Sublattice Magnetization Research Articles

Effects of charge ordering in electronic subsystem of quasi-2D BEDT-TTF conductors

The interplay of the inter-site Coulomb repulsion which induces charge ordering, and the correlated hopping of electrons, which causes peculiar dependence of energy characteristics on a doping, is studied in a model of BEDT-TTF organic conductor electronic subsystem. By applying the projection method in the Green function equation of motion approach the single-particle energy spectrum, sublattice magnetization in charge-ordered phase, and the transition temperature have been calculated. Dependence of Verwey temperature on the band filling shows that inter-site Coulomb correlation significantly influences the width of the concentration interval in which a charge order stabilizes. The correlated hopping causes the decrease of the critical value of electron concentration and increase of Verwey temperature in whole concentration interval of charge ordering stability.

Study on structural, magnetic and electrical properties of ReFeO3 (Re= La, Pr, Nd, Sm & Gd) orthoferrites

Nanoparticles of ReFeO3 (Re= La, Pr, Nd, Sm & Gd) orthoferrites were synthesized by sol-gel combustion method. Detailed structural and microstructural parameters of ReFeO3 nanoparticles (NPs) were reported in this research. The magnetic and electrical properties of ReFeO3 NPs were investigated by Magnetization-magnetic field and Polarization-electrical field hysteresis loops. Based on this research SmFeO3 exhibits a ferromagnetic trend with an incomplete S-like hysteresis curve and the other compounds show a weak ferromagnetic behavior. The magnetic behaviors of ReFeO3 NPs were discussed based on the magnetization of Re and Fe sublattices and a core-shell model. The dielectric properties versus the frequency of ReFeO3 NPs were measured in the frequency range from 500 to 10 MHz. The temperature dependence of dielectric constant (ε) and dielectric loss (tanδ) of the samples have been investigated at 1000 Hz to determine the Neel temperature (TN) of these compounds.

Unifying femtosecond and picosecond single-pulse magnetic switching in Gd-Fe-Co

Many questions are still open regarding the physical mechanisms behind the magnetic switching in GdFeCo alloys by single optical pulses. Phenomenological models suggest a femtosecond scale exchange relaxation between sublattice magnetization as the driving mechanism for switching. The recent observation of thermally induced switching in GdFeCo by using both several picosecond optical laser pulse as well as electric current pulses has questioned this previous understanding. This has raised the question of whether or not the same switching mechanics are acting at the femo- and picosecond scales. In this work, we aim at filling this gap in the understanding of the switching mechanisms behind thermal single-pulse switching. To that end, we have studied experimentally thermal single-pulse switching in GdFeCo alloys, for a wide range of system parameters, such as composition, laser power and pulse duration. We provide a quantitative description of the switching dynamics using atomistic spin dynamics methods with excellent agreement between the model and our experiments across a wide range of parameters and timescales, ranging from femtoseconds to picoseconds. Furthermore, we find distinct element-specific damping parameters as a key ingredient for switching with long picosecond pulses and argue, that switching with pulse durations as long as 15 picoseconds is possible due to a low damping constant of Gd. Our findings can be easily extended to speed up dynamics in other contexts where ferrimagnetic GdFeCo alloys have been already demonstrated to show fast and energy-efficient processes, e.g. domain-wall motion in a track and spin-orbit torque switching in spintronics devices.

Large negative magnetoresistance in the antiferromagnet BaMn2Bi2

A very large negative magnetoresistance (LNMR) is observed in the insulating regime of the antiferromagnet BaMn$_2$Bi$_2$ when a magnetic field is applied perpendicular to the direction of the sublattice magnetization. High perpendicular magnetic field eventually suppresses the insulating behavior and allows BaMn$_2$Bi$_2$ to re-enter a metallic state. This effect is seemingly unrelated to any field induced magnetic phase transition, as measurements of magnetic susceptibility and specific heat did not find any anomaly as a function of magnetic fields at temperatures above $2\,\mathrm{K}$. The LNMR appears in both current-in-plane and current-out-of-plane settings, and Hall effects suggest that its origin lies in an extreme sensitivity of conduction processes of holelike carriers to the infinitesimal field-induced canting of the sublattice magnetization. The LNMR-induced metallic state may thus be associated with the breaking of the antiferromagnetic parity-time symmetry by perpendicular magnetic fields and/or the intricate multi-orbital electronic structure of BaMn$_2$Bi$_2$.

