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MAGNETIC SYSTEMS 2009Magnetization at low temperatures and high magnetic fields on LuFe 2 O 4LuFe 2 O 4 is thought to be a multiferroic [Ikeda et al. Nature436, 1136-1138 (2005)], with a novel ferroelectric mechanism,based on charge order. The electrically active Fe withaverage valence of 2.5+ is contained in trigonal Fe-O doublelayers, a highly frustrated arrangement. Below ∼ 320 Kthe Fe valences order, resulting in the double layers becomingpolar, apparently with an antiferroelectric stacking ofthe polarization of the individual double layers [M.Angst etal. Phys. Rev. Lett. 101, 227601 (2008)].The frustration also effects the spin ordering occurring below240 K [Christianson et al. Phys. Rev. Lett. 100,107601 (2008)]. Several indications for magnetoelectricityhave been observed, though the microscopic details ofthis coupling remain to be elucidate. Apart from magnetoelectriccoupling, the magnetism in LuFe 2 O 4 has also attractedattention due to a giant magnetic coercivity, recentlyattributed a freezing of nano-scale pancake-like (Ising) ferromagneticdomains. In low fields we have observed magnetictransitions of sharpness and clear 3D spin order. Bymagnetization, spectroscopic, neutron and synchrotron experimentswe identified a further transition at 170 K, involvinga disruption of magnetic order and a structural distortionwhich can be tuned by a magnetic field.In our experiment at the LNCMI with a 10MW-magnet weperformed hysteresis loops with H||c and a magnetic fieldup to 22 T, within a temperature range between 60 K andhelium base temperature of 3 K. All the magnetization datawas measured with the extraction method. These measurementswere done after cooling down the sample in a zerofield (ZFC). From this ZFC we were able to obtain a virgincurve of the magnetization. The hysteresis loop at basetemperature (inset figure 116) shows a plateau in its virgincurve at a magnetic field of 15T, what is in good agreementwith the data from optical reflectance contrast [Xu etal. Phys. Rev. Lett. 101, 227602 (2008)]. At this valuethe transition is not complete and it remains up to a valueof about 20T until the crystal structure is switched (fromhexagonal to monoclinic) and the magnetization is saturated.We observe only one step in the virgin magnetizationcompared to [Iida et al. Physica B 155 307, (1989)] whereseveral occur. This is may be due to a better crystal qualitywith less different grains. From the in loop magneticbehaviour we were able to complete the magnetic phase diagramfor LuFe 2 O 4 for low temperatures, as shown in figure117, where it is shown that there is a coexistence betweena ferromagnetic (FM) phase and a antiferromagnetic(AFM) phase at low temperatures.We also measured the magnetization in as high fields as feasibleperpendicular to the c-axis. In this part of the experimentthere was no indication of a lacking of the magnetocrystallineanisotropy. This means that the magnetic momentsare all aligned in c-direction.Figure 116: Magnetic field dependence from the magnetizationwith H||c in LuFe 2 O 4 at different temperatures. All measurementswhere taken in zero field cooling (ZFC).Figure 117: Low temperature H-T phase diagram for H||c inLuFe 2 O 4 determined from data shown in figure 116 (arrows indicatethe direction of the transition as a function of the appliedfield).A. B. AntunesJ. de Groot, M. Angst (Forschungszentrum Jülich GmbH, 52428 Jülich, Germany)86