Showing papers on "Ferrimagnetism published in 2011"
TL;DR: It is found that the ultrafast spin reversal in GdFeCo, where spins are coupled antiferromagnetically, occurs by way of a transient ferromagnetic-like state, which provides a concept for the possibility of manipulating magnetic order on the timescale of the exchange interaction.
Abstract: The dynamics of spin ordering in magnetic materials is of interest for both fundamental understanding and progress in information-processing and recording technology. Radu et al. study spin dynamics in a ferrimagnetic gadolinium–iron–cobalt (GdFeCo) alloy that is optically excited at a timescale shorter than the characteristic magnetic exchange interaction between the Gd and Fe spins. Using element-specific X-ray magnetic circular dichroism spectroscopy, they show that the Gd and Fe spins switch directions at very different timescales. As a consequence, an unexpected transient ferromagnetic state emerges. These surprising observations, supported by simulations, provide a possible new concept of manipulating magnetic order on a timescale of the exchange interaction. Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism1,2,3,4. Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. Usually the dynamics are studied by observing the collective response of exchange-coupled spins, that is, spin resonances, after an external perturbation by a pulse of magnetic field, current or light. The periods of the corresponding resonances range from one nanosecond for ferromagnets down to one picosecond for antiferromagnets. However, virtually nothing is known about the behaviour of spins in a magnetic material after being excited on a timescale faster than that corresponding to the exchange interaction (10–100 fs), that is, in a non-adiabatic way. Here we use the element-specific technique X-ray magnetic circular dichroism to study spin reversal in GdFeCo that is optically excited on a timescale pertinent to the characteristic time of the exchange interaction between Gd and Fe spins. We unexpectedly find that the ultrafast spin reversal in this material, where spins are coupled antiferromagnetically, occurs by way of a transient ferromagnetic-like state. Following the optical excitation, the net magnetizations of the Gd and Fe sublattices rapidly collapse, switch their direction and rebuild their net magnetic moments at substantially different timescales; the net magnetic moment of the Gd sublattice is found to reverse within 1.5 picoseconds, which is substantially slower than the Fe reversal time of 300 femtoseconds. Consequently, a transient state characterized by a temporary parallel alignment of the net Gd and Fe moments emerges, despite their ground-state antiferromagnetic coupling. These surprising observations, supported by atomistic simulations, provide a concept for the possibility of manipulating magnetic order on the timescale of the exchange interaction.
827 citations
TL;DR: In this paper, an epitaxial spin-transfer torque memory was proposed to support thermally stable sub-10nm bits for spin transfer torque memories, measured using point-contact Andreev reflection.
Abstract: Ferrimagnetic Mn${}_{3}$Ga exhibits a unique combination of low saturation magnetization (${M}_{s}=0.11$ MA m${}^{\ensuremath{-}1}$) and high perpendicular anisotropy with a uniaxial anisotropy constant of ${K}_{\mathrm{u}}$ $=$ 0.89 MJ m${}^{\ensuremath{-}3}$. Epitaxial $c$-axis films exhibit spin polarization as high as $58%$, measured using point-contact Andreev reflection. These epitaxial films will be able to support thermally stable sub-10-nm bits for spin-transfer torque memories.
239 citations
TL;DR: The results allow us to explain the high value of saturation magnetization and provide a better insight into the complex interplay between cationic distribution and magnetic disorder in ferrimagnetic nanoparticles.
Abstract: CoFe(2)O(4) nanoparticles ( D(NPD) ~6 nm), prepared by a thermal decomposition technique, have been investigated through the combined use of dc magnetization measurements, neutron diffraction, and (57)Fe Mossbauer spectrometry under high applied magnetic field. Despite the small particle size, the value of saturation magnetization at 300 K (M(s) = 70 A m(2) kg(-1)) and at 5 K (M(s) = 100 A m(2) kg(-1)) are rather close to the bulk values, making the samples prepared with this method attractive for biomedical applications. Neutron diffraction measurements indicate the typical ferrimagnetic structure of the ferrites, showing an inversion degree (γ(NPD) = 0.74) that is in very good agreement with cationic distribution established from low temperature (10 K) Mossbauer measurements in high magnetic field (γ(moss) = 0.76). In addition, the in-field Mossbauer spectrum shows the presence of a non-collinear spin structure in both A and B sublattices. The results allow us to explain the high value of saturation magnetization and provide a better insight into the complex interplay between cationic distribution and magnetic disorder in ferrimagnetic nanoparticles.
