AFM FM Research Articles

Electronic structure and magnetothermal properties of two-dimensional ScCl.

Two-dimensional ferromagnetic materials with intrinsic half-metallic properties have strong application advantages in nanoscale spintronics. Herein, density functional theory calculations show that monolayer ScCl is a ferromagnetic metallic material when undoped (n = 0), and the transition from metal to half-metal occurs with the continuous doping of holes. On the contrary, as the concentration of doped electrons increases, the system will exhibit metallic characteristics, which is particularly evident from a variation in spin polarizability. Furthermore, we have discussed how doped carriers affect the shape of the Fermi surface and the Fermi velocity of electrons. Most importantly, Monte Carlo simulations show that the ScCl monolayer is particularly regulated by carrier concentration (n) and magnetic field (h). Additionally, trends in energy and magnetic exchange coupling in different magnetic configurations (AFM phase and FM phase) with different doping concentrations are presented. When n < -0.16, the material is not only a half-metallic material that easily flips the magnetic axis, but also proves to be a candidate ferromagnetic material that works stably at room temperature in terms of dynamic stability. In addition, the origin of magnetocrystalline anisotropy is analyzed, and the contribution of different orbitals to spin-orbit coupling is presented. Moreover, we note that when magnetic field is small (h < 1 T), the change in size has a significant effect on ferromagnetic phase transition. However, when the system size is large (size >15 nm), TC is less sensitive to magnetic field. In addition, hole doping and size effect will greatly affect the hC of the system, but when the hole doping exceeds the critical value (n = -0.16), its influence on the hysteresis loop is no longer obvious. These interesting magnetic phenomena and easily adjustable physical properties show us that monolayer ScCl will be a promising functional material.

Calculation of magnetocaloric effect with regard for dependence of heat capacity on magnetic field

A specific heat of the magnetic solid exhibiting AFM–FM phase transition is computed using the Landau-type theory of phase transitions. The experimentally observed dependence of the specific heat value on the external magnetic field is modelled. It is shown, that this dependence has strong influence on the giant magnetocaloric effect (MCE), which is inherent to the solids exhibiting the phase transitions accompanied by the strong change of magnetization value: the disregard of this dependence leads to the noticeable overestimation of adiabatic temperature change, which is the practically important characteristic of MCE. The temperature change characterizing the giant MCE observed in Fe–Rh alloys is computed. The reasonable agreement between the available experimental data and obtained theoretical results is demonstrated.

Mechanical Properties Evaluation of Polymer-Binding C-S-H Structure from Nanoscale to Macroscale: Hydroxyl-Terminated Polydimethylsiloxane (PDMS) Modified C-S-H

Exploring and modifying the C-S-H structure at a micro-nano level is an effective solution to improve the performance of Portland cement. Compared with organics inserting C-S-H, the research on the performance of a polymer-binding C-S-H structure from nanoscale to macroscale is limited. In this work, the mechanical properties of a modified C-S-H, using hydroxyl-terminated polydimethylsiloxane (PDMS) as the binders, are evaluated. The PDMS-modified C-S-H structures are introduced into macro-defect-free cement to obtain stress-strain curves changes at a macro scale. The AFM-FM was adopted to measure the morphology and elastic modulus of C-S-H at a nano scale. The molecular dynamics (MD) simulation was performed to assess the toughness, tensile properties, and failure mechanism. The results show that the PDMS-modified C-S-H powders change the break process and enhance ductility of MDF cement. The elastic modulus of PDMS-modified C-S-H is lower than pure C-S-H. When PDMS molecules are located between the stacking crystal units, it can enhance the toughness of C-S-H aggregates. The PDMS-modified C-S-H stacking structure has better plasticity, and its tensile strains are higher than the pure C-S-H. PDMS molecules hinder the initial crack expansion, leading to the branching of the initial crack. In addition, the measurement of AFM-FM can identify and obtain the mechanical properties of basic units of C-S-H. This paper enhances the understanding of cement strength sources and modification methods.

The effect of vanadium substitution on the structural and magnetic properties of (Fe[formula omitted]V[formula omitted])[formula omitted]Ga[formula omitted

Fe3Ga4 displays a complex magnetic phase diagram that is sensitive and tunable with both electronic and crystallographic structure changes. In order to explore this tunability, vanadium-doped (Fe1−xVx)3Ga4 has been synthesized and characterized. High-resolution synchrotron X-ray diffraction and Rietveld refinement show that samples up to 20% V-doping remain isostructural to Fe3Ga4 and display a linear increase in unit cell volume as doping is increased. Magnetic measurements reveal a suppression of the antiferromagnetic helical spin-density wave (SDW) with V-doping, revealed by changes in both the low-temperature ferromagnetic–antiferromagnetic (FM–AFM) transition (T1) and high-temperature AFM–FM transition (T2). At 7.5% V-doping, the metamagnetic behavior of the helical AFM SDW phase is no longer observed. These results offer an avenue to effective tuning of the magnetic order in Fe3Ga4 for devices, as well as increased understanding of the magnetism in this system.

