Spin Glass Materials Research Articles

Relaxation in the system

Results of ageing experiments on the alloy system are reported. All alloys display a maximum in the temperature dependence of the maximal rate of the relaxation, . The temperature of this maximum, , is ( is the freezing temperature) for the investigated spin-glass and cluster-glass materials. depends on the concentration of the dopant, and may depend on the waiting time used in the experiment.

The New Neutron Powder Diffractometer with a Multi-Detector System for High-Efficiency and High-Resolution Measurements

We describe a multipurpose neutron powder diffractometer for high-efficiency and high-resolution measurements, HERMES, which has been installed at the JRR-3M reactor in Japan Atomic Energy Research Institute. To obtain high-quality data for a short counting time, or with a small sample amount, HERMES has a multi-detector system which consists of 150 3He neutron detectors and simple window type collimation. The best resolution is Δ2θ of 18' and Δd/d of 3 ×10-3, where 2θ and d are scattering-angle and d-spacing, respectively. The duration of an ordinary powder diffraction experiment is 1–5 h under the present conditions in JRR-3M. Some typical results of powder diffraction experiments and performance are given in this paper. We also show a result of 2D intensity mapping of magnetic diffuse scattering from a single crystal of a spin glass material.

Sorting out landscapes by mesoscopic noise

Although complex landscape models have been proposed for many disordered systems, insufficient attention has been paid to experimental methods of distinguishing among different landscape types. The very broad distribution of relaxation rates often cited as being a useful diagnostic is actually far too generic. We present experimental results on two spin-glass materials showing that non-Gaussian fluctuation statistics provide sensitive discrimination between different spin-glass models, and allow surprisingly detailed determination of parameters of the state space. Related experiments on highly non-equilibrium systems, such as vortex dynamics in superconductors, will be briefly mentioned.

Magnetic Property and Evaluation of MnZn-Ferrite Fine Particles in Langmuir Blodgett Film

We investigated the magnetic properties of thin film samples with different number of layers stacked by the LB technique. The film samples showed a magnetically complex behavior in low temperature region, which is insufficiently explained by the idea of the homogenous dipole interaction field analogous to the demagnetizing field. The film thickness dependent behavior is discussed in terms of a picture of the magnetic viscosity which is realized in reentrant spin glass materials.

Structure and magnetic properties of the pyrochlore Sc 2Mn 2O 7

Sc 2Mn 2O 7 has been prepared at 60 kbar and the crystal structure has been determined from Rietveld analysis of x-ray powder diffraction data. A cubic pyrochlore structure is found described in space group Fd3m with a=9.5965(4) A ̊ . Magnetic susceptibility data conform to the Curie-Weiss law in the range 175 K to 300 K with an effective moment of 3.6(1) μ B Mn and θ p = +77(3) K indicating the predominance of ferromagnetic interactions. Nonetheless, the material does not show long range magnetic order and instead a spin glass like transition is found below 17 K. Thus, Sc 2Mn 2O 7 resembles Y 2Mn 2O 7 and Lu 2Mn 2O 7, both spin glass materials, rather than In 2Mn 2O 7 and Tl 2Mn 2O 7 which are ferromagnetic below 120 K. The compound is red-orange in color and is an insulator in contrast to the other manganate pyrochlores which are semiconductors.

Disordered magnetism and spin glasses

After a cursory introduction to the history and applications of disordered magnets and especially the spin glasses, I briefly explain what is a spin glass. Then the following topical subjects are considered: (a) geometric frustration, (b) aging dynamics and (c) quantum spin glasses and liquids. What will the future bring for this area? My answer would be mesoscopics, i.e., the change of magnetic behavior caused by nano-structuring the spin-glass material.

Nonlinear susceptibility measurements at the spin-glass transition of the pyrochlore antiferromagnet Y2Mo2O7

We have measured the magnetic field and temperature dependence of the nonlinear dc susceptibility, χnl, of the frustrated pyrochlore antiferromagnet Y2Mo2O7 close to and above the temperature, Tg∼22 K, where this material exhibits irreversible (spin-glass like) magnetic behavior. Our results suggest that the observed irreversible magnetic properties in this material are due to a thermodynamic spin-glass transition signaled by a divergence of the nonlinear magnetic susceptibility coefficient χ3∼(T/Tg−1)−γ at Tg with γ=3.3±0.5. χnl shows two power-law behavior at Tg, χnl∼H2/δ, with δ∼2.8 for H<1000 Oe and δ∼4.1 for H≳2000 Oe. These values for γ and δ, as well as the crossover behavior of χnl at Tg, is consistent what is found in common disordered Heisenberg spin glass materials.