Mixed spin-3/2 and spin-2 nanowire: magnetic properties and hysteresis behaviors

PurposeThe purpose of this article is to investigate the magnetic properties and the hysteresis loops behavior of a ferrimagnetic cubic nanowire with mixed spins SA = 3/2 and SB = 2.Design/methodology/approachWe have used the Monte Carlo simulation to examine the influences of the exchange interaction JB, the crystal field ∆ and the temperature on the magnetic properties and hysteresis loops of the nanowire. More exactly, we have shown the temperature dependence of the sublattice magnetizations (mA and mB) and the total magnetization (M) for several values of the Hamiltonian parameters, as well as the corresponding phase diagrams. Finally, the effect of an external magnetic field is studied by plotting the hysteresis loops of the system for different values of exchange interaction, crystal field and temperature.FindingsThe obtained results show the existence of second-order phase transitions, as well as the compensation behavior. Moreover, according to the values of the Hamiltonian parameters, the system can exhibit one, two or three hysteresis loops.Originality/valueThe magnetic nanowires are of great interest in experimental works, but without theoretical explanations, the experimental results cannot be clarified in depth. For this, we contribute through this theoretical study to understand the nanowires, especially those with mixed spins (2, 3/2).

Distinct magnetic ground states of R2ZnIrO6 (R=La,Nd) determined by neutron powder diffraction

Double perovskite iridates $A_2$ZnIrO$_6$ ($A$ = alkaline or lanthanide) show complex magnetic behaviors ranging from weak ferromagnetism to successive antiferromagnetic transitions. Here we report the static ($dc$) and dynamic ($ac$) magnetic susceptibility, and neutron powder diffraction measurements for $A$ = La and Nd compounds to elucidate the magnetic ground state. Below 10~K, the $A$ = La compound is best described as canted iridium moments in an antiferromagnet arrangement with a propagation vector \textbf{k} = 0 and a net ferromagnetic component along the $c$-axis. On the other hand, Nd$_2$ZnIrO$_6$ is described well as an antiferromagnet with a propagation vector \textbf{k} = (1/2~1/2~0) below $T_\mathrm{N} \sim$ 17 K. Scattering from both the Nd and Ir magnetic sublattices were required to describe the data and both were found to lie almost completely within the $ab$-plane. $Dc$ susceptibility revealed a bifurcation between the zero-field-cooled and field-cooled curves below $\sim$13 K in Nd$_2$ZnIrO$_6$. A glassy state was ruled out by $ac$ susceptibility but detailed magnetic isotherms revealed the opening of the loop below 13~K. These results suggest a delicate balance exists between the Dzyaloshinskii-Moriya, crystal field schemes, and $d$-$f$ interaction in this series of compounds.

Role of 3d–4f exchange interaction and local anti-site defects in the magnetic and magnetocaloric properties of double perovskite Ho2CoMnO6 compound

The strong coupling between 3d and 4f based magnetic sublattices in double perovskite (DP) compounds results in various exotic complex magnetic interactions, and the ground state contains multiple fascinating and remarkable magnetic states. In this article, we have performed a detailed investigation of the crystal structure, magnetic, and magnetocaloric properties of the ordered monoclinic polycrystalline double perovskite Ho2CoMnO6 (HCMO) compound. A study of the magnetization dynamics employing temperature and magnetic field shows a powerful correlation between Ho and Co/Mn sublattices. Due to the presence of the ferromagnetic superexchange interaction in between Co2+−O−Mn4+ networks, the system undergoes an ordered state at the transition temperature, TC≈77K. Below TC, a clear compensation point continued by negative magnetization is noticed in the virgin state of the compound. The reduction of the saturation magnetization (MS) in the hysteresis curves (M-H) can be explained by the existence of local anti-site defects or disorders and anti-phase boundaries in the system. Temperature dependence of magnetic entropy change (−ΔS) curves shows a maximum value of 13.4 J/kg K for ΔH=70kOe at a low temperature along with a noticeable inverse magnetocaloric effect. Moreover, the material holds reasonable values of magnetocaloric parameters. The absence of thermal hysteresis along with a large value of |ΔS| makes the system a potential candidate for low temperature as well as liquid nitrogen temperature-based magnetic refrigeration. Additionally, our experimental findings should encourage further detailed studies on the complex 3d–4f exchange interaction in the double perovskite system.