144 citations
TL;DR: In this paper, the effects of dilution at the surface, the surface exchange interaction, and the shell coupling on the magnetization profiles are investigated, and they find a number of characteristic phenomena for them.
Abstract: Magnetic properties (phase diagram and magnetization) of a cylindrical Ising nanowire or nanotube are investigated by the use of the effective-field theory with correlations. Particular emphasis is given to the effects of the surface and its dilution on them. Much attention is paid to the thermal variation of the magnetization when the spins at the surface are coupled antiferromagnetically to the ferromagnetic core spins by the negative shell coupling. The effects of dilution at the surface, the surface exchange interaction, and the shell coupling on the magnetization profiles are investigated. We find a number of characteristic phenomena for them.
139 citations
TL;DR: In this paper, a computational model of crystallographically amorphous ferrimagnetic alloys using a stochastic Landau-Lifshitzitz-Gilbert equation of motion for atomistic spins and an atomistic spin Hamiltonian with Heisenberg exchange is presented.
Abstract: We present a computational model of crystallographically amorphous ferrimagnetic alloys using a stochastic Landau-Lifshitz-Gilbert equation of motion for atomistic spins and an atomistic spin Hamiltonian with Heisenberg exchange. The spontaneous equilibrium magnetization is calculated and a comparison with a mean field model is made. The simulations show excellent agreement with experiments on GdFeCo using x-ray magnetic circular dichroism to determine the individual sublattice magnetizations. The calculated temperature dependence of the magnetization shows a polarization of the Gd sublattice leading to a common Curie temperature, in agreement with the experimental data. The intersublattice exchange is shown to be an important energy transfer channel for ultrafast dynamics.
124 citations
TL;DR: In this paper, the authors used sol-gel combustion and co-precipitation techniques to produce nickel ferrite nanoparticles with high saturation magnetization and hysteresis.
Abstract: Nickel ferrite nanoparticles of very small size were prepared by sol-gel combustion and co-precipitation techniques. At the same annealing temperature sol-gel derived particles had bigger crystallite size. In both methods, crystallite size of the particles increased with annealing temperature. Sol-gel derived nickel ferrite particles were found to be of almost spherical shape and moderate particle size with a narrow size distribution; while co-precipitation derived particles had irregular shape and very small particle size with a wide size distribution. Nickel ferrite particles produced by sol-gel method exhibited more purity. Sol-gel synthesized nanoparticles were found to be of high saturation magnetization and hysteresis. Co-precipitation derived nickel ferrite particles, annealed at 400°C exhibited superparamagnetic nature with small saturation magnetization. Saturation magnetization increased with annealing temperature in both the methods. At the annealing temperature of 600°C, co-precipitation derived particles also became ferrimagnetic.
116 citations
TL;DR: In this paper, the authors measured dielectric permittivity and electrical polarization as a function of temperature for two polycrystalline ACr2O4 spinels (Fe and Co).
Abstract: Dielectric permittivity (ɛ′) and electrical polarization (P) have been measured as a function of temperature for two polycrystalline ACr2O4 spinels (A = Fe and Co). Anomalies on the ɛ′(T) curves are detected at the characteristic magnetic transition temperatures (TC, TS, and Tlock-in) for FeCr2O4 and CoCr2O4 and also at the Jahn-Teller (JT) transition for FeCr2O4. The P(T) curves of both spinels exhibit transitions at TC showing that polar and ferrimagnetic states coexist in these oxides. The link between the distortion of the spinel structure due to the Jahn-Teller Fe2+ and the larger polarization value at 8 K, P = 35 μC/m2, against P = 3.5 μC/m2 for CoCr2O4, is also discussed.
115 citations
TL;DR: In this paper, the half-metallic gap is wide and the Fermi level falls at the middle of the gap and thus, it presents high degree of spin-polarization for a wide range of lattice constants.
