Showing papers in "Journal of Low Temperature Physics in 1981"

Nonequilibrium theory of dirty, current-carrying superconductors: phase-slip oscillators in narrow filaments near T c

Abstract: General equations for the dynamic behavior of dirty superconductors in the Ginzburg-Landau regime T c-T ≪ T care derived from microscopic theory. In the immediate vicinity of T ca local equilibrium approximation leads to a simple generalized time-dependent Ginzburg-Landau equation. The oscillatory phase-slip solutions presented previously are discussed in greater detail.

Specific heat and dispersion curve for helium II

Abstract: The dispersion curve for elementary excitations in He II at low temperatures and at the vapor pressure is evaluated using inelastic neutron scattering data. It is found that this dispersion curve is consistent with recent specific heat measurements of high resolution. We represent the dispersion curve by means of cubic splines, and emphasize the need for further neutron scattering studies at low momentum transfer.

On Pinning of Superconducting Flux Lines by Grain Boundaries

Abstract: A simple model is developed which describes the reduction of the mean free path of conduction electrons in metals near a grain boundary. This leads to a decrease of the self-energy of flux lines in a layer which is considerably thicker than the perturbed zone of the boundary itself. The model yields pinning forces which agree, within an order of magnitude, with recent measurements on niobium bicrystals, and with observed values of grain boundary pinning in Nb/sub 3/Sn.

Pair susceptibility and mode propagation in superconductors: A microscopic approach

Abstract: We derive the full microscopic set of equations governing small oscillations: (1) in the magnitude of the superconducting order parameter (the Schmid mode), (2) the phase of the order parameter in a neutral superfluid (the Anderson-Bogoliubov mode), and (3) the coupled oscillations in the phase of the order parameter and in the electric field (the transverse, or Carlson-Goldman mode). The derivation is not limited by the restrictions of previous papers. No limitations are required for the magnitude of the frequency, the concentration of impurities, or the magnitude of the temperature. Special attention is given to the Carlson-Goldman (CG) mode, whose dispersion law frequency (Ω) vs. wave vector (k) and damping is calculated. The velocity of the CG mode in the propagation region % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqyYdCNaai% 4laiaacYhacaqGRbGaaeiFaaaa!3B5A!\[\omega /|{\text{k|}}\] is found to equal % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaae4yaiaab2% dacaqGBbGaaeOmaiabfs5aejaabseadaWgaaWcbaGaeq4Xdmgabeaa% kiaacIcacaaIYaGaeqiWdaNaaeivaiabes8a0jaabMcacaqGDbaaaa!448D!\[{\text{c = [2}}\Delta {\text{D}}_\chi (2\pi {\text{T}}\tau {\text{)]}}\], where D is the diffusion constant and χ is the function appearing in the theory of superconducting alloys. In the dirty (l « ξ0) and clean (l ≫ ξ0) limits, this expression reduces to those previously derived by Schmid and Schon, and by Artemenko and Volkov, respectively. At large values of k, the frequency of the CG mode approaches a limiting value of 2δ. The damping is small in this limit and tends to zero as ¦k¦ increases. p ]Our results are obtained by calculating the linear response of a superconductor to a perturbation in the magnitude and phase of the order parameter, and the electromagnetic potentials. The response of the superconductor to these perturbations is calculated by properly continuing the thermodynamic perturbation function of linear response from imaginary frequencies to the real ones, then inserting into the self-consistency BCS equation and Poisson's equation. The derivation is based on the self-consistent BCS scheme. No kinetic equations are introduced at any stage of the calculation.

Thermodynamics of strong coupling superconductors including the effect of anisotropy

Abstract: The thermodynamics of several elemental superconductors is computed from isotropic Eliashberg theory formulated on the imaginary frequency axis. A symmary of the available experimental literature is presented and a comparison with theory is given. The small disagreements that are found are all in the direction expected from anisotropy effects. We calculate the effect of a small amount of model anisotropy on the critical temperature, critical field, and high-temperature specific heat from an exact solution of the anisotropic Eliashberg equations. These are the first such results below the critical temperature; unlike previous analytical work, we include retardation, anisotropy in the mass enhancement, and the effect of the Coulomb repulsion in enhancing anisotropy, all of which are significant. We derive a new formula independent of any model anisotropy for the rate of decrease with impurity lifetime of the critical temperature. Finally we demonstrate how the commonly used formulas of Markowitz and Kadanoff and of Clem may give entirely misleading estimates of the gap anisotropy when used to interpret certain experiments.