Revealing noncollinear magnetic ordering at the atomic scale via XMCD

Mass-selected V and Fe monomers, as well as the heterodimer {text{Fe}}_1{text{V}}_1, were deposited on a Cu(001) surface. Their electronic and magnetic properties were investigated via X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. Anisotropies in the magnetic moments of the deposited species could be examined by means of angle resolving XMCD, i.e. changing the X-ray angle of incidence. A weak adatom-substrate-coupling was found for both elements and, using group theoretical arguments, the ground state symmetries of the adatoms were determined. For the dimer, a switching from antiparallel to parallel orientation of the respective magnetic moments was observed. We show that this is due to the existence of a noncollinear spin-flop phase in the deposited dimers, which could be observed for the first time in such a small system. Making use of the two magnetic sublattices model, we were able to find the relative orientations for the dimer magnetic moments for different incidence angles.

Structural Model and Spin-Glass Magnetism of the Ce3Au13Ge4 Quasicrystalline Approximant.

In a search for unconventional heavy-Fermion compounds with the localized 4f moments distributed quasiperiodically instead of a conventional distribution on a regular, translationally periodic lattice, we have successfully synthesized a stable Ce3Au13Ge4 Tsai-type 1/1 quasicrystalline approximant of the off-stoichiometric composition Ce3+xAu13+yGe4+z (x = 0.17, y = 0.49, z = 1.08) and determined its structural model. The structure is body-centered-cubic (bcc), with space group Im3̅, unit cell parameter a = 14.874(3) Å, and Pearson symbol cI174, and can be described as a bcc packing of partially interpenetrating multishell rhombic triacontahedral clusters. The cerium sublattice, corresponding to the magnetic sublattice, consists of a bcc packing of Ce icosahedra with an additional Ce atom in a partially occupied site (occupation 0.7) at the center of each icosahedron. The measurements of its magnetic properties and the specific heat have demonstrated that it is a regular intermetallic compound with no resemblance to heavy-Fermion systems. The partially occupied Ce2 site in the center of each Ce1 icosahedron, the mixed-occupied Au/Ge ligand sites between the Ce2 and Ce1 atoms, and the random compositional fluctuations due to nonstoichiometry of the investigated Ce3+xAu13+yGe4+z alloy introduce randomness into the Ce magnetic sublattice, which causes a distribution of the indirect-exchange antiferromagnetic interactions between the spins. Together with the geometric frustration of the triangularly distributed Ce moments, this leads to a spin-glass phase below the spin freezing temperature Tf ≈ 0.28 K.

Experimental observation of weak antiferromagnetic correlation between blocked spins in the nanocrystalline La0.45Ca0.55MnO3 compound at low temperature

In the present study, an attempt has been taken to probe the meta-stable nature of the magnetic ground state in the nanocrystalline La0.45Ca0.55MnO3 compound at the low temperature through magnetic and magneto-transport properties and compared the results from the observation with the bulk polycrystalline form of the La0.45Ca0.55MnO3 compound. The charge-ordered antiferromagnetic ground state in the bulk compound transformed into a ferromagnetic state in the nanocrystalline sample. Additionally, in the nanocrystalline form of the compound, a short-range antiferromagnetic correlation between the blocked spin is noticed at the low temperature region (T ⩽ 50 K). Such weak magnetic interaction between magnetic spins or sublattices is probed through the magneto-transport and magnetocaloric measurements study in the nanocrystalline La0.45Ca0.55MnO3 compound.

Spin order and entropy in antiferromagnetic films subjected to magnetic fields

Using systematic effective field theory, we explore the properties of antiferromagnetic films subjected to magnetic and staggered fields that are either mutually aligned or mutually orthogonal. We provide low-temperature series for the entropy density in either case up to two-loop order. Invoking staggered, uniform and sublattice magnetizations of the bipartite antiferromagnet, we investigate the subtle order–disorder phenomena in the spin arrangement, induced by temperature, magnetic and staggered fields—some of which are quite counterintuitive. In the figures we focus on the spin- square-lattice antiferromagnet, but our results are valid for any other bipartite two-dimensional lattice.

