First-order Magnetostructural Transformation Research Articles

Manipulation of thermal hysteresis and magnetocaloric effect in the Ni-Co-Mn-In alloys through lattice contraction: Effect of Ge substitution for In

First-order magnetostructural transformation can be utilized to generate giant magnetocaloric effect for the magnetic refrigeration applications, but the intrinsic hysteresis always cripples the refrigeration efficiency. In this work, the strategy of lattice contraction was exploited to manipulate the thermal hysteresis and magnetocaloric response in the Ni-Co-Mn-In magnetocaloric alloys. Such strategy was effectuated by using Ge to replace In on account of the relatively smaller atomic radius of Ge, yielding a significant reduction in the thermal hysteresis. In a Ni46Co3Mn37In10Ge4 alloy with the combination of low thermal hysteresis of ∼4 K and large magnetization difference of 83.1 Am2kg−1 associated with the magnetostructural transformation, giant effective refrigeration capacity RCeff of 311 Jkg−1 and reversible entropy change ΔSM up to 22.8 Jkg−1K−1 were demonstrated by varying the field of 5 T. Besides, a large reversible adiabatic temperature change ΔTad of –3.0 K was also obtained by using a low field change of 1.5 T. Based on the displacement gradient tensor determined by the orientation relationship governing the martensitic transformation, it was presented that Ge substation for In effectively decreased the lattice misfit, thus lowering the elastic energy accompanying the structural transformation and the related transformation hysteresis.

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.

Anomalous electrical transport behavior in the vicinity of the first-order magnetostructural transition in the giant magnetocaloric Gd4ScGe4

Magnetic, specific heat, and electrical transport measurements of ${\mathrm{Gd}}_{4}{\mathrm{ScGe}}_{4}$ single crystals reveal sharp, discontinuous, nearly anhysteretic first-order magnetostructural transformation at ${T}_{\mathrm{C}}=63\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The electrical resistivity exhibits two distinct regions where it increases with decreasing temperature: between ${T}_{\mathrm{C}}$ and 120 K, as well as below 3 K; electronic transport remains conventionally metallic at all other measured temperatures, up to 325 K. The dispersion of charge carriers due to electron-paramagnon scattering is the likely reason for the observed anomalous transport above ${T}_{\mathrm{C}}$. Additionally, the existence of intermediate lattice states near the transition recognized by the spike in the interslab Ge-Ge distances is expected to reduce the mean free path of the electrons contributing to the unusual behavior of the electrical resistivity between ${T}_{\mathrm{C}}$ and 120 K. Beyond conventional electronic and lattice terms, the third component of likely magnetic origin contributes to the low-temperature heat capacity; the presence of spin waves may be responsible for the increased electron-magnon scattering below 3 K. Minor magnetocrystalline anisotropy is observed with the $b$ axis as the easy magnetization axis in ${\mathrm{Gd}}_{4}{\mathrm{ScGe}}_{4}$. A negative deviation from linearity in the temperature dependence of the inverse magnetic susceptibility is detected below 150 K.

Unusual field-induced spin reorientation in FeCr2S4 : Field tuning of the Jahn-Teller state

The multiferroic spinel ${\mathrm{FeCr}}_{2}{\mathrm{S}}_{4}$ is a benchmark material for exploring the competition of spin-orbit (SO) and Jahn-Teller (JT) coupling. Our magnetic and thermodynamic studies of stoichiometric single-crystalline samples evidence a magnetic-field-induced spin-reorientation transition in the cooperative JT state below 10 K. At 2 K, at a critical magnetic field of 4.5 T, the magnetization measured along the hard magnetization axis $\ensuremath{\langle}111\ensuremath{\rangle}$ manifests a jump to the fully saturated state accompanied by a steplike decrease of the sound velocity and an abrupt increase of the magnetostriction. All these quantities reveal a hysteretic behavior pointing towards a first-order magnetostructural transformation. Below the JT transition, the specific heat shows a complex behavior upon the application of magnetic fields depending on the crystallographic directions. The observed reduction by 20% of the magnetic anisotropy below the JT transition is attributed to the competition of the SO and JT interactions tuned by external magnetic fields. The concomitant change of the structural symmetry results in a change of the splitting of the lowest levels of the ${}^{5}E$ doublet of the tetrahedrally coordinated ${\mathrm{Fe}}^{2+}$ ions.

