Low Field Dc Magnetization Research Articles

Magnetic Properties and Phase Diagram of CoxMn1-xTiO3with Exchange Frustration

Low-field DC magnetization measurements have been made on a random mixture Co x Mn 1- x TiO 3 with competing exchange interactions and competing anisotropies, and the magnetic phase diagram in the concentration ( x )-temperature ( T ) plane has been determined from these results. Typical behavior characterizing a paramagnetic to spin glass transition is observed in an intermediate range of concentration. In the samples of the Co-rich and Mn-rich region, antiferromagnetic orderings take place. The magnetizations perpendicular to the spin axes of the respective antiferromagnetic orderings behave just as paramagnets even below the Neel temperature. At low temperatures, the Co-rich and Mn-rich samples with certain x reenter from the antiferromagnetic phase to the spin glass phase. In spite of the strong planar anisotropy of Co 2+ , the XY -like spin glass behavior has not been found, and the spin glass and reentrant transition in S // and S ⊥ occur simultaneously, reasons for which are discussed.

Microwave absorption in YBa2Cu3O x granular thin films at the dc magnetic fields

Localization of microwave absorption in YBa2Cu3O x superconducting thin films in the presence of very low dc magnetic field was studied. The granular YBa2Cu3O x thin films of submicrometer thickness had random orientation of grains, were inserted into a microwave cavity for maximum rf magnetic field. The microwave absorption in low dc magnetic field at temperatures just below the critical temperature shows properties which may be ascribed to superconducting grains. At lower temperatures the hysteretic effects and flux-trapping occur in the intergrain regions of the film.

London field penetration in heavy fermion superconductors

We present results of a series of high resolution, low fielddc-magnetization measurements on the heavy fermion superconductors UPt3, UBe13, U1−x Th X Be13 and CeCu2Si2, from which values of the magnetic penetration depth λ can be extracted. A study of the temperature variation reveals aT 2 power law in all cases. This can not be reconciled with a BCS-like isotropic energy gap but may be explained by the presence of low energy quasiparticle states inside the gap. In the case of very pure superconductors, one such possibility is the assumption of point-nodes in the gap function. We argue, however, that an interpretation in terms of resonant impurity scattering in various anisotropic superconducting states is more likely to explain a broad range of experimental data. The results on differently oriented single crystals of UBe13 and UPt3 reveal no pronounced anisotropy related to the crystal lattice. Absolute values of λ atT=0 are deduced by a novel method in which the field is first screened out from the sample by means of an evaporated superconducting film (of lowerT c ). Above this lowerT c the sudden penetration of field into the sample can then be measured absolutely.

S-wave pairing: A study of low-field dc magnetization in micrometer-sized HTSC powders

An extensive study of the low-field (μ0H ≤ 8 mT) dc initial susceptibility χin has been performed using micrometer-sized HTSC powders of varying sizes and methods of preparation. It is found, as before, that χin(T) is essentially constant for T ≲ 0.4Tc. This supports s-wave pairing and implies no zeros of the gap on the fermi surface. Additionally, χin(T) finds reasonable description in a London model for superconducting spheres and a two-fluid model λ(T).

Spin-Glass Behavior in Co-Ferrite Amorphous Films

Amorphous Co-ferrite thin films not containing vitrifying elements were prepared by ion beam sputter deposition. Their magnetic properties were investigated by means of low-field dc magnetization and ac susceptibility measurements, which gave evidence of spin glass behavior. The temperature dependence of magnetization in a 50 Oe external field exhibited an irreversibility just below room temperature, which decreased with increasing field strength. A cusp in the ac susceptibility (80 Hz, 1 Oe) corresponding to the spin freezing temperature was observed at 284 K. Below this temperature, relaxation of the thermoremanent magnetization was observed. Besides this transition at 284 K, a sharp peak in the ac susceptibility at 86 K, suggestive of another transition, was observed. It is thought that spin glass ordering with a high freezing temperature is caused mainly by transition to an amorphous structure for strong superexchange interactions.

High temperature superconducting powders

The results of low field DC magnetization measurements on a 10 μm dia HTSC powder of composition Y 1 Ba 2 Cu 3 O 7 and a 2 μm sized aligned powder of the same composition are reported. In contrast to other reports, no evidence of strong long time magnetic relaxation is found. In addition, an equation of state for the thermoremanent magnetization is proposed.

Effect of a magnetic field on the resonant multistep selective photoionization of gadolinium isotopes

A multistep photoionization with three resonant polarized photons has been used to separate the odd and even isotopes of gadolinium. Due to their hyperfine structure ( I= 3 2 ), the 155,157Gd isotopes can be photoionized via a J=2→2→1→0 scheme with three п photons whereas the even isotopes cannot. If a time delay is introduced between the three laser pulses we show that the presence of a low intensity dc magnetic field affects the selectivity. The effect of this weak field on the 156Gd photoionization rate has been calculated independently of the optical pumping and the result is in good agreement with the experiment.

