J.S. Muñoz

Magnetic properties of nanophase CoFe2O4 particles

Abstract Fine CoFe 2 O 4 particles with average size of 50 A have been produced by coprecipitation from Fe(II) and Co(II) solutions followed by a low-temperature calcination. The ac and dc magnetic susceptibilities, as well as magnetization, were studied between 5 and 300 K. The disordered system of such particles behaves superparamagnetically above T B = 250 K and ferrimagnetically below this temperature. T B can be considered as mean blocking temperature at zero field in the timescale of ac (111.1 Hz) magnetic susceptibility measurements. The anisotropy field exceeds 50 kOe below 150 K. The maximal coercivity measured at 5 K is 14.5 kOe and the maximal measured magnetization is 14.6 emu/g, which is much lower than the saturation magnetization of bulk CoFe 2 O 4 .

Room-temperature coercivity enhancement in mechanically alloyed antiferromagnetic-ferromagnetic powders

The coercivity, HC, and squareness of Co powders have been enhanced at room temperature by mechanically alloying them with antiferromagnetic powders with Neel temperature, TN, above room temperature. The enhancement is maximum after field annealing above TN. The existence of loop shifts and the dependence of HC on the annealing and measuring temperatures indicate that exchange bias effects are responsible for this behavior.

Coercivity and squareness enhancement in ball-milled hard magnetic–antiferromagnetic composites

The room-temperature coercivity, HC , and squareness, MR / MS ~remanence/saturation magnetizations!, of permanent magnet, SmCo 5 powders have been enhanced by ball milling with antiferromagnetic NiO ~with Neel temperature, T N 5590 K!. This enhancement is observed in the as-milled state. However, when the milling of SmCo 5 is carried out with an antiferromagnet with T N below room temperature ~e.g., for CoO, TN5290 K!, the coercivity enhancement is only observed at low temperatures after field cooling throughTN . The ferromagnetic-antiferromagnetic exchange coupling induced either by local heating during milling (SmCo51NiO) or field cooling (SmCo51CoO) is shown to be the origin of the HC increase.

Exchange bias in ferromagnetic nanoparticles embedded in an antiferromagnetic matrix

The fabrication process and magnetic properties of three types of system consisting of ferromagnetic (FM) particles embedded in an antiferromagnetic (AFM) matrix are discussed. The preparation techniques are ball milling, H2 partial reduction of oxides and nanoparticle gas condensation. The magnetic properties of the FM/AFM composites are shown to depend strongly on the morphology of the system (e.g., nanoparticle size), the AFM anisotropy and the AFM-FM coupling. For example, all the studied systems exhibit coercivity enhancement below the Neel temperature of the AFM. However, while Co nanoparticles embedded in CoO exhibit loop shifts of thousands of Oe, Fe nanoparticles in Cr2O3 only show a few Oe shifts. An interesting effect evidenced in all systems is the increase of remanence (MR) which, in the case of Co-CoO, ultimately leads to an improvement of the superparamagnetic blocking temperature of the nanoparticles.

Isothermal tuning of exchange bias using pulsed fields

Exchange bias, HE, and coercivity, HC, of antiferromagnetic (AFM)/ferromagnetic bilayers can be adjusted, after deposition, at temperatures below the Neel temperature of the AFM by subjecting the samples to large pulsed fields (in excess of HPulse=550 kOe). The efficiency of the process depends on the AFM system and the direction of the applied field with respect of the unidirectional anisotropy direction. Textured (111) Fe19Ni81/Fe50Mn50 bilayers show an HE reduction and a HC increase when the pulse field is applied antiparallel to the unidirectional anisotropy, while they only exhibit a reduction in HC when the pulse is applied parallel to their unidirectional anisotropy. On the other hand, textured (111) NiO/Co bilayers exhibit a change of the angular dependence of HE when the pulse is applied away from the unidirectional anisotropy. The effects could be caused by field induced changes in the domain structure of the AFM or transitions in the AFM (spin–flop or AFM–paramagnetic).

Magnetic properties of ball milled Fe-40 Al at.% alloys

A direct correlation between the lattice parameter and the saturation magnetization, during the disordering (ball milling) and posterior reordering (annealing) processes, has been found in Fe-40 Al at.% compounds. These results indicate that the paramagnetic-ferromagnetic-paramagnetic transitions induced by ball milling and subsequent annealing could be related to the changes in volume, and not only to nearest neighbors effects as is commonly assumed. Moreover, these alloys have been found to become spin glass at low temperatures, independently of their structural state (ordered or disordered).

High-coercivity ultralight transparent magnets

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.

STRUCTURAL, MECHANICAL AND MAGNETIC PROPERTIES OF NANOSTRUCTURED FeAl ALLOYS DURING DISORDERING AND THERMAL RECOVERY

Abstract The microstructural, mechanical and magnetic properties of the ball milled and annealed Fe-40Al at% B2 alloys have been studied. The disordering process is characterized by an alloy hardening and a para-ferromagnetic transition, which depend mainly on the grain refinement and the creation of antisite defects. The reordering process shows a continuous softening of the nanocrystalline alloy and the recovery of the paramagnetic state, which is essentially governed by the annihilation of antisite defects. The correlation between the microstructural, mechanical and magnetic properties make magnetic measurements a suitable, non-destructive, tool to study the microstructural and mechanical state of FeAl nanocrystalline alloys.

Increasing the Curie temperature of Ca2FeMoO6 double perovskite by introducing near-neighbour antiferromagnetic interactions

We report on the magnetic, magnetotransport and structural characterization of (Ca1?yNdy)2Fe1+xMo1?xO6 (x 0) of Fe ions in the metallic sublattice produces a remarkable increase, by more than 90?K, of the Curie temperature. M?ssbauer spectroscopy data indicate a reinforcement of the magnetic interactions. We argue that this dramatic enhancement of the ferromagnetic order is due to the strong antiferromagnetic superexchange coupling between near-neighbour Fe?Fe occupying regular and antisite positions in the structure. Moreover, the results indicate that the excess of magnetic ions (Fe) is essential to overcome the dilution effects caused by antisite defects.

Using exchange bias to extend the temperature range of square loop behavior in [Pt∕Co] multilayers with perpendicular anisotropy

The temperature dependence of the magnetic properties of [Pt∕Co] multilayers (ML), exhibiting perpendicular anisotropy, with and without exchange biasing with an antiferromagnet (AFM) has been investigated. Upon heating, a loss of the out-of-plane anisotropy and, consequently, of the remanence to saturation ratio is observed in these systems. However, such effect occurs at higher temperatures in the [Pt∕Co] ML exchange coupled to the AFM than for the unbiased ML. This is attributed to the additional anisotropy induced to the ML by the ferromagnetic-antiferromagnetic exchange coupling.