Showing papers on "Curie temperature published in 1987"

Evidence in rare‐earth (R)–transition metal (M) intermetallics for a systematic dependence of R‐M exchange interactions on the nature of the R atom

Abstract: In rare earth (R)–transition metal (M) compounds, large R‐M magnetic interactions can occur, which give rise to higher values of the ordering temperature (TC,TN) for compounds with magnetic R elements than for compounds with R nonmagnetic, i.e., La, Lu, Y. Due to the localized character of the 4f shell, these R‐M interactions are indirect, mediated by the 5d, 6s conduction electrons. The highest value of the ordering temperature is obtained for Gd compounds, and as a first approximation it is reasonable to write the interaction energy as ER‐M=−nRMMSRMSM, where MSR and MSM are the rare‐earth and transition metal spin moments, respectively. The molecular field coefficient nRM is generally assumed to be a constant throughout a given series, owing to the similarities of band structure for all R elements. In this paper, the molecular field coefficient nRM has been obtained for a number of series of rare earth‐transition metal compounds. The analysis reveals that nRM is not a constant going across a given serie...

Magnetism of epitaxial bcc iron on Ag(001) observed by spin-polarized photoemission.

Abstract: Epitaxial bcc Fe films grown on Ag(001) are ferromagnetic. At $T=30$ K, the 3- to 4-monolayer films are perpendicularly magnetized at remanence, whereas thinner and thicker films have their magnetization in plane. Above $T=100$ K no perpendicular remanence has been observed for any film thickness. Films consisting of more than 5 monolayers have a Curie temperature equal to that of bulk bcc Fe.

High power self-regulating heater

Abstract: An improved performance ferromagnetic self-­regulating heater. Constant alternating current is applied to a layered structure including at least one ferromagnetic layer. One or more layers of non-magnetic material is added to the ferromagnetic layer in such a way that the power factor of the heater is very sig­nificantly increased above its value in the absence of at least one of the layers. The alternating current flows through the different layers in varying quantities depending on layer composition, temperature and Curie point of the ferromagnetic layer. The structure generates heat by resistive heating as a function of the power applied. In one embodiment a single layer of non-­magnetic, high-resistance material is in intimate electrical and thermal contact with one surface of the ferromagnetic material. Below the effective Curie temperature of the ferromagnetic layer the current is mainly confined in the non-magnetic layer which heats with greater efficiency due to better resistive and impe­dance characteristics. In a second embodiment a further non-magnetic, low-resistance layer is added to the opposite surface of the ferromagnetic material. Here the majority of the current is switched from the high-­resistance to the low-resistance layer as the heater approaches effective Curie. By these means impedance matching circuit losses can be substantially reduced and energy is saved in high power systems based on the power factor.

Magnetic properties of a series of novel ternary intermetallics (RFe10V2)

Abstract: We have studied the magnetic properties of a series of novel ternary compounds of the composition RFe 10 V 2 (R ≡ Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y) X-ray diffraction showed that these materials crystallize in the tetragonal ThMn 12 structure The Curie temperature falls into the range 483 to 616 K From high-field magnetic measurements made at 42 K on aligned powders it was derived that the iron sublattice anisotropy is about 4 T, while the rare earth sublattice anisotropy is comparatively low in most of the compounds studied

The Curie temperature of the ferromagnetic transition metals and their compounds

Abstract: The Curie temperature of Fe, Co and Ni as well as of their compounds with Y (including Y2Fe14B) are calculated by including the effects of spin fluctuations in the long wavelength limit via a renormalisation of Landau coefficients. The numerical values depend on the use of recent calculations of correlation effects and of the band structure. As a result, good agreement is found with the experimental values of TC of the materials covered.

Pressure effect on the magnetic transition temperatures in the intermetallic compounds Mn3MC (M=Ga, Zn and Sn)

Abstract: The pressure effect on the magnetic transition temperatures of Mn 3 GaC, Mn 3 ZnC and Mn 3 SnC were measured under pressures up to 13 kbar. The pressure derivatives of the transition temperatures are obtained to be d T t /d P =-3.5 K/kbar and d T C /d P =0.4 K/kbar for Mn 3 GaC, d T t /d P =-0.9 K/kbar and T C /d P =0 for Mn 3 ZnC and d T C /d P =-1.9 K/kbar for Mn 3 SnC, where T t and T C are the magnetic order-order transition temperature and the Curie temperature, respectively. An intermediate magnetic phase (I) is induced in Mn 3 GaC under pressures above 3 kbar. The triple point (Antiferro./I/Ferro.) was determined to be T c =158 K and P c =3 kbar. The temperature variations of magnetization, magnetic susceptibility and latlice parameter were also measured.

