Range Ferromagnetic Order Research Articles

On the defect induced ferromagnetic ordering above room-temperature in undoped and Mn doped ZnO thin films

AbstractEvidence for long range ferromagnetic order above room-temperature, RTFM, in pristine ZnO, In2O3, TiO2 nanoparticles and thin films, containing no nominal magnetic elements have been reported recently. This could question the origin of RTFM in doped dilute alloys if for example the ZnO matrix itself develops a defect induced magnetic order with a significant moment per unit cell. In this presentation we report a systematic study of the film thickness dependence of RTFM in pure ZnO deposited by DC Magnetron Sputtering. We observe a maximum in the saturation magnetization, MS, value of 0.62 emu/g (0,018 μB/unit cell), for a ˜480 nm film deposited in an oxygen ambience of appropriate pressure. Above a thickness of around 1 μm the films are diamagnetic as expected. We thus see a sequential transition from ferromagnetism to para- and eventual diamagnetism as a function of film thickness in ZnO. We also find that in such a ZnO matrix with a maximum intrinsic defect induced moment, on doping with Mn the maximum enhanced MS value of 0.78 emu/g is obtained for 1at.% Mn doping. With this approach of appropriate doping in a defect tailored matrix, we routinely obtain RTFM in both undoped and Mn- doped ZnO thin films.

Long range spin ferromagnetic order with zero magnetization in(111)Sm1−xGdxAl2 films

Bulk Sm1−xGdxAl2 (0.01<x<0.035)compound exhibits an exciting magnetic property: its magnetization drops to zero at a compensationtemperature Tcomp while it maintains a long range ferromagnetic order. We achieved recently the first epitaxial growthof (111)Sm1−xGdxAl2 films by molecular beam epitaxy. Macroscopic magnetization measurements confirmedthe existence of a magnetic compensated state, occurring at a temperaturethat depends on the concentration of Gd. Specific features observed in theSm1−xGdxAl2 films are a strong [111] perpendicular anisotropy and a verylarge coercivity. X-ray magnetic circular dichroism at Sm and GdL2,3 edgesallows us to directly evidence the existence of a long range magnetic polarization of the Sm and Gd sublatticesat Tcomp, and a parallel coupling between the total magnetic moment ofSm3+ and Gd3+. Moreover, it is possible to tailor the magnetic behavior atTcomp by changing the thermo-magnetic treatment.

Thermodynamic and transport studies of the ferromagnetic filled skutterudite compoundPrFe4As12

A variety of thermodynamic and transport measurements were made on high-quality single crystals of the Pr-based filled skutterudite compound ${\text{PrFe}}_{4}{\text{As}}_{12}$. Abrupt features in magnetization, ac susceptibility, specific heat, resistivity, thermoelectric power, and ultrasonic velocity reveal the onset of long range ferromagnetic order below ${\ensuremath{\Theta}}_{C}=18\text{ }\text{K}$. The low-temperature magnetic susceptibility is characterized by a Curie--Weiss law with an effective moment of $3.52{\ensuremath{\mu}}_{B}/\text{f}\text{.u}\text{.}$ and a saturation magnetization of $2.3{\ensuremath{\mu}}_{B}/\text{f}\text{.u}\text{.}$, which is consistent with a magnetic ${\ensuremath{\Gamma}}_{5}$ triplet ground state. A gaplike reduction of the large electronic specific heat coefficient of $340\text{ }\text{mJ}/\text{mol}\text{ }{\text{K}}^{2}$ and several other features point to a strongly correlated electron behavior that is likely coupled to a change in magnetic and/or structural order near ${T}^{\ensuremath{\ast}}\ensuremath{\simeq}12\text{ }\text{K}$. Furthermore, this complex magnetic state is found to be strongly field dependent, as evidenced by a change in the easy axis at low fields and an additional contribution to thermal conductivity appearing only at high fields.

Magnetic states and spin-glass properties inBi0.67Ca0.33MnO3: Macroscopic ac measurements and neutron scattering

We report on the magnetic properties of the manganite ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ $(x=0.33)$ at low temperature. The analysis of the field expansion of the ac susceptibility and the observation of aging properties make clear that a spin-glass phase appears below $T=39\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the presence of magnetic order. Neutron scattering shows both magnetic Bragg scattering and magnetic diffusion at small angles and confirms this coexistence. In contrast to ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ $(x=0.3--0.33)$, which exhibits a mesoscopic phase separation responsible for a field driven percolation, the glassy and short range ferromagnetic order observed here does not cause a colossal magnetoresistance.

