Synthesis and positron characterizations of ferromagnetic Zn0.98Mn0.02O and paramagnetic Zn0.98Mn0.02O samples

Room temperature ferromagnetic and paramagnetic Zn_0.98Mn_0.02O samples have been synthesized by the solid state reaction method and by selecting the final annealing temperature. Employing positron annihilation techniques, defects for both samples have been characterized. The results indicate that the presence of cation vacancy type defects is relatively less in the ferromagnetic Zn_0.98Mn_0.02O sample than in the paramagnetic Zn_0.98Mn_0.02O sample.     

Optimization of Pr(0.9)Ca(0.1)MnO(3) thin films and observation of coexisting spin-glass and ferromagnetic phases at low temperature

Optimization of thin films of small bandwidth manganite, Pr(1-x)Ca(x)MnO3 (for x = 0.1), and their magnetic properties are investigated. Using different pulsed laser deposition (PLD) conditions, several films were deposited from the stoichiometric target material on SrTiO3 (001) substrate and their thorough structural and magnetic characterizations were carried out using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), SQUID magnetometry and ac susceptibility measurements. A systematic investigation shows that irrespective of the growth temperature (between 550 and 750?°C), all the as-deposited films have twin boundaries and magnetic double phases. Post-annealing in partial or full oxygen pressure removes the extra phase and the twin boundaries. Zero-field-cooled magnetization data show an antiferromagnetic to paramagnetic transition at around 100 K whereas the field-cooled magnetization data exhibit a paramagnetic to ferromagnetic transition close to 120 K. However, depending on the oxygen treatments, the saturation magnetization and Curie temperature of the films change significantly. Redistribution of oxygen vacancies due to annealing treatments leading to a change in ratio of Mn3+ and Mn4+ in the films is observed from XPS measurements. Low temperature (below 100 K) dc magnetization of these films shows metamagnetic transition, high coercivity and irreversibility magnetizations, indicating the presence of a spin-glass phase at low temperature. The frequency dependent shift in spin-glass freezing temperature from ac susceptibility measurement confirms the coexistence of spin-glass and ferromagnetic phases in these samples at low temperature.     

Magnetic and electrical properties in amorphous GeSeFe films

Amorphous GeSeFe films prepared by r.f. sputtering have shown paramagnetic, and ferromagnetic properties for the films with low and high Fe content, respectively. The temperature dependence of the electrical conductivity has indicated the variable range hopping conduction for the paramagnetic films and the metallic conduction for the ferromagnetic films. The films with intermediate Fe content have behaved intermediate between semiconductive and metallic conduction. These films are likely to be antiferromagnetic.     

Defects induced magnetic transition in Co doped ZnS thin films: Effects of swift heavy ion irradiations

The effect of swift heavy ions (SHI) on magnetic ordering in ZnS thin films with Co ions substituted on Zn sites is investigated. The materials have been synthesized by pulsed laser deposition on substrates held at 600 °C for obtaining films with wurtzite crystal structure and it showed ferromagnetic ordering up to room temperature with a paramagnetic component. 120 MeV Ag ions have been used at different fluences of 1×10¹¹ ions/cm² and 1×10¹² ions/cm² for SHI induced modifications. The long range correlation between paramagnetic spins on Co ions was destroyed by irradiation and the material became purely paramagnetic. The effect is ascribed to the formation of cylindrical ion tracks due to the thermal spikes resulting from electron–phonon coupling.     

Domain structure in uniaxial ferromagnets in the vicinity of a continuous phase transition from a paramagnetic phase to a ferromagnetic one

On the basis of the Landau-Ginzburg theory, the phase transition from the paramagnetic phase to the ferromagnetic phase with domain structure in ferromagnetic films with uniaxial anisotropy has been described. Analytic expressions have been obtained which describe the dependence of the period of the domain structure on the thickness of the film and temperature near the temperature of the phase transition. The results that have been obtained are in qualitative agreement with experimental data obtained for films made of the orthoferrites YFeO₃ and Ho_0.5 Dy_0.5FeO₃.     

Fabrication of ferromagnetic (Ga,Mn)As by ion irradiation

Using Mn⁺ implantation following ion beam-induced epitaxial crystallization (IBIEC) annealing, high Curie temperature ferromagnetic (Ga,Mn)As thin film was fabricated. The crystalline quality of the Mn⁺ implanted layer was identified by X-ray diffraction (XRD) and transmission electron microscopy (TEM). A clear ferromagnetic transition at T_c 253 K was observed by magnetization vs. temperature measurement. We infer that IBIEC treatment is a useful method not only for the low-temperature annealing of (Ga,Mn)As thin films but also for other dilute magnetic semiconductor (DMS) samples.     

Evidence of cobalt-vacancy complexes in Zn(1-x)Co(x)O dilute magnetic semiconductors

We investigate the local structure of ferromagnetic Zn(1-x)Co(x)O epilayers by coupling polarization-dependent x-ray absorption spectroscopy and ab initio calculations of selected defect structures. We give clear evidence of the presence of oxygen vacancies, located close to the Co atoms in a specific complex configuration. We also establish the upper concentration limit of metallic parasitic nanophases and their contribution to magnetism. Our results lead to the conclusion that oxygen vacancies play an important role in originating the high temperature ferromagnetism of Zn(1-x)Co(x)O.     

