FM PM Research Articles

Direct and inverse magnetocaloric effects in FeRh alloy

There are two types of magnetocaloric effects that are the so-called direct and inverse effects originating from ferromagnetic–paramagnetic (FM–PM) and antiferromagnetic–ferromagnetic (AFM–FM) phase transitions, respectively. Here, the magnetocaloric effects of FeRh alloy, which exhibit both the direct and inverse effects, have been studied by combining first-principles calculations and Monte Carlo simulations. The magnetic exchange coupling constants in the AFM and FM phases are evaluated by first-principles calculations based on the Liechtenstein formula. The Monte Carlo calculations considering the two-phase mixtures of AFM and FM well reproduce the experimental magnetization curves at zero external magnetic field. It is also shown that the isothermal magnetic entropy changes near the AFM–FM transition temperature under magnetic fields are successfully reproduced based on the Maxwell relation. The entropy changes in a wide range of the magnetic fields near the AFM–FM and FM–PM transition temperatures are investigated and the direct and inverse magnetocaloric effects of FeRh are discussed. The giant relative cooling power of FeRh alloy is achieved due to the large saturation magnetizations and the first-order AFM–FM phase transition.

Top-down design methodology for a 2 ps rms Jitter at 2.56 GHz of an analog PLL based on Ring and LC Tank Oscillators

A top-down methodology is proposed to design Phase-Locked-Loops (PLL) using behavioural and transistor-level simulation in two cases: Ring-Oscillator (RO) and LC Tank Oscillator (LCTO) with the aim to achieve a low-jitter PLL clock generator in 130 nm process. The optimization of these two PLLs is obtained in three steps. The first one is to design a model in Verilog-A of each block with its intrinsic jitter parameter. Each block is simulated alone to verify the nature of its intrinsic jitter: Frequency Modulation jitter (FM jitter) or Phase Modulation jitter (PM jitter). The second step is to place each of these blocks in a global schematic to obtain a full behavioural PLL. In this way, one can study the PLL operation and check the effect of each block's jitter on the PLL output. The third step is to use the intrinsic FM jitter or PM jitter values to simulate at the transistor level of each block individually and then all of them together. To evaluate the loop bandwidth and the loop stability of each of the two PLLs, a linearized PLL is designed using ideal sources. This approach is important to check the compatibility of each block in terms of jitter and bandwidth with respect to the target PLL performance. The optimization results are used to design and produce two PLLs. Measurements of these two are found to have an absolute time jitter of the order of 2 ps rms.

Magnetocaloric properties of Fe-substituted La–Sr-manganites

The effect of Fe doping on the structural, magnetic and magnetocaloric properties was studied on La1-xSrxMn1-yFeyO3 (x = 0.10, 0.20; y = 0.00, 0.10, 0.15) manganites prepared by solid state reaction method. X-ray structural analysis showed that the substitution of manganese by iron does not change the phase state in the rhombohedral series (x = 0.20). In the orthorhombic series (x = 0.1), the substitution of manganese by iron leads to a phase transition from the Jahn-Teller O′ phase to the ordinary O phase. We explain this transition by a decrease in the concentration of Mn3+ ions, which are Jahn-Teller centers. In addition, we recorded an increase in the unit cell parameters in the iron-containing samples of both series. We explain this effect by the presence of a certain amount of Fe4+ ions. Magnetic measurements showed that all samples undergo a second order FM–PM transition at the Curie temperature (Tc). The Curie temperature TC and the Curie-Weiss temperature (Θ) decrease with increasing Fe content. The relationship between the variation of TC and structural parameters the Me–O bond length and the bond angle of Me–O–Me with increasing Fe content has been established. In the paramagnetic temperature range, the formation of magnetic clusters was suggested from isothermal magnetization data. For a magnetic field variation of 5 T, the entropy change is evaluated.

