M. Özdemir

Magnetic anisotropies in ultrathin iron films grown on the surface-reconstructed GaAs substrate

Magnetic anisotropies of epitaxial ultrathin iron films grown on the surface-reconstructed GaAs substrate were studied. Ferromagnetic resonance technique was exploited to determine magnetic parameters of the films in the temperature range of 4–300K. Extraordinary angular dependence of the FMR spectra was explained by the presence of fourfold and twofold in-plane anisotropies. A strong in-plane uniaxial anisotropy with magnetic hard axis along the [11¯0] crystallographic direction is present at the GaAs∕Fe(001) interface while a weak in-plane uniaxial anisotropy for the Fe grown on Au has its easy axis oriented along [11¯0]. A linear dependence of the magnetic anisotropies as a function of temperature suggests that the strength of the in-plane uniaxial anisotropy is affected by the magnetoelastic anisotropies and differential thermal expansion of contacting materials.

Ferromagnetic resonance studies of exchange coupled ultrathin Py/Cr/Py trilayers

Magnetic properties of ultrathin Py/Cr/Py trilayers have been investigated as a function of Cr spacer layer thickness by using ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques. The Cr spacer layer thickness was increased from 4 to 40 A with 1 A steps to determine the dependence of interlayer exchange coupling between ferromagnetic layers on the spacer layer thickness. Two strong and well resolved peaks were observed which correspond to a strong (acoustic) and weak (optic) modes of magnetization precession in the effective dc field due to the exciting external microwave field as the external dc field orientation comes close to the film normal. The separation of the two modes in the field axis depends on the thickness of Cr spacer layer. An interchange in the relative positions of the acoustic and optic modes has been observed for a particular thickness of Cr spacer layer as well. A computer program for magnetically exchange coupled N magnetic layers was written to simulate t...

Ferromagnetic resonance in double perovskite epitaxial thin films of La2NiMnO6 on SrTiO3 and NdGaO3 substrates

Ferromagnetic resonance (FMR) studies of epitaxial La2NiMnO6 (LNMO) thin films on (100) oriented SrTiO3 and (110) oriented NdGaO3 substrates at room temperature are presented. Observation of FMR spectra above the Curie temperature of this compound confirms the presence of magnetic ordering in LNMO thin films at room temperature. Best fitting of FMR spectra has been made on the assumption of the coexistence of two magnetic phases with different easy and hard axis periodicities in the film plane of LNMO. The spectra of the films on various substrates are characterized by different in-plane and out-of-plane symmetries, which indicates the existence of different epitaxial growth on the substrates with different crystal symmetries and orientations.

The role of Pt impurities on both bulk and surface anisotropies in amorphous NiMn films

Abstract Electron spin resonance (ESR) measurements have been carried out on amorphous Ni76Mn24 and Ni74Mn24Pt2 films in the temperature range 4–300 K. The spectra at each temperature have been taken for both parallel geometry, where the applied magnetic field is in the surface of the films and perpendicular geometry with the magnetic field perpendicular to the surface. Using computer simulations, the bulk (K) and surface anisotropy constants (Q and ks), the exchange stiffness constants (D) and the induced exchange fields (Hexc) were obtained for both films as a function of temperature. At lower temperatures, we were not able to analyse the ESR spectra due to very broad lines, which are associated with the frustration of spins. The frustration manifests itself at higher temperatures for Ni74Mn24Pt2 than for Ni76Mn24 and becomes more severe with Pt impurities. The induced exchange anisotropy, Hexc, which is created during cooling of the sample rotates freely towards the applied field. The temperature behaviour of this anisotropy is in agreement with the exponential law (exp(-α/T) for both samples. The exchange stiffness constants, D, of both samples show similar behaviour, D increases with decreasing temperature, slowly at first above certain temperature values (100 K for Ni76Mn24 and ∼ 150 K for Ni74Mn24Pt2) and then more rapidly at lower temperatures, being related to the frustration of the system. Furthermore, the deduced bulk and surface anisotropy coefficients from the ESR data analysis revealed that Pt impurities have a considerable influence on the domain structure and, accordingly, the surface anisotropies of the NiMn system.

