The Raman exponent of single-molecular magnetic relaxation may take various unexpected values because of rich phonon spectrum and spin-phonon coupling of molecular crystals. We systematically examine the origins of different abnormalities, and clarify misunderstandings in the past, particularly the appropriateness of the fitting procedures for the exponents. We find that exponential laws raised by optical phonons can yield spurious power laws with low exponents. This observation indicates long-standing misunderstandings for origins of low Raman exponents in a large bulk of single-molecule magnets. Resulting from spin-lattice coupling with optical modes, presence of these exponents suggests the importance of the local dynamical environment for the magnetic relaxation in this regime.
Group theory of Raman effect in magnetic materials
Anhui University（安徽大学）；Hefei Institutes of Physical Science, Chinese Academy of Sciences（中国科学院合肥物质科学研究院）；Northeastern University（东北大学）；Yunnan University（云南大学）
We use a combination of polarized Raman spectroscopy experiment and model magnetism-phonon coupling calculations to study the rich magneto-Raman effect in the two-dimensional (2D) magnet $\mathrm{CrI_3}$. We reveal a novel layered-magnetism-assisted phonon scattering mechanism below the magnetic onset temperature, whose Raman excitation breaks time-reversal symmetry, has an antisymmetric Raman tensor, and follows the magnetic phase transitions across critical magnetic fields, on top of the presence of the conventional phonon scattering with symmetric Raman tensors in $N$-layer $\mathrm{CrI_3}$…
Magnetic-Field-Dependent Raman Scattering in Multiferroic Bilayer Films: Evidence for Stress-Mediated Magnetoelectric Coupling
首次报道了Pb（Zr，Ti）O_3-CoFe_2 O_4双层多铁体系的磁场相关拉曼散射研究。纳米双层膜的声子频率随着磁场的变化而发生明显的变化，而单层Pb（Zr，Ti）O3膜则没有这种变化。CoFe 2 O 4层的磁致伸缩产生机械传递到Pb（Zr，Ti）O3层的应力，导致模式变化。观察到的磁场诱导软化的软模式的Pb（Zr，Ti）O3层承担着一个惊人的相似的直接磁电输出的双层，提供证据的应力介导的磁电耦合机制的多铁性双层。
Recently it was discovered that van der Waals-bonded magnetic materials retain long range magnetic ordering down to a single layer, opening many avenues in fundamental physics and potential applications of these fascinating materials. One such material is FePS3, a large spin (S=2) Mott insulator where the Fe atoms form a honeycomb lattice. In the bulk, FePS3 has been shown to be a quasi-two-dimensional-Ising antiferromagnet, with additional features in the Raman spectra emerging below the Neel temperature of approximately 120 K. Using magneto-Raman spectroscopy as an optical probe of magnetic …
Resonant Raman signatures of bright and momentum-dark exciton coupled by intervalley phonon scattering in monolayer WSe2
University of Münster（明斯特大学）；Center for Soft Nanoscience (SoN), University of Münster（明斯特大学软纳米科学中心）；Technical University of Munich（慕尼黑工业大学）；Walter Schottky Institut and Physics Department, Technical University of Munich（慕尼黑工业大学沃尔特·肖特基研究所与物理系）；MCQST, München（慕尼黑量子科学与技术中心）
Raman spectroscopy is utilized to study the magnetic characteristics of heteroepitaxial NiO thin films grown by plasma-assisted molecular beam epitaxy on MgO(100) substrates. For the determination of the Néel temperature, we demonstrate a reliable approach by analyzing the temperature dependence of the Raman peak originating from second-order scattering by magnons. The antiferromagnetic coupling strength is found to be strongly influenced by the growth conditions. The low-temperature magnon frequency and the Néel temperature are demonstrated to depend on the biaxial lattice strain and the degr…
Magnons and Magnetodielectric Effects in CoCr$_2$O$_4$: Raman Scattering Studies
磁电材料由于其所表现出的丰富的现象而引起了广泛的技术和科学兴趣，包括磁性和铁电性有序的共存，磁介电行为，以及诸如电磁振子的奇异混合激发。多铁性尖晶石材料CoCr $_2$ O $_4$是多铁性材料的一个特别有趣的例子，因为亚铁磁相的磁电行为的证据似乎与传统的非共线自旋驱动机制相冲突。用非弹性光散射方法研究了CoCr $_2$ O $_4$的磁振子和声子谱随温度、压力和磁场的变化.在CoCr $_2$ O $_4$的居里温度（$T_C \sim 94$ K）以下，我们观察到具有T $_{1g}$对称性的$ω\sim 16 \,\text{cm}^{-1}$ $\boldsymbol q=0$磁振子，它具有轴向矢量的变换性质。T $_{1g}$对称磁振子的极大拉曼强度是具有大磁光响应的材料的特征，并且可能由强烈调制CoCr $_2$ O $_4$中的介电响应的大的磁波动引起。的T $_{1g}$ -对称磁振子的拉曼磁化率表现出很强的磁场依赖性，这是一致的磁介电响应中观察到的CoCr $_2$ O $_4$，这表明磁介电行为在CoCr $_2$ O $_4$主要是从磁场依赖性抑制的磁波动，强烈耦合到长波长的声子。随着所施加的压力增加CoCr $_2$ O $_4$中的磁各向异性降低了CoCr $_2$ O $_4$中T $_{1g}$对称性磁振子拉曼磁化率的磁场依赖性，表明应变可用于控制…
Detecting Magnetic Phase Transitions in Ion-Irradiated CrSBr Through Resonant Raman Scattering
