Spectroscopy methods for molecular nanomagnets

Structure and Bonding 164 (2014) 231-292

ML Baker, SJ Blundell, N Domingo, S Hill

© 2015 Springer-Verlag Berlin Heidelberg. This chapter provides a detailed overview of some of the primary spectroscopic methods that have contributed to the current understanding of molecular nanomagnets (MNs). These include: electron paramagnetic resonance (EPR); optical spectroscopy, including magnetic and X-ray magnetic circular dichroism (MCD/XMCD); inelastic neutron scattering (INS); and muon spin rotation (μ +SR). For each technique, a historical survey of the most important discoveries is provided, up to and including the most recent developments. Each section gives an introduction to the theoretical principles underpinning the techniques, as well as a description of experimental requirements and protocols. A common theme among the described spectroscopies is the fact that state-of-the-art measurements typically have to be performed at major research facilities such as synchrotrons (terahertz EPR and XMCD), high magnetic field laboratories (EPR), and accelerator facilities or reactors (INS and μ +SR). Details of such facilities are given where appropriate. Forefront issues that are addressed in the chapter include: the fundamental properties of both mono- and poly-nuclear single-molecule magnets (SMMs); the deployment of MNs in quantum information processing applications; the addressing of individual magnetic molecules on surfaces or in devices; the probing of spin dynamics in MNs using EPR, INS, and μ +SR; and studies of long-range magnetic ordering in MN crystals. An extensive list of references is provided. The chapter is intended for physicists, chemists, and materials scientists, particularly junior researchers who are just starting work in the field.

Coherent electron spin manipulation in a dilute oriented ensemble of molecular nanomagnets: pulsed EPR on doped single crystals.

Chem Commun (Camb) 50 (2014) 91-93

F Moro, D Kaminski, F Tuna, GFS Whitehead, GA Timco, D Collison, REP Winpenny, A Ardavan, EJL McInnes

Doping a Cr7Zn molecular nanomagnet into a diamagnetic and isostructural host allows pulsed X-band EPR on single crystals, including relaxation and nutation experiments on the S = 3/2 ground state.

Probing the magnetic phases in the Ni-V alloy close to the disordered ferromagnetic quantum critical point with mu SR


A Schroeder, R Wang, PJ Baker, FL Pratt, SJ Blundell, T Lancaster, I Franke, JS Moeller

Quantum spin coherence in halogen-modified Cr7Ni molecular nanomagnets

PHYSICAL REVIEW B 90 (2014) ARTN 184419

D Kaminski, AL Webber, CJ Wedge, J Liu, GA Timco, IJ Vitorica-Yrezabal, EJL McInnes, REP Winpenny, A Ardavan

Lattice-site-specific spin dynamics in double perovskite Sr2CoOsO6.

Physical review letters 112 (2014) 147202-

B Yan, AK Paul, S Kanungo, M Reehuis, A Hoser, DM Többens, W Schnelle, RC Williams, T Lancaster, F Xiao, JS Möller, SJ Blundell, W Hayes, C Felser, M Jansen

Magnetic properties and spin dynamics have been studied for the structurally ordered double perovskite Sr2CoOsO6. Neutron diffraction, muon-spin relaxation, and ac-susceptibility measurements reveal two antiferromagnetic (AFM) phases on cooling from room temperature down to 2 K. In the first AFM phase, with transition temperature TN1=108  K, cobalt (3d7, S=3/2) and osmium (5d2, S=1) moments fluctuate dynamically, while their average effective moments undergo long-range order. In the second AFM phase below TN2=67  K, cobalt moments first become frozen and induce a noncollinear spin-canted AFM state, while dynamically fluctuating osmium moments are later frozen into a randomly canted state at T≈5  K. Ab initio calculations indicate that the effective exchange coupling between cobalt and osmium sites is rather weak, so that cobalt and osmium sublattices exhibit different ground states and spin dynamics, making Sr2CoOsO6 distinct from previously reported double-perovskite compounds.

