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When a light beam propagates in a crystal that is asymmetric with respect to both space inversion and time reversal, absorption coefficients between two-counter propagating light beams can be different. This nonreciprocal optical phenomenon is called directional dichroism, which has been intensively studied on the so-called multiferroic materials with broken space-inversion and time-reversal symmetries, in broad wavelength regions ranging from microwave, terahertz, visible, to x-ray. The directional dichroism can appear not only in ferromagnets but also in antiferromagnets when the symmetry requirement is fulfilled. Therefore, the directional dichroism can be employed as a unique working principle of magneto-optical devices based on antiferromagnets and as a useful probe of antiferromagnets. However, the magnitude of previously reported directional dichroism in near-infrared-to-visible (NIR-VIS) region is small, typically about 1% or less in the nonreciprocal to reciprocal components, except for some specific materials. Few reports on antiferromagnets make use of directional dichroism.

In this study, the authors investigated the directional dichroism of a multiferroic material Pb(TiO)Cu₄(PO₄)₄in high magnetic fields up to 49 tesla via magneto-optical spectroscopy in the NIR-VIS range combined with a pulse magnet technique. Measuring the optical absorption coefficient for counter-propagating light beams, directional dichroism signals were successfully observed in a magnetic-field-induced phase (16 to 45 tesla at a temperature of 2 K). The relative magnitude of the observed signals is significantly large, exceeding 13% at a photon energy of approximately 1.4 eV. Moreover, the magnetic-field dependence of the directional dichroism signals resembles that of a theoretically calculated antiferromagnetic order parameter of the field-induced phase. This strongly suggests that the nonreciprocal directional dichroism (NDD) in the field-induced phase originates from the antiferromagnetic order parameter.

In general, probing an antiferromagnetic order parameter is considerably difficult compared to probing a ferromagnetic order parameter, namely, macroscopic magnetization. There are few experimental probes of an antiferromagnetic order parameter in a high magnetic field regime which is hard to access by typical superconducting magnets. Therefore, the present work not only demonstrates a large NDD but also suggests that the measurement of NDD with a pulse magnet technique provides a unique way of investigating an antiferromagnetic order parameter in a high-field regime.
(Witten by K. Kimura on behalf of all authors)