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Cross polarization (CP) is a widely used solid-state nuclear magnetic resonance (NMR) technique for enhancing the polarization of dilute S spins from much larger polarization of abundant I spins such as 1H. To achieve such a polarization transfer, the I spin should either be spin-locked or be converted to the dipolar ordered state through adiabatic demagnetization in the rotating frame. In this work, we analyze the spin dynamics of the Hartmann-Hahn CP (HHCP) utilizing the 1H spin-locking, and the dipolar-order CP (DOCP) having the 1H adiabatic demagnetization. We further propose an adiabatic demagnetization CP (ADCP) where a constant radio-frequency pulse is applied on the S spin while 1H is adiabatically demagnetized. Our analyses indicate that ADCP utilizes the adiabatic passage to effectively achieve the polarization transfer from the 1H to S spins. In addition, the dipolar ordered state generated during the 1H demagnetization process could also be converted into the observable S polarization through DOCP, further enhancing the polarized signals. It is shown by both static and magic-angle-spinning (MAS) NMR experiments that ADCP has dramatically broadened the CP matching condition over the other CP schemes. Various samples have been used to demonstrate the polarization transfer efficiency of this newly proposed ADCP scheme.

We have investigated the magnetic and magnetocaloric properties of antiferromagnetic Ising spin chain CoV2O6 by magnetization and heat capacity measurements. Both monoclinic α-CoV2O6 and triclinic γ-CoV2O6 exhibit field-induced metamagnetic transitions from antiferromagnetic to ferromagnetic state via an intermediate ferrimagnetic state with 1/3 magnetization plateau. Due to the field-induced metamagnetic transitions, these systems show large conventional as well as inverse magnetocaloric effects. In α-CoV2O6, we observe field-induced complex magnetic phases and multiple magnetization plateaus below 6K when the field is applied along c axis. Several critical temperatures and fields have been identified from the temperature and field dependence of magnetization, magnetic entropy change, and heat capacity to construct the H-T phase diagram. As compared to α-CoV2O6, γ-CoV2O6 displays a relatively simple magnetic phase diagram. Due to the large magnetic entropy change and adiabatic temperature change at low or moderate applied magnetic field, γ-CoV2O6 may be considered as a magnetic refrigerant in the low-temperature region below 20 K. [ABSTRACT FROM AUTHOR]

The influence of Fe doping for Mn in CoMn1-xFexGe alloys on their structural and magnetic phase transitions was investigated. CoMn1-xFexGe alloys display a first-order magneto-structural transition from a high temperature paramagnetic hexagonal Ni2In-type crystal structure to a low temperature ferromagnetic orthorhombic TiNiSi-type structure. An entropy change of ΔSM=-32.8 J/kgK was measured and adiabatic temperature change of -8.0K was computed for the x=0.085 alloy under a magnetic field of 7 T in the vicinity of the phase transition at 317 K. With increasing Fe content, transition temperatures and magnetic entropy changes decrease. In addition to investigating the effect of replacing Mn with Fe, the accessibility of the giant measured entropy change was quantified by means of the minimum magnetic field required to start and to complete the magnetostructural phase transition. [ABSTRACT FROM AUTHOR]

We have investigated the magnetic and magnetocaloric properties of antiferromagnetic Ising spin chain CoV2O6 by magnetization and heat capacity measurements. Both monoclinic α-CoV2O6 and triclinic γ-CoV2O6 exhibit field-induced metamagnetic transitions from antiferromagnetic to ferromagnetic state via an intermediate ferrimagnetic state with 1/3 magnetization plateau. Due to the field-induced metamagnetic transitions, these systems show large conventional as well as inverse magnetocaloric effects. In α-CoV2O6, we observe field-induced complex magnetic phases and multiple magnetization plateaus below 6K when the field is applied along c axis. Several critical temperatures and fields have been identified from the temperature and field dependence of magnetization, magnetic entropy change, and heat capacity to construct the H-T phase diagram. As compared to α-CoV2O6, γ-CoV2O6 displays a relatively simple magnetic phase diagram. Due to the large magnetic entropy change and adiabatic temperature change at low or moderate applied magnetic field, γ-CoV2O6 may be considered as a magnetic refrigerant in the low-temperature region below 20 K. [ABSTRACT FROM AUTHOR]

Structural and magnetic properties of Fe/Pt core-shell nanostructure prepared by a sequential reduction process are reported. Transmission electron microscopy shows nearly spherical particles fitting a lognormal size distribution with Do=3.0 nm and distribution width λD=0.31. In x-ray diffraction, Bragg lines only from the Pt shell are clearly identified with line-widths yielding crystallite size=3.1 nm. Measurements of magnetization M vs. T (2K-350K) in magnetic fields up to 90 kOe show a blocking temperature TB=13K below which hysteresis loops are observed with coercivity HC increasing with decreasing T reaching HC=750Oe at 2 K. Temperature dependence of the ac susceptibilities at frequencies fm=10 Hz-5 kHz is measured to determine the change in TB with fm using the Vogel-Fulcher law. This analysis shows the presence of significant interparticle interaction, the Neel-Brown relaxation frequency fo=5.3×1010Hz and anisotropy constant Ka=3.6×106 ergs/ cm3. A fit of the M vs. H data up to H=90 kOe for T>TB to the modified Langevin function taking particle size distribution into account yields magnetic moment per particle consistent with the proposed core-shell structure; Fe core of 2.2 nm diameter and Pt shell of 0.4 nm thickness. [ABSTRACT FROM AUTHOR]