Monte Carlo study on bilayer thin films magnetic properties dependence with discontinouos anisotropy parameters

Abstract In the last few years there has been significant interest in the field of thin films, due to numerous specific phenomena related to the low dimension of these systems, and to the large opportunities in development of high technologies based on their specific magnetic and electronic properties. When dealing with systems of reduced dimensionality it is important to take into account the influence of magnetic anisotropies. In this paper we investigate the magnetic properties of bilayer thin film. This behavior is modeled using Monte Carlo simulations, in the Extended Anisotropic Heisenberg Model. The magnetization, out-of-plane and in-plane magnetic susceptibilities, and also the specific heat bearings according to temperature are investigated in order to find the potential magnetic ordering phases and the critical temperatures, for two sets parameter assignments. For quasi-uniform anisotropy parameters of the film we detect the ferromagnetism-paramagnetism transition and then, by changing the model parameters values, we relieve a short range ferromagnetic ordering phase arising from the antiferromagnetic base layer coupling influence and from easy-plane anisotropy discontinuity on the layers interface.

A-site deficiency effects on the structural and magnetic properties of La0.7Sr0.3−x□xCoO3 cobaltites

Abstract Structural and magnetic properties of La0.7Sr0.3−x□xCoO3 (0 ≤ x ≤ 0.15) samples have been investigated. Powder samples have been synthesized using the solid state reaction method at high temperature. All our samples crystallize in the rhombohedral structure with R 3 ¯ c space group. With increasing Sr-deficiency amount, the unit cell volume decreases from 341.4 A3 (x = 0) to 338.2 A3 (x = 0.15). Our samples exhibit thermo-magnetic irreversibility behavior in the magnetization data in a magnetic applied field of 50 mT, which indicates the absence of long range ferromagnetic order. The Curie temperature decreases from 225 K (x = 0) to 180 K (x = 0.1). Sample with high deficient amount (x = 0.15) becomes predominantly spin glass phase. The effective magnetic moments values obtained from the Curie constant are close to that of the Co3+ (intermediate spin configuration) and Co4+ (low spin configuration) for 0 ≤ x ≤ 0.125 and Co3+ (low spin configuration) and Co4+ (low spin configuration) for x = 0.15.

Role of long range ferromagnetic order in the electronic structure of Sr1−xCaxRuO3

We investigate the role of long range ferromagnetic order in the electronic structure of Sr1−xCaxRuO3 using high resolution photoemission spectroscopy. SrRuO3 is a ferromagnetic metal but isostructural, isoelectronic CaRuO3 is an enhanced paramagnet. Surface spectra of CaRuO3 exhibit temperature induced modifications. This is not significant in other compositions. This may be attributed to the structural changes observed in previous studies. Interestingly, the bulk spectra reveal unusual spectral changes exhibiting a large decrease in the coherent feature intensity corresponding to only ferromagnetic samples, although the Ru moment is very similar in all compositions.

Evidence of ferromagnetism in off-stoichiometric Fe 2.5− xV 1+ xAl 0.5 (X 2YZ) Heusler alloys

In this work we study the structural and magnetic properties of Fe 2.5− x V 1+ x Al 0.5 alloys. These compounds exhibit a ferromagnetic behavior in the ordered L2 1 phase (X 2YZ) at low temperature. The increase of the V concentration is detrimental to the onset of a long range ferromagnetic ordering in this system. The results indicate that the Fe atoms are carrying the largest magnetic moment. A re-entrant transition from a spin-glass to a ferromagnetic state was observed at low temperature.

Transport, magnetic and structural investigations of Co–Ni–Al shape memory alloy

The transport, magnetic and structural properties of the ferromagnetic shape memory alloy with nominal composition Co 1.28Ni 1.17Al have been investigated. The alloy shows long range ferromagnetic order below 269 K. The anomaly due to the martensitic transition is observed in the resistivity and magnetization data in the temperature range around 175 K, which is associated with clear thermal hysteresis. A significant magneto-thermal irreversibility between the zero-field-cooled and the field-cooled data is observed below the martensitic start temperature. The structural change is confirmed by temperature dependent X-ray diffraction, and our analysis indicate that both martensite and austenite phases mutually coexist in the region of hysteresis. Existence of distinct minor hysteresis loops in the resistivity data is also in accordance with a metastable region of phase coexistence across the martensitic transition.

Dilute magnetic semiconductors based on InN

We present growth and characterization of Cr-doped InN by plasma assisted molecular beam epitaxy. The films were grown on c-plane sapphire substrates, employing GaN intermediate layers to accommodate the lattice mismatch between InN and sapphire. X-ray diffraction and Hall measurements were used for structural and electrical characterization of the films. The magnetic properties were measured using superconducting quantum interference device magnetometry. Cr : InN displayed long range ferromagnetic order up to room temperature without indications of phase separation. A optimal Cr-doping regime was found around 1–2% where the effective magnetic moment per Cr atom was maximal and the paramagnetic background signal of the samples was minimal.

