Magnetization Direction Changes Research Articles

SUPER RESOLUTION READOUT OF A MAGNETO-OPTICAL DISK WITH AN IN-PLANE MAGNETIZATION LAYER

A new magneto-optical (MO) disk, which consists of magnetic double layers, for super resolution readout was developed. We developed a new characteristic GdFeCo film as a readout layer whose magnetization direction changes from in-plane to perpendicular as a temperature rises. Using such a GdFeCo film combined with a DyFeCo recording film, higher linear density and track density than those of the conventional MO disks were achieved without an external magnetic field for readout and with a conventional optical head.

Magnetic properties and thermal stability of Sm 2Fe 17N x with intermediate nitrogen concentrations

The existence of interstitial Sm 2Fe 17N x with intermediate nitrogen contents (0 ≤ x ≤ 2.94) is revealed by a continuous increase of the unit cell volume, the Curie temperature, the saturation polarization and the anisotropy field with increasing N concentration. The thermal stability of Sm 2Fe 17N x increases also with increasing x. The anisotropy constants K 1 and K 2 at room temperature were deduced by fitting magnetization curves of oriented powders. The easy magnetization direction changes from the basal plane ( x = 0) via an easy cone ( δ = 24 ° for x = 0.4) to the c-axis ( x ≥ 0.55). The intrinsic magnetic properties of Sm 2Fe 17N 2.94 were found to be T c = 473 ° C, J s = 1.51 T and H A = 21.0 T where K 1 = 8.4 and K 2 = 2.1 MJ/m 3.

Crystal and magnetic structure of the permanent magnetic material Tm 2Fe 14C

R 2Fe 14C (R, rare earth metal) compounds are considered as new permanent magnetic materials of technical interest. Investigations of the compound Tm 2Fe 14C by means of X-ray and neutron diffraction as well as by metallographic techniques were carried out. Tm 2Fe 14C crystallizes in the tetragonal space group P4 2/ mnm and is isotypic to Nd 2Fe 14B. The lattice constants are a = 8.7304(2) Å and c = 11.7640(4) Å at 295 K. The nuclear and magnetic structure of Tm 2Fe 14C has been investigated by means of neutron diffraction at 650 K (above the Curie temperature T c), 400 K (above the spin reorientation temperature T SR and below T c), room temperature and 10 K. The magnetic moments of all Tm and Fe atoms are always oriented collinear and antiparallel. Measurement of the magnetization curve by means of neutron diffraction yields a T SR of 308 K, where the magnetization direction changes from parallel to the c axis above T SR to perpendicular to the c axis below T SR. The nuclear structure of Tm 2Fe 17C x (Th 2Ni 17 structure type) has been refined simultaneously since this phase was also present in the sample.

Structural and intrinsic magnetic properties of (Sm 1−xNd x) 2Fe 17N ≈2.7 and Sm 1−xNd x) 2(Fe 1−zCo z) 17N ≈ 2.7

Formation and magnetic properties of interstitital (Sm 1−xNd x) 2Fe 17N ≈2.7 nitrides are reported. The thermal stability of Sm 2Fe 17N ≈2.7 is not much affected by substituting Nd for Sm, whereas the Curie temperature is slightly decreased from 746 K for x = 0 to 725 K for Nd 2Fe 17N ≈2.7. The saturation polarization increases from 1.52 T for x = 0 to 1.60 T for x = 1. The strong uniaxial anisotropy of Sm 2Fe 17N 2.9 ) H A = 21.0 T at room temperature) is significantly reduced by substituting Nd for Sm due to the strong planar anisotropy of Nd 2Fe 17N ≈2.7 ( K 1 = −6.8 MJ/m 3 and K 2 = 0.8 MJ/m 3). For intermediate Nd contents the easy magnetization direction changes from the c-axis over an easy cone to the basal plane with decreasing temperature. By substituting Co for Fe the decrease of the anisotropy field can be partly balanced and the saturation polarization and the Curie temperature are further increased, giving H A = 14.8 T, J s = 1.57 T and T C = 835 K for (Sm 0.7Nd 0.3) 2(Fe 0.8Co 0.2) 17N ≈2.7.

Observations of plasma boundaries and phenomena around Mars with Phobos 2

Magnetic field and plasma measurements on board the Soviet spacecraft Phobos 2 have been analyzed during five elliptical orbits around Mars. The existence of at least one separate plasma boundary and an adjacent plasma layer, called the planetopause and the transition region, between the bow shock and the ionopause seems to be a characteristic feature of the solar wind interaction with an almost nonmagnetized planetary ionosphere. It is suggested that the planetopause is a multiple‐ion discontinuity, where a large number of solar wind protons are deflected at an exospheric density ramp. Strong changes in magnetic field and plasma flow direction within the transition region are interpreted as signatures of current sheets or internal shocks. The detected eclipse boundary in the tail is perhaps an elongation of the ionopause found at Venus. New ideas concerning the formation of multiple‐ion flow boundaries by electrostatic plasma‐plasma interaction are discussed. Finally, a remarkable Deimos event has been detected during the fifth orbit. This is explained as an interaction of the subsonic solar wind with a thin cloud of charged dust particles.