Ferromagnetic resonance investigation of Py/Cr multilayer system

The multilayer thin films consisting of alternating stacks of ferromagnetic Permalloy (Py) and ultra thin non-magnetic (sub-nanometer) spacer (Cr) layers have been investigated by using ferromagnetic resonance (FMR) and dc magnetization measurement techniques. The non-magnetic spacer layer thickness changes from 0.5 A to 2.5 A by 0.5 A steps. Polycrystalline composite metallic films were grown on Si substrate by usual magnetron sputtering techniques at UHV conditions. The magnetic hysteresis curves were recorded by conventional dc magnetization. The ac and dc magnetic properties were investigated by using ferromagnetic resonance. FMR measurements were carried out for different directions of external dc magnetic field in order to search magnetic anisotropy. Two strong and well defined peaks (acoustic and optic mode) were observed in FMR spectrum as the magnetic field direction approaches to the film normal. Their positions and relative intensities helped to characterize coupling species. The magnetic parameters have been deduced by using a theoretical model. It has been found that the exchange coupling parameter between ferromagnetic layers through non-magnetic Cr spacer is ferromagnetic in nature and strongly decreases with increasing spacer layer thickness. The magnetic anisotropy parameters strictly depend on magnetic layer thickness while the dc magnetization is almost constant for a few nanometer thick Permalloy as well. We observed only ferromagnetic coupling between ferromagnetic layers in all samples.The multilayer thin films consisting of alternating stacks of ferromagnetic Permalloy (Py) and ultra thin non-magnetic (sub-nanometer) spacer (Cr) layers have been investigated by using ferromagnetic resonance (FMR) and dc magnetization measurement techniques. The non-magnetic spacer layer thickness changes from 0.5 A to 2.5 A by 0.5 A steps. Polycrystalline composite metallic films were grown on Si substrate by usual magnetron sputtering techniques at UHV conditions. The magnetic hysteresis curves were recorded by conventional dc magnetization. The ac and dc magnetic properties were investigated by using ferromagnetic resonance. FMR measurements were carried out for different directions of external dc magnetic field in order to search magnetic anisotropy. Two strong and well defined peaks (acoustic and optic mode) were observed in FMR spectrum as the magnetic field direction approaches to the film normal. Their positions and relative intensities helped to characterize coupling species. The magnetic param...

A spin-wave resonance study on reentrant Ni77Mn23 thin films

Abstract The magnetic properties of thin (300 A) reentrant Ni 77 Mn 23 films grown on a quartz substrate using the electron beam evaporation technique have been investigated. The spin-wave resonance (SWR) technique has been used to study the exchange parameter D and magnetic anisotropies as a function of temperature between 4 and 300 K. The SWR spectra exhibit directional character upon cooling the sample in an external field H c , which is a sign of a spin-glass property. At higher temperatures, in addition to the dominant surface SWR mode, there is a much weaker bulk mode on the lower field side. Using a uniaxial surface and effective bulk anisotropy together we simulated the experimental spectra and deduced the surface anisotropy and magnetic stiffness parameters as a function of temperature. The sample shows an easy plane surface anisotropy. Both the exchange and anisotropy parameters increase when the temperature is in the spin-glass regime. Furthermore, the surface anisotropy energy dominates the overall magnetic behavior of this system.

