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Register a new userFirst order transition from antiferromagnetism to ferromagnetism in Ce(Fe$_{0.96}$Al$_{0.04})_2$
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Results of dc magnetization study are presented showing interesting thermomagnetic history effects across the antiferromagnetic to ferromagnetic transition in Ce(Fe$_{0.96}$Al$_{0.04})_2$. Specifically, we observe (i)ZFC/FC irreversibility rising with increasing field; (ii) virgin curve lying outside the envelope M-H curve. We argue that these effects are quite different from the characteristics seen in spin-glasses or in hard ferromagnets; they can be understood as metastabilities associated with a first order magnetic phase transition.
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Taking the pseudobinary C15 Laves phase compound Ce(Fe$_{0.96}$Al$_{0.04}$)$_2$ as a paradigm for studying a ferromagnetic to antiferromagnetic phase transition, we present interesting thermomagnetic history effects in magnetotransport as well as magnetisation measurements across this phase transition. A comparison is made with history effects observed across the ferromagnetic to antiferromagnetic transition in R$_{0.5}$Sr$_{0.5}$MnO$_3$ crystals.
We report on a study of electronic transport in semi-metallic Bi$_{0.96}$Sb$_{0.04}$. At zero field, the system is a very dilute Fermi liquid displaying a T$^{2}$ resistivity with an enhanced prefactor. Quantum oscillations in resistivity as well as in Hall, Nernst and Seebeck responses of the system are detectable and their period quantifies the shrinking of the Fermi surface with antimony doping. For a field along the trigonal axis, the quantum limit was found to occur at a field as low as 3T. An ultraquantum anomaly at twice this field was detected in both charge transport and Nernst response. Its origin appears to lie beyond the one-particle picture and linked to unidentified many-body effects.
The very nature of the ground state of the pyrochlore compound Yb$_2$Ti$_2$O$_7$ is much debated, as experimental results demonstrate evidence for both a disordered or a long-range ordered ground state. Indeed, the delicate balance of exchange interactions and anisotropy is believed to lead to competing states, such as a Quantum Spin Liquid state or a ferromagnetic state which may originate from an Anderson-Higgs transition. We present a detailed magnetization study demonstrating a first order ferromagnetic transition at 245 mK and 150 mK in a powder and a single crystal sample respectively. Its first-order character is preserved up to applied fields of $sim$ 200 Oe. The transition stabilizes a ferromagnetic component and involves slow dynamics in the magnetization. Residual fluctuations are also evidenced, the presence of which might explain some of the discrepancies between previously published data for Yb$_2$Ti$_2$O$_7$.
In this report we show that in the perovskite manganite La_{1-x}Ca_{x}MnO_3 for a fixed x approx 0.33, the magnetic transition changes over from first order to second order on reducing the particle size to nearly few tens of a nanometer. The change-over is brought about only by reducing the size and with no change in the stoichiometry. The size reduction to an average size of about 15 nm retains the ferromagnetic state albeit with somewhat smaller saturation magnetization and the ferromagnetic transition temperature T_{C} is suppressed by a small amount (4%). The magnetization of the nanoparticles near T_{C} follow the scaling equation $M/|epsilon|^beta = f_pm(H/|epsilon|^{gamma+beta})$, where, $epsilon = |T-T_C|/T_C$. The critical exponents, associated with the transition have been obtained from modified Arrott plots and they are found to be $beta=0.47pm 0.01$ and $gamma=1.06pm 0.03$. From a plot of M vs H at T_{C} we find the exponent $delta=3.10 pm 0.13$. All the exponents are close to the mean field values. The change-over of the order of the transition has been attributed to a lowering of the value of the derivative dT_{C}/dP due to an increased pressure in the nanoparticles arising due to size reduction. This effect acts in tandem with the rounding off effect due to random strain in the nanoparticles.
Yb(Rh1-xCox)2Si2 is a model system to address two challenging problems in the field of strongly correlated electron systems: The first is the intriguing competition between ferromagnetic (FM) and antiferromagnetic (AFM) order when approaching a magnetic quantum critical point (QCP). The second is the occurrence of magnetic order along a very hard crystalline electric field (CEF) direction, i.e. along the one with the smallest available magnetic moment. Here, we present a detailed study of the evolution of the magnetic order in this system from a FM state with moments along the very hard c direction at x = 0.27 towards the yet unknown magnetic state at x = 0. We first observe a transition towards an AFM canted state with decreasing x and then to a pure AFM state. This confirms that the QCP in YbRh2Si2 is AFM, but the phase diagram is very similar to those observed in some inherently FM systems like NbFe2 and CeRuPO, which suggests that the basic underlying instability might be FM. Despite the huge CEF anisotropy the ordered moment retains a component along the c-axis also in the AFM state. The huge CEF anisotropy in Yb(Rh1-xCox)2Si2 excludes that this hard-axis ordering originates from a competing exchange anisotropy as often proposed for other heavy-fermion systems. Instead, it points to an order-by-disorder based mechanism.
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