Point contact spectroscopy of U2Zn17

We report on point contact measurements on the heavy fermion system U₂Zn₁₇. It is shown that the first derivative of the current-voltage characteristic of low resistance contacts is very sensitive to the magnetic phase transition. A point contact theory applicable to heavy fermion compounds is still missing. The results are discussed in the frame of the heating model, which includes electrical and thermal resistivity as well as thermopower effects.     

Spin fluctuations and Curie temperature in the compounds R2M17 with nonmagnetic elements

The magnetic properties of materials for permanent magnets based on binary compounds R₂M₁₇ (R=Y, Sm; M=Fe, Co), also including additions of the nonmagnetic elements N, Al, and Si, are investigated a the theory of dynamical fluctuations of the electronic spin density. It is shown that the Curie temperature is determined by the ratio of the exchange splitting energy (proportional to the magnetization at T=0) and the rms value of the fluctuations (proportional to the local spin susceptibility). The fluctuations are much larger in iron compounds than in cobalt alloys. This results not only in quantitative differences in their characteristics but also in a qualitatively different change in the properties of these materials on nitriding. Fiz. Tverd. Tela (St. Petersburg) 41, 77–83 (January 1999)     

Spin transport in antiferromagnetic insulators

Electrical spin, which is the key element of spintronics, has been regarded as a powerful substitute for the electrical charge in the next generation of information technology, in which spin plays the role of the carrier of information and/or energy in a similar way to the electrical charge in electronics. Spin-transport phenomena in different materials are central topics of spintronics. Unlike electrical charge, spin transport does not depend on electron motion, particularly spin can be transported in insulators without accompanying Joule heating. Therefore, insulators are considered to be ideal materials for spin conductors, in which magnetic insulators are the most compelling systems. Recently, we experimentally studied and theoretically discussed spin transport in various antiferromagnetic systems and identified spin susceptibility and the Néel vector as the most important factors for spin transport in antiferromagnetic systems. Herein, we summarize our experimental results, physical nature, and puzzles unknown. Further challenges and potential applications are also discussed.     

Spin fluctuations in intermetallics

A review is given of investigations of the quasiparticle spectra and of the magnetic fluctuations spectra of metals with strong longitudinal magnetic susceptibilities which were performed by means of the de Haas-van Alphen effect and inelastic neutron scattering. Among the systems investigated are the incipient ferromagnets Ni₃Ga and TiBe₂, the low temperature ferromagnets Ni₃Al, YNi₃, ZrZn₂ and MnSi, the light rare earth metals including PrCu₂ which are close to antiferromagnetic instabilities at low temperatures, and the actinide heavy fermion superconductors UPt₃ and UBe₁₃. The strong renormalisation of the quasiparticle masses and the temperature dependence of the heat capacity and of the magnetic equation of state of these systems are discussed in terms of a quantitative model based on the relaxation frequency of very low lying spin fluctuations.     

From antiferromagnetic order to a field-polarised state in the heavy-fermion compound YbAgGe

Magnetisation measurements between 75 mK and 1 K were used to study the evolution up to magnetic fields of 8 T of the magnetic properties of the heavy-fermion metal YbAgGe. The magnetisation data confirms a first-order transition between two antiferromagnetic states, which is suppressed at 2 T. Close to 5 T, which is a field region where the known antiferromagnetic long-range order is already suppressed, the static linear susceptibility is enhanced, accompanied by a strong increase of the linear specific-heat coefficient. However, the enhancement of the static linear susceptibility is not strong enough to be alone responsible for the strong increase of the quasi-linear resistivity observed previously.     

Spin waves in the block checkerboard antiferromagnetic phase

Motivated by the discovery of a new family of 122 iron-based superconductors, we present the theoretical results on the ground state phase diagram, spin wave, and dynamic structure factor obtained from the extended J₁-J₂ Heisenberg model. In the reasonable physical parameter region of K₂Fe₄Se₅, we find that the block checkerboard antiferromagnetic order phase is stable. There are two acoustic spin wave branches and six optical spin wave branches in the block checkerboard antiferromagnetic phase, which have analytic expressions at the high-symmetry points. To further compare the experimental data on neutron scattering, we investigate the saddlepoint structure of the magnetic excitation spectrum and the inelastic neutron scattering pattern based on linear spin wave theory.     

Thermodynamic fluctuations in two-dimensional degenerate antiferromagnetic structures

A model of a two-dimensional antiferromagnet with an arbitrary anisotropic interaction that allows for degeneracy of the ground state is proposed. The lifting of degeneracy by thermodynamic fluctuations and the accompanying effects are studied by a method of self-consistent calculations of Gaussian angular fluctuations that is asymptotically exact at low temperatures. Fluctuations are shown to lead to collinear ordering of the orientations of magnetic sublattices, an effect that initiates long-range orientational order in systems with anisotropic interaction but retains only short-range order in systems with isotropic short-range interaction. The temperature patterns of the orientational correlators are given for the particular cases of dipole and isotropic short-range interaction models. The nature of the Ising-like behavior of the system is discussed for the case of a strong anisotropy of the correlators, which corresponds to quasi-one-dimensional behavior. Zh. éksp. Teor. Fiz. 111, 669–680 (February 1997)     

Quantum temperature fluctuations in the magnetization of antiferromagnetic semimetals

It is shown that in semimetallic, low-temperature antiferromagnetic materials located in a quantizing magnetic field, the part of the band magnetization M _∼ which oscillates in H can have a nonmonotonic temperature dependence. This non-Fermi liquid behavior will show up experimentally in the form of quantum temperature fluctuations of the magnetization when the decrease with rising temperature is oscillatory, rather than the usual monotonic decrease. It is shown that the magnetization from an individual spin electron (or hole) subband has the form of weakly damped periodic oscillations as a function of T ². This result makes it possible to develop an efficient method for studying the electronic structure of antiferromagnetic semimetals based on an examination of the quantum temperature fluctuations. Calculations show that quantum temperature fluctuations can be observed, for example, in the cerium monopnictides CeP and CeAs, which are strongly correlated, antiferromagnetic, compensated semimetals with low Neel temperatures. Fiz. Tverd. Tela (St. Petersburg) 40, 1674–1680 (September 1998)     

Spin fluctuations in hydrogen-stabilized Laves phase UTi2H5

Uranium Laves phase UTi₂ does not exist in a pure form, but can be stabilised by the presence of hydrogen, which can be absorbed in concentration exceeding 5?H atoms/f.u. Low temperature specific heat, magnetic susceptibility, and resistivity indicate that UTi₂H₅ is a spin fluctuator close to the verge of magnetic ordering. Its susceptibility follows at high temperatures the Curie–Weiss law with U effective moment µ_eff[ ?= 3.1?µ_B/U and paramagnetic Curie temperature Θ_p = ?200 K. The temperature dependence of specific heat exhibits a pronounced and weakly field dependent upturn in C_p/T versus T below 10 K reflecting the effect of spin fluctuations. It can be described by an additional T^½ term. The Sommerfeld coefficient γ = 256?mJ/mol K² classifies the compound as a mid-weight heavy fermion. Spin fluctuations are affecting also electrical and thermal transport and thermoelectric power, which all resemble UAl₂. A lattice anomaly at ≈ 240?K, attributed to the melting of hydrogen sublattice, reflects in most of bulk properties.