Session Y45: Magnetic Phase Transitions

Discovery of remarkably high magnetic ordering temperatures in dilute rare earth intermetallic compounds

Dilute, rare earth intermetallic compounds are ordered structures in which the rare earth comprises less than $5\%$ atomic of the compound, but still fully occupies a unique crystallographic site. When none of the other elements are moment bearing, it is generally anticipated that such a compound will have magnetic ordering temperatures near 10 K for the R = Gd member of the series and systematically lower temperatures for heavier R members. In this talk we will present a series of dilute rare earth intermetallic compounds that have ferromagnetic ordering temperatures that range from $T_C \sim 90$ K for R = Gd to $T_C \sim 5$ K for R = Tm. The origin of this high ordering temperature as well as systematic trends across the series will be discussed.   

The effects of pressure and chemical substitution on the transition temperature and groundstate of Gd-based dilute rare earth intermetallic compounds

Dilute, rare earth intermetallic compounds are ordered structures in which the rare earth comprises less than $5\%$ atomic of the compound, but still fully occupies a unique crystallographic site. We have investigated a family of Gd-based, ternary, dilute, rare earth intermetallic compounds in an attempt to understand the remarkably high ferromagnetic ordering found for some of the members ($T_C$ as high as 90 K). By studies of the thermodynamic and transport properties as a function of temperature, field and hydrostatic pressure and combining these results with band-structure calculations we have been able to determine that some of these compounds have an enhanced DOS at the Fermi level that may be correlated with the enhanced ordering temperatures.   

Paramagnetic Semiconductor to itinerant Ferromagnet in Fe$_{1-x}$Co$_x$S$_2$

Carrier doping of ``fool's gold'', the paramagnetic insulator FeS$_2$ by partial substitution of Co for Fe, results in an insulator-to-metal transition at $x\le 0.001$. Magnetization and susceptibility measurements for samples with Co substitution beyond $x~0.01$ reveal the emergence of a highly itinerant ferromagnet with no discontinuous changes with $x$, field, or temperature ($T$) suggesting either a continuous crossover or quantum phase transition to ferromagnetism. For $x\le 0.01$ and low-$T$, the conductivity decreases with smaller $T$ and displays a positive magnetoconductance. Samples which order magnetically show similar magnetoconductance, but with a minimum in conductivity near $T_c$. We conclude that either Kondo or magnetic polaron effects are responsible for these low-$T$ anomalies. Specific heat measurements display extraordinarily large carrier masses that diverge logarithmically or with a small power law at low-$T$ close to the ferromagnetic critical concentration, similar to the behavior near quantum critical points.   

Ferromagnetic Transition in Heusler Alloys

The transition from a magnetically disordered (paramagnetic) to a magnetically ordered (ferromagnetic) state at finite temperature in zero magnetic field has long been regarded as one of {\it the} text book examples for a second order phase transition. However, this mean field Weiss model leaves out some of the physics that can become crucial for itinerant magnets when approaching the transition temperature. Co$_2$TiSn and Co$_2$CrAl are two Heusler alloys with Curie temperatures just above room temperature. Theoretical band structure calculations, ours as well as by others, predict them to be close to half metals. Magnetisation data as a function of temperature and magnetic field show deviations from the mean field Weiss model which we argue are due to band structure properties.   

Field and temperature dependence of the first order antiferromagnetic to ferromagnetic phase transition in FeRh thin films

Chemically ordered FeRh exhibits a first order antiferromagnetic (AF) to a ferromagnetic (F) phase transition upon heating from room temperature to above a transition temperature of approximately 370 K. This transition is accompanied by a volume increase of $\sim $1{\%}, a change in resistivity, and a large change in entropy. The phase transition occurs within a few ps allowing it to be used in thermal assisted recording. For example, it was recently proposed to use exchange coupled FeRh/FePt bilayers for thermally assisted magnetic recording media. Here we studied the thermodynamics of the transition in thin FeRh films epitaxially grown onto MgO and sapphire substrates. A shift to lower transition temperatures is observed in external magnetic fields and supercooling is easily observed by cycling either the temperature or the magnetic field. A temperature-field phase diagram was constructed revealing a linear -8 K/ Tesla temperature-field relationship. The observed field dependence of the shift in transition temperature is modeled with an Ising spin type model utilizing a mean field approach. The computational results are in excellent agreement with the experimental data.   

