Thermomagnetic Irreversibility Research Articles

Study of surface structure and interfacial effects on optical and magnetic properties of un-doped and Si-doped hematite films

Thermal evaporation technique has been used to synthesize the α-Fe2O3 (hematite) and Si-doped hematite films on three different substrates (SiO2/Si, Al2O3 and quartz). The post-deposition heat treatment at 800 °C has further improved the crystalline nature of the films. Rutherford backscattering spectroscopy has provided information for surface elemental composition and depth profile. Raman spectra have supported the formation of rhombohedral phase and inter-layer diffusion between substrate and films. The x-ray photoelectron spectroscopy has confirmed the mixed-valence oxidation states for Fe ions and +4 state for the Si ions in Si-doped hematite films. The optical reflectance spectra in the UV–Visible region indicated contributions from the films and film-substrate interface. The Si-doping has enhanced soft ferromagnetic properties in the hematite film with noticeable decrease of the magnetic coercivity and increase of the magnetic moment. The films showed blocking of magnetization at lower temperatures, typically below 125 K, and a wide thermomagnetic irreversibility between zero-field cooled and field cooled magnetization curves. The modified surface chemical structure, ferromagnetism and optical properties in the Si doped hematite films promise their potential applications in spintronics.

Magnetic studies of ultrafine CoFe2O4 nanoparticles with different molecular surface coatings.

Surface functionalized ultrafine CoFe2O4 nanoparticles (NPs), with mean diameter ∼5 nm, were investigated by means of DC magnetization and AC susceptibility over the temperature range of 4-400 K. All NPs present the same CoFe2O4 core, with different molecular surface coatings, increasing gradually the number of carbon atoms in the coating layer: glycine (C2H5NO2), alanine (C3H7NO2), aminobutanoic acid (C4H9NO2), aminohexanoic acid (C6H13NO2), and aminododecanoic acid (C12H25NO2). Samples were intentionally fabricated in order to modulate the core-core magnetic dipolar interaction, as the thickness of the coating layer increases with the number of carbon atoms in the coating molecule. The magnetic data of the uncoated CoFe2O4 NPs were also collected for comparison. All investigated CoFe2O4 NPs (coated and uncoated) are in a magnetically blocked state at room temperature as evidenced by ZFC/FC measurements and the presence of hysteresis with ∼700 Oe coercivity. Low temperature magnetization scans show slightly constricted hysteresis loops with coercivity decreasing systematically with a decreasing number of carbon atoms in the coating molecule, possibly resulting from differences in magnetic dipole coupling between NPs. Large thermomagnetic irreversibility, slow monotonic increase in the FC magnetization and non-saturation of the magnetization give evidence for the cluster glass (CG) nature in the CoFe2O4 NPs. The out of phase part (χ'') of AC susceptibility for all samples shows a clear frequency dependent hump which was analyzed to distinguish superparamagnetic (SPM), cluster glass (CG) and spin glass (SG) behavior by using Néel-Arrhenius, Vogel-Fulcher, and power law fittings. These analyses rule out the SPM state and suggest the presence of significant inter-cluster dipolar interaction, giving rise to CG cooperative freezing in the high-temperature region. In the low-temperature range, however, the disordered spins on the nanoparticle's surface play an important role in the formation of the SG-like state, as evidenced by Arrott plots and temperature dependency of dM/dH in the initial magnetization curves. In summary, the magnetic measurements showed that undercooling the system evolves from a SPM state of weakly interacting spin clusters, through the CG state induced by strong dipolar interaction, to the SG state resulting from the frustration of the disordered surface spins.