Institute for Condensed Matter Physics, TU Darmstadt（达姆施塔特工业大学凝聚态物理研究所）；Department of Inorganic Chemistry, University of Chemistry and Technology Prague（布拉格化学技术大学无机化学系）；Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research（德累斯顿罗森多夫亥姆霍兹中心离子束物理与材料研究所）
Vanadium tetracyanoethylene (V[TCNE]$_{x}$, $x\approx 2$) is an organic-based ferrimagnet with a high magnetic ordering temperature $\mathrm{T_C>600 ~K}$, low magnetic damping, and growth compatibility with a wide variety of substrates. However, similar to other organic-based materials, it is sensitive to air. Although encapsulation of V[TCNE]$_{x}$ with glass and epoxy extends the film lifetime from an hour to a few weeks, what is limiting its lifetime remains poorly understood. Here we characterize encapsulated V[TCNE]$_{x}$ films using confocal microscopy, Raman spectroscopy, ferromagnetic …
Antiferromagnetic materials with strong spin-phonon coupling have the potential for spintronic applications. Here, we report the structural, magnetic, and vibrational properties of 12R-BaTi0.4Mn0.6O3. Temperature-dependent magnetization data show the presence of weak antiferromagnetic interactions. Correlation between magnetic and vibrational properties are probed by using temperature-dependent Raman spectroscopic measurements. Two of the Raman active phonons show deviation from anharmonic behaviour at low temperatures which can be attributed to spin-phonon coupling. The strength of coupling i…
Magneto-optic phonon resonances in magnetic topological EuCd2As2 via helical Raman spectroscopy
University of California, Los Angeles（加州大学洛杉矶分校）；Oak Ridge National Laboratory（橡树岭国家实验室）；Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）；Indian Institute of Science Education and Research Bhopal（博帕尔科学教育研究院）
We report results of the Raman measurements for SrRu$_2$O$_6$ having extraordinary high N$\acute{\textrm{e}}$el temperature for a layered material. No additional phonon modes were detected at the temperature of magnetic transition thus excluding lowering of the symmetry in the magnetically ordered phase. An unusual increase in softening and damping of some phonons as the temperature approaches $T_N$ indicate the appearance of a continuum of interacting electronic excitations at $T\geq300K$. We also observe an intensive Raman response at 2050 cm$^{-1}$. Analysis of the polarization dependence a…
Multiferroics possess two or more switchable states such as polarization, magnetization, etc. Phonon excitations in multiferroic phase are strongly modified by magnetoelectric coupling, spin-phonon coupling, and anharmonic phonon-phonon interactions. Here, we have investigated the correlation between phonons and multiferroic order parameters in hexagonal BaMO3: M = Ti1-x(Mn/Fe)x systems using powder x-ray diffraction (PXRD), Raman spectroscopic, and magnetic measurements. The structural transformation from a polar tetragonal to a non-polar 6H-type hexagonal phase is observed as a function of d…
We report on the lattice and spin dynamics in the monopnictide CrAs using polarized Raman scattering. This system exhibits a first-order magneto-structural phase transition at T_N~ 265 K, below which the magnetic moments of Cr form an incommensurate double helical magnetic structure while the unit cell volume abruptly expands. Across the transition, the frequencies of the fully symmetric Ag phonons strongly renormalize, which along with the results of first-principle calculations suggest the presence of a sizeable coupling of the phonons to the magnetic degrees of freedom in this compound. In …
Mutual interactions in many-body systems bring about a variety of exotic phases, among which liquid-like states failing to order due to frustration are of keen interest. Recently, an organic system with an anisotropic triangular lattice of molecular dimers has been suggested to host a dipole liquid arising from intradimer charge-imbalance instability, possibly offering an unprecedented stage for the spin degrees of freedom. Here we show that an extraordinary unordered(unfrozen) spin state having soft-matter-like spatiotemporal characteristics is substantiated in this system. $^1$H NMR spectra …
