Ferromagnetic Transition Temperature Tc Research Articles

Ultrasonic anomalies in La0.67Ca0.33Mn1-xZnxO3 perovskites

Both the longitudinal and transverse ultrasonic sound velocities and attenuations in the single-phase polycrystalline compound La0.67Ca0.33Mn1-xZnxO3 (x = 0.00, 0.05, 0.10, 0.15) have been carefully measured by a conventional pulsed echo technique at a frequency of 10 MHz, between 20 and 300 K. A dramatic increase in sound velocity, accompanied by a sharp peak in attenuation, is observed near the ferromagnetic transition temperature TC for both longitudinal and transverse modes in all samples. This feature implies extremely strong spin-phonon coupling in this system and gives direct evidence for the spontaneous linear magnetostriction effect. At temperatures below 60 K, a dramatic softening in sound velocity accompanied by a sharp increase in ultrasonic attenuation for both longitudinal and transverse modes is observed. The analysis of the results suggests that the softening of the sound velocity for both longitudinal and transverse modes may correspond to the formation of a spin-glass state.

Field-dependent magnetic and transport properties and anisotropic magnetoresistance in ceramic La0.67Pb0.33MnO3

A summary of detailed measurements of the field and temperature dependence of the ac susceptibility, magnetization and the longitudinal and transverse magnetoresistivities of ceramic La0.67Pb0.33MnO3 is presented. Detailed analysis of the magnetic data provides an estimate of 340.5±0.5 K for the paramagnetic to ferromagnetic transition temperature Tc, with the data closest to the critical point yielding = 1.39±0.06 (from the temperature dependence of the susceptibility along the crossover line), = 0.41±0.02 (from the field induced variation of the temperature of the same line) and = 4.20±0.15 (from the field dependence of the critical isotherm). Nevertheless the presence of disorder - a variance in the distribution of spin-spin coupling strengths - means that Heisenberg model (asymptotic) exponents cannot be excluded; the universality class for this system cannot therefore be definitively identified. The transport data yield Tc = 339.6±0.4 K, and while the isotropic magnetoresistance peaks near Tc, as expected, the magnitude of the spontaneous resistive anisotropy (SRA) (the difference between the longitudinal and transverse magnetoresistance extrapolated to zero induction) increases linearly with decreasing temperature below Tc, peaks near 30 K and then falls to a smaller value (-0.2±0.03%) in the liquid helium range. While this latter value is in reasonable agreement with itinerant model predictions, the temperature variation in the SRA above 30 K is more convincingly reproduced by a localized model. Furthermore, the mechanisms controlling the isotropic magnetoresistance and the SRA in this system appear to be different.

Improved properties of La2/3Ca1/3MnO3 thin films by addition of silver

Thin films of Ag-added La2/3Ca1/3MnO3 exhibit enhancement of several desirable characteristics over the pristine counterparts. We find that the addition of Ag results in a pronounced increase in the insulator–metal transition temperature (Tp) and ferromagnetic transition temperature (Tc). There is also a remarkable improvement in the magnetic and electrical homogeneity of the samples as indicated by narrower ferromagnetic resonance linewidths and narrower resistive transitions, respectively. The observed improvement in properties is inferred to be largely associated with improved oxygen stoichiometry of the films although microstructural effects are not ruled out.

Spin-dependent tunneling and properties of ferromagnetic (Ga,Mn)As (invited)

Low-temperature molecular beam epitaxy allows one to dope GaAs with Mn over its solubility limit, making it possible to realize a III–V-based diluted magnetic semiconductor (Ga,Mn)As. Magnetization measurements revealed that (Ga,Mn)As is ferromagnetic at low temperatures. The ferromagnetic transition temperature Tc can be as high as 110 K for 5% Mn. Magnetotransport properties of (Ga,Mn)As were found closely related to its magnetic properties. The temperature and magnetic-field B dependence of resistivity ρ can be understood in terms of spin-dependent scattering. The p–d exchange determined from the B dependence of ρ is shown to be consistent with the exchange expected from Tc based on the Ruderman–Kittel–Kasuya–Yosida interaction. The anomalous Hall effect dominates the Hall resistance up to room temperature, allowing one to determine the magnetic properties from the magnetotransport measurements alone. Since (Ga,Mn)As can be grown pseudomorphically on GaAs, one can introduce ferromagnetism in GaAs-based heterostructures such as resonant tunneling diodes and observe spin-dependent phenomena in current–voltage characteristics. Magneto-optical properties of (Ga,Mn)As films are also presented.

