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Register a new userIntrinsically weak magnetic anisotropy of cerium in potential hard-magnetic intermetallics
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Cerium-based intermetallics are currently attracting much interest as a possible alternative to existing high-performance magnets containing scarce heavy rare-earth elements. However, the intrinsic magnetic properties of Ce in these systems are poorly understood due to the difficulty of a quantitative description of the Kondo effect, a many-body phenomenon where conduction electrons screen out the Ce-4f moment. Here, we show that the Ce-4f shell in Ce-Fe intermetallics is partially Kondo screened. The Kondo scale is dramatically enhanced by nitrogen interstitials completely suppressing the Ce-4f contribution to the magnetic anisotropy, in striking contrast to the effect of nitrogenation in isostructural intermetallics containing other rare-earth elements. We determine the full temperature dependence of the Ce-4f single-ion anisotropy and show that even unscreened Ce-4f moments contribute little to the room-temperature intrinsic magnetic hardness. Our study thus establishes fundamental constraints on the potential of cerium-based permanent magnet intermetallics.
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The Fe(1+x)Sb compound has been synthesized close to stoichiometry with x = 0.023(8). The compound was investigated by 57Fe Mossbauer spectroscopy in the temperature range 4.2 - 300 K. The antiferromagnetic ordering temperature was found as 232 K i.e. much higher than for the less stoichiometric material. Regular iron was found to occupy two different positions in proportion 2:1. They differ by the electric quadrupole coupling constants and both of them exhibit extremely anisotropic electric field gradient tensor (EFG) with the asymmetry parameter equal one. The negative component of both EFGs is aligned with the c-axis of the hexagonal unit cell, while the positive component is aligned with the <120> direction. Hence, a model describing deviation from the NiAs P63/mmc symmetry group within Fe-planes has been proposed. Spectra in the magnetically ordered state could be explained by introduction of the incommensurate spin spirals propagating through the iron atoms in the direction of the c-axis with a complex pattern of the hyperfine magnetic fields distributed within a-b plane. Hyperfine magnetic field pattern of spirals due to major regular iron is smoothed by the spin polarized itinerant electrons, while the minor regular iron exhibits hyperfine field pattern characteristic of the highly covalent bonds to the adjacent antimony atoms. The excess interstitial iron orders magnetically at the same temperature as the regular iron, and magnetic moments of these atoms are likely to form two-dimensional spin glass with moments lying in the a-b plane. The upturn of the hyperfine field for minor regular iron and interstitial iron is observed below 80 K. Magneto-elastic effects are smaller than for FeAs, however the recoilless fraction increases significantly upon transition to the magnetically ordered state.
We investigated the anisotropic magnetic properties of CePd$_2$As$_2$ by magnetic, thermal and electrical transport studies. X-ray diffraction confirmed the tetragonal ThCr$_2$Si$_2$-type structure and the high-quality of the single crystals. Magnetisation and magnetic susceptibility data taken along the different crystallographic directions evidence a huge crystalline electric field (CEF) induced Ising-type magneto-crystalline anisotropy with a large $c$-axis moment and a small in-plane moment at low temperature. A detailed CEF analysis based on the magnetic susceptibility data indicates an almost pure $langlepm5/2 rvert$ CEF ground-state doublet with the dominantly $langlepm3/2 rvert$ and the $langlepm1/2 rvert$ doublets at 290 K and 330 K, respectively. At low temperature, we observe a uniaxial antiferromagnetic (AFM) transition at $T_N=14.7$ K with the crystallographic $c$-direction being the magnetic easy-axis. The magnetic entropy gain up to $T_N$ reaches almost $Rln2$ indicating localised $4f$-electron magnetism without significant Kondo-type interactions. Below $T_N$, the application of a magnetic field along the $c$-axis induces a metamagnetic transition from the AFM to a field-polarised phase at $mu_0H_{c0}=0.95$ T, exhibiting a text-book example of a spin-flip transition as anticipated for an Ising-type AFM.
Nanoparticles (size 20, 40 and 60 nm) of Pr_0.5Sr_0.5MnO_3 are prepared by sol-gel technique and their magnetic properties are studied using ferromagnetic resonance and magnetization measurements. A comparison with the properties of the bulk material shows that the ferromagnetic transition at 265 K remains unaffected but the anti-ferromagnetic transition at T_N = 150 K disappears in the nanoparticles. Further, the temperature dependence of magnetic anisotropy shows a complex behavior, being higher in the nanoparticles at high temperatures and lower at lower temperatures in comparison with the bulk.
Electrical resistivity rho, magnetic susceptibility chi, magnetization M and specific heat measurements are reported on a singlecrystalline sample of CePd5Al2, showing successive antiferromagnetic orderings at T_N1=4.1 K and T_N2=2.9 K. The temperature dependence of T_N1 shows a Kondo metal behavior with large anisotropy, rho_c/rho_a=3.2 at 20 K, and opening of a superzone gap along the tetragona c-direction below T_N1. Both T_N1 and T_N2 gradually increase with applying pressure up to 2.5 GPa. The data of chi(T) and M(B) in the paramagnetic state were analyzed using a crystalline electric field (CEF) model. It led to a Kramers doublet ground state with wave functions consisting primarily of |+-5/2>, whose energy level is isolated from the excited states by 230 and 300 K. This CEF effect gives rise to the large anisotropy in the paramagnetic state. In the ordered state, the uniaxial magnetic anisotropy is manifested as M_c/M_a=20 in B=5 T and at 1.9 K, and chi_c/chi_a=25 in B=0.1 T and at 4 K. This huge uniaxial magnetic anisotropy in the antiferromagnetic states can be interpreted in terms of isotropic magnetic interaction among the Ce^{3+} moments governed by the strong CEF. In powder neutron diffraction experiments, magnetic reflections were observed owing to the antiferromagnetic ordered states below T_N1, however, no additional reflection was found below T_N2.
Local configurational disorder can have a dominating role in the formation of macroscopic functional responses in strongly correlated materials. Here, we use entropy-stabilization synthesis to create single crystal epitaxial ABO3 perovskite thin films with equal atomic concentration of 3d transition metal cations on the B-site sublattice. X-ray diffraction, atomic force microscopy, and scanning transmission electron microscopy of La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 (L5BO) films demonstrate excellent crystallinity, smooth film surfaces, and uniform mixing of the 3d transition metal cations throughout the B-site sublattice. The magnetic properties are strongly dependent on substrate-induced lattice anisotropy and suggest the presence of long-range magnetic order in these exceptionally disordered materials. The ability to populate multiple elements onto a single sublattice in complex crystal structures opens new possibilities to design functionality in correlated systems and enable novel fundamental studies seeking to understand how diverse local bonding environments can work to generate macroscopic responses, such as those driven by electron-phonon channels and complex exchange interaction pathways.
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