Large Thermal Hysteresis Research Articles

Rational design of MnCoGe alloys for enhanced magnetocaloric performance and reduced thermal hysteresis

MnCoGe alloys are widely recognized as an important family of rare-earth-free magnetocaloric materials by engineering its magnetostructural coupling for giant entropy changes. However, its practicability for magnetic refrigeration is largely hindered by the large thermal hysteresis. In this work, we show that the co-doped MnCoGe compound, namely Mn0.95Cu0.03CoGe with 2 both mol% Mn vacancies and 3 mol% Cu-doping for Mn, displays a maximum entropy change of 29.0 J kg−1K−1 at 295 K under a magnetic field of 5 T, together with a relative cooling power as high as 314.5 J kg−1 and a record low thermal hysteresis of 16 K. The co-doping strategy in MnCoGe finely tunes the structural transition temperature within the range of Curie temperature window, leading to a strong magnetostructural coupling and giant magnetocaloric effect. Meanwhile, Mn-deficiency and Cu-doping considerably reduce the energy difference between martensitic and austenitic MnCoGe, rendering a minimal thermal hysteresis. Our co-doped MnCoGe alloys are robust candidates for near-room-temperature magnetic refrigeration.

Understanding Polyproline's Unusual Thermoresponsive Properties Using a Polyproline-Based Double Hydrophilic Block Copolymer.

Polyproline is a unique thermoresponsive polymer characterized by large thermal and conformational hysteresis. This article employs polyproline-based double hydrophilic block copolymers (PNIPAMn-b-PLPm) to gain insight into polyproline's thermoresponsive mechanism. The amine-terminated poly(N-isopropylacrylamide) (NH2-PNIPAMm) was used as the macroinitiator for ring-opening polymerization of proline-NCA monomers, resulting in various block copolymers (PNIPAMn-b-PLPm) with varying PLP block lengths. Block copolymers' thermal phase transitions were compared with their homopolymer counterparts using turbidimetry, variable-temperature NMR, dynamic light scattering, and circular dichroism spectroscopy. These experiments revealed that regardless of their compositions, all block copolymers exhibited a two-stage collapse (TCP(PLP) > TCP(PNIPAM)) during the heating cycle. In contrast, only one clearing temperature (TCL) was observed during cooling. The observed clearing temperature is closely correlated to the clearing temperature of PNIPAM blocks, suggesting the role of water-soluble PNIPAM blocks in resolving the PLP blocks. Moreover, thermal and conformational hysteresis related to the polyproline block is significantly suppressed in the presence of a PNIPAM block. Linking PNIPAM blocks has two significant effects on PLP segments' thermoresponsive behavior. For example, during the heating cycle, the precollapsed PNIPAM chains (as TCP(PNIPAM) < TCP(PLP)) prevent orderly aggregation within the PLP block. Meanwhile, during the cooling cycle below the clearing temperature of the PNIPAM block, the PNIPAM chains impart water solubility (as TCL(PNIPAM) > TCL(PLP)) to the collapsed PLP chains. Overall, the PNIPAM block imparts water solubility and perturbs PLP chains to form the native aggregate structure, suppressing the hysteresis effect. Accordingly, the large thermal and conformational hysteresis associated with native PLP chains appears to result from a noninterfering aggregation above the critical temperature.

Adapting the Mn content within a Fe-Mn-Si-based shape memory alloy by in situ parameter variations in laser-based additive manufacturing

Shape memory alloys exhibit unique properties ideal for functionally integrated components, such as actuators. While commonly used Ni-Ti alloys are well established, especially in biomedicine and aerospace, their high cost limits wider applications. Fe-based shape memory alloys present an affordable alternative, suitable for diverse applications, with a larger thermal hysteresis but lower recovery strain. Nonetheless, their functional properties can be enhanced through optimized processing methods like laser powder bed fusion and adjustments to their alloy composition. In the present study, laser powder bed fusion was used for modifying the composition and microstructure of a Fe-30Mn-6Si-5Cr alloy. For this purpose, laser parameters were varied to evaporate up to 47.3% of the initial Mn in the manufacturing process. In this context, the amount of Mn evaporated was dependent on both the line energy and more considerably on the volume energy density introduced in the manufacturing process. Subsequently, the impact of the resulting compositional changes on the microstructure was analyzed, and the functional properties were visualized using a demonstrator. Lowering the Mn content below 24.1% led to a higher degree of ferrite in the otherwise austenitic microstructure. This also affected the functional properties. These findings highlight the potential of laser-based additive manufacturing for modifying the composition and microstructure of shape memory alloys in situ and, thus, tailoring their functional properties.