Abstract: Extensive ab-initio electronic structure calculations on Heusler alloys suggest that Cr2CoGa is the alloy of choice to achieve the half-metallic fully-compensated ferrimagnetism since (1) it has been already grown experimentally [T. Graf et al., Z. Anorg. Allg. Chem. 635, 976 (2009)], (2) half-metallic XA structure is favored energetically over all the studied lattice constant range with respect to the L21 which is not half-metallic, (3) the half-metallic gap is wide and the Fermi level falls at the middle of the gap and thus, it presents high degree of spin-polarization for a wide range of lattice constants, and (4) the Curie temperature is extremely high reaching the 1520 K.
113 citations
TL;DR: In this paper, the phase diagrams of a ferrimagnetic cubic nanoparticle (nanocube) with a spin-3/2 core surrounded by a spin 1 shell layer with antiferromagnetic interface coupling were investigated.
103 citations
TL;DR: In this paper, a single phase, high crystalline and cubic structure of NiFe2O4 with a Fd3m space group was confirmed using X-ray diffraction pattern.
100 citations
TL;DR: In this paper, the changes in the magnetic dead layer (MDL) and saturation magnetization of the CoFeB layers are investigated as a function of the annealing temperature for four different unit structures, that are relevant to the synthetic ferrimagnetic free structure in MgO-based magnetic tunnel junctions.
Abstract: The changes in the magnetic dead layer (MDL) and saturation magnetization of the CoFeB layers are investigated as a function of the annealing temperature for four different unit structures, that are relevant to the synthetic ferrimagnetic free structure in MgO-based magnetic tunnel junctions. The MDL results for these unit structures are then converted into those for the constituent interfaces of the free structure. Most of the changes in the MDL thickness occur during annealing at a low temperature of 150 °C while those in the saturation magnetization occur at a high annealing temperature of 350 °C. These results for the MDL and saturation magnetization are critically tested by using the synthetic ferrimagnetic free structures with various thickness asymmetries. The observed switching properties of these tested structures are in good agreement with those expected from the results for the MDL and saturation magnetization, confirming the accuracy of the present results. The accuracy of the saturation magne...
TL;DR: Density functional calculations performed on the Ti 2 CoAl Heusler compound confirm it to be a half-metallic ferrimagnet with the spin-down energy gap of 0.49 eV as discussed by the authors.
TL;DR: In this article, the generalized Heusler compounds Mn2CoZ (Z = Al, Ga, In, Si, Ge, Sn, Sb) with the Hg2CuTi structure are studied by means of first principles calculations of the Heisenberg exchange coupling parameters, and Curie temperatures are calculated from those.
Abstract: The generalized Heusler compounds Mn2CoZ (Z = Al, Ga, In, Si, Ge, Sn, Sb) with the Hg2CuTi structure are of great interest due to their half-metallic ferrimagnetism. The complex magnetic interactions between the constituents are studied by means of first principles calculations of the Heisenberg exchange coupling parameters, and Curie temperatures are calculated from those. Due to the direct Mn–Mn exchange interaction in Mn2CoZ, the Curie temperature decreases, although the total moment increases when the valence electron number Z is increased. The exchange interactions are dominated by a strong direct exchange between Co and its nearest neighbor Mn on the B site, which is nearly constant. The coupling between the nearest neighbor Mn atoms scales with the magnetic moment of the Mn atom on the C site. Calculations with different lattice parameters suggest a negative pressure dependence of the Curie temperature, which follows from the decreasing magnetic moments. Curie temperatures of more than 800 K are predicted for Mn2CoAl (890 K), Mn2CoGa (886 K), and Mn2CoIn (845 K).
TL;DR: A comprehensive theory of the temperature and disorder dependence of their magnetic properties is presented by deriving and validating a new effective spin Hamiltonian for these materials, amenable to large-scale three-dimensional simulations.
Abstract: Double perovskites such as Sr(2)FeMoO(6) are rare examples of materials with half-metallic ground states and a ferrimagnetic T(c) above room temperature We present a comprehensive theory of the temperature and disorder dependence of their magnetic properties by deriving and validating a new effective spin Hamiltonian for these materials, amenable to large-scale three-dimensional simulations We show how disorder, ubiquitous in these materials, affects T(c), the magnetization, and the conduction electron polarization We conclude with a novel proposal to enhance T(c) without sacrificing polarization
TL;DR: In this article, the authors synthesize nanocrystalline zinc ferrite by high-energy ball-milling after 12h from a powders mixture of zinc oxide (ZnO) and hematite (α-Fe2O3) with balls to powders mass ratio of 20:1.
