J. Dubowik

The influence of sublayer thickness on GMR and magnetisation reversal in permalloy/Cu multilayers

Abstract It has been shown that in Py/Cu (Py = Ni 87 Fe 17 , permalloy) multilayers obtained by face-to-face sputtering technology, two well-separated antiferromagnetically coupled ranges can be found centered at d Cu ≈ 1 and 2 nm, respectively. However, d Cu values corresponding to the largest field sensitivity of the GMR effect are different from those for which the strongest antiferromagnetic exchange coupling has been found. With decreasing Py thickness, the magnetic properties become strongly influenced by the magnetisation fluctuations. We attribute this effect to the presence of compositionally intermixed interfaces. Changes in the magnetic properties and magnetoresistance as a function of Py thickness are analysed in terms of evolution from paramagnetic through superparamagnetic to ferromagnetic behaviour of these intermixed regions.

Magnetic field induced transition from weak to strong ferromagnetic coupling in NiFe∕Au∕Co∕Au multilayers

We report on a specific magnetostatic coupling in sputter deposited (Ni80Fe20∕Au∕Co∕Au)10 multilayers of alternating in-plane and out-of-plane magnetic anisotropies. We demonstrate on the basis of complementary studies (magnetoresistance, conventional magnetometry, and element specific soft x-ray resonant magnetic scattering hysteresis measurements) that the magnetization reversal of the Ni–Fe layers is strongly influenced by a magnetostatic coupling originating from the out-of-plane stripe domain stray fields of the Co layers.

Martensitic transformation in Ni2MnGa films: A ferromagnetic resonance study

Off-stoichiometric Ni2MnGa polycrystalline films, deposited by the flash-evaporation technique on the heated mica substrates, show a martensitic phase transformation at 310 K. At room temperature, the films have a tetragonal structure (a=b=0.598 nm, c=0.576 nm) close to the bulk Ni2MnGa with c/a=0.96. The austenite to martensite transformation brings about an anomalous minimum in the effective magnetization and a strong increase in the ferromagnetic resonance linewidth in the martensitic phase just below the transformation temperature. The results are discussed in terms of a qualitative model that combines the ferromagnetic resonance response with a specific microstructure of the polycrystalline Ni2MnGa film.

Electronic structure and magnetic properties of Ni2MnGa alloy films with different structural orders

The structural dependence of magnetic and transport properties of nearly stoichiometric Ni2MnGa alloy films prepared by flash evaporation is investigated. The films deposited at 720 K have the L21 Heusler structure and behave nearly the same as the bulk Ni2MnGa alloy, while the films deposited at 150 K have an almost amorphous structure. Disordered Ni2MnGa alloy films do not exhibit magnetic ordering down to 4 K, but rather have the characteristics of highly resistive Pauli paramagnets. Annealing of disordered films restores the L21 structure. The magnetic and transport properties of the annealed films are similar to those of the films deposited at 720 K. A possible explanation of the loss of magnetic moment in the disordered film is discussed with the aid of first-principles electronic-structure calculations.

Magnetoresistance and its relation to magnetization in Ni50Mn35Sn15 shape-memory epitaxial films

The magnetoresistance (MR) of Heusler alloy Ni50Mn35Sn15 epitaxial films on MgO substrates is studied as a function of temperature T and magnetic field H. The large negative MR extends over martensitic transformation with maximum of −22% at 110 K. In martensitic and austenitic phase, the MR is −3% and −5%, respectively. We show that the MR is governed mainly by magnetization paraprocess at high magnetic fields and scales as the square of magnetization Δm(H,T)2.