Influence of Ge-doping on the collinear and non-collinear antiferromagnetic phases of Mn[formula omitted]Si[formula omitted] alloy

Effect of Ge-doping at the non-magnetic site (Si-site) of the Mn-based binary alloy Mn5Si3 has been explored through temperature-dependent x-ray diffraction, dc-magnetic, and electrical transport measurements. All the doped alloys show D88 type hexagonal structure with space group P63/mcm at room temperature and undergoes two structural distortions, hexagonal (P63/mcm) → orthorhombic (Ccmm) → orthorhombic (Cc2m), on cooling. The magnetic characters of these orthorhombic (Cc2m), orthorhombic (Ccmm), and hexagonal (P63/mcm) phases are non-collinear antiferromagnetic (AFM1), collinear antiferromagnetic (AFM2), and paramagnetic (PM), respectively. Doping of Ge results in a significant increase in the AFM1 to AFM2 transition temperature (TN1). However, the AFM2 to PM transition point remains unchanged. In addition, a reasonable increase in the critical field values of the AFM1 to AFM2 transition via another non-collinear antiferromagnetic phase (AFM1′) with increasing Ge concentration has been observed, indicating the strengthening of AFM1 and AFM1′ phases over the AFM2 phase. Such behaviors make the observation of unusual magnetic properties of undoped Mn5Si3 alloys, like inverted hysteresis loop (IHL) and thermomagnetic irreversibility (TI), more evident for these Ge-doped alloys. Further, this study unfolds the presence of conventional and inverse magnetocaloric effect and they found to decrease with Ge doping. An interesting interplay for positive and negative magnetoresistance has also been observed in all the studied alloys.

Colossal anomalous Hall conductivity and topological Hall effect in ferromagnetic kagome metal Nd3Al

We report a polycrystalline kagome metallic ferromagnet Nd3Al with a large unconventional positive magnetoresistance (∼80%) and a colossal anomalous Hall conductivity of 3 × 104 S/cm. We find that, though it is predominantly ferromagnetic, the low temperature phase is rather complex. The reduction in the effective moment, thermomagnetic irreversibility, anomalous temperature dependence of magnetization, large and non-saturating positive magnetoresistance, and existence of the finite topological Hall effect make this compound quite interesting. Various experimental proofs point toward topological band structure and topological spin texture in the frustrated kagome lattice. Ab initio calculations broadly confirm the presence of flatbands and Weyl points originating from the itinerant Nd-moments. The non-trivial band structure, enhanced skew scattering, and topological spin texture in a frustrated kagome lattice are found to be responsible for the colossal Hall conductivity and the topological Hall effect.

Ni-doping assisted modification of the non-collinear antiferromagnetic ordering in Mn5Si3 alloy

Ni-doped Mn5Si3 alloys of nominal compositions Mn5−xNixSi3 (for x = 0.05, 0.1, and 0.2) have been investigated through detailed neutron powder diffraction (NPD) studies in zero magnetic field and ambient pressure. At room temperature, all three Ni-doped alloys crystallize with D88 type hexagonal structure having P63∕mcm space group. These alloys undergo paramagnetic → collinear antiferromagnetic → non-collinear antiferromagnetic transitions on cooling from room temperature. A significant decrease in collinear to non-collinear antiferromagnetic transition temperature has been observed with increasing Ni concentration. The magnetic structure of both antiferromagnetic phases can be described by the magnetic propagation vector k = (0,1,0). However, the moment size and the orientation in the non-collinear antiferromagnetic phase are found to be notably affected by the Ni-doping. Approaching near-parallel arrangement of Mn-moments with increasing Ni-doping is found to be responsible for the gradual disappearance of unusual magnetic properties (inverted hysteresis loop, thermomagnetic irreversibility, etc.) observed in Mn5Si3 alloy.

