Second-order Magnetic Phase Transition Research Articles

Magnetic properties and X-ray absorption spectra of La0.7Ca0.3MnO3 nanoparticles

The magnetic properties and structural characterization of La0.7Ca0.3MnO3 nanoparticles (NPs) prepared by mechanical milling were investigated. By changing the milling time (tm) from 1 to 24 h, we obtained NPs with average crystallite sizes of 10–40 nm. The magnetic studies reveal that the Curie temperature (TC), magnetization, and maximum magnetic-entropy change of NPs decrease rapidly with increasing tm. Concurrently, there is a transformation of the first-order to second-order magnetic phase transition. The structural analyses based on X-ray absorption (XAS) spectroscopy prove the tm increase leading to a slight shift of the Mn K-shell absorption edge towards lower energies of Mn3+. It means more Mn3+ ions introduced to NPs as increasing tm, making the concentration ratio of Mn3+/Mn4+ in NPs greater than 7/3 if comparing to the as-prepared sample with tm = 0 h. Particularly, in the first region of extended XAS fine structure spectra, there is a shift of a hump towards higher energies. This is different from the situation observed in the Mn K-edge absorption, and thus assigned to surface-related effects, where a high density of lattice defects and/or distortions is present. Such the circumstances lead to ferromagnetic interactions in NPs become weaker than that in the as-prepared sample.

Anomalous superfluid density in quantum critical superconductors.

When a second-order magnetic phase transition is tuned to zero temperature by a nonthermal parameter, quantum fluctuations are critically enhanced, often leading to the emergence of unconventional superconductivity. In these "quantum critical" superconductors it has been widely reported that the normal-state properties above the superconducting transition temperature T(c) often exhibit anomalous non-Fermi liquid behaviors and enhanced electron correlations. However, the effect of these strong critical fluctuations on the superconducting condensate below T(c) is less well established. Here we report measurements of the magnetic penetration depth in heavy-fermion, iron-pnictide, and organic superconductors located close to antiferromagnetic quantum critical points, showing that the superfluid density in these nodal superconductors universally exhibits, unlike the expected T-linear dependence, an anomalous 3/2 power-law temperature dependence over a wide temperature range. We propose that this noninteger power law can be explained if a strong renormalization of effective Fermi velocity due to quantum fluctuations occurs only for momenta k close to the nodes in the superconducting energy gap Δ(k). We suggest that such "nodal criticality" may have an impact on low-energy properties of quantum critical superconductors.

Correlation between A-site randomness and magnetic phase transition in half-doped manganite Pr0.5Ba0.5MnO3

The interplay between A-site randomness and magnetic phase transition in half-doped manganite Pr0.5Ba0.5MnO3 (PBMO) was extensively investigated. The H/M versus M2 isotherms show that the A-site ordered PBMO undergoes a second-order magnetic phase transition from paramagnetism to ferromagnetism, whereas the A-site disordered PBMO exhibits a fluctuation-driven first-order transition arising from a competing order phase possibly existing in the paramagnetic state. A step-like transition in magnetization and resistivity with a sharp width of ΔH/H ∼ 10−3 was only observed in the A-site partially ordered PBMO at 2 K, indicating that the metamagnetic transition is associated with a competition between randomly distributed short-range ferromagnetic and antiferromagnetic phases. These findings provide evidence that the A-site randomness not only suppresses A-type antiferromagnetism also moderately weakens long-range ferromagnetism in the A-site ordered PBMO.

Investigation of the magnetic phase transition and magnetocaloric properties of the Mn2FeSbO6 ilmenite

The magnetic phase transition and magnetocaloric properties of both mineral and synthetic melanostibite Mn2FeSbO6 with ilmenite-type structure have been studied. Mn2FeSbO6 orders ferrimagnetically below 270K and is found to undergo a second-order magnetic phase transition. The associated magnetic entropy change was found to be 1.7J/kgK for the mineral and 1.8J/kgK for the synthetic melanostibite for 5T field change. For the synthetic Mn2FeSbO6 the adiabatic temperature change was estimated from magnetic- and specific heat measurements and amounts to 0.2K in 1T field change. Perspectives of the functional properties of Mn2FeSbO6-based materials are discussed.

