Remanence Properties Research Articles

Hard Magnetic Properties of Nanocrystalline Fe-Rich Fe–Nd–B Alloys Prepared by Partial Crystallization of Amorphous Phase

When Fe-rich Fe-Nd-B amorphous alloys containing 88 to 90 at% Fe are annealed for 60-300 s at 923-1023 K, the annealed alloys have the nanostructure consisting of bcc-Fe, Fe 14 Nd 2 B and remaining amorphous phases and exhibit rather good hard magnetic properties. The best hard magnetic properties of remanence (B r ), coercive field ( i H c ) and maximum energy product ((BH) max ) are 1.14 T, 260 kA/m and 117 kJ/m 3 , respectively, for Fe 90 Nd 7 B 3 1.28 T, 252 kA/m and 146 kJ/m 3 , respectively, for Fe 89 Nd 7 B 4 and 1.22 T, 240 kA/m and 130 kJ/m 3 for Fe 88 Nd 8 B 4 . The mean particle sizes of these crystallites in the optimum annealing treatment are 20 to 40 nm and the thickness of the intergranular amorphous layer is 10 to 30 nm. The amorphous layer contains Nd concentrations much higher than the nominal concentration and the enrichment seems to be the reason for the residual existence of the amorphous phase at the high temperatures. The nanoscale Fe 14 Nd 2 B particles are surrounded by the bcc-Fe and amorphous phases. The three constituent phases have ferromagnetism and their Curie temperatures for the Fe 89 Nd 7 B 4 alloy annealed for 300 s at 923 K are about 1040 K for bcc-Fe and 630 K for Fe 14 Nd 2 B. The further increase in annealing temperature and time causes the decrease in hard magnetic properties presumably because of the grain growth of bcc-Fe and Fe 14 Nd 2 B phases resulting from the disappearance of the residual amorphous phase. The coexistence of bcc-Fe, Fe 14 Nd 2 B and amorphous phases on a subnanoscale is important for the achievement of the rather good hard magnetic properties and hence the bcc-Fe and amorphous phases seem to act as an effective magnetic exchange-coupled medium. The simultaneous achievement of the high B r and (BH) max values in the residual existence of the amorphous phase for the Fe-rich alloys containing about 90 at% Fe is believed to be the first evidence and has significant engineering importance because of the expectations of high deformability and high cost performance.

Influence of magnetic anisotropy distribution on interactions in Co-Cr-Ta thin films

In-plane remanence properties of Co-Cr-Ta thin films have been previously investigated in order to quantify magnetic interaction effects parallel to, and transverse to, the transfer direction. Samples were prepared at two deposition rates of the Cr underlayer. Magnetic layers deposited on high deposition rate Cr underlayer had lower peak heights of /spl Delta/I and small coefficients of.

Experimental analysis of vector Preisach modeling for anisotropic recording media

An experimental analysis of the rotational remanence properties of various recording samples is presented. The locus of the saturation remanence is an ellipse. For smaller rotating fields, the trajectory is a spiral whose limit cycle is an ellipse; however, depending on the initial magnetization, the aspect ratio and the center of the ellipse for small fields are different from those for the saturating field case.

Magneto-optic properties of macroscopic ferrimagnets

A new class of two phased materials, called macroscopic ferrimagnets, displays many properties associated with ferrimagnetism including magnetic compensation points. A typical material system which shows this phenomenon is cobalt containing /spl cong/100 /spl Aring/ precipitates of EuS. The negative exchange in these systems is relatively weak so the remanent magneto-optic properties show the ferrimagnetic state, but at fields above 2 to 3 T, alignment of both the Co and EuS with the field can occur. In addition to the usual magnetic compensation point at which the sign of the magneto-optic loop reverses, we observe a second, magneto-optic, compensation point which occurs when the Kerr rotation of the two phases are equal and opposite. >

Strong hysteresis versus zero-root dynamics

This paper presents the properties of remanence and of dependence on history pertaining to strong hysteresis through a foreign trade model. These properties reveal that it is impossible to assimilate zero-root dynamics to hysteresis.

