Magnetic properties of the nanocrystalline DyMnO3

We report on the X-ray and neutron diffraction and magnetic measurements of the nanosamples of DyMnO₃ annealed at temperatures of 800 °C, 850 °C and 900 °C. The diffraction data indicate that all the samples crystallize in an orthorhombic crystal structure (space group Pnma). The crystal structure parameters change slightly with preparation. All the samples are antiferromagnets at low temperatures. The Mn magnetic moments order near 40 K, while those of Dy below 8.4 K. The macroscopic magnetic and neutron diffraction data indicate a small difference of properties between the DyMnO₃ samples synthesized at different temperatures. The observed broadening of magnetic peaks connected with the Dy sublattice suggests a cluster-like character of magnetic ordering.     

Antiferroelectric phase transitions in single crystals PbZrO3:Sn revisited

This paper is a review of earlier results which have been completed with recent investigations on the nature and sequence of phase transitions evolving in the antiferroelectric PbZr_1–xSn_xO₃ single crystals (0 < x < 0.35). It was found that these crystals undergo several first-order phase transitions. To investigate these transitions, five experimental methods were used to characterize the crystal properties: birefringence, Raman light scattering, dielectric, thermodynamic and electromechanical methods, and the atomic force microscopy-piezoresponse force microscopy (AFM-PFM) method to focus also on nanoscale properties. Temperature dependencies have been tentatively measured in a wide range, including a region above T_C, where precursor dynamics is observed in the form of non-centrosymmetric regions existing locally in crystal lattices.

New experimental data (Raman light scattering and PFM) are presented mainly to revise the phase diagram and, at the same time, to complement other studies on the problem of pre-transitional effects. The phase transition mechanism in PbZr_1–xSn_xO₃ is discussed taking into account the incommensurability of crystal lattices, ferroelastic behavior of multicell cubic phases, electric field influence on antiferroelectric to ferroelectric transformation, and flexoelectric effect.     

Possible new phase transitions in hexagonal RbxWO3

Two correlated phase transitions have been observed in hexagonal Rb_xWO₃ in the concentration range 0.16≤x≤0.33. One of these is a concentration-dependent transition which occurs near x=0.25, and precipitously affects the superconducting transition temperature. The other is a temperature-dependent transition which appears as anomalies in the resistivity, Hall coefficient, and Seebeck coefficient between 100K and 280K. The temperature at which the anomalies occur depends strongly on the x-value.     

Magnetostructural phase transitions in manganese arsenide single crystals

The effect of an external magnetic field with a strength up to 140 kOe on the phase transitions in manganese arsenide single crystals has been investigated. The existence of unstable magnetic and crystal structures at temperatures above the Curie temperature T _C = 308 K has been established. The displacements of manganese and arsenic atoms during the magnetostructural phase transition and the shift in the temperature of the first-order magnetostructural phase transition in a magnetic field have been determined. It has been shown that the magnetocaloric effect in a magnetic field of 140 kOe near the Curie temperature T _C is equal to ??T ?? 13 K. A model of the superparamagnetic state in MnAs above the temperature T _C has been proposed using the data on the magnetic properties and structural transformation in the region of the first-order magnetostructural phase transition. It has been demonstrated that, at temperatures close to T _C, apart from the contribution to the change in the entropy from the change in the magnetization there is a significant contribution from the transformation of the crystal lattice due to the magnetostructural phase transition.     

Magnetic properties of hexagonal oxide ferrimagnets with orientational transitions

We study the magnetic structure of different types of hexaferrites, BaCo_xTi_xFe_12–2xO₁₉(M), BaCo_2–xZn_xFe₁₆O₂₇(W), and Ba₃Co_2.4Ti_0.4Fe_23.2O₄₁ (Z), and determine the concentration and temperature regions whre magnetically ordered phases such as the cone, plane, and axis of easy magnetization phases exist and the order of the magnetoorientational phase transitions. The constants of magnetic crystal anisotropy, including those of a higher order, were determined using polycrystalline textured samples. We estimate the exchange integrals for the main structural types of hexaferrites, M, W, Y, and Z. Based on these values, we show that it is possible to make a quite reliable prediction about the concentration and temperature dependence of such basic characteristics of hexaferrites as magnetization and anisotropy fields.V. D. Kuznetsov Siberian Physicotechnical Institute, State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 25–32, September, 1992.     

Phase transitions in AgTaO3 single crystals

AgTaO₃ single crystals have been obtained by the molten salt method; two different solvents, AgCl and V₂O₅, were used. X-ray investigations have been carried out in the temperature region from 300 K to 800 K. At room temperature AgTaO₃ possesses a pseudocubic perovskite-type unit cell with rhombohedral distortion. Two structural phase transitions, to tetragonal at 650 K and to cubic symmetry at 770 K, have been observed. In the tetragonal phase a jump of the lattice constants has been observed at approximately 680 K. DTA and domain structure investigations confirm the presence of these phase transitions.     

Magnetic phase transitions and large mass enhancement in single crystal CaFe4As3

High quality single crystal CaFe₄ As₃ was grown by using the Sn flux method. Unlike layered CaFe₂ As₂ , CaFe₄ As₃ crystallizes in an orthorhombic three-dimensional structure. Two magnetic ordering transitions are observed at ～ 90 K and ～ 27 K, respectively. The high temperature transition is an antiferromagnetic（AF） ordering transition. However, the low temperature transition shows complex properties. It shows a ferromagnetic-like transition when a field is applied along b-axis, while antiferromagnetism-like transition when a field is applied perpendicular to b-axis. These results suggest that the low temperature transition at 27 K is a first-order transition from an AF state to a canted AF state. In addition, the low temperature electron specific heat coefficient reaches as high as 143 mJ/mol·K 2 , showing a heavy fermion behavior.     