Canonical spin glass and cluster glass behavior in the polymorphs of LiFeSnO4

We report a comprehensive and comparative study of structural, static, dynamic, and nonequilibrium magnetic properties on the polymorphs of $\mathrm{LiFeSn}{\mathrm{O}}_{4}$. It exhibits two polymorphs: (i) orthorhombic phase (HT) stabilized at high temperature and (ii) hexagonal phase (LT) obtained at lower synthetic temperature. The HT phase crystallizes in the orthorhombic structure (space group: Pmcn) and exhibits a site disordered, nonfrustrated zig-zag network of ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Sn}}^{4+}$ ions. On the other hand, the LT phase shows a disordered frustrated kagome network of ${\mathrm{Li}}^{1+}, {\mathrm{Fe}}^{3+}$, and ${\mathrm{Sn}}^{4+}$ ions crystallizing in the hexagonal structure (space group: $P{6}_{3}mc$). The low-temperature thermo-magnetic irreversibility and the absence of heat capacity anomaly in both polymorphs indicate the absence of long-range magnetic ordering and a possible glassy state. Zero-field-cooled and field-cooled magnetic memory effect and spin relaxation measurements stipulate the spin glass nature of the polymorphs. Further, spin glass behavior is confirmed by the AC susceptibility measurements. Interestingly, these polymorphs reveal different classes of spin glass states. The site disordered HT phase exhibits a single spin-flip time ${\ensuremath{\tau}}_{0}\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ sec, indicating a canonical spin glass state. In contrast, LT phase, which is disordered and geometrically frustrated, shows the spin-flip time of ${\ensuremath{\tau}}_{0}\ensuremath{\sim}9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec, suggesting a cluster spin glass state. In addition, the exchange bias effect was observed due to magnetic inhomogeneity in both polymorphs.

Non-equilibrium magnetic response in concentrated spin-glass Au0.89Fe0.11 alloy

We report a detailed study of dc magnetization and ac susceptibility performed on the zero field cooled (ZFC) and field cooled (FC) state of polycrystalline Au0.89Fe0.11 alloy. The temperature variation of ZFC and FC dc magnetization at low fields shows a distinct peak around Tf = 33 K, which indicates the cooperative freezing of the finite size spin clusters. A weak thermomagnetic irreversibility between ZFC and FC magnetization appears at a temperature Tir, which is slightly below Tf. Further down the temperature at Tsh<Tir, the ZFC and FC magnetization curves exhibit strong thermomagnetic irreversibility. The ac susceptibility recorded after cooling in presence of field shows significant dispersion below Tf, and also, the FC state of Au0.89Fe0.11 alloy exhibits a pronounced memory effect in the dc magnetization which clearly underline that the FC state is not an equilibrium state. In contrast to the general perception obtained through the mean-field theories of thermodynamic phase transition in spin-glass envisaging the FC state to be an equilibrium state, the present experimental results clearly indicate that the energy landscape of the FC state of Au0.89Fe0.11 alloy is a nontrivial one.

Random magnetic anisotropy in a new compound Sm2AgSi3

We report the experimental study on the structural and magnetic properties of a new ternary intermetallic compound Sm2AgSi3. The properties of the sample were investigated in detail by X-ray diffraction, dc–magnetization, and heat capacity measurements. The polycrystalline compound of Sm2AgSi3 crystallizes in the ThSi2-type tetragonal structure (space group I41/amd). The temperature dependent dc–magnetization and heat capacity results demonstrate that the compound undergoes ferromagnetic behaviour with a Curie temperature (TC) of 14 K. The large coercive field in hysteresis loops and the thermomagnetic irreversibility in the ferromagnetic region revealed that the compound exhibits a large magnetic anisotropy. The magnitude of the applied field and the coercivity obtained from the M-H loops corroborate with the thermomagnetic irreversibility in the magnetization data. The magnetic contribution of the heat capacity reveals a broad Schottky–type anomaly above TC, due to the presence of the crystal electric field effect of Sm3+ in Sm2AgSi3.

Chemically inducing room temperature spin-crossover in double layered magnetic refrigerants Pr1.4+Sr1.6-Mn2O7 (0.0 ≤ x ≤ 0.5)

