正交LiMnO2中的重入自旋玻璃行为

用水热技术合成了单相正交LiMnO2(o-LiMnO2)粉末,X射线衍射表明其空间群为Pmnm.X射线衍射精修结果指出该材料存在少量的阳离子无序,但这种无序对材料的磁性没有表现出明显的影响.静态和动态的磁性研究结果表明o-LiMnO2中存在重入自旋玻璃行为,即在T≤TC≈118 K,反铁磁态转向铁磁态,在T≤Tf≈50 K,铁磁态又转向自旋倾斜玻璃态.     

Eu掺杂TbMnO3多晶材料的介电性质

采用固相反应法制备了Tb_0.8Eu_0.2MnO₃多晶材料.对样品的X射线衍射（XRD）分析表明Eu³⁺固溶于TbMnO₃中.测量了样品在低温(100 K ≤T≤ 300 K)和低频下(200 Hz≤f≤100 kHz)的复介电性质.在此温度区间内发现了两个介电弛豫峰.经分析认为低温峰（T≈170 K）起源于局域载流子漂移引起的偶极子极化效应,而高温峰（T≈290 K）则是由离子电导产生的边界和界面层的电容效应引起的.电阻率的测量显示在低温下（T≈230 K）存在明显的导电机制转变.     

钙钛矿型Pr1-xRbxMnO3 (0.05≤x≤0.08)的合成和性质

将Rb⁺掺杂到PrMnO₃体系,合成出Pr_1-xRb_xMnO₃ (0.05≤x≤0.08),X射线衍射表明该系列化合物都为正交相, 空间群为Pnma. 磁性研究发现, 该系列化合物在高于200 K时符合居里-外斯定律; 在低温时表现出一定的自旋玻璃态的性质, 说明在低温下存在着铁磁和反铁磁的相互竞争. 同时测试了化合物Pr_0.92Rb_0.08MnO_3.02在0和2 T下的电阻-温度曲线,计算出该化合物为半导体带隙约0.3 eV,116 K时其磁阻变化率大,约为30%.     

Mg2+掺杂Zn2SiO4:Mn2+的溶胶-凝胶法合成及真空紫外发光特性研究

采用溶胶-凝胶法（sol-gel method）于不同气氛条件下成功合成了Zn_1.92-xMg_xSiO₄:0.08Mn²⁺（0≤x≤0.12）系列粉末样品.利用X射线衍射(XRD)、光致发光(PL)谱等分析手段对Zn_1.92-xMg_xSiO₄:0.08Mn²⁺系列     

高性能MgB2长线材制备及性能表征

采用原位粉末装管工艺，分别以Mg粉(99.5%)，无定形B粉(99.9%)为原料，以纳米SiC(10—30nm)作为掺杂材料制备铁基MgB₂线.首先将已混合的原料在丙酮介质中球磨，真空干燥后，将粉末填入铁管内，然后通过孔型轧制、旋锻和拉拔等冷加工工艺得到11m长外径Ф1.75mm铁基MgB₂超导线.用扫描电镜，电子能谱，X射线衍射仪和超导量子干涉仪测试发现，样品微观结构整齐，晶粒大小均匀，内部仅含微量MgO，T_C(onset)=35.1K，ΔT_C＝5.3K.纳米SiC掺杂后，其中C造成MgB₂晶格畸变，形成有效磁通钉扎中心，C元素在MgB₂中分布均匀.标准四引线测试结果表明，11m线均分10段后，各点的J_c(4.2K,10T)均超过1.0×10⁴A/cm^{2，最高值达到1.2×104A/cm2.在10—18T范围各点临界电流值分布均匀，变化率小于10%.}     

Sr掺杂Eu1-xSrxMnO3体系的核磁共振和磁结构研究

对Eu_1-xSr_xMnO₃ (x=0.4, 0.5, 0.6, 0.7) 体系的磁结构进行了系统的研究. 通过核磁共振实验, 磁化测量, 并结合电输运测量结果表明, Sr的掺入使得 EuMnO₃反铁磁母相中出现铁磁相. 铁磁相和反铁磁相的竞争导致样品在低温下的自旋玻璃行为.分析认为, Eu_0.4Sr_0.6MnO₃和Eu_0.3Sr_0.7MnO₃的磁结构在低温下呈现更加复杂的特征, 主要源于铁磁团簇的形成以及无序相的存在.     