Managing hysteresis of Gd5Si2Ge2 by magnetic field cycling

The influence of magnetic field cycling through the first-order magnetostructural transformation on the magnetic and magnetocaloric properties, as well as hysteresis of polycrystalline Gd5Si2Ge2, has been studied using magnetometry. The cycling has a minimal effect on the magnetic field-induced entropy change and the phase transformation temperature of the material. On the other hand, magnetic hysteresis decreases by 30% after approximately ten cycles and remains low unless the sample is moved far into the paramagnetic regime. Factors playing a role in the history dependence of hysteresis have been discussed.

Influence of Cr and Fe on magnetic phase transition and magnetocaloric effect in Co2(Cr1-xFex)Al, (x ≤ 0 ≤ 1) soft magnetic Heusler alloys

ABSTRACTThe structural measurement indicates that the system possesses A2 to B2 phase transformation at the extent of A2 phase at room temperature. The present system shows first order magneto-structural transformation (FOMST). The substitution of Cr by Fe causes a vital role for an increase in magnetocaloric properties. The change in magnetic entropy (ΔSM) and relative cooling power (RCP) are evaluated under an applied field of 20 KOe that shows drastic changes near the blocking temperature under the specific temperature regime. In the vicinity of these observed properties, critical exponent parameters such as, β, ϒ and δ were also observed using field dependence magnetic entropy change.

Linear dependence of magnetocaloric effect on magnetic field in Mn0.6Fe0.4NiSi0.5Ge0.5 and Ni50Mn34Co2Sn14 with first-order magnetostructural transformation

The study on the field dependence of magnetocaloric effect (MCE) is considered to be of fundamental and practical importance, since it not only guides us in understanding and optimizing the MCE, but also helps us estimate the MCE for higher magnetic field which is not available in some laboratories. The magnetic field (0H) dependence of magnetic entropy change (△SM) has been studied extensively in many materials with second-order magnetic transition. However, the field dependence of MCE for first-order magnetic transition (FOMT) materials has not been sufficiently studied due to their complexity and diversity. In the present work, polycrystalline Mn0.6Fe0.4NiSi0.5Ge0.5, Ni50Mn34Co2Sn14, and LaFe11.7Si1.3 compounds with FOMT are prepared, and the magnetic and magnetocaloric properties are investigated systematically. In order to avoid a spurious △SM, the M-0H curves are measured in a loop process. The M-0H curves are corrected by taking into account the demagnetization effect, i.e. Hint=Hext-NdM. It is found that the -△SM follows a linear relationship -△SM=-△S0 +0H with the variation of magnetic field in Mn0.6Fe0.4NiSi0.5Ge0.5 compound when 0H 1 T. In addition, it is also noted that the △SM is approximately proportional to the square of 0H at low field. The origin of this linear relationship between △SM and 0H at high field and the deviation at low field are discussed by numerically analyzing the Maxwell relation. In addition to the △SM peak value, it is found that other △SM values at different temperatures also follow the linear relation at high field by performing the same numerical analysis. Moreover, it is found that the fitted △SM curve matches the experimental data very well. This result indicates that the linear relationship between △SM and 0H could be utilized to predict the △SM for higher magnetic field change when the field is lower than the saturation field. The applicability of this linear relationship is also verified in other systems with first-order magnetostructural transformation, such as Ni50Mn34Co2Sn14. However, it fails to describe the field dependence of △SM in LaFe11.7Si1.3, which exhibits a strong field dependence of transition temperature. Consequently, our study reveals that a linear dependence of △SM on 0H could occur in magnetostructural transition materials, which show the field independence of transition temperature.