Local magnetism and spin correlations in the geometrically frustrated cluster magnet LiZn <inf>2</inf> Mo<inf>3</inf> O<inf>8</inf>

Physical Review B - Condensed Matter and Materials Physics 89 (2014)

JP Sheckelton, FR Foronda, L Pan, C Moir, RD McDonald, T Lancaster, PJ Baker, NP Armitage, T Imai, SJ Blundell, TM McQueen

LiZn2Mo3O8 has been proposed to contain S=Mo3O13 magnetic clusters arranged on a triangular lattice with antiferromagnetic nearest-neighbor interactions. Here, microwave and terahertz electron spin resonance, Li7 nuclear magnetic resonance, and muon spin rotation spectroscopies are used to characterize the local magnetic properties of LiZn2Mo3O8. These results show the magnetism in LiZn2Mo3O8 arises from a single isotropic S=electron per cluster and that there is no static long-range magnetic ordering down to T = 0.07 K. Further, there is evidence of gapless spin excitations with spin fluctuations slowing down as the temperature is lowered. These data indicate strong spin correlations, which, together with previous data, suggest a low-temperature resonating valence-bond state in LiZn2Mo3O8. © 2014 American Physical Society.

Upper critical field of NaFe1-xCoxAs superconductors

PHYSICAL REVIEW B 89 (2014) ARTN 054502

S Ghannadzadeh, JD Wright, FR Foronda, SJ Blundell, SJ Clarke, PA Goddard

Stripe disorder and dynamics in the hole-doped antiferromagnetic insulator la 5 / 3 Sr 1 / 3 CoO 4

Physical Review B - Condensed Matter and Materials Physics 89 (2014)

T Lancaster, SR Giblin, G Allodi, S Bordignon, M Mazzani, R De Renzi, PG Freeman, PJ Baker, FL Pratt, P Babkevich, SJ Blundell, AT Boothroyd, JS Möller, D Prabhakaran

We investigate the magnetic ordering and dynamics of the stripe phase of La5/3Sr1/3CoO4, a material shown to have an hourglass magnetic excitation spectrum. A combination of muon-spin relaxation, nuclear magnetic resonance, and magnetic susceptibility measurements strongly suggest that the physics is determined by a partially disordered configuration of charge and spin stripes whose frustrated magnetic degrees of freedom are dynamic at high temperature and which undergo an ordering transition around 35 K with coexisting dynamics that freeze out in a glassy manner as the temperature is further reduced. © 2014 American Physical Society.

Controlling Magnetic Order and Quantum Disorder in Molecule-Based Magnets


T Lancaster, PA Goddard, SJ Blundell, FR Foronda, S Ghannadzadeh, JS Moeller, PJ Baker, FL Pratt, C Baines, L Huang, J Wosnitza, RD McDonald, KA Modic, J Singleton, CV Topping, TAW Beale, F Xiao, JA Schlueter, AM Barton, RD Cabrera, KE Carreiro, HE Tran, JL Manson

Dipolar ordering in a molecular nanomagnet detected using muon spin relaxation

PHYSICAL REVIEW B 89 (2014) ARTN 144420

FL Pratt, E Micotti, P Carretta, A Lascialfari, P Arosio, T Lancaster, SJ Blundell, AK Powell

Quantifying magnetic exchange in doubly-bridged Cu-X(2)-Cu (X = F, Cl, Br) chains enabled by solid state synthesis of CuF(2)(pyrazine).

Chemical communications (Cambridge, England) 49 (2013) 3558-3560

SH Lapidus, JL Manson, J Liu, MJ Smith, P Goddard, J Bendix, CV Topping, J Singleton, C Dunmars, JF Mitchell, JA Schlueter

Solid state techniques involving pressure and temperature have been used to synthesize the fluoride member of the CuX(2)(pyrazine) (X = F, Cl, Br) family of coordination polymers that cannot be crystallized by solution methods. CuF(2)(pyrazine) exhibits unique trans doubly-bridged Cu-F(2)-Cu chains that provide an opportunity to quantify magnetic superexchange in an isostructural Cu-X(2)-Cu series.

$μ$SR study of magnetic order in the organic quasi-one-dimensional ferromagnet F4BImNN

Phys. Rev. B American Physical Society 88 (2013) 064423-064423

SJ Blundell, JS Möller, T Lancaster, PJ Baker, FL Pratt, G Seber, PM Lahti

Magnetic fluctuations and spin freezing in nonsuperconducting LiFeAs derivatives

PHYSICAL REVIEW B 88 (2013) ARTN 060401

JD Wright, MJ Pitcher, W Trevelyan-Thomas, T Lancaster, PJ Baker, FL Pratt, SJ Clarke, SJ Blundell

Another dimension: Investigations of molecular magnetism using muon-spin relaxation

Physica Scripta 88 (2013)

T Lancaster, SJ Blundell, FL Pratt

We review examples of muon-spin relaxation measurements on molecule-based magnetic coordination polymers, classified by their magnetic dimensionality. These include the one-dimensional s = 1/2 spin chain Cu(pyz)(NO 3)2 and the two-dimensional s = 1/2 layered material [Cu(HF2)(pyz)2]BF4. We also describe some of the more exotic ground states that may become accessible in the future given the ability to tune the interaction strengths of our materials through crystal engineering. © 2013 The Royal Swedish Academy of Sciences.