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.

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.

Tuned AFM–FM coupling by the formation of vacancy complex in Gd0.6Ca0.4MnO3 thin film lattice

The effect of in situ oxygen and vacuum annealings on the low bandwidth manganite Gd1−x Ca x MnO3 (GCMO) thin film with x = 0.4 was investigated. Based on the magnetic measurements, the AFM–FM coupling is suppressed by the vacuum annealing treatment via destroying the double exchange interaction and increasing the unit cell volume by converting the Mn4+ to the Mn3+. Consequently, resistance increases significantly compared to pristine film. The results are explained by a model obtained from the positron annihilation studies, where the vacuum annealing increased the annihilation lifetime in A and B sites due to the formation of vacancy complexes V A,B–V O, which was not the case in the pristine sample. The positron annihilation analysis indicated that most of the open volume defects have been detected in the interface region rather than on the subsurface layer and this result is confirmed by detailed x-ray reflection analysis. On the other hand, the effect of oxygen annealing on the unit cell volume and magnetization was insignificant. This is in agreement with positron annihilation results which demonstrated that the introduction of oxygen does not change the number of cation vacancies significantly. This work demonstrates that the modification of oxygen vacancies and vacancy complexes can tune magnetic and electronic structure of the epitaxial thin films to provide new functionalities in future applications.

Real-space imaging of magnetic phase transformation in single crystalline Sm–Ca–Sr based manganite compound

Low-temperature-high-magnetic field magnetic force microscopy studies on colossal magnetoresistance material Sm0.5Ca0.25Sr0.25MnO3 have been carried out. These measurements provide real-space visualization of antiferromagnetic–ferromagnetic (AFM–FM) transition on sub-micron length scale and explain the presence of AFM–FM transition in the temperature-dependent magnetization measurements, but the absence of corresponding metal–insulator transition in temperature-dependent resistivity measurements at the low magnetic field. Distribution of transition temperature over the scanned area indicates towards the quench disorder broadening of the first-order magnetic phase transition. It shows that the length scale of chemical inhomogeneity extends over several micrometers.

Overcoming the asymmetry of the electron and hole doping for magnetic transitions in bilayer CrI3.

Electrical control of magnetism has great potential for low-power spintronics applications and the newly discovered two-dimensional van der Waals magnetic materials are promising systems for this type of applications. In fact, it has been recently shown experimentally (Jiang et al., Nat. Nanotechnol., 2018, 13, 549-553) that upon electrostatic doping by electrons bilayer CrI3 undergoes an antiferromagnetic-ferromagnetic (AFM-FM) phase transition, even in the absence of magnetic field. Doping by holes, on the other hand, does not induce the same transition in the experiment, which points to an intrinsic asymmetry in the hole and electron doping that limits the control of the transition by doping. We here show, based on first-principles calculations, that the asymmetry originates in the relativistic nature of the valence-band-edge states of the pristine bilayer, which inhibits the magnetic transition upon hole doping. Based on this finding, we propose an approach to overcome the asymmetry and predict the existence of the AFM-FM transition for both hole and electron doping upon moderate uniaxial compression along the soft direction of the bilayer.

Doping‐Driven Antiferromagnetic to Ferromagnetic Phase Transition in Tetragonal Cr2B2 Monolayer

Novel two‐dimensional (2D) tetragonal (tetra‐) (X = Mn, V) sheets are studied using density functional theory (DFT) with Monte Carlo (MC) simulations. Both, tetra‐ and tetra‐ sheets exhibit metallic ferromagnetism with great stability at elevated temperatures. The magnetic moment for tetra‐ (1.63 μ B per unit cell) and tetra‐( μ B per unit cell) sheets are found to be far less than that of pure tetra‐ ( μ B per unit cell). Interestingly, an antiferromagnetic–ferromagnetic (AFM–FM) transition occurs as a result of alloying the pure with and V atoms. The tetra‐ exhibits a sizable magnetic anisotropy energy (MAE) of 136.25 per atom, larger than that of tetra‐ (17.5 per atom), and both the sheets prefer out‐of‐plane magnetization. The MAE can be tuned via the application of an electric field for both, tetra‐ and tetra‐ cases. MC simulations based on 2D Heisenberg model predict a Curie temperature () of 140 and 20 K for tetra‐ and tetra‐ sheets, respectively. These novel 2D sheets hold promise for controllable spintronic devices application.