Crystallization-dependent magnetic properties of Mn1.56Co0.96Ni0.48O4 thin films

The effects of magnetic property dependence of the Mn_1.56Co_0.96Ni_0.48O₄ (MCN) films on crystallization are investigated in the growth temperature of 450-750 °C. With the growth temperature increase, both the crystalline quality and the grain size improve. The MCN films exhibit paramagnetic to ferromagnetic transition and the paramagnetic parts fit to the modified Curie-Weiss law. The ferromagnetic couplings of the magnetic ions in the MCN films enhance at elevated growth temperature. The saturation magnetization at 5 K increases with increasing growth temperature, but coercive field decreases monotonously. The magnetic properties of the MCN films strongly depend on their microstructures.     

Enhancement of room temperature ferromagnetism of Fe-doped ZnO epitaxial thin films with Al co-doping

Epitaxial films of ZnO doped with magnetic ion Fe and, in some cases, with 1% Al show clear evidence of room temperature ferromagnetic ordering. The Al doped optimized samples with carrier concentration n_c∼8.0×10²⁰ cm⁻³ show about 3 times enhanced saturation magnetization (0.58 μ_B/Fe²⁺) than the one with n_c∼3.0×10²⁰ cm⁻³ (0.18 μ_B/Fe²⁺). A clear correlation between the magnetization per transition metal ion and the ratio of the number of carriers to the number of donors have been found as is expected for carrier-induced room temperature ferromagnetism. The transport mechanism of the electrons in all the DMS films at low temperature range has been identified with the Efros's variable range hopping due to the electron-electron Coulomb interaction.     

Magnetic phase diagram of La(Fe0.873Co0.007Al0.12)13 cluster intermetallic compound

The magnetic properties of the La(Fe_0.873Co_0.007Al_0.12)₁₃ intermetallic compound have been studied. The compound is a cluster antiferromagnet showing the metamagnetic transition. The magnetic phase diagram of the compound has been constructed. The temperature dependence of the inverse paramagnetic susceptibility of La(Fe_0.873Co_0.007Al_0.12)₁₃ obeys the Curie–Weiss law. The large positive paramagnetic Curie point indicates the presence of predominantly ferromagnetic exchange interactions. The Neel temperature can be considered as the temperature of the ferromagnetic ordering of the Fe magnetic moments in clusters coupled antiferromagnetically.     

Spatially resolved inhomogeneous ferromagnetism in (Ga,Mn)as diluted magnetic semiconductors: a microscopic study by muon spin relaxation

Thin epitaxial films of the diluted magnetic semiconductor (DMS) GaMnAs have been studied by low energy muon spin rotation and relaxation (LE-microSR) as well as by transport and magnetization measurement techniques. LE-microSR allows measurements of the distribution of magnetic field on the nanometer scale inaccessible to traditional macroscopic techniques. The spatial inhomogeneity of the magnetic field is resolved: although homogeneous above Tc, below Tc the DMS consists of ferromagnetic and paramagnetic regions of comparable volumes. In the ferromagnetic regions the local field inhomogeneity amounts to 0.03 T.     

Crystal structure and magnetic properties of copper formate anhydrate [α-Cu(HCOO)2]

The crystal structure of anhydrous copper formate [α-Cu(HCOO)₂] has been refined from neutron powder data by the profile analysis technique at three different temperatures. Hydrogen atoms were located and refined with anisotropic temperature factors. The copper ion has a Jahn-Teller distorted tetragonal pyramidal coordination. Magnetic measurements showed a paramagnetic behaviour in the temperature region 10–300 K. The positive Curie temperature of θ_{c = 9.8(3}) suggests, however, a ferromagnetic ordering at low temperatures.     

The influence of iron,fluorine and boron implantation on the magnetic properties of graphite

Iron, fluorine and boron ions were implanted into highly oriented pyrolytic graphite (HOPG). The samples were characterized before and after ion implantation as well as after heat treatments in vacuum by measurements of the magnetic moment and element analysis. Whereas the main magnetic contribution remains diamagnetic the paramagnetic one clearly increases with implantations and correlates with the amount of implanted ions. It is shown that a large part of the paramagnetic contribution is caused by the structural disorder created by particle bombardment using iron, fluorine or boron. All implanted HOPG samples show practically no change of the small ferromagnetic signal observed in their virgin state. No particular influence of iron on the ferromagnetic properties of HOPG is observed, up to ∼4000 μg/g Fe-concentration in the implanted region. For comparison, ferrous sulphates were added to ultra-clean graphite powder. This iron addition increases the number of paramagnetic spins proportional to the iron content in the untreated samples. In heat-treated samples however, a clear ferromagnetic behaviour is observed due to the formation of a ferromagnetic iron compound.     