• A two-stage adiabatic demagnetization refrigerator is built with GGG as the first stage and CPA as the second stage. • A 3D printed PEEK suspension system is used at the second stage. • A no-load lowest temperature of 66.1 mK is achieved at the second stage when starting from 1 K@4 T. • Simulations estimate each PEEK suspension has a 1.10 μW thermal leak between 4 K and 0.3 K and a further reduction with a 1 K heat sink in the middle. • Experiments validate the effectiveness of 1 K heat sink. The salt pill suspensions play a critical role in determining the performance of adiabatic demagnetization refrigerators (ADR). Kevlar suspensions are commonly used to hold the salt pill in place and to minimize the thermal leak. However, traditional solutions such as tensioning the Kevlar and fixing it on mechanical frame with the help of epoxy or running it around a pulley are quite challenging. This paper presents an innovative suspension using 3D printing technology and a polyether ether ketone (PEEK) multi-ring suspension has been designed and tested. Numerical analysis shows that a single piece of the suspension may contribute about 1.10 μW thermal leak between 4 K and 0.3 K. To test its performance, a two-stage ADR is built with Gadolinium Gallium Garnet (GGG) as the first stage and chromium potassium alum (CPA) salt pill as the second stage which is supported by the PEEK suspension. The GGG stage is mainly used to provide an initial temperature for CPA stage. Typically, starting from 1 K@4 T, a no-load lowest temperature of 66.1 mK is achieved at a demagnetization rate of 0.005 T/s in the experiments. The thermal leak of the PEEK suspension with and without 1 K heat sink has been analyzed and compared, with the former one showing a big reduction of the thermal leak. This paper confirms the feasibility of PEEK as an ADR suspension material and provides a potential 3D printing method for complex suspension designs. [ABSTRACT FROM AUTHOR]

The influence of interchangeable substitution of similar heavy rare-earth-elements (HRE), i.e., Gd-Ho, Gd-Er, and Ho-Er, on the magnetic and magnetocaloric properties of HRE55Al27.5Co17.5 metallic glasses was evaluated. The magnetic transition temperature (TC) can be tuned in a wide temperature range from 8K to 93K by adjusting the substitutional concentration in the resulting metallic glasses. A roughly linear correlation between peak value of magnetic entropy change (∣ΔSM pk∣) and TC -2/3 was obtained in the three systems. This kind of substitutional adjustment provides a useful method for designing desirable candidates in metallic glasses with high magnetic entropy change, large magnetic cooling efficiency, and tunable TC for magnetic refrigerant in nitrogen and hydrogen liquefaction temperature ranges. © 2014 AIP Publishing LLC. [ABSTRACT FROM AUTHOR]

Magnetic and magnetocaloric properties of a spin-1 Heisenberg diamond cluster with two different coupling constants are investigated with the help of an exact diagonalization based on the Kambe's method, which employs a local conservation of composite spins formed by spin-1 entities located in opposite corners of a diamond spin cluster. It is shown that the spin-1 Heisenberg diamond cluster exhibits several intriguing quantum ground states, which are manifested in low-temperature magnetization curves as intermediate plateaus at 1/4, 1/2, and 3/4 of the saturation magnetization. In addition, the spin-1 Heisenberg diamond cluster may also exhibit an enhanced magnetocaloric effect, which may be relevant for a low-temperature refrigeration achieved through the adiabatic demagnetization. It is evidenced that the spin-1 Heisenberg diamond cluster with the antiferromagnetic coupling constants J¹/kB = 41.4 K and J2/kB = 9.2 K satisfactorily reproduces a low-temperature magnetization curve recorded for the tetranuclear nickel complex [Ni4(μ-CO3)2(aetpy)8](ClO4)4 (aetpy = 2-aminoethyl-pyridine) including a size and position of intermediate plateaus detected at 1/2 and 3/4 of the saturation magnetization. A microscopic nature of fractional magnetization plateaus observed experimentally is clarified and interpreted in terms of valence-bond crystal with either a single or double valence bond. It is suggested that this frustrated magnetic molecule can provide a prospective cryogenic coolant with the maximal isothermal entropy change -ΔSM=10.6 J·K-1·kg-1 in a temperature range below 2.3 K. [ABSTRACT FROM AUTHOR]

When the magnetocaloric effect is obtained using indirect or direct techniques, the result is the isothermal entropy change ΔSiso(T,ΔH) or the adiabatic temperature change ΔTad(T,ΔH). Evaluation of the linking relation dTad=-(T/Cp)×dSiso (Cp, specific heat; T, temperature) as a way to determine the magnetocaloric effect, however, is typically not performed because it requires detailed knowledge of Cp(T,H). Here, we outline the procedure for numerical integration of the magnetocaloric effect-this is important for a physics understanding and as a practical guide on implementing the procedure. The process is notably different than the well-known method used to calculate ΔSiso from the Maxwell equation. We test the procedure by using it to calculate Cp (with dTad and dSiso as input data) and comparing it with directly measured Cp, finding excellent agreement for both first-order transition in LaFe11.6Si1.4 and second-order transition in Gd. We establish the merits and limitations of the method. In particular, the presence of a demagnetizing effect in experimental data can reduce accuracy of the method. The procedure can be applied to accurately model the magnetocaloric effect in a magnetic cooling cycle. [ABSTRACT FROM AUTHOR]

