Key Magnetic Properties Research Articles

Spin-cycloid instability as the origin of weak ferromagnetism in the disordered perovskiteBi0.8La0.2Fe0.5Mn0.5O3

Powder neutron diffraction and magnetometry studies have been conducted to investigate the crystallographic and magnetic structure of ${\mathrm{Bi}}_{0.8}{\mathrm{La}}_{0.2}{\mathrm{Fe}}_{0.5}{\mathrm{Mn}}_{0.5}{\mathrm{O}}_{3}$. The compound stabilizes in the $Imma$ orthorhombic crystal symmetry in the measured temperature range of 5 to 380 K, with a transition to antiferromagnetic order at ${T}_{\mathrm{N}}\ensuremath{\approx}240$ K. The spin cycloid present for BiFeO${}_{3}$ is found to be absent with 50% Mn${}^{3+}$ cation substitution, leading to $G$-type antiferromagnetic order with an enhanced out-of-plane canted ferromagnetic component, evident from measurable weak-ferromagnetic hysteresis. Structural modifications do not solely explain this behavior, indicating that modified electron exchange interactions must be taken into account. A classical spin simulation was developed to investigate the effect of random substitution in a disordered pseudocubic perovskite. The calculations took into account the nearest-neighbor, next-nearest-neighbor, and Dzyaloshinskii-Moriya interactions, along with the local spin anisotropy. Using this framework to extend the established Hamiltonian model for BiFeO${}_{3}$, we show that only certain types of perturbations at a magnetic defect and the surrounding molecular fields trigger a simultaneous collapse of cycloidal order and the emergence of the long-range weak-ferromagnetic component. By adopting values for the Mn molecular fields appropriate for $\mathit{RE}$MnO${}_{3}$ ($\mathit{RE}=$ rare earth), simulations of BiMn${}_{0.5}$Fe${}_{0.5}{\mathrm{O}}_{3}$ exhibit the key magnetic properties of our experimental observations.

Tuning the Magnetic Properties of Nanoparticles

The tremendous interest in magnetic nanoparticles (MNPs) is reflected in published research that ranges from novel methods of synthesis of unique nanoparticle shapes and composite structures to a large number of MNP characterization techniques, and finally to their use in many biomedical and nanotechnology-based applications. The knowledge gained from this vast body of research can be made more useful if we organize the associated results to correlate key magnetic properties with the parameters that influence them. Tuning these properties of MNPs will allow us to tailor nanoparticles for specific applications, thus increasing their effectiveness. The complex magnetic behavior exhibited by MNPs is governed by many factors; these factors can either improve or adversely affect the desired magnetic properties. In this report, we have outlined a matrix of parameters that can be varied to tune the magnetic properties of nanoparticles. For practical utility, this review focuses on the effect of size, shape, composition, and shell-core structure on saturation magnetization, coercivity, blocking temperature, and relaxation time.

Physico-chemical investigation on mixed alkali metal ferrites prepared by solution combustion method – A comparative study

Mixed alkali metal nanoferrites of the compositions M 0.5− X/2 Zn X Mn 0.05Fe 2.45− X/2 O 4 (M = Li, Na and K), where x varies from 0→0.5 in steps of 0.1, have been prepared by solution combustion method. Powder X-ray diffraction analysis for all the samples show the formation of single phase cubic spinel structure. The lattice parameter increases linearly with Zn content, which is attributed to ionic size differences of the cations involved. Both X-ray as well as experimental densities show upward trend with increasing ‘ x’ due to increase in the molecular weight of the ferrite composition. Mössbauer spectra display the superimposition of paramagnetic doublet over ferrimagnetic sextet with increasing diamagnetic ‘Zn’ content. The key magnetic properties of the ferrite obtained, such as saturation magnetization and Curie temperature have also been studied. The combustion method used for the synthesis is a rapid approach for direct conversion of the stoichiometric reactant solutions into fine nanoparticles of ferrite product at a temperature (600 °C) much lower than that of the conventional ceramic method. Scanning electron micrographs confirm the formation of nanosized ferrites.

