Negative Magnetic Entropy Research Articles

Spin switching and magnetocaloric effect of high-entropy orthoferrite single crystal

The concept of high-entropy oxides/alloys was introduced into the rare-earth orthoferrite (RFeO3, R represents a rare-earth ion) system. Doping five different rare-earth ions with equal molar ratios at the R-site, a single crystal of high-entropy orthoferrite (Y0.2Sm0.2Eu0.2Gd0.2Er0.2)FeO3 (HERFO) was synthesized using optical floating zone method. At high temperatures, the weak ferromagnetic (wFM) moments induced by the distortion of the Fe sublattices align along the c axis, with a spin configuration of Γ4. As the temperature decreases, a spin reorientation transition (SRT) occurs from Γ4 to Γ2 due to the Fe3+-R3+ super-exchange interactions, with an immediate state Γ24. The wFM moments gradually rotate from the c axis to the a axis within the ac plane. In the zero-field-cooling (ZFC) mode, both the type-I and type-II spin switching (SSW) are observable along the a axis. Under low magnetic fields, the type-II SSW is magnetically controlled. The trigger temperature and variation in net moment of the type-II SSW exhibit systematic changes concerning the applied magnetic field. The rich magnetic phase transitions in HERFO make it a promising candidate for spintronic applications aimed at spin manipulation. Additionally, HERFO single crystal exhibits a large magnetocaloric effect (MCE) and magnetic entropy anisotropy, with the negative magnetic entropy change (‐ΔSm) along the c axis (14.45 J/(kg·K)) being greater than that along the a and b axes (9.57 J/(kg·K) and 10.32 J/(kg·K)). Consequently, HERFO holds great promise as a novel material for magnetic refrigeration applications aimed at achieving rotational magnetic cooling.

Metamagnetic phase transition induced large magnetocaloric effect in a Dy0.5Ho0.5MnO3 single crystal.

Magnetic refrigeration based on the magnetocaloric effect is gaining interest in orthogonal or hexagonal rare-earth manganite. However, a more comprehensive understanding of the underlying mechanism is still required. We grew a high-quality single crystal of Dy0.5Ho0.5MnO3 using the optical floating zone method, since the parent crystals DyMnO3 and HoMnO3 have orthogonal and hexagonal structures, respectively. The magnetic and magnetocaloric properties and refrigeration mechanisms are thoroughly investigated. Doping modifies the magnetism according to the results obtained from the investigation of magnetic and dielectric properties and heat capacity. The spin reorientation transition shifts towards low temperature in comparison to HoMnO3. Near the Néel temperature of rare-earth sublattices (5 K), the highest changes in negative magnetic entropy under 0-70 kOe are 18 J kg-1 K-1 and 13 J kg-1 K-1 along the a- and c-axes, respectively. The low-temperature metamagnetic phase transition caused by the alterations in the magnetic symmetry of Ho3+ contributes to an increased magnetocaloric effect in comparison to the parent crystals, rendering it a promising choice for magnetic refrigeration applications. Dy0.5Ho0.5MnO3 exhibits a clear magnetocrystalline anisotropy with enhanced refrigeration capacity and negative magnetic entropy change along the a-axis. The adiabatic temperature change of Dy0.5Ho0.5MnO3 is 8.5 K, larger than that of HoMnO3, rendering it a promising choice for low-temperature magnetic refrigeration applications.

Anisotropic Metamagnetic Spin Reorientation and Rotational Magnetocaloric Effect of Single Crystal NdAlGe.

Magnetic anisotropy strongly influences the performance of the magnetocaloric effect. We investigated the magnetocaloric properties of the NdAlGe single crystal with I41md structure. The temperature-dependent magnetization revealed significant anisotropic properties; stable antiferromagnetic transition at TN = 6 K for H//a and meta-magnetic spin reorientation at low temperature (T ≤ 5 K) within an intermediate field (H = 2 T) for H//c. During the metamagnetic spin reorientation, the abrupt change of the magnetic entropy leads to a significant magnetocaloric effect with negative magnetic entropy change (∆SM) by -13.80 J kg-1 K-1 at TC = 5.5 K for H = 5 T along the H//c axis. In addition, the antiferromagnetic state for H//a shows the inverse magnetocaloric effect(I-MCE) by positive entropy change ∆SM = 2.64 J kg-1 K-1 at TN = 6 K for H = 5 T. This giant MCE accompanied by the metamagnetic transition resulted in a significantly large relative cooling power (158 J/kg at H = 5 T) for H//c. The giant MCE and I-MCE can be applied to the rotational magnetocaloric effect (R-MCE) depending on the crystal orientations. NdAlGe exhibits rotational entropy change ∆Sc-a = -12.85 J kg-1 K at Tpeak = 7.5 K, H = 5 T. With comparison to conventional MCE materials, NdAlGe is suggested as promising candidate of R-MCE, which is a novel type of magnetic refrigeration system.