Palaeomagnetism, rock magnetism and evolution of the Great Barrier Reef

Palaeomagnetic and rock magnetic properties have been determined for sediment cores collected during ODP Leg 133 from the continental slope adjacent to the Great Barrier Reef. The magnetic remanence properties of the sediments are poor, with a pervasive viscous overprint that obscures most reversal boundaries. However, an inter-core magnetic susceptibility stratigraphy has been established that permits some refinement of the existing biostratigraphy. The quality of the palaeomagnetic remanence record relates to the residence time of magnetic particles in the corrosive, near-surface, sulphate-reducing zone. Magnetic susceptibility variations appear to reflect glacio-eustatic sea level cycles, and show a strong inverse correlation with both d18O and carbonate content. For the last several glacial maxima there are marked changes in magnetic properties, largely grain size-related, with sharp peaks in susceptibility and a reversal of the d18O-susceptibility phase relationship. One possible explanation can be found in terms of fluvio-deltaic processes and inter-reefal lagoonal reservoirs that develop during times of low sea level and become reworked during transgressions. Changes in the pattern of susceptibility variation occur at approximately 0.8 Ma and 0.4 Ma. Boundaries at these ages are seen in seismic profiles, they occur in the d18O frequency spectrum, and there is a rise in sea surface temperature at ~0.4 Ma. We suggest that these boundaries relate through palaeoclimatic and palaeoceanographic changes to stages in evolution of the Great Barrier Reef.

Magnetic properties of arrays of oriented iron particles as a function of particle size, shape and spacing

Hysteresis and remanence properties are reported for elongated iron particles deposited electrochemically in cylindrical pores of diameter d and spacing d c oriented perpendicular to the plane of an aluminium oxide substrate. Particle size, spacing and axial ratio were in the ranges d = 17–32 nm, d c = 26.5–64.5 nm and l/ d = 3–20 in three series of samples. For d = 17 nm and l/ d ≥ 10, hysteresis loops measured parallel to particle axes were almost rectangular and had parameters characteristic of single-domain (SD) particles reversing incoherently by fanning: M rs M s = 0.75–0.95, H cr ≈ H c ≈ 2 kOe, H sat ≈ 3 kOe . Perpendicular to particle axes, there was little hysteresis and H sat ≈ 6 kOe, typical of coherent reversals. Larger and less elongated particles had reduced parallel coercive forces ( H c = 1.4–1.7 kOe) and remanence ratios ( M rs M s = 0.3–0.4 ). Anhysteretic remanent magnetization produced in a weak field (10 Oe) had an inflected alternating field (a.f.) demagnetization curve characteristic of SD behaviour. The a.f. decay curve for 0.9 kOe isothermal remanent magnetization (IRM) was more exponential in form. The effect of interactions was tested by varying particle spacing d c. The distribution of critical fields for IRM acquisition was insensitive to both particle interaction and orientation of the field parallel or perpendicular to particle axes. However, each sample had a critical field distribution for a.f. demagnetization that was different from that for acquisition of IRM. For the most strongly interacting sample ( d = 17 nm, d c = 26.5 nm), median magnetizing fields for 0.9 kOe IRM were H | ≈ H ⊥ ≈ 450 Oe, whereas median a.f. destructive fields were H ⊥ ≈ 200 Oe, H | ≈ 400 Oe. Strong negative interactions are indicated by this observed deviation from the Wohlfarth relations.

The contribution of loess magnetism in China to the retrieval of past global changes—some problems

In the loess plateau of China, the stratigraphical position of the Brunhes / Matuyama boundary was not found in loess unit L8 at Luochuan and Jixian as is commonly accepted. The interbedded palaeosols may be differentiated from the parent loess not only by their high magnetic susceptibility but also by their enhanced remanence properties. These contrasts mainly result from concentration changes of the inherited magnetic minerals, grain-size changes and authigenic formation of magnetic minerals, which are all linked to climate-controlled soil formation processes.

Magnetometry and recording on very high coercivity cobalt alloy disk media

The hysteretic and remanent properties, the in-plane and out-of-plane anisotropy, and the high-density recording characteristics of very high coercivity (1450 Oe nominal) state-of-the-art commercial 95-mm thin-film disks from two different sources were investigated. It was found that these disks had comparable high density signal and noise properties, but different combinations of remanence and anisotropy parameters. It is concluded that disk manufacturers can reduce modulation noise and improve signal-to-noise ratio by controlling the coupling between the grains, the degree of perpendicular anisotropy present, and the susceptibility to recording demagnetization. >

