Ferromagnetic Quantum Criticality Studied by Hall Effect Measurements in UCoAl

Abstract

Hall effect measurements were performed under pressure and magnetic field up to 2.2 GPa and 16 T on a single crystal of UCoAl. At ambient pressure, the system undergoes a first order metamagnetic transition at the critical field Bm = 0.7 T from a paramagnetic ground state to a field-induced ferromagnetic state. The Hall signal is linear at low field and shows a step-like anomaly at the transition, with only little change of the Hall coefficient. The anomaly is sharpest at the temperature of the critical end point T0 = 12 K above which the first order metamagnetic transition becomes a crossover. Under pressure Bm increases and T0 decreases. The step-like anomaly in the Hall effect disappears at PM ≈ 1.3 GPa and the metamagnetic transition is not detected above the quantum critical end point (QCEP) at P∆ ≈ 1.7 GPa, Bm ≈ 7 T. Using magnetization data, we analyse our Hall resistivity data at ambient pressure in order to quantitatively account for both ordinary and anomalous contributions to the Hall effect. Under pressure, a drastic change in the field dependence of the Hall coefficient is found on crossing the QCEP. A possible Fermi surface change at Bm remains an open question.KEYWORDS: quantum critical end point, anomalous Hall effect, metamagnetism, ferromagnetism, UCoAl IntroductionThe metamagnetism in strongly correlated electron systems with Ising-type ferromagnetic (FM) behaviour is intensively studied because it produces a variety of unconventional effects. In some itinerant ferromagnets, such as UGe 2 1, 2 or ZrZn 2 , 3 one can drive the Curie temperature T C to 0 K by tuning an external control parameter like pressure, and the ground state is found to be paramagnetic (PM) above the quantum critical point (QCP). In theory, it has been suggested -and in some cases experimentally shown -that the second order ferromagnetic transition changes to first order at a tricritical point. 4 By applying a magnetic field above the critical point in the paramagnetic phase, such systems eventually recover their ferromagnetic state by undergoing a first-order metamagnetic transition at B m , drawing a wing-shape first order transition plane in the temperature (T ) -pressure (P ) -field (B) phase diagram. The first order transition terminates at high pressure and high field at T = 0 at the so-called quantum critical end point (QCEP). In this critical region, only few experiments were carried out because of the severe experimental conditions of low temperature, high pressure, and high field. [1][2][3]5 Due to the recent focus on quantum criticality, the metamagnetism in itinerant ferromagnets has been recently revisited theoretically (see e.g. Ref. 6,7). The main debate is whether a Lifshitz-like transition is associated with the occurence of metamagnetism.A good candidate to investigate itinerant metamagnetism is the heavy fermion compound UCoAl, with ZrNiAl-type hexagonal structure, space group P62m. At ambient pressure, its ground state is paramagnetic, with strong uniaxial magnetic anisotropy. By applying mag-

Ferromagnetic critical behavior inU(Co1−xFex)Al(0≤x≤0.02Karube1, Hattori2, Ishida3 et al. 2015Phys. Rev. B10250View full textAdd to dashboardCite

“…At ambient pressure, its ground state is paramagnetic (PM), but it undergoes a first-order metamagnetic transition by small magnetic fields only applied along its easy magnetization axis (c axis) of µ 0 H c ∼ 0.6 T at low temperatures. The first-order transition changes to crossover above the CEP at (T, µ 0 H c ) ∼ (12 K, 1.0 T) [6][7][8][9][10][11] . Under hydrostatic pressure (P hydro ), the metamagnetic transition field increases 6 and the CEP reaches the QCEP at (T, µ 0 H c , P hydro ) ∼ (0 K, 7 T, 1.5 GPa) 7,8 .…”

“…The first-order transition changes to crossover above the CEP at (T, µ 0 H c ) ∼ (12 K, 1.0 T) [6][7][8][9][10][11] . Under hydrostatic pressure (P hydro ), the metamagnetic transition field increases 6 and the CEP reaches the QCEP at (T, µ 0 H c , P hydro ) ∼ (0 K, 7 T, 1.5 GPa) 7,8 . In contrast, uniaxial pressure along the c axis (P c ) plays a role of a "negative pressure" and induces the FM state without magnetic field above as small as P c = 0.04 GPa [12][13][14] .…”

Scrutinizing Hall Effect inMn1−xFexSi: Fermi Surface Evolution and Hidden Quantum Criticality

“…In the case of localized magnetic moments (LMM) a collapse of the Fermi surface (FS) expected exactly at the quantum critical point (QCP) results in an abrupt change of the Hall constant at zero temperature [1,3]. In contrast, no direct evidence of the Lifshitz transition at QCP [2] is provided for itinerant magnets in the spin density wave model of quantum criticality [3].…”

“…Thirdly, despite the fact that the Mn 1−x Fe x Si solid solutions are often considered as itinerant magnets [9,10], LDA calculations [13] and recent magnetic resonance and magnetoresitance studies [14,15] favor the alternative explanation based on Heisenberg LMM of Mn ions. Therefore, different behavior of the Hall effect in QC systems with LMM and itinerant magnets [1][2][3] makes it possible to shed more light on the microscopic mechanisms of quantum criticality in Mn 1−x Fe x Si. This Letter addresses the aforementioned problems through the study of the Hall effect in the paramagnetic (P) phase of Mn 1−x Fe x Si (Fig.…”

“…The change of electron and hole concentrations is considered as a "driving force"for tuning the QC regime in Mn1−xFexSi via modifying of RKKY exchange interaction within the Heisenberg model of magnetism. Studying of the ordinary Hall effect (OHE) in quantum critical (QC) regime is an important tool, which allows choosing between various scenarios of non-Fermi liquid behavior in various strongly correlated electron systems [1][2][3]. In the case of localized magnetic moments (LMM) a collapse of the Fermi surface (FS) expected exactly at the quantum critical point (QCP) results in an abrupt change of the Hall constant at zero temperature [1,3].…”