The height of total entropy (ST) for a magnetic refrigerant material is essentially concerned with the magnetic and structural transitions. However, the participation of such transitions in layered materials is not well understood. Therefore, the purpose of this work is to investigate the interplay between double layer lattice with their single perovskite counterpart, to achieve optimal magnetocaloric performance. A series of self-doped Pr1.4+xSr1.6-xMn2O7 (0.0 ≤ x ≤ 0.5) Ruddlesden-Popper (R-P) perovskite have been prepared through the solid-state sintering method. With increasing the Pr-stoichiometry, the lattice faults have increased and the double layer lattice dramatically disintegrates into single perovskite structure. Due to the reduction of bilayer R-P phase into single perovskite the spin crossover occurs from weak bilayer (TC = 304 K) interactions towards the strong three-dimensional (TC = 308 K) interactions respectively. This series consistently develops thermomagnetic irreversibility in zero-field cooled (ZFC)-field cooled (FC) magnetization, which is indicative of a spin-glass state. The glassy nature has been ascribed collectively to the lattice strain produced because of dislocations and to an antiferromagnetic phase segregated at the surface. The maximum value of temperature average entropy change (TEC) and adiabatic temperature (ΔTad) has enhanced nearly by 4 folds from 0.53 J kg−1 K−1, 0.59 K (for x = 0.0) up to 1.85 J kg−1 K−1, 10 K (for x = 0.5) at 2.5 T, respectively. Additionally, the room temperature relative cooling power has improved from 26.94 J/kg up to 77.84 J/kg with an applied field of 2.5 T. Our findings in this work suggest that the controlled reduction of double layer lattice into single perovskite and/or existence of both phases simultaneously in bilayer R-P manganites may be very effective in obtaining the desirable characteristics of magnetocaloric effects.

Vertical inhomogeneous magnetic order in FeRh film

FeRh films, known as antiferromagnetic materials at low temperatures, exhibit unexpected ferromagnetic (FM) characteristics, unlike the bulk. The detailed temperature-dependent physical properties of FeRh films were examined to elucidate the causes. Temperature-dependent magnetic and electrical measurements showed that the coexistence of antiferromagnetic and residual FM states induces thermomagnetic irreversibility (e.g., spin-glass-like behavior) and negative magnetoresistance. Polarized neutron reflectometry (PNR) revealed the presence of non-uniform FM states at the interfaces. Additional temperature-dependent PNR profile and room temperature structural and spectroscopic analyses confirmed that the bottom FM region is correlated with structural distortion. In contrast, the top FM region is temperature-dependent and originates from non-stoichiometry.

Magneto-structural coupling induced multiple magnetic transitions and magnetocaloric effect of Ni43Mn46Sn9Ge2 alloy

Multiple magnetic and structural phase transitions and magnetocaloric effect of Ni43Mn46Sn9Ge2 Heusler alloy were detailedly investigated. The martensitic and inverse-martensitic transitions occur at temperatures slightly below room temperature, which induces a ferromagnetic - antiferromagnetic transition and large inverse magnetocaloric effect. A paramagnetic - ferromagnetic and antiferromagnetic - ferromagnetic transition was observed in high-temperature austenitic and low-temperature martensitic phase, respectively. The evident thermomagnetic irreversibility at low temperatures of the martensitic phase suggests a reentrant spin glass state, which originates from the frustration of the competing ferromagnetic and antiferromagnetic spins and results in the visible exchange bias, large coercivity and double-shifted hysteresis behaviors. A T-μ0H phase diagram was finally constructed.

The Influence of Yb Doping and Sintering Conditions on the Magnetocaloric and Mechanical Properties of EuS

For this work, europium monosulfide (EuS) powders were prepared by sulfurizing Eu2O3 powder with CS2 gas. The synthesized EuS powders were sintered by SPS at temperatures in the 800–1600 °C range for 0.33–1 h at 50 MPa under vacuum conditions. The influences of Yb doping and sintering conditions on the magnetocaloric and mechanical properties of EuS were investigated systematically. An increase in sintering temperature caused the rise of lattice parameters of EuS, whereas Yb doping caused them to drop. SEM showed that the grain size of the EuS increased with sintering temperatures in the 1000–1400 °C range. Higher sintering temperatures can enlarge the magnetizability and saturation magnetization of EuS compact. On the contrary, Yb doping can weaken the magnetizability and saturation magnetization of EuS compact. All sintered polycrystalline EuS compacts had weaker thermomagnetic irreversibility and lower magnetic anisotropy.

