Magnetic and thermal properties of Ferromagnetic (FM) Ce_{2.15}(Pd_{1-x}Ag_x)_{1.95}In_{0.9} alloys were studied in order to determine the Quantum Critical Point (QCP) at T_C => 0. The increase of band electrons produced by Pd/Ag substitution depress
es T_C(x) from 4.1K down to T_C(x=0.5)=1.1K, with a QCP extrapolated to x_{QCP}~ 0.6. Magnetic susceptibility from T>30K indicates an effective moment slightly decreasing from mu_{eff}=2.56mu_B to 2.4mu_B at x=0.5. These values and the paramagnetic temperature theta_P~ -10K exclude significant Kondo screening effects. The T_C(x) reduction is accompanied by a weakening of the FM magnetization and the emergence of a specific heat C_m(T) anomaly at T*~ 1K, without signs of magnetism detected from AC-susceptibility. The magnetic entropy collected around 4K (i.e. the T_C of the x=0 sample) practically does not change with Ag concentration: S_m(4K)~ 0.8 Rln2, suggesting a progressive transfer of FM degrees of freedom to the non-magnetic (NM) component. No antecedent was found concerning any NM anomaly emerging from a FM system at such temperature. The origin of this anomaly is attributed to an entropy bottleneck originated in the nearly divergent power law dependence for T>T*.
The highest antiferromagnetic (AFM) temperature in Ce based compounds has been reported for CeScGe with Tn=47K, but its local or itinerant nature was not deeply investigated yet. In order to shed more light into this unusually high ordering temperatu
re we have investigated structural, magnetic, transport and thermal properties of CeTi{1-x}Sc{x}Ge alloys within the range of stability of the CeScSi-type structure: 0.25<x<1. Along this concentration range, this strongly anisotropic system presents a complex magnetic phase diagram with a continuous modification of its magnetic behavior, from ferromagnetism (FM)for 0.25<x<0.50 (with 7K<Tc<16K) to AFM for 0.60<x<1 (with 19K<Tn<47K). The onset of the AFM phase is associated to a metamagnetic transition with a critical field increasing from H{cr}=0 at x~0.55 to ~6Tesla at x=1, coincident with an increasing contribution of the first excited crystal electric field doublet. At a critical point x{cr}=0.65 a second transition appears at Tl<Tn. In contrast to observations in itinerant systems like CeRh{2}Si{2} or CeRh{3}B{2}, no evidences for significant hybridization of the 4f electrons at large Sc contents were found. Therefore, the exceptionally large Tn of CeScGe can be attributed to an increasing RKKY inter-layer interaction as Sc content grows.
Structural, magnetic (M) and thermal (C_m) studies on Ce_2(Ni_{1-y}Pd_y)_2Sn alloys are presented within the 0<y<0.55 range of concentration, showing evidences for itinerant to local electronic transformation. At variance with RKKY type interactions
between localized moments mu_{eff}, the substitution of Ni by isoelectronic Pd leads the antiferromagnetic transition to decrease from T_N~3.8K to ~1.2K between y=0 and 0.48, while M(H) measured at H=5Tesla and 1.8K rises from 0.12 up to 0.75mu_B/Ce-at. Furthermore, the C_m(T_N) jump increases with concentration whereas |theta_P| decreases. The magnetic entropy S_m(T) grows moderately with temperature for y=0 due to a significant contribution of excited levels at low energy, while at y=0.5 it shows a incipient plateau around S_m=Rln2. All these features reflect the progressive ground state transformation of from itinerant to a local character. Another peculiarity of this system is the nearly constant value of C_m(T_N) that ends in an entropy bottleneck as T_N decreases. Consequently, the system shows a critical point at y_{cr}~0.48 with signs of ferromagnetic behavior above H_{cr}~0.3T. A splitting of the C_m(T_N) maximum, tuned by field and concentration, indicates a competition between two magnetic phases, with respective peaks at T_N~1.2K and T_I~1.45K.
Most of magnetic transitions related to Ce ordering are found below T_ord~12K. Among the few cases exceeding that temperature, two types of behaviors can be distinguished. One of them is related to the rare cases of Ce binary compounds formed in BCC
structures, with a quartet ground state, whose degeneracy is reduced by undergoing different types of transitions mostly structural. The other group shows evidences of itinerant character with the outstanding example of CeRh_3B_2 showing the highest T_ord=115K. The second highest ordering temperature has been reported for CeScGe with T_ord=47K, but the nature of this magnetic state has not been investigated very deeply. In order to shed more light into this unusual high temperature ordering we studied the structural, magnetic, transport and thermal properties of CeTi_1-xSc_xGe alloys in the stability range of the CeScSi-type structure 0.25<x<1 This system presents a rich variety of magnetic behaviors along this concentration range, with the magnetic ordering growing from ferromagnetic (FM) T_C~7K up to an antiferromagnetic (AFM) transition at T_N=47K. The different regions show the following characteristics: i) on the Ti rich side (0.25<x<0.50) it exhibits a FM ground state (GS) with large saturation magnetization values M_sat up to ~1.15 mu_B. ii) Around x=0.60, the first crystal electric field excited doublet starts to contribute to the GS magnetic properties. Furthermore an AFM component with a connected metamagnetic transition appears. iii) At x=0.65 a clear change in the GS nature is associated to a critical point above which the GS properties can be described like for an itinerant system (with decreasing M_sat) and an effective GS degeneracy N_eff=4. iv) For x>0.65, the magnetic phase boundary splits into two transitions, with an intermediate phase presenting incommensurate spin density waves features.