Magnetocaloric effect in Mn4FeGe3-xSix compounds has been studied by dc magnetization measurements. For the parent compound Mn4FeGe3, the paramagnetic to ferromagnetic transition temperature TC is above room temperature (320 K), which initially remains constant for small Si substitution at the Ge site and then decreases marginally with an increase in Si concentration. A large change in magnetic entropy at the TC, under a magnetic field variation of 50 kOe, with typical values of 5.9, 6.5, 5.9, and 4.4 J kg-1 K-1for x = 0, 0.2, 0.6, and 1 samples, respectively, along with a broad operating temperature range and a negligible hysteresis make Mn4FeGe3-xSix series a promising candidate for magnetic refrigerant material around room temperature. Mn4FeGe3-xSix series is found to undergo a second-order magnetic phase transition. The field dependence of the magnetic entropy change has been brought out and implemented to deduce the critical exponents. The critical behavior study shows that the magnetic interactions for x = 0 and 0.2 samples have two different behaviors below and above TC. Below TC, it follows the mean field theory with long-range magnetic interaction and above TC it follows the Heisenberg three-dimensional model with short-range or local magnetic interaction. The magnetic exchange interactions for the x = 0.6 and 1 samples follow the mean-field theory. [ABSTRACT FROM AUTHOR]

We performed micromagnetic simulations of the current-induced domain wall motion in permalloy nanowires with rectangular cross-section. In the absence of the nonadiabatic spin-transfer term, a threshold current, Jc is required to drive the domain wall moving continuously. We find that Jc is proportional to the maximum cross product of the demagnetization field and magnetization orientation of the domain wall and the domain wall width. With varying both the wire thickness and width, a minimum threshold current in the order of 106 A/cm2 is obtained when the thickness is equivalent to the wire width. With the nonadiabatic spin-transfer term, the calculated domain wall velocity ν equals to the adiabatic spin transfer velocity u when the current is far above the Walker limit Jw. Below Jw, ν=β/αu, where β is the nonadiabatic parameter and α is the damping factor. For different β, we find the Walker limit can be scaled as Jw=α/β-αJc. Our simulations agree well with the one dimensional analytical calculation, suggesting the findings are the general behaviors of the systems in this particular geometry. [ABSTRACT FROM AUTHOR]

Generating dipolar order under magic-angle spinning (MAS) conditions is explored for different pulse sequences and dipolar-coupling networks. It is shown that under MAS second-order dipolar order can be generated reliably with 10% to 30% efficiency using the Jeener-Broekaert sequence in systems where the second-order average Hamiltonian is a (near) constant of the motion. When using adiabatic demagnetization and remagnetization, second-order dipolar order can be generated and reverted back to Zeeman order with up to 60% efficiency. This requires a maximum field strength with a nutation frequency that is less than one-quarter of the rotor frequency, and that the spin system can be properly spinlocked under such conditions. A simple coherent description accounts for the principal features of the spin dynamics, even using the smallest possible system of three coupled spins. For the systems investigated, the lifetime of second-order dipolar order under MAS was found to be on the order of T1. [ABSTRACT FROM AUTHOR]

We study with finite temperature Monte Carlo simulation under periodic boundary conditions the demagnetization behavior of exchanged-coupled hard/soft magnets composed of hard cube inclusions (Nd2 Fe14B, or SmCo5) in a soft (FeCo) matrix. The easy axis of the hard and soft phases are parallel to the applied field. We study the effect of finite temperature and the soft phase content on the coercivity, the remanence, and the energy product of the composites. We find that because of the difference in magnetization between the two phases, to lower the dipolar energy the magnetization of the two phases is not completely parallel to each other. This results in a much smaller increase of the remnant magnetization with increasing the soft magnet content than anticipated, especially at higher temperatures. This, together with the lower coercivity of Nd2 Fe14B, greatly affects the energy product in Nd2 Fe14B/FeCo in comparison with SmCo5/FeCo. We underline the essential difference in the induction dependence on the applied field in these compounds. [ABSTRACT FROM AUTHOR]

The effects of a demagnetizing field and an external static magnetic field on the effective Snoek’s product of textured Z-type hexagonal ferrite, which is the product of effective susceptibility and resonance frequency, were examined in order to apply the Z-type hexagonal ferrite to high-frequency devices. As a result, the demagnetizing field due to the thin plate geometry increased the resonance frequency and the effective Snoek’s product when a high-frequency magnetic field was applied parallel to the c-plane of the Z-type hexagonal ferrite. Moreover, the resonance frequency, effective susceptibility, and effective Snoek’s product of 7.7 GHz, 10.7, and 82.2 GHz, respectively, were obtained when an external magnetic field that was sufficient to saturate the Z-type hexagonal ferrite slab was applied parallel to the c-plane and perpendicular to the high-frequency magnetic field. Furthermore, the Landau–Lifshitz equation suggested that a large Snoek’s product of over 50 GHz could be obtained with an external static field as small as 31 kA/m, which is enough to saturate the Z-type hexagonal ferrite. [ABSTRACT FROM AUTHOR]