Magnetic properties of (La 1− xNd x)Co 5 hydrides

Magnetization measurements have been performed on the pseudobinary compounds (La 1− x Nd x )Co 5 and their hydrides (La 1− x Nd x )Co 5H y . The substitution of La for Nd leads to a lowering of the temperature ( T R) at which the easy magnetization direction changes ( c-axis → a-axis). The value of T R decreases with La-concentration (1− x). The influence of the hydrogen absorption on T R is the same as that of the substitution of La for Nd. The lowering of T R can be interpreted by the weakening of the Nd=Co coupling which is induced by the substitution of La for Nd and the hydrogen absorption.

NMR study of Y 2(Co 1− xFe x) 14B compounds

59Co and 89Y NMR of Y 2(Co 1− x Fe x ) 14B compounds has been measured at 4.2 K. The line positions shift rapidly to higher frequency for x⩽0.3 and are almost constant for 0.3<x⩽0.9. These results may be due to the change of the easy magnetization direction from plane to axis at x = 0.3. The evaluated Co orbital moments at 8j 1 and 8j 2 sites give a large contribution to the anisotropy of the 3d sublatice.

Magnetostriction and magnetomechanical coupling in Ho0.85Tb0.15Fe2−xCox system

Magnetostriction and magnetomechanical coupling coefficient have been measured on Ho0.85Tb0.15Fe2−xCox (x=0, 0.5, and 1.2) alloys. The alloys were prepared by arc melting as well as by arc melting followed by zoning using induction furnace. Saturation magnetostriction (λs) and magnetomechanical coupling coefficient are found to decrease with the increase of cobalt concentration. The λs values of the zoned samples of x=0 and 0.5 are seen to be more than those of arc melted samples whereas, it is otherwise in the case of x=1.2. Magnetomechanical coupling coefficient values for all the samples are seen to increase with zoning. Change in the easy direction of magnetization in the case of x=1.2 is reflected in the magnetostriction of zoned samples.

Magnetic properties of Tm 2Fe 17N x studied by means of the 169Tm and 57Fe mössbauer effect

The 57Fe Mössbauer effect in Tm 2Fe 17N χ was measured in the temperature range 4.2–600 K. The temperature dependence of the 57Fe Mössbauer spectra shows a hyperfine field anomaly around the spin reorientation temperature ( T SR ≈ 190 K) where the easy magnetization direction changes from parallel to c ( T < T SR) to perpendicular to c ( T > T SR). At 4.2 K the 57Fe hyperfine field was found to have slightly increased after charging Tm 2Fe 17 with nitrogen. The 169Tm Mössbauer spectrum of Tm 2Fe 17N χ was measured at 4.2 K. From the quadrupolar splitting it was derived that interstitial solution of nitrogen atoms considerably increased the second-order crystal field parameter ( A 2 o≈300 Ka o −2 ) although the value reached is not much larger than found before in Tm 2Fe 17C.

Intrinsic magnetic properties of R2Fe17CyNx compounds: (R=Y, Sm, Er, and Tm)

Samples of R2Fe17CyNx (R=Y, Sm, Er, Tm) were prepared by arc melting appropriate amount of R, Fe, and C, vacuum annealing at 1373 K and finally annealing at 740 K in nitrogen for 10 h. The magnetic properties of these compounds were investigated by means of ac initial susceptibility, magnetization measurements, and x-ray diffraction. The thermal stability of the nitride phase was studied by differential scanning calorimetry. It was found that, when heated above 600 K, R2Fe17CyNx irreversibly decomposes N which is irrespective of the carbon concentration and rare-earth element. The Curie temperatures of R2Fe17CyNx are independent of the carbon concentration and are approximately 400 K higher than those of the corresponding pure R2Fe17 compounds. However, the Curie temperatures cannot be correlated to the composition x of the initial R2Fe17CyNx compounds at room temperature because some N was lost during the heating to Tc. In the Er and Tm compounds spin reorientation transitions were found, marking the change of the easy magnetization direction from the c axis to the basal plane with increasing temperature. The Tm compounds show an additional magnetic transition at low temperatures (below 40 K). A coexistence of the hexagonal and the rhombohedral structural modifications was found in Er2Fe17CyNx when y&amp;lt;1.5, characterized by two different spin reorientation temperatures. The anisotropy fields of Sm2Fe17CyNx are higher than that of Sm2Fe17Nx. Indications of a magnetic phase transition were found also in Sm2Fe17C0.7Nx and Sm2Fe17C0.9Nx.

