Showing papers on "Antiferromagnetism published in 2010"

Robust isothermal electric control of exchange bias at room temperature

Abstract: Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with high-speed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr(2)O(3) has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr(2)O(3) single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Neel temperature.

Magnetic Materials : Fundamentals and Applications

Abstract: Machine generated contents note: Part I. Basics: 1. Review of basic magnetostatics; 2. Magnetization and magnetic materials; 3. Atomic origins of magnetism; 4. Diamagnetism; 5. Paramagnetism; 6. Interactions in ferromagnetic materials; 7. Ferromagnetic domains; 8. Antiferromagnetism; 9. Ferrimagnetism; 10. Summary of basics; Part II. Magnetic Phenomena: 11. Anisotropy; 12. Nanoparticles and thin films; 13. Magnetoresistance; 14. Exchange bias; Part III. Device Applications and Novel Materials: 15. Magnetic data storage; 16. Magneto-optics and magneto-optic recording; 17. Magnetic semiconductors and insulators; 18. Multiferroics; Solutions to selected exercises.

Interface Ferromagnetism and Orbital Reconstruction in BiFeO3-La0.7Sr0.3MnO3 Heterostructures

Abstract: We report the formation of a novel ferromagnetic state in the antiferromagnet BiFeO3 at the interface with ferromagnet La0.7Sr0.3MnO3. Using x-ray magnetic circular dichroism at Mn and Fe L2,3 edges, we discovered that the development of this ferromagnetic spin structure is strongly associated with the onset of a significant exchange bias. Our results demonstrate that the magnetic state is directly related to an electronic orbital reconstruction at the interface, which is supported by the linearly polarized x-ray absorption measurement at the oxygen K edge

Antiferromagnetic Mott insulating state in single crystals of the hexagonal lattice material Na2IrO3

Abstract: We have synthesized single crystals of Na_2IrO_3 and studied their structure, transport, magnetic, and thermal properties using powder x-ray diffraction (PXRD), electrical resistivity, isothermal magnetization M versus magnetic field H, magnetic susceptibility \chi versus temperature T, and heat capacity C versus T measurements. Na_2IrO_3 crystallizes in the monoclinic \emph{C2/c} (No. 15) type structure which is made up of Na and NaIr_2O_6 layers alternately stacked along the c axis. The \chi(T) data show Curie-Weiss behavior at high T > 200K with an effective moment \mu_eff = 1.82(1) \mu_B indicating an effective spin S_eff = 1/2 on the Ir^4+ moments. A large Weiss temperature \theta = - 116(3)K indicates substantial antiferromagnetic interactions between these S_eff = 1/2, Ir^4+ moments. Sharp anomalies in \chi(T) and C(T) data indicate that Na_2IrO_3 undergoes a transition into a long-range antiferromagnetically ordered state below T_N = 15 K. The magnetic entropy at T_N is only about 20% of what is expected for S_eff = 1/2 moment ordering. The reduced entropy and the small ratio T_N/\theta \approx 0.13 suggest geometrical magnetic frustration and/or low-dimensional magnetic interactions in Na_2IrO_3. In plane resistivity measurements show insulating behavior. This together with the local moment magnetism indicates that bulk Na_2IrO_3 is a Mott insulator.

Slow relaxation processes and single-ion magnetic behaviors in dysprosium-containing complexes.

Abstract: A series of one-dimensional complexes [Ln(L1)3(HOCH2CH2OH)]n (L1 = 2-furoate anion; Ln = Nd (1), Sm (2), Gd (3), Tb (4), Dy (5), Er (6)) have been synthesized. The complexes were crystallized in the monoclinic space group P2(1)/c and show a chain-like structure determined by single-crystal X-ray diffraction. Magnetic properties indicate that carboxyl group of 2-furoate mediates different magnetic couplings in light and heavy rare earth complexes, namely, antiferromagnetic interaction between light rare earth ions and ferromagnetic interaction between heavy ones. Noticeably, complex 5 displays a strong frequency dependence of alternating current (AC) magnetic properties. Further magnetic studies show a distribution of a single relaxation process in 5. While 1,10-phenanthroline and phthalate anion (L2) were employed, [Dy2(L2)6(H2O)]n (7) was isolated by hydrothermal reactions and characterized magnetically. Research results also show the frequency dependence of AC magnetic susceptibilities, although the pht...