The evolution of the elastic moduli (Young modulus-E; adiabatic bulk modulus-BS and shear modulus-G and Poisson’s ratio-ν) in the temperature range T∈[300;1000] K or T/Tm∈[0.2;0.65] of isotropic polycrystalline beryllium samples was investigated using an ultrasonic method. The key point is to perform the relevant and consistent analysis of the complete set of elastic data taking account of the specific low values of ν (at ambient and at high temperature). The evolution of the elastic moduli is shown to be governed by the density evolution through the thermal expansion coefficient. Then, the elastic moduli versus temperature are accurately discussed and are at variance with the only known experimental data from 1972 [W. D. Rowland, J. Phys. F: Met. Phys. 2, 231 (1972)]. The analysis is completed by a comparison with computations performed by first-principle calculations [G. Robert et al. (unpublished)]. [ABSTRACT FROM AUTHOR]

The effect of demagnetization on the magnetic properties of a rectangular ferromagnetic prism under nonuniform conditions is investigated. A numerical model for solving the spatially varying internal magnetic field is developed, validated, and applied to relevant cases. The demagnetizing field is solved by an analytical calculation and the coupling between applied field, the demagnetization tensor field, and spatially varying temperature is solved through iteration. We show that the demagnetizing field is of great importance in many cases and that it is necessary to take into account the nonuniformity of the internal field, especially for nonconstant temperature distributions and composite magnetic materials. [ABSTRACT FROM AUTHOR]

Transport properties including electrical resistivity, thermopower, and thermal conductivity of polycrystalline Mo2.5Ru0.5Sb7-xTex compounds for 0⩽x⩽1 have been investigated in the 2-1000 K temperature range. Additional information on the concentrations and the scattering mechanisms of the charge carriers as well as on the magnetic properties has been obtained through Hall effect and magnetic susceptibility measurements performed in the 5-300 K temperature range. The enhancement in the Te content results in a decrease in the carrier concentration which is at the origin of the simultaneous increase in the electrical resistivity and thermopower. A single parabolic band model with acoustic phonon scattering enables to explain the compositional and temperature dependence of the thermopower while such a simple model fails to adequately describe the electronic thermal conductivity. This characteristic together with the unusual dependence of the thermal conductivity upon alloying might be a direct consequence of strong phonons-dimers interactions displayed by Mo3Sb7. Magnetic susceptibility data demonstrate that the antiferromagnetically-coupled dimers tend to disappear as x is enhanced, lending further support to a crucial role played by these interactions to understand the thermal transport in these materials. Low thermal conductivity coupled with high thermopower values result in a high dimensionless figure of merit ZT~0.70 at 1000 K in Mo2.5Ru0.5Sb6.5Te0.5 positioning this material as an interesting candidate for power generation applications. [ABSTRACT FROM AUTHOR]

We report on an approach to tailor the magnetic exchange in a conventional granular CoCrPt:SiO2 recording medium by irradiation with Co+ ions. Irradiation at low fluences enhances the intergranular exchange resulting in a narrowing of the switching field distribution (SFD). The modification of magnetic exchange coupling was evidenced by measuring the angular dependence of the switching field and is supported by an increase in the magnetic domain size. Further, micromagnetic simulations of a granular magnetic medium confirm the correlation between intergranular exchange and SFD. At high fluences, however, the irradiation damages lead to the degradation of the magnetic layer as magnetic anisotropy and saturation magnetization decrease. Ion irradiation simulations suggest that this is caused by strong intermixing at the grain and layer interfaces. [ABSTRACT FROM AUTHOR]

Transport properties including electrical resistivity, thermopower, and thermal conductivity of polycrystalline Mo2.5Ru0.5Sb7-xTex compounds for 0⩽x⩽1 have been investigated in the 2-1000 K temperature range. Additional information on the concentrations and the scattering mechanisms of the charge carriers as well as on the magnetic properties has been obtained through Hall effect and magnetic susceptibility measurements performed in the 5-300 K temperature range. The enhancement in the Te content results in a decrease in the carrier concentration which is at the origin of the simultaneous increase in the electrical resistivity and thermopower. A single parabolic band model with acoustic phonon scattering enables to explain the compositional and temperature dependence of the thermopower while such a simple model fails to adequately describe the electronic thermal conductivity. This characteristic together with the unusual dependence of the thermal conductivity upon alloying might be a direct consequence of strong phonons-dimers interactions displayed by Mo3Sb7. Magnetic susceptibility data demonstrate that the antiferromagnetically-coupled dimers tend to disappear as x is enhanced, lending further support to a crucial role played by these interactions to understand the thermal transport in these materials. Low thermal conductivity coupled with high thermopower values result in a high dimensionless figure of merit ZT~0.70 at 1000 K in Mo2.5Ru0.5Sb6.5Te0.5 positioning this material as an interesting candidate for power generation applications. [ABSTRACT FROM AUTHOR]

