14 Search Results

Aging in a single-crystal spin glass (Cu_0.92⁢Mn_0.08) has been measured using ac susceptibility techniques over a temperature range of 0.3–0.8𝑇_𝑔. In these studies, traditional aging experiments (or “quench” aging protocols) are compared to aging curves constructed from finite-cooling-rate curves. By comparing the growth rates of spin glass order between the two types of aging curves, it is determined that quantitative comparisons between protocols that are taken by quenching and protocols using a finite cooling rate are not possible without a deeper understanding of the interplay between aging and rejuvenation. Finally, we then demonstrate that the data presented indicate that rejuvenation,more » rather than cumulative aging, is the cause of the discrepancies between the two growth rates.« less

Abstract The London penetration depth, $\lambda \left(T\right)$ , was measured in various forms of niobium, including foils, thin films, single crystals, and samples from superconducting radio-frequency (SRF) cavities. We observed a significant difference in $\lambda \left(T\right)$ at low temperatures, $T<T_{\mathrm{c}}/3$ , due to low-energy quasiparticles. In particular, an unusual downturn of $\lambda \left(T\right)$ on cooling in the SRF cavity samples required to take into account deepmore » in-gap bound states. Theoretical modeling using the generalized Dynes density of states shows that such in-gap states lead to a downturn or a peak in $\lambda \left(T\right)$ upon cooling. Combined, experimental and theoretical findings provide a method for detecting two-level systems or states related to magnetic impurities in the bulk of niobium. This result is particularly relevant for the quantum informatics sciences technologies used in qubits and circuit quantum electrodynamics architecture based on SRF cavities.« less

Rejuvenation and memory, long considered the distinguishing features of spin glasses, have recently been proven to result from the growth of multiple length scales. This insight, enabled by simulations on the Janus II supercomputer, has opened the door to a quantitative analysis. We combine numerical simulations with comparable experiments to introduce two coefficients that quantify memory. A third coefficient has been recently presented by Freedberg et al. Here, we show that these coefficients are physically equivalent by studying their temperature and waiting-time dependence.

Here, the memory effect in a single crystal spin glass (Cu_0.92Mn_0.08) has been measured using 1 Hz ac susceptibility measurements over a reduced temperature range of 0.4 - 0.7 T_g and a model of the memory effect has been developed. A double-waiting-time protocol is carried out where the spin glass is first allowed to age at a temperature below T_g, T_{w$$_{1}$$}, followed by a second aging 4 K lower, T_{w$$_{2}$$}. The 4 K separation is sufficient to ensure rejuvenation has occurred. The model is based on calculating overlaps between the growth of the correlation lengths at the two temperatures. Itmore » accounts for the absolute magnitude of the memory effect as a function of both waiting times and temperatures. The data can be explained by the memory loss being a function of the relative change in the correlated volume at the first waiting temperature due to growth in the correlations at the second waiting temperature.« less

The effect of annealing on the superconducting properties of niobium single crystals was studied using optical, magnetic, and scanning tunneling microscopy (STM) methods. Pieces of the same crystal boule were studied before and after the annealing at 800 °C, 1400 °C, and near the melting point of niobium (2477 °C). The initial samples had a high hydrogen content and low-temperature imaging revealed large hydrides (hundreds of micrometers) appearing below 190 K. The formation of these large precipitates is already completely suppressed by annealing at 800 °C . However, the overall superconducting properties of the annealed samples did not improve and,more » in fact, worsened. In particular, the superconducting transition temperature decreased, the upper critical field increased, and the pinning strength increased. In the STM study, the sample was annealed initially at 400 °C, measured, annealed at 1700 °C, and measured again. The STM revealed a 'dirty' superconducting gap with a significant spatial variation in tunneling conductance after annealing at 400 °C. The clean gap was recovered after annealing at 1700 °C. This is likely due to oxygen redistribution near the surface, which is always covered by oxide layers in as-grown crystals. Our results indicate that vacuum annealing at least up to 1400 °C, while removing a large percentage of hydrogen, introduces additional nanosized defects, likely hydride precipitates, that act as efficient pair-breaking and pinning centers. The dimensionless scattering rate is estimated to have increased from Γ = 0.2 to about Γ = 0.4 after annealing at 1400 °C. Finally, these results on single crystals differ drastically from those obtained in polycrystalline bulk niobium (i.e. cut from superconducting radio-frequency cavities), where annealing is known to have a significant positive effect that is attributed to the improvement of the crystalline structure masking the more subtle influence of the hydrides.« less

