Showing papers on "Magnetic anisotropy published in 1992"

Magnetic live surface layers in Fe/Cu(100).

Abstract: The magnetic properties of Fe films epitaxially grown on Cu(100) have been correlated to their structure and morphology. Strained fcc iron films with thicknesses between 5 and 11 monolayers (ML) are ferromagnetically ordered at the surface with a perpendicular orientation of the magnetic moment, whereas the bulk of the films remains paramagnetic. The surface magnetism is related to an expanded interlayer distance at the surface. With the onset of dislocation formation at about 11 ML, the films become magnetic in the bulk and the magnetization switches to in-plane orientation.

Magnetization direction switching in Fe/Cu(100) epitaxial films: Temperature and thickness dependence.

Abstract: The magnetization direction switching is investigated in epitaxial Fe/Cu(100) films by spin-polarized scanning electron microscopy. We follow the transition from perpendicular to in-plane magnetization with both increasing film thickness and varying temperature. No variation of magnetic moment with magnetization direction change is found. A perpendicular stripe domain configuration is identified which evolves from the low-temperature single-domain state during the reorientation phase transition.

Prediction and confirmation of perpendicular magnetic anisotropy in Co/Ni multilayers.

Abstract: On the basis of first-principles (local-spin-density-approximation) calculations of the magnetocrystalline anisotropy energy, we have predicted a perpendicular orientation of the magnetization in a [111${]}_{\mathrm{fcc}}$ ${\mathrm{Co}}_{1}$/${\mathrm{Ni}}_{2}$ multilayer. This multilayer was then grown by vapor deposition and found to have the predicted properties. The saturation magnetization of 1 T is much larger than that of known magnetic multilayers with perpendicular anisotropy.

Suppression of tunneling by interference in half-integer-spin particles.

Abstract: Within a wide class of ferromagnetic and antiferromagnetic systems, quantum tunneling of magnetization direction is spin-parity dependent: it vanishes for magnetic particles with half-integer spin, but is allowed for integer spin. A coherent-state path-integral calculation shows that this topological effect results from interference between tunneling paths.

Magnetic and magneto‐optical properties of cobalt‐platinum alloys with perpendicular magnetic anisotropy

Abstract: Co1−xPtx alloys with Pt contents in the range 0.45≤x≤0.9 show sizable perpendicular magnetic anisotropy, 100% perpendicular remanence and coercivities in the range 160 kA/m. Thin films of this material are grown by electron beam evaporation onto fused silica or Si, at substrate temperatures between 150 and 350 °C. Spectroscopic investigations of the polar Kerr rotation show a significant enhancement of the Pt related UV peak. A comparison of the static signal levels R×(θk2+ek2)1/2 of Co/Pt multilayers and alloys shows an overall 50% enhancement in the case of alloys. Curie temperatures around 200 °C are observed for Co∼22Pt∼78 compositions. These properties, together with the potentially high chemical stability and ease of manufacturing make Co1−xPtx alloys very attractive materials for short wavelength magneto‐optic recording.

Exchange anisotropy in coupled films of Ni81Fe19 with NiO and CoxNi1−xO

Abstract: Shifted hysteresis loops were used to investigate exchange anisotropy in 500 A CoxNi1−xO/300 A Ni81Fe19 polycrystalline bilayer couples. Bilayers of Ni81Fe19 with NiO have a room‐temperature exchange field, He, of 30 Oe in the as‐deposited state. A maximum in the exchange field at room temperature was observed near x=0.4, indicating an optimal alloying of the properties of the high anisotropy CoO and the high Neel temperature NiO. The blocking temperatures of the exchange couples vary linearly with x, suggesting a linear dependence of the oxide Neel temperature with x.

Recent progress in random magnets

Abstract: Random anisotropy and phase transitions in magnetic glasses, D.J. Sellmyer does ferromagnetism persist in the re-entrant spin glass phase?, I. Mirebeau towards a comprehensive description of the slow dynamics of spin glasses and other complex systems, E. Vincent magnetic anisotropy of spin glasses, J.S. Kouvel exchange frustration and transverse spin freezing, D.H. Ryan and M. Grant phase transitions in magnets with random uniaxial anisotropy, Y. Goldsmith the random field enigma, Y. Shapir.