Giant spontaneous resistivity jumps in La0.825Ce0.175Fe12B6

We report the observation of spontaneous and very sharp resistivity jumps at low temperatures in the antiferromagnetic itinerant-electron system La0.825Ce0.175Fe12B6. This intermetallic compound undergoes multiple magnetic transformations, antiferromagnetic–ferromagnetic (AFM–FM) and ferromagnetic–paramagnetic (FM–PM), triggered by changes in both the temperature and the applied magnetic field. The magnetoresistance isotherms display irreversible abrupt steps at T ≤ 4 K, whereas the field dependence of the resistivity becomes smooth above 4 K. Meanwhile, the evolution with time of the electrical resistivity exhibits a huge spontaneous jump after an incubation time when both the applied magnetic field and temperature are constant. A giant negative magnetoresistance (MR = −71%) is discovered associated with the magnetic-field-induced first-order AFM-FM transition.

Giant volume magnetostriction in La0.85Ce0.15Fe12B6

We report the discovery of giant magnetostriction and huge negative thermal expansion (NTE) in the itinerant-electron metamagnetic compound La0.85Ce0.15Fe12B6. This system presents multiple magnetic transformations, antiferromagnetic–ferromagnetic (AFM–FM) and ferromagnetic–paramagnetic (FM–PM), driven by changes in both external field and temperature. The magnetic-field–dependent magnetostriction exhibits irreversible abrupt jumps at T ≤ 5 K, while it evolves smoothly with applied field above 5 K. A giant positive volume magnetostriction of ΔV/V (20 K, 6 T) = 0.80% is observed across the field-induced first-order AFM–FM metamagnetic transition. It is further revealed that the FM–PM transition is accompanied by a remarkably large NTE with a coefficient of linear thermal expansion αL = −34 × 10−6 K−1 over a temperature window of ΔT ∼ 63 K.

Colossal magneto-resistive relaxation effects in La0.9Ce0.1Fe12B6

The study of the magnetic, electronic transport, and magnetotransport properties of La0.9Ce0.1Fe12B6 itinerant-electron system has been performed by combining magnetization, electrical resistivity, and magnetoresistance experiments. Along with the antiferromagnetic (AFM) ordering at TN = 35 K, two consecutive magnetic transformations, antiferromagnetic–ferromagnetic (AFM–FM) and ferromagnetic–paramagnetic (FM–PM), occur upon heating under certain magnetic field values. At fixed temperatures, it is revealed that both AFM and PM phases can be converted into the FM phase irreversibly and reversibly via a first-order metamagnetic transition associated with a large hysteresis. Below 8 K, the metamagnetic transition is discontinuous, manifesting itself by multiple sudden jumps in magnetoresistance and magnetization. A giant negative magnetoresistance effect of about −78% is found. We further demonstrate that the time dependencies of the electrical resistivity and the magnetization exhibit colossal spontaneous steps after an incubation time in conditions where both the applied magnetic field and temperature are constant. Another intriguing observation in the phase diagram is the presence of a critical point at the crossover of the three distinct PM, FM, and AFM magnetic states.

Nd-Zn Co-substituted M-Type Strontium Hexaferrites with Enhanced Magnetic Properties

In this work, we report the synthesis of Nd-Zn co-substituted M-type strontium hexaferrites Sr1-xNdxFe12-xZnxO19 (0.0 ≤ x ≤ 0.5) by the solid-state method. The analysis of X-ray diffraction (XRD) shows that the single M-type hexaferrite structure can be obtained as the doping concentration is lower than 0.3, while the other side phase (α-Fe2O3) will appear in the XRD patterns for x ≥ 0.3. Furthermore, the specific hexagonal structure is also confirmed by Fourier transform infrared (FTIR) spectroscopy, and field emission scanning electron microscopy (FE-SEM) also discloses the micromorphology of the series samples over the entire range of substitution. Through the systematical magnetic measurements, enhanced magnetic properties can be acquired due to the effective substitution, especially for intrinsic coercivity (Hcj), which increases from 1800 Oe (x = 0.0) to 3230 Oe (x = 0.5) at room temperature. In addition, the doping sample also exhibits a considerable thermal stability because of a high ferromagnetic–paramagnetic (FM–PM) transition temperature (i.e., Curie temperature TC). In consideration of the magnetic performance and high FM–PM phase temperature, Sr1-xNdxFe12-xZnxO19 series of samples can be considered as promising candidates for magnetic recording devices because of such high saturation magnetization and considerable coercivity.