Properties of the distorted flux-line lattice near a planar surface

Abstract: The magnetic field, current density, and energy of an arbitrary array of curved or straight flux lines in a type II superconductor with a planar surface are calculated from the London theory. The general expressions and their expansion with respect to displacements from the equilibrium flux-line positions are given. The elastic energy of the distorted flux-line lattice near a planar surface is presented and discussed. The equilibrium configuration becomes unstable to the growth of helical perturbations if a current exceeding a critical value is applied parallel to the external magnetic field.

Noise characteristics of an ideal shunted Josephson junction

Abstract: Detailed computer simulations have been made of the effect of intrinsic noise current on the I-V characteristic and voltage noise spectral density S v (f) of an ideal Josephson element shunted by a capacitor and resistor. The minimum value of the parameter Β c at which hysteresis appears in the quasistatic I-V characteristic is greater than the noise-free value of ≈1. Moreover, as Β c is increased, a region of large differential resistance R d appears in the I-V characteristic that is associated with a large increase in S V(0). In this regime the noise current is seen to cause random switching between the superconducting and nonzero voltage states. For all bias conditions, S V(f) > R d 2SI(f) where S I(f) is the spectral density of the intrinsic current noise. A comparison is made between these results and previous calculations. The implications for low-noise superconducting devices are discussed.

Longitudinal critical current in type II superconductors. I. Helical vortex instability in the bulk

Abstract: The vortex lattice in type II superconductors is unstable against the growth of helical perturbations if the current along the vortices exceeds a critical value. The longitudinal critical current, the pitch, and the spatially varying amplitude of the elliptically polarized helices are calculated from the London theory at the onset of instability in planar current distributions far from the surface. For weak pinning (αLλ2 ≪ c 66) the wavelength and width of the mode extend over the entire specimen, and the critical current is 2H(c 66/c 11)1/4. For moderate pinning (c 66 αLλ2 ≪ c 11) the wavelength and width are close to Campbell's pinning length (c 11/αL)1/2, and the critical current times its mean density is 2H 2(αL/c 11)1/2. For strong pinning (αLλ2 ≪ c 11) helical instability occurs at pin-free vortex sections, the helix wavelength is 2.2d, and the critical current density is 0.47Hd/λ2 (H, d, c 11 and c 66), and αL are the magnetic field, spacing, elastic moduli, and pinning parameter of the vortex lattice, and λ is the magnetic penetration depth).

A thermal activation model for noise in the dc SQUID

Abstract: A thermal activation model is described for the dc SQUID. The equations of motion for the junction phase differences are shown to develop in time like the coordinates of a particle performing Brownian motion in a viscous medium in a two-dimensional potential field. Expressions are derived relating the average voltage, transfer function, and current and voltage noise spectral densities to the features of the potential determined by the device parameters. Comparison with a numerical simulation is presented. Calculations of the current noise as a function of loop inductance and critical current asymmetry are performed. An anomalously large current noise is predicted at certain values of the device characteristics. The correlation spectral density is also calculated as a function of loop inductance, and related to the optimal source resistance for a tuned SQUID amplifier. A theory of low-frequency noise sources in the SQUID is developed in a fashion compatible with the thermal activation model. Equilibrium temperature fluctuations as a possible source of 1/f noise in the SQUID are discussed. A scheme for optimizing the resolution at low frequencies is presented. Proper exploitation of low-capacitance Pb-PbOx-Pb junction technology is shown to increase the resolution at 1 Hz by at least a factor of 8, provided that the temperature fluctuations are the dominant source of noise.

