Showing papers on "Curie temperature published in 1998"

Direct evidence for a half-metallic ferromagnet

Abstract: Half-metallic materials are characterized by the coexistence of metallic behaviour for one electron spin and insulating behaviour for the other. Thus, the electronic density of states is completely spin polarized at the Fermi level, and the conductivity is dominated by these metallic single-spin charge carriers. This exotic physical property could have a significant effect on technological applications related to magnetism and spin electronics. Some ferromagnetic systems, such as Heusler compounds1 and chromium dioxide2, have been predicted theoretically to be half-metallic. However, a half-metallic system has not been demonstrated directly and the predictions are still in doubt3,4. Here we report spin-resolved photoemission measurements of a ferromagnetic manganese perovskite, La0.7Sr0.3MnO3, which directly manifest the half-metallic nature well below the Curie temperature. For the majority spin, the photoemission spectrum clearly shows a metallic Fermi cut-off, whereas for the minority spin, it shows an insulating gap with disappearance of spectral weight at ∼0.6 eV binding energy.

Magnetic phase transitions and the magnetothermal properties of gadolinium

Abstract: A study of four Gd samples of different purities using ac susceptibility, magnetization, heat capacity, and direct measurements of the magnetocaloric effect in quasistatic and pulse magnetic fields revealed that all techniques yield the same value of the zero-field Curie temperature of 294(1) K. The Curie temperature determined from inflection points of the experimental magnetic susceptibility and heat capacity is in excellent agreement with those obtained from the magnetocaloric effect and Arrot plots. Above 2 T the temperature of this transition increases almost linearly with the magnetic field at a rate of $\ensuremath{\sim}6\mathrm{K}/\mathrm{T}$ in fields up to 7.5 T. The spin reorientation transition, which occurs at 227(2) K in the absence of a magnetic field, has been confirmed by susceptibility, magnetization, and heat-capacity measurements. Magnetic fields higher than 2--2.5 T apparently quench the spin reorientation transition and Gd retains its simple ferromagnetic structure from the ${T}_{C}(H)$ down to $\ensuremath{\sim}4\mathrm{K}.$ The nature of anomaly at $T\ensuremath{\cong}132\mathrm{K},$ which is apparent from ac susceptibility measurements along the $c$ axis, is discussed. The presence of large amounts of interstitial impurities lowers the second-order $\mathrm{paramagnetic}\ensuremath{\leftrightarrow}\mathrm{ferromagnetic}$ transition temperature, and can cause some erroneous results in the magnetocaloric effect determined in pulsed magnetic fields. The magnetocaloric effect was studied utilizing the same samples by three experimental techniques: direct measurements of the adiabatic temperature rise, magnetization, and heat capacity. All three techniques, with one exception, yield the same results within the limits of experimental error.

Ferromagnetic resonance of ultrathin metallic layers

Abstract: The contribution that the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behaviour of ultrathin single films is reviewed. Experimental methods to measure FMR in situ in ultrahigh vacuum are presented. The temperature dependence of the magnetization, of the magnetic relaxation rate in the vicinity of the Curie temperature, and of the second- and fourth-order magnetic anisotropy energy (MAE) constants can be measured by FMR in situ for magnetic monolayers. Using the cases of Ni/Cu(001) and Gd/W(110) as examples, the role of the MAE for the quantitative description of temperature- and thickness-dependent reorientation transitions of the magnetization is discussed. Initial results for the anisotropy of the g-factor which is related to the anisotropy of the orbital moment (and the MAE) are presented.

Domain patterns in epitaxial rhombohedral ferroelectric films. I. Geometry and experiments

Abstract: Possible domain patterns are developed for (001) oriented (pseudocubic indexing) epitaxial rhombohedral perovskite ferroelectric (FR) films. We assume that the films are grown above their Curie temperature (TC) in a cubic paraelectric (PC) state. The rhombohedral distortion consists of a “stretch” along one of the four 〈111〉 crystallographic directions of the cubic perovskite unit cell. Domain pattern formation is concurrent with the PC→FR transformation on cooling from the growth temperature. The domain patterns form to minimize elastic energy in the film, at the energetic expense of both forming domain boundaries and developing local stresses in the substrate. Eight possible domains may form, half of which are related by inversion, thus leading to four mechanically distinct variants. The possible domain walls are determined by mechanical and charge compatibility and follow closely from the analysis of Fousek and Janovec [J. Appl. Phys. 40, 135 (1969)]. Domain patterns may develop with either {100} or {1...

