arXiv subjects

Rachel Nickel

Publications and source records attributed to Rachel Nickel.

5 recordsLinked to original sources

arXiv2026

Positive Temperature Coefficient of Anisotropy due to $d^6L$ Groundstate in $\epsilon$-Fe$_2$O$_3$

Positive anisotropy temperature coefficients ($dK/dT>0$) usually arise from hybridization between magnetic 3d states and strongly spin-orbit-coupled (SOC) subsystems. Yet $\epsilon$-Fe$_2$O$_3$ shows $dK/dT>0$ (125-200~K) without an obvious SOC partner. We study pure and Cr-doped (with weakened Fe-O hybridization) $\epsilon$-Fe$_2$O$_3$ across the transition from low temperature incommensurate to high-anisotropy magnetic phases. We identify a $d^6L$ groundstate in $\epsilon$-Fe$_2$O$_3$ while reduced hybridization yields $d^6 + d^6L^2$ in the Cr-doped system, highlighting metal-ligand hybridization as a route to tune magnetic, electronic and orbitronic properties.

cond-mat.str-el↗

arXiv2026

Direct High-Magnetic-Field Coupling to Stripe Order in a Cuprate Superconductor

Superconductivity in cuprates emerges out of a complex normal state that hosts density waves, pseudogap physics, and strange metal properties. Here, we access this normal state by synchronizing free-electron laser x-rays with high-magnetic-field pulses up to 44 T. We observe a linear increase in charge order amplitude and correlation length that persists far above the vortex melting transition. This behavior is incompatible with standard phase competition between charge order and superconductivity. By means of conventional hard x-ray diffraction and magnetostriction, we show that applied fields also enhance monoclinic lattice distortions. However, this magnetoelastic response is weaker and an epiphenomenon of the stripe order enhancement. Combined with recent observations of field-linear spin freezing, our results point to a direct coupling between magnetic field and the spin component of stripe order in the high-field normal state -- a mechanism independent of superconductivity suppression that has so far remained hidden from scattering probes.

cond-mat.str-el↗

arXiv2026

Ligand Mediated Magnetic Coupling Across Metamagnetic Transitions in CrPS4

Chromium thiophosphate (CrPS4) is a long-known material: a layered semiconducting antiferromagnet. Its recently discovered gate-tunable metamagnetic phase transitions, the remarkable positive and oscillating magnetoresistance as a tunnel barrier, and its Fano-resonance luminescence, elusive among the multitude of Cr3+ compounds, call for revisiting the understanding of its electronic structure, especially regarding how it relates to magnetic order. Here, we employ X-ray magnetic circular dichroism, implemented in both absorption and resonant inelastic X-ray spectroscopies, together with quantum many-body calculations, to unveil the role of metal-ligand covalency in mediating the metamagnetic transitions in CrPS4, using crystal-field and charge-transfer excitations as fingerprints of the evolving magnetic order. We reveal the role of extended superexchange paths involving P and S atoms, coupling interactions between the Cr spins across the different magnetic phases: antiferromagnetic, canted, and ferromagnetic. Our results elucidate the electronic states involved in these phases and provide prescriptions for engineering the metamagnetic phase diagram of CrPS4.

cond-mat.mtrl-sci↗

arXiv2025

Chiral Altermagnon in MnTe

Altermagnetism has surfaced as a novel magnetic phase, bridging the properties of ferro- and anti-ferromagnetism. The momentum-dependent spin-splitting observed in these materials reflects their unique symmetry characteristics, which also establish the conditions for chiral magnons to emerge. Here we provide the first direct experimental evidence for a chiral magnon in the altermagnetic candidate MnTe revealed by circular-dichroism resonant inelastic X-ray scattering (CD-RIXS). This mode which we term chiral altermagnon exhibits a distinct momentum dependence of its spin polarization consistent with the proposed altermagnetic $g-$wave symmetry of MnTe. Our polarization-resolved results corroborate the existence of a new class of magnetic excitations, demonstrating how altermagnetic order shapes spin dynamics and paves the way for advances in spintronic and quantum technologies.

cond-mat.mtrl-sci↗

arXiv2019

Temperature and field evolution of site-dependent magnetism in $ε$-Fe$_2$O$_3$ nanoparticles

8~nm epsilon-Fe2O3 nanoparticles exhibit a spin reorientation transition that begins at 150 K which is a hallmark of this unique iron-oxide polymorph. We find that the change from the high to low temperature magnetic structures has been suppressed by ~50 K. At the spin reorientation temperature, a change of the field-dependent response of the tetrahedral sites in intermediate field strengths (0.25 - 1.5 T) indicates that a collective tetrahedral distortion occurs to which the octahedral sites adjust, altering the magnetic anisotropy. An abrupt step in the hyperfine parameters' temperature dependencies, especially at 125 K for the hyperfine field associated with the Fe4 tetrahedral sites, suggests strongly a change in the superexchange pathways are responsible for the spin reorientation.

cond-mat.mes-hall↗