Direct and indirect measurement of the magnetocaloric effect in the La0.5Ca0.5−xPbxMnO3 (0 ≤ x ≤ 0.2) manganites

The magnetocaloric properties in La0.5Ca0.5−xPbxMnO3 (0 ≤ x ≤ 0.2) manganites are considered by both direct and indirect measurement techniques. The ac susceptibility shows that the compounds with low Pb doping exhibit mixture of CE-type AFM ordering and FM phase, while at high Pb doping FM phase is dominated. For samples x = 0 and 0.05, the M-H curves at different temperatures exhibit the magnetization first increases and then decreases with decreasing temperature, indicating the growth of the AFM phase below TN. In the curves for samples x = 0.1 and 0.2, with decreasing temperature the magnetization increases and the FM phase is overcoming at low temperature. The magnetic entropy change (ΔSM) is enhanced continuously with increasing Pb concentration up to x = 0.1, then slightly decreases. The magnetocaloric effect in the vicinity of TC for x = 0.1 and 0.2 was investigated by the direct field-induced adiabatic temperature change (ΔTad). The results of the direct method show the absolute values of obtained ΔTad are almost the same for both samples (0.62 and 0.53 K under 1.96 T for x = 0.1 and 0.2, respectively). The outcomes demonstrate that doping by Pb is a promising material for the magnetic refrigeration.

Thermodynamic properties and excitation spectrum of spin chain with antiferromagnetic - ferromagnetic interactions

Thermodynamic properties of antiferromagnetic spin chain with various AFM nearest-neighbor and FM next-nearest-neighbor exchange interactions are studied using functional integral method in J1 – J2 Heisenberg model. Besides, the excitation spectrum of the system is also given. The results show that the system has two distinct excitations with no energy gap from the ground state for any permissible values of J1 and J2 without single-ion anisotropy. Changing the exchange parameters influences on the thermodynamic properties and the excitation spectra of the system.

Enhancement of dielectric and magnetic properties in phase pure, dense BiFeO3 nanoceramics synthesized by spark plasma sintering techniques

Phase pure, dense BiFeO3 (BFO) ceramics with average grain sizes of ~ 110 nm, ~ 450 nm, and ~ 1.15 µm were fabricated by spark plasma sintering method. BFO ceramics exhibited grain-size-dependent magnetic properties, which ascribed to the antiferromagnetism–ferromagnetism (AFM–FM) transition. For BFO nanoceramics (~ 110 nm), such transition was much significant, and contributed to a large exchange bias field of HEB = 500 Oe at 5 K. In addition, BFO nanoceramics (~ 110 nm) exhibited lower leakage current and higher resistivity compared to the larger-grained BFO ceramics (~ 450 nm and ~ 1.15 µm). The calculated activation energies (Ea) and X-ray photoelectron spectroscopy analyses revealed the existence of different types of defects in BFO ceramics with different grain sizes.

Exchange Bias in Layered GdBaCo2O5.5 Cobaltite

It is established that excess oxygen content δ influences the exchange bias (EB) in layered GdBa-Co2O5 + δ cobaltite. The EB effect arises in p-type (δ > 0.5) cobaltite and disappears in n-type (δ < 0.5) cobaltite. The main parameters of EB in GdBaCo2O5.52(2) polycrystals are determined, including the field and temperature dependences of EB field H EB , blocking temperature T B , exchange coupling energy J i of antiferromagnet–ferromagnet (AFM–FM) interface, and dimensions of FM clusters. The training effect inherent in systems with EB has been studied. The results are explained in terms of exchange interaction between the FM and AFM phases. It is assumed that the EB originates from the coexistence of Co3+ and Co4+ ions that leads to the formation of monodomain FM clusters in the AFM matrix of cobaltite.

Hall detection of anisotropic domain walls during magnetic phase transition

We investigate the electrical detection of anisotropic antiferromagnetic/ferromagnetic (AFM/FM) domain walls in (2 at%) Pd-doped FeRh films. The Hall signals keep almost at zero at both AFM and FM states, while a sizeable Hall conductivity up to 300 (Ω cm)−1 emerges during the AFM–FM phase transition of Pd-doped FeRh films around room temperature. Such Hall signals could be ascribed to the transverse current scattered by the anisotropic AFM/FM domain walls, which are mainly aligned around the [1 0 0] axis of the Pd-doped FeRh films. The highly reproducible Hall signals in both the temperature and magnetic field-induced magnetic phase transition are supported by the observation of a nonrandom formation of AFM/FM domain walls. Our findings not only advance the understanding of the magnetic phase transitions, but also propose a considerable way for the detection of AFM/FM domain walls by an electrical means.

