Frustrated magnets exhibit novel and useful properties, including dramatic field-sensitive properties and suppressed magnetic ordering temperatures. To elucidate the microscopic origins of this behavior we (i) grew the prototypical spinel material, Mn3O4, in our laboratory and (ii) studied it with temperature- and field-dependent light scattering. There are several key results of our study ((M. Kim et al., Phys. Rev. Lett. 104, 136402

(2010)):

(1). A phonon mode splitting at the 33 K ferrimagnetic transition in Mn3O4 (Fig. (a)) indicates that magnetic frustration in Mn3O4 is resolved via a tetragonal-to-monoclinic structural transition.

(2). The spin frustrated phase was recovered below 33 K in Mn3O4 by applying a magnetic field transverse to the ordered moment (Fig. (b)), revealing a quantum phase transition to a rare state of matter that remains disordered down to T=0K.

(3). The structural distortions in Mn3O4 can be controlled with an applied field (Fig. (c)), which may have use in field-tuned “shape memory” devices.

Our results show the important role of spin-lattice coupling in governing the novel magnetic phases and properties of magnetically frustrated materials.

0

700

]011[

]110[

(a) Temperature dependent light scattering intensity of the 295 cm-1 phonon mode in Mn3O4, showing a tetragonal-to-monoclinic distortion near 33K. (b) Field-dependence of 295 cm-1 phonon intensity at T=3K, showing that the spin-frustrated tetragonal phase can be recovered by applying a magnetic field transverse to the magnetic ordering direction. (c) Field-temperature phase diagram of Mn3O4 inferred from these data for H || [1-10].

monoclinic ferrimagnet

En

erg

y S

hift

(cm

-1)

300

290

295

3 51H (T)

En

erg

y S

hift

(cm

-1)

300

290

295

50 100 150 200 300Temperature (K)

T2=33K

(a) (b)

(c)

T (

K)

H || [1-10]

monoclinic ferrimagnet

tetragonal spin-disordered

]011[

]110[

Field-tuning magnetic frustration in the magneto-dielectric spinel Mn3O4

S. Lance Cooper, U. of Illinois at Urbana-Champaign, DMR 0856321

The research supported by this grant has had several noteworthy “broader impacts” this year:

(1). Single-crystal growth and distribution - We have used floating zone methods to grow high quality single crystal samples of the spinel material Mn3O4 (Fig. (a)), which we have distributed to many other groups in the condensed matter community, including Prof. Peter Abbamonte, U. of Illinois (temperature-dependent X-ray), Dr. Christie Nelson, Brookhaven National Laboratory (magnetic-field-dependent X-ray), Prof. Gang Cao, U. of Kentucky (magnetic susceptibility), and Prof. Raffi Budakian, U. of Illinois (magnetic force resonance microscopy).

(2). Student training - This grant supported the PhD thesis work of two female graduate students from the U. of Illinois (Fig. (b)), Minjung Kim and Yewon Gim, and a Research Experience for Undergraduates (REU) student, Jake Seeley (Haverford College). These students learned a variety of techniques, including single crystal growth, X-ray diffraction and magnetic characterization, cryogenics, high pressure- and magnetic-field methods, and optical spectroscopic methods. Further, these students had extensive practice giving scientific presentations via group meetings and conference presentations.

(3). Outreach activities (Fig. (c)) – To encourage their interest in science, on June 24, 2010, I led a tour of our high-field/pressure optical spectroscopy laboratory to K-12 students as part of the Math Science Partnership (EnLiST) program. In July 2010, I gave a presentation on our NSF supported work, “Controlling exotic matter at extreme temperatures, pressures, and magnetic fields,” to REU students.

Grad student Minjung Kim

Large single crystal rod of Mn3O4 grown in our lab using a floating zone method

REU student Jake Seeley

(a)

(b)

Grad student Yewon Gim

PI Cooper giving a tour of our high-field, high-pressure optical spectroscopy laboratory to students, parents, and teachers

(c)

Field-tuning magnetic frustration in the magneto-dielectric spinel Mn3O4

S. Lance Cooper, U. of Illinois at Urbana-Champaign, DMR 0856321