Magnetic Gd Research Articles

Valence electron structures dependences of structural stability and properties of REX3 (RE = rare earth; X = In, Tl) and RE(In, Co)3 alloys

Abstract Intermetallic compounds REIn3 (RE = rare earth) have attracted much attention due to their unique characteristics: crystal field effect, Kondo effect, superconductivity, heavy fermion, and antiferromagnetism, and their cobalt diluted alloys exhibit the ferromagnetic half-metallic characteristics at room temperature. In this study, an empirical electron theory (EET) is employed to investigate systemically the valence electronic structure, the thermal and magnetic properties of REX 3 and their cobalt diluted alloys for revealing the mechanism of physical properties. The calculated bond length, melting point, and magnetic moment match the experimental ones very well. The study reveals that structural stability and physical properties of REX 3 and their cobalt dilute alloys are strongly related to their valence electron structures. It is suggested that the structural stability and cohesive energy depend upon the covalent electron, the melting point is modulated by covalent electron pair, and the magnetic moment is originated from 3d magnetic electron. The ferromagnetic characteristics of Co-diluted REIn3 alloys is originated from the introduction of strong ferromagnetic Co atom, but, a competition is caused between the electron transition from valence electron to magnetic electron on d orbit and its reversal electron transformation with increasing the content of cobalt, which results in the formations of diluted magnetic Gd(In,Co)3 alloy with minor amount of cobalt and strong magnetic Nd(In,Co)3 alloy with doping more Co atoms.

Atmospheric heating and magnetism driven by 22Ne distillation in isolated white dwarfs

The origin of atmospheric heating in the cool, magnetic white dwarf GD 356 remains unsolved nearly 40 years after its discovery. This once idiosyncratic star with Teff ≈ 7500 K, yet Balmer lines in Zeeman-split emission is now part of a growing class of white dwarfs exhibiting similar features, and which are tightly clustered in the HR diagram suggesting an intrinsic power source. This paper proposes that convective motions associated with an internal dynamo can power electric currents along magnetic field lines that heat the atmosphere via Ohmic dissipation. Such currents would require a dynamo driven by core 22Ne distillation, and would further corroborate magnetic field generation in white dwarfs by this process. The model predicts that the heating will be highest near the magnetic poles, and virtually absent toward the equator, in agreement with observations. This picture is also consistent with the absence of X-ray or extreme ultraviolet emission, because the resistivity would decrease by several orders of magnitude at the typical coronal temperatures. The proposed model suggests that i) DAHe stars are mergers with enhanced 22Ne that enables distillation and may result in significant cooling delays; and ii) any mergers that distill neon will generate magnetism and chromospheres. The predicted chromospheric emission is consistent with the two known massive DQe white dwarfs.

Achieving Magnetic Refrigerants with Large Magnetic Entropy Changes and Low Magnetic Ordering Temperatures.

Adiabatic demagnetization refrigeration (ADR) is a promising cooling technology with high efficiency and exceptional stability in achieving ultralow temperatures, playing an indispensable role at the forefront of fundamental and applied science. However, a significant challenge for ADR is that existing magnetic refrigerants struggle to concurrently achieve low magnetic ordering temperatures (T0) and substantial magnetic entropy changes (-ΔSm) at ultralow temperatures. In this work, we propose the combination of Gd3+ and Yb3+ to effectively regulate both -ΔSm and T0 in ultralow temperatures. Notably, the -ΔSm values for Gd0.1Yb0.9F3 (1) and Gd0.3Yb0.7F3 (2) in the 0.4-1.0 K range exceed those of all previously reported magnetic refrigerants within this temperature interval, positioning them as the most efficient magnetic refrigerants for the third stage to date. Although the -ΔSm values for Gd0.5Yb0.5F3 (3) in 1-4 K are less than those of the leading magnetic refrigerant Gd(OH)F2, the -ΔSm values for Gd0.7Yb0.3F3 (4) in 1-4 K at 2 T surpass those of all magnetic refrigerants previously documented within the same temperature range, making it the superior magnetic refrigerant for the fourth stage identified thus far.