层状钙钛矿La1.3Sr1.7Mn2-xCuxO7的磁性及电特性

通过固相反应烧结法成功制备了层状钙钛矿La_1.3Sr_1.7Mn_2-xCu_xO₇多晶，主要研究了其磁电特性.结果表明，样品为Sr₃Ti₂O₇型钙钛矿结构.随着温度的降低，其磁性经历了一个很复杂的转变过程.当x=0时，在T^*=231K出现二维短程铁磁有序，在<     

多自旋态离子Co替代诱导La2/3Ca1/3MnO3体系的输运反常

利用具有多自旋态的Co离子进行Mn位替代，制备了La_2/3Ca_1/3Mn_1-xCo_xO₃ (0≤x≤0.15) 系列样品并研究了体系的结构和输运特性.结果表明，在替代范围内，样品呈现很好的单相结构，各晶格参数随替代量的增大而减小；Co替代导致体系出现电输运反常，具体表现为在居里温度T_C以下电阻-温度曲线的二次金属-绝缘转     

内掺过渡金属非典型富勒烯M@C22(M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni) 几何结构、电子结构、稳定性和磁性的密度泛函研究

采用密度泛函理论中广义梯度近似对非典型富勒烯C₂₂和过渡金属内掺衍生物M@C₂₂(M=Sc,Ti,V,Cr,Mn,Fe,Co和Ni)的几何结构和电子结构进行计算研究.发现非典型富勒烯C₂₂的基态结构是含有一个四碳环的单重态笼状结构.过渡金属原子的掺入明显提高了体系的稳定性. C-M键既有一定共价性又有一定离子性.磁性、能级图、轨道分布和态密度图分析表明: M原子的3d轨道和碳笼的C原子的原子轨道之间存在较强的轨道杂化. Ti, Cr, Fe和Ni内掺的结构出现磁性完全猝灭现象. Sc和碳笼间是弱反铁磁作用, V,Mn和Co与碳笼间是弱铁磁作用.     

SrMn0.5Fe0.5O3的自旋玻璃态和过渡金属离子价态的研究

采用固相反应法制备了SrMn_0.5Fe_0.5O₃陶瓷样品,并对样品的晶体结构,磁性和离子价态进行了系统的表征与分析. X射线衍射谱的Rietveld拟合表明样品属于理想的立方钙钛矿型结构,Mn离子和Fe离子随机占据B位的O八面体中心. X射线光电子能谱表明Mn离子为+3和+4的混合原子价态,Fe离子为+3价. 样品在大于230K的高温区域呈现Curie顺磁特性,在小于230 K的低温区域样品表现出自旋玻璃态行为,这种特性源于Mn离子和Fe离子之间的交换作用及自身价态和分布的不均匀性. 由于Fe³⁺离子占据O八面体的中心,对顺磁区的Mssbauer谱测量表现为四级分裂.     

层状钙钛矿La1.3Sr1.7Mn2-xCuxO7的磁性及电特性

通过固相反应烧结法成功制备了层状钙钛矿La_1.3Sr_1.7Mn_2-xCu_xO₇多晶,主要研究了其磁电特性.结果表明,样品为Sr₃Ti₂O₇型钙钛矿结构.随着温度的降低,其磁性经历了一个很复杂的转变过程.当x=0时,在T^*=231K出现二维短程铁磁有序,在< 关键词： 层状钙钛矿 磁性 电特性     

Size-controlled hydrothermal synthesis and electrochemical behavior of orthorhombic LiMnO2 nanorods

Large amount of uniform orthorhombic LiMnO₂ (o-LiMnO₂) nanorods was fabricated by a hydrothermal route in 180 °C using γ-MnOOH nanorods as precursors through an isomorphous ion exchange process. The size of as-obtained o-LiMnO₂ nanorods was determined by that of γ-MnOOH precursors, which could be deliberately controlled. The electrochemical performance of o-LiMnO₂ nanorods was characterized via galvanostatic tests, which suggested that the size of as-obtained products played an important role in their electrochemical performances.     