Abnormality of Magnetic Behavior and Resistivity of La0.7xDyxSr0.3MnO3 (0.00x0.30) System at Low Temperature

By measuring M-T curves, -T curves and M R-T curves of the samples under different temperatures, the influence of Dy doping (0:00 x 0:30) on the magnetic and electric properties of La0.7xDyxSr0.3MnO3 has been studied. The experimental results show that, with the increase of the Dy content, the system undergoes a transition from long range ferromagnetic order to the cluster-spin glass state and further to antiferromagnetic order. For the samples with x=0.20 and 0.30, their magnetic behaviors are abnormal at low temperature, and their resistivities at low temperature have a minimum value. These peculiar phenomena not only come from the lattice effect induced by doping, but also from extra magnetic coupling induced by doping.

Double perovskite Sr2FeMoO6 films: Growth, structure, and magnetic behavior

The structural and magnetic properties of Sr2FeMoO6 thin films prepared by pulsed laser deposition were studied as a function of the laser energy density used to ablate a noncommercial stoichiometric target. Films deposited at a laser fluence as low as 1.6J∕cm2 exhibit a high degree of double perovskite lattice ordering, a ferromagnetic-paramagnetic transition around 355K, and a saturation magnetization of ∼3.4μB∕f.u. On the contrary, films deposited at φ values &amp;gt;3J∕cm2 show a vertically elongated lattice unit cell and the lack of long range ferromagnetic order with a severe decrease of the saturation magnetization (Ms∼1μB∕f.u.). The structural and magnetic properties observed in the latter samples are attributed to lattice disorder and to secondary phases resulting from the ablation process performed at high laser energy density. In these samples the presence of Fe ions not arranged in the Sr2FeMoO6 is confirmed by x-ray absorption measurement at the Fe L2,3 edge.

Ferromagnetism in the Fe-substituted spinel semiconductorZnGa2O4

Motivated by the recent experimental observation of long range ferromagneticorder at a relatively high temperature of 200 K in the Fe-dopedZnGa2O4 semiconducting spinel, we propose a possible mechanism for the observedferromagnetism in this system. We show, supported by band-structure calculations,how a model similar to the double-exchange model can be written down for thissystem and calculate the ground state phase diagram for the two cases where Fe isdoped either at the tetrahedral position or at the octahedral position. We findthat in both cases such a model can account for a stable ferromagnetic phasein a wide range of parameter space. We also argue that in the limit of highFe2+ concentration at the tetrahedral positions a description in terms of a two-band model is essential. Thetwo eg orbitals and the hopping between them play a crucial role in stabilizing the ferromagneticphase in this limit. The case when Fe is doped simultaneously at both the tetrahedral andthe octahedral position is also discussed.

Electric transport and magnetic properties of perovskitesLaMn1−xCoxO3 up to 900 K

The structural, magnetic and transport properties of theLaMn1−xCoxO3 system were investigated over a wide temperature range, 10–900 K. Two structural types, depending onx, weredetected—orthorhombic Pbnm () and rhombohedral (x>0.6), separated by thebi-phasic sample x = 0.6.At both ends of the LaMn1−xCoxO3 system, the respective substituents are unambiguously characterized asCo2+ () and Mn4+ (). On the Mn-rich side, up to x<0.5, we infer on the basis of high temperature transport and magnetic data the gradual increase of theCo2+ valence towardsthe prevailing Co3+, while for the Co-rich samples (), the Co3+ states tend to disproportionate at high temperatures toCo2++Co4+, which probably controls both the transport and magnetic properties. At low temperatures,the long range ferromagnetic order was confirmed by means of neutron diffraction for thex = 0.4 sample. A non-uniform magnetic state was detected at higher cobalt substitution up tox = 0.8 and was associated with FM clusters that are formed below K and coagulate below K.

Magnetic Properties of Ferromagnetic Nanostructures at Surface Studied by Surface Magneto-optical Kerr Effect

Formation process of ferromagnetic nanodot arrays on a periodically nanopatterned Cu(001) surface with nitrogen atoms are overviewed, and their magnetic properties studied by surface magneto-optical Kerr effect (SMOKE) in ultra high vacuum are described. The square lattice nanopattern induced by the strain due to nitrogen adsorption is formed by the strain-relief mechanism of the Cu lattice near the surface. On top of that, the diffusion and the adsorption of the deposited atoms depend on the substrate lattice strain, and the ferromagnetic metal island preferentially grow at the less strained clean Cu surface. As a result, a square nanodot array of 7 nm × 7 nm is formed on the surface. The hysteresis loops of the arrays were in-situ measured by SMOKE between 400 K and 95 K. The results indicate long range ferromagnetic order is established among the superparamagnetic Co dots of two-monoatomic-layers high by the magnetic interaction through the monolayer Co strips.