Molecular hydrogen polarization images of OMC-1

An image of the polarization of the shocked H2 v = 1-0 S(1) line emission in the core of OMC-1 has been obtained. Along the molecular outflow of the source, the line is dichroically polarized by a medium of aligned grains located between the earth and the shock fronts. The polarization pattern traces the magnetic field direction, which is parallel to the outflow axis and to the large-scale field direction determined from far-IR continuum measurements. Close to the IR source IRc2, the likely source of the outflow, the aligned vectors twist, indicating that the magnetic field direction changes. Modeling the line ratios of scattered H2 lines in the reflection nebula, it is concluded that the size distribution of grains there is typical of the small grains in the diffuse interstellar medium. By contrast, the scattered continuum radiation from the core region suggests that the grains there are larger than this.

166Er and 57Fe Mössbauer effect in Er 2Fe 17N x

The 57Fe Mössbauer effect in Er 2Fe 17N x was measured in the temperature range 4.2–580 K. The temperature dependence of the 57Fe Mössbauer spectra shows a hyperfine field anomaly around the spin reorientation temperature ( T SR ∼ 110 K) where the easy magnetization direction changes from parallel to c ( T < T SR) to perpendicular to c ( T > T SR). The 166Er Mössbauer spectrum of Er 2Fe 17N x was measured at 4.2 K. From the quadrupolar splitting it was determined that interstitial solution of N atoms strongly increases the second order crystal field coefficient ( A 0 2-400 K a -2 0).

Ion burst event in the Earth's dayside magnetosheath

The MEPA instrument on the AMPTE/CCE Spacecraft provided ion angular distributions as rapidly as every 6 sec for H, He, and O at energies of 10 keV to 2 MeV in the dayside magnetosheath whenever the solar wind pressure compressed the magnetopause within 8.75 RE, the CCE apogee. In this report we discuss a burst of energetic particles in the subsolar magnetosheath and its association with rapid changes in the local magnetic field direction in such a way that the magnetic field connected the spacecraft to the magnetopause during the enhancement. We find that magnetosheath angular distributions outside the burst peaked at 90° pitch angles, whereas during the burst they exhibited field aligned streaming either parallel or antiparallel to the magnetic field combined with a clear earthward gradient. The clear earthward gradients at E ≥ 10 KeV, the streaming, and the slope change in the burst‐time magnetosheath spectrum at ∼10 KeV suggest magnetospheric source for the burst‐time ≥10 KeV ions and heated solar wind for E &lt; 10 KeV.

Magnetic and crystallographic structural changes in ternacies of the type Er2Fe17Cx

The compounds Er2Fe17Cx with x=0, 0.4, 0.6, 0.8, 1.0 and 1.5 were investigated by means of AC initial susceptibility measurements and X-ray diffraction. Spin reorientation transitions, marking the changes in easy magnetization direction (EMD) with increasing temperature from parallel to the c-axis to perpendicular to the c-axis,were observed in Er2Fe17Cx for x⩾0.8. From X-ray diffraction measurements it was found that the crystal structure of Er2Fe17Cx changes with increasing carbon concentration from hexagonal (Th2Ni17-type) to rhombohedral (Th2Zn17-type). In samples (x>0.8) in which both crystallographic forms were found we detected two different spin reorientation temperatures. The difference in spin reorientation temperatures between the two crystallographic forms was explained on the basis of differences in the crystal structure and concomitant differences in crystal field effects.

Hyperfine field anisotropy and spin reorientation in ternary carbides of the type R 2Fe 17C x

The compounds Sm 2Fe 17C x and Er 2Fe 17C x were investigated using 57Fe Mössbauer spectroscopy at various temperatures. The compound Er 2Fe 17C gives rise to a spin reorientation at T SR = 82 K, at which temperature the easy magnetization direction changes from perpendicular to the c axis ( T > 82 K) to parallel to the c axis ( T < 82 K). Owing to a strong hyperfine field anisotropy, the temperature dependence of the effective hyperfine field shows a discontinuity which is fairly pronounced for the dumb-bell iron site in particular. From magnetic measurements it was derived that the Curie temperature increases significantly with x in Sm 2Fe 17C x . In Sm 2Fe 17C no spin reorientation was observed, showing that the easy magnetization remains parallel to the c axis up to the Curie temperature.

Magnetic anisotropy of GdCo 4− xFe xB compounds

When substituting Fe atoms for Co atoms in GdCo 4B a change of the easy magnetization direction from the c-axis to the base plane was observed. This behaviour of magnetocrystalline anisotropy confirms the opposite signs of the individual site anisotropy contributions coming from iron and cobalt.

