Ferrimagnetic Phase Research Articles

Inducing ferromagnetism and magnetoelectric coupling in the ferroelectric alloy system BiFeO3–PbTiO3 via additives

There is a growing interest in BiFeO3-based alloys because of the possibility it offers for developing high-temperature high-performance piezoelectric materials and for their interesting multiferroic properties. Often such ceramics are synthesized with additives either to reduce/suppress leakage current that the system inherits from the parent compound BiFeO3 or to promote sintering via formation of the liquid phase. We demonstrate here the propensity for stabilizing ferromagnetism in the ferroelectric solid solution BiFeO3–PbTiO3 (BF–PT) when synthesized with additive MnO2. Detailed investigation revealed that the ferromagnetic property of the ceramic is extrinsic and caused by the additive enabled precipitation of trace amount of the ferrimagnetic Pb-hexaferrite phase, not easily detected in conventional x-ray diffraction measurements. We also show that the ferromagnetic property is induced in Co-modified BF–PT. However, in this case, the additive stabilizes the CoFe2O4 spinel ferrite phase. While our findings offer a strategy to develop particulate magnetoelectric multiferroic composites using additive assisted precipitation of the ferrimagnetic phase(s) in BiFeO3-based ferroelectric alloys, it also helps in better understanding of the electromechanical response in BFO-based alloys.

Effects of annealing temperature on the magnetic properties of highly crystalline biphase iron oxide nanorods

We report on the effects of annealing temperatures ranging from 225 °C to 325 °C on the magnetic properties of high aspect ratio iron oxide nanorods consisting of a ferrimagnetic Fe3O4 phase and an antiferromagnetic α-Fe2O3 phase in an as-prepared state. Annealing at the aforementioned temperatures under a constant flow of O2 for 3 h leads to an increment of the volume fraction of the antiferromagnetic α-Fe2O3 phase and concomitant enhancement of the crystallinity of the ferrimagnetic Fe3O4 phase. These opposing effects compete with each other, resulting in a decrease in global magnetization with increasing the annealing temperature. The desirable magnetic properties are achieved for the sample annealed at 250 °C. For all samples investigated, we observed an increase in low field magnetization at low temperatures after the sample is field cooled in the presence of a 1T magnetic field, which we attribute to the ordering of macro-spins of the weakly ordered antiferromagnetic α-Fe2O3 phase in the presence of the cooling field. Our study will pave the way for determining the optimal conditions to enhance the magnetic characteristics in iron oxide nanorods, which will enable its use in spintronics and biomedical applications.

Size-dependent magnetic properties of Mn-Co-NiO based heterostructured nanoparticles

In this work, we investigate the synthesis, along with the structural and magnetic properties, of novel Mn-Co-NiO-based heterostructured nanocrystals (HNCs). The objective is to develop novel, well-structurally ordered inverted antiferromagnetic (AFM) NiO–ferrimagnetic (FiM) spinel phase overgrowth HNCs. Inverted HNCs are particularly promising for magnetic device applications because their magnetic properties are more easily controlled by having well-ordered AFM cores, which can result in magnetic structures having large coercivities, tunable blocking temperatures, and other enhanced magnetic effects. The synthesis of the HNCs is accomplished using a two-step process: In the first step, NiO nanoparticles are synthesized using a thermal decomposition method. Subsequently, Mn-Co overgrowth phases are grown on the NiO nanoparticles via hydrothermal nanophase epitaxy, using a fixed pH level (∼5.3) of the aqueous medium. This pH level was selected based on previous work in our laboratory showing that NiO/Mn3O4 HNCs of constant size have optimal coercivity and exchange bias when synthesized at a pH of 5.0. The crystalline structure and gross morphology of the Mn-Co-NiO-based HNCs have been analyzed using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques, respectively. Analysis using these techniques shows that the HNCs are composed of a NiO core and a CoMn2O4 overgrowth phase. Rietveld refinement of XRD data shows that the NiO core has the rocksalt (Fm3̄m) cubic crystal structure and the CoMn2O4 overgrowth has the spinel (I41/amd) crystal structure. Moreover, an increased relative amount of the CoMn2O4 overgrowth phase is deposited with decreasing NiO core particle size during the synthesis of the HNCs. The results from PPMS magnetization and high-resolution transmission electron microscopy (HRTEM) characterization of the Mn-Co-NiO-based HNCs are discussed herein.