Magnetostructural transformation and large magnetocaloric effect in Mn0.9Cu0.1CoGe1−xSix alloys

The structural transition temperature Tt increases largely with the introduction of Si in Mn0.9Cu0.1CoGe1-xSix alloys due to the enhancement of degree of hexagonal distortion. Thus, the first-order magnetostructural transformation from ferromagnetic orthorhombic to paramagnetic hexagonal phase can be obtained in the range of 0.15 < x < 0.21. Large magnetocaloric effect (MCE) under low magnetic field change of 2 T is obtained due to the magnetostructural transformation, e.g., the maximum magnetic entropy change (-ΔSM) value for x = 0.16 is 10.3 J/kg K at 297 K, which is comparable to or even larger than those of some typical room-temperature magnetocaloric materials. The nature of magnetostructural transition has been studied by different methods, and it is found that the Arrott plots fail to determine the order of phase transition. In contrast, the universal curve of ΔSM is proved to be a more effective criterion to distinguish the order of phase transition. Besides, a linear relationship between -ΔSM and Δμ0H is found, and so the -ΔSM values for higher field changes can be estimated by linear fitting. Consequently, large MCE induced by magnetostructural transformation suggests that Mn0.9Cu0.1CoGe1-xSix alloys could be promising candidates for room-temperature magnetocaloric materials.

Enhancing Magnetic Functionality with Scandium: Breaking Stereotypes in the Design of Rare Earth Materials

Replacement of strongly magnetic gadolinium with weakly magnetic scandium unexpectedly enhances ferromagnetic interactions in (Gd1–xScx)5Ge4. Based upon this counterintuitive experimental finding we demonstrate the unique role 3d1 electrons of scandium atoms play in mediating magnetic interactions between the gadolinium atoms from the neighboring layers in the Sm5Ge4-type crystal lattice. Scandium substitutions at and below 20% rapidly increase the Curie temperature, TC, of the Gd5Ge4 parent, eliminate both the kinetic arrest and hysteresis, and drastically improve reversibility of the first-order magnetostructural transformation at TC. In agreement with first-principles predictions, higher than 20% Sc leads to the formation of a closely related Pu5Rh4-type structure where the first-order magnetostructural transformation is replaced by a conventional second-order ferromagnetic ordering that remains accompanied by a continuous rearrangement of the crystal lattice. Comparison of two materials with similar s...

Magnetocaloric and negative thermal expansion effects for Ni55.5Mn19.5Ga25 Heusler alloy with magneto-structural transition

In the present work, the crystal structure, transforming property, magnetocaloric and negative thermal expansion effects were investigated in polycrystalline Ni55.5Mn19.5Ga25 alloy, respectively. The structure measurement indicates that it possesses unmodulated martensitic structure (L10) at room temperature. Here, the present alloy shows a first-order magneto-structural transformation from paramagnetic (PM) austenite to ferromagnetic (FM) martensite on cooling. Benefiting from this transformation, a considerable refrigerant capacity (RC) with a value of about 55 J/kg K was obtained around room temperature for the magnetic field change of 3 T. Moreover, the unique negative thermal expansion (NTE) effect was observed not only for the transforming range, but also for the pure martensitic state of the studied alloy. As distinct from a large NTE effect caused by magneto-structural transformation, such an effect at martensite is not accompanied by any thermal hysteresis, performing an excellently linear feature with a coefficient over 8 × 10−6/K within a broad temperature range (from 250 K to 50 K).

Temperature and Field Induced Strain Measurements in Single Crystal Gd5Si2Ge2

The first-order magneto-structural transformation that occurs in Gd5Si2Ge2 near room temperature makes it a strong candidate for many energy harvesting applications. Understanding the single crystal properties is crucial for allowing simulations of device performance. In this study, magnetically and thermally induced transformation strains were measured in a single crystal of Gd5Si2.05Ge1.95 as it transforms from a high-temperature monoclinic paramagnet to a lower-temperature orthorhombic ferromagnet. Thermally induced transformation strains of −8500 ppm, +960 ppm and +1800 ppm, and magnetically induced transformation strains of −8500 ppm, +900 ppm and +2300 ppm were measured along the a, b and c axes, respectively. Using experimental data coupled with general thermodynamic considerations, a universal phase diagram was constructed showing the transition from the monoclinic to the orthorhombic phase as a function of temperature and magnetic field.