Magnetic phase diagrams and large magnetocaloric effects of the two-dimensional antiferromagnetic triangular lattice of Gd3+ ions in KBaGd(BO3)2

We report a detailed study of the magnetic properties of KBaGd(BO$_3$)$_2$, in which magnetic Gd$^{3+}$ ($S=7/2$) ions form into two-dimensional triangular layers. Magnetization, specific heat and magnetocaloric effect (MCE) measurements have been performed on KBaGd(BO$_3$)$_2$ single crystals. The results show that a long-range antiferromagnetic state is established below $T_{\rm N}=0.24$ K. In zero fields, only about half of the full entropy is released at $T_{\rm N}$, indicating that not all the magnetic moments are frozen below the ordering temperature, as expected from the geometrical frustration of the triangular spin lattice. Further studies under external fields were performed down to 50 mK, and the magnetic phase diagrams are established with magnetic fields applied both within and perpendicular to the triangular plane. KBaGd(BO$_3$)$_2$ serves as an example of a two-dimensional triangular lattice with large spin values ($S=7/2$) and can be directly compared with the iso-structure KBaR(BO$_3$)$_2$ (R = Dy-Yb) family of doublet ground states, which exhibit effective spins of $S=1/2$.

Room temperature ferromagnetism in Gd-doped AlN hierarchical microstructures: Experimental and theoretical insights

Diluted magnetic semiconductor materials have attracted extensive attention as candidate materials for a new generation of multifunctional spintronic devices. At present, doping is an effective method to adjust the physical properties of semiconductor materials. Here, the Gd doped AlN (AlN:Gd) hierarchical microstructures were synthesized from mixtures of Al and Gd2O3 powder and N2 working medium by improved arc discharge method. The samples were analyzed by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscopy, photoluminescence (PL) spectroscopy and vibrating sample magnetometer. The analysis results show that Gd ions are successfully doped into AlN hierarchical microstructures. The PL spectrum excited by 200 nm exhibits a sharp emission line at 318 nm corresponding to 6P7/2-8S7/2 transitions of the Gd3+ ion, and a broad emission band round at 395 nm, which is related to with Al vacancy and O impurity defects. The magnetization curves suggest AlN:Gd hierarchical microstructures process room-temperature ferromagnetic behavior. The latter first-principle calculations indicate that the main origin of the magnetism in AlN:Gd arise from Al vacancies rather than magnetic Gd dopants. Our results suggest that the AlN:Gd hierarchical microstructures are promising materials for spintronic applications.

Experimental and Theoretical Study on the Nonmagnetic Li/Mg Cosubstitution in the Gd5-x(Li/Mg)xGe4 (x = 1.04, 1.17, 1.53) System.

Three Li- and Mg-cosubstituted compounds in the Gd5-x(Li/Mg)xGe4 (x = 1.04(2), 1.17(2), 1.53(2)) system have been successfully prepared by conventional high-temperature reactions. According to powder and single-crystal X-ray diffraction analyses, all three compounds adopt a Gd5Si4-type phase with the orthorhombic Pnma space group (Pearson code oP16, Z = 4) and six crystallographically independent atomic sites. The crystal structure can be described as a combination of two-dimensional Mo2FeB2-type ∞2[Gd2(Li/Mg)Ge2] layers and [Ge2] dimers. Interestingly, as 64% of Li and 26% of Gd at the RE3 and RE2 sites, respectively, were exclusively substituted by Mg in Gd3.47(1)Li0.36(2)Mg1.17(3)Ge4, the lattice parameter b was selectively shortened as a result of the RE3-Ge1 bond shrinkage in comparison to that in Gd4LiGe4, while lattice parameters a and c remained nearly intact. A series of theoretical calculations using the tight-binding linear muffin-tin orbital (TB-LMTO) method indicated that the reduction of the particular RE3-Ge1 bond distance in the title compounds could also be explained by an optimization of bonding based on the corresponding RE3-Ge1 crystal orbital Hamilton population (COHP) curve. Moreover, the specific site preference of Mg for the RE3 site was supported by both size-factor as well as electronic-factor criteria on the basis of the smallest atomic size and the highest electronegativity of Mg among the three cations. Therefore, the overall electronic structure was further interrogated by a density of states (DOS) analysis. The influence of nonmagnetic Li/Mg cosubstitution for the magnetic Gd atoms in the title Gd5-x(Li/Mg)xGe4 system on the magnetic characteristics was also thoroughly studied by isofield magnetization at 100 Oe and 10 kOe and isothermal magnetization measurements at 4 K using two of the title compounds: Gd3.83(1)Li0.48Mg0.69(3)Ge4 and Gd3.47(1)Li0.36(2)Mg1.17(3)Ge4.

