Enhanced Saturation Magnetization Research Articles

Effect of epitaxy and lattice mismatch on saturation magnetization of γ′-Fe4N thin films

We report the effect of epitaxial growth and lattice mismatch on the enhancement of saturation magnetization (Ms) of ferromagnetic γ′-Fe4N thin films deposited on different single crystal substrates having lattice mismatches from 0% to 11%. It was found that Ms in the γ′-Fe4N film increased with increasing degree of epitaxy and minimizing lattice mismatch between the film and the substrate. Maximum saturation magnetization of 1980±20emu∕cm3 (about 24% higher than previous result) was obtained with LaAlO3(100) substrate having zero lattice mismatch after postannealing of 30min, which is believed to originate from magnetovolume effect in well-ordered epitaxially grown films.

Magnetic moments of Fe clusters embedded in an Fe-Co alloy

Motivated by the search for high moment materials, we present first-principles calculations of the magnetic moments for Fe clusters embedded in an Fe-Co alloy as well as in a pure Co matrix. Both spin and orbital contributions have been calculated with the use of a real-space linear muffin-tin orbital method within the atomic sphere approximation. The cluster sizes considered range from 1 to 51 atoms. We find an enhancement of the spin moments of the atoms close to the interface between the cluster and the embedding matrix. The spin moment enhancement is decreasing with increasing Fe content in the surrounding matrix. The slightly lower magnetic moments for the Fe clusters embedded in the Fe rich matrices can nevertheless result in a material with an enhanced saturation magnetization due to the higher moments in the matrix. We argue that the largest average magnetic moments for embedded Fe clusters are obtained for an ${\mathrm{Fe}}_{0.5}{\mathrm{Co}}_{0.5}$ alloy in the matrix surrounding the clusters.

Antiferromagnetic coupling and enhanced magnetization in all-ferromagnetic superlattices

The structural and magnetic properties of a series of superlattices consisting of two ferromagnetic metals La0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO) grown on (001) oriented SrTiO3 are studied. Superlattices with a fixed LSMO layer thickness of 20unit cells and varying SRO layer thickness show a sudden drop in magnetization on cooling through a temperature where both LSMO and SRO layers are ferromagnetic. This behavior suggests an antiferromagnetic coupling between the layers. In addition, the samples having thinner SRO layers (n<6) exhibit enhanced saturation magnetization at 10K. These observations are attributed to the possible modification in the stereochemistry of the Ru and Mn ions in the interfacial region.

Enhancement of saturation magnetization in sputtered FeCoPd alloy and [FeCo-Pd]/sub n/ super-lattice films at room temperature

The saturation magnetization of Fe/sub 70/Co/sub 30/:Pd films and that of Fe/sub 70/Co/sub 30/ sublayers in [Fe/sub 70/Co/sub 30/-Pd]/sub n/ super-lattice films with various Fe/sub 70/Co/sub 30/ and Pd sublayer thicknesses are investigated. The saturation magnetization of these films was increased by alloying or laminating Fe/sub 70/Co/sub 30/ with Pd and the Fe/sub 70/Co/sub 30/:Pd and super-lattice films revealed high-saturation magnetizations of 2.45 and 2.57 T, respectively, while the saturation magnetization of Fe/sub 70/Co/sub 30/ film was only 2.38 T at room temperature. These results seem to be caused by the enhancement of the magnetic moment of the Fe atoms neighboring Co and Pd atoms.

Effects of oxygen incorporation in tensileLa0.84Sr0.16MnO3−δthin films duringex situannealing

Structural changes in epitaxial ${\mathrm{La}}_{0.84}{\mathrm{Sr}}_{0.16}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ (LSMO) $(001)∕\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}(001)$ thin films are studied using x-ray scattering during thermal annealing. As oxygen atoms are incorporated during annealing ex situ in air, the unit-cell volume contracts, which is attributed to the increased ratio of ${\mathrm{Mn}}^{4+}$ ions as indicated by enhanced saturation magnetization and ferromagnetic transition temperature. The unit-cell contraction occurs rather abruptly, suggesting that a structural phase transition happens during annealing. A $200\text{\ensuremath{-}}\mathrm{\AA{}}$-thick film remains almost fully strained even after annealing. In this case, a fourfold structural modulation is observed which accommodates the augmented lattice strain energy.

Properties of magnetite nanoparticles synthesized through a novel chemical route

We have developed a simple precipitation route to synthesize magnetite (Fe 3O 4) nanoparticles with controlled size without any requirement of calcination step at high temperatures. The study of these nanoparticles indicates an enhancement in saturation magnetization with reduction in size down to ∼10 nm beyond which the magnetization reduces. The latter is attributed to surface effects becoming predominant as surface to core volume ratio increases. From the view-point of applications, 10 nm size of magnetite particles seems to be the optimum.

Enhanced magnetization of nanostructured granular Ni/[Cu(II)–C–O] films

Enhanced saturation magnetization was observed in granular nanostructured Ni/[Cu(II)–O–C] films deposited by the nonaqueous, electroless polyol process. The saturation magnetization of Ni–Cu complex films at room temperature was 112 emu/g, which was about two times that of bulk Ni (54.4 emu/g). It is suggested that the enhancement was caused by the magnetic interaction between the Ni particles and the amorphous Cu(II) complex. The observed shifted Curie temperature, large anisotropy field, and strong temperature dependence of saturation magnetization support this argument.

