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In magnetic random access memory (MRAM) devices, soft magnetic thin film elements such as permalloy (Py) are used as unit cells of information. The epitaxial integration of these elements with the technologically important substrate Si (1 0 0) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni 82.5 Fe 17.5 (permalloy, Py) on Si (1 0 0) using a TiN/MgO buffer layer. Initial stages of growth are characterized by the formation of discrete islands that gradually merge into a continuous film as deposition times are extended. Interestingly, we find that the magnetic features of Py films in early stages of island coalescence are distinctly different from the films formed initially (discrete islands) and after extended deposition times (narrow distribution of equiaxed granular films). Isothermal in-plane and out-of-plane magnetic measurements performed on these transitional films show highly anisotropic magnetic behavior with an easy magnetization axis lying in the plane of the film. Importantly, when this sample is zero-field cooled, a positive exchange bias and vertical loop shift are observed, unusual for a soft ferromagnet like Py. Repeated field cycling and hysteresis loops up to the fields of 7T produced reproducible hysteresis loops indicating the existence of strongly pinned spin configurations. Classical interface related exchange bias models cannot explain the observed magnetic features of the transitional Py films. We believe that the anomalous magnetic behavior of such Py films may be explained by considering the highly irregular morphology that develops at intermediate growth times that are possibly also undergoing a transition from Bloch to Neel domain wall structures as a function of Py island size. This study broadens the current understanding of magnetic properties of Py thin layers for technological applications in magneto-electronic devices, integrated with Si (1 0 0).

The effects of substituting Zr by Hf on the structural and the magnetic properties of the nanocrystalline rapidly solidified Zr18-xHfxCo82 ribbons (x = 0, 2, 4, and 6) have been studied. X-ray diffraction and thermomagnetic measurement results indicated that upon rapid solidification processing four magnetic phases occur: rhombohedral Zr2Co11, orthorhombic Zr2Co11, hcp Co, and cubic Zr6Co23 phases. Microstructure analysis results showed the reduction in the percentage of the soft-magnetic phase (Co) compared to the hard-magnetic phase (Zr2 Co11 (rhombohedral)) with the increase in the Hf concentration. All the samples under investigation have ferromagnetic nature, at 4.2 K and at room temperature. The coercive force (Hc) and the saturation magnetization are (Ms) found to linearly increases with x (x ≤ 2), then Hc slightly increases and Ms slightly decreases with increasing x. The maximum energy product (BH)max at room temperature is found to increases with increasing x reaching a maximum value for x = 4. The magnetocrystalline anisotropy parameter of these samples are calculated to be K = 1.1 MJ/m3 and independent of Hf concentration. The above results indicate that the replacement of Zr by Hf improves the hard-magnetic properties of this class of rear-earth-free nanocrystalline permanent magnet materials.

Antiferromagnetically coupled magnetic thin films are promising candidates for the design of new magnetic storage and logic devices. The ability to control the interlayer thickness, therefore the magnetic reversal response, of exchange-coupled magnetic layers is of paramount importance in nanotechnology, especially in magnetic sensing element design and applications. In this work, magnetic force microscopy (MFM) with improved sensitivity and high spatial resolution probes was used to obtain a more detailed view of magnetization reversal behavior and domain evolution in the indirect exchange-coupled trilayer system: Co/Ru/Co. The effect of the variable Ru interlayer thickness on the exchange coupling and thus the magnetic domain structure during the ferromagnetic (FM)/antiferromagnetic (AF) coupling transition in Co/Ru/Co films is well demonstrated. The MFM images display a distinct signature of AF coupling for the films with Ru thickness of 0.4 nm. MFM has proven to be an effective tool for detecting FM/AF interlayer coupling and exploring magnetic domain structures in exchange-coupled layered thin films.