Sub-picosecond exchange–relaxation in the compensated ferrimagnet Mn2Ru x Ga

We study the demagnetization dynamics of the fully compensated half-metallic ferrimagnet Mn2Ru x Ga. While the two antiferromagnetically coupled sublattices are both composed of manganese, they exhibit different temperature dependencies due to their differing local environments. The sublattice magnetization dynamics triggered by femtosecond laser pulses are studied to reveal the roles played by the spin and intersublattice exchange. We find a two-step demagnetization process, similar to the well-established case of Gd(FeCo)3, where on a 5 ps timescale the two Mn-sublattices seem to have different demagnetization rates. The behaviour is analysed using a four-temperature model, assigning different temperatures to the two manganese spin baths. Even in this strongly exchange-coupled system, the two spin reservoirs have considerably different behaviour. The half-metallic nature and strong exchange coupling of Mn2Ru x Ga lead to spin angular momentum conservation at much shorter time scales than found for Gd(FeCo)3 which suggests that low-power, sub-picosecond switching of the net moment of Mn2Ru x Ga is possible.

Destruction of long-range magnetic order in an external magnetic field and the associated spin dynamics in Cu2GaBO5 and Cu2AlBO5 ludwigites

The quantum spin systems Cu$_2$M'BO$_5$ (M' = Al, Ga) with the ludwigite crystal structure consist of a structurally ordered Cu$^{2+}$ sublattice in the form of three-leg ladders, interpenetrated by a structurally disordered sublattice with a statistically random site occupation by magnetic Cu$^{2+}$ and nonmagnetic Ga$^{3+}$ or Al$^{3+}$ ions. A microscopic analysis based on density-functional-theory calculations for Cu$_2$GaBO$_5$ reveals a frustrated quasi-two-dimensional spin model featuring five inequivalent antiferromagnetic exchanges. A broad low-temperature $^{11}$B nuclear magnetic resonance points to a considerable spin disorder in the system. In zero magnetic field, antiferromagnetic order sets in below $T_\text{N}$ $\approx$ 4.1 K and ~2.4 K for the Ga and Al compounds, respectively. From neutron diffraction, we find that the magnetic propagation vector in Cu$_2$GaBO$_5$ is commensurate and lies on the Brillouin-zone boundary in the (H0L) plane, $\mathbf{q}_\text{m}$ = (0.45 0 -0.7), corresponding to a complex noncollinear long-range ordered structure with a large magnetic unit cell. Muon spin relaxation is monotonic, consisting of a fast static component typical for complex noncollinear spin systems and a slow dynamic component originating from the relaxation on low-energy spin fluctuations. Gapless spin dynamics in the form of a diffuse quasielastic peak is also evidenced by inelastic neutron scattering. Most remarkably, application of a magnetic field above 1 T destroys the static long-range order, which is manifested in the gradual broadening of the magnetic Bragg peaks. We argue that such a crossover from a magnetically long-range ordered state to a spin-glass regime may result from orphan spins on the structurally disordered magnetic sublattice, which are polarized in magnetic field and thus act as a tuning knob for field-controlled magnetic disorder.

Mixed Spin-1/2 and Spin-3/2 Blume-Capel Ising Ferromagnetic System in a Longitudinal Magnetic Field

By means of the recursion relations method, the exact formulations of magnetizations for a mixed spin-1/2 and spin-3/2 Blume-Capel Ising ferromagnetic model on Bethe lattice in a longitudinal magnetic field are presented. The exact expressions for sublattice magnetizations and average magnetization per site are obtained, respectively, and the temperature dependence of the magnetizations for the Bethe lattice with coordination number 4 is numerically studied. The effects of the longitudinal field and crystal-field interaction on the magnetizations are also studied. It is found that the sublattice magnetizations and average one have two different saturation values in the case of low temperature, and decrease from three different saturation values to the same constant as D/J decreases in the case of high temperature.

Study of the Morphology and Magnetic Properties of Fe Island Films with Antiferromagnetic Layers

Arrays of magnetic nanocontacts are fabricated by growing Fe island films and filling the space between the islands with antiferromagnetic layers. The magnetic structures of the islands and their dependences on their dimensions are studied using atomic-force microscopy and micromagnetic calculations. Micromagnetic numerical calculations of the influence of the spin-polarized current flowing from the ferromagnetic edge into the antiferromagnetic interlayer on magnetization in magnetic antiferromagnet sublattices are carried out. The magnetization skew angle in magnetic antiferromagnet sublattices is found as a function of the current density.

