Magnetic structures, phase diagram and spin waves of magneto-electric LiNiPO4 – Niels Bohr Institute - University of Copenhagen

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Niels Bohr Institute > Calendar > 2007 > Magnetic structures

Magnetic structures, phase diagram and spin waves of magneto-electric LiNiPO4

Ph.d.-defence: Thomas B. Stibius Jensen

Risø National Laboratory - Niels Bohr Institute

Materials with both magnetic and ferroelectric phases, known as multiferroics, have received growing attention in recent years. For most such systems the ferroelectric and magnetic phases have very different ordering temperatures, showing that the coupling between the magnetism and electric polarization is negligible. However, for some the magnetic and electric orderings are closely related.  One group of multiferroics where the magnetic and electric properties are strongly coupled are the so-called magneto-electric (ME) compounds. These systems have ferro- or antiferromagnetic (FM or AFM) phases where the application of a magnetic field will induce electric polarization. The reverse is also true; applying an electric field will induce a macroscopic magnetization. The lithium-phosphates, LiMPO4, where M=Mn, Fe, Co, Ni, have the strongest magneto-electric effects of any non-composite materials ever to be measured – yet the mechanisms connecting the magnetism and the electrical polarizations in these systems are not known.


Hoping to gain understanding of the microscopic interactions in the lithium-phosphates we have performed a series of neutron experiments on a single crystal of LiNiPO4. This compound is known to have a commensurate (C) AFM phase below 20.8 K, and to have long-range incommensurate (IC) order between 20.8 and 21.8 K. In our elastic experiments we have determined the magnetic structure of the IC phase, and have corrected the existing picture of the C structure. We have also applied a magnetic field along the crystallographic c-axis, determining the magnetic HT-phase diagram and the corresponding magnetic structures. Performing a series of zero field inelastic neutron measurements has allowed us to establish a credible model of the spin Hamiltonian by analysing the spin wave spectrum and intensities in linear spin wave theory.


In this talk I will show the results of our elastic and inelastic neutron experiments, and based on the magnetic structures in the different phases will propose a microscopic explanation for the ME effect in LiNiPO4.

The committee of the PhD defense:
Gerry Lander (ILL, Grenoble)
Olav Syljuåsen (Nordita, University of Oslo)
Jens Jensen (NBI, chairman)

Niels Hessel Andersen
Per Hedegård