Cis ferromagnetic, with a room-temperature coercivity of 70 Oe and a magnetization of 0.8 emu/g at 5.5 T,although the hysteresis loop is not saturated. The theoretical prediction that interacting locally nickel-richregions may be responsible for ferromagnetic behavior is supported by the observation of magnetically glassybehavior at low magnetic fields.I. INTRODUCTION

The effect of Hf on the permanent magnetism of nanocrystalline Zr18-xHfxCo82 ribbons (x = 0, 2, 4, and 6) was investigated by magnetic properties measurement and magnetic force microscopy (MFM). Emphasis is on the local magnetic domain structures in polycrystalline rapidly solidified Zr18-xHfxCo82 ribbons for four different samples with small fractions of Hf dopants (x ≤ 6). The investigation of the magnetic properties of the Zr18-xHfxCo82 ribbons revealed that all the samples under investigation are ferromagnetic at room temperature, and the corresponding MFM images show bright and dark contrast patterns with up-down magnetic domain structures. It is found that the saturation magnetization and the coercivity depend on Hf doping concentration x in the samples. For a sample with Hf concentration x = 4, the maximum energy product (BH)max value is 3.7 MGOe. The short magnetic correlation length of 131 nm and smallest root-mean-square phase shift value of 0.680 were observed for x = 4, which suggests the refinement of the magnetic domain structure due to weak intergranular exchange coupling in this sample. The above results indicate that suitable Hf addition is helpful for the magnetic domain structure refinement, the coecivity enhancement, and the energy-product improvement of this class of rare-earth-free nanocrystalline permanent-magnet materials.

Structural inhomogeneity on length scales comparable to magnetic interaction lengths can produce complex magnetic behavior. Crystalline GdAl2 is a ferromagnet with a Curie temperature of 170 K, while amorphous GdAl2 thin films exhibit classic spin-glass behavior with a freezing temperature of 16 K. Nanostructured GdAl2, made by mechanically milling initially crystalline GdAl2, exhibits ferromagnetic and spin-glass-like transitions; however, the spin-glass-like transition occurs at a higher temperature than the freezing temperature of amorphous GdAl2 thin films. Curie–Weiss analysis suggests that the paramagnetic-to-ferromagnetic transition is due to the ferromagnetic ordering of small GdAl2 clusters and that the glassy transition is most likely due to spin-glass-like ordering of a surface/interface phase.

The synthesis of hard-magnetic CoPt nanoparticle films by hydrogen reduction of Co nitride and Pt chloride mixture is reported. Thin porous alumina films are adopted as the carrier of the initial aqueous solution and of the final reducing products of CoPt nanoparticles. It is found that chemical ordering of L10–CoPt occurs above 400 °C. Partial phase transformation occurs in the alumina substrate when the treatment temperature is higher than 600 °C. The film coercivity increases with increasing annealing temperature and reaches a maximum value of 24.2 kOe when the reduction is carried out at 700 °C for 2 h.

WOS:000237682900052 We report on the use of direct laser interference patterning to form an "anisotropy" lattice in Co/Pt thin film multilayers. Co/Pt multilayers have been extensively studied and, for the compositions studied here, are characterized by strong perpendicular magnetic anisotropy in which the magnetic moment is perpendicular to the film plane. In direct laser interference patterning, two-to-four coherent laser beams from a pulsed Nd:YAG laser strike the sample surface simultaneously, and for sufficiently intense beams the sample properties are modified locally where interference maxima occur. Kerr rotation, magnetic force microscopy, and atomic force microscopy measurements after patterning by one pulse from the laser show that the films have a regular array of "dots" with in-plane magnetization in a background matrix of perpendicular magnetization. Such patterning holds promise for the study of model nanoscale magnetic systems.

Interlayer exchange coupling has been studied in a series of ${[\mathrm{Pt}({t}_{\mathrm{Pt}}\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})∕\mathrm{Co}(4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})]}_{n}∕\mathrm{NiO}({t}_{\mathrm{NiO}})∕{[\mathrm{Co}(4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})∕\mathrm{Pt}({t}_{\mathrm{Pt}}\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})]}_{n}$ multilayers with perpendicular anisotropy. The coupling oscillates between antiferromagnetic and ferromagnetic as a function of ${t}_{\mathrm{NiO}}$ with a period of $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, and the oscillatory behavior is related to the antiferromagnetic ordering of the $\mathrm{NiO}$ layer. This interlayer coupling between two $\mathrm{Co}∕\mathrm{Pt}$ multilayers is shown to occur domain by domain by magnetic force microscopy imaging. For the strongest antiferromagnetic coupling at ${t}_{\mathrm{NiO}}=11\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, an oscillation with a period of $\ensuremath{\sim}6\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ is superposed onto the exponential decay of the coupling strength as a function of ${t}_{\mathrm{Pt}}$. The exponential decay with ${t}_{\mathrm{Pt}}$ is ascribed to the exponential decay of the coupling between the $\mathrm{Co}$ layers across the $\mathrm{Pt}$ layers in each $\mathrm{Co}∕\mathrm{Pt}$ multilayer, and the superposed oscillatory behavior can be attributed to multiple reflections of electron waves at the $\mathrm{Co}∕\mathrm{Pt}$ interfaces and their interference. A linear dependence of the antiferromagnetic coupling strength on $1∕n$, (where $n$ is the number of repeats), is suggestive of a surface interaction for this interlayer coupling.