Active transient cooling by magnetocaloric materials

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

Large magnetocaloric effects due to the coincidence of martensitic transformation with magnetic changes below the second-order magnetic phase transition in Mn1−xFexCoGe

The magnetic properties, phase transitions, and related magnetocaloric effects in Mn1−xFexCoGe (x=0.12, 0.13) have been studied using magnetization and differential scanning calorimetry measurements. Instead of single first-order magnetostructural transition, two-step transitions have been detected in our studied system: one is second-order character followed by a first-order phase transition at lower temperature. The observed first-order phase transition as dominated by the crystallographic transition arises due to the coincidence of martensitic transformation with magnetic changes below the second-order magnetic phase transition in Mn1−xFexCoGe. Large magnetic entropy changes (−ΔSM) have been observed in our studied system due to a first-order phase transition between two high magnetization states. The estimated value of −ΔSM for x=0.12 (−ΔSM=16.5J/kgK for ΔH=5T around 242K) is comparable to the giant-magnetocaloric materials, such as Gd5Si2Ge2 and MnFeP0.45As0.55, which undergo a disorder-to-order magnetostructural transition.

Magnetism and large magnetocaloric effect in HoFe2−xAlx

The structural, magnetic and magnetocaloric properties of several HoFe2−xAlx compounds were investigated. The compounds in the Fe-rich region (0.36⩽x⩽0.4) crystallize in the cubic MgCu2 (C15) structure, while for the ones in the intermediate region (0.75⩽x⩽1.125) the hexagonal MgZn2 (C14) structure was observed. Electronic structure calculations were performed, showing a good agreement between theory and experiment. The Curie temperatures were found to decrease with Al content. For the Fe-rich compounds, these are close to room temperature, while for the compounds in the intermediate region, transition temperatures are well below 300K. No magnetic hysteresis was found around the Curie temperature for applied magnetic fields of up to 4T. All of the investigated compounds undergo a second-order magnetic phase transition at the Curie temperature. A maximum magnetic entropy change value of 7.6J/kgK was obtained for the sample with x=1.125, all of the samples displaying rather large RCP values. The possibility of incorporating these materials in magnetic refrigeration devices is discussed.

Magnetic properties and magnetocaloric effect of Ti-doped La0.7Sr0.3MnO3

We have studied the influence of Ti doping on the magnetic properties and the magnetocaloric effect of La0.7Sr0.3Mn0.92Ti0.08O3 prepared by using solid-state reaction. The temperature dependence of magnetization reveals that the presence of the Ti dopant strongly reduces the Curie temperature (TC) to a value of ∼235 K. The magnetic-entropy change (ΔSm) has been assessed by means of isothermal-magnetization data versus the magnetic field, M-H curves, recorded at different temperatures. Around TC, −ΔSm reaches maximum values of 1.1, 2.0, and 2.7 J/(kg·K) for applied fields of 10, 20, and 30 kOe, respectively. A relative cooling power (RCP) of 135 J/kg is, thus, achieved for an applied field of 30 kOe. Detailed analyses of the M-H curves around TC indicate that the sample exhibits a second-order magnetic phase transition, where the variations of the saturation magnetization (Ms) and the inverse initial susceptibility (χ0−1) versus temperature obey asymptotic relations. Using the Arrott-Noakes method, we have obtained the critical exponents β = 0.425 ± 0.016, γ = 1.017 ± 0.055, and δ = 3.393 ± 0.039. These exponents are in good agreement with the magnetic equation of state. The β value determined in our case is between those expected for the mean-field (β = 0.5) and the 3D Heisenberge (β = 0.365) models, demonstrating the existence of ferromagnetic clusters.