Magnetic, structural and magnetocaloric properties of Y0.9Gd0.1Fe2Hx hydrides

At 300 K, Y$_{0.9}$Gd$_{0.1}$Fe$_{2}$H$_{x}$ hydrides crystallize sequentially with increasing H concentration in various structures related to a lowering of the cubic MgCu$_{2}$ type structure of the parent alloy: cubic C1, monoclinic M1, cubic C2, monoclinic M2, cubic C3, orthorhombic O. Above 300 K, they undergo a first-order transition at a T$_{O-D}$ temperature driven by order-disorder of hydrogen atoms into interstitial sites. Their magnetic, structural and magnetocaloric properties have been investigated through magnetic measurements, and high-resolution synchrotron diffraction experiments. The magnetization at 5 K decreases slightly from 4 to 3.8 ${\mu}_{B}$ for x = 3 to 3.9 H f.u., then with a larger slope for higher H content. A discontinuous decrease of the magnetic transition temperature is observed: M1 and C2 hydrides are ferrimagnetic with T$_{C}$ near 300 K, M2 hydride displays a sharp ferromagnetic-antiferromagnetic transition at T$_{FM-AFM}$ =144 K, whereas C3 and O hydrides present only a sharp increase of the magnetization below 15 K and a weak magnetization up to RT. Negative magnetic entropy variations (${\Delta}$S$_{M}$) are measured near T$_{C}$ for the M1 and C2 phases, near T$_{FM-AFM}$ for the M2 phase, whereas positive ${\Delta}$S$_{M}$ peaks due to inverse MCE effect are found near T$_{O-D}$. A structural and magnetic phase diagram is proposed.

The Gd-Ni-Ga system at 870 ​K as a representative of rare-earth nickel gallides: Crystal structure and magnetic properties

The Gd-Ni-Ga system has been investigated at 870 ​K by X-ray and electron microprobe analyses. The known compounds, GdNi3Ga2 (YCo3Ga2-type), Gd3Ni6Ga2 (Ce3Ni6Si2-type), GdNiGa3 (BaNiSn3-type), GdNiGa4 (YNiAl4-type), GdNiGa2 (NdNiGa2-type), Gd2Ni2-2.3Ga1-0.7 (Mo2NiB2-type), GdNiGa (TiNiSi-type), GdNi0.5-0.66Ga1.5-1.34 (CeCu2-type), GdNi0.25Ga1.75 (CaIn2-type) and Gd26Ni9-6.5Ga8-10.5 (Sm26Co11Ga6-type), have been confirmed in this system. New ternaries, ∼GdNi3Ga5 (unknown type), Gd2Ni12.7-9Ga4.3-8 (Th2Zn17-type, R-3m, N 166–2, hP57), GdNi2.2-2Ga2.8-3 (CaCu5-type, P6/mmm, N 191, hP6), Gd2Ni3Ga5 (novel Gd2Ni3Ga5-type, Pnnn, N 48, oP80) and Gd12Ni4.85-5Ga3.15-3 (Er12Ni5Ga3-type, I4/mcm, N 140, tI80), were detected in the Gd-Ni-Ga system. Quasibinary solid solutions were detected for Th2Ni17-type Gd2Ni17, CaCu5-type GdNi5, Ce2Ni7-type Gd2Ni7, CeNi3-type GdNi3, MgCu2-type GdNi2, CrB-type GdNi, AlB2-type GdGa2, Gd3Ga2-type Gd3Ga2 and Cr5B3-type Gd5Ga3 while no appreciable solubility was observed for other binary compounds.Also, new Th2Zn17-type {Nd, Dy, Ho}2Ni9Ga8, Sm2Ni8.5Ga8.5, Tb2Ni9.5-9Ga7.5-8 and Er12Ni5Ga3-type Tb12Ni5Ga3 compounds were synthesized as prolongation of isostructural rows.Gd2Ni9Ga8 is paramagnetic down to 2 ​K GdNiGa2, GdNiGa3 and Gd2Ni3Ga5 are antiferromagnets with the Néel temperatures (TN) of 22 ​K, 14 ​K and 28 ​K, respectively. GdNiGa2 and Gd2Ni3Ga5 show negative and positive magnetic entropy change around TN. GdNi4Ga is a soft ferromagnet with Curie temperature (TC) of 22 ​K and magnetic entropy change of −10.9 ​J/kg⋅K for a field change of 50 ​kOe at TC. Gd12Ni5Ga3 shows ferromagnetic ordering below TC ​= ​140 ​K.

