On the reorientation transition of ultra-thin Ni/Cu(001) films

The reorientation transition of the magnetization of ferromagnetic films is studied on a microscopic basis within a Heisenberg spin model. Using a modified mean field formulation it is possible to calculate properties of magnetic thin films with non-integer thicknesses. This is especially important for the reorientation transition in Ni/Cu(001), as there the magnetic properties are a sensitive function of the film thickness. Detailed phase diagrams in the thickness-temperature plane are calculated using experimental parameters and are compared with experimental measurements by Baberschke and Farle (J. Appl. Phys. 81, 5038 (1997)).Comment: 7 pages(LaTeX2e) with one figure(eps), accepted for publication in JMMM. See also http://www.thp.Uni-Duisburg.DE/Publikationen/Publist_Us_R.htm

ORIENTATED FePt NANOCRYSTALS DEPOSITED ON POROUS SILICON

FePt nanocrystals with L10 chemical order have high magnetic anisotropy. To form the hard magnetic L10 phase as prepared fcc FePt nanocrystals need to be heated to 600°C. We demonstrate that the morphology of chemically etched porous silicon (PS) substrates and the presence of a magnetic field during the annealing process (600 °C, 1 h) affect the particle arrangement and orientation. X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) show the presence of the L10 ordered FePt particles (average diameter 15 nm) uniformly distributed on the substrate. The presence of perpendicular magnetic field during annealing increases the order parameter degree of L10 FePt NPs. These effects can be investigated from variations of the XRD peaks intensity ratio. Presence of magnetic field of 20mT in the perpendicular direction to the substrate surface increases the (001) peak intensity ratio with respect to (111) peak from 0.27 to 0.58. This effect is due to the superlattice formation at (001) direction

Characteristics of 5M modulated martensite in Ni-Mn-Ga magnetic shape memory alloys

The applicability of the magnetic shape memory effect in Ni-Mn-based martensitic Heusler alloys is closely related to the nature of the crystallographically modulated martensite phase in these materials. We study the properties of modulated phases as a function of temperature and composition in three magnetic shape memory alloys Ni 49.8Mn25.0Ga25.2, Ni 49.8Mn27.1Ga23.1 and Ni 49.5Mn28.6Ga21.9. The effect of substituting Ga for Mn leads to an anisotropic expansion of the lattice, where the b-parameter of the 5M modulated structure increases and the a and c-parameters decrease with increasing Ga concentration. The modulation vector is found to be both temperature and composition dependent. The size of the modulation vector corresponds to an incommensurate structure for Ni 49.8Mn25.0Ga25.2 at all temperatures. For the other samples the modulation is incommensurate at low temperatures but reaches a commensurate value of q ≈ 0.400 close to room temperature. The results show that commensurateness of the 5M modulated structure is a special case of incommensurate 5M at a particular temperature

Magnetic phase transitions in Ta/CoFeB/MgO multilayers

We study thin films and magnetic tunnel junction nanopillars based on Ta/Co$_{20}$Fe$_{60}$B$_{20}$/MgO multilayers by electrical transport and magnetometry measurements. These measurements suggest that an ultrathin magnetic oxide layer forms at the Co$_{20}$Fe$_{60}$B$_{20}$/MgO interface. At approximately 160 K, the oxide undergoes a phase transition from an insulating antiferromagnet at low temperatures to a conductive weak ferromagnet at high temperatures. This interfacial magnetic oxide is expected to have significant impact on the magnetic properties of CoFeB-based multilayers used in spin torque memories

Prediction of a surface state and a related surface insulator-metal transition for the (100) surface of stochiometric EuO

We calculate the temperature and layer-dependent electronic structure of a 20-layer EuO(100)-film using a combination of first-principles and model calculation based on the ferromagnetic Kondo-lattice model. The results suggest the existence of a EuO(100) surface state which can lead to a surface insulator-metal transition.Comment: 9 pages, 5 figures, Phys. Rev. Lett. (in press

Theory of the Spin Reorientation Transition of Ultra-Thin Ferromagnetic Films

The reorientation transition of the magnetization of ferromagnetic films is studied on a microscopic basis within Heisenberg spin models. Analytic expressions for the temperature dependent anisotropy are derived from which it is seen that the reduced magnetization in the film surface at finite temperatures plays a crucial role for this transition. Detailed phase diagrams in the temperature-thickness plane are calculated.Comment: 6 pages(LaTeX2e), one figure(eps), accepted for publication in JMM

Anisotropy of ultra-thin ferromagnetic films and the spin reorientation transition

The influence of uniaxial anisotropy and the dipole interaction on the direction of the magnetization of ultra-thin ferromagnetic films in the ground-state is studied. The ground-state energy can be expressed in terms of anisotropy constants which are calculated in detail as function of the system parameters and the film thickness. In particular non-collinear spin arrangements are taken into account. Conditions for the appearance of a spin reorientation transition are given and analytic results for the width of the canted phase and its shift in applied magnetic fields associated with this transition are derived.Comment: 6 pages, RevTeX

Electron Spin Resonance of the ferromagnetic Kondo lattice CeRuPO

The spin dynamics of the ferromagnetic Kondo lattice CeRuPO is investigated by Electron Spin Resonance (ESR) at microwave frequencies of 1, 9.4, and 34~GHz. The measured resonance can be ascribed to a rarely observed bulk Ce3+ resonance in a metallic Ce compound and can be followed below the ferromagnetic transition temperature Tc=14 K. At T>Tc the interplay between the RKKY-exchange interaction and the crystal electric field anisotropy determines the ESR parameters. Near Tc the spin relaxation rate is influenced by the critical fluctuations of the order parameter.Comment: This is an article accepted for publication in Journal of Physics: Condensed Matte

Magnetoelastic mechanism of spin-reorientation transitions at step-edges

The symmetry-induced magnetic anisotropy due to monoatomic steps at strained Ni films is determined using results of first - principles relativistic full-potential linearized augmented plane wave (FLAPW) calculations and an analogy with the N\'eel model. We show that there is a magnetoelastic anisotropy contribution to the uniaxial magnetic anisotropy energy in the vicinal plane of a stepped surface. In addition to the known spin-direction reorientation transition at a flat Ni/Cu(001) surface, we propose a spin-direction reorientation transition in the vicinal plane for a stepped Ni/Cu surface due to the magnetoelastic anisotropy. We show that with an increase of Ni film thickness, the magnetization in the vicinal plane turns perpendicular to the step edge at a critical thickness calculated to be in the range of 16-24 Ni layers for the Ni/Cu(1,1,13) stepped surface.Comment: Accepted for publication in Phys. Rev.