用高分辨粉末同步辐射和中子衍射研究了尖晶石型化合物ZnCr $_2$ S $_4$和ZnCr $_2$ Se $_4$的晶体结构和磁性。ZnCr $_2$ Se $_4$在$T_N=21$ K处表现出一级相变，转变为无公度螺旋磁结构。$T_N$以上的磁波动耦合到晶格，表现为负热膨胀。复杂的磁结构和异常的结构行为都可能与磁挫挫有关。外加磁场的应用使有序化温度和负热膨胀状态向低温方向移动。因此，自旋排序变为锥形结构。ZnCr $_2$ S $_4$在$T_{N1}=15$ K和$T_{N2}=8$ K处表现出两个伴随着结构相变的磁转变。晶体结构在高温顺磁状态下从立方尖晶石型（空间群$Fd$ \={3} $m$）转变为$T_{N2} < T < T_{N1}$的四方畸变中间相（空间群$I4_1$ / $amd$），转变为$T < T_{N2}$的低温正交相（空间群$I m m a$）。硫离子通过交换伸缩的协同置换是这些结构相变的起源。ZnCr $_2$ S $_4$的低温结构与Verwey转变以下的磁铁矿的正交结构相同。当施加5 T的磁场时，该系统显示出在ZnCr $_2$ Se $_4$中观察到的中间磁相中的诱导负热膨胀。
英文摘要：
The crystal and magnetic structures of the spinel compounds ZnCr$_2$S$_4$ and ZnCr$_2$Se$_4$ were investigated by high resolution powder synchrotron and neutron diffraction. ZnCr$_2$Se$_4$ exhibits a first order phase transition at $T_N=21$ K into an incommensurate helical magnetic structure. Magnetic fluctuations above $T_N$ are coupled to the crystal lattice as manifested by negative thermal expansion. Both, the complex magnetic structure and the anomalous structural behavior can be related to magnetic frustration. Application of an external magnetic field shifts the ordering temperature and the regime of negative thermal expansion towards lower temperatures. Thereby, the spin ordering changes into a conical structure. ZnCr$_2$S$_4$ shows two magnetic transitions at $T_{N1}=15$ K and $T_{N2}=8$ K that are accompanied by structural phase transitions. The crystal structure transforms from the cubic spinel-type (space group $Fd$\={3}$m$) at high temperatures in the paramagnetic state, via a tetragonally distorted intermediate phase (space group $I4_1$ / $amd$) for $T_{N2} < T < T_{N1}$ into a low temperature orthorhombic phase (space group $I m m a$) for $T < T_{N2}$. The cooperative displacement of sulfur ions by exchange striction is the origin of these structural phase transitions. The low temperature structure of ZnCr$_2$S$_4$ is identical to the orthorhombic structure of magnetite below the Verwey transition. When applying a magnetic field of 5 T the system shows an induced negative thermal expansion in the intermediate magnetic phase as observed in ZnCr$_2$Se$_4$.
Intrinsic susceptibility and bond defects in the novel 2D frustrated antiferromagnet Ba$_{2}$Sn$_{2}$ZnCr$_{7p}$Ga$_{10-7p}$O$_{22}$
本文报道了高阻挫反铁磁体Ba $_{2}$ Sn $_{2}$ ZnCr $_{7p}$ Ga $_{10-7p}$ O $_{22}$的微观和宏观磁性，分别用NMR和SQUID实验进行了探测. Cr $^{3+}$受抑kagomé双层的固有磁化率的$T$ -变化，$χ_{kag}$，在45 K附近显示最大值。详细研究了$0.29 \leq p \leq0.97$的磁晶格稀释。新的稀释无关的缺陷，可能与磁键无序，证明和讨论。我们将我们的结果与SrCr $_{9p}$ Ga $_{12-9p}$ O $_{19}$进行比较。键缺陷和自旋空位都不影响kagomé双层膜的平均磁化率。
The crystal and magnetic structures of stoichiometric ZnCr2Se4 have been investigated using synchrotron X-ray and neutron powder diffraction, muon spin relaxation (muSR) and inelastic neutron scattering. Synchrotron X-ray diffraction shows a spin-lattice distortion from the cubic spinel to a tetragonal I41/amd lattice below TN = 21 K, where powder neutron diffraction confirms the formation of a helical magnetic structure with magnetic moment of 3.04(3) μB at 1.5 K; close to that expected for high-spin Cr3+. MuSR measurements show prominent local spin correlations that are established at temper…