Effects of Hydrogenation on the Structure, Transport, and Magnetic Properties of Multilayers Composed of Transition Metals and Rare-Earth Metals

The effects of hydrogenation on the structure, transport, and magnetic properties of TM/RE (TM = Fe, Co; RE = La, Y, Gd) multilayers were investigated. The following effects were found to be common to all TM/RE multilayers: (a) transition of RE layers from metal to semiconductor, (b) expansion of RE layers, and (c) enhancement of saturation magnetization. The hydrogenation technique is practicable for changing the interface state and producing a semiconductor layer inside the multilayer, and thus shows potential for producing new functional magnetic multilayers.

Surface structure and surface-spin induced magnetic properties and spin-glass transition in nanometer Co-granules of FCC crystal structure

Separated Co-granules, of an average diameter as small as D = 2.0 nm, of FCC crystal structure have been synthesized by co-reducing Co2+ cations dispersed in a liquid. They exhibit an enhancedsaturation magnetization σs by as much as ∼34% with a more than an order of enhanced magnitude for the effective anisotropy constant Keff over the bulk values at 4.2 K. An irreversibility in the ZFC-FC (zero field cooled-field cooled) thermomagnetograms occurs at temperatures T ≤ TB, where TB is their blocking temperature. The ZFC thermomagnetogram peaks at TB according to their Keff and volume V. TB = 152 K has been found for D = 10 nm granules in an applied magnetic field of H = 1 kA/m. The sample, which is superparamagnetic(coercivity Hc = 0) in nature at T ≥ TB, develops Hc at T < TB with a unique dependence on temperature, Hc(T) = Hc(0)[1 − T/TB]1/2, with Hc(0) = 40.0 kA/m. The results are discussed with a two-phase modelstructure of granules. In this model, the grain-surface atoms have amodified magnetic structure of the core atoms. An inter-couplingbetween the magnetic spins in the two regions occurs in aferromagnetic or antiferromagnetic manner according to theirinterface that mediates their exchange interactions through it. Thestudies of σ, Keff, or Hc as a function of temperature (4.2 to 380 K) and/or size D (2 to 20 nm) demonstrate their strong correlation with the dynamics of the surface spins (DSS). An enhanced surfaceanisotropy with large total interface-energy in small granulesgoverns the DSS. An average value of the surface anisotropy constantKs = 2.28 mJ/m2 is determined by a linear plot of Keff with D−1 at D ≤ 2.9 nm. Larger granules follow a modified Keff − D−1 plot with an order of smaller Ks-value. The surface spins form a surface-spin-glass, which undergoes a magnetic transition to a spin-frozen state at a critical temperature TF = 71 K. The TF evolves following the well-known de Almeida-Thouless line, δ TF ∝ H2/3, at H≤ 42 kA/m.

Compositional separation in Co-Mn magnetic thin films

The compositional distribution in fcc Co-Mn magnetic thin films was studied in order to examine the possibility of a compositional separation, similar to that observed previously in hcp Co-Cr thin film magnetic recording media, occurring in an fcc alloy system. Spin-echo 59Co NMR study of Co75Mn25 films revealed a change from a homogeneous compositional distribution in a film deposited at a substrate temperature (Ts) of RT to a compositionally separated state in a film deposited at a Ts of 300 °C. Vibrating sample magnetometry revealed an associated three-fold enhancement in saturation magnetization in the films deposited at a Ts of 300 °C.

Enhanced saturation magnetization, electronic structure, and compositional segregation in epitaxially grown Co–Cr thin films

We have used the recently established linear correlation between the total intensities of the L3,2 ‘‘white lines’’ in electron energy-loss spectroscopy and the number of unoccupied 3d states to probe the local electronic structure in epitaxially grown Co–Cr thin films. The significant enhancement of saturation magnetization measured for these epitaxial films, when compared to homogenized bulk alloys, has been correlated directly to the unoccupied 3d states to the Co atoms in these materials. These measurements are the first direct electronic structure evidence that may support models associating the enhancement of saturation magnetization with intragranular segregation in Co–Cr thin films.

Epitaxial growth and enhanced saturation magnetization of single crystal Co/sub 1-x/Cr/sub x/ media suitable for perpendicular magnetic recording

Highly c-axis oriented, single crystal films of Co/sub 1-x/Cr/sub x/ have been grown epitaxially on mica substrates by e-beam evaporation. The method, using a Ru underlayer, is simple and applicable to similar c-axis oriented, large area growth of the wide range of Co alloys suitable for perpendicular recording. The films show narrow peak widths in X-ray diffraction (/spl Delta//spl Theta//sub 00.2//spl sim/1.0-1.5/spl deg/) and their magnetic properties are composition dependent. Moreover, for any given Cr concentration, these single crystal films exhibit the largest saturation magnetization when compared with either sputtered/evaporated samples or films grown under identical conditions on glass/silicon substrates. It is shown that this enhanced M/sub s/ is directly correlated with the dispersion of the c-axis about the film normal and their narrowest values are observed for these epitaxially grown films. X-ray microanalysis shows some inter/intra-granular variation in Cr content, but systematic segregation trends appropriate for explaining these results have not yet been observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Surface chemical state of sputtered Co-Cr films

Surface oxidation states of sputtered Co 80Cr 20 (at%) films were studied by performing Auger Electrin Spectroscopy (AES) and X-ray Photoemission Spectroscopy (XPS). Seven samples having different thicknesses (from 50 to 1000 nm) which were grown under the same conditions have been studied. It was found from these experiments that among all the films the initial oxidation occured in the Cr; then the following diffusion of the Co was observed at the top layer. Consequently the surface consists of a four-layered structure. A weak thickness dependence in the films of the oxidized region was found from the results of AES measurements. The change of the saturation magnetization through the sample was calculated using the results of AES and XPS data. These calculations show that a region with enhanced saturation magnetization can be expected under the non-ferromagnetic top layer.