Nucleation and evolution of domain walls as well as the magnetic reversal process in perpendicularly oriented hard/soft trilayers have been investigated within a micromagnetic approach, concentrated on the situation with very thin hard layer. The formula for the nucleation field has been derived, which falls as the soft layer thickness rises. Negative nucleation field occurs at thick soft layer when the shape anisotropy surpasses the crystalline one, where the magnetization in the center of the soft layer obeys the coherent rotation model. The microscopic and macroscopic hysteresis loops have been calculated numerically, with the angular distribution of the magnetization between the nucleation and pinning fields obtained. The area of the hysteresis loop decreases while the coercivity mechanism changes from nucleation to pinning as the soft layer thickness increases. The effect of the hard layer thickness is contrary to that of the soft layer, however, the former affects the demagnetization process and hysteresis loops significantly only when the layers are thin enough. [ABSTRACT FROM AUTHOR]

The field dependence of the adiabatic temperature change ΔTad of second order phase transition materials is studied, both theoretically and experimentally. Using scaling laws, it is demonstrated that, at the Curie temperature, the field dependence of ΔTad is characterized by H1/Δ. Therefore, as the magnetic entropy change ΔSM follows a H(1-α)/Δ power law, these two dependencies coincide only in the case of a mean field model. A phenomenological construction of a universal curve for ΔTad is presented, and its theoretical justification is also given. This universal curve can be used to predict the response of materials in different conditions not available in the laboratory (extrapolations in field or temperature), for enhancing the resolution of the data and as a simple screening procedure for the characterization of materials. [ABSTRACT FROM AUTHOR]

The modified transverse Ising model taking into account the four-spin exchange interaction and quantum fluctuation, as well as the mechanical constraint of the substrate, is constructed and applied to investigate the electrocaloric effect (ECE) in BaTiO3 thin films. It is found that the temperature dependence of ECE strongly depends on both the four-spin exchange interaction and quantum fluctuation. Most importantly, we achieve the adiabatic temperature change ΔT=11.78 K at Tm=490 K, which can be comparable with that observed experimentally in PbZr0.95Ti0.05O3 thin films and ferroelectric polymers. Furthermore, the internal stresses resulting from the clamping effect of the substrate play a crucial role in the ECE of ferroelectric films. Control of the misfit stress by appropriate choice of substrate provides an effective means to improve the adiabatic temperature change for use in cooling or thermodielectric power conversion devices. [ABSTRACT FROM AUTHOR]

Complete active space self-consistent field (CASSCF) calculations and subsequent second-order perturbative treatment (CASPT2) have been carried out on a series of metallic complexes based on a Fe(II)N6 core. These compounds play a determinant role in the elaboration of potential bistable architectures such as spin-crossover materials. The adiabatic energies between the high-spin (HS) (S=2) and low-spin (LS) (S=0) states are evaluated with respect to the value of the shift ionization potential–electronic affinity (IPEA shift) recently introduced in the zeroth-order Hamiltonian [Ghigo et al., Chem. Phys. Lett. 396, 142 (2004)]. Based upon a series of experimental data, it is concluded that the commonly applied IPEA shift value (0.25 a.u.) is not satisfactory to properly discriminate the open-shell HS and closed-shell LS states. We suggest that a 0.50–0.70 a.u. value would be preferable for these specific adiabatic gap calculations. [ABSTRACT FROM AUTHOR]

The effect of pressure on the magnetic and the magnetocaloric (MC) properties around the martensitic transformation temperature in NiCoMnSb Heusler alloy has been studied. The martensitic transition temperature has significantly shifted to higher temperatures with pressure, whereas the trend is opposite with the application of applied magnetic field. The maximum magnetic entropy change around the martensitic transition temperature for Ni45Co5Mn38Sb12 is 41.4 J/kg K at the ambient pressure, whereas it is 33 J/kg K at 8.5 kbar. We find that by adjusting the Co concentration and applying suitable pressure, NiCoMnSb system can be tuned to achieve giant MC effect spread over a large temperature span around the room temperature, thereby making it a potential magnetic refrigerant material for applications. [ABSTRACT FROM AUTHOR]

In this work the magnetocaloric effect in ferrimagnetic rare-earth iron garnets R3Fe5O12 where R=Y and Dy was theoretically investigated considering a model Hamiltonian, which takes into account three coupled magnetic sublattices in the mean field approximation. In Dy3Fe5O12 the inverse magnetocaloric effect was predicted and associated with the compensation temperature. Our theoretical results for the adiabatic temperature change in both compounds are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

The magnetic and magnetocaloric properties of LaFe11.9-yCoySi1.1 compounds with y=0.8 and 0.9 have been investigated. The temperature dependence of magnetization data revealed that LaFe11.1Co0.8Si1.1 and LaFe11Co0.9Si1.1 exhibit a second-order magnetic transition near room temperature at 282 and 294 K, respectively. Under a magnetic field change of 5 T, the maximum values of isothermal entropy change -ΔS are found to be 15 and 13.5 J/kg K for y=0.8 and 0.9, respectively. The refrigerant capacity has been calculated to be 320 J/kg for y=0.8 and 266 J/kg for y=0.9 under a field change of 5 T. Direct measurements of the temperature change for LaFe11.9-yCoySi1.1 compounds around room temperature were also performed. Measurements of ΔT were performed in adiabatic conditions with practice-oriented method by using a specially developed test bench. For both materials, the normalized temperature change was found to be about 1 K/T at their Curie temperatures. [ABSTRACT FROM AUTHOR]

