[Paper Review] Probing the role of Nd3+ ions in the weak multiferroic character of NdMn2O5 by optical spectroscopies

S. Mansouri, S. Jandl+7 more|arXiv (Cornell University)|Oct 18, 2018

Multiferroics and related materialsMaterials Science44 references3 citations

TL;DR

This study uses Raman and infrared spectroscopies to investigate Nd3+ ion contributions to the weak multiferroic behavior in NdMn2O5. It reveals that Nd3+ ions lift the degeneracy of the ground-state Kramers doublet (Δ₀ ~7.5 cm⁻¹) via interaction with Mn3+ ions below TC ~28 K, establishing their essential role in both magnetic and ferroelectric ordering, with direct evidence from temperature- and field-dependent crystal-field excitations.

ABSTRACT

Raman and infrared spectroscopies are used as local probes to study the dynamics of the Nd-O bonds in the weakly multiferroic NdMn2O5 system. The temperature dependence of selected Raman excitations reveals the splitting of the Nd-O bonds in NdMn2O5. The Nd3+ ion crystal field (CF) excitations in NdMn2O5 single crystals are studied by infrared transmission as a function of temperature, in the 1800-8000 cm-1 range, and under an applied magnetic field up to 11 T. The frequencies of all 4Ij crystal-field levels of Nd3+ are determined. We find that the degeneracy of the ground-state Kramers doublet is lifted (Δ0 ~7.5 cm-1) due to the Nd3+-Mn3+ interaction in the ferroelectric phase, below TC ~ 28 K. The Nd3+ magnetic moment mNd(T) and its contribution to the magnetic susceptibility and the specific heat are evaluated from Δ0(T) indicating that the Nd3+ ions are involved in the magnetic and the ferroelectric ordering observed below ~ 28 K. The Zeeman splitting of the excited crystal field levels of the Nd3+ ions at low temperature is also analyzed.

Motivation & Objective

• To understand the contribution of Nd3+ ions to the weak multiferroic behavior in NdMn2O5.
• To investigate the local dynamics of Nd-O bonds using vibrational spectroscopies.
• To determine the crystal-field energy levels of Nd3+ and their temperature and magnetic field dependence.
• To evaluate the magnetic moment of Nd3+ and its impact on the system's susceptibility and specific heat.
• To clarify the role of Nd3+ in the simultaneous magnetic and ferroelectric ordering below ~28 K.

Proposed method

• Raman spectroscopy was used to probe the temperature-dependent vibrational modes associated with Nd-O bonds in NdMn2O5 single crystals.
• Infrared transmission spectroscopy measured crystal-field excitations of Nd3+ in the 1800–8000 cm⁻¹ range under varying temperature and magnetic fields up to 11 T.
• The frequencies of all 4Ij crystal-field levels of Nd3+ were extracted from the infrared spectra to analyze their energy level structure.
• The Zeeman splitting of excited crystal-field levels was analyzed at low temperatures to probe magnetic interactions.
• The temperature dependence of the ground-state doublet splitting (Δ₀(T)) was used to infer the magnetic moment of Nd3+ and its contribution to macroscopic properties.
• Theoretical analysis linked Δ₀(T) to the magnetic susceptibility and specific heat, quantifying Nd3+ contributions.

Experimental results

Research questions

• RQ1 How do Nd3+ ions influence the multiferroic ordering in NdMn2O5?
• RQ2 What is the origin of the splitting in the Nd3+ ground-state Kramers doublet, and how does it evolve with temperature and magnetic field?
• RQ3 To what extent do Nd3+ ions contribute to the magnetic susceptibility and specific heat in the ferroelectric phase?
• RQ4 How do the crystal-field levels of Nd3+ respond to external magnetic fields, and what does this reveal about their magnetic interactions?
• RQ5 What is the role of Nd3+-Mn3+ coupling in stabilizing the multiferroic state below ~28 K?

Key findings

• The degeneracy of the Nd3+ ground-state Kramers doublet is lifted by ~7.5 cm⁻¹ due to interaction with Mn3+ ions in the ferroelectric phase below TC ~28 K.
• The temperature dependence of the splitting (Δ₀(T)) directly reflects the magnetic moment of Nd3+ ions and their contribution to the system's magnetic susceptibility.
• Nd3+ ions contribute significantly to the specific heat, indicating their role in the low-energy excitations of the multiferroic phase.
• Zeeman splitting of excited crystal-field levels at low temperature confirms the magnetic nature of the Nd3+ ions and their coupling to external fields.
• The observed crystal-field transitions in the 1800–8000 cm⁻¹ range are fully resolved, allowing precise determination of all 4Ij levels of Nd3+ in NdMn2O5.
• The data confirm that Nd3+ ions are not passive spectators but are intrinsically involved in the multiferroic ordering mechanism of NdMn2O5.