在1.25 GPa的静水压下，利用中子衍射测量建立了多铁性化合物HoMn $_{2}$ O $_{5}$的微观磁性的压力（$p$）-温度（$T$）相图.在常压下，随着温度的降低，无公度-公度-无公度磁相变相继发生。加压后，在最低温度下的无公度相几乎减少，公度相出现。建立的$p$ - $T$相图与最近报道的$p$ -$T$介电相图非常一致，其中铁电性是由施加压力引起的。我们还发现，$p$ - $T$磁相图是非常相似的，以前获得的磁场-温度相图。
英文摘要：
The pressure ($p$) -- temperature ($T$) phase diagram for microscopic magnetism in the multiferroic compound HoMn$_{2}$O$_{5}$ was established using neutron diffraction measurements under a hydrostatic pressure up to 1.25 GPa. At ambient pressure, incommensurate--commensurate--incommensurate magnetic phase transitions occur successively with decreasing temperature. Upon applying pressure, the incommensurate phase at the lowest temperature almost decreases and the commensurate phase appears. The $p$ -- $T$ phase diagram established shows excellent agreement with the recently reported $p$ -- $T$ dielectric phase diagram, where ferroelectricity is induced by applying pressure. We also found that the $p$ -- $T$ magnetic phase diagram is quite similar to the previously obtained magnetic field-temperature phase diagram.
Orthorhombic HoMnO3 is a multiferroic in which Mn antiferromagnetic order induces ferroelectricity. A second transition occurs within the multiferroic phase, in which a strong enhancement of the ferroelectric polarization occurs concomitantly to antiferromagnetic ordering of Ho 4f magnetic moments. Using the element selectivity of resonant X-ray diffraction, we study the magnetic order of the Mn 3d and Ho 4f moments. We explicitly show that the Mn magnetic order is affected by the Ho 4f magnetic ordering transition. Based on the azimuthal dependence of the (0 q 0) and (0 1-q 0) magnetic reflec…
Field-induced ferroelectric and commensurate spin state in mutiferroic HoMn$_{2}$O$_{5}$
对多铁性化合物HoMn $_{2}$ O $_{5}$进行了强磁场下的中子衍射测量。在零磁场下，高温无公度磁（HT-ICM）-公度磁（CM）-低温无公度磁（LT-ICM）顺序随温度的降低而发生，其中铁电极化仅出现在CM相。在施加磁场时，LT-ICM相完全消失，并且在最低温度下诱导CM相。这种场致CM状态与这种材料中的场致电极化的强度完全相关[Higashiyama { \it et al}.，Phys. Rev. B { \bf 72}，064421（2005）.]，强有力地表明，在多铁性$R$ Mn $_{2}$ O $_{5}$系统中，公度自旋态是铁电性的必要条件。
We used Raman scattering to study the lattice and magnetic excitations in the hexagonal HoMnO3 single crystals. The E2 phonon mode at 237 cm-1 is affected by the magnetic order. This mode is related to the displacement of Mn and O ions in a-b plane and modulates the Mn-O-Mn bond angles in a-b plane and the in-plane Mn-Mn superexchange interaction. The mode at 269 cm-1 associated to the displacement of the apical Ho3+ ions along the c direction presents an abrupt change of slope at TN showing that the role of the rare earth ions can not be neglected in the magnetic transition. We have identifie…
Low temperature dielectric anomalies in HoMnO_3: The complex phase diagram
在低温（1.8 K< T <10 K）和外加磁场作用下，多铁性六方HoMnO_3的介电常数呈现出前所未有的异常多样性，这与Ho磁矩、Mn自旋和铁电极化耦合系统中的磁相变有关.推导出的相图是复杂得多，比以前假设的，包括重入相，相变与不同的热和场滞后，以及几个多临界点。磁弹性相互作用在磁相变处引入晶格异常。对HoMnO_3的T-H相图的重新评价是必要的。
The structural distortion and magnetoelastic coupling induced through commensurate magnetism has been investigated by neutron diffraction in structurally related MnWO$_4$ and NaFe(WO$_4$)$_2$. Both systems exhibit a competition of incommensurate spiral and commensurate spin up-up-down-down ordering along the magnetic chains. In the latter commensurate phases, the alternatingly parallel and antiparallel arrangement of Fe$^{3+}$ respectively Mn$^{2+}$ moments leads to sizeable bond-angle modulation and thus to magnetic dimerization. For NaFe(WO$_4$)$_2$ this structural distortion has been determ…
An x-ray resonant diffraction study of multiferroic DyMn2O5
本文用X射线共振散射测量了Dy离子在多铁性DyMn $_{2}$ O $_{5}$中的磁序。磁序具有复杂的行为。当温度变化时，有几种不同的有序波矢，既有无公度的，也有公度的。此外，在两倍的相称的信号之一的波矢量的非磁性信号被观察到，其最大强度发生在相同的温度作为一个局部最大值的铁电极化。某些结果与RMn $_{2}$ O $_{5}$系列多铁性材料的其他成员的行为相似，可以用基于所谓的偏心自旋密度波的理论来解释。
Noncentrosymmetric Commensurate Magnetic Ordering of Multiferroic ErMn2O5