Three-dimensional Heisenberg critical phenomena in La0.6Bi0.1Sr0.3−xCaxMn0.9Cu0.1O3 manganites (x = 0 and 0.05)

In this paper, our main objective is to study the effect of Ca doping on the critical properties of La0.6Bi0.1Sr0.3−xCaxMn0.9Cu0.1O3 (x = 0 and 0.05). The X-ray diffraction analysis reveal that both specimens are indexed in the rhombohedral structure with R–3c space group. As for the magnetic studies, a second-order ferromagnetic–paramagnetic (FM–PM) phase transition takes place around the transition temperature (TC). Based on different methods such as extrapolation of the Arrott curve, Kouvel-Fisher procedure and critical isothermal analysis depending on the data from magnetic investigations, the examination of the critical behavior was performed. The values of the deduced critical exponent (β, γ, δ) for both compounds proved to be close to those predicted by Heisenberg's 3D model, revealing an inhomogeneous magnetic exchange coupling at short distances. These critical exponents show a good agreement with their corresponding values proven by Widom's law of scale, as well as the theory of scale. Additionally, a typical soft FM behavior with a low coercive field was observed at 10 K according to the hysteresis loops. These outcomes make our compounds good candidates for memory devices and magnetic recording.

Sintering Temperature Effects on Structural, Magnetic, Magnetocaloric and Critical Properties of Nd0.67Pb0.33Mn0.9Al0.1O3 Manganites

We have reported the results of structural, magnetic, and magnetocaloric characterizations as well as the critical behaviors of the polycrystalline Nd0.67Pb0.33Mn0.9Al0.1O3 manganites. These samples, denoted as S900 and S1100, were prepared using the sol–gel method and sintered separately at 900 °C and 1100 °C, respectively. XRD analysis shows good crystallization for the samples in the $$ R\overline{3}c $$ rhombohedral structure. From M(T) curves, second-order ferromagnetic–paramagnetic (FM–PM) phase transitions appear at the Curie temperatures (TC) around 130 K and 155 K for S900 and S1100, respectively. The estimated values of magnetic entropy change (−ΔSm) at μ0H = 5 T were found to be 3.23 and 4.41 J/kg.K for S900 and S1100, respectively. The corresponding values of relative cooling power (RCP) are equal to 242 and 245 J/kg. These values are relatively higher, making our samples promising candidates for magnetic refrigeration technology. We found that the critical exponents (β, γ, and δ) agree well with those of the mean-field and the 3D-Heisenberg models for S900 and S1100, respectively.

Study of the magnetic and magnetocaloric properties of new perovskite-type materials: La0.6Ba0.2Sr0.2Mn1−xFexO3

In this paper, we investigated the effect of iron doping on the structural, magnetic and magnetocaloric properties of La0.6Ba0.2Sr0.2Mn1−xFexO3 (0 < x < 0.06) samples. All the samples crystallized in a rhombohedral system with \({\text{R }}\bar{3}{\text{c}}\) space group. Magnetic measurements showed that our samples undergo a FM–PM transition around the Curie temperature (Tc) which is equal to 349 K, 289 K, and 249 K for x = 0, 0.03, and 0.06, respectively. This decrease in Tc with the increase in Fe content can be related to the weakness of the Mn3+–O–Mn4+ double exchange interaction. The magnetocaloric effect was estimated by the calculation of the magnetic entropy change (\(- \;\Delta S_{{\text{M}}}^{{\max}}\)) as a function of temperature at different applied magnetic fields. The maximum values of the magnetic entropy change are found to be 2.43 J kg−1 K−1, 2.16 J kg−1 K−1, and 1.65 J kg−1 K−1 under a magnetic field of 5 T for x = 0, 0.03, and 0.06, respectively.