A single-phased PrCo2B2 compound was prepared by an arc-melting method. The magnetic properties and magnetocaloric effect (MCE) were studied by the magnetization and heat capacity measurement. The compound exhibits a single paramagnetic to ferromagnetic transition at the Curie temperature TC of 18 K. A large reversible MCE was observed which is related to a second order magnetic transition from paramagnetic to ferromagnetic state. The values of maximum magnetic entropy change (-ΔSMmax) reach 4.9 and 9.6 J Kg-1 K-1 for the field change of 2 and 7 T with no obvious hysteresis loss in the vicinity of the Curie temperature, respectively. The corresponding maximum adiabatic temperature changes (ΔTadmax) are evaluated to be 4.3 and 9.8 K. The magnetic transition and the origin of large MCE in PrCo2B2 were discussed. [ABSTRACT FROM AUTHOR]

Magnetocaloric effect associated with the first-order martensitic transition (TM) is investigated in polycrystalline Ni50-xMn35+xIn15 alloys with varying Mn content but fixing In content. It is found that when Mn content reaches x=3, a field-induced metamagnetic transition takes place. An external magnetic field can reduce TM to a lower temperature at a rate of 3.5 K/T, thereby yielding a large magnetic entropy change ΔS near room temperature. The ΔS magnitude attains to 33 J/kg K under a magnetic field of 5 T at 285 K for the sample with x=3, while the temperature span of the ΔS peak can reach 15 K because of the reduction in TM by the external magnetic field. The calculated refrigerant capacity reaches 279 J/kg for the sample with x=3. These values of the magnetocaloric parameters suggest that these alloys are suitable candidates for magnetic refrigerants. [ABSTRACT FROM AUTHOR]

In this paper, we construct a phase-space surface hopping algorithm for use in systems that exhibit strong nonadiabatic coupling. The algorithm is derived from a representation of the electronic basis which is a function of the nuclear phase-space coordinates rather than the nuclear position coordinates. This phase-space adiabatic basis can be understood in the context of Berry’s superadiabatic basis formalism as the first-order superadiabatic correction to the conventional position-space adiabatic basis. This superadiabatic representation leads to nuclear dynamics described not by Newton’s equations of motion but by generalized Hamilton’s equations of motion. The phase-space surface hopping algorithm captures physical effects that cannot be described by traditional algorithms. For a simple model problem, we show that phase-space surface hopping is more accurate than position-space surface hopping, especially when the nonadiabatic coupling is strong. [ABSTRACT FROM AUTHOR]

Magnetic properties and magnetocaloric effect of the ferrimagnetic compounds Gd4Co3 and (Gd0.9Dy0.1)4Co3 have been studied. In Gd4Co3, two successive magnetic-entropy changes occur due to a spin-reorientation transition at TSR=163 K and a ferrimagnetic-magnetic ordering transition at TC=220 K. For a field change from 0 to 5 T, -ΔSMmax at TC is 5.74 J/kg K, the full width at half maximum (δTFWHM) of the -ΔSM versus T curve has a high value of 131 K, and the refrigerant capacity is as high as 575 J/kg. TC can easily be tuned by Dy substitution for Gd and the magnetic-entropy change is increased simultaneously. -ΔSMmax of (Gd0.9Dy0.1)4Co3 at TC=145 K for the same field change has increased to 8.47 J/kg K. Our studies suggest that the successive magnetic transitions could enlarge the temperature interval, which makes this type of material meaningful for magnetic refrigeration in wider temperature regions than conventional materials. [ABSTRACT FROM AUTHOR]

Magnetic entropy change and refrigerant capacity of Gd-based amorphous Gd71Fe3Al26 and Gd65Fe20Al15 alloys are investigated. The refrigerant capacities reach 750 and 726 J kg-1 for Gd71Fe3Al26 and Gd65Fe20Al15, respectively, which are much larger than those of all magnetocaloric materials ever reported. The peak values of magnetic entropy change under a field change of 0–5 T are 7.4 J kg-1 K-1 at 117.5 K and 5.8 J kg-1 K-1 at 182.5 K for Gd71Fe3Al26 and Gd65Fe20Al15, respectively. A very large refrigerant capacity and a considerable magnetic entropy change jointly make them attractive candidates for magnetic refrigerant. [ABSTRACT FROM AUTHOR]

The heat capacity of the Mn3Sn2 compound was measured between 2 and 300 K in magnetic fields up to 7 T. The isothermal magnetic entropy change (ΔSM) and the adiabatic temperature change (ΔTad) were calculated from these data. In agreement with previous magnetic measurements, three transitions are detected in the zero-field heat capacity versus temperature curve, two λ-like peaks close to TC1∼262 K and TC2∼227 K due to ferromagneticlike second-order transitions and a third anomaly at Tt∼197 K ascribed to the development of a weak antiferromagnetic interaction. The Debye temperature is estimated to be ∼303 K from the low-temperature data. The thermal dependences of ΔSM and ΔTad present a two-peak response with peak maxima near TC1 and TC2. For a field variation of 5 T, the values of ΔTad are found to be ∼2.1 and ∼1.6 K, respectively. The results are discussed and compared to previously published data concerning other magnetic refrigerants working near room temperature. [ABSTRACT FROM AUTHOR]