Magnetism and transport behavior of Ni42Co8Mn38Sb12: Magnetization, electrical resistivity and Hall effect measurements

Magnetic and transport behavior of Ni42Co8Mn38Sb12 alloy is discussed using the combined results of magnetization, electrical resistivity and Hall effect measurements. The alloy undergoes a ferromagnetic transition at 364 K, followed by a martensitic transition at 110 K. Large positive Curie-Weiss temperature, soft-ferromagnetic behavior with negative magnetic entropy change below 50 K and a well-scaled normalized magnetic-entropy onto a single universal curve confirm the predominant ferromagnetic exchange interactions in the alloy. Normal Hall coefficients were found to be nearly constant in ferromagnetic region in the temperature range of 120–300 K. The electrical transport properties are largely governed by electrons with a carrier concentration of about 1023/cc. Quadratic dependence of anomalous Hall resistivity (ρxy) on longitudinal resistivity (ρxx) indicates the dominant intrinsic contribution. In a magnetic field of 50 kOe, magnetoresistance is found to be about 34% near the martensitic transition temperature while it is about 2.8% near the second-order transition.

Magnetocaloric effect manipulated through interchain exchange coupling in nanochain arrays

Magnetocaloric effect in nanochain arrays is numerically studied when interchain exchange couplings (Jinter) are taken into account. With increasing Jinter, moment-reorientation phase transition temperature driven by anisotropy is enhanced and magnetic ordering phase transitions governed by Jinter may happen independently at higher temperatures, resulting in temperature induced multiple phase transitions that separate distinct dynamic properties of magnetization. As a result, maximum values of positive and negative magnetic entropy change (ΔSM) are both close to 0.4 J kg−1 K−1, with their peak temperature 100–300 K and 250–550 K, depending on Jinter, and a large refrigeration capacity is obtained due to a wide working temperature range of ΔSM. In nanostructures, the proper magnetic viscosity arising from Jinter makes materials become semi-bulk, i.e., in which ΔSM maximum value and peak temperature are highly enhanced, associated with a considerably wide working temperature range.

Large rotational magnetocaloric effect in GdVO4 single crystal

The magnetic properties and magnetocaloric effect (MCE) in GdVO4 single crystal have been investigated systematically. In addition to the previously reported spin-flop transition at 1 T along the c-axis, other spin-flop transitions also occur along the a and b axes at 1.7 T. Meanwhile, prominent magnetic anisotropy is observed below the paramagnetic temperature, which leads to evident distinction of negative magnetic entropies along the a, b, and c axes. The maximum negative magnetic entropy −ΔSM along the three axes are 45.91, 46.17, and 56.03 J/kg × K, respectively, at H = 7 T. The anisotropic MCE between the c and a, b axes also induces large rotational MCE (RMCE) in the bc plane, with the maximum value −ΔSR,bc reaching 10.1 J/kg × K at H = 7 T. These remarkable MCE and RMCE values at low temperatures make the GdVO4 single crystal a potential candidate for applications in cryogenic magnetic refrigeration areas.