Spin-orbit coupling induced anisotropy effects in bimetallic antiferromagnets: A route towards antiferromagnetic spintronics

Abstract: Magnetic anisotropy phenomena in bimetallic antiferromagnets ${\text{Mn}}_{2}\text{Au}$ and MnIr are studied by first-principles density-functional theory calculations We find strong and lattice-parameter-dependent magnetic anisotropies of the ground-state energy, chemical potential, and density of states, and attribute these anisotropies to combined effects of large moment on the $\text{Mn}\text{ }3d$ shell and large spin-orbit coupling on the $5d$ shell of the noble metal Large magnitudes of the proposed effects can open a route towards spintronics in compensated antiferromagnets without involving ferromagnetic elements

Evidence for a Lifshitz transition in electron-doped iron arsenic superconductors at the onset of superconductivity

Abstract: A study shows that in electron-doped Ba(Fe1−xCox)2As2, the presence or absence of superconductivity and antiferromagnetism can be linked directly to changes in the Fermi surface—as measured by angle-resolved photoemission spectroscopy.

Structure-related frustrated magnetism of nanosized polyoxometalates: aesthetics and properties in harmony

Abstract: The structural versatility characterizing polyoxometalate chemistry, in combination with the option to deliberately use well-defined building blocks, serves as the foundation for the generation of a large family of magnetic clusters, frequently comprising highly symmetric spin arrays. If the spin centers are coupled by antiferromagnetic exchange, some of these systems exhibit spin frustration, which can result in novel magnetic properties of purely molecular origins. We discuss here the magnetic properties of selected nanosized polyoxometalate clusters featuring spin triangles as their magnetic ‘building blocks’ or fragments. This includes unique porous Keplerate clusters of the type {(Mo)Mo5}12M30 (M = FeIII, CrIII, VIV) with the spin centers defining a regular icosidodecahedron and the {V15As6}-type cluster sphere containing a single equilateral spin triangle; these species are widely discussed and studied in the literature for their role in materials science as molecular representations of Kagome lattices and in relation to quantum computing, respectively. Exhibiting fascinating and unique structural features, these magnetic molecules allow the study of the implications of frustrated spin ordering. Furthermore, this perspective covers the impact of spin frustration on the degeneracy of the ground state and related problems, namely strong magnetic anisotropy and the interplay of antisymmetric exchange and structural Jahn–Teller effects.

From single- to double-first-order magnetic phase transition in magnetocaloric Mn1−xCrxCoGe compounds

Abstract: Substitution of some Cr for Mn atoms in MnCoGe was employed to control the magnetic and structural transitions in this alloy to coincide, leading to a single first-order magnetostructural transition from the ferromagnetic to the paramagnetic state with a giant magnetocaloric effect observed near room temperature. Further increase in the Cr content in the Mn1−xCrxCoGe alloys can induce another first-order magnetoelastic transition from the antiferromagnetic to the ferromagnetic state occurring at lower temperature. The giant magnetocaloric effect as well as the simultaneous tunability of the two magnetic transitions make these materials promising for future cooling applications.

[ReCl4(CN)2]2-: a high magnetic anisotropy building unit giving rise to the single-chain magnets (DMF)4MReCl4(CN)2 (M = Mn, Fe, Co, Ni).

Abstract: An S = 3/2, high-anisotropy building unit, trans-[ReCl4(CN)2]2−, representing the first paramagnetic complex with a mixture of just cyanide and halide ligands, has been synthesized through the reaction of (Bu4N)CN with ReCl4(THF)2. This species is characterized in detail and employed in directing the formation of a series of one-dimensional coordination solids of formula (DMF)4MReCl4(CN)2 (M = Mn (2), Fe (3), Co (4), Ni (5)). Variable-temperature dc magnetic susceptibility measurements demonstrate the presence of intrachain antiferromagnetic (2) and ferromagnetic (3−5) exchange coupling within these solids. In addition, probing the ac magnetic susceptibility as a function of both temperature and frequency reveals that all of the chain compounds exhibit slow relaxation of the magnetization. The relaxation time is shown to be thermally activated, with energy barriers to relaxation of Δτ = 31, 56, 17, and 20 cm−1 for 2−5, respectively. Notably, the field-dependent magnetization of the iron congener exhibits ...

