Electric control of magnetism at room temperature

Fabrication, characterization, and magnetic properties of exchange-coupled porous BaFe8Al4O19/Co0.6Zn0.4Fe2O4 nanocomposite magnets

Fabrication of exchange-coupled nanocomposite magnets has been considered to be the most effective method to achieve the high energy product for advanced permanent magnet applications. In this work, we report a facile auto-combustion synthesis to prepare porous exchange-coupled hard-soft ferrite-based magnetic BaFe8Al4O19-x wt% Co0.6Zn0.4Fe2O4 nanocomposites (where x = 10, 20, 30 and 40), which realize an effective exchange-coupled interaction when the x value is less than 30. Compared with BaFe8Al4O19, the optimized nanocomposite with 20% Co0.6Zn0.4Fe2O4 shows a 70.3% increase in Ms and a 60.4% enhancement in Mr and maintains a high Hc value of 8.8 kOe. The work demonstrates that the auto-combustion synthesis is a promising approach for the fabrication of high-performance ferrite-based permanent magnets.

Structure and magnetic properties of W-type hexaferrites

W-type hexaferrites (WHFs) (SrMe 2Fe16O27, Me = Mg, Co, Ni and Zn) are hard magnetic materials with high potential for permanent magnet applications owing to their large crystalline anisotropy and high cation tunability. However, little is known with regards to their complex structural and magnetic characteristics. Here, the substitution of metals (Me = Mg, Co, Ni and Zn) in WHFs is described and their crystal and magnetic structures investigated. From joined refinements of X-ray and neutron powder diffraction data, the atomic positions of the Me atoms were extracted along with the magnetic dipolar moment of the individual sites. The four types of WHFs exhibit ferrimagnetic ordering. For Mg, Ni and Zn the magnetic moments are found to be ordered colinearly and with the magnetic easy axis along the crystallographic c axis. In SrCo2Fe16O27, however, the spontaneous magnetization changes from uniaxial to planar, with the moments aligning in the crystallographic ab plane. Macromagnetic properties were measured using a vibration sample magnetometer. The measured saturation magnetization (M s) between the different samples follows the same trend as the calculated M s extracted from the refined magnetic moments of the neutron powder diffraction data. Given the correlation between the calculated M s and the refined substitution degree of the different Me in specific crystallographic sites, the agreement between the measured and calculated M s values consolidates the robustness of the structural and magnetic Rietveld model.

Spin–orbit coupling in magnetoelectric Ba3(Zn1−xCox)2Fe24O41 hexaferrites

Z-type hexaferrites Ba₃(Zn_1−xCo_x)₂Fe₂₄O₄₁ (x = 0.2, 0.4, 0.6, 0.8, defined as Z1–Z4) were synthesized by a sol–gel method.

Effects of Al3+ Substitution on Structural and Magnetic Behavior of CoFe₂O₄ Ferrite Nanomaterials

A sol-gel autocombustion method was used to synthesize Al³⁺ ion-substituted cobalt ferrite CoAl_xFe_2-xO₄ (x = 0⁻1.5). According to X-ray diffraction analysis (XRD), cobalt ferrite was in a single cubic phase after being calcined at 1000 °C for 3 h. Moreover, the lattice constant decreased with increase in aluminum substituents. When the sample was analyzed by Scanning Electron Microscopy (SEM), we found that uniformly sized, well-crystallized grains were distributed in the sample. Furthermore, we confirmed that Al³⁺ ion-substituted cobalt ferrite underwent a transition from ferrimagnetic to superparamagnetic behavior; the superparamagnetic behavior was completely correlated with the increase in Al³⁺ ion concentration at room temperature. All these findings were observed in Mössbauer spectra. For the cobalt ferrite CoAl_xFe_2-xO₄, the coercivity and saturation magnetization decrease with an increase in aluminum content. When the annealing temperature of CoAl_0.1Fe_1.9O₄ was steadily increased, the coercivity and saturation magnetization initially increased and then decreased.

Improved magnetic and magnetoelectric properties in BaFe12O19 nanostructures

Improved magnetism and magnetodielectric coupling are obtained in plate-like BaFe₁₂O₁₉ nanoparticles because of their larger structural distortion.

Nanoparticle-Based Magnetoelectric BaTiO3-CoFe2O4 Thin Film Heterostructures for Voltage Control of Magnetism

Multiferroic composite materials combining ferroelectric and ferromagnetic order at room temperature have great potential for emerging applications such as four-state memories, magnetoelectric sensors, and microwave devices. In this paper, we report an effective and facile liquid phase deposition route to create multiferroic composite thin films involving the spin-coating of nanoparticle dispersions of BaTiO₃, a well-known ferroelectric, and CoFe₂O₄, a highly magnetostrictive material. This approach offers great flexibility in terms of accessible film configurations (co-dispersed as well as layered films), thicknesses (from 100 nm to several μm) and composition (5-50 wt % CoFe₂O₄ with respect to BaTiO₃) to address various potential applications. A detailed structural characterization proves that BaTiO₃ and CoFe₂O₄ remain phase-separated with clear interfaces on the nanoscale after heat treatment, while electrical and magnetic studies indicate the simultaneous presence of both ferroelectric and ferromagnetic order. Furthermore, coupling between these orders within the films is demonstrated with voltage control of the magnetism at ambient temperatures.