Curie Temperatures and Tricritical Points in Mixed Ising Ferromagnetic Systems

Abstract: The critical behavior of a mixed ferromagnetic Ising spin system consisting of spin\(-\tfrac{1}{2}\) and spin-1 with a crystal-field interaction is investigated by the use of the effective-field theory with correlations. The general expressions for evaluating the Curie temperature and the tricritical point are obtained. We find that the tricritical point exists in the system with Z >3, where Z is the coordination number.

The electrical properties of ferroelectric LiTaO3 and its solid solutions

Abstract: ac impedance techniques have been used to characterize ceramic samples of LiTaO3 and the solid solutions Li1−5xTa1+xO3 as a function of temperature in the range 350–800 °C. LiTaO3 has a low conductivity and the principal charge carriers appear to be electrons. At temperatures well removed from its ferroelectric Curie temperature, TC, 640 °C, a single arc or semicircle appears in the complex impedance plane representation of the ac impedance data and is attributed to the electronic resistance of the sample in parallel with the geometric capacitance. At temperatures close to TC an additional semicircle appears. This is attributed to a circuit element that includes the high capacitance values associated with the ferroelectric polarization processes. A method for extracting capacitance values from such data is given. It is shown that, under certain circumstances, fixed frequency measurements of capacitance can give rise to incorrect estimates of both TC and the magnitude of the capacitance as a function of te...

Critical evaluation of the thermodynamic properties of cobalt

Abstract: The thermodynamic properties and the pressure-temperature phase diagram of Co have been evaluated from experimental information using thermodynamic models for the Gibbs energy of the various phases. For hcp and fcc Co the model describes the magnetic contribution to the molar volume, expansivity, and compressibility and the efect of pressure upon the Curie temperature. Experimental data of many different types are satisfactorily described by the evaluated model parameters.

Magnetic properties of novel epitaxial films (invited)

Abstract: The surface magneto‐optic Kerr effect (SMOKE) is used to explore the magnetism of ultra thin Fe films extending into the monolayer regime. Both bcc α‐Fe and fcc γ‐Fe single‐crystalline, multilayer films are prepared on the bulk‐terminated (1×1) structures of Au(100) and Cu(100), respectively. The characterizations of epitaxy and growth mode are performed using low‐energy electron diffraction and Auger electron spectroscopy. Monolayer range Fe/Au(100) is ferromagnetic with a lower Curie temperature than bulk α‐Fe. The controversial γ‐Fe/Cu(100) system exhibits a striking, metastable, surface magnetic phase at temperatures above room temperature, but does not exhibit bulk ferromagnetism.

Thermomagnetic recording method applying power modulated laser on a magnetically coupled multi-layer structure of perpendicular anisotropy magnetic film

Abstract: A thermomagnetic recording method is disclosed using a thermomagnetic recording medium where first, second and third magnetic thin films each formed of rare-earth and transition metals are sequentially superposed to form layers in a magnetically coupled manner. The recording is carried out by heating the thermomagnetic recording medium under a predetermined magnetic field perpendicular to the film plane thereof while selectively modulating, in accordance with data to be recorded, a first heating state at a first temperature T1 substantially above the Curie point Tc1 of the first magnetic thin film and adequate to hold the sublattice magnetization of the transition metal of the second magnetic thin film in a predetermined direction, and a second heating state at a second temperature T2 substantially above the Curie point Tc1 and adequate to invert the sublattice magnetization of the transition metal of the second magnetic thin film to the reverse of the predetermined direction. In a cooling step subsequent to the first and second heating states, maintaining the sublattice magnetization of the third magnetic thin film in a predetermined direction while orienting the sublattice magnetization of said second magnetic thin film to be directionally coincident with that of the third magnetic thin film at a temperature below the first temperature T1 without directionally inverting the sublattice magnetization of the first magnetic thin film.