Site Specific X-ray Anomalous Dispersion of the Geometrically Frustrated Kagomé Magnet, Herbertsmithite, ZnCu3(OH)6Cl2

Abstract: Structural characterization, exploiting X-ray scattering differences at elemental absorption edges, is developed to quantitatively determine crystallographic site-specific metal disorder. We apply this technique to the problem of Zn−Cu chemical disorder in ZnCu3(OH)6Cl2. This geometrically frustrated kagome antiferromagnet is one of the best candidates for a spin-liquid ground state, but chemical disorder has been suggested as a mundane explanation for its magnetic properties. Using anomalous scattering at the Zn and Cu edges, we determine that there is no Zn occupation of the intralayer Cu sites within the kagome layer; however there is Cu present on the Zn intersite, leading to a structural formula of (Zn0.85Cu0.15)Cu3(OH)6Cl2. The lack of Zn mixing onto the kagome lattice sites lends support to the idea that the electronic ground state in ZnCu3(OH)6Cl2 and its relatives is nontrivial.

Photoinduced metal-to-metal charge transfer toward single-chain magnet.

Abstract: Single-chain magnets (SCMs) that exhibit slow relaxation of their magnetization are attracting considerable attention To tune the properties of such materials with external stimuli such as light, heat, and pressure is a challenge Through the exploitation of light and heat induced transformation between diamagnetic Fe(II)(LS)(mu-CN)Co(III)(LS) (LS = low spin) units and paramagnetic Fe(III)(LS)(mu-CN)Co(II)(HS) (HS = high spin) units, we show the photoswitched transformation from a paramagnetic state to an antiferromagnetic ordered SCM state and the thermally induced reverse transformation, thus providing an effective way to control the spin topology of the SCM via light or a thermally induced metal-to-metal charge transfer

Quantum Kagome Antiferromagnet ZnCu3(OH)6Cl2

Abstract: The frustration of antiferromagnetic interactions on the loosely connected kagome lattice associated to the enhancement of quantum fluctuations for S =1/2 spins was acknowledged long ago as a keypoint to stabilize novel ground states of magnetic matter. Only very recently, the model compound Herbersmithite, ZnCu 3 (OH) 6 Cl 2 , a structurally perfect kagome antiferromagnet, could be synthesized and enables a close comparison to theories. We review and classify various experimental results obtained over the past years and underline some of the pending issues.

Exotic phases induced by strong spin-orbit coupling in ordered double perovskites

Abstract: We construct and analyze a microscopic model for insulating rocksalt ordered double perovskites, with the chemical formula ${A}_{2}B{B}^{\ensuremath{'}}{\text{O}}_{6}$, where the ${B}^{\ensuremath{'}}$ atom has a $4{d}^{1}$ or $5{d}^{1}$ electronic configuration and forms a face-centered-cubic lattice. The combination of the triply degenerate ${t}_{2g}$ orbital and strong spin-orbit coupling forms local quadruplets with an effective spin moment $j=3/2$. Moreover, due to strongly orbital-dependent exchange, the effective spins have substantial biquadratic and bicubic interactions (fourth and sixth order in the spins, respectively). This leads, at the mean-field level, to three main phases: an unusual antiferromagnet with dominant octupolar order, a ferromagnetic phase with magnetization along the [110] direction, and a nonmagnetic but quadrupolar ordered phase, which is stabilized by thermal fluctuations and intermediate temperatures. All these phases have a two-sublattice structure described by the ordering wave vector $\mathbit{Q}=2\ensuremath{\pi}(001)$. We consider quantum fluctuations and argue that in the regime of dominant antiferromagnetic exchange, a nonmagnetic valence-bond solid or quantum-spin-liquid state may be favored instead. Candidate quantum-spin-liquid states and their basic properties are described. We also address the effect of single-site anisotropy driven by lattice distortions. Existing and possible future experiments are discussed in light of these results.

