Thermomagnetic Hysteresis Research Articles

Thermomagnetic hysteresis effects in NiMn and NiMnPd thin films

dc magnetization measurements, for zero-field cooled (MZFC) and field-cooled (MFC) cases, have been carried out for flash-evaporated Pd-doped NiMn thin films. These included reentrant phases (Ni76−xPdx)Mn24, for 0⩽x⩽5, and Ni75Mn23Pd2, a pure spinglass phase. The studies were performed over the temperature range 3–300 K. Low-field magnetization measurements show the irreversibility effect (MZFC and MFC diverge) at temperatures below the Curie temperature Tc. In Ni75Mn23Pd2, MZFC falls below MFC, as usually observed. However, in reentrant compositions, MZFC crosses MFC upon warming into the ferromagnetic regime, where it stays above MFC at temperatures below Tc. This unusual behavior is attributed to a model of Imry and Ma in which, in a ferromagnet with antiferromagnetic impurities, the impurities can couple to the host ferromagnetic alignment and force the system to break into domains antiferromagnetically coupled to each other. Field-cooled hysteresis measurements indicate the uniaxial anisotropy in these samples to be small, in contrast with the rigid uniaxial anisotropy reported for the corresponding polycrystalline bulk samples. Since the lattice-orbit coupling is weak in the amorphous phase, this clearly demonstrates that the physical origin of the unidirectional anisotropy is associated with the spin-orbit coupling.

ANOMALIAS DE SUSCETIBILIDADE MAGNÉTICA NO BATÓLITO GRANÍTICO DE TEIXEIRA (PROVÍNCIA DA BORBOREMA, NORDESTE DO BRASIL) E SUA RELAÇÃO COM A ZONA DE CISALHAMENTO DE ITAPETIM

The batholith granitic of Teixeira forms a large ENE-trending dyke of 80 km long and 10 km wide emplaced between pelitic metasediments and Cariris Velhos orthogneisses. The batholith was locally deformed by shear zones, the most important the NE-trending Itapetim Shear Zone (ISZ) which cross cut the eastern part: of the dyke. In the gneiss situated in the southern part of the batholith, the ISZ host gold bearing quartz veins of the Itapetim Distnct A detailed magnetic study on the batholith show high susceptibilities (> 10 -3 SI) near fault and fractures zones that decrease m magnitude (< 0.5 x 10 -3 SI) f ar tromiaulted zones Thermomagnetic data and hysteresis parameters of the high susceptibility rocks revealed that the mam carrier of susceptibility is a multidomian Ti-poor magnetite. The mean coercivity around 4.8 mT indicates that the magnetic properties are yielded by fmest magnetite particles (< 10 um) MEV backscaterring examination found nearly equant magnetite grains varymg from 180 um down to 5 um. Some grams display fine discontinuous stripes of hematite occupying octahedral planes. Barite veins cross cutting the sihcate framework and cavities mhlled bv microcrystalline quartz and TR-enriched allanite hosted in magmatic quartz crystal indicates that an oxidizmg hydrothermal environment was associated with shear zone deformation. The relation between fluid fluxes within this segment of the Itapetim shear zone and evolution of the gold mineralization is discussed.

Metastable nanocrystalline A 1 phase and coercivity in Fe–Nd alloys

Transmission electron microscopy observations of as-cast hypoeutectic Fe x Nd 100− x alloys ( x>25) show that metastable Fe-rich and Nd-rich phases form the eutectic areas in its microstructure. The heat treatment of as-cast alloys at temperatures lower than 600°C does not result in the change of thermomagnetic data and hysteresis properties. Thermomagnetic data and increased H cJ (up to 80–400 kA/m of hypoeutectic alloys after heat treatment at temperatures 600–740°C indicate an increase in the amount of the hard magnetic phase ( T c=245°C) in its microstructure. High-resolution electron microscopy investigations of heat-treated alloys indicate the formation of the nanocrystalline A x phase inside the Fe-rich component of the metastable eutectic. The appearance of the A x phase, which is the same as the well-known highly anisotropic A 1 phase, increases the coercivity of hypoeutectic Fe–Nd alloys after heat-treatment.

