Pressure-Tuned Spin and Charge Ordering in an Itinerant Antiferromagnet

Abstract

Elemental chromium orders antiferromagnetically near room temperature, but the ordering temperature can be driven to zero by applying large pressures. We combine diamond anvil cell and synchrotron x-ray diffraction techniques to measure directly the spin and charge order in the pure metal at the approach to its quantum critical point. Both spin and charge order are suppressed exponentially with pressure, well beyond the region where disorder cuts off such a simple evolution, and they maintain a harmonic scaling relationship over decades in scattering intensity. By comparing the development of the order parameter with that of the magnetic wave vector, it is possible to ascribe the destruction of antiferromagnetism to the growth in electron kinetic energy relative to the underlying magnetic exchange interaction. DOI: 10.1103/PhysRevLett.99.137201 PACS numbers: 75.10.Lp, 75.30.Fv, 78.70.Ck, 81.30.Bx Electrons carry not only charge but also spin, and how magnetic order develops in metals where charge carriers remain itinerant continues to be a central problem in both condensed matter and device physics. As technology progresses and device dimensions shrink, quantum effects become more pronounced and a variety of potential ground states can emerge with coupled charge, spin, and orbital order [1]. These effects are most acute near quantum phase transitions, where magnetism first emerges at the absolute zero of temperature [2,3]. In particular, antiferromagnetic coupling between interacting mobile electrons is believed to underlie some of the most profound puzzles in modern metal physics, most notably exotic superconductivity, heavy fermions, and other non-Fermi-liquid phenomena [4,5].Nevertheless, definitive characterization of quantum critical behavior in itinerant magnets has proved elusive. Direct order parameter studies of stoichiometric, itinerant ferromagnets suggest that the quantum phase transition is always first order, shrouding the critical behavior [6]. Quantum critical behavior in itinerant antiferromagnets has been observed using indirect probes such as electrical transport and specific heat [5], but no direct studies of the order parameter of stoichiometric antiferromagnets exist. As a further complication, the effects of chemical doping and substitution are amplified at a quantum phase transition, where materials become ''hypersensitive'' to disorder [7].Directly observing the emergence of antiferromagnetism in a model stoichiometric system without the application of a symmetry-breaking field or dopant disorder would reveal fundamental aspects about the magnetic order itself. To this end, we present a direct x-ray diffraction study of the spin and charge order parameters in elemental chromium, the archetypical itinerant spin-density-wave (SDW) antiferromagnet, as the magnetic order is suppressed with pressure towards its quantum phase transition. Cr is attractive as a model system [8][9][10][11][12] and is particularly amenable to theoretical exposition given its simple bcc crystal lattice and well...

A clamp-type pressure cell for high energy x-ray diffraction

“…Although this is below the sensitivity reported for a DAC in a study on chromium in Ref. 12, it is sufficient for many problems we are facing in transition metal oxides with strongly correlated electrons.…”

“…9,10 Steady progress has pushed the achievable sensitivity to about 10 −9 . 11,12 The small dimensions of DAC are also perfectly suited for experiments at low temperatures. A disadvantage of the DAC technique can be the small sample size, typically below 100 m in diameter and 20 m in thickness, which makes it more sensitive to surface effects.…”

Hidden one-dimensional spin modulation in a three-dimensional metal

“…Non-resonant X-ray magnetic diffraction under pressure was performed at beamline 4-ID-D of the Advanced Photon Source 19,20 . A focused 20,000 keV X-ray beam was reduced to 100 Â 100 mm 2 size by motorized slits to match typical sample sizes around 80 Â 80 Â 30 mm 3 under pressure.…”

“…The typical pressure range for neutron magnetic diffraction is 25 GPa for powders 16 and o10 GPa for single crystals 17 . Single-crystal X-ray magnetic diffraction has become competitive over the past decade as a direct probe of antiferromagnetism under pressure [18][19][20] .…”

Diffraction line-shapes, Fermi surface nesting, and quantum criticality in antiferromagnetic chromium at high pressure (invited)

“…Both the SDW and CDW are amenable to measurement by x-ray diffraction using a third generation synchrotron source. 8 Relevant to the question of nesting are the diffraction line-shapes, which can be measured throughout the temperature-pressure phase diagram using high energy synchrotron x-ray diffraction and diamond anvil cell techniques. An important element in our discussion is the connection between the CDW order parameter and the underlying magnetic bandstructure.…”