Synthesis and magnetic hyperthermia properties of zwitterionic dopamine sulfonate ligated magnesium ferrite and zinc ferrite nanoparticles

$${\text{MgFe}_{2}\text {O}_{4}}$$ and $$\text {ZnFe}_{2}\text {O}_{4}$$ nanoparticles have shown relatively good biocompatibility and high superparamagnetic hyperthermia value, on par with the well-studied $$\text {Fe}_{3}\text {O}_{4}$$ nanoparticles. However, different studies have reported different values of hyperthermia on both the compounds. To elucidate which compound is superior hyperthermia compound, well-crystallined and narrow size distributed $$\text {MgFe}_{2}\text {O}_{4}$$ and $$Z\text {nFe}_{2}\text {O}_{4}$$ spherical nanoparticles (16 nm diameter) were synthesized by solvothermal reflux method and studied their magnetic hyperthermia values under identical conditions. To further enhance superparamagnetic hyperthermia in $$\text {MgFe}_{2}\text {O}_{4}$$ and $$\text {ZnFe}_{2}\text {O}_{4}$$ nanoparticles, hydrodynamic diameter of nanoparticles were reduced by completely removing the long chain oleic acid surfactant monolayer on nanoparticles and ligated short chain zwitterionic dopamine sulfonate surfactant on bare nanoparticles. These nanoparticles show good aqueous colloidal stability with zeta potential of − 32 mV. The study reveals that $$\text {MgFe}_{2}\text {O}_{4}$$ nanoparticles shows higher hyperthermia value (265 W/g) than that (216 W/g) of $$\text {ZnFe}_{2}\text {O}_{4}$$ nanoparticles at 1 mg/mL concentration under 35.33 kA/m and 316 kHz field parameters. This is high value at 1 mg/mL concentration compared to literature. High hyperthermia value arises from higher saturation mass magnetization (45.5 emu/g) of $$\text {MgFe}_{2}\text {O}_{4}$$ than that (36.5 emu/g) of $$\text {ZnFe}_{2}\text {O}_{4}$$ nanoparticles. High saturation magnetization of $$\text {MgFe}_{2}\text {O}_{4}$$ results from the preferential tetrahedral sites occupation of nonmagnetic $${\text {Mg}^{2+}}$$ due to small ionic radii than $${\text {Zn}^{2+}}$$ ions. If tetrahedral magnetic sublattice is occupied by more nonmagnetic cations, the net ferrimagnetic moment increases. The samples were characterized by X-ray diffraction, infrared spectra, thermogravimetry analysis, differential scanning calorimetry, field emission scanning electron microscope, zeta potential, vibrating sample magnetometer, calorimetric hyperthermia methods.

Room temperature synthesis of two-dimensional multilayer magnets based on α-CoII layered hydroxides

Research on two-dimensional (2D) materials is one of the most active fields in materials science and nanotechnology. Among the members of the 2D family, layered hydroxides (LHs) represent an exceptional case of study due to their unparalleled chemical versatility which allows the modulation of their physicochemical properties at will. Nowadays, LHs based on earth-abundant metals are key materials in the areas of energy storage and conversion, hybrid materials or magnetism. ɑ-Co hydroxides ( Simonkolleite -like structures) are promising phases with tuneable electronic and magnetic properties by ligand modification. However, even in the simple case of ɑ-Co II hydroxychlorides, the preparation of well-defined large 2D crystals is not straightforward, hindering the development of fundamental studies. Herein, we present the synthesis of 2D hexagonal crystals with outstanding size-thickness relationship (diameter > 5 μm and thickness of 20 ± 7 nm) by a simple homogeneous synthesis taking place at room temperature. In structural terms, no differences are observed between our layered materials and those obtained hydrothermally. However, dynamic susceptibility measurements alert about different arrangements of the magnetic sublattices, which have been rationalized with structural DFT calculations. This work provides an extremely easy bottom-up method to obtain high-quality 2D crystals based on ɑ-Co II hydroxides, paving the way for the development of fundamental studies and applications.