Observation of magnetocaloric and magneto-electric anomalies in R2Cu2O5 family of layered oxides

R 2 Cu 2 O 5 (R = Tb-Lu, Sc, In) (RCO) compounds are interesting magnetic materials due to their quasi-low dimensional crystal structure, which also lacks centro-symmetry. Previous investigations on these compounds indicated antiferromagnetic or canted-antiferromagnetic ground state with Néel temperatures lying below about 30 K. The present work tries to provide finer details of the intricate magnetic characteristics of the RCO compounds, which were not addressed in the previous works. These include thermomagnetic irreversibility, thermal hysteresis , hysteresis in the isothermal magnetization measurements and so on. Most interestingly, we observe large magnetocaloric effect as calculated from our magnetization data and we find that it is primarily connected to the metamagnetic transitions observed in the samples. Our work also recognizes anomalies in the dielectric constant at the magnetic transition temperatures indicating possible multiferroicity in the samples. Efforts have been made to understand the diverse magnetic properties of the RCO samples on the basis of the crystal structure and the rare-earth moment. • A comprehensive magnetic scenario of complete R 2 Cu 2 O 5 series in a single report. • Observation of sizable magnetocaloric effect in R 2 Cu 2 O 5 associated with metamagnetism. • Dielectric anomaly at magnetic transition indicates the magneto-electric coupling. • Studies of thermo-magnetic irreversibility, thermal hysteresis in R 2 Cu 2 O 5 oxides.

Interesting magnetic response of the nuclear fuel material UO2

ABSTRACT We report results of dc magnetization measurements on UO2. Our study reveals a deviation from standard Curie-Weiss (CW) paramagnetic behavior below 280 K, and that is followed by a transition to antiferromagnetic state with marked thermomagnetic irreversibility below TN = 30.6 K. The zero field cooled (ZFC) magnetization exhibits distinct structures not usually observed in the antiferromagnets, whereas field cooled (FC) magnetization looks more like a ferromagnet. The thermomagnetic irreversibility continues to exist in a subtle way even in the paramagnetic regime well above TN . This behavior along with the deviation from CW law highlight non-standard nature of the paramagnetic state. Magnetic response below TN changes significantly with the increase in the applied magnetic field. In the isothermal magnetic field variation of magnetization, a subtle signature of a magnetic field-induced phase transition is observed. All these experimental results highlight the non-trivial nature of the magnetic state in UO2.

Preparation and Characterization of Nanosized Substituted Perovskite Compounds with Orthorhombic Structure

In this work, five substituted perovskite such as (Gd0.9Sr0.1) Mn0.8Co0.2O3, Tb0.8Sr0.2FeO3, Gd0.6Sr0.4RuO3, SrCe0.95Y0.05O3, and Mn0.6Co0.4SnO3 were synthesized by tartrate and hydroxide precursor method. The resulting samples were characterized by inductively coupled plasma spectroscopy, energy dispersive X-ray analysis, infrared spectroscopy, thermal analysis, X-ray powder diffraction, transmission electron microscope (TEM), selected field of electron diffraction (SAED), d.c. electrical conductivity, Hall effect, dielectric measurements, and low-temperature magnetization measurements. The X-ray diffraction pattern for all compounds was indicated the formation of single-phase perovskite with orthorhombic structure except Tb0.8Sr0.2FeO3 and Mn0.6Co0.4SnO3 perovskite. These compounds showed a cubic and rhombohedral structure, respectively. The lattice parameter and the unit cell volume slightly decreased as ionic radii decrease in agreement with the lanthanide contraction. The average size of cation ˂ RA ˃, mismatch factor (σ2), and tolerance factor (t) gives the combined effects of disorder and inhomogeneity in these compounds. The average particle size determined from TEM was in the range of 22 to 77 nm for all compounds. The temperature dependence of electrical conductivity for all compounds showed a definite break in 500 K to 610 K. except the Gd0.6Sr0.4RuO3 compound, which corresponds to semiconducting behavior. While the Gd0.6Sr0.4RuO3 sample shows a metallic-like semiconductor. The thermoelectric power and Hall effect measurements for all compounds were n-type semiconductivity except the SrCe0.95Y0.05O3 compound. It showed p-type semiconductivity. The frequency dependence of the dielectric constant and dielectric loss in these substituted perovskites were discussed using the Maxwell-Wagner model. Magnetic studies showed that the thermo-magnetic irreversibility for all compounds.