Effects of electron irradiation and annealing on ferroelectric vinylidene fluoride-trifluoroethylene copolymers

Abstract: It is shown that the Curie point of the ferroelectric vinylidene fluoride‐trifluoroethylene copolymer can be shifted to the low temperatures by electron radiation. This effect was studied by in situ dielectric measurements as a function of the dose and of the irradiation temperature. An additional effect of the irradiation, in particular when performed at high temperature, was to noticeably decrease e’. We also have studied annealing at temperatures around the melting point and shown that most of the irradiation damage was nonreversible and cumulative whereas part of it could be annealed. We have shown that it was apparently impossible to shift the Curie temperature lower than a limit temperature of the order of 0 °C. We discuss this effect and suggest the hypothesis of a competition with the glassy transition occuring in this temperature region.

Magnetic phase diagram of disordered Ni-Mn near the multicritical point.

Abstract: From detailed magnetization data taken under various field-cooling conditions, the magnetic phase diagram of temperature versus composition is determined for disordered Ni-Mn of Mn concentration (x) near 25 at. %. With increasing x, the ferromagnetic Curie-point (T/sub c/) line descends and meets the ascending line for the spin-glass reentrance temperature (T/sub f//sub g/) at a multicritical point (MCP) located at x = 23.9, T = 102 K, from which the spin-glass freezing-temperature (T/sub g/) line emerges and reaches a maximum at higher x. The reentrant spin-glass (SG) ordering is accompanied by a net ferromagnetic (FM) moment, thus describing a mixed ferro-spin-glass (FSG) state, which is separated from the normal SG state by a boundary line that extends essentially vertically down in temperature from the MCP. Moreover, it is shown that the SG ordering of the FSG state probably persists into the FM regime above T/sub f//sub g/, where T/sub f//sub g/ (like T/sub g/) remains defined operationally by the appearance of irreversible and time-dependent magnetic effects.

Magnetic and dielectric properties of the spinel ferrite system Ni0.65Zn0.35Fe2−xMnxO4

Abstract: The recent development of a high‐magnetization nickel–zinc ferrite with stress‐insensitive square hysteresis loop achieved through substitution of Mn3+ ions has prompted a study of the family Fe3+065Zn2+035 [Ni2+065Fe3+135−xMn3+x]O4 The magnetic and dielectric properties of compositions with x ranging from 0 to 04 were measured to determine the effects of the manganese substitutions over this concentration range As anticipated, the saturation magnetizations decreased gradually with increasing x because of reduced magnetic moments in the B sublattice Of greater importance, however, was the observation that maximum hysteresis loop squareness and minimum stress sensitivity occurred with x∼02 A single‐ion magnetostriction model is used to interpret this result Since the electrical resistivity remained above 106 Ω cm and the dielectric loss tangent stayed below 10−3 for the entire series, Fe2+ ion formation is probably negligible

Strain induced martensite formation in stainless steel

Abstract: The Conversion Electron and X-ray Mossbauer studies of the surface of Type 316 stainless steel at 400 K, 300 K, and 100 K show that both the substitutional and interstitial elements perturb the cubic symmetry at the iron site. The single peak of austenite is a superposition of at least five quadrupole split doublets whose magnitudes and intensities depend on the type and concentration of the impurity elements. However, when the surface of the stainless steel is plastically deformed, a layer of martensite about 5000 A thick is formed on the austenite base. This layer consists of a mixture of 31 pct martensite with the rest being the original austenite. The magnetic environment of the iron in this martensite is controlled by the concentration of alloying elements, and the distribution of the hyperfine fields is determined by the number of nearest and next nearest neighbor impurity atoms. The magnetic field decreases linearly at first as the number of nearest neighbors increases and then follows a nonlinear trend for a number of nearest neighbors. The temperature dependence of the sublattice magnetization is different for each number of neighbors, and a Curie temperature has been estimated for each site.