Critical behavior, universality class and magneto-transport properties of Ni2MnIn

Structural, magnetic, transport and critical properties of Ni2MnIn are investigated using the combined results of x-ray diffraction, magnetization and electrical resistivity. Ni2MnIn crystallizes in cubic structure with Fm-3m space group. The alloy is metallic and shows Fermi liquid behavior. Critical exponents are extracted using the modified Arrott’s plot (MAP), Kouvel-Fisher (KF) and magnetocaloric effect (MCE) methods and are found to be mutually consistent. The alloy belongs to the three-dimensional (3D)-Heisenberg universality class with short range magnetic interactions, as inferred from the exchange interaction J (r) ∼ r−4.91. The suppression of spin-fluctuations by the applied magnetic field results in negative magnetoresistance (MR) and negative magnetic entropy change. Collapse of normalized magnetic entropy curves and the normalized magnetoresistance curves onto a single universal curve confirms the second-order nature of the magnetic phase transition.

Low-temperature coercivity of Mo2NiB2-type Tb2Co2Ga and Tb2Co2Al-based solid solutions

The magnetic ordering of polycrystalline Mo2NiB2-type Tb2Co2Ga and Tb2Co2Al-based Sm2-xTbxCo2Al, Tb2Co2Al1-xGax and Tb2Co1.8{Ni, Cu}0.2Al solid solutions has been established using bulk magnetic measurements. These compounds undergo a high-temperature ferromagnetic ordering and field sensitive transformation of magnetic ordering at low temperatures. The high-temperature ferromagnetic ordering is followed via negative magnetic entropy change and soft magnet properties, whereas low-temperature transformation of magnetic ordering indicates positive magnetic entropy change and excellent permanent magnet properties.

Negative Magnetic Entropy Change and Critical Behavior of Manganite La0.8Sr0.2Mn1−xCoxO3 (x = 0, 0.2)

La0.8Sr0.2Mn1−xCoxO3 (x = 0, 0.2) polycrystalline samples were prepared by solid-state reaction, and their structural, Griffiths phase, magnetic entropy change, critical behavior, and electrical transport properties were systematically investigated. The results show that all polycrystalline samples are rhombohedral symmetry structures; the Griffiths phase exists above the low-temperature magnetic transition temperature (TC2) of the two samples; the magnetic field is applied to the La0.8Sr0.2Mn1−xCoxO3 (x = 0, 0.2) samples. The maximum magnetic entropy change ΔSmax for 7 T is − 2.28 and − 2.36 J/(kg K), respectively. The doping of Co makes ΔSmax increase, and an obvious negative entropy change occurs at low temperature and low field; the critical behavior of La0.8Sr0.2MnO3 (LSMO) fits best with the mean field model, and the critical behavior of the sample after doping with the 3D Heisenberg model fits best; LSMO is a semiconductor material, and the metal insulator transition appears near the low-temperature magnetic transition temperature (TC2) when the Co element doping amount reaches 0.2. The conductivity of the two samples in the high-temperature region satisfies the small polaron model.

Influence of Annealing Conditions on Magnetic Properties, Magnetocaloric Effect, and Critical Parameters of Ni–Mn–Sn Ribbons

In this paper, we investigated the influence of annealing conditions on magnetic properties, magnetocaloric effect, and critical parameters of Ni50Mn50− x Sn x ( $x = 13$ , 13.5, and 14) alloy ribbons prepared by using the melt-spinning method. The ribbons were annealed at 1123 K for various times. A martensitic–austenitic (M–A) structural phase transformation was observed on both the as-quenched and annealed samples. Temperature of the M–A phase transition ( $T_{M-A}$ ) of the ribbons can be regulated in room temperature region by changing the annealing time. Maximum positive and negative magnetic entropy changes, $\vert \Delta S_{m}\vert _{\mathrm {max}}$ , larger than 3 and 1 $\text {J}\cdot \text {kg}^{-1}\cdot \text {K}^{-1}$ , respectively, were achieved on the Ni50Mn37Sn13 sample after annealing for 0.5 h. Critical parameters were determined to elucidate magnetic orders in the alloy. The obtained parameters are very close to those of the mean field theory of long-range ferromagnetic orders.

