Field-Induced Magnetization Steps in Intermetallic Compounds and Manganese Oxides: The Martensitic Scenario

Field-induced magnetization jumps with similar characteristics are observed at low temperature for the intermetallic germanide Gd5Ge4and the mixed-valent manganite Pr0.6Ca0.4Mn0.96Ga0.04O3. We report that the field location -and even the existence- of these jumps depends critically on the magnetic field sweep rate used to record the data. It is proposed that, for both compounds, the martensitic character of their antiferromagnetic-to-ferromagnetic transitions is at the origin of the magnetization steps.Comment: 4 pages,4 figure

Magnetic anisotropy, first-order-like metamagnetic transitions and large negative magnetoresistance in the single crystal of Gd2_{2}PdSi3_3

Electrical resistivity ($\rho$), magnetoresistance (MR), magnetization, thermopower and Hall effect measurements on the single crystal Gd$_{2}$PdSi$_3$, crystallizing in an AlB$_2$-derived hexagonal structure are reported. The well-defined minimum in $\rho$ at a temperature above N\'eel temperature (T$_N$= 21 K) and large negative MR below $\sim$ 3T$_N$, reported earlier for the polycrystals, are reproducible even in single crystals. Such features are generally uncharacteristic of Gd alloys. In addition, we also found interesting features in other data, e.g., two-step first-order-like metamagnetic transitions for the magnetic field along [0001] direction. The alloy exhibits anisotropy in all these properties, though Gd is a S-state ion.Comment: RevTeX, 5 pages, 6 encapsulated postscript figures; scheduled to be published in Phy. Rev. B (01 November 1999, B1

Tuning morphology and magnetism in epitaxial L10-FePt films

In this work, well-ordered epitaxial FePt thin ¿lms have been grown by RF sputtering on two different substrates (MgO (100) and SrTiO3 (100)) and the effect of different lattice parameters between the substrate and FePt ¿lm on morphology and magnetic behavior has been considered. Growth conditions have been optimized to obtain different morphologies and magnetic behaviors

Characteristic length scale of the magnon accumulation in Fe3O4/Pt bilayer structures by incoherent thermal excitation

The dependence of Spin Seebeck effect (SSE) with the thickness of the magnetic materials is studied by means of incoherent thermal excitation. The SSE voltage signal in Fe3O4/Pt bilayer structure increases with the magnetic material thickness up to 100¿nm, approximately, showing signs of saturation for larger thickness. This dependence is well described in terms of a spin current pumped in the platinum film by the magnon accumulation in the magnetic material. The spin current is generated by a gradient of temperature in the system and detected by the Pt top contact by means of inverse spin Hall effect. Calculations in the frame of the linear response theory adjust with a high degree of accuracy the experimental data, giving a thermal length scale of the magnon accumulation (¿) of 17¿±¿3¿nm at 300¿K and ¿¿=¿40¿±¿10¿nm at 70¿K

Chemical solution synthesis and ferromagnetic resonance of epitaxial thin films of yttrium iron garnet

We report the fabrication of epitaxial Y3Fe5O12 (YIG) thin films on Gd3Ga5O12 (111) using a chemical solution method. Cubic YIG is a ferrimagnetic material at room temperature, with excellent magneto-optical properties, high electrical resistivity, and a very narrow ferromagnetic resonance, which makes it particularly suitable for applications in filters and resonators at microwave frequencies. But these properties depend on the precise stoichiometry and distribution of Fe3+ ions among the octahedral/tetrahedral sites of a complex structure, which hampered the production of high-quality YIG thin films by affordable chemical methods. Here we report the chemical solution synthesis of YIG thin films, with excellent chemical, crystalline, and magnetic homogeneity. The films show a very narrow ferromagnetic resonance (long spin relaxation time), comparable to that obtained from high-vacuum physical deposition methods. These results demonstrate that chemical methods can compete to develop nanometer-thick YIG films with the quality required for spintronic devices and other high-frequency applications

Hydrostatic pressure control of the magnetostructural phase transition in Gd5Si2Ge2 single crystals

Magnetic and structural properties of single crystalline Gd5Si2Ge2 under hydrostatic pressure have been characterized by using magnetization, linear thermal expansion, and compressibility measurements. A strong dependence of Curie temperature on pressure, dTC∕dP=+4.8 K∕kbar, is observed in contrast with the smaller values of about 3 K∕kbar found in polycrystalline specimens. This difference reflects the role the microstructure may play in pressure-induced magnetic-crystallographic phase changes, likely related to stress relaxation at the grain boundaries, domain pinning and/or nucleation of defects. The pressure dependence of the critical magnetic field, d(dHC∕dT)∕dP, drops at the rate −0.122(5)kOe∕K kbar, which points to an enhancement of the magnetoelastic coupling with pressure. The latter affects the magnetocaloric behavior of the material at the rate d(ΔSM)∕dP≅1.8 J∕kg K kbar. The linear thermal expansion confirms the strongly anisotropic change of the lattice parameters through the orthorhombic to monoclinic crystallographic transformation with Δa∕a=+0.94%, Δb∕b=−0.13%, and Δc∕c=−0.22%. The structural transition temperature varies with pressure synchronously with the Curie temperature, and the size and shape of the strain anomalies remain nearly unaffected by the hydrostatic pressure, indicating, respectively, that the structural and magnetic transformations remain coupled, and the anisotropic behavior of the lattice is preserved as pressure increases. The room temperature linear compressibility data show that the magnetostructural transformation can be triggered isothermally at ∼6 kbar and that the compressibility is anisotropic

High-coercivity ultralight transparent magnets

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.Magnetic silica-aerogel composites have been synthesized by dispersing hard magnetic Nd2Fe14B particles in a sol during a fast sol-gel process and subsequently supercritically drying the resulting gels. The composites are found to retain most of the outstanding properties of their constituents: the large coercivity and moderate remanence of the magnetic powders and the transparency and low density of silica aerogels. Moreover, aerogels synthesized in the presence of a magnetic field exhibit the alignment of the particles, forming needle-like structures along the direction of the applied magnetic field, which results in optical and magnetic anisotropies. Due to their unique combination of properties, these types of materials may be appealing for magneto-optics and magnetic actuator applications