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Register a new userMagnetic field dependence of the nonlinear magnetic response and tricritical point in the monoaxial chiral helimagnet Cr$_{1/3}$NbS$_{2}$
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Eleanor M. Clements
Publication date
2018
fields
Physics
and research's language is
English
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We present a comprehensive study of the magnetization dynamics and phase evolution in Cr$_{1/3}$NbS$_{2}$, which realizes a chiral soliton lattice (CSL). The magnetic field dependence of the ac magnetic response is analyzed for five harmonic components, $M_{nomega}(H)$ $(n =1-5)$, using a phase sensitive measurement over a frequency range, $f = 11 - 10,000$ Hz. At a critical field, the modulated CSL continuously evolves from a helicity-rich to a ferromagnetic domain-rich structure, where the crossover is revealed by the onset of an anomalous nonlinear magnetic response that coincides with extremely slow dynamics. The behavior is indicative of the formation of a spatially coherent array of large ferromagnetic domains which relax on macroscopic time-scales. The frequency dependence of the ac magnetic loss displays an asymmetric distribution of relaxation times across the highly nonlinear CSL regime, which shift to shorter time-scales with increasing temperature. We experimentally resolve the tricritical point at $T_{TCP}$ in a temperature regime above the ferromagnetic Curie temperature which separates the linear and nonlinear regimes of the CSL at the phase transition. A comprehensive phase diagram is constructed which summarized the features of the field and temperature dependence of the magnetic crossovers and phase transitions in Cr$_{1/3}$NbS$_{2}$.
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In this letter we describe the ground-state magnetic structure of the highly anisotropic helimagnet Cr$_{1/3}$NbS$_2$ in a magnetic field. A Heisenberg spin model with Dyzaloshinkii-Moriya interactions and magne- tocrystalline anisotropy allows the ground state spin structure to be calculated for magnetic fields of arbitrary strength and direction. Comparison with magnetization measurements shows excellent agreement with the predicted spin structure.
Understanding the role of spin-orbit coupling (SOC) has been crucial to controlling magnetic anisotropy in magnetic multilayer films. It has been shown that electronic structure can be altered via interface SOC by varying the superlattice structure, resulting in spontaneous magnetization perpendicular or parallel to the plane. In lieu of magnetic thin films, we study the similarly anisotropic helimagnet Cr$_{1/3}$NbS$_2$, where the spin polarization direction, controlled by the applied magnetic field, can modify the electronic structure. As a result, the direction of spin polarization can modulate the density of states, and in turn affect the in-plane electrical conductivity. In Cr$_{1/3}$NbS$_2$, we found an enhancement of in-plane conductivity when the spin polarization is out-of-plane, as compared to in-plane spin polarization. This is consistent with the increase of density of states near the Fermi energy at the same spin configuration, found from first principles calculations. We also observe unusual field dependence of the Hall signal in the same temperature range. This is unlikely to originate from the non-collinear spin texture, but rather further indicates strong dependence of electronic structure on spin orientation relative to the plane.
We show that the stability (existence/absence) and interaction (repulsion/attraction) of chiral solitons in monoaxial chiral magnets can be varied by tilting the direction of magnetic field. We, thereby, elucidate that the condensation of attractive chiral solitons causes the discontinuous phase transition predicted by a mean field calculation. Furthermore we theoretically demonstrate that the metastable field-polarized-state destabilizes through the surface instability, which is equivalent to the vanishing surface barrier for penetration of the solitons. We experimentally measure the magnetoresistance (MR) of micrometer-sized samples in the tilted fields in demagnetization-free configuration. We corroborate the scenario that hysteresis in MR is a sign for existence of the solitons, through agreement between our theory and experiments.
The topologically-protected, chiral soliton lattice is a unique state of matter offering intriguing functionality and it may serve as a robust platform for storing and transporting information in future spintronics devices. While the monoaxial chiral magnet Cr$_{1/3}$NbS$_2$ is known to host this exotic state in an applied magnetic field, its detailed microscopic origin has remained a matter of debate. Here we work towards addressing this open question by measuring the spin wave spectrum of Cr$_{1/3}$NbS$_2$ over the entire Brillouin zone with inelastic neutron scattering. The well-defined spin wave modes allow us to determine the values of several microscopic interactions for this system. The experimental data is well-explained by a Heisenberg Hamiltonian with exchange constants up to third nearest neighbor and an easy plane magnetocrystalline anisotropy term. Our work shows that both the second and third nearest neighbor exchange interactions contribute to the formation of the helimagnetic and chiral soliton lattice states in this robust three-dimensional magnet.
The crystal structure of a disordered form of Cr$_{1/3}$NbS$_2$ has been characterized using diffraction and inelastic scattering of synchrotron radiation. In contrast to the previously reported symmetry (P6$_3$22), the crystal can be described by a regular twinning of an average P6$_3$ structure with three disordered positions of the Cr ions. Short-range correlations of the occupational disorder result in a quite intense and structured diffuse scattering; a static nature of the disorder was unambiguously attributed by the inelastic x-ray scattering. The diffuse scattering has been modeled using a reverse Monte-Carlo algorithm assuming a disorder of the Cr sub-lattice only. The observed correlated disorder of the Cr sub-lattice reduces the temperature of the magnetic ordering from 130 K to 88 K and drastically modifies the field dependence of the magnetization as it is evidenced by the SQUID magnetometery. We conclude, that in contrast to the helicoidal spin structure assumed for P6$_3$22 form, the compound under study is ferromagnetically ordered with a pronounced in-plane anisotropy.
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