Electron spin resonance in superconducting materials

The ESR of Gd in the superconducting phase of the type II superconductors CeRu₂ and LaOs₂ shows a shift for the field for resonance and inhomogeneous broadening of the lineshape. Both effects strongly depend on the three different microwave frequencies (resp. magnetic fields). The broadening of max. 800 G is attributed to a non resolved finestructure splitting. The local field distribution in vortex state of these type II superconductors is less than 100 G and is the main contribution for shift of the field for resonance. In addition ESR results of Gd and Eu doped into La are discussed forT >T _c. The comparison of depression of the superconducting transition temperature and exchange spin-flip scattering rate determined from ESR shows a perfect agreement.     

Electron spin resonance in superconducting materials

The ESR of Gd in the superconducting phase of the type II superconductors CeRu₂ and LaOs₂ shows a shift for the field for resonance and inhomogeneous broadening of the lineshape. Both effects strongly depend on the three different microwave frequencies (resp. magnetic fields). The broadening of max. 800 G is attributed to a non resolved finestructure splitting. The local field distribution in vortex state of these type II superconductors is less than 100 G and is the main contribution for shift of the field for resonance. In addition ESR results of Gd and Eu doped into La are discussed forT >T _c. The comparison of depression of the superconducting transition temperature and exchange spin-flip scattering rate determined from ESR shows a perfect agreement.     

Electron spin resonance of impurities in magnetic insulators

It is possible to observe and very useful to study the electron spin resonance of impurities in paramagnetic hosts. Among the interesting phenomena observable and measurable in these studies are: (a) symmetric and antisymmetric exchange interactions between the impurities and the hosts determined from the indirect superhyperfine interactions, pseudo-Zeeman interactions and pseudo crystal field effects, (b) the Jahn-Teller induced random strains effects resulting in the magnetic field dependence of the fine structure linewidths, and (c) the nature of the dynamical interactions of the hosts manifested in the lifetime broadenings of the impurity spectra.     

Magnetic resonance experiments in NiBr2 at high frequencies and high magnetic fields

By performing magnetic resonance experiments in NiBr₂ at high frequencies and magnetic fields, we have observed a resonance line which we relate to a helimagnetic phase, and an antiferromagnetic resonance line corresponding to an antiferromagnetic phase with strong out-of-plane anisotropy and small in-plane anisotropy.     

Electron spin resonance and Rashba field in GaN-based materials

We discuss problem of Rashba field in bulk GaN and in GaN/Al_xGa_1−xN two-dimensional electron gas, basing on results of X-band microwave resonance experiments. We point at large difference in spin-orbit coupling between bulk material and heterostructures. We observe coupled plasmon-cyclotron resonance from the two-dimensional electron gas, but no spin resonance, being consistent with large zero-field spin splitting due to the Rashba field reported in the literature. In contrast, small anisotropy of g-factor of GaN effective mass donors indicates rather weak Rashba spin-orbit coupling in bulk material, not exceed 400 G, α_BIA<4×10⁻¹³ eVcm. Furthermore, we observe new kind of electron spin resonance in GaN, which we attribute to surface electron accumulation layer. We conclude that the sizable Rashba field in GaN/Al_xGa_1−xN heterostructures originates from properties of the interface.     

Quenching of spin fluctuations by high magnetic fields

The quenching of spin fluctuations by magnetic fields has been observed in heat capacity and electrical resistivity measurements at low temperatures for a series of highly exchange enhanced magnetic materials. These include: the weak itinerant electron ferromagnets Sc₃In, Zr_1−xHf_xZn₂ (0 x 0.2) and Ni₃Al; the strong Pauli paramagnets RCo₂ (R = Sc, Y and Lu), TiBe₂ and Pd_1−xNi_x (0 x 0.01); and the heavy fermion systems CeSn₃, CeSi_x (x ≈ 1.85) an d UAl₂. The reported quenching of spin fluctuations in scandium and palladium by magnetic fields is reviewed, and it appears that the initial observations and conclusions are incorrect, and that fields greater than 10 and 40 T, respectively, will be necessary to quench spin fluctuations in these metals. The behaviors of these spin fluctuators have been grouped into six classes.     

Electron spin resonance of magnetic ions in metals an experimental review

The contribution that the technique of electron spin resonance has made to our understanding of the behaviour of localized moments in metals is reviewed. The main areas of application of the technique are discussed at a simple theoretical level and its limitations are pointed out. A comprehensive review of the experimental results obtained from electron spin resonance is presented and, in appendices to the article, a comprehensive list of experimental results is given indexed to author, impurity and host and to areas of application. In a final section of the review an attempt is made to suggest those areas in which electron spin resonance still has an important part to play in adding to our understanding of magnetism in metals.     

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.     

