Magnetostriction in the vicinity of spin-reorientation phase transitions in singlecrystal DyFe11Ti

The field, temperature, and angular dependences of longitudinal λ_‖ and transverse λ_⊥ magnetostriction in single-crystal DyFe₁₁Ti are investigated. Tensometric measurements were made in the temperature range from 78 to 300 K in magnetic fields up to 12 kOe. Measurements of the magnetostriction of single-crystal DyFe₁₁Ti, YFe₁₁Ti, and LuFe₁₁Ti imply that the sublattice of 3d transition metals makes only a small contribution to the magnetostriction in compounds RFe₁₁Ti, and that the primary contribution to the magnetostriction of these compounds comes from the rare-earth metal sublattice. The primary microscopic mechanism for magnetostriction is single-ion magnetostriction caused by the interaction of the anisotropic orbital electron cloud around the Dy³⁺ magnetic ion with the crystal field of the lattice. Fiz. Tverd. Tela (St. Petersburg) 41, 1647–1649 (September 1999)     

Effect of hydrogenation on spin-reorientation phase transitions and magnetic anisotropy constants of RFe11Ti single crystals (R=Lu, Ho, and Er)

The magnetic anisotropy and spin-reorientation phase transitions in single crystals of the RFe₁₁Ti (R=Lu, Ho, and Er) compounds and their hydrides are investigated. Measurements are carried out on capacitance and torque magnetometers. The magnetic anisotropy constants K ₁ and K ₂ are determined by the mathematical processing of experimental magnetization curves in terms of the phenomenological theory of the anisotropic ferromagnet magnetization. It is demonstrated that the hydrogenation strongly affects the magnitude and the sign of magnetic anisotropy constants, as well as the spin-reorientation phase transitions. The hydrogenation of the HoFe₁₁Ti compound leads to the change in sign of the magnetic anisotropy constant K ₁. The inference is made that a change in the atomic volume and the axial ratio c/a cannot result in the observed effects. A change in the magnetic anisotropy constants upon hydrogenation is primarily due to the change in the interaction of the quadrupole moment of a 4f electron subshell of rare-earth ions with surrounding ions of the crystal lattice and also with valence and conduction electrons.     

Direct experimental investigation of spin-reorientation phase transitions in an antiferromagnetic CoNiCu/Cu superlattice

The characteristic features of phase transitions induced by an external magnetic field and of the corresponding changes in the relative orientations of the spins in the ferromagnetic CoNiCu layers of a multilayer film, which are coupled by an antiferromagnetic exchange interaction via nonmagnetic Cu interlayers, are studied using a magnetooptic method for visualizing the fringing fields. It is established that the magnetization reversal process in this nanocomposite material proceeds by a spin-flop orientational phase transition on account of the formation and motion of specific domain walls as well as by incoherent rotation of the spins toward the applied field. It is shown that, depending on the direction of the external magnetic field with respect to the easy axis, asymmetric canted phases also arise as a result of such transitions. Pis’ma Zh. Tekh. Fiz. 64, No. 11, 778–782 (10 December 1996)     

Some features of magnetic domain structure reorganization during spin-reorientation phase transitions of the first and second order

On the basis of magnetic domain structure investigations in single crystals of R ₂Fe₁₄B, RFe₁₁Ti, RCo₅ intermetallic compounds (where R is a rare earth metal) in the temperature region 4.2–400 K, the features of domain structure transformation during spin reorientation transitions of the first and second order are analyzed.     

Evidence for a spin-reorientation type phase transition in FeGeO3

Using single crystal susceptibility, Mössbauer and magnetoelectric measurements, the nature of the two low temperature phase transitions in the clinopyroxine metagermanate FeGeO₃ is studied. It is concluded that FeGeO₃ orders antiferromagnetically along the unique b axis at T_N = 47 ± 1°K and undergoes a spontaneous, second-order phase transition of the spin reorientation type at T_R = 17 ± 3°K. Only below T_R does FeGeO₃ exhibit magnetoelectricity.     

Ultrasonic velocities near the spin-reorientation transitions in ErFeO3 at high pressure

Longitudinal and shear sound velocities have been measured near the spin-reorientation transitions in ErFeO₃ at hydrostatic pressure up to 2.4 kbar. The results, which are analyzed in terms of a phenomenological theory with magnetoelastic coupling terms, show that hydrostatic pressure has a very small effect on the magnetic anisotropy constants.     

Investigation of magnetic phase transitions in dilute copper ferrites

Mössbauer spectra of copper ferrites CuGa _x Al_2x Fe_{2 ? 3x} O₄ (x = 0.3, 0.4, 0.5) have been measured at temperatures in the range from 90 to 295 K. The degree of disturbance of long-range magnetic order has been determined as a function of the temperature. The Curie temperature has been found for the composition with x = 0.5, and the dependence of the Curie temperature on the concentration of the nonmagnetic impurity has been constructed.     