Noise characteristics of barium ferrite particulate rigid disks

This paper discusses the relationship between the noise characteristics and magnetic properties of longitudinal barium ferrite (Ba-F) rigid disks with different switching field distributions (SFD). The magnetomotive force dependencies of reverse dc-erase (RDC) noise are measured and compared with SFD values. Coated disks with acicular magnetic particles have dips and thin-film disks peaks in the RDC. In Ba-F disks, both cases are observed depending on the SFD values, though the depths or heights of the RDC noise are much smaller than those of coated disks with acicular particles or thin-film disks. Disks with small SFD values have peaks, and disks with large SFD values have dips. In order to find the relationship between noise properties and magnetic properties, interparticle interactions in Ba-F disks are investigated. Reverse dc remanence Id(H) and ac-demagnetized isothermal remanence Ir(H) are measured. Both are normalized by the saturation remanence. The deviation from the noninteracting system, ΔM = Id(H) − [1ΔM=Id(H)−[1− 2Ir(H)] and an interaction field factor (IFF) given by (H′r − Hr)/Hc, are derived from these remanent properties. Here, H′r is the field corresponding to 50% of the remanent magnetization, Hr is remanence coercivity. In Ba-F disks, ΔM shows positive interactions, and the peak heights of ΔM increase and IFF decrease with decreasing SFD values. Positive interactions between Ba-F particles seem to be caused by particle stacking. Therefore, particle stacking results in small SFD values and peak-type RDC noise.

Chemical Remanent Magnetization of Red Beds and Synthetic Hematite

Magnetic minerals produced during diagenesis add one or more components of chemical remanence to the new magnetization of the rock. The magnetic signature of chemically magnetized rocks may thus differ from the orientation of the Earth's magnetic field at the time of sediment deposition. Interpretation of paleomagnetic data obtained from chemically magnetized sediments requires an understanding of the way in which magnetic minerals record the orientation and intensity of the magnetic field applied during their growth and the reliability with which successive generations of these minerals grown in different applied field orientations record those fields. To understand the properties of chemical remanence produced during the growth of one or more generations of hematite, the authors precipitated hematite in known conditions of the applied magnetic field. Hysteresis properties, scanning electron microscopy, and thermogravimetric analysis demonstrate that the synthetic material is analogous to pigmentary hematite present in the Siwalik red beds of northern Pakistan; thermal demagnetization and isothermal remanence acquisition behavior of both are also similar. Hematite precipitated in a single episode parallels the growth field, and preliminary work suggests a linear relationship between the field intensity applied during precipitation and the mineral's remanent intensity. The directional properties of hematite produced inmore » successive generations are far more complex and argue for careful assessment of the reliability of paleomagnetic data from red beds. They show that the magnetic fabric analysis is useful in this evaluation.« less

Analysis of the paleomagnetism and rock magnetism of the Surco intrusion, Peru: an attempt to obtain a southern hemisphere reversal record

Investigation of the Surco pluton, a granodioritic body approximately 12 km in diameter and age 20 Ma, located near Lima, Peru, was initiated in order to obtain a geomagnetic reversal record. More than 300 samples provide a complete section through the intrusion. Preliminary results obtained using conventional paleomagnetic techniques indicate that the western part of the pluton records the ambient field behavior well; a consistent reversed direction is followed by a partial reversed to normal transition as one progresses into the core of the intrusion. Plots of NRM vs. IRM(S) demagnetization indicate a steady decrease in the geomagnetic field intensity prior to the reversal. The intrusion does not appear to have recorded the recovery of the normal intensity after the reversal. The existence of a complete section through the intrusion permitted, in principle, the observation of a symmetrical record of the reversal from each side of the pluton. However, the results from the eastern Surco section present a confusing picture and reveal significant variations in rock magnetic and remanence properties along section. Magnetic properties of mineral separates were studied in an attempt to understand variability in the rock as a paleomagnetic recorder. In general, the feldspars contain an unstable magnetization, while the mafic minerals hold a more stable magentization. In the eastern Surco, there are considerable variations in their magnetic properties.

Properties of chemical remanence in synthetic hematite: Testing theoretical predictions

We synthesized hematite under controlled conditions of the applied magnetic field in order to test the behavior of chemical remanent magnetization (CRM) predicted on the basis of theories developed to explain thermoremanent magnetization (TRM). We restrict this investigation to the remanence; acquired during mineral crystallization (grain‐growth CRM) and do not consider chemical remanence acquired during mineral alteration. The material produced by our technique is hematite, up to 10 μm in diameter. Thermal demagnetization to 200°C removes the viscous component acquired under identical field/temperature conditions to those prevalent during synthesis. As predicted by analogy to TRM, the CRM acquired parallels the orientation of the magnetic field applied during growth. The intensity of chemical remanence is a linear function of the field in which it grew, BCRM up to 7.5 mT, the maximum growth field investigated. Furthermore, the ratio of intensities of CRM to TRM acquired in the same field is independent of the strength of that field. The intensity of BCRM controls the growth and orientation of hematite crystals, a result not predicted from theory derived by analogy to TRM. The control on crystal growth by the field is reflected in the dependence on BCRM of the orientation of ellipsoids of anisotropy of isothermal remanence and the acquisition of isothermal and anhysteretic remanence.

