V.M.T.S. Barthem

The physics of coercivity

Abstract In hard magnets, coercivity is not a material intrinsic property. Magnetization reversal is constituted by a series of local processes, of which the first, termed as “true nucleation”, takes place within a defect. An original analysis is proposed in this article which relates the coercive field H c ( T ) to another experimental parameter, the activation volume. The obtained results exclude that coercivity is governed by true nucleation. Another process is necessarily involved before possible wall pinning in the bulk of the grains. It corresponds to the passage and expansion of the magnetic wall from the defect to the main hard phase. During passage/expansion, the wall energy increases progressively as the wall moves into regions with properties that approach those of the main hard phase and its surface area increases. At the same time, its behaviour tends to be governed by main phase properties. This scenario allows coercivity in hard magnets to be quantitatively interpreted.

Magnetic and magnetoelastic properties of the hexagonal TmNi5 compound

Abstract An extensive experimental study by magnetic resistivity, magnetostriction and thermal expansion measurements on a single crystal of the hexagonal TmNi5 compound is presented. In this compound, which orders ferromagnetically at 4.5 K, the magnetic and magnetoelastic properties are dominated by the strong crystal field (CEF) effects which lead to a quasi-doublet ground state well below the excited states and for which and are very close to saturation. These CEF effect s are responsible for the huge uniaxial anisotropy. One of the most noteworthy results is that, thanks to this CEF splitting, it is possible to observe separately the single ion magnetoelastic effects (field dependences at 1.2 K) and the exchange striction (thermal expansion below Tc). These results are quite satisfactorily interpreted within a model involving CEF effects, quadrupolar interactions and exchange striction for Tm3+ ions and taking into account the Ni contribution.

Revisiting magnetization processes in granular hard magnetic materials

We show that the generally accepted demagnetizing field corrections are not applicable to the broad class of modern granular magnetic materials, where magnetization reversal occurs via discrete switching of the moments of individual grains, and thus the self-demagnetizing field of a given grain is proportional to the spontaneous magnetization Ms, not to the sample magnetization M. This leads to the fact that the self-demagnetizing field must be considered as a contribution to the coercive field and that the slope of the magnetization variation is higher than the classical demagnetizing field slope. This description of demagnetization processes is confirmed by numerical modeling.

Magnetic properties of Fe–Cu alloys prepared by pulsed electrodeposition

FexCu100−x metastable alloys were prepared by pulsed electrodeposition for 5 50) are reminiscent of those observed in Fe–Cu alloys prepared by other methods. The Curie temperature decreases regularly with decreasing x. In the Fe-poor alloys (x≤30), the observed properties indicate that Fe-rich clusters form within a Cu-rich matrix. In the x=10 alloy, the Fe clusters are found to be superparamagnetic at room temperature, but a superferromagnetic order develops below a critical temperature of about 120 K. It is suggested that the intercluster coupling is mediated by Ruderman–Kittel–Kasuya–Yosida interactions which are predominantly positive due to the very short intercluster distances.

Texturing Nd–Fe–B magnets under high magnetic field

An original approach is explored in the preparation of anisotropic hard magnetic alloys. This constitutes a proof of principle toward the preparation of anisotropic bonded magnets. Nd–Fe–B ribbons (50% Nd2Fe14B+50% Nd–Cu alloy), constituted of Nd2Fe14B grains embedded in a Nd–Cu eutectic matrix, were annealed under an applied magnetic field up to 16 T at various temperatures above the Nd–Cu melting temperature. The grain orientation mechanism is described in terms of a competition between the aligning magnetic field torque acting on the solid magnetic grains and the friction counter torque at the interface between the grains and the liquid matrix. The large temperature effect on the orientation behavior is attributed to the associated increase in the liquid phase volume fraction.

Magnetic properties of lithium ferrite doped with aluminum and gallium

Measurements are reported on high‐field magnetization of powder samples of ordered lithium ferrite doped with Ga and Al [LiFe4.5(Ga,Al)0.5O8]. We found a linear behavior of the spontaneous magnetization with the difference in Ga‐Al concentrations. The data were interpreted with the Neel model to give the cation distribution and with the method of Rado–Folen to estimate the molecular field parameters (n, α, β) and the bilinear exchange integrals ( Jij).

Observation of magnons in Mn2Au films by inelastic Brillouin and Raman light scattering

The intermetallic antiferromagnetic compound Mn2Au has been attracting considerable interest for antiferromagnetic spintronics due to its high Neel temperature and strong spin–orbit coupling. We report on the experimental investigation of the zero-wave number magnon frequencies in Mn2Au films using Brillouin and Raman inelastic light scattering techniques. The derived effective anisotropy field values are in close agreement with theoretical calculations. With the values of the anisotropy and exchange fields, the full magnon dispersion curves in Mn2Au were calculated. Due to the weak in-plane anisotropy, the k ∼ 0 frequency of the lower magnon branch, 121 GHz, is among the lowest for 3D antiferromagnets, suggesting that Mn2Au is a good candidate for realizing the generation of spin currents by antiferromagnetic resonance driven spin-pumping, as proposed theoretically.

Texturing hard Nd-Fe-B powdered ribbons in high magnetic field

A new route to texture NdFeB alloys in magnetic field was presented in a recent paper [1], constituting an attempt towards the preparation of anisotropic bonded magnets. NdFeB ribbons composed of Nd2Fe14B grains embedded in a Nd-Cu eutectic matrix, were annealed under an applied magnetic field up to 16T, at temperatures above the Nd-Cu melting temperature. A crystallographic texture was found to progressively develop at annealing temperatures, above 700 °C. In this paper, it is proposed that the grain orientation mechanism involves a competition between the aligning magnetic field torque acting on the magnetic grains and thermal disordering effects which becomes more and more significant as the temperature is increased. A simple model is developed to evaluate these effects. It is shown as well that a lowering of the alloy coercivity takes place during annealing.

Magnetic properties of electrodeposited Fe-poor Fe-Cu alloys

In Fe-Cu alloys prepared by pulsed electrodeposition, Fe clusters form directly within a non-magnetic Cu matrix. The clusters contain around 140 Fe atoms and are superparamagnetic at room temperature. Below 160 K, it is proposed that the clusters order into a so-called superferromagnetic arrangement. This behaviour is ascribed to the persistence of weak exchange interactions mediated by isolated Fe atoms dispersed within the non-magnetic Cu matrix.

An analysis of coercivity in various hard magnetic materials relating coercive field and activation volume

In usual analyses, the coercive field of hard magnetic materials is related to the main phase anisotropy. This may be questioned since coercivity is not an intrinsic material property. In a recent paper, we have shown that a relation may be established between two experimental parameters characterising coercivity, the coercive field and the activation volume. The temperature dependence of the coercive field in various hard magnets is discussed within this model. The coercivity mechanism which emerges from the analysis is passage or expansion.