Structural origins of magnetic anisotropy in sputtered amorphous Tb-Fe films

Abstract: Using x-ray-absorption fine-structure measurements we have obtained clear evidence for structural anisotropy in amorphous sputter-deposited TbFe films exhibiting perpendicular magnetic anisotropy. Modeling of the data shows that perpendicular anisotropy in these films is associated with Fe-Fe and Tb-Tb pair correlations which are greater in plane and Tb-Fe correlations which are greater perpendicular to the film plane. Upon annealing at 300 \ifmmode^\circ\else\textdegree\fi{}C the measured structural anisotropy disappears and the magnetic anisotropy decreases to a level consistent with magnetoelastic interactions between the film and substrate.

Surface-step-induced magnetic anisotropy in thin epitaxial Fe films on W(001)

Abstract: Ultrathin epitaxial Fe films grown on W(001) surfaces prepared to yield uniform 25-\AA{}-wide terraces are shown to exhibit layer-dependent in-plane uniaxial magnetic anisotropy having an easy axis perpendicular to the steps. Hysteresis loops measured by magneto-optical techniques when steps are aligned parallel to the applied field H manifest the flip of magnetization that occurs near H=0. The absence of magneto-optic effects for films less than or equal to one monolayer thick is consistent with recent Iab initioR calculations that predict significant moment reduction resulting from strong film-substrate hybridization.

Magnetic anisotropies of ultrathin Co(001) films on Cu(001).

Abstract: We present experimental evidence that ferromagnetic order at room temperature in ultrathin epitaxial Co(001) layers on Cu(001) substrates is stabilized by in-plane magnetic anisotropies. All relevant anisotropy contributions have been determined as a function of Co layer thickness with and without an additional Cu overlayer.

Symmetry-induced uniaxial anisotropy in ultrathin epitaxial cobalt films grown on Cu(1 1 13).

Abstract: Uniaxial anisotropy has been found in ultrathin cobalt films grown on a Cu(1 1 13) surface. Our studies using scanning electron microscopy with polarization analysis clearly show that the easy axis of magnetization is parallel to the direction of the step edges of the Cu(1 1 13) substrate. In spite of the different anisotropy behavior, the domain structures in Co/Cu(001) and Co/Cu(1 1 13) are similar, which indicates that the domain pattern in ultrathin films is little affected by the anisotropy.

Investigation of the unusual magnetic spiral arrangement in BiFeO3

Abstract: In the ferroelectric-antiferromagnet, BiFeO3, the cycloidal spiral arrangement of the magnetic moments of the Fe3+ ions whose length is 620 A was confirmed. This magnetic ordering remains up to the Neel temperature (643K).

Magnetic properties of MnBi prepared by rapid solidification.

Abstract: The magnetic properties of the low-temperature phase of MnBi, prepared by rapid solidification, have been measured in a pulsed field over the temperature range 80--625 K. The anisotropy field, obtained by the singular-point-detection technique, is found to increase with temperature and has a maximum value of 9 T at 530 K. A fit to the saturation-magnetization measurement gives a virtual Curie point of 775 K. The coercive field is fitted by a hybrid domain-wall-pinning theory which yields, at 300 K, a domain-wall energy of 15.6 erg/${\mathrm{cm}}^{2}$ and a wall thickness of 70 \AA{}. Below 200 K, the presence of a ferrimagnetic phase is detected. Its critical field associated with spin reversal is a convex function of temperature having a maximum value of 8 T at 120 K.

Origin of the geometric forces accompanying Berry's geometric potentials.

Abstract: We consider the dynamics of a particle carrying a magnetic moment in a strong magnetic field whose direction varies slowly in space. In particular, we discuss the geometric Lorentz-type and electric-type forces that were discovered in studies of Berry's phase. We show that the Lorentz-type force felt by the particle is caused by a small misalignment of the magnetic moment with respect to the magnetic field while the electric-type force is the time average of a strong oscillatory force induced by the precession of the magnetic moment.

Surface and 2D magnetism

Abstract: The development of surface science and specifically of spin-polarized electron spectroscopy has been the driving force for a new era of surface and 2D magnetism. Classical primary magnetic quantities such as the temperature dependence of the spontaneous magnetization, the Curie point, the magnetic anisotropies, the variation of the quantum mechanical exchange interaction at clean or modified surfaces, and special surface-induced magnetic structures can now be determined with spin-polarized electron spectroscopies. But a variety of new fields are also appearing, for instance ultrafast time-resolved magnetometry employing laser-induced photoemission of spin-polarized electrons, and imaging of magnetic domains with unprecedented spatial resolution via spin-polarized cascade electrons. Furthermore, by analysing the spin of the photoemitted electrons or by observing the Bremsstrahlung emitted when a spin-polarized electron beam strikes the surface, the majority- and minority-spin electron states in ferromagnets can be investigated separately. Three types of spin-split electron state appear at the surface of a ferromagnet: the bulk magnetic bands, the surface states of Schottky type, and the image potential surface states. With ultrathin ferromagnetic films, magnetometry employing the measurement of the spin polarization of low-energy cascade electrons produces magnificent images of the magnetic domains and reveals the conditions for their occurrence. The dramatic response of the spontaneous magnetization to external disturbances such as an applied magnetic field or an exchange field transferred from a substrate is also obtained. The critical phenomena observed in surface and 2D magnetism fit well into the framework set by the theory many years ago, while the theoretically predicted exotic 2D ferromagnetism in elements that do not exhibit magnetism in 3D has not yet been verified beyond doubt.