Room-temperature weak collinear ferrimagnet with symmetry-driven large intrinsic magneto-optic signatures

Here we present a magnetic thin film with a weak ferrimagnetic (FIM) phase above the N\'eel temperature (${T}_{N}=240\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) and a noncollinear antiferromagnetic (AFM) phase below, exhibiting a small net magnetization due to strain-associated canting of the magnetic moments. A long-range ordered FIM phase has been predicted in related materials, but without symmetry analysis. We now perform this analysis and use it to calculate the magneto-optical Kerr effect (MOKE) spectra in the AFM and FIM phases. From the good agreement between the form of the measured and predicted MOKE spectra, we propose the AFM and FIM phases share the magnetic space group $C{2}^{\ensuremath{'}}/{m}^{\ensuremath{'}}$ and that the symmetry-driven magneto-optic and magneto-transport properties are maximized at room temperature in the FIM phase due to the nonzero intrinsic Berry phase contribution present in these materials. A room temperature FIM with large optical and transport signatures, as well as sensitivity to lattice strain and magnetic field, has useful prospects for high-speed spintronic applications.

In-depth magnetometry and EPR analysis of the spin structure of human-liver ferritin: from DC to 9 GHz.

Ferritin, the major iron storage protein in organisms, stores iron in the form of iron oxyhydroxide most likely involving phosphorous as a constituent, the mineral form of which is not well understood. Therefore, the question of how the ca. 2000 iron atoms in the ferritin core are magnetically coupled is still largely open. The ferritin core, with a diameter of 5-8 nm, is encapsulated in a protein shell that also catalyzes the uptake of iron and protects the core from outside interactions. Neurodegenerative disease is associated with iron imbalance, generating specific interest in the magnetic properties of ferritin. Here we present 9 GHz continuous wave EPR and a comprehensive set of magnetometry techniques including isothermal remanent magnetization (IRM) and AC susceptibility to elucidate the magnetic properties of the core of human liver ferritin. For the analysis of the magnetometry data, a new microscopic model of the ferritin-core spin structure is derived, showing that magnetic moment is generated by surface-spin canting, rather than defects. The analysis explicitly includes the distribution of magnetic parameters, such as the distribution of the magnetic moment. This microscopic model explains some of the inconsistencies resulting from previous analysis approaches. The main findings are a mean magnetic moment of 337μB with a standard deviation of 0.947μB. In contrast to previous reports, only a relatively small contribution of paramagnetic and ferrimagnetic phases is found, in the order of maximally 3%. For EPR, the over 30 mT wide signal of the ferritin core is analyzed using the model of the giant spin system [Fittipaldi et al., Phys. Chem. Chem. Phys., 2016, 18, 3591-3597]. Two components are needed minimally, and the broadening of these components suggests a broad distribution of the magnetic resonance parameters, the zero-field splitting, D, and the spin quantum number, S. We compare parameters from EPR and magnetometry and find that EPR is particularly sensitive to the surface spins of the core, revealing the potential to use EPR as a diagnostic for surface-spin disorder.