Magnetic and Table-Like Magnetocaloric Properties of Polycrystalline Pr0.7Ba0.1Sr0.2MnO3

The magnetic and magnetocaloric properties of Pr0.7Ba0.1Sr0.2MnO3 polycrystalline are presented. The sample is single phase of a monoclinic structure with a = 5.4726 A, b = 5.5045 A, c = 7.7339 A, α = γ = 90°, and β = 90.003°. X-ray absorption spectrum shows a co-existence of Mn4+ and Mn3+ ions in the compound. A ferromagnetic–paramagnetic (FM–PM) phase transition occurs at around 232 K. An analysis of the critical behavior near the FM–PM transition indicates a short-range magnetic ordering in Pr0.7Ba0.1Sr0.2MnO3 with critical exponents dependent on an applied magnetic field. The critical parameters are found to be β = 0.220 ± 0.03, 0.224 ± 0.002, and 0.227 ± 0.002, γ = 0.732 ± 0.011, 0.818 ± 0.003, and 0.958 ± 0.013, TC = 232.7, 234.6, and 236.6 K for field ranges of 1–3 T, 3–4 T and 4–5 T, respectively. The maximum entropy change (− ΔSmax) is found to be 5.67 J/kg K near the TC for a 5 T of field change. The table-like magnetocaloric effect observed in this sample is desirable for active magnetic refrigeration.

Structure, magnetic and magnetocaloric properties of nano crystalline perovskite La0.8Ag0.2MnO3

Abstract Nano crystalline La0.8Ag0.2MnO3 perovskite compound was prepared using sol–gel method. The synthesized nano compound was characterized using X-ray Diffraction (XRD) and High Resolution - Transmission Electron Microscope (HR–TEM). The magnetic and magnetocaloric properties were studied in a broad temperature range, and confirmed the presence of ferromagnetic–paramagnetic (FM–PM) transition around Curie temperature (TC = 306 K). We reported a maximum magnetic entropy change |ΔSM|max = 0.96 J/kg.K with relative cooling power (RCP) = 39.98 J/kg at magnetic field 2 T. Such results could be a significant practical application in magnetic refrigeration. The observed magnetic phase transition in the sample was testified a second order. The critical exponent analysis which define the magnetic transition revealed values 0.4025 for β, 3.33 for δ and 0.93 for γ. These values are closely related to those obtained by the mean field model. The second order magnetic phase transition is further confirmed by constructing a universal curve for variation of |ΔSM|/|ΔSmax| vs rescaled temperature (θ) for various magnetic field values.

Anomalous behavior above the Curie temperature in (Nd1-x Gd x )0.55Sr0.45MnO3 (x = 0, 0.1, 0.3 and 0.5).

Magnetic properties were studied just above the ferromagnetic–paramagnetic (FM–PM) phase transition of (Nd1−xGdx)0.55Sr0.45MnO3 with x = 0, 0.1, 0.3 and 0.5. The low-field inverse susceptibility (χ−1) of Nd0.55Sr0.45MnO3 exhibits a Curie–Weiss-PM behavior. For x ≥ 0.1, we observe a deviation in χ−1(T) behavior from the Curie–Weiss law. The anomalous behavior of the χ−1(T) was qualified as Griffiths phase (GP)-like. The study of the evolution of the GP through a susceptibility exponent, the GP temperature and the temperature range of the GP reveals that the origin of the GP is primary due to the accommodated strain. Likewise, the magnetic data reveal distinct features visible only for x = 0.5 at a low magnetic field that can be qualitatively understood as the result of ferromagnetic polarons, entailed by the strong effect of chemical/structural disorder, whose concentration increases upon cooling towards the Curie temperature. We explained the magnetic properties at a high temperature for the heavily Gd-doped sample (x = 0.5) within the phase-separation scenario as an assembly of ferromagnetic nanodomains, antiferromagnetically coupled by correlated Jahn–Teller polarons.

