High Magnetic Field Studies of the Hidden Order Transition inURu2Si2

Abstract

We studied in detail the low temperature/high magnetic field phases of URu2Si2 single crystals with specific heat, magnetocaloric effect, and magnetoresistance in magnetic fields up to 45 T. Data obtained down to 0.5 K, and extrapolated to T = 0, show a suppression of the hidden order phase at Ho(0) = 35.9 ± 0.35 T and the appearance of a new phase for magnetic fields in excess of H1(0) = 36.1 ± 0.35 T observed only at temperatures lower than 6 K. In turn, complete suppression of this high field state is attained at a critical magnetic field H2(0) = 39.7 ± 0.35 T. No phase transitions are observed above 40 T. We discuss our results in the context of itinerant vs. localized f -electron behavior and consider the implications for the hidden order phase.During the past few years there has been a true renaissance of interest in the unusual phase transition [1] that occurs in the superconducting heavy fermion system URu 2 Si 2 at T o ≈ 17 K, where all of the thermodynamic and transport properties exhibit a mean-field-like anomaly [2, 3, 4]. These early experimental results led to the conclusion that a magnetic phase transition, possibly of a spin density wave-type, took place. However, when probed with microscopic measurements, e.g., neutron diffraction and muon spin rotation (µSR), only a very tiny magnetic moment of ≈ 0.02 µ B /U was found. Such a small moment could not account for the large changes in behavior at the phase transition and it gradually became apparent that an unconventional type of magnetic order is at play. Indeed, recent neutron diffraction [5], nuclear magnetic resonance [6], and µSR [7] under pressure show the apparent tiny homogeneous moment to be due to a metallurgical minority phase of large moments (≈ 0.3 µ B ), that coexists with a majority (bulk) phase that has no magnetic moments. After more than 15 years of investigation the nature of the bulk phase transition remains unidentified, and the term hidden order (HO) has recently been coined to describe this phenomenon [8]. Besides pressure, yet another external parameter is known to affect the ordered state, i.e. external magnetic fields. Pulsed-field measurements of magnetization [9,10,11], resistivity [12,13], Hall effect [13,14], and ultrasonic velocity changes (elastic moduli, c ij ) [15] exhibit a three-step transition between 35 and 40 T for which a satisfactory explanation is still pending.In this Letter we present the first measurements of the temperature -field dependences of the specific heat, magnetocaloric effect and resistivity from 0.5 K to 20 K with fields up to 45 T to attempt to address the outstanding questions discussed above. The measurements indicate four regimes of anomalous behavior as URu 2 Si 2 emerges from its hidden ordered state: (I) The continuous phase transition becomes sharper and symmetric in temperature as magnetic field is increased above 32 T. (II) There is no phase transition to be seen down to 0.5 K at ∼36 T. (III) Between 36 and 39 T a first order-like transition reappears, and (IV) Above 40 T a Schottky...

Group theory and octupolar order inURu2Si2

“…There have been extensive studies of the effect of an external magnetic field on the phase diagram of URu 2 Si 2 [25,26]. The system is relatively insensitive to fields applied in the x-y plane, while fields B ẑ have substantial effect: hidden order can be suppressed completely with B cr,1 = 34.7T.…”

“…Next we discuss the high-field behavior at B > B cr,1 , and interpret the disjoint highfield phase observed in experiments [25,26] as a mixed quadrupolar-dipolar phase (the E phase in Fig. 2).…”

Γ3-Type Lattice Instability and the Hidden Order of URu2Si2

“…17,[19][20][21][22] These cascade transitions occur in the vicinity of the upper phase boundary of the HO phase and are associated with meta-magnetic-like increases in the c-axis magnetization. 19,23) Intriguingly, the crossover temperatures of T ;max and T ;max are gradually reduced with increasing magnetic field and vanishe in the vicinity of the HO phase boundary in the magnetic-field-temperature phase diagram.…”

Irreversible Heating Measurement with Microsecond Pulse Magnet: Example of the α–θ Phase Transition of Solid Oxygen

“…The surplus energy can be obtained from µ 0 BdM, which mainly results in the heating as hysteresis loss. 20,21 In previous studies, [3][4][5][6][7][8][9][10][11][12][13][14][15] the authors were mainly interested in ∆T rev , which reveals isentropes in the magnetic fieldtemperature (B-T ) plane. The discontinuity in the isentropes indicates the phase boundary in the B-T phase diagram.…”