Direct measurement of strain effects on magnetic and electrical properties of epitaxial SrRuO3 thin films

Abstract: By lifting an epitaxial thin film off its growth substrate, we directly and quantitatively demonstrate how elastic strain can alter the magnetic and electrical properties of single-domain epitaxial SrRuO3 thin films (1000 A thick) on vicinal (001) SrTiO3 substrates. Free-standing films were then obtained by selective chemical etching of the SrTiO3. X-ray diffraction analysis shows that the free-standing films are strain free, whereas the original as-grown films on SrTiO3 substrates are strained due to the lattice mismatch at the growth interface. Relaxation of the lattice strain resulted in a 10 K increase in the Curie temperature to 160 K, and a 20% increase in the saturation magnetic moment to 1.45 μB/Ru atom. Both values for the free-standing films are the same as that of the bulk single crystals. Our results provide direct evidence of the crucial role of the strain effect in determining the properties of the technologically important perovskite epitaxial thin films.

Crystal growth and low coercive field 180° domain switching characteristics of stoichiometric LiTaO3

Abstract: We grew LiTaO3 single crystals with a composition close to stoichiometry by using a double crucible Czochralski method. The switching field required for 180° ferroelectric domain reversal and the internal fields originating from nonstoichiometric point defects were compared for the stoichiometric and conventional commercially available crystals. The switching fields for the domain reversal in the stoichiometric crystal with a Curie temperature of 685 °C was 1.7 kV/mm. This is about one thirteenth of the switching field required for the conventional LiTaO3 crystals with a Curie temperature near 600 °C. The internal field in the stoichiometric crystal drastically decreased to 0.1 kV/mm.

Adiabatic spin dynamics from spin-density-functional theory: Application to Fe, Co, and Ni

Abstract: The adiabatic theory of spin-density waves is developed on the basis of spin-density-functional theory. The wave-number-dependent exchange constant matrix is obtained from spin-density-functional calculations with constrained moment directions. The central assumption considers a fast electronic and a slow magnetic time scale, and postulates negligible correlation of the fast motion between different ionic sites. The parameter-free calculated magnon spectra for Fe, Co, and Ni are in excellent agreement with available experimental data. In the case of Fe, they show strong Kohn anomalies. Using Planck statistics at low temperature, the temperature dependence of the magnetization is well described up to half the Curie temperature. It is conjectured that correlated local-moment clusters survive the Curie transition. On this basis, calculated Curie temperatures are obtained within $10%$ deviation from experiment for Fe and Co, but $30%$ to low for Ni.

Tunable magnetocaloric effect in ceramic perovskites

Abstract: A large entropy variation (magnetocaloric effect) has been discovered in ceramic perovskites with the formulas La0.65Ca0.35Ti1−xMnxO3−z and La0.5+x+yLi0.5−3yTi1−3xMn3xO3−z. Both Curie temperature and entropy change were studied from 4.2 to 400 K for different stoichiometric compositions and applied magnetic fields. Our conclusion is that these materials are excellent candidates for working materials in magnetic refrigeration and liquefaction devices in a wide temperature range.

Band structure theory of magnetism in 3d-4f compounds

Abstract: The band structure theory of magnetism in 3d-4f compounds is reviewed. Among the open-shell electrons, a hierarchy of interactions is present which governs the intrinsic magnetic properties of these materials. Density functional theory (DFT) is an appropriate tool to describe and quantitatively investigate both ground state properties and model interaction parameters, which are necessary to calculate the temperature-dependent behaviour. The cornerstones of the DFT are explicitly sketched, as well as recent developments needed to cope with the treatment of localized 4f and itinerant 3d magnetism in one and the same framework. This includes the open-core scheme, self-interaction corrected DFT, relativistic DFT, and orbital polarization. On this basis, the exchange coupling among itinerant and localized states can be understood, together with the size of Curie temperature and ground state spin and orbital magnetic moments. Finally, the problem of magnetocrystalline anisotropy is addressed, concerning both the band and the 4f crystal field contribution.