Random-anisotropy mixed-spin Ising on a triangular lattice

We have studied the mixed spin-1/2 and 1 Ising ferrimagnetic system with a random anisotropy on a triangular lattice with three interpenetrating sublattices A, B, and C. The spins on the sublattices are represented by σA (states ±1/2), σB (states ±1/2), and SC (states ±1, 0). We have performed Monte Carlo simulations to obtain the phase diagram temperature kBT/|J| versus the strength of the random anisotropy D/|J|. The phase boundary between two ferrimagnetic FR1 and FR2 phases at lower temperatures are always first-order for p < 0.25 and second-order phase transition between the FR1, FR2 and the paramagnetic P phases. On the other hand, for values of p ⪆ 0.5, the phase diagram presents only second-order phase transition lines.

Магнитные свойства стекол, синтезированных на основе горных пород различного генезиса

The structural-phase, chemical composition and magnetic properties of artificial glasses obtained by high-temperature melting of rock mixtures of different genesis: volcanogenic-sedimentary rocks, quartzitic shales, psamite-silt-pelitic complexes were studied. Different in duration cooling and glass transition conditions were used for glass synthesis. The magnetic state of the iron-containing phase and the analysis of the presence and contribution of iron oxides to the magnetic properties were assumed to be the characteristic criterion determining the parameters of the glass formation process. It was shown that the total iron content in artificial glasses depends exclusively on the composition of the initial charge. In this case, the iron-containing component is mainly determined by sedimentary rocks. The formation of magnetic minerals is determined both by the composition of the charge and by the rate of cooling of the melt. During “fast” cooling, the formed magnetic particles are mostly (up to 90% or more of the total content of the magnetic phase in the sample) in a superparamagnetic state. During “slow” cooling, a mixture of particles of different sizes and, accordingly, in different magnetic states is formed: from superparamagnetic to low-domagnetic. The ferrimagnetic phase crystallizing in artificial glasses is represented by chemically heterogeneous aggregates of iron oxides, mainly non-stoichiometric magnetite.

Manganese substitution induced magnetic transformation and magnetoelectricity in SrFe12O19.

In this study, manganese substituted strontium hexaferrite (SrFe12-xMnxO19; x = 0, 3, 5, and 7) prepared by the sol-gel auto-combustion method are studied. We observed that the substituted Mn preferentially goes to the 2a and 12k sites of Fe. Raman modes related to the 12k site suggest the stiffening of the lattice. The transformation of the grain's shape from hexagonal (x = 0 and 3) to rhombohedral (x = 7) was observed, as shown in the micrographs obtained from FESEM. The thermomagnetic curves show the shift of TC to lower temperatures with the increase in the Mn content. From x = 5 onwards, the growth of another magnetic phase (FiM2) of lower coercivity apart from the parent phase (FiM1) of higher coercivity is seen. The FiM2 phase was found to increase with the Mn content in the sample (16.4(3)% for x = 5 but 66.2(5)% for x = 7). Although the magnetization for both FiM1 and FiM2 decreases with the increase in temperature, both magnetic phases behave in contrast to each other for x = 5 and x = 7. The study suggests a transformation of the compound from high magnetic anisotropy (x = 0) to low magnetic anisotropy (x = 7). The x = 5 composition sample displays the highest value of the first-order ME coefficient (0.83(2) mV × cm-1 × Oe-1). The observed value for x = 5 composition is ∼2.5 times higher than that of the parent x = 0 composition sample (0.33(2) mV × cm-1 × Oe-1). The studies thus suggest that the x = 5 composition is one of the viable candidates for magnetoelectric applications.

Half-metallic quaternary CrYCoX (X= Si, Ge, Sn, Pb) alloys: DFT calculations and Monte Carlo simulation