Crystallographic, magnetic and magnetocaloric properties in novel intermetallic materials R3CoNi (R = Tb, Dy, Ho, Er, Tm, Lu)

A family of novel intermetallic R3CoNi with heavy rare earth ions has been synthesized (R = Tb, Dy, Ho, Er, Tm, Lu) and a study of the crystal structure of these phases performed. All the compounds adopt the rhombohedral Er3Ni2-type structure [Pearson’s symbol hR45; space group R-3h (N. 148)]. A thorough investigation of their magnetic and magnetocaloric properties has been undertaken. Magnetization and ac-susceptibility measurements as a function of temperature show that the samples with Tb, Dy, Ho, Er, and Tm present a paramagnetic to ferromagnetic (PM-FM) transition at temperatures in the range 96–6 K; different reorientation transitions take place below the respective TC, in most cases with thermomagnetic irreversibility. Thermal and magnetic measurements have been used to retrieve the set of critical exponents (α, β, γ, δ) for the PM-FM transition to assign a universality class to Tb3CoNi, Dy3CoNi, and Ho3CoNi, with the result that, in the first case, it is close to the Mean Field model, in the second one it is between the Chiral Heisenberg and the XY-Chiral, and in the third one it is close to the XY-Chiral model. Therefore, in Tb3CoNi the transition is governed by long-range order interactions, whereas in Dy3CoNi and Ho3CoNi there must be some kind of frustrated non-collinear ferromagnetism. The magnetocaloric measurements in five members of the family indicate that all of them present highly competitive magnetocaloric properties in their respective temperature ranges, with high magnetic entropy changes (from 12.8 to 18.5 J/Kg.K at μ0ΔH = 5 T) and refrigerant capacities (from 412 to 699 J/Kg at μ0ΔH = 5 T). These results assess and highlight the applicative interest of these compounds, besides suggesting the possibility of tuning the range of the operating temperature by modifying the rare earth ion. Finally, universal curves for the magnetocaloric properties have been found for Tb3CoNi, Dy3CoNi, and Ho3CoNi; the scaling of the magnetocaloric variables confirms the validity of the assigned universality classes.

Local inspection of magnetic properties in GdMnIn by measuring hyperfine interactions

GdMnIn is reported to crystallize in the hexagonal MgNi2-type structure presenting a spin-glass behavior with no magnetic order attributed to the triangular spin frustration of magnetic ions. In the present work, FC-ZFC magnetization, specific heat and AC susceptibility measurements along with the local magnetic exchange measured by hyperfine interactions at In sites are used to investigate the magnetic behavior in GdMnIn compound. The ZFC-FC magnetization curves exhibit an inflection which was ascribed to the antiferromagnetic transition at TN= 145 K. These curves also give an indication of thermomagnetic irreversibility at 118 K, which along with the absence of inflection in specific heat results might be associated to spin-glass behavior. Results of AC susceptibility and magnetic hyperfine field measured using 111In(111Cd) probe nuclei carried out by perturbed angular correlations (PAC) technique did not show evidence of spin-glass behavior. The thermomagnetic irreversibility in FC-ZFC curves along with results of hyerfine interactions suggest the presence of magneto-crystalline anisotropy effects and a weak long-range coupling in GdMnIn.

