236 research outputs found

### Magnetization of undoped 2-leg S = 1/2 spin ladders in La4Sr10Cu24O41

Magnetization data of single crystalline La4Sr10Cu24O41 are presented. In
this compound, doped spin chains and undoped spin ladders are realized. The
magnetization, at low temperatures, is governed by the chain subsystem with a
finite interchain coupling which leads to short range antiferromagnetic spin
correlations. At higher temperatures, the response of the chains can be
estimated in terms of a Curie-Weiss law. For the ladders, we apply the
low-temperature approximation for a S=1/2 2-leg spin ladder by Troyer et al.Comment: 2 pages, 2 figure

### Revisiting and modeling the magnetism of hole-doped CuO_2 spin chains in Sr{14-x}Ca_xCu_{24}O_{41}

Magnetization measurements of Sr{14-x}Ca_xCu_{24}O_{41} with 0 <= x <=12 in
magnetic fields up to 16 T show that the low-temperature magnetic response of
the CuO_2 spin chains changes strongly upon Ca doping. For x=0 quantum
statistical simulations yield that the temperature and field dependence of the
magnetization can be well described by an effective Heisenberg model in which
the ground state configuration is composed of spin dimers, trimers, and
monomers. For x>0 a constant contribution to the low-temperature magnetic
susceptibility is observed which cannot be explained in terms of simple chain
models. Alternative scenarios are outlined.Comment: 2 pages, submitted to the proceedings of the ICM, Kyoto, Japan,
August 200

### Pressure-induced melting of the orbital polaron lattice in La1-xSrxMnO3

We report on the pressure effects on the orbital polaron lattice in the
lightly doped manganites $\mathrm{La_{1-x}Sr_xMnO_{3}}$, with $x\sim 1/8$. The
dependence of the orbital polaron lattice on $negative$ chemical pressure is
studied by substituting Pr for La in
$\mathrm{(La_{1-y}Pr_y)_{7/8}Sr_{1/8}MnO_{3}}$. In addition, we have studied
its hydrostatic pressure dependence in
$\mathrm{(La_{0.9}Pr_{0.1})_{7/8}Sr_{1/8}MnO_{3}}$. Our results strongly
indicate that the hopping $t$ significantly contributes to the stabilization of
the orbital polaron lattice and that the orbital polarons are ferromagnetic
objects which get stabilized by local double exchange processes. The analysis
of short range orbital correlations and the verification of the Grueneisen
scaling by hard x-ray, specific heat and thermal expansion data reinforces our
conclusions.Comment: 7 figure

### High-magnetic field phase diagram and failure of magnetic Gr\"uneisen scaling in LiFePO$_4$

We report the magnetic phase diagram of single-crystalline LiFePO$_4$ in
magnetic fields up to 58~T and present a detailed study of magneto-elastic
coupling by means of high-resolution capacitance dilatometry. Large anomalies
at \tn\ in the thermal expansion coefficient $\alpha$ imply pronounced
magneto-elastic coupling. Quantitative analysis yields the magnetic Gr\"uneisen
parameter $\gamma_{\rm mag}=6.7(5)\cdot 10^{-7}$~mol/J. The positive
hydrostatic pressure dependence $dT_{\rm N}/dp = 1.46(11)$~K/GPa is dominated
by uniaxial effects along the $a$-axis. Failure of Gr\"uneisen scaling below
$\approx 40$~K, i.e., below the peak temperature in the magneto-electric
coupling coefficient [\onlinecite{toft2015anomalous}], implies several
competing degrees of freedom and indicates relevance of recently observed
hybrid excitations~[\onlinecite{yiu2017hybrid}]. A broad and strongly
magnetic-field-dependent anomaly in $\alpha$ in this temperature regime
highlight the relevance of structure changes. Upon application of magnetic
fields $B||b$-axis, a pronounced jump in the magnetisation implies
spin-reorientation at $B_{\rm SF} = 32$~T as well as a precursing phase at 29~T
and $T=1.5$~K. In a two-sublattice mean-field model, the saturation field
$B_{\rm sat,b} = 64(2)$~T enables the determination of the effective
antiferromagnetic exchange interaction $J_{\rm af} = 2.68(5)$~meV as well as
the anisotropies $D_{\rm b} = -0.53(4)$~meV and $D_{\rm c} = 0.44(8)$~meV

### Pr magnetism and its interplay with the Fe spin density wave in PrFeAsO

We have studied the magnetism of the Pr3+ ions in PrFeAsO_1-xF_x (x = 0;
0.15) and its interaction with the Fe magnetic order (for x = 0). Specific heat
data confirm the presence of a first excited crystal electric field (CEF) level
around 3.5 meV in the undoped compound PrFeAsO. This finding is in agreement
with recent neutron scattering experiments. The doped compound is found to have
a much lower first CEF splitting of about 2.0 meV. The Pr ordering in PrFeAsO
gives rise to large anomalies in the specific heat and the thermal expansion
coefficient. In addition, a field-induced transition is found at low
temperatures that is most pronounced for the magnetostriction coefficient. This
transition, which is absent in the doped compound, is attributed to a reversal
of the Fe spin canting as the antiferromagnetic Pr order is destroyed by the
external magnetic field.Comment: 8 pages, 6 figure

### Quenched charge disorder in CuO2 spin chains: Experimental and numerical studies

We report on measurements of the magnetic response of the anisotropic CuO_2
spin chains in lightly hole-doped La_x (Ca,Sr)_14-x Cu_24 O_41, x>=5. The
experimental data suggest that in magnetic fields B >~ 4T (applied along the
easy axis) the system is characterized by short-range spin order and
quasi-static (quenched) charge disorder. The magnetic susceptibility chi(B)
shows a broad anomaly, which we interpret as the remnant of a spin-flop
transition. To corroborate this idea, we present Monte Carlo simulations of a
classical, anisotropic Heisenberg model with randomly distributed, static
holes. Our numerical results clearly show that the spin-flop transition of the
pure model (without holes) is destroyed and smeared out due to the disorder
introduced by the quasi-static holes. Both the numerically calculated
susceptibility curves chi(B) and the temperature dependence of the position of
the anomaly are in qualitative agreement with the experimental data.Comment: 10 pages, REVTeX4. 11 figures; v2: Fig.2 replaced, small changes in
Figs.1 and 11; minor revisons in Sec. III.C; accepted by Phys. Rev.

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