Initial Magnetic Susceptibility Research Articles

A study of the spin reorientation transitions in (Er xHo 1 −x) Fe 10V 2 intermetallics

AC initial magnetic susceptibility (χ AC) measurements on the (Er x Ho 1- x )Fe 10V 2 series show peak anomalies for x≥ 0.25, which are an indication of the existence of spin reorientation phase transitions (SRT). No anomaly is observed for HoFe 10V 2, the SRT temperature decreasing with increasing Ho concentration. The critical exponents have been determined, with a value of γ = 1.20 ± 0.05 for all the compounds studied.

Flow duration of a dike constrained by palaeomagnetic data

SUMMARY We have conducted a palaeomagnetic study of a Permian red sandstone layer intruded by a 70cm wide Plio-Pleistocene basaltic dike, with the ultimate goal of estimating the flow duration of the magma before its solidification. 31 samples were collected at various distances from the dike/sandstone interface and were thermally demagnetized in order to separate the overprinted PlioPleistocene magnetization component from the primary Permian one and to determine for each sample the unblocking temperature, Tub, necessary to erase totally the overprint. This temperature can be used to determine the maximum palaeotemperature reached at any distance from the contact, provided the secondary magnetization is a (P)TRM and the thermal fluctuation effects and actual cooling rates are taken into account. Examination of various magnetic parameters (NRM intensity, 15 days VRM intensity, initial magnetic susceptibility and hysteresis curves) allowed us to establish the nature of the overprinted magnetization and consequently to exclude samples carrying a CRM overprint. An apparent palaeotemperature profile and two corrected ones were thus deduced from the samples in which the dike emplacement produced no crystallization of new magnetic minerals. These profiles were compared with the calculations of a new conductive thermal model which includes the basalt's latent heat of crystallization, the flow duration of the dike, the width of the circulating magma, and the temperature dependence of the thermal conductivity, k, in the basalt and in the sandstone, or alternatively convection simulated by an increase of the k values. Good fits with the experimental data are obtained for a flowing width consistent with field observations and when the latent heat is taken into account, in the two following cases. First, when k is temperature dependent in the basalt and in the sandstone, which implies a magma flow duration of 5 days. Second, when a limited convection process is assumed in the sandstone only; here, a flowing time of only 1 day is required. The corrected palaeotemperature profiles argue in favour of the second hypothesis, suggesting that when a dike is emplaced, heat transfer by fluids may play an important role in the surrounding rock.

First-order magnetic phase transition in quasi-bulk Gd(0001)

The UHV-compatible AC initial magnetic susceptibility ( chi AC identical to chi ( omega =constant, T)) technique is used to study (with nanometer resolution) the magnetic behaviour of Gd(0001)/W(110) thin films. The abruptness of the drop in chi AC within 4 K above the Hopkinson maximum is interpreted as indicating the presence of a first-order magnetic phase transition.

Magnetic properties and M�ssbauer spectra of gamma ferric oxide and doped gamma ferric oxide

Magnetic hysteresis, Mossbauer spectra and temperature variation of initial magnetic susceptibility of thirteen samples of doped γ-Fe2O3 containing cobalt or gadolinium are determined. The samples containing more than 1.0% cobalt are found to have a multi-domain configuration, and undoped γ-Fe2O3, gadolinium-doped γ-Fe2O3 and doped γ-Fe2O3 containing less than 1.0% (except 0.3%) cobalt have a single domain configuration. Mossbauer spectra of gadoliniumdoped samples suggest that gadolinium occupies A and B sites. In cobalt-doped samples, the effective magnetic fields at A and B sites are different at room temperature and liquid nitrogen temperature. The samples which have a multi-domain configuration display an additional central doublet in Mossbauer spectra indicating that these samples contain multi-domain clusters. The saturation magnetization of gadolinium-doped γ-Fe203 is much lower, and the coercive force of cobalt-doped samples is much higher than of gadolinium-doped and undoped samples.

Spin reorientation phenomena in RFe 10V 2 intermetallic compounds (RDy and Nd).

