Magnetic Domain State Research Articles

Electrochemical fabrication and magnetic properties of highly ordered silver–nickel core-shell nanowires

Ordered silver–nickel core-shell nanowire arrays were successfully fabricated by electrodeposition. The ordered silver nanowire arrays embedded in a porous alumina template were first fabricated from an aqueous solution of Ag(NO 3) 2 and Ac(NH 3). After removing out the template, the obtained silver nanowire arrays were subsequently electrodeposited with nickel at 1.6–2.6 V and 60 °C using the electrolyte composed of NiSO 4, NiCl 2 and H 3BO 3. Transmission electron microscopy observation revealed that a ∼15 nm thick nickel film was coated on the surface of the silver nanowires with about 200 nm in diameter. It was found that the silver nanowires with nickel coating showed enhanced magnetic properties in comparison to that of pure silver nanowires. The magnetic force microscope image of silver–nickel core-shell nanowires showed magnetic domain state. In addition, the hysteresis loops of the silver–nickel nanowire arrays showed a coercive field of 180 Oe, almost independent of the applied magnetic field. However, it was observed that a larger magnetic domain was found in parallel direction than that in perpendicular direction.

Micromagnetism and magnetization reversal of embedded ferromagnetic elements

A unique type of ferromagnetic microelement is explored. Unlike conventional ferromagnetic elements, the entities studied here are not separated topographically from each other, but embedded into a surrounding, continuous film. Fabrication of such elements is achieved by local irradiation of antiferromagnetically coupled $\mathrm{Fe}∕\mathrm{Cr}∕\mathrm{Fe}$ trilayers with $30\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ ${\mathrm{Ga}}^{+}$ ions, which cause a local destruction of the Cr interlayer in these systems. As a result, a transition to ferromagnetic properties is induced within micron-sized irradiated areas, which act as ferromagnetic elements. Since the surrounding area of these elements consists of magnetic material, i.e., two Fe layers which are still antiferromagnetically coupled, interesting coupling phenomena in lateral direction can be observed. In particular, the magnetic configuration within such systems leads to the formation of complex domain walls at the boundary between irradiated and nonirradiated areas exhibiting different types of fine structure. In addition, it is found that the capability of storing information in the form of magnetic single domain states in remanence depends on the geometry of the patterned elements. The fabrication method presented here is an efficient way to create magnetic model systems on the micron scale of different geometries and sizes for comparative studies of micromagnetics and magnetization reversal processes.

Effect of particle size on the structure and magnetic properties of La0.6Pb0.4MnO3 nanoparticles

We studied the effect of particle size on the structure and magnetic properties of La0.6Pb0.4MnO3 compounds with particle diameters varying from 5to100nm. With decreasing particle size, the increase of the rhombohedral distortion narrows the bandwidth WO2p and hence results in the decrease of ferromagnetic transition temperature. At the same time, the magnetic domain structure evolves from multidomain to single domain, and finally a superparamagnetic behavior is detected. The critical single domain size determined by experiment is about 25nm, and the critical superparamagnetic behavior size obtained by calculation is 5.8nm. It is found that the variation of coercivity is related to the evolution of magnetic domain states with particle size, and the saturation magnetization decreases significantly due to the increasing contribution of the surface effect with decreasing particle size.

Reliable absolute palaeointensities independent of magnetic domain state

Knowledge of palaeointensity variation is required for determining the full vector variation of the geomagnetic field as a function of geological time. This provides essential constraints for numerical geodynamo models. To date, most palaeointensity determination methods are laborious, characterised by rather low success rates, and demand substantial processing time. The rocks under investigation must obey stringent criteria to yield faithful palaeointensities: the magnetic particles must be single domain, the natural remanent magnetisation must be a thermoremanent magnetisation, and during the successive heating steps in the laboratory no chemical alteration should occur. Here, we describe a new method that allows all magnetic domain states to be processed, i.e. it does not require single domain particles. The method makes use of the linearity of partial thermoremanent magnetisation (pTRM) with the applied laboratory field. Multiple specimens are used so that every sample is exposed only once to a laboratory field, warranting that all samples experienced the same magnetic history. Through the limited number of thermal steps alteration effects are reduced as well. The laboratory pTRM and natural remanent magnetisation (NRM) of the specimens are oriented parallel to minimise the effects of high-temperature tails that affect multidomain minerals. The pTRM acquisition temperature is selected below the temperature at which chemical alteration sets in and above the temperature trajectory where secondary viscous NRM components occur. The procedure requires a lower number of steps than any other palaeointensity method, reducing significantly the total time needed per rock unit. We propose to name the new protocol ‘multispecimen parallel differential pTRM method’. It provides the correct answer to ∼ 5% for artificial samples and natural rocks containing multidomain magnetic particles that were given a laboratory TRM of known intensity. Application to the Paricutin September–December 1943 lava flow (three sites, 7 specimens per site) yields a weighted mean of 45.9 ± 1.25 μT, within uncertainty margin of the expected value of 45.0 μT.