Electronic transport in reactively sputtered Mn3GaN films prepared under optimized nitrogen flow

Abstract Mn-based nitrides with antiperovskite structures have several properties that can be utilized for antiferromagnetic spintronics. Their magnetic properties depend on the structural quality, composition and doping of the cubic antiperovskite structure. Such nitride thin films are usually produced by reactive physical vapor deposition, where the deposition rate of N can only be controlled by the N2 gas flow. We show that the tuning of the N content can be optimized using low temperature resistivity measurements, which serve as an indicator of the degree of structural disorder. Several Mn3GaN x films were prepared by reactive magnetron sputtering under different N2 gas flows. Under optimized gas flow conditions, we obtain films that exhibit a metal-like temperature dependence of the resistivity, a vanishing logarithmic increase of the resistivity towards zero, the highest resistivity ratio, and a lattice contraction of 0.4% along the growth direction when heated above the Néel temperature T N . The retarded formation of an additional magnetic phase appearing at a temperature T ∗ ≪ T N gives rise to a large thermal hysteresis of the resistivity and anomalous Hall effect.

Insights into reduction of hysteresis in (Mn, Fe)2(P, Si) compounds by experimental approach and Landau theory

The giant magnetocaloric effect in (Mn, Fe)2(P, Si) based alloys arises from a magnetoelastic ferromagnetic-paramagnetic (FM-PM) phase transition accompanied by a large thermal hysteresis. The thermal hysteresis leads to an undesirable irreversibility of the magnetocaloric effect (MCE) during cyclic operation, impeding the practical application in solid-state magnetic refrigeration. Here, we present pre-existing PM nuclei play a role in reducing hysteresis. A combinatorial analysis, using in situ X-ray diffraction (XRD) and magneto-optical Kerr effect (MOKE) microscopy, reveals that the residual PM phases at the ferromagnetic state of the compound acts as the nuclei for the growth of the PM phases. This is kinetically favorable for the FM-PM phase transition and contributes to a smaller hysteresis from an extrinsic perspective. In addition, the smaller changes in the lattice constants during the phase transition indicates a weakened first-order phase transition. Through a combined effect of intrinsic and extrinsic contributions, a giant MCE in the magnetic entropy change of 16 J kg–1 K−1 under 2 T and a low thermal hysteresis of 3.0 K were achieved in a basic Mn–Fe–Si–P quaternary system for room temperature applications. Furthermore, based on Landau theory and experimentally obtained data, we established an H-T phase diagram and unveiled the crucial role of the magnetoelastic coupling in manipulating thermal hysteresis. Overall, the findings in this work offer a strategy to mitigate hysteresis while retaining a large MCE through extrinsic control.

Kinetics of the plastic crystal transition in neopentyl glycol

The thermal hysteresis exhibited in plastic crystal compounds greatly reduces their cyclic efficiency, limiting their potential for replacing current environmentally harmful refrigerants. A mechanistic understanding of the origins of this hysteresis has yet to be established. Here, we systematically investigate the transformation kinetics of the model plastic crystal, neopentyl glycol (NPG), through microscopic and calorimetric techniques. We reveal an asymmetry between the forward (heating) and reverse (cooling) transitions. We also demonstrate that the forward transformation is rate-limited by the rate of growth of rotationally disordered domains. In contrast, the reverse transformation is rate-limited by the nucleation of the ordered crystal domain, demonstrated by the sharp exothermic peaks in calorimetry and rapid self-nucleation phenomena observed optically. This nucleation limitation is largely responsible for the large thermal hysteresis in NPG, which we observe to be as large as 16.7 °C for an approximately 10 mg sample cooled at 0.5 °C min−1. These findings demonstrate the underlying origin of the thermal hysteresis and introduce a direction to mitigate hysteresis in plastic crystal transformations.

Hydrogen-bond engineering induced ferroelastic phase transition in copper-based organic-inorganic hybrid thermochromic materials.

