Low-frequency Ac Susceptibility Research Articles

19F nuclear spin relaxation and spin diffusion effects in the single-ion magnet LiYF4:Ho3+

Temperature and magnetic field dependences of the 19F nuclear spin-lattice relaxation in a single crystal of LiYF4 doped with holmium are described by an approach based on a detailed consideration of the magnetic dipole-dipole interactions between nuclei and impurity paramagnetic ions and nuclear spin diffusion processes. The observed non-exponential long time recovery of the nuclear magnetization after saturation at intermediate temperatures is in agreement with predictions of the spin-diffusion theory in a case of the diffusion limited relaxation. At avoided level crossings in the spectrum of electron-nuclear states of the Ho3+ ion, rates of nuclear spin-lattice relaxation increase due to quasi-resonant energy exchange between nuclei and paramagnetic ions, in contrast to the predominant role played by electronic cross-relaxation processes in the low-frequency ac-susceptibility.

Interfaces in disordered media, energy distribution and ac response

We investigate the contribution of domain wall motion to low-frequency ac susceptibility. At low frequency the response is due to the stochastic motion of pinned domain walls, which can be viewed as hopping movements of an elastic interface between different minima of the energy landscape. Using the concept of waiting time distributions, which is related to the energy distribution of an elastic interface in disordered media, we derive the power-law behaviour of the ac response observed recently in some ferroelectrics below the Curie temperature.

Superfluid response in monolayer high-Tccuprates

We have studied the doping dependence of the in-plane and out-of-plane superfluid density, \rho^s(0), of two monolayer high-Tc superconductors, HgBa_2CuO_{4+\delta} and La_{2-x}Sr_xCuO_4, using the low frequency ac-susceptibility and the muon spin relaxation techniques. For both superconductors, \rho^s(0) increases rapidly with doping in the under- and optimally doped regime and becomes nearly doping independent above a critical doping, p_c = 0.20.

Search for the chiral superconducting state in Sr2RuO4–Ru eutectic

Sr2RuO4 has become one of the most intensively studied transition metal oxides displaying unconventional superconductivity (Tc=1.5 K). It is known that its eutectic system Sr2RuO4–Ru shows interface superconductivity with an enhanced onset of ∼3 K. It is predicted that in the 3-K phase, a second transition to a chiral state occurs at a temperature between 1.5 and 3 K. The second transition is thought to be observable by ac-susceptibility measurements. We report the low-frequency ac-susceptibility of Sr2RuO4–Ru eutectic measured using a SQUID ac magnetometer.

Aging, rejuvenation and memory effects in re-entrant ferromagnets

We have studied the slow dynamics of the ferromagnetic phases of the re-entrant CdCr_{2x}In_{2-2x}S_4 system for 0.85<x<=1 by means of low frequency ac susceptibility and magnetization measurements. Experimental procedures widely used in the investigation of the out-of-equilibrium dynamics of spin glasses (such as the x=0.85 compound) have been applied to search for aging, rejuvenation and memory effects, and to test their dependence on the disorder introduced by dilution of the magnetic ions. Whereas the rejuvenation effect is found in all studied samples, the memory effect is clearly enhanced for increasing dilutions. The results support a description of aging in both ferromagnetic and re-entrant spin-glass phases in terms of hierarchical reconformations of domain walls pinned by the disorder.

Domain growth by isothermal aging in 3D Ising and Heisenberg spin glasses.

Nonequilibrium dynamics of three-dimensional model spin glasses, the Ising system Fe0.50Mn0.50TiO3 and the Heisenberg-like system Ag(11 at % Mn), has been investigated by measurements of the isothermal time decay of the low frequency ac susceptibility after a quench from the paramagnetic to the spin-glass phase. It is found that the relaxation data measured at different temperatures can be scaled according to predictions from the droplet scaling model, provided that critical fluctuations are accounted for in the analyses.

Curie Temperature and Critical Exponent ? in a Fe2/V5 Superlattice

For a superlattice of 2 monolayers (ML) Fe and 5 ML V with a repetition rate of 50 [(Fe 2 /V 5 ) 50 ] the low-frequency ac susceptibility X exp was measured. To determine the correct Curie temperature T C and the real critical exponent y it is important to use (i) a very slow temperature scan rate of 5 mK/s, (ii) very small oscillatory magnetic fields below 4 A/m (50 mG) and (iii) not to perform a power law fit for the determination of T C and y, but rather determine both quantities independently. The onset of hysteresis losses defines T C = 304.75 K. The analysis yields a critical exponent γ = 1.72(10), which is in excellent agreement with the theoretically predicted value of the two-dimensional (2D) Ising model. No crossover to 3D behavior close to T C is observed. This suggests a vanishing interlayer exchange coupling at T C .

