[en] The influence of the Y doping on the magnetic properties of La_0.7-xY_xCa_0.3MnO₃ compounds (x=0; 0.07; 0.1; 0.15) and on the low-field and high-field magnetoresistance responses are investigated by means of transport, susceptibility and magnetisation measurements. The evolution of the magnetic state with yttrium doping is first described and then correlated with the low-temperature magnetoresistance effects. Y doping does not influence in a monotonic way the coercive field or the initial susceptibility of the samples, but modifies the magnetic state from a ferromagnetic (x=0) to a cluster glass (x=0.15). The key observation is that by relatively low temperatures (50 K) the resistivity change in low magnetic fields is controlled by the bulk magnetisation value only, independent of the Y content. It is further shown that the high-field behaviour of resistivity depends on the increased spin disorder resulting directly from the presence of yttrium

[en] The bismuth doping of the (Sm, Sr)₁Mn0₃ perovskites has been studies starting from the nominal composition Sm_0.54Bi_0.16Sr_0.3Mn0₃ sintered at various temperatures from ranging from 1,100 to 1,400 deg.C. The best results are obtained at 1,350 deg. C, leading to the final composition Sm_0.58Bi_0.04Sr_0.36Mn0₃ due to bismuth oxide volatility. The phase so obtained exhibits colossal magnetoresistance properties with a resistance ratio, more than an order of magnitude larger that that obtained for all other (Sm, Sr)₁MnO₃ perovskites without bismuth: R_o/R_B = 2 x 10⁴ at T_max = 80 K in a magnetic field of 5 T. (authors)

[en] A systematic study of the doping of the manganese site with chromium in the charge ordered Mn(IV)-rich perovskites Ca_1-xSm_xMnO₃ (0.15≤x≤0.5) has been performed. Whatever x, the charge ordering disappears for various chromium levels (2-13% Cr). By Cr doping, a ferromagnetic semimetallic component is induced at the expense of the antiferromagnetic state, at low temperature. An important feature deals with the expansion of colossal magnetoresistance (CMR) in all the diagram Ca_1-xSm_xMnO₃ in the range 0.15≤x≤0.50. This spectacular ability of chromium to induce CMR and ferromagnetism has been extended to other lanthanides, i.e. Eu, Gd and Ho, showing the influence of the A-site average size. Moreover, this study demonstrates that the charge ordering is the most stable for manganese valency close to 3.70. (orig.)

[en] The manganites are one of a number of materials whose magnetic properties may be particularly useful because they appear to be 100% spin-polarised metals at low temperature. Recently, it has been demonstrated that tunnelling experiments can access the spin-split density of states in these materials. In the present work, topographic and spectroscopic information was obtained on thin-film La_1-xCa_xMnO₃ material using a low-temperature UHV scanning tunnelling microscope. This paper will outline the results of this study and in particular the variation in observed features on the I-V curves as a function of surface treatment

[en] The doping of a manganese site with various M elements M=Fe³⁺, Al³⁺, Ga³⁺, Ti⁴⁺, Sn⁴⁺, and Mg²⁺ was studied in insulating charge ordered manganite Pr_0.6Ca_0.4MnO₃ that exhibits a canted CE-type magnetic structure. Whatever M, the charge ordering disappears for low x levels (x≥0.02) in doped manganite Pr_0.6Ca_0.4Mn_1-xM_xO₃. But the most important feature deals with the fact that a transition to a ferromagnetic metallic state is induced for M=Fe³⁺, Al³⁺, Ga³⁺, and Mg²⁺ in spite of the small size of the interpolated cation (Pr,Ca). In contrast, doping with a tetravalent cation does not drastically modify the magnetic and transport properties of the phase. copyright 1997 American Institute of Physics

[en] We have studied the variation of the paramagnetic to ferromagnetic transition temperature T_C and of the ferromagnetic to antiferromagnetic transition temperature T_N as a function of the average size of the A site cations (Ln, Sr) in the manganite perovskites Ln_0.5Sr_0.5MnO₃ (Ln=Gd, Sm, Y, Pr, La). For our investigation we used dc resistivity, magnetization, susceptibility and magnetoresistance measurements. Results show that the Curie temperature T_C increases from 85 to 310 K when left-angle r_A right-angle varies from 1.221 to 1.263 Angstrom, whereas T_N remains stable around 150 K. A magnetic phase diagram temperature-left-angle r_A right-angle has been established, allowing five regions with different magnetic behaviours to be evidenced: paramagnetic insulating, ferromagnetic metallic, antiferromagnetic insulating, canted antiferromagnetic metallic, and weak ferromagnetic insulating. The extension of these regions is determined by the value of the applied magnetic field. copyright 1997 American Institute of Physics