Magnetocaloric, electronic, magnetic, optical and thermoelectric properties in antiferromagnetic semiconductor GdCrO3: Monte Carlo simulation and density functional theory

Density functional theory and Monte Carlo simulation are performed on the magnetocaloric, electronic, optical, thermoelectric and magnetic properties of GdCrO3. The density of states diagrams is discussed. The values of the exchange energy calculated between the magnetic configurations Gd and Cr atoms was calculated. The GdCrO3 has semiconductor character and antiferromagnetic behavior. The dielectric function, absorption and reflectivity are also investigated. The thermoelectric response is evaluated by calculating electrical conductivity, thermal conductivity and seebeck coefficient (S) by using BoltzTraP code. The magnetic moment of Cr and Gd are calculated. The Néel temperature has been obtained. The presence of spin reorientation transition is also observed in this system. The maximum magnetic entropy change and relative cooling power are found to be respectively, 2.64 J.K−1.kg−1 and 25 J.kg−1 for h = 5 T.

Singular magnetic dilution behavior in a quasicrystal approximant

We report the effect of magnetic dilution on the physical properties of $({\mathrm{Gd}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}){\mathrm{Cd}}_{6}$ approximant crystals (ACs), close siblings of their corresponding quasicrystal (QC). Compared to the pure system ${\mathrm{GdCd}}_{6}$, we observe remarkable changes in the thermodynamic and magnetic bulk properties near and below the static-ordering temperatures from diluting the magnetic Gd atoms with nonmagnetic Y atoms by only 1--3% $(x=0.01--0.03)$. On the other hand, the corresponding QC system exhibits a monotonic change in its spin-glass behavior upon the magnetic dilution. We discuss the origin of the magnetic-dilution behavior in the present AC system in terms of possible magnetic frustration and short-range magnetic correlation that can be linked to its peculiar structure.

Controllable synthesis of Gd-doped SmB6 nanobelt arrays for modulating their surface transport behaviors

Samarium hexaboride (SmB6) nanobelts are believed as better platforms to study the interaction between strongly correlated electron states and topological states than their bulk topological Kondo insulator (TKI) counterparts because they have more abundant surface states. To date, there are few reports focused on the modulation of surface transport behaviors of SmB6 nanobelts because of the absence of high-quality controllable synthesis and doping techniques, despite which is very essential for their future applications. In this paper, the undoped and Gd-doped SmB6 nanobelt arrays have been successfully fabricated on the Si substrate by a one-step co-evaporation method. Both of the undoped and Gd-doped nanobelts were indexed as single-crystalline cubic structures with a growth direction of [110]. The results show that the low-temperature resistivity of the Gd-doped SmB6 nanobelts becomes unsaturated when the magnetic Gd content is increased to about 2.0 at.%. Different from the undoped SmB6 nanobelts, the extraordinary non-linear behaviors of the Hall resistivity–temperature curves are firstly found in the Gd-doped SmB6 nanobelts, attributing to the anomalous Hall effect (AHE). Both the breaking of the surface time-reversal symmetry of the SmB6 nanobelts and the resultant magnetism by the Gd doping are suggested to be responsible for the occurrence of the AHE. Our study may shed light on comprehending the interaction between correlated electron states and topological surface states in TKI nanostructures and modulating their surface transport behaviors for future quantum information applications.