On Some Effects in a System of Two Interacting Josephson Junctions

Abstract: We consider two long, weakly interacting Josephson junctions in an external magnetic field. A uniformly distributed direct current applied to the system excites the movement of a vortex chain within each junction. The weak interaction between the vortex chains results in monochromatical radiation from the system and some peculiarities in theI–V curve.

Point-Contact Spectroscopy of Electron-Phonon Interaction in Normal-Metal Single Crystals

Abstract: Recent studies of the phonon spectra of metals using “point-contact spectroscopy” (PCS) are reviewed, with particular emphasis on the possibility of studying the electron-phonon interaction in single crystals. Various regimes of current flow in contacts are discussed, including ballistic, diffusion, and thermal regimes. The former (the Knudsen limit) proves to be the most appropriate for spectroscopic purposes; the PC spectrum (the derivative d 2 V/dI 2)in this case reveals a complicated structure containing information about singular points of the phonon density of states as well as the anisotropy (and energy dependence) of matrix elements of the electron-phonon interaction. Experimental results are presented for Cu, the most suitable metal for obtaining contacts of sufficiently good quality. The origins of the PC spectrum background at high energies and the singularities of PC spectra at low energy are analyzed. They are attributed to the nonequilibrium phonons reabsorbed in the junction, and to the influence of electron-impurity scattering upon the effectiveness of electron-phonon collisions.

Viscosity measurements in superfluid 3 He-B from 2 to 29 bar

Abstract: Measurements from a high-Q torsional oscillator and a vibrating wire are presented which give a consistent view of the viscosity of the normal fraction of 3He-B at high pressure. At low pressures discrepancies are largely eliminated by taking into account a mean-free-path size effect. We compare our corrected viscosities with recent calculations. The superfluid fraction, also measured in this experiment, is reexamined in the light of recent specific heat measurements.

Specific heat of normal and superfluid 3 He

Abstract: Extensive measurements of the heat capacity of liquid 3 He in the normal and superfluid phases are reported. The experiments range from 0.8 to 10 mK and cover pressures from 0 to 32.5 bar in zero magnetic field. The phase diagram of 3 He, based on the platinum NMR temperature scale, is presented. In the normal liquid at low pressures and near the superfluid transitionT c an excess specific heat is found. The effective massm* of3He is at all pressures about 30% smaller than the values reported earlier. The calculated Fermi liquid parameters F0 and F1 are reduced asm*/m, while the spin alignment factor (1 + Z0/4)−1 is enhanced from 3.1–3.8 to 4.3–5.3, depending on pressure. The specific heat discontinuity ΔC/C atT c is forP = 0 close to the BCS value 1.43, whereas at 32.5 bar ΔC/C is 1.90±0.03 in the B phase and 2.04±0.03 in the A phase, revealing distinctly the pressure dependence of strong coupling effects. The temperature dependence of the specific heat in the B phase agrees with a model calculation of Serene and Rainer. The latent heatL at the AB transition is 1.14±0.02 µJ/mole forP = 32.5 bar and decreases quickly as the polycritical point is approached; at 23.0 bar,L = 0.03 ± 0.02 µJ/mole.

Low-temperature adsorption of gases on heterogeneous solid surfaces: Surfaces with random topography

Abstract: Equations for low-temperature adsorption isotherms, isosteric heats of adsorption, and heat capacities of the adsorbed phase are developed for monolayer localized adsorption of monoatomic gases on heterogeneous solid surfaces. The model assumes that adsorption sites having various adsorption energies are distributed on the adsorbent surface completely at random. The interactions between adsorbed molecules are treated in the quasichemical approximation. The equations are illustrated by numerous model calculations.

Ejection of positive cluster ions from large electron-bombarded 3 He or 4 He clusters

Abstract: Electron bombardment of freely flying helium clusters of some 105-107 atoms is observed to yield positive cluster ions of about 684He atoms or 853He atoms, respectively, aside from ionized clusters of about the original size. The ejection of the small cluster ions from the surface of the large clusters is ascribed to the release of energy resulting from the electrostrictive polarization of the vicinity of a newly formed ion. The observed final sizes of the small cluster ions are found to agree with those following from a macroscopic charged droplet model.