Rock-magnetic properties confirm the eolian origin of Miocene sequences from the west of the Chinese Loess Plateau

Diagnostic rock-magnetic characteristics of Miocene sequences from the western part of the Chinese Loess Plateau are used to confirm the eolian nature of sections the origin of which has been a matter of recent dispute (Alonso-Zarza et al., 2009, 2010; Guo et al. 2010). The key magnetic properties include (i) detailed correlation of both the orbitally timed and supra-orbital variations in magnetic properties between widely separated sections, (ii) the bi-modal distribution of magnetic properties with respect to particle size, reflecting in situ neoformation alongside the deposition of detrital minerals derived from the source areas, (iii) the consistent presence, in the clay fractions, of fine ferrimagnetic grains close to the superparamagnetic/stable single domain transition, all within the size range characteristic of those formed by weathering and pedogenesis under at least partially aerobic conditions, and (iv) absence of evidence for lacustrine indicators such as bacterial magnetosome assemblages, sub-oxic diagenesis or greigite formation.

Accurate and computationally efficient third-nearest-neighbor tight-binding model for large graphene fragments

Owing to the large sizes involved, most calculations of the electronic properties of graphene and its fragments involve empirical tight-binding models restricted to nearest-neighbor interactions only. Such approaches fail to predict key electronic and magnetic properties, however, and rely on assumed geometries. While alternative approaches based on density-functional theory are much more successful in predicting properties, they are often computationally prohibitive to apply. We introduce a simple third-nearest-neighbor $\ensuremath{\pi}$-only tight-binding approach that maintains the computational efficiency of the empirical method while achieving the accuracy of the density-functional methods to which it is parametrized. It yields both nuclear geometries and electronic structures of graphene fragments, providing an efficient and accurate replacement for traditional tight-binding models of graphene.

Materials for Magnetoresistive Random Access Memory

MRAM technology is based on the storage of data in stable magnetic states using devices that have a large magnetoresistance effect, so that the data can be read by determining the resistance of the device. MRAM is inherently nonvolatile and the magnetic states can be switched extremely fast and with no wear out. In this article, we review the fundamentals of MRAM technology, explain the innovations that have overcome barriers to commercialization, and describe areas where further innovation can advance the technology most significantly. The influence of key structural and magnetic properties of materials used in magnetic tunnel junctions, and optimization of those devices for read and write performance in memory arrays, is discussed in detail. The various approaches to writing data in MRAM arrays are described and compared, with emphasis on toggle MRAM, the type that is in commercial production, and on spin-torque MRAM, which shows great promise for future improvements in power and density. In the final section, we summarize recent demonstrations of advanced MRAM technology.

Relevance of pinning, nucleation, and interaction in nanograined epitaxial hard magneticSmCo5films

Epitaxial growth of ${\text{SmCo}}_{5}$ films on substrates with two different symmetries was used to tailor film texture. A geometry where the easy axes of all grains are aligned in parallel is compared with a geometry where two sets of grains have a perpendicular alignment. For these two types of films with well defined nanoscaled microstructures, the role of pinning, nucleation, and intergrain interactions for the key permanent magnetic properties, namely, coercivity and remanence, is studied. The angular dependency of switching field and remanence reveals that the depinning of domain walls rules the magnetization reversal. In addition, these measurements indicate that the magnetization processes occur independently within each set of perpendicularly aligned grains. Although a large positive $\ensuremath{\delta}J$ indicates strong intergrain interactions, remanence enhancement is absent in both samples. An analysis of the irreversible susceptibility during the magnetizing and demagnetizing processes gives a complete picture of the relevant processes, and shows that both pinning and nucleation of domains are involved.

Microstructure study and hyper frequency electromagnetic characterization of novel hexagonal compounds

The physical characteristics and preparation of novel Z-type hexagonal compound, which have iron deficient composition of Ba3Co2(0.8−x)Cu0.4Zn2xFe23.5O41 (x≤0.30), was investigated. The results show that Zn has little effect on the morphology of compact and homogeneous ferrites with average 10–15μm of lathy grain size, but make the formation temperature range narrow. In the range of Zn incorporation, the hexagonal cell lattice parameters (a and c) of ferrites reserve stable. However, the key magnetic and electrical properties, such as initial permeability, quality factor, resistivity, dielectric constant and magnetic hysteresis as well as Curie temperature were also characterized. The mechanisms involving in generating these variations were also discussed in this paper.

Fundamental Limits of Soft Magnetic Particle Composites for High Frequency Applications

A phenomenological model of magnetic nanoparticle composites consisting of ideal lossless particles with identical properties embedded in a non-magnetic matrix has been developed. The input material parameter for this model include the saturation magnetization (Ms) and the crystal anisotropy field (Hk) of the particles. The relationships between key magnetic properties like the low frequency effective permeability and the FMR frequency, and the material and physical attributes (such as shape, volume fraction, and packing type) of the particles have been identified using an iterative extension of the Bruggeman effective medium theory.