ErMn $_2$ O $_5$的非中心对称磁性结构已被证明与HoMn $_2$ O $_5$的非中心对称磁性结构非常相似（Vecchini { \it等人}，2008 Phys. Rev. B { \bf 77} 134434）。在25 K的磁调制的传播矢量$\vec k$ =（1/2，0，1/4）和对称性强加的磁配置允许非常少的限制。只有结合体磁化强度测量、粉末和单晶中子衍射以及球形中子极化分析的结果，才有可能清楚地区分不同的模型。磁化率测量结果表明，铒的磁矩平行于c轴排列，表明强的单离子各向异性。球形中子极化法表明在ErMn $_2$ O $_5$单晶中存在两个不相等的手性畴。利用同步辐射对ErMn $_2$ O $_5$粉末进行X射线衍射测量，结果表明Mn-O-Mn键的屈曲角在向铁电相转变的下方变化。
The crystal and magnetic structure of multiferroic LiFe(WO$_4$)$_2$ were investigated by temperature and magnetic-field dependent specific heat, susceptibility and neutron diffraction experiments on single crystals. Considering only the two nearest-neighbour magnetic interactions, the system forms a $J_1$, $J_2$ magnetic chain but more extended interactions are sizeable. Two different magnetic phases exhibiting long-range incommensurate order evolve at $T_{\text{N}1}\approx 22.2 \text{ K}$ and $T_{\text{N}2}\approx 19 \text{ K}$. First, a spin-density wave develops with moments lying in the $a…
Non-resonant and Resonant X-ray Scattering Studies on Multiferroic TbMn2O5
全面的X射线散射研究，包括共振散射在锰的L-边缘，Tb的L-和M-边缘，进行了单晶TbMn 2 O 5。在铁电相的布拉格禁戒位置观察到X射线强度，除了晶格和磁调制峰。它们的强度随温度的变化关系以及调制波矢之间的关系为交换伸缩引起的铁电性提供了直接的证据。共振X射线散射的结果表明，存在多个磁序，表现出不同的温度依赖性。在24 K附近的超导体到无公度的相变归因于自旋阻挫几何中磁序的相位滑移引起的位错。我们提出低温无公度相由反相畴壁分隔的公度磁畴组成，反相畴壁在相变时突然降低自发极化。
We report a neutron diffraction study of the multiferroic mechanism in (ND4)2FeCl5D2O, a molecular compound that exhibits magnetically induced ferroelectricity. This material exhibits two successive magnetic transitions on cooling: a long-range order transition to an incommensurate (IC) collinear sinusoidal spin state at TN=7.3 K, followed by a second transition to an IC cycloidal spin state at TFE=6.8 K, the later of which is accompanied by spontaneous ferroelectric polarization. The cycloid structure is strongly distorted by spin-lattice coupling as evidenced by the observations of both odd …
Multiferroicity induced by dislocated spin-density waves
我们发现了一个新的途径走向多铁性，显示如何磁性可以驱动铁电性，而不依赖于反转对称性破缺的磁序。我们的自由能分析表明，任何相称的自旋密度波有序的相位位错，即使它是共线的，引起电极化的强度。由于位错的存在，电子中心、磁性中心和反转中心不重合，这是多铁性耦合的充分条件。新的机制解释了形成的多铁性相的磁性可磁化性转变，如观察到的YMn $_{2}$ O $_{5}$和相关化合物。我们预测，在这些多铁性材料中，振荡的电极化是伴随着均匀极化。在此基础上，我们提出了具有铁电性的新型磁性材料。
Using single crystal neutron scattering we show that the magnetic structure Ni$_3$TeO$_6$ at fields above 8.6 T along the $c$ axis changes from a commensurate collinear antiferromagnetic structure with spins along c and ordering vector $Q_C$= (0 0 1.5), to a conical spiral with propagation vector $Q_{IC}$= (0 0 1.5$\pmδ$),$δ\sim$0.18, having a significant spin component in the ($a$,$b$) plane. We determine the phase diagram of this material in magnetic fields up to 10.5 T along $c$ and show the phase transition between the low field and conical spiral phases is of first order by observing a di…
Observation of different spin behavior with temperature variation and Cr substitution in a multiferroic compound YMn$_2$O$_5$
本文通过对YMn $_{2-x}$ Cr $_x$ O $_5$（x= 0.0，0.05）系列的低场体磁测量，探讨了自旋的集体响应。YMn $_2$ O $_5$的低场交流磁化率和直流磁化强度与电学测量中所观察到的类似，表现出多重跃迁。使用各种时间相关的磁化协议，已经观察到，在公度和非公度相位的自旋的行为是截然不同的。YMn $_{1.95}$ Cr $_{0.05}$ O $_5$与经历反铁磁有序的母体相比经历亚铁磁有序，具有增强的磁有序温度。由于母体化合物是一种重要的多铁性材料，在原子结构没有任何重大变化的情况下出现自发磁化是相当重要的。此外，在Cr取代化合物中观察到磁记忆效应，而在母体化合物中不存在。
In order to clarify the mechanism associated with pressure/magnetic-field-induced giant ferroelectric polarization in TbMnO3, this work investigated changes in magnetic ordering brought about by variations in temperature, magnetic field, and pressure. This was accomplished by means of neutron diffraction analyses under high pressures and high magnetic fields, employing a single crystal. The incommensurate magnetic ordering of a cycloid structure was found to be stable below the reported critical pressure of 4.5 GPa. In contrast, a commensurate E-type spin ordering of Mn spins and a noncollinea…