Magnetic properties of oxides and silicon single crystals

The magnetic properties of single crystals Si, SrTiO₃, LaAlO₃, MgO, and (La,Sr)(Al,Ta)O₃ were investigated systematically. Three origins of the magnetizations of these crystals, namely, an intrinsic diamagnetic, a paramagnetic, and a ferromagnetic contribution, have been found to influence the magnetic signals measured on the crystals, in some important application scenarios such crystals being served as substrates with the magnetic thin film epitaxially grown on. Quantitative analyses methodologies were developed and thorough investigations were performed on the crystals with the intrinsic materials parameters thus revealed, especially that the intrinsic diamagnetic susceptibility differential dχ_dia/dT were identified quantitatively for the first time in SrTiO₃, LaAlO₃, MgO, and (La,Sr)(Al,Ta)O₃. The paramagnetic contribution is found to be the key in terms of the magnetic properties of the crystals, which in turn is in fact a consequence of the 3d impurities doping inside the crystal. All the intrinsic materials parameters are given in this paper as datasets, the datasets are openly available at https://www.doi.org/10.57760/sciencedb.j00113.00028.     

Magnetic and magnetoelastic properties of GdZn single crystals

The magnetic and magnetoelastic properties of GdZn have been investigated by various ways: magnetization, magnetic susceptibility, magnetocaloric effect, pressure dependence of the Curie temperature, spontaneous magnetostriction, parastriction and sound velocity measurements. The γ-mode strain is large for an S-state ion, and two orders of magnitude larger than the ?-one. Both the first-order B^γ,2 and the three isotropic second-order magnetoelastic coefficients exhibit a m²(1 ? 0.5m²) thermal variation (where m(T, H) is the reduced magnetization), which differs from the classical behaviour. All the results suggest a noticeable contribution of the conduction band to the magnetism of GdZn.     

Field-induced phase transitions and giant magnetoresistance in Dy D{y_3}CoCo single crystals

Electrical resistivity and calorimetric measurements on Dy ₃ Coshow that below the Néel temperature (T N =44 K) the non-collinear antiferromagnetic structure exhibits field-induced magnetic phase transitions of a first-order type along all principal axes, accompanied by a strongly anisotropic giant magnetoresistance and by a change of the Sommerfeld coefficient of the specific heat. Quantum tunnelling of the magnetization appears to be possible for T < 0.6 K. Received 15 July 1999 and Received in final form 6 December 1999     

Magnetic phase transitions in Nd7Rh3 single crystal

Magnetic phase transitions in rare earth intermetallic compound Nd₇Rh₃ have been investigated using a single crystal. Measurement results of magnetization, magnetic susceptibility, specific heat, and electrical resistivity reveal that Nd₇Rh₃ has two magnetic phase transitions at T_N=34 K, T_t2=9.1 K and a change of the magnetic feature at T_t1=6.8 K in the absence of an external magnetic field. Antiferromagnetic orderings exist in all the three magnetic states; a large magnetic anisotropy between the c-axis and the c-plane is observed. In the magnetic phase below T_t2, an irreversible field-induced magnetic phase transition takes place in the c-plane; after removing external magnetic field, a coexistence state of ferro- and antiferromagnetic ordering or a ferrimagnetic state having a remanent magnetization M_R is stabilized. The M_R decays to a certain value for several hours after the first process; a magnetic field cooling effect was also observed in the c-plane below T_t2. In the antiferromagentic state above T_t2, the irreversibility disappears and an ordinary antiferromagnetic state takes place. As the origin of this phenomenon, a kind of martensitic structural transition that is observed in Gd₅Ge₄ can be considered.     

Birefringence and phase transitions in liquid crystals

The birefringence of liquid-crystalline phases is the result of the parallel order of molecules exhibiting a polarizability anisotropy. The magnitude and sign of the birefringence are determined by the structure and order of the liquid-crystalline phase types as well as by the polarizability properties of the constituent molecules. The characteristic change of the birefringence at phase transitions between liquid-crystalline phases indicates more or less pronounced structural changes. The temperature dependence of the birefringence is due to the temperature change of the molecular order.

It is shown that the structural variety of the liquid crystalline state is reflected by a big variety of their optical anisotropy properties.     

Magnetic phase transitions in garnets

The phenomenological theory broadly applicable to magnetic transitions in ferrimagnetic garnets is discussed briefly. The experimental techniques, particularly nuclear magnetic resonance and Mössbauer effect spectroscopy, are then reviewed. Finally, there is a review of the results on specific garnets which undergo such transitions. Some remaining problems are pointed out.     

磁性量子相变

量子相变广泛存在于关联电子材料体系中,与非常规超导和奇异金属行为有着紧密的联系。近年来,人们对量子相变的认识正在不断深入,不同类型的量子相变相继被发现。揭示量子相变的普适分类,发展和完善量子相变理论,探索量子临界点附近的呈展量子态及其产生机理是当前量子相变研究的热点。文章简要介绍磁性量子相变的一些最新研究进展以及面临的挑战。     

Ordering and phase transitions in liquid crystals

Structures and properties of different types of liquid crystals are discussed from a uniform point of view. Apart from the traditional mesophases (nematics, cholesterics, smectics), some new examples, including polymeric, metalloorganic and ferromagnetic liquid crystals, are also investigated. Systems with several macroscopic scales on which the type of ordering may differ, are described and analysed. Phase transitions and certain types of critical behaviour in different liquid crystals are studied.