Abstract: Nanocrystalline zinc ferrite (ZnFe2O4) is synthesized by high-energy ball-milling after 12 h from a powders mixture of zinc oxide (ZnO) and hematite (α-Fe2O3) with balls to powders mass ratio of 20:1. X-ray diffraction, vibrating sample magnetometer (VSM), the Mossbauer spectrometry and photoluminescence (PL) are used to characterize the samples. Rietveld analysis and VSM measurements show that the powder has an average crystallites size of 10 nm and a ferrimagnetic behavior with a saturation magnetization of 30 emu/g. After annealing at 700 °C, the lattice parameter reduces from 8.448 to 8.427 A and the sample transforms into a superparamagnetic behavior, which was confirmed as well by the room temperature Mossbauer spectrometry. Different mechanisms to explain the obtained results and the correlation between magnetism and structure are discussed. Finally, the broadband visible emission band is observed in the entire PL spectrum and the estimated energy band gap is about 2.13 eV.
TL;DR: The magnetic properties of Mn3−xCoxGa Heusler alloys exhibit a very interesting feature as discussed by the authors, which is that they have soft-magnetic hysteresis loops and follow the generalized Slater-Pauling curve indicating half-metallic ferrimagnetism.
Abstract: The magnetic properties of Mn3−xCoxGa Heusler alloys exhibit a very interesting feature. While the Mn-rich alloys (x = 0.1−0.4) are similar to Mn3Ga as they crystallize in a tetragonally distorted variation of the Heusler structure and exhibit comparable hard-ferrimagnetic properties highly useful for spin-transfer torque applications, the Co-rich samples (x = 0.6–1) exhibit cubic Heusler structures, have soft-magnetic hysteresis loops, and follow the generalized Slater-Pauling curve indicating half-metallic ferrimagnetism. The Curie temperatures of all alloys are above 700 K allowing for high operating temperatures.
TL;DR: In this article, the properties of thin films of the full Heusler compound Co{sub 2}TiSn prepared by DC magnetron co-sputtering were reported, and they were obtained by deposition on MgO (001) substrates at substrate temperatures above 600 C.
Abstract: In this article we report on the properties of thin films of the full Heusler compound Co{sub 2}TiSn prepared by DC magnetron co-sputtering. Fully epitaxial, stoichiometric films were obtained by deposition on MgO (001) substrates at substrate temperatures above 600 C. The films are well ordered in the L2{sub 1} structure, and the Curie temperature exceeds slightly the bulk value. They show a significant, isotropic magnetoresistance and the resistivity becomes strongly anomalous in the paramagnetic state. The films are weakly ferrimagnetic, with nearly 1 {mu}{sub B} on the Co atoms, and a small antiparallel Ti moment, in agreement with theoretical expectations. From comparison of x-ray absorption spectra on the Co L{sub 3,2} edges, including circular and linear magnetic dichroism, with ab initio calculations of the x-ray absorption and circular dichroism spectra we infer that the electronic structure of Co{sub 2}TiSn has essentially non-localized character. Spectral features that have not been explained in detail before, are explained here in terms of the final state band structure.
TL;DR: In this paper, the initial susceptibility and reduced total magnetization of a cylindrical Ising nanowire (or nanotube) are investigated by the use of the effective field theory with correlations, in order to clarify their distinctions between the ferromagnetic and ferrimagnetic behaviors when the core-shell exchange coupling takes a different sign.
TL;DR: In this paper, phase diagrams of a cylindrical ferrimagnetic nanotube with a negative core-shell interaction, described by the transverse Ising model, are investigated by the use of the effective field theory with correlations.
TL;DR: The magnetic ground state of the Heusler compound Mn${}_{2}$CoSn was predicted to be nearly half-metallic ferrimagnetic with a high spin polarization by ab initio electronic structure calculations as mentioned in this paper.
