Ferromagnetic Transition Temperature Tc Research Articles

NUMERICAL STUDY OF FERROMAGNETISM IN DILUTED MAGNETIC SEMICONDUCTOR QUANTUM-WELLS

We study the ferromagnetism in III-V diluted magnetic semiconductor (DMS) quantum-wells theoretically and numerically taking into account the occupation of multiple subbands by holes in quantum wells. Starting from the mean-field theory of carrier-induced ferromagnetism in III-V DMS along with the exchange-correlation interaction of holes within the local spin density approximation, we found that the ferromagnetic transition temperature Tc of DMS quantum-wells exhibits step-function-like dependence on the hole density, reflecting the quasi-two-dimensional nature of systems. Moreover, the temperature dependence of the spin polarization shows quite distinct characteristics depending on the hole density.

Zinc-blende-type MnAs nanoclusters embedded in GaAs

Structural, magnetic, and magneto-optical properties of GaAs:MnAs granular thin films have been investigated. Zinc-blende(ZB)-type MnAs clusters were found to be formed by annealing GaAs:MnAs at 500°C with a ramp rate of 5°C∕min under a nitrogen atmosphere. Magnetic and magneto-optical properties of this GaAs:MnAs granular material with ZB-type MnAs clusters were different from those of the GaAs:MnAs with NiAs-type hexagonal MnAs clusters. The ferromagnetic transition temperature TC of the GaAs:MnAs of ZB type is estimated to be 360K, which is higher than TC(=313K) of the NiAs-type hexagonal MnAs.

A convenient way of determining the ferromagnetic transition temperature of metallic (Ga,Mn)As

We have studied magnetic and transport properties of insulating and metallic (Ga,Mn)As layers before and after annealing. A dramatic increase of the ferromagnetic transition temperature TC by postgrowth annealing has been realized in both insulating and metallic (Ga,Mn)As. The as-grown insulating (Ga,Mn)As can be turned into metallic by the low-temperature annealing. For all the metallic (Ga,Mn)As, a characteristic feature in the temperature dependence of sheet resistance appears around TC. This phenomenon may provide a simple and more convenient method to determine the TC of metallic (Ga,Mn)As compared with superconducting quantum interference device (SQUID) measurement. Moreover, the TC of the metallic (Ga,Mn)As obtained by this way is in good agreement with that measured by a SQUID magnetometer.

Optical and magnetic properties of Mn+-implanted neutron-transmutation-doped GaAs bulks

( Ga 1−x Mn x ) As thin films with both semiconducting and magnetic properties were formed by using Mn+ ion implantation and thermal annealing with a goal of producing (Ga1−xMnx)As with a high ferromagnetic transition temperature Tc. Energy dispersive x-ray fluorescence measurements showed that the Mn+-implanted neutron-transmutation-doped (NTD) GaAs samples were (Ga1−xMnx)As thin films, and photoluminescence spectra showed that the annealed (Ga1−xMnx)As thin films were p-type semiconductors. The magnetization curve as a function of the magnetic field showed that ferromagnetism existed in the (Ga1−xMnx)As thin films, and the magnetization curve as a function of the temperature showed that the Tc was ≈140 K. The present results indicate that high-Tc (Ga1−xMnx)As thin films with both semiconducting and magnetic properties can be formed from NTD-GaAs bulks by using Mn+ implantation and annealing method.