Field-induced spin-reorientation transitions in magnetic superlattices with uniaxial anisotropy and biquadratic exchange

Phase transitions induced by an external field are investigated in magnetic multilayer systems with uniaxial anisotropy and biquadratic exchange. A magnetic field directed perpendicular to the plane of the layers changes the effective anisotropy and exchange constants, determining the orientation of the magnetization in the plane of the layers, and can give rise to spin-reorientation transitions. All possible types of such transitions are investigated for the case of uniaxial anisotropy, which differs substantially from the case of cubic anisotropy by the different renormalization of the effective anisotropy constants. Fiz. Tverd. Tela (St. Petersburg) 41, 461–463 (March 1999)     

Imaging spin-reorientation transitions in consecutive atomic Co layers on Ru(0001)

By means of spin-polarized low-energy electron microscopy, we show that the magnetic easy axis of one to three atomic-layer thick cobalt films on Ru(0001) changes its orientation twice during deposition: One-monolayer and three-monolayer thick films are magnetized in plane, while two-monolayer films are magnetized out of plane. The Curie temperatures of films thicker than one monolayer are well above room temperature. Fully relativistic calculations based on the screened Korringa-Kohn-Rostoker method demonstrate that only for two-monolayer cobalt films does the interplay between strain, surface, and interface effects lead to perpendicular magnetization.     

Investigation of quantum phase transitions using multi-target DMRG methods

In this paper we examine how the predictions of conformal invariance can be widely exploited to overcome the difficulties of the density-matrix renormalization group near quantum critical points. The main idea is to match the set of low-lying energy levels of the lattice Hamiltonian, as a function of the systems size, with the spectrum expected for a given conformal field theory in two dimensions. As in previous studies this procedure requires an accurate targeting of various excited states. Here we discuss how this can be achieved within the DMRG algorithm by means of the recently proposed Thick-restart Lanczos method. As a nontrivial benchmark we use an anisotropic spin-1 Hamiltonian with special attention to the transitions from the Haldane phase. Nonetheless, we think that this procedure could be generally valid in the study of quantum critical phenomena.Received: 20 May 2004, Published online: 5 November 2004PACS: 75.40.Mg Numerical simulation studies - 05.10.Cc Renormalization group methods - 75.10.Pq Spin chain models     

Magnetoresistance and magnetic phase transitions in a Pr11B6 antiferromagnet

In isotopically pure praseodymium hexaboride (Pr¹¹B₆) single-crystal samples, the transverse magnetoresistance Δρ/ρ has been measured in a temperature range of 2–20 K in magnetic fields up to 80 kOe. The field and angular dependences Δρ/ρ(H, ?, T ₀) reveal a new magnetic phase in the AFM state of Pr¹¹B₆ which is observable only for the external magnetic field orientation in a narrow angular range near H ‖ 〈110〉. The data remove the previous contradictions in the Pr11B6 magnetic phase diagram representation and can be explained under the assumption that the spin-polarized regions (ferrons) are involved in the formation of the complex magnetic structure in the Pr¹¹B₆ AFM state in the 5d band in the vicinity of the rare-earth ions.     

Investigation of magnetic phase transitions in diluted ferrites: Nickel ferrite

The Mössbauer spectra of nickel ferrite NiGa _x Al _x Fe_{2 ? 2x} O₄ were measured with significant dilution by nonmagnetic ions (with x = 0.5, 0.7, and 0.8). In the temperature range of 90–295 K, the ratio of the near- and long-range forces was experimentally found and the Curie temperatures were determined.     

Magnetic anisotropy and spin-reorientation transitions in RFe11Ti (R = Nd, Tb, Dy, Er) rare-earth intermetallics

RFe₁₁Ti rare-earth intermetallics (R = Nd, Tb, Dy, Er) which show temperature-induced spin-reorientation transitions (SRT) were investigated. The temperature dependences of the rare-earth anisotropy constants were calculated using the single-ion model. By fitting the experimental data a set of crystal-field and exchange-field parameters for R³⁺ ions wassb deduced. The temperature dependence of the first-order magnetization process fields was calculated for NdFe₁₁Ti and ErFe₁₁Ti within the model. The calculated spin-reorientation transition in NdFe₁₁Ti observed at T_s = 189 K is of first order with a discontinuous jump of the magnetization angle. Present calculations show that a first-order transition occurs in DyFe₁₁Ti at low temperatures. A second-order SRT from ‘easy axis’ to ‘easy cone’ exists in ErFe₁₁Ti. A first-order SRT ‘easy axis’-‘easy plane’ is predicted theoretically in TbFe₁₁Ti. The calculated curves of the temperature dependence of the anisotropy fields and the critical fields of the first-order magnetization processes for RFe₁₁Ti using the fitted crystal- and exchange-field parameters are in good agreement with the experimental data over a wide temperature range.     