Optical properties of pyrochlore oxides R2Mo2O7- delta (R: Sm, Gd, and Ho)

The temperature dependence of the reflectance of the spin glass materials R2Mo2O7- delta (R: Sm, Gd, and Ho) has been measured for frequencies from 40 to 40 000 cm-1 The AC conductivity of Sm2Mo2O7- delta derived from Kramers-Kronig analysis, indicates a Drude-like behaviour as the temperature is lowered. Between 150 and 40 K the scattering rate shows a sharp drop which is attributed to the scattering of conduction electrons by short-range ordered moments of the Mo ions. Below the spin glass transition temperature Tf ( approximately=40 K) the scattering rate saturates due to the freezing of the moments. Gd2Mo2O7- delta behaves like a poor metal at room temperature, but at low temperatures shows a linear increase in the conductivity for frequencies up to 400 cm1 suggesting a localized hopping conductivity. The localization remains in the spin glass state for T<Tf ( approximately=25 K). The conductivity of Ho2Mo2O7- delta is semiconductor-like with a small and slightly temperature-dependent gap around 0.25 eV. Our optical results support a qualitative band model proposed by Sleight and Bouchard for the pyrochlore oxides.

NOVEL BINARY SPIN GLASS ALLOYS SYNTHESIZED BY MECHANICAL MILLING

A few novel binary spin glass alloys have been successfully synthesized by mechanical milling of ordered intermetallic compounds in a high energy ball mill. These alloys are amorphous Co 2 Ge , atomically disordered crystalline GdAl 2, and ball-milled crystalline and amorphous CoZr. The characteristic features of spin glasses are observed; these are (a) the sharp cusps at the freezing temperature T f in both ac and dc magnetic susceptibility versus temperature curves and their peculiar sensitivity to the external magnetic field; (b) the irreversibility, i.e. the difference in value and shape between the magnetisation versus temperature curves after zero field cooling (ZFC) and field cooling (FC) at temperatures below T f and (c) the displacement of the FC magnetisation curve relative to ZFC curve and the corresponding remanence in the FC curve at low temperatures. The freezing temperatures are 41, 65, 35, and 11 K, respectively, which are defined by the sharp cusps in the lowest field ac susceptibility versus temperature curves. The freezing temperatures are lowered with increasing external field. The common feature of these materials is that all are binary alloys with a rather high concentration of the magnetic component. The discovery of these novel spin glasses is of significance because they not only represent new classes of spin glass materials but also demonstrate the use of mechanical milling as a novel technique to synthesize various new materials such as spin glasses. The results are reviewed briefly.

High-Resolution Transmission Electron Microscopy Study of Metallic Spin-Glass/Amorphous Silicon Multilayers

ABSTRACTHigh-resolution transmission electron microscopy has been used to investigate the structure and growth behavior of three separate multilayer systems composed of spin-glass alloys(AuFe.03,CuMn.15, and AgMn.09) alternating with amorphous silicon. Each of the three systems was fabricated with two different sample configurations. The first consisted of bilayers with 3 nm spinglass alloy and 7 nm amorphous siliconlayers. The second consisted of 7 nm spin-glass alloy and 7 nm amorphous silicon layers. HRTEM images of ion-milled cross-sectioned samples revealed variations in the degree of crystallinity of the spin-glass material. Variations in the amount and symmetry of interlayer formation were also observed. Systematic studies of such variations should help to explain differences in their measured spin-glass properties.

Phase Diagram of a Reentrant Ising Spin Glass Fe0.6Mn0.4TiO3on the Magnetic Field-Temperature Plane

Behavior of the reentrant spin glass (RSG) material Fe 0.60 Mn 0.40 TiO 3 in magnetic fields H <7.1 kOe is studied by neutron scattering technique, and the H - T phase diagram was constructed. For H ≥0.93 kOe, the random field (RF) induced history dependent behavior was observed between zero-field-cooled (ZFC) measurements and field-cooled (FC) measurements below the Neel temperature T N ( H ). By examining the behavior of Bragg and diffuse scattering in ZFC measurements, the RSG transition temperature T RSG is determined. One finds that it appears within a gradual crossover region from weak irreversibility to SG-induced strong irreversibility observed in the previous magnetization measurements. The initial slope of the T RSG line is perpendicular to the temperature axis.