New bulk amorphous Fe–(Co,Ni)–M–B (M=Zr,Hf,Nb,Ta,Mo,W) alloys with good soft magnetic properties

Abstract: We have found that an amorphous phase with a wide supercooled liquid region reaching 85 K before crystallization is formed in Fe–(Co, Ni)–(Zr, Nb, Ta)–B, Fe–Co–(Zr, Nb)–(Mo, W)–B and Co–Fe–Zr–B systems. The high stability of the supercooled liquid enabled the production of bulk amorphous alloys with diameters up to 5 mm by copper mold casting. These amorphous Fe–(Co, Ni)–M–B alloys exhibit good soft magnetic properties, i.e., saturation magnetization of 0.95 to 1.1 T, low coercivity of 1 to 8 A/m, Curie temperature of 560 to 590 K and low magnetostriction of 8–14×10−6. The effective permeability of the Co–based alloys exceeds 25 000 at 1 kHz and keeps high values above 5000 at the high frequency of 1 MHz. The permeability at 1 MHz is much higher than those for any kinds of soft magnetic materials. The frequency at which the imaginary part of permeability shows a maximum is also about 1 MHz. The success of synthesis of new Fe- and Co-based amorphous alloys with good soft magnetic properties and high glass-...

Phase transitions in ferrimagnetic and ferroelectric ceramics by ac measurements

Abstract: Ac conductivity measurements were carried out on polycrystalline samples of a ferrimagnetic spinel (Zn0.44Mn0.56Fe2O4) and a ferroelectric perovskite (Sr0.25Bi4Ti3.25O12.75), in the temperature range 20–160 and 20–660 °C, respectively, and in the frequency range 5 Hz–13 MHz. The impedance response in both cases could be resolved into two contributions, associated with the bulk (grains) and the grain boundaries. An analysis by means of the ac conductivity power law showed evidence of a critical temperature of 132 and 536 °C, for the ferrimagnetic and the ferroelectric samples, respectively, which corresponds to the Curie temperature for each type of material. These results are interpreted in terms of the disorder increase approaching the phase transition.

Structure and magnetic order of EuB 6

Abstract: We present a study of the structural and magnetic properties of single-crystalline EuB{sub 6}. Temperature-dependent x-ray diffraction found no significant anomalies at the onset of ferromagnetic order. The resistivity, dc susceptibility, magnetization, and specific heat prove the crystal to be of extraordinarily high quality. Two ferromagnetic transitions at T{sub c1}=15.3 K and T{sub c2}=12.5 K are observed in all investigated properties. The ordered state displays an unexpected anisotropy, and we derive the magnetic phase diagrams for the three principal cubic directions. We argue that the two magnetic phases are connected by spin reorientation, enabled by a reduction of the crystalline symmetry from cubic. An additional anomaly in the specific heat is observed at low temperatures, which we interpret as arising from the splitting of the Eu ground-state multiplet in internal magnetic fields. {copyright} {ital 1998} {ital The American Physical Society}

Metal–insulator transition induced by oxygen isotope exchange in the magnetoresistive perovskite manganites

Abstract: Perovskite manganites derived from LaMnO3 have recently become the subject of intensive study following the discovery of ‘colossal’ magnetoresistance (a magnetically induced change in electrical resistance of up to several orders of magnitude) in several members of this family of compounds1. The manganites exhibit a broad range of electronic and magnetic phases, ranging from low-resistance ferromagnetic metals to high-resistance insulators, which are extremely sensitive to variation of composition2, temperature and pressure3. A recent study showed that such sensitivity also extends to oxygen isotope exchange4: replacing 16O with 18O in La0.8Ca0.2MnO3 produces an unusually large change in the magnetic properties (a 21-kelvin decrease in the Curie temperature). The magnitude of this isotope shift is evidence for the essential role played by electron–phonon coupling5 in determining the transport properties of these materials. Here we show that this sensitivity to oxygen isotope exchange can be even more extreme. In its normal state, the compound La0.175Pr0.525Ca0.3MnO3 undergoes an insulator-to-metal transition as it is cooled below ∼95 K. But we find that, after substituting 18O for 16O, the compound remains an insulator down to 4.2 K, so providing a vivid demonstration of the importance of lattice vibrations in these materials.