We report two organic-inorganic hybrid thermochromic crystals, [(2A5MP)2CuCl4] (2A5MP = 2-amino-5-methylpyridine) (1) and [(2A4MP)2CuCl4] (2A4MP = 2-amino-4-methylpyridine) (2). Through hydrogen bond engineering optimization, the ferroelastic material 2 undergoes an isomorphic phase transition with a large thermal hysteresis of 36 K at 397 K/361 K.

Designing multifunctional organic thermochromic ferroelectric materials: remarkable melt-cool large thermal hysteresis of reversible single crystal to single crystal transformation

A comprehensive study of chiral N-benzylideneaniline analogues for their structural and material properties.

Impact of fast-solidification on all-d-metal NiCoMnTi based giant magnetocaloric Heusler compounds

Recently, the all-d-metal Ni(Co)MnTi based Heusler compounds are found to have a giant magnetocaloric effect (GMCE) near room temperature and manifest different functionalities like multicaloric effects, which can be employed for solid-state refrigeration. However, in comparison to other traditional Heusler compounds, the relatively large thermal hysteresis (ΔThys) and moderately steep ferromagnetic phase transition provides limitations for real applications. Here, we present that fast solidification (suction casting) can sufficiently tailor the GMCE performance by modifying the microstructure. Compared with the arc-melted sample, the magnetic entropy change of the suction-casted sample shows a 67% improvement from 18.4 to 29.4 Jkg−1K−1 for a field change (∆μ0H) of 5 T. As the thermal hysteresis has maintained a low ΔThys value (5.5 K) for the enhanced first-order phase transition, a very competitive reversible magnetic entropy change of 21.8 Jkg−1K−1 for ∆μ0H = 5 T is obtained. Combining high-resolution transmission electron microscopy (HRTEM) and positron annihilation spectroscopy (PAS) results, the difference in lattice defect concentration is found to be responsible for the significant improvement in GMCE for the suction-cast sample, which suggests that defect engineering can be applied to control the GMCE. Our study reveals that fast solidification can effectively regulate the magnetocaloric properties of all-d-metal NiCoMnTi Heusler compounds without sacrificing ΔThys.

DNA i‐motif formation at physiological pH revealed by Raman spectroscopy

AbstractThe intercalated motif (iM) is a four‐stranded structure consisting of two parallel homoduplexes formed by hemiprotonated C.C+ pairs and intercalated antiparallel to each other. iM is generally formed at acidic pH, as cytosine protonation is required for its formation. However, sequences with long cytosine tracts can form iM at physiological pH conditions. Here, we use both off‐resonant Raman and UV‐resonant Raman (using synchrotron radiation) spectroscopy (RS) to study the sequence (C9T3)3C9, which has been recently thoroughly analysed by other spectroscopic methods (CD, 1H NMR). In line with these methods, RS has shown that (C9T3)3C9 adopts iM at neutral pH and the iM forms with slow kinetics, melts in multiple steps and exhibits a large thermal hysteresis between the folding and refolding processes. The presence of isosbestic points in the temperature‐dependent Raman spectra (in line with the SVD analysis indicating just two independent spectral components) suggests that the (C9T3)3C9 is found as a mixture of disordered and ordered structures, the proportion of which is temperature dependent. The ordered iM species are believed to be composed of the same type of C.C+ pairs but differing in their number and arrangement along the cytosine tracts. The related sequence (C9A3)3C9 shows the same behaviour as (C9T3)3C9, indicating that the substitution of adenine bases for thymine bases has little effect.

Organic radical ferroelectric crystals with martensitic phase transition

Organic martensitic compounds are an emerging type of smart material with intriguing physical properties including thermosalient effect, ferroelasticity, and shape memory effect. However, due to the high structural symmetry and limited design theories for these materials, the combination of ferroelectricity and martensitic transformation has rarely been found in organic systems. Here, based on the chemical design strategies for molecular ferroelectrics, we show a series of asymmetric 1,4,5,8-naphthalenediimide derivatives with the homochiral amine and 2,2,6,6-tetramethylpiperidine-N-oxyl components, which adopt the low-symmetric polar structure and so allow ferroelectricity. Upon H/F substitution, the fluorinated compounds exhibit reversible ferroelectric and martensitic transitions at 399 K accompanied by a large thermal hysteresis of 132 K. This large thermal hysteresis with two competing (meta)-stable phases is further confirmed by density functional theory calculations. The rare combination of martensitic phase transition and ferroelectricity realizes the bistability with two different ferroelectric phases at room temperature. Our finding provides insight into the exploration of martensitic ferroelectric compounds with potential applications in switchable memory devices, soft robotics, and smart actuators.

