Terahertz dynamics in the high TC multiferroic CuO

Fathimath Faseela, Vivek Dwij, Ruturaj Puranik, Utkarsh Pandey, Vibhavari Parkar, Snehal Haldankar, S. S. Prabhu

Tata Institute of Fundamental Research, Mumbai, India

Competing magnetic interaction leads to an exotic ground state and thus, has become the playground of modern research. For example, spiral magnetic ordering from the spin frustration can induce a multiferroic ground state. The spin origin of the ferroelectricity provides a strong coupling between magnetism and ferroelectric ordering pivotal for future advanced applications where the magnetization of the material can be controlled with electric fields. However, most of such multiferroic states are limited to low temperatures below 60 K [1-4]. In this context, CuO is a unique material exhibiting two magnetic transitions from paramagnetic to an incommensurate antiferromagnetic state below 230 K and to a commensurate antiferromagnetic state below 213 K respectively [1,2,4]. The incommensurate antiferromagnetic state is characterized by a spin spiral with a modulation vector along (0.006, 0, 0.017) which results in spin-induced ferroelectricity [4]. Recent Raman and IR spectroscopic investigation revealed the signature of strong spin-phonon coupling in this ferroelectric state indicating possible electric field control of the magnetic state [3,4]. Furthermore, external pressure or strain has been found to induce a multiferroic state at room temperature which makes the CuO extremely important for understanding high TC multiferroicity [5]. In this context, we have tried to examine the Terahertz response across the ferroelectric temperature window of 213-230 K in polycrystalline CuO using Terahertz time-domain spectroscopy (THz-TDS). Figure 1 (a) shows the Terahertz Time-domain spectrum collected on the polycrystalline CuO pallet. As such multiferroic state is usually characterized by quasiparticle excitations in the terahertz range. Thus, we attempted to examine the Terahertz absorption across these transition temperatures to interrelate the Terahertz response with the multiferroic in the CuO.

Figure 1: (a) Temperature variation in the Terahertz TDS spectrum of CuO (b) THz absorption spectrum as δα (T-T202 K) across 202-240 K. (c) Quantified variation in the intensity of the THz absorption as δα (T-T202 K) at 0.75 THz. The arrow and line are guide to eyes.

We examined the δα (T-T202 K) in the temperature window of 202-240 K. Due to the polycrystalline nature of the CuO, we could not find direct signature of such quasiparticles. However, detailed examinations revealed strong modulation in the intensity in the 0.2-1 THz region for the absorption spectrum (Figure 1 (b) marked by the arrow) while intensity variation for high frequency (>1 THz) range was not significant. Earlier THz investigations suggested the existence of an electromagnon at 0.7 THz in the multiferroic temperature range of 213-230 K. We attribute the modulation in the intensity at the low-frequency region to the electromagnon and associated lattice modulation due to ferroelectric polarization in the multiferroic temperature window.

[1] Jones, S., Gaw, S., Doig, K. et al. “High-temperature electromagnons in the magnetically induced multiferroic cupric oxide driven by intersublattice exchange,” Nat Commun. 5, 3787 (2014); https://doi.org/10.1038/ncomms4787
[2] Rocquefelte, X., Schwarz, K., Blaha, P. et al. “Room-temperature spin-spiral multiferroicity in high-pressure cupric oxide,” Nat Commun. 4, 2511 (2013); https://doi.org/10.1038/ncomms3511
[3] A. B. Kuz’menko, D. van der Marel, P. J. M. van Bentum, E. A. Tishchenko, C. Presura, and A. A. Bush, “Infrared spectroscopic study of CuO: Signatures of strong spin-phonon interaction and structural distortion,” Phys. Rev. B 63, 094303 (2001); DOI:10.1103/PhysRevB.63.094303.
[4] B. K. De, V. Dwij, R. Misawa, T. Kimura, & V. G. Sathe, “Femtometer atomic displacement, the root cause for multiferroic behavior of CuO unearthed through polarized Raman spectroscopy,” Journal of Physics: Condensed Matter, 33(12), 12LT01 (2021); DOI: 10.1088/1361-648X/abd738.
[5] Noriki Terada, Dmitry D. Khalyavin, Pascal Manuel, Fabio Orlandi, Christopher J. Ridley, Craig L. Bull, Ryota Ono, Igor Solovyev, Takashi Naka, Dharmalingam Prabhakaran, and Andrew T. Boothroyd, “Room-Temperature Type-II Multiferroic Phase Induced by Pressure in Cupric Oxide,” Phys. Rev. Lett. 129, 217601 (2022); DOI: 10.1103/PhysRevLett.129.217601.

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