Heavy fermions in a high magnetic field

We give an overview on experimental studies performed in the last 25 years on heavy-fermion systems in a high magnetic field. The properties of field-induced magnetic transitions in heavy-fermion materials close to a quantum antiferromagnetic-to-paramagnetic instability are presented. Effects of a high magnetic field to the Fermi surface, in particular the splitting of spin-up and spin-down bands, are also considered. Finally, we review on recent advances on the study of non-centrosymmetric compounds and ferromagnetic superconductors in a high magnetic field.     

Studies on multiferroic materials in high magnetic fields

In this article, studies on the magnetoelectric effects of multiferroic materials in high magnetic fields, particularly pulsed magnetic fields, are discussed and results for some representative materials are presented. In the discussions on representative materials, the relationship between the crystallographic symmetry and the linear magnetoelectric effect in Cr₂O₃ is introduced. Then drastic changes in polarization caused by magnetic transitions are discussed through a case study of manganites with a perovskite-type structure. In addition, high field studies on the magnetoelectric effects in BiFeO₃, which is an exceptional multiferroic material, are presented and discussed in the framework of the Landau-Ginzburg theory.     

A note on the field-induced phase transitions of dilute metamagnets

The magnetization processes and the magnetic-field-induced phase transitions of dilute two-sublattice and totally disordered metamagnets with mixed ferro- and antiferro-magnetic exchange interactions are discussed. It appears, in particular, that the minimum value of the tricritical concentration, at which a change of the phase transition from first to second order occurs, is identical to the critical concentration of magnetic atoms.     

Magnetic disorder in cupric oxide

Investigations of the abnormal magnetic properties of cupric oxide reveal discrepancies between both experimental results and theoretical explanations. Through iron-doping cupric oxide by ball-milling and thermal treatments we have been able to obtain Mössbauer results that are an experimental evidence of semi-disorder. The magnetic hyperfine field of the Cu_0.995Fe_0.005O solid solution displays a spin-glass-like thermal dependence that undergoes two transitions, one at about 150 K, that can be assigned to the long-range ordering of the cupric oxide spins, and the second one at some temperature between 4.2 and 15 K, that exposes either the freezing of the Fe³⁺ spins into a local canted state or of magnetic clusters in the CuO matrix.     

Magnetic phase transitions and phase diagrams of DyMn2Ge2

The experimental studies of magnetic phase transitions in the layered tetragonal intermetallic compound DyMn₂Ge₂ are continued. The existence of spontaneous phase transitions is confirmed by the results of measurements of the temperature dependences of lattice parameters and the initial magnetic susceptibility. The measurements in strong (up to 50 T) and ultrastrong (up to 150 T) fields revealed two new field-induced magnetic transitions. The inclusion of the exchange interaction between next-to-nearest layers of manganese and the crystal field effects for the rare-earth subsystem along with the antiferromagnetic exchange interaction of nearest Mn layers has made it possible to describe the magnetic properties of DyMn₂Ge₂ in a wide range of magnetic fields. The parameters of these interactions are determined from a comparison of the experimental and theoretical magnetization curves and H-T phase diagrams.     

Quasiparticle properties of strongly correlated electron systems with itinerant metamagnetic behavior

A brief account of the zero temperature magnetic response of a system of strongly correlated electrons in strong magnetic field is given in terms of its quasiparticle properties. The scenario is based on the paramagnetic phase of the half-filled Hubbard model, and the calculations are carried out with the dynamical mean field theory (DMFT) together with the numerical renormalization group (NRG). As well known, in a certain parameter regime one finds a magnetic susceptibility which increases with the field strength. Here, we analyze this metamagnetic response based on Fermi liquid parameters, which can be calculated within the DMFT-NRG procedure. The results indicate that the metamagnetic response can be driven by field-induced effective mass enhancement. However, also the contribution due to quasiparticle interactions can play a significant role. We put our results in context with experimental studies of itinerant metamagnetic materials.     

Specific features of magnetic ordering in the SmFeGe2O7 compound

The results of the experimental investigation of the magnetic properties of the SmFeGe₂O₇ compound have been presented. It has been found that the temperature dependence of the susceptibility exhibits two features that coincide with the anomalies in the temperature dependence of the specific heat and indicate magnetic phase transitions in SmFeGe₂O₇. The external magnetic field induces a magnetic transition, the critical field of which depends on the temperature.     