A high Curie point and a high uniaxial anisotropy near room temperature are the most interesting features of the new iron rich R Fe 12-xM x alloys (R=rare earth, MTi, V, Cr…) in order to be used as starting materials for permanent magnets. When R is a rare earth ion with planar anisotropy spin reorientation transition (SRT) phenomena will take place due to the variation with temperature and competition of the anisotropy of the R and Fe magnetic sublattices. We have studied the SRT phenomena in RFe 10V 2 (RDy and Nd) using two different experimental techniques. A.C. initial magnetic susceptibility measurements have allowed us to determine the SRT temperature associated with the critical fluctuation of the SRT angle, being T SR=140 K and T SR=112 K for DyFe 10V 2 and Nd 2Fe 10V 2 respectively. On the other side we have measured the angular dependence of the parallel and perpendicular component of the magnetization to the applied magnetic field, which has permitted to obtain the SRT angle. From the aforementioned experiments we have obtained that at low temperatures a cone magnetic structure is stabilized in both compounds and from that we can conclude the relevance of the high order term in the crystal electric field interaction.

The influence of internal stress and grain size on the behaviour of domain wall interaction in pure nickel and Ni-Cu alloys

The magnetic anisotropy,K, and the initial magnetic susceptibility, χa, were used to study the effect of internal stress and grain size on the interaction of lattice defects with magnetic domain walls under moderately strong magnetic fields in pure nickel and Ni-Cu alloys. The observed dependence ofK and χa on the internal stress was attributed to the existence of structural defects producing a configuration of potential energy controlling the location of magnetic domains and the strength of magnetic pressure in the nickel matrix. The effect of grain size onK and χa was investigated, and the domain wall thickness was calculated. The existence of copper solute atoms in the nickel matrix was found to have an effect on both the magnetic anisotropy and initial magnetic susceptibility. This is assumed to be due to the induced magnetic free poles resulting from the magnetization variations produced by solute atoms in the matrix.

The effects of serpentinization on density and magnetic susceptibility: a petrophysical model

Abstract New measurements are presented of initial magnetic susceptibility and density for serpentinized harzburgites from the Josephine Peridotite (Oregon). An inverse correlation between density and susceptibility, which is typical of these rocks and similar rocks from other localities, is not explained by simple serpentinization reactions. Calculations of density and susceptibility as a function of reaction progress for reactions that produce magnetite show susceptibilities that are larger than those observed at low degrees of serpentinization. Susceptibilities of Fe-bearing serpentine and brucite are calculated from a molecular field model initially developed for the olivine and orthopyroxene solid solutions. At high degrees of serpentinization, susceptibilities that are lower than those observed are predicted by reactions where iron is sequestered by serpentine and brucite. Observed density-susceptibility trends of serpentinized harzburgites fall between the values predicted by these limiting reactions. The observed properties may be modelled by multiple reactions involved in multi-stage serpentinization processes such that the production of magnetite increases with the degree of serpentinization. The empirical and calculated properties provide a complementary characterization of density and susceptibility values of serpentinites.

The initial magnetic susceptibilities of Gd 3Fe 5O 12 and Tb 3Fe 5O 12 in the compensation point region

The initial magnetic susceptibilities of polycrystalline GdIG and TbIG have been measured by Gouy's method around their compensation points (288 K for GdIG; 248 K for TbIG). The temperature dependences of the susceptibilities are characterized by small jump and inflection at T comp. ± (10−12) K. A peak of d.c. susceptibility at 5 K above T comp. appears only in first run (cooling to low temperatures in the absence of external field and then measuring during the heating of the sample). The two parts of the complex susceptibility are determined for TbIG (single and polycrystals) and for polycrystalline GdIG, and are found close to zero in the T comp. vicinity. The peak of d.c. susceptibility and zero values of χ′ near T comp. are attributed to the domain structure changes.

Spin reorientation in (Er 0.6Ho 0.4) 2Fe 14B pseudoternary compound

A.C. initial magnetic susceptibility measurements on the ( Er 0.6 Ho 0.4) 2 Fe 14 B intermetallic compound show two anomalies at T sr1 = 164 ± 2 K and T sr2 = 43.4 ± 0.5 K that have been associated with spin reorientation transition processes. T sr1 corresponds with a temperature in which the easy magnetization direction begins to tilt away from the c-axis and giving rise to a conical magnetic structure. T sr2 is the temperature at which high order CEF terms begin to be important and a conel-cone2 SRT takes place, giving rise to a new cone2 phase in which the easy magnetization direction tends towards the c-axis. The orientation of the easy magnetization direction has been determined by means of the measurements of the parallel and perpendicular components of the magnetization to the applied magnetic field and also by polar plots of the magnetization in the range 3.8–300 K.