Magnetization and Susceptibility Studies on BaZrO3-Doped YBa2Cu3O7 − x Bulk Superconductors

We studied the YBa2Cu3O7 − x bulk superconductor doped with BaZrO3 up to 50 wt.%, obtained by solid-state reaction powder technology. From DC magnetization loops and low frequency AC susceptibility measurements we determined the influence of the BaZrO3 doping level on the critical temperature, critical current density, field for full penetration, and intergrain lower critical field. The results show that even high content of BaZrO3 does not lead to degradation of the superconducting properties of bulk YBa2Cu3O7 − x.

Aging, rejuvenation, and memory effects in Ising and Heisenberg spin glasses

We have compared aging phenomena in the Fe_{0.5}Mn_{0.5}TiO_3 Ising spin glass and in the CdCr_{1.7}In_{0.3}S_4 Heisenberg-like spin glass by means of low-frequency ac susceptibility measurements. At constant temperature, aging obeys the same `$\omega t$ scaling' in both samples as in other systems. Investigating the effect of temperature variations, we find that the Ising sample exhibits rejuvenation and memory effects which are qualitatively similar to those found in other spin glasses, indicating that the existence of these phenomena does not depend on the dimensionality of the spins. However, systematic temperature cycling experiments on both samples show important quantitative differences. In the Ising sample, the contribution of aging at low temperature to aging at a slightly higher temperature is much larger than expected from thermal slowing down. This is at variance with the behaviour observed until now in other spin glasses, which show the opposite trend of a free-energy barrier growth as the temperature is decreased. We discuss these results in terms of a strongly renormalized microscopic attempt time for thermal activation, and estimate the corresponding values of the barrier exponent $\psi$ introduced in the scaling theories.

Low-frequency AC susceptibility due to the interplay of loss mechanisms in HTS

The analysis of low-frequency ac susceptibility is complicated because the hysteretic losses due to pinning and the eddy-current losses are not simply additive. We show that the electromagnetic response of both ideal and nonideal type-II superconductors can be well described with a common nonlinear response function. This function makes a consistent description of the crossover from the power-law E(J) of flux-creep to the ohmic flux-flow regime. We interpret the scaling behavior of ac susceptibility with this nonlinear equation.

Memory effects in an interacting magnetic nano-particle sample

The non-equilibrium dynamics in a Fe–C nano-particle sample, showing a low-temperature spin-glass-like phase, have been studied by low-frequency AC-susceptibility experiments. A temporary stop in the cooling during an AC-susceptibility measurement yields a decrease of χ with time. Upon the following re-heating χ traces the previous curve with a dip at the temperature where the stop occurred. This effect has recently been discovered and discussed for different spin-glass systems, but has never been detected before in a nano-particle system.

Non-equilibrium dynamics in 3d spin glasses

The random spin structure of a quenched spin glass experiences a continuous re-structuring in the low-temperature spin glass phase; the spin glass ages. Some recent experimental studies of relaxation and temperature dependence of the low-frequency AC-susceptibility and DC-magnetisation have given essential new information on which are the governing physical parameters for the spontaneous re-construction of the spin configuration. These results are briefly reviewed and put into the context of a phenomenological real space picture of the spin glass phase. This picture includes: domains of spin glass ordered regions that use spontaneous droplet excitations as the only mechanism for their growth at constant temperature, chaos with temperature and overlap on short length scales between the equilibrium states at two different temperatures.

Nonequilibrium dynamics in an interacting Fe-C nanoparticle system

Nonequilibrium dynamics in an interacting Fe-C nanoparticle sample, exhibiting a low-temperature spin-glass-like phase, has been studied by low-frequency ac susceptibility and magnetic relaxation experiments. The nonequilibrium behavior shows characteristic spin-glass features, but some qualitative differences exist. The nature of these differences is discussed.