Phase diagrams of liquid helium mixtures and metamagnets: Experiment and mean field theory

The phase behavior at low temperature, in particular the critical and tricritical properties, of liquid ³He⁴He mixtures and certain types of metamagnets — a class of highly anisotropic antiferromagnets, such as FeCl₂ etc. — is investigated. Since both systems exhibit two successive phase transitions, one of which is a λ-type of transition while the other is a first order transition, special attention is given to the similarity in the behavior of the two systems. In part A first the experimental and earlier theoretical work are briefly reviewed. Then the phase behavior of each system is calculated on the basis of a simple model treated in mean field approximation. The results obtained are in general qualitative agreement with experiment. For the helium mixtures an isotopic mixture of hard-spheres following Fermi and Bose statistics is used, while for the metamagnet a two sublattice spin $\text{1}\text{2}$ Ising model with nearest neighbor and next-nearest neighbor interactions is employed. A simple physical argument for the analogous behavior of the two models is given. In part B, in an attempt to obtain a deeper understanding of the similarity of the two systems, the Hamiltonians of the two models are extended to include symmetry breaking fields: a particle source field for the helium mixtures and a staggered magnetic field for the metamagnet. The phase diagrams and critical behavior of the two models in an extended thermodynamic space that includes the symmetry breaking fields are discussed in mean field theory.We find that although the two models (treated in the mean field approximation) are very similar, there are differences near T = 0°K and in some of the critical exponents.     

Magnetic-field-induced phase transitions in molecular ferrimagnets with two compensation points

Phase transitions induced by an external magnetic field in materials analogous to iron earth with two compensation points are studied for the first time. The H-T phase diagrams are plotted. The dependences of the magnetic properties of these materials on the composition are investigated. It is shown that the net magnetization in the region between the two compensation points is small for some compositions and at these compositions, the magnetic properties are similar to antiferromagnetic ones.     

Magnetic resonant and non-resonant investigations of LiLnF4 (Ln = Y, Tm) powders

The properties of dielectric powders of the Van Vleck paramagnet LiTmF₄ and its diamagnetic analogue LiYF₄ have been investigated by both resonant methods (EPR, NMR, and the mass-spectroscopy) and non-resonant ones (conductometry and magnetization measurement). On the basis of experimental data and theoretical calculations a self-consistent model for the magnetic and other properties of these powders is suggested. Two structural phase transitions induced by the magnetic field are discovered in fine LiTmF₄ powder at low temperature in a high magnetic field.     

Critical behaviours and magnetic properties of three-dimensional bond and anisotropy dilution Blume——Capel model in the presence of an applied field

This paper studies the critical behaviours and magnetic properties of three-dimensional bond and anisotropy dilution Blume--Capel model (BCM) in the presence of an applied field within the effective field theory. The trajectory of tricritical point, reentrant transitions and degenerate patterns of anisotropy are obtained both for the bond and the anisotropy dilutions. The global phase diagrams demonstrate unusually reentrant phenomena. The temperature dependences of magnetization curves undergo remarkable spin glass behaviour at low temperatures, and transform from ferromagnetism to paramagnetism at high temperature in applied fields. Temperature dependence of magnetic susceptibility curve is in qualitative agreement with experimental result.     

掺镨(Pr3+)和铥(Tm3+)氟化镓铟玻璃标准的和改进后的贾德-奥菲尔特理论研究

贾德-奥菲尔特(Judd-Ofelt,J-O)理论是半定量地研究稀土离子掺杂基质中4f能级间跃迁光学性质的经典理论,但标准的贾德-奥菲尔特理论表达式本身含有一定的简化近似和假设,应用于Pr3 等具有特殊能级的稀土离子时会产生异常结果。制备了掺Pr3 和Tm3 的氟化镓铟玻璃,研究了玻璃的吸收光谱。应用标准的和改进后的贾德-奥菲尔特理论分别拟合得到了不同的贾德-奥菲尔特强度参量和跃迁振子强度。通过分析对比可知,“超敏感跃迁”是产生异常贾德-奥菲尔特理论计算的重要因素;当加入了“超敏感跃迁”所对应奇数项约化矩阵元后,运用改进的贾德-奥菲尔特理论可计算出氟化镓铟玻璃中Pr3 较合理的贾德-奥菲尔特强度参量,并可对Tm3 的振子强度参量进行更好的拟合。     

Metamagnetic behavior and Kondo breakdown in heavy-fermion CeFePO

We report that nonmagnetic heavy-fermion (HF) iron oxypnictide CeFePO with two-dimensional XY-type anisotropy shows a metamagnetic behavior at the metamagnetic field H(M)?4 T perpendicular to the c axis and that a critical behavior is observed around H(M). Although the magnetic character is entirely different from that in other Ce-based HF metamagnets, H(M) in these metamagnets is linearly proportional to the inverse of the effective mass, or to the temperature where the susceptibility shows a peak. This finding suggests that H(M) is a magnetic field breaking the local Kondo singlet, and the critical behavior around H(M) is driven by the Kondo breakdown accompanied by the Fermi-surface instability.     

