In-plane Hysteresis Loops Research Articles

Influence of dipolar interaction on small magnetic dot arrays

The effects of dipolar interaction in an array of small magnetic dots with perpendicular anisotropy are studied numerically within the framework of the Landau–Lifshitz–Gilbert equation. In the absence of a magnetic field, three typical configurations of the magnetic moments are found, depending on the dipolar coupling strength. Magnetization processes both parallel and perpendicular to the array are studied. The hysteresis loops are found to be highly sensitive to the dipolar coupling strength. The in-plane hysteresis loops are dominated by the strong dipolar interaction between particles, while the out-of-plane hysteresis loops are dominated by the perpendicular anisotropy. The coercive field, saturation field, and remanence are also sensitive to the strength of dipolar interaction. Dipole interactions also affect the characteristic switching time in a coupled array. Depending on the packing density of the dots in an array, the switching time may be shortened or lengthened.

Mn- and Cr-Doped InN: A Promising Diluted Magnetic Semiconductor Material

Cr- and Mn-doped InN films were successfully grown by plasma-assisted molecular beam epitaxy on c-plane sapphire substrates. Low temperature GaN buffer layers grown by metal-organic vapor-phase epitaxy were used to accommodate the large lattice mismatch between InN and sapphire. A high n-type carrier concentration of 1.5×1020 cm−3 was measured in InN films with 3% Cr-doping. Films of this type exhibit a well-defined in-plane magnetic hysteresis loop and remanence for temperatures varying from 5 to 300K. The Mn-doped films, however, turned out to exhibit less clear magnetic properties. Thus, ferromagnetism in Cr-doped InN can be concluded from our measurements.

Direct observation of strain fields in epitaxial growth Fe3O4 thin films on MgO substrates

Fe 3 O 4 thin films with a thickness of about 140 nm have been deposited on MgO (001) substrates by reactive sputtering. Microstructural studies indicated that monocrystalline Fe3O4 thin films were epitaxially grown on the MgO substrates with a cubic-on-cubic orientation relationship with respect to the substrates. Strain field due to slight lattice mismatch between the films and substrates was observed. The saturation moment and the coercivity for in-plane magnetization hysteresis loop were 235 emu/cc and 500 Oe, respectively. The lower saturation moment value of the Fe3O4 thin film suggested that the strain field may play a role in the reduction of magnetization.

Thickness-dependent magnetic excitations in Permalloy films with nonuniform magnetization

The static and dynamic properties of Permalloy films with thicknesses varying from 200 nm to 920 nm have been investigated in detail. For these films possessing a perpendicular anisotropy, the critical thickness for stripe domain nucleation has been determined using magnetic force microscopy imaging and in-plane hysteresis loop measurements. The zero-field dynamic permeability spectra measured over the frequency range 0.1--6 GHz reveal one resonance line below the critical thickness, whereas multiple resonance peaks whose number increases with increasing film thickness are observed above the critical thickness. The two-dimensional dynamic micromagnetic simulations reproduce successfully the thickness evolution of the experimental permeability spectra and give access to the thickness-dependent spatial localization of the main modes. However, the experimental resonance linewidths exceed the computed ones for the thickest films. Extended dynamic micromagnetic simulations including the screening of the pumping field due to eddy currents account only partially for resonance line broadening. The existence of additional relaxation mechanisms due to magnetic inhomogeneities (micromagnetic and structural) is discussed on the basis of parallel ferromagnetic resonance measurements versus frequency.

Disorder influence on the magnetic properties of La0.55Sr0.45MnO3/SrTiO3 superlattices

The structural and physical properties of La0.55Sr0.45MnO3/SrTiO3 superlattices grown by magnetron sputtering are studied. Two deposition temperatures and different mismatched substrates and buffer layers were used. The structure was determined by refinement through X-ray diffraction pattern fitting. The results indicate 1 unit cell (u.c.) interdiffusion at the interfaces and a 1 u.c. layer thickness fluctuation, i.e. roughness. In-plane hysteresis loops show the expected ferromagnetic behavior, while the perpendicular-to-plane hysteresis loops show anomalies which could evidence magnetic domain size inhomogeneities and stress.

Disorder influence on the magnetic properties of La0.55Sr0.45MnO3/SrTiO3 superlattices

Abstract The structural and physical properties of La 0.55 Sr 0.45 MnO 3 /SrTiO 3 superlattices grown by magnetron sputtering are studied. Two deposition temperatures and different mismatched substrates and buffer layers were used. The structure was determined by refinement through X-ray diffraction pattern fitting. The results indicate 1 unit cell (u.c.) interdiffusion at the interfaces and a 1 u.c. layer thickness fluctuation, i.e. roughness. In-plane hysteresis loops show the expected ferromagnetic behavior, while the perpendicular-to-plane hysteresis loops show anomalies which could evidence magnetic domain size inhomogeneities and stress.