Characterization of electronic properties of novel materials is of great importance for exploratory materials development and also for the discovery of new correlated phases. As several novel compounds are available in powder form only, contactless methods, which also work on air sensitive samples, are higly desired. We present that the microwave cavity perturbation technique is a versatile tool to study conductivity in such systems. The examples include studies on semiconducting-metallic crossover in carbon nanotubes upon alkali doping, study of vortex motion in the K$_3$C$_{60}$ superconduct…
The Fe electronic structure and magnetism in (i) monoclinic Ca$_2$FeReO$_6$ with a metal-insulator transition at $T_{MI} \sim 140$ K and (ii) quasi-cubic half-metallic Ba$_2$FeReO$_6$ ceramic double perovskites are probed by soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). These materials show distinct Fe $L_{2,3}$ XAS and XMCD spectra, which are primarily associated with their different average Fe oxidation states (close to Fe$^{3+}$ for Ca$_2$FeReO$_6$ and intermediate between Fe$^{2+}$ and Fe$^{3+}$ for Ba$_2$FeReO$_6$) despite being related by an isoelectron…
Superconductivity in the pseudo-binary pnictides Ru0.55Rh0.45P and Ru0.75Rh0.25As is probed by muon spin relaxation and rotation (muSR) measurements in conjuction with magnetic susceptibility, heat capacity and electrical resistivity measurements. Powder x-ray diffraction confirmed the MnP-type orthorhombic structure (space group Pnma) and showed a nearly single phase nature with small impurity phase(s) of about 5% for both the samples. The occurence of bulk superconductivity is confirmed with Tc = 3.7 K for Ru0.55Rh0.45P and T = 1.6 K for Ru0.75Rh0.25As. The superconducting state electronic h…
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The XXZ model on a square lattice in the presence of a transverse magnetic field is studied within the spin wave theory to investigate the resulting canted antiferromagnet. The small and large field regimes are probed separately both for easy-axis and easy-plane scenarios which reveal an unentangled factorized ground state at an intermediate value of the field. Goldstone modes are obtained for the field-free $XY$ antiferromagnet as well as for the isotropic antiferromagnet with field up to its saturation value. Moreover, for an easy-plane anisotropy, we find that there exists a non-zero field,…
We report the comprehensive experimental results identifying the magnetic spin ordering and the magnetization dynamics of a double perovskite Pr2CoFeO6 by employing the (dc and ac) magnetization, powder neutron diffraction (NPD) and X-ray magnetic circular dichroism (XMCD) techniques. X-ray diffraction and neutron diffraction studies revealed that Pr2CoFeO6 adopts a B-site disordered orthorhombic structure with space group Pnma. Additionally, ab initio band structure calculations performed on this system suggested an insulating anti-ferromagnetic (Fe-Fe) ground state. Magnetometry study showed…
Physical Properties of Metallic Antiferromagnetic CaCo{1.86}As2 Single Crystals
本文报道了CaCo{1.86} As_2单晶的研究。电子结构探测角分辨光电子能谱（ARPES）测量的CaCo{1.86} As 2和全电位线性增强平面波计算的超晶胞Ca 8 Co 15 As 16（CaCo{1.88} As 2）。我们的XRD晶体结构精修与先前的X射线和中子粉末衍射数据的组合精修一致，显示了在对应于组成CaCo{1.86} As 2的Co位点上具有7（1）%空位的四重-四重ThCr 2Si 2型结构[D. G. Quirinale等人，B88，174420（2013）]。各向异性磁化率chi（T）数据与Quirianale等人的磁中子衍射数据一致，后者证明了在Neel温度TN = 52（1）K以下存在A型共线反铁磁有序，易磁化轴为四轴。然而，没有明确的证据，从电阻率ρ（T）和热容Cp（T）的数据在TN的磁性转变的观察。从能带计算和rho（T），Cp（T）和ARPES数据中证明了金属基态，自旋极化计算表明A型AFM和FM基态之间存在竞争. Cp（T）数据显示出大的Sommerfield电子系数，反映了费米能级D（EF）处的大态密度，这与能带结构计算一致，该计算也表明Co 3d能带产生大的D（EF）。在1.8K下，H的M（H）数据||c在3.5T的外加磁场下表现出良好的一阶自旋翻转跃迁。在低T下从M（H）数据得到的0.3 μ B/Co的小有序矩、chi的大交换增强以及…