Thermomagnetic hysteresis in amorphous TbCr alloys

Structure and magnetization of Tb x Cr 100− x amorphous alloys ( x = 16, 28.5, 43 and 59 at%) have been studied. It was observed that two types of a local atomic environment, characterized by a different short-range order, coexist in the alloys. Thermomagnetic hysteresis, indicating a transition from a paramagnetic to a spin-glass-like state was observed in all alloys. The transition temperature depends strongly on the composition, linearly rising with increasing Tb concentration. It is shown that the magnetic ordered state is governed by the random magnetic anisotropy associated with Tb.

Relative paleointensity of the geomagnetic field over the last 200,000 years from ODP Sites 883 and 884, North Pacific Ocean

Ocean Drilling Program Sites 883 and 884 were cored as part of a three-site depth transect down the slopes of Detroit Seamount in the North Pacific Ocean. Continuous mineral magnetic and paleomagnetic measurements were made using u-channel samples for sediments that span the last 200 kyr. Thermomagnetic and high-field hysteresis data indicate that the magnetic mineralogy of the clay-rich sediments at the two sites is dominated by pseudo-single domain magnetite in a narrow range of grain sizes and concentrations, thereby meeting established criteria for relative paleointensity studies. The natural remanent magnetization (NRM) has been normalized using the anhysteretic remanent magnetization (ARM) and the low field magnetic susceptibility ( χ). The NRM/ARM and NRM/ χ records are nearly identical for both cores. Furthermore, coeval horizons in the two cores can be correlated by matching >100 magnetic susceptibility features over the 15 m length of both cores. Coherence function analysis indicates that the records are not significantly affected by local environmental conditions. These factors suggest that the large-scale variations in normalized remanence are most likely due to geomagnetic paleointensity fluctuations. Dating constraints are provided by a δ 18O stratigraphy from Site 883. Our North Pacific paleointensity versus age curve is similar to a published record from the western Caroline basin and a recently proposed global paleointensity curve. Offsets (up to 10 kyr) in the timing of paleointensity features between our composite North Pacific record and the global curve may result from imprecisions in the dating of our record. Nevertheless, the correspondence between the paleointensity records suggests that, in suitable sediments, paleointensity of the geomagnetic field can give a globally coherent, dominantly dipolar, signal.

Magnetic Properties of Li1−xNi1+xO2(0 ≲x≲ 0.08)

Magnetic properties of Li1−xNi1+xO2(0 ≲x≲ 0.08) crystallizing in the α-NaFeO2structure were studied by SQUID magnetometry. A sample withx≲ 0.01 prepared from Li2O2and Ni0.77O at 650°C follows the Curie–Weiss law from 350 to 80 K with μeff(effective Bohr magneton) of 1.91 μBand Θ (Weiss temperature) of +29.5 K. A spin glass-like thermomagnetic hysteresis with a cusp at 8 K was observed. Samples with largerxvalues exhibit ferromagnetic moments, typically 0.3 μB/Ni (x∼ 0.08) at 5 K. From these and other results we conclude that the dominant intra-Ni layer interaction is ferromagnetic and that excess Ni ions located in the Li layers introduce strong ferromagnetic inter-Ni layer interactions. Possible importance of orbital–spin coupling and frustration in orbital ordering will be raised to the magnetism of ideal LiNiO2, in which Ni3+(S= 1/2) ions with twofold orbital degeneracy form a ferromagnetic triangular lattice.