Continuous spin-reorientation phase transition in the surface region of an inhomogeneous magnetic film

A continuous spin-reorientation transition from a uniform magnetic state with the in-plane orientation of the moments of all atomic layers to a nonuniform canted state in the surface region is considered. This transition was discovered in experiments on the divergence of magnetic susceptibility in a perpendicular magnetic field at a temperature of about 240 K, which is lower than the Curie point of gadolinium, equal to 292.5 K. These experiments were carried out on an ultrathin iron magnetic film deposited on the (0001) surface of a thin gadolinium film. It is shown that, in the vicinity of the spin-reorientation transition, the thermodynamic potential has a form characteristic of the Landau theory of second-order phase transitions. The orientation angle of the moment of the surface atomic layer with respect to the plane of the film, which is chosen as an order parameter, exhibits anomalous behavior and increases with temperature. Expressions are derived for the magnetic susceptibility of each atomic layer. It is shown that, in the vicinity of the transition, the irregular part of the magnetic susceptibility of each atomic layer exhibits behavior characteristic of the susceptibility in the Landau theory: it is less by a factor of two in the low-symmetry phase and diverges at the transition point. The regular part of the magnetic susceptibility of each atomic layer makes an additional contribution to the asymmetry of the total susceptibility in the vicinity of the transition point; this result follows from the fact that the inhomogeneous magnetic system considered is semi-infinite.     

Effect of plastic deformation on phase transitions in a Ti50Ni47Fe3 alloy

The results of x-ray structural studies of the effect of previous plastic deformation by rolling on the occurrence of phase transitions in a TiNi(Fe) alloy are presented in the present work. The temperature dependences of the Bragg reflection intensities and half-widths and the dependence of the rhombic angle in the R phase with various degrees of deformation were obtained. Analysis of the results found gave the following rules. Plastic deformation substantially shifts the characteristic temperatures of the martensitic transitions (MT) R-B19′ and B19′-B19″, increases the temperature range of the transformation and can result in a “stepwise” transformation. It was observed that deformation weakly affects the temperature T_R for the B2-R transformation. Tomsk State Architecture and Construction Academy. V. D. Kuznetsov Siberian Physicotechical Institute at Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 11–20, July, 1996.     

The magnetization processes,spin reorientation transitions and magnetic domain structure in DyFe10CoTi single crystal

The tetragonal ThMn₁₂-type, single crystalline DyFe₁₀CoTi sample has been investigated by torque and magnetization measurements and observation of domain structure at various temperatures between 10–300 K and in magnetic field from B=0 to 0.15 T. These examinations showed that the magnetic structure of DyFe₁₀CoTi changes from “easy axis” (c-axis) type to conical at 225 K and to “easy plane” (ab plane) type at 100 K.     

Investigation of phase transitions of solid ionic conductors by automatic impedance spectroscopy

The first order phase transitions of the solid ionic conductors AgI, Ag₂HgI₄, Cu₂HgI₄, and CuTeBr at 420, 320, 343, and 348 K respectively, were investigated by automatic low frequency (100 Hz to 10 MHz) impedance spectroscopy. Measurements showed all four transitions covering a rather broad interval of temperatures, i.e. 3 to 5 K with AgI and CuTeBr, and about 8 K with Ag₂HgI₄ and Cu₂HgI₄, as well as possessing a hysteresis between 2.5 K (CuTeBr) and 4.4 K (AgI).     

Investigation of phase transitions and thermal expansion of some complex tungsten-containing oxides

The phase transitions have been investigated and the thermal expansion coefficients have been determined in compounds with a structure of brannerite, defect pyrochlore, and tungsten bronze of the general formula M ^I A ^VWO_{6 − δ} (M ^I = □, H₃O, Li, Na, K, Rb, and Cs; A ^V = V, Nb, Sb, and Ta; δ = 0, 0.5) by high-temperature X-ray diffraction and differential thermal analysis. It has been shown that brannerites and tungsten bronze do not undergo phase transitions and the compounds are characterized by a significant anisotropy of the thermal expansion, whereas pyrochlores undergo phase transitions of three types, and thermal deformations in them are determined by the size of M ^I atoms.     

Magnetocaloric effect, magnetic domain structure and spin-reorientation transitions in HoCo5 single crystals

A study on the magnetocaloric effect (MCE) has been carried out on the HoCo₅ single crystal using a direct method in the temperature range 78-300 K and in a magnetic field of 1.3 T at various orientations of the single crystal with respect to the direction of external magnetic field. Our research has shown that magnetic anisotropy strongly affects magnitude and sign of the MCE in the regions of spin-reorientation phase transitions. In addition, the magnetic domain structure of the HoCo₅ single crystal in (0 0 1) plane in the temperature range from room temperature down to 4.2 K was investigated.