Electron spin resonance in polyphenylacetylene

By polymerizing phenylacetylene in the absence of initiator and in vacuo while observing electron spin resonance, it was established that the paramagnetism in the thermal polymer (PPA-T) cannot be due to interaction with oxygen but must be a property of the chemically uncontaminated macromolecule. The ESR behavior of a concentrated solution of PPA(T) in monomer and of solid (amorphous) PPA(T) displays a Curie dependence, whereas the ESR absorption of crystalline polymer made with coordination catalysts increases by one to two orders of magnitude over the temperature of about 60 to 140°C. The major portion of the XRD crystallinity disappears more sharply, at about 120°C. The paramagnetism in PPA must therefore be associated with “conformational defects” in the individual macromolecules.     

Electron spin resonance in superconductors

The internal field distribution in type II superconductors substantially alters the electron spin resonance g value and line shape. The resulting changes can be used to determine both the width and approximate shape of the internal field distribution.     

The effects of large oscillating fields on spin precession in magnetic resonance

Magnetic resonance of a spin system which is acted upon by a large near-resonance oscillating magnetic field transverse to a static field has been studied experimentally and theoretically for many years. The technique of DEMUR (Double Electron Muon Resonance) has many advantages for such studies. This paper will describe the results of an experiment to study the precession of the muonium triplet near magnetic resonance using DEMUR.     

Electron spin resonance shift in spin ladder compounds

We analyze the effects of different coupling anisotropies in a spin-1/2 ladder on the electron spin resonance (ESR) shift. Combining a perturbative expression in the anisotropies with density matrix renormalization group computation of the short range correlations at finite temperature, we provide the full temperature and magnetic field evolution of the ESR paramagnetic shift. We show that for well chosen parameters the ESR shift can be in principle used to extract quantitatively the anisotropies and, as an example, discuss the material BPCB.     

Nuclear acoustic resonance in tantalum at high magnetic fields

Nuclear acoustic resonance has been observed at much higher magnetic fields than previously reported for nonmagnetic materials. Marked differences between the Δm = 1 and the Δm = 2 line shapes in tantalum were noted.     

Electron spin resonance in pyrolyzed lignin

An investigation has been made into electron spin resonance (ESR) occurring in the ranges of carbons prepared from periodate lignin by vacuum pyrolysis. Line shape, line width, g-factors, saturation effects, and free spin concentration have been measured and the effect of oxygen on these parameters has been examined. A g-shift and lines with inhomogeneous broadening have been observed. In the presence of oxygen the lower-temperature carbons showed an unexpected increase in the detectable free spin concentration. The discussion includes factors affecting the nature of the free spins in the pyrolyzed material.     

Electron-active cyclotron resonance in p-doped InMnAs in high magnetic fields

We present a theoretical and experimental study of electron-active cyclotron resonance in p-doped InMnAs in high magnetic fields. Results are based on an 8-band Pidgeon–Brown model generalized to include finite k_z effects and s(p)–d exchange interaction between itinerant carriers and Mn d-electrons. The e-active transitions in the valence band in p-doped samples take place due to the nature of multiple valence bands (heavy and light holes). We have calculated the absorption spectra in high magnetic fields and identified optical transitions which contribute to the cyclotron resonance for both e-active and h-active polarizations. Calculations show agreement with experimental results.     

Electron spin resonance and magnetic susceptibility suggest superconductivity in Na doped WO3samples

In WO₃ doped with Na (WO₃:Na) an Electron Spin Resonance (ESR) signal with unresolved fine and/or hyperfine structure is detected and used as a probe for the state of the carriers. Using the saturation method we determined the spin-lattice relaxation rate 1/ T ₁ of these paramagnetic centers. Upon cooling below 100 K, 1/ T ₁ decreases markedly, as known to occur in NMR when a gap opens in the superconducting state. At low temperatures, 1/ T ₁ quantitatively follows BCS behavior with a gap K or 2 . The magnetic susceptibility exhibits a strong difference between magnetic-field cooled and zero-field cooled regimes below which also supports a so far unknown superconducting phase resulting from Na doping. Received 14 April 1999 and Received in final form 6 September 1999     

Electron paramagnetic resonance experiments at high frequencies and low temperatures in one dimensional TMMC

We report the existence of collective excitations of spins in TMMC at high magnetic fields and low temperatures, but higher than T_N at zero magnetic field. The main features of the resonances can be attributed to those of AFMR. Three kinds of crystallographic domains obtained from twining are established. Two magnon excitations may have also been observed.     

Electron spin resonance of proton-irradiated graphite

In the case of colossal magnetoresistance in the perovskite manganites, "double exchange" mediated by the itinerant spins is believed to play a key role in the ferromagnetism. In contrast, the conventional "Heisenberg" interaction, i.e., direct (unmediated) interaction between the localized spins produced by the proton irradiation, is identified as the origin of proton irradiation-induced ferromagnetism in graphite.