Large reversible magnetocaloric effect in a Ni-Co-Mn-In magnetic shape memory alloy

Reversibility of the magnetocaloric effect in materials with first-order magnetostructural transformation is of vital significance for practical magnetic refrigeration applications. Here, we report a large reversible magnetocaloric effect in a Ni49.8Co1.2Mn33.5In15.5 magnetic shape memory alloy. A large reversible magnetic entropy change of 14.6 J/(kg K) and a broad operating temperature window of 18 K under 5 T were simultaneously achieved, correlated with the low thermal hysteresis (∼8 K) and large magnetic-field-induced shift of transformation temperatures (4.9 K/T) that lead to a narrow magnetic hysteresis (1.1 T) and small average magnetic hysteresis loss (48.4 J/kg under 5 T) as well. Furthermore, a large reversible effective refrigeration capacity (76.6 J/kg under 5 T) was obtained, as a result of the large reversible magnetic entropy change, broad operating temperature window, and small magnetic hysteresis loss. The large reversible magnetic entropy change and large reversible effective refrigeration capacity are important for improving the magnetocaloric performance, and the small magnetic hysteresis loss is beneficial to reducing energy dissipation during magnetic field cycle in potential applications.

Magnetostructural transformation and magnetocaloric effect in melt-spun and annealed Mn1−xCuxCoGe ribbons

The melt-spun and annealed Mn1−xCuxCoGe ribbons are prepared. The magnetic and magnetocaloric properties of these ribbons was investigated by magnetic, calorimetric and structural measurements. In contrast with the melt-spun ribbons, a coupled magnetostructural transformation between the ferromagnetic TiNiSi-type and paramagnetic Ni2In-type state is achieved in the annealed Mn1−xCuxCoGe (x=0.02, 0.04) ribbons. Accordingly, the large enhancement of magnetic entropy change peak values from the melt-spun to annealed ribbons is observed. The origins of the first-order magnetostructural transformation in annealed ribbons are discussed.

First-order magnetostructural transformation in Fe doped Mn–Co–Ge alloys

• We have achieved FOMT by substituting Mn by Fe atoms in off-stoichiometric MnCoGe. • A giant MCE (△ S m = −9.74 J kg −1 K −1 at △ H = 0–5 T for Mn 0.89 Fe 0.11 CoGe) was obtained. • Large lattice strain up to ∼12% accompanied by the transformation was observed. Substitution of Fe for Mn atoms in Mn–Co–Ge alloys was employed for tailoring concurrent structural and magnetic phase transitions. First-order magnetostructural transition from paramagnetic hexagonal Ni 2 In-type parent phase to ferromagnetic orthorhombic TiNiSi-type martensite was achieved. A giant magnetocaloric effect of △ S m = −9.74 J kg −1 K −1 was obtained under magnetic field change of 5 T in the vicinity of phase transition temperature. This result indicates that Fe doped Mn–Co–Ge alloys are ideal candidates for magnetic refrigeration applications. Furthermore, the magnetic field-induced martensitic transformation, associated with a large lattice strain of ∼12%, was directly evidenced by in situ synchrotron high-energy X-ray diffraction.

The magnetostructural transformation and magnetocaloric effect in Co-doped MnNiGe1.05 alloys

A series of MnNi1−xCoxGe1.05 (x = 0, 0.03, 0.05, 0.07, 0.09, and 0.11) alloys were prepared by the arc-melting method. With increasing content of Co, a first-order magnetostructural transformation between the antiferromagnetic TiNiSi-type phase and the ferromagnetic Ni2In-type phase was observed. A magnetic and crystallographic phase diagram for MnNi1−xCoxGe1.05 alloys was proposed in this paper. Owing to the abrupt and large jump of magnetization around the magnetostructural transformation, MnNi1−xCoxGe1.05 (x = 0.07, 0.09, 0.11) alloys show large and positive magnetic entropy changes at relatively low field.

Temperature and magnetic field-dependent x-ray powder diffraction study of dysprosium

An x-ray powder diffraction study of polycrystalline Dy was performed over the temperature interval from 5 to 300 K in applied magnetic fields up to 40 kOe. A complex first-order magnetostructural transformation, which occurs below {approx}8 kOe and above {approx}20 kOe, is decoupled into a first-order magnetic (helix-fan) transition and a second-order magnetostructural (hexagonal-orthorhombic) transition between {approx}135 and {approx}175 K in magnetic fields between {approx}8 and {approx}20 kOe. The transition to the ferromagnetic (FM) state is always accompanied by an orthorhombic distortion of the hexagonal lattice, including a transition from the paramagnetic (PM) to the FM state in magnetic fields exceeding 20 kOe.