Anomalous anisotropy of re-entrant film

Re-entrant polycrystalline films, 600 A thick, grown on a quartz substrate by using an electron beam evaporation technique, have been investigated. A spin-wave resonance (SWR) technique has been used to study the magnetic properties of the films. The SWR spectra exhibit highly anisotropic behaviour with respect to the film normal. When the sample is rotated around the film normal in a fixed applied field, both the resonance fields and the line-shapes are changed. This unexpected behaviour - for a polycrystalline film - has been attributed to a growth-induced geometric (oblique) anisotropy. The SWR spectra have been successfully analysed by using this oblique anisotropy term in addition to usual magnetocrystalline effective bulk, surface, and unidirectional anisotropy terms in the magnetic free energy. A substantial and temperature-dependent growth-induced geometric anisotropy has been obtained, beside the usual bulk and surface anisotropies. A strong correlation between the surface and oblique anisotropy has been found. This correlation has been attributed to a manifestation of the influence of easy-plane surface anisotropy along the fibre axes of individual grains on the effective magnetic anisotropy energy.

Localization in amorphous films

The electrical resistivity has been measured as a function of temperature between 1.5 - 300 K for amorphous alloys with x = 0.176, 0.22, 0.26. The resistivities all show square-root temperature dependences below the minima temperatures. The high-field magnetoresistance (H varies between 0 - 120 kOe) can be accounted for in theoretical models of localization in the presence of strong spin - orbit interaction. In addition, the spin-flip scattering rate due to local spin fluctuations decreases with increasing temperature and then levels off at about T = 50 K in a manner consistent with the magnetic state of the sample, while the inelastic scattering rate in this range remains almost of the same order. Furthermore, the magnetic anisotropy of the resistivity together with the magnetization data show that the magnetic order is progressively suppressed with increasing Fe content.

An ESR study on amorphous NiFe films

Abstract Amorphous Ni 90 Fel 10 alloy films with thickness 500 A and 2000 A deposited on quartz substrates by flash evaporation method have been studied by electron spin resonance (ESR) technique at x-band. The ESR spectra were taken as a function of both temperature and the applied magnetic field whose direction was varied from normal to parallel with respect to the film plane. The field derivative absorption line mainly consists of two major peaks for the thicker film and only a single major peak for the thinner one. The spectra have been analyzed by using the classical spin wave resonance (SWR) theory and the magnetic parameters of the system have been deduced. The surface anisotropy energy with uniaxial and biaxial terms have been used to simulate the experimental spectra. It has been found that noncrystalline nature of the sample considerably effects the magnetic behavior. The exchange stiffness constant was found to be seriously reduced compared to those of the polycrystalline films in the literature. However with room temperature aging, this value has increased visibly. Surface anisotropy has been determined to have an easy-plane character.

Superconductivity in Pt- and La-Doped BaFe 2 As 2 Compounds Prepared by Solid-State Reaction

We report magnetization, resistivity, and heat capacity measurements on samples of BaFe₂As₂, BaFe_1.9Pt_0.1As₂, and Ba_0.7La_0.3Fe_1.9Pt_0.1As₂ that have been prepared by solid-state reaction. Standard four-probe transport measurements in the temperature range 4.2-300 K showed that the substitution of non-isoelectronic Pt for Fe produced an electron-doped situation resulting in a superconductive phase transition at T_c = 24 K. Furthermore, we observe that the coupled spin-density-wave/antiferromagnetic transition which occurs for the BaFe₂As₂ sample at 140 K, as expected, also occurs in the Pt-doped BaFe_1.9Pt_0.1As₂ sample with only a small decrease in critical temperature to 138 K. We have also investigated in detail the superconducting state with magnetization measurements in the temperature range 5-400 K, up to a field of 9 T. From the magnetization data for BaFe_1.9Pt_0.1As₂, we observe that T_c = 21.7 K, which is in good agreement with the resistivity measurement. The zero-field cooled results correspond to a large Meissner fraction. In Ba_0.7La_0.3Fe_1.9Pt_0.1As₂, we also see a large diamagnetic response, with a reduction in T_c to 19.6 K. In addition, we analyzed heat capacity measurements for the electronic and phonon behavior of these materials. The analysis shows an excess entropy associated with the change in electronic behavior that is consistent with what is expected for Fe local moments, pointing to an electronic density of states that is based on a spin-fluctuation mechanism. We discuss the results in relation to a varying electronic density of states present in the samples and a possible pseudogap induced by addition of Pt.