Vector-magnetometry determination of magnetization processes in Pr$_{6}$Ni$_{2}$Si$_{3}$ and Nd$_{6}$Ni$_{2}$Si$_{3 }$

Hexagonal Pr$_{6}$Ni$_{2}$Si$_{3}$ and Nd$_{6}$Ni$_{2}$Si$_{3}$ order magnetically at 40 and 85 K, repectively. At 5 K, the magnetic order of both compounds appears to have a ferromagnetic component parallel to the hexagonal c-axis, and an antiferromagnetic component perpendicular to it. In both compounds, at 5 K and in a magnetic field of $\sim $2-3 T, applied perpendicular to the c-axis, a magnetic transition occurs. Using a QD-MPMS equipped with a transverse pickup coil system, we have studied the behavior of the magnetization vector in single-crystal samples during this transition. In both cases, the magnetization vector becomes anomalously \textit{longer} during the transition. This indicates the transition may be related to an antiferromagnetic component of the magnetic order, which is consistent with the magnetic order having both antiferro and ferromagnetic components.   

Observation of two distinct energy scales in the magnetization measurements of the anisotropic antiferromagnet TmAgGe

TmAgGe is an antiferromagnet ($T_{N}$ = 4.2 K) that crystallizes in a variant of the hexagonal Fe$_{2}$P structure (three Tm atoms per unit cell). We have studied the magnetization of the TmAgGe single crystals in fields $\mu _{0}H$ of up to 65 T as a function of the field orientation and the temperature $T$. With \textbf{H} in the basal \textbf{ab}-plane, a number of metamagnetic transitions are observed for $\mu _{0}H \quad <$5 T and $T \quad < \quad T_{N. }$However, when \textbf{H}$\vert \vert $\textbf{c}, three steps in the magnetization occur between 30 and 35 T, persisting to $T \quad \cong $ 60 K. On tilting \textbf{H} away from \textbf{c, }both sets of features (high-field steps and low-field metamagnetism transitions) are seen, showing that they arise from two distinct mechanisms. The dependence of the high-field steps on $T$ and field orientation suggests that they are associated with crystalline electric field (CEF) level crossing; the CEF confines the moments to the \textbf{ab}-plane. By contrast, it is the rearrangement of the moments within the basal plane that gives to the low-field metamagnetic transitions. To the best of our knowledge, TmAgGe is the first intermetallic system in which these two energy scales (CEF and in-plane exchange) can be unambiguously distinguished in this way.   

Anomalous Magnetic Properties near the Spin-Flop Bicritical Point in Mn$_{2}A$S$_{4}$ ($A$ = Si and Ge)

The magnetic properties of the single crystalline Mn$_{2}A$S$_{4}$ ($A$ = Si and Ge) with an olivine structure, which are the uniaxially anisotropic antiferromagnets (the $b$-axis as an easy axis), were investigated. Near the N\'{e}el temperature, both compounds exhibit the contrastive magnetic responses along the $c$-axis, namely, the spontaneous weak ferromagnetism in $A$ = Si and the significant enhancement of the differential susceptibility (\textit{dM/dH}) under the small magnetic field in $A$ = Ge. When $A$ = Ge, we also observed the evolution of \textit{dM/dH} along the $a$-axis at low temperatures. We discuss these phenomena on the basis of the magnetic field-temperature ($H-T)$ phase diagram with the spin-flop bicritical point ($H_{BP}$, $T \quad _{BP})$. The role of the thermal or quantum fluctuation was stressed. (Ref.) K. Ohgushi and Y. Ueda, Phys. Rev. Lett. \textbf{95}, 217202 (2005)   

Composition dependent properties of mixed magnetic Fe/Ni dichloride dihydrate

This mixed magnet is composed of two approximate 3D-Ising antiferromagnets ordering at 22.3 K (Fe component) and 7.3 K (Ni component). The pure substances also display metamagnetic transitions at low temperature near 39 kG (Fe) and 19 kG (Ni). Ferromagnetically coupled metal-dichloride-metal...chemical and structural chains occur in each component, with predominantly antiferromagnetic interchain interactions. Homogeneous mixtures have been obtained and examined by dc susceptibility and magnetization measurements over the entire composition range. Curie and Weiss constants are obtained from fits to high temperature data. In many mixtures two magnetic transitions at different temperatures are observed. In several mixtures marked nonlinearities in magnetization vs field appear, with signficant temperature dependence, and suggestive of metamagnetic transitions with associated hysteresis. Most interestingly, the inferred transition fields are lower than those of either component. The overall behavior can be compared with that in the previously studied and closely related Co/Ni mixed system. Some similarities appear, but also notable differences.   