Dielectric properties of epitaxial BaTiO3 thin films

Abstract: The dielectric response of epitaxial BaTiO3 thin films deposited on MgO was measured through surface electrodes as a function of applied bias, frequency, and temperature. The room temperature value of the dielectric constant was ∼500 with a dissipation factor, tan(δ), of 0.05 at 100 kHz. Measurements varying the bias field showed hysteresis of the dielectric response and a tunability of 30% for a maximum applied field of ∼7 MV/m. The frequency response of the dielectric constant is well described by a Curie–von Schweidler power law with an exponent ∼0.04 in the range 1 kHz–13 MHz. The films undergo a diffuse phase transition at temperatures higher than the bulk Curie temperature. The behavior of the dielectric response is attributed to the presence of residual strain in the epitaxial thin films.

Effect of annealing on the structure and magnetic properties of graphite encapsulated nickel and cobalt nanocrystals

Abstract: We report the structure and magnetic studies of carbon coated nanocrystals of nickel and cobalt synthesized in a special low carbon to metal ratio arc chamber. Powder x-ray diffraction profiles show peaks associated with single phase of fcc nickel or cobalt and major peaks of graphite with no evidence of carbides or solid solutions of carbon in the metal. Measured lattice spacing of crystalline particles and that of graphite coating from high-resolution transmission electron microscope images also confirm such findings. Magnetization measurements as a function of temperature in the range 20–900 °C give a Curie temperature equal to that of bulk metal within the experimental error. Upon heating and recooling of the particles a larger magnetization as high as 57% of bulk Co and 53% of bulk Ni was measured. Also M–H hysteresis loop of the particles have been measured at room temperature after annealing in the temperature range 20–650 °C for Ni, and 20–900 °C for Co. The dependence of room temperature saturation magnetization, remanent magnetization, and coercive field of the particles on annealing temperature is reported. These data are described by transition of particles form single domain to multidomain as a result of particle growth due to annealing. We also present the particle size distribution measurements that show log-normal behavior, and indicate substantial particle size growth due to annealing.

The correlation of the magnetic properties and the magnetocaloric effect in (Gd1−xErx)NiAl alloys

Abstract: A study of the magnetic properties of several (Gd1−xErx)NiAl alloys (where x=0, 0.30, 0.40, 0.46, 0.50, 0.55, 0.60, 0.80, and 1.00) was undertaken using both ac and dc magnetic and heat capacity measurements in an attempt to understand the table-like magnetocaloric effect previously observed in (Gd0.54Er0.46)NiAl. Results indicate the presence of both antiferromagnetic and ferromagnetic ordering processes in all alloys containing Gd. For ErNiAl, a metamagnetic transition from an antiferromagnetic ground state was observed. Within each alloy, several magnetic transitions occur over a temperature range from 10 K [in (Gd0.20Er0.80)NiAl] up to 35 K (in GdNiAl), with all but the lowest temperature transition shifting to higher temperatures with increasing Gd content. The change in magnetic entropy (ΔSmag) induced by a change in field is observed to peak around the Neel temperature for ErNiAl while gradually broadening and shifting toward the Curie temperature as the Gd content is increased. For Gd-rich alloys,...

Sm2(Co,Fe,Cu,Zr)17 magnets for use at temperature ⩾400 °C

Abstract: Sintered Sm2(Co,Fe,Cu,Zr)17 magnets with improved high temperature performance have been obtained by reducing the iron content in the magnet alloys. A record intrinsic coercive force, IHC, of 8.3 kOe at 400 °C was obtained when the iron content was decreased to 7 wt%. At 400–600 °C, 2:17 magnets with low iron content have demonstrated lower irreversible loss of magnetic flux, higher maximum energy product, and lower temperature coefficient of IHC. A temperature coefficient of IHC=−0.12%/°C (20–400 °C) was obtained for the low Fe magnets, compared to −0.23%/°C for commercial 2:17. Reducing iron content increases both Curie temperature and anisotropy field. Therefore, it is anticipated that new 2:17 magnet materials capable of operating at 400 °C or higher temperatures can be developed by reducing or eliminating the iron content and making other adjustments in composition and heat treatment.

