Showing papers on "Curie temperature published in 2008"

Magnetism in ultrathin film structures

Abstract: In this paper, we review some of the key concepts in ultrathin film magnetism which underpin nanomagnetism. We survey the results of recent experimental and theoretical studies of well characterized epitaxial structures based on Fe, Co and Ni to illustrate how intrinsic fundamental properties such as the magnetic exchange interactions, magnetic moment and magnetic anisotropies change markedly in ultrathin films as compared with their bulk counterparts, and to emphasize the role of atomic scale structure, strain and crystallinity in determining the magnetic properties. After introducing the key length scales in magnetism, we describe the 2D magnetic phase transition and survey studies of the thickness dependent Curie temperature and the critical exponents which characterize the paramagnetic–ferromagnetic phase transition. We next discuss recent experimental and theoretical results on the determination of the exchange constant, followed by an overview of measurements of the magnetic moment in the elemental 3d transition metal thin films in the various crystal phases that have been successfully stabilized, thereby illustrating the sensitivity of the magnetic moment to the local symmetry and to the atomic environment. Finally, we discuss briefly the magnetic anisotropies of Fe, Co and Ni in the fcc crystalline phase, to emphasize the role of structure and the details of the interface in influencing the magnetic properties. The dramatic effect that adsorbates can have on the magnetic anisotropies of thin magnetic films is also discussed. Our survey demonstrates that the fundamental properties, namely, the magnetic moment and magnetic anisotropies of ultrathin films have dramatically different behaviour compared with those of the bulk while the comparable size of the structural and magnetic contributions to the total energy of ultrathin structures results in an exquisitely sensitive dependence of the magnetic properties on the film structure.

Strong magneto-chiral dichroism in enantiopure chiral ferromagnets

Abstract: As materials science is moving towards the synthesis, the study and the processing of new materials exhibiting well-defined and complex functions, the synthesis of new multifunctional materials is one of the important challenges. One of these complex physical properties is magneto-chiral dichroism which arises, at second order, from the coexistence of spatial asymmetry and magnetization in a material. Herein we report the first measurement of strong magneto-chiral dichroism in an enantiopure chiral ferromagnet. The ab initio synthesis of the enantiopure chiral ferromagnet is based on an enantioselective self-assembly, where a resolved chiral quaternary ammonium cation imposes the absolute configurations of the metal centres within chromium-manganese two-dimensional oxalate layers. The ferromagnetic interaction between Cr(III) and Mn(II) ions leads to a Curie temperature of 7 K. The magneto-chiral dichroic effect is enhanced by a factor of 17 when entering into the ferromagnetic phase.

Cupric oxide as an induced-multiferroic with high-TC.

Abstract: Induced multiferroics, where ferroelectricity arises through the magnetic order, have attracted significant interest, despite maximum Curie temperatures of only 40 K. The discovery of multiferroic coupling up to 230 K in CuO therefore represents a major advance towards high-TC multiferroics. Materials that combine coupled electric and magnetic dipole order are termed ‘magnetoelectric multiferroics’1,2,3,4. In the past few years, a new class of such materials, ‘induced-multiferroics’, has been discovered5,6, wherein non-collinear spiral magnetic order breaks inversion symmetry, thus inducing ferroelectricity7,8,9. Spiral magnetic order often arises from the existence of competing magnetic interactions that reduce the ordering temperature of a more conventional collinear phase10. Hence, spiral-phase-induced ferroelectricity tends to exist only at temperatures lower than ∼40 K. Here, we propose that copper(II) oxides (containing Cu2+ ions) having large magnetic superexchange interactions11 can be good candidates for induced-multiferroics with high Curie temperature (TC). In fact, we demonstrate ferroelectricity with TC=230 K in cupric oxide, CuO (tenorite), which is known as a starting material for the synthesis of high-Tc (critical temperature) superconductors. Our result provides an important contribution to the search for high-temperature magnetoelectric multiferroics.

Room temperature ferromagnetism in ZnO films due to defects

Abstract: ZnO films were prepared by pulsed laser deposition on a-plane sapphire substrates under N2 atmosphere. Ferromagnetic loops were obtained with the superconducting quantum interference device at room temperature, which indicate a Curie temperature much above room temperature. No clear ferromagnetism was observed in intentionally Cu-doped ZnO films. This excludes that Cu doping into ZnO plays a key role in tuning the ferromagnetism in ZnO. 8.8% negative magnetoresistance probed at 5K at 60kOe on ferromagnetic ZnO proves the lack of s-d exchange interaction. Anomalous Hall effect (AHE) was observed in ferromagnetic ZnO as well as in nonferromagnetic Cu-doped ZnO films, indicating that AHE does not uniquely prove ferromagnetism. The observed ferromagnetism in ZnO is attributed to intrinsic defects.