Palaeomagnetism and high‐resolution dating of ceramic kilns in Thailand: A progress report

Abstract The production of high‐temperature stoneware ceramics began in Thailand some time after AD 900. It developed into a large‐scale industry following a change from in‐ground to surface kilns between the thirteenth and fifteenth centuries, before declining in the sixteenth and seventeenth centuries. High‐resolution dating of the industry is difficult, since historical references, inscriptions on ceramics, and associated coins are sparse. Direct dating methods, like radiocarbon and thermoluminescence, have an accuracy of one or two centuries, and are generally suitable for dealing with questions on a broad time‐scale. Palaeomagnetic measurements on cores from kiln walls give data on the direction of the earth's magnetic field at the time of last firing, and appear capable of resolving intervals of about twenty‐five years, within a sequence, for parts of the time‐scale. Changes in magnetic direction are, however, irregular in time and place, and other methods must be used to calibrate the palaeomagneti...

Magnetic phase transitions and magnetic anisotropy in Nd 2Fe 14 − xCo xB compounds

The temperature dependence of the anisotropy field of Nd 2Fe 14 − x -Co x B has been measured between 77 K and the Curie temperature, using the singular point detection (SPD) technique. The anisotropy field at room temperature is found to decrease with increasing cobalt content. Measurements of the temperature dependence of the initial susceptibility show that at a temperature T 1, which is below room temperature, the easy magnetization direction changes. The origin of the change is attributed to the neodymium-sublattice anisotropy. For the cobalt-rich samples ( x ⩾ 5) it is found that at temperatures T 2 higher than ambient temperatures a second spin reorientation takes place, which is attributed to the competing effect of the neodymium sublattice anisotropy and the 3d sublattice anisotropy.

Influence of hydrogen on the magnetic characteristics of R 2Fe 14B (R  Ce, Pr, Nd, Sm or Y) systems

The effect of absorbed hydrogen on the magnetic behavior of R 2Fe 14B (R  Ce, Pr, Nd, Sm and Y) has been investigated. These compounds are found to absorb 4.4 to about 6 H per formula unit. Hydrogen absorption does not change the structure of these compounds but is accompanied by an expansion in unit cell volume ranging from 4% to 6%. The measurements show that the hydrogen absorption leads to an increase in the iron moment in all the hydrides investigated and also a pronounced change in the room temperature easy direction of magnetization of the yttrium- and praseodymium-containing compounds. Anisotropy fields for Nd 2Fe 14B and Ce 2Fe 14B at 300 K are significantly reduced on hydrogenation. The results are taken to imply a weakening of 4f-3d exchange interaction with the existence of reasonably large 4f crystal field interactions at low temperatures.

The Steens Mountain (Oregon) geomagnetic polarity transition: 1. Directional history, duration of episodes, and rock magnetism

The thick sequence of Miocene lava flows exposed on Steens Mountain in southeastern Oregon is well known for containing a detailed record of a reversed‐to‐normal geomagnetic polarity transition. Paleomagnetic samples were obtained from the sequence for a combined study of the directional and intensity variations recorded; the paleointensity study is reported in a companion paper. This effort has resulted in the first detailed history of total geomagnetic field behavior during a reversal of polarity. A comparison of the directional variation history of the reversed and normal polarity intervals on either side of the transition with the Holocene record has allowed an estimate of the duration of these periods to be made. These time estimates were then used to calculate accumulation rates for the volcanic sequence and thereby provide a means for estimating time periods within the transition itself. The polarity transition was found to consist of two phases, each with quite different characteristics. At the onset of the first phase, a one‐third decrease in magnetic field intensity may have preceded the first intermediate field directions by about 600 years. Changes in field direction were confined near the local north‐south vertical plane when the actual reversal in direction occurred and normal polarity directions may have been attained within 550±150 years. The end of the first phase of the transition was marked by a brief (possibly 100–300 years) period with normal polarity and a pretransitional intensity which suggests a quasi‐normal dipole field structure existed during this interval. The second phase of the transition was characterized by a return to very low field intensities with the changes in direction describing a long counterclockwise loop in contrast to the earlier narrowly constrained changes. This second phase lasted 2900±300 years, and both normal directions and intensities were recovered at the same time. Both directional and intensity data document very erratic geomagnetic field behavior during the polarity transition. Changes in magnetic field direction were variable and occurred either (1) in a regular, progressive manner, (2) with sudden, extremely rapid angular changes (58°±21°/year), or (3) with little or no movement for periods of the order of 600±200 years. Changes in magnetic intensity occurred in a like manner and were sometimes correlated with changes in direction, but during other periods both directional and intensity changes occurred independently. Directional changes following the polarity transition occurred in a seemingly normal manner, although intensity fluctuations attest to some instability of the newly reestablished dipole.