It is demonstrated that ultrafast generation of ferromagnetic order can be achieved by driving a material from an antiferromagnetic to a ferromagnetic state using femtosecond optical pulses. Experimental proof is provided for chemically ordered FeRh thin films. A subpicosecond onset of induced ferromagnetism is followed by a slower increase over a period of about 30 ps when FeRh is excited above a threshold fluence. Both experiment and theory provide evidence that the underlying phase transformation is accompanied, but not driven, by a lattice expansion. The mechanism for the observed ultrafast magnetic transformation is identified to be the strong ferromagnetic exchange mediated via Rh moments induced by Fe spin fluctuations.

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We report the temperature dependence of the magnetic anisotropy in both compressive and tensile strained films of Pr 0.67 Sr 0.33 MnO 3 ͑PSMO͒. Compressive strain induced by growth on LaAlO 3 ͑LAO͒ substrates results in a spontaneous out-of-plane magnetization, while tensile strain ͑grown on SrTiO 3 ͒ results in in-plane magnetization. The coefficient of linear proportionality between the magnetic anisotropy energy and the tetragonal strain for both compressive and tensile strained PSMO films is larger than that found previously in strained La 0.67 Ca 0.33 MnO 3 films. From the data, we estimate a 20 unit-cell magnetic domain wall width for PSMO/ LAO. Scattering from such a narrow domain wall could produce a potentially significant contribution to the resistivity.

FePt/iron oxide core/shell nanoparticles are synthesized by a two step polyol process with 1,2hexadecanediol as the reducing reagent. Monodispersed 2.6-nm FePt nanoparticles are first obtained by reduction of iron(III) acetylacetonate and platinum(II) acetylacetonate. These preformed FePt nanoparticles are then used as seeds and an iron oxide shell is formed in the second synthesis step. The role of the iron oxide shell on sintering of FePt nanoparticles is investigated. Annealing studies show that these FePt/ iron oxide core/shell structures are stable after annealing at 550 °C for 30 min at which 2.6-nm FePt nanoparticles without oxide shell coating start to sinter. Low-temperature magnetic hysteresis behavior of the annealed core/shell nanoparticles suggests exchange coupling between the magnetically hard FePt core and the magnetically soft iron oxide shell.

We have studied the lattice parameter changes of L10 FePt nanoparticles annealed to near equilibrium as a function of composition by x-ray diffraction. We have found that the (111) diffraction peak shifts linearly with composition, however, the c parameter mostly changes in the Pt rich compositions and the a parameter mostly changes in the Fe rich compositions with respect to the equiatomic composition. This causes the tetragonality of the L10 structure to be maximized near the Fe 50%/Pt 50% composition. The magnetic properties were measured at room temperature and at 5 K and are correlated to the structural changes occurring as a function of composition.