Evolution of magnetic interactions in a pressure-induced Jahn-Teller driven magnetic dimensionality switch

Phys. Rev. B American Physical Society 87 (2013) 241102-241102

S Ghannadzadeh, JS Möller, PA Goddard, T Lancaster, F Xiao, SJ Blundell, A Maisuradze, R Khasanov, JL Manson, SW Tozer, D Graf, JA Schlueter

AC magnetic measurement of LiFeAs at pressures up to 5.2 GPa: The relation between T<inf>c</inf>and the structural parameters

Journal of the Korean Physical Society 63 (2013) 445-447

S Yamaguchi, N Yamaguchi, M Mito, H Deguchi, PJ Baker, SJ Blundell, MJ Pitcher, DR Parker, SJ Clarke

The pressure effects on the 111-type Fe-based superconductor LiFeAs were investigated through AC susceptibility measurements and X-ray diffraction experiments, and revealed a correlation between the superconducting transition temperature (Tc) and the As-Fe-As bond angle (α) rather than the height of As from the Fe layers (hAs). As the pressure was increased, Tcof 17 K at P = 0 GPa decreased down to 10 K at P = 5.2 GPa. According to a previous report from an X-ray diffraction experiment, α changes from 101.5° at 0 GPa to 97.8° at 17 GPa. The obtained change in Tcis consistent with Lee et al.'s plot of Tcas a function of α, and from this result, we conclude that Tcwill fall to zero at around α = 98°. © 2013 The Korean Physical Society.

Magnetic transition and spin dynamics in the triangular Heisenberg antiferromagnet $α$-KCrO$_2$

Phys. Rev. B American Physical Society 88 (2013) 180401-180401

F Xiao, T Lancaster, PJ Baker, FL Pratt, SJ Blundell, JS Möller, NZ Ali, M Jansen

Mn(dca)<inf>2</inf>(o-phen) {dca = dicyanamide; O-phen = 1,10-phenanthroline}: Long-range magnetic order in a low-dimensional Mn-dca polymer

Polyhedron 52 (2013) 679-688

JL Manson, CM Brown, Q Huang, JA Schlueter, T Lancaster, SJ Blundell, J Singleton, JW Lynn, FL Pratt

The crystal structure, phonon density-of-states, and magnetic properties of Mn(dca)2(o-phen) (dca = dicyanamide, N(CN)2-; o-phen = 1,10-phenanthroline) have been studied using several methods including magnetization, muon-spin relaxation, and neutron and X-ray scattering. From X-ray crystallography the structure was shown to consist of octahedral MnN 6 sites that are connected via four μ1,5-bridging dca ligands while the two remaining cis-positions are occupied by N-donors of the o-phen ligand. Two Mn2+ ions are bridged together by two dca anions to afford [Mn2(dca)2(o-phen)2]2+ "dimers." These dimers are linked in 2D via μ1,5-dca ligands to afford a polymeric structure whereas the o-phen ligands act as spacers. Bulk magnetic susceptibility data show a characteristic broad maximum at 2.65 K while neutron scattering and muon-spin relaxation data establish TN = 1.85 K. For T ≪ TN, the magnetization, as obtained in pulsed-fields, saturates at 5.75 T and reaches a moment typical of an isotropic S = 5/2 ion. The magnetic structure was determined and found to consist of antiferromagnetically ordered Mn2+ moments oriented in the ac-plane with no spin-canting being evident. The magnetic space group symmetry for the Mn2+ moments is P21′/c with each Mn 2+ ion carrying a magnetic moment of 4.7(1) μB, in good agreement with the value of 5.0 μB expected for an isotropic, high-spin S = 5/2 ion. The fact that LRO is observed in Mn(dca) 2(o-phen) is unusual among low-dimensional dicyanamide complexes especially since only μ1,5-dca modes are involved.

Correlated Electrons in Quantum Matter

CONTEMPORARY PHYSICS 54 (2013) 117-118

SJ Blundell

Weak magnetic transitions in pyrochlore Bi$_2$Ir$_2$O$_7$

Phys. Rev. B American Physical Society 87 (2013) 180409-180409

PJ Baker, JS Möller, FL Pratt, W Hayes, SJ Blundell, T Lancaster, TF Qi, G Cao