Dynamic magnetic phenomena in fine-particle goethite.

Abstract: The distinctive M\"ossbauer spectra of fine-particle goethite (\ensuremath{\alpha}-FeOOH) have been attributed by many authors to superparamagnetism. However, measurements of the magnetic anisotropy energy and particle volume show that the superparamagnetic blocking temperature for most samples is much greater than the N\'eel temperature. A model involving magnetic ordering of clusters created by high concentrations of vacancy defects is proposed, in which the cluster moments slowly relax, thus producing a Boltzmann distribution in the z component of the magnetization. The model provides excellent fits to the temperature-dependent hyperfine-field distributions observed in the M\"ossbauer spectra, and may have wider applicability to other diamagnetically substituted iron oxide materials. A linear relation is observed between ${\mathit{T}}_{\mathit{N}}$ and the inverse mean crystallite dimension in the [111] direction of -1060(130) K nm, and it is shown that goethite particles must have a volume of less than 1000 ${\mathrm{nm}}^{3}$ in order to show superparamagnetic M\"ossbauer spectra at room temperature.

Evaporated CoPt alloy films with strong perpendicular magnetic anisotropy

Abstract: CoPt alloy films with large perpendicular magnetic anisotropy, perpendicular coercivity, and saturated perpendicular remanence are reported. These films were fabricated by e‐beam evaporation at substrate temperatures near 200 °C and above. Well‐(111)‐textured Pt underlayers are shown to dramatically increase the perpendicular magnetic anisotropy of CoPt alloy films. The large perpendicular magnetic anisotropy is shown to be strongly related to good CoPt(111) texture, and not to the ordered tetragonal Co50Pt50 phase.

Current biased magnetoresistive spin valve sensor

Abstract: A magnetoresistive read sensor based on the spin valve effect in which a component of the read element resistance varies as the cosine of the angle between the magnetization directions in two adjacent magnetic layers is described. The sensor read element includes two adjacent ferromagnetic layers separated by a non-magnetic metallic layer, the magnetic easy axis of each of the ferromagnetic layers being aligned along the longitudinal axis of the ferromagnetic layers and perpendicular to the trackwidth of an adjacent magnetic storage medium. The sense current flowing in the sensor element generates a bias field which sets the direction of magnetization in each ferromagnetic layer at an equal, but opposite, angle θ with respect to the magnetic easy axis thus providing an angular separation of 2θ in the absence of an applied magnetic signal. The magnetizations of both ferromagnetic layers are responsive to an applied magnetic field to change their angular separation by an amount 2δθ.

Preparation and magnetic properties of epitaxial barium ferrite thin films on sapphire with in-plane, uniaxial anisotropy

Abstract: For the first time, epitaxial barium‐ferrite thin films, predominantly of the magnetoplumbite structure, with an easy axis of magnetization in‐plane have been prepared on single‐crystal sapphire substrates by reactive sputtering and post‐deposition annealing. Lattice images by high resolution transmission electron microscopy reveal the probable origin of the magnetic anisotropy to be epitaxy between the Ba‐ferrite and sapphire, causing the c‐axis of the Ba‐ferrite to lie nearly in‐plane. Coercivities of more than 9 kOe have been achieved in the easy‐axis direction in Al substituted films with magnetization reduced to approximately 25% of the value for unsubstituted films.

Growth-induced magnetic anisotropy in amorphous Tb-Fe.