We present a comprehensive investigation of the electronic structure, magnetization, specific heat, and crystallography of TbSi (FeB structure type) and TbSi_0.6⁢Ge_0.4 (CrB structure type) compounds. Both TbSi and TbSi_0.6⁢Ge_0.4 exhibit two antiferromagnetic (AFM) transitions at T_N⁢1≈ 58 and 57 K, and T_N⁢2≈ 36 and 44 K, respectively, along with an onset of weak metamagneticlike transition around 6 T between T_N⁢1 and T_N⁢2. High-resolution specific heat (C_P) measurements show the second- and first-order nature of the magnetic transition at T_N⁢1 and T_N⁢2, respectively, for both samples. However, in the case of TbSi, the low-temperature (LT) AFM to high-temperature (HT) AFM transitionmore » takes place via an additional AFM phase at the intermediate temperature (IT), where both LT to IT AFM and IT to HT AFM phase transitions exhibit a first-order nature. Both TbSi and TbSi_0.6⁢Ge_0.4 manifest significant magnetic entropy changes (Δ⁢S_M) of 9.6 and 11.6 J/kg-K, respectively, for Δ⁢μ₀⁢H=7 T, at T_N⁢2. The HT AFM phase of TbSi_0.6⁢Ge_0.4 is found to be more susceptible to the external magnetic field, causing a significant broadening in the peaks of Δ⁢S_M curves at higher magnetic fields. Temperature- and field-dependent specific-heat data have been utilized to construct the complex HT phase diagram of these compounds. As a result, temperature-dependent x-ray diffraction measurements demonstrate substantial magnetostriction and anisotropic thermal expansion of the unit cell in both samples.« less

Magnetic compounds possessing an intrinsic combination of near-zero magnetization with high magnetic anisotropy are highly desirable for spinronic applications and memory recording. A comprehensive study of Sm₇Pd₃ binary compound uncovered a unique combination of strong magnetoelastic behavior, very low net magnetization, and exceptionally high magnetic coercivity. The temperature-dependent X-ray synchrotron powder diffraction study indicates the abrupt changes in the compound's lattice parameters at the magnetic ordering temperature of T_C=169 K, although the crystal structure remains non-centrosymmetric hexagonal Th₇Fe₃-type down to 6 K. Density functional theory calculations confirm high intrinsic magnetocrystalline anisotropy of Sm₇Pd₃, which explains the extremely large coercivity ofmore » the polycrystalline sample, up to H_cr = 130 kOe at 2 K. In conclusion, this discovery brings to life a novel class of highly anisotropic materials that are distinctly different from known spintronic materials, making them interesting future systems for magnetic memory research.« less

Alumina-forming austenitic (AFA) alloys are relatively inexpensive high performance materials which combine the creep resistance of low-cost austenitic alloys and the oxidation resistance of expensive alumina forming alloys. However, a fundamental understanding of the role of key alloying elements such as Ni, Cr, Al, Nb, Ti, V, B and C in the experimentally observed oxidation behavior of these alloys is still lacking. The present work is a first in a series of studies aiming to quantitatively describe the role of Ni in promoting or disrupting protective Al₂O₃ scale formation on AFA alloys. Here, ternary Fe-Al-xNi model alloys with three differentmore » Ni contents were isothermally exposed in an atmosphere with a low partial pressure of oxygen between 800–1000 °C for 24 h to evaluate the role of Ni in the observed internal oxidation behavior. Increasing Ni contents had no impact on the internal oxidation behavior of the alloys. The experimental and theoretical analyses in the present work suggested a negligible effect of the internal oxide precipitates on the inward diffusion of oxygen, typically expected in these systems, while simultaneously highlighting the barriers in the development of reliable models for computation-assisted design of these alloys.« less

Here, magnetic, specific heat, and electrical transport measurements of Gd₄ScGe₄ single crystals reveal sharp, discontinuous, nearly anhysteretic first-order magnetostructural transformation at $$T_C$$ = 63 K. The electrical resistivity exhibits two distinct regions where it increases with decreasing temperature: between $$T_C$$ and 120 K, as well as below 3 K; electronic transport remains conventionally metallic at all other measured temperatures, up to 325 K. The dispersion of charge carriers due to electron-paramagnon scattering is the likely reason for the observed anomalous transport above $$T_C$$. Additionally, the existence of intermediate lattice states near the transition recognized by the spike in the interslabmore » Ge-Ge distances is expected to reduce the mean free path of the electrons contributing to the unusual behavior of the electrical resistivity between $$T_C$$ and 120 K. Beyond conventional electronic and lattice terms, the third component of likely magnetic origin contributes to the low-temperature heat capacity; the presence of spin waves may be responsible for the increased electron-magnon scattering below 3 K. Minor magnetocrystalline anisotropy is observed with the b axis as the easy magnetization axis in Gd₄ScGe₄. A negative deviation from linearity in the temperature dependence of the inverse magnetic susceptibility is detected below 150 K.« less

We study the field cooled magnetization of a CuMn spin glass (SG) under temperature perturbations. The T-cycling curves are compared with the reference curve without temperature cycling. There is a crossover from the cumulative aging region to noncumulative aging region as the temperature change is increased. The cumulative aging range scales with the chaos length, ℓ_c, becoming comparable to the correlation length, ξ, at the crossover boundary. The extracted chaos exponent, ζ=1.1, is in agreement with theoretical predictions. Furthermore, our results strongly suggest temperature chaos exists in real SG systems.