Griffiths phase, magnetic memory and ac susceptibility of an antiferromagnetic titanate-based perovskite Er0.9Sr0.1Ti0.975Cr0.025O3 system

In this study, we investigated different physical properties of Er0.9Sr0.1Ti0.975Cr0.025O3 titanate prepared by a solid-state reaction that produces a cubic structure with Fd-3m (227) as a space group. Zero-Field Cooled and Field Cooled measurements show a second-order antiferromagnetic transition at Neel temperature TN = 23 K, and the existence of Griffiths phase at around TGP = 132 K. This soft magnetic material depicts a magnetic memory since it ‘remembers’ its thermal history. The relative cooling power of this titanate-based sample was then measured to be around 292.27 J kg−1 at 5 Tesla and 400 J kg−1 at 6 Tesla. However, these values are lower than the RCP value reported for the most magnetic refrigerant Gd, although these results are high enough compared to different perovskite systems. Therefore, Er0.9Sr0.1Ti0.975Cr0.025O3 is a very suitable, environment-friendly magnetic refrigerant.

Cationic disorder: A pathway for demonstrating inverse exchange bias in Gd2CoRuO6

We report here the observation of the inverse exchange bias phenomenon in the ferrimagnetic double-perovskite ${\mathrm{Gd}}_{2}{\mathrm{CoRuO}}_{6}$. The compound crystallizes in a monoclinic structure (space group $P{2}_{1}/n$), and it undergoes long-range magnetic ordering below ${T}_{C}=60$ K. The ferrimagnetic state of the sample is manifested through magnetization reversal at low temperature. In order to corroborate the role of local structure on the magnetic properties of ${\mathrm{Gd}}_{2}{\mathrm{CoRuO}}_{6}$, we have performed a combined magnetization and x-ray absorption spectroscopy study on two different samples prepared through different cooling protocols (slow cooling and furnace cooling). Extended x-ray absorption fine-structure analysis indicates the presence of strong antisite disorder between Co and Ru in both samples. However, the slow-cooled sample shows completely random antisite disorder, whereas, small Co- and Ru-rich patches appear in the furnace-cooled sample. In both samples, a significant amount of mixed valency in Co and Ru ions is evident from our x-ray absorption near-edge structure. The antiparallel ordering along with different temperature dependence of the three magnetic sublattices, Gd, Co, and Ru, can be responsible for negative magnetization, whereas the inverse exchange bias effect can be attributed to the mixed interfacial coupling between adjacent magnetic nanoclusters. The difference in local structure can nicely be accounted for by the observed variations of the magnetic properties between two samples of ${\mathrm{Gd}}_{2}{\mathrm{CoRuO}}_{6}$.

The structure, magnetic and magnetocaloric effect of misch-metal (MM)2Fe17−xAlx (x = 0–2) compounds

In this study, the structural, magnetic and magnetocaloric effects (MCE) of misch-metal MM2Fe17−xAlx (0 ≤ x ≤ 2) compounds are investigated. The XRD patterns indicate that all the samples crystallize primarily in a rhombohedral Th2Zn17 type crystal structure coexisting with a small amount of precipitation phase α-Fe. The lattice parameters and the Curie temperature (TC) increase linearly with the increase of Al content. This allows the MM2Fe17−xAlx compounds to achieve magnetic refrigeration over the entire room temperature range. The maximum values of entropy changes (ΔSM) for MM2Fe17−xAlx (x = 0, 0.5, 1 and 1.5) compounds with a magnetic field variation of 0–2 T and 0–5 T are determined to be −2.1 J Kg-1 K−1, −1.3 J Kg-1 K−1, −1.9 J Kg-1 K−1, 1.9 J Kg-1 K−1 and −4.0 J Kg-1 K−1, −2.7 J Kg-1 K−1, −3.5 J Kg-1 K−1, −3.5 J Kg-1 K−1, respectively. The relative cooling power (RCP) for all the studied compounds is larger than 90 and 270 Jkg−1 while the full-width half maxima (δTFWHM) is larger than 60 K and 100 K with the field changes of 0–2 T and 0–5 T, respectively. These results indicate that MM2Fe17−xAlx (0 ≤ x ≤ 2) compounds could have higher magnetocaloric cost-performance comparable to the room-temperature magnetic refrigeration materials R2Fe17 (R = Pr, Nd) and Gd with the second-order phase transition (SOPT).