Magnetic excitations in layered magnetic materials that can be thinned down the two-dimensional (2D) monolayer limit are of high interest from a fundamental point of view and for applications perspectives. Raman scattering has played a crucial role in exploring the properties of magnetic layered materials and, even-though it is essentially a probe of lattice vibrations, it can reflect magnetic ordering in solids through the spin-phonon interaction or through the observation of magnon excitations. In bulk CrSBr, a layered A type antiferromagnet (AF), we show that the magnetic ordering can be di…
Spin-phonon coupling, the interaction of spins with surrounding lattice is a key parameter to understand the underlying physics of multiferroics and engineer their magnetization dynamics. Elementary excitations in multiferroic materials are strongly influenced by spin-phonon interaction, making Raman spectroscopy a unique tool to probe these coupling(s). Recently, it has been suggested that the dielectric and magnetic properties of 15R-type hexagonal BaMnO3 are correlated through the spin-lattice coupling. Here, we report the observation of an extensive renormalization of the Raman spectrum of…
Polycrystalline ceramic samples and a single crystal of EuTiO3 have been investigated by Raman spectroscopy in the temperature range 80-300 K. Although synchrotron XRD data clearly indicated the cubic to tetragonal phase transition around 282 K, no mode from the symmetry allowed Raman active phonons was found in the tetragonal phase, contrary to the case of the homologous SrTiO3. In order to study the evolution of this unique characteristic, ceramics of EuxSr1-xTiO3 (x=0.03-1.0) characterized by synchrotron XRD for the structural phase transition have been also investigated by Raman spectrosco…
The quadruple Calcium manganite (CaMn7O12) is a multiferroic material that exhibits a giant magnetically-induced ferroelectric polarization which makes it very interesting for magnetoelectric applications. Here, we report the Raman spectroscopy study on this compound of both the phonon modes and the low energy excitations from 4 K to room temperature. A detailed study of the Raman active phonon excitations shows that three phonon modes evidence a spin-phonon coupling at TN2 = 50 K. In particular, we show that the mode at 432 cm-1 associated to Mn(B)O6 (B position of the perovskite) rotations a…
The recent discovery of 2D magnets has revealed various intriguing phenomena due to the coupling between spin and other degree of freedoms (such as helical photoluminescence, nonreciprocal SHG). Previous research on the spin-phonon coupling effect mainly focuses on the renormalization of phonon frequency. Here we demonstrate that the Raman polarization selection rules of optical phonons can be greatly modified by the magnetic ordering in 2D magnet CrI$_3$. For monolayer samples, the dominant A$\rm_{1g}$ peak shows abnormally high intensity in the cross polarization channel at low temperature, …
Raman signatures of a non-reciprocal magnetic phase transition in Ca$_2$RuO$_4$
University of Erlangen-Nürnberg（埃尔朗根-纽伦堡大学）；University of Trieste（的里雅斯特大学）；CNR-SPIN, University of Salerno（萨莱诺大学 CNR-SPIN 研究所）；Elettra Sincrotrone Trieste（的里雅斯特电子同步辐射中心）；CNR-Istituto Officina dei Materiali (IOM)（CNR 材料工坊研究所）
The type-II terminated $1T$-TaS$_2$ surface of a three-dimensional $1T$-TaS$_2$ bulk material realizes the effective spin-$1/2$ degree of freedom on each David star cluster with ${\mathcal{T}^2=-1}$ such that the time-reversal symmetry is realized anomalously, despite the fact that bulk three-dimensional $1T$-TaS$_2$ material has an even number of electrons per unit cell with ${\mathcal{T}^2=+1}$. This surface is effectively viewed as a spin-$1/2$ triangular lattice magnet, except with a fully gapped topological bulk. We further propose this surface termination realizes a spinon Fermi surface …
Damping of Landau levels in neutral graphene at low magnetic fields: A phonon Raman scattering study
在电中性和准中性石墨烯的朗道能级展宽机制进行了研究，通过微磁拉曼实验在三个不同的样品，即，天然单层石墨烯片和背栅单层设备，都沉积在Si/SiO2衬底，和多层外延石墨烯作为参考样品。通过对与光学活性朗道能级跃迁相关的磁声子共振穿过E_2g喇曼活性声子能量的定量分析，估算了带间朗道能级跃迁宽度。与多层石墨烯相反，单层石墨烯样品显示出对低场共振的强阻尼，这与随机应变场产生的朗道能级能量的额外加宽贡献一致。在我们的单层样品中，这种额外的贡献被适当地量化为伪磁场分布Δ B = 1.0-1.7 T。
Magnetic circular dichroism of THz modes and selection rules of Raman-active optical phonons in the polar altermagnet candidate \ce{Mn2Mo3O8}
University of Augsburg（奥格斯堡大学）；Kazan (Volga region) Federal University（喀山（伏尔加沿岸）联邦大学）；Max Planck Institute for Solid State Research（马克斯·普朗克固体研究所）；Budapest University of Technology and Economics（布达佩斯技术与经济大学）；HUN-REN Wigner Research Centre for Physics（匈牙利研究与教育网络维格纳物理研究中心）
