Transition In Gd Research Articles

Large reversible magnetocaloric effects originating from ferromagnetic phase transition in Gd(Cu1−xNix)2 (x = 0.12, 0.15)

Here, the significant increase in MCE without any hysteresis loss was achieved through the substitution of nickel for a portion of the copper in GdCu2 compound. The values of maximum magnetic entropy changes (−ΔSMmax) and refrigeration capacity (RC) for Gd(Cu1-xNix)2 (x = 0.12, 0.15) compounds are 10.3 J/kg K, 455.3 J/kg, 9.3 J/kg K and 461.2 J/kg under varying magnetic fields from 0 to 5 T, respectively. The refrigeration capacity of Gd(Cu1−xNix)2 (x = 0.12, 0.15) is improved by an order of magnitude compared to the GdCu2 compound under varying magnetic fields from 0 to 5 T. While providing an ideal magnetocaloric refrigerated material for hydrogen liquefaction, our work also suggests a promising approach to the development of new magnetocaloric refrigerated materials.

Reversible single-crystal-to-single-crystal transition in Gd(III) metal-organic frameworks induced by heat and solvents with a significant magnetocaloric effect.

The design and synthesis of a Gd(III) metal-organic framework with the formula [Gd4(BTDI)3(DMF)4]n (JXUST-40, H4BTDI = 5,5'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)diisophthalic acid) are reported hererin. Interestingly, a reversible single-crystal-to-single-crystal transition between JXUST-40 and {[Gd4(BTDI)3(H2O)4]·6H2O}n (JXUST-40a) was achieved under the stimulation of heat and solvents. Both JXUST-40 and JXUST-40a exhibited good stability when soaked in common solvents and aqueous solutions with pH values of 1-12. Magnetic studies showed that JXUST-40a has a larger magnetocaloric effect with -ΔSmaxm = 26.65 J kg-1 K-1 at 2 K and 7 T than JXUST-40 due to its larger magnetic density. Structural analyses indicated that the coordinated solvent molecules play a crucial role in the coordination environment around the Gd(III) ions and the change in the framework, ultimately leading to the changes in the pore size and magnetism between JXUST-40 and JXUST-40a. In addition, both isomorphic [Dy4(BTDI)3(DMF)4]n (JXUST-41) and {[Dy4(BTDI)3(H2O)4]·6H2O}n (JXUST-41a) displayed slow magnetic relaxation behaviour.

Detection of electromagnetic phase transitions using a helical cavity susceptometer.

Fast and sensitive phase transition detection is one of the most important requirements for new material synthesis and characterization. For solid-state samples, microwave absorption techniques can be employed for detecting phase transitions because it simultaneously monitors changes in electronic and magnetic properties. However, microwave absorption techniques require expensive high-frequency microwave equipment and bulky hollow cavities. Due to size limitations in conventional instruments, it is challenging to implement these cavities inside a laboratory cryostat. In this work, we designed and built a susceptometer that consists of a small helical cavity embedded into a custom insert of a commercial cryostat. This cavity resonator operated at sub-GHz frequencies is extremely sensitive to changes in material parameters, such as electrical conductivity, magnetization, and electric and magnetic susceptibilities. To demonstrate its operation, we detected superconducting phase transition in Nb and YBa2Cu3O7-δ, metal-insulator transitions in V2O3, ferromagnetic transition in Gd, and magnetic field induced transformation in meta magnetic NiCoMnIn single crystals. This high sensitivity apparatus allows the detection of trace amounts of materials (10-9-cc) undergoing an electromagnetic transition in a very broad temperature (2-400K) and magnetic field (up to 90 kOe) ranges.

First-order ferromagnetic transitions of lanthanide local moments in divalent compounds: An itinerant electron positive feedback mechanism and Fermi surface topological change

Around discontinuous (first-order) magnetic phase transitions the strong caloric response of materials to the application of small fields is widely studied for the development of solid-state refrigeration. Typically strong magnetostructural coupling drives such transitions and the attendant substantial hysteresis dramatically reduces the cooling performance. In this context we describe a purely electronic mechanism which pilots a first-order paramagnetic-ferromagnetic transition in divalent lanthanide compounds and which explains the giant non-hysteretic magnetocaloric effect recently discovered in a Eu$_2$In compound. There is positive feedback between the magnetism of itinerant valence electrons and the ferromagnetic ordering of local $f$-electron moments, which appears as a topological change to the Fermi surface. The origin of this electronic mechanism stems directly from Eu's divalency, which explains the absence of a similar discontinuous transition in Gd$_2$In.