Outstanding Comprehensive Performance of La(Fe, Si)13Hy/In Composite with Durable Service Life for Magnetic Refrigeration

AbstractMagnetic refrigeration based on magnetocaloric effect (MCE) has become a promisingly alternative technology to the conventional vapor‐compression refrigeration. A great number of magnetic materials have been reported to exhibit larger MCE than that of the benchmark magnetic refrigerant Gd. However, these materials still have severe shortcomings on the “Non‐MCE” properties, such as hard preparation and fabrication, low thermal conductivity λ, and poor corrosion resistance and cycling stabilities, which hinder the practical application of these materials. In this paper, a novel La(Fe, Si)13Hy/In composite which is prepared by a readily available hot pressing method is demonstrated to exhibit an outstanding comprehensive performance with durable service life in various aspects. Noteworthily, the ΔSM does not decrease but increases with increasing nonmagnetic In metal. This advantageous anomaly is related to the strengthening of the first‐order itinerant electron metamagnetic transition induced by residual compression stress and surrounded constraints in the composite. The present results make La(Fe, Si)13Hy/In composite the most attractive alternative to Gd for magnetic refrigeration. Moreover, this work also provides a feasible way to solve the serious issues toward applications for La(Fe, Si)13‐based materials and other brittle magnetocaloric materials.

Magnetostructural phase transitions and magnetocaloric effect in (Gd5-xScx)Si1.8Ge2.2

Future advancements in magnetocaloric refrigeration/heat pumping technologies depend on the discovery of new materials that demonstrate large, tunable magnetocaloric effects (MCEs) in the vicinity of coupled magnetic and structural phase transitions that occur reversibly with minimum hysteresis. With this in mind, we investigate phase transitions, microstructure, magnetic, thermal, magnetocaloric, and transport properties of (Gd5-xScx)Si1.8Ge2.2 compounds. Replacement of magnetic Gd with non-magnetic Sc in Gd5-xScxSi1.8Ge2.2 increases the ferromagnetic to paramagnetic first order phase transition temperature, TC, with only a minor reduction in MCE when x ≤ 0.2. We also demonstrate that hydrostatic pressure further increases TC and reduces the hysteresis of the first order phase transition in Gd4.8Sc0.2Si1.8Ge2.2 from 7 to 4 K. Temperature-dependent x-ray powder diffraction study of Gd4.8Sc0.2Si1.8Ge2.2 confirms the monoclinic ↔ orthorhombic structural transformation at TC, in agreement with magnetic, calorimetric, and electrical transport measurements. In addition to the substantial magnetocaloric effect, a large magnetoresistance of ∼20% is also observed in Gd4.8Sc0.2Si1.8Ge2.2 for ΔH = 50 kOe in the vicinity of the magnetostructural transition. In a drastic reversal of the initial doping behavior further additions of Sc (x > 0.2) suppress formation of the monoclinic phase, change the nature of the transition from first-to second-order, and reduce both the transition temperature and magnetocaloric effect.

Electrical studies on double rare earth modified Bi6Fe2Ti3O18

ABSTRACTSingle-phase bismuth layered structure ferroelectromagnet (BLSFEM) LaGdxB5-xFe2Ti3O18 of composition (x = 0.1, 0.2, 0.4, 0.6 and 0.8), novel ceramics were prepared by solid state route. Electrical and impedance measurements were made on the ceramics. Complex Impedance spectroscopic plots showed the signature of magnetic ion (Gd) and the results were corroborated to the Debye and Gaussian fitting curves. The variation of dielectric and modulus with temperature plots were used to explain dielectric phenomenon. An attempt is made to understand the effect of magnetic (Gd) ion on A site of (Bi2O2)2+ (LaGdxBi3-xTi3Fe2O16)2−, layered perovskite (title) compound.