Photoemission electron microscopy study of remanent magnetic domain states in ferromagnetic wedge films deposited on substrates with micrometer-sized square plateaus

We present a detailed study of the remanent magnetic domain configurations in demagnetized polycrystalline ferromagnetic thin film wedges of cobalt and Permalloy deposited on prepatterned silicon substrates with micrometer-sized square plateaus, which have a height of 125 nm, using photoemission electron microscopy. We have observed the continuous evolution of the magnetic domain states in the square ferromagnetic elements on top of the plateaus as a function of film thickness. At high film thickness we observe the Landau state, which is the expected lowest energy state, but at lower thickness we see a variety of metastable states which are trapped as a result of local pinning. In a small thickness range below 10 nm, the square elements contain 360° walls and small domains which are likely to be a result of local effects such as magnetocrystalline anisotropy and edge roughness. We are able to simultaneously observe the development of the magnetic domains in the continuous polycrystalline film surrounding the plateaus and, rather than the expected large domains, we observe at intermediate film thickness a significant modification of the domain configuration to small domains. Here the roughness of the silicon substrate surrounding the plateaus, which is due to the reactive ion etching process used to prepare the prepatterned substrates, gives rise to local stray fields in the ferromagnetic film which play an important role in determining the resulting domain structure.

Multiplicity of magnetic domain states in circular elements probed by photoemission electron microscopy

A systematic study of the remanent magnetic states in Co and ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ disks $1.65\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in diameter with varying film thickness $(5--38\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ imaged by a noninvasive imaging technique (namely, photoemission electron microscopy) is presented. The magnetic configurations observed range from the vortex state, to other well defined multidomain states, namely a state with two vortex cores (``diamond'' state) and a high remanence ``triangle'' state, which are mapped according to material and thickness range. In most instances, more than one magnetic state coexist in different disks of the same array, subjected to the same field history. These observations are correlated with magneto-optical Kerr effect measurements and micromagnetic simulations to show that the observed magnetic metastable states are stabilized by defects or attained in the nucleation process following the removal of the applied magnetic field.

Synthesis and magnetic properties of Ni nanocylinders in self-aligned and randomlydisordered grown titania nanotubes

A novel ferromagnetic/semiconductor oxide nanocomposite formed by arrays of Ninanocylinders grown by the electrodeposition technique in a semiconductor oxide matrix ofself-aligned and randomly disordered titanium dioxide nanotubes has been synthesized.X-ray diffraction, EDX, SEM, AFM, rf-GDOES and VSM magnetometry techniqueshave been used to investigate the structural, compositional and morphologicalproperties, as well as its specific magnetic behaviour. Titania nanotubes have beengrown through a single anodization process, by using HF acidic electrolytes in apotentiostatic mode. The thus-obtained titanium dioxide nanotube outer diameter rangesbetween 90 and 150 nm, wall thickness about 25–40 nm and 300 nm in depth. Theelectrodeposited Ni nanocylinders reach above 100 nm diameter and 240 nm length,giving rise to coercive fields of 98 and 200 Oe, well within the hysteresis loopsperpendicular or parallel to the nanocylinder axis, respectively, which could beascribed to the formation of magnetic vortex domain states. It is expected that thisnovelty nanocomposite, based on ferromagnetic Ni nanocylinders embedded in asemiconductor titanium dioxide nanotube template, will become a promising candidate formany applications in a broad range of scientific and technological areas, suchas ultrahigh density magnetic storage media or spin-based electronic devices.