[en] A systematic study of the magnetic properties of the manganites Ln_0.5(A,A^'')_0.5MnO₃ (A,A^''=Ba, Ca, Sr) has been carried out. The variations of T_C and T_N vs two parameters, the average size of the interpolated cations left-angle r_A right-angle and the mismatch effect represented by the variance σ², have been studied for the series of manganites La_0.5(A,A^'')_0.5MnO₃, Pr_0.5(A,A^'')_0.5MnO₃, Nd_0.5(A,A^'')_0.5MnO₃ and Sm_0.5(A,A^'')_0.5MnO₃ (A,A^''=Ba, Ca, Sr), involving at most three different cations on the perovskite A site. The results obtained for other (Ln, Ln^'')_0.5Sr_0.5MnO₃ (Ln, Ln^''=Gd, Sm Y, Pr) have been taken into consideration. A σ²-left-angle r_A right-angle diagram has been established, which displays the different types of magnetic behaviors that can be exhibited by Ln_0.5A_0.5MnO₃ manganites, i.e., when the temperature is decreasing, either a paramagnetic (PM) to ferromagnetic (FM) transition, or PM to FM followed by FM to antiferromagnetic transitions, or a PM to weak ferromagnetic transition. copyright 1997 American Institute of Physics

[en] The structural transitions in the Pr_0.5Sr_0.5MnO₃ CMR perovskite have been studied, coupling neutron powder diffraction (NPD) and electron microscopy (EM) techniques. Both techniques show the existence of a Fmmm type structure at low temperature (T<135 K) that corresponds to the antiferromagnetic insulating (AFMI) state. No evidence of charge ordering is observed. At 135 K a transition to a ferromagnetic I4/mcm structure is observed by NPD. Further increasing the temperature (T≥265 K), Pr_0.5Sr_0.5MnO₃ becomes paramagnetic but keeps its I4/mcm structure. By EM, the transition occurring around 135 K is also observed, but it corresponds to a change from the Fmmm to an Imam structure, with a simultaneous disappearance of the twinning domains present in the Fmmm structure. This effect is reversible. The EM study also evidences that at high temperature (T∝430 K) another transition from the Imam to an I4/mcm structure occurs. The divergence between the NPD and EM observations is attributed to the fact that the structure of Pr_0.5Sr_0.5MnO₃ may be sensitive to the magnetic field that exists in the electron microscope (1 or 2 T). (orig.)

[en] The half-doped manganites with the perovskite structure, Ln_0.5A_0.5MnO₃, have been the purpose of numerous investigations, due to their interesting magnetic properties. All these results suggest that the Pr_0.5Sr_0.5-xCa_xMnO₃ system displays a great complexity, i.e., its average size of the A-site cations left-angle r_A right-angle governs the structural and physical properties. In the present paper, the authors establish the magnetic and structural phase diagrams of these half-doped manganites, and they study the relationships between their magnetic and structural transitions

[en] We carried out a comparative study of the A=Ca, Sr, Ba compounds of the α-ACr₂O₄ series, a family of layered chromites, which can be classified as S=3/2 quasi-2D triangular Heisenberg antiferromagnets (2DTHAF). The sizeable distortion, with respect to the perfect triangle lattice, of the spin layer topology can be progressively decreased by increasing the size of A²⁺, which leads to an increase of both the average distance between nearest-neighbouring Cr³⁺ and of the interplane spacing. The evolution with A²⁺ of the antiferromagnetic transition T_N, the intraplane coupling J, and the interplane coupling J′ has been determined on the basis of magnetization and heat capacity measurements and is discussed in the framework of the standard theoretical models describing quasi 2DTHAF. - Highlights: ► Increasing the ionic radius of A²⁺ has a non-monotonous impact on T_N. ► The small single-ion anisotropy can split the antiferromagnetic transition. ► The intraplane coupling is consistent with direct exchange. ► The interplane coupling derived from standard 2DTHAF models is questionable.