Landé factors of collective mode multiplets in 3 He-B and coupling strengths to sound waves

Abstract: In a previous paper we predicted a linear splitting of collective modes in3He-B with magnetic field, in particular, a fivefold splitting of the (8/5)1/2 Δ mode. A fivefold splitting of about the same size has been recently observed by Avenel, Varoquaux, and Ebisawa for the new sound attenuation peak. In this paper all 18 collective modes are classified in terms of total angular momentumJ=0, 1, 2, andJ z by means of a unitary transformation to the nine eigenstates¦J, J z〉. The Lande factors for the field splitting of the real and imaginary mode triplets and quintuplets [including the (12/5)1/2Δ mode quintuplet] are calculated for all temperatures. We also determine the sound dispersion relation. The angular dependence of the coupling constants of theJ=2 quintuplets is given by vY 2 2 J z (θ)v, in agreement with the results of Avenel, Varoquaux, and Ebisawa for the new mode. We find a coupling constant of the (8/5)1/2 Δ mode to sound which is an order of magnitude smaller than that given by Koch and Wolfle. By comparing this with the experimental strengths we conclude that the particle-hole asymmetry parameter is about four times larger than that derived from the free-particle density of states.

The microelectronic structure of platinum particles investigated by NMR

Abstract: The properties of195Pt nuclear magnetic resonance of small particles have been studied over a range of particle sizes from 33 to 200 A using pulsed NMR techniques. This work was initiated to study size-dependent phenomena and to elucidate their physical origin. An anomalous linewidth, more than an order of magnitude larger than that of bulk platinum for the smallest size sample, was discovered. This was found to be inhomogeneous broadening and to have a size dependence (d)−1, whered is the mean particle diameter of the sample. Within the temperature range of 1.7–77 K, no temperature dependence was observed. As a consequence, the broadening was attributed to an intraparticle Knight shift distribution resulting from electron spin density oscillations associated with the metal surface. Spin-echo envelopes from the small platinum particles were found to decay nonexponentially, indicating the presence of a nuclear spin diffusion process in the magnetic field gradients associated with the Knight shift distribution. The diffusion process was measured by the Hahn (90°–180°) pulse technique and the Carr-Purcell-Meibocm-Gill techniques and analyzed using a spin diffusion constant of platinum computed with the moment method of Redfield and Yu. The computed spin diffusion constant wasD z=4×10−12 cm2/sec and from this an rms magnetic field gradient was determined. Upon analyzing the size-dependent spin-echo results, the field gradient was found to be characteristic of a surface region of thickness 1.5 ± 0.5 lattice constants independent of the size of the particles. In contrast to these anomalous properties of small platinum particles, the peak position of the resonance lines and the spin-spin and spin-lattice relaxation times were found to be identical to the values for bulk material. A simple model ascribing free electron behavior tos conduction electrons was applied to study quantitatively the effects of electron spin density oscillation on NMR properties. Applying an infinite-barrier boundary condition to the electrons, we computed numerically the electron charge and spin density distributions for platinum particles of diameter up to 300 A. The computations revealed Friedel oscillations of these densities near the particle surface. From the computed electron spin density of thes electrons, the spatial distribution of the contact Knight shift was obtained, which could account for all the observed NMR properties of small platinum particles—the resonance position, line shape, linewidth, relaxation times, rms field gradient, and the thickness of the surface region.

Tunneling spectroscopy on the superconducting Chevrel-phase compounds Cu 1.8 Mo 6 S 8 and PbMo 6 S 8

Abstract: The Chevrel-phase compounds PbMo6S8 and Cu1.8Mo6S8 have been investigated by tunneling spectroscopy. These superconductors show a very strong electron-phonon interaction expressed by 4≤2δ0/kT c≤5. Structures in the tunneling density of states have been observed at most energies where the phonon spectra show maxima. The coupling of high-energy modes of the Mo6S8 units and of modes associated with displacements of the Pb (Cu) atoms are discussed.