Behavior of the Transverse Spin Component in the Short-Range Ising Spin-Glass Fe0.5Mn0.5TiO3

The spin-glass Fe 0.5 Mn 0.5 TiO 3 ( T SG =21.5 K) has been studied by low-field dc-magnetization, neutron scattering and Mössbauer measurements. The results obtained by the magnetization and neutron scattering measurements indicate that only the longitudinal spin component freezes below T SG . This fact shows that Fe 0.5 Mn 0.5 TiO 3 behaves as a typical Ising spin-glass in both of the measurements. However, the hyperfine fields H hf at Fe nuclei observed by the Mössbauer measurement point to directions other than the hexagonal c -axis in the spin-glass state. This indicates that the transverse spin component contributes to H hf as does the longitudinal one. Combining the results obtained by the three methods, it is concluded that the transverse spin components fluctuate rather incoherently with one another with characteristic times longer than our Mössbauer time scale of ∼10 -7 s.

Complex magnetic properties of the rare-earth copper oxides, R2CuO4, observed via measurements of the dc and ac magnetization, EPR, microwave magnetoabsorption, and specific heat.

We report the results of an extensive investigation of the magnetic properties of a large series of undoped ${R}_{2}$${\mathrm{CuO}}_{4}$ single crystals with R==Pr, Nd, Sm, Eu, and Gd (which are the host compounds for the newly discovered series of electron cuprate superconductors) and mixture versions of the form ${A}_{2\mathrm{\ensuremath{-}}x}$${B}_{x}$${\mathrm{CuO}}_{4}$, with A==Pr, Nd, Sm, Eu, or Gd, and B==Gd, Tb, or Dy. We have measured dc and ac magnetization, microwave magnetoabsorption, EPR, and specific heat. These measurements reveal two characteristic transition temperatures associated with a novel complex magnetic behavior, including weak ferromagnetism, two sharp peaks in the low-field dc magnetization, an unusual anisotropy in the EPR resonance field for R=Gd, and two additional anisotropic microwave absorption modes. The higher characteristic transition temeperature at \ensuremath{\sim}270 K is associated with antiferromagnetic ordering of the Cu moments which are strongly coupled within the ${\mathrm{CuO}}_{2}$ layers. The lower, at \ensuremath{\le}20 K, cannot be attributed to antiferromagnetic ordering of the R moments and is tentatively attributed to a spontaneous canted spin reorientation. An understanding of this magnetic behavior is important in order to ascertain its relationship to possible mechanisms of high-temperature superconductivity.

Ｃｏフェライトアモルファス膜のスピングラス挙動

Amorphous Co-ferrite thin films not containing glass formers were prepared by ion beam sputter deposition. Their magnetic properties were invetigated by means of low field dc magnetization and ac susceptibility measurements, which gave evidence of spin glass behavior. Temperature dependence of magnetization in an external field of 50 Oe exhibited an irreversibility below nearly room temperature, which decreased with increasing field. A cusp corresponding to spin freezing temperature was observed at 284 K in ac susceptibility measured at 80 Hz, 1 Oe. Below this temperature, relaxation of thermoremanent magnetization was observed. Besides this transition at 284 K, a sharp peak which suggests another transition was observed at 86 K in ac susceptibility. It is considered that the spin glass ordering with a high spin freezing temperature is mainly caused by amorphousization for strong super-exchange interactions.

Hc1 and penetration depth of Bi2Sr2CaCu2O8 crystals

Low-field dc-magnetization and ac-suseptibility measurements on Bi2Sr2CaCu2O8 crystals were performed using a SQUID sensor. We find very low values for Hc1(0), namely 2.5 and 125 Oe for fields parallel and perpendicular to the ab-plane respectively. The temperature dependence of λ can be fitted very well with BCS weak-coupling theory, giving λab (0) ∼ 300 nm.

Superconducting properties of grain boundaries in dense, fine grained YBa2Cu3Ox

YBa2Cu3Ox samples have been produced in the form of uniform spheres 50 to 100 μm in diameter. The grain size within the spheres may be varied by appropriate annealing over the range of 1–2 μm up to the sphere diameter. The superconducting properties of fine grained spheres (1.7 and 2.0 μm) have been investigated using low field DC magnetization measurements. The critical current of the grain boundary was found to be significantly larger for the 1.7 μm material than the 2 μm material.