Orbital ordering and unfrustrated ( π , 0 ) magnetism from degenerate double exchange in the iron pnictides

Abstract: The magnetic excitations of the iron pnictides are explained within a degenerate double-exchange model. The local-moment spins are coupled by superexchanges ${J}_{1}$ and ${J}_{2}$ between nearest and next-nearest neighbors, respectively, and interact with the itinerant electrons of the degenerate ${d}_{xz}$ and ${d}_{yz}$ orbitals via a ferromagnetic Hund exchange. The latter stabilizes $(\ensuremath{\pi},0)$ stripe antiferromagnetism due to emergent ferro-orbital order and the resulting kinetic-energy gain by hopping preferably along the ferromagnetic spin direction. Taking the quantum nature of the spins into account, we calculate the magnetic excitation spectra in the presence of both, superexchange and double exchange. A dramatic increase in the spin-wave energies at the competing N\'eel ordering wave vector is found, in agreement with recent neutron-scattering data. The spectra are fitted to a spin-only model with a strong spatial anisotropy and additional longer-ranged couplings along the ferromagnetic chains. Over a realistic parameter range, the effective couplings along the chains are negative corresponding to unfrustrated stripe antiferromagnetism.

Competing order and nature of the pairing state in the iron pnictides

Abstract: We show that the competition between magnetism and superconductivity can be used to determine the pairing state in the iron arsenides. To this end we demonstrate that the itinerant antiferromagnetic (AFM) phase and the unconventional ${s}^{+\ensuremath{-}}$ sign-changing superconducting (SC) state are near the borderline of microscopic coexistence and macroscopic phase separation, explaining the experimentally observed competition of both ordered states. In contrast, conventional ${s}^{++}$ pairing is not able to coexist with magnetism. Expanding the microscopic free energy of the system with competing orders around the multicritical point, we find that static magnetism plays the role of an intrinsic interband Josephson coupling, making the phase diagram sensitive to the symmetry of the Cooper-pair wave function. We relate this result to the quasiparticle excitation spectrum and to the emergent SO(5) symmetry of systems with particle-hole symmetry. Our results rely on the assumption that the same electrons that form the ordered moment contribute to the superconducting condensate and that the system is close to particle-hole symmetry. We also compare the suppression of SC in different regions of the FeAs phase diagram, showing that while in the underdoped side it is due to the competition with AFM, in the overdoped side it is related to the disappearance of pockets from the Fermi surface.

Spin and orbital Ti magnetism at LaMnO3/SrTiO3 interfaces

Abstract: In systems with strong electron-lattice coupling, such as manganites, orbital degeneracy is lifted, causing a null expectation value of the orbital magnetic moment. Magnetic structure is thus determined by spin-spin superexchange. In titanates, however, with much smaller Jahn-Teller distortions, orbital degeneracy might allow non-zero values of the orbital magnetic moment, and novel forms of ferromagnetic superexchange interaction unique to t(2g) electron systems have been theoretically predicted, although their experimental observation has remained elusive. In this paper, we report a new kind of Ti(3+) ferromagnetism at LaMnO(3)/SrTiO(3) epitaxial interfaces. It results from charge transfer to the empty conduction band of the titanate and has spin and orbital contributions evidencing the role of orbital degeneracy. The possibility of tuning magnetic alignment (ferromagnetic or antiferromagnetic) of Ti and Mn moments by structural parameters is demonstrated. This result will provide important clues for understanding the effects of orbital degeneracy in superexchange coupling.

Ferromagnetism at the interfaces of antiferromagnetic FeRh epilayers

Abstract: The nanoscale magnetic structure of FeRh epilayers has been studied by polarized neutron reflectometry. Epitaxial films with a nominal thickness of 500 A were grown on MgO 001 substrates via molecular-beam epitaxy and capped with 20 A of MgO. The FeRh films show a clear transition from the antiferromagnetic AF state to the ferromagnetic FM state with increasing temperature. Surprisingly the films possess a FM moment even at a temperature 80 K below the AF-FM transition temperature of the film. We have quantified the magnitude and spatial extent of this FM moment, which is confined to within 60‐80 A of the FeRh near the top and bottom interfaces. These interfacial FM layers account for the unusual effects previously observed in films with thickness 100 A. Given the delicate energy balance between the AF and FM ground states we suggest a metastable FM state resides near to the interface within an AF matrix. The length scale over which the FM region resides is consistent with the strained regions of the film.