Critical Behavior Near the Paramagnetic to Ferromagnetic Phase Transition Temperature in Polycrystalline La0.57Nd0.1Sr0.33Mn1−x Al x O3 (0.0≤x≤0.1)

The critical parameters provide important information concerning the interaction mechanisms near the paramagnetic to ferromagnetic transition. In this paper, we have investigated the effect of non-magnetic aluminum Al doping in Mn-site on the critical behavior in La0.57Nd0.1Sr0.33MnO3 compounds using magnetization methods near the Curie temperature TC through various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis. The values of critical exponents, derived from the magnetic data yield 0.326 ≤β≤0.366,1.265≤γ≤1.342 and 4.13 ≤δ≤5.07. The critical exponents found in this study obey the Widom scaling relation δ=1+γ/β, implying that the obtained values of β and γ are reliable. Experimental results have revealed that the samples exhibited the second-order magnetic phase transition with exponents which match well with those predicted for the three-dimensional Heisenberg (3D) around the critical temperature.

Effect of High Pressure on Magnetic Properties of C Doped Alkaline-Earth Oxides

The first-principles plane wave pseudopotential method within the generalized gradient approximation (GGA) is used to calculate the magnetic moment, volume, total energy of C doped alkaline-earth oxides X4CO3 (X=Ca, Sr, and Ba) and band structure, electronic state density of Ca4CO3 at ambient as well as at high pressure. Results show that the X4CO3 ferromagnetism decreases, and finally the second-order magnetic phase transition occurs from ferromagnetic (FM) to nonmagnetic (NM) state when increasing the pressure. The main reason of the pressure-induced magnetic transition and the magnetic collapse result from band widening in anion p states. At 69 GPa, the spin contribution of C atoms in Ca4CO3 decreases remarkably. This is due to the C–p in which majority spin states are increased while minority spin states are decreased.

Critical behavior in Ga-doped manganites La0.75(Sr,Ca)0.25Mn1−xGaxO3 (0≤x≤0.1)

The critical properties of perovskite manganite AMn1−xGaxO3 (A=La0.75Ca0.08Sr0.17, x=0, 0.05 and 0.1) at the ferromagnetic–paramagnetic transition have been analyzed. Experimental results revealed that all samples exhibit a second-order magnetic phase transition. The estimated critical exponents derived from the magnetic data using various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical magnetization isotherms M(TC, H). The critical exponent values for the undoped compound were found to match well with those predicted for the three-dimensional Heisenberg model (β=355±0.007, γ=1.326±0.002, δ=4.90±0.01). While on non-magnetic Ga substitution it tends towards mean-field with long-range interaction. The mean-field model might be due to the random dilution of the Mn sublattice by non-magnetic ion Ga3+ and/or the development of the physical size of the clusters which enhance the dipole–dipole interaction.

Preparation of New Composite Magnetocaloric Compounds by Modifying the Annealing Temperature of La0.8Ca0.2−x □ x MnO3 Perovskite

In this paper, a new method to broaden the range of the magnetic refrigeration temperature by changing the annealing temperature was proposed. Series of La0.8Ca0.2−x□xMnO3(0.00≤x≤0.20) compounds were prepared by solid-state reaction and annealed firstly at a temperature of 1473 K (S1) and then at 1073 K (S2). Morphologic and structural studies have revealed that the decrease of the annealing temperature modifies the grain size and the structure of these compounds. The magnetic measurements have shown that the annealing at low temperature (1073 K) increases the magnetization and enhances the one-electron bandwidth, which induces an increase of the Curie temperature for 183 K (S1) to 241 K (S2) for the x=0.00 sample. The magnetocaloric investigation has exposed that the decrease of the annealing temperature induces a change from a second-order magnetic phase transition to first-order one for S1 and S2 compounds, respectively. Also, we have found that the Relative Cooling Power (RCP) factor remains almost constant as a function of calcium-deficiency concentration (x) and the annealing temperature. Finally, we have deduced that we can use composite magnetocaloric compounds, exploiting a mixing of the same compounds annealed at two different temperatures (1473 K; S1) and (1073 K; S2), for refrigeration over the temperature range 175–264 K.