Nonequilibrium dynamics in a three-dimensional spin glass

Nonequilibrium dynamics in a Ag(11%Mn) spin glass has been studied by low frequency ac-susceptibility and magnetic relaxation experiments. The results unequivocally show that spin structures that memorize the cooling process are imprinted in the system. These imprinted structures disclose themselves through dramatic changes of the dynamics on reheating the spin glass through the temperatures where intermittent halts or changes of the cooling rate have been imposed. We can qualitatively interpret our results in terms of a real space picture based upon spin-glass domains, droplet excitations and chaos.

Relaxation of the field-cooled magnetization of an Ising spin glass

The time and temperature dependence of the field-cooled magnetization of a three dimensional Ising spin glass, Fe_{0.5}Mn_{0.5}TiO_{3}, has been investigated. The temperature and cooling rate dependence is found to exhibit memory phenomena that can be related to the memory behavior of the low frequency ac-susceptibility. The results add some further understanding on how to model the three dimensional Ising spin glass in real space.

Transverse susceptibility of magnetic inks. milling process

Transverse susceptibility ( χ tr) tests the dispersion quality of magnetic inks by probing the small amplitude, low frequency AC susceptibility in the presence of a perpendicular DC field. The existing theoretical model of this experiment considers only individual particles and fails to predict the changes of the susceptibility observed when aggregates are broken by milling. We have extended the model to describe milling of magnetic inks as a continuous transition from an initial state of completely aggregated, immobile particles to a final state of monodispersed particles. Each intermediate state of the ink is represented as a combination of the initial and final states. The particles in the aggregates contribute to χ tr only by the oscillation of their magnetic moment with the AC field. Monodispersed particles contribute to χ tr by displacement of their magnetic moment and by their physical motion. This model describes quantitatively the changes of the susceptibility with milling. The increase of χ tr is attributed to the increased number of monodispersed particles which also effectively increases the magnetic interaction between them and accounts for the shift of the maximum susceptibility to higher DC fields.

Memory and Chaos Effects in Spin Glasses

New low frequency ac susceptibility measurements on two different spin glasses show that cooling/heating the sample at a constant rate yields an essentially reversible (but rate dependent) $\ensuremath{\chi}(T)$ curve; a downward relaxation of $\ensuremath{\chi}$ occurs during a temporary stop at constant temperature (aging). Two main features of our results are (i) when cooling is resumed after such a stop, $\ensuremath{\chi}$ goes back to the reversible curve (chaos), and (ii) upon reheating, $\ensuremath{\chi}$ perfectly traces the previous aging history (memory). We discuss implications of our results for a real space (as opposed to phase space) picture of spin glasses.

Ac susceptibility response of bulk YBCO superconductors in the presence of a temperature gradient

Low-frequency ac susceptibility measurements on ceramic YBCO superconductors carried out at the presence of a quasi-one-dimensional temperature gradient are compared with those made without the temperature gradient. The values of the different characteristic temperatures measured on samples without and with a temperature gradient are identical but in the second case itch characteristic temperature is a medium value. When the temperature gradient is constant on the sample the arithmetic medium value of the local temperatures is the effective characteristic temperature different phenomena.

The vibrating sample magnetometer: Experiences of a volunteer (invited)

On its 40th anniversary, I describe how the vibrating sample magnetometer (VSM) was developed and, later, how the very low frequency VSM, a flux-integration device, was developed. The important features of the moving sample technique, detection coil symmetry, calibration, sensitivity, and image effects are discussed briefly. Some VSM adaptations discussed include operation at 3He and dilution refrigerator temperatures, at high hydrostatic pressures, in superconducting, high-power water-cooled and hybrid magnets, for very low frequency ac susceptibility, and with SQUID detectors.

Spin glass behavior of Mn2+ ions in zeolite-Y at very low temperature

Magnetic properties of manganese ion spin systems in zeolite-Y have been measured. These ions act as almost free ions in the zeolite. Low frequency ac-susceptibility can be described with the Curie-Weiss law down to 50 mK. The susceptibility has a maximum around 30 mK showing a magnetic anomaly. Characteristic relaxation times deduced from frequency dependence of the maxima are studied. They diverge towardsTc = 0 K according to the following formula, τ = τ0exp{(b/T)ψυ+1} where τ0 = 3 x 10−10 sec, b = 0.28K, and ψν = 0.4, respectively. It is expected from ESR measurements that the dominant interaction between the manganese ions is dipole-dipole like. This divergence is interpreted as that of a dipolar spin glass.