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Magnetic Phases in Geometrically Frustrated Magnet, ZnCr2O4, Revealed by Ultra-High Magnetic Fields Up to 600 T

Takeyama Group

Chromium spinel oxides, ACr2O4 (A = Zn, Cd, Hg), in which Cr3+ ions form a pyrochlore lattice are regarded as three-dimensional geometrically frustrated magnets. In these compounds, a spin-lattice coupling plays a crucial role to cause the emergence of diverse magnetic phases with simultaneous lattice distortion under magnetic fields. Interestingly, our recent studies exhibited existence of an unusual magnetic structure (a canted 2:1:1 magnetic structure) at lower magnetic field side of a 1/2 plateau phase only in ZnCr2O4, compared with HgCr2O4 and CdCr2O4 [1]. This is because a spin-lattice coupling of ZnCr2O4 places in a small limit. In this light, elucidation of magnetic phases in ZnCr2O4 up to a saturated magnetization moment has been highly demanded. However, full-magnetization processes of ZnCr2O4 can never be observed unless we overcome difficulties to achieve the multiple extreme conditions, ultra-high magnetic fields in the range of several hundred Tesla and very low temperature down to a few Kelvin. We have developed an original hand-made miniature liquid-He flow-type cryostat made totally of a “Stycast” resin and a cryogenic system for the electro-magnetic flux compression (EMFC) method as shown in Fig. 1, which have enabled us to conduct magneto-optical measurements at cryogenic temperatures of down to 4.6 K and under extremely high magnetic fields of up to 600 T [2].

Fig. 1. (a) Photo picture of a hand-made miniature liquid-He flow-type cryostat made totally of a “Stycast” resin, and (b) photo picture of a total cryogenic system set to a primary coil for the EMFC method.

Fig. 2. (a) The magnetization curve evaluated from the FR of ZnCr2O4 in magnetic fields up to 600 T and at temperature 4.6 K, and (b) the optical absorption intensity of the incident laser used for the FR measurements as a function of magnetic field. The dotted line shows a magnetic field at which an anomaly occurred in the absorption intensity. The inset shows magneto-optical absorption spectra around the energy at which the EMP and intra-d-d transitions take place in magnetic fields of up to 540 T and at 12 K.

Table 1. Physical analogy between magnetic phases of ZnCr2O4 and the theory developed for the quantum phases of 4He.

The magnetization processes of ZnCr2O4 up to a full saturation moment were unveiled by the precise Faraday rotation (FR) measured up to 600 T and at 4.6 K, of which typical data are shown in Fig. 2 (a). In addition, we could also obtain an optical absorption intensity of ZnCr2O4 as a function of magnetic field in the signal of the Faraday rotation as shown in Fig. 2 (b). A ferromagnetic phase transition was observed at 410 T and 4.6 K, above which magnetization of ZnCr2O4 was saturated at a value of 3 μB/Cr3+. In addition, an abrupt change in FR absorption intensity (a green line in Fig.2 (b)) was found at 350 T, while the magnetization monotonically increased. In order to clarify this anomaly, magneto-optical absorption spectral measurements were carried out by a streak spectroscopy as shown in the inset of Fig. 2 (b). As a result, anomalies of the optical absorption intensity were observed in both the intra-d band and the exciton-magnon-phonon transitions, in which the spectral shape and the peak position are very susceptible to the crystal and the magnetic structure. This anomaly indicates definite existence of a novel magnetic phase accompanied by changes of both crystal and magnetic structure. This new phase is beyond the prediction of the theory taking account of the spin-lattice coupling developed by Penc et al. [3]. The most feasible phase is realization of an umbrella-like magnetic structure between the canted 3:1 and the ferromagnetic phase.

If we assume the new phase as the umbrella-like spin alignment, then there found an interesting analogy with the theory developed for the quantum phases in 4He by Matsuda and Tsuneto [4], and Liu and Fisher [5]. According to their theories, the supersolid phase is sandwiched by a solid and a superfluid phase, and is unlikely to be adjacent to a liquid phase because a liquid-supersolid transition takes place by a simultaneous breaking of both the translational and spins rotational symmetry. Therefore, our proposed magnetic phases agree quite well with the sequence of phases; the canted 3:1 phase (i.e., supersolid) is sandwiched by the 1/2 plateau phase (i.e., solid phase) and the umbrella-like magnetic structure (i.e., superfluid phase). This analogy is well demonstrated in Table 1, presenting a beautiful and symmetric correspondence.


References
  • [1] A. Miyata et al., J. Phys. Soc. Jpn. 80, 074709 (2011).
  • [2] A. Miyata, et al. , Phys. Rev. Lett. 107, 207203 (2011).
  • [3] K. Penc et al., Phys. Rev. Lett. 93, 197203 (2004).
  • [4] H. Matsuda and T. Tsuneto, Prog. Theor. Phys. Suppl. 46, 411 (1970).
  • [5] K. S. Liu and M. E. Fisher, J. Low Temp. Phys. 10, 655 (1973).
Authors
  • A. Miyata and S. Takeyama