Vanadium-doping effects on magnetic and magnetocaloric efficiency of La0.7Sr0.2(CaLi)0.05Mn1−xVxO3 [x = 0.00 and x = 0.05] manganites

The La0.7Sr0.2Ca0.05Li0.05Mn1−xVxO3 [S1(x = 0) and S2(x = 0.05)] polycrystalline compounds are found to exhibit a ferromagnetic—paramagnetic transition (FM–PM) when temperature increases with a decrease of Curie temperature TC when substituting Mn with V from TC = 271 K to TC = 266 K. The Arrott plots near Curie temperature show positive slopes under an applied magnetic field varying from 0 to 5 T showing a second order magnetic transition for our samples. Basing on the magnetic-field dependences of magnetization measured around TC, maximum magnetic-entropy changes \(\left| {\Delta S_{M}^{{\hbox{max} }}} \right|,\) under the applied field of 5 T, are about 5.4 and 4.8 J kg−1 K−1 for S1 and S2 respectively. The efficiency of our samples for magnetic refrigeration application has been evaluated throw calculating the relative cooling power (RCP) values, which were equal to 211.5 J kg−1 for S1 and 195.5 J kg−1 for S2. The study of the universal curves show that the rescaled magnetic entropy change curves for different applied fields collapse onto a same curve confirming that both S1 and S2 reveal a second order transition.

Structural, magnetic and magnetocaloric properties of electron-doped manganite La0.9Hf0.1MnO3

We have prepared an electron-doped manganite La0.9Hf0.1MnO3 (LHMO) by a solid state reaction method, and then systematically investigated its structural, magnetic and magnetocaloric properties. The analysis of XRD data based on Rietveld refinement reveals that the LHMO sample crystallizes in the rhombohedral structure with a space group of RC. M(T) data show that the ferromagnetic–paramagnetic (FM–PM) transition temperature (TC) is 240 K. Under a magnetic field change of 5 T, the magnetic entropy change (−ΔSM) exhibits a peak value of = 4.27 J kg−1 K−1 with a relative cooling power value of 228.36 J kg−1 in the vicinity of the TC. These results indicate that the electron-doped manganite LHMO can be regarded as a suitable material to be used in active magnetic refrigeration near sub-room temperatures. Both an Arrott plots analysis and phenomenological universal curves confirm that the FM–PM transition in the LHMO is of a second order. In addition, the temperature-dependent −ΔSM can be well understood according to Landau’s theory of phase transition.

Effect of Cu substitution on magnetocaloric and critical behavior in Ni47Mn40Sn13−Cu alloys

Abstract A series of Heusler phases Ni47Mn40Sn13−xCux (x = 0, 0.5, 1, 1.25), which are magnetic shape-memory alloys, has been synthesized by mechanical alloying (MA) and characterized. The martensite-austenite phase transition was investigated by X-ray diffraction, differential scanning calorimetry, and DC magnetization measurements. With greater Cu content, the martensitic transition temperature TM increases while the magnetic transition temperature of austenite T c A remains almost unchanged. The maximum inverse magnetic entropy changes Δ S M for the x = 0.5 and x = 1 samples are nearly twice as high as for the undoped sample. Furthermore, the martensitic transformation of the doped samples occurs around room temperature, which is important for applications. Effective refrigerant capacities (RCeff) reduced due to the higher hysteresis losses in the x = 0.5 and x = 1 samples compare to the undoped one. Refrigerant capacities (RC) were examined in T c A . The largest value of RC = 207 J/kg was measured for the undoped sample. Critical properties near the ferromagnetic–paramagnetic (FM–PM) phase transition were analyzed in detailed. The critical parameters β, γ, δ and T c A of the x = 0 and x = 0.5 samples, which were determined through use of modified Arrott plots, the Kouvel-Fisher method, and the Widom scaling relation, differed from the mean-field values. The critical behavior indicates that long-range ferromagnetic order dominates in these Cu-doped samples.

Structural and phase transition of Mg-doped on Mn-site in La0.7Sr0.3MnO3 bulk/nanostructured perovskite characterised through online ultrasonic technique

Bulk and nanoshaped perovskite manganite materials were blended to form samples of La0.7Sr0.3Mg1−xMnxO3 (LSMMO) with x = 0.05, 0.075, and 0.100 employing solid state and sonochemical methods, respectively. The well-formed LSMMO component was characterized by comprehensive characterization techniques and rhombohedral structure was observed. The mean size of the bulk and nano LSMMO manganite perovskites ranges between 260–850 nm and 23–86 nm, respectively. The structural and phase transition of the manganite perovskites are explored by on-line ultrasonic velocity and attenuation measurements between 300 and 400 K. A deliberate change in the ultrasonic velocity, attenuation and elastic moduli shows an abnormal behavior with temperature in perovskites and was associated with the occurrence of ferromagnetic–paramagnetic (FM–PM) transition temperature (TC). In addition, the shifts in TC magnitude and width by increase in x are used to study the etiquette of TC and the change in the nanostructure of the bulk perovskites.