Magnetic properties of Er-doped ZnO films prepared by reactive magnetron sputtering

All Zn_1−x Er _x O (x=0.04, 0.05, and 0.17) films deposited on glass substrates by radio-frequency reactive magnetron sputtering exhibit the mixture of ferromagnetic and paramagnetic phases at room temperature. The estimated magnetic moment per Er ion decreases with the increase of Er concentration. The temperature dependence of the magnetization indicates that there is no intermetallic ErZn buried in the films. The ferromagnetism is attributed to the Er ions substitution for Zn²⁺ in ZnO lattices, and it can be interpreted by the bound-magnetic-polaron model.     

Three-dimensional bulk fermiology of CeRu2Ge2 in the paramagnetic phase by soft x-ray hnu-dependent (700-860 eV) ARPES

By virtue of the soft x-ray angle-resolved photoelectron spectroscopy, the three-dimensional bulk fermiology has been successfully performed for a strongly correlated Ce compound, ferromagnet CeRu2Ge2 in the paramagnetic phase. A clear difference of the Fermi surface topology from either band calculation or de Haas-van Alphen results in the ferromagnetic phase is observed and interpreted by considering the difference of the 4f contribution to the Fermi surfaces in the paramagnetic phase.     

Phase domain structures in cylindrical magnets under conditions of a first-order magnetic phase transition

The magnetic and resonance properties of cylindrical magnets at first-order phase transition from paramagnetic to ferromagnetic state were theoretically studied. It has been shown that in the external magnetic field directed perpendicularly to the rotation axis, formation of a specific domain structure of paramagnetic and ferromagnetic layers can be energetically favorable. The parameters of cylindrical phase domains as well as their dependences on temperature, magnetic field and material characteristics have been calculated. Peculiarities of the magnetic resonance spectra appearing as a result of the phase domain formation have been considered. Dependence of the resonance field of the system of ferromagnetic domains on magnetization and temperature has been obtained.     

Finite size effect and phase diagram of ultra-thin La0.7Sr0.3MnO3

La_0.7Sr_0.3MnO₃ (LSMO) ultra-thin films have been grown by pulsed laser deposition in conditions optimized for cation stoichiometry and bulk-like physical properties in the thick limit. Through electrical transport and magnetic measurements, a phase diagram is constructed as a function of film thickness. With decreasing thickness, the LSMO films cross over at high temperatures from a paramagnetic metal to a paramagnetic insulator, and at low temperatures from a ferromagnetic metal to a ferromagnetic insulator, in close similarity to that observed for varying the electronic bandwidth in bulk manganites.     

Micromagnetic imaging to determine the nature of the ferromagnetic phase transition in La(0.7)Ca(0.3)MnO3

There is considerable controversy surrounding the nature of the paramagnetic to ferromagnetic phase transition in La(0.7)Ca(0.3)MnO3. We have used transmission electron microscopy to determine whether the phase transition is first or second order. On warming through the transition point, the ferromagnetic phase retreats from the sample surface as it is replaced by the paramagnetic phase. This coexistence of ferromagnetic and paramagnetic phases indicates a primarily first order transition. However, there is also continuous loss of magnetization which precedes the phase transition. We compare this with the phase transition in nickel, an archetypal second order ferromagnet.     

Microfabrication of FeMnPt films involving magnetic phase change due to structural transformation caused by ion irradiation

Correlations between crystal structures and magnetic properties of Fe_1–xMn_x Pt films were studied. The disordered films with x ≥ 0.44 had paramagnetic properties and the ordered films with x ≥ 0.46 had antiferromagnetic properties, viz. a difference of 0.02 in the x ‐value (i.e., 1.0 at%) was found. At x = 0.44 with the ordered structure, the uniaxial magnetocrystalline anisotropy was about 2.1 × 10⁷ erg cm^–3. A microfabrication process involving the slight composition difference of 0.02, which results in ferromagnetic–paramagnetic phase change due to the structural transformation caused by ion irradiation, was investigated. Only the area irradiated by Mn ions changed from ferromagnetic to paramagnetic phase. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

On the origin of the ferromagnetism in Zn0.8Mn0.2O having a higher Curie temperature than Zn0.8Co0.2O

Zn_0.8Co_0.2O and Zn_0.8Mn_0.2O films were deposited on substrates by a sol–gel technique. X-ray diffraction, field-emission scanning electron microscopy, photoluminescence, and ferromagnetism measurements were used to characterize these dilute magnetic semiconductors. It is shown that the ferromagnetic properties might be related to the formation of acceptor-like defects in the Zn_0.8Co_0.2O and Zn_0.8Mn_0.2O films. It is found that ferromagnetic Zn_0.8Mn_0.2O has a higher Curie temperature than Zn_0.8Co_0.2O. In addition, the higher ratio of grain-boundary area to grain volume of Zn_0.8Mn_0.2O than Zn_0.8Co_0.2O indicates that grain boundaries and related acceptors are the intrinsic origin for ferromagnetism.