Structural dependence of some physical properties of the Ni2MnGe heusler alloy films

Structural ordering of the flash-evaporated Ni2MnGe Heusler alloy films has been studied by magneto-optical (MO), optical, and magnetic methods. The properties of the films have been compared with the corresponding properties of the bulk Ni2MnGe samples with L21 and tetragonally distorted L21 structures, respectively. Depending on a substrate temperature (Ts) and/or postdeposition annealing temperature (Ta) the films have been amorphous (Ts=150K), disordered (Ta 720K). The ordered films have exhibited optical and MO properties close to those of the bulk Ni2MnGe, while the amorphous films showed no ferromagnetic ordering. The optical and MO properties of Ni2MnGe with L21 structure have been proved almost the same as those of Ni2MnGa and Ni2MnIn. Changes in the optical properties of Ni2MnGe alloy films have been discussed in terms of Ni2MnGe band structure.

FMR study of coherent fine magnesioferrite particles in MgO-line shape behavior

Abstract Ferromagnetic resonance (FMR) in fine coherent precipitates (10 nm 3 v 3 nm 3 ) of magnesium ferrite embedded in MgO matrix has been studied. Room temperature results show that with decreasing particle size the resonance line width increases but at very small particle sizes ( v 3 ) shows an anomalous decrease. Asymmetric and distorted resonance line shape become more regular in consequence of motional averaging of crystalline anisotropy at higher temperatures and for smaller particle sizes. Simple model of independent superparamagnetic grains explains the influence of particle size distribution on the FMR line shape.

Effect of structural disorder on some physical properties of the Cu2MnAl Heusler alloy films

The structural dependence of the magnetic, transport, optical, and magneto-optical (MO) properties of Cu2MnAl Heusler alloy (HA) thin-film samples was investigated. Disordered thin films of Cu2MnAl HA fabricated by vapor-quenching deposition onto substrates cooled by liquid nitrogen behave as highly resistive Pauli paramagnets down to 5 K, while an annealing of the disordered films at 580 K for 1 h restores the L21-type ordered structure, as well as the ferromagnetic order and other physical properties typical for the ordered Cu2MnAl HA film. The film deposition onto substrates held at T⩾690K leads to the formation of a multiphase crystalline structure. A significant influence of the structural disorder on the optical and MO properties of Cu2MnAl was experimentally observed and discussed in terms of the electronic structure of the alloy.

Antiferromagnetic magnetostatic coupling in Co/Au/Co films with perpendicular anisotropy

Magnetization reversal processes in Au/Co/Au-wedge/Co/Au pseudo-spin-valve structures characterized by perpendicular anisotropy of 0.6-nm-thick Co layers were investigated by magneto-optical Kerr effect. The samples were deposited on a Ti/Au buffer layer of differing Au-layer thickness (different surface roughness and crystallite size). The distinctive influence of the buffer layer thickness on the dependences of switching fields, and energy of the effective interlayer coupling, versus Au spacer thickness is presented. In particular, increasing the buffer-layer thickness results in a decrease in the oscillation amplitude of the Ruderman–Kittel–Kasuya–Yosida-type interaction and in the enhancement of the antiferromagnetic coupling related to magnetostatic (orange peel) interactions.

Influence of the Ar-ion irradiation on the giant magnetoresistance in Fe/Cr multilayers

The influence of 200 keV Ar-ion irradiation on the interlayer coupling in Fe/Cr multilayers exhibiting the giant magnetoresistance (GMR) effect is studied by the conversion electron Mossbauer spectroscopy (CEMS), vibrating sample magnetometer hysteresis loops, magnetoresistivity, and electric resistivity measurements and supplemented by the small-angle x-ray diffraction. The increase of Ar-ion dose causes an increase of interface roughness, as evidenced by the increase of the Fe step sites detected by CEMS. The modification of microstructure induces changes in magnetization reversal indicating a gradual loss of antiferromagnetic (AF) coupling correlated with the degradation of the GMR effect. Distinctly weaker degradation of AF coupling and the GMR effect observed for irradiated samples with a thicker Cr layer thickness suggest that observed effects are caused by pinholes creation. The measurements of temperature dependence of remanence magnetization confirm increase of pinhole density and sizes during im...