Structural and magnetic properties of annealed ZnO–Co digital alloys

Diluted magnetic semiconductor (DMS) materials, which can utilize both the spin and charge properties of carriers, have attracted attention because the combination of two degrees of freedom holds promise for potential applications in spintronic devices [1, 2]. Among the various DMS materials, Ga1−x Mnx As and In1−x Mnx As DMS thin films grown on GaAs substrates have been the most extensively studied [3–6]. However, until now, the highest ferromagnetic transition temperature (Tc) obtained from III–V Ga1−x Mnx As DMS thin films has been 110 K [7]; thus, various studies on increasing the Tc of DMS materials have driven considerable efforts with the aim of realizing spintronic devices operating at higher temperature. Among several candidate DMS materials with a high Tc, (Zn1−x Cox )O DMSs are of current interest because they are theoretically expected to have high Tc values, as a consequence of the large energy gap and effective mass of (Zn1−x Cox )O [8, 9]. Since there are inherent problems due to possible existence of excess Co and CoO, the direct formation of (Zn1−x Cox )O films is very difficult. Thus, an alternative growth method of the (Zn1−x Cox )O films necessary for improving the crystallinity of films, and the improvement of the crystallinity of the films is very important in enhancing the value of Tc. Even though few works concerning the formation and the magnetic properties of the (Zn1−x Cox )O films on sapphire substrates were performed [10, 11], studies on the formation and the magnetic properties of the (Zn1−x Cox )O films on Si substrates have not been performed yet. Since Si substrates with large areas and good qualities are relatively cheap and extensively available in comparison with sapphires, Si technologies offer the potential applications for fabricating spintronic devices. This letter reports the effects of thermal annealing on the structural and optical properties of ZnO–Co digital alloys grown on p-Si (100) substrates by using the radio-frequency magnetron sputtering method. X-ray diffraction (XRD) measurement were performed to investigate the crystallization of the as-grown and the annealed ZnO–Co digital alloys, and superconducting quantum interference device (SQUID)

Growth of an ordered array of oriented manganite nanowires in alumina templates

We report the growth of oriented nanowires of manganites within the pores ofanodic alumina (AAO) templates. The processing of stoichiometric nanowires ofLa0.67Sr0.33MnO3 (LSMO) at relatively low temperatures (870 K) is greatly aided by the linear chain polymercation complex formed by ethylene glycol. The nanowires were found to grow preferentiallyalong the direction of cubic LSMO. The growth of oriented nanowires was achieved by:(a) electrostatically confining the sol particles to the core regions of the cylindrical pores ofthe template—this avoids heterogeneous nucleation of LSMO on amorphous pore walls and(b) introducing a polymer matrix that serves as a backbone structure to the growingcrystallites—this facilitates reorientation of the grains during heat treatment. The nanowiresare ferromagnetic at room temperature and exhibit a ferromagnetic transition temperature(Tc) of 370 K.

Magnetic and electrical transport properties of (La0.9Bi0.1)2/3Ca1/3MnO3

(La0.9Bi0.1)2/3Ca1/3MnO3 manganite perovskite was synthesized, the electrical and magnetic transport properties were investigated. Considerable thermal hysteresis in resistance (R) and magnetization (M) have been observed, and two peaks were observed in R–T curve. With an applied field of 5T, the thermal hysteresis in the resistance was suppressed, and only one peak was observed. The temperature-dependent magnetization (M–T) curve shows that below the paramagnetic–ferromagnetic transition temperature TC, a ferromagnetic–canted ferromagnetic transition might occur. The anomalous temperature-dependent behavior of the magnetization could account for the thermal hysteresis of the resistance.

Influence of the Mn interstitial on the magnetic and transport properties of (Ga,Mn)As

We report on measurements of the hole density, ferromagnetic transition temperature TC, and magnetization in a series of as-grown and annealed (Ga,Mn)As samples. Estimating the fraction of incorporated Mn occupying interstitial and substitutional sites allows a direct comparison of the predictions of mean field theory with experiment, and a determination of the magnetic moment per substitutional Mn. The saturation of TC at high Mn concentration is consistent with the mean field prediction. The estimated magnetic moment per Mn is close to the expected 5μB for all samples studied if an antiferromagnetic coupling between interstitial and substitutional Mn is assumed.