Cubic spline fits to thermodynamic and transport parameters of liquid 4 He above the λ transition

Abstract: A survey has been made of the more recent experimental measurements of the viscosity, density, thermal expansion coefficient, thermal conductivity, and specific heat of liquid 4He from the λ region up to 4.2 K. Cubic spline fits to these data are provided using a mean squares approach. The fits are used to plot the temperature dependence of the kinematic viscosity, the thermal diffusivity, and the Prandtl number.

Longitudinal critical current in type II superconductors. II. Helical vortex instability near the surface

Abstract: The calculation of the critical current for helical instability of the flux-line lattice, performed in a previous paper for a bulk superconductor, is extended to include the effect of a planar surface. The extension is of interest since an applied longitudinal current in general flows near the surface before the helical instability of the flux-line lattice triggers the transition to the flux-flow state. The magnetic stray field of the surface increases the critical current by a factor 1.41 (1.34) for weak (moderate) pinning, and it modifies the axis ratio of the helices. The pitch length and the extension of the helical mode into the specimen are decreased from their bulk values by surface pinning, but they are slightly increased if bulk pinning dominates.

Distorted steps in the I-V characteristic of Josephson junctions

Abstract: Observation of distorted steps (“wiggle-like” structure) in the I-V characteristic of Pb/In-oxide-Pb and other Josephson junctions subjected to microwave radiation is reported. Such structure is also found with an electronic analog based on the R(C)SJ model if the Stewart-McCumber parameter β c % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqOSdi2aaS% baaSqaaiaadogaaeqaaOGafyOpa4JbaGaacaaIWaGaaiOlaiaaiwda% aaa!3BED!\[\beta _c \tilde > 0.5\]0.5 and simultaneously the Hamilton-Johnson condition for substeps, Ω rCR∼-1, is fulfilled. The frequency spectrum of the analog's Josephson oscillation exhibits new lines and considerable noise when wiggles appear.

Observation of the coexistence of superconductivity and long-range magnetic order in TmRh4B4

Abstract: The ac electrical resistance, heat capacity, static magnetic susceptibility, thermal conductivity, and linear thermal expansion coefficient have been measured for the superconducting ternary compound TmRh4B4. The results indicate that the Tm3+ magnetic moments order at about 0.4 K, while bulk superconductivity, which occurs at 9.8 K, persists to temperatures below 60 mK, which was the low-temperature limit of the apparatus. Various possibilities for the type of magnetic order are examined.

Time-Dependent Voltage Generated by Flux Motion Across a Superconducting Strip

Abstract: A brief review is given of the theory predicting that the instantaneous voltage generated by single-quantum vortices or multiquanta flux tubes moving through a superconducting specimen depends upon both the specimen shape and the spatial configuration of the measuring circuit. This theory is then applied to obtain the voltage pulses expected from two different measuring circuits attached to a superconducting strip. The time-dependent voltage is predicted to be proportional not only to the vortex or flux-tube velocity but also to a geometry-dependent function of the position of the vortex or flux tube relative to the measuring circuit.

Transverse acoustic impedance of normal liquid3He

Abstract: Both the real and imaginary parts of the transverse acoustic impedance of normal liquid3He have been measured at excitation frequencies of 10 and 30 MHz, fluid pressures from 0.7 to 27 bar, and temperatures from 3 mK to 1 K. The impedance is obtained from the changes in resonance frequency and Q of a quartz crystal, which is electrically driven to oscillate in a thickness shear mode while immersed in liquid3He. These results are compared with the predictions of Fermi liquid theory, which takes into account two contributions to the impedance: (1) incoherent single-quasiparticle excitations, and (2) the excitation of the collective transverse sound mode. At 0.7 bar, our measurements of the impedance are in agreement with the predictions of Fermi liquid theory and imply that the symmetric Fermi liquid parameterF 2=1.25±0.4 ifF 1=6.3. At higher pressures, we also observe agreement in the region ωτ<0.3, where ω is the excitation frequency and τ is the quasiparticle scattering time. However, above 8 bar in the zero-sound regime (ωτ≳1), the impedance is observed to be frequency dependent, at constant ωτ. This frequency dependence cannot be explained within the present framework of Fermi liquid theory.