Magnetic behaviour of arrays of Ni nanowires by electrodeposition into self-aligned titania nanotubes

Arrays of Ni nanowires electrodeposited into self-aligned and randomly disordered titania nanotube arrays grown by anodization process are investigated by X-ray diffraction, SEM, rf-GDOES and VSM magnetometry. The titania nanotube outer diameter is about 160 nm, wall thickness ranging from 60 to 70 nm and 300 nm in depth. The so-obtained Ni nanowires reach above 100 nm diameter and 240 nm length, giving rise to coercive fields of 98 and 200 Oe in the perpendicular or parallel to the nanowires axis hysteresis loops, respectively. The formation of magnetic vortex domain states is also discussed.

Exsolved magnetite inclusions in silicates: Features determining their remanence behavior

Research Article| June 01, 2005 Exsolved magnetite inclusions in silicates: Features determining their remanence behavior Joshua M. Feinberg; Joshua M. Feinberg 1Department of Earth and Planetary Science, University of California, Berkeley, 340 McCone Hall, Berkeley, California 94720, USA Search for other works by this author on: GSW Google Scholar Gary R. Scott; Gary R. Scott 2Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA Search for other works by this author on: GSW Google Scholar Paul R. Renne; Paul R. Renne 3 Department of Earth and Planetary Science, University of California, Berkeley, 340 McCone Hall, Berkeley, California 94720, USA, and Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA Search for other works by this author on: GSW Google Scholar Hans-Rudolf Wenk Hans-Rudolf Wenk 4Department of Earth and Planetary Science, University of California, Berkeley, 340 McCone Hall, Berkeley, California 94720, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Joshua M. Feinberg 1Department of Earth and Planetary Science, University of California, Berkeley, 340 McCone Hall, Berkeley, California 94720, USA Gary R. Scott 2Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA Paul R. Renne 3 Department of Earth and Planetary Science, University of California, Berkeley, 340 McCone Hall, Berkeley, California 94720, USA, and Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA Hans-Rudolf Wenk 4Department of Earth and Planetary Science, University of California, Berkeley, 340 McCone Hall, Berkeley, California 94720, USA Publisher: Geological Society of America Received: 13 Oct 2004 Revision Received: 19 Jan 2005 Accepted: 26 Jan 2005 First Online: 03 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2005) 33 (6): 513–516. https://doi.org/10.1130/G21290.1 Article history Received: 13 Oct 2004 Revision Received: 19 Jan 2005 Accepted: 26 Jan 2005 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Joshua M. Feinberg, Gary R. Scott, Paul R. Renne, Hans-Rudolf Wenk; Exsolved magnetite inclusions in silicates: Features determining their remanence behavior. Geology 2005;; 33 (6): 513–516. doi: https://doi.org/10.1130/G21290.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Submicroscopic, needle-shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts, making them excellent candidates for paleomagnetic studies. A suite of samples containing clinopyroxene- and plagioclase-hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions' magnetic domain state. Alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others. The two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions. Magnetite-ulvöspinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103–105 magnetite prisms separated by thin ulvöspinel lamellae. The conversion of multidomain-sized needles into assemblages of interacting single domains increases the coercivity (and hence relaxation time) of the inclusions, and results in a thermochemical magnetic remanence. In samples without this exsolution microstructure, the inclusions' diameters determine coercivity and their magnetization is thermoremanent. Both styles of high-coercivity inclusions successfully record paleomagnetic directions in Mesozoic rocks, and their ubiquity within silicate minerals (clinopyroxene and plagioclase) of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field, marine magnetic anomalies, and crustal kinematics. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Magnetic properties of different grain-sized particles of sediments from the Okinawa Trough and their relationships to sedimentary environment

Multiple magnetic parameters were measured for nine different grain-sized fractions separated from the sediment samples that are representatives of four different sedimentary environments of the Okinawa Trough. Based on the measured results, the contributions of different grain-sized particles to total magnetic susceptibility of bulk sediments, the magnetic mineral assemblage and magnetic domain state as well as their relationships to sedimentary environment were discussed. Our research shows that the magnetic mineral is dominated by magnetite with a small amount of hematite and is primarily in pseudo-single-domain state. That indicates that the different sedimentary environ- ments in the Okinawa Trough have certain correlation in material provenance. The magnetic minerals enrich in dif- ferent grain-sized particles in response to different sedimen- tary environments. The contribution of the grain sizes from coarse to fine to coarse and fine to the magnetic susceptibility from the west to the east is in accordance with terrigenous material transportation from continental shelf of the East China Sea to the Okinawa Trough. It also shows difference in magnetic properties as a result of some environmental fac- tors.