Electro-Elastic Modeling of Multi-Step Transitions in Two Elastically Coupled and Sterically Frustrated 1D Spin Crossover Chains

One-dimensional spin crossover (SCO) solids that convert between the low spin (LS) and the high spin (HS) states are widely studied in the literature due to their diverse thermal and optical characteristics which allow obtaining many original behaviors, such as large thermal hysteresis, incomplete spin transitions, as multi-step spin transitions with self-organized states. In the present work, we investigate the thermal behaviors of a system of two elastically coupled 1D mononuclear chains, using the electro-elastic model, by including an elastic frustration in the nearest neighbors (nn) bond length distances of each chain. The chains are made of SCO sites that are coupled elastically through springs with their nn and next-nearest neighbors. The elastic interchain coupling includes diagonal springs, while the nn inter-chain distance is fixed to that of the high spin state. The model is solved using MC simulations, performed on the spin states and the lattice distortions. When we only frustrate the first chain, we found a strong effect on the thermal dependence of the HS fraction of the second chain, which displays an incomplete spin transition with a significantly lowered transition temperature. In the second step, we frustrate both chains by imposing different frustration rates. Here, we demonstrate that for high frustration values, the thermal dependence of the total HS fraction exhibits multi-step spin transitions. The careful examination of the spin state structures in the plateau regions showed the coexistence of special dimerized ferro–antiferro patterns of type LL-HH-LL-HH along the first chain and HH-LL-HH-LL (H=HS and L=LS) along the second one, revealing that the two chains are antiferro-elastically coupled. This type of spatial modulation of the spin state and bond length distances is very attractive because it anticipates the possible existence of periodic structures in 2D lattices, made of alternate 1D SCO strings with HLHLHL structures, coupled in the ferro-like fashion along the interchain direction.

Characterization of Agarose Gels in Solvent and Non-Solvent Media.

Agarose is known to form a homogeneous thermoreversible gel in an aqueous medium over a critical polymer concentration. The solid-liquid phase transitions are thermoreversible but depend on the molecular structure of the agarose sample tested. The literature has mentioned that agarose gels could remain stable in non-solvents such as acetone or ethanol. However, there has been no characterization of their behavior nor a comparison with the gels formed in a good solvent such as water. In the first step of this article, the structure was characterized using 1H and 13C NMR in both D2O and DMSO-d6 solvents. DMSO is a solvent that dissolves agarose regardless of the temperature. First, we have determined a low yield of methyl substitution on the D-galactose unit. Then, the evolution of the 1H NMR spectrum was monitored as a function of temperature during both increasing and decreasing temperature processes, ranging from 25 to 80 °C. A large thermal hysteresis was obtained and discussed, which aided in the interpretation of rheological behavior. The hysteresis of NMR signals is related to the mobility of the agarose chains, which follows the sol/gel transition depending on the chains' association with H-bonds between water and the -OH groups of agarose for tightly bound water and agarose/agarose in chain packing. In the second step of the study, the water in the agarose gel was exchanged with ethanol, which is a non-solvent for agarose. The resulting gel was stable, and its properties were characterized using rheology and compared to its behavior in aqueous media. The bound water molecules that act as plasticizers were likely removed during the exchange process, resulting in a stronger and more brittle gel in ethanol, with higher thermal stability compared to the aqueous gel. It is the first time that such gel is characterized without phase transition when passing from a good solvent to a non-solvent. This extends the domains of application of agarose.