Structural transition in Gd doped LaCrO3 isovalent by in-situ ultrasonic measurements

The Gadolinium substituted LaCrO3 isovalent perovskite was investigated in the present study. The samples of La1-xGdxCrO3 (x = 0, 0.025, 0.05, 0.10, 0.15 and 0.20) were prepared employing solid state reaction method. The X-Ray Diffraction (XRD) patterns confirmed that the prepared samples have orthorhombic structure with pnma space group at room temperature. The crystalline size of the samples is 98, 89, 76, 72, 60 and 55 nm respectively for the composition x = 0, 0.025, 0.05, 0.10, 0.15 and 0.20 and the same is decreasing with an increase in substitution of Gd in the samples. The scanning electron microscope images of the samples have spherical like morphology with a particle size of 150, 131, 129, 120, 116 and 94 nm for the composition x = 0, 0.025, 0.05, 0.10, 0.15 and 0.20 respectively. In-situ ultrasonic measurements were made over a wide range of temperature from 300 to 700 K to determine structural transition. The measured temperature dependent ultrasonic parameters are used to explore the structural phase transition from orthorhombic to rhombohedral phase in the prepared sample at high temperatures.

Garnet-to-Perovskite Transition in Gd3Sc2Ga3O12 at High Pressure and High Temperature

The structural phase transition of gadolinium-scandium-gallium garnet (Gd(3)Sc(2)Ga(3)O(12), GSGG) has been studied at high pressure and high temperature using the synchrotron X-ray diffraction technique in a laser-heated diamond anvil cell. The GSGG garnet transformed to an orthorhombic perovskite structure at approximately 24 GPa after laser heating to 1500-2000 K. The garnet-to-perovskite phase transition is associated with an ∼8% volume reduction and an increase in the coordination number of the Ga(3+) or Sc(3+) ion. The orthorhombic perovskite GSGG has bulk modulus B(0) = 194(15) GPa with B(0)' = 5.3(8), exhibiting slightly less compression than the cubic garnet structure of GSGG with B(0) = 157(15) GPa and B(0)' = 6.5(10). Upon compression at room temperature, the cubic GSGG garnet became amorphous at ∼65 GPa. Coupled with the amorphous-to-perovskite phase transition in Y(3)Fe(5)O(12) and Gd(3)Ga(5)O(12) at high-pressure-temperature conditions, we conclude that amorphization should represent a new thermodynamic state resulting from hindrance of the garnet-to-perovskite phase transition, whereas the garnet-to-amorphous transition in rare-earth garnets should be kinetically hindered at room temperature.

Infrared spectroscopy and the ferromagnetic transition in Gd

The low energy electronic structure of Gd has been investigated experimentally by infrared reflectance spectroscopy, and theoretically from first principles, using the fully relativistic Dirac linear-muffin-tin-orbital (LMTO) method in the local spin density approximation (LSDA) as well as within the LSDA + U approach. The reflectance of a Gd single crystal was measured with the electric field in the plane perpendicular to the c-axis for temperatures between 50 K and slightly above the Curie temperature (293 K) in the frequency range between 100 and 12 000 cm−1 (0.013–1.5 eV). As Gd enters the ferromagnetic state, the dissipative part of the optical conductivity exhibits interesting spectral weight transfers over the whole spectral range measured. It is shown that the ab initio calculations reproduce well the experimental spectra for the ferromagnetic state and allow one to explain the microscopic origin of the optical response of Gd in terms of interband transitions.

Active transient cooling by magnetocaloric materials

The magnetocaloric effect (MCE) has been intensively studied for novel energy efficient thermal management systems. The present study demonstrates a proof-of-concept magnetic cooling setup for active cooling of the thermal spikes of a heated resistor. Using Gd as the MCE material, the device was capable of actively cooling thermal spikes within one cycle since the dynamics of magnetic phase transition in Gd (a second-order magnetic phase transition material) are favorable to effect a fast MCE response. Enhanced cooling rate of the heated resistor of up to ∼85% for active cooling by MCE compared to passive cooling was achieved. The cooling curve of the resistor was found to follow an exponential decrease. Our results show that magnetic cooling systems can be an efficient solution to cool thermal spikes in active transient cooling systems.

Procedure for numerical integration of the magnetocaloric effect

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.

