Thermomagnetic Behavior Research Articles

Ultrafast thermomagnetic writing processes in rare-earth transition-metal thin films

We use time-resolved spin-polarized photoemission to investigate thermomagnetic writing of domains in magneto-optic media, focusing on the relaxation time of the magnetization and the dynamic behavior of the nucleation process. In our initial studies, we examine a 90-nm-thick GdTbFe film using a pulsed excimer laser (pulse duration: 16 ns) as the light source for the photoemission process. We find that the thermomagnetic switching behavior is different above and below the compensation temperature Tcomp. When the sample temperature is held above Tcomp, the spin polarization of the electrons emitted during the writing pulse has the sign of the initial state even though subsequent examination shows that a reversed magnetization domain has been formed. Therefore, the domain is thermomagnetically nucleated during the trailing edge of the 16 ns writing pulse or even later when the irradiated domain cools down. On the other hand, if the initial temperature is slightly below Tcomp, the electrons emitted during the writing pulse have reversed polarization showing that the reversal of the magnetization takes place quickly compared to the pulse duration. This difference shows that compensation-point writing is much faster than Curie-point writing. Based on these measurements we propose a model to interpret the different thermomagnetic switching processes which take place above and below Tcomp. The results can be explained by different thermal relaxation times between the excited electrons and the lattice and between the electrons and the spin system.

On the origin of inclination shallowing in redeposited sediments

We studied the detrital remanent magnetization of laboratory redeposited lake sediments, previously shown to be excellent recorders of the paleomagnetic field. The redeposited and original/natural sediments have indistinguishable stabilities to alternating fields (AF) demagnetization and remanence intensities, indicating that their remanences reside in the same magnetic particles. Thermomagnetic behavior, hysteresis parameters, and relatively high AF stabilities of the detrital and anhysteretic remanences are essentially identical throughout the sediment column, suggesting uniform distributions of submicron magnetite grains. The declination is accurately recorded, but inclination shallowing averaging 18° is observed for routinely hand‐stirred, rapidly redeposited sediments. However, sonically disaggregated sediments settle more slowly with anomalous shallowing averaging 7°, decreasing upward in the redeposited column. These results indicate that thorough disaggregation of the sediment reduces the magnitude of the inclination shallowing probably by more efficiently isolating the magnetic grains from the matrix, thus diminishing the dominance of the gravitational over geomagnetic aligning torques during deposition. The progressive decrease of the inclination shallowing upward in the sediment column might be due to a parallel upward increase in the fraction of isolated magnetic particles, which undergo Brownian motion and alignment by the geomagnetic field. The dominance of gravitational over geomagnetic aligning torques during deposition might be a cause for inclination shallowing in some coarser sediments. In contrast, inclination shallowing would be minimized for slow deposition of isolated submicron magnetite grains during natural sedimentation.

Magnetic properties of mechanically alloyed Fe-Si

The formation of both an amorphous Fe-Si phase and a crystalline solid solution of Si in Fe was demonstrated through differential scanning calorimetry (DSC), X-ray diffraction and thermomagnetic measurements of ball-milled pure-element powders. X-ray results are inconclusive about the effective alloying of Fe and Si and about their amorphization, although they seem to be compatible with both facts. Partial amorphicity of as-prepared samples seems to be related to the irreversible exothermic peaks observed at 195 degrees C on DSC thermograms. This is confirmed by comparing X-ray diffractograms of thermally treated samples with those of untreated material. Also, an anomaly in the thermomagnetic behavior which is not related to any transformation present in the equilibrium phase diagram was observed at 200 degrees C. >

Thermomagnetic switching on rare-earth transition-metal alloy magneto-optic disks

The thermomagnetic switching behavior of magneto-optic disks with amorphous TbFeCo and GdTbFe films was investigated on a test recorder. Domain length, write and erase fields, and erase laser power required for total domain erasure was studied for various film compositions, which differ in compensation temperature Tcomp and Curie temperature TC. With increasing TC the domain length at fixed writing laser pulse energy is reduced and the power required to erase domains with a certain size is increased. The optimum external field, for minimal write noise, decreases with increasing TC−Tcomp. The experimental dependencies agree with calculated results obtained from the force balance controlling the domain formation.