Short-range ferromagnetic order due to Ir substitutions in single-crystalline Ba(Co1− x Ir x )2As2 (0 ⩽ x ⩽ 0.25)

The ternary-arsenide compound BaCo2As2 was previously proposed to be in proximity to a quantum-critical point where long-range ferromagnetic (FM) order is suppressed by quantum fluctuations. Here we report the effect of Ir substitutions for Co on the magnetic and thermal properties of Ba (0 ⩽ x ⩽ 0.25) single crystals. These compositions all crystallize in an uncollapsed body-centered-tetragonal ThCr2Si2 structure with space group I4/mmm. Magnetic susceptibility measurements reveal clear signatures of short-range FM ordering for x ⩾ 0.11 below a nearly composition-independent characteristic temperature T cl ≈ 13 K. The small variation of T cl with x, thermomagnetic irreversibility between zero-field-cooled and field-cooled magnetic susceptibility versus T, the occurrence of hysteresis in magnetization versus field isotherms at low field and temperature, and very small spontaneous and remanent magnetizations <0.01 μ B/f.u. together indicate that the FM response arises from short-range FM ordering of FM spin clusters as previously inferred to occur in Ca(Co1−x Ir x )2−y As2. Heat-capacity C p(T) data do not exhibit any clear feature around T cl, consistent with the very small moments of the FM clusters. The C p(T) in the paramagnetic temperature regime 25–300 K is well described by the sum of a Sommerfeld electronic contribution and Debye and Einstein lattice contributions where the latter lattice contribution suggests the presence of low-frequency optic modes associated with the heavy Ba atoms in the crystals.

Mn-site doping and its effect on inverted hysteresis and thermomagnetic irreversibility behavior of antiferromagnetic Mn5Si3 alloy

The structural and magnetic behavior of Mn-site doped intermetallic manganese silicide alloys of nominal compositions Mn5−xAxSi3 (x = 0.05, 0.1, 0.2 and A = Ni, Cr) have been investigated with a focus to the inverted hysteresis behavior and thermomagnetic irreversibility. Room temperature x-ray powder diffraction data confirm that all the doped alloys crystallize in hexagonal D88 type structure with space group P63/mcm. The doped alloys are found to show paramagnetic–collinear antiferromagnetic (AFM2)–noncollinear antiferromagnetic (AFM1) transitions during cooling from room temperature. A significant decrease in the critical values of both AFM1–AFM2 transition temperatures and fields have been observed with the increasing Ni/Cr concentration. Inverted hysteresis loop, field-induced arrest, and thermomagnetic arrest, the key features of the undoped Mn5Si3 alloy, are found to be significantly affected by the Mn-site doping and eventually vanishes with 4% doping.

Magnetocaloric effect and critical exponent analysis around magnetic phase transition in NdCo2 compound

The magnetic properties and magnetocaloric effect of ferromagnetic NdCo2 compound have been investigated around the magnetic phase transition using magnetic and heat capacity measurements. The thermomagnetic irreversibility between the zero field-cooled and field-cooled magnetization curves is detected below Curie temperature in the low magnetic field, and it is attributed to the domain wall pinning effect. The maximum magnetic entropy change () of 7.33 Jkg−1K−1 and large relative cooling capacity of 529.96 Jkg−1 was obtained over the wide working temperature range of 78 K under magnetic field change of 5 T. The corresponding maximum adiabatic temperature change () is evaluated to be 3.1 K under a same magnetic field change. Meanwhile, negligible thermal and magnetic hysteresis loss was observed in paramagnetic (PM) to ferromagnetic (FM) transition region. Such a magnetocaloric effect is attributed to a second-order magnetostructural transition from a PM cubic phase to a FM tetragonal phase around the Curie temperature of 102 K. The observed magnetocaloric performance is comparable or even better than some earlier reported rare-earth based Laves compounds under the same magnetic field change, implying that the NdCo2 is one of the potential refrigerant material working at low temperature. The critical exponent analysis was carried out to understand the nature and underlying mechanism of the PM to FM phase transition. The critical exponents (β, γ) obtained from modified Arrott plots, the Kouvel–Fisher plot, and the critical isotherm are consistent with each other and obey the Widom scaling relation (δ = 1+γ/β). The values of critical exponents suggest that the NdCo2 belongs to the three-dimensional-Ising model with short-range interaction. The present results may give some clues for searching novel materials with excellent magnetocaloric performance for refrigeration technology.