Investigations of the molar heat capacity at low temperatures in the TiH x system

Abstract: Results from heat capacity measurements in the temperature range 1.8–10 K on TiH x samples with different x values are communicated. From the γ coefficients of the electronic heat capacity the dependence of the density of states N(E F) on the x value for the cubic and the tetragonal TiH x phases is discussed, and is compared with similar results from susceptibility measurements. The estimated value for the electron-phonon interaction parameter λ excludes superconductivity for TiH x . An additional lattice contribution to the heat capacity of TiH x samples with x < 1.7 was observed and seems to be related to a low-frequency anomaly in the phonon spectra.

Quasiclassical Green's function in the BCS pairing theory

Abstract: We study a generalization of the quasiclassical Green's function which allows us to include the transfer of momentum to the particles. In this approach, we may handle Galilei transformations, rotations, and gauge transformations in a systematic way. As an example, we calculate the quasiparticle flow pattern which arises during the motion of the orbital vector in the ABM phase, and discuss the meaning of the intrinsic angular momentum of the Cooper pairs. Finally, we consider charged particles in a magnetic field, and derive a Boltzmann equation for a superconductor which applies to the Hall effect in the case of moving vortices.

Specific heat of 5% and 3% 3 He- 4 He solutions under pressure

Abstract: Specific heat measurements have been made under pressure on a 5% 3He-4He solution between 10 and 93 mK, yielding values for the specific heat mass, m *, of 2.45 m 3, 2.74 m 3, and 3.07 m 3 at pressures of 0, 10, and 20 atm, respectively; corresponding values of the zero-concentration effective mass m 0 are 2.38 m 3, 2.64 m 3, and 2.90 m 3 for the above pressures. Measurements were also made on a 3% solution. Above 35 mK, all the measurements show deviations from a free-Fermi-gas behavior.

Model calculations of flux-tube nucleation in thin-film type I superconductors

Abstract: In the current-induced resistive state, multiquantum flux tubes are nucleated at the edge of thin-film strips of type I superconductors. The nucleation process can be theoretically described by an extended Gibbs free-energy barrier model. Within this model the size of the nucleated flux tubes is obtained by minimizing the total energy of the flux tube. The dependence of the flux-tube size on the nucleation rate and the transport current is calculated including the influence of the preceding flux tubes and the positive domain wall energy of type I superconductors. The interaction with the preceding flux tubes generates a steplike decrease of the flux-tube size with increasing nucleation frequency, whereas the wall energy establishes a minimum value of the flux-tube size. These calculations are in good agreement with recent experiments on the nucleation process of multiquantum flux tubes.

Vortices in 3He-A in a weak magnetic field

Abstract: The structure and the energy of vortices in rotating /sup 3/He-A are considered in the presence of a weak magnetic field. It is shown, using the logarithmic approximation for the free energy, that there will be a sequence of textural transitions when the magnetic field is increased. The transition from a nonsingular texture to a singular texture is studied in detail. It is shown that the l vector is almost uniform outside the cores of vortices. To verify these results, measurements by NMR and by ultrasound are suggested.

Low-temperature magnetic behavior of HoRh4B4

Abstract: Detailed measurements of the magnetization of HoRh4B4 and GdRh4B4 near the magnetic transition temperature are presented. In contrast to the ferromagnetic behavior of GdRh4B4, no spontaneous magnetization is found to develop below T min HoRh4B4. This raises doubts about the nature of the magnetic order in that material as well as in DyRh4B4 and TbRh4B4. Anisotropy and time dependence of the magnetization are found in these last three compounds. The behavior of the very small remanent magnetization and of the paramagnetic susceptibility of HoRh4B4 is shown. These results point to a complex magnetic order, possibly helical or sinusoidal antiferromagnetism with a long wavelength.