Demagnetization Correction Research Articles

Palaeomagnetism of some lithological units from the Lower Palaeozoic of the Vendée, France

Results of the first palaeomagnetic study carried out in the west Vendean region at the southern edge of the Armorican Massif are presented. The lithological units sampled range from Middle to Upper Cambrian and Silurian. After alternating field demagnetization and thermal cleaning, La Lombardiére basaltic rocks of Silurian age yielded only one residual component of magnetization ( D = 214°, I = + 11°) after tilt correction. The corresponding palaeopole is located at 28°S, 321°E. La Meilleraie sandstone and quartzite of Lower Ordovician age, which are equivalent to the Armorican sandstone, also contain one single residual component of magnetization ( D = 244°, I = + 5°) after demagnetization and tilt correction. The corresponding palaeopole is 15°S, 293°E. Finally, quartzites and rhyolites of Middle to Upper Cambrian age are characterized by two and possibly three components of magnetization. The mean direction of the main residual component after demagnetization and tilt correction is D = 319°, I = + 9°; the corresponding palaeopole position is 40°N, 222°E. The apparent path wander of the poles obtained in the Vendée is compared with the one of stable Europe (western) and Gondwana: a convergence of the poles from the Vendée and stable Europe is observed from the beginning of Ordovician time and a reasonable similarity at Silurian time. This study partly fills the gap of palaeomagnetic data for the period: Upper Cambrian-Silurian for the Armorican plate. As early as Upper Cambrian, the Vendée was an integral part if the Armorican plate. The plate tectonics scenario proposed in this paper is the following: accelerated separation of the Armorican block from Gondwana at Middle-Upper Cambrian time, motion of the Armorican block towards stable Europe at Ordovician time and proximity of Armorican plate to the northwest European craton at Silurian time. If this scenario is right, the nature of the Hercynian orogen is more likely the result of a collision, the Armorican plate acting as a buffer zone. Palaeomagnetic studies of the Vendean Bocage rock sequence are considered important for future work.

Correction of the Knight Shift for Demagnetizing Effects

AbstractThe NMR line shift contributions originating from demagnetizing fields due to classical macroscopic magnetic effects are discussed for ellipsoidal samples. It is shown that by measuring the line shifts along two main axes of the ellipsoid, the measured difference is sufficient to correct for demagnetizing effects in materials with homogeneous magnetization. It is also found that for special sample geometries even the demagnetizing factors are unnecessary for calculating the demagnetization correction term. Based on line shift and magnetization measurements, the measured and calculated differences of the NMR line positions are compared for sandwich‐type samples. Alloys with inhomogeneous magnetization are also briefly discussed.

On the demagnetization correction for boulder samples measured with a vessel probe

On the demagnetization correction for boulder samples measured with a vessel probe

Critical Behaviour of Spin Relaxation in EuS

We have measured both real and imaginary part of magnetic susceptibility of EuS near the Curie temperature T c by means of the high frequency magnetic susceptibility measurement method, in which no demagnetization corrections have been required. Except for the immediate vicinity of T c , the critical relaxation spectra obtained are described by a Debye form of relaxation and the critical index of the relaxation time is determined to be Δ =1.26±0.03. The value of Δ is slightly smaller than that of the static susceptibility, γ=1.41±0.01. For temperatures closer to T c , the critical relaxation spectra become to exhibit a polydispersive character.

EPR and magnetization of GdGa 2

Paramagnetic resonance and magnetic measurements were performed on powdered samples of GdGa 2. The magnetic data indicated ferrimagnetic behavior with T c ⋍ 181° K . Above 250° K the susceptibility obeys the Curie-Weiss law χ g = 2.66 2 × 10 −2 (T − 27.6) emu/g-Oe which corresponds to an effective moment of 7.95 Bohr magnetons. Over the range from 190 to 300°K the data obey a Néel type law, χ g −1 = 35.95 (T − 12.5) − 2.20 × 10 4 (T − 177) , which is indicative of ferrimagnetic order. The resonance measurements were performed at 9.013 gHz at 247, 296, and 349°K. The spectra were analyzed with a computerized curve-fitting technique that involves a linear combination of Lorentzian absorption and dispersion susceptibility components. Following demagnetization corrections, the g-factor was found to be 1.983 2 while the half-power, half-linewidth was 592.7 Oersteds.

Magneto-Optical Properties of Terbium Aluminum Garnet at Liquid-Helium Temperatures

The Verdet constant of a terbium aluminum garnet crystal (flat disk) has been measured at liquid-helium temperatures in the visible spectral range. Values appropriate to a sample with no demagnetizing field have been calculated from internal fields of the specimen rather than the external field, using standard demagnetization corrections and volume-susceptibility data. Photographs taken of the crystal in field values $H$ in the vicinity of a 180\ifmmode^\circ\else\textdegree\fi{} rotation illustrate the significance of demagnetizing fields. From such observations the distribution of internal magnetic fields in nonellipsoidal specimens can be mapped out. These new Verdet-constant data serve to verify further the proportionality of Verdet constant and magnetic susceptibility postulated by Van Vleck and Hebb. The onset of magnetic saturation at relatively low fields is due to the low temperatures of the measurement and to the large splitting factor associated with the ground state of ${\mathrm{Tb}}^{3+}$. The frequency dependence of the Verdet constant is in satisfactory agreement with the simple theory proposed by Rosenfeld.

STANDARD CURVES FOR INTERPRETATION OF MAGNETIC ANOMALIES OVER LONG TABULAR BODIES

The magnetic anomalies in vertical, horizontal, and total intensity for the thin infinite dike are shown to belong to a single mathematical family of curves for all values of dip and strike of the dike and all values of inclination of the magnetizing field. The complete family of standard curves has been constructed and is incorporated into an interpretational scheme based on superposition with observed magnetic profiles. This technique should give more reliable interpretations than methods based on only a few isolated points of a profile curve. By integration of the general thin‐dike response a general expression of similar form has been derived for thick dikes, and ten sheets of curves for dikes of varying width indices have been constructed. By employing the method of subtraction of curves these serve for constructing anomaly profiles over bodies of finite depth extent. Additionally, for thin dikes the much‐neglected demagnetization corrections have been incorporated in the interpretational method following verification by model studies. One important disclosure of this work is that the depth and location of the apex of an infinite tabular body may be determined without knowing the intensity or direction of magnetization within the body, assuming only that these quantities are constant throughout.