arXiv subjects
Publications and source records attributed to P. Chaddah.
We shall discuss magnetization and transport measurements in materials exhibiting a broad first-order transition. The phase transitions would be caused by varying magnetic field as well as by varying temperature, and we concentrate on ferromagnetic to antiferromagnetic transitions in magnetic materials. We distinguish between metastable supercooled phases and metastable glassy phase.
Coexisting ferromagnetic (FM) and antiferromagnetic (AFM) phases over a range of temperature (T) and magnetic field (H), have been reported in many materials. The 1st order FM-AFM transition is completed over a broad T (or H) range; this is ascribed to a landscape of free-energy density in systems with frozen disorder. Kinetic arrest, akin to glass transition, has been reported in doped CeFe$_2$, La-Pr-Ca-Mn-O, Gd$_5$Ge$_4$, etc., such that the coexisting fraction is frozen. The de-arrest, or melting, of this glass-like arrested state is seen to occur over a range of temperatures, implying a landscape (of T$_K$ or T$_g$). We argue that measuring magnetization along various specific paths in H-T space can help infer whether the landscapes of free-energy and for kinetic arrest are correlated. This will help to determine whether disorder affects glass formation, and the underlying 1st order transition, in contrasting (or similar) ways.
We report a detailed study of surface and interface properties of pulsed-laser deposited NiMnSb films on Si (100) substrate as a function of film thickness. As the thickness of films is reduced below 35 nm formation of a porous layer is observed. Porosity in this layer increases with decrease in NiMnSb film thickness. These morphological changes of the ultra thin films are reflected in the interesting transport and magnetic properties of these films. On the other hand, there are no influences of compositional in-homogeneity and surface/interface roughness on the magnetic and transport properties of the films.
Field variation of heat capacity at fixed temperatures is investigated to identify the origin of the field induced first order phase transition in polycrystalline Ce(Fe$_{0.094}$Al$_{0.04}$)$_2$ sample. The heat capacity at 4.5K and 3.5K shows hysteresis in different field cycles and the virgin curve stays outside the envelope curve. This is analogous to the magnetization and magneto-resistance behavior observed in this system, where the amount of hysteresis, and the magnitude of zero-field irreversibility are attributed to the degree of supercooling/superheating, and the extent of kinetic arrest of the reverse transition from ferro- to antifrromagnetic state in field reducing cycle respectively. However, contrary to the magnetization and magneto-resistance, in heat capacity both these features have decreased with reducing temperature signifying the importance of structural contribution.
We present results of dc magnetization and magnetic relaxation study showing the kinetic arrest of a first order ferromagnetic to antiferromagnetic transition in Ce(Fe$_{0.96}$Ru$_{0.04}$)$_2$. This leads to the formation of a non-ergodic glass-like magnetic state. The onset of the magnetic-glass transformation is tracked through the slowing down of the magnetization dynamics. This glassy state is formed with the assistance of an external magnetic field and this is distinctly different from the well known 'spin-glass' state.
We report results of magnetization, zero field resistivity and magnetoresistance measurements in ferromagnetic Heusler alloy Co$_2$TiSn. There is a striking change in the character of electron transport as the system undergoes the paramagnetic to ferromagnetic transition. In the paramagnetic state the nature of the electron transport is like that of a semiconductor and this changes abruptly to metallic behaviour at the onset of ferromagnetic ordering. Application of external magnetic field tends to suppress this semiconducting like transport leading to a negative magnetoresistance which reaches a peak in the vicinity of Curie temperature. Comparison is made with the similar unusual behaviour observed in other systems including UNiSn and manganites.
Very sharp magnetization step is observed across the field induced antiferromagnetic to ferromagnetic transition in various doped-CeFe$_2$ alloys, when the measurement is performed below 5K. In the higher temperature regime (T$>$5K) this transition is quite smooth in nature. Comparing with the recently observed similar behaviour in manganites showing colossal magnetoresistance and magnetocaloric material Gd$_5$Ge$_4$ we argue that such magnetization step is a generalized feature of a disorder influenced first order phase transition.
First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in doped-CeFe$_2$ alloys is studied with micro-Hall probe technique. Clear visual evidence of magnetic phase-coexistence on micrometer scales and the evolution of this phase-coexistence as a function of temperature, magnetic field and time across the first order FM-AFM transition is presented. Such phase-coexistence and metastability arise as natural consequence of an intrinsic disorder-influenced first order transition. Generality of this phenomena involving other classes of materials is discussed.
Relaxation in magnetisation of metastable states is reported. The states are prepared by supercooling a Ce(Ru, 5% Nd)2 alloy sample to different extents through similar paths in the H-T space, and then by applying a field jerk of 20 Oe of either sign. Based on standard theory, a possible explanation is put forward.