Quaternary Heusler alloys with high spin polarization and high Curie temperature, and lower Gilbert damping parameters have broad application prospects in spintronics field, which motivates us to design the alloys with above-mentioned properties. In the current manuscript, we systematically studied the electronic structure, magnetic properties and Gilbert damping parameters of quaternary Heusler alloys CrYCoX (X= Si, Ge, Sn, Pb). Electronic calculations show that the CrYCoX alloys are half-metallic ferrimagnets, and their integer total magnetic moments obey the Slater–Pauling rule: Mt = Zt-18. Origin of half-metallic gap is discussed by hybridization, occupation and distribution of electronic states. According to the Heisenberg model, we calculated the exchange coupling parameters of CrYCoX alloys, and find that the competition and cooperation between d−d exchange, super-exchange and RKKY exchange lead to the emergence of ferrimagnetic phase. Curie temperatures of CrYCoX alloys calculated by mean-field approximation and Monte Carlo simulation are significantly higher than room temperature. Finally, Gilbert damping parameters of CrYCoX alloys calculated by via linear response theory are localized in 0.0051∼0.0136 at 300 K, indicating they have lower energy loss and higher response speed as microwave and spintronic materials. Our results show that the CrYCoX alloys are promising candidates in room microwave and spintronic devices.

Creating and controlling Dirac fermions, Weyl fermions, and nodal lines in the magnetic antiperovskite Eu3PbO

The band topology of magnetic semimetals is of interest both from the fundamental science point of view and with respect to potential spintronics and memory applications. Unfortunately, only a handful of suitable topological semimetals with magnetic order have been discovered so far. One such family that hosts these characteristics is the antiperovskites, ${A}_{3}B\mathrm{O}$, a family of 3D Dirac semimetals. The $A={\mathrm{Eu}}^{2+}$ compounds magnetically order with multiple phases as a function of the applied magnetic field. Here, by combining band structure calculations with neutron diffraction and magnetic measurements, we establish the antiperovskite ${\mathrm{Eu}}_{3}\mathrm{Pb}\mathrm{O}$ as a new topological magnetic semimetal. This topological material exhibits a multitude of different topological phases with ordered Eu moments which can be easily controlled by an external magnetic field. The topological phase diagram of ${\mathrm{Eu}}_{3}\mathrm{Pb}\mathrm{O}$ includes an antiferromagnetic Dirac phase, as well as ferro- and ferrimagnetic phases with both Weyl points and nodal lines. For each of these phases, we determine the bulk band dispersions, the surface states, and the topological invariants by means of ab initio and tight-binding calculations. Our discovery of these topological phases introduces ${\mathrm{Eu}}_{3}\mathrm{Pb}\mathrm{O}$ as a new platform to study and manipulate the interplay of band topology, magnetism, and transport.

The Effect of Co-Doping on the Structural and Magnetic Properties of Single-Domain Crystalline Copper Ferrite Nanoparticles

Nanoparticles of Co-doped copper ferrite, Cu0.75Co0.25Fe2O4, were successfully synthesized by hydrothermal method. The preparation conditions were optimized to produce small nanoparticles with crystallite size of 20 nm that fall into the single-domain regime. The influence of Co-doping on the structure and magnetic properties of pure copper ferrite, CuFe2O4, was investigated. The prepared ferrite nanoparticles were found to be in a single structural phase with a spinel-type structure, according to the XRD and FT-IR measurements. When compared to pure Cu ferrite, the addition of Co increased the lattice constant and decreased the density. The TEM results confirmed the spherical morphology of the prepared ferrite nanoparticles. For the entire temperature range of the ferrite nanoparticles, the magnetization measurements showed a single ferrimagnetic phase. It was observed that the coercivity and remanent magnetization increased with decreasing temperature. Magnetic anisotropy was found to increase with Co-doping in comparison to pure Cu ferrite. The ZFC–FC magnetization curves showed that the blocking temperature (TB) of the prepared nanoparticles is above room temperature, demonstrating that they are ferrimagnetic at room temperature and below. Additionally, it was found that decreasing the magnetic field lowers TB. The FC curves below TB were observed to be nearly flat, indicating spin-glass behavior that might be attributed to nanoparticle interactions and/or surface effects such as spin canting and spin disorder.