Structural, magnetic and thermal measurements performed on CeCo{1-x}Fe{x}Si alloys are reported. Three regions can be recognized: i) Co-rich (x < 0.20) with a decreasing long range antiferromagnetic order which vanishes at finite temperature, ii) an
intermediate region (0.20 < x < 0.30) showing a broad magnetic anomaly (C_A) in specific heat and iii) the non-magnetic region progressively changing from a non-Fermi-liquid type behavior towards a Fermi liquid one as Fe concentration increases. The C_A anomaly emerges as an incipient contribution above T_N already at x = 0.10, which indicates that this contribution is related to short range correlations likely of quasi-two dimensional type. Both, T_N transition and C_A anomaly are practically not affected by applied magnetic field up to B ~ 10 Tesla.
Structural, magnetization and heat capacity measurements were performed on Ce$_2$(Pd$_{1-x}$Ni$_x$)$_2$Sn ($0 leq x leq 0.25$) alloys, covering the full range of the Mo$_2$FeB$_2$ structure stability. In this system, the two transitions observed in C
e$_2$Pd$_2$Sn (at $T_N=4.8$,K and $T_C=2.1$,K respectively) converge into a tri-critical point at $T_{cr}approx 3.4$,K for $xapprox 0.3$, where the intermediate antiferromagnetic AF phase is suppressed. The $T_N(x)$ phase boundary decrease is due to an incipient Kondo screening of the Ce-4f moments and local atomic disorder in the alloy. Both mechanisms affect the formation of Ce-magnetic dimers on which the Shastry-Sutherland lattice (SSL) builds up. On the contrary, the $T_C(x)$ transition to the ferromagnetic ground state increases as a consequence of the weakening of the AF-SSL phase. Applied magnetic field also suppresses the AF phase like in the stoichiometric compound.
Low temperature magnetic and thermal (C_m) properties of the ferromagnetic (FM) alloys Ce_2.15 (Pd_1-x Rh_x)_1.95 In_0.9 were investigated in order to explore the possibility for tuning a quantum critical point (QCP) by doping Pd with Rh. As expected
, the magnetic transition observed at T = 4.1K in the parent alloy decreases with increasing Rh concentration. Nevertheless it splits into two transitions, the upper being antiferromagnetic (AF) whereas the lower FM. The AF phase boundary extrapolates to T_N = 0 for x_cr ~ 0.65 whereas the first order FM transition vanishes at x ~ 0.3. The QC character of the T_N => 0 point arises from the divergent T dependence of the tail of C_m/T observed in the x = 0.5 and 0.55 alloys, and the tendency to saturation of the maximum of C_m(T_N)/T as observed in exemplary Ce compounds for T_N => 0. Beyond the critical concentration the unit cell volume deviates from the Vegards law in coincidence with a strong increase of the Kondo temperature.
Structural, magnetization and heat capacity studies were performed on Ce$_2$(Pd$_{1-x}$Ni$_x$)$_2$Sn ($0 leq x leq 1$) alloys. The substitution of Pd atoms by isoelectronic Ni leads to a change in the crystallographic structure from tetragonal (for $
x leq 0.3$) to centered orthorhombic lattice (for $x geq 0.4$). The volume contraction thorough the series is comparable to the expected from the atomic size ratio between transition metal components. The consequent weak increase of the Kondo temperature drives the two transitions observed in Ce$_2$Pd$_2$Sn to merge at $x = 0.25$. After about a 1% of volume collapse at the structural modification, the system behaves as a weakly magnetic heavy fermion with an enhanced degenerate ground state. Notably, an incipient magnetic transition arises on the Ni-rich size. This unexpected behavior is discussed in terms of an enhancement of the density of states driven by the increase of the $4f$-conduction band hybridization and the incipient contribution of the first excited crystal field doublet on the ground state properties.
The magnetic phase diagram of Ce$_2$Pd$_2$Sn is investigated through the field dependence of thermal, transport and magnetic measurements performed at low temperature. The upper transition, $T_M=4.8$ K is practically not affected by magnetic field up
to B=1 T, whereas the lower one $T_C(B)$ rapidly increases from 2.1 K joining $T_M$ in a critical point at $T_{cr}=(4.2pm 0.3$)K for $B_{cr}=(0.12pm 0.03)$ T. At that point the intermediate phase, previously described as an unstable Shastry-Sutherland phase, is suppressed. A detailed analysis around the critical point reveals a structure in the maximum of the $partial M/partial B(B)$ derivative which could be related to the formation of a novel phase in that critical region.