Electronic Phase Transitions in F-electron Metals at High Pressures: Synchrotron X-ray Spectroscopic Studies on GD to 100 GPa

AbstractUnusual phase transitions driven by electron correlation effects occur in many f-electron metals (lanthanides and actinides alike) from localized phases to itinerant phases at high pressures. The dramatic changes in atomic volumes and crystal structures associated with some of these transitions signify equally important changes in the underlying electronic structure of these correlated f-electron metals. Yet, the relationships among the crystal structure, electronic correlation and electronic structure in f-electron metals have not been well understood. In this study, utilizing recent advances in third-generation synchrotron x-ray spectroscopies and high-pressure diamond-anvil cell technologies, we describe the pressure-induced spectral changes across the volume collapse transition in Gd at 60 GPa and well above. The spectral results suggest that the f-electrons of high-pressure Gd phases are highly correlated even at 100 GPa – consistent with the Kondo volume collapse model and the recent experimental evidence of strong electron correlation of α-Ce.

Tracking and understanding the first-order structural transition inEr5Si4

Temperature-dependent single crystal x-ray-diffraction studies revealed a reversible first-order phase transition in Er 5 Si 4 . The high-temperature phase adopts the orthorhombic Gd 5 Si 4 -type structure, and the low-temperature phase has the monoclinic Gd 5 Si 2 Ge 2 -type structure. Unlike the magnetic/martensitic transition in Gd 5 Si 2 Ge 2 , the structural change in Er 5 Si 4 is not coupled with a magnetic transition, and the structural sequence below room temperature is just the reverse. A vibrational mode that breaks half of the interslab silicon dimers and rotates slabs in the monoclinic structure, thus lowering the symmetry from Pnma to P112 1 /a, has been identified using Landau theory. While the monoclinic phase is electronically stabilized at low temperatures, the orthorhombic phase is entropically preferable at high temperatures.

Specific Mass Shift in Gd I and Dy I

High-resolution diode-laser spectroscopy has been performed on atomic beams of natural Gd and Dy. Isotope shifts of the even-mass isotopes have been measured for two transitions in Gd I and one transition in Dy I. Specific mass shifts, as well as field shifts, have been derived for transitions of 4 f 7 5 d 6 s 2 –4 f 7 6 s 2 6 p in Gd I, and 4 f 10 6 s 2 –4 f 9 5 d 6 s 2 in Dy I; the specific mass shift is much larger than the normal mass shift. It has been found that the specific mass shift of the 4 f 10 6 s 2 –4 f 9 5 d 6 s 2 transition in Dy I is about one order of magnitude larger than that of the 4 f 7 5 d 6 s 2 –4 f 7 6 s 2 6 p transition in Gd I. This shows that the specific mass shift, related to the correlation effect between electrons, strongly depends on the orbital angular momentum of electrons.

Magnetic properties of Gd 5(Si 1.5Ge 2.5) near the temperature and magnetic field induced first order phase transition

The temperature (from 5 to 300 K) and DC magnetic field (from 0 to 90 kOe) dependencies of the DC magnetization and magnetic susceptibility, and the temperature (from 5 to 350 K) dependency of the AC magnetic susceptibility of Gd 5(Si 1.5Ge 2.5) have been studied. The temperature and/or magnetic field induced magnetic phase transition in Gd 5(Si 1.5Ge 2.5) is a first order ferromagnet↔paramagnet transition. The temperature of the magnetic transition in low AC magnetic field is 206 and 217 K for cooling and heating, respectively. The DC magnetic field increases the transition temperature by ∼0.36 K/kOe indicating that the paramagnetic phase can be reversibly transformed into the ferromagnetic phase. When the magnetic field is removed, the ferromagnetic phase transforms into the paramagnetic phase showing a large remanence-free hysteresis. The magnetic phase diagram based on the isothermal magnetic field dependence of the DC magnetization at various temperatures for Gd 5(Si 1.5Ge 2.5) is proposed. The magnetic field dependence of the magnetization in the vicinity of the first order phase transition shows evidence for the formation of a magnetically heterogeneous system in the volume of Gd 5(Si 1.5Ge 2.5) specimen where the magnetically ordered (ferromagnetic) and disordered (paramagnetic) phases co-exist.

Magnetic field and temperature-induced first-order transition in Gd 5(Si 1.5Ge 2.5): a study of the electrical resistance behavior

Magnetic field (0–4 T) and temperature dependencies (4.2–320 K) of the electrical resistance of Gd 5(Si 1.5Ge 2.5), which undergoes a reversible first-order ferromagnetic↔paramagnetic phase transition, have been measured. The electrical resistance of Gd 5(Si 1.5Ge 2.5) indicates that the magnetic phase transition can be induced by both temperature and magnetic field. The temperature dependence of the electrical resistance, R( T), for heating at low temperatures in the zero magnetic field has the usual metallic character, but at a critical temperature of T cr=216 K the resistance shows a ∼20% negative discontinuity due to the transition from the low-temperature high-resistance state to the high-temperature low-resistance state. The R( T) dependence for cooling shows a similar but positive ∼25% discontinuity at 198 K. The isothermal magnetic field dependence of the electrical resistance from 212⩽ T⩽224 K indicates the presence of temperature-dependent critical magnetic fields which can reversibly transform the paramagnetic phase into the ferromagnetic phase and vice versa. The critical magnetic fields diagram determined from the isothermal magnetic field dependencies of the electrical resistance of Gd 5(Si 1.5Ge 2.5) shows that the FM↔PM transition in zero magnetic field on cooling and heating occurs at 206 and 213 K, respectively. The full isothermal magnetic filed hysteresis for the FM↔PM transition is ∼2 T, and the isofield temperature gap between critical magnetic fields is ∼7 K.