Magnetocaloric effect and its implementation in critical behavior study of Mn4FeGe3−xSix intermetallic compounds

Magnetocaloric effect in Mn4FeGe3−xSix compounds has been studied by dc magnetization measurements. For the parent compound Mn4FeGe3, the paramagnetic to ferromagnetic transition temperature TC is above room temperature (320 K), which initially remains constant for small Si substitution at the Ge site and then decreases marginally with an increase in Si concentration. A large change in magnetic entropy at the TC, under a magnetic field variation of 50 kOe, with typical values of 5.9, 6.5, 5.9, and 4.4 J kg−1 K−1for x = 0, 0.2, 0.6, and 1 samples, respectively, along with a broad operating temperature range and a negligible hysteresis make Mn4FeGe3−xSix series a promising candidate for magnetic refrigerant material around room temperature. Mn4FeGe3−xSix series is found to undergo a second-order magnetic phase transition. The field dependence of the magnetic entropy change has been brought out and implemented to deduce the critical exponents. The critical behavior study shows that the magnetic interactions for x = 0 and 0.2 samples have two different behaviors below and above TC. Below TC, it follows the mean field theory with long-range magnetic interaction and above TC it follows the Heisenberg three-dimensional model with short-range or local magnetic interaction. The magnetic exchange interactions for the x = 0.6 and 1 samples follow the mean-field theory.

Complex magnetoelastic properties in the frustrated kagome-staircase compounds (Co1−xNix)3V2O8

High resolution heat capacity and thermal expansion experiments performed on single crystalline kagome-staircase compounds (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$ are presented. The parent compounds Co$_3$V$_2$O$_8$ and Ni$_3$V$_2$O$_8$ undergo a complex sequence of first- and second-order magnetic phase transitions. The low-temperature ($T<40$ K) magnetic entropy evolves monotonously with the doping content $x$, from the full S=1 Ni$^{2+}$ magnetic entropy in Ni$_3$V$_2$O$_8$ to half of the S=3/2 Co$^{2+}$ magnetic entropy in Co$_3$V$_2$O$_8$. Thermal expansion coefficients $\alpha_i$ ($i = a$, $b$ and $c$) show a strong anisotropy for all (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$ compounds. The low-temperature magnetic distortion indicates that Co-doping (Ni-doping) has similar effects to applying a uniaxial pressures along $a$ or $b$ ($c$). Linear Gr\"{u}neisen parameters $\Gamma_i$ are extracted for the three main axes $i$ and exhibit a complex behavior with both temperature and doping. For each axis, $\Gamma_i$ and $\alpha_i$ exhibit a sign change (at low temperature) at the critical concentration $x_c\simeq0.25$, at which the incommensurate magnetic propagation vector changes. Beyond our study, an understanding of the multiple and complex parameters (magnetic frustration, magnetic anisotropy, mixture of S=1 and S=3/2 ions, etc.) is now necessarily to bring light to the rich magneto-elastic properties of (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$.

Large reversible magnetocaloric effect in HoTiO3 single crystal

Large magnetocaloric effect has been observed in perovskite-type HoTiO3 single crystal accompanied by a second-order magnetic phase transition at TC ≈ 53 K. The values of maximum magnetic entropy change are about 5.96 and 11.56 J kg−1 K−1 under the magnetic field change of 2 and 5 T, respectively, without any detectable thermal and magnetic hysteresis loss. The large magnetic entropy change is attributed to the sharp magnetization jump, related to anomalies of the lattice parameters just at the Curie temperature. Such a large reversible magnetocaloric effect makes the perovskite HoTiO3 attractive for pursuing new materials for magnetic refrigeration.