Rock magnetic studies of Miocene volcanics in eastern Otago and Banks Peninsula, New Zealand: Comparison between Curie temperature and low temperature susceptibility behaviour

Abstract Samples were collected from 135 sites in 4 centres of Miocene alkali volcanic activity in the South Island of New Zealand: the Dunedin volcano, the Lyttelton volcano, the Akaroa volcano, and the Diamond Harbour Group. Strong field thermomagnetic behaviour has been used to determine the Curie temperatures and hence the magnetic mineralogy of the samples. The results show that differences in the degree of high temperature deuteric oxidation of the titano-magnetites occur between volcanic centres, flows, and even within flows, and that low temperature oxidation of Ti-rich titanomagnetites is extremely common. The use of low temperature susceptibility as a tool for identifying the magnetic mineralogy is also evaluated. Comparison with the thermomagnetic results shows that samples whose susceptibility continuously increases from liquid nitrogen temperature to room temperature can possess a wide range of Curie temperatures, contrary to previously published interpretations. The use of low temperature susceptibility for preselection of samples for paleointensity is rejected. Use of the technique alone can give only limited information about the magnetic mineralogy.

The thermomagnetic behaviour of non-compensated GdCo films

The thermomagnetic behaviour of a number of GdCo films with compositions in the range 22–45 at% Gd and close to those developing large coercivities has been studied. The polar Kerr effect has been used to examine the hysteretic properties for films at temperatures in the range -20 to +200°C. For the compositions selected, clear transitions in magnetic subnetwork dominance are observed at well defined transition temperatures. Only for very small composition ranges are these transition temperatures above room temperature. These results are not compatible with previously published interpolations of compensation temperatures for GdCo films in the composition range studied.

Paleomagnetism of the Puente Piedra Formation, Central Peru

Paleomagnetic samples were collected from 15 sites in the early Cretaceous Puente Piedra Formation near Lima, Peru. This formation consists of interbedded volcanic flows and marine sediments and represents the oldest known rocks of the Andean coastal province in this region. The Puente Piedra Formation is interpreted as a submarine volcanic arc assemblage which along with an overlying sequence of early Cretaceous clastic and carbonate rocks represents a terrane whose paleogeographic relationship with respect to the Peruvian miogeocline in pre-Albian time is unknown. Moderate to high coercivities, blocking temperatures below 320°C, and diagnostic strong-field thermomagnetic behavior indicate that pyrrhotite is the dominant magnetic phase in the Puente Piedra Formation. This pyrrhotite carries a stable CRM acquired during an event of copper mineralization associated with the intrusion of the Santa Rosa super-unit of the Coastal Batholith at about 90 ± 5 m.y. B.P. The tectonically uncorrected formation mean direction of: D = 343.2°, I = −28.6°, α 95 = 3.4° is statistically concordant in inclination but discordant in declination with respect to the expected direction calculated from the 90-m.y. reference pole for cratonic South America. The observed declination indicates approximately 20° of counterclockwise rotation of the Puente Piedra rocks since about 90 m.y. This is consistent with other paleomagnetic data from a larger crustal block which may indicate modest counterclockwise rotation during the Cenozoic associated with crustal shortening and thickening in the region of the Peru-Chile deflection.

Thermomagnetic behaviour of magnetically nonsaturated fluids

Magnetic relaxation processes are described in magnetic colloid suspensions by means of the thermodynamics of so-called internal variables. Both “orientational” and “extensional” magnetic dissipative processes are taken into account. In general, a nonlinear, frame-invariant, magnetomechanical behaviour accompanied by hysteresis is obtained. At magnetic thermodynamical equilibrium, however, a typical nonlinear “paramagnetic” pyromagnetic behaviour is recovered.

Some aspects of magneto-chemistry in NiO-Al<inf>2</inf>O<inf>3</inf>catalysts

Susceptibility-composition isotherm and thermomagnetic behavior of NiO-Al 2 O 3 catalysts, prepared by impregnating α-alumina in nickel nitrate solution and calcining at 450°C. have been studied. The samples were evaluated for ethylene hydrogenation at 200°C and 300°C, respectively. Isothermal reductions at 390°C, with and without magnetic field, for a poorly dispersed catalyst have been reported. In another set, six NiO-Al 2 O 3 catalysts, prepared from different nickel salts and having identical composition, were evaluated for thermomagnetic behavior, surface area, crystallite size, and activities for heptane-steam reaction at 700°C, respectively. Impregnated catalysts exhibited widely varying magnetic parameters. At low nickel concentration, NiO ensembles appeared to be bound to alumina by orbital angular moments. The unpaired 3d-electrons of Ni2+ion are stabilized by vacant p-orbitals of Al3+ions. As the nickel content increased, antiferromagnetism developed with the appearance of Neel point, negative Weiss constant, and spin compensation. At higher nickel concentration magnetic moment increased slightly due to overcoming of the influence of the support and suggested the formation of NiO multilayers. The changes of magnetic susceptibility with time during reduction above paramagnetic Curie point of nickel showed several peaks. The susceptibility increased due to the formation of nickel nuclei and decreased when the unpaired electrons were delocalized in the conduction band of aggregated nuclei. Superparamagnetic nickel showed optimum activities for both ethylene hydrogenation and heptane-steam reaction. So, these may be grouped as structure-sensitive reactions.