The structure, magnetic and dielectric properties of the CaBaCo4-xNixO7 (0 ≤ x ≤ 0.1) compounds

We report the structure, magnetic and dielectric behaviors of the polar magnet CaBaCo4-xNixO7 (0 ≤ x ≤ 0.1). The Rietveld refinements results for polycrystalline samples shows that the orthorhombic distortion gradually decreases with the increase of Ni2+ content. Different from the undoped sample, two antiferromagnetic phases and a ferrimagnetic phase can be observed in M-T curves for Ni-doped samples. For x = 0.06, the antiferromagnetic phase around 79K competes with another antiferromagnetic phase that appears around 62K, giving a magnetic plateau between 62K and 79K. With increasing Ni-doping, the ferrimagnetic phase transition peak moves to lower temperature and it fades gradually, while the antiferromagnetic phase transition peak around 79K shifts to higher temperature. The magnetization behaviors could be attributed to the enhanced antiferromagnetic interactions and the reorientation of the adjacent spins of cobalt ions due to the introduction of Ni2+ doping. As a result, the Ni-doped samples present stronger magnetoelectric coupling than the undoped sample, as well as an enhanced polarization.

Anisotropic magnetization and electronic structure of the first-order ferrimagnet ErCo2 studied by polarization dependent hard X-ray photoemission spectroscopy

The first-order ferrimagnet ErCo2 attracts interest not only because of metamagnetism and magnetocaloric effect just above TC ≈ 32–34 K but also because it is closely related with the itinerant metamagnetism of YCo2 and LuCo2. We study the electronic structure of single crystals with hard X-ray photoemission spectroscopy (HAXPES). Magnetization measurements reconfirm the first-order magnetic transition, metamagnetism, and strong magnetic anisotropy. Calculated ErCo2 band structures of the ferrimagnetic and paramagnetic phases are presented in detail. In the ferrimagnetic state, the density of states just below EF is smaller than in the paramagnetic phase. Valence band spectra in the paramagnetic state show strong polarization dependence. Furthermore, the change across the first-order ferrimagnetic transition in the valence band electronic structures is observed. These experimental data are well described by the band structure calculation incorporated with the polarization dependent cross-sections of orbitals. We further discuss possible effects of electron correlation and spin fluctuation.

Emergent magnetism and exchange bias effect in iron oxide nanocubes with tunable phase and size

We report a systematic investigation of the magnetic properties including the exchange bias (EB) effect in an iron oxide nanocube system with tunable phase and average size (10, 15, 24, 34, and 43 nm). X-ray diffraction and Raman spectroscopy reveal the presence of Fe3O4, FeO, and α-Fe2O3 phases in the nanocubes, in which the volume fraction of each phase varies depending upon particle size. While the Fe3O4 phase is dominant in all and tends to grow with increasing particle size, the FeO phase appears to coexist with the Fe3O4 phase in 10, 15, and 24 nm nanocubes but disappears in 34 and 43 nm nanocubes. The nanocubes exposed to air resulted in an α-Fe2O3 oxidized surface layer whose thickness scaled with particle size resulting in a shell made of α-Fe2O3 phase and a core containing Fe3O4 or a mixture of both Fe3O4 and FeO phases. Magnetometry indicates that the nanocubes undergo Morin (of the α-Fe2O3 phase) and Verwey (of the Fe3O4 phase) transitions at ∼250 K and ∼120 K, respectively. For smaller nanocubes (10, 15, and 24 nm), the EB effect is observed below 200 K, of which the 15 nm nanocubes showed the most prominent EB with optimal antiferromagnetic (AFM) FeO phase. No EB is reported for larger nanocubes (34 and 43 nm). The observed EB effect is ascribed to the strong interfacial coupling between the ferrimagnetic (FiM) Fe3O4 phase and AFM FeO phase, while its absence is related to the disappearance of the FeO phase. The Fe3O4/α-Fe2O3 (FiM/AFM) interfaces are found to have negligible influence on the EB. Our findings shed light on the complexity of the EB effect in mixed-phase iron oxide nanosystems and pave the way to design exchange-coupled nanomaterials with desirable magnetic properties for biomedical and spintronic applications.

