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Bramwell ST

Giblin SR

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Instrument: MUSR         Days Requested: 4         Experiment Number: 910392         Page: 1
 
Experiment Proposal
Experiment Number: 910392
Principal investigator(*) Professor S T Bramwell, University College London, United Kingdom
Co-investigator Mr R Aldus, University College London, United Kingdom
Co-investigator Dr T Fennell, University College London, United Kingdom
Co-investigator Dr S R Giblin, STFC, United Kingdom
Co-investigator Mr S Calder, University College London, United Kingdom
Co-investigator -
Co-investigator -
Co-investigator -
Co-investigator -
Experiment Title The search for magnetic monopoles in spin ice.
Instrument MUSR Days Requested: 4
Access Route Direct Access - New Previous RB Number: -
Science Areas Physics
Sponsored Grant No Sponsor: -
Grant Title -
Grant Number -
Start Date: - Finish Date: -
EU Access? No
Similar Submission? No
Abstract In recent work Moessner and colleagues have demonstrated that thermally induced defects in spin
ice behave as emergent magnetic monopoles. Within the material there exist magnetic "charges"
and we suggest that a signature of this may be found by measuring the muon relaxation.
Specifically it has been predicted that a high density of magnetic poles will be swept to the surface
under applied field. Therefore we would like to measure the field lines emerging from the poles in
a thin silver foil adjacent to the sample after a magnetic field has been pulsed on. Moreover once
the magnetic field is pulsed off a measurable relaxation is expected. We have previously shown
that this method of exterior muon implantation can assist interpreting conventional MuSR
measurements as well as providing a useful contrast to bulk magnetization measurements.  
Publications Phys Rev Lett 99 (26) 267203 (2007)
Phys Rev B 75 220408 (2007)
Nature Physics 3 566 (2007)
Phys Rev B 72 224411 (2005)
Science 294 1495 (2004)
Instrument: MUSR         Days Requested: 4         Experiment Number: 910392         Page: 2
ISIS Sample record sheet
 
Principal contact Professor S T Bramwell, s.t.bramwell@ucl.ac.uk, Tel: 020 7679 4648
Instrument MUSR, 4 days, preferred contact is Giblin, S R (s.r.giblin@rl.ac.uk)
Special requirements pulsed magnetic field 0-200G
SAMPLES
Material Dy2Ti2O7 
Formula Dy2Ti2O7 
Forms Single crystal
Volume -
Weight 1000 mg
Container / substrate -
Storage requirements -
Xtal details
SAMPLE ENVIRONMENT
Equipment T < 0.3K cryostat
Temperature range 50-5000 mK
Pressure range -
Magnetic field range -
Special equipment pulsed magnetic field box
required and coils
SAFETY
Hazards -
Hazard details -
Sample sensitivity -
Experimental hazards -
Sample prep hazards -
Equipment hazards -
Prep lab needed No
Special equip reqs -
Sample will be Removed By User
Instrument: MUSR         Days Requested: 4         Experiment Number: 910392         Page: 3
 
1. Background  
In spin ice materials like Ho2Ti2O7 and Dy2Ti2O7 [1] the Ising-like rare earth spins are 
analogous to hydrogen displacement vectors in water ice. Spin ice shares with water 
ice the Pauling "zero point" entropy and shows several interesting properties in 
applied magnetic field, including liquid-gas type transitions a so-called Kasteleyn 
transition and a giant entropy spike (see, for example [2]). Chemical modifications of 
spin ice include the "Berry phase" anomalous Hall effect material, Nd2Mo2O7 [3], 
while an alternative approach to spin ice has utilised nanomagnetic arrays [4]. 
 