Abstract: The magnetic ground state of the Heusler compound Mn${}_{2}$CoSn was predicted to be nearly half-metallic ferrimagnetic with a high spin polarization by ab initio electronic structure calculations. Mn${}_{2}$CoSn was synthesized, and the magnetic behavior of the compound was studied using a superconducting quantum interference device and x-ray magnetic circular dichroism. The experimental values were found to be in fair accordance with the theoretical predictions. The electronic structure and the crystal structure of Mn${}_{2}$CoSn were characterized comprehensively using x-ray powder diffraction, $^{119}\mathrm{Sn}$ M\"ossbauer spectroscopy, nuclear magnetic resonance, and hard x-ray photoelectron spectroscopy.
TL;DR: In this article, the electronic structure and magnetic properties of half-metallic full-Heusler compound with CuHg 2 Ti-type structure were examined by density functional theory (DFT) calculations.
TL;DR: In this article, it was shown that bulk La 0.67 Ca 0.33 MnO 3 (LCMN) sample of micron size particles is a typical ferromagnet and magnetic properties of LCMN samples with particle size in the nanometer scale are better described in the frame work of a ferrimagnetic structure.
TL;DR: The quaternary full Heusler compound Mn{sub 2-x}Co{sub x}VAl with x = 1 is predicted to be a half-metallic antiferromagnet as mentioned in this paper.
Abstract: The quaternary full Heusler compound Mn{sub 2-x}Co{sub x}VAl with x = 1 is predicted to be a half-metallic antiferromagnet. Thin films of the quaternary compounds with x = 0-2 were prepared by dc and RF magnetron co-sputtering on heated MgO (0 0 1) substrates. The magnetic structure was examined by x-ray magnetic circular dichroism and the chemical disorder was characterized by x-ray diffraction. Ferrimagnetic coupling of V to Mn was observed for Mn{sub 2}VAl (x = 0). For x = 0.5, we also found ferrimagnetic order with V and Co antiparallel to Mn. The observed reduced magnetic moments are interpreted with the help of band structure calculations in the coherent potential approximation. Mn{sub 2}VAl is very sensitive to disorder involving Mn, because nearest-neighbour Mn atoms couple antiferromagnetically. Co{sub 2}VAl has B2 order and has reduced magnetization. In the cases with x {ge} 0.9 conventional ferromagnetism was observed, closely related to the atomic disorder in these compounds.
TL;DR: In this article, an easy synthesis route of magnetite (Fe 3 O 4 ) nanopowder is developed by using thermal decomposition of Fe-urea complex ([Fe(CON 2 H 4 ) 6 ](NO 3 ) 3 ).
TL;DR: The role of the Z element is investigated: it influences the properties of the compounds mainly via its valence electron number and its atomic radius, which determines the lattice parameter.
Abstract: The Heusler compounds Mn(2)TiZ (Z = Al, Ga, In, Si, Ge, Sn, P, As, Sb) are of great interest due to their potential ferrimagnetic properties and high spin polarization. Here, we present calculations of the structural and magnetic properties of these materials. Their magnetic moment follows the Slater-Pauling rule m = N(V) - 24. None of them is actually a perfect half-metallic ferrimagnet, but some exhibit more than 90% spin polarization and Curie temperatures well above room temperature. The exchange interactions are complex; direct and indirect exchange contributions are identified. The Curie temperature scales with the total magnetic moment, and it has a positive pressure dependence. The role of the Z element is investigated: it influences the properties of the compounds mainly via its valence electron number and its atomic radius, which determines the lattice parameter. Based on these results, Mn(2)TiSi, Mn(2)TiGe, and Mn(2)TiSn are proposed as candidates for spintronic applications.
TL;DR: In this article, the magnetic and magnetoelectric properties of Ba2-xSrxNi2Fe12O22 single crystals over a wide composition range (0,≤ x≤ 1.5) were studied.