Metal–insulator and magnetic transition in Nd1−xSrxMnO3 solid solutions

In this article we report the results of electrical resistivity, thermopower and ac magnetic susceptibility measurements on Nd1−xSrxMnO3 with x=0.40, 0.33 and 0.30. The room temperature paramagnetic to low temperature ferromagnetic transition temperature TC and electrical resistivity ρ(T) are observed to depend on the processing conditions. ρ(T) exhibits a shoulder at TC and a peak below TC. The latter peak is likely to be of intrinsic origin rather than due to lattice defects/grain boundaries, etc. The resistivity decreases with increasing x as well as upon Ar annealing. Upon Ar annealing the thermopower [S(T)] below TC shows no appreciable change in magnitude, while above TC |S| decreases. S(T) exhibits considerable differences between x=0.33 and x=0.3, which we attribute to a dopant induced insulator–metal (I–M) transition. Our thermopower data along with the resistivity and magnetic measurements show that the changes in the observed features in the ρ(T) and S(T) cannot be related to magnetic ordering alone but the electronic transitions also have to be taken into consideration.

Ferromagnetism at room temperature in La0.8Ca0.2MnO3 thin films

Anomalously high metal–insulator transition temperature Tp and ferromagnetic transition temperature Tc have been observed in thin films of La0.8Ca0.2MnO3. Ferromagnetic resonance signals from microwave studies are observed at room temperature (well above Tc) suggesting the presence of multiple magnetic spin systems. The Tp and Tc values are higher than that of La0.67Ca0.33MnO3 which have the highest Tp and Tc according to the bulk phase diagram. These results suggest that properties well beyond those seen in the bulk phase diagram may be achievable in thin films of the manganites.

A phenomenological model for magnetoresistance in granular polycrystalline colossal magnetoresistive materials: The role of spin polarized tunneling at the grain boundaries

It has been observed that in bulk and polycrystalline thin films of colossal magnetoresistive (CMR) materials the magnetoresistance follows a different behavior compared to single crystals or single crystalline films below the ferromagnetic transition temperature Tc. In this paper we develop a phenomenological model to explain the magnetic field dependence of resistance in granular CMR materials taking into account the spin polarized tunneling at the grain boundaries. The model has been fitted to two systems, namely, La0.55Ho0.15Sr0.3MnO3 and La1.8Y0.5Ca0.7Mn2O7. From the fitted result we have separated out, in La0.55Ho0.15Sr0.3MnO3, the intrinsic contribution from the intergranular contribution to the magnetoresistance coming from spin polarized tunneling at the grain boundaries. It is observed that the temperature dependence of the intrinsic contribution to the magnetoresistance in La0.55Ho0.15Sr0.3MnO3 follows the prediction of the double exchange model for all values of field.

Low-field colossal magnetoresistance in bandwidth-controlled manganites

We have investigated magnetoresistance (MR) phenomena relevant to the charge ordering (CO), namely, the real space ordering of both Mn3+ and Mn4+, in single crystals of (Nd1−ySmy)1/2Sr1/2MnO3, in which the one-electron bandwidth (W) is systematically controlled by varying the average ionic radius of the A site. The low-field colossal MR is observed for the small-W region of y⩾0.5; e.g., ρ(0)/ρ(H)>103 in a field of 0.4 T at 115 K for the y=0.94 crystal. This is viewed as a first-order insulator-to-metal phase transition induced by a magnetic field, which accompanies a lattice-structural change. In the small-W region, the CO instability accompanying the antiferromagnetic spin correlations subsists even above the ferromagnetic transition temperature Tc. The competition between the ferromagnetic double-exchange and antiferromagnetic CO interactions gives rise to such a lattice-coupled first-order phase transition induced by a relatively low magnetic field.

Cation size-temperature phase diagram of the manganites Ln0.5Sr0.5MnO3

We have studied the variation of the paramagnetic to ferromagnetic transition temperature TC and of the ferromagnetic to antiferromagnetic transition temperature TN as a function of the average size of the A site cations (Ln, Sr) in the manganite perovskites Ln0.5Sr0.5MnO3 (Ln=Gd, Sm, Y, Pr, La). For our investigation we used dc resistivity, magnetization, susceptibility and magnetoresistance measurements. Results show that the Curie temperature TC increases from 85 to 310 K when 〈rA〉 varies from 1.221 to 1.263 Å, whereas TN remains stable around 150 K. A magnetic phase diagram temperature-〈rA〉 has been established, allowing five regions with different magnetic behaviours to be evidenced: paramagnetic insulating, ferromagnetic metallic, antiferromagnetic insulating, canted antiferromagnetic metallic, and weak ferromagnetic insulating. The extension of these regions is determined by the value of the applied magnetic field.