Synthesis, Phase Transition of La0.69Sr0.31MnO3 Nanoperovskites Toward Its Versatile Structural, Electrical and Mechanical Properties Employing Ultrasonic Measurements

The present investigation correlates the propagation of ultrasonic waves with various micro structural features of nano La0.69Sr0.31MnO3 (LSMO) perovskites with different grain sizes. A solid state reaction followed by the ball milling technique was used to synthesize the nano LSMO perovskite samples with various grain sizes. The occurrence of ferro-paramagnetic transition temperature (Tc) was explored through the observed anomalous behaviour in ultrasonic velocities, attenuations and elastic moduli. As the particle size gets reduced, lower Tc and broader transitions are observed due to the distribution of grain boundaries. The diffusion of the FM–PM phase transition along with decrease in Tc is correlated due to the effect of particle size. Furthermore structural, vibrational and electrical properties of nanosized LSMO perovskite samples with different grain sizes were studied by XRD, FTIR, BET surface area measurement, HRSEM, TEM and four probe conductivity studies. The phase of the synthesized nano LSMO perovskite samples are perfectly indexed to pure rhombohedral perovskite type crystal structure. The conductivity study proves the semiconductor nature of the nano LSMO perovskite samples. In addition to this, the peak broadening in ultrasonic studies at phase transition is in line with the observation made from the XRD pattern for the prepared nano LSMO perovskite samples.

Critical Behavior Near the Paramagnetic to Ferromagnetic Phase Transition in (La 0 . 6 Ca 0 . 4 MnO3)/x(CuO) Ceramic Composites

We report magnetic properties of (La0.6Ca0.4MnO3)/x(CuO) compounds in terms of isothermal magnetization analysis and scaling behavior with various critical exponents in the vicinity of the PM to FM transition region. According to the Banerjee criterion, we found that the FM–PM phase transition is the type of SOMT. Moreover, the critical exponents β, γ, and δ are determined using the modified Arrott-plot (MAP), the Kouvel–Fisher method (KF), as well as the critical isotherm (CI) analysis. The estimated results are close to those expected for three-dimensional Heisenberg class with short-range interaction. From the field dependence of RCP and ΔSM, it was possible to evaluate the critical exponents of the magnetic phase transition. These values are in good agreement with those obtained from the critical exponents using the Kouvel–Fisher plot (KF). Finally, we used the scaling hypotheses to scale the heat capacity changes to a single universal curve for (La0.6Ca0.4MnO3)/x(CuO) composites.

Magnetic inhomogeneity and Griffiths phase in Bi substituted La0.65−xBixCa0.35MnO3 manganites

In the present communication, we report the effect of Bi substitution on structural, magneto-transport, magnetic and thermal properties of La0.65−xBixCa0.35MnO3 (0≤x≤0.2) compounds. Rietveld refined XRD patterns confirm that all the samples are single phase and crystallize in rhombohedral symmetry with R-3C space group. It was observed from electrical and magnetic studies that with increasing Bi content both the metal-insulator transition temperature (TMI) and ferromagnetic–paramagnetic (FM–PM) transition temperature (TC) shift towards lower temperatures. Deviation of the temperature dependence of inverse susceptibility curves from the Curie–Weiss law suggests the existence of Griffiths-like phase. The electrical resistivity data were analyzed by utilizing various theoretical models. It is revealed that the electron–electron scattering is dominating in the metallic region, while the insulating region is well-described by the polaron hopping model. Analysis of thermoelectric power data further reveals that the small polaron hopping (SPH) mechanism is operative in the high-temperature insulating regime. The entropy change associated with the FM–PM transition is found to decrease with increasing x, which is presumably due to the increase in magnetic inhomogeneity with increasing Bi content.