The effect of disorder on the ferromagnetic transition is investigated in mechanically milled $\mathrm{Gd}{\mathrm{Al}}_{2}$. $\mathrm{Gd}{\mathrm{Al}}_{2}$ is a ferromagnet when crystalline and a spin glass when amorphous. Mechanical milling progressively disorders the alloy, allowing observation of the change from ferromagnetic to a disordered magnetic state. X-ray diffraction and pair-distribution-function analysis are used to determine the grain size, lattice parameter, and mean-squared atomic displacements. The magnetization as a function of temperature is described by a Gaussian distribution of Curie temperatures. The mean Curie temperature decreases with decreasing lattice parameter, where lattice parameter serves as a measure of defect concentration. Two different rates of change with lattice parameter are observed: the first, slower rate occurs while grain size is changing, and the grain size is constant in the second regime. The breadth of the Curie temperature distribution is linear with lattice parameter. Milled and annealed $\mathrm{Gd}{\mathrm{Al}}_{2}$ shows that the mean Curie temperature has the same general dependence on lattice parameter as milled, but unannealed $\mathrm{Gd}{\mathrm{Al}}_{2}$, while the breadth of the Curie temperature distribution is different in annealed samples compared to the unannealed materials.

Measurements of DC susceptibility, AC susceptibility, and AC susceptibility with an applied DC bias field were performed on mechanically milled GdAl2. A paramagnetic phase exists above a temperature T ≈ 140 K. However, there are significant deviations from the Curie-Weiss Law in this temperature regime, suggesting multiple magnetic components. Fits to the high temperature data show that two Curie-Weiss terms represent the data quite well. Below 140 K one of these magnetic components becomes ferromagnetic as indicated by a shoulder in the AC susceptibility and DC susceptibility data. This ferromagnetic component is suppressed by the application of sufficiently strong DC bias field. Accompanying this shoulder is a peak at lower temperatures (T < 50 K), which suggests the existence of another component that is magnetically glassy in nature. The two-component behavior of mechanically milled GdAl2 can be explained in terms of the nanostructure of the material, which consists of nanometer-sized grains and a disordered interphase.

Mechanical milling provides a unique means of studying the influence of grain size and disorder on the magnetic properties of nanostructured alloys. This paper compares the role of milling in the nanostructure evolution of two ferromagnets – SmCo5 and GdAl2 – and the subsequent impact of nanostructure on magnetic properties and phase transitions. The ferromagnetic properties of SmCo5 are enhanced by short (< 2 hours) milling times, producing up to an eight-fold increase in coercivity and high remanence ratios. The coercivity increase is attributed to defect formation and strain. Additional milling increases the disorder and produces a mix of ferromagnetic and antiferromagnetic interactions that form a magnetically glassy phase. GdAl2, which changes from ferromagnetic in its crystalline form to spin-glass-like in its amorphous form, is a model system for studying the dependence of magnetically glassy behavior on grain size and disorder. Nanostructured GdAl2 with a mean grain size of 8 nm shows a combination of ferromagnetic and magnetically glassy behavior, in contrast to previous studies of nanostructured GdAl2 with a grain size of 20 nm that show only spin-glass-like behavior.

FePt L10-structured clusters were synthesized by hydrogen reduction of Fe(NO3)3∙9H2O and H2PtCl6∙6H2O mixtures within the pores of alumina. They were released by dissolving the alumina matrix and capped with organic surfactants. After a series of chemical treatments, clusters with an average diameter of about 11nm were precipitated. The clusters were drop casted onto ordered nanoporous arrays. Clusters reaching the bottom of the pores formed an ordered magnetic array. The corresponding magnetic film exhibits distinct anisotropic behavior caused by the external magnetic field.