Thermomagnetic study of Finemet type nanocrystalline alloy by in situ hysteresis measurements

The origin of the excellent magnetic properties of nanocrystallized Fe 73.5Cu 1Nb 3Si 13.5B 9 have been explained essentially by its nanograin weak anisotropy and its global zero magnetostriction. However, it has been suspected that a more complex problem of two-phase magnetic coupling must be taken into consideration. It is proposed herein to investigate in this way by in situ hysteresis measurements up to saturation at high temperature (reaching 700°C). Details of the Thermo-Magnetic Hysteresis method (TMH) are given. Dynamic and isothermal crystallization are investigated. Isothermal tests point out that induction arising could result from crystallization and from a slower mechanism such as atomic diffusion. It is also shown that the crystallization stage occurs, at its beginning, in a superparamagnetic phase and reaches a superferromagnetic phase. During cooling, the complex superferro-ferromagnetic transition shows that the magnetic coupling between grains and the amorphous matrix plays a leading role in Finemet magnetic properties.

A palaeomagnetic and rock magnetic study of the northern Rajmahal Volcanics, Bihar, India

A new palaeomagnetic study of mid Cretaceous lavas from 23 localities in the northern part of the Rajmahal Hills gives a mean virtual geomagnetic pole at 10.4°N, 296.6°E, confirming the results of earlier workers. Stepwise thermal and alternating field demagnetisation indicates that 21 out of 23 sites are normally magnetised, whereas two sites in the northwest of the area have recorded two different transitional directions. We have measured the strong-field thermomagnetic behaviour, DC and AC hysteresis parameters, and the variation at low temperature of low-field susceptibility of these and other samples from the Rajmahal Volcanics. These rock magnetic studies reveal that the basaltic lavas contain titanomagnetites which have hardly been oxidised at high temperature, but have undergone low temperature oxidation. The reddened lavas of Gandeswari Hill, on the other hand, show evidence of extensive high temperature deuteric oxidation of titanomagnetite.

Magnetic studies of Ce4+ compensated Co-doped yttrium iron garnet thin film grown by chemical vapor deposition

A low pressure metalo-organic chemical vapor deposition (LPMOCVD) technique to form Co2+ and Ce4+ doped yttrium iron garnet (Y3−xCexFe5−xCoxO12: Co,Ce:YIG) films is described. A large concentration of Co2+ doping with x=0.3–0.7 results in uniaxial anisotropy perpendicular to the plane of the film with high coercivity values and sufficiently high saturation magnetization values. This has been possible by an alternate doping scheme where commonly used compensator, Ge4+ at Fe3+ sites has been replaced by a new compensator Ce4+ at Y3+ sites in the Co-doped YIG films. The structural and compositional aspect of stabilized garnet phase in Co2+,Ce4+:YIG thin films and optical, thermomagnetic and magnetic hysteresis properties are presented.

Magnetic properties of DyFe/sub 6/Al/sub 6/ compound studied by magnetostatic and Mossbauer methods

Magnetic investigations of DyFe/sub 6/Al/sub 6/ compound showed that it is a ferrimagnet with the Curie temperature T/sub C/=316 K and with a relatively strong magnetocrystalline anisotropy. Below 130 K all phenomena characteristic for spin glass behaviour were observed i.e. marked thermomagnetic hysteresis, time dependence of magnetization, inflection point in magnetic isotherms, an unusual shape of the hysteresis loop and a very large rotational hysteresis. Mossbauer absorption spectra below the Curie temperature were best described by the magnetic fields distribution which proves the magnetic disordering in the Fe sublattice. From the analysis of temperature dependence of magnetic fields distributions it follows that about 30% of the Fe atoms start to order below 130 K. The results obtained suggest that magnetic ordering in the Fe sublattice is responsible for the magnetic behaviour of DyFe/sub 6/Al/sub 6/ compound below 130 K.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetic moment and spin glass behaviour in an Al65Mn20Ge15 quasicrystal