Evolution from Itinerant Antiferromagnet to Unconventional Superconductor with Fluorine Doping in LaFeAs(O1-xFx) Revealed by 75As and 139La Nuclear Magnetic Resonance

Abstract: We report experimental results of 75 As and 139 La nuclear magnetic resonance (NMR) in the iron-based layered LaFeAs(O 1- x F x ) ( x = 0.0, 0.04, and 0.11). In the undoped LaFeAsO, 1/ T 1 of 139 La exhibits a distinct peak at T N ∼142 K below which the spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the 4% F-doped sample, 1/ T 1 T exhibits a Curie–Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the 11% F-doped sample, in contrast, pseudogap behavior is observed in 1/ T 1 T both at the 75 As and 139 La site with a gap value of Δ PG ∼172 K. The spin dynamics vary markedly with F doping, which is ascribed to the Fermi-surface structure. As for the superconducting properties for the 4 and 11% F-doped samples, 1/ T 1 in both compounds does not exhibit a coherence peak just below T c and follows a T 3 dependence at low temperatures, which suggests unconventional super...

Characterization of Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 ferroelectric crystal with enhanced phase transition temperatures

Abstract: The full set of material constants for relaxor-based ternary single crystals Pb(In(12)Nb(12))O(3)-Pb(Mg(13)Nb(23))O(3)-PbTiO(3) (PIN-PMN-PT) were determined and compared to binary Pb(Mg(13)Nb(23))O(3)-PbTiO(3) (PMNT) crystals The Curie temperature for rhombohedral compositions of PIN-PMN-PT was found to be in the range of 160-200 degrees C with ferroelectric rhombohedral to tetragonal phase transition on the order of 120-130 degrees C, more than 30 degrees C higher than that found for PMNT The piezoelectric coefficients (d(33)) were in the range of 1100-1500 pCN, with electromechanical coupling factors (k(33)) about 89%-92% comparable to PMNT crystals The coercive field of the ternary crystal was found to be 55 kVcm, double the value of the binary counterparts The dielectric behavior under varying dc bias exhibited a similar trend as observed in PMNT with a much broader usage temperature range Together with its enhanced field induced phase transition level, the ternary PIN-PMN-PT crystals are promising candidates for high temperature and high drive transducer applications

Role of conduction electrons in mediating exchange interactions in Mn-based Heusler alloys

Abstract: Because of large spatial separation of the Mn atoms in Heusler alloys the Mn 3d states belonging to different atoms do not overlap considerably. Therefore an indirect exchange interaction between Mn atoms should play a crucial role in the ferromagnetism of the systems. To study the nature of the ferromagnetism of various Mn-based semi- and full-Heusler alloys we perform a systematic first-principles calculation of the exchange interactions in these materials. The calculation of the exchange parameters is based on the frozen-magnon approach. The calculations show that the magnetism of the Mn-based Heusler alloys depends strongly on the number of conduction electrons, their spin polarization and the position of the unoccupied Mn 3d states with respect to the Fermi level. Various magnetic phases are obtained depending on the combination of these characteristics. The Anderson's s-d model is used to perform a qualitative analysis of the obtained results. The conditions leading to diverse magnetic behavior are identified. If the spin polarization of the conduction electrons at the Fermi energy is large and the unoccupied Mn 3d states lie well above the Fermi level, an RKKY-type ferromagnetic interaction is dominating. On the other hand, the contribution of the antiferromagnetic superexchange becomes important if unoccupied Mn 3d states lie close to the Fermi energy. The resulting magnetic behavior depends on the competition of these two exchange mechanisms. The calculational results are in good correlation with the conclusions made on the basis of the Anderson s-d model which provides useful framework for the analysis of the results of first-principles calculations and helps to formulate the conditions for high Curie temperature.

Achieving high Curie temperature in (Ga,Mn)As

Abstract: We study the effects of growth temperature, Ga:As ratio, and postgrowth annealing procedure on the Curie temperature TC of (Ga,Mn)As layers grown by molecular beam epitaxy. We achieve the highest TC values for growth temperatures very close to the two-dimensional–three-dimensional phase boundary. The increase in TC, due to the removal of interstitial Mn by postgrowth annealing, is counteracted by a second process, which reduces TC and which is more effective at higher annealing temperatures. Our results show that it is necessary to optimize the growth parameters and postgrowth annealing procedure to obtain the highest TC.

