Electronic transport studies of heavy fermion ferromagnet YbNi₄P₂

Abstract

The ferromagnetic heavy fermion metal YbNi₄P₂, with Curie temperature T ≈ 160 mK and Kondo temperature T ≈ 8 K, is studied through magnetoresistance and Hall effect measurements. The study is motivated by the observation of a ferromagnetic quantum critical point (FM QCP) in the series YbNi4(P_1–xAs_x)₂, a situation which theory predicts should not exist in metals with d > 1 dimensionality. The parent compound YbNi₄P₂ has been explored here in a large range of temperatures and magnetic fields; measurements made in Bristol probe from room temperature down to 35 mK with magnetic fields up to 12 T, and high field measurements made in the High Magnetic Field Laboratory (HFML) in Nijmegen study up to 35 T. A primary interest of electronic transport measurements in this series is to uncover the role of the f electrons in the FM QCP, crucially the Kondo hybridisation with the conduction electrons that gives rise to the heavy fermion physics. This problem is intimately linked to the Fermi surface, which is probed in this thesis through Hall effect and quantum oscillation measurements. Hall effect measurements determine that no change occurs in the ordinary Hall effect on polarising YbNi₄P₂ with a transverse magnetic field (B || c), confirming the presence of heavy fermion quasiparticles in the ferromagnetic phase. High field quantum oscillations probe a 3D Fermi surface with strongly suppressed Kondo hybridisation, and display an unusual spin-splitting which is modelled here by a field tuned quasiparticle g-factor.
Hall effect measurements also act as a sensitive probe of the magnetic order through the anomalous Hall effect (AHE). This is used to map features of the magnetisation in YbNi₄P₂. Behaviour inconsistent with common scaling laws is observed, preventing the magnitude of the magnetisation being studied through an AHE, and likely indicating field/temperature tuned terms of AHE (e.g. changes in Berry curvature). The phase diagram is also studied through magnetoresistance measurements, finding new additions to the many Lifshitz transitions of YbNi₄P₂, a potential sign of transverse field induced quantum criticality, and novel scaling laws over a relatively large temperature span above T_C.

Date of Award	5 Dec 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Sven Friedemann (Supervisor) & Stephen M Hayden (Supervisor)