Critical behavior at paramagnetic to ferrimagnetic phase transition in A-site ordered perovskite CaCu3Cr2Nb2O12

A new perovskite-type oxide, CaCu3Cr2Nb2O12, has been synthesized at 10 GPa and 1100 °C. The compound crystallizes in space group Im3¯ with 1:3 ordered Ca and Cu cations at the A sites and disordered Cr and Nb cations at the octahedral B sites. CaCu3Cr2Nb2O12 exhibits a paramagnetic–ferrimagnetic transition at around 192.3 K and the Arrott plots manifest this transition is a second-order transition. We investigate the critical behavior of CaCu3Cr2Nb2O12 using the modified Arrott plots, Kouvel–Fisher plot, and the scale theory. The estimated critical exponents β, γ and δ, which yield β = 0.465 ± 0.006, γ = 1.312 ± 0.019 and δ = 3.833 ± 0.019 with TC = 197 K, fall within the predicted values from the mean-field model and three-dimensional Heisenberg model. The renormalization group analysis suggests that the exchange interaction decays as J(r)∼ r−4.871, indicating the magnetic interactions may be long-range coupling in CaCu3Cr2Nb2O12.

Synthesis of antiferromagnetic Weyl semimetal Mn3Ge on insulating substrates by electron beam assisted molecular beam epitaxy

The antiferromagnetic kagome semimetals Mn3X (X = Ge, Sn, Ga) are of great interest due to properties arising from their Berry curvature, such as large anomalous Nernst and anomalous Hall coefficients, and spin to charge conversion efficiencies at ambient temperatures. However, the synthesis of epitaxial thin films of Mn3Ge in the desired hexagonal phase has been challenging because they do not wet insulating substrates, necessitating the use of a metallic buffer layer. Furthermore, a ferrimagnetic tetragonal phase also forms readily under typical growth conditions, interfering with hexagonal phase properties. We have synthesized atomically smooth and continuous epitaxial thin films of hexagonal Mn3Ge directly on insulating LaAlO3 (111) substrates using electron beam assisted molecular beam epitaxy, using a three-step process that mitigates the formation of the tetragonal phase. The anomalous Nernst coefficient is found to be more than six times larger in our films than in sputtered thin films of Mn3Ge and significantly larger than that of Fe. Our approach can be used to grow thin layers of kagome materials, without interference from a buffer layer in transport properties, and may be applicable to a broader range of materials with large surface energies that do not grow readily on insulating substrates.

Assessment of heavy metal contamination of an electrolytic manganese metal industrial estate in northern China from an integrated chemical and magnetic investigation.

Heavy metal concentrations (Al, V, Mn, Fe, Co, Ni, Cu, Zn, and Pb) and the magnetic properties of soil and sediment samples in/around an electrolytic manganese metal (EMM) industrial estate in northern China were investigated. Potential enrichment of Mn, Zn, and Pb was found in/around the core area of the EMM industrial estate; however, the pollution load index (PLI) values did not indicate severely polluted levels. For adults, all hazard index (HI) values of noncarcinogenic risks in the soil samples were below the safe level of 1.00. For children, none of the HI values exceeded the safe level, except Mn (HI = 1.23) in one industrial estate sample. The particle size of magnetic materials was mostly in the range of stable single-domain, and coarser ferrimagnetic phases enhanced the magnetic parameters in the industrial estate soils. Highly positive correlations were found between magnetic parameters, heavy metal concentrations, and PLI values, demonstrating that the magnetic parameters are an efficient proxy for assessing heavy metal contamination. Enrichment of Mn, Zn, and Pb was mainly derived from the EMM industry. The data showed that the EMM industrial estate under cleaner production had limited adverse impacts on the adjacent environment from the perspective of heavy metal contamination.

