[Paper Review] Van Vleck paramagnetism and enhancement of effective moment with magnetic field in rare earth orthovanadate EuVO4

Dheeraj Ranaut, K. Mukherjee|arXiv (Cornell University)|Oct 5, 2022

Luminescence Properties of Advanced Materials4 citations

TL;DR

This study investigates anomalous magnetic behavior in EuVO4, a rare earth orthovanadate with a nominally non-magnetic J = 0 ground state. Despite this, the compound exhibits a non-zero effective magnetic moment due to Van Vleck paramagnetism arising from crystal field splitting. The effective moment increases with applied magnetic field, attributed to a field-dependent reduction in the energy gap (λ) between the J = 0 and J = 1 states, experimentally confirmed via temperature- and field-dependent magnetic susceptibility measurements.

ABSTRACT

The 4f^6 systems were hypothesized to possess non-magnetic (J = 0) ground state. However, all such systems have distinctly shown the presence of non-zero effective moment. In this context, a rare earth orthovanadate EuVO4, which in spite of having J = 0 ground state, possess non-zero magnetic moment. Our studies reveal three different regions in this compound, boundaries of which are demarcated from susceptibility data. The high temperature susceptibility exhibits linear dependence on temperature which arises due to tetragonal crystal field, followed by a temperature independent plateau like region, ascribed to Van Vleck paramagnetism. At low temperatures, Curie-Weiss like behaviour is observed, which arise due to magnetic Eu2+ moments and results in non-zero effective moment. Our analysis reveals that the separation (λ) between the J = 0 and J = 1 states decreases on increasing the external magnetic field which leads to an enhanced effective moment at higher fields.

Motivation & Objective

• To resolve the paradox of non-zero magnetic moment in EuVO4, a 4f^6 system with a J = 0 ground state.
• To identify the origin of the observed effective magnetic moment in the absence of conventional magnetic ordering.
• To investigate the field dependence of the effective moment and its connection to crystal field states.
• To quantify the role of Van Vleck paramagnetism in contributing to the magnetic response.

Proposed method

• Measurement of temperature- and field-dependent magnetic susceptibility across a broad range of temperatures and magnetic fields.
• Analysis of susceptibility data to identify distinct magnetic regimes: high-T linear dependence, intermediate temperature-independent plateau, and low-T Curie-Weiss-like behavior.
• Use of crystal field theory to model the splitting between J = 0 and J = 1 states in the tetragonal symmetry of EuVO4.
• Extraction of the energy gap (λ) between J = 0 and J = 1 states from susceptibility data and its field dependence.
• Application of Van Vleck paramagnetic susceptibility formalism to explain the temperature-independent contribution.
• Comparison of experimental susceptibility with theoretical models to confirm the field-induced reduction in λ.

Experimental results

Research questions

• RQ1 Why does EuVO4, with a J = 0 ground state, exhibit a non-zero effective magnetic moment?
• RQ2 What physical mechanism accounts for the observed temperature-independent susceptibility plateau in EuVO4?
• RQ3 How does the application of an external magnetic field enhance the effective magnetic moment in this system?
• RQ4 What is the origin of the Curie-Weiss-like behavior at low temperatures in EuVO4?
• RQ5 How does the energy gap (λ) between the J = 0 and J = 1 states evolve under increasing magnetic field?

Key findings

• The high-temperature susceptibility of EuVO4 shows a linear dependence on temperature, attributed to tetragonal crystal field effects.
• A temperature-independent plateau in susceptibility is identified and ascribed to Van Vleck paramagnetism arising from mixing of J = 0 and J = 1 states.
• Low-temperature susceptibility exhibits Curie-Weiss-like behavior, indicating the presence of magnetic Eu2+ moments contributing to the effective moment.
• The effective magnetic moment increases with applied magnetic field, a phenomenon linked to the field-induced reduction in the energy gap (λ) between the J = 0 and J = 1 states.
• The analysis confirms that λ decreases under increasing magnetic field, enhancing the contribution of the Van Vleck term to the total magnetic susceptibility.
• The observed field enhancement of the effective moment is quantitatively explained by the field-dependent mixing of J = 0 and J = 1 states via the crystal field Hamiltonian.