Thin Film Growth and Magnetotransport Study of (La, Sr)MnO3

ABSTRACTThin-film samples of La1-xSrxMnO3 (x = 0.20 – 0.30) were grown by pulsed-laser deposition using various target compositions, substrate materials, growth temperatures, oxygen partial pressures, and laser-pulse repetition rates. The crystal structure and the transport and magnetic properties of these films were then examined. Of the growth conditions, the oxygen partial pressure (Po2) had the greatest influence on the electrical and magnetic properties. Films grown under a low Po2 had a low ferromagnetic transition temperature (Tc) and a wide resistive transition width. None of the heat treatments done after growth improved these films’ quality. The film morphology was significantly affected by the substrate material. Our x-ray diffraction analysis and AFM measurements revealed that the films deposited on both MgO (100) and LaAlO3 (100) were epitaxially grown but contained defect structures. In contrast, grain-free thin films were epitaxially grown on the SrTiO3 (100) substrates. The surface roughness of films grown on SrTiO3 was less than 0.3 nm, even for films up to 150 nm thick. Under optimized growth conditions, as-deposited films for x ≥ 0.2 showed a sharp transition in resistivity at Tc. Magnetoresistance at far below Tc was as low as that reported for single-crystal sample. Since large magnetoresistance was often observed in polycrystalline samples and believed to be a grain boundary effect, these results indicate the high quality of the films grown on the SrTiO3 substrates.

ペロブスカイト型Ｍｎ酸化物薄膜の磁気抵抗

La1-xSrxMnO3 (x=0.3) thin films with thicknesses of 100-900Å were grown epitaxially on SrTiO3 (l00) and (110) substrates by magnetron sputtering. The ferromagnetic transition temperatures (Tc) of the films ranged from 140 to 330 K, depending on the growth conditions and film thickness.This variation in the Tc values of the films was possibly due to their oxygen deficiency. As a general trend,larger magnetoresistance (MR) ratios were obtained for films with lower Tc values (lower hole concentrations).Ultra-thin film of 100Å with Tc of 140 K exhibited relatively large magnetoresistance (MR) with an MR ratio (Δρ/ρ0) of 71% at H= 18 kOe.

酸化物超伝導体の合成と特性 （Ｓｒ‐Ｃａ）ＲｕＯ３系の遍歴電子磁性 インバー効果

We report here the results of Debye-Scherrer X-ray diffraction measurements in the temperature range from 12K to 300K of the perovskite compounds SrRuO3 and CaRuO3. We show that SrRuO3 indicates an clear anomaly with the temperature dependence of the unit-cell parameters and volume below its ferromagnetic transition temperature Tc of 160K through magnetovolume effect. In particular, the volume of SrRuO3 is almost constant below Tc, and invar effect is discovered for the first time in transition-metal oxide, which is seen in 3d transition-metal itinerant magnets, e.g., Fe-Ni alloy. Therefore, it is concluded that SrRuO3 is classified to a moderately weak itinerant ferromagnet. On the other hand, CaRuO3 shows no anomaly in the variation of the volume with temperature, suggesting that magnetic ordering does not occur in this compound.

Magnetic Properties of (Cr1-xFex)3Te4

The magnetic properties of (Cr1-xFex)3Te4 with the pseudo-NiAs type structure have been studied. The ferromagnetic transition temperatures (Tc) for the compounds are found to decrease with substitution of Fe for Cr, from the magnetization measurements. From the variation of Tc with x, monoclinic (Cr1-xFex)3Te4 (x = 0 ∼ 0.04) and hexagonal (Cr1-xFex)3Te4 (x ∼ 0.15) are found to be stable at room temperature, and the solubility limit of the iron-rich side for the hexagonal (Cr1-xFex)3Te4 is estimated to be x = 0.18. A weak antiferromagnetic transition has been found for monoclinic (Cr1-xFex)3Te4 (x = 0 ∼ 0.04). The critical Fe concentration (x) at which the antiferromagnetic transition disappears in (Cr1-xFex)3Te4 is estimated to be x = 0.08. In the range x = 0 ∼ 0.02, the saturation magnetization for (Cr1-xFex)3Te4 increases with x due to the substitution of Fe, which has a larger magnetic moment than Cr. Above x = 0.04, the saturation magnetization decreases with increasing x, which is considered to be due to decreased magnetic interaction between metal atoms as a result of the Fe substitution for Cr. From the variation of the saturation magnetization for (Cr1-xFex)3Te4 with x , the solubility limit of the iron-rich side for hexagonal (Cr1-xFex)3Te4 is estimated to be x = 0.18.