Investigation on physical characteristics of novel Z-type Ba 3Co 2(0.8− x) Cu 0.40Zn 2 xFe 24O 41 hexaferrite

The influence of Zn incorporation into Cu-modified Co 2Z (Ba 3Co 2Fe 24O 41) ferrite on physical characteristics of low-sintered materials, which have stoichiometric compositions of Ba 3Co 2(0.8− x) Cu 0.4Zn 2 x Fe 24O 41 ( x≤0.50), is investigated. The experimental results show that Zn has little effect on the morphology of Cu-modified ferrites. The average 10–15 μm of lathy grain size for Ba 3Co 2(0.8− x) Cu 0.4Zn 2 x Fe 24O 41 is just like that of Ba 3Co 2(1− x) Cu 0.4Fe 24O 41. In the range of solid solubility of Zn, the lattice parameters ( a and c) of ferrites with Z-type hexagonal phase structure remain stable, compared with the sole Cu-modified ferrite. The variation of key magnetic properties, such as saturation magnetization, remnant magnetization, coercive force, magnetic hysteresis loop and initial permeability, magnetic loss, dielectric constant and Curie temperature of the ferrites as well as the variation mechanism are also addressed in this paper.

The most violent super-active regions in the 22nd and 23rd cycles

We survey 14 super-active regions (SARs) in the 22nd cycle and 15 SARs in the 23rd cycle. Each produced major flares and major solar storms. Among them, the 25 most violent super active regions (VSARs) are selected based on five parameters: the largest area of sunspots, X-ray flare index (XRI), 10.7 cm radio flux, proton flux and geomagnetic A p index. In order to understand the VSARs, we have investigated a few key magnetic properties of those regions, i.e., net magnetic flux, tilt angle and force-free parameter αbest. The following results are found: (1) Most VSARs (84%) in our samples have net magnetic flux greater than 1021 Mx, implying that those are seriously unbalanced flux regions. Unbalanced flux active regions probably provide a nest to relate the small-scale to the large-scale magnetic field. (2) Most of the VSARs (68%) are of abnormal magnetic structure, violating the Hale–Nicholson Law. For most of the normal VSARs, the tilt angles are larger than 40°. 84% of the VSARs follow the hemispheric helicity rule. Generally, they have large magnetic twist and writhe helicity. (3) We also enlarge our samples to study the locations of VSARs by adding the top 10 of the major flares, proton events and severe magnetic storms from 1976 to 2001. It is found that 77% in our 30 samples of VSARs were preferentially located in 4 longitude bands, i.e., l c=80°±15° l c=170°±15° l c=260°±15° and l c=350°±15°. The interval of those longitude bands is roughly 90°. From the above results, we suggest that there probably is a special magnetic environment in the sub-photosphere of the four longitude bands where it is preferred to produce abnormal and complex active regions which easily produce major flares and major solar storms. Area, magnetic class, net magnetic flux, Carrington longitude and tilt angle of an active region may serve to predict likelihood of the active region producing hazarded space weather.

Correlation of thermal stability and signal-to-noise ratio of thin film recording media

The thermal stability of data stored in thin film recording media is closely tied to signal-to-noise ratio (SNR), yet is not correlated in a simple way. Indeed grain size strongly affects both SNR and thermal stability, but macroscopic magnetic properties as well as recording conditions are equally influential. This is illustrated by measurements of SNR and signal thermal decay along with a simple model analysis. Dependence of areal density on key magnetic properties, under SNR and stability constraints, is formulated.