Magnetocrystalline anisotropy energy in L10-type CoPt single crystals

L10-type Co100−xPtx single crystals were prepared by atomic ordering under compressive stress and the magnetic properties were investigated. The second order magnetocrystalline anisotropy constant K1 at 298K exhibits a maximum value of 4.1×107ergcm−3 at x=50. On the other hand, the fourth order magnetocrystalline anisotropy constant K2 becomes minimum at the same composition. The large uniaxial magnetocrystalline anisotropy constant KU with a low saturation magnetization MS in L10-type Co100−xPtx would bring about the thermal stability of the single magnetic domain state, as well as the sharp coherent magnetization reversal.

Magnetic domain states in nano-sized Co nuclei electrodeposited onto monocrystalline silicon

In the present work, nano-sized magnetic nuclei of Co have been electrodeposited onto p-Si (111). The deposition follows a mechanism of progressive nucleation and growth controlled by diffusion. MFM studies showed that the transition between magnetic domain states is strongly dependent on the size and shape of the nuclei. A critical height h 0 is defined below which the nuclei presented always a single-domain configuration. The limiting lower boundary for the single-domain state calculated from the theory is quantitatively coincident with the experimental results.

Integrated magnetic studies of the El Romeral iron-ore deposit, Chile: implications for ore genesis and modeling of magnetic anomalies

Mineralogical and rock-magnetic studies of iron ores and host rocks in El Romeral Mine are carried out to characterize the magnetic mineralogy and the processes that affect the natural remanent magnetization during emplacement and evolution of the iron-ore deposit. Extremely important is the identification of magnetic mineralogical composition (magnetite and/or titanomagnetite, hematite and/or titanohematite, and titanomaghemite) and grain size. These data permit investigation of magnetic domain state and magnetization acquisition processes and to assess their significance as a source of magnetic anomalies. Chemical remanent magnetization (CRM) seems to be present in most of investigated ore and wall-rock samples, substituting completely or partially the original thermoremanent magnetization (TRM). Magnetite (or Ti-poor titanomagnetite) and titanohematite are commonly found in the ores. Although hematite may carry a stable CRM, no secondary components are detected above 580 °C, which probably attests that oxidation occurred soon after the extrusion and cooling of the ore-bearing magma. The microscopy study under reflected light shows that magnetic carriers are mainly titanomagnetite with significant amounts of ilmenite–hematite minerals. Magmatic titanomagnetite, found in igneous rocks, shows trellis texture, which is compatible with high temperature (deuteric) oxy-exsolution processes. Hydrothermal alteration in ore deposits is indicated by goethite and hematite oxide minerals. Grain sizes range from a few microns to >100 μm, and possible magnetic states from single to multidomain, in agreement with hysteresis measurements. Thermal spectra, continuous susceptibility measurements, and isothermal remanent magnetization acquisition suggest a predominance of spinels as magnetic carriers, most probably titanomagnetites with low-Ti content. For quantitative modeling of the magnetic anomaly, we used data on bulk susceptibility and natural remanent intensity for quantifying the relative contributions of induced and remanent magnetization components, and this allows greater control of the geometry of source bodies. The position and geometry of these magnetic sources are shown as ENE-striking tabular bodies, one steeply inclined (75°) to the south and another lying horizontal.

Rock-Magnetic and Oxide Microscopic Studies of the El Laco Iron Ore Deposits, Chilean Andes, and Implications for Magnetic Anomaly Modeling