Physical properties of Hastelloy® C-276™ at cryogenic temperatures

Abstract: In recent years, the Ni–Mo–Cr superalloy Hastelloy® C-276™ has been used as a substrate material for fabricating superconducting tapes such as YBCO and MgB2 coated conductors. With increasing piece length, these coated conductors are within reach of large scale commercial applications. However, data on the physical properties of Hastelloy C-276 at temperatures relevant for these applications are not yet available. In this work, physical properties including magnet succeptibility, specific heat, electrical resistivity, and the Seebeck coefficient are measured from 2to300K and thermal conductivity from 2to200K. Our results show that Hastelloy C-276 exhibits Curie paramagnetism between 4 and 300K with a Curie constant C=0.091K. A spin-glass-like behavior is observed below 3K. The electrical resistivity has a minimum at ∼12K, and shows a linear weak T dependence at higher temperatures. The specific heat Cp between 15 and 40K follows Cp=γT+AT3. Below ∼10K, an upturn in Cp∕T with decreasing T is interpreted by ...

Zero Thermal Expansion in PbTiO3-Based Perovskites

Abstract: PbTiO3−Bi(Zn1/2Ti1/2)O3 and related solid solutions exhibit an unusually negative thermal expansion with considerably enhanced tetragonality and Curie temperature. A zero thermal expansion is found over a wide temperature range of room temperature to 500 °C. Direct experimental evidence is given for the fact that the negative thermal expansion of PbTiO3-based solid solutions is strongly correlated with the temperature dependence of the ferroelectricity, which is a new negative thermal expansion mechanism.

Magnetostriction in Mn3CuN

Abstract: Discovery of large magnetostriction in an antiperovskite Mn3CuN is reported Mn3CuN undergoes the first-order transition from high-temperature (high-T) paramagnetic to low-temperature ferromagnetic (FM) phase at the Curie temperature TC=143K, accompanied by cubic-to-tetragonal structural deformation In the tetragonally distorted FM phase, Mn3CuN, even in a polycrystalline form, expands 02% and shrinks 01% in the direction parallel and perpendicular to the external field of 90kOe, respectively This magnetostriction is possibly due to rearrangement of thermoelastic martensite variants by magnetic field, similar to FM Heusler alloys such as Ni2MnGa

Piezoelectric properties of low-temperature sintered Li-modified (Na, K)NbO3 lead-free ceramics

Abstract: LiNbO3-doped (Na, K)NbO3 lead-free piezoceramics were prepared by conventional sintering at a temperature as low as 950 °C using excess Na2O additives. The crystal structure changed from orthorhombic to tetragonal with increasing LiNbO3 amount since the phase transition temperature TO−T shifted downward. In the region of two-phase coexistence, enhanced piezoelectric constant d33 (280 pC/N) and electromechanical coupling factor kp (48.3%) with a high Curie temperature TC (475 °C) were obtained in the nominal composition 0.92(Na0.535K0.48)NbO3–0.08LiNbO3. Our results open up the way to low-temperature sintering of (Na, K)NbO3-based lead-free piezoceramics with high performance.

Oxide Dilute Magnetic Semiconductors—Fact or Fiction?

Abstract: Magnetism in oxides was thought to be well-understood in terms of localized magnetic moments and double-exchange or superexchange rules. This understanding was shaken by the publication of an article in 2001 stating that thin films of anatase TiO2 with only 7 at.% Co substitution had a Curie point in excess of 400 K [Matsumoto et al., Science 291, 854 (2001)]. Room-temperature ferromagnetism had previously been predicted for p-type ZnO with 5 at.% Mn [Dietl et al., Science 287, 1019 (2000)]. A flood of reports of thin films and nanoparticles of new oxide “dilute magnetic semiconductors” (DMSs) followed, and high-temperature ferromagnetism has been reported for other systems with no 3d cations. The expectation that these materials would find applications in spintronics motivated research in this area. Unfortunately, the data are plagued by instability and a lack of reproducibility. In many cases, the ferromagnetism can be explained by uncontrolled secondary phases; it is absent in well-crystallized films and bulk material. However, it appears that some form of high-temperature ferromagnetism can result from defects present in the oxide films [Coey, Curr. Opin. Solid State Mater. Sci. 10, 83 (2007); Chambers, Surf. Sci. Rep. 61, 345 (2006)], although they are not DMSs as originally envisaged.