Dynamic magnetic properties in 2-dimensional kinetic spin-7/2 Ising system

The dynamic magnetic properties, such as dynamic phase transitions (DPTs) and dynamic phase diagrams (DPDs), are reported in 2-Dimensional (2D) kinetic spin-7/2 Ising system under an oscillating external magnetic field. With this aim in mind, the Glauber type stochastic dynamics are utilized to obtain the mean-field dynamic equation determining kinetic system's motion on a square lattice. The obtained mean-field dynamic equation is solved numerically. First, to observe the kinetic system's dynamic phases, the average sublattice magnetization time variations are investigated. After that, to determine the type (first- or second-order) of the DPTs and obtain the DPTs, the dynamic sublattice magnetizations' thermal behavior is examined. Finally, the DPDs are calculated in two different planes. The DPDs contains paramagnetic (P), ferromagnetic-1/2 (F1/2), ferromagnetic-7/2 (F7/2) fundamental phases and six mixed regions (or phases) as well as private points and reentrant phenomena. It has been appeared that the DPTs and DPDs are strongly dependent on the interaction parameters.

The thermal properties of the mixed spin-1/2, 1, 3/2 Ising model on the Bethe lattice

A triple mixed-spin Ising system defined on the Bethe lattice is numerically investigated by means of exact recursion relations (ERRs) calculations. The lattice is constituted by three types of magnetic atoms A, B, C with spins [Formula: see text], [Formula: see text], [Formula: see text] respectively arranged in the form ABCABC. The effects of bilinear exchange and crystal-field interactions as well as those of thermal fluctuations on the order parameters and phase diagrams are thoroughly studied and specified. First-order transitions and tricritical points are present for the coordination number [Formula: see text] whereas at [Formula: see text] they are absent. Global compensation phenomena are absent for the magnetic system. Instead, it is shown that it can only occur between the sublattice magnetizations B and C of the system. Several novel kinds of reentrance of the phase boundaries while varying the values of model parameters have been reported.

Magnetic ordering of Ho and its role in the magnetization reversal and coercivity double peaks in the Ho3Fe5O12 garnet

Rare-earth based garnet system, Ho3Fe5O12 shows magnetization compensation phenomenon at Tcomp = 138 K and a magnetic transition at 50 K. In the present work, we reveal the physics of these low temperature magnetic transitions by carrying out detailed dc magnetization, ac susceptibility, neutron depolarization, and neutron diffraction studies. Interestingly, our dc magnetization study under low (≲ 50 Oe) magnetic fields has shown a previously unknown sign reversal of magnetization (below the Tcomp) that persists down to 5 K, the lowest measured temperature. Magnetic measurements under moderate fields between 50 Oe and 1 kOe reveal two compensation temperatures (in the range of 105 – 150 K depending upon the applied field value) leading to double magnetization reversals. Interestingly, for magnetic fields ≥ 1 kOe, these two compensation temperatures merge to a single Tcomp (=138 K). A modified Stoner-Wohlfarth model has been used to explain the asymmetrical nature of double peak around the Tcomp in the temperature dependent coercivity data. The ac susceptibility study supports the existence of both the transitions with an evidence of deep minimum at the Tcomp (138 K) and a broad peak at 50 K. Mesoscopic neutron depolarization study has revealed a zero domain-magnetization state at the Tcomp. Temperature dependent neutron diffraction study has revealed that Ho carries an induced moment above the Tcomp due to polarizing effect under the internal magnetic field of the two magnetically ordered Fe sublattices at 567 K, whereas a single umbrella type ordering of Ho sublattice moments appears below the Tcomp, resulting in the distortion of magnetic unit cell from cubic to rhombohedral symmetry. However, below 50 K, Ho3+ sites are divided into two inequivalent magnetic sublattices, having different moments and canting angles, and it leads to double umbrella type magnetic structure. Neutron diffraction study has revealed an asymmetric variation of the tetrahedral, octahedral, and dodecahedral sublattice moments resulting in a magnetization reversal at ~ 138 K which is very well supported by the mean field theory calculations. In the present study, we have resolved the ambiguity of magnetic ordering of the rare-earth (Ho3+) sublattice and its role in multiple magnetic transitions. The utility of such compensated ferrimagnetic materials in spin polarizers/analyzers and fast switching memory devices has been emphasized.