Microscopic and rock-magnetic studies of the ores and host rocks of the El Laco iron oxide deposits permit us to characterize the magnetic mineralogy and the processes affecting natural remanent magnetization (NRM) during emplacement and evolution of the deposits. Particular attention was devoted to identifying the magnetic mineral composition (magnetite and/or titanomagnetite, and hematite and/or titanohematite, and titanomaghemite) and grain size variations of both ores and host rock. Rock-magnetic data are used to clarify magnetic domain states and remanence acquisition processes, and to assess their significance as a source of magnetic anomalies. Microscopy under reflected light demonstrates that magnetic carriers are mainly magnetite, with significant amounts of ilmenite-hematite minerals. Magmatic titanomagnetites in the andesitic rocks show trellis textures, compatible with high-temperature oxy-exsolution processes. Supergene reactions in ore deposits under eruption conditions are indicated by goethite and hematite oxide minerals. Grain sizes range from a few microns to >100 μm. Hysteresis measurements point to pseudo-single-domain states. Thermal spectra, continuous temperature-dependent susceptibility measurements, and isothermal remanent magnetization (IRM) acquisition suggest predominance of spinels (titanomagnetite or titanomaghemite) with low-Ti contents as magnetic carriers. Although the presence of (titano)hematites is indicated by hysteresis and IRM studies, their contribution to the total remanence seems to be minor. The Fe-oxides in the ore are typically poor in Ti, whereas in the rocks they are Ti-bearing. For the modeling of the magnetic anomalies, we used data on bulk susceptibility and NRM intensity and direction in order to constrain the relative contributions of induced and remanent magnetization components, and to obtain improved control regarding depth and geometry of source bodies. The deep magnetic source corresponds to an ENE-striking tabular body, steeply inclined 65° to the north.

Noninvasive observation of magnetization reversal in micron-sized Co bars

We demonstrate how room temperature scanning Hall probe microscopy (SHPM) can be applied to noninvasively probe the (meta-)stable magnetic domain states of individual mesoscopic Co bars as a function of applied field. These data are correlated with magneto-optical measurements of the hysteresis loop as well as higher spatial resolution magnetic force microscopy imaging. In particular, we show how a combination of visualization techniques is essential for fully understanding the mechanism of magnetization reversal in such microstructures. Micromagnetic calculations of the magnetic states and their corresponding stray fields are correlated with the observations. These experiments prove the usefulness of a noninvasive and quantitative technique such as room temperature SHPM in an applied magnetic field to study magnetic domain stability and switching field distributions.

Observation of inverse magnetoresistance in epitaxial magnetite/manganite junctions

We have fabricated and characterized epitaxial Fe3O4/CoCr2O4/La0.7Sr0.3MnO3 magnetic trilayer junctions (MTJs) grown on (110) and (100) SrTiO3 substrates. Large inverse junction magnetoresistance (JMR) as high as −25% has been observed at a field of 4 kOe in (110) oriented MTJs. The improvement of the JMR over that observed in previous epitaxial magnetite junctions is attributed to the choice of the spinel structure CoCr2O4 barrier, which minimizes structural disorder at the barrier/Fe3O4 interface. As a comparison, studies on (100) MTJs elucidate the effect of the magnetic domain state of the ferromagnetic electrodes on the JMR.

Secular variation of the geomagnetic field intensity during the past 2000 years in Japan

We report the results of paleointensity determination with volcanic rocks of Izu-Oshima and Fuji Volcano, Japan. Samples used for the present study were basaltic rocks from nine lava flow units of Oshima Volcano, which erupted between AD 590 and 1950, and Aokigahara lava flow derived from Fuji Volcano in AD 864. Demagnetization and several rock magnetic experiments show that the samples have stable, single-component remanences, which are carried by magnetite or titanomagnetite with the PSD magnetic domain state. Thellier’s method was used for the paleointensity experiments, and reliable determinations for 10 eruptive units were obtained from 58 samples in total. The unit-mean values generally show good consistency with the previous archaeointensity and paleointensity results, allowing us to construct a reference curve of the field intensity variation for the past 2000 yr in Japan. The reference curve has a general trend of decreasing intensity since AD 500, showing two peaks of the field strength around AD 500 and 1400 and two troughs around AD 1200 and 1700. Since around AD 800, the field intensity predictions for Japan by the inversion spherical harmonic models show good consistency with the reference curve, suggesting that the peak and the troughs in this period have probably been produced mainly by non-dipole sources. The geocentric axial dipole variation extracted from the reference curve with the inversion model shows good agreement with the global averages of the virtual axial dipole moment, except that our reference curve yields a significant peak around AD 500. This suggests that a temporal strengthening of the dipole moment might have occurred over a few hundred years around AD 500.

