Research ArticlePHYSICS

Weak-field induced nonmagnetic state in a Co-based honeycomb

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Science Advances  24 Jan 2020:
Vol. 6, no. 4, eaay6953
DOI: 10.1126/sciadv.aay6953


  • Fig. 1 Crystal structure and anisotropic magnetic susceptibility.

    (A) Schematic of the BaCo2(AsO4)2 crystal structure, showing the honeycomb plane stacking along the c axis. (B) An individual honeycomb layer made of edge-sharing CoO6 octahedra; the [AsO4]3− tetrahedra sit in the middle of each honeycomb. The lines in (A) and (B) indicate the unit cell. (C) Room temperature x-ray diffraction pattern of the crushed BaCo2(AsO4)2 single crystals, indicating the high quality of the crystals. Calculated diffraction peak positions are marked by short blue ticks. The inset in (C) shows a photo of a dark pink single crystal. a.u., arbitrary units. (D and E) dc magnetic susceptibility χ and the inverse susceptibility 1/χ as a function of temperature measured for a BaCo2(AsO4)2 single crystal, under magnetic fields (0.4 T) applied both in plane [(D), H ⊥ c] and out of plane [(E), H ∥ c]. The magnetic transition is shown in detail in the insets. The CW fitting (black lines) results in Curie temperatures of Θ|| = −167.7 K and Θ = 33.0 K for out-of-plane and in-plane magnetic fields, respectively. emu, electromagnetic unit.

  • Fig. 2 Field-induced phase transitions.

    (A) Left: ac magnetic susceptibility as a function of in-plane dc field up to 1 T, measured at multiple temperatures. The dashed curve indicates the critical field Hc2 at different temperature, obtained from the peak positions of χ′ac. The box area has been zoomed into the right panel. Right: The dashed lines illustrate the critical field Hc1 at each temperature. (B and C) Magnetic hysteresis measured under an in-plane dc field |μ0H| ≤ 1 T at 1.8 K (B) and 5 K (C), respectively. The arrows in (B) indicate the field sweeping direction. Hc1 and Hc2 obtained in (A) at each temperature are indicated by the dashed lines, separating multiple magnetic phases with colorful shading. The two insets show the magnetic hysteresis at each temperature within a full field range of 9 T.

  • Fig. 3 Weak in-plane field manipulation of the magnetic structure.

    (A and B) Magnetization as a function of temperature, measured with an in-plane field (A) μ0H ≤ 0.26 T and (B) μ0H > 0.26 T. TN1 and TN2 with red arrows indicate both phase transitions. (C) Low-temperature specific heat Cp/T at several magnetic in-plane fields. TN with red arrow indicates the observed single peak at all temperatures associated with the first-order transition. (D) dc magnetization as a function of temperature measured under several out-of-plane fields. (E) Magnetic hysteresis loop measured under out-of-plane fields up to 9 T. (F) Low-temperature specific heat Cp/T measured with dc fields applied perpendicular to the hexagonal plane.

  • Fig. 4 Temperature and field dependence of the thermal conductivity.

    (A) Thermal conductivity over temperature κxx/T against temperature at various magnetic fields. The inset shows a schematic of the experiment setup. Two thermometers measuring temperature at A and B are marked as TA and TB. External field B is applied in-plane. w and l represent the sample’s width (1.5 mm) and length (3 mm), respectively. (B) Thermal conductivity κxx versus magnetic field at various temperatures. Hc1 and Hc2 marked in the figure are obtained from ac susceptibility shown in Fig. 2A. The magnetic field is applied in the ab plane for all thermal conductivity measurements.

  • Fig. 5 A phase diagram showing the evolution of the AFM order under an in-plane field in BaCo2(AsO4)2.

    TN1 (black dots) is defined as the temperature where the low-temperature broad transition has a maximum in Fig. 3A. The corresponding error bars illustrate the full width at half maximum of the hump. The Néel temperature TN2 (blue circles) is determined from magnetization in Fig. 3B. TN is determined from specific heat data (black squares) shown in Fig. 3C or thermal conductivity measurements (red diamonds) shown in Fig. 4A. The boundary between different phases, Hc1 and Hc2, is determined from the ac susceptibility in Fig. 2A.

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