The coexistence of cluster glass behavior and long-range ferromagnetic ordering in La0.7Sr0.25Na0.05Mn0.7Ti0.3O3 manganite

The electron-doped La0.7Sr0.25Na0.05Mn0.7Ti0.3O3 (LSNMTi0.3) sample was synthesized by a conventional solid-state reaction. Rietveld analysis of the X-ray diffraction (XRD) data showed that the compound crystallized in the space group R3¯c. Magnetic characterization present a signature of a coexisting AFM–FM ordering and a cluster-glass phase. The M2 vs. H/M curves prove that the samples exhibit a second-order magnetic phase transition and the critical properties near the ferromagnetic–paramagnetic phase transition temperature have been analyzed from data of the static magnetization measurements for the sample, through various techniques such as the modified Arrott plot and the critical isotherm analysis. The critical exponent values estimated from the isothermal magnetization measurements are found to be consistent and comparable to those predicted by the long-range mean-field theory.

Multiferroic behavior, enhanced magnetization and exchange bias effect of Zn substituted nanocrystalline LaFeO3 (La(1−x)ZnxFeO3, x=0.10, and 0.30)

Nanocrystalline Zn-substituted lanthanum orthoferrite La(1−x)ZnxFeO3 (x=0.1 and 0.3) (LZFO1 for x=0.1 and LZFO2 for x= 0.3) was prepared by a chemical co-precipitation method. The as-prepared samples were annealed at 500°C for 12h. The formation of the pure crystallographic phase of LaFeO3 (LFO) and the substitution of Zn2+ in the doped samples were confirmed by X-ray diffractograms. Magnetization curves and hysteresis loops in the range of 300–2K showed that magnetization of the doped samples was considerably enhanced compared to that of the pristine LFO. The maximum enhancement factor of ∼2.5 for the saturation magnetization measured at room temperature was found in LZFO2. Interestingly, an exchange bias effect was observed in the nanoparticles LFO and LZFOs, which suggest the formation of antiferromagnetic–ferromagnetic (AFM–FM) core-shell structure of the AFM nanoparticles of LFO and LZFOs. The hysteretic behavior observed in the polarization–electric field curves recorded at room temperature indicates the onset of ferroelectric ordering of the samples. The dielectric constant measured in the presence and absence of a magnetic field confirmed that the samples are type-II multiferroics with positive magneto-coupling effect.

Irreversible metamagnetic transition and magnetic memory in small-bandwidth manganite Pr1−xCaxMnO3 (x = 0.0–0.5)

The present paper reports detailed structural and magnetic characterization of the low-bandwidth manganite Pr1−xCaxMnO3 (with x = 0.0–0.5) (PCMO) polycrystalline samples. With increasing Ca content, reduction of the unit cell volume and improvement in perovskite structure symmetry was observed at room temperature. Magnetic characterization shows the signature of coexisting AFM–FM ordering and spin-glass phase at the low doping range (x = 0.0–0.2) while increased hole doping (x = 0.3–0.5) leads to charge ordering, training effect and an irreversible metamagnetic phenomenon. The large irreversible metamagnetism in the CO phase of PCMO and the corresponding spin memory effect is a direct consequence of hysteretic first-order phase transition arising from the weakening of the CO state under the external magnetic field and trapping of the spins due to a strong pinning potential in the material.

Structural and Magnetic Studies of Ca2−x Sm x MnO Compounds (x=0–0.4)

The Ca2−x Sm x MnO4 (x=0–0.4) compounds were synthesized by a solid–solid method and characterized by XRD at room temperature. The XRD profiles were indexed with a tetragonal structure (I/4mmm space group) for x≤0.3 and orthorhombic one (Pnma space group) for x=0.4. The magnetic measurements were studied as a function of temperature (T=2–300 K) and applied field (μ 0 H=0–10 T). When the temperature decreases, each compound has shown first a ferromagnetic–paramagnetic (FM–PM) transition and then an antiferromagnetic–ferromagnetic (AFM–FM) one. The transition temperatures are found to be Sm-doping dependent. For all compounds, a spin-glass phenomenon is evidenced by FC/ZFC magnetization curves.