Three-Dimensional Antiferromagnetic Order of Single-Chain Magnets: A New Approach to Design Molecule-Based Magnets

Abstract: Two one-dimensional compounds composed of a 1:1 ratio of MnIII salen-type complex and NiII oximato moiety with different counter anions, PF6 and BPh4, were synthesized: [Mn(3,5 Cl2saltmen)NiACHTUNGTRENUNG(pao)2- ACHTUNGTRENUNG(phen)]PF6 (1) and [Mn(5- Clsaltmen)NiACHTUNGTRENUNG(pao)2ACHTUNGTRENUNG(phen)]BPh4 (2), where 3,5-Cl2saltmen2=N,N'-(1,1,2,2- tetramethylethylene)bis(3,5-dichlorosalicylideneiminate); 5-Clsaltmen2= N,N'-(1,1,2,2-tetramethylethylene)- bis(5-chlorosalicylideneiminate); pao=pyridine-2-aldoximate; and phen=1,10-phenanthroline. Singlecrystal X-ray diffraction study was carried out for both compounds. In 1 and 2, the chain topology is very similar forming an alternating linear chain with a [-MnIII-ON-NiII-NO-] repeating motif (where -ON- is the oximate bridge). The use of a bulky counteranion, such as BPh4, located between the chains in 2 rather than PF6 in 1, successfully led to the magnetic isolation of the chains in 2. This minimization of the interchain interactions allows the study of the intrinsic magnetic properties of the chains present in 1 and 2. While 1 and 2 possess, as expected, very similar paramagnetic properties above 15 K, their ground state is antiferromagnetic below 9.4 K and paramagnetic down to 1.8 K, respectively. Nevertheless, both compounds exhibit a magnet-type behavior at temperatures below 6 K. While for 2, the observed magnetism is well explained by a Single-Chain Magnet (SCM) behavior, the magnet properties for 1 are induced by the presence in the material of SCM building units that order antiferromagnetically. By controlling both intra- and interchain magnetic interactions in this new [MnIIINiII] SCM system, a remarkable AF phase with a magnet-type behavior has been stabilized in relation with the intrinsic SCM properties of the chains present in 1. This result suggests that the simultaneous enhancement of both intrachain (J) and interchain (J') magnetic interactions (with keeping J @ J'), independently of the presence of AF phase might be an efficient route to design high temperature SCM-based magnets.

Ferroelectricity induced by spin-dependent metal-ligand hybridization in Ba₂CoGe₂O₇.

Abstract: We have investigated the variation of induced ferroelectric polarization under a magnetic field with various directions and magnitudes in a staggered antiferromagnet Ba₂CoGe₂O₇. While the ferroelectric polarization cannot be explained by the well-accepted spin current model nor the exchange striction mechanism, we have shown that it is induced by the spin-dependent p-d hybridization between the transition metal (Co) and ligand (O) via the spin-orbit interaction. On the basis of the correspondence between the direction of electric polarization and the magnetic state, we have also demonstrated the electrical control of the magnetization direction.

The spin-1/2 J1–J2 Heisenberg antiferromagnet on the square lattice:Exact diagonalization for N=40 spins

Abstract: We present numerical exact results for the ground state and the low-lying excitations for the spin-1/2 J1-J2 Heisenberg antiferromagnet on finite square lattices of up to N=40 sites. Using finite-size extrapolation we determine the ground-state energy, the magnetic order parameters, the spin gap, the uniform susceptibility, as well as the spin-wave velocity and the spin stiffness as functions of the frustration parameter J2/J1. In agreement with the generally excepted scenario we find semiclassical magnetically ordered phases for J2 < $J_2^{c_1}$ and J2 > $J_2^{c_2}$ separated by a gapful quantum paramagnetic phase. We estimate $J_2^{c_1}$ ≈0.35J1 and $J_2^{c_2}$ ≈0.66J1.

