0.05 ) 2 O 4. The Fig (e), (f), (g), (h) were collected at 65 K, 80 K, 110 K, 120 K, which show the growth of short range magnetic ordering. Fig. 9.

Cubic Fd3m Li (¼, ¼, ¼), Mn ( ½, ½, ½), O (x, x, x) Temp. (K )a ( Å )O (x)Mn-O( Å ) Mn-O-Mn ( ° ) χ² (14) (6)1.9518(4)96.283(28) (6) (5)1.9520(4)96.275(25) (6) (5)1.9521(4)96.279(25) (6) (5)1.9528(4)96.257(25) (6) (5)1.9529(4)96.294(25) (5) (5)1.9535(3)96.308(21) (8) (4)1.9542(5)96.270(30)1.620 _ Nanophysics Laboratory, Department of Physics, National Central University Table. 1.