Non-volatile ferroelectric control of ferromagnetism in (Ga,Mn)As

Abstract: Diluted magnetic semiconductor devices where magnetism can be controlled by an electric field are of significant interest for applications, as they combine the appealing properties of multiferroics with existing semiconductor technology. By using a ferroelectric polymer as the gate of a transistor device, non-volatile electric control over the magnetism of (Ga,Mn)As has now been achieved. Multiferroic structures that provide coupled ferroelectric and ferromagnetic responses are of significant interest as they may be used in novel memory devices and spintronic logic elements1,2,3,4. One approach towards this goal is the use of composites that couple ferromagnetic and ferroelectric layers through magnetostrictive and piezoelectric strain transmitted across the interfaces5,6,7. However, mechanical clamping of the films to the substrate limits their response1,8. Structures where the magnetic response is modulated directly by the electric field of the poled ferroelectric would eliminate this constraint and provide a qualitatively higher level of integration, combining the emerging field of multiferroics with conventional semiconductor microelectronics. Here, we report the realization of such a device using (Ga,Mn)As, which is an archetypical diluted magnetic semiconductor with well-understood carrier-mediated ferromagnetism, and a polymer ferroelectric gate. Polarization reversal of the gate by a single voltage pulse results in a persistent modulation of the Curie temperature of the ferromagnetic semiconductor. The non-volatile gating of (Ga,Mn)As has been made possible by applying a low-temperature copolymer deposition technique that is distinct from pre-existing technologies for ferroelectric gates on magnetic oxides. This accomplishment opens a way to nanometre-scale modulation of magnetic semiconductor properties with rewritable ferroelectric domain patterns, operating at modest voltages and subnanosecond times.

Large-scale simulation of the spin-lattice dynamics in ferromagnetic iron

Abstract: combined application of the Langevin spin dynamics and the fluctuation-dissipation theorem. We investigate several applications of the method, performing microcanonical ensemble simulations of adiabatic spin-lattice relaxation of periodic arrays of 180° domain walls, and isothermal-isobaric ensemble dynamical simulations of thermally equilibrated homogeneous systems at various temperatures. The predicted isothermal magnetization curve agrees well with the experimental data for a broad range of temperatures. The equilibrium as well as time-correlation functions of spin orientations exhibit the presence of short-range magnetic order above the Curie temperature. Furthermore, short-range order spin fluctuations are shown to contribute to the thermal expansion of the material. Our analysis illustrates the significant part played by the spin degrees of freedom in the dynamics of motion of atoms in magnetic iron and iron-based alloys. It also shows that the spin-lattice dynamics algorithm developed in this paper offers a viable way of performing large-scale dynamical atomistic simulations of magnetic materials.

Martensitic phase transformation in rapidly solidified Mn50Ni40In10 alloy ribbons

Abstract: Heusler alloy Mn50Ni40In10 was produced as preferentially textured ribbon flakes by melt spinning, finding the existence of martensitic-austenic transformation with both phases exhibiting ferromagnetic ordering. A microcrystalline three-layered microstructure of ordered columnar grains grown perpendicularly to ribbon plane was formed between two thin layers of smaller grains. The characteristic temperatures of the martensitic transformation were MS=213K, Mf=173K, AS=222K, and Af=243K. Austenite phase shows a cubic L21 structure (a=0.6013(3)nm at 298K and a Curie point of 311K), transforming into a modulated fourteen-layer modulation monoclinic martensite.

Magnetism without magnetic impurities in ZrO2 oxide

Abstract: We present an ab initio study of the magnetism induced in ZrO2 dioxide by substitution of the cation by an impurity from the groups 1A or 2A of the Periodic Table (K and Ca). It is demonstrated that the K impurity leads to a robust induced magnetic moment on the surrounding O atoms in the cubic ZrO2 host whilst Ca impurity leads to a nonmagnetic groundstate. The estimated Curie temperature is above room temperature.

Ab-initio design of half-metallic fully-compensated ferrimagnets: the case of Cr2MnZ (Z= P, As, Sb, Bi) compounds

Abstract: Electronic structure calculations from first-principles are employed to design some new half-metallic fully-compensated ferrimagnets (or as they are widely known half-metallic antiferromagnets) susceptible of finding applications in spintronics. Cr2MnZ (Z= P, As, Sb, Bi) compounds have 24 valence electrons per unit cell and calculations show that their total spin moment is approximately zero for a wide range of lattice constants in agreement with the Slater-Pauling behavior for ideal half-metals. Simultaneously, the spin magnetic moments of Cr and Mn atoms are antiparallel and the compounds are ferrimagnets. Mean-field approximation is employed to estimate their Curie temperature, which exceeds room temperature for the alloy with Sb. Our findings suggest that Cr2MnSb is the compound of choice for further experimental investigations. Contrary to the alloys mentioned above half-metallic antiferromagnetism is unstable in the case of the Cr2FeZ (Z= Si, Ge, Sn) alloys.