Magnetizing rare-earth alloy magnets at elevated temperatures

It is well known that high magnetic fields are needed to fully magnetize rare-earth based magnets. Even with the conventional magnetizing process employing a large amplitude magnetic pulse (mp), this poses practical difficulties and limits the number of poles and various application specific pole designs required in a magnet. On investigating magnetizability of these magnets, it was concluded that magnetizing can be effectively performed at much lower magnetic fields than normally needed if the temperature of the magnet during the magnetizing process is raised and maintained within a predetermined range. In an experiment, an anisotropic melt-spun Nd-Fe-B magnet of dimension 1×1×0.73 cm3 (easy-axis along the shorter dimension) was fully magnetized at a temperature between 90 and 110 °C using a mp of amplitude 22.8 kOe. The same sample at room temperature required a mp of amplitude above 42 kOe to get fully magnetized. Similar data for sintered Sm-Co and sintered as well as rapidly quenched isotropic Nd-Fe-B alloys are presented. The optimum ranges of temperatures are explained as a result of thermal deterioration on the magnet’s intrinsic coercivity and remanence properties, and the limiting effect of demagnetizing field on the magnetizing process.

Remanence and longitudinal recording properties of advanced particulate media and metal thin-film media

A comparison is made of the magnetic remanence and longitudinal recording properties of advanced particulate media with metal thin-film media. The remanence properties were studied parallel and orthogonal to the track direction and were diagnostic of recording performance. The recording properties were measured using thin-film heads with inductive-write and magnetoresistive-read elements. Longitudinal barium ferrite and isotropic cobalt alloy media show substantially narrower switching field distributions (SFDs) compared to acicular cobalt-modified gamma -Fe/sub 2/O/sub 3/ media. Narrow SFD, high H/sub c/, and small M/sub r/ delta lead to small, well-defined magnetic transitions and high bit densities. Indeed, there is a linear correlation between the calculated transition length, a, and the linear density. Results confirm that particulate barium ferrite and cobalt alloy media have very small transition lengths and high linear densities. Cobalt-modified gamma -Fe/sub 2/O/sub 3/ media show lower linear densities; however, their intrinsic signal-to-noise ratios (SNR) are much higher compared to the barium ferrite and cobalt alloy media. The trend of noise with increasing density or frequency differs for the three types of media.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetism and transmission electron microscopy of Fe‐Ti oxides and pyroxenes in a granulite from Lofoten, Norway

Granulites from southwest Lofoten, Norway, often carry intense and stable components of natural remanent magnetization (NRM). Coercivities in excess of 100 mT and blocking temperatures up to 580°–645°C are common. Several phases in these rocks can carry stable components of NRM. Clinopyroxenes (augite with exsolved pigeonite and orthopyroxene) usually contain abundant Fe‐Ti oxides, the majority of which are too small to be studied conclusively with visible light. Additionally, these rocks typically contain coarse‐grained (∼100–1000 μm) exsolution intergrowths of rhombohedral Fe‐Ti oxides and coarse‐grained magnetite. We have studied the magnetism and mineralogy of the titanohematite and the inclusion‐bearing augites in one granulite sample using transmission electron microscopy (TEM) and magnetic experiments to understand better the remanence properties of naturally occurring Fe‐Ti oxides and their relationships to silicate minerals. The coarse‐grained titanohematite has a coercivity of 350 mT and a laboratory saturation remanence that unblocks at 570°–600°C. The hysteresis characteristics of this titanohematite are typical of much smaller ∼10μm‐sized single domains, which apparently reflects the presence of exsolved ilmenite lamellae, as small as &lt;10 nm in thickness. These lamellae presumably inhibit (1) domain wall nucleation and (2) movement of walls that manage to nucleate. The inclusions in the augites serve as carriers of lower coercivity remanence with blocking temperatures less than about 560°C. Magnetite, hematite, and ilmenite are present in the augite, and intergrowth of the rhombohedral phases, revealed by TEM, persists down to the 20‐nm scale. The distribution of the magnetite inclusions in the augite appears to be unrelated to any silicate microstructure, while that of the rhombohedral Fe‐Ti oxides is strongly controlled by silicate microstructure, with intergrowths of these oxides commonly decorating lamellae of pigeonite, orthopyroxene, and clinoamphibole within the augite. The NRM of this sample possibly dates from the amphibolite facies metamorphism that the region experienced about 1000 m.y. ago. Paleomagnetic studies of these rocks could readily recover virtual geomagnetic poles dating from this event. In addition to the included oxides, the augites display a number of unusual features that probably result from the protracted polymetamorphic history of these granulites: pigeonite lamellae are not parallel to the b‐crystallographic axis, pigeonite lamellae cut across those of orthopyroxene and numerous included minerals occur. Rare earth elements are concentrated within inclusions that reside in the augite.