A modified Landau–Devonshire thermodynamic theory is employed to investigate the effect of grain size on the electrocaloric effect of dense BaTiO3 nanoceramics. Theoretical analysis indicates that both the magnitude of the electrocaloric effect and the temperature corresponding to the maximum of electrocaloric effect depend on not only the grain size but also the electric field. Most importantly, BaTiO3 nanoceramics have the giant electrocaloric coefficient and adiabatic temperature change. Moreover, it is worth noting that the decrease of grain size reduces the temperature to a great extent where the electrocaloric effect is maximized. Therefore, control of the grain size may be used to adjust the electrocaloric properties of dense BaTiO3 nanoceramics. [ABSTRACT FROM AUTHOR]

The magnetocaloric properties of powder samples of the monoclinic YFe2H4.2, YFe2D4.2, and Y0.5Tb0.5Fe2D4.2 compounds have been studied at their itinerant electron metamagnetic transition (TM0=131, 84, and 127 K, respectively). Large reversible entropy changes are observed, up to -ΔSM=10.83 J K-1 kg-1 for a field variation of 5 T in YFe2D4.2, making these alloys candidates for magnetic refrigeration applications. The results are compared with previously published data on other potential magnetic refrigerants with itinerant electron metamagnetic transitions. [ABSTRACT FROM AUTHOR]

Gadolinium displays a strong magnetocaloric effect at temperatures close to room temperature making it useful in the field of room temperature magnetic refrigeration. We discuss the importance of including the effects of the demagnetization field when considering the magnetocaloric properties of gadolinium. The adiabatic temperature change ΔTad of gadolinium sheets upon application of a magnetic field has been measured at a range of applied magnetic fields and sample orientations. A significant dependence of ΔTad on the sample orientation is observed. This can be accounted for by the demagnetization factor. Also, the temperature dependence of ΔTad has been measured experimentally and modeled by mean field theory. Corrections to mean field theory modeling due to the demagnetization field are proposed and discussed. [ABSTRACT FROM AUTHOR]

The magnetocaloric properties of powder samples of the monoclinic YFe2H4.2, YFe2D4.2, and Y0.5Tb0.5Fe2D4.2 compounds have been studied at their itinerant electron metamagnetic transition (TM0=131, 84, and 127 K, respectively). Large reversible entropy changes are observed, up to -ΔSM=10.83 J K-1 kg-1 for a field variation of 5 T in YFe2D4.2, making these alloys candidates for magnetic refrigeration applications. The results are compared with previously published data on other potential magnetic refrigerants with itinerant electron metamagnetic transitions. [ABSTRACT FROM AUTHOR]

It is shown that 2E states in tetrahedral systems exhibit a linear E×T Jahn–Teller effect which is of purely relativistic origin (that is, it arises from the spin-orbit-coupling operator). The electrostatic interactions give rise to a Jahn–Teller effect which is quadratic in the T displacements. The 4×4 Hamiltonian matrix in a diabatic spin-electron basis is derived by an expansion of the electrostatic electronic Hamiltonian and the Breit–Pauli spin-orbit operator in powers of the Jahn–Teller active normal mode and taking account of symmetry selection rules for the matrix elements. The adiabatic potential-energy functions of the 2E×T system are doubly degenerate (Kramers degeneracy). For small displacements from the tetrahedral reference geometry, the adiabatic potential-energy surfaces represent a double cone in four-dimensional space, which is a novel topography of Jahn–Teller potential-energy surfaces. The topological phases of the adiabatic electronic wave functions are discussed. [ABSTRACT FROM AUTHOR]

Adiabaticity plays a central role in modern magnetic resonance experiments, as excitations with adiabatic Hamiltonians allow precise control of the dynamics of the spin states during the course of an experiment. Surprisingly, many commonly used adiabatic processes in magnetic resonance perform well even though the adiabatic approximation does not appear to hold throughout the process. Here we show that this discrepancy can now be explained through the use of Berry’s superadiabatic formalism, which provides a framework for including the finite duration of the process in the theoretical and numerical treatments. In this approach, a slow, but finite time-dependent Hamiltonian is iteratively transformed into time-dependent diagonal frames until the most accurate adiabatic approximation is obtained. In the case of magnetic resonance, the magnetization during an adiabatic process of finite duration is not locked to the effective Hamiltonian in the conventional adiabatic frame, but rather to an effective Hamiltonian in a superadiabatic frame. Only in the superadiabatic frame can the true validity of the adiabatic approximation be evaluated, as the inertial forces acting in this frame are the true cause for deviation from adiabaticity and loss of control during the process. Here we present a brief theoretical background of superadiabaticity and illustrate the concept in the context of magnetic resonance with commonly used shaped radio-frequency pulses. [ABSTRACT FROM AUTHOR]

Previous analyses [Z. Yan and J. Chen, J. Appl. Phys. 72, 1 (1992); J. Chen and Z. Yan, ibid., 84, 1791 (1998); Lin et al., Physica B 344, 147 (2004); Yang et al., ibid., 364, 33 (2005); Xia et al., ibid., 381, 246 (2006).] of irreversibilities in magnetic refrigerators overlooked several important losses that could be dominant in a real active magnetic regenerative refrigerator (AMRR). No quantitative expressions have been provided yet to estimate the corresponding entropy generations in real AMRRs. The important geometric parameters of AMRRs, such as the aspect ratio of the active magnetic regenerator and the refrigerant diameter, are still arbitrarily chosen. Expressions for calculating different types of entropy generations in the AMRR were derived and used to optimize the aspect ratio and the refrigerant diameter. An optimal coefficient of performance (15.54) was achieved at an aspect ratio of 6.39 and a refrigerant diameter of 1.1 mm for our current system. Further study showed that the dissipative sources (e.g., the fluid friction and the unbalanced magnetic forces) in AMRRs, which were overlooked by previous investigations, could significantly contribute to entropy generations. [ABSTRACT FROM AUTHOR]