Thickness as a control parameter for magnetocaloric effect in Cr75−xFe25+x (x = 0, 5) thin films

The results of a detailed investigation of the magnetocaloric effect (negative isothermal magnetic entropy change, -ΔSM(T,H)) in Cr70Fe30 and Cr75Fe25 thin films are presented. The generalized magnetic scaling equation of state in nonlinear scaling variables, which makes use of the previously reported critical exponents, not only reproduces the observed -ΔSM(T) at constant magnetic fields (1kOe⩽H⩽70kOe) over a wide temperature range around the ferromagnetic-paramagnetic phase transition temperature, T=Tc, but also describes correctly the functional dependence of -ΔSM on H at T=Tc. Reasonably large relative cooling power (RCP) and magnetic refrigerant capacity (RC), primarily due to the unusually large (130 K–180 K) full-width at half-maximum (FWHM) of the -ΔSM(T)|H curve, is observed for the magnetic field change ranging between 20 kOe and 70 kOe. This work clearly bears out that the film thickness can be used as a control parameter to tune both the peak value of -ΔSM as well as the FWHM, and hence RCP and RC. Another important result is the observation of giant isothermal magnetic entropy change at T = 2 K within the reentrant regime (where long-range ferromagnetic order coexists with cluster spin glass order) when the film thickness is reduced to ≃20nm.

Magnetic transition and magnetocaloric effect in Nd6Fe13Pd compound

The crystal structure, magnetic transitional behaviors and magnetocaloric effect were investigated in Nd6Fe13Pd compound. The results show that Nd6Fe13Pd crystallizes in the tetragonal Nd6Fe13Si-type structure (space group I4/mcm) with the lattice parameters a = b = 8.0765 Å and c = 22.3747 Å and two magnetic transitions from the ferromagnetic to the antiferromagnetic states with Ttr = 173 K and from the antiferromagnetic to the paramagnetic states with the Néel temperature of 369 K occurred in the Nd6Fe13Pd compound. The magnetic transition between the ferromagnetic and the antiferromagnetic states can be induced by either changing temperature or applying a magnetic field with very small thermal and magnetic hystereses and the transition temperature is very sensitive to the applied field. The experimental results indicate a broad working temperature span and negative magnetic entropy changes around Ttr = 173 K. For a magnetic field change of 2 T, the maximum magnetic entropy change −ΔSM reaches to 4.4 J kg−1 K−1 with the corresponding refrigeration capability of about 125 J kg−1.

Structure and cryogenic magnetic properties in Ho2BaCuO5 cuprate

Here we report on a detailed investigation of the crystal structure, cryogenic magnetic and magneto-caloric properties in Ho2BaCuO5 cuprate. The Ho2BaCuO5 crystallized in the orthorhombic crystal structure with Pbnm space group and undergoes a paramagnetic (PM) to antiferromagnetic (AFM) transition at the Néel temperature (TN) of 12.5K. An obvious signal change in the magneto-caloric effect (MCE) was observed, which is correlated with the fact of the AFM ground state inducing a small negative magnetic entropy change (-ΔSM) at/below TN and a field-induced metamagnetic transition (first ordered) from AFM to ferromagnetic (FM) producing a large positive -ΔSM above TN. For a magnetic field change of 0–7T, the -ΔSMmax ≈ 10.4J/kgK, RCP ≈ 263J/kg and RC ≈ 199J/kg for the maximum magnetic entropy change, relative cooling power and refrigerant capacity are evaluated, respectively.

Exploring the Inverse Magnetocaloric Effect in Discrete MnII Dimers

The inverse magnetocaloric effect (IMCE) in molecular solids is explored for two antiferromagnetically coupled MnII dinuclear complexes. Magnetic studies reveal that both of them demonstrate the IMCE with negative magnetic entropy changes (−ΔSM = −3.5 J kg–1 K–1), which are in line with an analytic function derived from the quantized phenomenological model proposed in this work.

Effects of solidification rate and excessive Fe on phase formation and magnetoclaoric properties of LaFe 11.6x Si 1.4

The effects of solidification rate and excessive Fe on phase formation and magnetocaloric properties of LaFe11.6xSi1.4 (x=1.1, 1.2) were investigated by XRD, SEM and VSM measurements. The XRD results show that the amount of LaFeSi phase in the as-cast melt-spun ribbons prepared by a copper wheel at a speed of 10 m/s is less than that in the as-cast arc melting buttons with the same x values. The annealed melt-spun ribbons contain smaller amount of La(Fe, Si)13 (1:13) phase than the corresponding annealed arc melting buttons. Although the melt-spun sample has finer crystalline grains of α-Fe, as indicated by SEM analysis, its crystalline size has not reached nano-scale. Therefore, the magnetic exchange-coupling between 1:13 phase and α-Fe phase has not been observed in melt-spun ribbons. Further, the maximum negative magnetic entropy change (-SMax) and relative cooling power (RCP) of annealed melt-spun ribbons under a field change of 0-2 T are weaker than those of the corresponding annealed arc melting buttons.