Structure, magnetic and electrical properties of soft magnetic Co–C amorphous thin films

Amorphous Co x C 1− x thin films, with x in the range of 60–75% in atomic percentage, have been prepared by pulsed filtered vacuum arc deposition. The structures of the films were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The in-plane magnetic hysteresis loops were measured by a superconducting quantum interference device magnetometer at room temperature. The electrical transport properties were measured by the four-probe technique at various temperatures ranging from 20 to 300 K. The films were found to be magnetically soft with coercivities in the range of 2 to 12 Oe, resistivities in the range of 130 to 300 μ Ω cm, and magnetic saturation flux densities in the range of 6 to 13 kG. The films also showed good thermal stability in their structural, electrical and magnetic properties upon annealing up to 200 °C in a vacuum furnace.

Dielectric Characteristics of In-Plane Polarized Lead Zirconate Titanate Thin Films on Oxide Layers

Sol-gel derived lead zirconate titanate (PZT) films have been prepared on STO-passivated silicon substrates. The STO buffer layer of thickness about 55 nm is prepared by rf-magnetron sputtering. XRD results reveal that the PZT film has well-crystallized perovskite phase, indicating that the thin STO layer can effectively prevent reaction and inter-diffusion between the PZT film and silicon substrate. Interdigitated electrodes (IDEs) have then been deposited on the PZT film by magnetron sputtering and patterned using the standard photolithography. With the IDEs, the dielectric and ferroelectric properties of the PZT film under transverse or in-plane electric fields have been investigated. By assuming a uniform distribution of electric field (in-plane electric field model), the estimated relative permittivity of the PZT film is about 2100, while the dielectric loss is less than 1%. Good in-plane polarization hysteresis loop is observed, showing an observed remanent polarization value of 21 μC/cm2, which is comparable to that of a PZT film with both top and bottom electrodes. The in-plane polarized PZT/STO/SiO2/Si film can be used to fabricate d33-mode unimorph bending transducers, which will have much better performance than the conventional bending transducers driven electromechanically through the piezoelectric d31 mode.

Permalloy thin films on MgO(001): Epitaxial growth and physical properties

Permalloy thin films were deposited onto MgO(001) substrates by standard sputtering technique at growth temperatures ranging from 200 to 800 °C. Both reflection high-energy electron diffraction (RHEED) experiments and atomic force microscopy observations reveal that the substrate temperature for two-dimensional epitaxial growth should not exceed 350 °C. A series of permalloy films with thicknesses ranging from 3.5 to 100 nm was prepared at 300 °C. All films were grown as (001) single crystal with the same crystallographic orientations as the MgO(001) substrates. As evidenced by RHEED and x-ray diffraction, films with thicknesses larger than about 20 nm present a structural relaxation. The in-plane magnetization hysteresis loops obtained by longitudinal Kerr loop measurements exhibit a 〈100〉 fourfold cubic anisotropy and an enhancement of the magnetic softness with decreasing film thickness, both likely originating from the large interfacial strain configuration.

Magnetically Soft Co-C Granular-like Amorphous Thin Films With High Resistivity and High Saturation Flux Density

AbstractGranular-like amorphous CoxC1-x nanocomposite thin films, with x in the range of 60-75% in atomic percentage, have been prepared by pulsed filtered vacuum arc deposition. The structures of the films were characterized by non-Rutherford backscattering spectrometry, transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and Raman spectroscopy. The in-plane magnetic hysteresis loops were measured by a superconducting quantum interference device magnetometer at room temperature. The electrical transport properties were measured by the four-probe technique at various temperatures ranging from 20 to 300 K. The films were found to be magnetically soft with coercivities in the range of 2 to 12 Oe, resistivities in the range of 130 to 300 μΩcm, and magnetic saturation flux densities in the range of 6 to 13 kG. The films also showed good thermal stability in their structural, electrical and magnetic properties upon annealing up to 200°C in a vacuum furnace.