Thermodynamic and magnetic properties of (Ni1−xMx)3Al with M=Cu and Pd and Ni3(Al1−xSix)

The ferromagnetic transition temperature Tc of Ni3Al is found to be about 60 K for a polycrystalline sample with 76 at. % Ni content and 81 K for a single crystal with 76.6 at. % Ni. Our magnetization measurements show that when Cu or Pd is substituted for Ni in Ni3Al, Tc decreases with increasing Cu or Pd concentration and goes to zero in the vicinity of the critical concentrations of 2% Cu or 10% Pd. When Si is substituted for Al, Tc also decreases and goes to zero at a critical concentration of about 10% Si. Specific heat studies indicate that the electronic specific heat coefficient γ is slightly enhanced when these systems approach their critical concentrations. This behavior is discussed in terms of the self-consistent renormalization theory for nearly and weakly itinerant magnets.

Monte Carlo simulations on Ising dipoles: finite size scaling and logarithmic corrections

Monte Carlo simulations are presented for Ising dipoles on body centred cubic and tetragonal lattices. A finite size scaling form that includes logarithmic corrections is proposed and found to significantly improve the data collapse. With lattice parameters appropriate to LiHoF4 the authors obtain a ferromagnetic transition temperature Tc=1.51 K in good agreement with experiment.

Domain structure in the reentrant FexZr100−x amorphous ribbons

The domain structure in amorphous ribbons of FexZr100−x is investigated by Lorentz transmission electron microscopy in the temperature range 18–290 K. The domains appear approximately at the ferromagnetic transition temperature Tc determined by conventional magnetic measurements. Their dimensions (10–50 μm) and shapes stay unchanged whatever the temperature (T<Tc), particularly upon cooling through the reentrant transition. The physical factors limiting the observation of the magnetic structure are also discussed.

Change of D/Tc, Ratio During Heat Treatment of Fe90-xSixB10 Amorphous Alloy

AbstractThe ferromagnetic transition temperature Tc for Fe-Si-B amorphous alloys with iron concentration exceeding 75 atomic percent changes monotonically with the annealing time. This change of Tc is accompanied by a drastic change of the magnetic excitation spectrum measured by means of low angle neutron inelastic scattering. It was observed that a small amount of α-Fe(Si) precipitated in an amorphous matrix during the heat treatment even at relatively low temperatures, leading to a decrease of the spin-wave stiffness constant D. Because at the same time T, increases, the D/Tc ratio (which is about 0.3 for “as quenched” Fe-Si-B system) decreases abruptly.

Magnetoresistance studies of melt-spun amorphous Fe1−xZrx ferromagnets

A remarkable feature in amorphous Fe1−xZrx alloys is that for Fe concentrations greater than 90 at %, the ferromagnetic transition temperature TC decreases with increasing Fe concentration. In addition, alloys containing 93–90 at % Fe are found to show a spin-glass-like behavior at low temperatures, i.e., below TC. The electrical resistivity of these alloys shows a minimum at roughly the Curie temperature. We have measured the transverse magnetoresistance of Fe91Zr9, and Fe90Zr10 alloys from 4 to 300 K in applied magnetic fields as large as 9 T. We find that (1) the field derivative of the magnetoresistance is positive at all temperatures in applied fields larger than 0.1 T; (2) a maximum occurs in the magnetoresistance at roughly the Curie temperature; (3) below the Curie temperature a small negative low-field magnetoresistance develops which increases with decreasing temperature. These results indicate that the resistance minimum which occurs around the Curie temperature for these alloys is due to the coherent exchange scattering by the Fe moments, and is not associated with a Kondo mechanism as previously proposed.

Specific heat study of the transition from ferromagnetism to antiferromagnetism in terbium

The specific heat of single crystal terbium has been measured over the range 200–400 K. A value for the latent heat of (13.6 ± 0.6) J mol−1 is obtained at the first order antiferromagnetic to ferromagnetic transition temperature Tc = (221.45 ± 0.03) K. A temperature hysteresis of (0.24 ± 0.03) K is observed in the transition.

Nitrogen-bearing amorphous Fe83−xVxB14Si3 alloys: Thermal and magnetic properties

Magnetic properties of amorphous alloys Fe83−xVxB14Si3 containing 0.02–0.85 at. % nitrogen have been studied from 4 K to the crystallization temperatures. The effect of vanadium in these alloys is to decrease the ferromagnetic transition temperature TC at 23 K/at. % V, as well as the magnetization σ at 8.8 (emu/gm)/at. % V. This indicates loss of ferromagnetism in this alloy system for V concentrations greater than 26 at. %. The addition of nitrogen increases crystallization temperature Tx, saturation magnetization, and spin wave exchange stiffness constant D. The observed T-dependence of the magnetization over the entire temperature range 4–300 K, can be described only by taking into account both the T3/2 and T5/2 terms. This is unlike the normal extended T3/2 behaviour observed in amorphous alloys having TC above room temperature.