Magnetic and magneto‐optical properties of Tb‐Fe‐Co amorphous films

Abstract: Characteristics of Tb‐Fe‐Co amorphous films have been systematically investigated in order to optimize compositions suitable for magneto‐optical recording medium with high readout signal. In order to optimize alloy composition, Curie temperature TC and compensation temperature Tcomp are summarized as functions of both Tb and Co contents. Crystallization temperature Tx of Tb‐Fe‐Co amorphous films has been also measured to be about 400 °C and tends to increase with replacing Fe by either Co or Tb. To evaluate Tb‐Fe‐Co films as a magneto‐optical recording medium, carrier C and modulation noise Nmod (defined by the difference in noise before and after writing) are measured as a function of external field Hex. It is found that written bits of large Nmod show an irregular shape, while those of small Nmod are of regular one. To reduce Nmod and obtain high readout C/N, films with lower saturation magnetization Ms (that is, low demagnetization field) at just below writing temperature are preferred.

Structural and magnetic properties of RE2Fe14BH(D)x; REY, Ce, Er

Abstract: The structural and magnetic characteristics of RE 2 Fe 14 BH x hydrides with RE ≡ Y, Ce, Er have been studied as a function of x . Both Curie temperature and magnetization are increased on hydriding the ternary bondes. Neutron diffraction experiments performed on deuterated samples have permitted the location of the host sites and the measurement of the magnetic moments of iron and RE. Different anomalies reported in the structural and magnetic behaviour of Ce 2 Fe 14 BH x are related to the change of electronic state on cerium.

Theoretical temperature and grain-size dependence of domain state in X = 0.6 titanomagnetite

Abstract: Domain state calculations have been made for x = 0.6 titanomagnetite (TM60) as a function of grain size (a), temperature (T), stress (σ), and exchange constant (A), based on the equilibrium domain models of Amar and Kittel. Grains were assumed to be rectangular parallelepipeds, containing a simple array of uniformly spaced domains separated by planar, 180° Bloch walls, in zero magnetic field. To investigate the effects of residual stress upon domain number N, the domain wall energy was given in terms of either magnetocrystalline or uniaxial stress anisotropy. The effects of temperature upon N were modeled through the thermal variation of the material constants of TM60 which described magnetostatic (saturation magnetization) and domain wall (magnetocrystalline, magnetostriction, and exchange) energies. Calculations confirmed that both the Amar and Kittel models yielded very similar results at room temperature, regardless of whether stress or magnetocrystalline anisotropy was dominant. Rapid divergence between the two models occurred only close to the Curie temperature. Thus, significant discrepancies which have been noted between the predicted number of domains and the observed number of domains are not due to a lack of refinement in previous models, but must reflect uncertainties of a more fundamental nature. Systematic failure of particles to achieve absolute energy minimum states may not be sufficient by itself to explain this discrepancy. Higher levels of residual stress or lower values of the exchange constant, or both, may be necessary in order to reconcile theory with observation. The thermal models predicted that N will either increase or decrease, with heating, according to whether the wall energy falls more or less rapidly than the magnetostatic energy with temperature. Furthermore, the thermal dependence of N should be accounted for in models of thermoremanent magnetization. A simple model for calculating domain blocking temperatures (Tdb) was developed and predicted that (1) particles governed by magnetocrystalline anisotropy had Tdb values which were truncated well below the Curie temperature, while those particles governed by stress anisotropy had much higher Tdb values and (2) in all cases, very large multidomain grains had distributed Tdb values, while particles in the two- and three-domain range, had higher and more discrete Tdb values. Domain calculations also confirmed that TM60 particles should exhibit less thermal sensitivity in their domain structure when controlled by stress anisotropy than when controlled by magnetocrystalline anisotropy. This may help to explain recent domain observations on natural TM60, which suggested that few domain state changes occurred with heating. The model results also predicted that the exchange constant in TM60 should vary with temperature approximately as Ms1−3.

Beneficial effect of Co substitution on the magnetic properties of rapidly quenched Nd‐Fe‐B

Abstract: Isotropic Nd15(Fe1−xCox)77B8 ribbons are produced by conventional melt spinning. For moderate Co additions the coercivity HcI increases while the remanence Jr remains almost constant. Co contents above x=0.3 lead to decreasing HcI and Jr values. The beneficial effect of Co on coercivity is ascribed to the occurrence of a Nd‐Fe‐Co minority phase which serves as pinning center for domain walls. The temperature characteristic of the coercivity and the remanence is substantially improved due to the increased Curie temperature Tc and the pinning‐type behavior of the material.