Magnetic domain structures in submicron-size particles of epitaxial Fe (001) films: Shape anisotropy and thickness dependence

In this work the influence of shape anisotropy on the magnetic hysteresis and zero-field magnetic domain state of submicron-size particles of Fe was investigated. Arrays of particles having circular (diameter dc=550 or 200 nm), rectangular (900 nm by 300 nm), or elliptical (450 nm by 150 nm) shape were prepared by electron lithography and ion-beam milling of epitaxial Fe (001) films of thicknesses t=50, 30, 15, and 10 nm. The samples were characterized by magnetization measurements and magnetic-force microscopy (MFM). All films have cubic anisotropy, for t=50, 30, and 15 nm with the same anisotropy constant as bulk Fe: K1=(4.30.1)104 J m-3. For t=10 nm the effective anisotropy is increased, K1=5.7104 J m-3, due to surface effects. The effects of the interplay between the magnotocrystalline and shape anisotropies are observed as the lateral extension of the films is decreased. The circular particles with dc=550 nm have closed magnetic domain structures with a fourfold symmetry, compatible with the magnetocrystalline anisotropy, for all thicknesses. In the rectangular particles a gradual change is observed as the thickness decreases. For t=50 nm a diamond structure comprising three closed-domain substructures is formed in the demagnetized state. The rectangles with t=30 and 15 nm are multidomains with the number of closed substructures decreasing to 2 and 2 or 1, respectively. The thickness dependence of the domain structure and an accompanying change of character in the MFM contrast are explained by an increasing amount of Neel component in the domain walls with a width that increases with decreasing film thickness. The rectangles with t=10 nm are quasisingle domains, forming a flower state. The small circular particles (dc=200 nm) and the elliptical ones, both with t=10 nm, are considered to be stable single domains in zero field. Judging from the hysteresis curves, magnetization reversal does not occur by coherent rotation in any of the particles

First-order reversal curve (FORC) diagrams for pseudo-single-domain magnetites at high temperature

The recently developed first-order reversal curve (FORC) technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations. However, there are still some gaps in the basic understanding of FORC diagrams, in particular the behavior of pseudo-single-domain (PSD) grains and the contribution of magnetostatic interactions. In this paper we address some of these problems. We report the first FORC diagrams measurements on narrowly sized and well-characterized synthetic PSD through multidomain (MD) magnetite samples. The FORC diagrams evolve with grain size from single-domain (SD)-like to MD-like through the PSD grain size range. Since each sample contains grains of essentially a single size, individual PSD grains evidently contain contributions from both SD-like and MD-like magnetic moments, in proportions that vary with grain size; the evolving FORC diagrams cannot be due to physical mixtures of SD and MD grains of widely different sizes. The FORC diagrams were all asymmetric. Small PSD samples have FORC diagrams with a distinctive closed-contour structure. The distributions of the larger MD grains display no peak, and lie closer to the interaction-field axis. To assess the effect of magnetostatic interactions, we measured FORC diagrams between room temperature and the Curie temperature. On heating the FORC distributions contract without changing shape until ∼500°C. Above this temperature the diagrams become more MD-like, and in addition become more symmetric. The temperature dependence of the interaction-field parameter is proportional to that of the saturation magnetization, in accordance with Néel’s interpretation of the Preisach diagram. The decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions. The magnetic hysteresis parameters as a function of temperature were determined from the FORC curves. As the grain size decreased the normalized coercive force was found to decrease more rapidly with temperature.

Magnetic hysteresis and domain states in Li1+ substituted Ni–Zn ferrites

Compositions of Ni0.32Zn0.68-2xLixFe2+xO4 with x=0.0, 0.025, 0.050, 0.075, 0.100, 0.125, 0.150, 0.175, 0.200 were prepared by oxalate precursors. The Ms−T curves resemble type R. The magnetic moment (nB) values were found to increase with increase of Li1+ content up to x=0.175, and thereafter it decreased. The former explained on the basis of the exchange interactions, whereas the decrease of nB is related to the migration of Ni to tetrahedral (A) sites. There is a decrease in Yafet–Kittle (Y–K) angles at octahedral (B) sites, suggesting that lithium does not favor Hc canting. Actually, Li1+ tends to reduce canting, resulting in the increase of nB coercive force (Hc) and remanance ratio (R) variations have been related to increase of anisotropy constant (Kl) with increasing Li1+ content. The values of R are comparable with the theoretical value, 0.87, which is probably due to the very high density of the samples. Thermal variations of Hc and R reveal that neither parameter changes rapidly with increasing temperature, indicating that the material contains multi-domain (MD) grains in predominance. These results are also supported by a.c. susceptibility studies. The values of wall susceptibility are higher than those of rotational susceptibility, suggesting that the susceptibility is contributed mainly by the domain wall motion.