Features of the temperature dependence of the initial magnetic susceptibility in FeBO3

On etudie la variation en temperature de la susceptibilite magnetique initiale dans FeBO 3 afin d'obtenir des informations sur l'existence d'une transition de phase. On note la presence d'etats d'impurete dans FeBO 3 , confirmes par le comportement a haute temperature de la partie imaginaire de la susceptibilite

EXACT SOLUTIONS FOR A 4-SPIN-INTERACTION ISING MODEL ON THE d=3 PYROCHLORE LATTICE

The pyrochlore lattice is a three-dimensional (d=3) periodic array of corner-sharing regular tetrahedra. An Ising model is defined upon this lattice as having a 4-spin “volume” interaction among the corner Ising spins of each elementary tetrahedron. The model is solved exactly and shown to be devoid of any finite-temperature phase transition, thereby remaining in a disordered (high-temperature) phase. Exact solutions are found for all multisite correlations which are then used to obtain exact solutions for specific heat, initial and non-linear parallel magnetic susceptibilities as well as initial perpendicular susceptibility, inelastic neutron scattering function, and the joint configurational probabilities of the Ising spins. All solutions are simple algebraic functions of K and tanh K (K being the multispin coupling parameter) and exhibit such behaviors as rounded extrema, crossing points, sigmoidal curves having non-vertical inflection points, and the like. Despite their “analytical quiescence”, most of these simple solutions are evidently the first exact explicit expressions for the selected physical quantities to appear in the literature for classical and quantal d=3 Ising models having multispin “volume” interactions, with the prospect of offering some guidance and insights upon a larger class of multisite-interaction model systems.

A.C. initial magnetic susceptibility and spin reorientation transitions in (Er xR 1−x) 2Fe 14B magnets (R=Nd and Dy)

A.C. initial magnetic susceptibility has been measured in the series (Er xNd 1−x) 2Fe 14B and (Er xDy 1−x) 2Fe 14B in the range of temperatures between 3.8 and 350 K. The results clearly show that these measurements constitute a very useful tool in order to detect spin reorientation phenomena. The interplay of different kinds of anisotropies: axial, conical and planar in these systems produces a dependence of the spin reorientation transition (SRT) on concentration, x. In the case of Er-Nd compounds SRT processes were found for all the concentrations in the range of temperatures measured, and for intermediate concentrations two peak anomalies were observed indicating the existence of two SRT's. In the case of the Er-Dy systems, no SRT was observed for concentrations x < 0.6.

Spin reorientation processes in hard magnetic pseudoternaries (ErxNd1−x)2Fe14B

The effect of the different kinds of anisotropies (axial, conical, and planar) competing in the same compound with different grades of influence with the concentration, x, produces a wide range of peculiar behaviors. We have studied the magnetic anisotropy in this series by means of the thermal evolution of the spin reorientation transition (SRT) with concentration. We have measured the ac initial magnetic susceptibility for the concentrations x=0, 0.2, 0.4, 0.6, 0.8, 0.9, and 1. The results clearly show peaklike anomalies which allow us to obtain the critical temperature for the phase transition. We have also measured the magnetization parallel and perpendicular to the applied magnetic field on magnetically aligned powder samples in order to determine the thermal evolution of the orientation of the easy magnetization direction. The compounds with intermediate concentrations (0.2≤x≤0.8) present two peaklike anomalies in the range of temperature measured (3.5–350 K), which have been associated with critical phenomena related to the SRT (axial→cone 1 and cone 1→cone 2). The remaining concentrations display SRT axial→planar in the case of Er-rich compounds and axial→cone in the Nd-rich ones.

Thermal expansion and ac initial magnetic susceptibility in RECu intermetallic compounds (abstract)

We have carried out a systematic study of the series RECu cubic intermetallic compounds where RE=Y, Gd, Tb, Dy, and Ho by means of the thermal expansion and ac initial magnetic susceptibility measurements. The measurements clearly show in some of these compounds (RE=Y, Gd, and Tb) very large anomalies associated with a martensitic structural phase transition, from the cubic CICs at room temperature to the tetragonal FeB structure. These anomalies exhibit very large hysteresis effects which make it necessary to reobtain the original CICs crystal structure to anneal the samples up to about 600 K. In the case of TbCu a magnetic-phase transition occurs simultaneously to the structural one; the trigger mechanism which drives this transition has been studied on the basis of the strong influence of the magnetoelastic energy using a single-ion crystal electric-field model within the mean-field approximation. In the case of RE=Dy and Ho only slight anomalies have been observed in the thermal expansion measurements at the region of the magnetic-phase transition and no hysteresis effects were observed. The existence or nonexistence of the spontaneous magnetostriction at the order temperature constitutes a suitable test of the nature of the magnetic structure (collinear or multiaxis) in these systems.