Hydrothermal phase formation of orthorhombic LiMnO2 and its derivatives as lithium intercalation compounds

Different from a conventional solid state reaction, a hydrothermal reaction mechanism is very difficult to illuminate and sometimes it remains undisclosed. Making an attempt to understand the hydrothermal phase formation process of o-LiMnO₂ obtained between the reaction of spinel type Mn₃O₄ precursor and LiOH aqueous solution, the possible reaction route was postulated and experimentally testified. Firstly, the selective dissolution of Mn²⁺ from the tetrahedral site of [Mn^II]^tet_4a[Mn^III₂]^oct_8dO₄, which is considered as to be an ionic exchange reaction with Li⁺, and an additional Li⁺ intercalation into the host structure of precursor would give rise to the formation of meta-stable Li₂Mn₂O₄ ([Li^I]^tet_4a[Li^I]^oct_8c[Mn^III₂]^oct_8dO₄). Secondly, the phase would be simultaneously transformed to thermodynamically stable o-LiMnO₂ phase under hydrothermal state during hydrothermal reaction. Through the above reaction process, the solid solution range of o-LiCo_xMn_1−xO₂ was as large as x ≤ 0.14, and that of o-LiFe_xMn_1−xO₂ was x ≤ 0.05. Co doped o-LiMnO₂ has higher capacity and good cyclability upon cycling, being substantially more stable to cycle than the unsubstituted and Fe doped materials.     

The low field longitudinal magnetoresistance of PdMn,PdFe and AuFe near the percolation limit

The low field longitudinal magnetoresistance, Δp/p_o, of Au + 14.5%at.Fe is measured as a function of field (- 1kG < H < 1kG), temperature (1.2K, 140K) and annealing conditions. When the sample is annealed and quenched from 950 C, the low field (～ 100 G) Δp/p_o versus H curve exhibits two sharp peaks at about 35 and 75 K respectively. The low temperature peak is associated with a spin glass transition (T_g) whereas the high temperature one would correspond to a ferromagnetic transition (T_c). The spin-glass state such as T ? T_g is characterised by a clear quadratic behavior of Δp/p_o as a function of H whereas in the ferromagnetic state (T_g < T < T_c), the magnetoresistance is completely dominated by a linear term. Quenching the sample from 550C considerably reduces the overall magnetoresistance, suppresses the linear term and transforms the double peak structure into a a single rather broad cusp of a spin-glass type. To help interpreting the above results, we also carried out low field magnetoresistance measurements on Pd + 0.2 at.%Fe, Pd + 2 at.%Mn and Pd + 10 at.%Mn at low temperatures.     

Weak ferromagnetism in copper metaborate CuB2O4

CuB₂O₄ single crystals have been grown and their magnetic and resonance properties have been investigated for the first time. The temperature dependence of the susceptibility was found to contain features at T=21 and 10 K. The CuB₂O₄ single crystal transformed at T=21 K to a weakly ferromagnetic state. The sharp drop in susceptibility at T<10 K is caused by a transition of the magnetic system of CuB₂O₄ to an antiferromagnetic state. The effective magnetic moment of the Cu₂₊ ion, determined from the high-temperature part of the magnetic susceptibility, is 1.77 μ _B. The room-temperature g factors are, respectively, 2.170 and 2.133 for magnetic field parallel and perpendicular to the c axis of the crystal. The antiferromagnetic resonance parameters in the weakly ferromagnetic and antiferromagnetic phases were measured. Fiz. Tverd. Tela (St. Petersburg) 41, 1267–1271 (July 1999)     