Electrical and pyroelectric properties of in-plane polarized lead lanthanum titanate thin film

Pb 0.9 La 0.1 Ti 0.975 O 3 (PLT10) thin films were deposited on SiO2/Si(100) substrates coated with a ZrO2 buffer layer. Studies by x-ray diffraction and scanning electron microscopy reveal that the ZrO2 film consists of both tetragonal and monoclinic phases, with the tetragonal phase being the dominant one. The PLT10 film has a perovskite structure and the grains in the film have a rather uniform size of about 50 nm. By using interdigital transducer (IDT) electrodes the in-plane electrical properties, hysteresis loop, and pyroelectric coefficient of the PLT10 film were measured. The dielectric constant and loss factor vary only slightly with frequency in the range 103–106 Hz, with the loss factor being less than 0.01 over the entire range. The leakage current density is lower than 2×10−8 A/cm2 at a bias field of 5 kV/cm. The remnant polarization and coercive field are 12.6 μC/cm2 and 9.93 kV/cm, respectively. The film exhibits a reasonably high pyroelectric coefficient (95 μC/m2 K) after it has been poled by applying 120 V ac at 0.1 Hz across the IDT electrodes.

Magnetic anisotropy of epitaxial Fe (0 0 1) micrometric squares

Arrays of single crystalline Fe (0 0 1) squares separated by submicrometric distances have been fabricated by electron beam lithography and ion beam etching to study the influence of size reduction on the magnetic anisotropy properties. This magnetic behaviour has been analysed by in-plane magneto-optical hysteresis loops. For large square sizes, the angular dependence of the remanence is consistent with the Fe cubic magnetic anisotropy. On the other hand, a dramatic change is found for square sizes smaller than 3 μm, where an uniaxial magnetic behaviour is observed.

Temperature dependent chemical ordering in FePt(001) and FePt(110) films

The growth temperature is one of the critical parameters in the formation of the chemically ordered L10 phase in FePt. To uniquely determine the influence of growth temperature, while other growth parameters remain fixed, we present the “temperature wedge method” where a calibrated temperature gradient of several hundred Kelvin is established across the substrate during co-deposition of Fe and Pt. Samples were deposited in a temperature range of 250 °C–600 °C simultaneously on MgO(001) and MgO(110) substrates. The Fe≈50Pt≈50 composition was verified by Rutherford backscattering. The chemical order parameter, S, at different positions along the samples, i.e., at different growth temperatures, was determined by x-ray diffraction measurements, and it was found that at all temperatures S(001)>S(110). Perpendicular and in-plane magneto-optic Kerr hysteresis loop measurements indicate large magnetic anisotropy at the “hot ends” of the samples, corresponding to large values of S.

The magnetic properties of cobalt films produced by glancing angle deposition

The magnetic properties of columnar cobalt films fabricated by glancing angle deposition were investigated. Varying the azimuthal rotation speed and polar angle of incidence of the vapor flux produces films with different microstructures. The magnetic anisotropies and squareness ratio between perpendicular and in-plane hysteresis loops are measured for various films and found to have a weak dependence on the azimuthal rotation speed of the substrate at the polar angle of 75/spl deg/. The squareness ratio is strongly affected by the variation of the polar angle of incidence of the vapor flux. At an azimuthal rotation speed of 0.092 rev/s and polar angle of 83/spl deg/, the Co film is composed of well-separated columns with the packing fraction of 0.3. The hysteresis loops of this film exhibit a perpendicular anisotropy with the squareness ratio of 1.6. The X-ray diffraction spectrum shows that the Co columns possess a strong (21~1~2) preferred orientation, which indicates that the perpendicular anisotropy is from shape anisotropy and not from crystalline anisotropy.

Origin of the magnetization reversal of an Fe thin film on Si(111)

The magnetic behavior of a 60-\AA{} Fe film grown on Si(111) is reported and discussed. Scanning tunneling microcopy images showed Fe stripes along one of the $〈110〉$ directions in the (111) plane, which favors easy magnetization axis parallel to the stripes. Inverted in-plane hysteresis loops and loops with unusual local peaks were measured and interpreted. A phenomenological model was proposed to interpret the magnetization data and an excellent concordance between the experimental and the calculated curves was obtained. The peculiarities in the magnetization curves are directly related to the very small misorientation of the sample's surface from the (111) plane, and to competing anisotropies. The model calculations showed that the magnetization leaves the sample's plane for small fields, forming an obtuse angle with the field for some configurations, thus resulting in negative remanence, which was confirmed by the nonzero values of the normal to the plane magnetization component from the magneto-optic Kerr effect polarimetry measurements.