Tunable magnetism in metal adsorbed fluorinated nanoporous graphene

Developing nanostructures with tunable magnetic states is crucial for designing novel data storage and quantum information devices. Using density functional theory, we investigate the thermodynamic stability and magnetic properties of tungsten adsorbed tri-vacancy fluorinated (TVF) graphene. We demonstrate a strong structure-property relationship and its response to external stimuli via defect engineering in graphene-based materials. Complex interplay between defect states and the chemisorbed atom results in a large magnetic moment of 7 μB along with high in-plane magneto-crystalline anisotropy energy (MAE) of 17 meV. Under the influence of electric field, spin crossover effect accompanied by a change in the MAE is observed. The ascribed change in spin-configuration is caused by the modification of exchange coupling between defect states and a change in the occupation of d-orbitals of the metal complex. Our predictions open a promising way towards controlling the magnetic properties in graphene based spintronic and non-volatile memory devices.

Structural studies of CaAl12O19, SrAl12O19, La2/3+δ Al12–δO19, and CaAl10NiTiO19 with the hibonite structure; indications of an unusual type of ferroelectricity

Various oxides with the hibonite structure were synthesized and structurally analyzed using powder neutron diffraction. The structure of CaAl12O19 at 298 and 11 K shows dipoles that are apparently too dilute to order unless subjected to a suitable electric field. Magnetoplumbites, such as BaFe12O19, are isostructural with hibonite. These compounds possess ferromagnetic properties, which combined with the electric dipoles may influence multiferroic behavior. Our SrAl12O19 sample showed two distinct hexagonal phases, a major phase with the normal hibonite structure and a minor phase having a closely related structure. Our sample of the defect hibonite phase La2/3+δAl12–δO19 shows a distinctly higher δ value (0.25) vs. that reported (~0.15) for samples made from the melt. Finally, we used to advantage the negative scattering length of Ti to determine the site occupancies of Ni and Ti in CaAl10NiTiO19.

Intrinsic interfacial phenomena in manganite heterostructures

We review recent advances in our understanding of interfacial phenomena that emerge when dissimilar materials are brought together at atomically sharp and coherent interfaces. In particular, we focus on phenomena that are intrinsic to the interface and review recent work carried out on perovskite manganites interfaces, a class of complex oxides whose rich electronic properties have proven to be a useful playground for the discovery and prediction of novel phenomena.

Electric field control of the magnetocaloric effect

Through strain-mediated magnetoelectric coupling, it is demonstrated that the magnetocaloric effect of a ferromagnetic shape-memory alloy can be controlled by an electric field. Large hysteresis and the limited operating temperature region are effectively overcome by applying an electric field on a laminate comprising a piezoelectric and the alloy. Accordingly, a model for an active magnetic refrigerator with high efficiency is proposed in principle.

Thermally induced magnetization switching in Fe/MnAs/GaAs(001): selectable magnetic configurations by temperature and field control

Spintronic devices currently rely on magnetization control by external magnetic fields or spin-polarized currents. Developing temperature-driven magnetization control has potential for achieving enhanced device functionalities. Recently, there has been much interest in thermally induced magnetisation switching (TIMS), where the temperature control of intrinsic material properties drives a deterministic switching without applying external fields. TIMS, mainly investigated in rare-earth–transition-metal ferrimagnets, has also been observed in epitaxial Fe/MnAs/GaAs(001), where it stems from a completely different physical mechanism. In Fe/MnAs temperature actually modifies the surface dipolar fields associated with the MnAs magnetic microstructure. This in turn determines the effective magnetic field acting on the Fe overlayer. In this way one can reverse the Fe magnetization direction by performing thermal cycles at ambient temperatures. Here we use element selective magnetization measurements to demonstrate that various magnetic configurations of the Fe/MnAs/GaAs(001) system are stabilized predictably by acting on the thermal cycle parameters and on the presence of a bias field. We show in particular that the maximum temperature reached during the cycle affects the final magnetic configuration. Our findings show that applications are possible for fast magnetization switching, where local temperature changes are induced by laser excitations.

Absence of evidence ≠ evidence of absence: statistical analysis of inclusions in multiferroic thin films

Assertions that a new material may offer particularly advantageous properties should always be subjected to careful critical evaluation, especially when those properties can be affected by the presence of inclusions at trace level. This is particularly important for claims relating to new multiferroic compounds, which can easily be confounded by unobserved second phase magnetic inclusions. We demonstrate an original methodology for the detection, localization and quantification of second phase inclusions in thin Aurivillius type films. Additionally, we develop a dedicated statistical model and demonstrate its application to the analysis of Bi(6)Ti(2.8)Fe(1.52)Mn(0.68)O18 (B6TFMO) thin films, that makes it possible to put a high, defined confidence level (e.g. 99.5%) to the statement of 'new single phase multiferroic materials'. While our methodology has been specifically developed for magnetic inclusions, it can easily be adapted to any other material system that can be affected by low level inclusions.

Multiferroic iron oxide thin films at room temperature

Multiferroic behaviour at room temperature is demonstrated in ε-Fe2 O3 . The simple composition of this new ferromagnetic ferroelectric oxide and the discovery of a robust path for its thin film growth by using suitable seed layers may boost the exploitation of ε-Fe2 O3 in novel devices.