Comprehensive magnetic phase diagrams of the polar metal Ca3(Ru0.95Fe<

Abstract

Polar metals exist as a rather unique class of materials as they combine two seemingly mutually exclusive properties (polar order and metallicity) in one system. So far only a few polar metals have been unambiguously identified; the magnetic ones are exceptionally rare. Here we investigate a 5% Fe doped polar metal Ca3Ru2O7, via electrical transport, magnetization, microstrain and optical second harmonic generation measurements. We report the full magnetic phase diagrams (in the field-temperature space) for magnetic field H//a and H//b, which exhibit distinct field-dependent magnetizations behavior. In particular for H//a we found a new ferromagnetic incommensurate spin structure, which is absent in the pure Ca3Ru2O7. We propose a microscopic spin model to understand this behavior, highlighting the role of Fe doping in tipping the delicate balance of the underlying exchange interaction energy in this system.

Spontaneous cycloidal order mediating a spin-reorientation transition in a polar metal

“…Such behavior is reminiscent of the highly robust, tunable soliton lattices seen in chiral helimagnets under field [32]. Within this framework, we can also explain previous observations of incommensurate structures in doped Ca 3 Ru 2 O 7 [17][18][19]. Here, a reduced anisotropy from the introduction of magnetic dopants should allow an easier turning of the moments away from their easy-axis by the DMI, stabilizing cycloidal structures with shorter repeat distances and over larger temperature ranges.…”

“…We show how the frustration that renders the cycloid sensitive to temperature also allows it to be delicately tuned by magnetic field, under which higher harmonic modulations of the fundamental order appear to be generated. This necessitates a reinterpretation of an incommensurate structure previously observed under field [16], unifying it with the magnetic response to doping [17][18][19]. Our results thus demonstrate a transition at the juncture of polar metals, spin-orbit-coupled systems, and magnetic frustration.…”

Spontaneous cycloidal order mediating a spin-reorientation transition in a polar metal

“…Within this framework, we can also explain previous observations of incommensurate structures in doped Ca 3 Ru 2 O 7 [33][34][35]. Here, a reduced anisotropy from the introduction of magnetic dopants should allow an easier turning of the moments away from their easy-axis by the DMI, stabilising cycloidal structures with shorter repeat distances and over larger temperature ranges.…”

μSR investigation of the Fe-doped Ca$_{3}$Ru$_{2}$O$_{7}$ polar metal

“…A partial substitution of Ru with Fe (at only 5%) is able to induce several dramatic effects. First, it enhances the PM-to-AFM-a transition temperature up to 86 K [5][6][7][8]. Secondly, it lowers the T MI temperature down to 40 K, corresponding to an isostructural transition [8].…”

“…First, it enhances the PM-to-AFM-a transition temperature up to 86 K [5][6][7][8]. Secondly, it lowers the T MI temperature down to 40 K, corresponding to an isostructural transition [8]. Below 40 K, the coexistence of an incommensurate-(IC) and an AFM-b order (IC + AFM-b) sets in.…”