Magnetic Anisotropy and Ferromagnetic Resonance in Co/Pb Multilayers

The magnetic properties of sputtered Co/Pb multilayers have been studied by vibrating-sample magnetometer (VSM), torque magnetometer and ferromagnetic resonance spectroscopy (FMR). Magnetic measurements show that with decreasing Pb layer thickness the perpendicular anisotropy field H u increases from 5.7 to 8.3 kOe. The interfacial anisotropy is obtained to be as small as K s = 0.006 erg/cm 2 . It is demonstrated that the increase of H u with decreasing Pb layer thickness is due to the variation of texture in h.c.p. Co along the [00.2] direction with spacer thickness. The in-plane magnetic hysteresis loops show a higher saturation field and smaller remanence ratio for thin spacer films which cannot be accounted for solely by H u and indicates the existence of antiferromagnetic (AFM) coupling between Co layers through Pb spacer. The interlayer coupling is also implied by the observation of spin wave resonance (SWR) in perpendicular FMR. From the SWR spectrum the effective exchange constant is obtained as A eff = 3.4 x 10 -7 erg/cm. A resonance peak appears on the high-field side of the uniform FMR mode, which is attributed to the AFM coupling of adjacent ferromagnetic layers producing an optical mode. The Pb layer thickness (t Pb ) dependence of the separation of the resonance fields, ΔH, between the optical mode anti uniform mode in the perpendicular FMR spectrum is fitted by ΔH = 2.80 exp (-(t Pb - 1))/1.78 (kOe) with a characteristic thickness of 1.78 nm. Thus, the coupling strength (J = M s t Co ΔH/2), in proportion to ΔH, decreases exponentially with Pb layer thickness, which is consistent qualitatively with the variation of the in-plane remanence ratio with spacer thickness.

Co-sputtered Thin Films Consisting of Cobalt Nano-Grains Embedded in Graphite-like Carbon and Their Magnetic Properties

The microstructure and the magnetic properties of co-sputtered cobalt-carbon thin films were analyzed. Cobalt-carbon thin films consisting of nano-crystalline cobalt grains separated by graphite-like carbon were obtained after subsequent annealing. The phase of the cobalt grains depended on the carbon concentration; a hexagonal close-packed phase and a heavily faulted close-packed phase were observed. The grain size ranged from 4 to 20 nm. The in-plane magnetic hysteresis loops measured at room temperature showed that the coercivity and the squareness were drastically changed with the carbon concentration. These changes can be related to a change in the magnetocrystalline anisotropy of the cobalt grains due to a difference in the phase and/or size effect that can be explained by the presence of superparamagnetic particles. The highest coercivity (660 oersteds) was measured in a film with a large grain size as well as a hexagonal close-packed cobalt phase. The annealing temperature was found to have an optimum value; the magnetic properties were improved upon annealing due to carbide decomposition, but at high annealing temperatures, coalescence of the grains resulted in sample degradation.

Ultrathin magnetic films: Electrochemistry versus molecular-beam epitaxy

Ultrathin Co films have been electrodeposited on Cu(001) from an aqueous ${\mathrm{Na}}_{2}$${\mathrm{SO}}_{4}$/${\mathrm{CoSO}}_{4}$ electrolyte under cleanliness conditions equivalent to ultrahigh vacuum of 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ mbar. Films thicker than 1.5 ML show in-plane magnetization and square hysteresis loops. The saturation magnetization increases linearly with film thickness. The magnetization vanishes below 1.5 ML in agreement with molecular-beam epitaxy (MBE) films at 300 K. The coercivity varies approximately linearly with film thickness and is ten times larger than usually observed in MBE-grown Co. ``Magnetically dead'' layers have not been found.

Small angle neutron scattering in CoCr films in perpendicular and non-perpendicular transmission

Small angle neutron scattering data for 0.4 μm CoCr films are presented for 4 magnetic fields along the in-plane hysteresis loop. In magnetic saturation, the contrast observed in the magnetic scattering appears the same as seen in the nuclear scattering. This is attributed to the same shape of the inhomogeneities.

Magnetic properties and microstructure of evaporated Co oxide tape media

The magnetic properties and microstructure of Co oxide tape media were studied. The intrinsic in-plane and out-of-plane anisotropy constants of metal evaporated CoO tape media were measured from the in-plane torque and the out-of-plane torque curves. It was found that after considering the tilting effect of the column, the total intrinsic anisotropy constant, calculated from the in-plane torque curve, is much less than that calculated from the out-of-plane anisotropy constant. TEM electron diffraction studies indicate that the Co c-axes are randomly oriented. However, from the in-plane hysteresis loop measurements, the easy axes are pretty well parallel to incident beam plane. The cross sectional TEM micrograph shows that the column is tilted 30° out-of-plane and has a diameter of 10 nm.