Pressure-induced transition from a spin glass to an itinerant ferromagnet in the half-doped manganiteL0.5Ba0.5MnO3(L=Smand Nd) with quenched disorder

The effect of quenched disorder on the multiphase competition has been investigated by examining the pressure phase diagram of half doped manganite Ln0.5B0.5MnO3 (Ln = Sm and Nd) with A-site disorders. Sm0.5Ba0.5MnO3, a spin glass insulator at ambient pressure, switches to a ferromagnetic metal with increasing pressure, followed by a rapid increase of the ferromagnetic transition temperature Tc. The rapid increase of Tc was confirmed also for Nd0.5Ba0.5MnO3. These observations indicate that the unusual suppression of the multicritical phase boundary in the A-site disordered system, previously observed as a function of the averaged A-site ionic radius, is essentially controlled by the pressure and hence the band width. The effect of quenched disorder is therefore much enhanced with approaching the multicritical region.

Magnetotransport Properties of the Diluted Ferromagnetic Semiconductor (Ga,Mn)As

We have studied experimentally the magnetotransport properties of a p-type diluted ferromagnetic semiconductor Ga1–xMnxAs by measuring the resistivity and Hall-effect as a function of temperature in various magnetic fields. The Ga1–xMnxAs thin films with thickness 1µm were grown on semi-insulating GaAs substrates by using a low temperature molecular beam epitaxy (MBE) technique and varying the Mn concentration between x=0.02 and x=0.05. The samples with x ⩽ 0.03 showed insulating behaviour at low temperatures (resistivity ∼1/T), whereas the samples with x ⩾ 0.04 showed metallic behaviour (a finite resistivity at low temperatures). In the metallic samples the exchange interaction between the hole spins in the valence band and the spins of the Mn 3d electrons was manifested in the anomalous Hall-effect (AHE), from which the ferromagnetic transition temperature TC = 30K was estimated. Further, a resistivity peak appeared at TC in the metallic samples. The AHE can be explained by the wellknown theory for the Hall-effect in ferromagnetic conductors, where the AHE is proportional to the average spin-polarization of the magnetic lattice. The resistivity vs. temperature can be described accurately by a model, where a transport mechanism for disordered heavily doped semiconductors is combined with the spin disorder scattering processes due to the exchange interaction. This model explains both the resistivity peak at TC in the metallic samples and the temperature dependence of the resistivity in the semiconducting samples.

Role of the rare-earth ion on the strength of the ferromagnetic exchange interactions inRMn0.5M0.5O3 (M = Co,Ni)

The role of the trivalent rare-earth ion on the paramagnetic to ferromagnetic transition temperature(Tc) of the substituted perovskite-type rare-earth (R) manganese oxides,RMn0.5Co0.5O3 and RMn0.5Ni0.5O3 (R = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er and Yb), is compared. For a particularR3+ with largerionic size, the Tc of RMn0.5Ni0.5O3 is largerthan that of RMn0.5Co0.5O3 and vice versa for smaller ionic size, though the ionic sizes of trivalent low-spin Co and Ni ions are comparable.Tc values of both Co and Ni containing compositions decrease with the decrease in the ionic size ofR3+, showing a faster decrease for heavier rare-earth ions beyondGd3+. Though the decreasing size of the rare-earth ion is responsible for the decrease in the magnetictransition temperature, due to structural distortions, an anomalous change in the rate of decrease ofTc beyondGd3+ is due to contributions from the single-ion anisotropy of the heavier rare-earth ions.

Formation and Characterization of (Zn1-xMnx)O Diluted Magnetic Semiconductors Grown on (0001) Al2O3 Substrates

(Zn1-xMnx)O diluted magnetic semiconductors (DMSs) grown on (0001) Al2O3 substrates by radio frequency magnetron sputtering were investigated with the aim of producing a material with a high ferromagnetic transition temperature (Tc). X-ray diffraction, photoluminescence, and Hall-effect measurements showed that the grown (Zn1-xMnx)O thin films were p-type crystalline semiconductors with single phases. Magnetization curve as a function of magnetic field at 5 K indicated that ferromagnetism existed in the (Zn1-xMnx)O thin films, and magnetization curve as a function of temperature showed that the Tc of the (Zn0.93Mn0.07)O thin film was 70 K. These observations can improve the understanding of the increase in Tc for (Zn1-xMnx)O DMSs grown on (0001) Al2O3 substrates.