Abstract An icosahedral quasicrystal of composition Al65Mn20Ge15 has been produced by rapid solidification processing. X-ray diffraction reveals this to be a single-phase quasicrystal to the limits of resolution (⩽ 5 %) of X-ray powder diffractometry. Magnetic susceptibility measurements disclose a large local moment on Mn in this material as determined from fits to a Curie-Weiss law. A 2.10 μ B local moment is the largest observed for Mn in Al-based quasicrystalline alloys with similar concentrations (Mn content). D.c. magnetization measurements show a nonlinear field dependence of the magnetization in the field range between 1 and 5 T. A spin glass transition has also been observed at low temperatures and studied using d.c. and a.c. susceptibility measurements and field-dependent specific heat measurements. A spin glass freezing temperature, T f ∼ 8 K, in a 1 T applied field, has been determined from thermomagnetic hysteresis as the temperature at which the field cooled (FC) and zero field cooled (ZFC) ...

Evidence for the reversible structural transformation in amorphous Fe18Co72Zr10

The temperature cyclical scanning of magnetic anisotropy with temperature from room temperature to 620 K has been investigated for Fe18Co72Zr10. The sample shows a thermomagnetic hysteresis in magnetic anisotropy which, together with the differential scanning calorimetry and the observation of the temperature dependence of field-induced anisotropy, provides the evidence for a first-order reversible structural transformation. Two short-range orders (SRO) with h.c.p.-like and f.c.c.-like symmetry have been proposed to explain the observations.

Concentrated spin-glass behavior in Al37Mn30Si33 quasicrystals.

Detailed magnetic measurements on icosahedral ${\mathrm{Al}}_{37}$${\mathrm{Mn}}_{30}$${\mathrm{Si}}_{33}$ quasicrystals are presented. It has been established that this high-Mn-content quasicrystal is not a real ferromagnet and instead shows a concentrated spin-glass behavior with reentrant ferromagnetism. Low-field-magnetization measurements show a Brillouin-function-like behavior, suggesting a ${\mathit{T}}_{\mathit{c}}$ of 120 K. High-field M-T data approach Curie-like paramagnetism because of the large high-field susceptibility. Arott plots show that no spontaneous moment exists. Thermomagnetic hysteresis and time dependence are also observed in the magnetization. For the first time several magnetic quasicrystals with varying Mn and Si or Ge content can be located on the exchange-fluctuation axis of a magnetic phase diagram.

On spin reorientation phenomena - metastability

All possible spin reorientation transitions in anisotropic magnetic materials are described for the case where the first ( K 1) and second order ( K 2) anisotropy constants are monotonic functions of temperature and are assumed to change sign only once. The corresponding free energy diagrams are used to show the spin reorientation process. The effect of metastability (where K 2 is negative) and the expected thermomagnetic hysteresis in such cases is also discussed.

Low field magnetic properties and relaxation effects of Co(S 1− xSe x) 2 spin-glass

High resolution SQUID magnetometry measurements on Co(S 1 minus; x Se x ) 2 frustrated system with competing exchange interactions in composition range x( x=0.14, 0.25 and 0.35) above ferromagnetic percolation limit are presented. Cusp of low field d.c. susceptibility, thermomagnetic hysteresis and time dependent phenomena were observed at low temperatures. The time dependence of the zero-field cooled magnetization ( M ZFC) and remanences (TRM and IRM) can generally be described by a power law M= M 0± At a (−0.2⩽ a⩽0.3), where the exponent a depends on field, temperature and history of the sample. In most cases exponent a is small enough and ln t law is a good approximation for experimental data. These results can be qualitatively understood in terms of blocking model of spin-glasses.

