arXiv subjects

Johannes Werner

Publications and source records attributed to Johannes Werner.

4 recordsLinked to original sources

arXiv2025

Two-stage evolution of magnetic correlations in spiral spin liquid material, Ca$_{10}$Cr$_{7}$O$_{28}$

We present an X-band and tunable high-frequency/high-field electron spin resonance (HF-ESR) study of single-crystalline Ca$_{10}$Cr$_{7}$O$_{28}$, which constitutes alternating antiferromagnetic and ferromagnetic kagome bilayers. At high temperatures, a phonon-assisted relaxation process is evoked to account for the pronounced increase of the linewidth in an exchange-narrowing regime ($k_{\rm B}T\gg J$). In contrast, at low temperatures ($k_{\rm B}T\lesssim J$), a power-law behavior in line narrowing is observed. Our data reveal two distinct power-law regimes for the linewidth which crossover at $T^*\approx 7.5$~K. Notably, the intriguing evolution of the ESR linewidth in this alternating kagome bilayer system with opposite sign of exchange interactions highlights distinct spin dynamics compared to those in a uniform kagome antiferromagnet.

cond-mat.str-el↗

arXiv2020

Magnetic phase diagram and magneto-elastic coupling of NiTiO3

We report high-resolution dilatometry on high-quality single crystals of NiTiO3 grown by means of the optical floating-zone technique. The anisotropic magnetic phase diagram is constructed from thermal expansion and magnetostriction studies up to B=15T and magnetization studies in static (15T) and pulsed (60T) magnetic fields. Our data allow to quantitatively study magneto-elastic coupling and to determine uniaxial pressure dependencies. While the entropy changes are found to be of magnetic nature, Grüneisen analysis implies only one relevant energy scale in the whole low-temperature regime. Thereby, our data suggest that the observed structural changes due to magneto-elastic coupling and previously reported magnetodielectric coupling[1] are driven by the same $magnetic$ degrees of freedom that lead to long-range magnetic order in NiTiO3, which in turn, establishes a linear magnetodielectric coupling in this compound.

cond-mat.str-el↗

arXiv2019

Synthesis and Magnetism of a Li$_2$FeSiO$_4$ Single Crystal

A macroscopic single crystal of $γ_{\rm II}$-Li$_2$FeSiO$_4$ has been grown by means of the high-pressure optical floating-zone technique. Static magnetic susceptibility $χ$ implies that the tetrahedrally-coordinated Fe$^{2+}$ ions are in the high-spin, $S=2$, state. While a sharp decrease in $χ$ implies long-range antiferromagnetic order below \tn\ = 17.0(5)~K, the presence of a broad maximum at $T_{\rm m} = 28$~K suggests quasi-low-dimensional magnetism. Applying magnetic fields along the easy magnetic $a$-axis yields additional contributions to the susceptibility $\partial M/\partial B$ and magnetostriction for $B > 7$~T, and an anomaly at $B\approx 14.8$~T.

cond-mat.str-el↗

arXiv2017

Anisotropy governed competition of magnetic phases in the honeycomb quantum magnet Na$_3$Ni$_2$SbO$_6$ studied by dilatometry and high-frequency ESR

Thermodynamic properties as well as low-energy magnon excitations of $S=1$ honeycomb-layered Na$_3$Ni$_2$SbO$_6$ have been investigated by high-resolution dilatometry, static magnetisation, and high-frequency electron spin resonance studies in magnetic fields up to 16 T. At $T_{\rm N}$ = 16.5 K, there is a tricritical point separating two distinct antiferromagnetic phases AF1 and AF2 from the paramagnetic regime. In addition, our data imply short-range antiferromagnetic correlations at least up to $\sim 5\cdot T_{\rm N}$. Well below $T_{\rm N}$, the magnetic field $B_{\rm C1}\approx$ 9.5 T is needed to stabilize AF2 against AF1. The thermal expansion and magnetostriction anomalies at $T_{\rm N}$ and $B_{\rm C1}$ imply significant magnetoelastic coupling, both of which associated with a sign change of $\partial L/\partial B$. The transition at $B_{\rm C1}$ is associated with softening of the antiferromagnetic resonance modes observed in the electron spin resonance spectra. The anisotropy gap $Δ= 360$ GHz implies considerable uniaxial anisotropy. We conclude the crucial role of axial anisotropy favoring the AF1 spin structure over the AF2 one. While the magnetostriction data disprove a simple spin-flop scenario at $B_{\rm C1}$, the nature of a second transition at $B_{\rm C2}\approx$ 13 T remains unclear. Both the sign of the magnetostriction and Grüneisen analysis suggest the short-range correlations at high temperatures to be of AF2-type.

cond-mat.str-el↗