Abstract: Coherent magnetic anisotropy is shown to be induced in vapor-deposited amorphous Tb-Fe by a thermally activated growth process. This process is hypothesized to involve rearrangement of local adatom configurations into energetically favorable orientations which minimize surface energy during the growth, a process analogous to the frequently observed crystallographic texturing of polycrystalline thin films. The anisotropy is found to be independent of the state of stress in the film during the growth, and does not depend on film thickness; these results are inconsistent with recently proposed models

Structure and magnetic properties of R2Fe17Cx (x∼2.5)

Abstract: A series of rare‐earth iron carbides were prepared by heat treatment of R2Fe17 compounds in methane. The nominal composition of these carbides is R2Fe17Cx, x∼2.5. The carbides retain the original Th2Zn17 or Th2Ni17 structure with a 6.5% volume expansion over the carbon‐free unit cell. For Sm2Fe17C2.5, the Curie temperature is 760 K and the easy magnetization direction is along the c axis. The anisotropy field of Sm2Fe17C2.5 at room temperature is 15±0.5 T.

Magnetization reversal mechanism evaluated by rotational hysteresis loss analysis for the thin film media

Abstract: Rotational hysteresis loss analysis has been carried out for CoCrTa, CoNiPt, and CoCrPt thin-film media to evaluate the influence of microstructure on coercive force through the measurement of magnetic anisotropy. In each medium, the magnitudes of H/sub k//sup grain/ defined as a magnetic field where rotational hysteresis loss W/sub r/ diminishes remain almost a constant value independently of the values of coercive force. With increasing coercive force, the magnitude of the coercive force gradually becomes smaller than that of the switching field of magnetization H/sub p/ evaluated by torque analysis. Through the analysis of H/sub k//sup grain/ and H/sub p/, it is suggested that the coercive force in each examined medium is strongly dependent on the degree of intergranular exchange coupling and/or magnetostatic interactions. While on CoNiPt and CoCrPt media coercive force depends on the rotational hysteresis integral, no correlation between them was observed in CoCrTa films. >

Perpendicular anisotropy and antiferromagnetic coupling in Co/Ru strained superlattices.

Abstract: Despite the large lattice mismatch (\ensuremath{\simeq}8%), we demonstrate the growth of epitaxially ordered Co/Ru superlattices consisting of hcp (0001) Co and Ru sublayers. For small Ru interlayer thickness, a large antiferromagnetic exchange coupling between the Co sublayers is observed that influences strongly the magnetization process, as the magnetization within Co sublayers can no longer be considered to be fully locked ferromagnetically. For small Co and Ru thicknesses, the magnetization is oriented along the film normal, while the adjacent Co layers are strongly coupled antiferromagnetically. For this peculiar magnetic structure, a first-order spin-flop transition from an antiferromagnetic state to a ferromagnetic canted state is observed and clearly revealed by use of a ferromagnetic-resonance technique.

A torque transducer utilizing a circularly polarized ring

Abstract: A novel type of magnetoelastic torque transducer is described. It consists of two elements: a circumferentially magnetized magnetostrictive ring, rigidly attached to a shaft carrying the torque to be measured, and a Hall effect or similar magnetic field intensity sensor mounted in proximity to the ring. Stresses in the ring, associated with the torque being transmitted, alter the effective anisotropy orientation from circular to more or less steeply helical. Discontinuity of the axial component of magnetization at the ring ends creates a magnetic field in the space around the ring. A simple analysis predicts both a linear range in the relationship between field intensity and torque and a polarity which depends on the sense of the torque. The electrical output of the field sensor is thus a linear analog of the torque. Experimental transducers exhibit these expected features as well as a small, notably negative, hysteresis. The stability of the circular magnetization is both theoretically supportable and experimentally verified. >

Orientational dependence of the interface magnetic anisotropy in epitaxial Ni/Co/Ni sandwiches.

Abstract: Magneto-optical Kerr effect measurements have been used to investigate the orientational dependence of the magnetic anisotropy in ultrathin Ni/Co-wedge/Ni sandwiches deposited by molecular beam epitaxy on single-crystal Cu substrates. The results show a marked dependence of both the volume and interface anisotropy terms on the growth direction of the samples: [100], [110], and [111]. In addition, the (111) interface term was found to be significantly larger than existing literature values for Co/Ni multilayers. This effect is discussed in terms of the microstructure of the studied films.

Mn substitution effect on magnetostriction temperature dependence in Tb0.3Dy0.7Fe2

Abstract: Magnetostriction temperature dependencies in Tb0.3Dy0.7 (Fe1−xMnx)2 were investigated. Mn substitution lowers the spin reorientation temperature, at which magnetostriction shows a sharp drop. Moreover, Mn containing compounds show larger magnetostriction than that for a Mn‐free compound at low temperature. Mossbauer measurements show that easy magnetization direction for the Mn containing compound is in the 〈111〉 direction at 300 K, while it is in the 〈100〉 at 77 K. These results indicate that the tetragonal distortion λ100 increases by Mn addition in Tb0.3Dy0.7Fe2.