Observation of complex magnetic behavior in the perovskite rare earth copper oxide systems, R 2CuO 4

EPR, microwave absorption, and dc magnetization measurements were made on single crystals of the form R 2CuO 4, which are the host compounds for the newly discovered series of electron cuprate superconductors. These measurements reveal two characteristic transition temperatures associated with a novel complex magnetic behavior, including weak ferromagnetism, two sharp peaks in the low field DC magnetization, an unusual anisotropy in the EPR resonance field for R = Gd, and two additional anisotropic microwave absorption modes. The higher characteristic transition temperature, at ≈270K, is attributed to AF ordering of the Cu moments, and the lower, at ≤20K, to a spontaneous canted spin reorientation. An understanding of this magnetic behavior is important in order to ascertain its relationship to possible mechanisms of high temperature superconductivity.

Direct measurement of the temperature-dependent magnetic penetration depth in Y-Ba-Cu-O crystals.

The temperature dependence of the magnetic penetration depth $\ensuremath{\lambda}$ is determined directly from low-field dc magnetization measurements on single-crystal Y-Ba-Cu-O (${T}_{c}=89.7$ K, $\ensuremath{\Delta}{T}_{c}=0.2$ K). The results are consistent with the behavior expected from a BCS ($s$ wave) superconductor.

Initial susceptibility and microwave absorption in powder samples of Y 1Ba 2Cu 3O 6.9

We report a systematic study of the temperature dependences of microwave absorption and low field dc magnetization in powder samples of the superconductor Y 1Ba 2Cu 3O 6.9. Both measurements indicate a transition temperature near 90 K. The temperature dependence of the diamagnetic susceptibility, as well as its value at T⪡ T c are close to that of a conventional BCS superconductor.

Magnetization studies of the high-Tc compound Y 1Ba2Cu3Oz

The superconductive state magnetization of the high-Tc compound Y1Ba2Cu3Oz has been investigated. Single-phase, polycrystalline samples of this archetype were prepared by solidstate reaction. The onset of the superconductive transition, measured magnetically, was 90.7 K. In addition to low-field dc magnetization results, measurements of the magnetic critical current density Jc (H,T) are presented as a function of magnetic field and temperature. The low values of Jc, especially at high temperature, are discussed in terms of intergranular supercurrents that flow within individual grains in superconductive layers of reduced dimensionality.

Ferromagnetic transition in Pd-Fe and Pd-Co alloys (abstract)

New low-field dc magnetization measurements on dilute Pd-Fe and Pd-Co samples between 0.2 and 1.0 at. % Fe(Co) are presented. It is demonstrated that defects produced during plastic deformation and surface inhomogeneities produced during strain relieve annealing (by either preferential Pd evaporation and/or surface segregation) have a dramatic effect on the low-field response of the samples. Plastic deformation increases the Curie temperature by as much as 20%. Strain relieve anneal produces a well-defined surface layer of the order of 0.02 mm thick with a Curie temperature slightly higher than that of the bulk. This surface layer couples via dipolar forces to the bulk of the sample significantly altering the low-field response of the latter. Both effects considerably ‘‘smear’’ the magnetic transition of the samples. This explains, for example, why the low field susceptibility of ferromagnetic Pd-Co alloys never reach the demagnetization limit.1 A systematic study of the two effects will be presented and new conclusions about the ferromagnetic properties of homogeneous Pd-Fe and Pd-Co will be drawn.

Magnetism in amorphous Ru xFe 80-xB 20 alloys: Magnetization and Mössbauer study

Amorphous alloys of Ru xFe 80-xB 20, 0 ≦ x ≦ 22 have been investigated using low field ac susceptibility, dc magnetization and 57Fe Mössbauer spectroscopy. The magnetic phase diagram of Ru xFe 80-xB 20 is confirmed. Mössbauer measurements reveal a low temperature transition in Ru 18Fe 62B 20 below T c at which the average Fe hyperfine field rises anomalously and magnetization measurements indicate a non-collinear magnetic structure. The effect of substitution of Ru on distribution in exchange is discussed.

The magnetic phase diagram of the disordered spinels Zn xCo 1- xFeCrO 4 ( x=0.0, 0.2, 0.4, 0.5, 0.8)

A magnetic phase diagram is proposed for the disordered spinels Zn x Co 1- x FeCrO 4 ( x=0.0, 0.2, 0.4, 0.5, 0.8) on the basis of the results of ac susceptibility, low field dc magnetization and Mössbauer studies. For small or zero dilution, the system undergoes a para- to unstable ferrimagnetic transition followed by a cluster spin glass ordering. As the magnetic dilution increases, the ferrimagnetic phase gradually disappears and a ‘liquid-like’ phase appears prior to the freezing.

Spin-glass properties of the diluted antiferromagnet Eu xSr 1- xTe

Measurements of ac-susceptibility and low-field dc-magnetization on the diluted antiferromagnet Eu x Sr 1- x Te reveal frequency-, field- and history dependencies indicating spin-glass-like behaviour for x ≲ 0.5 similar to Eu x Sr 1- x S. For x ≳ 0.5 different dc-magnetization behaviour indicates a crossover to putative antiferromagnetic ordering.