The latest twist in the story of spin ice is the recent discovery by Moessner, 
Castelnovo and Sondhi [5] that thermally induced defects in the spin ice state behave 
as unbound magnetic monopoles. These are an emergent material property 
(monopoles in the H-field) rather than elementary cosmic particles (monopoles in the 
B-field), but from a practical point of view they are expected to behave just like 
magnetic charges, confined in the material samples. The theory of effective H-
monopoles is described in Jackson's textbook (Classical Electrodynamics): while 
there it is envisaged that these effective poles are essentially macroscopic objects, the 
difference in spin ice is that they are well defined almost down to the atomic scale. 
The magnetic scalar potential at a point r exterior to a sample containing such 
monopoles can be transformed, using Green's theorem to give the following relation:  
 
dS
r
dv
r
 
where  is the surface pole density and  the bulk pole density, and the integral is 
over surface (S) and volume (v) respectively. This means that a muon implanted 
exterior to a sample of spin ice will sense a magnetic field distribution arising from 
the bulk as well as surface poles. In the zero field cooled phase of spin ice surface 
poles are completely absent (there is zero magnetization), but there is a thermal 
population of poles in the bulk, which must generate an exterior field distribution.  As 
a field is applied these bulk poles will be swept to the surfaces, magnetizing the 
sample.  
 
In recent work (PRL, accepted subject to minor amendments) we used exterior 
implantation to investigate a spin-ice related material, and demonstrated that a good 
signal could be obtained: the method avoids any problem of sample perturbation by 
the muons and muon relaxation by hyperfine effects, which is likely to be important 
in spin ice materials (all previous SR on spin ice materials is extremely hard to 
interpret, probably for this reason). For our purposes, magnetic field detection by SR 
is superior to a bulk method, as the slowly moving poles (estimated relaxation time 1 
s) are completely static on the timescale of bulk ac-susceptibility, but partly dynamic 
on the SR time scale. Recent theoretical work (Holdsworth, submitted to Nature 
Physics) has demonstrated strong signatures of monopole diffusion in the dynamic 
response. Our plan is to do pulsed field SR in red/green mode, which is magnetic 
pulse on, magnetic pulse off, thus giving a differential measurement. When the field is 
removed the monopoles on the surface (like surface charges) slowly diffuse into the 
bulk, which should roughly maintain the rms field at the muon site but introduce an 
observable dynamic component which can be compared with theory. 
 
Instrument: MUSR         Days Requested: 4         Experiment Number: 910392         Page: 4
 
2. The Proposal 
In the experiment we will implant muons into silver positioned near to a sample of 
spin ice, Dy2Ti2O7 as in our recent work on Tb2Sn2O7 (RB620576). To illustrate that 
we can measure external fields accurately we show in Fig. 1 the field derived from the 
precession frequency of muons implanted in silver at various distances from a 
magnetized block of nickel, versus the exact theoretical expression for the stray field 
distribution in the silver[6] (arising from the nickel: some parameters were adjusted 
within acceptable bounds to get the precise fit). It should be emphasised that spin ice 
is roughly as magnetically dense as nickel so when magnetised by a very weak field 
the response will be similar.  
 
 
3. Experimental Details 
We will use a single crystal in the temperature range 0.05 - 5 K and the field range 0 - 
0.02 T (the field direction will be parallel to [110], an unimportant detail at these low 
fields where the susceptibility tensor is isotropic). We have already tested the dilution 
fridge with a pulsed transverse field of 0.02T to ensure that no significant heat load is 
induced on the fridge, the temperature was logged every 0.25s and no increase in 
temperature was observed. 
We request 4 days of time on MuSR in order to accurately map out and optimise the 
response of the muons as a function of pulsed fields and temperature.    
 
 
 
Fig. 1. With a magnetized piece of Nickel and muons implanted into a silver sample a 
known distance away from the Ni, the dipolar field can be calculated[6] and seen to 
match the experimental data. 
 
 
 [1] Bramwell and Gingras, Science 16, 1495 (2001).  [2] Moessner and Sondhi, Phys. 
Rev. B 68 064411 (2003). [3] Taguchi et al., Science 291, 2573 (2001). [4] Wang et 
al., Nature, 439, 303 (2006). [5] Castelnovo et al., Nature (2008). [6] Engel-Herbert 
et al., J. Appl. Phys, 97, 74504 (2005). 


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EXperimental Proposal - The Search for Magnetic Monopoles in Spin Ice

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