Effect of low-temperature annealing on (Ga,Mn)As trilayer structures

The effect of low-temperature annealing on (Ga,Mn)As/GaAs/(Ga,Mn)As trilayer structures is studied. Low-temperature annealing significantly increases the ferromagnetic transition temperature TC of top (Ga,Mn)As layers, reaching as high as 160 K, whereas no apparent effect is observed on bottom (Ga,Mn)As layers. The annealing effect on Be-doped trilayers is also presented.

Enhanced temperature-independent magnetoresistance below the metal-insulator transition temperature of epitaxial La0.2Nd0.4Ca0.4MnO3 thin films

Epitaxial La0.2Nd0.4Ca0.4MnO3 thin films have been deposited at 800°C on LaAlO3 substrate using pulsed laser deposition technique. The structural and magnetotransport properties of the films have been studied. The sharp peak in the temperature dependence of the resistance corresponding to metal-to-insulator transition (Tp) has been observed at a temperature of Tp=82 K, 97 K and 110 K for 0 Oe, 20 kOe and 40 kOe magnetic fields, respectively. The film exhibits a large nearly temperature-independent magnetoresistance around 99% in the temperature regime below Tp. The zero field-cooled (ZFC) and field-cooled (FC) magnetization data at 50 Oe shows irreversibility between the ZFC and FC close to the ferromagnetic transition temperature Tc=250 K. The ZFC temperature data of the film displays ferromagnetic behavior for higher temperature regime Tc=250 K>T>Tp=82 K, and a decrease in magnetization with decreasing temperature up to 5 K below 82 K exhibiting a sort of antiferromagnetic behavior in the low temperature regime (T<82 K=Tp=TN).

Alternating current susceptibility study of the low doped regime of La1−xSrxMnO3 perovskites

The dynamic response of La1−xSrxMnO3 perovskites has been studied in the low doped regime (x⩽0.14). The frequency dependence of the ac susceptibility gives clear indications in favor of the existence of electronic phase separation not only in the O″ phase but also at temperatures between the ferromagnetic transition temperature TC, and the O′–O″ structural transition occurring at TFI. The influence of the Jahn–Teller distortion on the magnetic susceptibility is also detected as a drop in the magnetic susceptibility on crossing down the O–O′ structural transition, while a jump occurs at O′–O″ transition. A close similarity, both structural and magnetic, between the O and O″ phase is strongly suggested by the experimental results.

Correlation between electroresistance and magnetoresistance in La0.82Ca0.18MnO3 single crystal

The resistivity of La0.82Ca0.18MnO3 single crystal has been investigated as a function of external magnetic field and separately under an applied current flow. The measurements were carried out at various temperatures below and above the ferromagnetic transition temperature TC. It has been found that the dynamic electroresistance exhibits stunning similarities to the colossal magnetoresistance at the corresponding temperatures. The correlation observed between the electric- and magnetic-field effects is attributed to electrically induced magnetoresistance.

Reduction in conduction noise of La–Ca–Mn–O polycrystalline films due to Ag addition

Ag-added La0.67Ca0.33MnO3(Ag–LCMO) polycrystalline films have been prepared on SrTiO3 substrates by the ultrasonic spray pyrolysis technique and their properties are compared with the La0.67Ca0.33MnO3(LCMO) films prepared under similar conditions. Addition of Ag in LCMO films improves their surface morphology and reduces their lattice constant. During the synthesis process, Ag in Ag–LCMO film melts and segregates to grain boundaries. In the Ag–LCMO film, normalized voltage noise reduces by more than one order of magnitude, and peak temperature (Tp) and ferromagnetic transition temperature (Tc) increase by 10 and 20 K, respectively. Improvement in surface morphology and better oxygenation of the film due to the presence of Ag leads to reduction in conduction noise of the LCMO film.

