Giant magnetoresistance and topological Hall effect in the EuGa₄ antiferromagnet

Abstract

We report on systematic temperature- and magnetic field-dependent studies of the EuGa4 binary compound, which crystallizes in a centrosymmetric tetragonal BaAl4-type structure with space group I4/mmm. The electronic properties of EuGa4 single crystals, with an antiferromagnetic (AFM) transition at T N ∼ 16.4 K, were characterized via electrical resistivity and magnetization measurements. A giant nonsaturating magnetoresistance was observed at low temperatures, reaching ∼ 7 × 1 0 4 % at 2 K in a magnetic field of 9 T. In the AFM state, EuGa4 undergoes a series of metamagnetic transitions in an applied magnetic field, clearly manifested in its field-dependent electrical resistivity. Below T N, in the ∼4–7 T field range, we observe also a clear hump-like anomaly in the Hall resistivity which is part of the anomalous Hall resistivity. We attribute such a hump-like feature to the topological Hall effect, usually occurring in noncentrosymmetric materials known to host topological spin textures (as e.g., magnetic skyrmions). Therefore, the family of materials with a tetragonal BaAl4-type structure, to which EuGa4 and EuAl4 belong, seems to comprise suitable candidates on which one can study the interplay among correlated-electron phenomena (such as charge-density wave or exotic magnetism) with topological spin textures and topologically nontrivial bands.

Spin order and fluctuations in the EuAl4 and EuGa4 topological antiferromagnets: A μSR study

“…Single crystals of EuAl 4 and EuGa 4 were grown by a molten Al-and Ga-flux method, respectively, the details of growth being reported elsewhere [6,7]. The crystal orientation was checked by x-ray diffraction (XRD) measurements using a Bruker D8 diffractometer with Cu K α radiation.…”

“…Among these materials, the Eu-4 f electrons bring new intriguing aspects to the topology. Both EuAl 4 and EuGa 4 are antiferromagnets below their critical temperatures T N = 15.6, and 16.4 K, respectively, with the former also undergoing a CDW transition at T CDW ∼ 140 K [6][7][8][9][10][11][12]. Further, while EuGa 4 exhibits only one antiferromagnetic (AFM) transition, EuAl 4 undergoes four subsequent AFM transitions below T N .…”

“…Further, while EuGa 4 exhibits only one antiferromagnetic (AFM) transition, EuAl 4 undergoes four subsequent AFM transitions below T N . More interestingly, by applying a magnetic field along the c axis, both EuAl 4 and EuGa 4 undergo a series of metamagnetic transitions in the AFM state [6,7,10]. Within a field range of ∼1-2.5 T (EuAl 4 ) or ∼4-7 T (EuGa 4 ), a clear humplike anomaly was observed in the Hall resistivity, most likely a manifestation of the topological Hall effect (THE) [6,7].…”

“…More interestingly, by applying a magnetic field along the c axis, both EuAl 4 and EuGa 4 undergo a series of metamagnetic transitions in the AFM state [6,7,10]. Within a field range of ∼1-2.5 T (EuAl 4 ) or ∼4-7 T (EuGa 4 ), a clear humplike anomaly was observed in the Hall resistivity, most likely a manifestation of the topological Hall effect (THE) [6,7]. Very recently, a THE has been observed also in Al-doped EuGa 4 [13], which exhibits comparable critical fields to EuAl 4 [6].…”

“…THE has been observed mostly in magnetic compounds whose crystal structure lacks an inversion center, while centrosymmetric compounds that host magnetic skyrmions are rare [15][16][17][18][19]29,30]. Eu(Al,Ga) 4 represent such rare cases, where to look for the possible existence of magnetic skyrmions [6,7,13]. According to neutron diffraction studies, in the AFM state, the magnetic q vector of EuAl 4 changes from q 1 = (0.085, 0.085, 0) at T N = 13.5 K to q 2 = (0.170, 0, 0) at 11.5 K and slightly to q 3 = (0.194, 0, 0) at 4.3 K [31].…”