Insight into the structural and magnetotransport properties of epitaxial α−Fe2O3/Pt(111) heterostructures: Role of the reversed layer sequence

We report on the chemical structure and spin Hall magnetoresistance (SMR) in epitaxial $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(\mathrm{hematite})(0001)\text{/}\mathrm{Pt}(111)$ bilayers with hematite thicknesses of 6 and 15 nm grown by molecular beam epitaxy on a MgO(111) substrate. Unlike previous studies that involved Pt overlayers on hematite, the present hematite films were grown on a stable Pt buffer layer and displayed structural changes as a function of thickness. These structural differences (the presence of a ferrimagnetic phase in the thinner film) significantly affected the magnetotransport properties of the bilayers. We observed a sign change of the SMR from positive to negative when the thickness of the hematite increased from 6 to 15 nm. For $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(15\phantom{\rule{0.16em}{0ex}}\mathrm{nm})\text{/}\mathrm{Pt}$, we demonstrated room-temperature switching of the N\'eel order with rectangular, nondecaying switching characteristics. Such structures open the way to extending magnetotransport studies to more complex systems with double asymmetric metal/hematite/Pt interfaces.

Insight into ground-state spin arrangement and bipartite entanglement of the polymeric coordination compound [Dy[formula omitted]Cu[formula omitted]][formula omitted] through the symmetric spin-1/2 Ising–Heisenberg orthogonal-dimer chain

The ground-state spin arrangement and the bipartite entanglement within Cu2+-Cu2+ dimers across the magnetization process of the 4f-3d heterometallic coordination polymer [Dy(hfac)2(CH3OH)2Cu(dmg)(Hdmg)2]n (H2dmg = dimethylglyoxime, Hhfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) are theoretically examined using the symmetric isotropic spin-1/2 Ising–Heisenberg orthogonal-dimer chain. The numerical results point to five possible ground states of the compound with three different degrees of the quantum entanglement within Cu2+-Cu2+. Besides the standard ferrimagnetic and saturated phases without quantum entanglement of Cu2+ ions, which are manifested in low-temperature magnetization curve as wide plateaus at the non-saturated magnetization 16.26μB and at the saturation value 20.82μB, respectively, one also finds an intriguing singlet-like phase with just partial entanglement within Cu2+-Cu2+ and two singlet phases with fully entangled Cu2+-Cu2+ dimers. The former quantum phase can be identified in the low-temperature magnetization process as very narrow intermediate plateau at the magnetization 9.27μB per unit cell, while the latter ones as zero magnetization plateau and intermediate plateau at the magnetization 18.54μB. Non-monotonous temperature variations of the concurrence, through which the entanglement within cooper dimers is quantified, point to the possible temporary thermal activation of the entangled states of Cu2+-Cu2+ also above non-entangled ferrimagnetic and saturated phases.

D-d hybridization controlled large-volume-change martensitic phase transition in Ni-Mn-Ti-based all-d-metal Heusler compounds

Martensitic phase transition in all-d-metal Heusler compounds was investigated by using first principles calculations. The large change in volume during martensitic phase transition and its physical origin were revealed in representative compound Ni2MnTi. It was found that the enhanced d-d interatomic hybridization due to intrinsic volume shrinkage during martensitic phase transition led to significant increase in thermodynamic driving force and great improvement in the deformation ability. Their strong correlations induced large volume change during martensitic phase transition. The tetragonal martensitic phase in Ni2MnTi preferred the weaker magnetic state. Herein, it was proposed that a metamagnetic martensitic phase transition from cubic ferromagnetic phase to tetragonal ferrimagnetic phase with large volume change can be realized by tuning d-d interatomic hybridization level. The investigations indicate that careful control of interatomic hybridization level in Ni-Mn-Ti-based compounds can serve as an inherent tuning parameter to design and explore high-performance candidates as phase transition materials for solid-state refrigeration.