Hysteresis properties of magnetic spherules and whole rock specimens from some Paleozoic platform carbonate rocks

It is widely established that many Paleozoic carbonate rocks were remagnetized during the late Paleozoic Kiaman reversed superchron. Yet the paleomagnetic recorders of this event have remained elusive. Magnetic spherules have been candidates for this role, because they are probably authigenic. Therefore we have studied the hysteresis properties of individual magnetic spherules from two formations, the remagnetized Onondaga (Devonian, New York) and the effectively unremagnetized Wabash (Silurian, Indiana). Also, we have compared the hysteresis properties of spherules to those of remagnetized limestones from the Onondaga, Trenton (Ordovician, New York), and Helderberg (Devonian, New York) formations and from the unremagnetized Wabash formation. The spherules studied here vary from about 20 µm to over 100 µm in diameter. Nevertheless, on the basis of hysteresis behavior this spherule population spans the full range of domain states, from truly multidomain (MD) to pseudo‐single‐domain (PSD) to single domain (SD). Because these spherules are so large, it is remarkable that more than half display the highly PSD‐like behavior typical of many remagnetized carbonate rocks. Equally as remarkable is the lack of dependence on spherule size of various hysteresis properties such as coercive force (Hc), remanent coercive force (Hcr), the ratio of saturation remanence to saturation moment (Mrs/Ms), and the ratio of remanent coercive force to coercive force (Hcr/Hc). These findings dispel the preconception that large spherules are too magnetically soft to be stable carriers of ancient magnetization. Surprisingly, Onondaga and Wabash spherules yield virtually identical suites of hysteresis properties. On the one hand, this high degree of similarity could indicate that spherules are irrelevant to remagnetization. More intriguing, however, is that this similarity could mean that the Wabash formation, although effectively unremagnetized, nevertheless contains magnetic mineralogical traces of a regional remagnetization event. It is significant that for these spherules a bilogarithmic plot of Mrs/Ms versus Hcr/Hc does not define a simple linear trend. Instead, Mrs/Ms increases more rapidly with decreasing Hcr/Hc for spherules in the PSD range than in the MD range. When a power law is fit solely to the hysteresis ratios of PSD‐like to SD‐like spherules, extrapolation to the SD limit gives Mrs/Ms = 0.75. Similarly, a power law fit to the hysteresis ratios obtained here from remagnetized carbonate rocks yields Mrs/Ms = 0.72 at the SD boundary. These results could indicate that a large percentage of SD‐like spherules, like the SD carriers in remagnetized carbonate rocks, are governed by magnetocrystalline anisotropy. Finally, the present data point to values of Hc near 350 Oe both for SD‐like spherules and for the SD grains thought to carry remanence in remagnetized carbonates. Clearly, PSD‐ and SD‐like spherules share several key rock magnetic properties with remagnetized carbonate rocks. Thus spherules still are likely candidates for carrying the remagnetization signal.

Magnetic properties of cubic R xY1−xAl2 (R=Dy, Tb) intermetallic random anisotropy magnets (invited)

A short review is made of the key magnetic properties of dilute cubic RxY1−xAl2 (R=Tb, Dy) intermetallics, in order to show their main magnetic features. Dilution by Y introduces a weak random magnetic anisotropy (RMA). The rich magnetic phase diagram is described, including paramagnetic (P), spin glass (SG), correlated spin glass (CSG), random ferromagnetic (RFM), and ferromagnetic (F) phases, with a triple point and two multicritical ones. The paper deals with the induced macroscopic magnetic anisotropy cooling in a magnetic field below TSG or TCSG transition temperatures, which can be either unidirectional or uniaxial or both. High-field (3 T) magnetostriction in Tb series shows, for x=0.48, 0.59, 0.87, a decrease of the Callen α exponent (λt∼mα, m=reduced magnetization) below 3. The Sompolinsky irreversibility parameter Δ has been determined for the Tb series in the SG regime, and a replica model is presented to explain the Δ(T) dependence. The character of the P→SG or P→CSG transitions is addressed, through the scaling of nonlinear susceptibility (SG regime) or a ferromagnetic-like scaling of magnetization, respectively. From quasielastic neutron scattering around Q=[1,1,1] we determine the temperature dependence of the magnetic correlation length in Dy0.8Y0.2Al2, which peaks, but does not diverge at Tc. A magnon excitation at 3.5 meV is reported for x=0.8.

Effect of bismuth oxide on the phase composition and magnetic properties of the 12Fe2O3 � 3BaO � 2CoO system

A study was made of the effect of the addition of the low-melting-point bismuth oxide on the phase composition and magnetic properties of the 12Fe2O3·3BaO·CoO system. The optimum Bi2O3 concentration, at which the key magnetic properties attain their maxima, is 1 wt.%. The influence exerted by bismuth oxide on the magnetic properties of the ferrite is associated with qualitative and quantitative changes in the phase composition and microstructure of specimens and possibly also with partial dissolution of the barium hexaferrite structure.