Curie point singularity in the temperature derivative of resistivity in (Ga,Mn)As.

Abstract: We observe a singularity in the temperature derivative $d\ensuremath{\rho}/dT$ of resistivity at the Curie point of high-quality (Ga,Mn)As ferromagnetic semiconductors with ${T}_{c}$'s ranging from approximately 80 to 185 K. The character of the anomaly is sharply distinct from the critical contribution to transport in conventional dense-moment magnetic semiconductors and is reminiscent of the $d\ensuremath{\rho}/dT$ singularity in transition metal ferromagnets. Within the critical region accessible in our experiments, the temperature dependence on the ferromagnetic side can be explained by dominant scattering from uncorrelated spin fluctuations. The singular behavior of $d\ensuremath{\rho}/dT$ on the paramagnetic side points to the important role of short-range correlated spin fluctuations.

Effect of CuO on the Sintering Temperature and Piezoelectric Properties of (Na0.5K0.5)NbO3 Lead-Free Piezoelectric Ceramics

Abstract: When a small amount of CuO was added to (Na 0.5 K 0.5 )NbO 3 (NKN) ceramics sintered at 960°C for 2 h, a dense microstructure with increased grains was developed, probably due to liquid-phase sintering. The Curie temperature slightly increased when CuO exceeded 1.5 mol%. The Cu 2+ ion was considered to have replaced the Nb 5+ ion and acted as a hardener, which increased the E c and Q m values of the NKN ceramics. High piezoelectric properties of kp = 0.37, Q m = 844, and e T 3 /e 0 = 229 were obtained from the specimen containing 1.5 mol% of CuO sintered at 960°C for 2 h.

Fundamental magnetic parameters from pure synthetic greigite (Fe3S4)

Abstract: Pure ferrimagnetic greigite (Fe3S4) has been synthesized by reacting ferric chloride with thiourea and formic acid at 170°C. Sample purity was confirmed by X-ray diffraction, neutron diffraction and Mossbauer spectroscopy, coupled with magnetic measurements. Scanning electron microscope observations indicate clear cubo-octahedral and polyhedral crystal morphologies. The grain sizes are as large as 44 ?m. Detailed low- and high-temperature magnetic measurements document the previously poorly known magnetic properties of greigite. The synthetic greigite samples are dominated by pseudo-single-domain and multi-domain behavior. The saturation magnetization (M s ) at room temperature is ?59 Am2kg?1 (3.13 ? B per formula unit), which is higher than any value previously reported for greigite in the literature largely because of the high purity of this sample compared to others. No low-temperature magnetic transition has been detected; however, a local coercivity minimum is observed at around 130 K, which is probably associated with domain walls present in the studied samples. The high-temperature magnetic properties of greigite are dominated by chemical decomposition above around 250°C, which precludes determination of the Curie temperature, but our evidence indicates that it must exceed 350°C. On the basis of the Bloch spin wave expansion, the spin wave stiffness of greigite was determined for the first time as ?193 meV·A2 from low-temperature M s measurements, with the corresponding exchange constant J AB of ?1.03 meV.

Determining exchange splitting in a magnetic semiconductor by spin-filter tunneling.

Abstract: A large exchange splitting of the conduction band in ultrathin films of the ferromagnetic semiconductor EuO was determined quantitatively, by using EuO as a tunnel barrier and fitting the current-voltage characteristics and temperature dependence to tunneling theory. This exchange splitting leads to different tunnel barrier heights for spin-up and spin-down electrons, and is large enough to produce a near fully spin-polarized current. Moreover, the magnetic properties of these ultrathin films (<6 nm) show a reduction in Curie temperature with decreasing thickness, in agreement with theoretical calculation [R. Schiller et al., Phys. Rev. Lett. 86, 3847 (2001)].

Size-dependent ferroelectric behaviors of BaTiO3 nanowires

Abstract: Landau-Ginsburg-Devonshire theory is introduced to investigate the size-dependent ferroelectric properties of BaTiO3 nanowires, with the consideration of polarization orienting along the radial direction It is shown that the Curie temperature, mean polarization, and area enclosed by hysteresis loop decrease with the reducing diameter of the nanowire However, this size effect becomes obvious only when the diameter is below 20nm Above this, the ferroelectric behaviors of nanowires are almost the same as those of bulk materials Furthermore, there exists a temperature-dependent critical diameter for BaTiO3 nanowire below which the hysteresis loops vanish and it is 36nm at room temperature