Magnetic properties of Cenozoic ophiolites in the Hayama-Mineoka and Setogawa belts, South-Central Honshu, Japan.

Intensity and directions of natural remanent magnetization and their stability against alternating field demagnetization as well as thermomagnetic behavior, opaque mineralogy and chemical composition of constituent ferromagnetic minerals have been extensively investigated with pillow and dyke basalts, dolerites, gabbros and ultramafic rocks composing the Hayama-Mineoka and Setogawa ophiolite complexes.It has been demonstrated by the present work that degree of alteration of most basalts in these ophiolite complexes is low and comparable to that of the oceanic basalts. Titanomagnetite is subjected to low-temperature oxidation mostly owing to bleaching of ferrous ions but directions of the original TRM appear to be unaffected by the oxidation, although its intensity may be reduced and additional CRM may be acquired. Hydrothermal alteration recognized in a restricted area substantially reduces intensity of NRM and causes scatter in directions. Ultramafic rocks have stable and relatively large NRM, which is carried by fine magnetite particles precipitated along margins of olivine crystals.

New magnetic results from Allende C3(V)

Detailed characterization of the temperature-dependent behavior of the composition and structure-sensitive magnetic parameters, as well as the record of continuous thermal demagnetization of NRM, provides new insight into the complex magnetic behavior in Allende, a C3(V) chondrite. There is no ∼ 120 K Verwey transition in the Allende specimen studied, indicating that the magnetite present is possibly non-stoichiometric or else is a minor magnetic phase. Magnetic transitions have been identified as 50–75, ∼ 150, ∼ 320, and 590–610°C in careful study of the I S( T) behavior in five separate specimens. Up to 90% of the NRM and irreversible H R( T) behavior ( H R = remanent coercive force) is associated with magnetic phases having transitions or undergoing thermally induced transformation at temperatures ≲320°C. These phases contribute about 15% of the total I S value. Published paleofield estimates (the Thellier-Thellier test or modifications thereof) should be reviewed with caution as the presented experimental data identify complex irreversible thermomagnetic behavior beginning at temperatures as low as 50°C. The thermomagnetic data suggest that phases carrying the dominant part of the NRM are intergrown, magnetically interacting, and thermally unstable. The ratio (REM) of the NRM/SIRM (natural remanence/saturation remanence) ranges from 0.004 to 0.006 for the bulk meteorite and from 0.0004 to 0.005 for chondrules of all sizes. The chondrules studies by Lanoix et al. (1977, 1978) and Sugiura et al. (1979), which yielded large paleofield estimates, have REM>0.1. The chondrules giving the large paleofields have anomalous magnetic properties, different from other groups of chondrules studied by Sugiura et al. (1979) and the present work. The H R( T) behavior suggests coexisting magnetic phases with differing Curie points or with the low temperature transitions being due to transformation of a thermally metastable phase in much the same way as the conversion of γFe 2 O 3 → αFe 2 O 3 in epitaxy with Fe 3O 4 was demonstrated to be responsible for similar thermomagnetic behavior in massive iron ores (Wasilewski, 1979). This behavior is irreversible.

Magnetostatic interactions in a duplex ferromagnetic alloy

The chill-cast alloy, Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">75</inf> Si <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</inf> B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</inf> , consists of submicron rods of Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> B, a magnetically "hard' phase with high T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> , in a matrix of Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Si, a "soft" phase with low T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> . Magnetic properties change markedly on cooling through 590°C, the T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> of Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Si, and 245°C, where the magnetocrystalline anisotropy of Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> B changes from easy-axis to easy-plane. The unusual magnetic and thermomagnetic behavior observed is discussed in terms of the short-range and long-range magnetostatic interactions between the two phases.

Thermomagnetic behavior of a duplex ferromagnetic alloy, Fe75Si15B10

The chill-cast alloy Fe75Si15B10 consists of submicron Fe2B rods in a Fe3Si matrix. The magnetically ’’hard’’ Fe2B has a higher Curie temperature than the magnetically ’’soft’’ Fe3Si, which leads to unusual thermomagnetic behavior during cooling in low fields. When the thermoremanent magnetization (TRM) of the Fe2B produces a demagnetizing field greater than the applied field, the Fe3Si magnetizes opposite to the applied field, thereby reducing the net internal field. However, the magnetic heterogeneity of the sample prevents the net internal field from reaching zero. Significant magnetization changes are also noted in the temperature range where the magnetic anisotropy of Fe2B changes from easy-axis to easy-plane.