Effect of substitution of nickel on the magnetic properties of Sm2Co17

Structural, metallographic and magnetic properties of Y2Co17−xNix and Sm2Co17−xNix have been established to ascertain the effect of nickel on the permanent magnet properties of Sm2Co17. The results indicate that the 2 : 17 compositions exist in the concentration limits 0&amp;lt;x&amp;lt;8 in Y2Co17−xNix and 0&amp;lt;x&amp;lt;6 in Sm2Co17−xNix. Metallographic examination of these materials confirms the absence of secondary phases. Magnetic properties were examined in the temperature range 4.2–1200 K in external fields up to 21 kOe. The ordering temperatures of Sm2Co17−xNix and Y2Co17−xNix decrease with increasing concentration of nickel. Examination of saturation magnetization results indicates a smooth variation with the addition of nickel. The easy direction of magnetization at room temperature was established for the samples by examining the x-ray diffraction patterns of aligned powders. The results show that the axial anisotropy exhibited by Sm2Co17 is retained with the substitution of nickel. For a few nickel-rich compositions of Sm2Co17−xNix, thermomagnetic hysteresis effects were observed. The results are interpreted on the basis of a strong competition between the exchange field and the crystal field interaction. Calculations were performed to support this interpretation.

Geomagnetic secular variation recorded in the stable magnetic remanence of recent sediments

Continuous cores of sediment covering the period 13 000 yr BP to the present day have been collected for magnetic investigations from Lake Windermere using a pneumatically operated piston corer. The total remanent magnetization of one 6 metre core and six 1 metre cores has been measured using a computerized slow-speed spinner magnetometer. The stability of the remanence has been confirmed by alternating magnetic field tests and repeat measurements of the remanence. The magnetic mineral content of the sediments has been deduced from thermo-magnetic and rotational hysteresis studies. Low-temperature magnetic transition experiments on the NRM of the sediment shows that it least half of the remanence is carried by haematite. The inclinations of the magnetic vectors are consistently low. This is attributed to compaction of the sediment after deposition. The time variations of the geomagnetic field are discussed and their correlations with the results from the Lake Windermere sediments are indicated. A comparison is also made of the results from the Pleistocene Narita bed (Japan) with those from Lake Windermere.

Societies and Academies

LONDON Physical Society, May 19. W. F. FLOYD: Interference tones in superheterodyne receivers. The problem involves the reception of at least three signals, double detection, and a filter action between the two detectors. In the case of rectification by detectors with generalised characteristics, quantitative analysis is extremely complex. The form of the result, however, shows how large is the number of possible sources of interference tones. The specific case of square-law rectification is also considered. W. G. MARLEY: A method of measuring the specific heats of poor conductors. A brief review of existing methods of determining specific heats is given with reference to their suitability for poor conductors. The method of cooling, which has received scant attention in the past, is developed to afford an accurate and ready method. D. BROWN: The demonstration of eddy currents in conductors of various shapes. A method is described whereby it is possible to demonstrate visually the existence of eddy or Foucault currents in masses of conducting material, and the way in which the flow of the currents may be distorted or baffled by suitable slots or laminations. S. TOLANSKY: The absence of fine structure from the arc spectrum of silver. The arc spectrum of silver, produced in a water-cooled hollow-cathode discharge, has been examined for fine structure with a variable-gap silvered Fabry-Perot interferometer in the region 8300-4000 A. Results for the resonance lines at approximately 3300 A. are quoted also. Silver consists of two odd isotopes 107, 109, and although 5s, 6s, 7s, 5p, 6p, and 5d terms have been studied, no trace of fine structure has been found, in spite of very excessive over-exposing in many lines. All lines are extremely sharp and are worth considering as wave-length standards, being well distributed. Attention is directed to the fact that the analogous spectrum of CdII also shows no fine structure, so that it is concluded that the absence of structure is probably due to the electron configuration’ and not necessarily to smallness of the nuclear magnetic moment. This is remarkable, for penetrating s electrons are involved in some configurations. W. Y. CHANG and WILLIAM BAND: Thermomagnetic hysteresis in steel. The temperature variation of a new hysteresis of the thermomagnetic electromotive force in a steel wire is described. The hysteresis is of a negative or abnormal form with a maximum amplitude of about 2 microvolts between up and down branches. The accuracy of the apparatus and method is critically examined, and an error of more than 0.5 microvolt in any reading is considered to be unlikely. A qualitative explanation of the phenomenon is given.