Magnetic properties of an easy-plane trigonal NdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> antiferromagnet

The field and temperature dependences of magnetization and the temperature dependences of the initial magnetic susceptibility have been theoretically studied for three crystallographic directions in a trigonal NdFe₃(BO₃)₄ antiferromagnetic crystal. The calculations were performed using a molecular field approximation and a crystal field model for the rare-earth subsystem. The obtained theoretical expressions are applied to the interpretation of recent experimental data [1–4] on the magnetic properties of NdFe₃(BO₃)₄. The results of calculations show a good agreement with experiment. The proposed theory adequately describes (i) anomalies of the Schottky type in the temperature dependence of the magnetic susceptibility, (ii) nonlinear curves of magnetization in the basal plane in a magnetic field up to 1 T (showing evidence of the first-order phase transitions) and their evolution with the temperature, and (iii) the field and temperature dependences of magnetization in a magnetic field up to 9 T.     

Correlated Electron Materials and Field Effect Transistors for Logic: A Review

Correlated electron systems are among the centerpieces of modern condensed matter sciences, where many interesting physical phenomena, such as metal-insulator transition and high-T _c superconductivity appear. Recent efforts have been focused on electrostatic doping of such materials to probe the underlying physics without introducing disorder as well as to build field-effect transistors that may complement conventional semiconductor metal-oxide-semiconductor field effect transistor (MOSFET) technology. This review focuses on metal-insulator transition mechanisms in correlated electron materials and three-terminal field effect devices utilizing such correlated oxides as the channel layer. We first describe how electron-disorder interaction, electron-phonon interaction, and/or electron correlation in solids could modify the electronic properties of materials and lead to metal-insulator transitions. Then we analyze experimental efforts toward utilizing these transitions in field effect transistors and their underlying principles. It is pointed out that correlated electron systems show promise among these various materials displaying phase transitions for logic technologies. Furthermore, novel phenomena emerging from electronic correlation could enable new functionalities in field effect devices. We then briefly review unconventional electrostatic gating techniques, such as ionic liquid gating and ferroelectric gating, which enables ultra large carrier accumulation density in the correlated materials which could in turn lead to phase transitions. The review concludes with a brief discussion on the prospects and suggestions for future research directions in correlated oxide electronics for information processing.     

Anomalies in Ising metamagnets

Ising metamagnets in a field with weak ferromagnetic intralayer interactions and highly coordinated antiferromagnetic interlayer couplings are studied using mean field theory and Monte Carlo simulations. In the antiferromagnetic phase anomalies in the magnetization and the specific heat are observed, reflecting the competing ordering tendencies of the external field and the interlayer couplings. Results are compared to recent experimental findings on FeBr₂.     

Effects of the randomly distributed magnetic field on the phase diagrams of Ising nanowire I: Discrete distributions

The effect of the random magnetic field distribution on the phase diagrams and ground state magnetizations of Ising nanowire is investigated using effective field theory with correlations. Trimodal distribution has been chosen as a random magnetic field distribution. The variation of the phase diagrams with that distribution parameters has been obtained and some interesting results have been found such as reentrant behavior and first order transitions. Also for the trimodal distribution, ground state magnetizations for different distribution parameters have been determined which can be regarded as separate partially ordered phases of the system.     

Giant magnetostriction materials

One of the significant technical developments in magnetism of the early 1970's was the discovery of a new class of rare earth intermetallic compounds, the RFe₂ Laves phases, which were found to exhibit room temperature magnetostrictive strains approaching 2 × 10⁻³, an order of magnitude larger than any previously known. Since that time both the fundamental and technical properties of these materials have been of intense interest, and they remain the subject of active research even today. The large strains available are useful in such applications as production of high amplitude, low frequency sound waves in water, certain types of strain gages, vibration compensation and compensation for temperature induced strains in large laser mirrors. Because the performance of these materials depends critically on such fundamental properties as the magnetic anisotropy, magnetization and grain orientation of the material, there has been a very strong interplay between fundamental studies and applications. In this article we briefly review the fundamental magnetic and magnetostrictive properties of the RFe₂ Laves phases, focusing especially on the complex behavior of the anisotropy and the success of crystal field theory in explaining it. We also present neutron measurements of magnetic excitation spectra and explain how they provide an understanding of the remarkable success of mean field theory for these systems.