Ch. Binek

Freezing field dependence of the exchange bias in uniaxial FeF2–CoPt heterosystems with perpendicular anisotropy

Abstract The exchange bias effect is measured for the first time in FeF 2 –CoPt heterosystems with perpendicular anisotropy. The exchange field exhibits a strong dependence on the axial freezing field. This behavior is explained in terms of the microscopic spin structure at the interface, which is established on cooling to below T N . We calculate the dependence of the spin structure on the freezing field within the framework of an Ising model. It takes into account the Zeeman energy as well as an antiferromagnetic exchange coupling between the adjacent layers at the interface.

Exchange Bias in a generalized Meiklejohn-Bean approach

Abstract A generalized Meiklejohn–Bean model is considered in order to derive an analytic expression for the dependence of the exchange bias field on the layer thickness involved in ferromagnetic/antiferromagnetic heterosystems, on the orientation of the applied magnetic field with respect to the magnetic easy axes and on the quenched magnetization M AF of the antiferromagnetic pinning layer. While M AF is a well-known feature of field-cooled dilute antiferromagnets, it seems to occur quite generally also in pure AF pinning substrates. The new analytic expressions are successfully compared with recent experimental results and Monte Carlo investigations.

Optimization of magneto-optical Kerr setup: Analyzing experimental assemblies using Jones matrix formalism

We present a comparative study on an experimental and theoretical optimization of magneto-optical Kerr setups based on photoelastic modulation and phase sensitive detector methodology. The first and second harmonics, I omega,2 omega, of the reflected light intensity are measured for a CoO/Co magnetic reference film. The magnetic field dependence of the optical off-diagonal Fresnel reflection coefficients rps and rsp follows the sample magnetization. Different Kerr setups provide various dependencies of I omega,2 omega on the reflection coefficients and, hence, on the Kerr ellipticity epsilon K and rotation theta K. Jones matrix formalism has been used to analyze the impact of a systematic variation of relative analyzer and polarizer orientations with respect to each other and with respect to the retardation axis of the modulator involved in longitudinal Kerr setups for incoming s-polarized light. We find one particular setup which maximizes I(omega) as well as I2 omega and maximizes the signal-to-noise ratio. Inefficient setups are characterized by I omega,2 omega intensities involving large nonmagnetic contributions of rp and rs.

Spin polarization asymmetry at the surface of chromia

We demonstrate boundary spin polarization at the surface of a Cr2O3 single crystal using spin-polarized low-energy electron microscopy (SPLEEM), complementing prior spin polarized photoemission, spin polarized inverse photoemission, and x-ray magnetic circular dichroism photoemission electron microscopy measurements. This work shows that placing a Cr2O3 single crystal into a single domain state will result in net Cr2O3 spin polarization at the boundary, even in the presence of a gold overlayer. There are indications that the SPLEEM contrast for the two polarization states may be different, consistent with scanning tunneling microscopy spectroscopy results obtained from ultrathin films of Cr2O3.

Transient spin structures at the antiferro-to-paramagnetic phase boundary of FeBr2

Abstract Excess magnetization and anomalous susceptibility loss is observed on antiferromagnetic FeBr 2 in an axial field H below and above its H-T -phase boundary between the multicritical temperature ( T m = 4.6 K) and the Neel temperature ( T N = 14.1 K). These and other unusual properties of FeBr 2 are attributed to frustration-induced intraplanar non-critical spin fluctuations, which can be simulated within the framework of a triaxial version of the 2D-ANNNI model.

Scaling of the Magnetoelectric Effect in Chromia Thin Films

Scaling behavior of chromia (0001)-oriented films is investigated for film thicknesses ranging from the nanoscale up to the mesoscopic regime. Unusually prominent finite-size scaling of the critical point shift is observed which is accompanied by finite-size effects of magnetic response functions. We investigate the scaling of the antiferromagnetic ordering temperature by reviewing data from the literature and complementing them with our findings. Particular attention is paid to the hitherto unexplored regime of mesoscopic film thicknesses where conventionally no finite-size effects are expected. Results of previously unreported direct measurements of the magnetic susceptibility of chromia thin films grown by molecular beam epitaxy are presented. The magnetometry data reveal qualitative changes in the temperature dependence and peak value of the susceptibility in comparison with the susceptibility of a bulk single crystal. These non-trivial finite-size effects may originate from oxygen defects which accompany reactive sputtering and evaporation growth processes. Significant implications for the scaling behavior of the magnetoelectric effect and consequences for potential spintronic applications are discussed.

Magnetic entropy changes in nanogranular Fe:Ni61Cu39

Artificial environment-friendly Gd-free magnetic nanostructures for magnetic cooling are investigated by temperature-dependent magnetic measurements. We consider two-phase nanocomposites where nanoclusters (Fe) are embedded in a Ni61Cu39 matrix. Several composite films are produced by cluster deposition. The average Fe cluster size depends on the deposition conditions and can be tuned by varying the deposition conditions. The quasiequilibrium Curie temperature of the Fe particles is high, but slightly lower than that of bulk Fe due to finite-size effects. Our experiments have focused on ensembles of 7.7 nm Fe clusters in a matrix with a composition close to Ni61Cu39, which has a TC of 180 K. The materials are magnetically soft, with coercivities of order 16 Oe even at relatively low temperature of 100 K. The entropy changes are modest, −ΔS = 0.05 J/kg K in a field change of 1 T and 0.30 J/kg K in a field change of 7 T at a temperature of 180 K, which should improve if the cluster size is reduced.

Neutron scattering study of transverse magnetism in the metamagnet FeBr2

Abstract:In order to clarify the nature of the additional phase transition at H1(T) < Hc(T) of the layered antiferromagnetic (AF) insulator FeBr2 as found by Aruga Katori et al. (1996) we measured the intensity of different Bragg-peaks in different scattering geometries. Transverse AF ordering is observed in both AF phases, AF I and AF II. Its order parameter exhibits a peak at T1 = T (H1) in temperature scans and does not vanish in zero field. Possible origins of the step-like increase of the transverse ferromagnetic ordering induced by a weak in-plane field component when entering AF I below T1 are discussed.

METAMAGNETIC DOMAINS AND DYNAMIC FLUCTUATIONS IN FEBR2

The mixed phase and the regime of non-critical fluctuations of the magnetic phase diagram of FeBr2 is investigated by SQUID susceptometry and light diffraction techniques. The experiments seem to evidence instability of the tricritical point as conjectured recently. The observation of stripe domains and light diffraction below and above Tm=4.6 K and the virtual continuation of the phase transition line to above Tm are in agreement with the occurrence of a critical endpoint at Tm and of a bicritical endpoint at T>Tm.

Isothermal entropy changes in nanocomposite Co:Ni67Cu33

The temperature-dependent magnetic properties of artificial rare-earth, free-magnetic nanostructures are investigated for magnetic cooling. We consider two-phase nanocomposites, where 2 nm nanoclusters of cobalt are embedded in a Ni67Cu33 matrix. Several composite films were produced by cluster deposition. The average Co nanocluster size can be tuned by varying the deposition conditions. Isothermal magnetization curves were measured at various temperatures 150 K < T < 340 K in steps of 10 K. The isothermal entropy changes ΔS were calculated using the Maxwell relation. The entropy changes measured were, –ΔS = 0.15 J/kg·K in a field change of 1 T at 260 K and 0.72 J/kg·K in a field change of 7 T at 270 K.