Energy transfer from Gd 3+ to Cr 3+ in Cr 3+:Gd 3Sc 2Ga 3O 12 (GSGG)

The temporal characteristics and temperature dependence of the Gd 3+ to Cr 3+ energy transfer in Cr 3+: GSGG are presented. Following pulsed excitation of the 8S 7 2 → 6P 7 2 transition in Gd 3+, the risetime of the resulting Cr 3+ fluorescence was used to determine the Gd 3+ to Cr 3+ energy transfer rate. The results indicate a rapid energy migration among the Gd 3+ ions followed by a Gd 3+ to Cr 3+ energy transfer step over a single fixed distance.

Study of the structural phase transition in Gd 2− xCe xCuO 4

The structural phase transition in Gd 2− x Ce x CuO 4 ( x = 0, 0.12) characterized by the rotation of the CuO 4-squares around the c axis has been studied by different diffraction techniques. The order parameter of this transition is found to increase continuously below 658(1) K for Gd 2CuO 4 and below 707.2(7) K for Gd 1.88Ce .12CuO 4 with an unusual temperature dependence. The rotation of the CuO 4-squares is coupled to a spontaneous strain along the c-axis. Structure analysis of the undistorted high temperature T′-phase in Gd 2CuO 4 indicates persisting local distortions.

Local magnetization and magnetotransport in magnetic semiconductors

We describe the influence of local magnetization on electron localization in magnetic semiconductors. This includes a review of the magnetic field induced insulator-metal transition in Gd 3-xv xS 4 (where v=vacancy) and the observation of activated transport, i.e. a ‘hard’ gap, in the hopping regime for the diluted magnetic semiconductor Cd 1-xMn xTe:In.

A neutron study of the spin reorientation transition in Gd

The temperature dependence of the spin orientation in Gd has been reexamined with particular emphasis on the spin-reorientation phase transition near 235 K. The spin orientation and the relative magnetization were determined in the 9–310 K region from Bragg intensity measurements on the (100) and (002) reflections from an isotopic single crystal of 160Gd. At low temperatures, the angle between the spin and the c axis is 40°. This angle ϑ increases with increasing temperature up to 71° at 130 K and then remains constant out to 220 K where it begins a precipitous decrease on approaching the reorientation transition at Tr⩾235 K. Unexpected intensity behavior was observed both below Tr, where the relative magnetization dips about 3% below the smooth curve in the 220–234 K region, and above Tr, where Bragg intensity remains at the (002) position out to about 285 K. Since the anisotropy constants of Gd indicate that ϑ = 0 above Tr, we attribute this (002) intensity to long wavelength orientation fluctuations and assume that only the average ϑ is zero above Tr. In spite of this complication, the transition exhibits Landau behavior in that the order parameter sin ϑ follows a (Tr−T)1/2 dependence in the range 223–234 K.

Isotope shift studies in gadolinium spectra

Isotope shift studies have been carried out in the gadolinium spectrum using a recording Fabry-Pérot spectrometer and gadolinium samples enriched in 156Gd and 160Gd isotopes. Isotope shifts Δσ(156–160) have been recorded in 134 lines in the region 3930–4140 Å. Some of these lines involve the recently identified even configuration 4f 85d6s of Gd I and the newly classified transition 4f 86s–4f 86p of Gd II. From the isotope shift measurements of lines involving the 4f 86s–4f 86p transition in Gd II, the isotope shift, ΔT(156–160) = 87 mK, has been obtained for the 4f 86s configuration. Electronic configurations have been suggested for a number of energy levels and configuration mixing has been pointed out in certain cases.

Brillouin linewidth and anomalous damping of acoustic phonons near the phase transition in Gd 2(MoO 4) 3

Anomalous damping of acoustic phonons has been observed from the measurements of the linewidths of Brillouin components near the transition temperature in Gd 2(MoO 4) 3 and the temperature dependence of damping constants has been fitted by a power law. Just below T 0, the phonons propagating along the x-axis can only travel one wavelength in their relaxation time.