Possible mechanisms causing failure of Thellier palaeointensity experiments in some basalts

Abstract: SUMMARY The normally magnetized zone of the Jurassic Lesotho basalts, although providing apparently quite reliable palaeofield directions (Kosterov & Perrin 1996), shows anomalous behaviour when studied in vacuum using the Thellier palaeointensity method: typically the slope of the natural remanent magnetization‐thermoremanent magnetization (NRM‐TRM) curves is very steep at intermediate temperatures (200 to 400‐460°C). In order to elucidate the reasons for such an anomalous behaviour, six representative samples (from a total of 74 studied using this method) were subjected to a variety of analyses. These experiments indicate that the magnetic properties are dominated by pseudo-single-domain (PSD) magnetite grains some 1 mm in size, resulting from high-temperature oxidation of titanomagnetite. Laboratory heatings in vacuum up to the Curie point do not change significantly the roomtemperature hysteresis characteristics or the initial susceptibility k. Similarly, the k(T ) curves in vacuum are (with a single exception) rather reproducible. Since the laboratory TRMs yield almost ideal NRM‐TRM plots, the anomalous NRM‐TRM plot is presumably due to some peculiarity of the natural TRM. The partial TRM (pTRM) acquisition capacity in the moderate temperature range (cooling from 200 to 20°C) is generally very strongly reduced after heating to 270°C, which indicates that some magnetic alteration has already occurred at these temperatures. Hysteresis measurements between room temperature and the Curie temperature T c show that some small (less than 10 per cent) but significant irreversible changes in hysteresis characteristics also occur during heating. In particular, the coercive force H c0 at room temperature is typically reduced after heating at a moderate temperature (175°C) but increases after treatments at 475°C and, more pronouncedly, at 580°C. The saturation magnetization J s0 remains unchanged, except for a very small decrease (less than 5 per cent) occurring in some samples after the two latter treatments. These changes are most clearly seen on H c (T )‐J s (T ) bilogarithmic plots, which show that the moderate-temperature change in coercivity can extend up to 200‐250°C. Thus hysteresis measurements as a function of temperature oVer a promising tool for sample pre-selection for Thellier experiments. Alternating-field demagnetization and cycling of pTRMs at liquid-nitrogen temperature suggest that the blocking mechanism is largely multidomain-like near room temperature but becomes less so as the Curie point is approached. The main reason for the failure of the Thellier experiments is the loss of a fraction of the NRM (natural TRM) at temperatures apparently lower than the blocking temperatures in nature. It is suggested that this anomalous behaviour results from the reorganization of the domain structure of the PSD grains during heating. This transformation, which seems to be triggered by the coercivity decrease observed at very moderate temperatures, can reduce the NRM intensity without requiring any correlated pTRM acquisition.

Ferromagnetism in the Hubbard model on fcc-type lattices

Abstract: The Hubbard model on fcc-type lattices is studied in the dynamical mean-field theory of infinite spatial dimensions. At intermediate interaction strength finite temperature Quantum Monte Carlo calculations yield a second order phase transition to a highly polarized, metallic ferromagnetic state. The Curie temperatures are calculated as a function of electronic density and interaction strength. A necessary condition for ferromagnetism is a density of state with large spectral weight near one of the band edges.

Softening and Broadening of the Zone Boundary Magnons in Pr0.63Sr0.37MnO3

Abstract: We have studied the spin dynamics in Pr0.63Sr0.37MnO3 above and below the Curie temperature T-C = 301 K. Three distinct new features have been observed: a softening of the magnon dispersion at the zone boundary for T T-C, and no evidence for residual spin-wave-like excitations just above T-C. The results are inconsistent with double exchange models that have been successfully applied to higher T-C samples, indicating an evolution of the spin system with decreasing T-C.