Reversible Thermal Hysteresis in Heating‐Cooling Cycles of Magnetic Susceptibility: A Fine Particle Effect of Magnetite

AbstractThermomagnetic curves of magnetic susceptibility (κ) are key to characterizing magnetic properties. We report hump‐shaped κ‐T curves of magnetite‐bearing basalt during heating‐cooling cycles to ∼340°C, with a large thermal hysteresis and similar starting and ending values, even in multiple repeated cycles, ruling out changes in magnetic mineralogy. Based on FORC diagrams and published results of engineered materials, we propose that thermal hysteresis arises from configurations of magnetic moments in clusters of single‐domain particles due to dipolar coupling, with different collective behavior during heating and cooling. This effect modifies the hump‐shaped thermal relaxation behavior of the individual nanoparticles. FORC and κ‐T results indicate an increase in effective particle sizes after 700°C‐heating. Our results are a warning against premature interpretation of a decreasing trend in κ‐T curves by maghemite inversion. Instead, fine particle behavior should be considered when a hump‐shaped κ‐T behavior is detected.

Thermal hysteretic behavior and negative magnetoresistance in the charge density wave material EuTe4

EuTe4 is a newly-discovered van der Waals material exhibiting a novel charge-density wave (CDW) with a large thermal hysteresis in the resistivity and CDW gap. In this work, we systematically study the electronic structure and transport properties of EuTe4 using high-resolution angle-resolved photoemission spectroscopy (ARPES), magnetoresistance measurements, and scanning tunneling microscopy (STM). We observe a CDW gap of about 200 meV at low temperatures that persists up to 400 K, suggesting that the CDW transition occurs at a much higher temperature. We observe a large thermal hysteretic behavior of the ARPES intensity near the Fermi level, consistent with the resistivity measurement. The hysteresis in the resistivity measurement does not change under a magnetic field up to 7 T, excluding the thermal magnetic hysteresis mechanism. Instead, the surface topography measured with STM shows surface domains with different CDW trimerization directions, which may be important for the thermal hysteretic behavior of EuTe4. Interestingly, we observe a large negative magnetoresistance at low temperatures that can be associated with the canting of magnetically ordered Eu spins. Our work shed light on the understanding of magnetic, transport, and electronic properties of EuTe4.

Zero-dimensional organic-inorganic hybrid manganese bromide with coexistence of dielectric-thermal double switches and efficient photoluminescence.

Zero-dimensional (0D) hybrid metal halides have attracted much attention due to their rich composition, excellent optical stability, large exciton binding energy, etc. Photoelectric switchable multifunctional materials can integrate multiple physical properties (e.g., ferroelectricity, photoluminescence, magnetic, etc.) into one device and are widely used in many fields such as smart switches, sensors, etc. However, multifunctional materials with thermal energy storage, stimulant dielectric response, and light-emitting properties are rarely reported. Here, we synthesized a new organic-inorganic hybrid metal halide single crystal [TEMA]2MnBr4 (1) (TEMA+ = triethylmethylammonium). Compound 1 undergoes a reversible phase transition at a high temperature of 344/316 K, having a large thermal hysteresis of 28 K and exhibits high stability dielectric switching characteristics near the phase transition temperature. The single crystal exhibits green emission at 513 nm under UV excitation, originating from the 4T1g(G) → 6A1g(S) transition of Mn2+ ions. Excitingly, this single crystal's photoluminescence quantum yield (PLQY) is as high as 80.78%. This work provides a strategy for the development of organic-inorganic hybrid optoelectronic multifunctional materials with high-efficient light emission and switchable dielectric properties.

Enhanced elastocaloric effect and specific adiabatic temperature variation in Ni-Mn-In-Si-Cu shape memory alloys

In this work, we demonstrate a giant elastocaloric effect in a Ni50Mn33In14Si1Cu2 alloy. Owing to the large transformation entropy change and low thermal hysteresis, giant adiabatic temperature change (ΔTad) of − 18.2 K is obtained on unloading from a maximum compressive strain of 6 % with a low stress of 145 MPa. Moreover, large specific adiabatic temperature variation (ΔTad/σmax) up to 126 K/GPa has been achieved. This ΔTad/σmax value significantly exceeds those in the typical shape memory alloys. The present results demonstrate that the target alloy exhibits a distinct advantage in the strength of elastocaloric effect, being crucial for practical applications of solid-state refrigeration. In addition, it is found that the cyclic stability can be effectively enhanced by one-step overstress superelastic training for the first cycle.