Linear birefringence studies of incommensurate systems: II. KFeF4

Abstract

The temperature dependence of the linear birefringence (LB), Δn _c , is measured in the range 5 K ? T ? 500 K on samples of KFeF₄, which originate from hydrothermal, flux and Bridgman growth techniques. Pronounced anomalies are found at the orthorhombic-orthorhomic phase transition at T _c ∽ 400 K. It is weakly discontinuous with a near-tricritical exponent β ∽ 0.2. Weak anomalies near T _i ∽ T _c + (25 … 40 K) seem to indicate a transition into an intermediate incommensurate phase. Its XY-model character is reflected by the critical LB exponent β = 0.8 ± 0.1. A smooth LB anomaly below 200 K is due to 2-dimensional ferromagnetic spin-order.     

Magnetic properties of hematite with large coercivity

The magnetic properties of hematite powders produced by a solid state nucleation-and-growth process are studied as a function of temperature T and applied field H. Independently of the temperature, there exists a soft magnetic contribution that is assigned to the canting of spins at the superficial shell of each particle and is not affected by the Morin transition. At 220<T<T _M a magnetic contribution with high coercivity is observed, due to spin–flop in the anti ferromagnetic state and above T _M =248 K the weakly ferromagnetic state has a coercivity that ranges from 6 kOe to 4 kOe when raising T up to room temperature. Different sub-grain structures were obtained by means of isochronal and isothermal annealing. Changes in the susceptibility are directly related to the sub-particle size. It is concluded that sub-boundaries are the defects responsible for the high coercivities observed in the weakly ferromagnetic state.     

Ferromagnetism in the high-pressure phases of (GaSb)1−x Mnx

The electrophysical and magnetic properties of recently discovered high-pressure phases in the GaSb-Mn system with simple cubic and tetragonal structures have been examined. It has been shown that samples with the primitive cubic structure for low temperatures are in the ferromagnetic state and the Curic temperature depends on the initial manganese content and reaches T _c = 280 K for x = 0.6. It has been shown that these samples for a manganese content x ≤ 0.5 are in the semiconducting state with large impurity conduction and pass to the metallic state as x increases. The GaSbMn phase with the tetragonal structure has ferromagnetic properties up to temperatures of T ～450 K (at which the phase begins to decay) and exhibits metallic properties. The magnetization at T = 77.3 K is equal to M = 0.58 μ_B and 0.28 μ_B per manganese ion for the simple cubic and tetragonal phases, respectively.     

Changes in the magnetic structure of (La0.25Pr0.75)0.7Ca0.3MnO3 upon the isotopic substitution of 18O for 16O

A giant isotope effect, wherein the low-temperature metallic state is replaced by an insulator state, was recently observed when ¹⁸O was substituted for ¹⁶O in (La_0.25Pr_0.75)_0.7Ca_0.3MnO₃ [N. A. Babushkina, L. M. Belova, O. Yu. Gorbenko et al., Nature (London) 391, 159 (1998)]. In the present work, the temperature evolution of the magnetic structure of two samples of this compound is studied by neutron diffraction. One sample contained a natural mixture of oxygen isotopes, 99.7% ¹⁶O, while the other was enriched to 75% with the isotope ¹⁸O. It is established that the samples are crystallographically identical at room temperature. As the temperature decreases, the sample with ¹⁶O undergoes successive antiferromagnetic (T _AFM=5150 K) and ferromagnetic (T _FM=110 K) transitions, which lead to the establishment of a noncollinear ferromagnetic structure, while a purely antiferromagnetic (T _AFM=150 K) order arises in the sample with ¹⁸O. The temperature dependences of the intensities of the diffraction peaks associated with the charge ordering differ substantially in samples with ¹⁶O and ¹⁸O, and they correlate with the behavior of their electric resistance and magnetic structure. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 46–51 (10 January 1999)