Ground-state properties and high-pressure behavior of plutonium dioxide: Density functional theory calculations

Abstract: Plutonium dioxide is of high technological importance in nuclear fuel cycle and is particularly crucial in long-term storage of Pu-based radioactive waste. Using first-principles density-functional theory, in this paper we systematically study the structural, electronic, mechanical, thermodynamic properties, and pressure induced structural transition of PuO$_{2}$. To properly describe the strong correlation in the Pu $5f$ electrons, the local density approximation$+U$ and the generalized gradient approximation$+U$ theoretical formalisms have been employed. We optimize the $U$ parameter in calculating the total energy, lattice parameters, and bulk modulus at the nonmagnetic, ferromagnetic, and antiferromagnetic configurations for both ground state fluorite structure and high pressure cotunnite structure. The best agreement with experiments is obtained by tuning the effective Hubbard parameter $U$ at around 4 eV within the LDA$+U$ approach. After carefully testing the validity of the ground state, we further investigate the bonding nature, elastic constants, various moduli, Debye temperature, hardness, ideal tensile strength, and phonon dispersion for fluorite PuO$_{2}$. Some thermodynamic properties, e.g., the Gibbs free energy, volume thermal expansion, and specific heat, are also calculated. As for cotunnite phase, besides the elastic constants, various moduli, and Debye temperature at 0 GPa, we have further presented our calculated electronic, structural, and magnetic properties for PuO$_{2}$ under pressure up to 280 GPa. A metallic transition at around 133 GPa and an isostructural transition in pressure range of 75-133 GPa are predicted.

Magnetic quantum tunneling: insights from simple molecule-based magnets

Abstract: This perspectives article takes a broad view of the current understanding of magnetic bistability and magnetic quantum tunneling in single-molecule magnets (SMMs), focusing on three families of relatively simple, low-nuclearity transition metal clusters: spin S = 4 NiII4, MnIII3 (S = 2 and 6) and MnIII6 (S = 4 and 12). The MnIII complexes are related by the fact that they contain triangular MnIII3 units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the MnIII centers, thereby leaving the single-ion physics more-or-less unaltered. This allows for a detailed and systematic study of the way in which the individual-ion anisotropies project onto the molecular spin ground state in otherwise identical low- and high-spin molecules, thus providing unique insights into the key factors that control the quantum dynamics of SMMs, namely: (i) the height of the kinetic barrier to magnetization relaxation; and (ii) the transverse interactions that cause tunneling through this barrier. Numerical calculations are supported by an unprecedented experimental data set (17 different compounds), including very detailed spectroscopic information obtained from high-frequency electron paramagnetic resonance and low-temperature hysteresis measurements. Comparisons are made between the giant spin and multi-spin phenomenologies. The giant spin approach assumes the ground state spin, S, to be exact, enabling implementation of simple anisotropy projection techniques. This methodology provides a basic understanding of the concept of anisotropy dilution whereby the cluster anisotropy decreases as the total spin increases, resulting in a barrier that depends weakly on S. This partly explains why the record barrier for a SMM (86 K for Mn6) has barely increased in the 15 years since the first studies of Mn12–acetate, and why the tiny Mn3 molecule can have a barrier approaching 60% of this record. Ultimately, the giant spin approach fails to capture all of the key physics, although it works remarkably well for the purely ferromagnetic cases. Nevertheless, diagonalization of the multi-spin Hamiltonian matrix is necessary in order to fully capture the interplay between exchange and local anisotropy, and the resultant spin-state mixing which ultimately gives rise to the tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn3 and Ni4). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc.) of the molecules highlighted in this study proves to be of crucial importance. Not only that, these simple molecules may be considered among the best SMMs: Mn6 possesses the record anisotropy barrier, and Mn3 is the first SMM to exhibit quantum tunneling selection rules that reflect the intrinsic symmetry of the molecule.

Magnetic Behavior Control in Niccolite Structural Metal Formate Frameworks [NH2(CH3)2][FeIIIMII(HCOO)6] (M = Fe, Mn, and Co) by Varying the Divalent Metal Ions

Abstract: By changing template cation but introducing trivalent iron ions in the known niccolite structural metal formate frameworks, three complexes formulated [NH(2)(CH(3))(2)][Fe(III)M(II)(HCOO)(6)] (M = Fe for 1, Mn for 2, and Co for 3) were synthesized and magnetically characterized. The variation in the compositions of the complexes leads to three different complexes: mixed-valent complex 1, heterometallic but with the same spin state complex 2, and heterometallic heterospin complex 3. The magnetic behaviors are closely related to the divalent metal ions used. Complex 1 exhibits negative magnetization assigned as Neel N-Type ferrimagnet, with an asymmetric magnetization reversal in the hysteresis loop, and complex 2 is an antiferromagnet with small spin canting (α(canting) ≈ 0.06° and T(canting) = 35 K), while complex 3 is a ferrimagnet with T(N) = 32 K.