Abstract: We studied the magnetic and magnetoelectric properties of Ba2-xSrxNi2Fe12O22 single crystals over a wide composition range (0 ≤ x ≤ 1.5). All the crystals show a ferrimagnetic order at around 660 K. While a Sr-free crystal is simply ferrimagnetic down to the lowest temperature, a transition from the ferrimagnetic into a screw magnetic ordered state was observed at temperatures below 300 K in Sr-substituted crystals. The transition temperature monotonically increases with increasing Sr content, meaning that the screw ordered state is stabilized by the Sr-substitution. By applying a magnetic field perpendicular to the hexagonal c axis, the samples showing the ground-state screw order undergo successive metamagnetic transitions and exhibit magnetically induced ferroelectricity in some of the intermediate magnetic phases. In an intermediate magnetic phase, the largest electric polarization emerges (2 × 102 μC/m2 for x = 1.5 crystal), i.e., magnetoelectric effect. The evolution of the magnetic structures related to the magnetoelectric effect in x = 1.5 crystal was clarified by means of in-field neutron diffraction measurements. Though the magnetoelectric effect in the as-grown crystal was measurable only below ∼100 K due to its low resistivity, a post-annealing drastically enhances the resistivity and allows us to observe the magnetically induced ferroelectricity up to ∼175 K.
TL;DR: In this paper, it was shown that one of the two Mn moments has local character, whereas the other Mn moment and the Co moment are itinerant, based on noncollinear spin DFT.
Abstract: Epitaxial thin films of the half-metallic Xa compound Mn(2)CoGa (Hg(2)CuTi prototype) were prepared by dc magnetron co-sputtering with different heat treatments on MgO (001) substrates. High-quality films with a bulk magnetization of 1.95(5) mu(B) per unit cell were obtained. The L(3,2) x-ray magnetic circular dichroism spectra agree with calculations based on density functional theory (DFT) and reveal the antiparallel alignment of the two inequivalent Mn moments. X-ray magnetic linear dichroism, in good agreement with theory as well, allows us to distinguish between itinerant and local Mn moments. Based on noncollinear spin DFT, it is shown that one of the two Mn moments has local character, whereas the other Mn moment and the Co moment are itinerant.
TL;DR: The structure, dc magnetization and ac susceptibility characteristics of the rare-earth ion-substituted cobalt-ferrites (CoOFe1925La0075O3 and CoOFe 1925Dy0075 O3) are evaluated in this paper.
Abstract: The structure, dc magnetization and ac susceptibility characteristics of the rare-earth (R = La,Dy) ionsubstituted cobalt-ferrites (CoOFe1925La0075O3 and CoOFe1925Dy0075O3) are evaluated R-substituted Co-ferrites crystallize in the cubic inverse spinel phase The irreversible temperature (Tirr) between zero field cooled (ZFC) and field cooled (FC) magnetization for CoOFe1925La0075O3 and CoOFe1925Dy0075O3 determined from the temperature variation of magnetization measurements are 283 and 292 K, respectively The broadening of ZFC magnetization and more than one maximum indicates the coexistence of short-range ferrimagnetic clusters of different size with a long-range ferrimagnetic phase Magnetization curves indicate no saturation up to 30 kOe suggesting the canted spin structure inside the clusters The relaxation times of spin clusters calculated using theVogel−Fulcher law for the frequency-dependent ac susceptibility measurements are on the order of ∼10−6 s
TL;DR: In this article, the authors aimed at the low temperature synthesis of iron deficient samples of Ni-Cu-Zn ferrite using the microwave sintering technique, and the samples were sintered at 950°C for 30 min.
TL;DR: In this article, the effects of the exchange interaction have been investigated based on the corresponding atomic configuration generated by the occupation selectivity of the doped Co atoms, and it has been indicated that the interatomic distance plays a critical role in producing the local ferromagnetic structure.
Abstract: Polymorphic magnetization behavior has been observed experimentally in the Heusler alloy Mn${}_{2}$NiGa in which Co has been substituted for Ni or Ga. The magnetization of the austenitic phase can be enhanced up to 132 emu/g, when more than 50$%$ of the antiferromagnetic couplings between Mn atoms are changed to ferromagnetic couplings at the largest composition tolerance for Co substituting for Ga. The effects of the exchange interaction have been investigated based on the corresponding atomic configuration generated by the occupation selectivity of the doped Co atoms. First-principles calculations indicate that a high level of d-electron hybridization can occur when Mn atoms are the nearest neighbors of a Co atom. This causes a strong ferromagnetic exchange interaction in specific atomic configurations and produces a local ferromagnetic structure in the native ferrimagnetic structure matrix. It has been indicated that, based on theoretical work of Stearns, the interatomic distance plays a critical role in producing the local ferromagnetic structure. This has also been used to explain the magnetization behavior through the martensitic transformation in Mn${}_{2}$NiCoGa alloys.