The influence of Sm doping at A site on the magnetic,electrical properties in La0.67Sr0.33MnO3

In this paper, the M_T curves, ESR, Infrared and Raman spectra, ρ_T cu rves and MR_T curves of La067-xSmxSr033MnO 3 (x=000, 010, 020, 030) are studied. The experimental results ind icate that with increasing Sm content, the compound undergoes a transition from long_range ferromagnetic order to the cluster_spin glass state, and further to a ntiferromagnetic order. The variation in magnetic structure and extra magnetic c oupling caused by Sm doping lead to colossal magnetoresistance effect.

Influence of heavy-doping with Dy on the magnetic and electrical properties in La0.7Sr0.3MnO3

The M_T curves, M_H curves, ρＴ curves and MR_T curves of La0.7-xDyxSr0.3MnO3 (x=000，030，040，050，060 and 070) are studied. Experimental results indicate that with increasing do ping concentration of Dy, the system changes from long_range ferromagnetic order to cluster_spin glass state, then to antiferromagnetic state and to subferromag netic state. For x=03, there appears a peak on the M_T curve near T N,however, it disappears at x=04. For x=05, and zero field cooli ng(ZFC) the curve shows a valley, and then a peak at TN; after the 001 T field cooling process, the initial value of M is negative at low temperatur es. For x=060 and 070, their ZFC and FC M_T curves are both similar to the ZFC one of x=050, and the transport properties at heavy doping beco me abnormal under a magnetic background. The peculiar phenomena shown above can be interpreted in terms of the Nel's double_lattice model combined with the re sultsof M_H hysteresis at the characteristic temperatures.

Ferromagnetism in diluted magnetic semiconductor quantum dot arrays embedded in semiconductors

We present an Anderson-type model Hamiltonian with exchange coupling between the localized spins and the confined holes in the quantum dots to study the ferromagnetism in diluted magnetic semiconductor (DMS) quantum dot arrays embedded in semiconductors. The hybridization between the quantum-confined holes in the DMS quantum dots and the itinerant holes in the semiconductor valence band makes hole transfer between quantum dots, which can induce the long range ferromagnetic order of the localized spins. In addition, it makes the carrier spins both in the DMS quantum dots and in the semiconductors polarized. The spontaneous magnetization of the localized spins and the spin polarization of the holes are calculated using both the Weiss mean field approximation and the self-consistent spin wave approximation, which are developed for the present model.

Field induced two-dimensional ferromagnetic ordering in a gadolinium stearate Langmuir-Blodgett film

Magnetic properties of materials confined to nanometer length scales are providing important information regarding low dimensional physics. Using gadolinium based Langmuir-Blodgett films, we have shown that a two-dimensional short range ferromagnetic order with no spontaneous magnetization occur as the field is applied along the normal to the growth (in-plane) direction. However, along the growth (out-of-plane) direction, we observe expected paramagnetic behavior down to a temperature of 2 K and a field of up to 70 kOe. We found that in-plane saturation moment increases and the field required to get this saturation decreases with decreasing temperature. These results are consistent with the theoretical predictions on isolated two-dimensional spin system having critical point at $T=0.$ We have provided preliminary explanations of the observed field induced two-dimensional ferromagnetic ordering by considering asymmetric exchange coupling.

Low-field transverse magnetovoltage in manganite films

The temperature and in plane magnetic field dependences of the transverse voltage, VY, were studied in patterned epitaxial films of La0.7Ca0.3MnO3(LCMO)/MgO and La0.7(Ca0.5Sr0.5)0.3MnO3(LSCMO)/MgO. Above the Curie temperature, TC, VY(T) shows the same insulating behavior as the longitudinal voltage, VX(T). With the onset of long range ferromagnetic order near TC, an additional contribution to VY appears, exhibiting a sin(2α) dependence on the angle α between the current and the magnetic field, H. An hysteretic VY(H) behavior with two minimums at the coercive field, ±HC, due to the magnetization reversal, was found in the ferromagnetic regime. A magneto voltage, ΔVY(0.5 kOe)/VY(0)=400%, and a low-field sensitivity of 20%/Oe (50 μV/Oe) for Hext=10–20 Oe in a LSCMO film were observed at room temperature. The results are explained as an anisotropic magnetoresistance (AMR) effect, which becomes maximal slightly below TC. The maximal AMR ratios, 1.7% and 1.5–2.3%, were obtained for LCMO and LSCMO films, respectively.