Ultra-low hysteresis in giant magnetocaloric Mn1-xVxFe0.95(P,Si,B) compounds

Large thermal hysteresis in the (Mn,Fe)2(P,Si) system hinders an efficient heat exchange and thus limits the magnetocaloric applications. Substitution of manganese by vanadium in the Mn1-x1Vx1Fe0.95P0.593Si0.33B0.077 and Mn1-x2Vx2Fe0.95P0.563Si0.36B0.077 compounds enable a significant reduction in the thermal hysteresis without losing the giant magnetocaloric effect. For the composition closest to the critical one, where first-order crossovers to second-order phase transition in the series of x2 = 0.02, Mn0.98V0.02Fe0.95P0.563Si0.36B0.077 exhibits a thermal hysteresis that is reduced from 1.5 to 0.5 K by 67%, yielding an adiabatic temperature change of 2.3 K and magnetic entropy change of 5.6 J/kgK for an applied field of 1 T, which demonstrates its potential for highly efficient magnetic heat pumps utilizing low-cost permanent magnets.

Large reversible magnetocaloric effect and magnetoresistance by improving crystallographic compatibility condition in Ni(Co)-Mn-Ti all- d -metal Heusler alloys

Recently, all-$d$-metal Ni(Co)-Mn-Ti Heusler systems have become the research hotspot due to their magnetoresponsive properties associated with a tunable first-order magnetostructural transformation (MST) and excellent mechanical stability for potential applications. However, the presence of large thermal hysteresis acts as an obstacle to the cyclic operation of this novel material. In this present paper, we investigate a large reversible magnetocaloric effect (MCE) and magnetoresistance (MR) in ${\mathrm{Ni}}_{37\ensuremath{-}x}{\mathrm{Co}}_{13+x}{\mathrm{Mn}}_{34.5}{\mathrm{Ti}}_{15.5}$ all-$d$-metal Heusler alloys that undergo a first-order MST accompanied by a large magnetization change between ferromagnetic austenite and antiferromagnetic martensite phases. Tuning the small at.% of Co doping in ${\mathrm{Ni}}_{37\ensuremath{-}x}{\mathrm{Co}}_{13+x}{\mathrm{Mn}}_{34.5}{\mathrm{Ti}}_{15.5}$ systems, we achieved an optimum composition with $x$ = 1 where low thermal hysteresis of $\ensuremath{\sim}4.7$ K, a narrow transformation interval of $\ensuremath{\sim}11.2$ K, and improved sensitivity of transformation temperature $\ensuremath{\sim}2.8$ K/T is observed. In addition, the origin of small hysteresis is studied based on geometric compatibility between cubic austenite and monoclinic martensite phases, calculated from the powder x-ray diffraction data. These optimized parameters lead to a reversible magnetic field-induced inverse martensitic transition under the field cycling, yielding a large reversible magnetic entropy change ($\mathrm{\ensuremath{\Delta}}{\mathrm{S}}_{M}$) of $\ensuremath{\sim}17.78$ J ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 277 K in a field change of 5 T. Moreover, a large reversible magnetoresistance (MR) of $\ensuremath{\sim}14%$ out of 32.6% is also obtained under 5-T magnetic field for $x$ = 1 sample in all-$d$-metal Heusler alloys. These reversible magnetoresponsive properties are comparable to other Ni-Mn-based Heusler alloys and have not been reported so far in the all-$d$-metal Heusler system. Therefore our present work demonstrates a pathway to design new cyclically stable multifunctional materials in all-$d$-metal Heusler systems for solid-state cooling devices and magnetic recording applications.

The Magnetic Phase Transition of FeRh Modulated by AC Magnetic Field

It is well known that CsCl‐type FeRh undergoes from antiferromagnetic (AFM) to ferromagnetic (FM) phase transition near room temperature with large thermal hysteresis. Therefore, it is particularly essential to find new methods to manipulate the magnetic order of FeRh and reduce thermal hysteresis. Herein, the magnetic phase transition of FeRh film by applying a low‐frequency in‐plane alternating current (AC) magnetic field is successfully manipulated. It is found that under the AC magnetic field, the phase transition of the FM–AFM is promoted, and the heating branch shifts at a rate of −8 K T−1, which causes the thermal hysteresis of FeRh reduction of about 2 K. These phenomena may be attributed to the Zeeman energy and the periodic vibration of domain walls. This work provides the possibility for the realization of low‐power spintronic devices.