We report the photoelectron spectra of EuSin- cluster anions (3≤=n≤=17). They reveal dramatic electronic rearrangements over the size range n=10–12. In particular, a marked increase in the adiabatic electron affinity of EuSi12 (2.8 eV) compared to its stoichiometric neighbor, EuSi11 (1.9 eV), is observed. We propose that a significant geometric reorganization due to the encapsulation of a europium atom occurs in this size range and is responsible for the detected changes in the electronic structure. In light of this interpretation, EuSi12 is the smallest fully endohedral europium-silicon cluster [ABSTRACT FROM AUTHOR]

In this paper we describe the results of a parameter survey of a 16 segmented Halbach cylinder in three dimensions in which the parameters internal radius, rin, external radius, rex, and length, L, have been varied. Optimal values of rex and L were found for a Halbach cylinder with the least possible volume of magnets with a given mean flux density in the cylinder bore. The volume of the cylinder bore could also be significantly increased by only slightly increasing the volume of the magnets, for a fixed mean flux density. Placing additional blocks of magnets on the end faces of the Halbach cylinder also improved the mean flux density in the cylinder bore, especially so for short Halbach cylinders with large rex. Moreover, magnetic cooling as an application for Halbach cylinders was considered. A magnetic cooling quality parameter, Λcool, was introduced and results showed that this parameter was optimal for long Halbach cylinders with small rex. Using the previously mentioned additional blocks of magnets can improve the parameter by as much as 15% as well as improve the homogeneity of the field in the cylinder bore. [ABSTRACT FROM AUTHOR]

The Nd49Al13Ni38 amorphous alloy has been prepared by melt-spinning in the form of ribbons. Its magnetic properties have been investigated via superconducting quantum interference device magnetometry and its magnetic phase diagram was established. Hysteresis and temperature-dependent magnetization measurements show the occurrence of a reentrant spin-glass behavior on cooling. With increasing applied field, the spin-freezing temperature decreases and disappears at very high field, and the Curie temperature increases, broadening the temperature range of the ferromagnetic state. The resulting magnetocaloric effect was evaluated and compared to other interesting magnetic refrigerant materials. [ABSTRACT FROM AUTHOR]

Time-dependent density functional theory (TDDFT) employing the exact-exchange functional (TDDFTx) has been formulated using the optimized effective potential method for the β static hyperpolarizabilities, where it reduces to coupled-perturbed Kohn-Sham theory. A diagrammatic technique is used to take the functional derivatives for the derivation of the adiabatic second kernel, which is required for the analytical calculation of the β static hyperpolarizabilities with DFT. The derived formulas have been implemented using Gaussian basis sets. The structure of the adiabatic exact-exchange second kernel is described and numerical examples are presented. It is shown that no current DFT functional satisfies the correct properties of the second kernel. Not surprisingly, TDDFTx, which corrects the self-interaction error in standard DFT methods and has the correct long-range behavior, provides results close to those of time-dependent Hartree-Fock in the static limit. [ABSTRACT FROM AUTHOR]

We present a synergetic experimental/theoretical study of hydrated hexafluorobenzene anions. Experimentally, we measured the anion photoelectron spectra of the anions, C6F6-(H2O)n (n=0–2). The spectra show broad peaks, which shift to successively higher electron binding energies with the addition of each water molecule to the hexafluorobenzene anion. Complementing these results, we also conducted density functional calculations which link adiabatic electron affinities to the optimized geometric structures of the negatively charged species and their neutral counterparts. Neutral hexafluorobenzene-water complexes are not thought to be hydrogen bonded. In the case of C6F6-(H2O)1, however, its water molecule was found to lie in the plane of the hexafluorobenzene anion, bound by two O–H...F ionic hydrogen bonds. Whereas in the case of C6F6-(H2O)2, both water molecules also lie in the plane of and are hydrogen bonded to the hexafluorobenzene anion but on opposite ends. This study and that of Schneider et al. [J. Chem. Phys. 127, 114311 (2007), preceding paper] are in agreement regarding the geometry of C6F6-(H2O)1. [ABSTRACT FROM AUTHOR]

Magnetic and magnetocaloric effects of (Mn1-xFex)5Ge3 compounds are studied systematically. The maximum of magnetic entropy changes of 8.01 J/kg K under an external field change of 5 T is obtained for (Mn0.9Fe0.1)5Ge3, which is the largest value in Mn5Ge3-based solid solutions. Moreover, the Fe substitution increases the refrigeration capacity (RC) value greatly. The largest RC value of 237 J/kg in (Mn0.8Fe0.2)5Ge3 even compares favorably to that of many well-known magnetic refrigeration materials. Thus the Fe-containing (Mn1-xFex)5Ge3 compounds are much-improved magnetic refrigerants for the application of room-temperature magnetic refrigeration. The increase of the RC value is probably resulted from the formation of magnetic nanostructure. [ABSTRACT FROM AUTHOR]