Effect of Structural Disorders on Magnetocaloric Effect in $\hbox{La}_{2/3-{\rm x}}\hbox{Gd}_{\rm x}\hbox{Sr}_{1/3-{\rm y}}\hbox{Ba}_{\rm y}\hbox{MnO}_{3}$

Cation disordering in ABO3 perovskite manganites plays the import role on their magnetism and transport properties. In the present, single phase of La2/3-xGdxSr1/3-yBayMnO3 ploycrystals are synthesized with fixing the averaged A-site ionic size of 1.247 Å. Subtle structural variations have been studied by Rietveld refinements using X-ray powder diffraction data. The unit cell symmetry remains unchanged with doping while the lattice parameters, Mn-O bond length, the Mn-O-Mn bond angle vary with the dopants. With increasing the Ba content, Mn-O bond length decreases while Mn-O-Mn bond angle increases, which indicates that the double-exchange interaction increases with Ba content. However, the ferromagnetic-to-paramagnetic transition temperature, Tc, decreases with Ba content, which confirms the strong suppression effect of cations disordering on magnetic ordering. The first-order to second-order magnetic phase transition is manipulated by a rival ship between A-site size-disorder degree and electron-lattice coupling for the sample system. In this system, we easy adjust the Tc above the room temperature. The transition temperature may be easily broadened by adjusting the cations disordering. It may be used as a promising candidate for magnetic refrigerant near room temperature.

Critical behavior and magnetic entropy change in La 0.7Ca 0.3Mn 0.9Zn 0.1O 3 perovskite manganite

We studied the critical behavior and magnetic entropy change in a perovskite-manganite compound of La 0.7Ca 0.3Mn 0.9Zn 0.1O 3 around its Curie temperature of T C = 206.75 K. Experimental results revealed that the sample exhibited the second-order magnetic phase transition with the exponents β = 0.474 and γ = 1.152 close to those expected from the mean-field theory ( β = 0.5 and γ = 1.0). In the vicinity of T C , the magnetic entropy change Δ S M reached maximum values of 1.1, 1.7, and 2.7 J/kg K under magnetic-field variations of 10, 20, and 35 kOe, respectively. These Δ S M values are much lower than those reported previously on the parent compound of La 0.7Ca 0.3MnO 3. The nature of this phenomenon is discussed by means of the characteristics of the magnetic phase transition, and critical exponents.

ChemInform Abstract: Magnetic Phase Transitions in Nanoclusters and Nanostructures

AbstractReview: 20 refs.

Giant magnetocaloric effects by tailoring the phase transitions

The MnCoGe alloy can crystallize in either the hexagonal Ni2In- or the orthorhombic TiNiSi-type of structure. In both phases MnCoGe behaves like a typical ferromagnet with a second-order magnetic phase transition. For MnCoGeBx with B on interstitial positions, we discover a giant magnetocaloric effect associated with a single first-order magnetostructural phase transition, which can be achieved by tuning the magnetic and structural transitions to coincide. The results obtained on the MnCoGe-type alloys may be extensible to other types of magnetic materials undergoing a first-order structural transformation and can open up some possibilities for searching magnetic refrigerants for room-temperature applications.

Magnetocaloric effect in Gd/W thin film heterostructures

In an effort to understand the impact of nanostructuring on the magnetocaloric effect, we have grown and studied gadolinium in MgO/W(50 Å)/[Gd(400 Å)/W(50 Å)]8 heterostructures. The entropy change associated with the second-order magnetic phase transition was determined from the isothermal magnetization for numerous temperatures and the appropriate Maxwell relation. The entropy change peaks at a temperature of 284 K with a value of approximately 3.4 J/kg K for a 0–30 kOe field change; the full width at half max of the entropy change peak is about 70 K, which is significantly wider than that of bulk Gd under similar conditions. The relative cooling power of this nanoscale system is about 240 J/kg, somewhat lower than that of bulk Gd (410 J/kg). An iterative Kovel–Fisher method was used to determine the critical exponents governing the phase transition to be β=0.51, and γ=1.75. Along with a suppressed Curie temperature relative to the bulk, the fact that the convergent value of γ is that predicted by the two-dimensional Ising model may suggest that finite size effects play an important role in this system. Together, these observations suggest that nanostructuring may be a promising route to tailoring the magnetocaloric response of materials.