Field-induced strain under load in Ni–Mn–Ga magnetic shape memory materials

Abstract: Single-crystal Ni2MnGa shows a nearly 0.2% strain under a magnetic field of 8 kOe at −8 °C. Polycrystalline samples have been prepared near stoichiometry to study the composition dependence of the magnetic and elastic properties. A narrow band of compositions has been found having a range of Curie and martensitic transformation temperatures, Tc and T0, extending to above room temperature. The compressive stress–strain characteristics in variable transverse field were studied in samples selected to have T0 just below room temperature. Stress-induced martensite was observed as expected and the magnetic field was applied under fixed load for various stresses. A transverse field of 3200 Oe caused the sample to strain under load doing work that increased up to 1.3 J/kg with increasing volume fraction of stress-induced martensite.

Magnetic moments and Curie temperatures of Ni and Co thin films and coupled trilayers

Abstract: We present x-ray magnetic circular dichroism (XMCD) measurements on single layers, bilayers, and trilayers consisting of Ni, Co, and Cu. Using XMCD sum rules, spin and orbital contributions to the total magnetic moment were determined for 2.1 ML Co and 4 ML Ni single layers on Cu(001). In accordance with theory for Co, both spin and orbital moments show an enhancement. In the case of Ni, both spin and orbital moments were reduced. The total magnetic moment of the 4 ML Ni film reduces by a factor of 2 as compared to the Ni bulk. The Curie temperature of both Ni and Co were found to reduce upon capping with Cu. In trilayers, the shape of the Ni magnetization curves were found to be influenced through exchange coupling due to the presence of Co.

Neutron-Diffraction Studies on the Magnetic Ordering Process in the Layered Mn Perovskite La2-2xSr1+2xMn2O7 (x = 0.40, 0.45 and 0.48).

Abstract: Comprehensive neutron-diffraction studies on La 2-2 x Sr 1+2 x Mn 2 O 7 ( x =0.40,0.45 and 0.48) single crystals have revealed that the low-temperature magnetic phase consists of planar ferromagnetic (FM) and A-type antiferromagnetic (AFM) components, indicating a canted AFM ordering. Upon increasing the hole concentration x , the canting angle between planes changes from 6.3° (nearly planar FM) at x =0.40 to 180° (A-type AFM) at x =0.48, while the ordering temperature of the FM component, T C , decreases from 120 K to 0 K, correspondingly. We have also discovered that the A-type AFM ordering remains above T C and shows an anomalous exponential decrease to T N ∼200 K. This newly found intermediate A-type AFM phase may play a significant role in the enhancement of CMR effects in this layered Mn perovskite system.

The influence of iron substitution on the magnetic properties of hausmannite, Mn2+(Fe,Mn)(2)(3+)O-4

Abstract: The occurrence of hausmannite with an apparent Curie temperature close to 750 K, instead of 41.8 K was recently described from hydrothermally altered manganese ore from the Kalahari manganese field, South Africa. The unusual magnetic properties were relat

Magnetic properties of REMe0.5Mn0.5O3 (RE = Rare Earth Element, Me = Ni, Co)

Abstract: Magnetic properties of REMe 0.5 Mn 0.5 O 3 with Me = Ni and Co are investigated for a series of rare earth (RE) elements. In both systems with Me = Ni and Co, the Curie temperature ( T C ), below which the magnetic moments of Me 2+ and Mn 4+ allign ferromagnetically, decreases remarkably as RE goes from La to heavier elements; from 295 K for RE = La to 50 K for Lu with Me = Ni, and from 250 K for La to 95 K for Ho with Me = Co. The decrease of T C is interpreted as due to the bond angle (θ) dependence of the superexchange interactions between Me 2+ and Mn 4+ through O 2- ; T C is roughly linear to cos 2 θ as predicted by a simple theory [C. Boekema et al. : Int. J. Mag. 3 (1972) 341], but a distinct deviation from the linear relation is observed for the late RE elements. The magnetic moments of RE ions appears only in a low temperature region, typically below 30 K. Qualitative discussion on the exchange interactions involving RE is presented.