Preparation and Magnetic Properties of Cr-Modified Mn2Sb Thin Films

The this films of Cr-modified Mn2Sb which exhibit a magnetic first-order phase transition were prepared by a diffusion anneal of double layers of Mn2Sb and Cr made by vacuum deposition. A sharp and complete transition was found to be obtained for the films prepared by using the Mn2Sb layer having oriented grains with their c axes perpendicular to the substrate. A diffusion anneal of 1 hour at 500°C was sufficient to prepare the films which exhibit the first-order transition. The saturation magnetization, magnetic hysteresis loop, remanence, coercive force and electrical resistivity were measured as a function of temperature. It was found that thermo-magnetic behavior observed in the prepared films is similar to, but more exaggerated than, that in powder samples.

The preparation, characterization and magnetic properties of synthetic analogues of some carriers of the palaeomagnetic record.

Synthetic analogues of two magnetic mineral systems have been prepared: (i) a titanomagnetite system Fe2.4-δAlδTi0.6O4 (0<δ<0.45). (ii) a titanomaghemite system prepared by oxidation of Fe2.4-δMgδTi0.6O4(0<δ<0.35). The following properties were determined: unit cell edge, Curie temperature, thermomagnetic behaviour, saturation magnetization, isothermal remanence and coercive force. The introduction of Al, Mg and vacancies into titanomagnetites reduces saturation magnetization and coercive force. Curie temperature is lowered by the presence of Al and Mg ions but rises with increasing concentration of vacancies.

Correlation of thermomagnetic behavior of carbonaceous chondrites with chemical petrographic classification.

Some degree of correlation was noted between the thermomagnetic behavior of the carbonaceous chondrites and the chemical-petrographic classification scheme of Van Schmus and Hayes.

Thermomagnetic behavior of hard magnetic thin films

The annealing behavior of hard magnetic films of the Co-M system (M = Ni, P) was followed using differential thermal analysis. These studies showed that the thermal behavior of electrodeposited Co, Co-P, Co-Ni, and Co-Ni-P films is different from that of chemically deposited Co-P films. The most pronounced reaction was found to be exothermic and occurred between 200°C and 300°C. After thermally stabilizing these films at 500°C for 1 hour in a vacuum, the temperature dependence of H c was found to depend on additive content and crystallite orientation. Films with low additive contents and hcp

The iron-titanium oxides of some Dunedin (New Zealand) lavas, in relation to their palaeomagnetic and thermomagnetic character (with an appendix on associated chromiferous spinel)

SummaryEarlier palaeomagnetic studies on the Dunedin volcano showed that a portion of the sequence is reversely or anomalously magnetized. Some of the rocks used for palaeomagnetic study were subsequently classified into groups according to their thermomagnetic behaviour (unpublished work).Cell dimensions and Curie-point curves have been measured for oxides separated from representative rocks of each group. The oxides are mostly homogeneous titanomagnetites containing between 40 and 55 mol. % of ulvöspinel, with a generally small degree of late low-temperature alteration. X-ray and thermomagnetic data also suggest that there was some early oxidation to titanomaghemite, probably during cooling.The thermomagnetic behaviour of rocks classified in the different groups is attributed to variable oxidation of the titanomagnetite during thermomagnetic treatment. The high Curie points of rocks used for palaeomagnetic studies may often be due merely to oxidation of titaniferous magnetite and not to nearly pure primary Fe3O4. Since there is insufficient oxidation in the lavas examined for any currently accepted self-reversal process to have been operative, the reversely magnetized part of the Dunedin sequence was probably erupted during a reversed polarity epoch, possibly between about 13 and 11 million years ago.The basanitic lavas contain minor amounts of chromiferous spinel, as inclusions in silicate phenocrysts and as cores to separate microphyric titanomagnetite; its significance is briefly discussed.

Heating experiments on New Zealand ironsands and the presence of pseudobrookite

Abstract Results of heating experiments, with microscopic, X-ray, thermomagnetic, and chemical data, are related to previous ironsand investigations. Reducing runs were made in coal gas, hydrogen, and buffered hydrous atmospheres. For the last, the end-product compositions agree well with published data for the synthetic Fe-Ti-O system. Earlier conclusions about thermomagnetic behaviour of ironsand titano-magnetites are confirmed. On heating in air homogeneous titanomagnetite is oxidised to homogeneous titanhematite. Inhomogeneous titanomagnetite grains remain so, the final product containing cubic residua strongly enriched in minor constituents (Mg, Mn, Al, and possibly V), and Ti is found only in the titanhematite phase. Possible economic implications are briefly discussed. Pseudobrookite is tentatively identified in rare grains in natural titanomagnetite concentrates and in products of the oxidising run. Its scarcity is attributed to the low TiO2 content of New Zealand ironsand magnetites.

Thermomagnetic Behaviour of Ferromagnetic Domain Nuclei

Certain features of the thermomagnetic behaviour of a single crystal specimen of silicon-iron when being magnetized in an easy direction of magnetization are explained in terms of the presence of domain nuclei in the specimen, following the work of Bates, Christoffel, Clow and Davis.