Spin-glass behavior of mechanically milled crystalline GdAl2.

We report on the discovery of spin-glass behavior of a crystalline compound with a rather high content of the magnetic component, namely mechanically milled ${\mathrm{GdAl}}_{2}$ (mm-${\mathrm{GdAl}}_{2}$). Both ac and dc magnetic susceptibility measurements show a single transition from the paramagnetic to the spin-glass state below 65 K (${\mathit{T}}_{\mathit{f}}$), which is further confirmed by the observation of thermal irreversibility of the magnetization below ${\mathit{T}}_{\mathit{f}}$. Discovery of spin-glass behavior in mm-${\mathrm{GdAl}}_{2}$ is of significance since it not only represents a new class of spin-glass material but also demonstrates the use of mechanical milling as a novel technique to synthesize various new spin glasses.

Photoinduced magnetic behavior of the amorphous metallic spin glass material Fe3(GaTe3)2

A new ternary material Fe3(GaTe3)2 is prepared by using a rapid precipitation metathesis reaction between the Zintl material K3GaTe3 and FeCl2 in aqueous solution. The dc specific resistivity of this material is in the boundary expected for metallic conductors (ρ=4.36×10−3 Ω cm). The dc magnetic susceptibility of Fe3(GaTe3)2 is reported over 6–250 K temperature region and the material is characterized as a spin glass with a freezing temperature of about 22 K. Magnetization data are also reported as both thermal remanent magnetization and isothermal remanent magnetization as a function of magnetizing field. When cooled well below the spin glass freezing temperature, the spin glass material has been observed to exhibit photomagnetic effects consistent with a disruption of the spin glass state caused by UV radiation.

Spin Freezing in High Mn Concentrated Cd1-xMnxTe System

Static scaling analysis was made on the nonlinear susceptibility of high Mn concentrated Cd1-xMnxTe with scaling function: χnl=τβg(H/τ(γ+β)/2). With increasing of Mn content the critical exponents β and γ become larger and depart from the values reported for typical spin glass materials. This behavior of the critical exponents suggests that the increase in Mn content causes a change in the spin freezing mechanism from spin glass like one to disordered antifcrromagnetic one at x∼0.46 in Cd1-xMnxTe.

Spin-relaxation model for the Mössbauer spectra of BaSnxTi2-xFe4O11: A ferric oxide spin glass.

A spin-relaxation model was used to interpret the $^{57}\mathrm{Fe}$ M\ossbauer spectra of the ferric oxide spin glass ${\mathrm{BaSn}}_{\mathit{x}}$${\mathrm{Ti}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{4}$${\mathrm{O}}_{11}$, with x=0.0, 0.4, 0.8, 1.2, 1.6, and 2.0. This interpretation is suggested as an alternative to distributions of static hyperfine nuclear fields, which require many free parameters to account for the temperature dependence of the spectra, and do not have an obvious physical interpretation. The spin-relaxation analysis used here reduces to the number of free parameters to essentially two: a spin-freezing molecular field and an electronic spin-flip frequency. The results verify the existence of a double magnetic transition in the sample with x=0.0, and the suppression of the upper transition in the samples with x=0.4 and 0.8. The results also suggest the presence of a double magnetic transition in the samples with xg1.0 which has not been observed using other measurements, and may be related to a known difference in structure between the x=0.0 and 2.0 samples. The least-squares fits obtained rival the results of distributions of static fields while requiring fewer free parameters that have direct physical significance. It is suggested that many analyses of spin-glass materials in terms of static-hyperfine-field distributions should be reconsidered.

Muon-spin-relaxation and neutron-scattering studies of magnetism in single-crystal La1.94Sr0.06CuO4.

