Unconventional magnetic phase separation in $\ensuremath{\gamma}{\text{-CoV}}_{2}{\mathrm{O}}_{6}$

Unconventional magnetic phase separation in $γ {-CoV}_{2} O_{6}$

L. Shen, E. Jellyman, E. M. Forgan, E. Blackburn, M. Laver, E. Canévet, J. Schefer, Z. He, and M. Itoh

Phys. Rev. B 96, 054420 – Published 14 August 2017

Abstract

We have explored the magnetism in the nongeometrically frustrated spin-chain system $γ − {CoV}_{2} O_{6}$ which possesses a complex magnetic exchange network. Our neutron diffraction patterns at low temperatures $(T \leq T_{N} = 6.6$ K) are best described by a model in which two magnetic phases coexist in a volume ratio 65(1) : 35(1), with each phase consisting of a single spin modulation. This model fits previous studies and our observations better than the model proposed by Lenertz et al. [J. Phys. Chem. C 118, 13981 (2014)], which consisted of one phase with two spin modulations. By decreasing the temperature from $T_{N}$ , the minority phase of our model undergoes an incommensurate-commensurate lock-in transition at $T^{*} = 5.6$ K. Based on these results, we propose that phase separation is an alternative approach for degeneracy-lifting in frustrated magnets.

Received 5 September 2016
Revised 31 May 2017

DOI:https://doi.org/10.1103/PhysRevB.96.054420

Physics Subject Headings (PhySH)

Frustrated magnetism Magnetic interactions Magnetic phase transitions Phase separation

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

L. Shen^*, E. Jellyman, E. M. Forgan, and E. Blackburn^†

School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom

M. Laver

School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, United Kingdom

E. Canévet and J. Schefer

Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

Z. He

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China

M. Itoh

Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan

^*Corresponding author: lxs233@alumni.bham.ac.uk
^†Corresponding author: e.blackburn@bham.ac.uk

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Vol. 96, Iss. 5 — 1 August 2017

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Figure 1
(a) Crystallographic structure of $γ CVO$ . Oxygen anions (omitted for clarity) occupy the corner of the shaded polyhedra. Possible NN spin exchange paths for Co(1) and Co(2) are displayed separately in two unit cells. (b) Neutron powder diffraction pattern measured at $λ = 4.5$ Å and $T = 1.5$ K. The red solid dots are experimental observations. The black and blue lines are the calculated pattern and the difference using the two-phase model. Black, pink, and green vertical bars mark the nuclear, $k_{1}$ - and $k_{2}$ -modulated Bragg positions, respectively. Upper inset: Enlarged view of the shaded area in the main panel. The black solid line is the Rietveld refinement to the long-range order. The purple solid line is a Gaussian fit to the short-range correlations. The dotted line is a $3^{\circ}$ polynomial fit to the background between 0.2 and 0.6 $Å^{- 1}$ . Lower inset: Differences between the observed and calculated intensities in the low- $Q$ region based on the two background treatment methods described in the main text. (c) The spin structures, reproduced by vesta [15], in the $k_{1}$ (upper panel) and $k_{2}$ (lower panel) phases, respectively. The black frames display the size of the corresponding magnetic unit cells.
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Figure 2
(a) Selected regions of the powder diffraction patterns between 5.4 K and 6.6 K. The blue curve and arrows mark the shifting reflections $(1, 0, 0) + k_{2}$ and $(- 2, 0, 1) + k_{2}$ , respectively. The peak positions in the intermediate region are fitted with Gaussian functions (solid lines). A constant vertical shift has been applied to patterns measured above $T^{*}$ . The remnant peak above $T^{*}$ is indexed as (0.5, $- 1$ , 0). (b) Temperature dependence of the (1.5, 0, 0) and $(- 0.5$ , 0, 1) reflections generated by $k_{1}$ , which in contrast do not shift. (c) Temperature dependencies of the $x$ and $z$ components of $k_{2}$ around $T^{*}$ . C denotes commensurate, IC denotes incommensurate, and PM denotes paramagnetic.
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Figure 3
(a) Normalized intensity versus temperature plots of reflections at $Q_{2} = (0.5, 0, 0)$ and $Q_{3} = (1, 0, 0) + k_{2}$ and the magnetic Bragg peak at $\sim 1.35$ $Å^{- 1}$ consisting of $Q_{4} = (1.5, 0, 0)$ and $Q_{5} = (- 0.5, 0, 1)$ reflections. (b) Intensity versus $Q$ curve around the $Q_{1} = (- 0.25, 0, 0.25)$ and the $Q_{2}$ reflections at 6.6 K. The nuclear scattering background, taken at 35 K, has been subtracted. The solid lines are numerical fits described in the text. (c) and (d) Evolution of the diffuse scattering signals in the low- $Q$ region as a function of temperature.
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Physical Review B

covering condensed matter and materials physics

Unconventional magnetic phase separation in $γ {-CoV}_{2} O_{6}$

L. Shen, E. Jellyman, E. M. Forgan, E. Blackburn, M. Laver, E. Canévet, J. Schefer, Z. He, and M. Itoh

Phys. Rev. B 96, 054420 – Published 14 August 2017

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Physical Review B

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Unconventional magnetic phase separation in γ-CoV2O6

L. Shen, E. Jellyman, E. M. Forgan, E. Blackburn, M. Laver, E. Canévet, J. Schefer, Z. He, and M. Itoh

Phys. Rev. B 96, 054420 – Published 14 August 2017

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Unconventional magnetic phase separation in $γ {-CoV}_{2} O_{6}$