Controlled nanocrystallinity in Gd nanobowls leads to magnetization of 226 emu/g

Gadolinium (Gd) metal is of great interest in applications such as contrast-enhanced MRI and magnetic cooling. However, it is generally difficult to produce oxide-free and highly magnetic Gd nanoparticles due to the aggressively reactive nature of Gd with oxygen. Herein, we utilized a nanofabrication route and optimization of experimental conditions to produce highly magnetic air-stable oxide-free Gd nanoparticles. The nanobowls displayed the highest saturation magnetization to date for Gd, reaching 226.4 emu/g at 2 K. The crystalline composition of Gd is found to affect the observed magnetization values: the higher magnetization is observed for nanoparticles that have a lower content of the paramagnetic face-centered cubic (fcc) phase and a greater content of the ferromagnetic hexagonal close-packed (hcp) phase. The relative fcc content was found to depend on the deposition rate of the Gd metal during the nanofabrication process, thereby correlating with altered magnetization.

Theranostic Fibers for Simultaneous Imaging and Drug Delivery.

New methods for creating theranostic systems with simultaneous encapsulation of therapeutic, diagnostic, and targeting agents are much sought after. This work reports for the first time the use of coaxial electrospinning to prepare such systems in the form of core-shell fibers. Eudragit S100 was used to form the shell of the fibers, while the core comprised poly(ethylene oxide) loaded with the magnetic resonance contrast agent Gd(DTPA) (Gd(III) diethylenetriaminepentaacetate hydrate) and indomethacin as a model therapeutic agent. The fibers had linear cylindrical morphologies with clear core-shell structures, as demonstrated by electron microscopy. X-ray diffraction and differential scanning calorimetry proved that both indomethacin and Gd(DTPA) were present in the fibers in the amorphous physical form. This is thought to be a result of intermolecular interactions between the different components, the presence of which was suggested by infrared spectroscopy. In vitro dissolution tests indicated that the fibers could provide targeted release of the active ingredients through a combined mechanism of erosion and diffusion. The proton relaxivities for Gd(DTPA) released from the fibers into tris buffer increased (r1 = 4.79-9.75 s(-1) mM(-1); r2 = 7.98-14.22 s(-1) mM(-1)) compared with fresh Gd(DTPA) (r1 = 4.13 s(-1) mM(-1) and r2 = 4.40 s(-1) mM(-1)), which proved that electrospinning has not diminished the contrast properties of the complex. The new systems reported herein thus offer a new platform for delivering therapeutic and imaging agents simultaneously to the colon.

Random-anisotropy ferromagnetic state in theCu5Gd0.54Ca0.42intermetallic compound

By applying the alloy design concept that stable intermetallic phases between two immiscible elements can be formed by adding a third element that forms stable compounds with both elements, we have synthesized the first known stable intermetallic compound of Cu, Gd, and Ca, where copper acts as the mediating element between the immiscible Gd and Ca. A compound with the composition $\mathrm{C}{\mathrm{u}}_{84}\mathrm{G}{\mathrm{d}}_{9}\mathrm{C}{\mathrm{a}}_{7}$ (equivalent to $\mathrm{C}{\mathrm{u}}_{5}\mathrm{G}{\mathrm{d}}_{0.54}\mathrm{C}{\mathrm{a}}_{0.42}$) was synthesized by the Czochralski technique in the form of a large single crystal of high structural perfection, and the structural model was determined by x-ray diffraction (XRD). The compound crystallizes in the hexagonal system, space group $P6$/mmm, and the crystal structure is isotypic to the $\mathrm{C}{\mathrm{u}}_{5.44}\mathrm{T}{\mathrm{b}}_{0.78}$. The unit cell contains inherent disorder due to partial occupation of the Cu3 site and the substitutional disorder at the Gd/Ca mixed site located at the vertices of the hexagonal unit cell, where Gd and Ca randomly substitute each other. The random substitution of magnetic Gd by nonmagnetic Ca atoms makes the magnetic Gd lattice disordered, which leads to interesting magnetic ordering at low temperatures that occurs below ${T}_{C}=24\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in zero and low external magnetic fields. By performing a large set of complementary experiments along two perpendicular crystallographic directions (the [001] hexagonal-axis direction and the [100] hexagonal-plane direction), we show that the zero-field collective magnetic state can be described as a random-anisotropy ferromagnetic state, where random magnetic anisotropies originate from the magnetic dipole interactions between the Gd moments in the magnetically disordered lattice. The random-anisotropy ferromagnetic state in the $\mathrm{C}{\mathrm{u}}_{84}\mathrm{G}{\mathrm{d}}_{9}\mathrm{C}{\mathrm{a}}_{7}$ is characterized by randomness and frustration of magnetic interactions, which are the two ingredients that allow classifying this state into the generic class of spin glasses. Our paper opens the possibility to search for new ternary intermetallic phases in the Cu--Gd--Ca phase diagram, including structurally complex phases and quasicrystals.