Hysteresis effect in $^{55}$Mn NMR of TbMn$_2$O$_5$

$^{55}$Mn zero-field NMR has been performed on antiferromagnetic magnetoelectric multiferroic compound TbMnO$_5$ from 60 K to 1.8 K. The broad and complex spectrum is distributed between 240 MHz and 280 MHz implying a superposition of NMR lines arising from the several inequivalent Mn ions due to the amplitude-modulated spin structure. When external perturbing field up to 3 T is applied, a dramatic change in the first moment and the signal enhancement is observed which are hysteretic in nature. The behavior is attributed to antiferromagnetic domain walls which are coupled to ferroelectric domains similar to that observed in weakly ferromagnetic $\alpha$-Fe$_2$O$_3$. The nature of strong coupling of magnetic and electric ordering and its subsequent effect on its multiferroic behavior are discussed.   

Magnetic Field Induced Phases in the Quantum Antiferromagnet Cs$_2$CuCl$_4$ as revealed by NMR.

We report nuclear magnetic resonance (NMR) measurements on the 2D frustrated quantum antiferromagnet Cs$_2$CuCl$_4$.$^{133}$Cs spectra at temperatures down to $60$\,mK and in external magnetic field up to $15$\,T are presented. At the lowest temperature we observe multiple field induced quantum phase transitions in a magnetic field applied within the spin plane. The phases are distinguished by their local spin textures. The possible nature of these phases and their underlying spin structure are discussed. Furthermore, simulations of spectra are presented.   

\textbf{Phase Transitions at the Cr}$_{1-x }$\textbf{V}$_{x }$\textbf{/W(110) System}

The rich phase diagram of thin films of chromium-vanadium (Cr$_{1-x}$V$_{x})$ alloys offers a unique possibility to study the phenomena associated with the spin-density wave (SDW) ground state and quantum phase transition (QPT). Using angular-resolved photoemission (ARP) we have studied the periodicity of SDW in the region of commensurate-incommensurate phase transition followed by quantum phase transition as a function of two non-thermal parameters: film thickness and vanadium concentration. That allows probing the interplay between spatial confinement and Fermi surface topology change resulting from modification of chromium electron structure by vanadium doping.   

The spin state transition in LaCoO$_{3}$; revising a revision

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-$L_{2,3}$ edge we demonstrate that the spin state transition in LaCoO$_{3}$ can be well described by a low-spin ground state and a high-spin first excited state which becomes populated at elevated temperatures. From the temperature dependence of the spectral lineshapes we find that LaCoO$_{3}$ at finite temperatures is an inhomogeneous mixed-spin-state system. Crucial is that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low-/intermediate-spin picture is at variance, and that instead the original low-/high-spin scenario is the better \textit{ansatz}.   

Transport and Magnetic Properties of Nd$_{2}$Ni$_{2}$Pb and NdNiPb

We report magnetic, transport and thermodynamic measurements for Nd$_{2}$Ni$_{2}$Pb and NdNiPb, members of recently-discovered $R$-Ni-Pb families of intermetallics. In Nd$_{2}$Ni$_{2}$Pb a $\lambda$-type specific heat jump was observed at 19 K corresponding to an antiferromagnetic transition. Magnetization measurements show this phase to have a canted structure, with a metamagnetic transition in $H$ = 3 T at low temperatures. We have further explored the metamagnetic transition using field dependent specific heat, concluding that the metamagnetic phase is a fully aligned phase. The single antiferromagnetic phase stands in contrast to the more complex magnetic structures observed in the heavy-rare-earth members of this family. Nd is the lightest RE forming this type strucure. At high temperatures the magnetization obeys a Curie law and the magnetic moment agrees with the free ion moment of Nd. Resistivity measurements showed metallic behavior with a kink at 19 K. We performed similar measurements on NdNiPb and observed an antiferromagnetic transition at 4 K. Specific heat indicated rather different critical behavior, with magnetic fluctuations extending well above the transition. This work was supported by the Robert A. Welch Foundation (grant A-1526), the National Science Foundation (DMR-0103455), and by the Texas A\&M University Telecommunications and Informatics Task Force.   

Magnetic Susceptibility Experiments on the Heavy Lanthanides Using Designer Diamond Anvils

The high pressure magnetic properties of the heavy lanthanide elements Gd, Tb, Dy, Ho, Er, and Tm have been investigated by ac magnetic susceptibility using designer diamond anvils. It is found that the magnetic transition temperatures monotonically decrease with increasing pressure. In addition, the amplitudes of the magnetic transition signals decrease with increasing pressure, with the signals all eventually disappearing at pressures by 20~GPa. The transition temperatures, $T_{Crit}$ are all found to drop at a rate proportional to their de Gennes factor, and the values of $T_{Crit}/T_{Crit}(P=0)$ vs $P/P_{Crit}$, where $P_{Crit}$ is the pressure where the magnetic transition disappears, all sit on a single phase diagram.