84 research outputs found
Switchable Hardening of a Ferromagnet at Fixed Temperature
The intended use of a magnetic material, from information storage to power
conversion, depends crucially on its domain structure, traditionally crafted
during materials synthesis. By contrast, we show that an external magnetic
field applied transverse to the preferred magnetization of a model disordered
uniaxial ferromagnet is an isothermal regulator of domain pinning. At elevated
temperatures, near the transition into the paramagnet, modest transverse fields
increase the pinning, stabilize the domain structure, and harden the magnet,
until a point where the field induces quantum tunneling of the domain walls and
softens the magnet. At low temperatures, tunneling completely dominates the
domain dynamics and provides an interpretation of the quantum phase transition
in highly disordered magnets as a localization/delocalization transition for
domain walls. While the energy scales of the rare earth ferromagnet studied
here restrict the effects to cryogenic temperatures, the principles discovered
are general and should be applicable to existing classes of highly anisotropic
ferromagnets with ordering at room temperature or above.Comment: 10 pages, 4 figure
Probing many-body localization in a disordered quantum magnet
Quantum states cohere and interfere. Quantum systems composed of many atoms
arranged imperfectly rarely display these properties. Here we demonstrate an
exception in a disordered quantum magnet that divides itself into nearly
isolated subsystems. We probe these coherent clusters of spins by driving the
system beyond its linear response regime at a single frequency and measuring
the resulting "hole" in the overall linear spectral response. The Fano shape of
the hole encodes the incoherent lifetime as well as coherent mixing of the
localized excitations. For the disordered Ising magnet,
, the quality factor for spectral holes
can be as high as 100,000. We tune the dynamics of the quantum degrees of
freedom by sweeping the Fano mixing parameter through zero via the
amplitude of the ac pump as well as a static external transverse field. The
zero-crossing of is associated with a dissipationless response at the drive
frequency, implying that the off-diagonal matrix element for the two-level
system also undergoes a zero-crossing. The identification of localized
two-level systems in a dense and disordered dipolar-coupled spin system
represents a solid state implementation of many-body localization, pushing the
search forward for qubits emerging from strongly-interacting, disordered,
many-body systems.Comment: 22 pages, 6 figure
Interplay of disorder and geometrical frustration in doped Gadolinium Gallium Garnet
The geometrically-frustrated, triangular antiferromagnet GGG exhibits a rich
mix of short-range order and isolated quantum states. We investigate the
effects of up to 1% Neodymium substitution for Gallium on the ac magnetic
response at temperatures below 1 K in both the linear and nonlinear regimes.
Substitutional disorder actually drives the system towards a more perfectly
frustrated state, apparently compensating for the effect of imperfect
Gadolinium/Gallium stoichiometry, while at the same time more closely
demarcating the boundaries of isolated, coherent clusters composed of hundreds
of spins. Optical measurements of the local Nd environment substantiate the
picture of an increased frustration index with doping.Comment: 5 pages, 5 figure
Barkhausen noise in the Random Field Ising Magnet NdFeB
With sintered needles aligned and a magnetic field applied transverse to its
easy axis, the rare-earth ferromagnet NdFeB becomes a
room-temperature realization of the Random Field Ising Model. The transverse
field tunes the pinning potential of the magnetic domains in a continuous
fashion. We study the magnetic domain reversal and avalanche dynamics between
liquid helium and room temperatures at a series of transverse fields using a
Barkhausen noise technique. The avalanche size and energy distributions follow
power-law behavior with a cutoff dependent on the pinning strength dialed in by
the transverse field, consistent with theoretical predictions for Barkhausen
avalanches in disordered materials. A scaling analysis reveals two regimes of
behavior: one at low temperature and high transverse field, where the dynamics
are governed by the randomness, and the second at high temperature and low
transverse field where thermal fluctuations dominate the dynamics.Comment: 16 pages, 7 figures. Under review at Phys. Rev.
Probing many-body localization in a disordered quantum magnet
Excitations in disordered systems are typically categorized as localized or delocalized, depending on whether they entail disturbances extending throughout the system or are confined to small, generally nanometer scale, subsystems. Such categorization is impossible to achieve using traditional spectroscopy where the response to a weak oscillating (ac) electromagnetic probe is measured as a function of frequency. However, the localized excitations can be separated from each other as well as the delocalized continuum by measuring a spectral "hole" in the ordinary response while a large amplitude pump is imposed at a fixed frequency. Localized excitations will result in a very sharp "hole," and any residual couplings to other excitations, both localized and extended, will determine its detailed shape. This technique probes incoherent lifetime effects as well as coherent mixing or quantum interference phenomena, describable in terms of the Fano effect. Here we show that in a disordered Ising magnet, LiHo0.045Y0.955F4, the quality factor Q for spectral holes, the ratio of the drive frequency to their width, can be as high as 100,000. In addition, we can tune the dynamics of the quantum degrees of freedom by sweeping the quantum mixing parameter through zero via the amplitude of the ac pump as well as a static external transverse field. The zero-crossing is associated with a dissipationless response at the drive frequency. The identification of such a point where localized degrees of freedom are minimally mixed with their environment in a dense and disordered dipolar coupled spin system implies control over the bath coupling of qubits emerging from strongly interacting many-body systems
Theory of magnon-polaritons in quantum Ising materials
We present a theory of magnon-polaritons in quantum Ising materials, and
develop a formalism describing the coupling between light and matter as an
Ising system is tuned through its quantum critical point. The theory is applied
to Ising materials having multilevel single-site Hamiltonians, in which
multiple magnon modes are present, such as the insulating Ising magnet
LiHoF . We find that the magnon-photon coupling strengths may be tuned by
the applied transverse field, with the coupling between the soft mode present
in the quantum Ising material and a photonic resonator mode diverging at the
quantum critical point of the material. A fixed system of spins will not
exhibit the diamagnetic response expected when light is coupled to mobile spins
or atoms. Without the diamagnetic response, one expects a divergent
magnon-photon coupling strength to lead to a superradiant quantum phase
transition. However, this neglects the effects of damping and decoherence
present in any real system. We show that damping and decoherence may block the
superradiant quantum phase transition, and lead to weak coupling between the
soft magnon mode and the resonator mode. The results of the theory are applied
to experimental data on the model system LiHoF in a microwave resonator.Comment: 22 pages, 6 figure
- …