A Miniature Magnetic-Piezoelectric Thermal Energy Harvester

In this paper, we report a miniature thermal energy harvester with a novel magnetic-piezoelectric design. The harvester consists of a soft magnetic Gd cantilever beam, a piezoelectric lead zirconate titanate sheet, an NdFeB hard magnet, silicon clamps, and a silicon frame. In this design, the harvester is driven by a temperature difference between a cold side and room temperature ambient air, unlike other magnetic-piezoelectric thermal energy harvesters that are driven by a temperature difference between a cold side and a hot side or between two hot sides. Experimental results show that with a temperature difference of 20 °C (cold side: 6.7 °C, hot side: 26.7 °C), the harvester produces a maximum peak-to-peak voltage of 37 mV and a root mean square voltage of 1.98 mV. The estimated maximum instantaneous power density and average power density is 21.7 nW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and 62.9 pW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , respectively. Moreover, the total volume of our harvester (length × width x height: 6 × 3.5 × 3 mm) is 217 times lower than that of previous experimental harvesters and 38 times smaller than that of previous theoretical-modeled harvesters. Therefore, our harvester is the smallest machined magnetic-piezoelectric thermal energy harvester designed to date. These features enable our harvester to be more easily implemented and integrated with micro wireless sensors and thereby increase more self-powered wireless-sensing applications.

Research of spin wave function and exchange coupling interactions in metal magnetic materials

Exchange coupling is one of the most important fundamental interactions in ferromagnetic systems. Understanding of the parameters in this interaction may help describe numerous properties of metal magnetic materials. However, in the localized electron theory or itinerant electron theory there are also certain difficulties when utilizing this approximation method to study magnetic ordering problems for multi-atom systems. In realistic magnets exchange coupling is also related to the coexistence of localized and itinerant degrees of freedom. In this case Heisenberg exchange relationship has some limitations. If the exchange relationship only depends on the structure of the magnet, and is not related to energy differences between the phases, we can better avoid the Heisenberg exchange limits. Based on this, we use the general principle of the exchange coupling theory to analyse the usual approximation, and discuss the opportunity to calculate the parameters of such coupling rigorously without specific assumptions about the range of magnetic order or any approximation about the form of magnetization density. We propose a method for calculating the exchange coupling parameter to any approximation. The range of applicability of the above relation is discussed quantitatively for real magnetic systems (magnetic metal materials Gd, Fe, Ni) and spin waves, and the relevance for the exchange coupling is also analysed. This analysis for metal magnetic system (Fe, Ni and Gd) shows that the most significant improvement is obtained for exchange coupling between nearest magnetic atoms and for spin wave spectrum at finite wave vectors. It can be described by the relationship between the exchange coupling approximation and spin wave spectrum, and also interaction between the nearest neighbor magnetic atoms in ferromagnetic systems; these will give reasonable description to the large wave vectors part of spin wave spectra in any magnet with not fully localized magnetism. This point of view from the magnetism theory is consistent with the experimental results.