D.X. Li

Magnetic properties of stoichiometric Gd monopnictides

We report on the systematic magnetization measurements performed on the stoichiometric Gd monopnictides, ( = N, P, As, Sb, and Bi), at temperatures between 1.6 and 350 K in magnetic fields up to 42 T. GdN shows the ferromagnetic behaviour with a Curie temperature . The other samples order antiferromagnetically at the Neel temperatures = 15.9, 18.7, 23.4, and 25.8 K for = P, As, Sb, and Bi, respectively. A spin - flop transition is observed at low temperature for except GdN. Based on molecular field approximation, the nearest-neighbour and next-nearest-neighbour exchange constants and of are calculated using the experimental data. Comparing with Eu monochalcogenides, the other group of S-state rare-earth compounds, the absolute values of and in are anomalously large, and the ferromagnetic RKKY interactions due to semimetallic carriers are very weak. These anomalous properties are discussed with a new theoretical model developed very recently by Kasuya.

Spin-glass behaviour with extended short-range ferromagnetic order in U2RhSi3

For U2RhSi3, which crystallizes in a disordered derivative of the hexagonal AlB2-type structure, we present a detailed investigation of magnetic properties by measurements of AC and DC susceptibility, magnetization, magnetic relaxation, specific heat and electrical resistivity. The results provide evidence for the formation of extended short-range ferromagnetic order at the temperature TC25 K, as indicated by a specific heat anomaly, and for a low-temperature spin-glass state with a freezing temperature Tf = 19.6 K. According to systematic specific heat experiments on isostructural spin-glass systems, short-range ferromagnetic correlations above the spin-freezing temperature are observed in U2RhSi3 (TC25 K, Tf = 19.6 K) and U2CoSi3 (TC10 K, Tf8 K), but not in U2PdSi3 (Tf = 13.5 K) and U2NiSi3 (Tf22 K). Different degrees of structural disorder of transition metal and silicon atoms, both on the same crystallographic site, seem to be the origin for the distinct different magnetic behaviour.

Charge-ordering in Sm4Bi3 and Yb4As3 with the anti-Th3P4 structure

Abstract Sm4Bi3 and Yb4As3 are mixed valence materials with the ratio of 1:3 of trivalent and divalent rare earth ions and are expected to show charge-ordering. In the specific heat of Sm4Bi3 and Yb4As3 we observed very sharp peaks at 260 K and at 293 K, respectively. Entropies estimated from these peaks are too small to be ascribed to full ordering, suggesting short range ordering at higher temperatures.

Evidence of formation of magnetic polaron states in semimetallic Gd-monopnictides

Abstract Large stoichiometric and nonstoichiometric single crystals of the gadolinium monopnictides GdX (X = P, As, Sb, Bi) have been grown for the first time by the mineralization method. Magnetic and transport properties of stoichiometric and nonstoichiometric samples are found to be very different. The weak ferromagnetism and the negative magnetoresistivity observed in nonstoichiometric samples of GdP and GdAs at low temperatures and low magnetic fields can be considered as clear evidence of the formation of trapped magnetic polaron states in gadolinium monopnictide systems.

Spin-glass behavior with short-range antiferromagnetic order in Nd2AgIn3

We present experimental results of complex susceptibility, static magnetization, magnetic relaxation, specific heat, and electrical resistivity measurements on an intermetallic compound Nd2AgIn3. The results indicated that Nd2AgIn3 undergoes a spin-glass transition at a freezing temperature Tf=12.6 K accompanied with the formation of short-range antiferromagnetic order. The mechanism of formation of spin-glass state in Nd2AgIn3 is different from that in diluted metallic spin-glass or uranium intermetallic compound. The existences of frustrated moments due to the triangles of nearest neighbors in Nd atomic layers and randomly distributed Ruderman–Kittle–Kasuya–Yosida interactions may be responsible for the spin glass state formed in Nd2AgIn3.

Spin-glass behavior in U2PtSi3

Abstract We present the DC magnetization, specific heat and magnetic relaxation measurements on well-annealed polycrystal-line sample of U 2 PtSi 3 . All the main characteristics of spin-glass behavior are observed. They are: (1) frozen-in magnetic moments below a freezing temperature T f and, hence, a peak in the field-dependent magnetization; (2) absence of visible singularity in temperature dependence of specific heat at T f ; (3) irreversibility in magnetization below T f ;(4) remanent magnetic relaxation on macroscopic time scales after changing the magnetic field below T f . The spin-glass state in U 2 PtSi 3 at low temperatures is considered to originate from the randomly frustrated U-U interactions resulting from the disordering of Pt and Si atoms on the crystallographic sites.

Magnetic properties of ferromagnetic GdN

Abstract A polycrystalline GdN sample was prepared by the direct reaction of gadolinium metal and nitrogen gas at 1600°C and at a pressure of 1300 atm. We measured magnetization and the magnetic field dependence of the susceptibility and specific heat of GdN. Susceptibility and magnetization show ferromagnetic behavior for GdN with the saturation value reached at about 20 kOe. Specific heat measurements revealed a ferromagnetic transition in GdN at T c = 58 K with an additional broad peak around 20 K. Experimental results were explained with an exchange interaction mechanism.

Giant and isotropic low temperature magnetocaloric effect in magnetic semiconductor EuSe

We present the giant and isotropic magnetocaloric effect (MCE) in antiferromagnetic semiconductor EuSe. Near the transition temperature TN = 4.6 K, the maximum value of the magnetic entropy change (−ΔSmmax) and the relative cooling power of EuSe for a field change of 5 T are evaluated to be 37.5 J kg−1 K−1 and 580 J kg−1, respectively, based on the magnetization and specific heat data. The −ΔSmmax value of EuSe obtained in this work is the largest among the low temperature (<20 K) magnetic refrigerant materials reported so far, which is even larger than that of the best known room temperature giant MEC materials.

Spin-glass-like behavior of ternary uranium compound U2AuSi3

Abstract DC magnetization, specific heat and magnetic relaxation measurements are reported for a polycrystalline U 2 AuSi 3 . The temperature dependence of the magnetization shows a cusp at a characteristic temperature T f that strongly depends on the applied magnetic field. The specific heat does not show visible singularity around T f , ruling out the existence of the usual long range spatial magnetic order. The pronounced difference between the zero field cooled (ZFC) and the field cooled (FC) magnetization, as well as the very slow decay of the remanence indicate the occurrence of a highly irreversible frozen state below T f . These features are the characteristics of spin glass behavior.

Mechanism of strong ferromagnetism in GdN

Abstract GdN and EuO are found to be strong ferromagnets with similar values of T c . They have similar electronic characteristics but have different 4f level positions and band gaps. Due to these differences, a mechanism for strong ferromagnetism in EuO is not applicable for GdN. Detailed theoretical investigation has been performed on the cross processes between the f-d mixing and the f-d exchange interactions, which are thought to give strong ferromagnetic exchange interaction. It is found that the mixing through 4f 8 configuration is important in GdN in good agreement with experimental results.