Magnetic Properties and Scaling Behavior in Perovskite–like La0.7(Ba1−xPbx)0.3CoO3 System

ABSTRACTIn this study, we used x-ray diffraction patterns and dc magnetic measurements to investigate the crystallographic structure, magnetic properties and scaling behavior of the distorted perovskite La0.7(Ba1−xPbx)0.3CoO3 (0 ≤ x ≤ 0.5) system with a constant ratio of Co4+/Co3+. Samples with x = 0.0 and 0.1 were crystallized in the cubic structure with a ∼ 7.76 Å whereas samples with x ≥ 0.2 were crystallized in an orthorhombic Pbnm space group with a ∼ b ∼ 5.50 Å and c ∼ 7.85 Å. For all our samples the spin-glass-like behavior were observed in low temperature and low field ranges. The Pb2+ substitution on Ba2+ site does not significantly affect the ferromagnetic transition temperature TC, but does introduce large variation in the magnetic strength. In both the ferromagnetic and paramagnetic states the minimum values of the average effective moments provided by every Co ion occur at x = 0.3. We also observed the scaling behaviors of magnetic data in all samples. The derived values of the critical exponents (β, γ, δ) were consistent with those predicted by mean field theory and a three-dimensional Heisenberg model.

Effect of Li substitution on the crystal structure and magnetoresistance of LaMnO3

The crystal structure and magnetoresistance of the polycrystalline La1−xLixMnO3 (x=0.10, 0.15, 0.20, 0.30) are investigated. The result of the Rietveld refinement of x-ray powder diffraction shows that the room temperature structural transition from rhombohedral (R3̄C) to orthorhombic (Pbnm) symmetry occurs at the Li-doped level x⩾0.2. Accompanying the occurrence of the structural transition, the lattice distortion and the bending of the Mn–O–Mn bond increase and the ferromagnetic transition temperature TC decreases. For x=0.10 and 0.15 samples, double metal–insulator (M–I) transitions accompanying a single ferromagnetic transition and a negative magnetoresistance as high as 26% in a magnetic field of 0.8 T are observed. For x=0.20 and 0.30, the samples manifest nonmetallic behavior throughout the measured temperature range. We suggest that the double M–I transitions phenomena of low Li-doped samples originate from the magnetic inhomogeneity due to the formations of the Mn3+ and Mn4+-rich regions induced by partial substitution of the monovalent Li1+ ions for the trivalent La3+ ions. The transport property of high Li-doped samples (x=0.20 and 0.30) can be explained according to the additional localization of eg electrons induced by a static coherent Jahn–Teller distortion of the MnO6 octahedra stemming from the structural transition from rhombohedral (R3̄C) to orthorhombic (Pbnm) and the reduced bandwidth of eg electrons due to the increased bending of the Mn–O–Mn bond.

Synthesis, X-ray, magnetic and electrical studies of substituted (Pr,Sr)MnO3 perovskites

A systematic study of the structural, magnetic and electrical properties of the manganites Pr1-x Sr x MnO3(0≤ x ≤ 0.5) has been carried out. X-ray diffraction investigation shows a structural change with composition, from orthorhombic (0 ≤ x ≤ 0.2) to rhombohedral (0.25 ≤ x ≤ 0.5). The magnetic properties of Pr1-x Sr x MnO3 samples could be explained on the basis of a double exchange mechanism between pairs of Mn3+ and Mn4+ ions. These properties are strongly dependent on the ratio of Mn3+/Mn4+. The maximum of the ferromagnetic transition temperature Tc is reached at x ≈ 0.35 corresponding to a value 1.85 of this ratio. The investigation of the electrical properties shows a semiconductor to metal transition as a function of temperature (0.25≤x≤0.4) with a metallic-like behaviour above a critical temperature Tp . A semiconducting-like one is observed for all the range of temperature (50–300 K) for (0 ≤ x ≤ 0.2 and x = 0.5). The evolution of activated energies with the carrier concentration has been investigated.