Combined muon-spin-relaxation (${\mathrm{\ensuremath{\mu}}}^{+}$SR) and elastic magnetic neutron-scattering experiments on a large single crystal of ${\mathrm{La}}_{1.94}$${\mathrm{Sr}}_{0.06}$${\mathrm{CuO}}_{4}$ are reported. ${\mathrm{\ensuremath{\mu}}}^{+}$SR data taken without an external field indicate that the Cu spins are fluctuating slowly or frozen below ${\mathit{T}}_{\mathit{f}}^{\mathrm{\ensuremath{\mu}}}$\ensuremath{\approxeq}6.0 K. At 3.9 K, measurements done with an applied longitudinal field show that the muon depolarization is almost completely due to static moments. Neutron measurements of the magnetic peaks Q=(1,0,0) and (0,1,1) show that no three-dimensional long-range order is present down to 1.8 K. However, the quasielastic scattering intensity from the two-dimensional ridge Q=(1,k,0) at k=-0.12 increases sharply between 40 and 20 K. Below ${\mathit{T}}_{\mathit{f}}^{\mathit{N}}$=20 K the scattering intensity is roughly constant. The difference between these two freezing temperatures is explained in terms of the different range of fluctuation frequencies to which each of these probes is sensitive. The dependence of the freezing temperature on the frequency windows of different probes is similar to that seen in traditional spin-glass materials like CuMn.

Magnetism in spin glasses

Experimental results on the relaxation of the magnetization of spin glasses at temperatures below the spin glass freezing temperature T g show very similar behaviour for different spin glass materials. Some data on relaxation are presented and discussed in the context of a droplet scaling theory for the spin glass phase elaborated by Fisher and Huse (Phys. Rev. B, in print).

The crystal structure of the spin-glass pyrochlore, Y 2Mo 2O 7

The crystal structure of the spin-glass material, Y 2Mo 2O 7, has been determined from powder neutron diffraction data using profile (Rietveld) methods. The data are consistent with the fully ordered cubic pyrochlore structure, a 0 = 10.230(1) Å with Y in 16 d, Mo in 16 c, O in 48 f( x = 0.3382(1)), and O′ in 8 b of Fd3 m. Attempts to refine models with O′ disordered over the 32 e sites or between the 8 a and 8 b sites resulted in convergence to the 8 b positions. Derived YO and MoO distances are in excellent agreement with those found in isostructural materials giving indirect evidence for YMo ordering over the cation sites.

Spin-glass-like behaviour and long-range order in dilute YTb alloys

Using neutron diffraction, we have examined the magnetic ordering of two YTb alloys whose magnetic properties are characteristic of spin-glass materials. For YTb 5 at % we have determined that a spiral state appears below the susceptibility cusp at 26.6 K, despite differences between field coolled and zero field cooled magnetization. No evidence for long-range order was found for YTb 3 at % below its cusp at 15.5 K. We conclude that irreversibility alone can not be used to characterize the spin-glass state.

Transient E.S.R. absorption in Eu0.4Sr0.6S, in the paramagnetic phase, near the spin-glass temperature : the exchange reservoir

In the insulating spin-glass material Eu 0.4 Sr 0.6 S, the transient E.S.R. absorption following a strong r.f. pulse has been studied at X band for T≥T g . The signal was found to depend strongly upon the experimental conditions (energy of the r.f. pulse, value of the driving magnetic field). At 4.2 K and with a weak perturbing r.f. energy (1 μJ), the recovery was exponential (τ∼40 ms). By coupling continuous saturation experiments with pulse experiments made at different values of the driving field, we have shown that the observed signal corresponds to a transient shift of the absorption line, reflecting a rise of the spin temperature. This signal is finally the dynamical manifestation of a behaviour already met in steady E.S.R. experiments. The experimental results are discussed within the Bloembergen-Wang exchange reservoir model; it is found that, due to the dipolar interactions, the r.f. energy absorbed by the Zeeman reservoir is rapidly (10 −10 s) transferred to the exchange reservoir. The energy is then given up to the phonons through the modulation of the exchange coupling by the phonons (direct process). The heat capacity of the spins mainly comes from the exchange energy, and overwhelms that of the phonons up to 15 K; this leads to a phonon-bottleneck: the phonons rapidly reach the spin temperature at the end of the pulse, then the temperature of the whole spin-phonon system slowly relaxes towards the bath temperature Etude experimentale en bande X a T≥T g , du signal transitoire de RPE consecutif a une impulsion intense de champ hyperfrequence: il depend de l'energie de l'impulsion et de la valeur du champ magnetique directeur et il correspond a un deplacement transitoire de la raie d'absorption associe a une elevation de la temperature de spin. Discussion des resultats experimentaux a l'aide du modele de reservoir d'echange de Bloembergen et Wang