We present results of magnetization measurements showing that the magnetic response of the antiferromagnetic state of SmMn_2Ge_2 depends on the path used in the field(H)-temperature(T) phase space to reach this state. Distinct signature of metastablity is observed in this antiferromagnetic state when obtained via field-cooling/field-warming paths. The isothermal M-H loops show lack of end-point memory, reminiscent of that seen in metastable vortex states near the field-induced first order phase transition in various type-II superconductors.
The intermetallic compound SmMn$_2$Ge$_2$, displaying multiple magnetic phase transitions, is being investigated in detail for its magnetization behavior near the 145 K first order ferromagnetic to antiferromagnetic transition occuring on cooling, in particular for thermomagnetic history effects in the magnetization data. The most unusual finding is that the thermomagnetic irreversibility, [= M$^{FCW}$(T)-M$^{ZFC}$(T)] at 135 K is higher in intermediate magnetic field strengths. By studying the response of the sample (i.e., thermomagnetic irreversibility and thermal hysteresis) to different histories of application of magnetic field and temperature, we demonstrate how the supercooling and superheating of the metastable magnetic phases across the first order transition at 145 K contribute to overall thermomagnetic irreversibility.
Resistivity and thermoelectric power (TEP) measurements on CeFe$_2$ and two of its pseudo-binaries Ce(Fe, 5% Ir)$_2$ and Ce(Fe, 7% Ru)$_2$ between 78K and 275K are reported. The resistivity data are analysed in terms of contributions from scattering due to phonon, magnon, spin fluctuation and lattice defects, and also from interband scattering. Attempts are made to analyze the TEP data in terms of these resistivity components. Thermal hysteresis is observed in the temperature dependence of TEP in the Ir and Ru doped CeFe$_2$ samples around the ferromagnetic to antiferromagnetic transition, indicating the first order nature of this transition.
The magnetotransport behaviour is investigated in detail across the first order magnetic phase transition from ferromagnetic to antiferromagnetic state in polycrystalline Ce(Fe$_{0.96}$Al$_{0.04}$)$_2$ sample. The study clearly brings out various generic features associated with a first order transition, viz., hysteresis, phase coexistence, supercooling and superheating, presence and limits of the metastable regimes. These results of magnetotransport study exhibit and support all the interesting thermomagnetic history effects that were observed in our earlier dc-magnetisation study on the same sample. Most notable here is the initial (or virgin) resistivity vs. field curve lying outside the hysteretic "butterfly shaped" magnetoresistivity loops obtained on cyclying the magnetic field between high enough positive and negative strengths. These findings, bearing one-to-one similarity with the data obtained in their magnetic counterpart (i.e., dc-magnetisation), have been ascribed an origin due to the arresting of this first order transition kinetics at low temperature and high magnetic field.
We discuss first-order phase transitions that are broadened by disorder, but still remain first order on the local mesoscopic level. Using vortex-matter as our paradigm, we argue that phase transitions in general can be broadened by two different causes viz. due to a distribution of the phase transition point (field or temperature) from one region of the sample to another, or due to a distribution in the local magnetic field experienced by different regions of the sample. We show that in the second case there is an apparent "extrinsic" hysteresis even if the transition is locally second-order. We show that bulk measurements under isothermal conditions can be used to infer whether the broadened transition is first-order at the local (mesoscopic) level. Keywords: vortex-matter, phase transitions, hysteresis
We highlight interesting thermomagnetic history effects across the transition line between the (quasi) ordered and disordered vortex states in single crystal YBa$_2$Cu$_3$O$_{7-δ}$, and argue that these features are indicative of the first order nature of the transition line. We suggest that the destruction of the ordered vortex state in YBa$_2$Cu$_3$O$_{7-δ}$ leading to vortex liquid (at high temperatures and low fields) and amorphous vortex solid (at low temperatures and high fields), takes place along a unified first-order transition line. The nonmonotonic behavior of this first order transition line gives rise to the possibility of more than one Kauzmann point where the entropies of the ordered and disordered vortex states are equal. In the high temperature region, one may order the vortex lattice by warming it, giving rise to an inverse melting effect.
A recent paper [Physical Review {\bf B 64} 014508 (2001)] claims to present an exact method to extract the critical current density J$_C$ from the M-H hysteresis curve. We show that this claim appears unjustified.
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.
Taking the pseudobinary C15-Laves phase compound Ce(Fe$_{0.96}$Al$_{0.04}$)$_2$ as a paradigm for studying a ferromagnetic(FM) to antiferromagnetic(AFM) phase transition, we present interesting thermomagnetic history effects in magnetotransport measurements across this FM-AFM transition. We argue that these distinctive hysteretic features can be used to identify the exact nature -first order or second order - of this kind of transition in magnetic systems where electrical transport is strongly correlated with the underlying magnetic order. A comparison is made with the similar FM-AFM transitions observed in Nd and Pr-based manganese compounds with perovskite-type structure.