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Seminar | Materials Science Seminar | Materials Science Division

Evolution of Magnetic Phases and Emergent Magnetic Monopoles in the Spin Ice Ho2Ti2O7

MSD Seminar

Abstract: The presence of geometric and magnetic frustration in the spin ice Ho2Ti2O7 (HTO) leads to a macroscopically degenerate ground state with a magnetic phase exhibiting short-range spin-ice correlation among the holmium ions. This macroscopic degeneracy gets lifted and the residual entropy is released when the external field is applied along certain crystallographic directions, leading to specific long-range ordered phases.

Developing such an ordered phase or transition from one to another magnetic phase in HTO leads to a dynamical transient state with a continuous loop of creating and annihilating emergent magnetic monopoles. Torque magnetometry provides a unique technique for studying the evolution of such magnetic phases and the associated magneto-crystalline anisotropy. The technique has much higher sensitivity, and any small change in the order parameter with varying magnetic field or the crystallographic orientation is subsequently reflected as a changed slope and curvature of the data.

A phenomenological model for spin-flips has been proposed to map this rich magnetic phase diagram of spin-ice HTO. A direct comparison between single crystals and thin films of varying thicknesses in HTO torque data has helped in understanding the possible tuning of spin-ice physics in thin films. Other magnetometric techniques, such as tunnel diode oscillators, which provide a scaled measurement of dynamical susceptibility, and standard DC magnetization measurement through the MPMS technique, strongly support the torque results and model predictions. This thorough study makes a solid stride toward developing a complementary measurement technique for studying a rich magnetic phase diagram, possibly a much more effective tool for thin-film studies where neutron scattering and MPMS techniques face serious limitations.