Effect of low-temperature oxidation on the remanence properties of titanomagnetites.

Changes in remanent magnetization and hysteresis parameters associated with low-temperature oxidation have been investigated using x=0.3, 0.5 and 0.7 synthetic titanomagnetite samples and natural submarine basalts. The intensity of remanence and the median destructive field (MDF) of TRM decreases with increasing oxidation. As oxidation proceeds, the intensity of CRM increases and its MDF decreases, except for x=0.7 titanomagnetite samples, which show maximum intensity and MDF around 0.6 of oxidation state (z). The intensity and stability of CRM become almost comparable to those of TRM in a high oxidation state. Saturation magnetization (Js) decreases with increasing oxidation and in a fully oxidized state Js is about 60% of initial values. The saturation remanence of titanomagnetite samples is almost constant until z-0.6 and tend to decrease as z increases further. The coercive force and coercivity of remanence decrease for x=0.3 and 0.5 samples monotonically with increasing z, whereas those for x=0.7 samples show maximum values for oxidation states of about 0.6. The correlation is not good between oxidation state z and the NRM intensity of submarine basalts, while the NRM becomes more stable with increasing low-temperature oxidation. Changes exhibited by DSDP samples against AF demagnetization show almost the same tendency for MDF of CRM for the x=0.7. However, other parameters are not always consistent with those of synthetic samples.

Magnetic remanence properties of particulate recording media: comparison of barium ferrite with acicular materials

The magnetic remanence properties of barium ferrite particulate media were studied and compared with those of acicular particulate materials to understand better the differences in recording properties. The loss of remanence after the application and removal of an applied magnetic field was measured for fields opposite and transverse to the direction of initial magnetization. The loss of remanence after application of a reverse field gives information about the distribution of switching fields; this distribution is narrower in barium ferrite than in acicular materials. The loss of remanence after the application of transverse fields gives information about the magnetic anisotropy; barium ferrite is much more resistant to demagnetization by transverse fields than are acicular materials, but only in oriented media. All results are analyzed in terms of their dependence on relevant magnetic squareness parameters to account for the differing degrees of particle orientation in the samples and thus to observer fundamental differences in material properties. >

Changes in TRM and ARM in a basalt due to laboratory heating

Laboratory heating (in air) often results in irreversible changes of remanence properties in natural rocks. Such an effect was traced for a recent basalt sample containing titanomagnetite ( x = 0.4) as the main remanence carrier for heating times between 3 and 300 min. It is shown that the change can be explained by the scheme of high-temperature oxidation Titanomagnetite(PSD, MD) → Iron-rich phase (SD) + Ilmenite Remanent magnetizations in both the original and heated samples are the mixtures of two components; high blocking temperature, high coercivity fraction and low blocking temperature, low coercivity fraction. The TRM and ARM show distinctly different behaviors in this change showing the limitation of using the ARM as an analog of the TRM.

Physical properties of skarns

A petrophysical laboratory study was carried out on samples of skarn, lightly mineralized skarn ores, associated calc silicates, carbonate host rocks, intrusive granitoids and country rocks from mineralized areas at King Island, Duckmaloi and Mt Moss. The skarn categories were found to be relatively dense and porous, and to have some radiometric character in the form of low potassium content. Rocks from the skarn environments had variable magnetic susceptibility and remanence properties that were not generally diagnostic. Electrical resistivities of skarns, although somewhat lower than the other rock types, tended to be high but this property was not distinctive. Induced polarization was, as expected, sensitive to the presence of minor sulphides. Thermal conductivities could not differentiate the rock types. Acoustic ultrasonic velocities were high for most rocks and could not be used to distinguish types.