The structural, magnetic and magnetic entropy changes on CoFe2O4/CoFe2 composites for magnetic refrigeration application

In this article, the magnetocaloric effect on thermally reduced CoFe2O4/CoFe2 composite was reported. Co-precipitated CoFe2O4 nanoparticles were thermally reduced at 500 and 800°C for 2h under H2 atmosphere. The reduction process resulted in different fractions of soft and hard (CoFe2O4/CoFe2) magnetic composites and the products were characterized through various procedures. The structure of CoFe2O4/CoFe2 composites was found to be cubic with an average crystallite size of 31 and 30nm for the samples reduced at 500 and 800°C, respectively. The stoichiometry of the composites confirmed that the percentage of soft CoFe2 magnetic phase was increased to 87% with temperature up to 800°C for 2h, besides showed accumulation of nanoparticles. The room temperature saturation magnetization of the CoFe2O4/CoFe2 composites increases from 184 to 190emu/g as a consequence of high concentration of CoFe2 phase. Conversely, the sample reduced at 800°C attained the maximum magnetization of 200 and 221emu/g at 300 and 5K, respectively. Field cooled (FC) and zero field cooled magnetization (ZFC) and the isothermal magnetic entropy change (ΔSm) on the CoFe2O4/CoFe2 composite (with 87% of soft CoFe2 magnetic phase) were estimated. The composite showed the interesting positive and negative magnetic entropy change (ΔSm) of 0.923 and −0.923J/kg.K−1 near room temperature at around 310 and 290K, respectively.

Effects of the partial substitution of Ni by Cr on the transport, magnetic, and magnetocaloric properties of Ni50Mn37In13

The structural, magnetic, and magnetotransport properties of Ni50-xCrxMn37In13 Heusler alloys have been synthesized and investigated by x-ray diffraction (XRD), field and pressure dependent magnetization, and electrical resistivity measurements. The partial substitution of Ni by Cr in Ni50Mn37In13 significantly improves the magnetocaloric effect in the vicinity of the martensitic transition (TM). This system also shows a large negative entropy change at the Curie temperature (TC), making it a candidate material for application in a refrigeration cycle that exploits both positive and negative magnetic entropy changes. The refrigeration capacity (RC) values at TM and TC increase significantly by more than 20 % with Cr substitution. The application of hydrostatic pressure increases the temperature stability of the martensitic phase in Ni45Cr5Mn37In13. The influence of Cr substitution on the transport properties of Ni48Cr2Mn37In13 is discussed. An asymmetric magnetoresistance, i.e., a spin-valve-like behavior, has been observed near TM for Ni48Cr2Mn37In13.

Effect of Fe substitution on charge order state and magnetocaloric effect in Pr0.5Sr0.5Mn1−xFexO3 (0.00 ≤ x ≤ 0.08) manganites

The charge order state and magnetocaloric effect have been studied in Pr0.5Sr0.5Mn1−xFexO3 (0.00 ≤ x ≤ 0.08) prepared by conventional solid-state reaction. Charge order state is weak in parent Pr0.5Sr0.5MnO3 and absent with increasing doping level. The long range charge order antiferromagnetic state (CO/AFM) reemerges for x = 0.06 and subsequently weak accompanied with rapid collapse in double exchange ferromagnetic order for x = 0.08. Magnetic measurements indicate that Fe does not participate in the double exchange interaction, the Curie temperature (TC) is systematically depressed with increasing Fe content from 265 K (x = 0.00) down to 185 K (x = 0.06). Negative magnetic entropy change (ΔSMmax) of −0.96 J/kg K and large positive value of 2.57 J/kg K in Pr0.5Sr0.5Mn0.94Fe0.06O3 are achieved when it undergoes paramagnetic to ferromagnetic (PM-FM) and ferromagnetic to antiferromagnetic (FM-AFM) magnetic phase transition in a magnetic change of 15.0 kOe. Though the materials with considerable magnetic hysteresis in FM-CO/AFM because of first order magnetic phase transition, it gives a strategy to obtain change order state in manganites.