Spin re-orientation transition and high field magnetostriction in ErFe 10V 2

A.C. initial magnetic susceptibility measurements on the intermetallic compound ErFe 10V 2 show the presence of a spin re-orientation transition from axis to cone which commences at 60K and culminates in a final cone magnetic structure of some 45 degrees to the c-axis at 4.2K. The axis to cone transition has been accounted for in a crystalline electric field-exchange model with inclusion of crystal field terms up to 4th order. The anisotropic and volume magnetostrictions have been measured in pulsed magnetic fields of up to 12 Tesla down to 77K. Both the anisotropic and volume magnetostrictions are found to be small at low temperatures. The volume magnetostriction increases with increasing temperature.

Some properties of melt-produced YBaCuO wire

YBaCuO ceramic, with a nominal metal ratio 1:2:3, was melted in an alumina furnace and squirted in the form of thin wire. The wire obtained is dense and hard but insulating. After homogenisation and short oxygenation the wire becomes superconducting with Tc (onset) equal to that of the usual YBa2Cu3O7 ceramics. Both the resistivity and the initial magnetic susceptibility measurements indicate an improvement of the inter-grain coupling in the wire, which may lead to an enhanced critical current.

Intra- and intergrain effects in a.c. susceptibility of YBaCuO ceramics

The dependence of the initial magnetic susceptibility on the magnetic field, frequency and density has been investigated for several ceramic YBa 2Cu 3O 7 samples. The superconducting transitions show invariably two steps, associated with the grain and bulk (shielding) effects respectively. The width of transition decreases, whereas the lower critical field increases, with increasing density.

Comparison of the hysteresis characteristics of synthetic samples with different magnetite and haematite contents

The values of the initial magnetic susceptibility κ were measured with different instruments. The values of saturated magnetic polarization J s, the coercive force H c, the alternating current demagnetization curves J rs J rso = F( H) and the hysteresis curves J = J( H) were determined for synthetic samples containing synthetic magnetite with grain sizes of < 40 μm, 40–80 μm, 80–120 μm and 120–160 μm, and concentrations of 0.1–5.0% by volume. Analogous parameters were also determined for synthetic samples containing natural and synthetic haematite with a grain size of 40–80 μm and concentration of 1.0–10.0% by volume. The results indicate a significant effect of the magnetic interactions between the grains of the magnetic minerals in the samples and the ways of determining the grain size and concentration of magnetite or haematite in rock samples.

Variation of the magnetic inhomogeneity in glassy Fe xNi 80- xB 18Si 2 alloys

A careful analysis of the magnetization measurements of amorphous Fe x Ni 80- x B 18Si 2 alloys (3 ⩽ x ⩽ 15) above the percolation threshold ( x c = 2.55) has been performed. The non-monotonic variation of the effective critical exponent of the initial magnetic susceptibility, γ ∗, with temperature is found to become less pronounced with increasing x - x c, and probably disappears around x = 20. The tendency towards a homogeneous ferromagnetism around that concentration is also obtained from the analysis of the Arrott-Belov-Kouvel plots. The results are compared with those for similar glassy ferromagnetic systems and a possible origin of the magnetic inhomogeneity is discussed.

Initial magnetic susceptibility of Sn films with Cr, Mn, Fe, Co and Ni

Sn alloys with additions of 3d elements are artificially produced by vapour quenching on a liquid helium cooled substrate. The magnetic susceptibility of the as produced films is measured in situ by a sensitive ac magnetic induction method. The experimental result demonstrates that all mentioned 3d atoms have no localized magnetic moment in Sn except crystalline SnMn ( S = 4 2 ), amorphous SnMn ( S = 3 2 ) and crystalline SnCr ( S = 1). SnFe and SnCo become magnetic at the percolation limit of 25 at % of the 3d element, SnNi does not. The Sn systems with localized magnetic moment show spinglass behaviour.