Unified picture for magnetic correlations in iron-based superconductors.

Abstract: The varying metallic antiferromagnetic correlations observed in iron-based superconductors are unified in a model consisting of both itinerant electrons and localized spins. The decisive factor is found to be the sensitive competition between the superexchange antiferromagnetism and the orbital-degenerate double-exchange ferromagnetism. Our results reveal the crucial role of Hund's rule coupling for the strongly correlated nature of the system and suggest that the iron-based superconductors are closer kin to manganites than cuprates in terms of their diverse magnetism and incoherent normal-state electron transport. This unified picture would be instrumental for exploring other exotic properties and the mechanism of superconductivity in this new class of superconductors.

Magnetic recording sensor with sputtered antiferromagnetic coupling trilayer between plated ferromagnetic shields

Abstract: A magnetic recording sensor for use in a data storage device is described. The sensor has a magnetoresistive sensing element and magnetic shields shielding the magnetoresistive sensing element. The magnetic shields include a first plated soft ferromagnetic layer, a second plated soft ferromagnetic layer, and an antiferromagnetic coupling (AFC) trilayer between the first plated soft ferromagnetic layer and the second plated soft ferromagnetic layer. The AFC trilayer includes a first AFC layer of sputtered ferromagnetic material; a second AFC layer of a nonmagnetic antiferromagnetic exchange material, and a third AFC layer of sputtered ferromagnetic material. Shields with AFC trilayers in bottom, side, and/or top shields, and well as between shields are provided. A method of fabricating is also provided.

Nonmagnetic Fe-site doping of BiFeO3 multiferroic ceramics

Abstract: In this paper, we show that a pure single phase by doping Fe-site of BiFeO3 (BFO) using tetravalent Zr4+ ions can be achieved by introducing cation (Bi3+) vacancies. The structural analysis reveals that the ferroelectric nature of BFO should be weakly affected by 10% of Zr4+ doping as the c/a ratio and the Curie temperature TC remain roughly unchanged compared to that of pure BFO. In contrast, the magnetic properties are affected as a weak ferromagnetism and a change of Neel temperature TN are observed. Beyond the double-exchange interactions arising from the creation of Fe2+, we propose another simple model inducing a local ferromagnetic coupling rather than an antiferromagnetic which considers the replacement of the magnetically active Fe3+, time to time, by a nonactive Zr4+.

Surface magnetism, Morin transition, and magnetic dynamics in antiferromagnetic α-Fe2O3 (hematite) nanograins

Abstract: The grain size of α-Fe2O3 decreases to ∼20 nm by 64 h mechanical milling of the bulk sample. X-ray diffraction pattern suggested identical crystal structure in bulk and mechanical milled samples. Magnetic study (at temperatures of 100–900 K and fields of 0–±15 kOe) showed many interesting features during the decrease in grain size in antiferromagnetic α-Fe2O3, e.g., suppression of Morin transition, enhancement in low temperature magnetization, magnetic blocking at high temperature, exchange bias effect, and unusual relaxation of magnetic spin moment. We understand the results in terms of core-shell spin structure of nanograins, where the core part essentially retained the magnetic structure of the bulk sample and the magnetic structure of the shell part is modified due to grain size reduction and surface modification during mechanical milling. Core-shell structure also plays an important role in exhibiting the increasing soft ferromagnetic character in the present hematite samples. The in field magnetic relaxation at room temperature revealed some interesting properties of the magnetic spin ordering in hematite system.

Giant reversible magnetocaloric effect in metamagnetic HoCuSi compound

Abstract: The magnetic properties and magnetocaloric effect (MCE) of antiferromagnetic HoCuSi compound have been studied. It is found that HoCuSi undergoes a field-induced first order metamagnetic transition from antiferromagnetic (AFM) to ferromagnetic (FM) states below the Neel temperature (TN). A giant MCE without hysteresis loss is observed in HoCuSi around TN. The maximal magnetic entropy change (−ΔSM) and refrigerant capacity are 33.1 J/kgK and 385 J/kg, respectively, for a field change of 0–5 T. The excellent magnetocaloric properties can result from the field-induced AFM-FM transition below TN and the increase in magnetization change caused by the change in lattice volume at TN.